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Stretchable and Flexible Snake Skin Patterned Electrodes for Wearable Electronics Inspired by Kirigami Structure

Stretchable and Flexible Snake Skin Patterned Electrodes for Wearable Electronics Inspired by... IntroductionRapid advances in wearable and stretchable electronics includes intelligent functions that enhances memory, in situ healthcare, communication, motion monitoring, and physical senses.[1,2] Wearable devices have diverse applications, including bioelectronics in close contact with the body, human–machine interfaces, smart clothing, and wearable healthcare devices.[3,4] With the spread of stretchable and foldable smartphones, the stretchability of materials is now a requirement for next‐generation electronics. As wearable electronics are worn externally on the human body, they must be made of that are highly stretchable and reliable.[5,6] In particular, developing highly stretchable electrodes with high conductivity is imperative because stretchable electrodes are critical components of optoelectronic devices and sensors. However, typical inorganic electrode materials, such as metals and transparent conductive oxides, cannot be used as stretchable electrodes in wearable electronics owing to their inherent brittleness. The materials for stretchable electrodes include carbon nanotubes or 2D materials such as graphene[7,8] and superlattices.[9] Structural approaches include microbuckling, prestraining, and wavy patterns.[10] In materials approaches, hybrid electrode materials are coated or mixed with stretchable substrates.[11] These methods provide high stretchability but are limited by the electrical and optical properties of the electrodes. In particular, the choice of electrode materials, such as Ag nanowires, PEDOT:PSS, and CNT, is limited and the solution process is unsuitable for large area coatings.[12,13] Structural approaches, including patterning of 2D and 3D electrodes using laser cutting or mold forming, buckling substrates, and wavy patterning have been suggested for stretchable electrodes. However, these processes incur high costs and are complicated.[14] In additionally, the presence of cutting lines and holes is a critical limitation for device applications.[15] Patterning using a shadow mask prevents the electrodes from being damaged, by avoiding holes, which occur during the laser patterning process when creating a stretchable net structure. This helps lower the process costs and increase the applicability of the device. Among the structural approaches, shadow mask patterning, which can complete the patterning and deposition process in one step without a cutting or molding process, was selected in this study for its wide application and cost‐effective and simple process. Specially, considering mass‐production of the stretchable electronics, large area coating and process compatibility of the stretchable electrodes is one of the critical issues to solve the problems of previously reported stretchable electrodes. The Kirigami structure and biomimicry concept were combined to develop a new pattern for stretchable electrodes. Kirigami is a Japanese word for origami, which involves cutting paper into a specific pattern or shape.[16] Kirigami is a unique structure that is being actively studied in academia owing to its outstanding mechanical properties. For example, if paper is cut in the Kirigami pattern, the deformability is improved and the paper, which has the property of tearing when pulled, opens, and stretches in three dimensions. Kirigami patterned paper has a negative Poisson's ratio owing to the perforation line, which increases the thickness of the stretched paper in 3D.[17] Designs inspired by the Kirigami patterns have been applied in various fields, including architecture, foldable and portable solar cells, and micro/nanoscale mechanical systems.[18–20] Stretchable devices are typically fabricated by applying Kirigami structures to 2D materials or by laser cutting and etching processes. This method has the same disadvantages as mentioned above, regardless of the usefulness of the Kirigami structure.[21,22] To increase the versatility of this promising structure, a Kirigami patterning without cutting was developed. This patterning method has become a cornerstone for developing a new type of stretchable electrode. Further, biomimetics (composed of two Greek words, “bios”‐ meaning life and ‐“mimesis” meaning imitation), were grafted.[23] Swimwear for athletes that reduce water resistance by imitating shark‐scale structures, and water‐repellent coatings that imitate the surface structure of lotus leaves are representative biomimicry technologies.[24,25] Biomimicry is an effective research method that has solved various human problems by studying the properties of living things. Although the Kirigami pattern and biomimicry have been applied separately in numerous cases, reports of stretchable electrodes combining Kirigami and biomimicry are lacking.In this study, a novel stretchable electrode was developed by applying a pattern that combines Kirigami and biomimicry. Among various animals with flexible and elastic movements, such as octopus and blowfish, the structure of the snake's abdominal skin is notably similar to that of the reticulated Kirigami structure. Therefore, herein, several types of stretchable patterns inspired by the skin pattern on the belly of a corn snake were designed. The structural conditions of the pattern were analyzed through finite element analysis (FEA) simulations.[26] By iterating the snake‐skin pattern design and FEA simulation, a snake‐skin pattern with optimal structural conditions was determined. The pattern was fabricated as a stretchable snake‐skin electrode using an AgPdCu (APC) target, polyurethane (PU) substrate, and sputter. The stretching and bending properties were measured, and the results were analyzed. Next, the stretchable electrode was applied as a temperature sensor and light‐emitting diode (LED) interconnector to prove the practicality of the snake‐skin electrode.Results and DiscussionThe corn snake has a skin pattern consisting of a diamond shape on the back and a mixture of hexagons and diamonds on the belly. Evidently, the structural change when the ventral pattern was stretched or shrunk was found to be similar to that of the Kirigami structure, which is often used as a stretchable pattern. As the snake's body stretched, the “V” line opened at both ends of the long hexagonal structure. Consequently, the snake's skin could be thinned and stretched without being torn or damaged. This is similar to the principle of Kirigami, wherein the paper is twisted up and down along the cutting line when pulled and stretched.[27,28] Herein, various types of stretch patterns inspired by snakes (Figure 1a) were designed, and FEA simulations were conducted for stretchability comparisons.[29] The purpose of the simulation was to compare the magnitude of the stress generated in each pattern when the same strain was applied to the electrode. The magnitude of the distributed stress in the snake‐skin pattern is shown in Figure 1b, and the maximum stress received by each pattern is graphically shown in Figure 1c. For simplicity, the sample patterns are referred to as P0, P1, P2, P3, and P4. In PX, X denotes the number of simulations; in PX.Y, Y is the index of each design of Simulation X. For example, P2.1 points to the first pattern in the second simulation. The maximum stress applied to the pattern can be calculated in two ways: by focusing on the region near the notch or by excluding the notch.[30] A comparison of the values calculated using the two methods with the actual stretching test results revealed that the maximum stress value in the region excluding the notch coincided with the actual stretching test result. Therefore, the simulation results were analyzed based on the stress values, excluding the notch region (Figure S1, Supporting Information). In Figure 1d, stretchability is the strain just before the resistance increases by more than ten times compared with the initial resistance when the 30 nm thick APC electrode is stretched. Evidently, all snake‐skin patterns exhibited better stretchability compared with the reference electrode P0. In addition, a difference in stretchability between the patterns was evident. In the case of P1.3 and P1.4, which have rigid and complex patterns, the stress was concentrated in the middle core line of the outer rhombus and the inner hexagon. This resulted in poor electrode stretchability. In the case of P1.2, the stress was slightly higher for the same strain. Because P1.2 is a pattern with rounded corners with a radius of 0.5 mm, the stress was expected to be dispersed by the rounded corners, thus resulting in better stretchability than that of P1.1.