A Sprayable Electrically Conductive Edible Coating for Piezoresistive Strain Sensing

Edible electronics leverages the electronic properties of food‐derived materials to deliver safer technologies that can be degraded (or digested) in the environment (or body) at the end‐of‐life. Sensors will be central to future smart edible robots, and edible strain sensors are particularly interesting as they can transduce deformation, providing real time feedback of the movement. Yet, to date edible strain sensors have been limited to the use of ionic conductive hydrogels, resulting in sensors not directly suitable for direct current operation and therefore not compatible with existing edible batteries. Here, the first edible strain sensor based on electronic conduction made of a novel conductive ink sprayed over an edible substrate is presented. The ink formulation consists of activated carbon (conductor), Haribo gummy bears (binder), and water−ethanol mixture (dispersant). The ink, deposited on multiple substrates by spray deposition, produces edible electrically conductive composite coatings with resistivity of ≈50 Ω cm. The coatings were used as a piezoresistive layer to fabricate strain sensors with gauge factors of 19−92 suitable for direct current operation. As a proof‐of‐concept of future edible systems, the sensor is validated by integrating it within a gelatin actuator to produce a sensorized gripper powered by an edible battery.


Introduction
Globally, 40 million tons per year of electronic waste (e-waste) accumulate in landfills. [1]As the production of electronic devices continues expanding, the e-waste accumulation rate is expected to increase to 120 million tons per year by 2050. [1,2]To mitigate this figure, sustainable materials development for electronics and robotics is emerging as one of the objectives for technological innovation. [3,4]Thus, researchers have recently started to consider unconventional materials for green electronics and robotics, such as wood, [5] fungi, [6] algae, [7] and insects [8] among others.In this perspective, edible electronics aims at leveraging the electronic properties of food-derived materials to deliver technology. [9]Edible devices are not subjected to a complex recycling stream, are mostly biodegradable, are free from environmentally hazardous substances used in the fabrication of traditional electronics, [3] and-above all-are inherently safe upon ingestion.Finally, at the end of their lifetime, edible devices can be disposed of or repurposed in the same way as food waste. [9,10]These peculiar properties open previously unconceived scenarios: in agrifood, edible sensors can be applied in direct contact with food for quality monitoring, [11] for instance tracking fruit growth using strain sensors; [12] in healthcare, miniaturized edible systems can be engineered to acquire diagnostically relevant information for gastrointestinal (GI) tract monitoring before being metabolized by the body, eliminating the risks associated with retention of ingestible technologies; [13−16] edible robots, equipped with a range of sensors, could also deliver nutrition to humans in an emergency, [11,17] or act as drug-loaded prey for wild animals. [18]dentifying edible materials with electronic properties compatible with industrial processing methods and suitable for implementing edible sensors remains an open challenge. [9]Among the edible sensors so far demonstrated, [9,[11][12][13][18][19][20][21][22][23][24][25] a large interest has been shown for strain sensors, [19−23] as they are widely adopted in robotics for the ability to provide feedback from actuators. [21] Piezresistivity (resistance variation due to a mechanical deformation) has been exploited to produce edible hydrogel-based strain sensors [19,[20][21][22]26] with gauge factor in the range of 0.308 [19] −1.49.[20] Edible hydrogel-based strain sensors benefit from being tunable, easy to prepare, and can be eaten in large amounts.However, ionic conductors [27−33] rely on hydration and the charge transport mechanism involves ions migration instead of electrons flow. Thus, they are influenced by environmental conditions and are not compatible with recently developed edible power supplies [34,35] as they typically cannot be easily operated in a direct current regime. Eletronic conduction, where current is due to the movement of electrons, as in a common metal wire, is instead suitable for direct current operation.Examples of edible electronic conductors are metals, such as gold and silver.[9,36,37] Such conductors display excellent electrical conductivity, but their use in edible electronics is limited by the cost, the complex sourcing process, and the low acceptable level of intake (microgram per kilogram of body weight per day). On te other hand, activated carbon (AC, E 153) is an emerging candidate as edible electronic conductor, because it is authorized as a food additive by the European Food Safety Authority (EFSA), [38] can be eaten in larger quantities (mg kg −1 of body weight per day), [9] is economically affordable (< 0.3 € g -1 ), and is distributed as a powder, [24] which makes it ideal for composite formulations with resistivity in the range 0.5−1000 Ω cm.[24,39−44] AC has been used in combination with polydimethylsiloxane (PDMS), [39,40] concrete, [42] foams, [43] and hydrogels [45] to produce strain sensors for wearable and structural health monitoring applications.Also, a large variety of edible electronics components leveraging AC have been already implemented (e.g., electrochemical sensors, [46] supercapacitors, [47] batteries, [34,35] tilt sensors, [18] triboelectric generators, [41] electrodes, [24] pressure sensors [13] ).However, a food-based strain sensor employing AC as the electronic conductor suitable to be operated in a direct current regime and therefore compatible with edible batteries [34,35] has not been documented yet.
