Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 25613−25623
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Ultrastretchable Wearable Strain and Pressure Sensors Based on Adhesive, Tough, and Self-healing Hydrogels for Human Motion Monitoring Jiajun Xu, Guangyu Wang, Yufan Wu, Xiuyan Ren,* and Guanghui Gao* Polymeric and Soft Materials Laboratory, School of Chemical Engineering and Advanced Institute of Materials Science, Changchun University of Technology, Changchun 130012, China Downloaded via GUILFORD COLG on July 18, 2019 at 01:48:22 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
S Supporting Information *
ABSTRACT: Currently, flexible wearable hydrogel-based sensors have attracted considerable attention due to their promising applications in a variety of fields. However, concurrently integrating toughness, adhesiveness, self-healing ability, and conductivity into the hydrogel is still a great challenge. Here, casein sodium salt from bovine milk (sodium casein, SC) and polydopamine (PDA, inspired by mussels) were successfully introduced into the polyacrylamide (PAAm) hydrogel system to fabricate a tough and adhesive SC−PDA hydrogel. The hydrogel exhibits splendidly reversible adhesive behavioral bonding toward various materials and even human skin. Moreover, based on the dynamic cross-linking of SC and PDA in the system, the hydrogel has superstretching ability, excellent fatigue resistance, and rapid self-healing ability. In addition, the existence of sodium ions also endowed the SC−PDA hydrogel with sensitive deformation-dependent conductivity to act as a flexible strain and pressure sensor for directly monitoring large-scale human motions (e.g., joint bending) and tiny physiological signals (e.g., speaking and breathing). Therefore, the strategy would broaden the path of a new generation of hydrogel-based sensors for wide applications. KEYWORDS: hydrogels, tough, adhesive, self-healing, strain and pressure sensor
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INTRODUCTION Flexible human-friendly devices form an imperative part for wearable electronic devices,1,2 which have been attracting tremendous attention in recent years. To meet the escalating requirement for complex human motion, a stretchable, recoverable, and conductive substrate material is urgently needed.3−5 Nowadays, a conductive hydrogel as a promising material candidate, which has electronic characteristics like human skin, is developed and assembled into electronic devices, such as flexible electrodes, flexible mechanical sensors, flexible displays, and other devices with the human body.6−11 Among them, the flexible sensors were able to detect subtle environmental changes such as temperature,12 humidity,13 pressure,14 and deformation15,16 and transform the changes into recordable electrical signals including current, resistance, and capacitance, for the applications of health monitoring, human motion detection, and electronic skin of soft robots.17,18 To date, many flexible and wearable hydrogel sensors using carbon nanotubes,19,20 metals/semiconductors,21,22 graphene,23,24 conductive polymers,25,26 and microfluidics27,28 as conductive materials combined with elastomeric substrates have been successfully fabricated and have shown high sensitivity toward signal detection. For example, Lee and coworkers29 reported extremely stretchable strain sensors based on a borax-cross-linked poly(vinyl alcohol)/single-walled © 2019 American Chemical Society
carbon nanotube composite hydrogel, which could measure and withstand strain up to 1000%, with high gauge factor and excellent cycling stability. Gu et al.30 developed a microporous conductive hydrogel as a strain sensor for human motion monitoring by the combination of stiff polypyrrole and a soft hydrogel network. However, most of the hydrogel sensors without adhesion need to be attached to the human skin or clothes with the help of adhesive tapes or binding bandages, which greatly hinders their application in human joint motion.31,32 Thus, pursuing a hydrogel sensor with desirable self-adhesive behavior is an ongoing task. Over the past years, some endeavor has been devoted to developing adhesive hydrogels.33−35 To some extent, however, due to contradiction between toughness and adhesion, most of adhesive hydrogels usually present weak toughness, which resulted in cohesion failure during peeling adhesion tests.36,37 Therefore, it has always been a challenge to fabricate a hydrogel sensor that could exhibit robust bonding to the surface of materials and simultaneously maintain excellent mechanical properties. In addition, self-healing capability is also a desirable attribute for hydrogel devices.38,39 Because of the suffered damage in the flexible sensors, the self-healing capability can endow sensors Received: May 14, 2019 Accepted: June 25, 2019 Published: June 25, 2019 25613
