Soft Chemiresistive Sensing Shields Soft Robotic Actuators from Mechanical Degradation due to Critical Solvent Exposure

Thermoplastic elastomers (TPEs) are popular for fabricating soft actuators thanks to their compatibility with thermoplastic processing methods, like material extrusion additive manufacturing. However, these TPEs are susceptible to nonpolar solvents, and upon exposure, the mechanical properties can diminish significantly. Herein, soft chemiresistive sensing elements based on thermoplastic elastomers and carbon black (CB) are developed for detecting the presence of nonpolar solvents. A low CB concentration (20% w/w) leads to a faster response for detecting the presence of nonpolar solvents. A threshold value of the electrical current is defined by tensile testing, based on exposure time that does not diminish the mechanical properties. Exposure to solvent vapor above 150 s decreases the elongation at fracture significantly (450%). To achieve nociceptive behavior for a sensorized soft actuator and shield the actuator from structural damage, this threshold value is used for programming a closed‐loop control motion system. When the threshold value is reached, the actuator withdraws from the solvent surface autonomously. However, this threshold decreases with parameters like the solvent temperature and sensor distance from the solvent surface. Overall, adding soft chemiresistive elements into elastomer‐based soft robots extends their lifetime, by preventing damage in harsh chemical atmospheres.


Introduction
In biological organisms, nociception is a function of signaling the presence of eminent or occurring damage. [1]4][5] However, structural damage can have other causes, like exposure to chemical species and this type of nociception has not yet been explored in artificial systems.Soft robots, like grippers and hands, are often used in industry. [6]In industrial settings, the presence of solvents is common.15] Using flexible nociceptive sensors for detecting the presence of these harmful chemicals can prevent any long-term damage that could endanger the function of the robots and at the same time maintain the flexibility that is required for performing dexterous tasks. [16]hemiresistive sensors can be used to detect the presence of chemical species, like solvents and vapors. [17][27] During the physisorption that is otherwise known as swelling, thanks to van der Waals interactions, the adsorbed chemical species adheres to the polymer surface causing a change in the volume and an increase in electrical resistance of the sensing materials.[30] Chemiresistive sensors can find application in the field of robotics for applications like electronic noses, [31,32] sensorized electronic skin, [33] or exploration of unknown environments. [34,35]he majority of chemiresistors are based on solid-state semiconductors and metal oxides.This type of chemiresistive sensing has been already explored for vehicle robotics used in harmful environments. [36]However, such chemiresistors are not suitable for soft robots, as they are for their rigid counterparts, as they require several assembly steps, and the integration is challenging.
In soft robotics, soft and stretchable electronics are often used.Soft resistors based on elastomers are an option for soft robots as they combine the sensing properties of a chemiresistor with the softness and flexibility of an elastomer. [37]However, studies for soft robots in harmful environments, like exposure to solvents have not yet been investigated.
[40][41] Wang et al. developed fiber vapor sensors based on styrene-based copolymer (TPS) rubber and carbon nanotubes for polar and nonpolar solvents. [39]The morphology of the fiber with a large surface area facilitated the absorption of the chemical species on the fiber.Segal et al. developed a chemiresistive sensor based on TPU and carbon black (CB) and also favored the structure of the form of filaments (fibers) with a diameter of 1.27 mm for the detection of methanol and ethanol. [38]In another attempt, a blend of TPU and TPS rubber was used for polar and nonpolar chemical sensors by Gao et al. [42] The blend obtained properties of detecting polar solvents thanks to the TPU content and nonpolar solvents thanks to the TPS solvent.It was discovered that the sensitivity of the sensor response depended on the degree of swelling of the polymer to each solvent, which was possible with a higher content of TPS in the blend.The mechanism was attributed to the higher elasticity of the TPS compared to the TPU, which allowed for higher expansion of the material during swelling.
[51] In this study, soft chemiresistive sensing elements based on TPS and CB are investigated for their use as chemical nociceptors.Based on the TPS matrix material, the chemiresistive sensing elements are optimized to detect nonpolar solvents (toluene, dichloromethane, chloroform).The majority of the investigations are performed with toluene solvent as a milder alternative to organochlorides, chloroform, and dichloromethane.The goal is to define an electrical threshold value for programming a closed-loop system that removes the soft actuator autonomously from the solvent exposure when needed to avoid diminishing the mechanical integrity of the soft actuator (Figure 1).

