Mechanoreceptor‐Inspired Dynamic Mechanical Stimuli

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ReseaRch aRticle

Mechanoreceptor-Inspired Dynamic Mechanical Stimuli Perception based on Switchable Ionic Polarization

Hong-Joon Yoon, Dong-Min Lee, Young-Jun Kim, Sera Jeon, Jae-Hwan Jung, Sung Soo Kwak, Jihye Kim, SeongMin Kim, Yunseok Kim, and Sang-Woo Kim*

Diverse touch experiences offer a path toward greater human–machine inter-action, which is essential for the development of haptic technology. Recent advances in triboelectricity-based touch sensors provide great advantages in terms of cost, simplicity of design, and use of a broader range of mate-rials. Since performance solely relies on the level of contact electrification between materials, triboelectricity-based touch sensors cannot effectively be used to measure the extent of deformation of materials under a given mechanical force. Here, an ion-doped gelatin hydrogel (IGH)-based touch sensor is reported to identify not only contact with an object but also defor-mation under a certain level of force. Switchable ionic polarization of the gelatin hydrogel is found to be instrumental in allowing for different sensing mechanisms when it is contacted and deformed. The results show that ionic polarization relies on conductivity of the hydrogels. Quantitative studies using voltage sweeps demonstrate that higher ion mobility and shorter Debye length serve to improve the performance of the mechanical stimuli-percep-tible sensor. It is successfully demonstrated that this sensor offers dynamic deformation-responsive signals that can be used to control the motion of a miniature car. This study broadens the potential applications for ionic hydrogel-based sensors in a human–machine communication system.

DOI: 10.1002/adfm.202100649

Dr. H.-J. Yoon, D.-M. Lee, Y.-J. Kim, S. Jeon, J.-H. Jung, Dr. S. S. Kwak, Dr. J. Kim, Dr. S. M. Kim, Prof. Y. Kim, Prof. S.-W. KimSchool of Advanced Materials Science and EngineeringSungkyunkwan University (SKKU)Suwon 16419, Republic of KoreaE-mail: [email protected]. S.-W. KimSchool of Advanced Institute of Nanotechnology (SAINT)Sungkyunkwan University (SKKU)Suwon 16419, Republic of KoreaProf. S.-W. KimSamsung Advanced Institute for Health Sciences & Technology (SAIHST)Sungkyunkwan University (SKKU)Suwon 16419, Republic of Korea

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.202100649.

(sensory cells) that account for our percep-tion of the texture and pressure of touched objects.[2] Among mechanoreceptors, slow-adapting type I (SA-I) receptors are essen-tial for the sensation of light touch and are sensitive to low-frequency dynamic skin deformation (<5 Hz).[3] Another tactile afferent, rapidly adaptive (RA) cells, adapt rapidly to a stimulus.[4] To develop artificial skin, it is crucial to understand and mimic the features of innervated biological skin. Despite advances in pressure sensors based on conventional approaches, such as piezoelectric, capacitive, optical, and resonant components, the sensation of pressure under dynamic deformation has remained challenging to replicate because both the magnitude of applied pressure and the level of induced deformation must be conveyed simultaneously.[5–8] A recent study, in which a skilled sensor fabrica-tion technique was emphasized, showed that an array of membrane-based strain sensors integrated with a pressure sensor are able to measure both strain and pres-sure.[9] In addition to a sophisticated

device design concept, the use of an alternative approach in terms of new materials, which adopt not only the softness of natural skin but also skin's sensory characteristics in a dynamic event, is desirable.

Hydrogels have been studied in a variety of recent technolo-gies, including engineered tissues, biomedical devices, and soft robotics, due to their permeability to various molecules, biocompatibility, and biodegradability.[10–12] Their mechanical smoothness in particular brings them closer to the mechanical properties of human skin and allows them to adapt to the sur-face of touched objects.[13] Recently, a hydrogel-elastomer lay-ered structure was proposed that could sense pressure based on capacitance change.[6] However, it may be of limited use in a confined space due to its thickness (>2 mm) and the presence of unsecured interfacial bonding during a dynamic mechanical event. Another promising approach uses triboelectric nano-generators (TENGs) as pressure sensors. TENGs can transmit information about the level of output signals (e.g., voltage and current) upon exertion of external pressure as well as informa-tion on an object's roughness (i.e., texture).[14] Hydrogels can benefit TENG-based pressure sensors because encapsulated hydrogels can be used to make a TENG stretchable, allowing

1. Introduction

We encounter countless tactile stimuli through our skin, which effortlessly perceive and convey qualities of the external world by touch.[1] Our skin is equipped with mechanoreceptors