DOI: 10.1021/acsami.9b08369 ACS Appl. Mater. Interfaces 2019, 11, 25613−25623
Research Article
ACS Applied Materials & Interfaces
and progressively changed to dark brown. The reaction was sustained for 24 h to obtain PDA solution. Preparation of the SC−PDA Hydrogel. The SC−PDA hydrogel was synthesized via simple free-radical polymerization. The procedure for the synthesis of hydrogel is described here. In the first step, SC, PDA solution, and AAm were dissolved in deionized water (controlling the total volume of the solution to 20 mL) under constant stirring at 40 °C. After the solution became homogeneous, MBA was added into the beaker and stirred for 1 h. Subsequently, KPS and TMEDA as the radical initiators were added to the solution and stirred for 10 min at room temperature. Then, the solution was injected into a reaction mold (100 × 20 × 6 mm3), which was composed of a pair of glass plates and a silica gel sideline with 6.0 mm thickness. Finally, the samples were placed in an oven at 40 °C for 5 h to obtain the SC−PDA hydrogel. For comparison, a sodium casein hydrogel (SC hydrogel) was fabricated under the same conditions without any PDA solution and a polydopamine hydrogel (PDA hydrogel) was fabricated under the same conditions without any sodium casein. To more clearly represent the preparation of the hydrogel, the formulations of the different component hydrogels are shown in Table S1. Dynamic Light Scattering Measurement. The particle size and particle size distribution of PDA were characterized by an American Brookhaven 90Plus particle size analyzer. The laser light-scattering measurements were set at 90°. The samples of PDA were diluted with deionized water to 1/1000 of the original concentration for analysis. Five tests were performed for each sample. Then, the average particle size and the size distribution were obtained. The particle size and distribution of PDA are shown in Figure S1. Fourier Transform Infrared (FTIR) Analysis. Chemical structures of the SC−PDA hydrogel, SC hydrogel, and PDA hydrogel were analyzed by a Fourier transform infrared (FTIR) spectrometer (Avatar-360, Nicolet, America). FTIR spectra were recorded in the spectral range of 400−4000 cm−1 with a resolution of 4 cm−1 and 64 scans for each sample. Before the measurement, the SC−PDA hydrogel, SC hydrogel, and PDA hydrogel samples were freeze-dried in a freeze vacuum dryer (FDU-2110, EYELA) under vacuum. Meanwhile, the samples were ground and dispersed in KBr by compression to form a sheet. The FTIR spectroscopy data for SC, PDA, and SC−PDA hydrogels are shown in Figure S2. Peeling Test. The adhesive strength of hydrogels on various material surfaces was measured using a texture analyzer (CT3-1000, made in the United States) and in 90° peeling mode. The samples were prepared with a length of 100 mm, width of 20 mm, and thickness of 6 mm. As a stiff backing for hydrogels, the poly(ethylene terephthalate) film was bonded onto hydrogels with cyanoacrylate adhesive. The prepared samples were tested with the standard 90° peeling test with a constant peeling speed of 5 mm/min, and all peeling tests were carried out directly without waiting time unless otherwise stated. The substrate materials included aluminum, glass, silica rubber, pig skin, plastic, and poly(tetrafluoroethylene) (PTFE). The interfacial toughness, Γ, was determined by dividing the plateau force, F, in the steady-state region of the peeling process by the width of the hydrogel sheet, W. At least, five tests were repeated for each sample to determine the peeling strength and interfacial toughness. Mechanic Contact Measurement. Mechanic contact measurements were used to evaluate the interacting adhesive performance based on the adhesion developed by Johnson, Kendall, and Roberts (JKR theory). The specific test was performed by a texture analyzer (CT3-1000, made in the United States). The test rate was a constant velocity of 0.5 mm/s. The maximum contact force was 0.1 N. The probe used in the test was a nylon ball of 25.4 mm diameter (14 g). The test process consists of loading−unloading periods and a holding time of 60 s. Each experiment was repeated at least five times. The work of adhesion (Wadh) was determined by the force (F), displacement (x), and the maximum contact area (Amax) as per the following equation