Development of Chemiresistive Composites
A thermoplastic elastomer based on styrene-based tri-block copolymer (TPS), with a Shore hardness of 50 A was obtained from Kraiburg TPE (Waldkraiburg, Germany), and the conductive filler CB was obtained from Imerys (Paris, France).The two components were mixed in different ratios using the torque rheometer Polylab Rheomix 600 from Thermofisher (Karlsruhe, Germany).After mixing, using a capillary rheometer RH7 from Netsch (Selb, Germany), filaments were extruded with a 1.75 mm diameter.To obtain the granules required for the MEX-AM process, the filaments were cut into 3 mm pellets.The fabrication process has been described in more detail elsewhere. [43,44,52]

Fabrication of Chemiresistive Sensor Strips
To investigate the chemiresistive response of the sensor composites, sensor elements were integrated onto a TPS substrate (10 Â 25 Â 0.6 mm) and we called them TPS/CR strips.As a substrate, TPS from Kraiburg TPE (Waldkraiburg, Germany) with a Shore hardness of 68 A was selected.The sensing elements were printed as a straight line with one, two, or three layers and in a u-shape geometry.For comparison, strips based on pure chemiresistive composites (CR strips) and substrate strips without sensing element (TPS strips) were fabricated.
The printing was performed with a pellet-based fused deposition modeling printer (FDM) from Indart3D (Gipuzkoa, Spain). [49] printing temperature of 230 °C, a speed of 30 mm s À1 , and a layer height of 0.2 mm were used for the sample production.The temperature of the printing bed was 45 °C.A 90°rectangular infill of 100% was used.The steps of the production of the strips from the mixing of the composites to the printing of the strips can be seen in Figure 1.

Fabrication of the Soft Bending Actuator with Integrated Sensing Elements
The soft robotic actuator, composed of a flexible hinge, was fabricated with MEX-AM, using the pellet-based FDM 3D printer from Indart3D and the printing parameters described in Section 2.2.In addition to the chemiresistive sensing element, developed in this study, a piezoresistive element reported by Georgopoulou et al. previously was integrated to monitor the bending action. [53]For the soft-bending actuator, a 0.1 mm tendon, based on stainless steel from Jenzi (Plünderhausen, Germany), was fixed on a Dynamixel AX-12 A servomotor from Robotis (Lake Forest, Illinois, USA), and an Arduino microcontroller was used for programming the servomotor.The closedloop control system was programmed using Arduino.

Testing the Swelling of the Strips
Toluene, chloroform, and dichloromethane were obtained in 99.8% purity from Thermo Fischer Scientific (Dreieich, Germany).To investigate the swelling behavior in toluene vapor and solvent, the dimensions of the samples were measured with a digital caliper before the testing and after exposure.
The relative volume was calculated with the Equation ( 1) where V is the volume after exposure time and V 0 is the value of the initial volume before testing.

