Self‐Healing, Recyclable, Biodegradable, Electrically Conductive Vitrimer Coating for Soft Robotics

Sensors and transducers enable the robots’ movements and interactions with humans and the environment. Particularly, tactile and motion sensors, even those inspired by the human skin, often miss many of its essential features. Indeed, the materials that constitute such sensors are often rigid and lack self‐healing and biodegradability. Furthermore, the large‐scale diffusion of these technologies propelled by robots spread in many aspects of the lives, from industrial to household settings, contributes heavily to the electronic and robotic waste problem. Recycling strategies for materials for robotics sensors are thus pivotal for future development. This work proposes self‐healable, recyclable, and biodegradable electrically conductive coatings. These coatings are based on conductive inks that combine graphene nanoplatelets and carbon nanofibers with a soft biodegradable vitrimer binder and are realized by spray coating. The use of the vitrimer ensures satisfying adhesion to diverse substrates, flexibility, conformability, self‐healing, and recyclability of the conductive coating. This material is a sustainable alternative to standard conductive inks for flexible electronics and soft robotics. Indeed, tests for the live monitoring of SoftHand3, the grasping system of many worldwide diffused robots, have yielded promising results. The use of biodegradable ingredients and the possibility of recycling makes it an appealing material to face the sustainability issue of today's electronics and robotics.


Introduction
Consumer electronics are pervasive in our society, representing a global market of about one trillion $ with thousands of companies and billions of users. [1]However, current electronics are designed in a linear economy context (i.e., produce, use, and dispose) without considering sustainability and end-of-life issues.][10][11] In parallel, robots are becoming ubiquitous, considering their advantages in repetitive and human-exhausting works and activities.Thus, robotic waste (bot-waste) is accumulating globally. [12,13]n this regard, the irruption of soft, compliant, bio-based, and biodegradable alternatives that are also recyclable will herald a paradigm change for the progress of environmentally friendly materials in robotics, reducing bot-waste. [12]ost humanoid robots are assembled with rigid, nonfunctional, and long-lasting materials.][26] The first approach is more straightforward in design and read out compared to the others, and can show a fast response time (usually few ms) in a wide range of forces. [28][37][38][39] Green piezoresistive composites are manufactured simply and through techniques compatible with the polymer industry. [26,40]Nevertheless, they are prone to mechanical hysteresis, especially using biopolymers that are not as deformable as their unsustainable counterparts. [41]els are often degradable but can dehydrate and since they conduct ions, they cannot sustain a direct current supply. [34,42]The functionalization of a substrate with electrically conductive materials can have the advantages of easy and large-scale manufacturing as well as controlling the hysteresis, which depends largely on the features of the substrate employed. [36,43,44]ubstrates are typically coated using metallic or nonmetallic conductive inks. [39,45]Metallic ones are the less resistive but also the priciest. [39]Nonmetallic inks include conductive polymers or nanocarbons.They are more electrically resistive than metal inks. [39]However, they have other crucial benefits: i) they do not need a post-coating sintering step, which can damage low thermal budget soft polymeric substrates, and ii) they are easier to disperse using water or alcohols. [39]Especially some carbon-based nanomaterials, such as graphene nanoplatelets (GnPs) and carbon nanofibers (CnFs), are already produced at a scale compatible with the polymer industry (i.e., thousands of tons per year).Moreover, they exhibit satisfying electrical properties at a low price (i.e., less than 0.1 $/g and a few $/g, respectively). [46]All the above combined with the facile formulation of these nanocarbon inks have promoted the prototyping of devices across a broad spectrum of domains, including soft strain sensors [44,[47][48][49][50][51][52] and motion/tactile sensing. [24,48]part from self-healing, degradation, motion detection, and temperature sensing, which represent the holy grail of soft robotic skins, reusability and recyclability are two aspects that should become a must-have for the incoming materials in electronics/robotics. [12,[53][54][55] Such circular economy concepts are limited in the soft robotic sector but would diminish the environmental impact of the designed robots and enhance their sustainability. [12,56]One example of robotic component recyclability is the work by Partridge A.J. et al., [56] in which elastomeric materials from broken soft actuators are recycled to remold actuators without loss of function.Another example is the one from Heiden A. et al. [57] in which they develop biogels that can be 3D printed several times to make soft actuators.Mechanical and chemical recycling of polyimine magnetic composites for robotic applications was presented by Zhu and coworkers. [58]ther recent approaches rely on the recycling of sensors.[61][62][63][64][65][66] The only few examples we found about recycling an electrically conductive composite with an application as a strain sensor are the recent works by Yuan D. et al. [67] and Guo Z. et al.