Here, we present an AC-based edible conductive ink and its scalable deposition method producing electrically conductive edible coatings that can be used as a functional piezoresistive layer for strain sensors.The ink is made of AC as the electronic con-ductor, commercial Haribo gummy bear as the binder, and water and ethanol as green dispersants (Figure 1a).The selected manufacturing method is spray-coating (Figure 1b), a non-contact and low-temperature technique, as it is well-established in both food [48] and electronics [49] industries, and it can accommodate the use of a large variety of substrates, including food (Figure 1c).The formulation can be deposited to create patterned conductive composite layers through masking, with a resolution of hundreds of microns.The edible conductive coating displays a resistivity as low as 53 Ω cm and piezoresistive behavior under deformation.The piezoresistive property of the coating was combined with stretchable edible substrates to produce edible strain sensors similar to those reported for wearable applications, [39,40] with a gauge factor in the range of 19.0−91.7 and linear response.The sensor presented here can operate with direct current and is therefore compatible with edible energy sources. [34,35]The sensor properties are validated by integrating it with an edible gripper [50] and an edible battery, [34] resulting in a sensorized edible gripper.

Ink Formulation and Deposition
The primary goal of the work resides in the edible ink formulation suitable for spray-coating, consisting of a conductor, a binder, and a dispersant, conceived to be produced at a large scale with widely available materials.AC was selected as the conductor.Haribo gummy was selected as a binder because of its wide availability, its viscoelastic properties (particularly useful in strain sensing [51−53] ), and its notoriety, which might contribute to increasing the acceptance rate of food-based electronics.Haribo gummy is a commercial candy product containing gelatine, glucose syrup, sucrose, dextrose, citric acid, vegetable oils, and fruit extracts. [54]Gelatin has already been adopted to produce degradable electronic and robotic components, [11,12,18,50] and Haribo gummy in particular has already been used for actuators, [51] sensors, [52] and structural components. [18,55]Water and ethanol were selected as dispersants to provide the volatility required for spray-coating.The ink was prepared by first dissolving gummies in deionized (DI) water at a concentration of 0.08 g mL −1 at 80 °C under stirring.Ethanol was then added to the solution in a volume ratio of 7:1, creating a dispersion of gummy nanoparticles with an average hydrodynamic diameter of ≈50 nm (Figure S1g, Supporting Information).
AC was added to the mixture as the last step of the formulation.The specific AC product herein adopted has micrometric size (Figure 1d), total pore volume of 1.14 cm 3 g −1 , specific surface area of 1245 m 2 g, average porous size of 3.4 nm, and typical graphitic properties (Figure S1, Supporting Information).To assess the electrical properties imparted to the composite by different AC loading, AC was added to the suspension in several filler:binder weight percentages, ranging from 0% to 200%, always using a fixed ink volume per unit area (0.6 mL cm −2 ).The suspension was then tip-sonicated before being freshly deposited onto the target substrate using a spray-coating setup.PDMS was used as the reference substrate for the analysis of the electrical and piezoresistive properties of the coating as it is largely characterized and adopted for flexible/stretchable electronics.The micromorphology of the 200% AC formulation deposited onto PDMS confirms the formation of a homogenous coating with clearly identifiable AC particles with a size in the range of 10−100 μm creating a rough surface (Figure 1d; Figures S1  and S2, Supporting Information).The coatings with 50% and 100% AC present a less uniform and dense AC covering compared to higher concentrations (Figure S2, Supporting Information).Spray-coating can also be used in combination with shadow masks to produce patterns.The smallest feature size (spatial resolution) achievable by spraying the formulation with 200% AC was assessed by fabricating patterns with progressively decreasing size with the aid of an aluminum shadow mask applied onto the target substrate.Deposited patterns were then visually inspected by SEM (Figure 1e,f; Figure S5, Supporting Information).
