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Ultrasensitive Wearable Soft Strain Sensors of Conductive, Selfhealing, and Elastic Hydrogels with Synergistic “Soft and Hard” Hybrid Networks Yan-Jun Liu,†,§ Wen-Tao Cao,† Ming-Guo Ma,*,†,‡ and Pengbo Wan*,§ †
Engineering Research Center of Forestry Biomass Materials and Bioenergy, Beijing Key Laboratory of Lignocellulosic Chemistry, College of Materials Science and Technology, Beijing Forestry University, Beijing 100083, P.R. China ‡ Key Laboratory of Pulp and Paper Science & Technology of Ministry of Education/Shandong Province, Qilu University of Technology, Jinan 250353, P.R. China § Center of Advanced Elastomer Materials, State Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, P.R. China S Supporting Information *
ABSTRACT: Robust, stretchable, and strain-sensitive hydrogels have recently attracted immense research interest because of their potential application in wearable strain sensors. The integration of the synergistic characteristics of decent mechanical properties, reliable self-healing capability, and high sensing sensitivity for fabricating conductive, elastic, self-healing, and strain-sensitive hydrogels is still a great challenge. Inspired by the mechanically excellent and self-healing biological soft tissues with hierarchical network structures, herein, functional network hydrogels are fabricated by the interconnection between a “soft” homogeneous polymer network and a “hard” dynamic ferric (Fe3+) cross-linked cellulose nanocrystals (CNCs−Fe3+) network. Under stress, the dynamic CNCs− Fe3+ coordination bonds act as sacrificial bonds to efficiently dissipate energy, while the homogeneous polymer network leads to a smooth stresstransfer, which enables the hydrogels to achieve unusual mechanical properties, such as excellent mechanical strength, robust toughness, and stretchability, as well as good self-recovery property. The hydrogels demonstrate autonomously self-healing capability in only 5 min without the need of any stimuli or healing agents, ascribing to the reorganization of CNCs and Fe3+ via ionic coordination. Furthermore, the resulted hydrogels display tunable electromechanical behavior with sensitive, stable, and repeatable variations in resistance upon mechanical deformations. Based on the tunable electromechanical behavior, the hydrogels can act as a wearable strain sensor to monitor finger joint motions, breathing, and even the slight blood pulse. This strategy of building synergistic “soft and hard” structures is successful to integrate the decent mechanical properties, reliable selfhealing capability, and high sensing sensitivity together for assembling a high-performance, flexible, and wearable strain sensor. KEYWORDS: wearable strain sensors, hybrid network hydrogels, self-healing, strain sensing, dynamic coordination
1. INTRODUCTION Stretchable wearable sensors, as essential components in wearable devices, have been attracting much attention in bioelectronics and robotics fields.1−6 Among them, strain sensors have initiated enormous research efforts because of their repeatable electrical changes upon external forces, and their sensing sensitivity similar to human skin tactile sensations, which is promising for biomechanics studies, human health monitoring, implantable sensors in medical treatments, or sensory skin in soft robotics.7−12 Although the highly sensitive strain sensors based on the strategy of integrating semiconductor on polymer foils2,8 or embedding conductive fillers into elastomeric matrix10 are well-developed, their low stretchability, poor durability, as well as rigid nature against seamless integration with skin, give rise to challenges for bodyattachable wearability. © XXXX American Chemical Society
