Ultrastretchable and Self-Healing Double-Network Hydrogel for 3D

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Ultra-stretchable and Self-healing Double Network Hydrogel for 3D Printing and Strain Sensor Sijun Liu, and Lin Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07445 • Publication Date (Web): 14 Jul 2017 Downloaded from http://pubs.acs.org on July 15, 2017

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ACS Applied Materials & Interfaces

Ultra-stretchable and Self-healing Double Network Hydrogel for 3D Printing and Strain Sensor Sijun Liu, Lin Li* School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore E-mail: [email protected]

Abstract On the basis of the thermoreversible sol-gel transition behavior of κ-carrageenan in water, a double network (DN) hydrogel has been fabricated by combining an ionically crosslinked κ-carrageenan network with a covalently crosslinked polyacrylamide (PAAm) network. The κ-carrageenan/PAAm DN hydrogel demonstrated the excellent recoverability and significant self-healing capability (even when notched). More importantly, the warm pregel solution of κ-carrageenan/AAm is able to be used as an ink of a 3D printer to print complex 3D structures with remarkable mechanical strength after UV-exposure. Furthermore, the κ-carrageenan/PAAm DN hydrogel exhibited a great strain sensitivity with a gauge factor of 0.63 at the strain of 1,000 %, and thus the hydrogel can be used as sensitive strain sensors for applications in robotics and human motion detection.

Keywords: 3D printing, strain sensor, recoverability, self-healing, double network hydrogel

1. Introduction Significant efforts have been devoted to develop highly stretchable hybrid double network (DN) hydrogels with improved mechanical properties by combining noncovalent and covalent cross-links in two networks. For example, Sun et al fabricated the

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alginate/polyacrylamide (PAAm) DN hydrogel by combining a Ca2+ crosslinked alginate network with a covalently crosslinked PAAm network, which exhibits a remarkable fracture toughness of ∼9000 J∙m-2 and a notch insensitivity.1 Chen et al prepared the agar/PAAm DN hydrogel with an excellent recoverability.2-3 Although a great success in developing stretchable DN hydrogels has been achieved, it is hard to fabricate DN hydrogel products with complex 3D structures because multiple steps and rigorous conditions are needed in the fabricaiton of DN hydrogels. 3D printing is an additive manufacturing process aimed at rapid production of complex 3D structures with high shape fidelity.4-5 Although it was first proposed by Hull in 1986,6 the introduction of DN hydrogels to 3D printing happened recently, where Hong et al constructed the stretchable alginate/polyethylene glycol (PEG) DN hydrogels into human organs, ear and nose, by using an extrusion-based 3D printing technique.7 However, in order to endow the printability of the alginate/PEG DN hydrogel, an inorganic nanofiller, nanoclay, had to be added into the pre-gel solution of alginate/PEG to control viscosity and shear thinning property. The presence of inorganic particles increases the complexity of 3D printing. For example, the excellent dispersion of inorganic particles is required to avoid clogging of the nozzle. Several methods have been developed to print hydrogels, including extrusion printing,8-9 stereolithography with UV photo-polymerization,10 fused deposition,11-12 and inkjet printing.13-14 It is possible to print stretchable DN hydrogels into complex 3D structures using an appropriate method. In the past decade, wearable and human friendly strain sensors for transducing mechanical deformation into electrical signals have been vigorously developed,15 which are promising for wide applications in robotics,16 sports,17 health monitor,18 and electronic skin,19 etc. Conventional metallic strain sensors are flexible, but they can sustain strains of only about 5 % due to the very limited stretchability of metal.20-21 Recently, several representative strain sensors using semiconductor,22-23 graphene,24-25 carbon nanotubes,26 conductive


