Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
www.acsami.org
Highly Stretchable and Transparent Thermistor Based on SelfHealing Double Network Hydrogel Jin Wu,† Songjia Han,† Tengzhou Yang,† Zhong Li,§ Zixuan Wu,† Xuchun Gui,† Kai Tao,*,‡ Jianmin Miao,§ Leslie K. Norford,∥ Chuan Liu,*,† and Fengwei Huo⊥ †
State Key Laboratory of Optoelectronic Materials and Technologies, Guangdong Province Key Laboratory of Display Material and Technology, School of Electronics and Information Technology, Sun Yat-sen University, Guangzhou 510275, China ‡ The Ministry of Education Key Laboratory of Micro and Nano Systems for Aerospace, Northwestern Polytechnical University, Xi’an 710072, China § School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798, Singapore ∥ Department of Architecture, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States ⊥ Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, P. R. China S Supporting Information *
ABSTRACT: An ultrastretchable thermistor that combines intrinsic stretchability, thermal sensitivity, transparency, and self-healing capability is fabricated. It is found the polyacrylamide/carrageenan double network (DN) hydrogel is highly sensitive to temperature and therefore can be exploited as a novel channel material for a thermistor. This thermistor can be stretched from 0 to 330% strain with the sensitivity as high as 2.6%/°C at extreme 200% strain. Noticeably, the mechanical, electrical, and thermal sensing properties of the DN hydrogel can be self-healed, analogous to the self-healing capability of human skin. The large mechanical deformations, such as flexion and twist with large angles, do not affect the thermal sensitivity. Good flexibility enables the thermistor to be attached on nonplanar curvilinear surfaces for practical temperature detection. Remarkably, the thermal sensitivity can be improved by introducing mechanical strain, making the sensitivity programmable. This thermistor with tunable sensitivity is advantageous over traditional rigid thermistors that lack flexibility in adjusting their sensitivity. In addition to superior sensitivity and stretchability compared with traditional thermistors, this DN hydrogel-based thermistor provides additional advantages of good transparency and self-healing ability, enabling it to be potentially integrated in soft robots to grasp real world information for guiding their actions. KEYWORDS: stretchable thermistor, double network hydrogel, self-healing, transparent, stretchable electronics
■
electronics using serpentine ultrathin metal (Au14 or Pt18), graphene,1,2,19,20 Si membrane,21 polyaniline (PANI) nanofibers,9 (PEDOT:PSS)−carbon nanotube (CNT) composite,22 and graphite.23 Furthermore, stretchable multifunctional sensors have been fabricated to detect multiple stimuli, such as strain, temperature, and humidity.18,21,24−27 However, these sensing devices have limited stretchability from 5 to 70% due to the utilization of nonstretchable sensing material, interconnect, or electrodes.2 Furthermore, these conventional nonstretchable structures are usually not transparent, making the fabricated sensors opaque.2 In addition, the structural engineering approach demands complicated processes and therefore makes the fabricated devices expensive and very hard to
INTRODUCTION Stretchable electronic sensors that can conformably attach to nonplanar and complex surfaces have given rise to many emerging applications in wearable electronics, artificial skin, human motion and health monitoring, human−machine interface, and soft robots.1−6 Temperature sensing is one of the key functions of integrated stretchable systems that can be used in soft artificial intelligence robots.1,7−13 The capability to detect environmental and surface temperature can give early warning to soft robots and therefore avoid high-temperatureinduced damage of inside temperature-sensitive electronic components.3,14,15 Furthermore, the thermistor can also help soft robots gather information about environmental surroundings.16 However, the fabrication of highly stretchable thermistors remains challenging to date.1,17 Flexible or stretchable thermistors based on structural engineering of nanomaterials have been developed, including epidermal © XXXX American Chemical Society
Received: March 1, 2018 Accepted: April 13, 2018
A
DOI: 10.1021/acsami.8b03524 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Schematic illustrating the network structures of PAM/carrageenan DN hydrogel. (b, c) Optical images showing the thermistor has 100% strain and 360° twist, respectively. (d) Optical transmittance of the DN hydrogel versus wavelength.
control.2,28 An alternative strategy to overcome the stretching limit is the utilization of intrinsically stretchable and transparent materials with high sensitivity to temperature. However, this kind of material has been rarely reported.2,16 In addition to good stretchability and transparency, the selfhealing ability is also highly desirable in soft electronics, such as soft robots.3,29 We expect the mechanical and electrical property of artificial skin can recover after deformation, cracking, and fracture, just like the self-healing property of human skin.3 Self-healing materials enable the self-healing electronics to mend themselves both electrically and mechanically29 and thus prolong the service life of devices by repairing themselves after mechanical damage.3,30 Although it is urgent to expand the market of stretchable and self-healing electronics, it is still challenging to develop this kind of a sensing device.29 Double network (DN) hydrogel composed of two types of polymer networks with different physical natures has attracted considerable attention recently due to its good mechanical strength, stretchability, and toughness.31−35 Especially, the DN hydrogel exhibits enhanced stretchability compared with corresponding single-network components, making it attractive for fabrication of ultrastretchable sensing devices.32,36 To date, the thermal responsive property of the polyacrylamide (PAM)/ carrageenan DN hydrogel has not been investigated. In this work, for the first time, we employ PAM/carrageenan DN hydrogel to develop a unique thermistor that combines
ultrahigh stretchability, good transparency, and the ability of self-healing. We found the ionic conductive DN hydrogel is highly sensitive to temperature, enabling it as stretchable ionic skin for temperature sensing. Because the DN hydrogel is intrinsically stretchable, self-healing, and transparent, no structural engineering is required to fabricate the stretchable devices. The DN hydrogel-based thermistor exhibits excellent stretchability (330%), which is much higher than those of previously reported temperature sensors based on other materials (70% for graphene and 30% for serpentine metal and Si diodes).1,2 Compared with liquid metal conductors and ionic liquid, the solid and elastic DN hydrogel bypasses the requirement of a container.29,37−41 Importantly, we find that the sensitivity of the thermistor can be facilely tuned by mechanical strain, which is different from conventional rigid ceramic-based thermistors that lack the flexibility to tune their sensing characteristics once the devices are made.1,42 In addition, the good flexibility of the DN thermistor enables it to be attached conveniently on nonplanar curvilinear surfaces for practical temperature measurements.