. [68] In the first, an epoxy resin waste is transformed into a vitrimer by transesterification catalyst infusion process that enabled recycling and compounding with carbon nanotubes.In the second, bulk carbon nanotubes composites can be reprocessed after damage.Nevertheless, such materials are not designed for human-mimicking robots and are bulk freestanding composites, not always the best choice for integration in existing electronics/robotics systems.Moreover, degradability, self-healing or temperature-sensitivity are not investigated.
A group of polymers known as vitrimers is a promising alternative to conventional thermosets because they exhibit dynamic cross-links that simplify processing and recycling. [69]At service temperatures, they behave like permanently cross-linked polymers, but at higher temperatures, the exchange reactions within their network accelerate, allowing flow, while keeping a consistent number of chemical bonds and cross-links.Therefore, they may be repaired, reshaped, and recycled using standard thermoplastic polymer processing techniques like extrusion, 3D printing, or melt mixing.Furthermore, environmentally friendly characteristics can be added to such materials by using biobased, waste-based, and/or degradable precursors to produce vitrimers.Nevertheless, these biobased, high-performance vitrimers are currently scarce.[72] Their large-scale research, engineering, production, and use may increase if they branch out into other industries like electronics and robotics.In this context, there are only a few works.Guo Z. et al. [68] and Zhang J. et al. [73] propose vitrimer carbon nanotube composites with application in human motion sensing.Deng J. et al. [74] use vitrimer silver nanowires composites for triboelectric nanogenerators and wearable electronics.These works are the only ones that present vitrimers in the field of electronics, while none is in the robotics one.Furthermore, the results deal mostly with bulky composites, that are rarely biobased and/or degradable or self-healable.
In the present work, we demonstrated for the first time the use of a biobased and biodegradable vitrimer as a binder for a conductive ink comprising GnPs and CnFs.The ink is green since it uses soybean oil as a precursor for the vitrimer and ethanol as a solvent.Many different substrates, such as natural rubber, paper and glass were easily coated through a simple, large-scale spraypainting method.The conductive coatings showed a low resistivity in the order of 10 −4 Ω m.Exploiting the unconventional mechanical properties and self-healing features of the vitrimer (T g of ≈10 °C), combined with the elasticity of the natural rubber substrate, human mimicking strain sensing soft robotics skin were fabricated.The vitrimer provided other desirable features of human skin, such as temperature sensitivity and degradability.Furthermore, it enabled recycling of the substrate and the coating itself simply using ethanol.The resulting device was tested for real-time monitoring on robotic handsets, SoftHand3, the grasping system of many worldwide diffused robots, with excellent re- sults.To the best of our knowledge, this is the first time a vitrimer has been used for applications in soft strain sensors for robots.