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polymer,27 as conductive materials combining with elastomeric substrates have been successfully fabricated. Although most of these strain sensors can be stretched to a strain of over 200 %, the breaking of contact condition of conductors during deformation usually leads to unstability of strain sensors. For example, Cai et al prepared a self-healing piezoresistive strain sensor using single wall carbon-nanotube as conductor, which showed a large hystersis in the response under the stretching-releasing cycles.28 It is worth noting that the important characteristics of strain sensors are their capability to repeatedly detect the change of electrocal signal under various extensions, and without incurring damage. Therefore, it is still a challenge to fabricate strain sensors with high stretchability and strain sensitivity for applications in robotics and human motion detection. Recently, we have chosen a polysaccharide, κ-carrageenan, to make a physical and thermoreversible network to synthesize a κ-carrageenan/PAAm DN hydrogel by means of one-pot method, and the resulting DN hydrogels exhibited the extraordinary mechanical properties and recoverability.29 In this work, we further study the toughening mechanism, self-healing

property,

3D

printing

capability and

strain

sensitivity of

our

κ-

carrageenan/PAAm DN hydrogel.

2. Experimental Section κ-Carrageenan/PAAm DN hydrogel fabrication All reagents, κ-carrageenan with Mw ≈ 3.0 × 105 g/mol, potassium chloride (KCl), acrylamide (AAm), N,N’-methylenebisacrylamide (MBA) used as a chemical crosslinker, and 2-hydroxy-4’-(2-hydroxyethoxy)-2-methylpropiophenone used as a UV-initiator were purchased from Sigma-Aldrich, Singapore, and they were added into deionized water with continuous magnetic stirring at 90 oC for 5 h, where the total concentration of κ-carrageenan and AAm was fixed at 18 wt%. The resulting solution was injected into a glass mold (length



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× width × height = 100 mm × 100 mm × 2.5 mm) and cooled at 4 °C for 30 min to form the κ-carrageenan network first. Subsequently, the glass mold was placed under a UV lamp (wavelength of 365 nm and intensity of 8 mW/cm2) to carry out the photo-polymerization reaction for 1 h to produce the κ-carrageenan/PAAm DN hydrogel. After the photopolymerization, the DN hydrogels were removed from the glass mold, and then used for mechanical and other tests.

Mechanical tests For tensile tests, the hydrogel samples were cut into a dumbbell shape, with a gauge length of 30 mm, a width of 5 mm, and a thickness of 2.5 mm. The tensile tests were carried out at 100 mm/min and room temperature using an Instron machine (Model 5567). The elastic modulus, E, was determined by the average slope over 10 – 30 % of strain from the stress-strain curve. In order to investigate the fracture process and toughening mechanism of the κ-carrageenan/PAAm DN hydrogel, the following tests were carried out. (1) The loadingunloading cycles were applied to the κ-carrageenan SN hydrogel, the PAAm SN hydrogel, and the κ-carrageenan/PAAm DN hydrogel under the tensile strain lower than their corresponding yielding strains. (2) The successive and progressive stretches: the specimens were stretched to various strains in the first loading, and then relaxed to zero force, followed by the second loading. E2nd/E1nd and Γ2nd/Γ1nd were determined and used to evaluate the effect of various stretches on the fracture process and toughening mechanism for the κcarrageenan/PAAm DN hydrogel. For the recovery experiments, the notched samples were tested by a cycle of loading-unloading at a fixed strain (ε = 400%), and then the deformed and relaxed notched samples were sealed in a polyethylene bag and stored in a water bath of 90 oC. Finally, the specimens were taken out at different time intervals and cooled down to room temperature for tensile tests again.


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Self-healing The DN hydrogel with a cylindrical shape was cut into eight small fractions, and the hydrogels with sheet and dumbbell shapes were cut into two pieces. The cut surfaces were brought together to form a contact and then sealed in polyethylene bags and immersed into water baths of various temperatures for a fixed time. Subsequently, the healed samples were taken out for tensile and other tests.

Rheological tests The rheological measurements were performed on a rotational rheometer (DHR, TA Instruments, USA) with a parallel plate geometry of 40 mm in diameter and a gap of 0.55 mm. Strain sweeps in the range of 0.1 - 100 % at frequencies of 0.1 - 2 Hz were carried out to determine the linear viscoelastic range of the samples. A low-viscosity silicone oil was placed to the sample’s perimeter to prevent water evaporation during a measurement. Rheological temperature sweeps at a scanning rate of 1 oC/min and time sweeps in the angular frequency of 1 Hz and oscillatory strain of 2.0 % were carried out to investigate the gelation behavior of the κ-carrageenan/AAm pre-gel solution.

