Ion-conductive self-healing hydrogels based on an interpenetrating polymer network for a multimodal sensor

Chemical Engineering Journal 371 (2019) 452–460

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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Ion-conductive self-healing hydrogels based on an interpenetrating polymer network for a multimodal sensor

T

Sung-Ho Shina, Woojoo Leea, Seon-Mi Kima, Minkyung Leea, Jun Mo Kooa,b, ⁎ ⁎ ⁎ Sung Yeon Hwanga,c, , Dongyeop X. Oha,c, , Jeyoung Parka,c, a

Research Center for Bio-based Chemistry, Korea Research Institute of Chemical Technology (KRICT), Ulsan 44429, Republic of Korea Department of Fibre and Polymer Technology, KTH Royal Institute of Technology, Teknikringen 58, SE-100 44 Stockholm, Sweden c Advanced Materials and Chemical Engineering, University of Science and Technology (UST), Daejeon 34113, Republic of Korea b

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

self-healing hydrogel is developed • Abased on an interpenetrating polymer network strategy.

cross-linked • Chemically/ionically Fe /PAA and physically cross-linked 3+

• • •

PVA organize the IPN. The hydrogel satisfies the three capabilities of robustness, self-healing, and conductivity. The multimodal sensing capabilities for strain, pressure, and temperature are determined. A dual sensor attached to a finger simultaneously detects folding/pressure-motions.

A R T I C LE I N FO

A B S T R A C T

Keywords: Self-healing polymer Hydrogel Sensor Soft electronics

Conductive self-healing polymer hydrogel and related soft sensor devices are receiving considerable attention from academia to industry because of their impacts on the lifetime and ergonomic design of soft robotics, prosthesis, and health monitoring systems. However, the development of such a material has thus far been limited considering performances and accessibility. Herein, robustness, self-healing, and conductivity for soft electronic skin are realized by an interpenetrating polymer network (IPN) system based on chemical/ionic crosslinked poly(acrylic acid) containing ferric ions, intercalated with physically cross-linked poly(vinyl alcohol). This IPN hydrogel successfully satisfies all three aforementioned capabilities; elongation at break greater than 1400%; recovery to original mechanical properties greater than 80% after 24 h; and 0.14 S m−1 of ionic conductivity, which is electrically healable. Such ionic conductivity of hydrogels enables multimodal sensing capabilities, i.e., for strain, pressure, and temperature. Particularly, a uniquely designed dual sensor attached to a finger simultaneously detects mechanical folding and pressure changes independently and can undergo large deformation 1000 times repeated and heating up to 90 °C.

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Corresponding authors. E-mail addresses: [email protected] (S.Y. Hwang), [email protected] (D.X. Oh), [email protected] (J. Park).

https://doi.org/10.1016/j.cej.2019.04.077 Received 17 January 2019; Received in revised form 20 March 2019; Accepted 11 April 2019 Available online 11 April 2019 1385-8947/ © 2019 Published by Elsevier B.V.

Chemical Engineering Journal 371 (2019) 452–460

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1. Introduction

Rhodamine B from Sigma-Aldrich (USA), acrylic acid (AA, 99.5%) from Alfa Aesar (USA), iron(III) chloride hexahydrate (FeCl3·6H2O, 99+%) from Acros Organics (Belgium), and poly(dimethylsiloxane) (PDMS) from Dow Corning (USA) were purchased and used without further purification. As a dielectric and elastomeric layer in the pressure sensor, a commercial tape (VHB™ 4910, 3 M, USA) was purchased and used.