with more reliability, extended lifetime, and enhanced durability during their life span. Many electronic hydrogels have developed and exhibited good self-healing behavior. Bao and her co-workers40 prepared a self-healing electronic senor based on a nickel-nanostructured particle supramolecular organic composite. Liu et al.41 also demonstrated a self-healing epidermal strain sensor owing to the reversible boron ester bond. Nevertheless, these healed hydrogels had limited stretchability at less than 100%. As a result, there is great imperative to simultaneously integrate self-healing, stretchability, adhesiveness, and conductivity into the hydrogel sensors. Recently, mussel-inspired polydopamine (PDA) exhibited superior binding ability to a wide range of solid material surfaces due to the structural similarity to the mussel-adhesive proteins with catechol and amine groups, which have shed new light on preparing adhesive hydrogel materials. Furthermore, a series of noncovalent interactions (hydrogen-bonding and π−π stacking) between PDA molecules can endow the hydrogels with self-healing capability.42−44 In addition, some proteintackifying hydrogel strategies have been developed in recent years.45,46 Sodium casein (SC) is the sodium salt of the main protein casein from bovine milk, which has excellent water solubility and long-term preservation compared to those of casein. Also, it possesses a large number of amino acid residues including amino and carboxyl groups and hydrophobic blocks. It can form a micellar structure by means of noncovalent intermolecular binding interactions.47,48 The casein micellar structure can impart excellent adhesive and mechanical properties to hydrogels. In this investigation, casein sodium salt from bovine milk (sodium casein, SC) and polydopamine (PDA, inspired by mussels) were successfully introduced into the polyacrylamide (PAAm) hydrogel system to fabricate the sodium casein− polydopamine (SC−PDA) hydrogel. The SC−PDA hydrogels showed surprising stretchability and self-healing capability while also exhibiting an excellently repeatable adhesive behavior at the surface of diverse materials, such as metal, glass, pig skin, rubber, plastic, etc. In addition, the presence of sodium salt in casein sodium also imparts certain conductivity to the SC−PDA hydrogel. Therefore, the developed SC−PDA hydrogel integrates several desirable features including high stretchability, self-healing property, adhesion ability, and conductivity, which make it extremely suitable for wearable sensors to detect human motion and even important physiological signals.
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EXPERIMENTAL SECTION
Materials. Dopamine hydrochloride (DA), ethanol (C2H5OH, ≥99.7%), ammonia solution (NH4OH, 25−28%), acrylamide (AAm, 99.0%), potassium persulfate (KPS, 99.5%), N,N′-methylenebisacrylamide (MBA, 97%), and N,N,N′,N′-tetramethylethylenediamine (TEMED, ≥99.5%) were supplied by Aladdin (Shanghai, China). Sodium casein (SC, ≥90%) was supplied by Beijing Yinuokai Technology Co., Ltd. Deionized (DI) water was used in the experiment. Synthesis of PDA. PDA nanospheres were prepared according to the previously reported methods.49 The synthesis was performed in an alcohol−water mixed solvent. Ethanol (40 mL) was mixed with ammonia aqueous solution (NH4OH, 1 mL, 25−28%) and deionized (DI) water (90 mL) under mild magnetic stirring at room temperature for 30 min. Dopamine hydrochloride (0.5 g) was dissolved in 10 mL of DI water and then injected into the above mixed solution. The color of this mixed solution turned pale brown 25614
DOI: 10.1021/acsami.9b08369 ACS Appl. Mater. Interfaces 2019, 11, 25613−25623
Research Article
ACS Applied Materials & Interfaces Wadh =
The resistance changes of the hydrogels were measured by the above electrochemical workstation. The relative change of the resistance was calculated by the following formula
∫ F dx A max
ΔR /R 0 = (R − R 0)/R 0
Amax was determined as per the following equation
where R0 and R are the resistances without and with the applied strain, respectively.