[31,32]1Figurea) Image of a snake skin that inspired the stretchable pattern for electrodes (upper). Schematic of changes in skin structure according to the shrinking and stretching movements of the snake (lower). b) FEA simulation images of stress distribution under the same strain for the first snake‐skin patterns (P0 and P1.1–P1.4). c) Maximum stress values of the snake‐skin patterns excluding the notch and adjacent parts. d) Stretchability test results of snake‐skin‐patterned electrodes.Based on the first simulation analysis results, the second group of patterns, excluding the complex and rigid designs (such as P1.3 and P1.4), was designed (Figure 2a). The P2.1 pattern exhibited the best stretchability in the first simulation. P2.2 and P2.3 were patterned with shortened and lengthened “V” parts to check whether the critical structure for stretching of the pattern was “V”‐shaped diagonals. To investigate the stretchability of the patterns within a fixed area, the ratio of the width, height, and length of the diagonal were varied, rather than varying the absolute diagonal length. In the P2.2 pattern, the width increased, whereas the length decreased in the same diagonal length. In the P2.3 pattern, the width decreased, whereas the length increased in the same diagonal length. Finally, by analyzing P1.3 and P1.4, stress was confirmed to be concentrated in the widest area of the pattern. Consequently, P2.4 was designed with a narrower pattern width to check the effect of the pattern width on stretchability. Simulations for the second group of patterns were performed under the same conditions as the first simulation. The results confirmed that P2.4, with a width of 1 mm, which is half the width of P2.1–P2.3, received significantly lower stress compared with other patterns. Comparing stress values of P2.1 and P2.4 as in Figure 2b,c, P2.4 received approximately half the stress of P2.1.[33] This result confirms that the smaller the pattern width, the better the stretchability of the pattern. This is consistent with the result of the first simulation wherein stress was concentrated in the thick structure in P1.3 and P1.4. Next, the comparative analysis of P2.1, P2.2, and P2.3 revealed that contrary to expectations, P2.3, which has the largest ratio of the length of the “V” diagonal, exhibited the lowest stretchability. Thus, the absolute width of the horizontal direction and the length of the diagonal are more important than the ratio of the length of the diagonals. Finally, P2.2, which is longer in width than P2.1 but has a smaller diagonal ratio, was found to exhibit poorer stretchability; thus, the diagonal length has a more significant effect on elasticity than the width and diagonal length in the horizontal direction.2Figurea) FEA simulation images of stress distribution for the same strain. b) Maximum stress values of the patterns excluding the notch and adjacent parts. c) Normalized stress distribution results.Based on the analyses of the first and second simulations, the third group of patterns was designed for the final simulation (Figure 3a). P2.4, which exhibited the best stretchability in the second simulation, was set to P3.1. To confirm the effect of the absolute transverse length on stretchability, P3.2 was designed with two more hexagonal structures than P3.1. Next, to confirm the effect of the absolute length of the diagonal, P3.3 was designed, which maximized the length of the “V”‐shaped diagonal from P3.2. Finally, to confirm the effect of the corner curvature, which was unclear in the first simulation, P3.4 was designed with a radius of curvature of 1 mm. A comparison of P3.1 and P3.2 confirmed that a longer transverse length improved the stretchability. The simulation results indicated that the stress received for the same strain was inversely proportional to the width of the pattern, in a ratio of almost 1:1. As shown in Figure 3b, the width of P3.2 was ≈1.6 times that of P3.1, and the stress on P3.2 was reduced by ≈1/1.5 times that of P3.1. Next, the comparison of P3.2 and P3.3 confirmed that the longer the length of the “V”‐shaped diagonal line, the less the stress applied to the pattern for the same strain. This is because this “V”‐shaped part is a key element in the stretching motion of the snake‐skin pattern. The exact stretching mechanism is illustrated in Figure 4. Finally, a comparison of P3.2 and P3.4 concluded that providing curvature to the corner improved the stretchability of the pattern. In the first simulation, the radius of curvature of 0.5 mm did not contribute to the elasticity. Thus, in the third pattern design, the radius of curvature was doubled to determine whether the small radius of curvature was a problem or whether the assumption that the curvature would affect the stretchability was incorrect. Evidently, P3.4 received ≈1/5 of the stress compared with P3.2, which had an angled corner. Therefore, the curvature clearly improved stretchability. Thus, P1.2 had poor elasticity compared with P1.1 in the first simulation because the diagonal length was shortened due to insufficient curvature, and proper stress distribution was not achieved. This is illustrated by the fact that P3.4 received less than half the stress for the same strain as P3.3, although the absolute length of the diagonal decreased, as shown in Figure 3c. The optimal conditions for the snake‐skin pattern were determined from the analysis of all the simulations. A narrow pattern width, rounded corner with a radius of 1 mm or more, long “V”‐shaped diagonal length, and longest transverse length within a given area are the critical conditions for designing a stretchable pattern.3Figurea) FEA simulation images of stress distribution. b) Maximum stress values of the snake‐skin patterns excluding the notch and adjacent parts. c) Normalized stress distribution results.4Figurea) Structural change upon stretching of the snake‐skin pattern, on a principle similar to the hinge of a door. b) FEA images of angled corner (left) and round corner (right). c) Surface FE‐SEM images of the 30 nm APC electrode after the 30% stretching fatigue test with (lower) or without patterning (upper).Figure 4a shows how the “V”‐ structure improved the stretchability of the snake‐skin pattern. In the pattern, the “V”‐structure acts like a hinge on a door.[34] Hereinafter, this section will be referred to as the hinge structure. Just as hinges open and close to allow the door to move, when the snake‐skin pattern is stretched and released, this hinge structure opens to enable a stretchable movement. The effect of the snake‐skin patterning is shown in Figure S2 of the Supporting Information. The width of the optimal snake‐skin electrode line was stretched by only 10% of its original width, whereas the reference electrode was stretched by 50% at the same 40% strain. This is possible because the hinge structure circumvents the elongation of the electrode. Another important structural element is the corner curvature. Given that the hinge structure renders the electrode stretchable, the stress applied to the corners must be distributed. As shown in Figure 4b, in the case of the left‐angled corner, the stress was highly concentrated in the inner corner, whereas the outer part of the pattern did not contribute to the stress distribution. Conversely, in the simulation image on the right side of Figure 4b, the stress applied to the pattern was slightly distributed in and out of the pattern. Even if the same amount of stress was applied, when a larger area received distributed stress, the average applied stress was lowered, and cracks in the electrode could be prevented. The effect of curvature applied to the corners is shown in Figure 5. Figure 4c shows the surface FE‐SEM image after the stretching fatigue test of the reference P0 electrode and the P3.4 snake‐skin electrode. The P3.4 is the pattern that was judged to be optimal as a result of the previous simulation. Both APC electrodes were prepared by DC magnetron sputtering with a thickness of 30 nm. To confirm whether patterning imparted structural stretchability to the electrodes according to the mechanism analyzed, FE‐SEM images of reference and snake‐skin‐patterned electrodes were taken after the stretching fatigue test. The stretching fatigue test was conducted by repeating 30% strain 1000 times. In the case of the reference APC electrode without the pattern, wide and deep vertical and horizontal cracks were formed owing to the brittleness of the APC film. Conversely, in the case of the snake‐skin pattern electrode, no cracks were observed, even at a magnification of 10 000 times. The cracks were negligibly observed with the naked eye.[35,36] This is because the stress reduction and distribution resulting from the snake‐skin pattern reduced the damage to the electrode to a recoverable level.5FigureResistance change data of P0, P3.3, P3.4, and P4 for a) 30% strain stretching and b) 50% strain stretching. Results of 1000 cycles of a 30% strain fatigue test for c) P0, the reference pattern, and d) P3.4, the optimal snake‐skin pattern.Figure 5 shows the results of a stretching test conducted to confirm the effect of snake‐skin patterning on the stretchability of the APC electrodes. The tested APC electrodes were electrodes with patterns P0, which is a straight reference pattern; P3.4, an optimal snake‐skin pattern; P3.3, with angled corners; and P4, with double the width of the P3.4 pattern. After depositing a 30 nm thick APC thin film, the resistance change rates were measured using a digital multimeter (DMM) while stretching the electrodes with a lab‐designed zig. The results of stretching by 30% and 50% are shown in Figure 5a,b. The initial sheet resistance of the snake‐skin patterned electrode is between 14 and 19.4 Ohm cm−2. The graphs exhibited similar trends. The optimal snake‐skin structure, P3.4, exhibited a resistance change of less than 1, even at 50% strain, whereas P0 exhibited a value more than six times larger. For the snake‐skin electrode, the conductivity was recovered even after deformation of more than 50%; however, this was not the case for the reference electrode. In the case of P3.3, with angled corners, and P4, with thick lines, which are comparable patterns, the resistance increase was lower compared with P0 but higher than that in P3.4. Stretching fatigue tests were performed on the P0 and P3.4 electrodes. Figure 5c,d shows the resistance change as a result of repeating the 30% strain stretching 1000 times. The dynamic fatigue test results revealed that the stretchability of the snake‐skin electrode was much better and more stable than that of the conventional electrode.