3D printing A 3D bioprinter (Biofactory, regenHU Ltd., Switzerland) was used for printing of the κcarrageenan/PAAm DN hydrogel. This 3D bioprinter is equipped with a pressure-controlled cartridge connected with a thermal controller. The ink (i.e. a κ-carrageenan/AAm pre-gel solution) was loaded into a sterile cartridge. In order to remain the sol state of the pre-gel solution inside the cartridge during the printing, a syringe heater was used to wrap the cartridge. The printability of the κ-carrageenan/AAm pre-gel solution was evaluated using a



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combination of these parameters: applied pressure (0.1 – 5 bars), temperature (40 – 70 oC), and platform moving speed (50 – 600 mm/s) with a constant nozzle (the tip size of 300 μm). To demonstrate its ability to fabricate a complex construct, the multilayered 3D structures with a hollow triangular prism and a hollow cube were fabricated respectively by 3D printing. Finally, the printed structures were UV cured for 1 h to photo-polymerize AAm.

Fabrication of a strain sensor A strain sensor was assembled by using the κ-carrageenan/PAAm DN hydrogel as conductor, VHB tape as elastomeric substrates and encapsulant. The top and bottom of the sensor were insulated using VHB to prevent water evaporation of the hydrogel. Before the strain sensor was assembled, the surfaces of the hydrogel were dried with N2 gas for 10 s to improve the adhesion between VHB and hydrogel. For the human motion detection, the sensors were connected to a Keithley electrometer (Model 420) to measure the resistances.

3. Results and Discussion 3.1. Mechanical properties of the DN hydrogel We recently prepared the hybrid κ-carrageenan/PAAm DN hydrogel by combining an ionically crosslinked κ-carrageenan network with a covalently crosslinked PAAm network, which exhibited excellent mechenical properties and recoverability.29 Herein, we further investigated the effects of a physical crosslinker (KCl) and a chemical crosslinker (MBA) on the mechanical properties of the κ-carrageenan/PAAm DN hydrogel, and explored the toughening mechanism. It was found that the elastic modulus (E) of the DN hydrogel increased with increasing K+ and MBA contents, respectively (Figures S1 and S2, Supporting Information), suggesting that the mechanical strength of the DN hydrogel comes from the contribution of both the κ-carrageenan network and the PAAm network. In addition, KCl and


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MBA also greatly affected the toughness of the DN hydrogel, and the highest fracture energy was obtained at 6 wt% KCl relative to κ-carrageenan and 0.05 wt% MBA based on AAm. After the peak values the fracture energy decreased with increasing KCl and MBA contents. Therefore, in this study, unless otherwise mentioned, we mainly focus on the best κcarrageenan/PAAm DN hydrogel (i.e. weight ratio of κ-carrageenan to AAm = 2:16, 6 wt% KCl relative to κ-carrageenan, 0.05 wt% MBA and 3 wt% UV-initiator based on AAm) that showed the maximum fracture energy of 6,150 J∙m-2 (Figures S1 and S2, Supporting Information), which is higher than the value (~ 1,000 J∙m-2) of articular cartilage. Through the loading and unloading experiments, we further evaluated the fracture process and toughening mechanism of the κ-carrageenan/PAAm DN hydrogel. At the small tensile strains (less than the yielding strain of DN hydrogel), the κ-carrageenan single network (SN) hydrogel exhibited a pronounced hysteresis and retained a significant permanent deformation after unloading. The κ-carrageenan/PAAm DN hydrogel also showed a pronounced hysteresis even though a negligible hysteresis was observed in the PAAm SN hydrogel (Figure 1a), suggesting that unzipping of the double-helical aggregates took place at small strains. After the yielding point, the fracture process of DN hydrogel was studied by loading several samples to different tensile strains of the first loading-unloading cycle in the two successive loading-unloading cycle measurements. The ratios of elastic modulus E2nd and fracture energy Γ2nd in the second loading-unloading cycle to those in the first loadingunloading cycle were used to evaluate the breaking extent of the κ-carrageenan network. The remarkable decreases in E2nd/ E1st and Γ2nd/Γ1st with increasing strain of the first loadingunloading cycle indicate that the elastically effective κ-carrageenan aggregates were broken as the extension increased (Figure 1b). Related to the previous results that the elastic modulus of the DN hydrogel was higher than the sum of the elastic moduli of the κ-carrageenan and PAAm SN hydrogels,[29] it can be inferred that the κ-carrageenan/PAAm DN hydrogel is not