Polymer hydrogels consist of a large amount of water and three-dimensional (3D) polymer networks, which not only have good flexibility similar to biotissues, but exhibit excellent stretchability, self-healing ability, and good self-recovery property over conventional “hard” polymeric materials, making them ideal candidates for soft matrices of wearable or implantable devices. In the recent studies, some successful strategies were employed to prepare mechanically excellent hydrogels by introducing effective energy dissipation domains or building multilayer network structures, including double network hydrogels,13,14 nanocomposite hydrogels,15−18 ABA triblock copolymer hydrogels,19−22 macromolecular microReceived: May 29, 2017 Accepted: July 11, 2017 Published: July 11, 2017 A
DOI: 10.1021/acsami.7b07639 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces Scheme 1. Schematic Illustration of the Synergistic “Soft and Hard” Hierarchical Network Structuresa
(a) “Soft” homogeneous polymer networks formed by host−guest polymerization based on rearrangement of the special side groups (PVA−OH and PVP−CO). (b) “Hard” cellulose nanocrystals (CNCs) as nanosized reinforcing domains bound together by a dynamic CNCs−Fe3+ binding unit. (c) Synergistic “soft and hard” hierarchical network structures containing extensive interchain hydrogen bonding. Chemical formula of (d) homogeneous polymer networks and (e) CNCs−Fe3+ binding unit respectively. a
sphere composite hydrogels,23,24 and free-radical polymerization hydrogels.25−27 Typically, Lin et al. utilized tridentate coordination interaction between Fe3+ and acrylic group as the secondary cross-link to design dual-cross-linked network hydrogels, exhibiting ultrahigh tensile strength (∼6 MPa), ultrahigh toughness (∼27 MJ m−3), large elongation (>7 times), good self-recovery property (∼4 h).14 Rauner and coworkers synthesized the amorphous calcium phosphate nanostructures, which are homogeneously distributed within polymer hydrogels to form ultrastiff and tough hydrogels (stiffness up to 440 MPa and fracture energy of 1300 J m−2).18 Nevertheless, integrating these performances of decent mechanical properties, reliable self-healing capability, and high sensing sensitivity together to design conductive, elastic, selfhealing, and strain sensing hydrogels for soft wearable sensors is still challenging. It is worth noting that most of biological soft tissues, such as ligament, tendon, and cartilage, are quitely strong, quickly selfhealing, and highly sensitive to external force changes.28,29 These biological soft tissues typically contain synergistic “soft and hard” hierarchical network structures with elastin, matrix and ordered nanoreinforcing domains.30 For example, ligament is formed from “hard” collagen fibrils bound together by a “soft” elastin and tendon also has analogous synergistic architecture containing “soft” and “hard” components.31 Inspired from these biological soft tissues with hierarchical network structures, it is highly desirable to simulate the complex architecture of biological soft tissues to design a synergistic “soft and hard” hierarchical network structure for fabricating functional network hydrogels. In this study, we fabricate the conductive, elastic, self-healing, and strain-sensitive hydrogels from a “soft” homogeneous polymer network via covalent cross-linking of poly(vinyl alcohol) (PVA) and poly(vinylpyrrolidone) (PVP), in which the “hard” Fe3+ cross-linked cellulose nanocrystals (CNCs)
network with the dynamic CNCs-Fe3+ coordination bonds is interconnected as nanoreinforcing domains (Scheme 1). The synergistic “soft and hard” hierarchical network structures by combining smooth stress-transfer and reversible coordination bonds enable the functional network hydrogels to achieve excellent mechanical strength (2.1 MPa of tensile stress), robust toughness (∼9.0 MJ m−3), reliable stretchability (830% elongation), reliable self-healing capability (within 5 min), and good self-recovery property (recovery in 30 min). Furthermore, the hierarchically porous network inside the hydrogels enables the hydrogels to display ultrasensitive, stable and repeatable resistance variations upon mechanical deformations for acting as a flexible and wearable strain sensor to diagnostically monitor human motion signal and important physiological signals, such as finger joint motions, breathing modes, and even the slight blood pulse changes in different motion states.
