Magnetic soft centirobot to mitigate biological threats

Received: 21 November 2023 | Revised: 7 March 2024 | Accepted: 13 March 2024 DOI: 10.1002/smm2.1289 RESEARCH ARTICLE Magnetic soft centirobot to mitigate biological threats Jayraj V. Vaghasiya 1 |Carmen C. Mayorga‐Martinez 2 |Jaroslav Zelenka 3 | Shelja Sharma 2 |Tomas Ruml 3 |Martin Pumera 1,2,4,5,6 1 Future Energy and Innovation Laboratory, Central European Institute of Technology, Brno University of Technology, Brno, Czech Republic 2 Center for Advanced Functional Nanorobots, University of Chemistry and Technology, Prague, Czech Republic 3 Department of Biochemistry and Microbiology, University of Chemistry and Technology, Prague, Czech Republic 4 Advanced Nanorobots & Multiscale Robotics Lab, Faculty of Electrical Engineering and Computer Science, VSB‐Technical University of Ostrava, Ostrava, Czech Republic 5 Department of Medical Research, China Medical University Hospital, China Medical University, Taichung, Taiwan, China 6 Department of Chemical and Biomolecular Engineering, Yonsei University, Seodaemun‐gu, Seoul, Korea Correspondence Carmen C. Mayorga‐Martinez, Center for Advanced Functional Nanorobots, University of Chemistry and Technology, Technická 5, Prague 166 28, Czech Republic. Email: [email protected] Martin Pumera, Future Energy and Innovation Laboratory, Central European Institute of Technology, Brno University of Technology, Purkyňova 123, Brno 61200, Czech Republic. Email: [email protected] Funding information Ministry of Education, Youth, and Sports (MSMT), Grant/Award Number: CZ.02.01.01/00/22_010/0002552; Europen Regional Development Fund (EFRR); ERDF/ESF project TECHSCALE, Grant/Award Number: CZ.02.01.01/00/ 22_008/0004587; European Union under the REFRESH‐Research Excellence for Region Sustainability and High‐tech Industries Project, Grant/Award Number: CZ.10.03.01/00/22_003/0000048; MEYS CR, Grant/Award Number: Czech Nano Lab Project (No. LM2023051) Abstract Soft robots have drawn a lot of interest in the field of human–robot interfaces because they can mimic the propulsion of soft bodies and archive complex tasks that cannot be made by rigid robots such as performing the complex motion, avoiding collisions by absorbing impacts, and shape adaptation by elastic deformation. Herein, drawing inspiration from creatures in the Cambrian period, such as Hallucigenia, we develop a centimeter‐sized soft robot with multiple magnetic legs (referred to as a soft centirobot). This robot is equipped with graphitic carbon nitride (g‐C 3 N 4 ) nanosheets to kill biological threats by photogenerated reactive oxygen species under black light illumination. The motion of g‐C 3 N 4 soft centirobot is controlled by magnetic actuation even in complex wastewater samples (with a relative speed of 0.12 body lengths per second). The magnetic multilegs work as a propeller to walk across and cover large regions, and water disinfection is more efficient than what could be achieved by nano/micrometer scale sheets of g‐C 3 N 4. Finally, factors affecting the accelerated propulsion of g‐C 3 N 4 soft centirobot such as design principle, structure geometry, body mass, driving mechanism, and magnetic sensitivity, have been investigated. We envision that such a photoactive 2D material‐based integrated centimeter‐sized robot shall find application in many areas where pathogen removal is required. KEYWORDS 2D materials, graphitic carbon nitride, photocatalyst, soft robotic, water remediation SmartMat. 2024;e1289. wileyonlinelibrary.com/journal/smm2 | 1of13 https://doi.org/10.1002/smm2.1289 This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. © 2024 The Authors. SmartMat published by Tianjin University and John Wiley & Sons Australia, Ltd. 1|INTRODUCTION Soft robotics is an emerging branch of modern robotics that garners great interest for its potential application in bioengineering, bionics, and industry. Soft robots can be actuated by electrical, light, and magnetic stimuli, and demonstrate shape adaptability and flexibility by the elastic deformation of the materials used in their preparation. 1,2 Moreover, they can be made for complex and multimodal motion modes, and reach confined spaces. Indeed, magnetically actuated soft robots are more promising because they are wireless, untethered, and controlled by magnetic fields or permanent magnets. 3 In recent years, many magnetic‐driven soft robots have been reported, which is inspired by a variety of nature animals such as millipede, 4 inchworm, 5 jellyfish, 6 octopus, 7 sea anemone, 8 and scallop. 9 However, the majority of reports have concentrated on structural engineering and locomotion behavior. 