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Tough and Recoverable Triple-Network Hydrogels Based on Multiple Pairs of Toughing Mechanisms With Excellent Ionic Conductivity as Stable Strain Sensors.

INTRODUCTION

Ionic conductive hydrogels with polymer networks swollen with electrolyte solutions have been explored for applications in flexible devices, including ionic skin sensors and hydrogel-based electrolytes for energy storage devices due to their excellent biocompatibility, intrinsic flexibility, and interesting electrochemical properties [1-10]. In particular, the resistance change of ionic gels is much lower than traditional electronic conductors under large strains which makes it very advantageous to be used as strain sensors [1]. The emerging application of hydrogels in flexible devices require it possess multifunctional properties including stable mechanical and functions under various deformations or external environments.

In recent years, one breakthrough to design tough hydrogels is by integrating sacrificial bonds to dissipate mechanical energy such as fracture of short polymer chains and reversible crosslinks (hydrogen bond, crystallization, and ionic interaction) into polymer networks [11-22]. The chemical and physical hybrid crosslinked double-network (DN) gels or single-network gels demonstrate not only tough but also self-recovery abilities from fatigue damage [21]. With the development of tough hydrogels, ionic conductive hydrogels with enhanced mechanical properties and other healable functions were developed by doping salt (e.g., NaCI and LiCl) in the current tough hydrogels or the salt ions also participate in physical crosslinking [2, 23-25]. Another approach is using the polyelectrolyte which contain a lot of charge groups to facilitate the ion conduction [26-28]. Suo and co-workers developed highly stretchable, tough alginate/PAM hybrid DN hydrogels with alginate network ionically crosslinked by various multivalent cations (e.g., [Ca.sup.2+], [Fe.sup.3+], and [Al.sup.3+]) [18, 29]. Zhang et al. synthesized the supramolecular sodium alginate nanofibrillar/PAM hydrogel ionic sensors with fracture strength of 0.75 MPa with low water content [30]. But the mechanical and sensing functions of alginate/PAM DN hydrogels may not stable to external various environments. For example, Suo found that the strength of alginate/PAM hybrid DN hydrogel is significantly reduced and the DN gel lost its energy dissipation mechanism when swells in a saline solution due to unzipping of the ionic crosslinks [18]. Practically, ionic conductive hydrogel sensors are required to be tough and recoverable from fatigue damage and keep mechanical and sensing functions stable under various loading conditions or external environments during applications. However, tough hydrogels and hydrogel ionic conductors developed so far rely on a single pair of toughing mechanisms to dissipate mechanical energy and maintain elasticity which may lose their toughness due to environmental and loading effects. For example, the change of a solution's pH or ionic strength may eliminate some physical crosslinking which may destroy mechanical and other functional performance.

Recently, we and other researchers show that it is an efficient way to fabricate very tough and stable hydrogels by integrating multi-bonds into multi-network to dissipate energy and maintain elasticity [31, 32]. Tough and rapidly recoverable PAM/agar/polyvinyl alcohol (PVA) triple-network (TN) hydrogels were fabricated by introducing different bonds with various strength into TNs in different length scale of hydrogels [32]. Therefore, it is very promising to design next-generation ionic conductive tough hydrogels which possess multifunctional properties by combination of multiple pairs of toughing mechanisms across multiple length scales to resist various environments and loading conditions. In this way, if one pair of mechanisms becomes ineffective, other pairs can still maintain high toughness and functions of the hydrogel devices.

Recently, Suo and co-workers designed another hybrid PVA/PAM DN hydrogels with hydrogen bonds and crystallization crosslinked PVA network and covalently crosslinked PAM network, which show an elastic modulus of 5 MPa and strength of 2.5 MPa and very fast self-recovery ability after deformation. In addition, PVA/PAM hybrid DN gels retain very stable in concentrated electrolyte solution [16]. We consider combining alginate/PAM and PVA/PAM DN hydrogels together to achieve a PVA/M-alginate/PAM TN hydrogel. It is expected that the TN hydrogels can simultaneously show robust elasticity, high stretchability, and fast self-recoverable or self-healing ability, as well as stable device functions.