Human skin is mechanically soft yet robust, self-healable, and protects inner organs from external threats. It is also capable of sensing outer stimuli such as physical pressure, temperature, or moisture without interference. Broad and comprehensive research has been conducted to develop artificial electronic skins (e-skins) that mimic the nature of real skin [1–5]. The potential applications of e-skins include soft robotics [6], prosthesis [7], health monitoring systems [8,9], human-machine interfaces [8,9], and smart sensor systems inserted in the internet of things [9]. To realize smart electronic device systems based on e-skins, the development of mechanically flexible and stretchable electrodes is essential. Various materials and strategies such as conductive layers on pre-strained elastomeric substrates [10,11] and elastomer composites of carbon [12,13] or metal nanomaterials [14,15] have been devised for developing stretchable electronic systems. However, such approaches lack a self-healing capability, which is among the distinctive characteristics to mimic human skin. Self-healing polymers for soft e-skins are required to autonomously repair both mechanical and electronic damage under ambient conditions [16–21]. To satisfy the stretchable, self-healing, and conductive capabilities, several material designs such as the adoption of healing agents [22], nano-scaled graphitic [23] or inorganic material-elastomer composites [24], liquid metals [25], and ion-conductive hydrogels [26–29] were developed. Among them, ion-conductive self-healing hydrogels have competitive merits compared to other material types as follows: (1) they have similar moduli to those of natural organisms such as skin or muscle tissue (Young’s modulus of 104–109 Pa) [30], such that they are able to effectively mimic the viscoelastic behavior of human skin; (2) because hydrogels act as electrolytes, they are applicable to various applications such as artificial muscles, triboelectric generators, and energy storage devices; and (3) they are cost-effective compared to the other groups of conductive self-healing materials [31–33]. Considering such prerequisite requirements, a covalently and ionically cross-linked double network (DN) of poly(acrylic acid) (PAA) hydrogel containing ferric ions (Fe3+) was reported to achieve both self-healing and conductivity [34–36]. These characteristics were feasible because of the diffusion of ions through loosely cross-linked polymer structures at the cost of mechanical robustness. Few strategies to improve the mechanical properties by adopting third polymers/additives were effective without a guarantee of electric or self-healing performances [37–43]. To achieve the improvement of fundamental robustness while preserving the mechanical and electrical self-healing for the reliable base materials of soft e-skins, herein, a physically cross-linked poly(vinyl alcohol) (PVA) matrix was sophisticatedly intercalated into the DN of Fe3+/PAA to prepare an interpenetrating polymer network (IPN). Free radical polymerization of acrylic acid (AA) monomer/chemical crosslinker/ferric chloride (FeCl3) premixed with a PVA aqueous solution built the DN, followed by a single freeze-thaw (F-T) cycle completed the IPN structured hydrogel that was simultaneously self-healable, robust, and conductive. This ion-conductive self-healing hydrogel using the IPN strategy was applied to a multimodal sensor that can detect the strain, pressure, and temperature using different transduction mechanisms. Furthermore, a distinctively designed sensor attached to a finger can detect changes in resistance by an unfolding/folding motion and capacitance by fingertip-pressing/releasing motion without mutual interference.

2.2. Preparation of hydrogels IPN hydrogels were basically prepared using a two-step crosslinking procedure consisting of free radical polymerization and a freezethaw (F-T) cycle. First, a PVA aqueous solution (10 and 20 wt%) was prepared by dissolving PVA powder in distilled water at 90 °C with vigorous stirring. An AA aqueous solution was created by mixing AA (10 wt% vs H2O), FeCl3·6H2O (2 mol% vs AA), and MBA (0.1 mol% vs AA). The best performance hydrogel (entry #7) was of a 7:6 wt ratio of PAA (in 10 wt%) and PVA (in 20 wt% aqueous solution) as shown in Table S1. After both aqueous solutions were homogeneously mixed, they were poured into glass vials following the sample entries provided in Table S1. Then, the mixture was mixed again, and APS (5 wt% vs AA) was added. For the free radical polymerization and chemical crosslinking of PAA, the glass vials were thoroughly sealed and placed under fluorescent lamps for 24 h at a room temperature of 25 °C. For the physical cross-linking of PVA based on the F-T cycle, the glass vials were placed into a −20 °C freezer for 6 h and afterwards in a room temperature condition for thawing. 2.3. Characterization of materials and self-healing performance Images of freeze-dried hydrogels were taken using a scanning electron microscope (SEM) instrument (MIRA 3, TESCAN, Czech). The beam intensity was 15 kV and the images were taken after platinum coating. For the preparation of freeze-dried hydrogel samples, they were rapidly frozen by liquid nitrogen followed by a drying process using a freeze dryer (IlShinBioBase, Korea). Steady and dynamic rheological experiments were conducted to evaluate the viscoelastic behavior and self-healing performance of the IPN hydrogels using an oscillatory rheometer (MCR302, Anton Paar, Austria). Measurements were performed on 25-mm-diameter circular plates at 25 °C unless specific information is provided. The relaxation time of the hydrogel was calculated using the following equation; λ = J′ × |η*| = 39.48 × G′ × |η*| × w−2, where λ is the relaxation time, J′ is the compliance, |η*| is the complex viscosity, G′ is the storage modulus, and w is the angular frequency. The mechanical properties were measured using a universal testing machine (UTM, Instron 5943, UK) loaded with a 1-kN load cell; the tensile rates are provided in each figure caption. For the cyclic uniaxial stretching-relaxation tests, there was an intermission of 30 s between each cycle. The tensile-testing specimen was created following an international standard (ASTM D638 type V). The dog bone-shaped specimen was 9.53 mm in length, 3.18 mm in width, and 3.40 ± 0.10 mm in thickness in its rectangular region. For the mechanical self-recovery test, the dog bone-shaped IPN hydrogel sample was cut in the middle using a sharp razor blade and contacted together at 25 °C under ambient conditions without additional forces or stimuli. 2.4. Characterization of electrical properties and physical sensor performances