Testing the Chemiresistive Response
For assessing the chemiresistive response, the electrical signal was recorded when the strips or soft bending actuators were exposed to a solvent or vapor environment.For the solvent testing, the sensor strips were immersed in the solvent, while the electrical current was recorded with the time.The experiments were performed at room temperature (25 °C).For the vapor testing, the solvent container was equipped with a lid to ensure saturated conditions.This was the case for both the sensor strips and the soft robotic actuators.The current response was recorded with time.The experiments were performed at room temperature (25 °C).Unless specified otherwise, the distance of the sensor from the surface of the solvent was 10 cm.To investigate the effect of the distance of the sensor from the solvent surface on the sensor response, the distances of 5, 10, and 15 cm were investigated.The solvent temperature was varied between 25 (RT) and 60 °C to identify the temperature effect on the sensor response.
A Keithley 2450 digital source meter from Keithley Instruments (Solon, USA) was used.The current was recorded, using the source meter, while a constant voltage of 12 V was applied during the measurement.For the experiments with the strips and the soft bending actuators, a sampling rate of 10 and 100 Hz was used, respectively.
The relative current was calculated using the Equation ( 2) where I is the current measured during exposure and I 0 is the initial current value before starting the experiments.
At the beginning of this study, the current threshold of I rel = À0.99 was defined for all experiments.The response time to reach the threshold value was investigated for the different strips.The measurement plotted in the figures represents the average value of three measurements.Based on mechanical tensile experiments after exposure time, the threshold value was adjusted to avoid embrittlement of the TPS material.Experiments with two different threshold values were performed on the soft bending actuator to demonstrate the nociceptive behavior.The reported curves and values represent the average of three sample measurements.

Optical Microscopy
The cross-section area of the strips was determined using a Zeiss Stereo Discovery microscope (Carl Zeiss Microscopy, Jena, Germany).For sample preparation, TPS/CR printed strips were immersed in liquid nitrogen and then cut to reveal the crosssection area.

Tensile Testing
Selected TPS/CR strips were used to investigate the effect of the exposure time on the stress-strain behavior of the thermoplastic elastomer-based samples.Therefore, tensile testing was performed until the point of fracture.The tensile tests were performed with a Zwick Roell Z005 universal testing machine from Zwick Roell (Ulm, Germany) with a strain rate of 200 mm min À1 .Pneumatic clamps applied the pressure of 4 bar to minimize slipping of the strips during testing.

Sensor Selectivity Assessment
To test the selectivity of the TPS/CR strips in the presence of interfering environmental stimuli, a climate chamber MKF (Binder, Tuttlingen, Germany) was used.Electrodes from the source meter were inserted in the chamber to record the current values with the applied conditions.The temperature was varied between 20 and 100 °C, while the relative humidity was constant RH = 50%.The electrical current value was recorded every 5 °C after 120 s of exposure.In addition, different values of relative humidity (50%-70%) were applied on the TPS/CR strips, for a constant temperature of 50 °C.The electrical current value was recorded every RH = 5% after 120 s of exposure.

Effect of Single Line and u-Shape Sensor Morphologies on the Response Time
The chemiresistive sensor behavior was based on the swelling of the TPS matrix.Diffusion of the solvent was expected to affect the swelling of the TPS polymer.Therefore, the effect of sensor thickness (the number of printed layers) on the sensor response was investigated.Thermoplastic elastomer (TPE)/CR strips with a different number of printed layers (i.e., 1, 2, and 3 layers) of the sensing material with 20% w/w CB were investigated (Figure 2a).To test the sensor response of the different sensor thicknesses, the strips were immersed in toluene solvent, while the electrical current was recorded (Figure 2b).
As shown in Figure 2c, the relative current decreased with the exposure time.This can be explained by the swelling of the TPS matrix material in toluene solvent that led to an increase in the interparticle distance, and therefore, a decrease of the electrical current.This is in good agreement with reported behavior in the literature. [42]The time to reach the selected relative current threshold of À0.99 for the one-, two-, and three-layer chemiresistive sensors was 136, 352, and 519 s, respectively (Figure 2d).A larger cross-section area resulted in a lower value of the resistivity (higher current) and thus, achieving the threshold required a longer time.
For the electrical signal readout, u-shape sensors have the advantage that both electrodes are on the same side, and therefore, wiring connections to the electronic circuit board are easier. [44,54,55]In this study, the u-shape design allowed the protection of the electrodes and electric cables from the organic solvent to avoid corrosion after long exposure times.The u-shape sensor was directly printed onto the TPS substrate.To ensure functionality (continuous conductive path), two layers of sensor material were printed.No significant differences in the cross-section area of the u-shaped sensor were observed, compared to the TPS/CR strip with two layers (Table S1, Supporting Information).Based on the results above (Figure 2d), the response time was significantly shorter, namely, 26 s.It is worthwhile to mention that for the u-shape, the total length of the sensing element was larger than the single line TPS/CR strip.Therefore, it can be concluded that in general, sensing elements with a higher electrical resistance (e.g., lower current value, see Figure 2f ) achieved the threshold value faster.For the following investigations, only the u-shape geometry of the sensor element is further investigated.

