Polymer based sensors fabricated by printing technologies

ii DIREITOS DE AUTOR E CONDIÇÕES DE UTILIZAÇÃO DO TRABALHO POR TERCEIROS Este é um trabalho académico que pode ser utilizado por terceiros desde que respeitadas as regras e boas práticas internacionalmente aceites, no que concerne aos direitos de autor e direitos conexos. Assim, o presente trabalho pode ser utilizado nos termos previstos na licença abaixo indicada. Caso o utilizador necessite de permissão para poder fazer um uso do trabalho em condições não previstas no licenciamento indicado, deverá contactar o autor, através do RepositóriUM da Universidade do Minho. Licença concedida aos utilizadores deste trabalho https://creativecommons.org/licenses/by-nc-nd/4.0/ iii ACKNOWLEDGMENTS This work wouldn’t be possible without the help of many people that contributed to it. First of all, I would like to thanks the Foundation for Science and Technology (FCT) (grant: SFRH/BDE/103649/2014) and Somatica, Materials & Solutions Lda. for the financial support to this project. A massive thank you to Professor Senentxu Lanceros-Méndez for being my supervisor and, above all, for being a truly friend on supporting my entrepreneur journey for the last 15 years. A huge thanks to Professor Gerardo Rocha to accept to be my supervisor and for all the support during this project. My gratitude to Professor Gabriela Botelho from the Chemistry Department, for her kindness in allowing me to have access to her laboratory which was fundamental to prepare the inks. Many thanks to everyone in the ESM group, but a special thanks to Juliana Oliveira, Bruna Gonçalves, Nélson Castro, Pedro Costa, Nélson Pereira, Carlos Costa, Pedro Libanio and Renato Gonçalves, who helped me to overcome different obtscules along this work. Finally, I would like to dedicate this work to my parents, specially to my mother, since she had a dream of me becaming a doctor after seeing the movie Doctor Zhivago . By the way, I still haven’t found Lara! To my parents. Knowing is not enough, We must apply. Willing is not enough, We must do. - Johann Wolfgang von Goethe - iv STATEMENT OF INTEGRITY I hereby declare having conducted this academic work with integrity. I confirm that I have not used plagiarism or any form of undue use of information or falsification of results along the process leading to its elaboration. I further declare that I have fully acknowledged the Code of Ethical Conduct of the University of Minho. v Sensores de base polimérica fabricados por tecnologias de impressão Sensores podem ser encontrados em diversos objetos do cotidiano tais como como telemóveis, equipamentos de cozinha, ecrãs tácteis, teclados, brinquedos e outras inúmeras aplicações. Desde o aparecimento do primeiro sensor em 1860, a tecnologia de sensores cresceu exponencialmente devido aos avanços realizados na ciência e engenharia de materiais, sendo capazes de serem duráveis, leves, finos, flexíveis, baratos e rápidos de produzir. Nas últimas décadas inumeros desenvolvimentos têm sido alcançados na formulação de tintas, tipos de substratos e técnicas de impressão, e atualmente os sensores flexíveis e impressos são aplicados em várias áreas tais como aeroespacial e automóvel, robótica, biomédica, eletrónica de consumo, embalagem e retalho, com uma fatia de mercado de US$ 3,6 mil milhões em 2018. Esta tese centra-se no desenvolvimento de tintas eletroativas para produzir sensores impressos em substratos poliméricos, nomeadamente tintas piezoresistivas, magnetoeléctricas e piezoeléctricas. Estas tintas foram desenvolvidas para as tecnologias de impressão por serigrafia e por spray, que são as técnicas mais utilizadas pela a indústria. A tinta piezoresistiva foi baseada no elastómero termoplástico estireno-etileno/butileno-estireno (SEBS), como matriz polimérica, e em nanotubos de carbono de paredes múltiplas (MWCNT) como “fillers” condutores, utilizando a técnica de impressão por doctor blade . Foram utilizados três solventes amigos do ambiente e as suas propriedades foram avaliadas, revelando uma boa reprodutibilidade e uma boa resposta da sensibilidade, ideal para a produção de sensores de pressão. Compósitos magnetoelétricos (ME) à base de poli(fluoreto de vinilideno-co-trifluoretileno)/ferrite de cobalto (P(VDF-TrFE)/CoFe2O4) foram impressos por spray e uma elevada resposta ME foi obtida, demonstrando assim a adequação dos materiais desenvolvidos para a produção de sensores a baixo custo e de grande escala de produção. Uma tinta piezoelétrica, baseada em poli(fluoreto de vinilideno-co-trifluoroetileno), (PVDF-TrFE), foi desenvolvida utilizando um solvente amigo do ambiente, e impressa através de três técnicas de impressão: doctor blade , spray e serigrafia, sendo posteriormente avaliadas as suas propriedades. Os filmes de PVDF-TrFE foram aplicados no desenvolvimento de uma tela táctil que permitiu a detecção da pressão e libertação dos toques exercidos. Finalmente, outro protótipo impresso foi desenvolvido, constituído por uma matriz capacitiva, e juntamente com uma eletrónica de leitura capacitiva foi obtido um touchpad capacitivo capaz de detectar vários toques em simultaneo e de identificar objetos previamente marcados. Em conclusão, foi demonstrada a possibilidade de fabricar sensores totalmente impressos utilizando diferentes tintas eletroativas e diversas técnicas de impressão. Dois protótipos totalmente funcionais foram desenvolvidos e testados, capazes de medir o toque/força e reconhecer objetos previamente marcados. Palavras-chave: Sensores impressos, piezoresistivo, piezoelétrico, magnetoelétrico, polimero vi Polymer based sensors fabricated by printing technologies Sensors can be found in everyday objects such as mobile phones, kitchen appliances, touchscreens, keyboards, toys and other applications. Since the development of the first sensor in 1860, the sensor technology has grown almost exponentially due to advances in materials science and engineering, being able to be durable, light, thin, flexible, cheap and fast to produce. In the last decades developments have been achieved in terms of ink formulation, substrate materials and printing techniques, and nowadays, flexible and printed sensors are used in many areas including aerospace and automotive, robotics, biomedical, consumer electronics, packaging and retail, with a market share of $3.6 billion in 2018. This thesis focus on the development of electroactive inks to produce printed sensors on polymeric substrates, namely piezoresistive, magnetoelectric and piezoelectric inks. These inks were developed for screen printing and spray printing technologies, which are the most used techniques in the industry. The piezoresistive ink was based on the thermoplastic elastomer styrene‐ethylene/butylene‐styrene (SEBS) as polymer matrix, and multi-walled carbon nanotubes (MWCNT) as conductive fillers, and printed by doctor blade. Three environmental friendly solvents were used and the proprieties evaluated, showing a good reproducibility and sensibility to produce pressure sensors. Magnetoelectric (ME) composites based on poly(vinylidene fluoride-co-trifluoroethylene)/cobalt ferrite (P(VDF-TrFE)/CoFe2O4) were printed by spray and a high ME response was obtained, demonstrating the suitability of the developed materials for large-scale and cost effective sensor production to be used in a wide range of applications. A piezoelectric ink, based on poly(vinylidene fluoride-co-trifluoroethylene), (PVDF-TrFE), was developed using an environmentally friendly solvent N,N0-dimethylpropyleneurea (DMPU). Three printing techniques were used: doctor blade, spray printing and screen printing, and the proprieties of the inks were evaluated. The PVDF-TrFE films were applied in the development of a touch screen which allowed the detection of both touch and release events, revealing as a good candidate for a new generation of touch panels with a low environmental impact. Finally, another printing prototype was developed capable of detecting touch and marked objects. This flexible screen printed sensor consisted on a capacitive matrix based on silver ink with diamond shape, and a polyethylene terephthalate (PET) substrate. Coupled with conventional capacitive reading electronics, a high functional capacitive touchpad with a resolution of 36 points/mm was obtained, capable of detecting several touches simultaneously and to identify marked objects. In conclusion, it was demonstrated the possibility of fabricating complete printed sensors with different electroactive inks, using several printing techniques. Two fully functional prototypes were developed and tested, capable of measuring the touch/force and recognizing marked objects. Keywords: Printed sensors, piezoresistive, piezoelectric, magnetoelectric, polymer vii Table of Contents List of figures ......................................................................................................................................... ix List of tables........................................................................................................................................... xii List of symbols ...................................................................................................................................... xiii List of acronyms ..................................................................................................................................... xv 1. Introduction ..................................................................................................................................... 1 1.1. Printed sensors .................................................................................................................... 2 1.2. Materials .............................................................................................................................. 7 1.2.1. Inks................................................................................................................................ 7 1.2.2. Substrates ..................................................................................................................... 9 1.3. Printing technologies .......................................................................................................... 10 1.3.1. Spray coating ............................................................................................................... 12 1.3.2. Screen printing ............................................................................................................ 13 1.4. Objectives ........................................................................................................................... 15 1.5. Thesis structure .................................................................................................................. 15 1.6. References ......................................................................................................................... 16 2. The influence of different solvents on the stability and performance of piezoresistive inks for flexible electronics............................................................................................................................................. 21 2.1. Introduction ........................................................................................................................ 22 2.2. Experimental ...................................................................................................................... 23 2.2.1. Materials ...................................................................................................................... 23 2.2.2. Sample preparation ..................................................................................................... 23 2.2.3. Sample characterization .............................................................................................. 24 2.3. Results and discussion ....................................................................................................... 25 2.4. Conclusions ........................................................................................................................ 31 2.5. References ......................................................................................................................... 32 3. Spray-printed magnetoelectric multifunctional composites ............................................................ 35 3.1. Introduction ........................................................................................................................ 36 3.2. Experimental ...................................................................................................................... 37 3.2.1. Materials ...................................................................................................................... 37 3.2.2. Ink preparation and printing conditions ........................................................................ 38 3.2.3. Sample characterization .............................................................................................. 38 3.3. Results and discussion ....................................................................................................... 