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it to adapt to the surface geometry of opposite materials (die-lectrics) when it comes into contact with objects.[15] This high degree of geometrical conformity enables TENG-based pres-sure sensors to detect a low level of pressure with ultrahigh sensitivity. Because a hydrogel is in particular an ionic polymer, an induced electrical response (i.e., internal polarization) due to the migration of internal ions upon a stress gradient correlates with the pressure at a local point.[16] Accordingly, ionic polymer-based pressure (or strain) sensors, also called piezoionic sen-sors, have been reported, which have intriguing applications in wearable/consumer electronics.[17,18] A broader understanding of the relevant variables, for example, the type and concen-tration of ions, that govern the performance of such sensors is necessary. It would be beneficial to take advantage of both i) triboelectricity, to perceive an object touching and ii) the piezoionic effect, to measure mechanical force as the hydrogel becomes deformed over the course of time.

Here, we propose an ion-doped gelatin hydrogel (IGH) and exploit its similarities to the skin and switchable ionic polari-zation in response to dynamic mechanical stimuli. To take advantage of these characteristics of IGHs, we developed a mechanoreceptor-inspired dynamic mechanical stimuli-perceptible device that is able to accept successive tactile infor-mation on contact and deformation events; the device plays a similar role as SA-I and RA cells in human skin. This IGH-based device relies on a combination of the triboelectric and piezoionic effects, which enable its application in a confined area and render its structure quite simple (basically a dielectric layer/bottom electrode). Upon contact with human skin, the triboelectric effect induces an ionic polarity to compensate for the surface charge of IGHs, which is electrically characterized by a negative voltage signal. When a deformative stimulus is applied, piezoionic effect-induced ion redistribution produces a switched polarity compared to when it first makes contact, which generates a positive electrical output. We investigated the tunability of the sensor's dynamic sensing performance with various ion concentrations and types. Theoretical insights on ion dynamics, including the Debye-Hückel theory and ionic mobility, provided an electrical voltage output that was improved by 44.6% under mechanical deformation. Advance-ments in new materials for the perception of successive tac-tile experiences allowed us to develop texture recognition and a dynamic sensing-based wireless communication system. We demonstrate a wristband type IGH-based communicator that transmits dynamic stimuli-based signals to a miniature car to manage its trajectory. We provide not only a full explanation of the dynamic mechanical events involved but also a demon-stration of a human-machine communication system that can transmit multiple signals using small devices.

2. Results

2.1. Understanding an Ion-Doped Gelatin Hydrogel and its Switchable Ionic Polarization

Advances in material design, particularly materials that are equipped with high sensitivity like that of human skin, are necessary for the development of touch sensors. Since gelatin

networks are highly porous, they can be used to manufacture ion-based electronics, producing human-machine interfaces.[19] Figure 1a shows the chemical structure of a gelatin network with its amine group- and carboxyl group-rich regions. Amine (-NH2) based functional groups, including an amidine group (-C(NH)NH2), provide electron-deficient sites to which anions are drawn. Meanwhile, the functional groups possessing a C-O double bond (carbonyl groups) offer an electron-rich area, attracting cations.[20] We verified these properties by using density-functional theory (DFT) to determine the electrostatic potential gradient around the functional groups (Figure 1b). The computational study demonstrates that the electrostatic poten-tial is distributed with positive values around the amine-based groups and negative values around the carbonyl-based groups. Figure 1c illustrates a schematic design of the polymeric net-work present in our IGH, which is comprised of crystalline and amorphous regions of gelatin networks. To identify the organi-zation of gelatin networks, X-ray diffraction (XRD) analysis was employed in the crystallographic study of gelatin hydrogel and a 0.2 m KCl doped IGH (Figure S1, Supporting Information). Their XRD peaks were found at 8° and 20° of 2θ values, thereby implying the presence of the triple-helical crystalline structures in their gelatin networks.[21,22] Based on the following equation, the degree of crystallinity (XC) was further calculated to be 3.19 and 2.94% for gelatin hydrogel and the IGH, respectively

C1

C

t

XA

Ai

n

i∑= = (1)

where ACi is the area under each crystalline peak, At is the total area that includes both amorphous background and crystalline regions. The crystalline regions consist of ordered and densely packed gelatin chains, while the amorphous regions feature randomly oriented and sparsely arranged chains. Thus, the crystalline regions of our gelatin networks contain more mobile charged ions than the sparsely formed regions owing to their higher concentration of ion-attracting sites, which gives them a similar role as natural mechanoreceptors.[23] The as-described IGHs feature an abundance of ions, allowing for their potential use as ion-based electronics.