■
EXPERIMENTAL SECTION
Materials. All chemicals including kappa-carrageenan (molecular weight around 3.0 × 105 g/mol), acrylamide (AAm), potassium chloride (KCl), N,N-methylenebisacrylamide (MBA), and 2-hydroxy4′-(2-hydroxyethoxy)-2-methylpropiophenone were purchased from Sigma-Aldrich. The MBA and 2-hydroxy-4′-(2-hydroxyethoxy)-2B
DOI: 10.1021/acsami.8b03524 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 2. (a) Optical images showing the DN thermal sensor at 0% (above) and 100% strains (below), respectively. (b) Resistance variation with temperature under different strains. (c) Sensitivity versus strain. The sensitivity at different strains is obtained by normalizing the data in (b), followed by calculating the slope after performing linear fitting. (d) Dynamic responses of this thermistor to 32, 43, 53, 64, 78, and 92 °C. (e) Response versus temperature using the data in (d). (f) Comparison of the dynamic responses of this thermistor to 62 and 82 °C at 0 and 100% strains. The heat from hot air flow was provided by a hair dryer with adjustable temperature settings. (g) Dynamic responses of the thermistor to 70 °C in 20 experimental cycles. (h) Plot of the quantitative response of the thermistor versus the experimental cycle. (i) One selected detection cycle in (g) reveals the response time of 13 s and recovery time of 120 s. methylpropiophenone were, respectively, deployed as cross-linker and UV initiator for synthesis of PAM. Synthesis of PAM/Carrageenan DN Hydrogel. The DN hydrogel was prepared by a one-pot polymerization method in the aqueous solution of acrylamide and carrageenan.32,36 The overall weight percentage of each chemical is: acrylamide/carrageenan/UV initiator/MBA/KCl/H2O = 14.7:3:0.73:0.006:0.89:80.7%. The acrylamide and carrageenan were dissolved in deionized water at 95 °C under continuous magnetic stirring in the first step. Then, UV initiator, MBA, and KCl were dissolved in the above aqueous solution. After magnetic stirring of the mixture at 95 °C for 3 h, the solution was poured into a plastic dish (12 cm × 12 cm × 1.5 mm for length, width, and height, respectively) and sealed to avoid water evaporation. The system was stored at 5 °C for 1 h for the formation of the first network of ionic cross-linked carrageenan. Then, the system was exposed to UV light with the wavelength of 360 nm for 1 h for the cross-linking of the second network of PAM. Finally, the DN hydrogel was peeled off from the plastic mold and cut in slices to investigate its mechanical and temperature sensing properties. Fabrication and Characterization of the Thermistor. To fabricate a stretchable thermistor, the DN hydrogel slice was placed on a glass slide, stretched to desired strains, and then fixed on the glass slide using a clip. The DN hydrogel slice was encapsulated in a plastic parafilm (Bemis Company, Inc.) to prevent the evaporation of water
inside the hydrogel. The resistance of the thermistor was measured by a Keithley 2602 Source Meter in the temperature sensing test. A Shimadzu UV-2501PC was employed to acquire the UV−vis spectra. The Fourier transform infrared (FTIR) spectroscopy measurement was carried out on a Shimadzu Fourier transform infrared spectrophotometer IR prestige-21. A Philips 1600 W Travel Hair Dryer with adjustable heat setting was employed to heat the DN thermistor. The temperature of the hot plate, hair dryer, and hot and cold cup was calibrated by a thermocouple (CENTER 308, K type).
■
RESULTS AND DISCUSSION Figure 1a shows the molecule structure of the PAM/ carrageenan DN hydrogel that is composed of covalently cross-linked PAM and an ionically cross-linked carrageenan network. The DN hydrogel was prepared by the one-pot method. All reactants were added to a water pot and formed a unique aqueous solution at 95 °C. The DN hydrogel was prepared sequentially in two steps. Upon cooling of the solution, the first ionic carrageenan network was formed through double helix association. Subsequently, the second covalent PAM network was generated upon initiation with UV illumination. The as-synthesized DN hydrogel sheet was peeled off from a mold and cut to a slice with the length × width × C
DOI: 10.1021/acsami.8b03524 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces thickness = 7 mm × 3 mm × 1.5 mm for fabrication of the thermistor. The ions such as K+ and Cl− and water were encapsulated in the DN hydrogel networks. The movement of K+ and Cl− ions in water media contributes to the conductivity of the DN hydrogel. The elastic DN hydrogel can be stretched, twisted, and bent to a large extent without breaking its structures (Figure 1b,c). After release, the shape recovers quickly. It demonstrates the good elasticity, flexibility, and mechanical adaptability. In the stretching process, the doublehelical aggregates of carrageenan could be unzipped and dissociated to release energy. The typical optical transmittance of the DN hydrogel in Figure 1d reveals the good transparency of the sensing material. The optical transmission of the DN hydrogel increases with the wavelength of light in the visible region, from 40% at 350 nm wavelength to 76% at 480 nm wavelength and 81% at 800 nm wavelength. There is a strong and broad peak located at 3428 cm−1 in the FTIR spectra of the DN hydrogel (Figure S1, Supporting Information). This peak is ascribed to the symmetrical stretching vibration of hydroxyl groups.32,43,44 The high intensity of the hydroxyl stretching groups may suggest the existence of hydrogen bond between PAM and carrageenan, which improves both the mechanical stretchability and selfhealing ability of the DN hydrogel. The photographs in Figure S2 (Supporting Information) indicate that the DN thermistor can be conveniently stretched