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only composed of simple interpenetration of two networks, but also has some strong interactions such as hydrogen bonding between κ-carrageenan and PAAm. This inference has been proved by FTIR (Figure S3, Supporting Information) where the symmetrical stretching vibration of -OH groups of κ-carrageenan at 3352 cm-1 in the κ-carrageenan/PAAm gel was broadened and enhanced as compared to the κ-carrageenan SN gel as well as the characteristic peaks of PAAm in the κ-carrageenan/PAAm gel shifted to lower wave-numbers in contrast to the PAAm SN gel, suggesting the formation of the hydrogen bonding between NH2 groups on the PAAm chains and -OH groups on the κ-carrageenan chains. The κ-carrageenan/PAAm DN hydrogel also exhibited the notch insensitivity (Figure S4, Supporting Information). As compared to the un-notched sample, the tensile strain slightly decreased from 2,000 to 1,760 % for the edge-notched sample and to 1,630 % for the centernotched sample. The notch blunting behavior is probably due to the synergic effect of two networks, where the PAAm network may bridge the crack and stabilizes the deformation while the double-helical aggregates in the κ-carrageenan network are unzipped and κcarrageenan chains are pulled out from double helices to dissipate energy. As a result, the stress concentration at the notch is weakened. On the other hand, it is well known that the energy dissipation (U) in the hybrid DN hydrogels mainly relies on the physically crosslinked network.30 The dissipated energy of such a DN hydrogel is recoverable after relaxed. In our previous work,29 we illustrated this property by stretching a un-notched sample to a strain of 400 %, and the elastic modulus and energy dissipation were recovered to 100 % and 98 % after the deformed and relaxed DN samples were stored at 90 °C for 20 min. Here, we further investigate the recoverability of a notched sample using the same method. As shown in Figure 1c, the elastic modulus and energy dissipation could be recovered to 91 % and 84 % respectively even for the notched sample, suggesting that the κ-carrageenan/PAAm DN hydrogel possesses an excellent recoverable capability.


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Figure 1. (a) Curves of loading-unloading cycles for the κ-carrageenan SN hydrogel, the PAAm SN hydrogel, and the κ-carrageenan/PAAm DN hydrogel when the applied strain is lower than the yielding strain of DN hydrogel. (b) Elastic modulus E2nd and fracture energy Γ2nd of the successive second loading normalized by that of the first loading with various strains for the κcarrageenan/PAAm DN hydrogel. (c) Ratios of elastic modulus and energy dissipation (U) during the second loading-unloading cycle to those during the first loading-unloading cycle for the relaxed and notched samples kept at 90 oC with various storage times. For the κ-carrageenan and PAAm SN hydrogels, the contents of κ-carrageenan and AAm were kept the same as those in the DN hydrogel.