The spatial resolution observed through SEM imaging was between 150 and 300 μm.As such, the method is suitable for the fabrication of micrometric electronic components that can find applications in GI tract monitoring systems.

Electrical Characterization
The resistivity of the coating with different AC loadings (0%, 25%, 50%, 100%, 150%, 200%) was assessed by measuring the sheet resistance in a four-probe method and the thickness of the coating (Figure S3, Supporting Information).The resistance was above the measurable limit of the instrument (1 GΩ) for AC loadings of 0% and 25%.Starting from 50% AC, ohmic I−V curves could be recorded, with a resistivity of 295 kΩ cm (Figure 2a), demonstrating effective network percolation in the deposited coating.Coating resistivity dramatically decreased when increasing AC loading, reaching 53.8 ± 16.6 Ω cm for 200% AC.The achieved resistivity value is comparable with the one of pristine AC.
The coating stability (200% AC) was assessed by measuring the coating resistance over time in constant temperature and humidity conditions (25°C, 15% RH) for three identical samples.The coating displayed a total average resistance decrease of ≈4% from the initial measured value over 21 day.However, most of the resistance change was observed in the first 3 days (Figure 2b).While the initial deviations are possibly associated with the adjustment of the percolative network, the average relative resistance variation remains within ± 1% from day 6 to day 21, therefore indicating considerable stability over time.
Electrical impedance spectroscopy was performed to assess the electrical conduction mechanism (Figure 2c,d).For AC loading of 0% and 25%, the behavior of the composite is comparable to an open circuit condition, corroborating the previous observation of no network percolation.From 50% to 200%, a progressive decrease in the impedance modulus is observed.The zero-phase observed at low frequencies for these coatings indicates a purely resistive behavior, consistent with the quasi-static characterization and compatible with an electronic conduction mechanism (excluding ionic contributions that could arise from the binder).The majority charge carriers are determined to be holes given the positive measured Seebeck coefficients of 11.25 ± 2.6 μV K −1 for the 200% AC coating (Figure S4, Supporting Information), as usually observed for other carbon-based materials. [24]

Edible Strain Sensor
Piezoresistivity was observed by depositing a thin layer of the piezoresistive coating onto a stretchable material, an already established method in literature for wearable sensors. [39,40]To characterize the piezoresistive performance with respect to stateof-the-art sensors and decouple the effect of the substrate, the coating was first applied onto a well-established substrate for strain sensing, namely PDMS.SEM analysis under strain indicates that the piezoresistivity mechanism is based on microcrack formation (Figure 3a,b; Figure S6, Supporting Information).As the stretchable substrate with the applied conductive coating is subjected to increasing strain, fractures with a width up to 100 μm start to appear and to spread throughout the network, yielding an increase in the electrical resistance.SEM imaging also shows that the micro-cracks on edible substrates slowly heal (Figure 3c; Figures S6 and S7, Supporting Information).Therefore, the piezoresistive effect is reversible within a certain range, indicating that the material can be used for sensing.Strain sensors were prepared in a dogbone shape (Figure S7c, Supporting Information) by applying coatings with AC loadings of 100%, 150%, or 200% directly onto PDMS.The resistance of the sensor was measured while controlled strain (0−8% range) was being applied using a tensile tester.The highest average gauge factor (as per Equation (1) in Experimental Section) of 88.2 was observed for the PDMS-based strain sensor with 200% AC loading (Figure 3d), which is in line with state-of-the-art strain sensors made of graphene or carbon nanotubes with the same substrate. [49]ence, fully edible strain sensors were implemented by depositing a thin layer of the piezoresistive coating onto edible viscoelastic materials.In particular, two viscoelastic substrates were investigated: the first was produced using only gummy and water (hereafter referred to as gummy); the second was produced by using a blend of gum arabic and gummy in a 1:3 weight ratio and water (hereafter referred to with the acronym GA as per gum arabic).In both cases, gummy remained the main component to ensure material matching between binder and substrate yielding improved adhesion of the coating.The latter substrate exhibits an increased Young's modulus, from 1.82 MPa (gummy) to 21 MPa (GA), due to the presence of gum arabic.As such, while the use of coatings with different AC loadings can be used to modulate the sensor resistance, the addition of gum arabic was effective in modulating the mechanical properties of the substrate, as shown in Figure S8 and Table S1 (Supporting Information).By selecting different substrates and AC loadings, gauge factors in the range of 19.0−91.7 were obtained from the edible strain sensors, which were comparable with the gauge factor obtained using PDMS as substrate (Table S2, Supporting Information).In particular, strain sensors implemented using the 200% AC coating on gummy and GA substrates exhibited average gauge factors of 23 and 51.2, respectively, and, in both cases, a high linearity (R 2 > 0.998) (Figure 3e,f; Table S2, Supporting Information).Cyclic testing was then performed to assess the response to multiple stretch-release cycles.These tests were performed on PDMS-based and fully edible sensors using a repeated testing stimulus of 1% strain and 5 s hold time before strain release.The highly elastic properties of PDMS yield a very fast response, with recovery time in the order of seconds (Figure S11, Supporting Information).Differently, edible strain sensors showed a viscoelastic behavior (Figure S8e,f, Supporting Information) therefore a higher recovery time in the order of tens of seconds was observed (Figure 3g,h).Nonetheless, a repeatable output was observed using both gummy and GA substrates.By comparison of the cyclic testing performed on PDMS, the higher recovery time is associated with the mechanical properties of the substrate rather than of the coating.Although the high recovery time represents a limitation for applications involving rapid deformation, the strain sensor is suitable for applications involving relatively slow deformation, including structural health monitoring, pneumatic actuation, and fruit growth. [12]Further substrate optimization will be required to accommodate application scenarios requiring a rapid response.Nonetheless, this work demonstrates that strain sensors based on electronic conduction and exhibiting a gauge factor comparable to the state-of-the-art [49] are indeed achievable using only food-derived materials (Figure 3f).The full dataset of the strain sensors produced in this work is reported in Figures S9−S11, and Tables S1 and S2 (Supporting Information).
Bending sensing was also demonstrated using the 200% AC coating on both gummy (Figure S11h, Supporting Information) and GA substrates (Figure 4a), featuring relative resistance variations up to 30% in a sensing range of 0°−15°.A simulated digestion test was performed to quantify the time required for the degradation of the strain sensor.For the test, a coating with 200% AC was deposited onto a GA substrate and immersed in a simulated gastric fluid at 37 °C under constant stirring.After 15 min the material was completely dissolved, demonstrating the transiency properties of the sensor (Figure 4b; Video S3, Supporting Information).
The coating functionality was validated in an edible robotics application by integrating the piezoresistive composite onto a soft inflatable edible gripper, [50] producing a sensorized gripper capable of detecting its deformation when pressurized (see Figure S12, Supporting Information).Strain sensors applied on actuators are well-established in robotics, as they can provide real time feedback of the movement. [21,23,25]The simple and facile deposition method allowed direct spraying of the composite (200% AC) onto the gripper in a one-step process producing a piezoresistive electronic skin (e-skin).
The e-skin was effective in reliably detecting multiple actuations (Figure 4d) with different frequencies, hold time in the pressurized state, and intensities (Figure S12, Supporting Information).Subsequently, the e-skin was powered by a previously developed edible battery, [34] which can provide a voltage of ≈0.65 V and a current in the microampere range using only food materials.Data indicates that also in this condition the batterypowered piezoresistive e-skin is effective in detecting consecutive actuations and patterns (Figure 4e,f).The current modulated by the actuations obtained by supplying a constant voltage from the battery can be fed directly into a processing unit dedicated to automatic sensing and actuation of the robot.Using the edible battery would not have been possible with ionic strain sensors as typically they do not operate in a constant current/voltage regime.As such, this work not only demonstrates the compatibility of the method herein presented with edible batteries operating in a direct current regime but also represents the first integration among edible sensors, actuators, and electronic energy sources.