A max = 2πRH
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where R is the radius of the ball probe and H is obtained from the distance at the maximum compression force. Mechanical Property. The uniaxial tensile test was carried out by a tensile tester (SHIMADZU, model AGS-X, 100 N, Japan) with a constant velocity of 100 mm/min. The samples were cut into dumbbell shape with a gauge length of 30 mm, a width of 4 mm, and a thickness of 3 mm. The corresponding dissipated energy was calculated from the area of the stress−strain loop curves. For tensile loading−unloading tests and successive loading−unloading tests, the samples were first stretched to a maximum extension strain of 1000% and then unloaded. In addition, for the successive loading−unloading tests, the hydrogels were applied to a layer of silicone oil after each loading−unloading test for avoiding water volatilization. The hysteresis energy (Uhys) was estimated by the area based on the loading−unloading curves. To investigate the compression properties of hydrogels, the compression performance was analyzed by a texture analyzer (CT31000, made in the United States) at room temperature. The samples were formed into cylindrical shapes (20 mm diameter × 15 mm thickness). The test velocity was 5 mm/s, and the maximum strain of the hydrogel was 85%. The hysteresis energy was calculated from the area between the loading and unloading curves. Measurements of each sample were repeated at least five times and were averaged for a given sample. Self-healing Performance. The hydrogel samples were cut into two parts with a scalpel, and the two halves were placed back together manually. At the same time, the wounded hydrogel was put into a sealed bag to prevent water evaporation during healing. Then, the healed samples were observed at room temperature without any stimulation. The healed hydrogels were investigated by a tensile test. The self-healing efficiency of hydrogels was evaluated by tensile stress−strain curves of the original hydrogels and self-healing hydrogels. Rheological Measurement. Dynamic rheological measurements were performed with a rheometer (Anton Paar, Austria MCR302) at 25 °C using 25 mm flat parallel plates. Before the measurement, a small portion of mineral oil was used to lay on the edge of fixture plates to prevent water evaporation from hydrogels. Dynamic oscillatory strain sweep experiments were performed on the hydrogels to determine the linear viscoelastic region. The dynamic strain sweep from 0.01 to 1000% was first performed at a constant frequency, ω = 10 rad/s. Consequently, frequency sweep tests were performed at a strain amplitude of γ = 0.5% over a frequency range of 0.01−100 rad/ s for all samples. Morphological Observation. The morphologies of hydrogels were performed by scanning electron microscopy (SEM, JSM-6510, JEOL, Japan) with a working distance of 15 mm and under an accelerating voltage of 10 kV. Before observation, all samples were freeze-dried and then coated with platinum to provide a conductive environment. The magnification factor was 200×. Conductive Property. The electrochemical property was recorded using a Metrohm Autolab electrochemical instrument (AUT86925). The resistance of hydrogels was measured by a fourprobe alternating current (AC) impedance method over a frequency range of 1−106 Hz. The ionic conductivity of the hydrogel can be obtained by the following formula σ=
RESULTS AND DISCUSSION To prepare a tough and adhesive hydrogel, sodium casein (SC) and polydopamine (PDA) were successfully introduced into the PAAm hydrogel system as a driving force. The hydrogel preparation processes and formation mechanism are shown in Figure 1a. After polymerization, SC and PDA can not only act
Figure 1. (a) Preparation process and formation mechanism of the SC−PDA hydrogel. (b) Exhibition of hydrogel with excellent mechanical and adhesive behavior.