to different strains (0−330%) using different tensile stress. The excellent stretchability is attributed to the unzipping of the double-helical aggregates of carrageenan and the polymer chain of PAM, as well as the hydrogen bond interaction between the two polymer networks.36 The DN hydrogel displayed a hysteresis loop on the tensile stress−strain curves that were obtained in a loading and unloading experiment (Figure S3, Supporting Information). The hysteresis reveals an effective energy dissipation mechanism.33,45 After 41 consecutive stretching cycles under 150% strain, the resistance variation of this thermistor is within 4%, demonstrating the good stability of this thermistor under repeated stretching (Figure S4, Supporting Information). The soft, flexible, and conformal features make this thermistor suitable for attaching on nonplanar surfaces for temperature sensing. The stretchable DN thermistor in relaxed (0% strain) and stretched (100% strain) states are shown in Figure 2a. The resistance of the DN thermistor decreased monotonically with increased temperature under all strains, suggesting the negative temperature coefficient (NTC) behavior of the DN hydrogel (Figure 2b).46 For example, the resistance of the thermistor decreased from 38.5 to 0.7 kΩ when the temperature increased from 26 to 64 °C under 200% strain. The sensitivity is defined as the normalized resistance change per degree centigrade: S = ΔR/(R0 × ΔT)% = (R0 − R)/(R0 × ΔT), in which R0 and R are the resistance of the thermistor at room temperature (25 °C) and tested temperature, respectively. The unit of sensitivity is %/°C. It is worth noticing that the sensitivity increased from 1.99 to 2.6%/°C when the strain increased from 0 to 200% (Figure 2c). The boosted sensitivity with increased strain may be attributed to the enlarged surface area of DN hydrogel that is exposed to the heat in the stretched state, which enables more efficient heat adsorption and increases the response. In addition, the alignments of both polymer chains and ionic conduction pathways in the stretched state may also contribute to increased thermal response. The relationship between resistivity and temperature is deduced using the data in Figure
2b (Figure S5, Supporting Information). Interestingly, strain aligns ionic conduction pathways that enable higher conductivity in the stretched state. In the practical application, the stretchable thermistor can be calibrated by integrating a temperature-insensitive strain sensor to measure the strains. Resistance change of the DN hydrogel with further strain is shown in Figure S6 (Supporting Information), indicating this material can also be employed to fabricate stretchable strain sensors for various applications, such as robotics, human motion monitoring, therapeutics, etc.47−49 The strain-dependent resistance change is mainly attributed to the geometric change of the sensor during stretching. The resistance increases with tensile strain as the length increases, while the crosssectional area decreases with strain.41 The stretchable DN hydrogel-based thermistor with facile tunable thermal sensitivity is advantageous over conventional rigid thermistors that have fixed device structures and unchangeable thermal sensitivity.1 Importantly, this stretchable thermistor based on DN hydrogel displays both much higher sensitivity and better stretchability compared with reported thermistors based on many other materials, including graphene,1 reduced graphene oxide (RGO),2 oligomers/single-wall carbon nanotubes (SWCNTs),3 ultrathin Au,14 Si nanoribbon,21 PAM/polyiodides (PPI),26 PANI nanofiber,9 and (PEDOT:PSS)−CNT composite22 (Figure 3 and Table S1, Supporting Information).
Figure 3. Sensitivity is plotted against stretchability for the thermistors based on different materials, including DN hydrogel (this work), graphene,1 reduced graphene oxide (RGO)/polyurethane,2 oligomers/ SWCNTs,3 ultrathin Au film,14 Si nanoribbons,21 PAM/PPI,26 and PANI nanofiber.9 The thermistors with and without self-healing ability are marked with empty and solid symbols, respectively.
The high sensitivity of this ionic conductive hydrogel is attributed to its ionic transporting behavior, which can be activated at elevated temperature.41 The ionic mobility increases with temperature.50 Furthermore, the enhanced ion dissociation at elevated temperature also leads to increased concentration of charge carriers.41 Both effects contribute to reduced resistance at high temperature and therefore the occurrence of NTC behavior. Noticeably, the thermistor based on DN hydrogel provides additional advantages of self-healing ability and good transparency. The remarkable stretchability, high sensitivity, and self-healing ability in conjunction with the high transparency of the DN hydrogel makes it a promising thermosensitive material.3,7,16 The dynamic responses of the thermistor obtained when touching a hot plate surface with different temperatures are shown in Figure 2d. The normalized resistance change is plotted as R/R0 (%) versus time. The response is defined as D
DOI: 10.1021/acsami.8b03524 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 4. (a, b) DN hydrogel can be used as a flexible electronic conductor to light an LED indicator even in 160° flexion state and 360° twist state, respectively. (c) Resistance variation with flexion angle. Inset: scheme showing the definition of flexion angle. (d) Resistance variation with twist angle. Inset: photograph shows a 360° twisted hydrogel slice. (e) Resistance variation with time obtained in the detection of 64 °C with the flexion angle of 0 and 180°, respectively. (f) Comparison of the responses of this thermistor to 64 °C at different flexion and twist angles.