[37] The snake‐skin‐patterned electrode exhibited a gradual increase in resistance during the stretching fatigue test; however, the increase was significantly less. Owing to the deformation of the PU substrate during the stretching test, a small increase in the resistance change was observed. However, when the resistance was measured again after several tens of minutes, it was restored. This recovery was confirmed by the FE‐SEM image in Figure 4c. The results of the 30% and 50% strain stretching experiments and the stretching fatigue test confirm that snake‐skin patterning improves the stretchability of metal thin‐film electrodes. Since the snake‐skin patterned electrode used in the experiment had a very low initial resistance comparable to silver metal electrode with a thickness of 30 nm, it maintained excellent conductivity even after repeated stretching. After fatigue tests, the snake‐skin patterned electrode showed a sheet resistance 40 Ohm cm−2, which could be acceptable in operation of various stretchable devices.[38]To demonstrate the outstanding flexibility of the snake‐skin‐patterned electrodes, bending tests were conducted.[39,40] Figure 6a shows the dynamic inner and outer bending fatigue test results for the optimal snake‐skin‐patterned APC electrode; all inset pictures show the bending sequence during the dynamic fatigue tests. Evidently, after 10 000 cycles of bending with an inner diameter of 2 mm and outer diameter of 4 mm, the resistance of the APC electrode with the snake‐skin pattern changed negligibly. The results of the dynamic folding test are shown in Figure 6b, with the inset picture indicating the folding sequence. The results of repeating 180° folding for 10 000 cycles with a rod of radius 5 mm revealed that the resistance change rate was less than 1. The slight increase in the resistance can be explained by the contact resistance of the area fixed to the jig of the electrode.[41] For bending, the electrode exhibits little risk in separating from the jig during the measurement. Therefore, during the bending test, the electrode was not fixed strongly; consequently, contact resistance was absent. However, in the case of the other tests, the electrode was fixed very strongly on the jig because of the high risk of detachment from the jig owing to the dynamic shape change of the electrode. Consequently, the damage to the thin film owing to strong fatigue caused by the repeated movement of the contact part temporarily increased the resistance. The rolling test results are shown in Figure 6c. As a result of repeating the winding and unwinding operations of the electrode for 10 000 cycles on a rod with a radius of 5 mm, the resistance increased slightly. However, this was likely to be a temporary change in resistance owing to contact resistance. The dynamic torsional fatigue test was conducted by twisting the snake‐skin electrode at an angle of 40° to the left and right, as shown in Figure 6d. The result was similar to the other aforementioned mechanical property analyses. Based on these results, snake‐skin electrodes are not only stretchable but also very flexible; thus, they are suitable for application in wearable and stretchable devices.6FigureThe 10 000 cycles fatigue test for a) inner and outer bending at inner and outer radii of 2 and 4 mm, respectively. b) Folding at a radius of 5 mm, c) rolling at a radius of 5 mm, and d) twisting at an angle of 40°.The snake‐skin‐patterned electrode exhibited excellent electrical conductivity and mechanical stability under various mechanical stresses, such as stretching, bending, folding, rolling, and twisting, thus demonstrating potential applicability to various stretchable or wearable devices. Therefore, to test their potential application, a wearable sensor for monitoring human temperature was fabricated. Such sensors are significant, particularly in pandemics. Figure 7a shows the structure and measurement method of a wearable temperature sensor based on snake‐skin APC electrodes.[42] By applying a constant voltage, the resistance changes in the sensor according to the temperature change were measured. Figure 7b shows the change in resistance when the temperature was increased in 2 °C intervals, from 20 to 40 °C. Notably, the resistance change was evident in the range close to body temperature. Figure 7c shows a constant increase of the resistance value with respect to a continuous temperature change in increments of 5 °C. The photographs inside the figure are examples of a snake‐skin‐patterned electrode attached to the human wrist. The softness and stretchability of the electrode fabricated on the PU substrate resulted in good adherence to the curved skin. Next, the temperature‐sensing properties between 30 and 40 °C were reexamined for the application of snake‐skin‐patterned electrodes to wearable healthcare devices (Figure 7d). These data were obtained using an electrode that had undergone a stretching fatigue test and therefore incurred some damage as a result of repeated stress. However, because of the effect of the oxygen ion beam treatment (IBT) on the substrate, the snake‐skin temperature sensor exhibited stable sensing properties. Evidently, the adhesion between the APC thin film and PU substrate was enhanced by IBT (Figures S3 and S4, Supporting Information). The temperature sensor with a snake‐skin‐patterned electrode exhibited stable sensing properties and had a high sensitivity to temporary temperature changes. As shown in Figure 7e, when a water droplet at 5 or 10 °C was dropped onto the sensor or when exhalation reached the surface of the sensor, the resultant temperature change was immediately sensed, and the resistance changed rapidly. The change in resistance differed according to the temperature of the water droplet. When colder water was dropped on the sensor, the resistance decreased more. Furthermore, the change in resistance owing to exhalation differed according to the distance between the sensor and the mouth. In the lower graph of Figure 7e, the peak heights were different. When the mouth was close to the sensor, the peak increased as the distance traveled by the breath decreased. This is represented by the second peak. However, when the mouth was far from the sensor, the peak was lower than the first and third peaks as the breath was cooled. The snake‐skin temperature sensor exhibited excellent performance and was able to sense the subtle temperature difference of exhalation according to distance. As shown in Figure 7b,d, the snake‐skin temperature sensor exhibited a linear response of resistance to temperature changes. The temperature resistance coefficient (TCR) representing the relative difference in resistance with temperature can be extracted from the thermal resistance curve, and the TCR equation is expressed as follows[43]1TCR=1R(T0)R(T)−R(T0)T−T0\[\begin{array}{*{20}{c}}{{\rm{TCR}} = \frac{1}{{R\left( {{T_0}} \right)}}\frac{{R\left( T \right) - R\left( {{T_0}} \right)}}{{T - {T_0}}}}\end{array}\]where R(T0) is the resistance at the initial temperature, T0; and R(T) is the resistance at a specific temperature, T. Based on Figure 7 and Equation (1), the wearable temperature sensor with a snake‐skin‐patterned electrode has a positive TCR value because the electrode material is metal.[44] Thus, based on the stable and sensitive temperature‐sensing performance, the snake‐skin‐patterned electrodes can be applied to various wearable sensors and biosignal sensing devices.[45] In this study, the snake‐skin pattern was applied to APC electrode material; nonetheless, any desired material can be used according to the needs of the researcher. This renders the snake‐skin pattern highly versatile. Snake‐skin‐patterned electrodes can be used as a medical device component that informs users of body temperature through real‐time monitoring. This could help diagnose fever associated with infectious diseases, such as COVID‐19, as shown in Figure 7f.7Figurea) Illustration of the layered structure of the temperature sensor and test method. b) Resistance changes of the sensor according to the temperature change. c) Graph of resistance changes according to temperature increase. d) Graph of resistance changes according to temperature changes after 30% strain stretching fatigue test, highlighting the range of human body temperature (box highlighted in red). e) Resistance changes as a reaction to water droplets and a person's exhalation. f) Illustration of heath care devices incorporating the snake‐skin‐patterned electrode.A stretchable snake‐skin‐patterned electrode can be used as a stretchable interconnector in wearable devices. To demonstrate its feasibility as a stretchable interconnector, a simple circuit to turn a blue LED on and off was constructed using copper tape, batteries, and blue LEDs. In this circuit, the Cu tape was cut off and connected to a snake‐skin‐patterned electrode, as shown in Figure 8a. Herein, the change in resistance according to the state of the interconnector could be intuitively observed via the change in the LED brightness.