1 Soft robots' contribution to water cleaning 10,11 applications is relatively limited compared to other areas. The use of a centimeter‐scale soft robot for water cleaning is a promising application as these robots can cover a larger area in less time. In addition, they can load large quantities of active nanomaterials into their bodies, facilitating their recovery and reuse at the same time. Inspired by Cambrian period animals, particularly those with soft bodies and multilegs (e.g., hallucigenia), we develop a centimeter‐scale magnetic soft robot coated with g‐C 3 N 4 (g‐C 3 N 4 soft centirobot) to control biological threats in water samples (Figure 1A). Biological threats such as pathogens (i.e., viruses and bacteria), as well as toxins released deliberately in water or food to cause disease and death in humans, are a serious global concern. The g‐C 3 N 4 nanosheets present on the body and legs of the soft centirobot serve as a photocatalyst to generate reactive oxygen species (ROS) 12 to kill bacteria in contaminated river water. 13,14 The magnetic multilegs work as a propeller to walk across and cover large regions of the water surface (Figure 1B). A comprehensive study was conducted to understand g‐C 3 N 4 soft centirobot mobility in water media. In addition, g‐C 3 N 4 soft centirobot structural engineering was optimized and improved through modeling and experiment. The soft centirobot reported in this study exhibited impressive locomotion ability in a variety of obstacle situations and navigated to the desired destination by applying a rotating magnetic field. Additionally, the potentialofthissoftcentirobottokillE. coli (model bacteria) was investigated. 2|EXPERIMENTAL SECTION 2.1 |Experimental materials details Neodymium iron boron (NdFeB) microparticles were purchased from Nanoshel (UK) Limited, United Kingdom. FIGURE 1 (A) Hallucigenia sparsa‐inspired g‐C 3 N 4 soft centirobots and their corresponding chemical structures. (B) Antimicrobial activity of g‐C 3 N 4 soft centirobot in wastewater and its ability to generate ROS under black light for killing E. coli bacteria. 2of13 | VAGHASIYA ET AL. 2688819x, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smm2.1289 by Technical University Ostrava, Wiley Online Library on [03/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Melamine was purchased from Merck. Neodymium permanent magnet was purchased from Unimagnet, Czech Republic. Polydimethylsiloxane (PDMS, Sylgard 184) and silicon elastomer curing agent were purchased from Biesterfeld Silicon s.r.o., Czech Republic. Plastic containers were purchased at a local shop in Prague, Czech Republic. 2.2 |Synthesis of g‐C 3 N 4 Photocatalyst was obtained by heating melamine in air. Two grams of melamine was heated at a rate of 5 °C/min from 25 °C to 550 °C in a muffle furnace. Afterward, the temperature was maintained at 550 °C for 2 h to produce the yellowish product g‐C 3 N 4 . 2.3 |Preparation of g‐C 3 N 4 soft centirobots The first step was to design and prepare a soft robot three‐dimensional (3D) mold. Afterward, a 10:1 mass ratio of Sylgard 184 to crosslinker were combined and the mixture was aggressively stirred. Then, different concentrations of NbFeB microparticles (15 wt% to 85 wt%) were added to the polymer mixture. The 3D‐printed mold was then filled with composite solution and left on a neodymium magnet for 30 min. After magnetization, it was heated in an oven for thermal curing at 70 °C for 16 h, resulting in a soft robot that could be magnetically actuated. Where type 1 involves a random distribution of NdFeB particles (without applying a magnetic field). Type 2 was developed by applying a magnetic field perpendicular to the direction of the legs, while in type 3, the magnetic field was applied to the tips of the legs. After that, the surface of the soft robot was enveloped with PDMS, and a layer of g‐C 3 N 4 (~25 mg) was applied using a paintbrush. Then g‐C 3 N 4 coated soft centirobot was heated 70 °C for 4 h to enhance the adsorption of g‐ C 3 N 4 on the surface of PDMS. Lastly, the obtained g‐ C 3 N 4 soft centirobot was rinsed thoroughly with ethanol to remove the unanchored g‐C 3 N 4 and air‐dried overnight. The dimensions of the soft centirobots are 3 cm in height and 2 cm in width. The length of the legs and the spacing between them vary to achieve optimal performance for soft centirobots. 2.4 |Characterization Morphological characterization of synthesized g‐C 3 N 4 and NdFeB were carried out by scanning electron microscopy (SEM, Maia 3, Tescan), high‐resolution transmission electron microscopy (HR‐TEM, EFTEM Jeol 2200 FS), and energy‐dispersive X‐ray spectroscopy (EDS, SDD detector X‐Max N 80TS). Sample crystallinity was confirmed by X‐ray powder diffraction (XRD, Bruker D8). Optical microscopy was used to examine the structural alteration and magnetization of NbFeB microparticles. 