((R0 − R)/R0) × 100%, where R0 and R are the resistance of the thermistor at 25 °C and tested temperature, respectively. The decreased resistance of the DN hydrogel upon touching a hot object is attributed to increased mobility of both polymer chains and conductive ions at elevated temperature. The response increased monotonically with temperature from 32 to 92 °C (Figure 2e). Note that the DN thermistor clearly responded when a hair dryer blew it with hot air, demonstrating the practical application of this thermistor (Figure 2f). Furthermore, different temperatures of the hair dryer can be distinguished by observing the magnitude of response of the thermistor. Noticeably the response of this thermistor to the hot air flow nearly doubled with increased strain from 0 to 100%. The boosted thermal response of the thermistor by stretching agrees well with the aforementioned results in Figure 2b. Furthermore, the recovery speed of the thermistor in the stretched state is slightly higher than that in the relaxed state, which may be attributed to enlarged surface area and accelerated heat exchange in the stretched state. We studied the repeatability of this thermistor by switching the temperature between 25 and 70 °C for 20 experimental cycles (Figure 2g−i). A nearly constant response of 22.1% with a small standard deviation (SD) of 0.58% was obtained over the 20 cycles, demonstrating the good repeatability. The minimal detectable temperature change of this thermistor can be calculated as (sensitivity)/(SD) = (2.0%)/(2.6%/°C) = 0.77 °C.41 A response time of 13 s was observed, and the sensor achieved full signal recovery within 120 s after natural cooling. The response time and recovery time of our thermistor are longer than those of the reported thermistors based on ultrathin Au film and graphene, which have extremely small thickness (Table S1, Supporting Information). However, our thermistor displays much shorter response and recovery time than the thermistors based on many other materials, such as
oligomers/SWCNTs, Si nanoribbons, ionic liquid, etc.41 Both the response and recovery rates of this thermistor are determined by two factors, which are the heat transfer through the middle glass slide and the amount of energy it takes to heat the sensor. The response speed is related to the rate of heat transfer from the underlying hot plate to the DN hydrogel through the large glass slide, and the recovery speed is influenced by the rate of heat transfer from the hydrogel to air through the glass slide. The relatively small thermal conductivity of the glass substrates (0.9−1.3 W/(m K) for a soda lime microscope glass at 25 °C) and the large size of the glass slide (5 mm × 2.5 mm) may lead to slow heat transfer, leading to prolonged response and recovery processes.1 In addition, the high heat capacity of water (4.1707 J/(g K)) inside the hydrogel51 and the large mass of sensor (0.11 g) also prolong the response and recovery processes. For example, the heat absorbed by the sensor is calculated as: (4.1707 J/(g K)) × (0.11 g) × 45 K = 20.6 J when the sensor is heated from 25 to 70 °C.51 In future work, the response and recovery time can be shortened by reducing the size and mass of the thermistor. For instance, it is reported that the ionic conductive hydrogel can be three-dimensional printed as a tiny cable with the small thickness of 50 μm.52 The recovery takes a longer time than the response does because the heat dissipation between hydrogel and air in the recovery process is slower than the heat transfer between glass slide and hydrogel in the response process. Flexibility is further examined as it becomes an increasingly important capability for the practical application of soft thermally responsive materials in wearable devices.3 A very small external stress can be employed to bend the DN hydrogel into the curled deformation with adjustable flexion angles. The DN hydrogel can be exploited as an electrical conductor to light an light-emitting diode (LED) indicator even when the device has a large flexion angle of 160° (Figure 4a) and twist angle of E
DOI: 10.1021/acsami.8b03524 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 5. LED illumination experiment demonstrates that the DN hydrogel was conductive before cutting (a), became nonconductive after cutting (b), and recovered its conductivity after self-healing (c). (d) The self-healed DN remained conductive at 52% strain. (e) The time evolution of the self-healing process, as measured by its resistance change. (f) Dynamic responses of the self-healed thermistor to three different temperatures given by the hair dryer.
light an LED indicator after cutting a DN hydrogel slice into two parts, followed by a self-healing process (Figure 5). Before the polymer slice was cut, the LED indicator was lighted (Figure 5a). The LED indicator was extinguished when the DN hydrogel was cut by a knife (Figure 5b). However, the LED indicator became lighted again after the two furcated parts were brought together and cured at 95 °C for 30 min, indicating the recovery of conductivity after self-healing (Figure 5c). Although the stretchability and mechanical strength of the self-healed DN hydrogel is not as good as those of the original one, the selfhealed hydrogel can still be stretched up to 52% strain without breaking its structure from the furcated place, demonstrating the good mechanical robustness after self-healing (Figures 5d and S3, Supporting Information). The deteriorated stretchability of the DN hydrogel after self-healing is attributed to the incomplete healing process. Because the covalently cross-linked PAM network is not easily self-healed, the self-healing mainly happened on the double-helical carrageenan above its sol−gel transition temperature (290−350 K). In future work, the selfhealing efficiency can be further improved by optimizing the ratio of PAM to carrageenan, curing temperature and time. In addition to strain, the self-healed DN hydrogel can withstand other mechanical deformations such as twist and bend without degrading its conductivity and thermal sensitivity. The resistance of the DN hydrogel slice was also measured quantitatively at different stages to further investigate its selfhealing property (Figure 5e). The resistance increased from 30.9 kΩ to infinity when the hydrogel slice was fractured to two
360° (Figure 4b). In the repeated bending, twisting, and relaxing processes of the flexible conductor, no noticeable brightness change was observed for the LED light. The reason is that the electrical resistance remained almost unchanged with the increased flexion angle from 0 to 180° and increased twist angle from 0 to 360°, revealing the insensitivity of the DN hydrogel to both flexion and twist (Figure 4c,d). For example, the resistance of this DN thermistor did not show an appreciable increasing or decreasing trend but only exhibited a small variation of 1.7% when the twist angle increased from 0 to 360°, demonstrating impressive flexibility, mechanical adaptability, and electrical stability. The immunity of this thermistor to bending is also demonstrated by the small resistance variation with different bending radius of curvatures (Figure S7, Supporting Information). We compared the dynamic responses of the flexible thermistor to 64 °C when it was relaxed (flexion 0°) and highly bent (flexion 180°), respectively (Figure 4e). Only a small degradation of response (3.7%) was observed when the flexion angle increased from 0 to 180°, indicating the highly robust flexible thermistor can maintain its sensitivity even at the large flexion angle of 180° (Figure 4e,f). Similar to flexion, the DN thermistor was also insensitive to twist in the practical temperature sensing application (Figure 4f). The good flexibility and immunity to mechanical deformation make this thermistor suitable for practical application on nonplanar surfaces. The self-healing capability of the DN hydrogel is demonstrated by employing it as a conductor in a circuit to F
DOI: 10.1021/acsami.8b03524 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 6. (a, b) Dynamic responses of the DN thermistor obtained when a glass cup was filled with 8.2 °C cold water and 67 °C hot water, respectively. The inset of (a) shows the thermistor contacted conformably with the curved glass cup surface. The inset of (b) is an infrared image of the cup that filled with hot water. The thermistor that attached on the cup is marked by blue dashed cycles. (c) Response−time curves obtained when the thermistor approached the hot cup with the gaps of 1, 0.8, and 0.6 cm, respectively. (d, e) Photo and corresponding infrared image, respectively, showing the sensor could detect the finger temperature. (f) Response of this thermistor to the gentle touch of a finger in two consecutive cycles.