3.2. Self-healing of the DN hydrogel Because of the thermoresponsive nature of κ-carrageenan in the DN hydrogel, a selfhealing capability is expected. However, the self-healing at room temperature was not observed because the double helices of κ-carrageenan are able to be dissociated into single chains only upon heating (above the gel-sol transition temperature) and then the κcarrageenan chains can be re-associated into double helices upon cooling. Thus, the selfhealing could be observed after the cut samples were stored at a high temperature above the gel-sol transition temperature for a given time. The experiments were conducted as follows: the DN hydrogel samples with a cylinder shape of diameter 10 mm and a sheet shape of the dimension of 50 mm × 24 mm × 2.5 mm were respectively cut into eight small columns and two pieces, and four small columns and one sheet were stained with methylene blue in order to distinguish them easily (Figure 2a1 and 2b1). Subsequently, the cut surfaces were brought



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together to form a contact and sealed into a polyethylene bag, and then submerged into a water bath of 90 oC and stored for 20 min. The cut samples were found to be well healed (Figure 2a2 and 2b2), where the self-healed cylinder is able to bridge between two "piers" to support the self-weight without any visible damage. Furthermore, the self-healed sheet could withstand a weight of 250 g as shown in Figure 2c, suggesting that our κ-carrageenan/PAAm DN hydrogel possesses an excellent self-healing capability. The self-healing property of the κ-carrageenan/PAAm DN hydrogel can also be demonstrated on a complete circuit composed of a LED indicator and the DN hydrogel as the conductor. The LED indicator was successfully lighted when a voltage of 5 V was applied (Figure 2d1). The LED indicator was extinguished when the DN hydrogel was cut off (Figure 2d2). If the self-healed DN hydrogel was used as the conductor, the circuit was restored and the LED indicator was lighted again (Figure 2d3). The self-healing efficiency of the DN hydrogel was defined by Rs/Ro, where Rs is the conductivity of the self-healed hydrogel and Ro is the original conductivity. It was found that Rs/Ro is as high as 99.2 %, which is comparable to the self-healed and conductively reduced graphene oxide/sodium polyacrylate (rGO/SP) nanocomposite hydrogel,35 indicating that the κ-carrageenan/PAAm DN hydrogel possesses an extraordinary electrical restoration performance. In addition, we performed a series of self-healing experiments to quantitatively examine the effects of storage time and water bath temperature on the mechanical properties of self-healed DN hydrogels. It is observed that the tensile strength and elongation at break increased with increasing storage time when the cut samples were stored at 90 oC (Figure 2e). However, the effect of water bath temperature at a fixed storage time 20 min on the mechanical properties of self-healed DN hydrogel is not obvious (Figure S5, Supporting Infomration).


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Figure 2. (a) Freshly cut DN hydrogel samples with the small columns, and the self-healed cylinder can bridge between two "piers" to support the self-weight. (b) Freshly cut DN hydrogel samples with sheet shape, and the self-healed sheet shaped sample. (c) Self-healed sheet shaped sample with a width of 24 mm and a thickness of 2.5 mm could hold a weight of 250 g. (d) Circuit comprises an LED indicator connected by undamaged, cut and self-healed DN hydrogel sheets, respectively. (e) Effect of storage time on the stress-strain curves of self-healed dumbbell-shaped samples kept at the water bath of 90 oC, and dependences of maximum stress and elongation at break on storage time.

3.3. 3D printing of the DN hydrogel Next, we demonstrate the capability of printing the κ-carrageenan/PAAm DN hydrogel into various complex 3D structures. In order to print 3D structures of a thermoresponsive hydrogel, the following points are critical. First, in order to ensure the string shape of pre-gel solution during extrusion, the precise temperature control in the syringe was needed. Second, in order to have high shape fidelity, the gelation process is required to be quick enough to 11 ACS Paragon Plus Environment


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prevent the ink from spreading once deposited on the substrate. Here, we evaluated the 3D printability of the pre-gel solution of κ-carrageenan/AAm by rheological experiments. As shown in Figure 3a, the κ-carrageenan/AAm solution exhibits a liquid state at high temperatures, where G" > G'. Upon cooling, G' and G" increase slightly with decreasing temperature. When the temperature was decreased closely to a critical value, G' and G" increase sharply, and then G' exceeds G" at 44.7 oC due to the formation of a gel network. The κ-carrageenan/AAm solution also showed a quick gelation rate (Figure 3b), which allowed the κ-carrageenan/AAm solution to form a gelled string on the substrate quickly. The significant increase in viscosity (0.7 to 2,000 Pa·s) was observed during the sol-gel transition (Figure 3c), which is desirable for the fabrication of precise 3D structures. In contrast to the κ-carrageenan/AAm system, however, the agar/AAm solution exhibited a larger gelation temperature range from 90 to 30 oC upon cooling,31 so that it is hard to precisely control printed structures when the agar/AAm solution is used as an ink of a 3D printer. For our κcarrageenan/PAAm DN hydrogel, the quick sol-gel transition and the narrow gelation temperature range allow the κ-carrageenan/AAm solution to be directly used as an ideal ink to print this DN hydrogel into complex 3D constructs.