2. Experimental

The ionic conductance was evaluated by measuring the linear resistance using a source meter (Keithley 2400, USA). For the strain sensing experiment, the dog bone-shaped hydrogel sample with the same dimensions as previously mentioned was uniaxially stretched using a UTM machine at a tensile rate of 100 mm min−1 while the copper wires were connected to both ends of the rectangular region. The resistivity was calculated based on the measured resistance of the

2.1. Materials Poly(vinyl alcohol) (PVA, 89,000–98,000 g mol−1, 99+% hydrolyzed), ammonium persulfate (APS, ≥98.0%), N,N′-methylenebisacrylamide (MBA, ≥99.5%), Reactive Blue 4, Direct Red 80, and 453

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rectangular-shaped hydrogel using an equation, R = ρ × L × A−1, where R is the resistance, ρ is the resistivity, L is the length, and A is the surface area of the specimen. The initial dimensions of the hydrogel for strain sensing were a length of 9.53 mm, a width of 3.18 mm, and a thickness of 3.40 ± 0.10 mm. To fabricate the pressure sensor, VHB™ tape (thickness = 1 mm) was sandwiched by IPN hydrogels to form a capacitor with a parallel plate (1 cm × 1 cm) geometry. The pressure sensor was encapsulated by thermally cured PDMS at 60 °C in a convection oven (weight ratio of base:curing agent = 30:1), while the copper wires were connected to the IPN hydrogels. The capacitance change by a compressive force was evaluated using a home-made capacitance meter based on the Arduino-based micro-processing unit system. To apply pressures (20–100 kPa), balance weights of 200–1000 g were loaded onto the sandwich structure of parallel plates of the ion-conductive hydrogel with an area of 1 cm × 1 cm. To test the temperature-sensing capability, the dog-bone-shaped IPN hydrogel encapsulated by PDMS was stored in the oven at a temperature of 20–90 °C for 30 min; subsequently, the ionic conductance was measured using a source meter. The fabrication of the dual sensor system is graphically illustrated in Fig. S1. VHB™ tape was sandwiched by the dog bone-shaped IPN hydrogel and a thin film-shaped IPN hydrogel, followed by encapsulation by PDMS. The active dimensions of the capacitive pressure sensor were a thickness = 1 mm, a width = 9.5 mm, and a length = 12 mm. The active dimensions of the resistive strain sensor were as thickness = 1.6 ± 0.1 mm, a width = 3.18, and a length = 9.53 mm. This dimension of the active area is the neck position of the longer dog boneshaped sample (ASTM D638 type V). The prepared sensor system was attached onto the lower side of the index finger to detect finger motions such as unfolding/folding and pressing/releasing by measuring the change in either resistance or capacitance.

