A Review on Sustainable Inks for Printed Electronics: Materials for Conductive, Dielectric and Piezoelectric Sustainable Inks

Citation: Sanchez-Duenas, L.; Gomez, E.; Larrañaga, M.; Blanco, M.; Goitandia, A.M.; Aranzabe, E.; Vilas-Vilela, J.L. A Review on Sustainable Inks for Printed Electronics: Materials for Conductive, Dielectric and Piezoelectric Sustainable Inks. Materials 2023,16, 3940. https://doi.org/10.3390/ ma16113940 Academic Editor: Parikshit Sahatiya Received: 26 April 2023 Revised: 18 May 2023 Accepted: 20 May 2023 Published: 24 May 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Review A Review on Sustainable Inks for Printed Electronics: Materials for Conductive, Dielectric and Piezoelectric Sustainable Inks Leire Sanchez-Duenas 1,* , Estibaliz Gomez 1, Mikel Larrañaga 2, Miren Blanco 1, Amaia M. Goitandia 1, Estibaliz Aranzabe 1and JoséLuis Vilas-Vilela 3 1Surface Chemistry & Nanotechnologies Unit, Fundación Tekniker, Inaki Goenaga 5, 20600 Eibar, Spain; [email protected] (E.G.); [email protected] (M.B.); [email protected] (A.M.G.); [email protected] (E.A.) 2Electronics and Communications Unit, Fundación Tekniker, Inaki Goenaga 5, 20600 Eibar, Spain; [email protected] 3Department of Physical Chemistry, Faculty of Science and Technology, University of the Basque Country, UPV/EHU, Barrio Sarriena s/n, 48940 Leioa, Spain; [email protected] *Correspondence: [email protected] Abstract: In the last decades, the demand for electronics and, therefore, electronic waste, has increased. To reduce this electronic waste and the impact of this sector on the environment, it is necessary to develop biodegradable systems using naturally produced materials with low impact on the environment or systems that can degrade in a certain period. One way to manufacture these types of systems is by using printed electronics because the inks and the substrates used are sustainable. Printed electronics involve different methods of deposition, such as screen printing or inkjet printing. Depending on the method of deposition selected, the developed inks should have different properties, such as viscosity or solid content. To produce sustainable inks, it is necessary to ensure that most of the materials used in the formulation are biobased, biodegradable, or not considered critical raw materials. In this review, different inks for inkjet printing or screen printing that are considered sustainable, and the materials that can be used to formulate them, are collected. Printed electronics need inks with different functionalities, which can be mainly classified into three groups: conductive, dielectric, or piezoelectric inks. Materials need to be selected depending on the ink’s final purpose. For example, functional materials such as carbon or biobased silver should be used to secure the conductivity of an ink, a material with dielectric properties could be used to develop a dielectric ink, or materials that present piezoelectric properties could be mixed with different binders to develop a piezoelectric ink. A good combination of all the components selected must be achieved to ensure the proper features of each ink. Keywords: printed electronics; sustainable materials for printed electronic inks; conductive ink; dielectric ink; piezoelectric ink; biobased ink; biodegradable ink; sustainable ink 1. Introduction Printed electronics are limited by parameters such as their cost, maintenance, or the production of significant quantities of electronic waste. In the last decades, due to the increasing demand for electronics and the speed-up of programmed obsolescence, there has been an increase in the quantity of produced electronic waste (e-waste) [ 1 ]. Moreover, the materials used to produce printed electronics are usually metals, such as gold, silver, palladium, tin, or copper, because of their chemical stability and non-degradability. Using metallic materials in printed electronics ensures an appropriate electrical behavior, showing electrical conductivities up to 10 7 S/m, while other materials, such as carbon, present electric conductivities up to 10 5 S/m. Nevertheless, this represents a high consumption of precious metals, as well as a problem for society, as some of them are considered critical raw materials (CRMs) [2]. The 2023 EU list of CRMs is shown in Table 1[3]. Materials 2023,16, 3940. https://doi.org/10.3390/ma16113940 https://www.mdpi.com/journal/materials Materials 2023,16, 3940 2 of 22 Table 1. 2023 European Union Critical Raw Materials. 2023 EU CRMs Antimony Bismuth Feldspar Helium Manganese Phosphorus Tantalum Arsenic Boron Fluorspar Heavy Rare Earth Elements (HREE) Natural graphite Platinum Group Metals (PGM) Titanium metal Bauxite Cobalt Gallium Light Rare Earth Elements (LREE) Nickel Scandium Tungsten Baryte Coking coal Germanium Lithium Niobium Silicon metal Vanadium Beryllium Copper Hafnium Magnesium Phosphate rock Strontium Therefore, there is a need to develop more environmentally friendly electronic systems. This need could be achieved in two different ways: by using sustainable or biobased materials or by using biodegradable and/or recyclable materials. Sustainable or biobased materials come from a living matter source (biomass) and reduce the environmental impact. Biodegradable and/or recycled materials can degrade partially or completely after their service life, as shown in Figure 1[4]. Materials 2023, 16, x FOR PEER REVIEW 2 of 23 present electric conductivities up to 105 S/m. Nevertheless, this represents a high consumption of precious metals, as well as a problem for society, as some of them are considered critical raw materials (CRMs) [2]. The 2023 EU list of CRMs is shown in Table 1 [3]. Table 1. 2023 European Union Critical Raw Materials. 2023 EU CRMs Antimony Bismuth Feldspar Helium Manganese Phosphorus Tantalum Arsenic Boron Fluorspar Heavy Rare Earth Elements (HREE) Natural graphite Platinum Group Metals (PGM) Titanium metal Bauxite Cobalt Gallium Light Rare Earth Elements (LREE) Nickel Scandium Tungsten Baryte Coking coal Germanium Lithium Niobium Silicon metal Vanadium Beryllium Copper Hafnium Magnesium Phosphate rock Strontium Therefore, there is a need to develop more environmentally friendly electronic systems. This need could be achieved in two different ways: by using sustainable or biobased materials or by using biodegradable and/or recyclable materials. Sustainable or biobased materials come from a living matter source (biomass) and reduce the environmental impact. Biodegradable and/or recycled materials can degrade partially or completely after their service life, as shown in Figure 1 [4]. Figure 1. Scheme of the processes of a disintegrable system. However, the development of biodegradable electronics (commonly called green electronics) with high conductivity manufactured using printing technologies in environmental conditions is currently a challenge [5,6]. There are available devices with conductive lines (such as magnesium, zinc, or iron, which are degradable in physiological environments, in certain conditions of pH and temperature). The conductive lines are deposited in a biodegradable substrate (sodium carboxymethyl cellulose, silk, etc.) and manufactured using chemical or physical vapor deposition [7]. These devices are designed to degrade in a water-based solution, making them inappropriate for applications in environments with considerable humidity [6]. To consider printed electronic devices sustainable, the substrate and the ink deposited on them should also be sustainable. Examples of various sustainable substrates i i n i in i n n i n i i i Figure 1. Scheme of the processes of a disintegrable system. However, the development of biodegradable electronics (commonly called green electronics) with high conductivity manufactured using