An outstanding advantage of the flexible thermistor is the convenience of being attachable on a nonplanar surface for practical temperature sensing. As an example, it can be attached on a curved glass cup surface to monitor the temperature change. The resistance of the device increased and decreased immediately when cold (8.2 °C) and hot water (67 °C) were poured into the cup, respectively, showing good ability of temperature sensing (Figure 6a,b). In addition, the resistance returned to the initial value after the water was removed. The signal recovery time is longer than the response time because the heat transfer process between glass and air is slower than that between water and glass. Remarkably, the thermistor is sufficiently sensitive to detect the temperature distribution of cold and hot objects without touching their surfaces (Figure 6c). For example, the thermistor showed evident response when it was brought to approach a hot cup with the temperature of 67 °C without touching the surface. Furthermore, the response increased with decreased distance between the thermistor and cup surface. This thermistor can also detect the objects with much lower temperature such as human skin. For example, the resistance decreased clearly when a human finger covered with plastic glove gently touched the sensor (Figure 6d−f). Because the signal can achieve complete recovery, the reversible thermistor can be utilized repeatedly without degrading its sensitivity (Figure 6f). The above examples demonstrate the capability of this thermistor to perform practical and accurate temperature measurement for real objects. The softness, stretchability, transparent, and selfhealing capabilities coupled with its excellent sensitivity to temperature make the DN hydrogel potentially useful for fabricating electronic skin.
parts, but the resistance recovered almost to the original level after a self-healing process, suggesting the high efficiency of the self-healing process. Interestingly, the self-healed hydrogel maintains an effective thermistor to detect the hot air flow provided by the hair dryer (Figure 5f). For example, the self-healed thermistor displayed the responses of 18.5, 26.3, and 30.7% to the temperatures of 63, 76, and 84 °C, respectively. The self-healing behavior is attributed to the thermoreversible sol−gel transition and the coil−helix structural transition of carrageenan in aqueous media above its sol−gel transition temperature.32,53 The double helices of carrageenan can disassociate into coils when the polymer was heated above its sol−gel transition temperature. In the cooling process, the carrageenan coils could reversibly reassociate to double helix structures, repairing the furcated surfaces. The hydrogen bond interaction between PAM and carrageenan also plays an important role in enhancing the selfhealing ability. The weak hydrogen bond is easily broken in the fracture process but can reform readily in the self-healing process, enabling the soft hydrogel to repair itself automatically at the broken interfaces.47 In a control experiment, the two furcated parts were brought together at room temperature without heating. The ions can transport in the hydrogel in this case,47 so the hydrogel can still conduct electricity. As such, the LED indicator can still be lighted. However, the DN slice had poor stretchability as it can be easily fractured from the furcated place in the stretching process. The mechanical strength of the DN hydrogel has not recovered at room temperature because new double helix structures have not formed at the furcated places to reconnect the polymer segments. Therefore, selfhealing for the conductivity is efficient at room temperature but self-healing for the stretchability needs proper heating. G
DOI: 10.1021/acsami.8b03524 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
ACS Applied Materials & Interfaces
■
■
CONCLUSIONS In summary, we have fabricated a DN hydrogel-based thermistor that combines good stretchability, transparency, and self-healing property. For the first time, we find the PAM/ carrageenan DN hydrogel has excellent thermal responsive property, making it promising for fabricating flexible electronics for highly sensitive temperature monitoring. Both the thermal sensitivity and stretchability of this DN hydrogel-based thermistor are better than those of previously reported flexible thermistors based on other materials. For instance, this thermistor demonstrates the high sensitivity of 2.6%/°C at the extreme strain of 200%. The minimal detectable temperature change of this thermistor is as small as 0.77 °C. Notably, the sensitivity of this stretchable thermistor can be programmed conveniently by controlling its strain. The tunable sensitivity is a unique advantage of this stretchable thermistor as compared with a conventional ceramic-based thermistor that only exhibits a fixed sensitivity. Note that a highly efficient self-healing process contributes to the recoverable electrical, mechanical, and thermal sensing properties of the DN hydrogel. The mechanical flexibility of the thermistor is demonstrated by extreme flexion and twist, which do not adversely affect its electrical and thermal sensing properties. The good flexibility allows the device to be attached on nonplanar surfaces for accurate temperature measurements. This novel, stretchable, transparent, and self-healing thermistor holds great potential in future wearable temperature sensing application. For example, it can be potentially integrated in soft robots to grasp real world information for guiding their actions.3,16
■
ACKNOWLEDGMENTS J.W. thanks the start-up Grant 76120-18831105 from Sun Yatsen University, Guangzhou, China. C.L. acknowledges the Guangdong Natural Science Funds for Distinguished Young Scholars under Grant 2016A030306046 and the Guangdong Youth Top-notch Talent Support Program (No. 2016TQ03X648).