2. EXPERIMENTAL SECTION 2.1. Materials. All chemicals used here were of analytical grade and used as received without further purification. All experiments were operated under an ambient atmosphere. Microcrystalline cellulose (MCC, molecular weight of 34, 843−38, 894, degree of polymerization (DP), DP = 215−240), was purchased from Sinopharm Group Chemical Reagent Co., Ltd. Poly(vinyl alcohol) (PVA, ∼88% hydrolyzed, DP = 1750 ± 50) was purchased from Sinopharm Chemical Reagent Co., Ltd., and poly(vinylpyrrolidone) (PVP, K-45) was purchased from Xilong Chemical. Ferric chloride with six crystalwaters as the source of Fe3+ was purchased from Xilong Chemical and sulfuric acid (98%) were purchased from Beijing Chemical Works. 2.2. Preparation of Cellulose Nanocrystals. Fifteen grams of MCC with a 60 wt % H2SO4 solution (150 mL) were in a deionized water bath at 45 °C for 30 min with stirring. Then, the suspension was diluted with 8-fold of deionized water. After 10 min, the suspension was washed repeatedly with deionized water and centrifugation (3800 rpm, 5 min for each cycle) until the pH of the supernatant was higher than 1. The samples were subsequently dialyzed against water for 72 h, B
DOI: 10.1021/acsami.7b07639 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces and then the samples underwent vacuum freeze-drying to get Cellulose nanocrystals (CNCs) powders. 2.3. Preparation of Functional Network Hydrogels. Typically, 10 g PVA was dissolved in distilled water (115 mL) at 85 °C for 15 min under mechanical stirring (800 rpm) to obtain 8.0 wt % PVA solution. Five grams of PVP was dissolved in distilled water (20 mL) at room temperature for 5 min under ultrasonic treatment (100 W) to obtain 20 wt % PVP solution. The precursor solution A (20 mL) was prepared by mixing PVA (8.0 wt %) and PVP (20 wt %) with volume ratio of 7:3 at room temperature for 5 min with stirring. The precursor solution B was prepared by mixing CNCs (15 mg) and Fe3+ (2.5 mg) in water (5 mL) at room temperature for 10 min with stirring. Slowly add precursor solution B into precursor solution A to be stirred until forming uniform solution. The uniform solution was then poured into a glass beaker (100 mL) for exposure to microwave (microwave energy at 600 W) using UWave-1000 (Shanghai Sineo) for 3 min. The uniform solution was removed from the microwave reactor and immediately add sulfuric acid (ω = 10 wt %, 1% of the reaction mixture (volume)) into the uniform solution under vigorous stirring conditions to be viscous. Then the viscous solution gradually cooled down to room temperature, and the functional networks hydrogels were obtained. At last, the F-hydrogels were stored in refrigerator at −25 °C for 24 h (complete details in Table S1). 2.4. Mechanical Test. The uniaxial tensile tests of the hydrogels (the original height of loading fixture is 20 mm and the sample diameter is 14 mm, water content is 76.05%−79.45%) were performed with a mechanical testing apparatus (UTM6530, Shenzhen Suns Technology Co,. Ltd.) at a crosshead speed of 80 mm min−1. As for the loading−unloading test and the successive loading−unloading tests, the crosshead velocity were both kept at 120 mm min−1. In addition, a layer of silicone oil was coated on the hydrogels surface and encased by the plastic wrap during the successive loading−unloading tests for avoiding water volatilization. All the tensile tests were carried out at room temperature. The tensile stress, tensile strain, and fracture energy were determined by the rupture point of the stress−strain curve. The elastic modulus was calculated from the slope over 10−20% of strain ratio of the stress−strain curve. The toughness was calculated from the area of stress−strain curves. 2.5. Electrical Test. The resistance changes of the hydrogels in a different state were obtained by CGS-8 Intelligent Sensing Analysis System. The relative change of the resistance is calculated on the basis of the current monitored: ΔR/R0 = (Rs − R0)/R0, where R0 and Rs are the resistance without and with applied strain, respectively.
bonding between soft networks and hard nanosized reinforcing domains to further sustain the whole hierarchical network structures and facilitate stress-transfer from the soft phase to hard phase (Scheme 1c). 3.2. Preparation of Functional Network Hydrogels. The components of Fe3+ ion and CNCs were added to the uniformly mixed solution containing PVA and PVP, and then the uniformly mixed solution was transferred to a beaker to prepare the functional network hydrogels (termed as Fhydrogel) via simultaneous host−guest polymerization and ionic coordination triggered by microwave-assisted treatment under acid catalysis (Figure 1). No organic solvents, initiators,
3. RESULTS AND DISCUSSION 3.1. Design of a Synergistic Soft and Hard Hierarchical Network Structure. Inspired by the mechanically excellent and self-healing biological soft tissues with hierarchical network structures (e.g., ligament, tendons, and cartilage), the functional network hydrogels were prepared from the synergistic soft and hard hierarchical network structures. The polymer chains of PVA and PVP served as soft matrix phase, which undergo self-assembly upon the rearrangement of the special side groups (PVA−OH and PVP−CO)32 to form a homogeneous network (Scheme 1a). The 1,3-dioxane skeleton as the cross-linking points were regularly distributed and the hidden lengths were involved between the cross-linking points of the polymer chains. Meanwhile, CNCs were served as “hard” nanosized reinforcing domains where a Fe3+ ion attached to three C6-OH groups by ionic coordination on different CNCs molecular chains to form a dynamic stability “Y” shaped coordinate (Scheme 1b). On the one hand, the homogeneous polymer network results in a smooth stress-transfer to maintain the elasticity. On the other hand, the dynamic CNCs−Fe3+ coordination bonds act as sacrificial bonds to dissipate energy efficiently toward improving toughness and self-recovery property. In addition, there are extensive interchain hydrogen
Figure 1. Preparation of functional network hydrogels: (a) Photographs of the components (PVA, PVP, CNCs, and Fe3+) for preparing the functional networks hydrogels. (b−d) The fabrication processes of three different types of functional network hydrogels triggered by microwave-assisted treatment under acid catalysis.