Fabrication Procedure and Morphology
The ink preparation and deposition process are schematized in Figure 1a.The boronic ester vitrimer developed from soybean oil in our previous work was selected as a binder thanks to its flexibility, recyclability, biodegradability and ability to self-heal after being damaged (i.e., cuts or cracks) at ambient conditions. [69]ven though the vitrimer is covalently crosslinked, it can be decrosslinked on-demand by the hydrolysis of boronic esters and dissolved in 90% vol ethanol.This solvent was selected since it belongs to the green solvent category and it can readily disperse carbon nanomaterials. [6,39,75]A mixture of GnP and CnF with a weight ratio of 1:1 was selected to achieve a good performance-cost balance.Indeed, although being more expensive, the tube-like shape of CnF can provide more connection points and therefore higher conductivity than GnP at lower nanofiller loading. [6,46]The conductive fillers were easily dispersed in different wt.% in the vitrimer solution using less than 3% of Tween80, a nontoxic, edible, and biodegradable surfactant derived from polyethoxylated sorbitan and oleic acid. [76]The obtained dispersions were used for the vitrimer conductive coatings (VCCs).Spray coating was employed as an upscalable and industry-relevant technique in contrast, for example, to commonly used spin coating suitable only for small surfaces.The coating was successfully deposited on several substrates with different mechanical properties and polarities, demonstrating its versatility.Natural rubber was used as an elastic, biobased, micro rough, and highly stretchable substrate, paper as a fold-able fibrous one, and glass was chosen as a rigid flat substrate (Figure 1a).For soft robotics applications and especially for the development of strain sensors, highly elastic substrates are needed.Therefore, most characterizations were performed with natural rubber as a substrate.The other important parameter of materials used for strain sensing is the minimal creep.The vitrimer relaxes stress even in ambient conditions due to the exchange of boronic ester crosslinks and does not return to its initial length after stretching.Therefore, it cannot be used as a freestanding material for strain sensing and must be deposited on an elastic substrate with minimal creep.Natural rubber has such features and can ensure reliable and long-lasting performance.The natural rubber used in this work has two sides (matt and shiny); the coating was deposited on the matt side, showing more roughness which is known to improve the mechanical interlocking of the deposited coating (Figure S1, Supporting Information). [24]EM images in Figure 1b show the top view and the crosssection before and after the hot press of the vitrimer conductive coating with 20 wt.% (VCC20) conductive nanofillers on natural rubber.Figures S2 and S3 (Supporting Information) show SEM images of coatings with lower and higher concentrations of CnFs and GnPs compared to Figure 1b.The increasing weight percentages of fillers in the samples decrease noticeably the compactness of the coating with a consequent increase in their thickness.As discussed next, such morphology can have disadvantages concerning the adhesion and electrical conductivity (Figure 2). [6,77]or this reason, coated substrates were hot pressed at 140 °C, obtaining coatings with no visible voids and a reduction of thickness from 50 to ≈10 μm.The vitrimer binder and the natural rubber did not degrade to volatile compounds at the pressing temperature, as demonstrated by the TGA analysis (Figure S4, Supporting Information).Hot pressing improved the adhesion of the coating to more than 2.5 mN m −1 on the natural rubber, as demonstrated by the tape test shown in Figure S5 (Supporting Information).

Electrical Characterization
The uncoated natural rubber and the pure vitrimer were electrically insulating, with resistances above 10 9 Ω, corresponding to the instrument limits.The coatings on natural rubber containing 10, 20, and 30 wt.% of the conductive fillers (GnPs:CnFs at 1:1 weight ratio) from now on will be referred to as VCC10, VCC20 and VCC30 respectively (Table S1, Supporting Information).Resistances of the spray-coated samples on natural rubber extracted from the current-voltage curves were converted to resistivity considering the sample geometry and plotted in Figure 2a.The VCCs showed Ohmic I-V curves (Figure S6, Supporting Information), which implies the achievement of the electrical percolation threshold.Indeed, thanks to the high aspect ratio and different geometries [78,79] of the GnPs and CnFs fillers, a network allowing electrical transport was created.
As shown in Figure 2a, for the different nanofiller concentrations, the resistivity was 3.8•10 −1 Ω m for VCC10, 3.0•10 −2 Ω m for VCC20, and 7.6•10 −3 Ω m for VCC30.The hot-pressing pro-cedure decreased the resistivity value by order of magnitude.In particular, the VCC10 reached 3.3•10 −2 Ω m, the VCC20 reached 9.7•10 −4 Ω m, and the VCC30 displayed 7.5•10 −4 Ω m.Such improvement can be attributed to an enhanced percolative network of the nanocarbons inside the vitrimer binder, which have a denser number of connections among themselves compared to unpressed ones, and to the lower thickness of the samples after hot pressing.In addition to natural rubber, paper and glass were coated with the conductive ink containing 20 wt.