[46] As shown on the right side of Figure 8a, when the interconnector was damaged by physical stress, the resistance of the circuit increased and the LED dimmed; otherwise, the LED remained bright. Figure 8b shows the LED lit when the snake‐skin‐patterned interconnector was straightened, bent to a bending radius of 5 mm, and stretched by 20%. Photographs were taken in both light and dark environments for accurate brightness comparison. The brightness of the LED was similar, despite the stress applied to the interconnector in various forms. Essentially, the resistance of the snake‐skin‐patterned interconnector did not change significantly, as expected from Figures 5 and 6. Therefore, the high performance of the stretchable temperature sensors and interconnectors demonstrates that the snake‐skin‐patterned electrode is a promising material for next‐generation stretchable electronics and wearable bioelectronics.8Figurea) Schematic of LED lighting according to interconnector connection and resistance change. Photographs of the LED when the snake‐skin‐patterned interconnector is b) in pristine state, c) bent, and d) 20% stretched.ConclusionHerein, the characteristics of snake‐skin patterning were investigated by combining the Kirigami structure and biomimicry. Using the Kirigami and snake‐skin structures together, a high‐performance stretch pattern was developed. To develop optimal snake‐skin patterns, FEA simulations were performed on several patterns to determine an APC pattern that received relatively little stress for the same strain. The static stretching test revealed that all the fabricated types (P3.3, P3.4, P4) of snake‐skin‐patterned electrodes exhibited a significantly lower resistance change compared with the reference electrode P0. In particular, the optimal P3.4 electrode exhibited a resistance change of less than 1.5, even with a severe strain of 50%. Thus, the snake‐skin pattern effectively imparts structural stretchability to the patterned APC electrode. Further, dynamic fatigue tests, such as bending, folding, rolling, and twisting, were performed. They revealed that the snake‐skin electrode exhibited an almost zero change in resistance, even after repeated stresses for 10 000 cycles. Applicability to actual wearable healthcare devices was evaluated by evaluating the performance of a temperature sensor. The wearable temperature sensor with a snake‐skin‐patterned electrode exhibited stable and sensitive temperature‐sensing properties, particularly in the temperature region near the human body temperature. Furthermore, the snake‐skin‐patterned electrode was applied to a stretchable interconnector, which is a key component of wearable electronics. Even under severe bending or stretching, the blue LED connected to the snake‐skin‐patterned interconnector displayed stable brightness. The results prove that the snake‐skin‐patterned electrode can maintain stable conductivity against deformation and stress that may occur when attached to the human body. Therefore, a snake‐skin‐patterned electrode designed by combining the Kirigami structure and biomimicry is a promising electrode for next‐generation stretchable electronics and wearable electronics.Experimental SectionDesign and Simulation of Snake‐Skin‐Like PatternTo determine the optimal snake‐skin pattern inspired by the Kirigami structure, FEA was conducted using Midas NFX (Midas IT Co.). Figure S5 of the Supporting Information shows one of the simulation models used. Here, stretchability was evaluated using a tensile tester with uniaxial displacement. Half of both ends of the electrodes were fixed using a jig. The simulation models were simplified because the mechanical deformations of the fixed regions are minor compared to those of the unfixed area. In addition, because the aspect ratio (length‐to‐thickness) was larger than 106:1, unexpected simulation errors could occur, thereby decreasing the analysis efficiency. Furthermore, the thickness ratio between the PU substrate and patterned electrodes was sufficiently large to cause simulation errors and reduce the analysis efficiency. Therefore, based on the experimental conditions and mechanical dimensions of the samples, simplified 2D‐models of the freestanding patterned electrodes were designed. Considering the stress concentration, fine elements were formed at each notch to obtain more accurate results, as shown in Figure S5 of the Supporting Information. Finally, all degrees of freedom were confined (i.e., fixed boundary) along the edge (Line 1), and a mechanical load was applied on the opposite edge (Line 2) in the form of a forced displacement of 9 × 10−3 mm. The effective stress (σeff) results were obtained to easily compare the formed mechanical stresses. The patterns applied to the experiment were named as follows.Fabrication of Snake‐Skin‐Like ElectrodeA 4‐in. magnetron sputtering system (19NNS002, NNSV) was used to fabricate a snake‐skin‐patterned electrode at room temperature. APC alloy was chosen as the electrode material owing to its high conductivity and good stability.[47] Using a 4‐in. APC target (Ag: 98.1, Pd: 0.9, Cu: 1 wt%; Dasom RMS), the APC film was sputtered on the PU substrate at a fixed DC power of 100 W, Ar flow rate of 10 sccm, and working pressure of 3 × 10−6 Torr. To ensure uniformity of the electrode, the z‐axis rotation speed of the sample‐loaded disk was set to 25 rpm. For snake‐skin patterning, 0.2 mm thick INVAR 36 (Ni: 36%, Fe: 64%) shadow masks were used. To prevent the shadowing effect caused by the gap between the mask and the substrate, a ferromagnetic mask and 1 mm thick rubber magnet were employed, as shown in Figure S6 of the Supporting Information. To exclude the effect of electrode thickness on elasticity, the snake‐skin electrode was deposited with electrode thicknesses of 20, 30, and 40 nm, and a preliminary stretching experiment was performed (Figure S7, Supporting Information). The 30 nm thick electrode exhibited the lowest change in resistance to strain; therefore, the thickness of the electrode was fixed at 30 nm for the snake‐skin‐patterned electrode.Properties of Snake‐Skin‐Like ElectrodeFor the analysis of stretchability, the resistance changes of the snake‐skin electrodes were measured using a lab‐designed zig and a digital multimeter (DMM; 34450A, Keysight). When analyzing the stretching property, the resistance value was measured using the average of the resistance values measured with the two tips of the DMM narrow, with one hexagon in between and three hexagons wide (Figure S8, Supporting Information). Other mechanical properties of the snake‐skin electrodes were analyzed using a lab‐designed bending, folding, rolling, and twisting tester to confirm the flexibility of the snake‐skin electrode. In addition, a dynamic fatigue test was performed for 10 000 cycles for all the above mechanical tests to confirm mechanical flexibility and reliability.Applications of Stretchable Snake‐Skin ElectrodesA wearable and stretchable temperature sensor was fabricated by coating a PU solution on the snake‐skin electrode using a spin‐coating system at 1000 rpm for 15 s (Figure S9, Supporting Information). The PU solution was covered to protect the temperature sensor from humidity and contaminants from the environment and to accurately measure the temperature using the stretchable temperature sensor. A copper tape of width 1 cm was attached to both ends of the snake‐skin electrode to protect the contact points from being covered by the PU solution. Next, the PU spin‐coated electrode was cured in an oven at 60 °C for 3 h. The temperature sensor was placed on a hot chuck, and the tips of the probe station placed in contact with the contact electrode parts. Thus, the resistance change of the snake‐skin temperature sensor as a function of temperature was measured using a hot chuck controller (MSTECH, MST‐1000H), and the semiconductor characteristics were analyzed (Tektronix, Keithley 4200‐SCS). Blue LEDs, Cu tape, batteries, and wires were used to fabricate the LED interconnecting circuit. The middle section of the copper tape was cut to confirm the stable performance of the snake‐skin electrode as an interconnector of the circuit. When a constant voltage was applied to the batteries, the snake‐skin interconnector was bent to a radius of 5 mm and stretched by 30%. A comparison of the brightness of the LED with the naked eye intuitively confirmed whether the resistance of the snake‐skin interconnector increased.AcknowledgementsThis research was supported by the Commercialization Promotion Agency for R&D Outcomes (COMPA) funded by the Ministry of Science and ICT(MSIT) (2022RMD‐S07). In addition, this work was supported by Korea institute for Advancement of Technology (KIAT) grant funded by the Korea government (MOTIE) (P0008458, HRD Program for Industrial Innovation) and the National Research Foundation of Korea (NRF‐2022M3D1A2083618).Conflict of InterestThe authors declare no conflict of interest.Data Availability StatementThe data that support the findings of this study are available from the corresponding author upon reasonable request.T. Li, Y. Li, T. Zhang, Acc. Chem. Res. 2019, 52, 288.L. Wang, D. Chen, K. Jiang, G. Shen, Chem. Soc. Rev. 2017, 46, 6764.A. Nag, S. C. Mukhopadhyay, J. Kosel, IEEE Sens. J. 2017, 17, 3949.J. Lee, S. Kim, J. 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Stretchable and Flexible Snake Skin Patterned Electrodes for Wearable Electronics Inspired by Kirigami Structure