39 xiv Qp Electrical charge F Applied force ωp Cut-off angular frequency s Complex Laplace’s domain  0 Newtonian viscosity of the solution  S Solvent Newtonian viscosity xv List of acronyms A AC Alternating current AI Artificial Intelligence B BPF Band-pass filter B2B Business to business C CPME Cyclopentyl Methyl Ether COM Communication port CAGR Compound annual growth rate CoFe2O4 Cobalt ferrite D DMPU N,N0-dimethylpropyleneurea DSC Differential scanning calorimetry DC Direct current DMPU N, N-dimethylpropyleneurea DMF N,N-dimethylformamide F FTIR Fourier transform infrared spectroscopy FE-PE Ferro-paraelectric G GF Gauge Factor I ITO Indium tin oxide I2C Inter-Integrated Circuit IC Integrated circuit IoT Internet of Things L LPF Low-pass filter M ME Magnetoelectric MEMS Microelectromechanical systems 2-MeTHF 2-Methyltetrahydrofuran MWCNT Multi-walled carbon nanotubes O OFET Organic field-effect transistor OLED Organic light-emitting diode OPV Organic photovoltaic xvi P PC Polycarbonate PCB Printed circuit board PDMS Polydimethylsiloxane PE Printed electronics PEN Polyethylene naphthalate PET Polyethylene terephthalate PU Polyurethane PVDF Polyvinylidene fluoride PVDF-TrFE Poly(vinylidene fluoride-co-trifluoroethylene) PI polyimide PEDOT:PSS poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) PCB Printed Circuit Board PS pressure sensitivity R RFID Radio frequency identification S SEBS Styrene‐ethylene/butylene‐styrene T TGA Thermogravimetric analysis TPU Thermoplastic polyurethane U UART Universal Asynchronous Receiver/Transmitter UV Ultraviolet (radiation) W WHO World Health Organization Chapter 1. Introduction 1 1. Introduction Printed and flexible sensors represent a growing market, which is estimated to reach $7.6 billion by 2027. The next generation of printed sensors will enable many applications, from human-machine interfaces to environmental sensing. These sensors benefit from the latest materials and technologies in the printed electronics industry. They have the capability to be manufactured on polymeric substrates, which offer advantages in terms of mechanical flexibility, thickness and weight reduction. This chapter introduces the concepts related to printed sensors, materials and the technologies to fabricate them. Also presents the main objectives and the structure of this thesis. Chapter 1. Introduction 2 1.1. Printed sensors When a printing method is used to develop electrical devices on a substrate, it is named Printed Electronics (PE) [1]. The first concept of printed electronics is credited to Albert Hanson when in 1903 filled a patent for “Printed wires”, but the real first printed circuit was produced only in 1936 by the Austrian engineer Paul Eisler as a part of a radio set. In 1943 the USA began to use the technology on a large scale production of radios during World War II. Printed circuit technology was released for commercial use in the USA in 1948 [2]. In the over 50 years, since its origin, printed electronics has progressed from the production of printed circuit boards (PCBs), through the ordinary use of membrane switches, to today’s radio frequency identification (RFID), smart labels, electronic skin patches, active clothing and flexible displays [3]. Due to a rapid emergence of an extensive range of nanomaterials and solution processability, the production of thin and flexible electronics to be operated at room temperature and with a cost-effective manufacturing has become possible [4]. Flexible and printed electronics continues to grow and it is being applied by most of the industries around the world, including automotive, aerospace, healthcare, consumer electronics, retail and packaging [5], among others. The main markets are the organic light-emitting diode (OLED) displays, photovoltaics, logic/memories, batteries, several types of displays, including e-paper and electroluminescent technologies, and sensors. Most of the OLED displays are not printed, however one of the main goals from the OLED industry is to have most of the technology fabricated on plastic substrates by 2020, overtaking those on glass substrates, obtaining thinner, lighter, curved and foldable displays [6] (figure 1.1 (a)). Organic photovoltaics (OPV) that can be printed is already a reality. They are very thin and light weight, which helps the installation of this device in a variety of locations, and the shape and color can also be customized [7] (figure 1.1 (b)). However the efficiency of OPV has not yet reached that of silicon solar cells. Until June of 2019, the best ratio of energy output to light input obtained was over 22 percent, by the addition of selenium on cadmium telluride solar cells [8]. Chapter 1. Introduction 3 Figure 1.1. (a) Foldable OLED display (reprinted from [9]). (b) Organic photovoltaic cells (reprinted from [10]). Flexible, thin and printed batteries has become a very active field thanks to the rise of Internet of Things (IoT), wearables and environmental sensors. All of these applications require ultra-thin, flexible, rollable or stretchable battery designs that traditional battery technologies cannot provide. Many applications are still emerging, and the requirements are fast evolving (figure 1.2) [11]. Figure 1.2. Applications of batteries with new form and structural factors [11]. Another area where printed electronics is applicable is OLED lighting, which is an emerging solidstate lighting technology. OLED lighting has the potential to emit warm light across large surfaces with good efficiency and to create new form factors into the lighting sector (figure 1.3 (a)) [12]. Chapter 1. Introduction 4 Printed and flexible sensors are a well-established subcategory of the PE market and is still growing. There are many types of printed sensors today, some with a very simple structure with a few electrodes, while others are much more complex implying multiple layer depositions. All these sensors have the capability to be manufactured on plastic substrates, which confers mechanical flexibility, thinness and weight reduction (figure 1.3 (b)) [13]. Figure 1.3. (a) Plastic flexible OLED lighting panel (reprinted from [14]). (b) Printed sensor (reprinted from ([15]). In 2018 the total market for printed, flexible and organic electronics was $31.6 billion, however the value of components fully produced by printing was $5.8 billion and it is expected to rise to $8.5 billion in 2029 (figure 1.4). Figure 1.4. Market share of printed, flexible and organic electronics in 2018 [6]. Chapter 1. Introduction 5 The largest applications using printing technologies comes from the printed sensors market, which will be the main topic in this thesis. The largest segment in the printed sensor industry, by market value, is the glucose test strips. This biosensor monitor the blood sugar levels in patients and transmit the captured data to doctors, enabling them to analyze the sensor-collected data and initiate personalized treatments and medications for patients [16]. The healthcare market is huge and a particular segment is growing in a fast pace, which is the one related to wearable electronics and sensors. According to research firm Allied Market Research , this market is projected to reach $2.26 billion by 2022, growing at a compound annual growth rate (CAGR) of 52.9% from 2016 to 2022 [17]. This growth can also be perceived by the number of publications released since 2010 (figure 1.5) [18]. Figure 1.5. Number of publications per year with titles including wearable electronics [18]. Wearable technology has a variety of applications which grows as the field itself expands and it is being incorporated into navigation systems, health fitness monitoring, advanced textiles, entertainment, and fashion industry [18]. Chapter 1. Introduction 6 Other market segments of the printed sensors are now moving into mass production, such as gas sensors which are expected to become the second largest segment in the next few years, according to the market research firm IDTechEx (figure 1.6). One of the main reasons is due to the fact that 91% of the world’s population lives in places where air quality exceeds World Health Organization (WHO) guideline limits, meaning that nearly 4.2 million deaths occurs every year as a result of exposure to ambient (outdoor) air pollution [19]. Figure 1.6. Printed and flexible sensor market segments forecast 2017-2027 according to IDTechEx . Printed gas sensors will be compact and cheap, and they will be able to detect a wide variety of gases, outdoor and indoor, in real time. This is why environmental sensing and pollution monitoring will play a huge role in the next decade [20]. Other types of printed sensors (figure 1.7) are also very important since they contribute to the development of new products and applications for different industries such as automotive, aerospace, consumer electronics, retail and packaging, among others. Chapter 1. Introduction 7 Figure 1.7. Example of different technologies used to produce printed sensors able to measure specific proprieties [13]. 1.2. Materials 1.2.1. Inks Advances in new material formulations are enabling many an increasing number of applications and with reduction of costs to produce it, leading to the growing of the flexible and printed electronics market [21]. Regarding to materials for printed electronics, both organic and inorganic are used and their selection will depend on several factors, such as cost, transparency, conductivity and other requirements needed for each type of application. Organic materials include conjugated polymers with specific characteristics such as conducting, semiconducting, photovoltaic and electroluminescent properties. Other polymers can be used but typically they are used as insulators and dielectrics. Inorganic materials, such as silver and gold nanoparticles, may be used and they achieve some goals that organics and polymers can’t provide, such as highly ordered interfaces and layers. Inks, in liquid form, must act as conductors, semiconductors, dielectrics or insulators [22]. Using one or several of these inks, it is possible to produce organic field-effect transistor (OFET), OLED, OPV, RFID, batteries, sensors, etc. [23]. Chapter 1. Introduction 14 Figure 1.12. Schematic representation of the screen printing setup [41]. Both printing techniques are widely used in the industry and have many advantages, however each one of them have their own limitations, which are illustrated in table 1.3. Table 1.3. Main characteristics of the printing techniques used on this work, and the corresponding requirements needed for the inks and substrates [31]. Printing technology Characteristics Ink’s limitations Substrate limitations Spray  Low resolution patterns  Simple process,  Low time to market;  Low-cost equipment.  Very low resolution up to 1 mm  Viscosity: 1-1000cPs  Max particle diameter 10µm;  Flash point: >45 ºC.  From small to very large size areas;  Any substrate;  Any roughness. Screen  Needs a different frame for each different pattern;  Simple process, widely used in the industry;  Ideal for small, medium and high production volumes;  Low time to market;  Typical resolution of 0.2 mm  Low-cost equipment.  Viscosity: 1k-40k cPs  Surface Tension: 25-36mN/m  Max particle diameter depends on the mesh  From small to medium size areas;  Flat and curved substrates with small roughness. Chapter 1. Introduction 15 1.4. Objectives The main objective of this work is to produce inks with appropriate characteristics for specific printing technologies and, as consequence, to obtain different types of printed sensors. The development of piezoelectric, piezoresistive and magnetoelectric inks, for screen-printing and spray coating will be performed and different printed sensors will be obtained, as well as proofs of concept of touchpads for interactive applications. In summary, the specific objectives are:  Development of inks based on different electroactive polymers with different fillers in order to tailor printability. Select the type and content of the solvent taking into account ink performance and green chemistry approaches.  Study the possibility of tailoring the desired properties with the inclusion of the selected nanofillers (conductive fillers for piezoresistive materials and magnetostrictive fillers for magnetoelectric materials).  Obtain fundamental knowledge on the printing materials through the relationship between processing, structural properties and morphology, and the micro and macroscopic response of the materials.  Study electrical and mechanical behavior, and the compatibility of the different printed layers.  Selecting the best materials and microstructures from the point of view of the selected applications.  Construction and test sensor prototypes on different substrates. 1.5. Thesis structure The presented thesis is divided in six chapters, four of them based on published or submitted scientific papers, and arranged in a sequential order corresponding to the progress of this work. Chapter 1 presents a general overview on printed sensors, materials used to develop the inks and the main printing technologies used in the industry. It is to notice that the specific state of the art related to the different works is provided in each chapters. The main objectives and the structure of the thesis are also reported in this chapter. Chapter 1. Introduction 16 Chapter 2 reports the influence of different solvents in the production of piezoresistive inks and the performance of pressure sensors based on thermoplastic elastomer styrene‐ethylene/butylene‐styrene (SEBS) and multi-walled carbon nanotubes (MWCNT). Chapter 3 is dedicated to the discussion of the performance of spray printed magnetoelectric composites, based on poly(vinylidene fluoride-co-trifluoroethylene)/cobalt ferrite P(VDF-TrFE)/(CoFe2O4), suitable for sensor and actuator applications. Chapter 4 presents the development of piezoelectric inks, based on poly(vinylidene fluoride-cotrifluoroethylene) (PVDF-TrFE), using green solvents to be used by doctor blade, spray coating and screen printing. The films obtained by these printing techniques were used to develop a touchscreen. Chapter 5 shows a flexible screen printed sensor matrix, fabricated with silver ink in a polyethylene terephthalate (PET) substrate. This functional capacitive touchpad is obtained using a diamond shaped capacitive electrodes coupled with conventional capacitive reading electronics, and also allows the identification of marked objects. Chapter 6 ends this thesis, presenting some final remarks, the main conclusions of this thesis and some suggestions for future work. 1.6. References 1. Pontius, N., “What is Printed Electronics? Learn about How Printed Electronics is Used, the Applications, Challenges, Benefits, and More”. Available on: https://www.pannam.com/blog/whatis-printed-electronics/. Accessed on 23/03/2019. 