aqueous solution was homogeneously mixed. After the addition of APS as a radical initiator, the gelation began by free radical polymerization and chemical cross-linking. In addition, ionic bonds formed between the ferric ions and carboxylic groups of PAA that act as the dynamic bonds inside the gel networks (Fig. 1b) [34,44,45]. After the hydrogel was initially formed, the strength and elasticity of the material were enhanced by a single F-T cycle. Using the F-T cycle method, the crystallization of PVA occurred inside the hydrogel networks and the formation of crystallites, which act as sites of physical cross-linking, enhanced the mechanical properties of the IPN hydrogels [46]. To identify the optimal conditions of the self-healable IPN hydrogels, various ratios of both PAA and PVA polymers were attempted and the experimental details can be found in the Supporting Information (Table S1). The fluidic property was tested by inclining the glass vials containing IPN hydrogels to selectively choose the optimal condition to create an IPN hydrogel (Fig. S2). In addition, rheological measurements were conducted to evaluate the viscoelastic properties of hydrogels of different ratios (Fig. S3). Consequently, it was found that the optimal condition for the self-healable IPN hydrogel was 7:6 wt ratio of PAA (in 10 wt%) and PVA (in 20 wt% aqueous solution) (entry #7 in Table S1). As reported in comparative studies, DN of Fe3+/PAA and single network of PVA hydrogels have disadvantages of low mechanical stability and insufficient self-healing efficiency, respectively (Table S2) [34,46]. In addition, the micro porous structures of the freeze-dried PAA, PVA, and IPN hydrogels were confirmed by scanning electron microscope, showing the lyophilized porous structures of the IPN hydrogel (Fig. S4). The porous structure of hydrogels that facilitates the transport of electrons and ions generally supports the ion-conductive property of the hydrogel [47,48]. The mechanical properties of IPN hydrogels were investigated using tensile tests. The maximum elongation of the IPN hydrogel was measured to be greater than 1400% with a tensile rate of 10 mm min−1 (Fig. 2a,b). The prepared IPN hydrogels have sufficient mechanical robustness to endure mechanical tests such as creating a knot and the notch-insensitivity test (Fig. S5). Then, the self-healing capability was confirmed by attaching four different pieces of IPN hydrogels (Fig. 2c). For a better contrast, each sample was colored with a different dye and physically in contact without additional forces or stimuli. After 24 h of autonomous self-healing, the samples were connected together and endured mechanical deformation. To quantitatively evaluate the mechanical self-healing capability,

3. Results and discussion 3.1. Material preparation and self-healing characterizations Ion-conductive self-healing IPN hydrogels were prepared via sequential chemical/ionic cross-linking of Fe3+/PAA chains and physical cross-linking of PVA chains, as schematically illustrated in Fig. 1a. First, for the AA monomer aqueous solution containing MBA as a chemical cross-linker and FeCl3 as an ionic cross-linker, pre-dissolved PVA

Fig. 1. Preparation of ion-conductive self-healing IPN hydrogels. (a) Schematic illustration: (i) FeCl3, MBA, and AA monomer aqueous solution, (ii) PVA aqueous solution, (iii) FeCl3/AA and PVA mixture, (iv) PAA polymerization for double network in the pre-formed PVA domains, and (v) freeze-thaw (F-T) cycle for the formation of PVA crystallites. (b) Completed molecular structures of Fe3+/PAA-PVA IPN hydrogels. 454

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Fig. 2. Tensile stress-strain analyses and mechanical self-healing properties. (a) Photograph of self-healable IPN hydrogel before and after applying a tensile strain greater than 1400%. (b) Stress-strain curves and various tensile rates. The unit of the numbers provided in the legend is mm min−1. (c) Photographs of pre-cut hydrogel samples for the manual self-healing experiment, stained with different dyes for the images with high contrast (leftup: Reactive Blue 4, right-up: Direct Red 80, rightdown: Rhodamine B, and left-down: pristine). After the samples were physically contacted for 24 h without additional stimulus, the autonomous selfhealing was confirmed by uniaxial stretching. (d) Stress-strain curves of self-healed IPN hydrogels at different healing times from 3 to 48 h compared to the pristine sample. The tensile rate was 10 mm min−1.