An exploded view shows the detailed components of the dynamic perceptible device (Figure 1d). A printed circuit board (PCB) layer, constituting a 0.4 mm thick FR-4 glass epoxy layer and a 35 µm Au/Cu electrode, was prepared as a substrate with dimensions of 4 cm by 4 cm. Our IGH was synthesized through the dissolution of gelatin powder into distilled water and the addi-tion of KCl salt. We used conventional methods for the formation of a decent polymeric structure (more details in “Experimental Section”). The overall thickness of our IGH-based dynamic tactile sensor is about 0.5 mm, so it can be used in confined spaces.

We further studied our IGHs to confirm their working prin-ciples in response to external stimuli, including contact and deformation events. The triboelectric effect plays a primary role in describing ion dynamics when an initial contact occurs (Figure 1e). When our IGH makes contact with a triboelectric material (especially human skin), electron transfer takes place on the surfaces of the two substances. Human skin is known to be tribo-positive, so it introduces a negative potential on the



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surface of the IGH.[24] We performed contact potential difference (CPD) measurements to characterize the surface potential of an IGH, where electron transfer takes place before/after friction events (Figure S2, Supporting Information). The CPD value of −0.08 V for the KCl-doped IGH confirms that its surface is nega-tively charged by triboelectrification. Mobile ions, bound to the crystalline region by Coulomb force, transfer to the surface of the IGH following contact electrification. At this point, cations move toward the negatively-charged regions, which results in asym-metric displacements of ions. Thus, ionic polarization has been created. The imposed stress starts to gradually deform the IGH, causing mobile charged ions to thermodynamically migrate from the stressed region to the tensile region.[17] Instantaneous ion

polarization takes place owing to the difference in ion mobility between K+ and Cl−, which runs counter to the triboelectric effect-induced polarity (Figure 1f). Accordingly, ionic polarization induces a flow of electrons through the underlying electrode, which provides the working mechanism of the switchable ionic polarization-based tactile sensor. These features are essential for the IGHs’ dynamic tactile sensing capability.

2.2. Theoretical Analysis of Switchable Ionic Polarization

The fundamental principles of ion dynamics can provide an understanding of the electrical characteristics induced by ion

Figure 1. Conceptual design of the IGH and ionic switchable polarization. a) Chemical structure of the gelatin network and expression of amine and carbonyl functional group-rich areas. b) DFT-simulated electrostatic potential plot of the gelatin network. c) Schematic illustration of the IGH, which constitutes cation-anion pairs ([K+][Cl−]) distributed in the gelatin networks. The gelatin networks are divided into crystalline and amorphous regions. d) Exploded view of the IGH-based dynamic perceptible device. e) Schematic illustration of ion dynamics inside of the IGH under the triboelectric effect. Inset shows ionic polarization in detail, indicating anions that are present beneath cations. f) Schematic illustration of ion dynamics induced by the piezoionic effect. Inset represents ionic polarization under a deformative circumstance.



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mobility in IGHs, including not only the well-defined tribo-electric effect but also the yet-unidentified capacity for ionic polarization under mechanical deformation. We introduce here underlying theoretical findings that explain the micro-scopic underpinnings of our IGH (Figure 2a). Hydrogels are aggregations of polymer networks and water, entailing contact electrification between the networks and water. Under this electrification, the surface group of gelatin networks physically absorbs charged ions, resulting in the formation of a surface charge.[25] Given the existence of the fixed surface charge in gelatin-water interfaces, an electric double layer (EDL), mostly composed of oppositely charged ions, is formed to maintain electroneutrality; this is referred to as a compact layer.[26] This layer impacts ion conductivity in that it determines the number of ions traveling through the diffuse layer. To understand the potential distribution of the compact layer, the Poisson–Boltzmann equation (shown below), describing variation in electrostatic potential with distance from the surface, needs to be explained.

exp / k22

20 r

Bz

en z z e z T

i

i i i∑ψ ψε ε

ψ[ ]( )∇ = ∂∂

= − −∞ (2)

where ∇2ψ = div(grad ψ), z is the surface normal direction, ε0 is permittivity of free space, εr is relative permittivity of the sol-vent, ni

∞ is the bulk volume density that can also be expressed as 1000 An N ci i=∞ (NA: Avogadro's number, ci: the molar con-centration of ion i), and zi and kB are the valency of ion i and the Boltzmann constant, respectively.[27] As shown in Figure 2a-i, the potential distribution ψ is plotted against the distance z from the gelatin wall. The zeta (ζ) potential in this plot repre-sents an experimentally determined potential at the slip plane, which is an imaginary interface where the diffuse layer begins and is equipped with mobile co-ions and counterions.[28] The ψi potential, known as the inner Helmholtz potential, takes the

maximum value throughout this electrostatic potential distri-bution. Inferring from the existence of positive potentials, we quantitatively confirmed that the relevant charged ions remain immobile within the compact layer.