2. LPE, “Insight into the history of PCBs”. Available on: https://www.lpe.hu/en/history-of-pcb. Accessed on 24/03/2019. 3. Electronics Media, “All you Need to Know About Printed Electronics”. Available on: https://www.electronicsmedia.info/2017/07/30/what-is-printed-electronics/. Accessed on: 26/03/2019. 4. Khan, S. and Lorenzelli, L., Recent advances of conductive nanocomposites in printed and flexible electronics. Smart Mater. Struct. 2017, 26, 083001. 5. 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Romero Gómez, J., et al., Magnetocaloric effect: a review of the thermodynamic cycles in magnetic refrigeration, Renew. Sustain. Energy Rev. 17 (2013), p. 74–82. 29. Khan, A., et al., Piezoelectric thin films: an integrated review of transducers and energy harvesting, Smart Mater. Struct. (2016), p. 25. Chapter 1. Introduction 19 30. Suarato, G., et al., Role of pH-responsiveness in the design of chitosanbased cancer nanotherapeutics: a review, Biointerphases (2016), p. 11. 31. Oliveira, J., Polymer-based smart materials by printing technologies: Improving application and integration, Additive Manufacturing 21 (2018), p. 269–283. 32. IST, Available on: https://www.ist-uv.com/en/technology/uv-technology/conventional-drying-vs-uvdrying/. Accessed on: 28/04/2019. 33. IST, Available on: https://www.youtube.com/watch?time_continue=139&v=c1zfBecDSog, Accessed on: 28/04/2019. 34. Mendes-Felipe, C., State-of-the-Art and Future Challenges of UV Curable Polymer-Based Smart Materials for Printing Technologies; Adv. Mater. Technol. 2019, 4, 1800618. 35. Widawski, G., Thermal and UV-curing Behavior of Inks, Adhesives, and Coatings by Photo-, In-situ DEA and DMA. Available on: https://pdfs.semanticscholar.org/3917/09e740a5517ee64f1c88b31982ce824307d4.pdf. Accessed on: 28/04/2019. 36. Locicero, A., “The Conductive Ink and Materials Market”. Available on: https://www.inkworldmagazine.com/issues/2019-03-01/view_features/the-conductive-ink-andmaterials-market-574489/. Accessed on: 28/04/2019. 37. Zang, X., et al., Laser‐Induced Molybdenum Carbide–Graphene Composites for 3D Foldable Paper Electronics. Advanced Materials, 30(26), 1800062, 2018. 38. Bollström, R., et al., Roll-to-roll printed electronics on paper; Materials and methods, Conference paper, Proceedings of PaperCon 2012. Available on: https://www.researchgate.net/publication/230673799_Roll_to_roll_printed_electronics_on_pap er. Accessed on: 29/04/2019. 39. Choudhary, R.B., et al., Electronics with Plastics, Foils and Fabrics: The Ensuing Flexible and Hybrid Electronics; Electronics & Communications, Volume 1, Issue 1, 2018. Available on: https://crimsonpublishers.com/cojec/pdf/COJEC.000505.pdf 40. Manuela, S., et al., Printing Technologies on Flexible Substrates for Printed Electronics; IntechOpen, Chapter 3, p. 47-70, 2018. Available on: http://dx.doi.org/10.5772/intechopen.76161 41. Saengchairat, N., et al., A review: additive manufacturing for active electronic components. Journal Virtual and Physical Prototyping, Volume 12, 2017 - Issue 1 Chapter 1. Introduction 20 42. Rogers, D., “Functional and Industrial Printing Market to Grow to $114.8 Billion by 2022”. Available on: https://www.printedelectronicsnow.com/contents/view_experts-opinion/2017-08-16/functionaland-industrial-printing-market-to-grow-to-1148-billion-by-2022/46378. Accessed on: 30/04/2019. 43. Das, R., “Printing Technologies find their Place in Printed Electronics”. Available on: https://www.idtechex.com/research/articles/printing-technologies-find-their-place-in-printedelectronics-00010920.asp. Accessed on: 30/04/2019. 44. Izdebska, J., and Thomas, S., Printing on Polymers: Fundamentals and Applications, Elsevier, 2016, p. 247-261 Chapter 2. The influence of different solvents on the stability and performance of piezoresistive inks for flexible electronics 21 2. The influence of different solvents on the stability and performance of piezoresistive inks for flexible electronics This work reports on polymer-based piezoresistive sensors based on the biocompatible thermoplastic elastomer styrene‐ethylene/butylene‐styrene (SEBS) as polymer matrix and multi-walled carbon nanotubes (MWCNT) as conductive fillers. The effect of different solvent for the preparation of screen printable ink is evaluated. The MWCNT/SEBS composites have been prepared with different environmental friendly solvents, including cyclopentyl methyl ether (CPME), 2-MeTHF and p-cymene, in order to reduce the environmental impact of the sensor development. The piezoresistive response of the printed sensors developed, using this greener approach, shows good reproducibility and a pressure sensibility in the range of 0.4 to 0.8 MPa-1. Overall, the results confirm that it is possible to obtain excellent piezoresistive performance in these composites, allowing the development of a new generation of environmentally friendlier pressure sensors. Chapter 2. The influence of different solvents on the stability and performance of piezoresistive inks for flexible electronics 22 2.1. Introduction Flexible and stretchable sensing elements to monitoring systems based on smart and multifunctional materials are playing an increasing role in the development of electronic devices [1]. Important innovations in this field are based on the development of a novel generation of multifunctional materials processable by additive manufacturing technologies and, therefore, with simple integration into electronic devices [1]. In this context, research in the area of polymer composite with tailored active response is strongly growing [2], in particular with respect to functional composites reinforced with conductive fillers for sensor applications [3-5]. The piezoresistive response of these composites, variation of the electrical resistivity with an applied stimulus, allows the development of stretchable/flexible force and deformation sensing systems [6, 7]. Most of those composites are based on multi-walled carbon nanotubes (MWCNT) as active filler to provide the necessary electrical conductivity of the composite [8], based on their high aspect ratio, mechanical and electrical properties [9]. In particular, the high aspect ratio of MWCNT allows to obtain high electrical conductivity in the composites at low percolation thresholds, maintaining therefore the main mechanical characteristics of the pristine polymers, such as flexibility and stretchability [10-12]. Commercial piezoresistive sensors are based on semiconductor or metal strain gauges [13], showing several drawbacks and difficulties to be integrated into devices, mainly due to the low mechanical flexibility and sensitivity to environment conditions [14, 15]. Based on the wide range of polymers-based composites, low-cost and easy processability with additive manufacturing technologies, it is possible to design improved piezoresistive materials with high levels of flexibility, stretchability and simple integration in large area applications [16]. Polymer-based materials have been largely studied in view their intrinsic properties and applicability. Conventional strain gauges are typically attached to a polymeric substrate and their piezoresistive sensibility is near Gauge Factor (GF) ≈2 [17]. Piezoresistive materials based in polymer can reach to GF≥ 100 [13], show improved mechanical properties as well as tailorable properties and electromechanical response, due to the large range selection possibilities of polymers and fillers.. Tailoring polymer-based materials for sensor applications to be implemented by solvent based printing technologies, such as screen [3, 18, 19] or spray printing [1, 4, 13, 20], involves proper selection of solvent the polymer solvent, allowing polymer dissolution, good dispersion of the fillers, and being human and environmental friendly. Chapter 2. The influence of different solvents on the stability and performance of piezoresistive inks for flexible electronics 23 Within polymers, thermoplastic elastomers combine the best properties of thermoplastics with the stretchability of the elastomers, without vulcanization process [21, 22]. Thermoplastic elastomer styreneethylene/butylene-styrene (SEBS) triblock copolymers present excellent mechanical properties, including high maximum strain and low mechanical hysteresis, and chemical stability [13], being therefore a suitable selection for the development of force and deformation sensors, from low to large mechanical deformations. In this context, the present work reports on the processing and characterization of nanocomposite films and inks suitable for screen printing based on MWCNTs as conductive filler and the thermoplastic elastomer, styrene-ethylene-butylene-styrene as polymer matrix. Different environment-friendly solvents, including cyclopentyl methyl ether (CPME), 2-MeTHF and p-cymene were used in order to produce piezoresistive strain sensors, that were compared with samples prepared with Toluene, the most used solvent in this context [5, 22]. 2.2. Experimental 2.2.1. Materials The thermoplastic elastomer Calprene CH-6120, a SEBS copolymer with a ratio of ethylenebutylene/styrene of 68/32 and a molecular weight of 245.33 g/mol, was supplied by Dynasol. MWCNTs were supplied by Nanocyl: reference NC7000, purity of 90%, length of 1.5 µm and diameter of 9.5 nm. Cyclopentyl methyl ether (CPME) and 2-methyltetrahydrofuran (2-MeTHF) were supplied from Carlo Erba. Toluene was supplied by Fisher and p-cymene by Acros Organics. 2.2.2. Sample preparation MWCNT/SEBS composites with filler weight contents ranging from 0 to 12 wt.% were prepared by dispersing the corresponding MWCNT loading in the solvents: CPME, toluene, 2-MeTHF and p-cymene solvents within an ultrasound bath (ATU, Model ATM40-3LCD) for 4 h with temperature ranging from 25 to 35 ºC for homogeneous dispersion and for avoiding agglomerates. After achieving a good dispersion of the nanofillers, SEBS was added with an appropriate polymer/solvent ratio based on the density of the solvent in order to achieve similar viscosities for the inks: 1g: 9ml for p-cymene and 2-MeTHF, and 1g: 6ml for toluene and CPME. Then, the solution was Chapter 2. The influence of different solvents on the stability and performance of piezoresistive inks for flexible electronics 30 Although the samples are composed of the same materials (with different solvent) the electromechanical performance is a little bit different. Toluene and CPME composites show a linear behavior under cycle’s compression, with a slight decrease of the electrical resistance. Composites with 2-Me-THF present a lower electrical resistance variation (due to larger filler content) and is more sensible at lower pressures than p-cymene composite. This last composite is the material that shows lower electrical resistance variation, compared to all sensor materials. The piezoresistive behavior was quantified by the pressure sensitivity (PS), calculated after equation 2.2: 𝑃𝑆 =∆𝑅 𝑅0 ⁄ 𝑃 (2.2) where 𝑅0 is the initial electrical resistance in the unloaded state, ∆𝑅 is the electrical resistance variation and 𝑃 is the applied pressure. Figure 2.4. shows the variation of the electrical resistance as a function of the applied pressure for the four different nanocomposites. 0.0 0.2 0.4 0.6 0.8 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 5CNT/SEBS-2M 3CNT/SEBS-T 5CNT/SEBS-p 3CNT/SEBS-CPME DR/R0 Pressure (MPa) Figure 2.4. Electrical resistivity variation as a function of the applied pressure for the different nanocomposites for applied pressures up to 1 MPa. Chapter 2. The influence of different solvents on the stability and performance of piezoresistive inks for flexible electronics 31 Table 2.2. Pressure sensibility of MWCNT/SEBS composites with different solvents, for 3 and 5 wt.% of MWCNT. Sample Pressure sensibility (MPa-1) Toluene (3 wt.%) 0.73 ± 0.03 CPME (3 wt.%) 0.62 ± 0.07 2-MeTHF (5wt.%) 0.53 ± 0.08 p-cymene (5 wt.%) 0.34 ± 0.09 It is shown that the ∆𝑅 𝑅 ⁄ shows a linear behavior and decreases as the pressure applied increases for all the nanocomposites. The pressure sensibility was calculated by a linear fitting of the result. Table 2 shows that the composite with toluene 3 wt.% is the one with the higher sensibility (0.73 MPa-1), followed by CPME 3 wt.% (0.62 MPa-1), 2-Me-THF 5 wt.% (0.53 MPa-1) and p-cymene 5 wt.% (0.34 MPa-1). Despite toluene 3 wt.% is the composite with higher sensibility, this is the one that uses the most hazardous solvent. The remaining three composites comprises environmentally “friendly” solvents and despite the lower value of sensibility, in case of CPME and 2-MeTHF the values are comparable with toluene and suitable for applications. The dispersion of the MWCNT in SEBS matrix affect the electrical and electromechanical behavior for several solvent used, with p-cymene present higher conductivity but smaller electrical resistance variation for pressure applications. Depending of the filler content, all MWCNT/SEBS composites materials in function of the hazard or environment-friendly solvents can works as pressure sensor material. 