90 °C near the boiling point of water, suggesting that the self-healing and mechanical properties of the IPN hydrogel were conserved at a temperature as high as 90 °C. This triple network system strongly binds the water molecules. The thermal tolerance enables the post process of fabrication such as thermal curing of PDMS as described in the following paragraph [53]. In addition, to test the mechanical tolerance of self-healing performance, the time-dependent G′ and G′′ of IPN hydrogel were measured by sequentially applying alternate small (10%) and large (600 and 800%) angular strains (Fig. 3d). At the large strain interval, G′′ > G′, suggesting the hydrogel was broken. However, G′ and G′′ instantly recovered to the original values at the beginning of the small strain interval. In its actual use as a conductor for e-skin, the selfhealing and mechanical properties of IPN hydrogel would recover after a large deformation such as bending or extending.

the dog bone-shaped specimens composed of the IPN hydrogel were cut using a sharp blade, re-contacted together, and maintained at the room temperature of 25 °C for a specified time (3–48 h) to induce autonomous self-healing. As shown in Fig. 2d, the repaired IPN hydrogels restored their original toughness by 81% and 108% after 24 and 48 h, respectively. A slight increase in toughness after 48 h compared to that of the original sample is attributed to the inevitable dehydration of the hydrogel sample. Even though PVA can self-heal owing to the hydrogen bonding among the hydroxyl groups, physical cross-linking of PVA chains by F-T cycles deteriorates self-healing at the cost of mechanical improvements. We found that a single F-T cycle is optimal for the best self-healing and mechanical performance. The main motivation for the self-healing capability of IPN hydrogels is attributed to the dynamic ionic bonding between the ferric ions and carboxylic groups of the PAA chains. The IPN hydrogel withstands tensile loading–unloading cycles of varying strain from 100 to 500% as well as five times of repeating 200% strain with minimal hysteresis (Fig. S6), implying its high potential as an elastomeric and mechanically durable material to be used as a soft material for mechanical sensors [49,50]. The self-healing ability of an IPN hydrogel under thermal and mechanical stresses occurring in actual usage and fabrication processes can be evaluated via a dynamic rheology study. Using a frequency-sweep mode, we measured the storage modulus (G′), loss modulus (G′′), and complex viscosity of the IPN hydrogel depending on angular frequency under different temperature conditions of 25, 60, and 90 °C (Fig. 3a,b). As shown in Fig. 3c, the relaxation time (λ) derived from the aforementioned frequency-sweep data is a good indicator to quantify the self-healing efficiency as well as a function of chain mobility with respect to analogous polymers [18,51,52]. All the examined rheological properties of the IPN hydrogel are independent of temperature even at

3.2. Multimodal sensor for the detection of strain, pressure, and temperature Self-healable IPN hydrogels with ionic conductivity might be applicable in numerous fields such as biomedical diagnostic devices, soft robotics, and biomimetic e-skins [54,55]. To emphasize the versatility of self-healable IPN hydrogels, the fabrication and characterization of the strain sensor, pressure, and temperature sensor are shown in Fig. 4. The ionic conductivity originates from the free migration of ferric and chloride ions in the hydrogel networks. The light-emitting diode (LED) demonstration intuitively shows the conductivity of the IPN hydrogel (Fig. S7). The ionic conductivity of the IPN hydrogel was measured as ∼0.14 S m−1. Although this value is not as high as that of other hydrogels, including metallic or carbon-based nanomaterials [7,31], it is sufficiently high for the hydrogel to be used in the aforementioned 455

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Fig. 3. Dynamic rheological analyses. (a) Storage modulus G′ (filled circle) and loss modulus G′′ (open circle). (b) Complex viscosity as a function of angular frequency. (c) Relaxation time as a function of angular frequency at 25, 60, and 90 °C, respectively. (d) Time-dependent analyses of G′ and G′′ alternately applying small (10%) and large (600% and 800%) oscillatory angular strain.