Herein, extremely tough and flexible PVA/M-alginate/PAM hydrogels with excellent self-recoverable properties and high ionic conductivity were successfully fabricated by introducing multiple pairs of supramolecular sacrificial bonds in TN through three steps methods of in situ chemical crosslinking, freezing/thawing, and soaking in salt solution with multivalent cation. The first covalently crosslinked PAM network maintains the elasticity of TN hydrogels, physically crosslinked second PVA and third SA networks break to dissipate energy under deformation and can be recombination upon unloading. In addition, the presence of abundant ionic species in the TN hydrogels endows the tough and recoverable TN hydrogels high ionic conductivity. The multifunctional TN hydrogels were demonstrated as strain sensors which show high sensitivity. Moreover, such TN hydrogels with multi-bond crosslinking in different networks was supposed to guarantee high and stable mechanical and device performance under various environments and loading conditions compared to alginate/PAM and PVA/PAM DN hydrogels. We believe this work will inspire the development of tough hydrogels with multifunctional properties to be used in flexible devices.

EXPERIMENTAL

Materials

PVA particle (Mw ~ 74,800 g/mol and degree of hydrolysis ~99%), acrylamide (AAm, 99.0%), and NJV--methylene diacrylamide (MBA, 97.0%) were supplied by Aladdin (Shanghai, China). Alginic acid sodium salt (SA, from brown algae, viscosity [greater than or equal to] 2000cp) was supplied by Sigma. Potassium persulfate ([K.sub.2][S.sub.2][O.sub.8]) was purchased from Sinopharm Chemical Reagent Co., Ltd. All purchased chemicals and solvents were the analytical purity.

Preparation of Hydrogels

The TN hydrogels were prepared by a simple one-pot method of in situ chemical crosslinking, freezing/thawing, and soaking in multivalent cation salt solution. A certain amount of SA and PVA was dissolved in deionized water at 90[degrees]C for 1 h to obtain a homogeneous and transparent solution. Then, AAm monomers, MBA crosslinker, and KPS initiator were added into the above aqueous solution. The solution was stirred to dissolve all the reactants and was mixed homogeneously. Then, the solution was degassed for three times and subjected to thermo-polymerization at 60[degrees]C for 3 h to form the first network of PAM gel. The gel samples containing PVA and SA were frozen at -20[degrees]C for 3 h and then allowed to thaw at room temperature for 6 h. The freeze/thaw process was repeated two times. In the end, the formed hydrogel is immersed in a 0.3 M multivalent cation salt (NaCl, Ca[Cl.sub.2], Fe[Cl.sub.3]) aqueous solution for 3 h. For control tests, the hydrogels without SA or PVA were synthesized using the same experimental process. The obtained samples were named as M-alginate/PAM DN gels, PVA/PAM DN gels, and PVA/M-alginate/PAM TN gels. All samples with various compositions are listed in Table 1.

XRD and FTIR Characterization

The samples were cut into films and then dried at 50[degrees]C until all water was removed. XRD measurements for the dried films were carried out by a Rigaku D max/RB X'Pert instrument, operating with Cu K[alpha] radiation in the range of 10[degrees] [less than or equal to] 20 [less than or equal to] 90[degrees] at room temperature. The FTIR attenuated total refection (ATR) absorption spectra of the dried films were recorded using a Nicolet Nexus 670 spectrometer in the range of 4,000-600 [cm.sup.-1].

Swelling Properties of the TN Hydrogels in Different pH Condition

The degree of swelling of hydrogels was evaluated by gravimetric measurement. A hydrogel was first dried in a vacuum oven at 60[degrees]C. The dry weight of the hydrogel was measured, and then, the sample was allowed to swell in distilled water or solution with different pH condition at room temperature (25[degrees]C). The swollen sample was taken out at various time intervals with excess water removed by filter paper. Then, the weight of the swollen hydrogel was measured. The swelling measurements were continued until equilibrium swelling of the sample was achieved. The equilibrium degree of swelling of a fully swollen hydrogel was thus calculated using the equation: equilibrium degree of swelling = [W.sub.e] - [W.sub.0]/ [W.sub.0], where [W.sub.e] is the weight of hydrogel after achieving equilibrium swelling and [W.sub.0] is the weight of the dry hydrogel sample. The pH sensitivity of theTN hydrogel was investigated in acidic and alkaline media. Acidic and alkaline solutions were prepared using HCl (pH 0-6) and sodium hydroxide (pH 8-14), respectively. The pH values were measured with a pH meter. The equilibrium degree of swelling at each pH was calculated using the above equation.