printing technologies in environmental conditions is currently a challenge [5,6]. There are available devices with conductive lines (such as magnesium, zinc, or iron, which are degradable in physiological environments, in certain conditions of pH and temperature). The conductive lines are deposited in a biodegradable substrate (sodium carboxymethyl cellulose, silk, etc.) and manufactured using chemical or physical vapor deposition [ 7 ]. These devices are designed to degrade in a water-based solution, making them inappropriate for applications in environments with considerable humidity [6]. To consider printed electronic devices sustainable, the substrate and the ink deposited on them should also be sustainable. Examples of various sustainable substrates are rice paper, biodegradable polymers such as PLA (polylactic acid) or its derivates, silk, or cellulose paper, among others [ 7 , 8 ]. Moreover, the used deposition technique must be environmentally friendly, making inkjet printing a good deposition technique because it wastes little material [9]. Materials 2023,16, 3940 3 of 22 Most of the inks employed to produce electronic devices using inkjet printing are a combination of organic polymers and functional materials, depending on the final characteristics of the device [10]. The most common classification of inks for printed electronics is based on their electrical properties. This classification includes conductive, semi-conductive, dielectric, and resistive inks. Nevertheless, as this review tries to collect the materials of the inks, the selected classification focuses on the ink’s composition. Taking this into account, it is possible to define four main components of an ink, as shown in Figure 2. Materials 2023, 16, x FOR PEER REVIEW 3 of 23 are rice paper, biodegradable polymers such as PLA (polylactic acid) or its derivates, silk, or cellulose paper, among others [7,8]. Moreover, the used deposition technique must be environmentally friendly, making inkjet printing a good deposition technique because it wastes little material [9]. Most of the inks employed to produce electronic devices using inkjet printing are a combination of organic polymers and functional materials, depending on the final characteristics of the device [10]. The most common classification of inks for printed electronics is based on their electrical properties. This classification includes conductive, semi-conductive, dielectric, and resistive inks. Nevertheless, as this review tries to collect the materials of the inks, the ifi i n f n h ink’ i i n. T kin hi in count, it is possible to define four main components of an ink, as shown in Figure 2. Figure 2. Ink composition. 1. Functional material The functional material is responsible for giving the ink its properties. Depending on h ink’ fin f n i n i , i is possible to classify inks into three groups: conductive inks, dielectric inks, and piezoelectric inks. Conductive inks are those that, after being processed, have electrical properties. They are composed of materials such as metals (for example, silver particles), carbon derivates, or conductive polymers. Dielectric inks are formulated to have the property of isolating the place where they are deposited. The used materials could be cellulose or silicon dioxide, among others. Piezoelectric inks can Figure 2. Ink composition. 1. Functional material The functional material is responsible for giving the ink its properties. Depending on the ink’s final functionality, it is possible to classify inks into three groups: conductive inks, dielectric inks, and piezoelectric inks. Conductive inks are those that, after being processed, have electrical properties. They are composed of materials such as metals (for example, silver particles), carbon derivates, or conductive polymers. Dielectric inks are formulated to have the property of isolating the place where they are deposited. The used materials could be cellulose or silicon dioxide, among others. Piezoelectric inks can produce an electrical current when suffering a slight deformation or being deformed when an electrical current is applied. Their composition materials must exhibit this property, such as the PVDF. Materials 2023,16, 3940 4 of 22 2. Solvent The functional material is dispersed in a solvent and then mixed with the polymeric resin. 3. Polymeric resin or binder The polymeric resin acts like the binder of the ink, being responsible for the ink’s mechanical properties, its stability, or avoiding particle agglomeration. 4. Additives The last component of the inks is the additive, used to improve or secure certain properties of the mixture, such as controlling the surface tension, its rheological properties, its adhesion and wettability, or preventing agglomeration [11]. All these elements of ink’s composition have to be used in the correct proportion to ensure the functionality of the ink and its properties. These properties, such as the solid content of the ink, may vary depending on the deposition method used. For example, in technologies such as inkjet printing, liquid inks are needed. The viscosity of these types of inks is lower than pastes’ viscosities, which are designed to be deposited by screen printing or similar methods. To reach the specific properties of each type of ink, the specific components must be adjusted. The solid content of each ink will vary depending on the desired final properties as well as the functional material used. The viscosity of the ink tends to increase with increasing solid content. Inks with solid contents varying between 20% and 80% have been found. The content of additives in an ink is usually less than 5–10%, and the remaining content is a combination of a polymeric resin and the solvent where the functional material is dispersed [ 12 – 14 ]. Table 2shows the percentages of each component of screen printing and inkjet printing inks. Table 2. Ink composition. Material Screen Printing Inkjet Printing Refs. Functional material 5–70% 10–30% [13,14] Polymeric resin/Binder 20–50% 5–30% [13,14] Solvent 15–65% 60–90% [13,14] Additives <5–10% <5–10% [13,14] To ensure the sustainability of the formulated inks, it is important to select the materials that make up most of the ink. In this review, the different functional materials that can be used to develop sustainable inks, as well as the solvents and binders used, are collected. Additives, being a small percentage of the whole ink, are not analyzed. To be considered sustainable, the formulated ink must follow one of these criteria: 1. It is a biobased ink. Most of its compounds are produced from a natural or renewable source. Some examples are materials produced from biomass (produced from agricultural waste), obtained from a biogenic process, or produced using a sustainable route, among others. As with biobased plastics, currently, there is no rule that measures the sustainability of an ink based on its biobased content [15]. 2. It is a biodegradable ink. Most of its compounds degrade partially or totally in a reasonable period. For polymers, the degradation is measured by the UNE-EN ISO 14855 rule [ 16 ], which measures the biodegradation of the polymer in ambient compost, or the UNE-EN ISO 14852 [ 17 ], measuring the biodegradability in aqueous media, at 20–25 ◦C for 6 months. 