Conclusion
Edible electronics is in its infancy, therefore novel components are needed to unlock its potential and move towards more complex systems.Here we introduced a piezoresistive edible strain sensor exhibiting electronic conduction that overcomes limitations of previous implementations based on ion-conducting hydrogels and can operate in direct current operation.To produce the sensor, we first developed a and versatile method for the formulation of an edible activated carbon-based conductive ink made with commercial candies as binders, and water and ethanol as solvents.The ink can be sprayed on several substrates obtaining electrically conductive composite coatings.Then, the piezoresistivity of the coating is exploited in combination with a food-based stretchable substrate to produce fully edible strain sensors.Among the sensors produced, the best-performing one was obtained using a gummy/gum arabic substrate and a coating formulation with a 200% filler:binder weight percentage, which operates in a 0-8% strain range and exhibits a gauge factor of 51.2.The edible sensor can be used standalone for foodcontact applications, such as fruit growth monitoring, and can be disposed of as food waste.A proof-of-concept integration with edible robotic components was also demonstrated by depositing the piezoresistive coating directly onto a gelatin actuator, producing a fully edible sensorized gripper.The resulting piezoresistive e-skin on the actuator can be operated by edible electronic power sources and is effective in tracking the movement of the actuator.Regardless of the sensing application herein demonstrated, our technique offers a scalable method for AC composites deposition onto different substrates, producing patternable electrically conductive layers.This method can therefore untap the potential of AC in edible electronics, supporting a wide range of applications in food-quality monitoring, healthcare, and robotics.
Ink Preparation: Gummy (0.4 g) was dissolved in 5 mL of DI water at 80 °C under stirring using a 50 mL beaker.Ethanol was then added to the solution in a volume ratio of 7:1 under stirring and with no heat applied, forming a mixture with a high turbidity, characteristic of dispersed systems.This dispersion was stirred for 2 min before adding AC in the desired conductor-to-binder ratio.The suspensions formed after adding AC were stirred for 10 min at room temperature.Afterwards, the suspension in the beaker was tip-sonicated for 15 min (Branson Ultrasonics 450 Digital sonifier, titanium probe Branson Ultrasonics 101-148-070).The inks were always prepared just before the spray deposition.Typically, samples were produced with the same ink volume per unit area of 0.6 mL cm −2 .
Spray Coating: The spray deposition of the inks was performed under a chemical hood by using a commercial airbrush (Paasche VL Series) with a 750 μm nozzle supplied with compressed air at 2 bar.The airbrush and the target substrate were positioned perpendicularly: the airbrush was positioned at a fixed distance of 15 cm from the target substrate, which was held vertically using a custom holder.Samples were left to dry in air under the fume hood at room temperature for 2 h.A shadow mask was used to define geometry and resolution limit.The shadow mask was obtained either by using a custom paper stencil or by using thermal evaporator aluminum masks.
PDMS Substrates: PDMS was prepared by mixing the polymer with its curing agent (25 g, 10:1 ratio), degassed in a vacuum chamber, and poured into a glass petri dish (diameter 9.5 cm).Curing was performed at 90°C for 2 h.Afterward, PDMS was peeled off from the Petri dish and cut to size using a surgical blade and a stencil.For the electrical characterization of the ink, substrates were 5 × 1 cm 2 rectangles.For strain sensing, substrates were dogbone shaped as in Figure S7c (Supporting Information) with an active area of 3.0 × 0.6 cm 2 and ≈2 mm thickness.Before the deposition of the coating, PDMS substrates were exposed to oxygen plasma (1 min, 20 W).