Mechanical Properties

The mechanical properties of all gel samples were measured by an electromechanical universal testing machine (CMT2103). The tensile tests of cylindrically shaped gel specimens (5.5 mm in diameter; standard distance 10 mm) were measured at a tension speed of 100 mm/min. The compressive tests of cylindrically shaped gel samples (22 mm in diameter; length 10 mm) were conducted at a crosshead speed of 20 mm/min. The elastic modulus (E) was calculated in the range of 5%-10% strain from the loading curve. The fracture energy (toughness) was estimated using the area below the tensile stress-strain curve. [[sigma].sub.f] denotes as fracture strength, [[epsilon].sub.f] denotes as fracture strain, E denotes as elastic modulus, and W denotes as fracture energy.

For cyclic mechanical loading-unloading measurement, gel samples were firstly stretched to a strain ratio and then unloaded. The dissipated energy (Uhys) can be obtained by calculating the area of the stress-strain hysteresis loop. Cyclic loading at a strain ratio of 300% for tension and strain at 80% for compression was conducted five times with the same gel specimen to investigate the Mullins effect of gels. For successive cyclic loading measurement as the increased strain deformation, the loading-unloading cycles were repeatedly conducted on the same specimen with increased strain. The elastic modulus (E) at each cycle was calculated in the linear range from the loading curve. The dissipated energy (Uhys) during each loading cycle is calculated from the area between the nth loading-unloading curves. For self-recovery tests, the gel specimens were firstly stretched to a strain of 300% and then unloaded at the speed of 100 mm/min. Then, after resting for various times (0-30 min) at room temperature, the same loading and unloading were performed again. The recovery rate (%) is defined as the ratio of dissipated energy (or elastic modulus ([E.sub.t]) at various recovery times to that of the first loading cycle.

Ionic Conductivity and Electrical Test on Strain Sensors

Ionic conductivity was measured in a blocking-type cell where hydrogel membranes were sandwiched between two stainless steel electrodes by the electrochemical impedance spectroscopy (EIS) method. The impedance measurements were taken under an electrochemical working station CHI660E (Chenhua, Shanghai) in the frequency range of 0.01 Hz to 100 kHz with an open circuit potential mode using AC excitation voltage of 5 mV at room temperature. The ionic conductivity of hydrogels can be calculated from Eq. 1 from the known area and thickness of the hydrogel membranes:

[[sigma].sub.H] = l/[R.sub.b]A (1)

where l is the hydrogel membrane thickness (cm), A is the activity area ([cm.sup.2]), and [R.sub.b] is the resistance of the hydrogel membrane.

The real-time electrical signals of the hydrogels at different strains were recorded by using a CHI660E Electrochemical workstation. The relative change of the resistance was calculated by Ohm's law (R = U/I) on the basis of a constant voltage applied to the strain sensors and changes in the electrical current under different strains.

RESULTS AND DISCUSSION

Hydrogels Formation and Structure Analysis

Scheme 1 shows a one-pot method of preparing the TN hydrogels by in situ chemical crosslinking, freezing/thawing, and soaking in multivalent cation salt solution. PVA and SA were first dissolved by heating forming solution containing the linear-random coil configured macromolecules. Afterwards, AAm, MBA, and KPS were added into the solution and mixed homogeneously, and then, the loosely crosslinked PAM first network was formed by in situ thermal initiated chemical crosslinking of AAm and MBA. The resulting gel samples containing PVA and SA were subjected to two times freeze/thaw process, which led to formation of the second physically crosslinked network via a number of intermolecular hydrogen bonds and the formation of crystallites of PVA. In the end, the formed hydrogel immersed in an salt solution containing multivalent cations. During the immersion process, the [Ca.sup.2+] or [Fe.sup.3] +ions were progressively diffused into the SA component to replace the [Na.sup.+] ions out of the gels, establishing multivalent cation-carboxylate ionic triplet crosslinks resulting in the third alginate network [29]. A PVA/Na-alginate/PAM TN hydrogel is also synthesized by immersing the above formed hydrogel into NaCl solution. In this case, the chain entanglement between PVA and SA macromolecular chains increased due to the salting-out affect which enhance the mechanical properties of TN gels [33]. Because of the different gelation mechanism, PVA/M-alginate/PAM TN hydrogels with three asymmetric independent polymer networks can be successfully achieved. The successfully introduced multiple pairs of supramolecular physical bonds can work as sacrificial bonds to toughen the materials when hydrogel deforms. The broken bonds can reform upon unloading endowing the recovery of hydrogels' properties and functions with the assistance of the elastic covalent network.