2.4. Conclusions Polymer-based printable piezoresistive sensors were developed using different solvents. In a step to develop environmental friendly materials for pressure sensor applications, three different “green” solvents were used, including CPME, 2-MeTHF and p-cymene. The composites, show an excellent variation of the electrical resistance with mechanical deformation, where the resistance decreases with applied deformation and increases with decreasing deformation, showing a piezoresistive response suitable for pressure sensing applications. In particular, all composites prepared with environmental friendlier solvents show a pressure sensibility values comparable with the Chapter 2. The influence of different solvents on the stability and performance of piezoresistive inks for flexible electronics 32 non-environmentally friendly toluene, being good candidates for a new generation of piezoresistive environmental friendly inks for pressure sensors applications. 2.5. References 1. Oliveira J., et al., Polymer-based smart materials by printing technologies: Improving application and integration. Additive Manufacturing. 2018; 21: p. 269-283. 2. Narayana K.J. and Gupta Burela R., A review of recent research on multifunctional composite materials and structures with their applications. Materials Today: Proceedings. 2 ed. 2018. p. 55805590. 3. Costa P., et al., High-performance graphene-based carbon nanofiller/polymer composites for piezoresistive sensor applications. Composites Science and Technology. 2017; 153: p. 241-252. 4. Park M., et al., Design of conductive composite elastomers for stretchable electronics. Nano Today. 2014; 9(2): p. 244-260. 5. Oliveira J., et al., Stretchable scintillator composites for indirect X-ray detectors. Composites Part B: Engineering. 2018; 133: p. 226-231. 6. Sun Q., et al., Functional biomaterials towards flexible electronics and sensors. Biosensors and Bioelectronics. 2018; 119: p. 237-251. 7. Teixeira J., et al., Piezoresistive response of extruded polyaniline/(styrene-butadiene-styrene) polymer blends for force and deformation sensors, Materials & Design. 2018; 141: p. 1-8. 8. Li Y., et al., Molecular mechanics simulation of the sliding behavior between nested walls in a multiwalled carbon nanotube. Carbon. 2010; 48(10): p. 2934-2940. 9. Sahoo N.G., et al., Polymer nanocomposites based on functionalized carbon nanotubes. Progress in Polymer Science. 2010; 35(7): p. 837-867. 10. Hu N., et al., Reinforcement effects of MWCNT and VGCF in bulk composites and interlayer of CFRP laminates. Composites Part B: Engineering. 2012; 43(1): p. 3-9. 11. Otero F., et al., Study and prediction of the mechanical performance of a nanotube-reinforced composite. Composite Structures. 2012; 94(9): p. 2920-2930. 12. Joshi U.A., et al., Influence of dispersion and alignment of nanotubes on the strength and elasticity of carbon nanotubes reinforced composites. Journal of Nanotechnology in Engineering and Medicine. 2011; 2(4). Chapter 2. The influence of different solvents on the stability and performance of piezoresistive inks for flexible electronics 33 13. Gonçalves B.F., et al., Green solvent approach for printable large deformation thermoplastic elastomer based piezoresistive sensors and their suitability for biomedical applications. Journal of Polymer Science, Part B: Polymer Physics. 2016; 54(20): p. 2092-2103. 14. Lee D., et al., A prototype high sensitivity load cell using single walled carbon nanotube strain gauges. Sensors and Actuators A: Physical. 2012; 180(0): p. 120-6. 15. Kanoun O., et al., Flexible carbon nanotube films for high performance strain sensors. Sensors (Switzerland). 2014; 14(6): p. 10042-71. 16. Hossain M.S., et al., Fabrication of smart parts using powder bed fusion additive manufacturing technology. Additive Manufacturing. 2016; 10: p. 58-66. 17. Costa P., et al., Piezoresistive polymer blends for electromechanical sensor applications. Composites Science and Technology. 2018; 168: p. 353-62. 18. Gonçalves B.F., et al., Development of water-based printable piezoresistive sensors for large strain applications. Composites Part B: Engineering. 2017; 112: p. 344-52. 19. Oliveira J., et al., Increasing X-ray to visible transduction performance of Gd2O3:Eu3+PVDF composites by PPO/POPOP addition. Composites Part B: Engineering. 2016; 91: p. 610-4. 20. Oliveira J., et al., Water based scintillator ink for printed X-ray radiation detectors. Polymer Testing. 2018; 69: p. 26-31. 21. Costa P., et al., Mechanical, electrical and electro-mechanical properties of thermoplastic elastomer styrene-butadiene-styrene/multiwall carbon nanotubes composites. Journal of Materials Science. 2013; 48(3): p. 1172-9. 22. Costa P., et al., Electro-mechanical properties of triblock copolymer styrene–butadiene– styrene/carbon nanotube composites for large deformation sensor applications. Sensors and Actuators A: Physical. 2013; 201: p. 458-67. 23. Bao W.S., et al., A novel approach to predict the electrical conductivity of multifunctional nanocomposites. Mechanics of Materials. 2012; 46: p. 129-38. 24. Das D. and Satapathy B.K., Microstructure-rheological percolation-mechanical properties correlation of melt-processed polypropylene-multiwall carbon nanotube nanocomposites: Influence of matrix tacticity combination. Materials Chemistry and Physics. 2014; 147(1-2): p. 127-40. 25. Chen J., et al., A review of the interfacial characteristics of polymer nanocomposites containing carbon nanotubes. RSC Advances. 2018; 8(49): p. 28048-85. Chapter 2. The influence of different solvents on the stability and performance of piezoresistive inks for flexible electronics 34 26. Al-Saleh M.H., et al., CNT/ABS nanocomposites by solution processing: Proper dispersion and selective localization for low percolation threshold. Composites Part A: Applied Science and Manufacturing. 2013; 46: p. 53-9. 27. Bessaguet, C., et al., Electrical behavior of a graphene/PEKK and carbon black/PEKK nanocomposites in the vicinity of the percolation threshold. Journal of Non-Crystalline Solids, 2019. 512: p. 1-6. Chapter 3. Spray-printed magnetoelectric multifunctional composites 35 3. Spray-printed magnetoelectric multifunctional composites This work reports the performance of spray printed magnetoelectric (ME) composites based on poly(vinylidene fluoride-co-trifluoroethylene)/cobalt ferrite, P(VDF-TrFE)/CoFe2O4. It is shown that for a 20 wt.% ferrite content the composite exhibits a fibrillar-porous structure, ≈1.8 GPa Young’s Modulus, saturation magnetization of 11.2 emu.g-1, 6.0 emu.g-1 magnetic remanence and a magnetic coercivity of 2050 Oe. Further, it is demonstrated a 34 dielectric constant (at 10 kHz) and a 27 pC.N-1 piezoelectric coefficient. Such high dielectric and piezoelectric responses explain the ME response of 21.2 mV∙cm−1∙Oe−1 at an optimum magnetic field of 2450 Oe, which is superior to the response of similar composites prepared by bar-coating. The high ME response and the simple and scalable printing method demonstrates the suitability of these materials for cost effective and large-scale sensor/actuator applications. This chapter is based on the following publication: J. Serrado Nunes, et al., submitted on Additive Manufacturing 2019. Chapter 3. Spray-printed magnetoelectric multifunctional composites 36 3.1. Introduction Additive manufacturing is rapidly expanding and modifying the way in which products are designed, optimized, manufactured and integrated [1]. With the possibility to transform digital information into physical components, this technology is leading to new routes in the manufacturing industry by producing complex geometries with tailored material properties, freedom on the design and environmental benefits, by transforming pre-defined files into fully functional products [2]. In this scenario, printing technologies are becoming increasingly popular for the development of functional devices [3]. Particularly interesting is the development of smart materials compatible with printing technologies, such as shape-memory materials, electroactive and magnetoactive materials [4]. Smart materials obtained through printing technologies are particularly suitable for the development and implementation of printed electronics field, a highly increasing research and technological field [4]. Despite several reports regarding the development of conductors, dielectric, and semiconductor inks for different electronic components, there are still few reports of fully-printed devices [3, 4]. In particular, for an effective next generation of fully printed sensing devices and systems, it is required the development of functional inks based on smart materials, including magnetic, ferroelectric, piezoelectric and in some cases magnetoelectric [3, 4, 5]. ME materials are particularly interesting as they provide effective coupling between electric and magnetic orders, being actively used in multifunctional integrated devices [4, 6]. Since the ME response in single-phase ME materials is very small at very low temperatures, their ME coupling cannot be used in technological applications [7]. This is why most studies are related with the development of ME composites consisting of two phases, one piezoelectric and the other magnetostrictive, mechanically coupled, and which give rise to ME coefficients 3 orders of magnitude higher than the ones found in single-phase ME materials [5, 8]. Theoretical research as well as some recent experimental studies have shown that piezoelectric polymers, in addition to solve some problems found in ceramic-based ME materials such as fragility, low electrical resistance and high dielectric losses [8, 9], can allow colossal ME response in composites due to large piezoelectric stress coefficients and great displacement transfer capability of the piezoelectric polymer [6]. Polymers also offer several advantages for an effective printing of smart materials due to the higher versatility than inorganic materials, higher flexibility, suitability to be implemented in a variety of substrates, possibility of tailoring their side-chains and molecular structure, opportunity to introduce active (nano)fillers, as well as particles with specific properties into the material, enabling materials to be fabricated with specific chemical and physical properties [4]. Despite this favourable context, many Chapter 3. Spray-printed magnetoelectric multifunctional composites 37 printing methods are very specific for a given application and only a few are applicable to a wide range of materials and surfaces [4]. An effective and simple solution-based method for the preparation of polymerbased ME composite films is spray printing due to its high production velocity, efficient use of materials, good reproducibility and compatibility with different substrates [10]. Despite this favourable context for the emergence of printed polymer-based ME materials, it is highly surprising that the only work found in the literature, as far as we know, is the report of Chlaihawi et al . [11]. Nevertheless, such ME material was produced by screen printing poly(vinylidene fluoride) (PVDF) piezoelectric polymer into a magnetostrictive Metglas® substrate, not being in this way a fully-printed ME material and therefore limiting the aforementioned advantages of printed devices. Additionally the flexibility, simple fabrication (solvent based), easy shaping, the possibility of miniaturization and/or to produce large uniform areas and the absence of degradation at the piezoelectric/magnetostrictive interface are obvious advantages of nanocomposites when compared to such laminated structures [7, 8]. In this context, this paper presents a poly(vinylidene fluoride-co-trifluoroethylene)/CoFe2O4-based ink, which is then printed in a glass substrate trough spray-printing and fully characterized with respect to its functional response, including piezoelectric, magnetic and magnetoelectric responses. Poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) was selected due to its high piezoelectric response (|d33|≈30 pC.N-1) at room temperature and physicochemical stability; while CoFe2O4 was selected due to its high magnetostrictive coefficients (λ=200 ppm), high Curie temperatures, chemical stability, wear resistance and simple processability [7, 12]. 3.2. Experimental 3.2.1. Materials CoFe2O4 nanoparticles (≈35-50nm) were purchased from Nanostructured & Amorphous Materials, Inc. (Texas, USA) . N,N-dimethylformamide (DMF), pure grade, was supplied by Fluka (New Jersey, USA) and P(VDF-TrFE) was supplied by Solvay (Brussels, Belgium). All the chemicals and particles were used as received from the suppliers. Chapter 3. Spray-printed magnetoelectric multifunctional composites 38 3.2.2. Ink preparation and printing conditions The composite solutions were prepared following the general guidelines presented in the protocol [13] for the development of piezoelectric PVDF composites. In short, in order to obtain a good dispersion of the CoFe2O4 particles within the P(VDF-TrFE) matrix, the desired amount of ferrite (20% in weight percentage – wt.%) was added to DMF (10 mL) and then placed in an ultrasound bath (ATU ATM 3LCD) during 8 h to avoid a magnetic agglomeration, then 2 g of P(VDF-TrFE) were subsequently added and a CoFe2O4/ P(VDF-TrFE)/DMF-based ink was obtained. The 20 wt.