■
REFERENCES
(1) Yan, C.; Wang, J. X.; Lee, P. S. Stretchable Graphene Thermistor with Tunable Thermal Index. ACS Nano 2015, 9, 2130−2137. (2) Trung, T. Q.; Ramasundaram, S.; Hwang, B. U.; Lee, N. E. An All-Elastomeric Transparent and Stretchable Temperature Sensor for Body-Attachable Wearable Electronics. Adv. Mater. 2016, 28, 502− 509. (3) Yang, H.; Qi, D.; Liu, Z.; Chandran, B. K.; Wang, T.; Yu, J.; Chen, X. Soft Thermal Sensor with Mechanical Adaptability. Adv. Mater. 2016, 28, 9175−9181. (4) Khan, Y.; Garg, M.; Gui, Q.; Schadt, M.; Gaikwad, A.; Han, D.; Yamamoto, N. A. D.; Hart, P.; Welte, R.; Wilson, W.; Czarnecki, S.; Poliks, M.; Jin, Z.; Ghose, K.; Egitto, F.; Turner, J.; Arias, A. C. Flexible Hybrid Electronics: Direct Interfacing of Soft and Hard Electronics for Wearable Health Monitoring. Adv. Funct. Mater. 2016, 26, 8764−8775. (5) Trung, T. Q.; Lee, N. E. Flexible and Stretchable Physical Sensor Integrated Platforms for Wearable Human-Activity Monitoring and Personal Healthcare. Adv. Mater. 2016, 28, 4338−4372. (6) Yan, C.; Wang, J.; Kang, W.; Cui, M.; Wang, X.; Foo, C. Y.; Chee, K. J.; Lee, P. S. Highly Stretchable Piezoresistive GrapheneNanocellulose Nanopaper for Strain Sensors. Adv. Mater. 2014, 26, 2022−2027. (7) Di Giacomo, R.; Bonanomi, L.; Costanza, V.; Maresca, B.; Daraio, C. Biomimetic Temperature-Sensing Layer for Artificial Skins. Sci. Rob. 2017, 2, No. eaai9251. (8) Keplinger, C.; Sun, J. Y.; Foo, C. C.; Rothemund, P.; Whitesides, G. M.; Suo, Z. G. Stretchable, Transparent, Ionic Conductors. Science 2013, 341, 984−987. (9) Hong, S. Y.; Lee, Y. H.; Park, H.; Jin, S. W.; Jeong, Y. R.; Yun, J.; You, I.; Zi, G.; Ha, J. S. Stretchable Active Matrix Temperature Sensor Array of Polyaniline Nanofibers for Electronic Skin. Adv. Mater. 2016, 28, 930−935. (10) Wang, X.; Dong, L.; Zhang, H.; Yu, R.; Pan, C.; Wang, Z. L. Recent Progress in Electronic Skin. Adv. Sci. 2015, 2, No. 1500169. (11) Honda, W.; Harada, S.; Arie, T.; Akita, S.; Takei, K. Wearable, Human-Interactive, Health-Monitoring, Wireless Devices Fabricated by Macroscale Printing Techniques. Adv. Funct. Mater. 2014, 24, 3299−3304. (12) Someya, T.; Kato, Y.; Sekitani, T.; Iba, S.; Noguchi, Y.; Murase, Y.; Kawaguchi, H.; Sakurai, T. Conformable, Flexible, Large-Area Networks of Pressure and Thermal sensors with Organic Transistor Active Matrixes. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 12321−12325. (13) He, Y.; Gui, Q.; Liao, S.; Jia, H.; Wang, Y. Coiled Fiber-Shaped Stretchable Thermal Sensors for Wearable Electronics. Adv. Mater. Technol. 2016, 1, No. 1600170. (14) Webb, R. C.; Bonifas, A. P.; Behnaz, A.; Zhang, Y.; Yu, K. J.; Cheng, H.; Shi, M.; Bian, Z.; Liu, Z.; Kim, Y. S.; Yeo, W. H.; Park, J. S.; Song, J.; Li, Y.; Huang, Y.; Gorbach, A. M.; Rogers, J. A. Ultrathin Conformal Devices for Precise and Continuous Thermal Characterization of Human Skin. Nat. Mater. 2013, 12, 938−944. (15) Kim, D. H.; Lu, N.; Ghaffari, R.; Kim, Y. S.; Lee, S. P.; Xu, L.; Wu, J.; Kim, R. H.; Song, J.; Liu, Z.; Viventi, J.; de Graff, B.; Elolampi, B.; Mansour, M.; Slepian, M. J.; Hwang, S.; Moss, J. D.; Won, S. M.; Huang, Y.; Litt, B.; Rogers, J. A. Materials for Multifunctional Balloon Catheters with Capabilities in Cardiac Electrophysiological Mapping and Ablation Therapy. Nat. Mater. 2011, 10, 316−323. (16) Chen, D.; Pei, Q. Electronic Muscles and Skins: A Review of Soft Sensors and Actuators. Chem. Rev. 2017, 117, 11239−11268. (17) Ren, X.; Pei, K.; Peng, B.; Zhang, Z.; Wang, Z.; Wang, X.; Chan, P. K. A Low-Operating-Power and Flexible Active-Matrix Organic-
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b03524. FTIR spectra of the DN hydrogel; photographs showing the double network (DN) thermistor stretched to different strains from 0 to 330%; tensile stress/strain curves obtained in the loading−unloading process for the original (black) and self-healed (red) DN hydrogels; plot of normalized resistance change ΔR/R0 (%) versus number of stretching cycles; plots of the resistivity of the DN hydrogel versus temperature at different strains; plots of the resistivity as a function of strain at different temperatures; calculation of the resistivity of the DN hydrogel slice, relative resistance variation of the thermistor with the strain; comparison of various flexible thermistors based on different materials; plot of resistance versus bending radius of curvature in the bending test (PDF)