Figure S1a (Supporting Information) showed FTIR-ATR spectra of of PVA/PAM DN, Ca-alginate/PAM DN, and PVA/Ca-alginate/PAM TN hydrogels. For PVA containing gels, the absorption at 1075 [cm.sup.-1] demonstrates the crystalline region of PVA. The broad band at 3300 [cm.sup.-1] is attributed to the combined absorption peaks of OH and N[H.sub.2] of the PVA, PAM, and alginate compositions. The bands at 1650 [cm.sup.-1] (C=O stretching) of the PAM and alginate were also detected. As shown in Supporting Information Fig. S1b, XRD analysis showed that the PVA/PAM DN gel samples prepared by freezing-thawing method exhibited a typical peak at 2[theta] = 19.1[degrees] and a weak peak at 2[theta] [approximately equal to]40[degrees], corresponding to the diffraction of PVA semi-crystalline [34, 35], PVA semi-crystalline can also be formed in PVA/Ca-alginate/PAM TN hydrogels. In addition, the decreased crystallization of PVA/Ca-alginate/PAM TN hydrogels was found as the weaken weak at 2[theta] = 19.7[degrees], which may due to molecular entanglement and hydrogen bonding between three networks [36]. The swelling behaviors of the prepared hydrogels were also studied through firstly freeze-dried the TN gel and then soaked in pure water at 25[degrees]C (Supporting Information Fig. S1c). It is well known that the equilibrium swelling degree of hydrogels depend on the crosslink densities of the polymer network. PVA/Ca-alginate/ PAM TN hydrogels show the lowest equilibrium swelling degree which indicate the increased crosslink densities compared with the PVA/PAM DN and Ca-alginate/PAM DN gels.

Mechanical Properties

As the Illustration in Fig. 1, PVA/Ca-alginate/PAM TN hydrogels were able to withstand different high-level deformations of knotted stretching (a and b) and twisted stretching (c and d) without breaking. Moreover, a small-cylindrical TN gels could bear 1,000 g weight. This indicates that the designed TN gels are very strong, tough, and flexible.

A comparative analysis of the compression and tensile properties of the PVA/PAM DN, Ca-alginate/PAM DN, and PVA/Ca-alginate/ PAM TN hydrogels was also carried out. As shown in Fig. 2a and c, PVA/PAM DN gels and Ca-alginate/PAM DN gels show low compression elastic modulus of 0.29 MPa and 0.42 MPa and compression strength of 1.14 MPa and 1.05 MPa, whereas PVA/Ca-alginate/PAM TN hydrogels exhibit greatly improved compression properties with compression elastic modulus of 0.82 MPa and compression stress of 2.29 MPa, respectively. As it can be seen from Fig. 2b and d, PVA/PAM gels (E of 34.58 kPa, [[sigma].sub.f] of 172.15 kPa, [[epsilon].sub.f] of 1,351%) and Ca-alginate/PAM DN gels (E of 109.49 kPa, [[sigma].sub.f] of 244.24 kPa, [[epsilon].sub.f] of 979%) exhibit good extensibility but poor strength. However, PVA/Ca-alginate/PAM TN hydrogels are both stiff and tough (E of 152.16 kPa, [[sigma].sub.f] of 512 kPa, [[epsilon].sub.f] of 871 %). Among all (Fig. 2b and e), the TN gels achieved the highest fracture energy of 3 MJ/[m.sup.3], whereas the fracture energy of Ca-alginate/PAM DN gels and PVA/PAM gels are 1.8 MJ/[m.sup.3] and 1.4 MJ/[m.sup.3], respectively.

Tensile properties after swelling in different PH condition of PVA/Ca-alginate/PAM TN hydrogel were also investigated (Supporting Information Fig. S4). It was found that tensile properties of TN hydrogel ([[sigma].sub.f] of 360 kPa, [[epsilon].sub.f] of 541%) decreased after equilibrium swelling compared to the as prepared TN gels ([[sigma].sub.f] of 512 kPa, [[epsilon].sub.f] of 871%) at pH = 7. Tensile properties of TN hydrogel also decreased seriously ([[sigma].sub.f] of 32 kPa, [[epsilon].sub.f] of 135%) at about pH = 13 as shown in the Supporting Information Fig. S4b. Most of the ionic crosslinked hydrogels are pH-responsive, and their water absorption capacity is controlled by the pH of the medium. This pH-responsive phenomenon has been observed and reported widely [37-40]. The negatively charged Ca-alginate network in the TN hydrogel is strongly sensitive to ions in environmental solution. In the low pH range, most of the carboxylate ions (-COO-) remain in the carboxylic acid (-COOH) form and the equilibrium degree of swelling of TN hydrogel is about 500% and does not change much from pH = 0 to pH = 11. However, it increased rapidly to 2,260% as the pH of the medium increased beyond pH 12 because of ionization of the -COOH group into -COO- (carboxylate ions), and the resulting ionic repulsion caused the sudden jump in swelling and the decrease of mechanical properties at around pH 13 of TN hydrogel.