3. Its compounds are not considered critical raw materials or harmful to the environment. The resultant ink could be also a combination of these criteria, as shown in Figure 3. Each material used, if sustainable, could be classified in a section of Figure 3. An aspect to consider when developing sustainable inks is the fabrication and material costs. If the developed ink is extremely expensive, it would not be competitive with commercial ones. Materials 2023,16, 3940 5 of 22 Being too expensive prevents these types of inks from achieving their main objective: to reduce electronic waste. Materials 2023, 16, x FOR PEER REVIEW 5 of 23 The resultant ink could be also a combination of these criteria, as shown in Figure 3. Each material used, if sustainable, could be classified in a section of Figure 3. An aspect to consider when developing sustainable inks is the fabrication and material costs. If the developed ink is extremely expensive, it would not be competitive with commercial ones. Being too expensive prevents these types of inks from achieving their main objective: to reduce electronic waste. Figure 3. Criteria to classify a sustainable ink. 2. Composition of Inks 2.1. Functional Material 2.1.1. Conductive Inks Conductive inks are generally composed of a functional material (or its precursor), which gives the ink its electrical conductivity properties. If the ink is composed of the precursor, the metal particles are prepared with bottom-up methods, decomposing the precursor’s molecules thermally or through the reduction of metal salts reacting with a reduction agent. Depending on the functional material used, conductive inks could be classified into three separate groups: (1) conductive inks formed of metallic particles, (2) carbon-based conductive inks, or (3) particle-free conductive inks. Most conductive inks use metallic materials, such as silver or copper, to achieve good electric conductivity. However, some of the used materials are considered critical raw materials or are harmful to some species (such as silver for submarine life), entailing a risk to the environment [18]. Carbon-based materials are the second family of materials used to give inks electrical conductivity. These materials have demonstrated good conductivity and can come from an inexhaustible source. Their biodegradability is secured in certain conditions; for example, carbon nanotubes (CNTs) degradation is produced by macrophages [19]. These allotropic forms of carbon (graphite, graphene, CNTs, or carbon black) can be used Figure 3. Criteria to classify a sustainable ink. 2. Composition of Inks 2.1. Functional Material 2.1.1. Conductive Inks Conductive inks are generally composed of a functional material (or its precursor), which gives the ink its electrical conductivity properties. If the ink is composed of the precursor, the metal particles are prepared with bottom-up methods, decomposing the precursor’s molecules thermally or through the reduction of metal salts reacting with a reduction agent. Depending on the functional material used, conductive inks could be classified into three separate groups: (1) conductive inks formed of metallic particles, (2) carbon-based conductive inks, or (3) particle-free conductive inks. Most conductive inks use metallic materials, such as silver or copper, to achieve good electric conductivity. However, some of the used materials are considered critical raw materials or are harmful to some species (such as silver for submarine life), entailing a risk to the environment [18]. Carbon-based materials are the second family of materials used to give inks electrical conductivity. These materials have demonstrated good conductivity and can come from an inexhaustible source. Their biodegradability is secured in certain conditions; for example, carbon nanotubes (CNTs) degradation is produced by macrophages [19]. These allotropic forms of carbon (graphite, graphene, CNTs, or carbon black) can be used separately or by combining their properties to ensure good conductivity. Carbon is an element present in nature that can be found in fossil form, in the air, or in the ocean, and can be processed and afterward recycled and returned to nature. The third family of inks contains those that do not have particles, such as Poly(2,3dihydrothieno-1,4-dioxin)-poly(styrenesulfonate) inks (PEDOT:PSS inks). This material is Materials 2023,16, 3940 6 of 22 not biodegradable at all, but, used in a small proportion, the final ink could be considered biodegradable. Depending on the selected material, the conductivity of the final ink will vary. Table 3 contains the values of conductivity of various functional materials. Table 3. Conductivity of some functional materials used in conductive inks. Material Conductivity Ref. Silver 6.8 ×107S/m [20] Copper 5.98 ×107S/m [20] Gold 4.3 ×107S/m [20] Aluminium 3.8 ×107S/m [20] Magnesium 2.2 ×107S/m [21] Wolframium 1.8 ×107S/m [22] Zinc 1.7 ×107S/m [21] Nickel 1.5 ×107S/m [20] Iron 1.04 ×107S/m [21] Platinum 9.5 ×106S/m [21] Palladium 9.4 ×106S/m [21] Tin 8.7 ×106S/m [21] Carbon 102–106S/m [23] PEDOT: PSS 2×10−1–2.1 ×105S/m [23] A strategy to synthesize biogenic silver particles has been developed in recent years. This sintering method is low cost, causes less toxic waste, and consumes less energy while a higher yield is obtained. The process is based on the ability of certain organisms, such as bacteria, yeasts, or fungi, to alter the chemical nature of metals and reduce them into nanoparticles [ 24 ]. Depending on the specific organism used and the environmental conditions, the nanoparticles obtained may have different physicochemical properties [ 24 ]. Nevertheless, the attainment of silver nanoparticles depends on numerous factors, such as the genetic properties of the organisms or the environmental conditions [ 11 ]. If sintering silver through this process, we could consider it a biobased material [ 25 ]. Even so, silver is a problem for submarine life and is not biodegradable, becoming a problem for the environment [18]. Carbon can be provided from biomass produced from agricultural waste. This source is abundant, sustainable, renewable, and rich in this material (up to 55% of biomass is carbon). It is necessary to apply thermal treatments to biomass to obtain graphitic structures [26]. Other carbon sources are vegetable oils, chicken oil, or camphor (C 10 H 16 O) after being correctly processed [ 27 – 30 ]. Carbon nanostructures could also be formed through the thermal treatment of cellulose using a nickel salt during the process [31]. It has been demonstrated that graphene could be obtained from daily materials such as food, waste, plastics, or plants, treating them at hot temperatures in an H 2 /Ag atmosphere to obtain high-quality graphene layers [30]. CNTs are generally obtained through the Chemical Vapor Deposition (CVD) process. The sustainability of this process can be improved by using more sustainable catalysts or renewable carbon sources. Metals or metallic oxides that can be found in nature, such as lava or sand, could be used as catalysts [ 32 , 33 ]. These sources are not commonly considered renewable but are abundant in nature and low cost. However, the use of these catalysts does not lead to uniformity in the morphology or the nanostructure of the CNTs [ 30 ]. CNTs could also be produced using iron extracted from plants such as sesame seeds as organic precursors [ 34 ]. The obtained CNTs present a uniform size [ 35 ]. Pol et al. described a process to obtain CNTs from polymer waste without solvents [ 36 ]. This process is based on a thermal dissociation in a closed system within autogenic pressure and catalysts. The procedure described could, in addition, solve another current environmental problem: plastic degradation [30]. Materials 2023,16, 3940 7 of 22 Focusing on the biodegradability of carbon-based materials, there are studies that demonstrate the degradation of CNTs. This degradation occurs not only chemically with strong oxidants or thermal treatments in an oxygen atmosphere but also through enzymatic oxidation with horseradish peroxidase. These studies demonstrate the total degradation of the CNTs in in vitro systems (using different animal tissues, cells, or molecules) without cytotoxicity. However, for