Furthermore, the Debye–Hückel equation, a simplified form of the Poisson–Boltzmann equation, can be derived from the minimized condition of surface potential (ψS) at every interface (specifically ziψi < 25.7 mV at 25 °C).

22

22

zzψ ψ κ ψ ( )∇ = ∂

∂= (3)

where κ is the Debye-Hückel parameter, which mainly depends on bulk volume density ni

∞.

2 2

0 r B

1/2e n z

k Ti

i i∑κ

ε ε=

∞

(4)

The Debye length (λD), defined as a reciprocal value of κ, is necessary to explain ion concentration dependence on the compact layer, since it indicates the distance from the gelatin surface to the slip plane. The λD with various amounts of solute (KCl) was calculated in the molality range from 0.1 to 0.4 (Figure 2b). It was computed at a constant temperature of 25 °C. Under these conditions, Equation (4) becomes

3.04 10 10

z cD

i i

λ = × −

(5)

Consequently, the λD exponentially decreases with a higher concentration of KCl, indicating that the substrate with the higher ion concentration has a more spacious diffuse layer. Considering the entangled and porous structure of the gelatin chain network, the width of the ion-mobile channel governs the

Figure 2. Theoretical analysis of ion-doping effect. a) Microscopic view of the gelatin network. Charge carrier species are dispersed corresponding to potential distribution, starting from the wall of the gelatin network. b) Diagram of λD plots in the ion concentration range studied (0.1–0.4 m). c) Diagram of relative ionic mobility columns dependent on alkali metal ion type.



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ionic conductivity on a large scale (Figure S4, Supporting Infor-mation). With superior ionic conductivity, strong polarization can be achieved.

Next, we studied the effects of alkali metal ions (Li+ and Na+) with anions fixed to the chloride ion because changes in the anion will cause variation in the surface potentials of aqueous electrolyte solutions.[29] The study of the effect of cations on ionic conductance does not include variations in other param-eters, instead using relative ionic mobilities. According to previous reports, if the value of κaP (where aPdenotes atomic radius) is less than 1, the molecule is considered to have a thick EDL, and ionic mobility can be written as

2

3

2

30 r 0 ra EPµ ε ε ζη

ε εη

= = (6)

where η represents the dynamic viscosity of a solvent.[30] The parameters other than aP remain constant, suggesting that potassium ions (K+), with the largest ionic radius, have the most substantial ionic mobility. The relative ionic mobilities of Li+, Na+, and K+ are plotted in Figure 2c. Since higher ionic mobility allows for more significant transport performance, these ions are responsible for an enhanced intensity of ionic polarization.

2.3. Microscopic Analysis of Ion Conduction

To verify the dependence of ionic transport on λD and relative mobility, we conducted a microscopic analysis using voltage sweep measurements (Figure 3a). Au electrode layers (50 nm thick), which were utilized to apply an electrical bias, were depos-ited on a prepared 2 cm by 2 cm SiO2 wafer (Figure S4, Sup-porting Information). Our synthesized IGHs were coated on the

substrate using a spin coater with a length and width of 5 mm by 5 mm and a thickness of 50 µm. To investigate the concen-tration dependence of ion conduction, we fabricated the devices with various concentrations of KCl electrolytes (0.1–0.4 m). We confirmed here that the addition of the KCl salts does not destroy the gelatin network (Figure S5, Supporting Informa-tion). Figure 3b indicates a series of representative current/bias (I/V) curves at the mentioned KCl concentrations, which are measured in the voltage range of 0 to +2 V (hydrogels applied over +2 V can become hydrolyzed). Interestingly, the tendency of the measured ionic conductance (equivalent to the slope of the I/V curve) in Figure 3b coincides with our theoretical expec-tations. That is, we observed that the highest concentration of KCl was associated with the greatest ionic conductance.[31] IGHs doped with different cations, including Li+ and Na+, were used to evaluate the effect of ion type on ion transport perfor-mance. With concentration fixed at 0.3 m, positive bias ranging from 0 to +2 V was applied to the system in the same manner. As a result, K+ showed superior ionic conductivity, followed by Na+ and Li+ (Figure 3c). This tendency of different cations coin-cides with their relative ionic mobilities.