Effect of the CB Concentration on the Response Rime
To investigate the effect of the CB concentration on the response time, conductive TPS/CR and CR strips with carbon filler content between 20% and 50% w/w were investigated and the results are presented in Figure 3.Only for high CB concentrations, namely 40% w/w and 50% w/w, a saturation of the swelling was observed.As the filler content increased, the amount of polymer matrix decreased.Based on the fact that only polymer matrix swells in the presence of the chemical species, the maximum degree of swelling decreased with increasing filler content.This is in good agreement with the literature. [40]Jun-Xue et al. described the swelling as the free volume (not occupied by CB particles) of polymer matrix material able to absorb the solvent.The mobility of the polymer chains in this free volume is reduced with higher filler content, resulting in a lower swelling effect. [56]Therefore, as expected, the TPS strips without any CB reached the highest value of the relative volume.
To evaluate the effect of the nonconductive TPS substrate on the response time, selected CR and TPS/CR strips were compared with each other.The response time of strips with CB concentrations 20% w/w, 30% w/w, and 40% w/w (Figure 3c,d,e) was investigated after dipping into the toluene solvent.In all cases, the relative current decreased with the time of exposure to the solvent due to the swelling of the matrix material, as reported for other TPS materials. [42,57]It was seen that the response time for the TPS/CR was shorter in comparison to CR strips.This behavior was expected, because of the higher polymer chain mobility, as explained above. [42]A similar observation was made by Sisk and Lewis for chemiresistive sensors based on polyethylene and polypropylene with CB concentration close to the percolation threshold. [58]As for the deviation values, it can be seen that the strips with 30% w/w concentration had the lowest relative error (20%).The relative errors were similar for the TPE/CR strips with 20% w/w (28%) and 40% w/w (29%) carbon filler.In addition to the solvent immersion experiments, the strips were fixed 10 cm above the solvent surface (i.e., vapor detection).A similar trend of the response time was observed (Figure 3f,g,h).As expected, the time required to reach the current threshold was significantly longer and can be explained by the lower concentration of the toluene molecules in the vapor phase.For the CR strips with higher CB content (e.g., 40% w/w), a lagging phase, where the electrical signal did not change with time was seen at the beginning of the curve (Figure 3e,h).It was assumed that the conductive filler network density with such a high carbon content was high.Therefore, the diffusion of the solvent was hindered and the rearrangement of the filler particles during swelling was slow.Due to the low mobility of the molecular polymer chains, the interparticle distance between the CB particles required longer time to increase resulting in the lagging phase.This lagging phase is undesirable for chemiresistive sensing elements, because the response time increases, increasing the risk of diminishing the mechanical properties.