% of ferrite content was chosen once it ensures a high ME coupling and flexibility [7]. Flexible films were obtained by spray-printing the CoFe2O4/ P(VDF-TrFE)/DMF ink, using a commercial airbrush (Ventus Titan Dual Action 0.25mm), at 10 cm of the glass substrate using a pressure of 3 psi. Solvent evaporation was performed inside an oven at controlled temperature of 210 °C for 10 minutes [13]. It is to notice that the solvent evaporation temperature is above the melting temperature of the polymer, but that it crystallizes in the electroactive phase when cooled down to room temperature [13]. Then, the films (≈50 μm thick) were removed from the oven and allowed to cool at room temperature. 3.2.3. Sample characterization The morphology of the P(VDF-TrFE)/CoFe2O4 printed layers were evaluated via scanning electron microscopy (SEM) with a Hitachi S-4800 set-up at 10 kV. Previouly, samples were coated with a thin gold layer using a sputter coating (Polaron, model SC502). The mechanical characterization was carried out in the tensile mode with a Shimadzu AGS-J 500N universal testing set-up. The samples were cut into rectangular geometry of 20 mm x 6 mm with a sample thickness of ≈50 μm. The Young´s modulus was calculated based on the evaluation of three replicas from the first linear slope of the stress-strain curves (in the range 0.5-1%). The standard deviation of the measurement was estimated to ±0.08 GPa. The measurements of the capacity and dielectric loss tan δ were performed with an Agilent E4980A Precision LCR Meter in the measuring frequency range of 1 kHz to 1 MHz at an applied voltage of 0.5 V. In order to obtain a plane parallel condenser geometry, Au contacts with 5 mm diameter were deposited on both sides of the samples using a Polaron SC502 sputter coater (40 nm of gold thickness). The real d from the electrical capacity (C) taking into account the geometry of the sample (thickness (d) and electrode area (A)) (equation 3.1): Chapter 3. Spray-printed magnetoelectric multifunctional composites 39 A dC '  (3.1) Magnetic hysteresis loops were measured at room temperature using a Microsenses ADE 3473-70 Technologies vibrating sample magnetometer (VSM). The poling of the samples was achieved, after an optimization procedure, (60 min of corona poling at 120 °C in a home-made chamber). In order to optimize the piezoelectric response, the electric field was kept applied when the samples were cooled to room temperature. The piezoelectric response (d33) of the poled samples was obtained with a wide range d33-meter (model 8000, APC Int. Ltd). The ME response was obtained by measuring the transversal ME voltage coefficient (α33) using the dynamic lock-in amplifier method14. A pair of Helmholtz coils was used to generate an AC magnetic field with amplitude of 1 Oe and frequency of ≈20 kHz (electromechanical resonance of the composite) that is superimposed to a DC bias field driven by an electromagnet. Both fields are applied out of plane of the nanocomposite film and the generated voltage across the sample thickness is measured using a digital Lock-in amplifier (Stanford Research SR530). The ME voltage coefficient (α33) was calculated from the measured voltage using equation 3.2: tH V AC 33  D  (3.2) where ΔV is the measured output voltage, HAC is the amplitude of the AC magnetic field, and t is the thickness of the nanocomposite film. 3.3. Results and discussion The morphology of the samples are shown in the SEM images presented in Figure 3.1. Both transversal (a) (obtained by cutting the samples after 1 minute immersion in liquid N2) and surface (b) images reveal a fibrillar microstructure induced by the printing procedure. Chapter 3. Spray-printed magnetoelectric multifunctional composites 46 17. Martins, P., et al., Effect of filler dispersion and dispersion method on the piezoelectric and magnetoelectric response of CoFe2O4/P(VDF-TrFE) nanocomposites. Applied Surface Science 2014, 313, 215-219. 18. (a) Brito-Pereira, R., et al., Magnetoelectric response on Terfenol-D/ P(VDF-TrFE) two-phase composites. Composites Part B: Engineering 2017, 120, 97-102; (b) Martins, P., et al., Linear anhysteretic direct magnetoelectric effect in Ni0.5Zn0.5Fe2O4/poly(vinylidene fluoride-trifluoroethylene) 0-3 nanocomposites. Journal of Physics D: Applied Physics 2011, 44 (48). 19. (a) Moradi, R., et al., Preparation and characterization of polyvinylidene fluoride/graphene superhydrophobic fibrous films. Polymers 2015, 7 (8), 1444-1463; (b) Yao, J.; Bastiaansen, C. W. M.; Peijs, T., High strength and high modulus electrospun nanofibers. Fibers 2014, 2 (2), 158-187. 20. Gonçalves, R., et al., Development of magnetoelectric CoFe2O4/poly(vinylidene fluoride) microspheres. RSC Advances 2015, 5 (45), 35852-35857. 21. Kisiel, A., et al., In Dielectric properties of polymer composites with the addition of ferrite nanoparticles, IOP Conference Series: Materials Science and Engineering, 2016. 22. Gutiérrez, J., et al., Synthesis, physical and magnetic properties of BaFe12O19/P(VDF-TrFE) multifunctional composites, European Polymer Journal 2015, 69, 224-231. 23. Martins, P., et al., Improved magnetodielectric coefficient on polymer based composites through enhanced indirect magnetoelectric coupling. Applied Physics Letters 2016, 109 (11). 24. Martins, P., et al., Dielectric and magnetic properties of ferrite/poly(vinylidene fluoride) nanocomposites. Materials Chemistry and Physics 2012, 131 (3), 698-705. 25. Zhou, J. P., et al., Magnetoelectric effects on ferromagnetic and ferroelectric phase transitions in multiferroic materials. Acta Materialia 2014, 76, 355-370. 26. Martins, P., et al., Determination of the magnetostrictive response of nanoparticles via magnetoelectric measurements. Nanoscale 2015, 7 (21), 9457-9461. 27. Martins, P., et al., Wide-range magnetoelectric response on hybrid polymer composites based on filler type and content. Polymers 2017, 9 (2), 62. Chapter 4. Environmental friendlier printable piezoelectric inks and their application for the development of highly sensitive all-printed touchscreens 47 4. Environmental friendlier printable piezoelectric inks and their application for the development of highly sensitive all-printed touchscreens This work reports the development of piezoelectric inks based on poly(vinylidene fluoride-cotrifluoroethylene), PVDF-TrFE, dissolved in the environmentally friendly solvent N, N0dimethylpropyleneurea (DMPU). The inks were prepared for different printing techniques, including doctor blade, spray-printing, and screen-printing, representing therefore a relevant step for the additive manufacturing of sensors, actuators and touch screens, among others. The rheological properties of the inks demonstrate that DMPU is a good solvent for this polymer and allows to obtain dense piezoelectric films with the different printing techniques, showing also similar physico-chemical and electrical properties. The highest piezoelectric coefficient |d33| = 19 pC/N is obtained for films processed by screen-printing, the piezoelectric coefficient being stable over more than six weeks. The corresponding PVDF-TrFE films were applied in the development of a touch screen by additive manufacturing, with two screen-printed silver electrodes in the form of an array of touch buttons. The all-printed piezoelectric touch-screen allows easy fabrication, therefore, improved integration into a variety of substrates, including flexible ones. This chapter is based on the following publication: J. Serrado Nunes, et al., submitted in 2019. Chapter 4. Environmental friendlier printable piezoelectric inks and their application for the development of highly sensitive all-printed touchscreens 48 4.1. Introduction Considering the rapid technological advances and the need for low cost, simple processing and large area applications, additive manufacturing technologies are becoming increasingly interesting for applications including sensors, light-emitting devices, flat panel displays, solar cells, and batteries, among others [1-3]. These applications require materials with specific properties, being piezoelectric materials particularly relevant for sensor and actuator applications [4, 5]. The most suitable piezoelectric polymer for sensor and actuator applications is poly (vinylidene fluoride) (PVDF) and its co-polymers due to their high piezoelectric d33 coefficient, ranging from −18 to −32 pC/N and high dielectric constant (ε=7-12) [6-8]. PVDF is a semi-crystalline polymer, exhibiting different crystalline phases, α, β, γ, δ and a morphology strongly dependent on the processing conditions (temperature and crystallization time), the β-phase exhibiting the largest piezoelectric properties [7-9]. PVDF shows a glass transition temperature, Tg, around -34 °C and a melting temperature, Tm, ranging between 165 ºC and 189 ºC, depending on the crystalline phase present in the polymer. The degree of crystallinity of the polymer depends on the processing method and can vary from 45% to 60% [7, 10, 11]. Within the PVDF co-polymers, poly(vinilidene fluoride-trifluoroethylene), PVDF-TrFE, is of particular interest in the context of printed electronics and additive manufacturing of sensors and actuators due to its high piezoelectric coefficient and, in particular, because at VDF/TrFE specific molar ratios, the polymer crystallizes in the polar phase, independently of the processing conditions [12]. P(VDF-TrFE) shows the ferro-paraelectric (FE-PE) transition (Curie Temperature, Tc) below the melting temperature, Tm, and for the molar ratio (VDF/TrFE) (75/25), the FE-PE transition is at a temperature of 140 ºC on heating, and the PE-FE transition is at 75ºC on cooling. Further, the glass transition temperature, Tg, is around -26 ºC and the melting temperature, Tm, at 147 ºC [13-15]. Several processing techniques have been used for the preparation of PVDF-TrFE films including solvent casting [16], electrospinning [17], Langmuir–Blodgett [18], spin-coating [19] or screen-printing [20], among others. Piezoelectric PVDF-TrFE has been used for several applications, such as, medical ultrasound transducers [21], energy harvesting [22], MEMS resonators [23], battery separator [24], sensors and actuators [25] and tissue engineering applications [26]. Among the different possible applications, piezoelectric materials and, in particular, piezoelectric polymers presents excellent characteristics for the development of touch sensors due to their accuracy, sensitivity and ability to measure dynamic events, when compared to the capacitive sensors. Thus, Chapter 4. Environmental friendlier printable piezoelectric inks and their application for the development of highly sensitive all-printed touchscreens 49 piezoelectric polymers PVDF and PVDF-TrFE are often used in this type of applications [27, 28], sometimes being processed by additive manufacturing technologies. Flexible pressure sensors have been thus developed by screen-printing with PVDF-TrFE and their nanocomposite with multi-walled carbon nanotubes (MWCNTs), the active materials being sandwiched between printed metal electrodes in a parallel plate structure placed on polyimide (PI) and polyethylene terephthalate (PET) substrates [29]. The nanocomposite presents a uniform dispersion of conductive nanofillers within the matrix and excellent response as foldable pressure sensor [29]. Further, an active matrix sensor array was also developed based on PVDF-TrFE films with top and bottom conductive PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid)) polymer layer ink deposited on PET substrates. The piezoelectric d33 response of the polymer was 32 pC.N-1 [30]. Only recently printable piezoelectric sensors based on a PVDF-TrFE polymer ink have been fabricated on flexible PET substrates, where silver was evaporated on the PET substrates to act as bottom electrodes and painted silver glue was used as the top electrode. These sensors show bending-mode sensitivity values up to 200 nCN−1, which is 4 orders of magnitude larger than the force sensitivity in the normal direction [20]. Considering the state-of-art for printable piezoelectric pressure sensors, the goal of this work is the development of piezoelectric inks for different additive manufacturing technologies, including spray printing and screen-printing, as well as to fabricate an all-printed touchscreen to demonstrate the suitability of the approach. In the area of solvent-based additive manufacturing techniques, environmental issues are particularly relevant and, therefore, inks are prepared based on the green solvent N,N0dimethylpropyleneurea (DMPU). The most common solvent used in PVDF-TrFE solutions is N,N-dimethylformamide (DMF) which it is classified as hazardous, being its replacement by environmental friendlier and less hazardous solvents a priority [31]. 4.2. Experimental 4.2.1. Materials PVDF-TrFE (Solvene 250, VDF/TrFE = 70/30) was acquired from Solvay and N,Ndimethylformamide (DMF) and N,N0-dimethylpropyleneurea (DMPU) were obtained from Merck and LaborSpirit, respectively. Chapter 4. Environmental friendlier printable piezoelectric inks and their application for the development of highly sensitive all-printed touchscreens 50 4.2.2. Ink preparation and processing of the films The inks were prepared following the general guidelines presented in [32]. PVDF-TrFE copolymer concentration in solution ranging from 6 to 23 wt.