Figure 3d depicts ionic migration inside the IGHs at various ion concentrations. When one of the probes applies a positive potential to an Au electrode, it attracts anions while pushing out cations. Since KCl at a higher concentration has a more extensive diffuse layer, more ions can travel through the layer. As the mobile ions move in different directions, ionic polari-zation occurs inside the IGH, and the polarization becomes more intense when more ions are present. The enhanced ionic polarization induces more charges in the underlying electrode, which leads to increased ionic conductance. Likewise, Figure 3e describes the variation in ionic mobility with different cation types. The degree of movement of cations depends on the mobility of the ions under the same voltage. Owing to the high

Figure 3. Microscopic analysis of ion dynamics. a) Schematic illustration of our measurement method using a probe station. b,c) Representative current/bias (I/V) curves measured with different KCl concentrations (b) and types of alkali metal ion (c). d,e) Schematic illustration of ion migration inside the IGH when a positive bias is applied. λD mainly affects concentration (d) dependent ion conduction. Higher ion concentration is associ-ated with shorter λD, which causes wider ion mobile channels. Ionic mobility (μ) of alkali metal ions (e) controls ionic conduction. The ionic mobility increases in the order of Li+, Na+, and K+.



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mobility of potassium ions, the IGH that contains them has the most substantial ionic polarization, which leads to the greatest ionic conductivity.

2.4. Device Scale Analysis of Dynamic Motion Sensing

Based on our theoretical study describing the changes in ionic polarization with different ion concentrations and types, we conducted electrical characterization at the device scale to investigate whether it coincides with our findings in the voltage sweep measurement. IGH-based devices (displayed in Figure 1d with dimensions of 4 cm by 4 cm were measured by the method shown in Figure S6, Supporting Information. We

chose an aluminum plate (1 cm by 1 cm) because it has tribo-positive property compared to our IGHs. Figure 4a shows a rep-resentative voltage output generated when the aluminum plate comes into contact with the IGH and deforms, respectively. To experimentally simulate i) contact and ii) deformation, forces of 0.1 kgf (10 kPa) and more than 0.5 kgf (50 kPa) were set. We performed a finite element method (FEM) simulation to measure the displacement fields upon force exertion (Figure S7, Supporting Information). According to the results of FEM simulation, we considered the deformation that occurred under 0.1 kgf as a contact mode. When 0.5 kgf was applied, the pie-zoionic effect occurred with a displacement of 4 mm. Under 0.1 kgf, which was meant to create contact, the output voltage had a negative sign, indicating that the triboelectric effect was

Figure 4. Electrical characterization of the switchable ionic polarization of an IGH. a) Representative characteristic peaks. Vcontact and Vdeformation denote voltage output under contact and deformation, respectively. The triboelectric effect dominantly affects Vcontact, whereas the piezoionic effect mainly controls Vdeformation. b) Schematic illustration of the operative mechanism under contact (above) and deformation (below). c) Vcontact amplitude output and electrical conductivity plots at different ion concentrations. Electrical conductivity is inversely proportional to the triboelectric voltage output. d) Vdeformation outputs corresponding to different ion concentrations and types of alkali metal ions. Higher concentration and ionic radius result in enhanced piezoionic voltage output. e,f) Electrical output of Vcontact and Vdeformation recorded with different surface textures of PFA (e) and nylon (f). Triboelectric output decreased while piezoionic output remained nearly unchanged. Insets are SEM images representing the roughness of the films. The scale bar is 20 µm.



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taking place. Meanwhile, the voltage output had a positive sign when the pressure was sufficient to induce deformation, revealing that the piezoionic effect was occurring. Figure 4b illustrates the operating mechanism for both events. When contact occurs, electron transfer takes place based on triboelec-trification. As shown in Figure S2, Supporting Information, the IGH has a positive surface potential in the initial state. When it is contacted, the electrical sign of positive/negative depends on whether the triboelectric material is an electron donor or acceptor relative to the IGH. The amount of transferred charge determines the amplitude of the electrical output. Generation of an output voltage during a deformation event is based on the redistribution of mobile charged ions. The imposed mechanical stress includes a localized ionic charge gradient due to the sep-aration of ions with different mobilities.[32] This phenomenon refers to the piezoionic effect.