Effect of Sensor Distance and Temperature on the Response Time
In the case of vapor detection, influences like the sensor distance to the solvent surface and solvent temperature can also affect the response time.These parameters were investigated for the TPS/ CR strips with a carbon filler content of 20% w/w.As shown in Figure 4a, three different distances from the surface of the solvent were used (5, 10, and 15 cm).It is known from the Langmuir isotherm model that the adsorption rate is linked with the concentration of the adsorbed species.Therefore, a gradient of concentration in the beaker appears when the lid is placed on top (time point 0 s) and a faster diffusion (e.g., swelling of the polymer) for smaller distances is expected (Figure 4b).Fick's law of diffusion describes the flux of the solute as a linear dependency of the distance from the surface of the solvent. [59]The fitting of the response time to reach the relative current threshold value of À0.99 is shown in Figure 4c.A linear relationship (R 2 = 0.99) with a slope of 1.91 and an intercept of À12.78 was analyzed.Thanks to the linear relationship between the time required to reach the current threshold of À0.99 and the distance, the sensor can be used to detect the spacing between the soft robot body and the solvent, too.
To investigate the effect of the solvent temperature on the response time, the beaker with the nonpolar solvent was placed on a heated plate.The experiment was conducted at four different temperatures (RT, 30, 45, and 50 °C) with a constant distance of 10 cm between the TPE/CR strip and the solvent surface (Figure 4d).It was seen that a higher temperature led to a shorter time to reach the relative current threshold (Figure 4f ).
The relation between the time required to reach the relative current threshold and the solvent temperature can be described by an exponential function.Based on the Clausius-Clapeyron equation for the equilibrium state, the temperature will affect the vapor pressure, and therefore, the adsorption rate. [60]For solvents like toluene, the adsorption capacity and rate increase with temperature, which results in a shorter time to reach the relative current threshold value. [61]In Figure 4f, a correlation coefficient (R 2 ) of 0.98 between the time to reach the relevant current threshold value and the temperature of the nonpolar solvent could be achieved.
To assess the stability of the sensors in different environmental conditions, the TPE/CR strips were exposed to different conditions of temperature and humidity.For temperatures below 60 °C, there was not a significant dependency of the electrical current on the temperature (Figure 4g).For temperatures between 60 and 100 °C, a decrease of 0.02 was observed for the relative current, associated with the positive temperature coefficient effect of elastomer-based resistive sensors. [62,63]Immersing the TPE/CR strips in liquid water had minimal effect on the electrical current (Figure 4h).This fact is in good agreement with what has been reported before. [64]In addition, exposure to different humidity values showed a minimal effect on the relative current (Figure 4i).Overall, in the investigated range, the sensor response of the strips was mostly unaffected by changes in the environmental conditions.The TPE/CR strips are expected to respond selectively for the detection of nonpolar solvent in the presence of interfering environmental stimuli.