% fillers and after hot-pressing showed resistivities comparable to the VCC20 on natural rubber (Figure S7, Supporting Information) demonstrating the broad applicability of the developed conductive coatings.These coatings are among the most conductive in the literature of 2D material-based coating with carbon based nanomaterials, whose resistivities usually range between 10 3 and 10 −5 Ω m. [39] The electrical response of the samples was tested versus tensile deformation, as shown in Figure 2b.The samples have been stretched till rupture that occurred ≈1000% elongation, as shown in the stress-strain curve in Figure S9 (Supporting Information).Notably, the relative resistance (R/R 0 ) changes in a nonlinear fashion with the applied strain for all three VCCs, while normally a monotonic increase of the resistance with strain is observed. [80,81]82][83][84] VCCs with various conductive fillers loadings show different behavior.For all three samples, the relative resistance increases in the first stage of the stretching (< 10%, labeled as "I") indicating that the conductive nanofillers network gets partially interrupted (scheme of Figure 2b).In the second stage (between 10% and 50% stretch, labeled as "II") the VCC10 and VCC20 samples show a decreasing R/R 0 [81] while for VCC30 the rate of increase of the relative resistance with strain slightly diminishes with respect to stage "I".A zoom in highlighting the described trends till 50% elongation is provided in Figure S8 (Supporting Information).The behavior of VCC10 and VCC20 during stage II might be attributed to the reorganization and alignment of the conductive nanofillers inside the soft vitrimer, which restores the conductive network and improves the fillers percolation.The VCC10 improves the network so much at this stage that the R/R 0 is lower than the initial value before stretching (R/R 0 < 1).Such a process is less effective as the concentration of the conductive fillers increases, as evident for the VCC30, since the densely packed nanofillers have less freedom to reorganize, align, and move.Then, for all the samples, when maximum rearrangement/alignment is reached (>50% stretch, labeled as "III"), the behavior is governed by sliding of the conductive fillers one on top of each other along the "flake-flake", "flakewire", and "wire-wire" direction that led to a slow change in resistance due to the high aspect ratio of the filler.The nanofillers slide on top of each other along their "long" direction.As such, the network breakdown in stage III is slower than the one in stage I of Figure 2b.When stretched to 200%, VCC10, VCC20, and VCC30 reach the relative resistivities of 1.07, 14.20, and 258.00, respectively.Further stretching results in a formation of macroscopic cracks and a complete breakdown of the conductive network of VCC30 (Figure S10, Supporting Information) while VCC10 and VCC20 remain conductive up to 500% elongation (Figure S11, Supporting Information).This behavior is due to the high load of nanofillers in the VCC30 coating, which makes the sample more brittle.

Application on Robotic SoftHand3
Next, we discuss how the electrical resistance changes with repeated stretch release cycles.Repetitive motions of humans' and humanoid robots' joints like fingers, elbows, and knees are subjected to cyclic strains up to ≈20%. [69]The repeated stretchrelease test was performed directly on a robotic hand designed to perform repeatable and well-controlled opening-closing motion.The natural rubber coated with VCCs, after their manufacturing, was applied and conformed on top of the fingers of SoftHand3 robotics hand.The hand was programmed to close and open ten consecutive times, and R/R 0 was monitored during the movement (Figure 2c and d; Figure S12, Supporting Information).All three materials showed sensitivity to the robotic hand movement with a stretch of ≈20% and good reproducibility, as shown in Figure 2d.Increasing the conductive fillers content, the relative resistance measured when the hand was fully closed (maximum stretch) decreased significantly from ≈1200 for VCC10 to 35 for VCC30.For all the samples, every closing-opening cycle resulted in almost the same R/R 0 values when the hand was fully closed and fully opened.Moreover, all the materials showed a very satisfying recovery of relative resistance after the 10th cycle was completed, with values 1.04 for VCC10, 1.48 for VCC20, and 1.24 for VCC30 after ≈300 s of relaxation time.Note that the variation of resistance observed in repeated stress release test at 20% strain is higher than the one observed in monotonic strain tests of Figure 2b at the same strain, since the strain rate in this second test is two orders of magnitude higher.Strain rate is known to strongly influence the piezoresistivity of materials. [85,86]Our work is the first example of a strain sensor based on a vitrimer conductive coating successfully applied in robotics.Previously, a disulfide vitrimer conductive composite has been used to detect human joints motion. [68]However, the proposed disulfide vitrimer is not a coating but a bulk material, making it less versatile and more cumbersome to apply in existing robotic designs.Moreover, it lacks self-healing at room temperature, a crucial property that our material has and that will be game-changing for electronic skins of the future, as discussed in Figure 4a.
This specific robotic hand was not able to make well controlled, complicated movements.As such, we used a tensile tester to simulate the real-life situation of variable movements and deformations that could be precisely controlled.Two deformation regimes were designed and tested, as described in the Supporting Information.The sensors could detect cyclic progressive deformations (Figure S13, Supporting Information) and step progressive ones (Figure S14, Supporting Information).From the progressive deformation test, where samples were kept at fixed elongation for 1 min between the steps, characteristic relaxation times could be calculated (Figure S15, Supporting Information).The relaxation times increase from 11 s for VCC10, 23 s for VCC20 to 55 s for VCC30.The longer relaxation times can be most likely attributed to the decreased mobility caused by the higher loading of nanofillers inside the vitrimer matrix.