Electrical Characterization: To measure the sheet resistance, the coating was deposited onto PDMS substrates.Three identical samples were produced for each filler:binder ratio under test.The sheet resistance was measured using a four-probe configuration to eliminate the contribution from the contact resistance.Four identical rectangular electrodes (W = 3 × 2 mm 2 ) with a constant distance of L = 5 mm were applied onto the coating using silver paint (RS PRO conductive paint 186-3600) as in Figure S3a (Supporting Information).The two external electrodes were connected respectively to ground and a sweeping current source using a B1500A Keysight Semiconductor Parameter Analyzer.The electrodes were contacted to the instrument using a probe station (Cascade Microtech) featuring precision micromanipulators and a microscope.The sheet resistance was then obtained by reading the voltage difference between the two internal electrodes, dividing it by the supplied current and multiplying for the geometrical factor W/L.The height of the coating was measured by imaging the section using a lateral microscope.The resistivity was then obtained by multiplying the sheet resistance with the coating height.
To assess the stability of the coating, three identical samples were introduced into an environmental chamber (Memmert HPP110) with fixed temperature and humidity conditions (25 °C, 15% RH), contacted using standard wires and connected to an external source meter (Keithley 2612A) through cable windows.The resistance of the three samples was measured on day 0 (i.e., immediately after fabrication and 2 h drying), 1, 2, 6, 9, 12, 14, and 21.Each datapoint in Figure 2b is obtained by calculating the relative resistance variation with respect to the previous datapoint.
To measure the impedance of the conductive coating, samples were prepared as above.Two rectangular electrodes (3 × 2 mm 2 ) were applied onto the coating using silver paint with a distance of 5 mm.The impedance was measured using a potentiostat (Multi-PalmSens 4) by applying a sinusoidal stimulus of 100 mV within a frequency range of 10 Hz−100 kHz.
Strain Sensor Fabrication: Strain sensing was demonstrated onto three different substrates, namely PDMS, gummy, and GA.PDMS substrates were fabricated as described above.Gummy substrates were fabricated by melting 12 g of gummies in 12 mL of DI water at 80°C under stirring.The solution was then cast into a Petri dish (diameter 9.5 cm) and air-dried under a fume hood for a minimum of 3 days.The petri dish was PDMS coated to aid the removal of the substrate after drying.GA substrates were manufactured by melting 9 g of gummy and 3 g of gum arabic into 12 mL of DI water at 80°C under stirring.The next steps were identical to gummy substrates.Substrates were cut into dogbone shapes for testing with exact dimensions reported in Figure S7c (Supporting Information) and ≈2 mm average thickness.Coatings with different AC loadings were directly applied to the specimens by spraying.
Strain Sensor Characterization: To test the mechanical properties of the substrates, an Instron tensile testing machine was used.To study the piezoresistivity of the coating, a Universal testing machine by Hongjin was adopted to apply controlled strain while the resistance was measured by applying a constant voltage (1 V) and measuring the current using a potentiostat and a National Instrument DAQ unit (NI USB-6211) controlled using a custom Matlab script with a sampling frequency of 100 Hz.The noise level of the setup was first quantified by performing an open circuit baseline and measurements using only the substrate without the conductive coating (Figure S9a, Supporting Information).The limit of detection of the setup was quantified to be 7.6 nA and the highest measurable resistance with this setup was quantified to be 130.7 MΩ.
The first test aimed at determining the strain range of interest.Electrodes were created onto the sample under test by applying silver paint onto the coating and mechanically clamping a flat electrode.One sample per edible substrate and AC loading was subjected to a linear strain up to 50% with a 1 mm min −1 rate while measuring the resistance (Figure S9, Supporting Information).Thus, a testing range of 0−8% strain was set (1 mm min −1 pull rate) to extract the calibration curves, as all the samples showed a saturating behavior starting from ≈10% strain.Experiments in the range 0−8% were performed in triplicates for each substrate and AC loading.The average was performed using curves fitted using fifth-order polynomial models.The gauge factor was calculated in the 1−3% strain range of the averaged curves according to the following equation: where ΔR/R 0 represents the relative variation of resistance from the zerostrain resistance value R 0 and ΔL/L 0 represents the strain applied to a specimen with a zero-strain length of L 0 .Cyclic testing was performed using the same setup.A repetitive stimulus composed of a 1% strain with a pull rate of 10 mm min −1 , a hold time of 5 s before strain release, and a wait time of 30 s (for PDMS) and 60 s (for edible sensors), was adopted.