as cross-linking centers to improve the mechanical properties of hydrogels but also impart excellent adhesion to SC−PDA hydrogels. This strong adhesion results from the presence of a large number of reactive functional groups from SC and PDA, allowing SC−PDA hydrogels to exhibit tough bonding to various substrates via possible physical or chemical interactions. As shown in Figure 1b, the hydrogel can easily adhere directly without waiting time and lift the weight of a 500 g volumetric flask. To evaluate the adhesive properties of the SC−PDA hydrogel, a 90° peeling test was implemented to measure the peeling force between hydrogels and the aluminum alloy substrate, and the schematic image of the peeling test is shown in Figure 2a. It was found that the different components would significantly influence the adhesive strength. As shown in Figure 2b−d, with the increase of SC, PDA, and MBA contents, the adhesive peeling force of hydrogels showed a trend of first increasing and then decreasing. When the contents of SC, PDA, and MBA were 25.0 wt %, 25.0 v/v %, and 0.023 mol %, respectively, the hydrogels show the excellent adhesive properties and the adhesion strength reached up to 675 N/m. This phenomenon is due to the influence of different components on the network structure of the hydrogel, which in turn affects the balance between cohesion and adhesion of the hydrogel. To vividly display the excellent adhesive behavior of the SC−PDA hydrogel, the images of hydrogels adhering to various materials are shown in Figure 2e, including rubber, plastic, glass, poly(tetrafluoroethylene) (PTFE), aluminum, pig skin, stone, and leaf. Notably, as shown in Figure 2f, the SC−PDA hydrogel was bonded between two iron blocks and soaked in water for 24 h. After 24 h, it does not exhibit detachment between the hydrogel and the iron block and still exhibits excellent adhesiveness. Therefore, it was envisioned that the SC−PDA hydrogel can be used in a humid or even an underwater environment.
L RS
where σ is the ionic conductivity (S/cm), L is the distance between each two electrodes (cm), R is the hydrogel resistance (Ω), and S is the cross-sectional area of the hydrogel. 25615
DOI: 10.1021/acsami.9b08369 ACS Appl. Mater. Interfaces 2019, 11, 25613−25623
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Figure 2. (a) Schematic image of the peeling test. (b−d) Peeling strength of SC−PDA hydrogels influenced by different contents of SC, PDA, and MBA, respectively. (e) Adhesive exhibition of the SC−PDA hydrogel adhering to various organic and inorganic materials, including rubber, plastic, glass, PTFE, aluminum, pig skin, stone, and leaf. (f) SC−PDA hydrogel was bonded between two iron blocks and would not separate after soaking in water for 24 h.
wide range of interface bonding energy will greatly satisfy practical applications. In addition, the reusable adhesion behavior of hydrogels had received much attention in practical applications. As shown in Figure 4a−f, the repeatable adhesive ability of SC−PDA hydrogels was investigated by performing the peeling test on aluminum, glass, silica rubber, plastic, pig skin, and PTFE, respectively. The SC−PDA hydrogel exhibits excellent adhesion behavior toward different substrates, and this adhesiveness is durable and repeated by physical interactions,
The effect of different contact times on the peel strength of the SC−PDA hydrogel was also discussed and is shown in Figure 3a. As the contact time increases, the peel strength of the SC−PDA hydrogel increases significantly, and the maximum adhesion strength reaches 2200 N/m after adhering for 30 min. Moreover, the comparison of interfacial adhesion energy for various adhesive materials is also summarized in Figure 3b.35,45,50−56 The adhesion energy of the SC−PDA hydrogel was significantly superior than that of other adhesive hydrogels, biological tissues, and commercial tapes. Also, this 25616
DOI: 10.1021/acsami.9b08369 ACS Appl. Mater. Interfaces 2019, 11, 25613−25623
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ACS Applied Materials & Interfaces
Figure 3. (a) Effect of different contact times on the peel strength of the SC−PDA hydrogel on the aluminum substrate. (b) Comparison of interfacial adhesion energy of various materials adhering to solid materials as a function of water concentrations, including adhesive hydrogels, biological tissues, and commercial tapes.
Figure 4. Repeatable and durable adhesion performance of SC−PDA hydrogels for different substrates: (a) aluminum, (b) glass, (c) silica rubber, (d) plastic, (e) pig skin, and (f) PTFE.
Figure 5. (a) Mechanical contact tests of SC, PDA, and SC−PDA hydrogels at the set maximum contact force of 0.1 N. (b) Corresponding adhesion force and adhesion energy. (c) Exhibition of the test process for SC, PDA, and SC−PDA hydrogels.
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Figure 6. (a−c) Tensile stress−strain curves of SC−PDA hydrogels for different contents of SC, PDA, and MBA, respectively. (d−f) Corresponding stress and toughness. (g) Exhibition of superior mechanical properties for SC−PDA hydrogels, including stabbing, stretching, loading, and blowing balloons.