Recyclability
Even though the vitrimer used as a binder is a covalently crosslinked material, the boronic ester crosslinks can be hydrolyzed by water in 90% vol ethanol, which was also used to prepare the inks for spray coating.The hydrolysis and subsequent boronic esters regeneration upon solvent evaporation and drying is reversible multiple times and can be employed to recycle and reuse the VCCs.A coated substrate can be simply placed in 90 vol % ethanol solution for a few hours at room temperature until complete hydrolysis and dissolution of the vitrimer matrix (Figure 3a).Subsequently, the substrate, after being washed with a small volume of the fresh solvent and dried, can be reused.The dissolved vitrimer with the remaining conductive fillers form a recycled ink that can be applied again by spray coating (Figure 3b).The recycling was performed using VCC20 on a glass substrate since it allows for a better visual demonstration.After spray-coating of the recycled ink, the resistance was measured again converted to resistivity.Such a process allowed to obtain resistivity almost equal to the original (/ 0 = 0.94 ± 0.10).In Figure 3c is shown SEM images of the neat and recycled VCC20 coating on glass.The two coatings are equivalent in terms of mor- phology.While recyclable by boronic ester hydrolysis the vitrimer matrix is at the same time resistant to water.Its hydrophobic nature prevents water absorption protecting boronic ester crosslinks from hydrolysis. [69]To trigger the hydrolysis for recycling purposes an organic solvent miscible with water that can swell and dissolve the vitrimer is needed to enable water penetration and hydrolysis of boronic esters.There are only a few examples of recyclable conductive vitrimer coatings in literature.The imine vitrimer conductive composite published by Zhang et al. [73] could be chemically recycled and reused but only using toxic DMF as a solvent and excess amine to break the cross-links and dissolve the vitrimer.To regenerate the material, terephthalaldehyde had to be added to rebalance the stoichiometric ratio between the amine and aldehyde groups.In another work, Guo et al. [68] recovered just the conductive filler from the vitrimer composite by breaking the cross-links and dissolving it in DMF-containing thiols and they did not recycle the whole material.

Mimicking of Human Skin
Human skin has self-healing properties and can repair scratches, cuts, and wounds.In addition, it can sense not only motion and touch but can also detect temperature.The other important aspect of human skin as a biological material is that it undergoes biodegradation in the natural environment.Thus, newgeneration electronic skin should also target these properties to mimic human skin accurately.Thanks to the intrinsic vitrimer feature, the VCCs present outstanding self-healing and recovery of electrical conductivity (Figure 4a) after physical damage.Indeed, cracks and gaps were formed after stretching the VCC20 to 80% elongation (Figure S16 Supporting Information for the experimental setup used), as revealed by SEM, and the relative resistance increased to 3.81.When the strain was released and the sample was left for 24 h at room temperature, the defects disappeared, and the relative resistance returned almost to the initial value (R/R 0 = 0.97).The self-healing of the conductive network without external stimuli like pressure and/or heat is possible thanks to the following: i.The low glass transition of the vitrimer matrix that is ≈10 °C.
ii.The presence of boronic ester cross-links that require low activation energy (E a = 29 kJ mol −1 ) and can exchange at room temperature.
The conductive vitrimer composite based on disulfide exchange proposed by Guo and coworkers [68] could be repaired however, hot pressing at 90 °C for 30 min had to be employed, most likely due to a high T g of almost 40 °C and a high activation energy of 83 kJ mol −1 .The other example of a conductive vitrimer composite able to repair cracks and cuts is a system utilizing imine exchange published by Zhang et al. [73] Nevertheless, hot pressing at 90 °C for 4 h was needed, and a few drops of a toxic DMF solution containing amines were used to heal through transamination.The VCCs showed to be sensitive to temperature in the tested range (30-120 °C), and the relative resistance R/R 0 decreased with increasing temperature (Figure 4b; Figures S17 and S18, Supporting Information).The VCC10 with the lowest conductive filler loading shows the highest sensitivity, followed by VCC20 and VCC30, respectively.The higher the conductive fillers loading, the less mobility they have for rearrangements at elevated temperatures.Moreover, when the conductive network rearranges, it has a less acute effect on the overall relative resistance since more alternative connections exist in the material with higher filler content.Ad-hoc characterization and measurements are needed in another follow-up study to corroborate such hypothesis.