To achieve significant responsiveness to the successive events of mechanical stimuli, we attempted to optimize the conditions of ion concentration and type. As the TENGs are based on the coupling effect of contact electrification and charge induction, larger charges induced in an underlying elec-trode would be preferable.[33,34] However, the higher the ionic conductivity, the less charge will be induced in the electrode due to the behavior of internal ions compensating for the tri-boelectric charges. We measured electrical conductivity at dif-ferent concentrations (0.1–0.4 m) using a standard four-point probe to compare them with triboelectric output performance (Vcontact amplitude). In general, more ions are associated with greater electrical conductivity and a reduction in electrical output (Figure 4c and Figure S8, Supporting Information). Our IGH with 0.1 m KCl had the smallest electrical conductivity of 7.73 m−1 and the highest voltage amplitude of 0.495 V. On the contrary, the IGH with the highest electrical conductivity of 42.28 µS m−1 was found to hold the least triboelectric output of 0.103 V, representing reduced sensitivity. Next, we catego-rized the piezoionic output performance series (Vdeformation) for both ion concentration and alkali metal ion type. Trends in the amplitudes agree with the results of our previous study. IGHs containing cations with larger ionic radii show increased elec-trical output. In addition, the IGHs with higher concentrations of KCl salts showed enhanced Vdeformation outputs owing to the reduced λD. These findings suggest that ionic conductivity can be further increased at higher ion concentrations and larger ionic radii, but the earlier discussion of contact electrification informs us that an appropriate ion concentration (0.2–0.3 m) is preferred. With these optimized conditions of our IGHs, we further investigated their mechanical durability under certain compressive stresses (Figure S9, Supporting Information). We confirmed that continuously applied pressure of 500 kPa did not cause any change in their responsiveness to dynamic mechanical stimuli, which offers sustainability required to be as materials for human-machine interfaces technology.

Since perceiving the texture of touched objects is also impor-tant, we studied the texture recognition capability of IGH-based sensors. We prepared textured perfluoroalkoxy alkane (PFA) and nylon film and measured output voltage, Vcontact, as the two made contact with our IGH-based sensor. Nontextured films were also used to measure Vcontact to verify the capacity of the device to recognize different textures. This feature is determined

by the level of triboelectrification, which generally relies on both i) differences in triboelectric series and ii) contact area between the materials. We prepared textured materials (1 cm by 1 cm) with sandpaper (more details in “Experimental Section”). We measured the voltage output of pairs of films with textured and nontextured surfaces by the method shown in Figure S6, Supporting Information, to draw a distinction between Vcontact and Vdeformation (Figure S10, Supporting Information). Since Vdeformation recognizes the applied stress gradient on the IGH-based sensor, we designed a set of experiments to measure Vdeformation to ensure that the same level of force was applied to the devices. The Vcontact amplitude with nontextured PFA film (50 µm thick), a well-known tribo-negative material, achieved 4.72 V, but this was reduced to 2.86 V when the PFA film was first rubbed with sandpaper (Figure 4e). Only −10.52 mV was generated when our IGH made contact with a nontextured nylon film (60 µm thick), a representative tribo-positive mate-rial (Figure 4f). The output decreased to −5.39 mV with tex-tured nylon. This may be attributable to the reduced surface area preventing sufficient electron transfer between the mate-rials. However, all the piezoionic outputs (Vdeformation) were sim-ilar because they are mainly affected by IGH displacement. By taking advantage of the triboelectric effect, we obtained infor-mation about counter-materials based on a triboelectric series and studied textures as well.

2.5. Dynamic Tactile Sensing-based Wireless Communication System

With the growing attention to pathogen contamination man-agement and convenient data exchange, technologies capable of transmitting and receiving information in a contactless manner are in the spotlight. The consolidation of dynamic tac-tile sensing with rapid changes in force, skin compliance, and structural flexibility of our IGH enabled us to efficiently utilize it as a human-machine interface for contactless communica-tion. We developed a wearable dynamic sensing-based com-municator (WDC) that controlled the movement of a miniature car as a prototype. The WDC consists of three 0.2 m KCl-doped IGH-based dynamic sensor arrays and a commercially available integrated circuit board with a Bluetooth module (Figure 5a). We chose a flexible and chemically inert polyethylene tereph-thalate (PET) layer (170 µm thick) as the WDC substrate, on which an elaborate Au (100 nm) electrode pattern was con-structed. An 8-bit microcontroller (ATMEGA4809) was adopted because of its minimal dimensions (4.3 cm by 1.7 cm) and ver-satility; it is capable of accepting electric signals from the sensor arrays, processing them into digitalized codes, and transmit-ting as-prepared sources from the WDC to our miniature car. A Bluetooth module (HC-06) supports the microcontroller in the signal transmission procedure (see the block diagram in Figure S11, Supporting Information). Figure 5b represents the working algorithm that governs the whole system. Tactile information (contact/deformation) from the sensor initializes the signaling and processing steps, for which no additional equipment, such as a low-pass RC filter, is required to reduce electrical noise. In signal processing, categorizing the electrical outputs with different signs from the sensors enabled faster