Figure 7. (a) Tensile loading−unloading tests of the SC−PDA hydrogel at different setting strains (200, 400, 600, 800, and 1000%). (b) Continuous 10 tensile cyclic loading−unloading curves of the SC−PDA hydrogels. (c) Continuous 10 compressive cyclic loading−unloading curves of the SC−PDA hydrogels. (d−f) Corresponding stress and hysteresis energy.
maintaining almost the original peeling adhesive strength without any loss during 10 cycles. Moreover, the adhesiveness of hydrogels and traditional scotch tapes was compared, as shown in Figure S3. The SC−PDA hydrogels exhibit more superior adhesive properties compared to those of conventional scotch tapes. Especially for pig skin, the peel strength of hydrogels reaches up to 30 times higher than that of the traditional transparent tape.
To better demonstrate the tackifier mechanism of the hydrogel, the mechanical contact test was executed with the JKR theory to estimate the adhesive performance of SC, PDA, and SC−PDA hydrogels. The JKR theory aims to evaluate the actual adhesion of hydrogels by small-area contact testing to eliminate energy dissipation caused by large deformations. As presented in Figure 5a,b, the adhesion force and adhesion energy (Wadh) of SC−PDA hydrogels were superior than those 25618
DOI: 10.1021/acsami.9b08369 ACS Appl. Mater. Interfaces 2019, 11, 25613−25623
Research Article
ACS Applied Materials & Interfaces of SC and PDA hydrogels. The excellent bonding strength depends on the balance of adhesion and cohesion. Figure 5c clearly shows the specific test process for the three hydrogels. It could be clearly found that the SC hydrogel showed insufficient adhesion and the PDA hydrogel showed insufficient cohesion (not separated from the probe). As a result, the excellent adhesion properties of SC−PDA hydrogels are the result of synergy between SC and PDA to achieve cohesion and adhesion balance. For the hydrogel, endowing adhesiveness while maintaining excellent mechanical properties is challenging but significant. SC and PDA not only tackify the hydrogel but also participate in the construction of a cross-linked network structure to enhance the mechanical properties of the hydrogel. Figure 6a− c shows the tensile stress−strain curves for different contents of SC, PDA, and MBA, respectively, and the corresponding toughness of SC−PDA hydrogels is also calculated and shown in Figure 6d−f. Remarkably, by carefully adjusting the contents of SC, PDA, and MBA, the SC−PDA hydrogels achieved extreme mechanical properties. The tensile maximum strength and toughness of the hydrogel reached up to 170 kPa and 1.1 MJ/m3, respectively. To better illustrate the effects of SC, PDA, and MBA on the mechanical properties of hydrogels, the mechanical properties of only SC, only PDA, and only MBA hydrogels were also explored and are displayed in Figure S4. By comparison, the excellent mechanical properties of SC− PDA hydrogels can be clearly demonstrated as a result of the synergistic effect of three components. At the same time, the superior mechanical properties of SC−PDA hydrogels were also demonstrated through a series of exhibitions in Figure 6g, including stabbing, stretching, loading, and blowing balloons. Additionally, the dynamic rheological measurement57 and scanning electron microscopy (SEM) analysis are shown in Figure S5. Then, the energy hysteresis behavior and fatigue resistance of SC−PDA hydrogels were investigated by tensile and compression cycling tests. The hysteresis curves of hydrogels at different strains during the loading−unloading cycle are shown in Figure 7a,d. With the strain increasing from 200 to 1000%, more cross-linking points were destroyed, dissipating a great amount of energy and resulting in a significant increase in hysteresis loops. Meanwhile, each loading−unloading curve at a different strain was always coincident, indicating that the hydrogel had undergone the self-recovery process and displayed the excellent energy dissipation ability during the unloading process. Antifatigue properties are also important for hydrogels. The continuous 10 tensile and compression cycling curves without any residence time of SC−PDA hydrogels are shown in Figure 7b,c. Also, the corresponding stress and hysteresis energy are calculated in Figure 7e,f. Obviously, the internal network of gels was largely weakened and cannot be recovered immediately after the first loading. However, during the subsequent nine loading−unloading cycles, the tensile and compressive strength, and hysteresis energy of the hydrogel were almost similar. The above results revealed that SC−PDA hydrogels exhibited an excellent fatigue resistance due to the temporary destruction and rapid recovery. Exhibition of SC, PDA, and SC−PDA hydrogels before and after self-healing is shown in Figure 8. It is evident that PDA and SC−PDA hydrogels displayed excellent self-healing behavior. Meanwhile, the healing properties of the three hydrogels were quantitatively analyzed by tensile testing and are shown in Figure 8b,c. Surprisingly, the results of each
Figure 8. (a) Exhibition of SC, PDA, and SC−PDA hydrogels before and after self-healing for 24 h at room temperature. (b) Tensile curves of SC, PDA, and SC−PDA hydrogels before and after self-healing for 24 h at room temperature. (c) Corresponding stress and strain healing efficiency.