2.5 |Magnetic actuation of g‐C 3 N 4 soft centirobot A single soft centirobot was placed in a 1‐L water tank (and different mediums such as river wastewater and highly viscous oil) with a permanent magnet system composed of a rectangular magnet and rotating motor to generate a rotating magnetic field. The velocity and direction of the soft centirobots are controlled by a rotating motor and reposition (xy‐axis) the magnetic beneath, respectively. The provided rotating magnetic field is effective within a limited radius of 3.5 cm. The magnetic field intensity can be modified by changing the distance between the workspace and the magnet beneath. A commercial camera was used to capture the motion of the soft centirobot. 2.6 |Simulation of the magnetic field surrounding a g‐C 3 N 4 soft centirobot The electromagnetic simulation (EMS) feature of the Solidworks software was used to mimic the magnetic field surrounding the g‐C 3 N 4 soft centirobot on a neodymium magnet. The purpose of the simulation was to determine how the magnetic flux density changed for the distance between the soft centirobot and magnet. The simulation uses a 60 mm × 30 mm × 15 mm sized magnet and operates in air geometry. 2.7 |E. coli‐contaminated solution preparation and treatment with g‐C 3 N 4 soft centirobot E. coli bacteria used in this study were obtained from the collection of opportunistic pathogens at the Department of Biochemistry and Microbiology, UCT, Prague. E. coli was propagated on Luria–Bertani (LB) agar at 37 °C and stored at 4 °C. To contaminate the Milli‐Q and river water samples, 100 μL of LB medium inoculated with E. coli (optical density of 1 McFarland) was added to 100 mL of the desired solution and the g‐C 3 N 4 soft centirobots were placed to the solution. VAGHASIYA ET AL. | 3of13 2688819x, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smm2.1289 by Technical University Ostrava, Wiley Online Library on [03/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 2.8 |Black light irradiation setup For irradiation of the g‐C 3 N 4 soft centirobots in water samples, a custom LED lamp with a 395 nm wavelength was used. This used 12 LZ1‐10UB0R‐00U6 LEDs and 12 LZ4‐20MA00‐0000 LEDs. The distance between the samples and the LED array was 20 cm. 2.9 |Bacterial viability evaluation Bacterial viability was evaluated using the Miles and Misra method. After the contamination of Milli‐Qor river water, the suspension was treated under different conditions. The resulting suspensions were serially diluted (1:10, 1:100, 1:1000, etc.) in water and 15 μLof each dilution was drop cast on Petri dishes containing LB agar. Finally, Petri dishes with bacteria solutions seeding were incubated overnight at 37 °C. Colony‐forming units were quantified from different dilutions to calculate bacterial viability. 2.10 |Statistical analysis The bacterial viability scores were analyzed by the averages of three separate measurements and data were shown as mean ± SD. To determine the statistical difference between treated and control biofilms, a two‐ tailed ttest was computed with a 95% confidence interval and 0.05 significance threshold (α). Microsoft Excel software was used to examine the data. 3|RESULTS AND DISCUSSION 3.1 |Design of the g‐C 3 N 4 soft centirobot The design of g‐C 3 N 4 soft centirobot was inspired by the Hallucigenia species, an extinct marine organism from the Cambrian period. Hallucigenia has a soft body (size ~0.5–5.5 cm) with several pairs of legs that allow it to walk underwater. 15 Utilizing these characteristics, we developed magnetic soft robots on the centimeter scale (3 cm × 2 cm). The surface of this soft centirobot was adorned with photocatalytic 2D materials like g‐C 3 N 4 for the application of mitigating biological threats from water. The steps for preparing a magnetically responsive g‐C 3 N 4 soft centirobot are schematically shown in Figure 2A. A detailed description of the process is found in Section 2. The fabrication of a soft centirobot with magnetically responsive multiple legs that demonstrates permanent magnetization was achieved by distributing a composite of NdFeB microparticles and PDMS polymer in a 3D mold. During PDMS polymerization, a suitable magnetic field is provided to the composite, which causes the uniform distribution of magnetic particles and the formation of magnetic‐sensitive soft legs. Further, the g‐ C 3 N 4 was loaded in the body of soft centirobots. 