% was dissolved in DMPU or DMF solvents at 30 °C (three hours under magnetic stirring) until a transparent and homogeneous solution was obtained. After the complete dissolution of PVDF-TrFE, different manufacturing techniques were used for film preparation, as schematically represented in Figure 4.1. Figure 4.1. Schematic representation of the different techniques and procedures used for the preparation of the films. Notice that the mask was not used in the present work, as only non-patterned films were prepared. As the result of a preliminary optimization study, the solution of PVDF-TrFE with weight fraction of 6 % was selected for spray-printing of the films on a clean glass substrate with a commercial airbrush (Airbrush Kit - Ventus), at a distance of 10 cm using an airbrush pressure of 3 psi. Solutions of PVDF-TrFE with weight fractions of 17 % and 23 % were used for the preparation of the films by screen-printing by pressing the ink with a squeegee over the screen (polyester mesh with 62 monofilaments per cm) placed at a 10 mm distance of a clean glass substrate. Finally, a solution with a weight fraction of 15 % of PVDF-TrFE was used for the preparation of the films by the doctor blade coating method. Note here that all films were prepared using solutions formulated with the green solvent DMPU. DMF based solutions were only prepared for comparison purposes with respect to the properties of the polymer solutions. Independently of the processing technique used to prepare films, the DMPU solvent was evaporated at 230 ºC in an air convection oven (Binder, ED23). Then, the PVDF-TrFE films were removed from the Chapter 4. Environmental friendlier printable piezoelectric inks and their application for the development of highly sensitive all-printed touchscreens 51 oven and cooled down to 25 ºC. The final thickness of the films was ~ 30 ± 5 μm for all printing techniques. In order to optimize the piezoelectric response, the films were poled using the electrode poling method in a hot plate home-made chamber, after an optimization procedure, with an applied voltage of 1 MV/cm, poling time of 15 minutes and poling temperature of 120 ºC [33]. In the following, the different prepared PVDF-TrFE films will be identified by the experimental technique by which they were produced. 4.2.3. Characterization techniques The rheological behavior of the PVDF-TrFE polymer solutions was characterized at 25 ºC using a stress-controlled rotational rheometer MCR-300 (Anton Paar, Austria) coupled to a Couette geometry. After loading the solutions, steady shear rates were ramped from 1500 s-1 down to 10 s-1 within 5 min, immediately followed by another ramp with steady shear rates from 10 s-1 up to 1500 s-1 also within 5 min. The viscosity data obtained from the two ramps nicely overlapped indicating that flow curves were obtained under steady-state conditions and that the polymer solutions showed no thixotropy under such experimental conditions. The morphology of the PVDF-TrFE films was examined with a scanning electron microscope, Jeol JSM-5410, after the deposition of a 20 nm gold conductive layer by magnetron sputtering (Polaron SC502 apparatus). The polymer phase of the films was determined by Fourier transformed infrared spectroscopy (FTIR), using a Spectrum 100 set up from Perkin-Elmer in ATR mode over a range of 650–1750 cm−1 with a resolution of 4 cm−1. 64 scans were performed in each sample. Differential scanning calorimetry (DSC) analysis was carried out with a Mettler-Toledo DSC821e apparatus. Samples were placed in 40 µL aluminum pans with perforated lids to allow the release and removal of decomposition products and measurements were performed between 25 and 200 ºC at a heating rate of 10 ºC.min-1 for cooling and heating under a flowing nitrogen atmosphere. Degree of crystallinity (ΔXcryst) of the samples was calculated after equation 4.1: ∆Χ𝐶=Δ𝐻𝑓 Δ𝐻100 (4.1) Chapter 4. Environmental friendlier printable piezoelectric inks and their application for the development of highly sensitive all-printed touchscreens 52 where Δ Hf is the measured melting enthalpy and Δ H100 is the melting enthalpy for a 100% crystalline sample (103.4 J.g-1) [8]. The degradation temperature of the films was determined by thermogravimetric analysis (TGA) with PerkinElmer instrument, Pyris1TGA. The samples were transferred to open ceramic crucibles and analysed using a thermobalance operating between 40 and 800 ºC at a heating rate of 10 ºC.min-1 under a nitrogen flow of 50 mL/min. Dielectric measurements were performed using a Quadtech 1920 LCR precision meter. The capacity of the samples and the dielectric losses were measured at room temperature in the frequency range of 20 Hz to 1 MHz with an applied voltage of 0.5 V in samples in the geometry of a parallel plate capacitor. Circular gold electrodes of 5 mm diameter were deposited by magnetron sputtering with as Polaron SC502 apparatus. The dielectric constant was then obtained taking into consideration the geometrical characteristics of the samples. The piezoelectric d33 coefficient was measured in the poled samples with an APC YE2730A d33 meter. 4.2.4. Piezoelectric touchscreen prototype fabrication A multi-touch detection 2 × 3 sensors matrix was designed in which each sensor was individually connected to the readout electronic system to reduce cross-talking between sensors. Figure 4.2 a) shows a schematic representation of the implemented layout, comprising the screen-printed piezoelectric film and electrodes on both surfaces of the film. The electrodes were screen-printed using a silver nanoparticle ink (Novacentrix HPS-021LV). The PVDF-TrFE film with 23 wt.% was used for this prototype, which was 30 μm thick and the printing method resulted in conductive layers with an average thickness of 12 μm in each side. After printing the electrodes, the film was heated up to 80 °C for an hour to cure the ink. Then, an insulating adhesive coating was applied as protection, and the connector incorporated into the printed connection lines (Amphenol FCI 65801-012LF). The final result is a thin and elastic enough touchscreen that keeps the functional properties even when the film is bent (Figure 4.2 b). Chapter 4. Environmental friendlier printable piezoelectric inks and their application for the development of highly sensitive all-printed touchscreens 53 Figure 4.2. a) Layout of the all printed touchscreen comprising a 30 μm thick PVDF-TrFE film and six silver paint printed electrodes (grey – top; light green – bottom; dark green – top and bottom). b) Photograph of the piezoelectric touchscreen. The resistance of the electrodes plus connecting wires was < 24 Ω for each sensor, while the capacitance is between 600 to 640 pF, corresponding to a saturation frequency of 10.4 MHz in the worstcase scenario. Electrical measurements were performed with a Fluke 117 multimeter. For the readout electronics, a charge amplifier was used to convert the electrical signal generated by the piezoelectric sensors and a digital acquisition system was implemented based on the ADC (analog-to-digital converter) of the microcontroller Microchip ATmega328P @ 8 MHz. The data was sent to a computer via UART (universal asynchronous receiver-transmitter) serial port, where a Java graphical application displayed the data. The ADC used 8 bits at a rate of 7.3 kS/s. The baud rate of the serial communication was 115,200 bit/s. 4.2.5. Equivalent circuit of the sensors The displaced charge in the piezoelectric sensors does not depend on the area of the electrodes nor the thickness of the film for a certain amount of force. However, larger electrodes represent the difficulty in uniform deformations along the electrodes. In fact, in the printed piezoelectric PVDF-TrFE films, the coefficient d 33 correlates the displaced electrical charge Qp (in C) with the applied force F (in N) by: 𝑄𝑝=𝑑33 ∙𝐹 (4.2) Chapter 4. Environmental friendlier printable piezoelectric inks and their application for the development of highly sensitive all-printed touchscreens 54 Therefore, the displaced charge Qp can be represented by a charge source, instead of a voltage source or a current source. As shown in Figure 4.3 – where the subscript p indicates the piezoelectric element –, the electrodes contribute with a capacitance Cp (in F) and a resistance Rp (in Ω) which influences the maximum voltage and current. The current Ip (in A) that flows through the circuit, given by the derivative of Qp over time t (in s): 𝐼𝑝=𝑑𝑄𝑝 𝑑𝑡 (4.3) allows representing the charge source as a current source. a) b) c) Figure 4.3. Equivalent model of a piezoelectric sensor: a) generic piezoelectric component; b) considering the capacitance and resistance of the electrodes; c) charge source represented by a controlled current source. Together, Rp and Cp create a low-pass filter (LPF) and the resulting transfer function of the sensor in Laplace’s domain (for Fourier transform use s = j ω) Hp ( s ) is: 𝐻𝑝(𝑠)=𝑄𝑜𝑢𝑡(𝑠) 𝑄𝑝(𝑠) =1 𝑠 𝜔𝑝+1, 𝜔𝑝=1 𝐶𝑝𝑅𝑝 (4.4) where Qp ( s ) and Qout ( s ) are the total charge displacement in the piezoelectric element and its actual output, respectively; s is the complex Laplace’s domain, and ω p the cut-off angular frequency (in rad/s) of the LPF. Qp Cp Rp 𝐼𝑝=𝑑𝑄𝑝 𝑑𝑡 Cp Rp Chapter 4. Environmental friendlier printable piezoelectric inks and their application for the development of highly sensitive all-printed touchscreens 55 4.3. Results and discussion 4.3.1. Rheological characterization Figure 4.4 a) shows the flow curves of two sets of PVDF-TrFE solutions produced with different concentrations of polymer dissolved in DMPU or DMF. As concentration increases, solution viscosity increases and evolves from a Newtonian solution to a shear thinning solution. 101102103 10-1 100 101a) DMF 25 wt% 22 wt% 19 wt% 16 wt% 12 wt% 9 wt% shear viscosity (Pa.s) shear rate (s-1) DMPU 23.9 wt% 16.7 wt% 12.7 wt% 10 wt% 8.3 wt% 7 wt% 6 wt% 4.8 wt% 4.3 wt% 3.6 wt% Figure 4.4. a) Flow curves of PVDF-TrFE solutions dissolved at different concentrations in DMPU (open symbols) and DMF (solid symbols). b) Concentration dependence of the specific viscosity of DMPU based solutions (open symbols) and DMF based solutions (solid symbols). Lines and numbers indicate slopes. Inset: mechanical spectrum (storage modulus G’: triangles; loss modulus G’’: squares) for a 23.9 wt.% PVDF-TrFE solution in DMPU. Lines in the inset highlight the terminal regime with slopes of 1 and 2 for G’’ and G’ respectively. b) Chapter 4. Environmental friendlier printable piezoelectric inks and their application for the development of highly sensitive all-printed touchscreens 62 0,0 0,5 1,0 1,5 0 1 2 3 4 5a) Voltage / V Time / s 0 2 4 6 8 10 12 14 16 18 20 0 1 2 3 4 5 Voltage / V Time / s b) Figure 4.7. a) Touchpad response upon finger pressing and release, b) piezoelectric sensor under cycling a pressing and releasing events and c) sensor matrix on touch event in one of the sensors and corresponding signals for each sensor. See also the supplementary video. The output signal is almost noise-free due to the BPF implemented with a narrow passing frequency. To verify the stability of the signal, Figure 4.7 b) shows one of the sensors being continuously pressed and released. The same behavior is observed for other sensors. It is observed in Figures 4.7 c) the existence of cross-talking, which is explained due to film vibration when sensor 4 is touched. Nonetheless, it is possible to reduce this error by comparison of the various sensors with their peak value (see supplementary video). Chapter 4. Environmental friendlier printable piezoelectric inks and their application for the development of highly sensitive all-printed touchscreens 63 Thus, figure 4.8 shows that the piezoelectric sensors present an excellent response without hysteresis and that, therefore, the developed inks can be applied for the development of touch panels produced by additive manufacturing techniques. 4.4. Conclusions Piezoelectric films of poly(vinylidene fluoride-co-trifluoroethylene), PVDF-TrFE, were prepared by different additive manufacturing techniques, including, doctor blade, spray-printing and screen-printing for touch-screen application. A green solvent N,N0-dimethylpropyleneurea (DMPU) was used and the rheological properties demonstrate that this is a good solvent for PVDF-TrFE. Regardless of the printing technique, dense films were obtained with suitable morphology, polymer phase, thermal and electrical properties for piezoelectric sensing applications. Screen-printing was used to prepare piezoelectric films that, after poling, showed a piezoelectric coefficient of |d33|= 19 pC/N, showing suitable performance as touchpad with screen-printed silver electrodes, allowing to detect both touch and release events in the piezoelectric film with low cross talk due to the implemented sensor geometry and electronic readout systems based on charge amplifier with a band-pass filter (BPF). It was observed cross-talking in the sensors due to film vibration on finger touch, however, it is possible to determine which sensor was stimulated by peak signal comparison. Thus, it is concluded that the all-printed piezoelectric touch-screen developed in this work is promising for applications in printed electronics for a new generation of touch panels produced by additive manufacturing techniques, taking also into account the environmental issues with respect to ink formulation. 4.5. References 1. Perelaer, J., et al., S. Printed Electronics: The Challenges Involved in Printing Devices, Interconnects, and Contacts Based on Inorganic Materials. Journal of Materials Chemistry 2010, 20 , 8446-8453. 2 Berggren, M., et al., Organic Materials for Printed Electronics. Nature Materials 2007, 6 , 3. 3. Arias, A. C., et al., Materials and Applications for Large Area Electronics: Solution-Based Approaches. Chemical Reviews 2010, 110 , 3-24. Chapter 4. Environmental friendlier printable piezoelectric inks and their application for the development of highly sensitive all-printed touchscreens 64 4. 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Capacitive Based Force Sensing in Capacitive Touch Panels. IEEE Access 2016, 4 , 3769-3774. 29. Khan, S., et al., Flexible Pressure Sensors Based on Screen-Printed P(Vdf-Trfe) and P(VdfTrfe)/Mwcnts. IEEE Transactions on Semiconductor Manufacturing 2015, 28 , 486-493. 