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Abstract

IntroductionRapid advances in wearable and stretchable electronics includes intelligent functions that enhances memory, in situ healthcare, communication, motion monitoring, and physical senses.[1,2] Wearable devices have diverse applications, including bioelectronics in close contact with the body, human–machine interfaces, smart clothing, and wearable healthcare devices.[3,4] With the spread of stretchable and foldable smartphones, the stretchability of materials is now a requirement for next‐generation electronics. As wearable electronics are worn externally on the human body, they must be made of that are highly stretchable and reliable.[5,6] In particular, developing highly stretchable electrodes with high conductivity is imperative because stretchable electrodes are critical components of optoelectronic devices and sensors. However, typical inorganic electrode materials, such as metals and transparent conductive oxides, cannot be used as stretchable electrodes in wearable electronics owing to their inherent brittleness. The materials for stretchable electrodes include carbon nanotubes or 2D materials such as graphene[7,8] and superlattices.[9] Structural approaches include microbuckling, prestraining, and wavy patterns.[10] In materials approaches, hybrid electrode materials are coated or mixed with stretchable substrates.[11] These methods provide high stretchability but are limited by the electrical and optical properties of the electrodes. In particular, the choice of electrode materials, such as Ag nanowires, PEDOT:PSS, and CNT, is limited and the solution process is unsuitable for large area coatings.[12,13] Structural approaches, including patterning of 2D and 3D electrodes using laser cutting or mold forming, buckling substrates, and wavy patterning have been suggested for stretchable electrodes. However, these processes incur high costs and are complicated.[14] In additionally, the presence of cutting lines and holes is a critical limitation for device applications.[15] Patterning using a shadow mask prevents the electrodes from being damaged, by avoiding holes, which occur during the laser patterning process when creating a stretchable net structure. This helps lower the process costs and increase the applicability of the device. Among the structural approaches, shadow mask patterning, which can complete the patterning and deposition process in one step without a cutting or molding process, was selected in this study for its wide application and cost‐effective and simple process. Specially, considering mass‐production of the stretchable electronics, large area coating and process compatibility of the stretchable electrodes is one of the critical issues to solve the problems of previously reported stretchable electrodes. The Kirigami structure and biomimicry concept were combined to develop a new pattern for stretchable electrodes. Kirigami is a Japanese word for origami, which involves cutting paper into a specific pattern or shape.[16] Kirigami is a unique structure that is being actively studied in academia owing to its outstanding mechanical properties. For example, if paper is cut in the Kirigami pattern, the deformability is improved and the paper, which has the property of tearing when pulled, opens, and stretches in three dimensions. Kirigami patterned paper has a negative Poisson's ratio owing to the perforation line, which increases the thickness of the stretched paper in 3D.[17] Designs inspired by the Kirigami patterns have been applied in various fields, including architecture, foldable and portable solar cells, and micro/nanoscale mechanical systems.[18–20] Stretchable devices are typically fabricated by applying Kirigami structures to 2D materials or by laser cutting and etching processes. This method has the same disadvantages as mentioned above, regardless of the usefulness of the Kirigami structure.[21,22] To increase the versatility of this promising structure, a Kirigami patterning without cutting was developed. This patterning method has become a cornerstone for developing a new type of stretchable electrode. Further, biomimetics (composed of two Greek words, “bios”‐ meaning life and ‐“mimesis” meaning imitation), were grafted.[23] Swimwear for athletes that reduce water resistance by imitating shark‐scale structures, and water‐repellent coatings that imitate the surface structure of lotus leaves are representative biomimicry technologies.[24,25] Biomimicry is an effective research method that has solved various human problems by studying the properties of living things. Although the Kirigami pattern and biomimicry have been applied separately in numerous cases, reports of stretchable electrodes combining Kirigami and biomimicry are lacking.In this study, a novel stretchable electrode was developed by applying a pattern that combines Kirigami and biomimicry. Among various animals with flexible and elastic movements, such as octopus and blowfish, the structure of the snake's abdominal skin is notably similar to that of the reticulated Kirigami structure. Therefore, herein, several types of stretchable patterns inspired by the skin pattern on the belly of a corn snake were designed. The structural conditions of the pattern were analyzed through finite element analysis (FEA) simulations.[26] By iterating the snake‐skin pattern design and FEA simulation, a snake‐skin pattern with optimal structural conditions was determined. The pattern was fabricated as a stretchable snake‐skin electrode using an AgPdCu (APC) target, polyurethane (PU) substrate, and sputter. The stretching and bending properties were measured, and the results were analyzed. Next, the stretchable electrode was applied as a temperature sensor and light‐emitting diode (LED) interconnector to prove the practicality of the snake‐skin electrode.Results and DiscussionThe corn snake has a skin pattern consisting of a diamond shape on the back and a mixture of hexagons and diamonds on the belly. Evidently, the structural change when the ventral pattern was stretched or shrunk was found to be similar to that of the Kirigami structure, which is often used as a stretchable pattern. As the snake's body stretched, the “V” line opened at both ends of the long hexagonal structure. Consequently, the snake's skin could be thinned and stretched without being torn or damaged. This is similar to the principle of Kirigami, wherein the paper is twisted up and down along the cutting line when pulled and stretched.[27,28] Herein, various types of stretch patterns inspired by snakes (Figure 1a) were designed, and FEA simulations were conducted for stretchability comparisons.[29] The purpose of the simulation was to compare the magnitude of the stress generated in each pattern when the same strain was applied to the electrode. The magnitude of the distributed stress in the snake‐skin pattern is shown in Figure 1b, and the maximum stress received by each pattern is graphically shown in Figure 1c. For simplicity, the sample patterns are referred to as P0, P1, P2, P3, and P4. In PX, X denotes the number of simulations; in PX.Y, Y is the index of each design of Simulation X. For example, P2.1 points to the first pattern in the second simulation. The maximum stress applied to the pattern can be calculated in two ways: by focusing on the region near the notch or by excluding the notch.[30] A comparison of the values calculated using the two methods with the actual stretching test results revealed that the maximum stress value in the region excluding the notch coincided with the actual stretching test result. Therefore, the simulation results were analyzed based on the stress values, excluding the notch region (Figure S1, Supporting Information). In Figure 1d, stretchability is the strain just before the resistance increases by more than ten times compared with the initial resistance when the 30 nm thick APC electrode is stretched. Evidently, all snake‐skin patterns exhibited better stretchability compared with the reference electrode P0. In addition, a difference in stretchability between the patterns was evident. In the case of P1.3 and P1.4, which have rigid and complex patterns, the stress was concentrated in the middle core line of the outer rhombus and the inner hexagon. This resulted in poor electrode stretchability. In the case of P1.2, the stress was slightly higher for the same strain. Because P1.2 is a pattern with rounded corners with a radius of 0.5 mm, the stress was expected to be dispersed by the rounded corners, thus resulting in better stretchability than that of P1.1.