in vivo systems (evaluating the degradation of CNTs in living organisms), the degradation that occurs is partial, and there is still long-term concern about its toxicity [19]. Taking all this into account, we can classify carbon as biobased and a material that could be biodegradable in certain conditions. Other metals that are considered to be biodegradable, such as magnesium (Mg), zinc (Zn), or iron (Fe), can be used to develop conductive inks [ 7 ]. These are corrodible metals, and they degrade relatively quickly. Mg and Zn are more often used due to their lower cost and ease of processing. However, they degrade more quickly than Fe, making the last one more suitable for applications with a longer lifetime [ 7 ]. Lee et al. developed bioresorbable systems using Zn microparticles sintered electrochemically. The substrate used to deposit them was a bioresorbable polymer: a sheet of polylactic-co-glycolic acid (PLGA) [ 37 ]. Hwang et al. manufactured biodegradable electronics using Mg, a biodegradable polymer, among other materials [38]. The last type of conductive inks found in the market are those composed of a conductive polymer, such as PEDOT:PSS. Their conductivity is lower than particle-based ones, but it could be interesting to study the possibility of developing a conductive ink based on a biopolymer. PEDOT:PSS is a conductive polymer exhibiting biocompatibility, electrochemical properties, good electric conductivity, and versatile processing, and it is commercialized in water dispersion. There are studies demonstrating the biodegradability of montmorillonite/PEDOT:PSS composites (MMT/PEDOT:PSS) after being processed by specific super worms [ 39 ]. PEDOT:PSS does not fulfill the biodegradability ISO 14852 rule [ 9 ], which measures its degradation in aqueous media. Nevertheless, Pietsch et al. developed PEDOT:PSS biodegradable electrodes according to the ISO 14855 rule, which measures de degradation of the complete system in compost media [ 9 ]. This happens because the quantity of the PEDOT:PSS in the device is less than the non-degradable quantity accepted by the ISO 14855 rule to consider it biodegradable. Therefore, to consider a PEDOT:PSS ink biodegradable, it is necessary to combine a low quantity of PEDOT:PSS with an appropriate biodegradable binder [ 40 ]. The main strategy for using the PEDOT:PSS as functional material is to mix it with a biodegradable polymer, keeping conductivity. Conductivities up to 4 7 × 10 −1 S/m have been reached in particle PEDOT systems dispersed in poly(L-lactic acid) (PLLA) [ 41 ]. Mantione et al. developed different PEDOT:biopolymer dispersions, achieving conductivities up to 7 × 10 2 S/m. The different PEDOT:biopolymer dispersions and the conductivities achieved are collected in Table 4[ 42 ]. These types of inks are particle-free inks. Table 4. PEDOT:Biopolymer dispersions and achieved conductivities. PEDOT:Biopolymer Conductivity (S/m) Ref. PEDOT:dextran sulphate 7×102[42] PEDOT:DNA 102[42] PEDOT:heparin 0.1–5 [42] PEDOT:chondroitin Sulphate 0.2–7.5 [42] PEDOT:hyaluronic acid 0.3–7.1 [42] PEDOT:sulphated cellulose 57.6 [42] PEDOT:pectin <1 [42] PEDOT:guar gum 2.8–12.9 [42] Materials 2023,16, 3940 8 of 22 2.1.2. Dielectric Inks Dielectric inks are insulator inks below a certain electric voltage, called the breakdown voltage. Therefore, dielectric inks should be formulated with insulator materials that are biobased or biodegradable and mixed with the appropriate solvents, binders, and additives, as shown in Figure 2. Dielectric inks define their isolation capacity depending on their dielectric constant. The dielectric constant of materials usually used in electronics is shown in Table 5. Table 5. Dielectric constants of materials used in electronics. Material Dielectric Constant Frequency Ref. Paper 2–4 106Hz [43] Mica 3–6 103Hz [44] Teflon 2 103Hz [45] Rubber 6.7 - [46] Polymers (general) ~2 103Hz [43] High-Density Polyethylene (HDPE) 2.3 103Hz [43] Low-Density Polyethylene (LDPE) 2.3 103Hz [43] Polypropylene (PP) 2.3 106Hz [43] Mylar 3.25 103Hz [43] Kapton 3.9 103Hz [43] Polyvinyl chloride (PVC) 3.4 103Hz [47] Glass (Pyrex) 5 - [46] Porcelain 6–8 - [46] Hereinafter, different biobased and/or biodegradable materials that exhibit dielectric properties and are good candidates for formulating dielectric inks are collected. There are biodegradable inorganic dielectric materials, such as silicon dioxide (SiO 2 ), magnesium oxide (MnO), or silicon nitride (Si 3 N 4 ), that have been used as dielectric materials due to their dielectric properties [ 7 ]. SiO 2 has a dielectric constant of 3.9, while the dielectric constant of Si 3 N 4 is 7.5 [ 48 ]. This type of material could be dispersed in a binder to develop a dielectric ink for printed electronics. Several types of biodegradable polymers can be found in nature, as shown in Figure 4a. These polymers exhibit dielectric properties. Due to their low dielectric loss and high voltage breakdown, natural polymers such as glucose, lactose, or adenine, among others, could be considered in the manufacture of biodegradable and biocompatible electronic devices. Likewise, biodegradable polymers could be synthesized, as shown in Figure 4b. As an advantage, these polymers’ physicochemical properties are more controlled, in comparison to natural ones, by controlling their synthesis conditions. Polymers such as polylactic acid (PLA), polyvinyl alcohol (PVA), poly(dimethyl siloxane) (PDMS), and polyurethane (PU) have been used in dielectric applications. The values of the dielectric constant of different biodegradable polymers are collected in Table 6. Cellulose is a natural biopolymer found abundantly on Earth and is obtained from a vegetal source (annually, between 10 11 and 10 12 tons of nanocellulose are produced) [ 49 ]. The nanocelluloses are cellulose-based materials characterized by having nanoscale dimensions taken from plants, such as wood, coconut husk, sisal, algae, etc., or obtained from animals or bacteria [ 50 , 51 ]. They can be classified into three groups: (1) cellulose nanocrystals (CNCs), obtained chemically from plants or animals, (2) cellulose nanofibrils (CNFs), obtained mechanically, also from plants or animals, or (3) bacterial nanocellulose (BNCs), obtained from bacteria. Materials 2023,16, 3940 9 of 22 Materials 2023, 16, x FOR PEER REVIEW 9 of 23 (a) (b) Figure 4. (a) Natural biodegradable polymers; (b) Synthetic biodegradable polymers. Cellulose is a natural biopolymer found abundantly on Earth and is obtained from a vegetal source (annually, between 1011 and 1012 tons of nanocellulose are produced) [49]. The nanocelluloses are cellulose-based materials characterized by having nanoscale dimensions taken from plants, such as wood, coconut husk, sisal, algae, etc., or obtained from animals or bacteria [50,51]. They can be classified into three groups: (1) cellulose nanocrystals (CNCs), obtained chemically from plants or animals, (2) cellulose nanofibrils (CNFs), obtained mechanically, also from plants or animals, or (3) bacterial nanocellulose (BNCs), obtained from bacteria. Figure 4. (a) Natural biodegradable polymers; (b) Synthetic biodegradable polymers. Table 6. Dielectric constants of potential biobased and/or biodegradable materials. Material Dielectric Constant Frequency Ref. Cellulose 3.9–7.5 103Hz [47] Keratin 8 3×106Hz [52] Chitosan 5.5 103Hz [53] Starch 40–65 2.45 ×109Hz [54] Silk Fibroin 6.1 3×105Hz [55] Poly(glutamic acid) 130 103Hz [56] Hyaluronic acid 60 9×109Hz [57] Alginate 18.35 106Hz [58] Dextran 45–60 1010 Hz [59] Collagen 4.5 103Hz [60] PLA 2.9 103Hz [61] PVA 12 103Hz [47] Polycaprolactone 3 109Hz [59] PDMS 2.6 103Hz [47] PBS 17.5 - [62] Materials 2023,16, 3940 16 of 22 Table 8. Cont. Binder Active Element Solvent Other Materials Tech. Sustainability Reason Ref. Aqueous solution of TEMPO Silver Water-based: Water/ Isopropyl alcohol CNCs NaBr Ethanol Screen printing Water as solvent and cellulose as additive [50] NaOCl NaOH AgNO3 NaBH4or Hydrazine Ethylene glycol Dispersing agent (Disperbyk 2012 1–5 wt.