30. Zirkl, M., et al., An All-Printed Ferroelectric Active Matrix Sensor Network Based on Only Five Functional Materials Forming a Touchless Control Interface. Advanced Materials 2011, 23 , 20692074. 31. Prat, D., et al., Chem21 Selection Guide of Classicaland Less Classical-Solvents. Green Chemistry 2016, 18 , 288-296. 32. Ribeiro, C., et al., Electroactive Poly(Vinylidene Fluoride)-Based Structures for Advanced Applications. Nature Protocols 2018, 13 , 681. Chapter 4. Environmental friendlier printable piezoelectric inks and their application for the development of highly sensitive all-printed touchscreens 66 33. Ramadan, K. 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M., et al., Composition-Dependent Physical Properties of Poly[(Vinylidene Fluoride)-CoTrifluoroethylene]–Poly(Ethylene Oxide) Blends. Journal of Materials Science 2013, 48 , 3494-3504. 40. Correia, D. M., et al., Physicochemical Properties of Poly(Vinylidene FluorideTrifluoroethylene)/Poly(Ethylene Oxide) Blend Membranes for Lithium Ion Battery Applications: Influence of Poly(Ethylene Oxide) Molecular Weight. Solid State Ionics 2014, 268 , 54-67. 41. Hahn, B., et al., Dielectric Relaxation of the Crystal-Amorphous Interphase in Poly(Vinylidene Fluoride) and Its Blends with Poly(Methyl Methacrylate). Macromolecules 1985, 18 , 718-721. 42. Ando, Y., et al., Quantitative Confirmation of the Crystal-Amorphous Interphase in Semicrystalline Poly(Vinylidene Fluoride) and Poly (Vinylidene Fluoride)/Poly (Ethyl Methacrylate) Blends. Journal of Polymer Science Part B: Polymer Physics 1994, 32 , 179-185. 43. Pethrick, R. A. and Richards, R. W., Static and Dynamic Properties of the Polymeric Solid State, Eds., Helfand, E., D. Reidel, New York, 1982, 475 Pp. Journal of Polymer Science: Polymer Letters Edition 1983, 21 , 1019-1019. 44. Sharma, M., et al., Temperature-Dependent Dielectric Response of (1-X)Pvdf/(X)Batio3 Nanocomposite Films. Physica B: Condensed Matter 2019, 563 , 23-29. 45. Nunes-Pereira, J., et al., A Green Solvent Strategy for the Development of Piezoelectric Poly(Vinylidene Fluoride–Trifluoroethylene) Films for Sensors and Actuators Applications. Materials & Design 2016, 104 , 183-189. Chapter 5. Marked Object Recognition Multitouch Screen Printed Touchpad for Interactive Applications 67 5. Marked Object Recognition Multitouch Screen Printed Touchpad for Interactive Applications In this work, a flexible screen printed sensor matrix is fabricated based on silver ink in a polyethylene terephthalate (PET) substrate. Diamond shaped capacitive electrodes coupled with conventional capacitive reading electronics enables fabrication of a highly functional capacitive touchpad, and also allows for the identification of marked objects. This chapter is based on the following publication: J. Serrado Nunes, et al., Marked Object Recognition Multitouch Screen Printed Touchpad for Interactive Applications; Sensors 2017, 17(12), 2786. Chapter 5. Marked Object Recognition Multitouch Screen Printed Touchpad for Interactive Applications 68 5.1. Introduction Innovations in the areas of materials development and production are strongly affecting information and communication technologies, with a strong impact in a large variety of human activities, ranging from industrial processes to entertainment. Ubiquitous computation paradigms, tangible user interfaces, and physical computation have been playing an increasing role in the human–computer interaction field, giving rise to a new perspective of the fusion between computer and materials [1]. Circuit printing methods and sensitive electronics are enabling materials and digital technology to work together as complementing parts of novel devices, allowing to present information in a large variety of forms, colours, and textures [2–4] and at the same time allowing for the development of a new generation of sensors and actuators. In this context, the touch sensors market and applications have experienced a strong growth mostly based on capacitive technologies, since the iPhone® had its debut back in 2007 [5, 6]. The projected capacitive technology can be implemented through two reading methods: the selfcapacitance method and the mutual-capacitance. The self-capacitance consists in analysing the capacitance of the electrodes to ground, when a finger is placed close together to the electrode. This allows the detection of just two fingers, though the hardware is of lower cost. On the other hand, the mutual-capacitance is a more robust method based on the mutual capacitance between two electrodes, where one electrode acts as a driver and the other one acts as a sensor. This method allows the detection of several touches, though it implies higher hardware costs. In both methods, the matrix of electrodes is scanned at high frequency [6, 7]. Capacitive touch technology and market share is covered quite well in the literature [6, 7]. Capacitive surfaces are composed by a dielectric substrate such as glass, acrylic, or polyester, and a coating based on a conductive transparent material. Indium tin oxide (ITO) conductive coating is the most used material in order to achieve the required conductivity and transparency, but the ITO deposition technology is not cost efficient [8, 9], paving the way for replacement by printed technology solutions. Together with transparent capacitive touch surfaces for big monitors, smartphones, and tablets, there is an increasing demand of opaque capacitive applications as well, such as keyboards [10], touchpads [11], position tracking surfaces [12, 13], proximity buttons [14], and pen-shaped haptic devices [15]. Chapter 5. Marked Object Recognition Multitouch Screen Printed Touchpad for Interactive Applications 69 Other applications that do not require transparency, such as humidity sensors, can also use capacitance variations to measure physical variables. These sensors can be produced by printing conductive ink over a polymer substrate [16, 17]. The scientific and technological communities are addressing polymer-based sensors and sensor matrices for an increasing number of applications, as they are characterized by a cost-effective production based on additive manufacturing, being able to be produced on large scale [18, 19]. Presenting further advantages, printing electronic components on plastic substrates offers devices the potential of being thin, foldable/rollable, lightweight, and wearable [20]. Thus, there is a growing trend on the development of polymers and polymer composites for the development of new devices [21–23]. In this work, we present a method for producing an opaque projected capacitive touch surface, based on the mutual-capacitance method, with the corresponding readout electronics and software data treatment. Using a screen printing method and silver ink, a Melinex® substrate, and an adhesive PET encapsulation, a functional capacitive surface was fabricated. This surface also incorporates an object recognition module and multi-functional touch input for smart books or smart surface applications, among others. These functionalities enhance multimedia interaction content for development purposes and demonstrate the important possibilities offered by polymer-based materials for a new generation of lowcost capacitive touchscreens. 5.2. Experimental 5.2.1. Printed Geometry Capacitive sensors may have many different geometries, each with advantages and disadvantages as it is explored in [24]. In the present work, the selected geometry is based on a diamond pattern, which is one of the most commonly used. In such a pattern, the capacitors are spread over two layers and arranged in a diamond-like structure [9, 25, 26]. The dielectric material is between rows and columns, thus providing a capacitance between each intersection, providing the possibility to determine the touch location when the capacitance changes with the approximation of any surface with enough conductivity, such as skin. Figure 5.1 shows the geometries used for the development of the touchpad with an area of 102 x 67 mm2, and the corresponding layers. Chapter 5. Marked Object Recognition Multitouch Screen Printed Touchpad for Interactive Applications 70 Every capacitive square in the electrodes has an area of 16 mm2 except on the borders, which are 8 mm2 triangles, and the corners with an area of 4 mm2. With this resolution, it is possible to detect human fingertip size conductive objects. A similar size matrix with higher resolution would require a capacitive controller with a higher number of channels than the one used in the present work. Lowering the resolution could still be suitable to detect larger conductive areas like hands or feet. For nontransparent printed touchpads, conductive silver ink is a suitable choice, as it can be simply cured in a normal oven at a constant temperature, without temperature curve requirements or special treatments. In this work, the selected silver ink was HPS-21LV from Novacentrix® (Austin, TX, USA), which was cured at 120ºC in a P Selecta oven model 2005165 for 30 min. The used substrate was Melinex® (Lohmann Technologies UK Ltd., Milton Keynes, UK), which is a PET sheet with an ink adhesion treatment for improved printing process. (a) (b) Figure 5.1. Illustration of the capacitive matrix silver electrodes geometry: (a) diamond electrodes pattern front view; (b) diamond electrodes pattern perspective view. 5.2.2. Capacitive Detection Circuit The capacitive variation between the intersections on the matrix can be detected by amplitude [27, 28] or frequency [29, 30] analysis, methods that can be used to detect single point-to-point variations. However, for a matrix of capacitive sensors, a multiplexing method for rows and columns would be needed in order to sample each intersection. Using a commercial controller, this requirement is bypassed with the high number of reading channels. The capacitive controller used was the IQS550 from Azoteq Ltd. Chapter 5. Marked Object Recognition Multitouch Screen Printed Touchpad for Interactive Applications 71 (Paarl, South Africa). According to manufacturer instructions, the complete interface circuit was designed as presented in Figure 5.2 b), to obtain the touch locations of the trackpad. The digital data provided by the controller is sent through a firmware communication via I2C (Inter-Integrated Circuit) to the microcontroller dsPIC33FJ128GP804, from Microchip Technology Inc. (Chandler, ZA, USA). (a) (b) Figure 5.2. Schematic representation of the hardware: Communication PCB as I2C-USB converter (a) and capacitive controller instrumentation circuit (b). In order to transmit the data from the IQS550, another Printed Circuit Board (PCB) was built to serve as interface to a USB terminal (Figure 5.2 a)). For a direct USB communication, a FT232RL from Future Technology Devices International Ltd. (Glasgow, UK) was used, trading data with the microcontroller through a UART (Universal Asynchronous Receiver/Transmitter) connection. 5.2.3. Touchpad Fabrication The printing method was performed by screen printing with a polyester mesh with 62/64 wires. First, the matrix was printed in one side of the polymer (top print) and dried out at room temperature for 6 h. Then, the bottom side of the polymer was printed with the aid of aligning points. The completed matrix was cured then for 30 min at 120 ºC in order to evaporate the solvent. The top layer print was not immediately cured in order to avoid mechanical shrinkage or deformation of the substrate, which would highly affect the alignment when printing the bottom layer pattern. Further, an Amphenol FCI connector was used for the electrical connection, and an adhesive PET layer was applied on each side of the film 3V3 100nF 5V USB 1 2 3 4 5 Shield Shield +5V DD+ ID GND 10 MHz 10μF 22pF 22pF dsPIC33FJ128GP804-i/PT TXD RXD SDA NRST INT0 SCL XTAL1 XTAL2 VCAP PWR VDD 2 3 7 17 29 39 16 6 18 21 22 31 3040 28 23 1 44 43 24 VSS VSS AVSS VSS VDD AVDD PGD PGC /MCLR 20 4 16 15 17 26 25 21 18 7 2 1 VCCIO TXD RXD TEST VCC GND D3V3OUT D+ FT232RL GND GND AGND 100nF LM3940 IN OUT GND Shield 470nF 33µF 10nF 4.7µF 100nF Fbead 100nF 100nF 4.7 k 4.7 k PWR SDA SCL INT0 NRST GND USB Communication PCB schematic IQS550 VSS TX14 48 47 46 45 44 43 42 41 40 39 38 37 13 14 15 16 17 18 19 20 21 22 23 24 36 35 34 33 32 31 30 29 28 27 26 25 1 2 3 4 5 6 7 8 9 10 11 12 VDDTX13TX12TX11 TX10 TX9 TX8 TX7 TX6 TX5 TX4 TX3 TX2 TX1 TX0 RX9A RX8A RX7A RX6A RX5ARX4ARX3ARX2ARX1ARX0A SDA SCL VDDHI VSS VREG NRST RDY 100nF 100nF 1µF33µF 100nF 1µF 100nF Transmitters Receivers Capacitive touch controller PCB schematic Chapter 5. Marked Object Recognition Multitouch Screen Printed Touchpad for Interactive Applications 78 12. Valtonen, M., et al., Capacitive indoor positioning and contact sensing for activity recognition in smart homes. J. Ambient Intell. Smart Environ. 2012, 4, 305–334. 13. Grosse-Puppendahl, T., et al., Swiss-cheese extended: An object recognition method for ubiquitous interfaces based on capacitive proximity sensing. In Proceedings of the SIGCHI Conference on Human Factors in Computing Systems, Paris, France, 27 April–2 May 2013; ACM: New York, NY, USA. 14. Kaneswaran, K. and Arshak, K. Capacitive Interfaces for Navigation of Electric Powered Wheelchairs. In 13th International Conference on Biomedical Engineering; Springer: Berlin/Heidelberg, Germany, 2009. 15. Tian, L., et al., Image-based haptic display via a novel pen-shaped haptic device on touch screens. Multimedia Tools Appl. 2017, 76, 14969–14992. 16. Rivadeneyra, A., et al., Comparative study of printed capacitive sensors. In Proceedings of the 2015 IEEE 10th Spanish Conference on Electron Devices (CDE), Madrid, Spain, 11–13 February 2015. 