Fig. 4. Strain, pressure, and temperature sensing performances of IPN hydrogels. (a) Schematic illustration of experimental set-up for strain sensing, in which the resistance of rectangular region is measured during the reversible uniaxial stretching. (b) Normalized change in linear resistance (△R/R0) versus tensile strain. (c) △R/R0 and tensile strain as a function of time. (d) Schematic illustration of experimental set-up for pressure sensing, in which IPN hydrogel is used as the conductive layers. (e) Normalized change in capacitance (△C/C0) versus applied normal pressure. Error bars indicate the standard deviations from the five repetitive experiments. (f) △C/C0 as a function of time at different applied pressures. (g) Schematic illustration of experimental set-up for temperature sensing. (h) △R/R0 versus temperature. (i) △R/R0 as a function of number of cycles applying low and high temperatures sequentially. 456

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diffusion-temperature dependence as stated follows; J = −D × (dc/ dx) = −D0 × exp(−Qd/RT) × (dc/dx), where J is the diffusion flux, D is the diffusion coefficient, D0 is the temperature-independent constant, Qd is the activation energy for diffusion, R is the gas constant, T is the absolute temperature, and dc/dx is the concentration gradient. Hence, the IPN hydrogel was exposed to cold and hot atmospheric conditions, as shown in Fig. 4g. The electrical resistance (6.3 k Ω at the initial state of 25 °C) decreased in proportion to the temperature, up to 90 °C (Fig. 4h). Moreover, the IPN hydrogel exhibited stable temperaturesensing capability with high thermal stability and negligible degradation, which were confirmed using repetitive temperature application cycles (Fig. 4i). Hence, the IPN hydrogel has potential as a conductive material in strain, pressure, and temperature sensor applications. The versatility of the IPN hydrogel is not limited only to its robust mechanical properties and self-healing capability, but also includes its applicability in the fields of physical sensors and artificial skins. In comparison to previously reported hydrogels, the IPN hydrogel exhibits noteworthy multi-sensing performance (Table S3) [35,55,67,68].

applications [56,57]. In addition, the IPN hydrogel could autonomously self-recover its ionic conductive property, in which the slight increase in resistivity is attributed to the dehydration of hydrogel and the loss of ions during the process of cut and re-attachment (Fig. S8). The theoretical model of strain sensors was previously reported [58], which states that R/R0 = (l/l0)2 = (ε + 1)2, where R is the resistance of the hydrogel after uniaxial stretching, R0 is the resistance of the hydrogel before uniaxial stretching, l is the length of hydrogel after uniaxial stretching, l0 is the length of the hydrogel before uniaxial stretching and ε is the mechanical strain of the hydrogel. That is, as the hydrogel is uniaxially stretched, the surface area of the hydrogel decreases and the distance of the conductive path increases (Fig. 4a). As a result, the normalized change in resistance (6.3 k Ω at the initial state) was found to be linearly proportional to the tensile strain up to 150% (Fig. 4b), which is consistent with the literature [37,40]. In addition, no hysteresis was observed between uniaxial stretching and relaxation cycles which is attributed to the elastic behavior of the IPN hydrogels. Moreover, the strain sensing performance was able to self-recover, as shown in Fig. S9. The time-dependent change in resistance under tensile strain was studied and it showed no time delay between the input signal (i.e., the tensile strain) and output data (i.e., the change of resistance) (Fig. 4c). Moreover, the pressure sensor was fabricated to detect tactile pressures, which is highly desirable to build human-machine interfaces (Fig. 4d) [59,60]. A 1-mm-thick commercial VHB™ tape was used as the elastomeric and dielectric layer. PDMS was used to prevent the dehydration of the hydrogel [61]. Capacitance changes can be illustrated by the definition of the capacitance in the structure of two parallel electrodes. In the definition, C = ε0 × εr × A × d−1, in which C is the capacitance between the parallel conductive plates, ε0 is the permittivity of a vacuum, εr is the relative permittivity, A is the area of the parallel conductive plates, and d is the distance between the parallel conductive plates. Thus, as the compressive pressure increases, the area (A) enlarges and the distance (d) decreases, resulting in the increment in capacitance. The normalized capacitance change (5.4 picofarad, pF, at the initial state) as a function of applied pressures is shown in Fig. 4e. The pressure sensitivity was calculated as ∼0.018 kPa−1. To investigate the time-dependent sensing performance, the capacitance change as a function of time was measured (Fig. 4f). As shown in the plot, recovering behavior with negligible hysteresis was shown during the loading-unloading repeated test with increasing pressures (20–60 kPa). Furthermore, the detection of light objects or subtle touch may widen the ranges of potential applications of the IPN hydrogel-based sensor system. Thus, the minimum pressure sensing level was estimated using the signal-to-noise ratio (SNR) as reported elsewhere [62]. SNR is a parameter used to distinguish the valid sensing results from circumstantial noise, in this case defined as SNR = μ × σ−1, where μ is the averaged value of the increased capacitance when 20 kPa was applied (within the region of 10–20 s), and σ is the standard deviation of the noise levels when the pressure was removed (within the region of 20–35 s). From the experimental data, μ was calculated as 0.40 and σ was calculated as 0.031, thus the SNR was 12.90 and the minimum pressure sensing level was 20 kPa/12.90 = ∼1.55 kPa. Since this minimum detection limit is smaller than the pressure of a gentle touch, which is ∼10 kPa [63,64], the application of this pressure sensor can be extended to touch sensing. Furthermore, the IPN hydrogels can also be used as temperature sensors owing to their unique conductivity, which originates from the migration of free ions in the water medium. As the temperature rises, the diffusion rate of free ions also increases, which results in a decrease in electrical resistance. Moreover, the dependence of diffusion rate of free ions in the hydrogel on the temperature can be electrochemically explained by incorporating well-known Fick’s first law and Arrhenius equation [65,66]. In other words, since the conductance of the hydrogel is determined by the diffusion of particles (i.e. ions) in the hydrogel, temperature-sensibility of the hydrogel can be elaborated by the