[31,32]1Figurea) Image of a snake skin that inspired the stretchable pattern for electrodes (upper). Schematic of changes in skin structure according to the shrinking and stretching movements of the snake (lower). b) FEA simulation images of stress distribution under the same strain for the first snake‐skin patterns (P0 and P1.1–P1.4). c) Maximum stress values of the snake‐skin patterns excluding the notch and adjacent parts. d) Stretchability test results of snake‐skin‐patterned electrodes.Based on the first simulation analysis results, the second group of patterns, excluding the complex and rigid designs (such as P1.3 and P1.4), was designed (Figure 2a). The P2.1 pattern exhibited the best stretchability in the first simulation. P2.2 and P2.3 were patterned with shortened and lengthened “V” parts to check whether the critical structure for stretching of the pattern was “V”‐shaped diagonals. To investigate the stretchability of the patterns within a fixed area, the ratio of the width, height, and length of the diagonal were varied, rather than varying the absolute diagonal length. In the P2.2 pattern, the width increased, whereas the length decreased in the same diagonal length. In the P2.3 pattern, the width decreased, whereas the length increased in the same diagonal length. Finally, by analyzing P1.3 and P1.4, stress was confirmed to be concentrated in the widest area of the pattern. Consequently, P2.4 was designed with a narrower pattern width to check the effect of the pattern width on stretchability. Simulations for the second group of patterns were performed under the same conditions as the first simulation. The results confirmed that P2.4, with a width of 1 mm, which is half the width of P2.1–P2.3, received significantly lower stress compared with other patterns. Comparing stress values of P2.1 and P2.4 as in Figure 2b,c, P2.4 received approximately half the stress of P2.1.[33] This result confirms that the smaller the pattern width, the better the stretchability of the pattern. This is consistent with the result of the first simulation wherein stress was concentrated in the thick structure in P1.3 and P1.4. Next, the comparative analysis of P2.1, P2.2, and P2.3 revealed that contrary to expectations, P2.3, which has the largest ratio of the length of the “V” diagonal, exhibited the lowest stretchability. Thus, the absolute width of the horizontal direction and the length of the diagonal are more important than the ratio of the length of the diagonals. Finally, P2.2, which is longer in width than P2.1 but has a smaller diagonal ratio, was found to exhibit poorer stretchability; thus, the diagonal length has a more significant effect on elasticity than the width and diagonal length in the horizontal direction.2Figurea) FEA simulation images of stress distribution for the same strain. b) Maximum stress values of the patterns excluding the notch and adjacent parts. c) Normalized stress distribution results.Based on the analyses of the first and second simulations, the third group of patterns was designed for the final simulation (Figure 3a). P2.4, which exhibited the best stretchability in the second simulation, was set to P3.1. To confirm the effect of the absolute transverse length on stretchability, P3.2 was designed with two more hexagonal structures than P3.1. Next, to confirm the effect of the absolute length of the diagonal, P3.3 was designed, which maximized the length of the “V”‐shaped diagonal from P3.2. Finally, to confirm the effect of the corner curvature, which was unclear in the first simulation, P3.4 was designed with a radius of curvature of 1 mm. A comparison of P3.1 and P3.2 confirmed that a longer transverse length improved the stretchability. The simulation results indicated that the stress received for the same strain was inversely proportional to the width of the pattern, in a ratio of almost 1:1. As shown in Figure 3b, the width of P3.2 was ≈1.6 times that of P3.1, and the stress on P3.2 was reduced by ≈1/1.5 times that of P3.1. Next, the comparison of P3.2 and P3.3 confirmed that the longer the length of the “V”‐shaped diagonal line, the less the stress applied to the pattern for the same strain. This is because this “V”‐shaped part is a key element in the stretching motion of the snake‐skin pattern. The exact stretching mechanism is illustrated in Figure 4. Finally, a comparison of P3.2 and P3.4 concluded that providing curvature to the corner improved the stretchability of the pattern. In the first simulation, the radius of curvature of 0.5 mm did not contribute to the elasticity. Thus, in the third pattern design, the radius of curvature was doubled to determine whether the small radius of curvature was a problem or whether the assumption that the curvature would affect the stretchability was incorrect. Evidently, P3.4 received ≈1/5 of the stress compared with P3.2, which had an angled corner. Therefore, the curvature clearly improved stretchability. Thus, P1.2 had poor elasticity compared with P1.1 in the first simulation because the diagonal length was shortened due to insufficient curvature, and proper stress distribution was not achieved. This is illustrated by the fact that P3.4 received less than half the stress for the same strain as P3.3, although the absolute length of the diagonal decreased, as shown in Figure 3c. The optimal conditions for the snake‐skin pattern were determined from the analysis of all the simulations. A narrow pattern width, rounded corner with a radius of 1 mm or more, long “V”‐shaped diagonal length, and longest transverse length within a given area are the critical conditions for designing a stretchable pattern.3Figurea) FEA simulation images of stress distribution. b) Maximum stress values of the snake‐skin patterns excluding the notch and adjacent parts. c) Normalized stress distribution results.4Figurea) Structural change upon stretching of the snake‐skin pattern, on a principle similar to the hinge of a door. b) FEA images of angled corner (left) and round corner (right). c) Surface FE‐SEM images of the 30 nm APC electrode after the 30% stretching fatigue test with (lower) or without patterning (upper).Figure 4a shows how the “V”‐ structure improved the stretchability of the snake‐skin pattern. In the pattern, the “V”‐structure acts like a hinge on a door.[34] Hereinafter, this section will be referred to as the hinge structure. Just as hinges open and close to allow the door to move, when the snake‐skin pattern is stretched and released, this hinge structure opens to enable a stretchable movement. The effect of the snake‐skin patterning is shown in Figure S2 of the Supporting Information. The width of the optimal snake‐skin electrode line was stretched by only 10% of its original width, whereas the reference electrode was stretched by 50% at the same 40% strain. This is possible because the hinge structure circumvents the elongation of the electrode. Another important structural element is the corner curvature. Given that the hinge structure renders the electrode stretchable, the stress applied to the corners must be distributed. As shown in Figure 4b, in the case of the left‐angled corner, the stress was highly concentrated in the inner corner, whereas the outer part of the pattern did not contribute to the stress distribution. Conversely, in the simulation image on the right side of Figure 4b, the stress applied to the pattern was slightly distributed in and out of the pattern. Even if the same amount of stress was applied, when a larger area received distributed stress, the average applied stress was lowered, and cracks in the electrode could be prevented. The effect of curvature applied to the corners is shown in Figure 5. Figure 4c shows the surface FE‐SEM image after the stretching fatigue test of the reference P0 electrode and the P3.4 snake‐skin electrode. The P3.4 is the pattern that was judged to be optimal as a result of the previous simulation. Both APC electrodes were prepared by DC magnetron sputtering with a thickness of 30 nm. To confirm whether patterning imparted structural stretchability to the electrodes according to the mechanism analyzed, FE‐SEM images of reference and snake‐skin‐patterned electrodes were taken after the stretching fatigue test. The stretching fatigue test was conducted by repeating 30% strain 1000 times. In the case of the reference APC electrode without the pattern, wide and deep vertical and horizontal cracks were formed owing to the brittleness of the APC film. Conversely, in the case of the snake‐skin pattern electrode, no cracks were observed, even at a magnification of 10 000 times. The cracks were negligibly observed with the naked eye.[35,36] This is because the stress reduction and distribution resulting from the snake‐skin pattern reduced the damage to the electrode to a recoverable level.5FigureResistance change data of P0, P3.3, P3.4, and P4 for a) 30% strain stretching and b) 50% strain stretching. Results of 1000 cycles of a 30% strain fatigue test for c) P0, the reference pattern, and d) P3.4, the optimal snake‐skin pattern.Figure 5 shows the results of a stretching test conducted to confirm the effect of snake‐skin patterning on the stretchability of the APC electrodes. The tested APC electrodes were electrodes with patterns P0, which is a straight reference pattern; P3.4, an optimal snake‐skin pattern; P3.3, with angled corners; and P4, with double the width of the P3.4 pattern. After depositing a 30 nm thick APC thin film, the resistance change rates were measured using a digital multimeter (DMM) while stretching the electrodes with a lab‐designed zig. The results of stretching by 30% and 50% are shown in Figure 5a,b. The initial sheet resistance of the snake‐skin patterned electrode is between 14 and 19.4 Ohm cm−2. The graphs exhibited similar trends. The optimal snake‐skin structure, P3.4, exhibited a resistance change of less than 1, even at 50% strain, whereas P0 exhibited a value more than six times larger. For the snake‐skin electrode, the conductivity was recovered even after deformation of more than 50%; however, this was not the case for the reference electrode. In the case of P3.3, with angled corners, and P4, with thick lines, which are comparable patterns, the resistance increase was lower compared with P0 but higher than that in P3.4. Stretching fatigue tests were performed on the P0 and P3.4 electrodes. Figure 5c,d shows the resistance change as a result of repeating the 30% strain stretching 1000 times. The dynamic fatigue test results revealed that the stretchability of the snake‐skin electrode was much better and more stable than that of the conventional electrode.