In case of accidental release of VCC into the environment, it is pivotal to understand what happens to such a class of materials in terms of degradation.To simulate the accidental release in the marine environment, we performed biological oxygen demand (BOD) analysis in seawater (Figure 4c).As previously demonstrated, the vitrimer binder shows high biodegradation starting only three days after the insertion in seawater and reaching a BOD value of 15.7 mg O 2 /100 mg of the material. [69]As a wellknown biodegradable reference, we tested printing paper showing slower biodegradation over 30 days.Indeed, it started to de-grade after a week and reached a BOD value of 0.7 mg O 2 /100 mg at the end of the test.Natural rubber substrate started biodegradation in seawater after 5 days, reaching 3.5 mg O 2 /100 mg after 30 days.Moreover, small holes and cracks were present on both surfaces (shiny and matt) of the natural rubber at the end of the test period, as demonstrated by SEM analysis, additionally confirming the biodegradation happening (Figures S19 and  S20, Supporting Information).It was previously demonstrated that natural rubber could undergo biodegradation in soil thanks to the presence of bacteria and fungi, which utilize it as a carbon and energy source. [87,88]To the best of our knowledge, this is the first time the degradation of natural rubber has been tested in seawater.The lower degradation of the paper compared to the rubber could be due to the higher crystallinity of cellulose fibers compared to the natural rubber.The introduction of GnP and CnF conductive fillers did not prevent the biodegradation of the vitrimer binder, and both paper and natural rubber coated with VCCs were still biodegradable.Lower BOD value recorded after 30 days for VCC20 on paper (3.2 mg O 2 /100 mg) than for VCC20 on natural rubber (6.1 mg O 2 /100 mg) is most likely due to lower BOD values of the paper substrate itself.Higher BOD values were observed after 30 days for VCCs containing more conductive fillers.This faster degradation with higher filler con-centration may be because their presence enables water to penetrate faster and more efficiently in contact with the vitrimer.

Conclusion
A recyclable conductive ink based on GnPs and CnFs filled vitrimer has been developed in this study, bearing in mind circular economy and green principles from the choice of materials to the fabrication and recycling process.The conductive coating was produced by combining the insulating vitrimer binder with the conductive fillers, using 90% vol ethanol as a green solvent.The ink could be applied by spray coating on substrates with different chemical and mechanical properties, such as natural rubber, paper, and glass, and subsequently, compression molded to increase the compactness of the coatings and their adhesion.I-V curves were ohmic, and the compression process highly reduced the resistivity of the coatings, reaching a low value of 7.5•10 −4 Ω m for the highest nanofiller loaded samples.Such coatings showed a nonmonotonic piezoresistive behavior.They were implemented directly on a real robotic hand with promising results.Moreover, the use of the vitrimer as the binder enabled the recyclability, self-healing and temperature sensitivity of the coating.Biological oxygen demand tests confirmed the high degradability of the vitrimer-containing samples.The systematic engineering of such technologies in the electronics and robotics sector could spur circular economy and green approaches in such expanding areas.Vitrimers could enable human-mimicking soft sensors for humanoid robots.

Experimental Section
Materials: Vitrimer based on epoxidized soybean oil acrylate crosslinked with diboronic ester dithiol was used as a binder. [69]CnFs (length 20 -100 μm, diameter 100 nm, and aspect ratio between 200-1000) were purchased from Merck.GnPs (6-8 nm thick, 25 μm wide with a surface area of 120 to 150 m 2 g −1 and aspect ratio between 3125-4167) were purchased from STREM Chemicals.Black natural rubber (thickness of 0.25 mm) was used as a highly elastic substrate purchased from 4Drubber.75 g m −2 printing paper was used as a fibrous substrate, and microscope glass slides were utilized as a flat substrate.Silver conductive paint with a resistivity of 10 −5 Ω m was purchased from RS Components.Analytical grade ethanol and Tween80 (a non-ionic surfactant based on sorbitane monooleate) were purchased from Merck.All chemicals were used as received unless otherwise stated.