Figure 3. (a) Storage modulus G' and loss modulus G" at an angular frequency of 1 Hz and a oscillatory strain of 2 % during cooling from 70 to 20 oC at 1 oC/min for the pre-gel solution of κcarrageenan/AAm. (b) Dynamic moduli G' and G" and (c) complex viscosity η* as a function of time during annealing at 25 oC. In the pre-gel solution of κ-carrageenan/AAm, the weight ratio of κ-


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carrageenan to AAm was fixed at 3:15, and the content of KCl was 6 wt% relative to κ-carrageenan, while the contents of MBA and UV-initiator were 0.05 wt% and 3 wt% based on AAm, respectively.

Because the pre-gel solution of κ-carrageenan/AAm has a quick sol-gel transition and high gel strength, we chose the κ-carrageenan/AAm solution containing K+, MBA and UVinitiator as the ink for our 3D printer (Biofactory, regenHU Ltd., Switzerland). The printing process is described in the Experimental Section. The κ-carrageenan/AAm hydrogel can be printed into various shapes, such as a mesh pattern with 5 layers (Figure S6, Supporting Information), a hollow triangular prism and a hollow cube (Figure 4a), a cone pattern (Figure 4b) and a dumbbell shape (Figure 4d). The success in printing the hollow triangular prism and the hollow cube with a height of about 8 mm indicates that the pre-gel solution of κcarrageenan/AAm is an ideal ink for the fabrication of 3D constructs with relatively high strength that can support their self-weights during printing. The printed DN hydrogels also exhibited a high deformability and toughness after UV-exposure for 1 h. For example, after the printed cone sample underwent a 90 % compressive strain, the sample regained 96 % of its original height within 10 min of unloading (Figure 4c). The printed dumbbell-shaped sample could be uniaxially stretched to 14 times of its original length, accompanying with a necking phenomenon (Figure 4e). In addition, we further investigated the recoverability of the printed dumbbell-shaped sample. It was found that the elastic modulus and energy dissipation

recovered to 92 % and 85 % respectively after the deformed and relaxed

dumbbell-shaped sample was stored at 90 oC for 20 min (Figure 4f). These results clearly indicate that the κ-carrageenan/PAAm DN hydrogel not only displays the remarkable selfhealing capability, but also can be printed into various complex 3D structures with high mechamical stength and recoverability.



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Figure 4. 3D printed κ-carrageenan/PAAm DN hydrogel, (a) hollow triangular prism and cube 3D patterns, and (b) a cone pattern. (c) The printed cone sample could return to its original shape after underwent a compressive strain of 90 % and relaxation. (d) A printed dumbbell-shaped sample. (e) Tensile test of the printed dumbbell-shaped sample. (f) Stress-strain loops for a freshly printed dumbbell-shaped sample (the black loop) after cured for 1 h and the recovered one (the pink loop) after relaxed and stored at 90 oC for 20 min. For the ink of 3D printing, the weight ratio of κcarrageenan to AAm was fixed at 3:15. The content of KCl was fixed at 6 wt% relative to κcarrageenan. The contents of MBA and UV-initiator were fixed at 0.05 wt% and 3 wt% based on AAm, respectively.