The effect of the variation of MBA crosslinker dose on tensile properties and swelling of TN hydrogel was investigated, and the results were shown in the Supporting Information Fig. S3. It is shown that the tensile strength and the elastic modulus of TN hydrogel increased and the equilibrium degree of swelling of TN hydrogel decreased with the increase of MBA crosslinker dose. The equilibrium swelling degree and the elastic modulus of hydrogels depend on the crosslink densities of the polymer network is a general theory in hydrogel materials.

Furthermore, the effect of concentration of PVA in the second network and various multivalent cation in the third network on mechanical properties of PVA/Ca-alginate/PAM TN hydrogels were systematically investigated. As shown in Figs. 2 and 3, It can be seen that by introducing PVA network, PVA/Ca-alginate/PAM TN hydrogels exhibit much better mechanical properties than PVA/PAM DN gels (E of 34.58 kPa, [[sigma].sub.f] of 172.15 kPa, 1.8 MJ/[m.sup.3]) (PVA 0 wt%). As the concentration of PVA was increased from 2 to 6 wt%, tensile strength increased from 383.21 to 512.75 kPa, modulus (Fig. 3b and d) increased from 111.94 to 152.16 kPa, and fracture energy (Fig. 3e) increased from 2 to 3 MJ/[m.sup.3], respectively. In addition, the fracture strains of TN gels almost keep constant as the increasing of PVA content which are highly stretchable but become stronger. As shown in Fig. 3a and c, similar to the trend of tensile properties, the compressive strength and compressive modulus also increased with the increase of PVA content. The effects of different type of cations such as [Na.sup.+], [Ca.sup.2+], and [Fe.sup.3+] on tensile properties of TN gels were explored. It is found that the alginate third network of the TN gels crosslinked by trivalent cations ([Fe.sup.3+]) (E of 354.74 kPa, [[sigma].sub.f] of 929 kPa, [W.sub.f] of 4.7 MJ/[m.sup.3]) is much stronger and tougher than those crosslinked by divalent cations ([Ca.sup.2+]) (E of 152.27 kPa, [[sigma].sub.f] of 509.78 kPa, [W.sub.f] of 3 MJ/[m.sup.3]) and containing [Na.sup.+] cations (E of 35.26 kPa, [[sigma].sub.f] of 156.27 kPa, [W.sub.f] of 0.76 MJ/[m.sup.3]) (Fig. 4), which is consistent with the results of the reported DN gels [29]. This indicates cations that have different charge and ion radius will affect the strength of interaction differently. Trivalent cations could interact with three carboxylic groups of different alginate chains, lead to a larger coordination number ([(COO).sub.3]M)) and the strongest interaction.

The above-mentioned results indicate that the TN gels are much tougher than DN gels and also exhibited a wide range of tunable mechanical properties not only due to multi-bonds crosslinking in the unique TNs but also enormous hydrogen bonds and chain entanglement within the three networks resulting to a continuous and stronger network. Most importantly, the presence of multiple pairs of toughing mechanisms contributes to the improvement of the mechanical strength and toughness of PVA/Ca-alginate/PAM TN hydrogels. Under deformation, the first chemical-crosslinked PAM networks retained the original configuration of TN gels, whereas multiple pairs of hydrogen bonds as well as crystalline in PVA network and multivalent cation ionic crosslinking in alginate network break to dissipate energy and toughen the TN gels [21].