3.2 |Soft centirobot locomotion analysis Following the principle of the magnetic soft robot's leg deflection in an external magnetic field, the steady locomotion of the soft centirobot is achieved by rotating the magnetic field. This magnetically induced deflection of the soft centirobot's legs was adequate to produce propulsion in water (Figure 2B,andMovieS1). Further, optimizing the propulsion of soft centirobots in aqueous media depends on several variables, the most crucial of which are the mass, length of legs, number of legs, applied magnetic field, and magnetic sensitivity. To produce soft centirobots, commercial NdFeB microparticles were used. The SEM image of NdFeB microparticles shows a size range from 5 to 20 µm as can be seen in Supporting Information S1: Figure 1.XRD analysis shows diffraction peaks between 30° and 60°. (004), (105), (006), and (008) crystal plans correspond to the NdFeB magnet (Supporting Information S1: Figure 2). 16 We initially fabricated three distinct soft centirobot, each with a unique distribution of NdFeB microparticles. Centirobot type 1 (Figure 2C,D): NdFeB microparticles are dispersed randomly throughout the legs; centirobot type 2 (Figure 2E,F): after magnetization of legs with an even distribution of NdFeB microparticles; and centirobot type 3 (Figure 2G,H): accumulated NdFeB microparticles at the tips of legs. Figure 2D,F,H display the optical microscopy photographs of the fabricated soft centirobot legs and associated NdFeB microparticle dispersion. The soft centirobot legs are depicted to be canonically shaped with a length of 15 mm and a diameter of 1.47 mm (top part of the leg). In centirobot type 1, there are only a few sporadic spots where the density of NdFeB microparticles is high or low (Figure 2D). When a magnetic gradient is applied during the cure of PDMS (centirobot type 2), NdFeB microparticles are evenly distributed and oriented perpendicular to the length of the soft centirobot legs (Figure 2F and Supporting Information S1: Figure 3). In centirobot type 3, accumulated NdFeB microparticles at the tips of the legs are created by the slow PDMS polymer curing step. There is a clear boundary observed between the pure PDMS portion and the NdFeB microparticle‐rich portion at the bottom of the leg 4of13 | VAGHASIYA ET AL. 2688819x, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smm2.1289 by Technical University Ostrava, Wiley Online Library on [03/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License (Figure 2G). To evaluate how changes in NdFeB microparticle distribution impact the deflection of soft centirobot legs, we moved the various types of legs with the help of a rotating magnetic field setup (Supporting Information S1: Figure 4). All three types of robots are placed on top of a rotating magnetic field. We observed that centirobot type 2 deflection is substantially greater than centirobot types 1 and 3 (Supporting Information S1: Figure 5). These results indicate that magnetic sensitivity increases significantly when the NdFeB microparticle distribution is aligned uniformly. 17,18 However, centirobot type 3 exhibits minimal deflection due to the organized alignment of NdFeB particles in these legs is perpendicular to the rotating magnetic field. In contrast, centirobot type 1 legs have an uneven distribution of NdFeB, leading to asymmetric motion and consequently reduced deflection of the legs. This outcome implies that the magnetic forces affecting the legs are directly related to the alignment of NdFeB particles. 19,20 Therefore, we choose soft centirobot type 2 for further research. Following the systematic investigation of the soft centirobot leg deflection, their velocity as a function of the distance between the magnetic field and soft centirobots was evaluated and depicted in Figure 3A.It is easy to notice that the g‐C 3 N 4 soft centirobot's velocity increases as the distance between it and the magnetic system increases by 1–5 cm. Soft centirobot legs become immobile when the centirobot is placed very close (>1 cm) to a rotating magnetic field. However, the highest velocity of 0.27 cm/s was attained at the optimum distance (~5 cm) between the soft centirobot and magnetic system; above 6 cm, the velocity gradually decreases due to the weaker magnetic field at a greater distance. To better understand the magnetic field effect on the motion of the g‐C 3 N 4 soft centirobot, we simulated the magnetic field and soft centirobot in Solidworks with EMS. The relationship between the magnetic flux density and the distance of the soft centirobot is depicted in Figure 3B. As shown in Figure 3C, the magnetic flux density strength significantly decreases as the distance