3.3. Dual sensor for the finger-motion detection As shown in Fig. 4, the prepared IPN hydrogels have capabilities as a strain-sensitive conductive material as well as a mechanically elastic material to be used in a tactile pressure sensor. To maximize this advantage, a dual sensor system simultaneously detecting strain and pressure was designed based on the independent transducing mechanisms (Fig. 5a). The detailed procedures of dual sensor fabrication are also depicted schematically in Fig. S1. We simply utilized the three types of active components (i.e., the top electrode, dielectric layer, and bottom electrode) to form the multimodal sensor system. The key-design is that the pressure-sensitive capacitor is formed between the bottom and top electrode hydrogel, and the top electrode is longer and dog-bone-shaped on which a strain-sensitive resistor is connected at the neck position. Because only the middle part of the dog-bone shape is stretched under strain and both ends are free of deformation, there is no interference between the two signals. Previously reported sensor systems detecting both strain and pressure have intervention between them. In other words, the sensor system cannot distinguish the type of external forces because they rely on the single transducing mechanism [69–72]. However, the uniquely designed sensor system presented here is able to distinguish the type of external force, and can detect finger motions. To test the aforementioned theoretical hypothesis, the fabricated sensor system was attached to the index finger on the bottom side (Fig. 5b). The sequences of finger motions were unfolding(i), pressing (ii), releasing(iii), and folding(iv). The change in electrical signals under different finger is shown in Fig. 5c. By the motion of finger unfolding, the resistance increased ∼0.2 fold (△R/R0 of 0.2) compared to the original values (16.7 k Ω) without the interference of the capacitance. For the next motion of fingertip pressing while preserving unfolding, the capacitance increased to ∼3.0 fold (△C/C0 of 3.0) that of the original value (6.2 pF) without the interference of the resistance. The fingertip releasing and finger folding motions correlated with each independent signal. Moreover, to investigate the stability of the dual sensor system, the device performance after repetitive mechanical fatigue and thermal stress was tested (Fig. 5d,e). The whole process of unfolding-pressing-releasing-folding was repeated 1000 times with negligible degradation of the device and reliable mechanical sensitivity, which is attributed to the stress-relaxation feature of the self-healing IPN hydrogels (Fig. 5d). Interestingly, thermal exposure of the device at 90 °C for 6 h did not affect the sensor performance as demonstrated in the rheological study, proving durable operations at a high temperature in actual use (Fig. 5e). The actual output values of resistance (Ω) and capacitance (pF) for the stability test of the dual-sensor system against repetitive mechanical fatigue and thermal stress are given in Tables S4 and S5. 457