■
Research Article
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (K.T.). *E-mail:
[email protected] (C.L.). ORCID
Jin Wu: 0000-0002-3065-6858 Xuchun Gui: 0000-0001-7430-3643 Fengwei Huo: 0000-0002-5318-4267 Notes
The authors declare no competing financial interest. H
DOI: 10.1021/acsami.8b03524 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces Transistor Temperature-Sensor Array. Adv. Mater. 2016, 28, 4832− 4838. (18) Kim, D. H.; Lu, N. S.; Ma, R.; Kim, Y. S.; Kim, R. H.; Wang, S. D.; Wu, J.; Won, S. M.; Tao, H.; Islam, A.; Yu, K. J.; Kim, T. I.; Chowdhury, R.; Ying, M.; Xu, L. Z.; Li, M.; Chung, H. J.; Keum, H.; McCormick, M.; Liu, P.; Zhang, Y. W.; Omenetto, F. G.; Huang, Y. G.; Coleman, T.; Rogers, J. A. Epidermal Electronics. Science 2011, 333, 838−843. (19) Kong, D.; Le, L. T.; Li, Y.; Zunino, J. L.; Lee, W. TemperatureDependent Electrical Properties of Graphene Inkjet-Printed on Flexible Materials. Langmuir 2012, 28, 13467−13472. (20) Bendi, R.; Bhavanasi, V.; Parida, K.; Nguyen, V. C.; Sumboja, A.; Tsukagoshi, K.; Lee, P. S. Self-Powered Graphene Thermistor. Nano Energy 2016, 26, 586−594. (21) Kim, J.; Lee, M.; Shim, H. J.; Ghaffari, R.; Cho, H. R.; Son, D.; Jung, Y. H.; Soh, M.; Choi, C.; Jung, S.; Chu, K.; Jeon, D.; Lee, S. T.; Kim, J. H.; Choi, S. H.; Hyeon, T.; Kim, D. H. Stretchable Silicon Nanoribbon Electronics for Skin Prosthesis. Nat. Commun. 2014, 5, No. 5747. (22) Harada, S.; Honda, W.; Arie, T.; Akita, S.; Takei, K. Fully Printed, Highly Sensitive Multifunctional Artificial Electronic Whisker Arrays Integrated with Strain and Temperature Sensors. ACS Nano 2014, 8, 3921−3927. (23) Dinh, T.; Phan, H.-P.; Dao, D. V.; Woodfield, P.; Qamar, A.; Nguyen, N.-T. Graphite on Paper as Material for Sensitive Thermoresistive Sensors. J. Mater. Chem. C 2015, 3, 8776−8779. (24) Yeo, W. H.; Kim, Y. S.; Lee, J.; Ameen, A.; Shi, L. K.; Li, M.; Wang, S. D.; Ma, R.; Jin, S. H.; Kang, Z.; Huang, Y. G.; Rogers, J. A. Multifunctional Epidermal Electronics Printed Directly Onto the Skin. Adv. Mater. 2013, 25, 2773−2778. (25) Ho, D. H.; Sun, Q.; Kim, S. Y.; Han, J. T.; Kim do, H.; Cho, J. H. Stretchable and Multimodal All Graphene Electronic Skin. Adv. Mater. 2016, 28, 2601−2608. (26) Yu, H.; Guo, Y.; Yao, C.; Perepichka, D. F.; Meng, H. A Smart Polymer with A High Sensitivity to Temperature and Humidity based on Polyacrylamide Hydrogel Doped with Polyiodide. J. Mater. Chem. C 2016, 4, 11055−11058. (27) Zhao, S.; Zhu, R. Electronic Skin with Multifunction Sensors Based on Thermosensation. Adv. Mater. 2017, 29, No. 1606151. (28) Tao, K.; Tang, L.; Wu, J.; Lye, S. W.; Chang, H.; Miao, J. Investigation of Multimodal Electret-Based MEMS Energy Harvester With Impact-Induced Nonlinearity. J. Microelectromech. Syst. 2018, 276−288. (29) He, Y.; Liao, S.; Jia, H.; Cao, Y.; Wang, Z.; Wang, Y. A SelfHealing Electronic Sensor Based on Thermal-Sensitive Fluids. Adv. Mater. 2015, 27, 4622−4627. (30) Cao, Y.; Morrissey, T. G.; Acome, E.; Allec, S. I.; Wong, B. M.; Keplinger, C.; Wang, C. A Transparent, Self-Healing, Highly Stretchable Ionic Conductor. Adv. Mater. 2017, 29, No. 1605099. (31) Sun, J. Y.; Zhao, X.; Illeperuma, W. R.; Chaudhuri, O.; Oh, K. H.; Mooney, D. J.; Vlassak, J. J.; Suo, Z. Highly Stretchable and Tough Hydrogels. Nature 2012, 489, 133−136. (32) Liu, S.; Li, L. Recoverable and Self-Healing Double Network Hydrogel Based on kappa-Carrageenan. ACS Appl. Mater. Interfaces 2016, 8, 29749−29758. (33) Yuan, N.; Xu, L.; Wang, H.; Fu, Y.; Zhang, Z.; Liu, L.; Wang, C.; Zhao, J.; Rong, J. Dual Physically Cross-Linked Double Network Hydrogels with High Mechanical Strength, Fatigue Resistance, NotchInsensitivity, and Self-Healing Properties. ACS Appl. Mater. Interfaces 2016, 8, 34034−34044. (34) Means, A. K.; Ehrhardt, D. A.; Whitney, L. V.; Grunlan, M. A. Thermoresponsive Double Network Hydrogels with Exceptional Compressive Mechanical Properties. Macromol. Rapid Commun. 2017, 38, No. 1700351. (35) Stevens, L.; Calvert, P.; Wallace, G. G.; in het Panhuis, M. IonicCovalent Entanglement Hydrogels from Gellan Gum, Carrageenan and An Epoxy-Amine. Soft Matter 2013, 9, 3009−3012.