FIGURE 2 (A) Fabrication procedure of g‐C 3 N 4 soft centirobot. (B) Schematic view of the soft centirobot's legs deflect upon a rotating magnetic field. (C and D) Digital and optical images, respectively, of centirobot type 1, showing the random dispersal of NdFeB microparticles throughout the legs. (E and F) Digital and optical images, respectively, of centirobot type 2, showing the even distribution of NdFeB microparticles upon exposure magnetic field. (G and H) Digital and optical images, respectively, of centirobot type 3 with NdFeB microparticles accumulated at the leg tips. VAGHASIYA ET AL. | 5of13 2688819x, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smm2.1289 by Technical University Ostrava, Wiley Online Library on [03/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License between the magnet and the soft centirobot increases. The distribution of the rectangular magnet's magnetic field is shown in Figure 3D, where the highest intensity of 1.24 T is recorded at the surface. As we mentioned in Section 2, the soft centirobot is driven in water by a rotating magnetic field system composed of a rectangular magnet and a rotating motor. As the magnet rotates, the magnetic field impacting on the g‐C 3 N 4 soft centirobot rotates its legs in a conical pattern, which assists in the forward propulsion of the g‐ C 3 N 4 soft centirobot. Next, we examined how the speed of the rotating magnetic field affects soft centirobot propulsion. Figure 3E shows the velocity of the soft centirobot as a function of motor speed in r/min. When the speed increased from 100 to 400 r/min, g‐C 3 N 4 centirobot velocity also increased, reaching up to 0.3 cm/s. However, the soft centirobot velocity noticeably dropped at >400 r/min as the centirobot lost control of its magnetically responsive legs and began to vibrate in one place. Numerous species in the Cambrian period have legs that vary in length and quantity, which greatly affects how quickly they move. 21 Therefore, we attempted to fabricate three distinct leg lengths and leg counts of soft FIGURE 3 g‐C 3 N 4 soft centirobot (type 2) velocity analysis: (A) velocity as a function of distance between workspace and magnetic system; (B and C) magnetic field strength on soft centirobot obtained by modeling; (D) dispersion of the magnetic field in x–yplan; (E) velocity versus speed of applied rotating magnetic field; (F) influence of leg length on soft centirobot velocity; (G) velocity as a function of leg rows; (H) snapshots of the soft centirobot's travel distance over 30 s; (I) comparative relative velocity versus body mass of soft centirobot and other reported soft robots. 6of13 | VAGHASIYA ET AL. 2688819x, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smm2.1289 by Technical University Ostrava, Wiley Online Library on [03/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License centirobots. Figure 3F and Supporting Information S1: Figure 6a show the relationship between the g‐C 3 N 4 soft centirobot leg length and velocity. The optimum leg length is 1.5 cm because longer legs (3 cm) generate more friction with water and shorter legs (1 cm) cannot deflect. The speeds of three distinct soft centirobots, featuring two, three, and four rows of legs, each row containing five legs, are illustrated in Figure 3G and Supporting Information S1: Figure 6b. The dimensions of the head part for all three soft centirobots are identical, the variation lies only in the number of legs and the spacing between them (Supporting Information S1: Figure 7). Our observations reveal that the three‐row‐legged centirobot exhibits higher speed than the two and four‐ row‐legged centirobots. The spacing between two legs in the four‐row configuration is very short (~3 mm), whereas the two‐row legs are aligned in a 10 mm space. Therefore, the generated deflection in the very short space arranged legs, high possibly to touch each other, resulting in a slowdown in speed. In the case of legs arranged at a larger distance, there is insufficient force generated in water, leading to a decrease in speed. However, the optimal spacing in the three rows of legs allows the soft robot to deflect freely, generating enough propulsion to move forward at high speed. In Figure 3H, snapshots capture the travel speeds of all three soft centirobots over a duration of 30 s. Moreover, experiments were carried out to examine the velocity of the soft centirobot concerning various concentrations of NdFeB. Supporting Information S1: Figure 8 demonstrates that the velocity of the soft centirobot escalates with the increasing concentration of NdFeB, reaching its peak at 55 wt% NdFeB within the polymer matrix. For NdFeB content exceeding 55 wt%, there was minimal change observed in velocity. However, the limitation of employing a high concentration of NdFeB (85 wt%) diminishes the softness of robot legs. Therefore optimized NdFeB concentration (55 wt%) has been used for all studies. Next, Figure 3I compares the relative velocity concerning body mass of the soft centirobot (black solid square) and previously reported soft robots. 