Hysteresis and Energy Dissipation

Tough hydrogels based on sacrificial bonds mechanism usually show excellent energy dissipation which can be reflected by the area of hysteresis in the stress-strain loading-unloading curve. As we can see from Fig. 5a and b, PVA/Ca-alginate/PAM TN gels exhibit the largest stress (458 kPa) and hysteresis loop, and the corresponding dissipated energy was 2,573 kJ/[m.sup.3] at a given strain of 600%, which is much higher than PVA/PAM DN gels (72 kPa and 475 kJ/[m.sup.3]) and Ca-alginate/PAM DN gels (250 kPa and 1,476 kJ/[m.sup.3]), respectively. The remarkable energy dissipation of PVA/Ca-alginate/PAM TN gels is attributed to the synergy of two mechanisms: unzipping the network of [Ca.sup.2+] crosslinks in the third alginate network and fracture of reversible non-covalent PVA second networks. The TN gels also show the highest energy dissipation of 416 kJ/[m.sup.3] among all the hydrogels under 80% strain compression small deformation (Supporting Information Fig. S2). Meanwhile, the hysteresis behavior of TN hydrogels with different PVA content (Fig. 5c and d) was investigated. As the content of PVA increase from 2% to 6% in TN hydrogels, the dissipated energy of TN gels increase from 1,660 to 2,720 kPa at strain of 600%. It indicates that the increase of PVA content can dissipate energy more effectively by introducing more reversible non-covalent bonds in TN gels.

Furthermore, cyclic tensile loading tests were performed on TN gels in a series of different maximum stains (Fig. 6). The hysteresis loops of TN gels increased with the increase of maximum strains. Consequently, the stress and dissipated energy increased from 253 kPa and 469 kJ/[m.sup.3] at 200% strain to 459 kPa and 2,573 kJ/[m.sup.3] at 600% strain. The strain-dependent energy dissipation behaviors and effective energy dissipation occurs at higher strain max of TN gels further indicate that the two types of reversible PVA and ionic crosslinked networks distribute across multiple length scales in TNs and break layer by layer at different strains. The combination of various weak and strong interactions endow TN gels with brilliant mechanical properties.

Self-Recovery of TN Gels under Fatigue Damage

The fatigue resistance and recovery ability of hybrid crosslinked TN hydrogels were expected under multiple cycles of compression and tension deformation due to the presence of multiple pairs of reversible interactions. The successive five compression cyclic loading cycles with no resting time on a TN gel at 80% strain was conducted, and the corresponding compression stress and dissipated energy of TN gels at different cycles were calculated (Fig. 7a and b). The gel exhibited a largest hysteresis loop with dissipated energy of 451.43 kJ/[m.sup.3] at 80% strain in the first compression loading cycle. The hysteresis loops of the TN gels decreased slightly after the first loading cycle but retained almost coincided (358 kJ/[m.sup.3]) of the following successive four compression cycles. Moreover, the successive five tension loading cycles on a TN gels at 300% strain and dissipated energies were also measured (Fig. 7c and d). It was found that the hysteresis loops and the maximum stress of the TN gels appeared a substantial decrease in the following tensile loading cycles. The decrease of energy dissipation and elastic modulus after the first loading cycle suggests a significant softening in the TN gels under large tensile strain deformation. These results indicate that the TN hydrogels do suffer from fatigue damage due to the internal fracture of the physical networks during the first loading process with large strain deformation and the damage could not be recovered without resting.

The internal damage process of TN gels was further investigated by conducting successive cyclic loading tests on the same PVA/Ca-alginate/PAM TN gels specimen at various strain ranging from 200% to 600% without resting. The dissipated energy of TN gels increased as the enhancement of deformation strain. It can be seen that any reloading curves of TN hydrogel always crossed the previous unloading curves (Fig. 8a), indicating that the network could partly be recovered based on the reversible interaction during the unloading process. Elastic modulus of TN hydrogels decreased from 114 kPa at strain of 200% to 43 kPa at strain of 600% (Fig. 8b). The softness of TN hydrogels were quantified by a parameter defined as (1 - [E.sub.X]/[E.sub.0]) x 100%, where [E.sub.X] and [E.sub.0] were elastic modulus at different strain and initial modulus, respectively [41]. Softness increased to 46% at strain of 400% and to 88% at strain of 600%.

It is expected that self-recovery properties of TN gels can be induced by reconstruction of the two reversible physically crosslinked networks with enough resting time. Cyclic loading tests were conducted at the strain of 300% with different resting time to evaluate self-recovery ability of the PVA/Ca-alginate/PAM TN gels at room temperature. It was found that the stress-strain curves can gradually recover to the original loading pathway with increase in resting time (Fig. 8c). The recovery efficiency of PVA/Ca-alginate/PAM TN gels could exceed 77% and 82% after 5 and 30 min, respectively (Fig. 8d). The results showed that the TN hydrogels exhibited very excellent self-recovery properties. The multiple physical interaction in TNs endow the recovery of PVA/Ca-alginate/PAM TN gels at room temperature from fatigue damage. The above results indicate that TN gels fabricated in this work have integrated high mechanical properties, including high strength, high toughness, and rapid self-recovery from fatigue damage.