%) Hydroxypropyl methylcellulose Rheological additive (Reobyk 7420) HCl - PEDOT: dextran sulfate Water-based - Inkjet printing Biopolymer to increase PEDOT conductivity and water-based [42] -PEDOT: DNA Water-based - Inkjet printing Biopolymer to increase PEDOT conductivity and water-based [42] -PEDOT: heparin Water-based - Inkjet printing Biopolymer to increase PEDOT conductivity and water-based [42] - PEDOT: chondroitin sulfate Water-based - Inkjet printing Biopolymer to increase PEDOT conductivity and water-based [42] - PEDOT: hyaluronic acid Water-based - Inkjet printing Biopolymer to increase PEDOT conductivity and water-based [42] - PEDOT: sulfated cellulose Water-based - Inkjet printing Biopolymer to increase PEDOT conductivity and water-based [42] -PEDOT: pectin Water-based - Inkjet printing Biopolymer to increase PEDOT conductivity and water-based [42] -PEDOT: guar gum Water-based - Inkjet printing Biopolymer to increase PEDOT conductivity and water-based [42] -Sodium Alginate Water-based - Screen printing Alginate as functional material and water-based [71] - CNCs Water-based - Aerosol printing CNCs as active material and water-based [68] 3. Conclusions In this review, recent advances in the development of sustainable inks (conductive, dielectric, and piezoelectric) are presented. The sustainability of the inks can be achieved in three ways: (1) using biobased materials, (2) using biodegradable materials, or (3) avoiding the use of critical raw materials. The sustainability of the ink is secured by selecting the appropriate materials and considering the quantity of each material in the inks to be considered biodegradable. The classification of the different materials collected to develop inks for electronic applications is shown in Figure 8. Electric conductive inks need a functional material that allows the electricity to pass through the printed lines. Three types of materials can be used: metallic materials, carbonbased materials, or polymeric materials. Some metallic materials, such as silver or gold, despite presenting the highest conductivities (10 5 S/cm), are considered critical raw materials and, therefore, are inappropriate for developing sustainable inks. However, there are studies that have developed a biobased Materials 2023,16, 3940 17 of 22 silver, reducing the impact on the environment due to lower energy consumption in its synthesis and less toxic produced waste. Materials 2023, 16, x FOR PEER REVIEW 17 of 23 Figure 8. Classification of the materials used to develop sustainable inks, considering the proper material and quantity of each one to be considered biodegradable. Electric conductive inks need a functional material that allows the electricity to pass through the printed lines. Three types of materials can be used: metallic materials, carbonbased materials, or polymeric materials. Some metallic materials, such as silver or gold, despite presenting the highest conductivities (105 S/cm), are considered critical raw materials and, therefore, are inappropriate for developing sustainable inks. However, there are studies that have developed a biobased silver, reducing the impact on the environment due to lower energy consumption in its synthesis and less toxic produced waste. Carbon-based materials, such as graphite, graphene, carbon nanotubes, or carbon black, present conductivity (103 S/cm) and can be used to develop sustainable inks. Carbon can be found in nature in multiple forms and is biobased and biodegradable. Taking all this into account, it is a good candidate for developing sustainable inks for printed electronics. Other conductive materials that can be used are conductive polymers, such as PEDOT:PSS. Despite the fact that this material is neither biobased nor biodegradable, an ink with a low content of it could be considered sustainable and not harmful to the environment. Another strategy to develop a sustainable ink based on a conductive polymer is to disperse the PEDOT in a biopolymer, reaching conductivities of 101 S/cm. In printed electronics, dielectric inks are also needed to isolate parts of the circuitry or to protect devices from the natural environment, such as moisture. It is also necessary to develop sustainable inks that ensure these functionalities. Thus, a functional material with dielectric properties should be dispersed in a binder, such as cellulose or natural resins, or natural proteins, such as keratin. Different material dispersions have been h i ( n n , n ) ih v n n h h hi n in i ni i v T i i n i n i i ( , , nin ) n h i ( A, A ) A n Figure 8. Classification of the materials used to develop sustainable inks, considering the proper material and quantity of each one to be considered biodegradable. Carbon-based materials, such as graphite, graphene, carbon nanotubes, or carbon black, present conductivity (10 3 S/cm) and can be used to develop sustainable inks. Carbon can be found in nature in multiple forms and is biobased and biodegradable. Taking all this into account, it is a good candidate for developing sustainable inks for printed electronics. Other conductive materials that can be used are conductive polymers, such as PEDOT:PSS. Despite the fact that this material is neither biobased nor biodegradable, an ink with a low content of it could be considered sustainable and not harmful to the environment. Another strategy to develop a sustainable ink based on a conductive polymer is to disperse the PEDOT in a biopolymer, reaching conductivities of 101S/cm. In printed electronics, dielectric inks are also needed to isolate parts of the circuitry or to protect devices from the natural environment, such as moisture. It is also necessary to develop sustainable inks that ensure these functionalities. Thus, a functional material with dielectric properties should be dispersed in a binder, such as cellulose or natural resins, or natural proteins, such as keratin. Different material dispersions have been reported and deposited on a substrate using different techniques, presenting suitable properties. The last type of sustainable inks required are piezoelectric inks. There are biobased polymers (natural and synthetic) that present this property in particular conditions. Because of this, these materials could be dispersed in an appropriate binder. After the deposition, the materials must be exposed to a poling process to align all the dipoles of the material in the correct direction to obtain its piezoelectric properties. Currently, few piezoelectric inks have been found in the market or reported in the literature. By correctly selecting the Materials 2023,16, 3940 18 of 22 materials and formulating an ink, it could be possible to develop a sustainable piezoelectric ink for printed electronics. All these functional materials should be dispersed in a solvent and mixed with a binder. To be sustainable, these materials should also be biobased or biodegradable. Several types of binders and solvents have been found, such as water, PLA, cellulose dispersions, or resins coming from nature, such as shellac or xantana. The materials used must be compatible with the functional material selected, ensuring conductivity and good behavior to be printed using the method they are purposed to. Different sustainable inks and dispersions have been found for screen printing, aerosol printing, and inkjet printing. The sustainability of these inks and/or dispersions depends on different parameters: the sustainability of the functional material, the sustainability of the binder, and the sustainability of the solvent. To formulate the different sustainable inks, it is necessary to verify the compatibility between the different materials and to select them correctly. Moreover, it is necessary to define the deposition method and adjust the final properties of the formulated inks. Currently, the development of biobased or biodegradable inks for printed electronics is a challenge. The inks must have good behavior and properties as well as competitive cost compared with commercial inks. Studies on developing this type of ink are being conducted. Another challenge that researchers may focus on is securing the recyclability of the ink and the global electronics systems in order to achieve sustainability objectives and green electronics. Author Contributions: Conceptualization, E.A. and J.L.V.