22–40 The soft centirobot can drive backward and forward on the surface of the water with a speed of 0.12 body lengths per second, which is faster than those of other similar soft centirobots. The excellent propulsion efficiency of the soft centirobot is due to its simple geometry, lightweight, and excellent magnetic responsiveness. For the biological threat application of the soft centirobot, we covered it with g‐C 3 N 4 . Before preparing the g‐C 3 N 4 soft centirobot, the morphology of g‐C 3 N 4 was examined by SEM and TEM. SEM and TEM images (Figure 4A,B)depict clearly the nanosheet structure of g‐C 3 N 4 . Moreover, the high‐resolution TEM image (Figure 4C) and selective area diffraction pattern of g‐C 3 N 4 (inset in Figure 4C) reveal the FIGURE 4 Morphological characterizations of g‐C 3 N 4 . (A) SEM image; (B) TEM image; (C) high‐resolution TEM image (inset depicts selective diffraction area pattern, SAED); (D) EDS elementary mappings (left to right): baseline TEM image, carbon, nitrogen, and oxygen, showing even distributions of each element. VAGHASIYA ET AL. | 7of13 2688819x, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smm2.1289 by Technical University Ostrava, Wiley Online Library on [03/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License polycrystalline nature of g‐C 3 N 4 reported before. 41,42 Figure 4D depicts the TEM image (left) and corresponding EDS element mappings for g‐C 3 N 4 . A bright area on the elemental map denotes a greater concentration of the associated element. Carbon, nitrogen, and oxygen were evenly distributed in the g‐C 3 N 4 . Further structural properties of g‐C 3 N 4 were confirmed by XRD. The diffraction peaks (100) and (200) of g‐C 3 N 4 found at 13.7° and 27.7° showed good agreement with previous reports (Supporting Information S1: Figure 2). 43 The sharp peak at 27.7° revealed layer stacking in a conjugated atomic system, whereas the peak at 13.7° is related to the structure of the tri‐s‐triazine with interlayer spacing (JCPDS 87–1526). 44 The most important factors for long‐term driving g‐ C 3 N 4 soft centirobot on complex routes are their flexibility, compact size, material binding, and lightweight. Figure 5A,B show, respectively, soft centirobots with and without g‐C 3 N 4 coating. In soft centirobots FIGURE 5 (A) Digital photograph of soft centirobot; (B) digital photograph of g‐C 3 N 4 ‐coated soft centirobot; (C–E) flexibility of g‐C 3 N 4 soft centirobots; (F) body mass of g‐C 3 N 4 soft centirobot; (G) Movie S3 snapshot of soft centirobot walking in forward and backward directions along the S‐shaped track; (H) Movie S4 snapshot of soft centirobot walking along the O‐shaped track. Note that in (G and H), the soft centirobots are depicted without g‐C 3 N 4 coating to show clearly their walking tendency. (I) Movie S5 snapshot of g‐C 3 N 4 soft centirobot walking smoothly in high viscosity media; (J) Movie S6 snapshot of g‐C 3 N 4 soft centirobot walking in real river wastewater. 8of13 | VAGHASIYA ET AL. 2688819x, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smm2.1289 by Technical University Ostrava, Wiley Online Library on [03/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License covered by g‐C 3 N 4 , the flexibility remains intact as can be seen in Figure 5C–Eand Supporting Information S1: Figure 6C, where both a soft centirobot and a g‐C 3 N 4 ‐ coated soft centirobot are shown in several bending modes. To examine the adherence of g‐C 3 N 4 to the surface of the soft centirobot, it was manually rubbed with both hands, and no visible traces of g‐C 3 N 4 were detected (Movie S2). This lack of residue can be attributed to the formation of hydrogen bonds (Supporting Information S1: Figure 9) with oxygen molecules within the PDMS (during the heating process), potentially augmenting the overall adhesion between g‐C 3 N 4 and PDMS. 45–47 Moreover, the total mass of the g‐C 3 N 4 soft centirobot is less than 1 g (Figure 5F). Because their flexibility and lighter weight reduce friction during the walking motion in a rotating magnetic field, they are able to move quickly. To showcase the driving performance of the soft centirobot, two distinct shaped routes (S and O shapes) were developed and the soft centirobot was moved