(36) Lu, X.; Chan, C. Y.; Lee, K. I.; Ng, P. F.; Fei, B.; Xin, J. H.; Fu, J. Super-Tough and Thermo-Healable Hydrogel-Promising for ShapeMemory Absorbent Fiber. J. Mater. Chem. B 2014, 2, 7631−7638. (37) Sun, J. Y.; Keplinger, C.; Whitesides, G. M.; Suo, Z. Ionic Skin. Adv. Mater. 2014, 26, 7608−76014. (38) Jia, H.; Tao, X.; Wang, Y. Flexible and Self-Healing Thermoelectric Converters Based on Thermosensitive Liquids at Low Temperature Gradient. Adv. Electron. Mater. 2016, 2, No. 1600136. (39) Tao, X.; Jia, H.; He, Y.; Liao, S.; Wang, Y. Ultrafast Paper Thermometers Based on a Green Sensing Ink. ACS Sens. 2017, 2, 449−454. (40) Guan, L.; Nilghaz, A.; Su, B.; Jiang, L.; Cheng, W.; Shen, W. Stretchable-Fiber-Confined Wetting Conductive Liquids as Wearable Human Health Monitors. Adv. Funct. Mater. 2016, 26, 4511−4517. (41) Ota, H.; Chen, K.; Lin, Y.; Kiriya, D.; Shiraki, H.; Yu, Z.; Ha, T.J.; Javey, A. Highly Deformable Liquid-State Heterojunction Sensors. Nat. Commun. 2014, 5, No. 5032. (42) Feteira, A. Negative Temperature Coefficient Resistance (NTCR) Ceramic Thermistors: An Industrial Perspective. J. Am. Ceram. Soc. 2009, 92, 967−983. (43) Wu, J.; Li, Z.; Liu, C.; Tao, K.; Xie, X.; Khor, K. A.; Miao, J.; Norford, L. K. 3D Superhydrophobic Reduced Graphene Oxide for Activated NO2 Sensing with Enhanced Immunity to Humidity. J. Mater. Chem. A 2018, 6, 478−488. (44) Wu, J.; Tao, K.; Zhang, J.; Guo, Y.; Miao, J.; Norford, L. K. Chemically Functionalized 3D Graphene Hydrogel for High Performance Gas Sensing. J. Mater. Chem. A 2016, 4, 8130−8140. (45) Azevedo, S.; Costa, A. M. S.; Andersen, A.; Choi, I. S.; Birkedal, H.; Mano, J. F. Bioinspired Ultratough Hydrogel with Fast Recovery, Self-Healing, Injectability and Cytocompatibility. Adv. Mater. 2017, 29, No. 1700759. (46) Cai, G.; Wang, J.; Qian, K.; Chen, J.; Li, S.; Lee, P. S. Extremely Stretchable Strain Sensors Based on Conductive Self-Healing Dynamic Cross-Links Hydrogels for Human-Motion Detection. Adv. Sci. 2017, 4, No. 1600190. (47) Wu, J.; Tao, K.; Miao, J.; Norford, L. K. Improved Selectivity and Sensitivity of Gas Sensing Using a 3D Reduced Graphene Oxide Hydrogel with an Integrated Microheater. ACS Appl. Mater. Interfaces 2015, 7, 27502. (48) Cai, L.; Song, L.; Luan, P.; Zhang, Q.; Zhang, N.; Gao, Q.; Zhao, D.; Zhang, X.; Tu, M.; Yang, F.; Zhou, W.; Fan, Q.; Luo, J.; Zhou, W.; Ajayan, P. M.; Xie, S. Super-Stretchable, Transparent Carbon Nanotube-Based Capacitive Strain Sensors for Human Motion Detection. Sci. Rep. 2013, 3, No. 3048. (49) Cohen, D. J.; Mitra, D.; Peterson, K.; Maharbiz, M. M. A Highly Elastic, Capacitive Strain Gauge Based on Percolating Nanotube Networks. Nano Lett. 2012, 12, 1821−1825. (50) Leys, J.; Wubbenhorst, M.; Menon, C. P.; Rajesh, R.; Thoen, J.; Glorieux, C.; Nockemann, P.; Thijs, B.; Binnemans, K.; Longuemart, S. Temperature Dependence of the Electrical Conductivity of Imidazolium Ionic Liquids. J. Chem. Phys. 2008, 128, No. 064509. (51) Zhou, S.-Q.; Ni, R. Measurement of the Specific Heat Capacity of Water-Based Al2O3 Nanofluid. Appl. Phys. Lett. 2008, 92, No. 093123. (52) Tian, K.; Bae, J.; Bakarich, S. E.; Yang, C.; Gately, R. D.; Spinks, G. M.; in het Panhuis, M.; Suo, Z.; Vlassak, J. J. 3D Printing of Transparent and Conductive Heterogeneous Hydrogel-Elastomer Systems. Adv. Mater. 2017, 29, No. 1604827. (53) Iijima, M.; Takahashi, M.; Hatakeyama, T.; Hatakeyama, H. Detailed Investigation of Gel−Sol Transition Temperature of κcarrageenan Studied by DSC, TMA and FBM. J. Therm. Anal. Calorim. 2013, 114, 895−901.
I
DOI: 10.1021/acsami.8b03524 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