3.4. A DN hydrogel based strain sensor From Figure 2d, we know that the κ-carrageenan/PAAm DN hydrogel is a good conductor. Herein, the resistance tests were carried out to further investigate the strain sensitivity of the κ-carrageenan/PAAm DN hydrogel. Figure 5a shows the dependence of resistance of the κ-carrageenan/PAAm DN hydrogel on applied strains under the stretching14 ACS Paragon Plus Environment

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releasing cycles. It is obvious that the resistance of the DN hydrogel increased with increasing strain, and then the resistance gradually decreased during releasing. The variation of resistance was also visually observed using a LED connected with the DN hydrogel sheet in a circuit. Obviously, the LED darkened with increasing strain, and then became brighter after the stretching was released as shown in the insets of Figure 5a. The gauge factor (S) is defined as the ratio of relative resistance change (∆R/R0) to applied strain (ε), S = (∆R/R0)/ε = [(R-R0)/R0]/ε, to evaluate the strain sensitivity of resistance for the κ-carrageenan/PAAm DN hydrogel, where R0 is the resistance at 0 % strain and R is the resistance under stretching. As shown in Figure 5b, the gauge factor of the DN hydrogel was 0.23 at the strain of 100 % and increased to 0.63 as the strain was increased to 1,000 %. Thus the change of gauge factor with tensile strain can be greatly fitted into a parabolic equation, S = 5.49×10-7ε2 – 1.94×10-4ε + 0.25

(1)

Usually, the metallic materials have a higher gauge factor. However, these materials only sustain small stretchability (less than 5%).32 Although, the gauge factor of the κcarrageenan/PAAm DN hydrogel is lower than that of conventional metal based strain sensors, it is still higher than 0.06 at 200 % strain for the piezoresistive electronic strain sensor33 and 0.348 at 700 % strain for the capacitive soft strain sensor,34 indicating that our κcarrageenan/PAAm DN hydrogel possesses an excellent strain sensitivity. On the other hand, because the deformation after the stretching-releasing cycles with the large tensile strains could not be recovered immediately at room temperature, a hysteresis (increase in resistance after releasing) was observed as shown in Figure 5a. If the resistance change, (Rr – R0)/R0, was used to investigate the effect of tensile strain, where Rr is the resistance after releasing. It can be found from Figure 5c that the resistance change increased sharply with tensile strain from 100 to 200 %, increased slightly from 200 to 400 %, and then was almost kept constant after 400 %. It is interesting to find that the strain range (100 ~



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200 %) where the resistance change increased sharply was almost consistent with that where the necking phenomenon took place. This is because the deformation induced by yielding led to the decrease in the amount of ions per unit time passing through the cross-section of hydrogel sample. However, even in this case, the resistance change, [(55.2 – 50.4)/50.4] = 9.6 %, is less than 10 % at the strain of 1000 %. Furthermove, the stability of the DN hydrogel was investigated by repeatedly applying the strains of 100% and 300%, and the resistance curves responding to the repeated stretching-releasing cycles were recorded. The curves of resistance vs time almost overlap for the 100 % strain, and the resistance change of the DN hydrogel remains almost constant within 10 cycles (Figure 5d). In the case of 300 % strain, the curves of resistance vs time overlap from the second cycle, and the increase of resistance after releasing was observed from the second cycle (Figure S7, Supporting Information). However, the resistance change is less than 5 % within the first 10 cycles, suggesting that the κ-carrageenan/PAAm DN hydrogel has a good conductive stability.


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Figure 5. (a) Dependence of resistance of the κ-carrageenan/PAAm DN hydrogel on applied strain under the stretching-releasing cycles with various tensile strains, and the inset pictures were the luminance variations of a LED in the stretching-releasing cycle. (b) Effects of various tensile strains on the gauge factor. (c) Effects of various tensile strains on the resistance change induced by the hysteresis in the response in the stretching-releasing cycle. (d) Stability of the DN hydrogel by repeatedly applying strain of 100 % for 10 cycles.