Simulated Digestion: The simulated gastric fluid (pH 1.1) was prepared by dissolving 2 g of NaCl, 3.2 g of pepsin, and 3.1 g of 1 m HCl in 995 mL of DI water.The simulated digestion experiment was conducted at 37 °C under stirring.
Sensorized Gripper: Gelatin grippers [50] and edible batteries [34] were produced as in previous works.AC was directly sprayed onto the gelatin grippers without any pre-treatment.The formulation with 200% AC was selected for this experiment.The actuator activation was manually produced using a syringe pump.For the experiments in Figure 4d and Figure S12c−i (Supporting Information), the sensorized gripper was not integrated with the edible battery and the coating resistance was measured using a source meter (Agilent B2912A) by applying a constant 100 mV supply while measuring the current.For the experiments in Figure 4e,f, the sensorized gripper was connected to the edible battery, which was used to provide a constant voltage to the e-skin, using silver paint and standard aluminum wires.The provided battery voltage and the current flowing in the sensor were measured using a source meter (Agilent B2912A).
Statistics: Resistivity (Figure 2a) and sheet resistance (Figure S3e, Supporting Information) were measured over three identical samples for each datapoint, and data is presented as average and standard deviation.
The coating stability (Figure 2b) was measured over three identical samples measured over time, and data is presented as average and standard deviation.The impedance (Figure 2c) was measured over one sample for each AC loading condition.The piezoresistive behavior (Figure 3d,e; S10a−i, Supporting Information) and the relative calculation of gauge factor (Table S2, Supporting Information) were measured over three identical samples for each AC loading and for each substrate.Cyclic testing (Figure 3g,h; Figure S11a−g, Supporting Information) was performed on a single sample for each AC loading and each substrate.
Piezoresistivity under bending (Figure 4a; Figure S11h, Supporting Information) was measured over three identical samples, and data is presented as average and standard deviation.Data from the sensing skin integrated onto the gripper (Figure 4d−f; Figure S12, Supporting Information) was collected using a single sensorized gripper.All the data analyses were performed using Matlab.

Figure 1 .
Figure 1.a) Materials employed for the formulation of edible sprayable ink.b) Spray-coating method.c) Conductive pattern sprayed onto a vegetable.d) Scanning electron microscopy (SEM) image showing the micromorphology of the coating with 200% of AC on PDMS.e) Test patterns deposited with progressively decreasing size to estimate spatial resolution.The size of the micropatterns is compared to a standard gelatin capsule.f) SEM analysis of the testing micropatterns.

Figure 2 .
Figure 2. a) Average electrical resistivity and standard deviation over three samples against filler loadings of the coating.b) Average and standard deviation of relative resistance variation of the edible coating (200% AC) deposited on glass and stored at 25°C and 15% of RH for 21 day over three samples.c) Electrical impedance spectroscopy (modulus); and d) phase of the coatings with different AC loadings and open circuit (o.c.).

Figure 3 .
Figure 3.The piezoresistivity mechanism is dominated by micro-cracks formation.200% AC coating onto the gummy substrate with: a) 0% strain; b) 50% strain; and c) back to 0% strain after 1 h wait.Average resistance variation and standard deviations versus strain for 200% AC coatings calculated over three identical samples onto several substrates, namely, d) PDMS; e) gummy; and f) GA-substrate.Cyclic testing for: g) one gummy-based (200% AC); and h) one GA-based (200% AC) strain sensor.f) List of ingredients of our edible strain sensors.

Figure 4 .
Figure 4. a) Average resistance variation and standard deviation versus bending angle for 200% AC on GA substrate over three identical samples.Inset: bending angle reference.b) Photograms of the simulated digestion test (see Video S3, Supporting Information) indicate an estimated digestion time of 15 min.c) First integration of an edible actuator, edible sensing skin, and an edible battery.d) Data from the sensing skin during consecutive actuation of the gripper (traditional power supply unit).The gripper was activated/pressurized manually using a syringe.e) Data from the e-skin during consecutive actuation of the gripper (red) when powered by the edible battery (blue).f) Data from the e-skin during patterned actuation of the gripper (morse code, red curve) when powered by the edible battery (blue).