hydrogel tensile healing were consistent with the above description. The outstanding self-healing abilities of hydrogels were mainly attributed to the following two aspects: first, the PDA molecules could be linked to PAAm based on the interactions (recoverable π−π stacks and hydrogen bonds) between catechol groups of PDA and amide groups of PAAm. Second, a large number of recyclable hydrogen bonds were formed in the hydrogel among PDA, SC, and PAAm.43,44 Afterward, the healing behavior of SC−PDA hydrogels with different PDA contents is shown in Figure S6. With the increase of the PDA content, the self-healing efficiency is obviously improved. Due to the existence of irreversible chemical cross-linking, the healing stress of hydrogels is hindered, reaching only about 40%. However, it is worth mentioning that the strain healing efficiency of hydrogels reached up to 95%. Besides, the healing behavior of hydrogels at different healing times was also explored and is exhibited in Figure S7. The SC−PDA hydrogels exhibited rapid healing properties, and the healing efficiency was significantly improved with the increase of healing time. It is well known that SC is the salt of the main protein casein in milk, so the existence of sodium ions also endows the SC− PDA hydrogel with conductivity. As demonstrated in Figure S8, the conductivity of SC−PDA hydrogels was measured by the four-probe AC impedance method and the conductivity value obviously increases as the content of SC increases. The significant leaps in conductivity were derived from the diffusion of more free ions into hydrogels. Furthermore, the curves of relative resistance change for conductive hydrogels under various tensile strains (0−300%) are investigated in Figure 9a. The electric resistance of the hydrogel exhibited a steplike increasing trend as the strain increased, and the resistance almost recovered the initial value after the hydrogel was released. Moreover, the resistance of the hydrogel changes smoothly and responds immediately during the stretching process. The conductive hydrogel was also connected to a complete circuit containing a power supply and a blue lightemitting diode bulb. The brightness of the small bulb gradually decreases with the increase of strain, as depicted in the inset image of Figure 9a. These results indicated the high strain sensitivity and excellent electrical stability of the SC−PDA hydrogel. Additionally, the pH values of the SC−PDA hydrogel precursor solution with different contents of PDA were also measured. As shown in Figure S9, the pH is around 25619
DOI: 10.1021/acsami.9b08369 ACS Appl. Mater. Interfaces 2019, 11, 25613−25623
Research Article
ACS Applied Materials & Interfaces
Figure 9. Demonstration of the SC−PDA hydrogels as wearable sensors with strain sensitivity: (a) relative resistance change curves of the hydrogel sensor at different tensile strains. The relative resistance change curves for detection of various human motions: (b) speaking, (c) deeply breathing, (d) knuckle bending, (e) elbow joint bending, and (f) knee joint bending. (g−i) Relative resistance change curves of knuckles, elbows, and knees at different bending speeds, respectively.