through these routes successfully under the guidance of the magnetic field (Figure 5G,H). The soft centirobot is moved forward and backward along the S‐ shaped route (route length = 36 cm) at speeds of ~0.14 (±0.03) cm/s (Movie S3). The soft centirobot speed is 0.05 (±0.02) cm/s to complete one round of the O‐shaped track with a 10 cm diameter (Movie S4). The speeds of soft centirobots in S‐and O‐shaped tracks diminish, because they require more time to change direction. These findings suggested that by the application of a magnetic rotating field, soft centirobots can navigate in complex modes and confined spaces. Additionally, we showed that the g‐C 3 N 4 soft centirobot is capable of moving through very viscous media (1.412 Pa/s) (Figure 5I and Movie S5). Also, attempted to maneuver g‐C 3 N 4 ‐coated soft centirobots through turbid river water with sediments that contained a variety of pollutants as barriers to smooth motion (Figure 5J and Movie S6). However, the g‐C 3 N 4 soft centirobot walked effectively and was navigated controllably. This revealed that g‐ C 3 N 4 soft centirobots are not only capable of water disinfection but can be used in a variety of applications including payloads and the removal of oil and organic solvents from contaminated water. The control experiment confirmed that the propulsion exhibited by the soft centirobot is attributed to the deflection of its legs. When all legs are trapped and continue to apply a rotating magnetic field, the propulsion of the centirobot is completely stopped (Movie S7). Lastly we examined the underwater movement patterns of g‐C 3 N 4 soft centirobots (Movie S8). The weight of the centirobot's head was augmented to three times its original value (2.815 g) for convenient placement inside the water. We observed that the speed of soft centirobot is ~0.22 cm/s. However, this outcome suggests potential future applications for transporting lightweight samples within aquatic environments. 3.3 |Antibacterial activity of g‐C 3 N 4 soft centirobot Many bacteria and viruses pose biological threats manifested as serious infectious illnesses that can spread and cause outbreaks. Germs can spread through close person‐to‐person contact or by contaminated food or water. Most pathogens found in drinking water are eliminated by conventional water disinfection techniques including UV radiation, ozone, and chlorination. 48 However, chlorination's disinfection by‐products (e.g., halogenated trihalomethanes and haloacetic acids) could potentially be harmful to human health. 49 UV radiation may also be harmful to people and occasionally ineffective against UV‐resistant bacteria. 50 There have been various photocatalysts that use visible or near‐UV (black) light to increase the safety and efficiency of the water treatment by photogeneration of ROS. 51 Especially, g‐C 3 N 4 has demonstrated excellent antibacterial activity toward Staphylococcus epidermidis and E. coli. 52–54 However, the majority of previously published studies on disinfection used g‐C 3 N 4 nano/ microparticles, which makes its recovery challenging and leads to secondary contamination. In this study, to avoid this fact we decorate the surface of soft centirobots with multiples g‐C 3 N 4 sheets. In this way, a centibot carries many sheets of g‐C 3 N 4 on its body surface and can efficiently destroy a greater amount of the pathogen of interest. Once centirobot disinfects the water, it is retable by using an external magnet and it can be reused. Based on this premise, the antibacterial activity of g‐C 3 N 4 soft centirobots against E. coli was evaluated. For this aim, a large volume (100 mL) of water was contaminated with E. coli growninLB medium (see details in the Section 2). First, we evaluated the effect of black light intensity and exposure time on bacterial viability (Figure 6A). It can be seen that the application of a 5 mW/m 2 dose of black light alone for 15 and 30 min decreased bacterial viability to about 10% and 15%. To verify the efficiency g‐ C 3 N 4 soft centirobots for water purification, the 5 mW/ m 2 dose of black light was applied at 15 min and 30 min (Figure 6B). It was clearly demonstrated that g‐C 3 N 4 soft centirobots exposed longer time (30 min) to black light decreases bacterial viability by 54%, while a shorter time (15 min) decreases bacterial viability by 29%. In addition, the role of g‐C 3 N 4 soft centirobots to improve the eradication of bacteria can be noted. VAGHASIYA ET AL. | 9of13 2688819x, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smm2.1289 by Technical University Ostrava, Wiley Online Library on [03/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License