[37] The snake‐skin‐patterned electrode exhibited a gradual increase in resistance during the stretching fatigue test; however, the increase was significantly less. Owing to the deformation of the PU substrate during the stretching test, a small increase in the resistance change was observed. However, when the resistance was measured again after several tens of minutes, it was restored. This recovery was confirmed by the FE‐SEM image in Figure 4c. The results of the 30% and 50% strain stretching experiments and the stretching fatigue test confirm that snake‐skin patterning improves the stretchability of metal thin‐film electrodes. Since the snake‐skin patterned electrode used in the experiment had a very low initial resistance comparable to silver metal electrode with a thickness of 30 nm, it maintained excellent conductivity even after repeated stretching. After fatigue tests, the snake‐skin patterned electrode showed a sheet resistance 40 Ohm cm−2, which could be acceptable in operation of various stretchable devices.[38]To demonstrate the outstanding flexibility of the snake‐skin‐patterned electrodes, bending tests were conducted.[39,40] Figure 6a shows the dynamic inner and outer bending fatigue test results for the optimal snake‐skin‐patterned APC electrode; all inset pictures show the bending sequence during the dynamic fatigue tests. Evidently, after 10 000 cycles of bending with an inner diameter of 2 mm and outer diameter of 4 mm, the resistance of the APC electrode with the snake‐skin pattern changed negligibly. The results of the dynamic folding test are shown in Figure 6b, with the inset picture indicating the folding sequence. The results of repeating 180° folding for 10 000 cycles with a rod of radius 5 mm revealed that the resistance change rate was less than 1. The slight increase in the resistance can be explained by the contact resistance of the area fixed to the jig of the electrode.[41] For bending, the electrode exhibits little risk in separating from the jig during the measurement. Therefore, during the bending test, the electrode was not fixed strongly; consequently, contact resistance was absent. However, in the case of the other tests, the electrode was fixed very strongly on the jig because of the high risk of detachment from the jig owing to the dynamic shape change of the electrode. Consequently, the damage to the thin film owing to strong fatigue caused by the repeated movement of the contact part temporarily increased the resistance. The rolling test results are shown in Figure 6c. As a result of repeating the winding and unwinding operations of the electrode for 10 000 cycles on a rod with a radius of 5 mm, the resistance increased slightly. However, this was likely to be a temporary change in resistance owing to contact resistance. The dynamic torsional fatigue test was conducted by twisting the snake‐skin electrode at an angle of 40° to the left and right, as shown in Figure 6d. The result was similar to the other aforementioned mechanical property analyses. Based on these results, snake‐skin electrodes are not only stretchable but also very flexible; thus, they are suitable for application in wearable and stretchable devices.6FigureThe 10 000 cycles fatigue test for a) inner and outer bending at inner and outer radii of 2 and 4 mm, respectively. b) Folding at a radius of 5 mm, c) rolling at a radius of 5 mm, and d) twisting at an angle of 40°.The snake‐skin‐patterned electrode exhibited excellent electrical conductivity and mechanical stability under various mechanical stresses, such as stretching, bending, folding, rolling, and twisting, thus demonstrating potential applicability to various stretchable or wearable devices. Therefore, to test their potential application, a wearable sensor for monitoring human temperature was fabricated. Such sensors are significant, particularly in pandemics. Figure 7a shows the structure and measurement method of a wearable temperature sensor based on snake‐skin APC electrodes.[42] By applying a constant voltage, the resistance changes in the sensor according to the temperature change were measured. Figure 7b shows the change in resistance when the temperature was increased in 2 °C intervals, from 20 to 40 °C. Notably, the resistance change was evident in the range close to body temperature. Figure 7c shows a constant increase of the resistance value with respect to a continuous temperature change in increments of 5 °C. The photographs inside the figure are examples of a snake‐skin‐patterned electrode attached to the human wrist. The softness and stretchability of the electrode fabricated on the PU substrate resulted in good adherence to the curved skin. Next, the temperature‐sensing properties between 30 and 40 °C were reexamined for the application of snake‐skin‐patterned electrodes to wearable healthcare devices (Figure 7d). These data were obtained using an electrode that had undergone a stretching fatigue test and therefore incurred some damage as a result of repeated stress. However, because of the effect of the oxygen ion beam treatment (IBT) on the substrate, the snake‐skin temperature sensor exhibited stable sensing properties. Evidently, the adhesion between the APC thin film and PU substrate was enhanced by IBT (Figures S3 and S4, Supporting Information). The temperature sensor with a snake‐skin‐patterned electrode exhibited stable sensing properties and had a high sensitivity to temporary temperature changes. As shown in Figure 7e, when a water droplet at 5 or 10 °C was dropped onto the sensor or when exhalation reached the surface of the sensor, the resultant temperature change was immediately sensed, and the resistance changed rapidly. The change in resistance differed according to the temperature of the water droplet. When colder water was dropped on the sensor, the resistance decreased more. Furthermore, the change in resistance owing to exhalation differed according to the distance between the sensor and the mouth. In the lower graph of Figure 7e, the peak heights were different. When the mouth was close to the sensor, the peak increased as the distance traveled by the breath decreased. This is represented by the second peak. However, when the mouth was far from the sensor, the peak was lower than the first and third peaks as the breath was cooled. The snake‐skin temperature sensor exhibited excellent performance and was able to sense the subtle temperature difference of exhalation according to distance. As shown in Figure 7b,d, the snake‐skin temperature sensor exhibited a linear response of resistance to temperature changes. The temperature resistance coefficient (TCR) representing the relative difference in resistance with temperature can be extracted from the thermal resistance curve, and the TCR equation is expressed as follows[43]1TCR=1R(T0)R(T)−R(T0)T−T0\[\begin{array}{*{20}{c}}{{\rm{TCR}} = \frac{1}{{R\left( {{T_0}} \right)}}\frac{{R\left( T \right) - R\left( {{T_0}} \right)}}{{T - {T_0}}}}\end{array}\]where R(T0) is the resistance at the initial temperature, T0; and R(T) is the resistance at a specific temperature, T. Based on Figure 7 and Equation (1), the wearable temperature sensor with a snake‐skin‐patterned electrode has a positive TCR value because the electrode material is metal.[44] Thus, based on the stable and sensitive temperature‐sensing performance, the snake‐skin‐patterned electrodes can be applied to various wearable sensors and biosignal sensing devices.[45] In this study, the snake‐skin pattern was applied to APC electrode material; nonetheless, any desired material can be used according to the needs of the researcher. This renders the snake‐skin pattern highly versatile. Snake‐skin‐patterned electrodes can be used as a medical device component that informs users of body temperature through real‐time monitoring. This could help diagnose fever associated with infectious diseases, such as COVID‐19, as shown in Figure 7f.7Figurea) Illustration of the layered structure of the temperature sensor and test method. b) Resistance changes of the sensor according to the temperature change. c) Graph of resistance changes according to temperature increase. d) Graph of resistance changes according to temperature changes after 30% strain stretching fatigue test, highlighting the range of human body temperature (box highlighted in red). e) Resistance changes as a reaction to water droplets and a person's exhalation. f) Illustration of heath care devices incorporating the snake‐skin‐patterned electrode.A stretchable snake‐skin‐patterned electrode can be used as a stretchable interconnector in wearable devices. To demonstrate its feasibility as a stretchable interconnector, a simple circuit to turn a blue LED on and off was constructed using copper tape, batteries, and blue LEDs. In this circuit, the Cu tape was cut off and connected to a snake‐skin‐patterned electrode, as shown in Figure 8a. Herein, the change in resistance according to the state of the interconnector could be intuitively observed via the change in the LED brightness.