-V.; writing—original draft preparation, L.S.-D.; writing—review and editing, E.G., M.L., M.B., A.M.G., E.A. and J.L.V.-V.; project administration, E.G.; funding acquisition, E.G. All authors have read and agreed to the published version of the manuscript. Funding: This publication is supported by the SUINK project funded by the European Union’s Horizon Europe research and innovation programme under Grant Agreement No. 101070112. Funded by the Basque Government ELKARTEK2021 (KK-2021/00040) and ELKARTEK2023 KK-2023/0005. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: No new data were created or analyzed in this study. Data sharing is not applicable to this article. Acknowledgments: This publication is supported by the SUINK project funded by the European Union’s Horizon Europe research and innovation programme under Grant Agreement No. 101070112. Views and opinions expressed are, however, those of the author(s) only and do not necessarily reflect those of the European Union or the European Commission. Neither the European Union nor the granting authority can be held responsible for them. The authors would like to acknowledge the Basque Government funding within the ELKARTEK2021 (KK-2021/00040) and ELKARTEK2023 KK-2023/00056 Programmes. Conflicts of Interest: The authors declare no conflict of interest. References 1. Huang, X.; Liu, Y.; Hwang, S.W.; Kang, S.K.; Patnaik, D.; Cortes, J.F.; Rogers, J.A. Biodegradable Materials for Multilayer Transient Printed Circuit Boards. Adv. Mater. 2014,26, 7371–7377. [CrossRef] [PubMed] 2. Poulin, A.; Aeby, X.; Siqueira, G.; Nyström, G. Versatile carbon-loaded shellac ink for disposable printed electronics. Sci. Rep. 2021,11, 23784. [CrossRef] [PubMed] 3. European Commission, Directorate-General for Internal Market, Industry, Entrepreneurship and SMEs. Proposal for A Regulation of The European Parliament and of The Council Establishing A Framework for Ensuring A Secure and Sustainable Supply of Critical Raw Materials and AMENDING Regulations (EU) 168/2013, (EU) 2018/858, 2018/1724 and (EU) 2019/1020; European Commission, Directorate-General for Internal Market, Industry, Entrepreneurship and SMEs: Brussels, Belgium, 2023. 4. Curran, M.A. Biobased Materials. In Kirk-Othmer Encyclopedia of Chemical Technology; John Wiley & Sons: New York, NY, USA, 2010; pp. 1–19. [CrossRef] Materials 2023,16, 3940 19 of 22 5. Najafi, M.; Zahid, M.; Ceseracciu, L.; Safarpour, M.; Athanassiou, A.; Bayer, I.S. Polylactic acid-graphene emulsion ink based conductive cotton fabrics. J. Mater. Res. Technol. 2022,18, 5197–5211. [CrossRef] 6. Atreya, M.; Dikshit, K.; Marinick, G.; Nielson, J.; Bruns, C.; Whiting, G.; Rady, P.M. Poly(lactic acid)-Based Ink for Biodegradable Printed Electronics With Conductivity Enhanced through Solvent Aging. ACS Appl. Mater. Interfaces 2020 ,12, 23494–23501. [CrossRef] [PubMed] 7. Hosseini, E.S.; Dervin, S.; Ganguly, P.; Dahiya, R. Biodegradable Materials for Sustainable Health Monitoring Devices. ACS Appl. Bio Mater. 2021,4, 163–194. [CrossRef] 8. Dreimol, C.H.; Guo, H.; Ritter, M.; Keplinger, T.; Ding, Y.; Günther, R.; Poloni, E.; Burgert, I.; Panzarasa, G. Sustainable wood electronics by iron-catalyzed laser-induced graphitization for large-scale applications. Nat. Commun. 2022,13, 1–12. [CrossRef] 9. Pietsch, M.; Schlisske, S.; Held, M.; Strobel, N.; Wieczorek, A.; Hernandez-Sosa, G. Biodegradable inkjet-printed electrochromic display for sustainable short-lifecycle electronics. J. Mater. Chem. C Mater. Opt. Electron. Devices 2020 ,8, 16716–16724. [CrossRef] 10. Nayak, L.; Mohanty, S.; Nayak, S.K.; Ramadoss, A. A review on inkjet printing of nanoparticle inks for flexible electronics. J. Mater. Chem. C 2019,7, 8771–8795. [CrossRef] 11. Ibrahim, N.; Akindoyo, J.O.; Mariatti, M. Recent development in silver-based ink for flexible electronics. J. Sci. Adv. Mater. Devices 2022,7, 100395. [CrossRef] 12. Chemical Components of Printing Inks—PCC Group. Available online: https://www.products.pcc.eu/en/k/printing-inks/ (accessed on 20 March 2023). 13. Tawiah, B.; Howard, E.K.; Asinyo, B.K. The chemistry of inkjet inks for digital textile printing-review. BEST Int. J. Manag. Inf. Technol. Eng. (BEST IJMITE) 2019,4, 61–78. 14. Kipphan, H. Handbook of Print Media: Technologies and Production Methods; Springer Science and Business Media: New York, NY, USA, 2001. [CrossRef] 15. Comisión Europea, Dirección General de Medio Ambiente. Communication from The Commission to The European Parliament, The Council, The European Economic and Social Committee and The Committee of The Regions. EU Policy Framework on Biobased, Biodegradable and Compostable Plastics; Comisión Europea, Dirección General de Medio Ambiente: Brussels, Belgium, 2022. 16. ISO 14855-2:2018; ISO Determination of The Ultimate Aerobic Biodegradability of Plastic Materials under Controlled Composting Conditions—Method by Analysis of Evolved Carbon Dioxide—Part 2: Gravimetric Measurement of Carbon Dioxide Evolved in A Laboratory-Scale Test. ISO: Geneva, Switzerland, 2019. 17. ISO 14852:2018; ISO Determination of The Ultimate Aerobic Biodegradability of Plastic Materials in An Aqueous Medium— Method by Analysis of Evolved Carbon Dioxide. ISO: Geneva, Switzerland, 2019. 18. Ratte, H.T. Bioaccumulation and toxicity of silver compounds: A review. Environ. Toxicol. Chem. 1999,18, 89–108. [CrossRef] 19. Yang, M.; Zhang, M. Biodegradation of Carbon Nanotubes by Macrophages. Front. Mater. 2019,6, 225. [CrossRef] 20. Taherian, R.; Kausar, A. The Theory of Electrical Conductivity. In Electrical Conductivity in Polymer-Based Composites; Elsevier Science & Technology Books: New York, NY, USA, 2018. [CrossRef] 21. Haynes, W.M.; Lide, D.R.; Bruno, T.J. CRC Handbook of Chemistry and Physics, 97th ed.; CRC Press: Boca Raton, FL, USA, 2016. 22. Wellner, M. Elements of Physics; Plenum Press: New York, NY, USA, 1991. [CrossRef] 23. Wang, Y.A.; Lu, S.M.; He, W.M.; Gong, S.A.A.; Zhang, Y.M.; Zhao, X.; Fu, Y.; Zhu, Z. Modeling and characterization of the electrical conductivity on metal nanoparticles/carbon nanotube/polymer composites. Sci. Rep. 2022 ,12, 10448. [CrossRef] [PubMed] 24. Guilger-Casagrande, M.; Germano-Costa, T.; Pasquoto-Stigliani, T.; Fraceto, L.F.; Lima, R.d. Biosynthesis of silver nanoparticles employing Trichoderma harzianum with enzymatic stimulation for the control of Sclerotinia sclerotiorum. Sci. Rep. 2019 ,9, 14351–14359. [CrossRef] [PubMed] 25. Wolny-Koładka, K.; Malina, D.; Suder, A.; Pluta, K.; Wzorek, Z. Bio-Based Synthesis of Silver Nanoparticles from Waste Agricultural Biomass and Its Antimicrobial Activity. Processes 2022,10, 389. [CrossRef] 26. Safian, M.T.; Haron, U.S.; Mohamad Ibrahim, M.N. A Review on Bio-based Graphene Derived from Biomass Wastes. Bioresources 2020,15, 9756–9785. [CrossRef] 27. Afre, R.A.; Soga, T.; Jimbo, T.; Kumar, M.; Ando, Y.; Sharon, M.; Somani, P.R.; Umeno, M. Carbon nanotubes by spray pyrolysis of turpentine oil at different temperatures and their studies. Microporous Mesoporous Mater. 