Soft Robotics Bending Actuator Module with Chemiresistive Sensing Elements
To prove the concept of the nociceptive function of the chemiresistive sensor elements for soft robotics, a tendon-based bending actuator was placed onto a beaker with different nonpolar solvents, like toluene, chloroform, and dichloromethane.All three solvents have chemical affinity to TPS.Such actuator could be potentially part of a soft robotic gripper and be used for performing tasks that require dexterity, such as cleaning, packaging, and chemical disposal.
To validate the appropriate relative current threshold, tensile testing up to point of fracture for the TPE/CR strips exposed to the nonpolar solvents was performed (Figure 5a).It was seen that exposure to toluene solvent even after a short time interval (30 s) resulted in a significant reduction of the tensile strength and elongation at break.Nonetheless, for the exposure to toluene vapor, the embrittlement of the TPE/CR strips was observed after long exposure time (500 s).Based on the tensile testing analysis, it was observed that 150 s of exposure to toluene vapor resulted in a 180% reduction of the elongation at the point of fracture.However, the TPE/CR strips maintained a high elongation at the point of fracture (940%) and the stress-strain response was similar to the original value at lower strains.The tensile test after exposure to vapor (150 s) for the three nonpolar solvents is summarized in Figure 5b, and no significant difference in the mechanical behavior was observed.
The solvent uptake was investigated for the three different solvent vapors, whereas the highest uptake was observed for the chloroform and the lowest for the toluene (Figure 5c).The solvent uptake for 150 s under saturated vapor conditions and the maximum uptake are presented in Table S2, Supporting Information.As discussed by Rahiman and Unnikrishnan, the difference in the polarity of the two solvents affects the interaction between the solvent and the styrene-based rubber, causing the differences in solvent uptake. [65]For the later closed-loop control, the relative current threshold at 150 s was analyzed and the values are reported in Figure 5d.The trend is in good agreement with the solvent uptake values in Figure 5c.In addition to the chemiresistive nociception sensor, a piezoresistive sensor based on TPS (Shore hardness 50 A) with 50% w/w CB [49,52,66] was integrated into the soft robotic actuator to monitor the bending actuation (Figure 5e).
Figure 6 presents the response of the chemi-and piezoresistive sensors for the three different nonpolar solvents during cyclic bending experiment with the soft robotic actuator.The results of the chemiresistive sensor response are in good agreement with the previous results and the sensor response was repeatable between the five cycles.As expected, the piezoresistive sensor can be used to monitor the movement of the soft robotic bending actuator.In the bent and extension position, relaxation behavior of the sensor signal was observed.The values were around 8% and similar for the exposure to the three different solvents.The drift of the signal was also similar for the exposure to the different solvents (3%).The drift between the second and fifth cycle, as well as the relaxation of the sensor signal have been summarized in Table S3, Supporting Information.
The chemiresistive sensor signal showed a different behavior.When the actuator remained in the bent position, the signal continued to decrease because of the swelling response.This resulted in significant relaxation (100%).When the finger moved to position extended, the chemiresistive sensor first increased and then decreased.The drift of the electrical signal was also three times higher than the piezoresistive sensor.Thus, it was not possible based on the chemiresistive sensor signal alone to monitor the position of the actuator.
To verify the nociception ability of the chemiresistive sensor with a relative current threshold value of À0.15, the value was changed to À0.99.As shown in Figure 6d, the exposure time was longer and after three cycles, the hinge in the soft robotic bending actuator broke, as shown in Figure 6e.In this case, neither the chemi-nor the piezoresistive sensor could be used to detect the eminent damage of the hinge.Further investigations with chemiresistive nociception function on soft robotic structures will further advance the nociceptive capabilities for structural damage that are conventionally detected after the damage incident occurs.

Conclusion
In this study, soft chemiresistive sensing elements were developed to detect mechanical embrittlement of thermoplastic elastomers when exposed to nonpolar solvents (toluene, chloroform, and dichloromethane).In comparison to piezoresistive sensor materials based on TPS material, a lower CB content improved the sensor behavior due to the higher free volume of the polymer, which allowed a higher degree of swelling.Geometrical dimensions of the sensor and setup parameters like distance to the solvent surface and temperature were investigated.In addition, the tendon-based soft robotic bending actuator module was used as a demonstrator for the nociception concept.
Nociception is a function of signaling the presence of eminent or occurring damage in natural organisms.To achieve this in artificial systems, a soft robotic bending actuator was programmed to move away from the chemical exposure autonomously before mechanical embrittlement.By setting a current threshold value the chemiresistive sensor shielded the soft robotic actuators from mechanical degradation due to critical solvent exposure.The defined threshold signal was used as a trigger for the closed-loop control system that automatically removed the soft bending actuator from the dangerous nonpolar solvent.To achieve a fast response and avoid a lagging phase at the beginning of the exposure, a low CB filler content (20% w/w) was selected.The sensor was not significantly affected by changes in the surrounding temperature and humidity.Exposure to the solvent vapor for 150 s preserved the mechanical properties, whereas exposure for 500 s resulted in embrittlement of the TPS.Programming the closed-loop control of the cyclic motion with a higher threshold value led to the mechanical disintegration of the soft robotic actuator after three cycles.Overall, the chemiresistive sensing elements could shield the soft bending actuator from critical exposure time in a nonpolar solvent vapor, acting as a type of chemoreceptive nociceptor for soft robots.