or cross-linking agents are involved in the preparation process. Moreover, the resultant hydrogels contained a large amount of water (82.3−87.5%) in the as-prepared state (Table S1). Two relatively independent reactions were included in the preparation of F-hydrogels. On the one hand, the self-assembly was performed between the alcoholic hydroxyl side groups of the PVA chain and ketone groups of the PVP chain under acid catalysis, obtaining the 1, 3-dioxane polymers.32 On the other hand, the C6-OH groups on different CNCs molecular chains coordinated to Fe3+ ions, resulting in the dynamic “Y” shaped coordinates. The different microwave power absorption values for the reactants (polymers and CNCs) with different dielectric constants were displayed in Table 1,33,34 attributing to the independent reactions of the rearrangement of the special side groups between PVA and PVP and the dynamic coordination of CNCs-Fe3+ by the selective microwave heating, respectively. C
DOI: 10.1021/acsami.7b07639 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
The Fourier transform infrared (FTIR) spectra of PVA, PVP, CNCs, and the as-prepared F-hydrogels were presented in Figure S3a. The characteristic peaks of PVA appeared at 3450 cm−1 for − OH stretching vibration and 1325 cm−1 for C−O stretching vibration. The characteristic peak at 1650 cm−1 was observed for the CO stretching vibration of the pyrrole ring of PVP. The characteristic peaks at 3470 cm−1 (O−H stretching vibration) and 1015 cm−1 (C−O stretching vibration) were obtained for CNCs.37 Compared with the FTIR spectra of PVA, PVP, and CNCs, the characteristic peaks at 1650 cm−1 (CO) and 3450 cm−1 (−OH) decreased after the formation of F-hydrogels probably from the synergistic selfassembly between the −OH side groups of the PVA chain and the ketone groups of the PVP chain. Two new peaks appeared at 1121 cm−1 for the C−O−C stretching and 627 cm−1 for the Fe−O stretching, respectively, verifying the rearrangement of the specific side groups of PVA and PVP, and the CNCs-Fe3+ coordination during the reaction, respectively. To further investigate the functional groups on the surface of as-prepared F-hydrogels, the X-ray photoelectron spectroscopy (XPS) measurement was conducted. The XPS spectrum of the asprepared F-hydrogels (Figure S3b) exhibited three obvious peaks of C 1s, N 1s and O 1s at 284.84, 399.33, and 532.3 eV, respectively. Figure S3c shows that the high-resolution XPS spectra of the F-hydrogels for C 1s, O 1s and N 1s, in which the peak for CO near 289 eV disappeared and the peak for C− O−C at 533.2 eV appeared,38 thus further verifying the ketalization reaction between the ketones of PVP and the alcoholic hydroxyl groups of PVA. To get more evidence about the internal structure of functional network hydrogels, the freezing/thawing experiment was carried out. Figure 2 shows the thawing processes for H-hydrogels, C-hydrogels and Fhydrogels, respectively. The thawing process for C-hydrogels lasted longer than that for H-hydrogels from the association effect of C6-OH groups on the CNCs molecular chains, which results in a larger amount of crystalline microdomains.30 By
Table 1. Dielectric Constant and Loss Tangent of PVA, PVP, and CNCs code
dielectric constant (ε)
loss tangent (tan δ)
PVA PVP CNCs
1.9−2.0