Ink Preparation: The vitrimer binder was hydrolyzed and dissolved in 1 mL H 2 O and 9 mL ethanol at RT for 24 h. [69]CnFs and GnPs were dispersed in 20 mL ethanol and 50 μL of the surfactant Tween80 each and sonicated for 1 h.The vitrimer solution was filtered to remove any possible solid residues (< 1%).Tween 80 improved ink stability over an extended period.Without the surfactant, the ink could still be used immediately after fabrication, yielding similar results.However, it was observed that without Tween 80, the ink tended to aggregate over a matter of hours.To ensure the ink's usability even after storing it for a few weeks, the decision to include Tween 80 was made, which effectively extends the lifetime of the fabricated inks.Three kinds of samples were prepared by mixing the aforementioned components, following the quantities reported in Table S1 (Supporting Information).The obtained mixtures contain respectively 10 wt.%, 20 wt.%, and 30 wt.% of the fillers with respect to the total weight of the coating material and are referred to as VCC10, VCC20 and VCC30 (vitrimer conductive coating).Each of them contains as a solvent 49 mL of ethanol, 1 mL of water, and 100 μL of Tween80.The prepared vitrimer-GnPs+CnFs inks were again sonicated for 1 h before spray coating.
Coating Method: An area of 100 cm 2 of the substrate was fixed onto a metal plate at an angle of 60°with the floor.An airbrush atomizer spray coater (VL Siphon feed, 0.73 mm internal nozzle diameter, Paasche airbrush) was used to spray 50 mL of a solution with a pressure of 2 bar, keeping its nozzle at a distance of ≈15-20 cm from the substrate.The obtained samples were left to dry, then hot pressed using a Carver press for one hour at 140 °C applying low load of 0.1 ton to avoid the substrate's deformation and improve compactness, adhesion and electrical conductivity of the coatings.
Characterization: At least three samples were measured for each test, unless specified differently.
SEM Analysis: SEM images were acquired using an analytical (low vacuum) scanning electron microscope using a JSM-6490LA SEM (JEOL) at 5 kV acceleration voltage.For the cross-section analysis, the samples were submerged in liquid nitrogen to allow easy breakage and preserve morphology and then bent until they broke.Afterwards, the samples were attached to an appropriate stub using carbon tape and sputter-coated in the Cressington 208HR sputter coater with a 10 nm-thick layer of gold to reduce charging effects.
Tape Test: A scotch tape with known adhesion force to steel of 2.5 mN m −1 was applied onto the coated substrate and peeled off after 10 s to probe the adhesion of the coating to the substrate.Specifically, this test was used to compare and evaluate the improvement of the adhesion before and after compression molding.
Electrical Characterization: Electrical properties were measured using Signatone 1160 probe station connected to a Keithley 2612A source meter, to record I-V curves with a two-point probe method.For simple resistivity tests, samples with an area of 10 mm 2 were fixed by double sided adhesive tape onto glass slides.Two electrodes were painted on opposite sides of the surface using silver conductive paint.The applied electrical voltage at the electrodes was varied from −1.0 to 1.0 V and backwards.The resistivity was calculated using Equation 1, where V and I are the applied voltage and the measured current, and w, t and L are the dimensions of the sample.
Electrical Characterization under Deformation: Tensile deformation was applied by Instron dual column tabletop universal testing system 3365 coupled with the Keithley 2612A source meter.All measurements were conducted on dogbone specimens ISO 527-2 type 5A.The applied voltage was kept at 1 V for all the duration of the measurements.
During the constant strain test the electrical resistance of the samples was measured during the tensile elongation up to 500% strain with a strain rate of 5 mm min −1 , corresponding to 0.2 min −1 strain rate.
To perform cyclic strain, test a strain, increasing by 5% at each cycle, with a strain rate of 5 mm min −1 was applied and then immediately released while the resistance was continually monitored.A release time of 1 min was used before proceeding with the next cycle, up to a final deformation of 50% strain.After reaching the maximum strain of 50%, the samples were brought back to their initial length and allowed to relax for 400 s.
While performing the progressive strain test the samples were stretched for eight progressive steps of 10% strain each, from 0% up to 80% strain at a rate of 5 mm min −1 while the electrical resistance was continually monitored.In between the steps, the samples were still left for 1 min before proceeding with the successive elongation step.