2006,96, 184–190. [CrossRef] 28. Ghosh, P.; Afre, R.A.; Soga, T.; Jimbo, T. A simple method of producing single-walled carbon nanotubes from a natural precursor: Eucalyptus oil. Mater. Lett. 2007,61, 3768–3770. [CrossRef] 29. Kumar, M.; Ando, Y. Single-wall and multi-wall carbon nanotubes from camphor—A botanical hydrocarbon. Diam. Relat. Mater. 2003,12, 1845–1850. [CrossRef] 30. Titirici, M.M.; White, R.J.; Brun, N.; Budarin, V.L.; Su, D.S.; Del Monte, F.; Clark, J.H.; MacLachlan, M.J. Sustainable carbon materials. Chem. Soc. Rev. 2014,44, 250–290. [CrossRef] 31. Sevilla, M.; Fuertes, A.B. Graphitic carbon nanostructures from cellulose. Chem. Phys. Lett. 2010,490, 63–68. [CrossRef] 32. Su, D.S.; Rinaldi, A.; Frandsen, W.; Weinberg, G. Nanocarbons: Efficient synthesis using natural lava as supported catalyst. Phys. Status Solidi (B) 2007,244, 3916–3919. [CrossRef] 33. Endo, M.; Takeuchi, K.; Kim, Y.A.; Park, K.C.; Ichiki, T.; Hayashi, T.; Fukuyo, T.; Iinou, S.; Su, D.S.; Terrones, M.; et al. Simple synthesis of multiwalled carbon nanotubes from natural resources. ChemSusChem 2008,1, 820–822. [CrossRef] Materials 2023,16, 3940 20 of 22 34. Chen, X.G.; Su, D.S.; Hamid, S.B.A.; Schlögl, R.; Li, P.; Yu, H.; Akinwande, A.I.; Milne, W.I. The morphology, porosity and productivity control of carbon nanofibers or nanotubes on modified activated carbon. Carbon 2007,45, 895–898. [CrossRef] 35. Zhao, J.; Guo, X.; Guo, Q.; Gu, L.; Guo, Y.; Feng, F. Growth of carbon nanotubes on natural organic precursors by chemical vapor deposition. Carbon 2011,49, 2155–2158. [CrossRef] 36. Pol, V.G.; Thiyagarajan, P. Remediating plastic waste into carbon nanotubes. J. Environ. Monit. 2010,12, 455–459. [CrossRef] 37. Lee, Y.K.; Kim, J.; Kim, Y.; Kwak, J.W.; Yoon, Y.; Rogers, J.A. Room Temperature Electrochemical Sintering of Zn Microparticles and Its Use in Printable Conducting Inks for Bioresorbable Electronics. Adv. Mater. 2017,29, 1702665. [CrossRef] 38. Hwang, S.; Song, J.; Huang, X.; Cheng, H.; Kang, S.; Kim, B.H.; Kim, J.; Yu, S.; Huang, Y.; Rogers, J.A. High-Performance Biodegradable/Transient Electronics on Biodegradable Polymers. Adv. Mater. 2014,26, 3905–3911. [CrossRef] 39. Lee, S.; Hong, Y.; Shim, B.S. Biodegradable PEDOT:PSS/Clay Composites for Multifunctional Green-Electronic Materials. Adv. Sustain. Syst. 2022,6, 2100056. [CrossRef] 40. Xu, C.; Guan, S.; Wang, S.; Gong, W.; Liu, T.; Ma, X.; Sun, C. Biodegradable and electroconductive poly(3,4-ethylenedioxythiophene)/carboxymethyl chitosan hydrogels for neural tissue engineering. Mater. Sci. Eng. C 2018,84, 32–43. [CrossRef] 41. Feig, V.R.; Tran, H.; Bao, Z. Biodegradable Polymeric Materials in Degradable Electronic Devices. ACS Cent. Sci. 2018 ,4, 337–348. [CrossRef] [PubMed] 42. Mantione, D.; Del Agua, I.; Sanchez-Sanchez, A.; Mecerreyes, D. Poly(3,4-ethylenedioxythiophene) (PEDOT) Derivatives: Innovative Conductive Polymers for Bioelectronics. Polymers 2017,9, 354. [CrossRef] [PubMed] 43. Simula, S.; Ikäläinen, S.; Niskanen, K.; Varpula, T.; Seppä, H.; Paukku, A. Measurement of the dielectric properties of paper. J. Imaging Sci. Technol. 1999,43, 472–477. [CrossRef] 44. Young, H.D.; Freedman, R.A.; Ford, A.L.; Zuleta Estrugo, K. University Physics, 15th ed.; Pearson: Harlow, CA, USA, 2019. 45. Ehrlich, P.; Amborski, L.E.; Burton, R.L. Dielectric properties of Teflon from room temperature to 314 ◦ C and from frequencies of 102 to 105 c/s. J. Res. Nat. Bur. Stand 1953,51, 185–188. [CrossRef] 46. Serway, R.A.; John, W.; Jewett, J. Physics for Scientists and Engineers with Modern Physics, 10th ed.; Brooks/Cole Pub.: Pacific Grove, CA, USA, 2019. 47. Luo, Q.; Shen, H.; Zhou, G.; Xu, X. A mini-review on the dielectric properties of cellulose and nanocellulose-based materials as electronic components. Carbohydr. Polym. 2023,303, 120449. [CrossRef] [PubMed] 48. Lenka, T.R.; Panda, A.K. AlGaN/GaN-based HEMT on SiC substrate for microwave characteristics using different passivation layers. Pramana J. Phys. 2012,79, 151–163. [CrossRef] 49. Hoeng, F.; Denneulin, A.; Bras, J. Use of nanocellulose in printed electronics: A review. Nanoscale 2016 ,8, 13131–13154. [CrossRef] 50. Martinez-Crespiera, S.; Pepió-Tàrrega, B.; González-Gil, R.M.; Cecilia-Morillo, F.; Palmer, J.; Escobar, A.M.; Beneitez-Álvarez, S.; Abitbol, T.; Fall, A.; Aulin, C.; et al. Use of Nanocellulose to Produce Water-Based Conductive Inks with Ag NPs for Printed Electronics. Int. J. Mol. Sci. 2022,23, 2946. [CrossRef] 51. Tuukkanen, S.; Rajala, S. Nanocellulose as a Piezoelectric Material. In Piezoelectricity—Organic and Inorganic Materials and Applications; IntechOpen: Rijeka, Croatia, 2018; pp. 1–14. [CrossRef] 52. Bibi, F.; Villain, M.; Guillaume, C.; Sorli, B.; Gontard, N. A Review: Origins of the Dielectric Properties of Proteins and Potential Development as Bio-Sensors. Sensors 2016,16, 1232. [CrossRef] 53. Bonardd, S.; Robles, E.; Barandiaran, I.; Saldías, C.; Leiva, Á.; Kortaberria, G. Biocomposites with increased dielectric constant based on chitosan and nitrile-modified cellulose nanocrystals. Carbohydr. Polym. 2018,199, 20–30. [CrossRef] 54. Ndife, M.K.; ¸Sumnu, G.; Bayindirli, L. Dielectric properties of six different species of starch at 2450 MHz. Food Res. Int. 1998 ,31, 43–52. [CrossRef] 55. Wang, C.; Hsieh, C.; Hwang, J. Flexible Organic Thin-Film Transistors with Silk Fibroin as the Gate Dielectric. Adv. Mater. 2011 , 23, 1630–1634. [CrossRef] [PubMed] 56. Takashima, S. Dielectric dispersion of polyglutamic acid solution. Biopolymers 1963,1, 171–182. [CrossRef] 57. Jani, S.K.; Dahiya, J.N.; Roberts, J.A. Dielectric changes in hyaluronate solutions at microwave frequencies as a function of concentration, system pH, and temperature. Biopolymers 1980,19, 931–944. [CrossRef] 58. Jeevan Kumar, R.; Rajasekhar, E.; Subhan, C.M.; Panduranga, P.; Gupta, N.V.S. Dielectric Studies of Acrylic Resin, Alginate, Dental Plaster, Dental Stone, Glass Ionomer and Silver Amalgam. Indian J. Adv. Chem. Sci. 2014,2, 98–103. 59. Zaslavsky, B.Y.; Miheeva, L.M.; Rodnikova, M.N.; Spivak, G.V.; Harkin, V.S.; Mahmudov, A.U. Dielectric properties of water in the coexisting phases of aqueous polymeric two-phase systems. J. Chem. Soc. Faraday Trans. 1 Phys. Chem. Condens. Phases 1989 , 85, 2857–2865. [CrossRef] 60. Tomaselli, V.P.; Shamos, M.H. Electrical properties of hydrated collagen. I. Dielectric properties. Biopolymers 1973 ,12, 353–366. [CrossRef] 61. Dichtl, C.; Sippel, P.; Krohns, S. Dielectric Properties of 3D Printed Polylactic Acid. Adv. Mater. Sci. Eng. 2017 ,2017, 6913835. [CrossRef] 62. Shkir, M.; Chandekar, K.V.; Hossain, M.M.; Palanivel, B.; Ahmad, N.; Ashraf, I.M.; Somaily, H.H.; Algarni, H.; AlFaify, S. Enhanced dielectric and electrical properties of PbS nanostructures facilely synthesized by low-cost chemical route: An effect of Ce doping concentrations. Mater. Chem. Phys. 2022,278, 125626. [CrossRef] 63. Curtis, A.J. Dielectric Properties of Polyamides: Polyhexamethylene Adipamide and Polyhexamethylene Sebacamide. J. Res. Natl. Bur. Stand. A Phys. Chem. 1961,65A, 185–196. [CrossRef] Materials 2023,16, 3940 21 of 22 64. Koizumi, N.; Hanai, T. Dielectric properties of polyethylene glycols: Dielectric relaxation in solid state (special issue on polymer chemistry, I). Bull. Inst. Chem. Res. Kyoto Univ. 1964,42, 115–127. 65. Lorenzini, R.G.; Kline, W.M.; Wang, C.C.; Ramprasad, R.; Sotzing, G.A. The rational design of polyurea & polyurethane dielectric materials. Polymer 2013,54, 3529–3533. [CrossRef] 66. Le Bras, D.; Strømme, M.; Mihranyan, A. Characterization of Dielectric Properties of Nanocellulose from Wood and Algae for Electrical Insulator Applications. J. Phys. Chem. B 2015,119, 5911–5917. [CrossRef] [PubMed] 67. Li, J.; Long, Y.; Yang, F.; Wang, X. Degradable piezoelectric biomaterials for wearable and implantable bioelectronics. Curr. Opin. Solid State Mater. Sci. 2020,24, 100806. [CrossRef] [PubMed] 68. Williams, N.X.; Bullard, G.; Brooke, N.; Therien, M.J.; Franklin, A.D. Printable and recyclable carbon electronics using crystalline nanocellulose dielectrics. Nat. Electron. 