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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Stretchable and Self-Healable Conductive Hydrogels for Wearable Multimodal Touch Sensors with Thermoresponsive Behavior O. Young Kweon,†,∥ Suman Kalyan Samanta,‡,∥ Yousang Won,§ Jong Heun Yoo,† and Joon Hak Oh*,§
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†
Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea ‡ Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur 721302, West Bengal, India § School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, Seoul 08826, Republic of Korea S Supporting Information *
ABSTRACT: Multifunctional hydrogels with properties including transparency, flexibility, self-healing, and high electrical conductivity have attracted great attention for their potential application to soft electronic devices. The presence of an ionic species can make hydrogels conductive in nature. However, the conductivity of hydrogels is often influenced by temperature, due to the change of the internal nano/microscopic structure when temperature reaches the sol−gel phase transition temperature. In this regard, by introducing a novel surface-capacitive sensor device based on polymers with lower critical solution temperature (LCST) behavior, near-perfect stimulus discriminability of touch and temperature may be realized. Here, we demonstrate a multimodal sensor that can monitor the location of touch points and temperature simultaneously, using poly(N-isopropylacrylamide) (PNIPAAm) in hybrid poly(vinyl alcohol) (PVA) and sodium tetraborate decahydrate cross-linked hydrogels doped with poly(sodium acrylate) (SA) [w/w/w = 5:2.7:1−3]. This multimodal sensor exhibits a response time of 0.3 s and a temperature coefficient of resistance of −0.58% K−1 from 20 to 40 °C. In addition, the LCST behavior of PNIPAAmincorporated PVA/SA gels is investigated. Incorporation of LCST polymers into high-end hydrogel systems may contribute to the development of temperature-dependent soft electronics that can be applied in smart windows. KEYWORDS: conductive hydrogel, stretchable electronics, self-healing, thermoresponsive gel, touch sensor
1. INTRODUCTION The largest sensory organ of the human body is human skin, and its sense of touch deals with the temporal and spatial perception of external stimulus through a numerous number of receptors (e.g., mechanoreceptors for pressure/vibration, thermoreceptors for “warm” and “cold” temperature) that are densely distributed all over an organ or tissue in our body.1,2 Human skin is composed of multilayers of muscles and fats and a complex structure supported on a deformable system. In particular, the Ruffini corpuscle, a type of slow-adapting mechanoreceptor, is present in the dermis and has a spindlelike structure tied to the collagen matrix, making it sensitive to skin stretching and slippage. The Ruffini corpuscle is also classically regarded as a thermoreceptor, because there will be pain in the case of deep burns as this receptor is burned off.3 Various artificial electronic skins (e-skin) and soft electronic devices with multimodality, high flexibility, and humanfriendliness have been intensively studied that could mimic the sensory aptitudes of human skin for their potential applications in soft robotics, prosthetics, and human−machine interfaces.4−7 Despite these efforts, precise stimulus detection and perfect discrimination of multi-signals have not yet been fully achieved in multimodal sensing platforms. Cho et al.8 © XXXX American Chemical Society
developed a stretchable and transparent all graphene multifunctional e-skin sensor matrix that can detect humidity, temperature, and pressure. Ko et al.9 reported multimodal ferroelectric sensors using microstructured hierarchically engineered elastic carbon nanotube fabrics, which were capable of simultaneously sensing external multistimuli, such as touch and temperature. Most researchers developed multimodal sensors by laminating or assembling each individual sensor layer that can provide electrical signals, such as changes in resistance, capacitance, and current. Moreover, with the continuous development of soft electronics, efficient integration of wearable, attachable, or implantable devices with the human body is required.10−15 Particularly, touch sensors can act as transducers that convert changes in external stimuli to measurable signals for elucidating a precise position where they are touched. Although developing a pixel-by-pixel-based sensing matrix is often employed for electronic devices or smart phones (e.g., projected capacitive touch technology), a multimodal sensor array can also be fabricated by introducing a Received: March 18, 2019 Accepted: June 25, 2019 Published: June 25, 2019 A
DOI: 10.1021/acsami.9b04440 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
behavior of these thermoresponsive coatings may lead to temperature-dependent transparency28 and conductivity of the hydrogel film, which has potential for application to smart windows. Herein, we report a new type of stretchable and self-healable touch sensor using ionic hydrogels composed of poly(vinyl alcohol) (PVA), sodium tetraborate decahydrate (borax), and sodium polyacrylate (SA). The developed PVA/SA-based multimodal sensor shows excellent discriminability between the touch position and surrounding temperature simultaneously, with advantages of high optical transparency (ca. 91%), self-healing capability (within 7.0 s), and outstanding bendability (pressure/bending-insensitive characteristics). The rapid self-healing process of the hydrogel is helpful in recovering the electrical properties of the sensors and avoiding degradation of performance during large deformations. Furthermore, using the surface-capacitive behavior of PNIPAAm-incorporated PVA/SA gel, we have fabricated a largearea stretchable device (12 cm × 9 cm) that exhibits a response time of 0.3 s with a temperature coefficient of resistance (TCR) of −0.58% K−1 from 20 to 40 °C in real time. To the best of our knowledge, this is the first demonstration of the fabrication of PVA-based hydrogels integrated with the LCST behavior of the PNIPAAm polymer and their application in multimodal touch sensors. The developed multimodal sensors not only exhibit excellent stimulus discriminability of touch and temperature by monitoring the signal pattern of electrical conductivity but also show temperature-variable optical transparency. On the basis of these key features, our PVA/ SA gels can be used for monitoring environmental stimuli and health conditions, as well as for energy-saving devices such as smart windows.