Monitoring of the Robotic Hand Motion: Real time activity of the robotic hand was monitored at finger joints, coupling the Keithley source meter with a cyclic motion of the robotic SoftHand3.The code for the cyclic motion of the hand was written in Matlab Simulink exploiting some preexistent packages from the Piaggio research center of Pisa.The sample was fixed onto the fingers with Bostix Poly Max transparent silicon glue.Two copper electrodes were then fixed at its ends with paper tape.A camera was used to record the real time evolution of the hand motion and the simultaneous measurement of the resistance changes at a fixed applied voltage of 1 V.

Dependence of Electrical Properties on Temperature:
The relative electrical resistance of the prepared samples was tested under the influence of temperature.A dogbone shaped sample was fixed in a petri dish with a tape and its two ends were connected using crocodile clamps (Figure S17, Supporting Information).The IV curves were recorded by the Keithley source meter applying a constant voltage of 1 V (Figure S18, Supporting Information).The sample was heated inside the SH-262 Benchtop atmospheric chamber (ESPEC NORTH AMERICA, INC) from RT to 120 °C applying ramps of 10 °C with a rate of 1 °C min −1 and 1 min equilibration times.
Reusability and Recycling: The glass substrate coated with VCC 20% was put into a solution of 90% vol ethanol and 10% vol water and left soaking for 3 h.After that, the glass substrate was removed, washed with a small amount of fresh solvent and dried at RT.The remaining solution containing the conductive fillers was sprayed on the same glass substrate, hot pressed and the final resistivity was measured.
Biodegradability in Marine Environment: Biodegradability of the samples was assessed by measuring biochemical oxygen demand (BOD), which can be determined by monitoring the oxygen consumption in a closed respirometer.In details, ≈100 mg of sample was added to 432 mL of seawater collected from the Genoa Sea harbor as the single carbon source.The seawater was chosen in order to mimic real environmental conditions.It already contains microbial consortia and the saline nutrients needed for their growth.The experiment was conducted at room temperature inside dark glass bottles with a volume of 510 mL, hermetically closed with the OxiTop® measuring head.A CO 2 scavenger was added in order to sequestrate carbon dioxide produced during the biodegradation, and biotic consumption of the oxygen present in the free volume of the system was measured as a function of the decrease in pressure.Raw data of oxygen consumption (mg O 2 /L) were corrected subtracting the mean values of the blanks, obtained by measuring the oxygen consumption of the seawater in absence of any test material.After this subtraction, values were normalized on the mass of the individual samples and referred to 100 mg of the material (mg O 2 /100 mg).

Figure 1 .
Figure 1.a) Schematic representation of the coating fabrication on natural rubber, paper, and glass and b) SEM images of the top view (left) and the cross-section (right) before and after the hot press of the vitrimer conductive coating with 20 wt.% (VCC20) conductive nanofillers on natural rubber.

Figure 2 .
Figure 2. a) Resistivity of the vitrimer conductive coatings (VCCs) before pressing (NP are the initials for Not Pressed) and after hot pressing.b) Change in the initial value of resistance (R 0 ) of the conductive coatings with continuous strain and schematic representation of the distribution of conductive fillers within the vitrimer matrix at different strains.c) Schematic representation of the VCC on natural rubber strain sensor and its placement on robotic SoftHand3.d) Variation of R 0 with cyclic deformation caused by closing and opening of the robotic SoftHand3 and schematic representation of the first two deformation steps.

Figure 3 .
Figure 3. a) Digital images of the VCC20 on glass substrate submerged in 90% ethanol at different time points.b) Schematic representation of the vitrimer conductive coating (VCC) recycling process allowing for full reuse of the coating c) SEM images of the neat and recycled VCC20 coating on glass.In the recycled image is also reported the ratio between the resistivity obtained after recycling and the initial one.

Figure 4 .
Figure 4. a) SEM images (above) and digital images (below) of the neat vitrimer conductive coating containing 20% of conductive fillers (VCC20), deformed to 80% strain and released and left to relax for 24 h.b) Relative resistance changes with increasing temperature.c) Biological oxygen demand (BOD) of the vitrimer binder, natural rubber substrate, VCC10, 20 and, 30, as well as the paper substrate and VCC20 on paper.