17. Molina-Lopez, F., et al., All additive inkjet printed humidity sensors on plastic substrate. Sens. Actuators B Chem. 2012, 166, 212–222. 18. Song, A., et al., A novel texture sensor for fabric texture measurement and classification. IEEE Trans. Instrum. Meas. 2014, 63, 1739–1747. 19. Reis, S., et al., Fabrication and Characterization of High-Performance Polymer-Based Magnetoelectric DC Magnetic Field Sensors Devices. IEEE Trans. Ind. Electron. 2017, 64, 4928– 4934. 20. Briand, D., et al., Making environmental sensors on plastic foil. Mater. Today 2011, 14, 416–423. 21. Reis, S., et al., Optimized anisotropic magnetoelectric response of Fe61.6Co16.4Si10.8B11.2/PVDF/Fe61.6Co16.4Si10.8B11.2 laminates for AC/DC magnetic field sensing. Smart Mater. Struct. 2016, 25, 055050. 22. Reis, S., et al. Characterization of Metglas/poly (vinylidene fluoride)/Metglas magnetoelectric laminates for AC/DC magnetic sensor applications. Mater. Des. 2016, 92, 906–910. 23. Martins, P., et al., Electroactive phases of poly (vinylidene fluoride): Determination, processing and applications. Prog. Polym. Sci. 2014, 39, 683–706. 24. Akhtar, H. and Kakarala, R., A methodology for evaluating accuracy of capacitive touch sensing grid patterns. J. Disp. Technol. 2014, 10, 672–682. 25. Bouman, C.A. and Kakarala, R., Capacitive touch sensing: Signal and image processing algorithms. 2011, 7873, 78730H. Chapter 5. Marked Object Recognition Multitouch Screen Printed Touchpad for Interactive Applications 79 26. Hwang, T.-H., et al., A highly area-efficient controller for capacitive touch screen panel systems. IEEE Trans. Consum. Electron. 2010, 56, 1115–1122. 27. Marioli, D., et al., Measurement of small capacitance variations. In Proceedings of the IEEE CPEM’90 Digest. Conference on Precision Electromagnetic Measurements, Ottawa, ON, Canada, 11–14 June 1990. 28. Baglio, S., et al., A high sensitivity conditioning circuit for capacitive sensors including stray effects compensation and dummy sensors approach. In Proceedings of the 21st IEEE Instrumentation and Measurement Technology Conference (IMTC), Como, Italy, 18–20 May 2004. 29. Pintér, Á. and Dénes, I., Interface circuit for measuring small capacitance changes in sensor networks. IET Sci. Meas. Technol. IET 2015, 9, 570–578. 30. Yang, C., et al., Configurable Hardware-Efficient Interface Circuit for Multi-Sensor Microsystems. In Proceedings of the 5th IEEE Conference on Sensors, Daegu, Korea, 22–25 October 2006. Chapter 6. Final remarks, conclusions and future work 80 6. Final remarks, conclusions and future work This chapter presents some final remarks related to the printed sensors global market, as well the main conclusions of the present work, which was devoted to the development of polymer-based sensors using different types of inks and printing technologies. A few suggestions for future works are also provided. Chapter 6. Final remarks, conclusions and future work 81 6.1. Final remarks In order to understand the big picture of this work in a global economic perspective, it is important to point out some facts that are linked with the printed and flexible sensor technology. As mentioned in chapter 1, the market of printed electronics is still growing, and mainly because of the Internet of Things (IoT). IoT is already being used in multiple application areas like autonomous driving, smart cities, industrial applications, home and building automation, among others [1]. As IoT growths, more and different types of sensors are needed, and some features are required such as lowcost, lightweight, ease of production and integration technology that can sense, store information securely, and transmit data [2]. For these reasons, printed memories, printed sensors, and communications devices will play a very important role as a part of the IoT market. With the increasing number of device connections it is expected to have a significant intensification of data transmission and required bandwidth, and to diminish this issue, the use of artificial intelligence (AI) in smart sensors will contribute to reduce the communication infrastructure [1]. This will allow to have sensor networks able to communicate with each other, collecting large amounts of data and actuating accordingly. These sensor networks could be applied in agriculture, environment, security, industrial technology and entertainment, among other areas [3]. The IoT allows for virtually endless opportunities and connections to take place, many of which we cannot even think of or fully understand the impact of today [4]. In 2018 the number of IoT devices reaches 7 billion, not including smartphones, tablets, laptops or fixed line phones. The worldwide connection growth is mainly driven by IoT devices, both by the consumer (e.g., Smart Home) as well as by enterprises/B2B (e.g., connected machinery). The number of IoT devices that are active is expected to reach 10 billion by 2020 and will double by 2025 (figure 6.1.) [4]. Chapter 6. Final remarks, conclusions and future work 82 Figure 6.1. Number of active IoT and Non-IoT device connections from 2015 to 2025. The global market for IoT (end-user spending on IoT solutions) was $150 billion on 2017 and, due to the market acceleration for IoT, it is expected to reach $1.6 trillion by 2025, at a compound annual growth rate (CAGR) of 39% (figure 6.2.) [4]. Figure. 6.2. Global IoT market forecast up to 2025. Chapter 6. Final remarks, conclusions and future work 83 The IoT will impact our lives, and many opportunities and challenges are yet to come as more and more devices start to join the IoT, and the role of smart materials is key [5]. This work was co-financed by Fundação para a Ciência e a Tecnologia (FCT) and by the company Somatica, Materials & Solutions, Lda, which is the market leader in Portugal of input devices for the industry (keypads, keyboards, control panels, trackballs, touchpads, etc.). With this work, Somatica had the goal to achieve new materials that could be used to manufacture its own input devices, and thus be more internationally competitive and increase its exports. Due to the successful results obtained, the company is already analyzing the possibility to implement some of the developed inks to produce customized keyboards for different industries. References: [1] Silicon Semiconductor, “AI In Sensors For IoT”. Available on: https://siliconsemiconductor.net/article/106227/AI_In_Sensors_For_IoT. Accessed on: 25/06/2019. [2] Pontius, N., “What is Printed Electronics? Learn about How Printed Electronics is Used, the Applications, Challenges, Benefits, and More”. Available on: https://www.pannam.com/blog/what-isprinted-electronics. Accessed on: 25/06/2019. [3] Brewer, D. “AI Needs Printed Electronics for Sensor Arrays”. Available on: https://www.eetimes.com/author.asp?section_id=36&doc_id=1334119#. Accessed on: 25/06/2019. [4] IOT Analytics, “State of the IoT 2018: Number of IoT devices now at 7B – Market accelerating”. Available on: https://iot-analytics.com/state-of-the-iot-update-q1-q2-2018-number-of-iot-devices-now-7b/. Accessed on: 25/06/2019. [5] Morgan, J., “A Simple Explanation Of 'The Internet Of Things'. Available on: https://www.forbes.com/sites/jacobmorgan/2014/05/13/simple-explanation-internet-things-thatanyone-can-understand/#3edf95631d09. 25/06/2019. Chapter 6. Final remarks, conclusions and future work 84 6.2. Conclusions The development of polymer based sensors has been demonstrated as a suitable approach for the fabrication of new flexible and printed sensors to be applied in many applications on printed electronics industry. These sensors can be made by using different types of inks, where specific materials and solvents are selected achieve specific requirements characteristics and proprieties. The selection of the polymer substrate is also an important parameter to take into account in order to guarantee a good adhesion of the ink and to control the resulting printed feature’s shape, position and structure. In this scope, the presented work was focused on the development of novel functional inks using different printing technologies, such as doctor blade, spray coating and screen printing, which are one of the most used techniques in commercial printed electronics devices in our days. Thus, polymer-based piezoresistive sensors were developed and printed using three different types of “green” solvents: CPME, 2-MeTHF and p-cymene. The composites revealed an excellent variation of the electrical resistance with mechanical deformation, increasing and decreasing inversely with the applied deformation, showing a piezoresistive response suitable for pressure sensing applications. All composites, prepared with environmental friendlier solvents, shown close results when comparing with the non-environmentally friendly toluene solvent, being noble candidates for a new generation of piezoresistive environmental friendly inks for pressure sensors. This successfully used method, for piezoresistive ink development, allows the creation of a novel solution in this area using a non-toxic solvent approach. Further, a functional all-printed ME materials were developed using a simple upscalable method based on spray-printing, in order to fully demonstrate the potential of the technological approach. The reinforced fibrillar structure of P(VDF-TrFE)/CoFe2O4, with 20 wt.% of ferrite content, exhibits improved dielectric and piezoelectric responses when compared to a composite with a similar composition obtained by bar-coating. The ME coupling coefficient was about 21.2 mV∙cm−1∙Oe−1 at an optimum magnetic field of 2450 Oe, which is suitable for sensor/actuator applications, demonstrating the good performance of the printed ME material. The simple printing method, easy integration into devices and the possibility to be obtained over flexible and large areas, validate the developed of ME materials for applications in areas of the printed electronics and energy harvester devices. Piezoelectric films of PVDF-TrFE, using also an environmental-friendly approach with DMPU as a green solvent, were prepared by different additive manufacturing techniques, including doctor blade, spray printing and screen printing for touch-screen application. Regardless of the printing technique used, Chapter 6. Final remarks, conclusions and future work 85 dense films were obtained with suitable morphology, polymer phase, thermal and electrical properties for piezoelectric sensing applications. Piezoelectric films were obtained by screen printing and showed a piezoelectric coefficient of |d33|= 19 pC/N after poling, showing suitable performance as touchpad to detect both touch and release events in the piezoelectric film with low cross talk due to the implemented sensor geometry and electronic readout systems based on charge amplifier with a band-pass filter (BPF). A BPF frequencies between 5 to 10 Hz successfully eliminated the electrostatic noise and 50/60 Hz grid noise. The observed cross-talking in the sensors, due to film vibration caused by finger touch, was determined by peak signal comparison. At the end, an all-printed piezoelectric touch-screen, using an environmental friendly solvent, was developed showing a good promising for a new generation of touch panels produced by additive manufacturing techniques. Finally, a touch and object detection printed capacitive matrix was fabricated with a PET substrate and two printed silver layer patterns, one on each side. A commercial capacitive controller IC was used in order to convert the capacitance in each intersection to digital values, followed by communication to a microcontroller, which treats the data to be sent to a computer software application. The resolution of 36 points/mm obtained from the digital controller, connected to the printed matrix, enables handwriting applications, touch buttons, gesture detection, and hovering direction sensors. The inexpensive nature of the materials/quantities required allows the fabrication of larger surfaces for object interaction by capacitive pattern recognition. This application demonstrated the suitability of printed technologies and polymer-based material for developing low-cost smart objects/surfaces for flexible multimedia applications. Thus, this work demonstrates that polymer-based sensors fabricated by different printing technologies can be produced to be applied in large areas and be used in many applications. These printed sensors can be fully printed using the most common printing technologies in the industries, with different functional inks and environmental friendly solvents. With the growth of the IoT and AI, flexible and printed sensors will be in greater demand for the coming years. Chapter 6. Final remarks, conclusions and future work 86 6.3. Future work In the present work it has been proven that polymer sensors can be achieved using the most common printing technologies in the industry. In this perspective, several studies can be carried out in order to upscale the ink preparation for industrial production. An interesting future work to explore would be:  Improve the stabilization of the inks, by adding appropriate additives, such as dispersants. This will maintain particles well dispersed during time and avoid their agglomerations.  Develop UV curable formulations of the developed inks, since ultraviolet curing process is increasingly used in the industry. This will allow to increase the speed production of printed sensors.  Optimize and characterize all inks to other types of printing technologies such as inkjet, gravure and offset printing. These three printing technologies are also very used in the industry.