[46] As shown on the right side of Figure 8a, when the interconnector was damaged by physical stress, the resistance of the circuit increased and the LED dimmed; otherwise, the LED remained bright. Figure 8b shows the LED lit when the snake‐skin‐patterned interconnector was straightened, bent to a bending radius of 5 mm, and stretched by 20%. Photographs were taken in both light and dark environments for accurate brightness comparison. The brightness of the LED was similar, despite the stress applied to the interconnector in various forms. Essentially, the resistance of the snake‐skin‐patterned interconnector did not change significantly, as expected from Figures 5 and 6. Therefore, the high performance of the stretchable temperature sensors and interconnectors demonstrates that the snake‐skin‐patterned electrode is a promising material for next‐generation stretchable electronics and wearable bioelectronics.8Figurea) Schematic of LED lighting according to interconnector connection and resistance change. Photographs of the LED when the snake‐skin‐patterned interconnector is b) in pristine state, c) bent, and d) 20% stretched.ConclusionHerein, the characteristics of snake‐skin patterning were investigated by combining the Kirigami structure and biomimicry. Using the Kirigami and snake‐skin structures together, a high‐performance stretch pattern was developed. To develop optimal snake‐skin patterns, FEA simulations were performed on several patterns to determine an APC pattern that received relatively little stress for the same strain. The static stretching test revealed that all the fabricated types (P3.3, P3.4, P4) of snake‐skin‐patterned electrodes exhibited a significantly lower resistance change compared with the reference electrode P0. In particular, the optimal P3.4 electrode exhibited a resistance change of less than 1.5, even with a severe strain of 50%. Thus, the snake‐skin pattern effectively imparts structural stretchability to the patterned APC electrode. Further, dynamic fatigue tests, such as bending, folding, rolling, and twisting, were performed. They revealed that the snake‐skin electrode exhibited an almost zero change in resistance, even after repeated stresses for 10 000 cycles. Applicability to actual wearable healthcare devices was evaluated by evaluating the performance of a temperature sensor. The wearable temperature sensor with a snake‐skin‐patterned electrode exhibited stable and sensitive temperature‐sensing properties, particularly in the temperature region near the human body temperature. Furthermore, the snake‐skin‐patterned electrode was applied to a stretchable interconnector, which is a key component of wearable electronics. Even under severe bending or stretching, the blue LED connected to the snake‐skin‐patterned interconnector displayed stable brightness. The results prove that the snake‐skin‐patterned electrode can maintain stable conductivity against deformation and stress that may occur when attached to the human body. Therefore, a snake‐skin‐patterned electrode designed by combining the Kirigami structure and biomimicry is a promising electrode for next‐generation stretchable electronics and wearable electronics.Experimental SectionDesign and Simulation of Snake‐Skin‐Like PatternTo determine the optimal snake‐skin pattern inspired by the Kirigami structure, FEA was conducted using Midas NFX (Midas IT Co.). Figure S5 of the Supporting Information shows one of the simulation models used. Here, stretchability was evaluated using a tensile tester with uniaxial displacement. Half of both ends of the electrodes were fixed using a jig. The simulation models were simplified because the mechanical deformations of the fixed regions are minor compared to those of the unfixed area. In addition, because the aspect ratio (length‐to‐thickness) was larger than 106:1, unexpected simulation errors could occur, thereby decreasing the analysis efficiency. Furthermore, the thickness ratio between the PU substrate and patterned electrodes was sufficiently large to cause simulation errors and reduce the analysis efficiency. Therefore, based on the experimental conditions and mechanical dimensions of the samples, simplified 2D‐models of the freestanding patterned electrodes were designed. Considering the stress concentration, fine elements were formed at each notch to obtain more accurate results, as shown in Figure S5 of the Supporting Information. Finally, all degrees of freedom were confined (i.e., fixed boundary) along the edge (Line 1), and a mechanical load was applied on the opposite edge (Line 2) in the form of a forced displacement of 9 × 10−3 mm. The effective stress (σeff) results were obtained to easily compare the formed mechanical stresses. The patterns applied to the experiment were named as follows.Fabrication of Snake‐Skin‐Like ElectrodeA 4‐in. magnetron sputtering system (19NNS002, NNSV) was used to fabricate a snake‐skin‐patterned electrode at room temperature. APC alloy was chosen as the electrode material owing to its high conductivity and good stability.[47] Using a 4‐in. APC target (Ag: 98.1, Pd: 0.9, Cu: 1 wt%; Dasom RMS), the APC film was sputtered on the PU substrate at a fixed DC power of 100 W, Ar flow rate of 10 sccm, and working pressure of 3 × 10−6 Torr. To ensure uniformity of the electrode, the z‐axis rotation speed of the sample‐loaded disk was set to 25 rpm. For snake‐skin patterning, 0.2 mm thick INVAR 36 (Ni: 36%, Fe: 64%) shadow masks were used. To prevent the shadowing effect caused by the gap between the mask and the substrate, a ferromagnetic mask and 1 mm thick rubber magnet were employed, as shown in Figure S6 of the Supporting Information. To exclude the effect of electrode thickness on elasticity, the snake‐skin electrode was deposited with electrode thicknesses of 20, 30, and 40 nm, and a preliminary stretching experiment was performed (Figure S7, Supporting Information). The 30 nm thick electrode exhibited the lowest change in resistance to strain; therefore, the thickness of the electrode was fixed at 30 nm for the snake‐skin‐patterned electrode.Properties of Snake‐Skin‐Like ElectrodeFor the analysis of stretchability, the resistance changes of the snake‐skin electrodes were measured using a lab‐designed zig and a digital multimeter (DMM; 34450A, Keysight). When analyzing the stretching property, the resistance value was measured using the average of the resistance values measured with the two tips of the DMM narrow, with one hexagon in between and three hexagons wide (Figure S8, Supporting Information). Other mechanical properties of the snake‐skin electrodes were analyzed using a lab‐designed bending, folding, rolling, and twisting tester to confirm the flexibility of the snake‐skin electrode. In addition, a dynamic fatigue test was performed for 10 000 cycles for all the above mechanical tests to confirm mechanical flexibility and reliability.Applications of Stretchable Snake‐Skin ElectrodesA wearable and stretchable temperature sensor was fabricated by coating a PU solution on the snake‐skin electrode using a spin‐coating system at 1000 rpm for 15 s (Figure S9, Supporting Information). The PU solution was covered to protect the temperature sensor from humidity and contaminants from the environment and to accurately measure the temperature using the stretchable temperature sensor. A copper tape of width 1 cm was attached to both ends of the snake‐skin electrode to protect the contact points from being covered by the PU solution. Next, the PU spin‐coated electrode was cured in an oven at 60 °C for 3 h. The temperature sensor was placed on a hot chuck, and the tips of the probe station placed in contact with the contact electrode parts. Thus, the resistance change of the snake‐skin temperature sensor as a function of temperature was measured using a hot chuck controller (MSTECH, MST‐1000H), and the semiconductor characteristics were analyzed (Tektronix, Keithley 4200‐SCS). Blue LEDs, Cu tape, batteries, and wires were used to fabricate the LED interconnecting circuit. The middle section of the copper tape was cut to confirm the stable performance of the snake‐skin electrode as an interconnector of the circuit. When a constant voltage was applied to the batteries, the snake‐skin interconnector was bent to a radius of 5 mm and stretched by 30%. A comparison of the brightness of the LED with the naked eye intuitively confirmed whether the resistance of the snake‐skin interconnector increased.AcknowledgementsThis research was supported by the Commercialization Promotion Agency for R&D Outcomes (COMPA) funded by the Ministry of Science and ICT(MSIT) (2022RMD‐S07). In addition, this work was supported by Korea institute for Advancement of Technology (KIAT) grant funded by the Korea government (MOTIE) (P0008458, HRD Program for Industrial Innovation) and the National Research Foundation of Korea (NRF‐2022M3D1A2083618).Conflict of InterestThe authors declare no conflict of interest.Data Availability StatementThe data that support the findings of this study are available from the corresponding author upon reasonable request.T. Li, Y. Li, T. Zhang, Acc. Chem. Res. 2019, 52, 288.L. Wang, D. Chen, K. Jiang, G. Shen, Chem. Soc. Rev. 2017, 46, 6764.A. Nag, S. C. Mukhopadhyay, J. Kosel, IEEE Sens. J. 2017, 17, 3949.J. Lee, S. Kim, J. 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Journal

Advanced Materials Interfaces – Wiley

Published: Apr 1, 2023

Keywords: interconnector; Kirigami; snake skin; stretchability; stretchable electrodes

References

  • Nanoporous Carbons for Soft and Flexible Energy Devices, Series: Carbon Materials: Chemistry and Physics
    Borghi, F.; Soavi, F.; Milani, P.