7.0, indicating that the SC−PDA hydrogel hardly caused damage to the skin after directly contacting with the skin. In view of the many excellent properties of SC−PDA hydrogels, such as superior toughness and recoverability, strong and reversible adhesiveness, outstanding self-healing ability, sensitive deformation-dependent conductivity, etc., the hydrogel was highly suitable for a wearable strain sensor attached directly to the human skin for monitoring human motions. The SC−PDA hydrogel sensor exhibits unique responsiveness to not only large-scale human body movements but also even subtle motions. As demonstrated in Figure 9b, the hydrogel strain sensors can accurately distinguish and record various voice signals when directly adhered to the throat, such as saying “hello” and “hydrogel”. Also, the electric resistance changes exhibited superior reproducibility for the same words. Moreover, the hydrogel strain sensors are also able to sense human deep breath when attached onto the rib cage during human breathing, as shown in Figure 9c. On the other hand, the hydrogel adhered directly to the knuckles, elbows, and knee joints without any additional tapes or bandages to detect large-scale human motion. Figure 9d−f displays the relative resistance changes of the bending and
stretching process of knuckles, elbows, and knee joints. Clearly, the hydrogel sensor can accurately detect the joint movement behavior of each joint. Besides, the relative resistance changes of the knuckles, elbows, and knees at different bending speeds were also explored and are shown in Figure 9g−i. Surprisingly, the hydrogel sensors can quickly respond to different strain speeds. As the bending frequency increased, the relative resistance change became denser. The results illustrated the ultrafast response and high strain sensitivity of the hydrogel sensor. To further investigate pressure sensing for the SC−PDA hydrogels, as shown in Figure 10a, a hydrogel sample was assembled by wires and sealing films. Then, the sensor was installed in the sole of a shoe and connected to the analyzer. A volunteer (about 60 kg) wore the shoe and performed a series of sports. During the movement, the resistance of the hydrogel changes under different pressures. As demonstrated in Figure 10b−d, for different motions, like standing, jumping, and walking, the sensor responded quickly and outputted the corresponding signals. Meanwhile, the pressure sensor had enough sensitivity to detect the same motion at different speeds, as shown in Figure 10e, such as walking. It is worth 25620
DOI: 10.1021/acsami.9b08369 ACS Appl. Mater. Interfaces 2019, 11, 25613−25623
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Research Article
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (X.R.). *E-mail:
[email protected] (G.G.). ORCID
Guanghui Gao: 0000-0003-2947-2311 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by grants from the National Natural Science Foundation of China (Nos. 51873024 and 51703012) and Jilin Provincial Development and Reform Commission (No. 2019C052-2).
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Figure 10. (a) Schematic illustration of the SC−PDA hydrogels as pressure sensors. The relative resistance change curves of the hydrogel pressure sensor during different motion states of a person: (b) standing, (c) jumping, and (d) walking. (e) Curves of the relative resistance changes of hydrogels as pressure sensors when a person walked at different speeds.
noting that there is almost no significant change in the baseline of relative resistance changes during the repeat motion, demonstrating the excellent fatigue resistance of hydrogel sensors.
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CONCLUSIONS In this work, a novel hydrogel with superior toughness and recoverability, strong and reversible adhesiveness, outstanding self-healing ability, and sensitive deformation-dependent conductivity was successfully fabricated by free-radical polymerization. The hydrogel exhibited a fracture stress of 170 kPa, a fracture strain of more than 2100%, and splendid reversible adhesive behavior with various materials and even human skin. Notably, the adhesiveness can still be maintained in a humid or even an underwater environment. Meanwhile, the SC−PDA hydrogel could rapidly self-heal without needing any external stimulation. Furthermore, the hydrogels displayed ultrasensitive and stable resistance responsiveness to act as a flexible and wearable strain or pressure sensor for directly monitoring large-scale human motions and tiny physiological signals, including speaking and breathing; the bending of knuckles, elbows, and knee joints; and even the different motion states of a person, like standing, jumping, and walking. In conclusion, the SC−PDA hydrogel could hold considerable promise for a variety of applications such as medical electrodes, energy storage devices, and wearable devices.
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b08369. Preparation of hydrogels, chemical characteristics of hydrogels, adhesion of hydrogels, tensile curves of control samples, rheological measurement and SEM, tensile curves before and after self-healing hydrogels, and conductivity of hydrogels (PDF) 25621
DOI: 10.1021/acsami.9b08369 ACS Appl. Mater. Interfaces 2019, 11, 25613−25623
Research Article
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