2021,4, 261–268. [CrossRef] [PubMed] 69. Singh, R.; Lin, Y.T.; Chuang, W.L.; Ko, F.H. A new biodegradable gate dielectric material based on keratin protein for organic thin film transistors. Org. Electron. 2017,44, 198–209. [CrossRef] 70. Lee, D.W.; Lim, C.; Israelachvili, J.N.; Hwang, D.S. Strong adhesion and cohesion of chitosan in aqueous solutions. Langmuir ACS J. Surf. Colloids 2013,29, 14222. [CrossRef] 71. Allgén, L.; Roswall, S. A dielectric study of sodium alginate in aqueous solution. J. Polym. Sci. 1957,23, 635–650. [CrossRef] 72. Vela Cabello, R. Caracterización de la respuesta piezoeléctrica de materiales compuestos basados en PVDF—BaTiO 3 . Master’s Thesis, Universidad Carlos III de Madrid, Madrid, Spain, 2013. 73. Gonçalves, S.; Serrado-Nunes, J.; Oliveira, J.; Pereira, N.; Hilliou, L.; Costa, C.M.; Lanceros-Méndez, S. Environmentally Friendly Printable Piezoelectric Inks and Their Application in the Development of All-Printed Touch Screens. ACS Appl. Electron. Mater. 2019,1, 1678–1687. [CrossRef] 74. Ismail, N.; Essalhi, M.; Rahmati, M.; Cui, Z.; Khayet, M.; Tavajohi, N. Experimental and theoretical studies on the formation of pure β -phase polymorphs during fabrication of polyvinylidene fluoride membranes by cyclic carbonate solvents. Green Chem. 2021,23, 2130–2147. [CrossRef] 75. Arkema Piezotech: Piezoelectric Materials. Available online: https://piezotech.arkema.com/en/Products/piezoelectriccopolymers/ (accessed on 20 March 2023). 76. Nanopaint. Functional Inks. Available online: https://nanopaint-tech.com/products/functional-inks/ (accessed on 20 March 2023). 77. Magron: Piezoelectric Inks. Available online: https://www.ferrozone.co.kr/blank-6 (accessed on 20 March 2023). 78. Hänninen, A.; Sarlin, E.; Lyyra, I.; Salpavaara, T.; Kellomäki, M.; Tuukkanen, S. Nanocellulose and chitosan based films as low cost, green piezoelectric materials. Carbohydr. Polym. 2018,202, 418–424. [CrossRef] [PubMed] 79. Valadorou, D.; Papathanassiou, A.N.; Kolonelou, E.; Sakellis, E. Boosting the electro-mechanical coupling of piezoelectric polyvinyl alcohol–polyvinylidene fluoride blends by dispersing nano-graphene platelets. J. Phys. D Appl. Phys. 2022 ,55, 295501. [CrossRef] 80. Yang, F.; Li, J.; Long, Y.; Zhang, Z.; Wang, L.; Sui, J.; Dong, Y.; Wang, Y.; Taylor, R.; Ni, D.; et al. Wafer-scale heterostructured piezoelectric bio-organic thin films. Science 2021,373, 337–342. [CrossRef] [PubMed] 81. Chiellini, E.; Corti, A.; D’Antone, S.; Solaro, R. Biodegradation of poly (vinyl alcohol) based materials. Prog. Polym. Sci. 2003 ,28, 963–1014. [CrossRef] 82. Tuukkanen, S.; Toriseva, J.; Pammo, A.; Virtanen, J.; Lahti, J. Piezoelectric properties of roll-to-roll fabricated polylactic acid films. In Proceedings of the 2020 IEEE 8th Electronics System-Integration Technology Conference (ESTC), Tonsberg, Norway, 15–18 September 2020; pp. 1–3. [CrossRef] 83. Li, Y.; Feng, J.; Zhang, J.; He, B.; Wu, Y.; Zhao, Y.; Xu, C.; Wang, J. Towards high-performance linear piezoelectrics: Enhancing the piezoelectric response of zinc oxide thin films through epitaxial growth on flexible substrates. Appl. Surf. Sci. 2021 ,556, 149798. [CrossRef] 84. Ting, Y.; Gunawan, H.; Zhong, J.; Chiu, C. A new poling method for piezoelectric ceramics with thick film. J. Eur. Ceram. Soc. 2014,34, 2849–2855. [CrossRef] 85. Frank, B.P.; Smith, C.; Caudill, E.R.; Lankone, R.S.; Carlin, K.; Benware, S.; Pedersen, J.A.; Fairbrother, D.H. Biodegradation of Functionalized Nanocellulose. Environ. Sci. Technol. 2021,55, 10744–10757. [CrossRef] 86. Brooke, R.; Fall, A.; Borràs, M.; Yilma, D.B.; Edberg, J.; Martinez-Crespiera, S.; Aulin, C.; Beni, V. Nanocellulose based carbon ink and its application in electrochromic displays and supercapacitors. Flex. Print. Electron. 2021,6, 045011. [CrossRef] 87. Hoeng, F.; Bras, J.; Gicquel, E.; Krosnicki, G.; Denneulin, A. Inkjet printing of nanocellulose-silver ink onto nanocellulose coated cardboard. RSC Adv. 2017,7, 15372. [CrossRef] 88. Zappi, D.; Varani, G.; Cozzoni, E.; Iatsunskyi, I.; Laschi, S.; Giardi, M.T. Innovative Eco-Friendly Conductive Ink Based on Carbonized Lignin for the Production of Flexible and Stretchable Bio-Sensors. Nanomaterials 2021,11, 3428. [CrossRef] 89. Stini, N.A.; Gkizis, P.L.; Kokotos, C.G. Cyrene: A bio-based novel and sustainable solvent for organic synthesis. Green Chem. 2022 , 24, 6435. [CrossRef] 90. Pan, K.; Fan, Y.; Leng, T.; Li, J.; Xin, Z.; Zhang, J.; Hao, L.; Gallop, J.; Novoselov, K.S.; Hu, Z. Sustainable production of highly conductive multilayer graphene ink for wireless connectivity and IoT applications. Nat. Commun. 2018 ,9, 5197. [CrossRef] [PubMed] 91. Goma Laca. Available online: https://es.wikipedia.org/w/index.php?title=Goma_laca&oldid=148444462 (accessed on 20 March 2023). Materials 2023,16, 3940 22 of 22 92. Sunderland, M. Sustainable Bio-Char-Based Ink Having Conductive Properties. U.S. Patent No. 11,053,405, 6 July 2021. 93. Rocha Camargo, J.; Almeida Silva, T.; Rivas, G.A.; Janegitz, B.C. Novel eco-friendly water-based conductive ink for the preparation of disposable screen-printed electrodes for sensing and biosensing applications. Electrochim. Acta 2022,409, 139968. [CrossRef] 94. Koga, H.; Saito, T.; Kitaoka, T.; Nogi, M.; Suganuma, K.; Isogai, A. Transparent, Conductive, and Printable Composites Consisting of TEMPO-Oxidized Nanocellulose and Carbon Nanotube. Biomacromolecules 2013,14, 1160–1165. [CrossRef] 95. Lee, S.; Koo, J.; Kang, S.K.; Park, G.; Lee, Y.J.; Chen, Y.Y.; Lim, S.A.; Lee, K.M.; Rogers, J.A. Metal microparticle—Polymer composites as printable, bio/ecoresorbable conductive inks. Mater. Today 2018,21, 207–215. [CrossRef] 96. Doble, M.; Kruthiventi, A.K. Alternate Solvents. In Green Chemistry and Engineering; Academic Press: Cambridge, MA, USA, 2007; pp. 93–104. [CrossRef] 97. Mavuri, A.; Mayes, A.G.; Alexander, M.S. Inkjet Printing of Polyacrylic Acid-Coated Silver Nanoparticle Ink onto Paper with Sub-100 Micron Pixel Size. Materials 2019,12, 2277. [CrossRef] 98. Lin, Z.; Le, T.; Song, X.; Yao, Y.; Li, Z.; Moon, K.; Tentzeris, M.M.; Wong, C. Preparation of water-based carbon nanotube inks and application in the inkjet printing of carbon nanotube gas sensors. J. Electron. Packag. 2013,135, 011001. [CrossRef] 99. Lee, Y.I.; Kim, S.; Lee, K.J.; Myung, N.V.; Choa, Y.H. Inkjet printed transparent conductive films using water-dispersible single-walled carbon nanotubes treated by UV/ozone irradiation. Thin Solid Film. 2013,536, 160–165. [CrossRef] 100. Kordás, K.; Mustonen, T.; Tóth, G.; Jantunen, H.; Lajunen, M.; Soldano, C.; Talapatra, S.; Kar, S.; Vajtai, R.; Ajayan, P. Inkjet Printing of Electrically Conductive Patterns of Carbon Nanotubes. Small 2006,2, 1021–1025. [CrossRef] 101. Gracia-Espino, E.; Sala, G.; Pino, F.; Halonen, N.; Luomahaara, J.; Mäklin, J.; Tóth, G.; Kordás, K.; Jantunen, H.; Terrones, M.; et al. Electrical transport and field-effect transistors using inkjet-printed SWCNT films having different functional side groups. ACS Nano 2010,4, 3318–3324. [CrossRef] 102. Fan, Z.; Wei, T.; Luo, G.; Wei, F. Fabrication and characterization of multi-walled carbon nanotubes-based ink. J. Mater. Sci. 2005 , 40, 5075–5077. [CrossRef] 103. Heurtematte, A.; Small, W.R.; Paunov, V.N. Inkjet printed water sensitive transparent films from natural gum–carbon nanotube composites. Soft Matter 2007,3, 840–843. [CrossRef] Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.