Ionic Conductivity of PVA/Ca-Alginate/PAM TN Gels and Sensing Functions

The presence of abundant of [Ca.sup.2+] and [Cl.sup.-] ionic species endows the tough and recoverable PVA/Ca-alginate/PAM TN gels high ionic conductivity. The ionic conductivity of PVA/Ca-alginate/PAM TN gels was measured, and the results were shown in Fig. 9a. Ionic conductivity of PVA/Ca-alginate/PAM TN gels is 1.3 S/cm at room temperature. The conductive ability of TN gels was also illustrated by serving as the conductor and lighted a LED with high brightness (Fig. 9c). The multifunctional PVA/M-alginate/PAM TN hydrogels with very excellent mechanical properties and ionic conductivity were demonstrated to be flexible strain sensors. The resistance change ratio upon loading is defined as [DELTA]R/[R.sub.0] = [absolute value of [R.sub.0] - R]/[R.sub.0], where [R.sub.0] and R are the initial resistance and that under strain. As shown in Fig. 9b, the resistance of the PVA/Ca-alginate/PAM TN gels gradually increased as the strain increased, it can reach 500% under the strain of 300%. The resistance change ratio also increases if the gel was applied by the increased stepwise strain (Fig. 9c) which change the brightness of LED under different strain. At the same time, if the strain was maintained for a while, the resistance change ratio would also remain unchanged, which indicate that the TN gels ionic sensors exhibited excellent resistance stability under different strain. Furthermore, the ionic conductive TN gels were demonstrated as sensors to detect various human motions in real time (Fig. 9d-f). The TN hydrogels were attached onto the skin directly, and the resistance change ratios upon movements were recorded. It shows that the resistance change ratio increased as increasing of bending angle of finger. Cyclic change of [DELTA]R/[R.sub.0] values of TN hydrogel sensors can also be found as the wrist, and elbow were cyclically bent and stretched. These results indicate a strong correlation between the movements and the output signals of the multifunctional PVA/Malginate/PAM TN hydrogel sensors, which suggests possible quantitative detection of the movement of human bodies.

CONCLUSION

A multifunctional PVA/M-alginate/PAM hydrogel with very excellent mechanical properties and sensing functions was fabricated by introducing multiple pairs of toughing mechanisms into TN. The multiple supramolecular physical networks work as sacrificial networks to toughen the materials when hydrogel deforms. The broken bonds can reform upon unloading endow the recovery of hydrogels' properties and functions with the assistance of the elastic covalent network. The TN hydrogels exhibit a high fracture strength of 512 kPa, a fracture toughness of 3 MJ/[m.sup.3], and excellent recoverable from fatigue damage (~70% toughness recovery after 5 min resting at room temperature) properties. The presence of abundant ionic species endows the tough and recoverable TN hydrogels high ionic conductivity and high sensitivity as strain sensors. Moreover, such TN hydrogels with multi-bond crosslinking in three networks can potentially guarantee stable mechanical and sensor functions under various deformations or external environments compared to the DN candidates. This work provides a simple strategy for fabricating multifunctional hydrogels with high stability to fulfill its flexible devices applications.

ACKNOWLEDGMENTS

This work was supported by the Natural Science Foundation of Jiangsu Province (BK20160992), the Natural Science Foundation of the Jiangsu Higher Education Institutions (15KJB430018), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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Shuai Li, (1) Ximan Bu, (1) Linlin Wu (iD,1) Xiaofeng Ma, (2) Wenjing Diao, (1) Zhenzhen Zhuang, (1) Yongmin Zhou (1)

(1) College of Materials Science and Engineering, Nanjing Tech University, Nanjing, 210009, People's Republic of China (2) College of Science, Nanjing Forestry University, Nanjing, 210037, People's Republic of China

Additional Supporting Information may be found in the online version of this article.

Correspondence to: L. Wu; e-mail: llwu@njtech.edu.cn; and Y. Zhou; e-mail: 13951832092@163.com)

Contract grant sponsor: the Natural Science Foundation of Jiangsu Province; contract grant number: BK20160992. contract grant sponsor: the Natural Science Foundation of the Jiangsu Higher Education Institutions; contract grant number: 15KJB430018. contract grant sponsor: Priority Academic Program Development of Jiangsu Higher Education Institutions. DOI 10.1002/pen.25164

Published online in Wiley Online Library (wileyonlinelibrary.com).