single conductive layer into a surface-capacitive array platform.3,16 Ionic conductive hydrogels are good candidates for soft electronics since they show excellent transparency and minimal variation in resistance under large deformable or stretching states.17−19 Such devices could include, for example, a touch panel on the surface of a printed circuit board or a flexible electronic device attached to human skin.19 Many hydrogels are biocompatible, so they can be used for transplantation into the body, healing therapy, and drug delivery. Some hydrogels show high transparency, allowing 99% transmission of the visible light; hence, they are useful for transmission of optical information.20 In addition, hydrogels containing a large amount of water are capable of dissolving ions, thus enabling the gels to serve as ionic conductors.21 Furthermore, these soft electronic devices sometimes require self-healing properties for repeatedly recovering their mechanoelectrical performances at room temperature, even at the point of damage, or under high degrees of stretching. Joo et al.22 reported uniformly dispersed poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) within hybrid organogels composed of polyacrylamide (PAAm) in ethylene glycol (EG). Upon increasing the PEDOT:PSS content to a smaller extent, the electrical percolation in the gels could be greatly improved and the fabricated PEDOT:PSS−PAAm organogels exhibited excellent electrical conductivity (0.01 S cm−1) with stretchability up to 50% strain. Park et al.23 reported a self-healing and conductive hydrogel made by the polymerization method of conductive pyrrole in agarose solution. These conductive hydrogels showed long-term stability within living organisms and conductivities of 0.65−0.41 S cm−1, under both bending and stretching tests over a range 0−35% of the original length. However, none of these hydrogel-based electronic devices can show excellent bending properties and stable electrical performances when stretched. In addition, the conductivity of hydrogels is often influenced by temperature due to the internal nano/microscopic structure changes when the temperature reaches the sol−gel phase transition temperature. This also provides an indirect model for variation of the internal percolation network with changes in temperature. This means that the state of aggregation and extent of hydration of such polymers can be modulated at a specific temperature, known as the lower critical solution temperature (LCST). The rational control of this property may lead to variations in the electrical properties of the polymer solutions or films. The LCST is an interesting behavior that is observed in certain polymer solutions. A temperature lower than the LCST enables a polymer to be entirely miscible in a particular solvent in all proportions, whereas phase separation takes place above the LCST. The LCSTs of few water-soluble polymers are particularly exciting, as they can cause phase separation from its solution upon heating. This may lead to the alteration of several key properties, including hydrophobicity, volume, as well as surface phenomena.24 One of the representative temperature-responsive polymers featuring an LCST in water is poly(Nisopropylacrylamide) (PNIPAAm).25,26 PNIPAAm is a temperature-responsive polymer showing a sharp transition behavior and an LCST of about 33 °C. Above this temperature, a reversible phase transition takes place from a swollen hydrated state to a shrunken dehydrated state,27 thus causing a phase separation from water. Incorporation of such LCST active polymers into a transparent hydrogel and the
2. EXPERIMENTAL SECTION 2.1. Preparation of PVA/SA Composite Hydrogels. PVA, sodium borate decahydrate (borax), and sodium polyacrylate (SA) were purchased from Sigma-Aldrich. The 5 wt % PVA (MW 85 000− 124 000 g mol−1) solution was prepared by dissolving PVA powder in deionized water at ∼85 °C with vigorous stirring for 3 h. After obtaining a viscous PVA solution, 35 wt % SA in distilled water was injected drop by drop into the PVA solution at ratios of 5:1, 5:2, and 5:3 PVA/SA (w/w) solution. Next, PNIPAAm was added to the total solution at ratios of 1, 2, and 4 mg mL−1. After cooling the mixture to room temperature, borax (10 wt % in H2O) solution (2.5 mL) was added with gentle stirring to form a gel. The total mixture was then heated at 80 °C until completely dissolved. The prepared gel was poured into a customized rectangular-shaped polyethylene plastic mold, and the mold was placed in a vacuum chamber to remove residual bubbles and create a flat-shaped gel. After the polymerization reaction was completed, the gel layer was dipped into an EG solution−distilled water mixture (25:75) for 48 h. 2.2. Preparation of Thermoresponsive Touch Sensors. The prepared rectangular-shape hydrogel film was placed on a 0.15 mm PET film. Copper electrodes for transmission of a current-change signal to the board were connected at each of the four corners of the film. A signal-processing controller that can provide touch applications was purchased from 3 M Corp; this controller has five electronic wires. Among them, four wires except the top-roof electrode were used to connect the prepared rectangular-shaped hydrogel film to the controller. We connected the wires clockwise. Downloaded EX II series capacitive touch system electronics was used to detect the appropriate current level of the hydrogels. 2.3. Characterization of PVA/SA Hydrogels. The physical and mechanical characteristics of the PVA/SA hydrogels were studied using a force test stand (M7-10, ESM303; Mark-10), field emission scanning electron microscopy (S-4800; Hitachi, Japan), and a UV−vis B
DOI: 10.1021/acsami.9b04440 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. Experimental setup and a schematic image of the PVA/SA multimodal sensor. Schematic illustration of (a) the device configuration and (b) the overall fabrication process of conductive hydrogel for multimodal touch/temperature sensor application. near infrared (NIR) spectrophotometer in the Korea Basic Science Institute (Daegu, Korea). FT-IR (VERTEX 70; Bruker) in transmission mode was used to characterize the molecular structure of the hydrogel. Electrochemical impedance spectroscopy (EIS) measurements were carried out using electrochemical instrumentation (IVIUM Stat; IVIUM Technologies, Eindhoven, The Netherlands). Heavily n-doped silicon wafers (