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and more accurate data classification. Our miniature car was designed to have different trajectories depending on the sign originating from the tactile information (Figure 5c,d). Move-ments were implemented by transmitting a wireless commu-nication signal between the Bluetooth modules. For instance, when a signal with a negative sign (based on contact informa-tion) is received, the car performs left, forward, and right turns according to where stimulation is applied (Video S1, Supporting Information). Dynamic tactile sensing-based communica-tion was demonstrated; the motion feedback performance of the system was wonderfully simple and occurred in real-time. During one demonstration, the car, which had turned left

because of the input from the WDC, changed its path and took a sharp left turn when an additional force was applied to the WDC (Figure 5e). Similarly, while the contact was made with the WDC, we effortlessly increased the level of force to order the car to stop moving forward (more details in Videos S2–S4, Supporting Information).

3. Conclusion

Triboelectricity-based tactile sensors have been limited to a group of soft materials that enable perception of a range

Figure 5. Wearable dynamic sensing-based wireless communication system. a) Photograph of the WDC, which consists of 3 elements, the IGH-based dynamic tactile sensor, microcontrollers, and Bluetooth modules. b) Flow chart of working algorithm of the WDC for manipulation of a hand-made miniature car. c,d) Schematic illustration of dynamic motion (c) and corresponding trajectories of the car (d). e) Demonstration of the real-time feed-back system depending on dynamic motion. The car, which was turning left upon receiving a contact signal, performs a sharp left turn after receiving a new deformation signal. Likewise, a car that is going forward stops as soon as it receives a deformation signal.



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of applied force as they become deformed, resulting in the increased contact area. Since their performance relies heavily on the extent of contact electrification, their sensing capacity has remained unrevealed, despite demonstrated textural rec-ognition with simple structures and a wide range of usable materials. Herein, we showed that an IGH, equipped with the combined characteristics of the triboelectric and piezoionic effects, offers a possible solution toward recognition of a deformative stimulus. A DFT simulation was performed to the-oretically prove that mobile ions can be attracted to the IGHs. We examined the ion-doping effect within a concentration range of 0.1–0.4 m to enhance piezoionic electrical output, which ensured remarkable sensitivity in deformative circumstances. Additionally, we manipulated ionic conduction by altering the type of cations present. The λD and μ served as essential param-eters for understanding these ion dynamics. We confirmed that the management of the ion transport properties increased piezoionic voltage output by about 44.6%, from 0.522 V (0.2 m LiCl) to 0.755 V (0.4 m KCl). We also characterized the contact electrification properties according to the electrical conductivity of dielectric materials, which has not yet been reported in an experimental study. Hence, we explored ways of optimizing ion doping effects to secure decent dynamic tactile sensing perfor-mance. With the optimized conditions of the 0.2 m KCl-doped IGH, we successfully demonstrated a WDC and used it to control the trajectory of a miniature car. Tactile sensing-based electronic skins have tremendous difficulty in discriminating between tactile and touchless modes in real-time.[35] We can fur-ther develop our findings into a next-generation dynamic tactile sensor that enables simultaneous recognition of pressure and texture through an in-depth analysis of ion dynamics. Also, we believe that our proposed research will expand the future uses of electronic skins in diverse robotics-related systems, including contactless information transmission systems, remote medical diagnostics, and artificial intelligence technology.

4. Experimental SectionPreparation of IGH-based Dynamic Tactile Sensor: Gelatin powder

(gelatin from bovine skin, type B, Sigma-Aldrich) was dissolved in deionized water at 15 wt%. The gelatin aqueous solution was stirred vigorously for 24 h at 60 °C. Then, salts including lithium chloride, sodium chloride, and potassium chloride (Sigma-Aldrich) were added in the range of 0.1–0.4 m. To ensure sufficient dissolution of the salts, the solution was stirred for another 24 h with the temperature maintained at 60 °C. After the gelatin solution was spin-coated on a PCB substrate at 500 rpm for 30 s, it was heated to 60 °C in an oven.