Caption: SCH 1 Schematic fabrication of PVA/M-alginate/PAM TN hydrogels. [Colorfigurecanbeviewedatwileyonlinelibrary.com]

Caption: FIG. 1. Illustration of the mechanical properties of PVA/Ca-alginate/PAM TN hydrogels which could withstand different high-level deformations of knotted stretching (a and b) and twisted stretching (c and d) without breaking; (e) a small cylindrical TN gels could bear 1,000 g weight. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 2. Compression (a) and tensile properties (b) of (1) PVA/PAM DN, (2) Ca-alginate/PAM DN, and (3) PVA/Ca-alginate/PAM TN hydrogels. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 3. (a) Compression stress-strain curves and (b) the corresponding stress and elastic modulus, (c) tensile stress-strain curves and (d) the corresponding tensile strength, elastic modulus, and (e) fracture energy of the PVA/Ca-alginate/ PAM TN gels at different PVA concentrations. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 4. (a) Tensile stress-strain curves and (b) the corresponding tensile strength, elastic modulus and (c) fracture energy of the PVA/Ca-alginate/PAM TN gels with various multivalent cation crosslinking. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 5. (a) Loading-unloading curves and (b) the corresponding stress and dissipated energy of various hydrogels performed at 600% tensile strain, (c) Loading-unloading curves and (d) the corresponding stress and dissipated energy of PVA/Caalginate/PAM TN hydrogels with different PVA concentration performed at 600% strain. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 6. (a) Cyclic loading curves and (b) the corresponding stress and dissipated energy of PVA/Ca- alginate/PAM TN gels at different maximum stretching ratio. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 7. (a) Cyclic compression loading curves without resting and (b) the corresponding stress and dissipated energy of PVA/Ca-alginate/PAM TN gels at 80% strain and (c) cyclic tensile loading curves without resting and (b) the corresponding stress and dissipated energy of PVA/Ca-alginate/PAM TN gels at 300% strain. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 8. (a) Successive loading-unloading curves of the same PVA/Ca-alginate/PAM TN gel at different increased strain without resting between any two consecutive loadings; (b) elastic modulus and softness of TN gels at different strain, (c) Self-recovery of TN gel at different resting times at room temperature; (d) toughness (Uhys) recovery rates of the TN gel in response to different resting times at room temperature. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 9. Electronic properties of strain sensors based on PVA/Ca-alginate/PAM TN gels, (a) Electrochemical impedance spectra and the ionic conductivity of the TN gel. (b) The dependence of resistance change ratio of hydrogels on tensile strain, (c) The change of resistance and the brightness of a LED light at different increased stepwise strain. The relative resistance changes versus time based on hydrogel sensors (d) on the finger bending with several angles, (e) on the wrist bending with cyclically bent and stretched; (f) on the elbow bending with cyclically bent and stretched. [Color figure can be viewed at wileyonlinelibrary.com]
TABLE 1. Hydrogels with various compositions.

Sample                 Network    AM (wt%)    SA (wt%)
                      structure

Ca-alginate/PAM          DN          20           2
PVA/PAM                  DN          20           0
PVA/Na-alginate/PAM      TN          20           2
PVA/Fe-alginate/PAM      TN          20           2
PVA/Ca-alginate/PAM      TN          20           2
PVA/Ca-alginate/PAM      TN          20           2
PVA/Ca-alginate/PAM      TN          20           2

Sample                PVA (wt%)   MBA (wt%)   KPS (wt%)

Ca-alginate/PAM           0        0.0133       0.11
PVA/PAM                   6        0.0133       0.11
PVA/Na-alginate/PAM       6        0.0133       0.11
PVA/Fe-alginate/PAM       6        0.0133       0.11
PVA/Ca-alginate/PAM       2        0.0133       0.11
PVA/Ca-alginate/PAM       4        0.0133       0.11
PVA/Ca-alginate/PAM       6        0.0133       0.11
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Author:Li, Shuai; Bu, Ximan; Wu, Linlin; Ma, Xiaofeng; Diao, Wenjing; Zhuang, Zhenzhen; Zhou, Yongmin
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:9CHIN
Date:Aug 1, 2019
Words:6656
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