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Fig. 5. Finger-attachable dual sensor system simultaneously detecting strain and pressure. (a) Structure of dual sensor system, in which strain and pressure sensors are transplanted to selectively detect each stimulus without intervention. (b) Sequential index finger motions as follows: (i) unfolding the finger, (ii) pressing the table with the fingertip, (iii) releasing the fingertip, and (iv) folding the finger. (c) Normalized change in resistance and capacitance as a function of time, reflecting the results of finger motion sensing. (d,e) Stability test of dual-sensor system against repetitive mechanical fatigue and thermal stress. Filled triangle represents the status of unfolding, the hollow triangle represents the status of folding, the filled circle represents the status of pressing, and the hollow circle represents the releasing. (d) Normalized changes in resistance and capacitance repeating the finger motion cycles 1/10/100/1000 times. (e) Normalized changes in resistance and capacitance after heating the device at 90 °C for 0/1/3/6 h. Zero point of the changes in resistance and capacitance was set as the resistance and capacitance values of the initial finger gesture.

4. Conclusions

Acknowledgments

In summary, a novel and facile method to prepare a hydrogel with ionic conductivity, mechanical robustness, and self-healing capability was presented. In the hydrogel, a PVA polymeric chain, which was physically cross-linked, was sophisticatedly intertwined with a PAA polymeric chain, which was chemically/ionically cross-linked by MBA and ferric ions, to form an interpenetrating polymer network, termed as IPN. Such IPN hydrogels showed mechanical robustness as well as an autonomous self-healing capability at the room temperature of 25 °C. The results of the rheological measurements indicate that the IPN hydrogel is highly stable to oscillatory deformation and a high temperature up to 90 °C. Moreover, the IPN hydrogel exhibits ionic conductivity derived from ions in the matrix; hence, it was adopted as an electrode to fabricate strain, pressure, and temperature sensors. As each type of mechanical sensor utilizes a different transduction mechanism, the fabrication of a finger-attachable dual sensor system consisting of a resistive strain sensor and capacitive pressure sensor in a unique design, even for mechanical fatigue and thermal stress at a temperature of 90 °C, was successfully demonstrated. As presented in this work, mechanical sensor systems based on conductive, self-healing, and mechanically robust hydrogels could provide insights into emerging industrial fields such as soft robotics, prosthesis, and health monitoring systems.

This research was supported by the KRICT Core Project (SI1941-20, KK1941-30), KRICT Excellence Research Group Project (BS.K18-604), and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2018R1C1B6000966). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.04.077. References [1] M.L. Hammock, A. Chortos, B.C.-K. Tee, J.B.-H. Tok, Z. Bao, 25th Anniversary article: the evolution of electronic skin (E-skin): a brief history, design considerations, and recent progress, Adv. Mater. 25 (2013) 5997–6038. [2] Z. Lei, P. Wu, A supramolecular biomimetic skin combining a wide spectrum of mechanical properties and multiple sensory capabilities, Nat. Commun. 9 (2018) 1134. [3] Y.J. Tan, J. Wu, H. Li, B.C.K. Tee, Self-healing electronic materials for a smart and sustainable future, ACS Appl. Mater. Interf. 10 (2018) 15331–15345. [4] X. Wang, L. Dong, H. Zhang, R. Yu, C. Pan, Z.L. Wang, Recent progress in electronic skin, Adv. Sci. 2 (2015) 1500169. [5] J.-Y. Sun, C. Keplinger, G.M. Whitesides, Z. Suo, Ionic skin, Adv. Mater. 26 (2014) 7608–7614. [6] S. Terryn, J. Brancart, D. Lefeber, G.V. Assche, B. Vanderborght, Self-healing soft pneumatic robots, Sci. Robot 2 (2017) eaan4268. [7] B.C.-K. Tee, C. Wang, R. Allen, Z. Bao, An electrically and mechanically self-healing

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