Material Characterizations: Pure gelatin hydrogel and IGHs were drop-coated on glass substrates and the infrared spectra were measured with a Fourier-transform infrared spectrometer (Nicolet iS5, Thermo Scientific). For XRD measurements, a multipurpose X-ray diffractometer was used (SmartLab, Rigaku). Surface potential data and topography were obtained using an atomic force microscope (XE100, Park Systems) with a Pt/Cr-coated silicon tip. A lock-in amplifier (SR830, Stanford Research) was used for Kelvin probe force microscopy analysis.

Voltage Sweep Measurement: A SiO2 wafer was cut into dimensions of 2 cm by 2 cm. PET film was also cut as shown in Figure S4, Supporting Information, such that it could be used as a mask. The cutting was done with a laser cutter. An Au electrode (50 nm thick) was deposited using electron-beam evaporation. Polyimide films were carefully attached to the surface of the prepared SiO2 substrate. IGH solution prepared

as mentioned above was poured and spin-coated at 500 rpm for 30 s. Then, the IGH-coated SiO2 substrate was cooled at 4 °C for 24 h. A probe station was used to measure the current/bias (I/V) curves of the as-prepared device.

Electrical Characterization: An oscilloscope (Tektronix DPO3052) and a voltage probe (Tektronix P5100A) with 40 MΩ input impedance were used to record voltage output. To precisely manipulate the applied forces, a pushing tester was used. For the four-point probe measurement, KCl-doped gelatin aqueous solutions at different concentrations in the range of 0.1–0.4 m were stirred for 24 h at 60 °C. Then, they were carefully coated onto a glass substrate (3 cm by 3 cm), which was cooled at 4 °C for another 24 h.

FEM Simulation: FEM simulation data, shown in Figure S7, Supporting Information, was obtained using COMSOL Multiphysics. The mechanical properties of a gelatin hydrogel were provided.[36] The dimensions of the prepared gelatin hydrogels were 40 mm × 20 mm × 10 mm. Then, the displacement field was computed with applied forces of 0.1 kgf, and 0.5 kgf on top of the gelatin structure.

DFT Simulation: The molecular structure of gelatin (PDB ID: 1CLG) was obtained from the Protein Data Bank.[37] Before quantum mechanical calculations, gelatin monomer was prepared with the addition of hydrogen atoms to the termini. Using the prepared gelatin structure, the calculations were carried out using density functional theory with the plane-wave based CASTEP code.[38,39] Generalized gradient approximation, Perdew–Burker–Ernzerhof, was adopted as an exchange-correlation function for all calculations.[40,41] The interaction between ion and valence electron was described with OTFG ultrasoft pseudopotentials for geometry optimization and electron density calculation. For IR calculation, norm-conserving OTFG pseudopotentials and EDFT/all band for minimization were used. Plane waves were included up to the kinetic energy cutoff of 800 eV. For Brillouin zone integration, a 1 × 1 × 1 Monkhorst-Pack special k-point grid was used. The calculations converged in energy to 5 × 10−6 eV atom−1 and the structures were relaxed until the forces were less than 10−2 eV Å−1. The gelatin monomer was separated by more than 10 Å of vacuum to eliminate spurious interactions.

Preparation of Textured Films: A 50 µm thick PFA film (Alphaflon) and a 60 µm thick nylon film (GFM) were cut into dimensions of 1 cm by 1 cm. After being carefully cleaned with methanol solvent, they were rubbed with sandpaper with an applied force of 3.0 kgf (300 kPa).

Dynamic Sensing-based Wireless Communication System: Electrical signals from the wristband (WDC) were transmitted to a channel of a microcontroller (Arduino Nano Every, Arduino Co.) by the Au electrode pattern. Another microcontroller (Arduino Uno, Arduino Co.) was built into the miniature car. Bluetooth modules (HC-06, Shenzhen Hengjiexin Electronics Co.) were connected to the WDC and the car. The microcontrollers and Bluetooth modules were powered with batteries.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsH.-J.Y. and D.-M.L. contributed equally to this work. This work was financially supported by Nano Material Technology Development Program (2020M3H4A1A03084600) and the Basic Science Research Program (2020R1A2B5B01001785) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT. In addition, this work was funded by Development of Acoustic Sensor based on Piezoelectric Nanomaterials Program (GRRC Sungkyunkwan 2017-B05) from GRRC program of Gyeonggi province.

Conflict of InterestThe authors declare no conflict of interest.



2100649 (10 of 10) © 2021 Wiley-VCH GmbH

Data Availability StatementThe data that support the findings of this study are available from the corresponding author upon reasonable request.

Keywordscommunicators, dynamic perception, ion dynamics, ionic hydrogels, mechanical stimuli, piezoionic effect, triboelectric effect

Received: March 1, 2021Published online:

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