To evaluate the performances of the κ-carrageenan/PAAm DN hydrogel as a stretchable electronic device in a natural environment, we assembled a wearable strain sensor using the κ-carrageenan/PAAm DN hydrogel as conductor, and Scotch tape (VHB 4010, 3M) as elastomeric substrates and encapsulant. The κ-carrageenan/PAAm DN hydrogel based strain sensors were directly attached on the skin to detect the bending and stretching of a human body. Figures 6a and 6b illustrate the detection of the forefinger’s bending. It can be seen that the resistance of the strain sensor raised to different levels with increasing the bending degree of the forefinger. Importantly, the resistance immediately fell back to the original level when the forefinger was completely stretched to relax the deformation of the sensor. Furthermore, the κ-carrageenan/PAAm DN hydrogel sample with the S shape fabricated by 3D printing was also assembled into the strain senosr to detect the motion of a human body (Figure S8, Supporting Information). It is observed that the printed DN hdyrogel based strain sensor is capable of distinguishing the different bending angles of the wrist. When the wrist was held at a certain angle, the resistance almost remained at a constant value and returned to the original value after straightening the wrist (Figure 6c). In addtion, we further challenged the self-healed DN hydrogel that was used as conductor to be assembled into a strain sensor. It can be seen that the self-healed DN hydrogel based strain sensor responded to the motion of the thumb finger rapidly without visible damage when the index finger was repeatedly bent



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(Figure 6d). These results indicate that our κ-carrageenan/PAAm DN hydrogel is an ideal candidate for fabricating the stretchable and wearable electronic skin.

Figure 6. Monitoring various human motion in real time, (a) the κ-carrageenan/PAAm DN hydrogel based strain sensor fixed on forefinger of a human hand to monitor its bending and (b) its change of resistance with time. (c) Change of resistance with time when the 3D printed DN hydrogel sample with the shape of number “S” was used as conductor, and the insets show the 3D printed DN hydrogel based strain sensor attached on the wrist at different bending radians, 0, π/4 and π/2, respectively. (d) Change of resistance with time when the self-healed DN hydrogel was used as conductor, and the insets show that the self-healed DN hydrogel based strain sensor attached on the thumb finger bending repeatedly.

4. Conclusion We have synthesized a stretchable DN hydrogel with excellent recoverable, self-healing and 3D printable properties by combining an ionically crosslinked κ-carrageenan network 18 ACS Paragon Plus Environment

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with a covalently crosslinked PAAm network. Because of thermoreversible feature of the solgel transition of the κ-carrageenan, the stiffness and toughness of the DN hydrogel even with a notch could be respectively recovered to 91 % and 84 % by heating at 90 oC for 20 min. The thermoreversible sol-gel transition is also responsible for the self-healing property of the DN hydrogel, where the cut surfaces could be healed via the gel-sol-gel transition in a heating-cooling cycle, and the self-healed DN hydrogel exhibited a high self-healing efficiency (99.2% in conductivity). More importantly, the warm pre-gel solution of κcarrageenan/AAm was sucessfully used as an ink of a 3D printer to print the hydrogel into complex 3D structures, and the printed DN hydrogel samples demonstrated the high mechanical strength after UV-exposure. Furthermore, the κ-carrageenan/PAAm DN hydrogel possesses a strain sensitivity, which endows our DN hydrogel to be used as wearable strain sensor to effectively monitor and distinguish multifarious motions of a human body. This study has for the first time demonstrated that the κ-carrageenan/PAAm DN hydrogel is not only stretchable and recoverable, but also self-healing, 3D-printable and strain-sensitive.

Supporting Information Effects of various weight percentages of KCl and MBA on mechanical properties of DN hydrogels; FTIR spectra of the κ-carrageenan, PAAm, and κ-carrageenan/PAAm gels; Photo images showing the notch insensitivity of DN hydrogel; Effects of water bath temperatures on the stretch-strain curves of self-healed DN hydrogels; 3D printed κ-carrageenan/PAAm DN hydrogel with mesh pattern; 3D printed κ-carrageenan/PAAm DN hydrogel based strain sensor.



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Acknowledgements This work was supported by the Academic Research Fund Tier 1 (RG100/13) from the Ministry of Education, Singapore

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Table of content


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Ultrastretchable and Self-Healing Double-Network Hydrogel for 3D

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