Figure 1 .
Figure 1.Schematic representation of the thermoplastic processing steps required for the sensor and sensorized actuator.The mechanism behind setting the threshold and the chemiresistive sensor response is illustrated.Optimizing the sensor response time and defining the exposure time threshold that does not embrittle the actuator material are required for establishing a closed-loop control that can shield the actuator from structural damage upon prolonged exposure.

Figure 2 .
Figure2.a) Schematic and microscope images of the printed sensor elements on TPS in a sensing element with one-, two-, and three-layer configuration, as well as for the u-shape sensor with two layers configuration.b) Schematic of the experimental setup used to assess the chemiresistive response for strips immersed in liquid solvent.c) Schematic of the response of the chemiresistive sensor with the exposure time.d) The response of the relative current upon exposure to toluene solvent for the TPS/CR strips with the different layer heights and the u-shape sensor.e) The response of the current upon exposure to toluene solvent for the TPS/CR strips with the different layer heights and the u-shape sensor.For the sensor material, a constant concentration of CB of 20% w/w matrix was selected.

Figure 3 .
Figure 3. a) The swelling response of the CR strips, expressed by the relative volume, with different CB concentrations and a schematic representation of rel.volume change based on the CB concentration.b) Picture of the CR strip and TPS/CR strips.The time required to reach the current threshold of CR and TPS/CR strips with a CB filler concentration of c) 20% w/w, d) 30% w/w, and e) 40% w/w immersed in toluene solvent.The response time of CR and TPS/CR strips with a CB filler concentration of f ) 20% w/w, g) 30% w/w, and h) 40% w/w exposed to toluene vapor (10 cm above the solvent surface).

Figure 4 .
Figure 4. a) The measurement setup to investigate the effect of the sensor distance from the solvent surface.b) The response time for TPS/CR strips with 5, 10, and 15 cm distance from the solvent surface at room temperature (25 °C).c) Fitting of the distance to the solvent surface versus the response time to reach the threshold value of À0.99.d) The measurement setup to investigate the effect of the solvent temperature on the response time to reach the threshold value of À0.99 for the relative current.e) The time required to reach the relative current threshold of the TPS/CR strips with 20% w/w CB concentration when heating the solvent to different temperatures (30, 45, 50 °C).f ) Fitting of the response time to reach the threshold value of À0.99 for the relative current versus the reverse of temperature).g) The change in relative current with applied temperature for a value of relative humidity (RH = 50%).The value was recorded after 120 s of exposure.h) The change in the relative current over time for immersing the TPS/CR strips in liquid water.i) The change in the relative current with applied relative humidity values for a temperature of 50 °C.The value was recorded after 120 s of exposure.

Figure 5 .
Figure 5. a) Stress-strain curve for TPS/CB strips after exposure 30 s of a toluene solvent and 150 and 500 s of toluene vapor.b) Stress-strain curve after 150 s exposure of the TPS/CR strips in toluene, dichloromethane, and chloroform vapor.c) The solvent uptake with the exposure time for the three solvents.d) Vapor exposure test for the soft tendon-based bending actuator with the integrated chemiresistive sensing element (CB 20% w/w) under saturated vapor conditions at room temperature (25 °C).e) The position of piezoresistive and chemiresistive sensors to detect the mechanical deformation by bending and the swelling by the nonpolar solvent, respectively.

Figure 6 .
Figure 6.a) The relative current response of the chemiresistive sensor (20% w/w CB) and the relative resistance response of the piezoresistive sensor during cyclic bending of the soft tendon-based robotic actuator module for toluene, b) dichloromethane, and c) chloroform vapor at room temperature (25 °C).d) The relative current response of the chemiresistive sensors and the relative resistance response of the piezoresistive sensor during cycling with a relative current threshold value À0.99 upon exposure to toluene solvent vapor at room temperature (25 °C).e) Optical analysis of the tendon-based bending actuator after exposure in toluene with a relative current threshold value of À0.99 (visible structural damage after the test) and f ) schematic of the setup of the soft-bending actuator with the closed-loop control.