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Feb 7, 2017 - E-mail: [email protected]. DOI: 10.1002/adma.201606900 crosslinkers or polymers could enable the multi-stimuli-responsive and ...
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Hong Chen, Fengyu Yang, Qiang Chen,* and Jie Zheng* crosslinkers or polymers could enable the multi-stimuli-responsive and mechanically strong properties in polymer networks. Unlike other mechanoresponsive materials[13,14] such as benzocyclobutene[15] and 1,2-dioxetane,[16] spiropyran (SP) is a multi-stimuli-responsive mechanophore, which can change its color and fluorescence in response to force, heat, and light.[17,18] Under these external stimuli, SP undergoes a reversible structural transformation between spiropyran (a ringclose) and merocyanine (MC, a ring-open) states (Figure 1), leading to the reversible optical property change. The mechanism of the SP↔MC-induced optical change has been well studied[14,17,19–23] and will not be discussed here. But, the challenges still remain. SP, as a highly hydrophobic molecule, is usually used to crosslink with hydrophobic polymers such as poly(methyl acrylate) (PMA),[17] poly (methyl methacrylate),[21] polydimethylsiloxane,[23] and polyurethane[19] in organic solutions (dimethyl sulfoxide, acetonitrile and dimethylformamide) to produce different SP-based polymers and elastomers in solid state. The poor solubility of SP in aqueous solution makes it very challenging to directly incorporate SP into highly hydrophilic hydrogels (50%– 90% water content), while still retaining SP mechanophore function. More importantly, no SP-based tough hydrogels have been reported to date, thus little is known about the SP-induced mechanotransduction and toughening mechanisms in the hydrogels. To overcome these challenges, here we developed a new micellar-copolymerization method to incorporate highly hydrophobic SP into highly hydrated polymer network, resulting in SP-crosslinked poly(AM-co-MA/SP) (polyacrylamide-co-methyl acrylate/spiropyran) hydrogels that exhibit SP-induced multimechanoresponsive/recovery and mechanically strong properties. Figure 1 shows a general procedure to prepare poly(AM-coMA/SP) gels using the micellar-copolymerization method. The mechanophore SP was synthesized using the well-established protocols[17,20] and its chemical structure was confirmed by the 1H NMR (Figure S1, Supporting Information). Briefly, SP and hydrophobic photoinitiator phenylbis (2,4,6-trimethylbenzoyl) phosphine oxide (PBPO, IAGURE 819, a white light photoinitiator) were dissolved first in methyl acrylate (MA) and then in TWEEN 80 aqueous solution (1 wt%), where hydrophobic SP and initiators were protected in the hydrophobic environment of TWEEN 80 micelles, resulting in an uniform emulsion solution. Then, hydrophilic monomer acrylamide (AM) was added to the emulsion solution, followed by photoinitiated micellar copolymerization of MA, SP, and AM under white light.

A newly developed polyacrylamide-co-methyl acrylate/spiropyran (SP) hydrogel crosslinked by SP mechanophore demonstrates multi-stimuliresponsive and mechanically strong properties. The hydrogels not only exhibit thermo-, photo-, and mechano-induced color changes, but also achieve super-strong mechanical properties (tensile stress of 1.45 MPa, tensile strain of ≈600%, and fracture energy of 7300 J m−2). Due to a reversible structural transformation between spiropyran (a ring-close) and merocyanine (a ring-open) states, simple exposure of the hydrogels to white light can reverse color changes and restore mechanical properties. The new design approach for a new mechanoresponsive hydrogel is easily transformative to the development of other mechanophore-based hydrogels for sensing, imaging, and display applications. Stimuli-responsive materials as “smart” materials are able to adapt their physicochemical properties and structural conformations in response to external changes in pH,[1] temperature,[2] light,[3] ionic strength,[4] solvent,[5] and magnetic/electric fields,[6] while polymer hydrogels as “soft and wet” materials have advantages (e.g., high content water, 3D porous networks, and flexible shapes) to mimic the complex microenvironments in biological systems. So, integration of stimuli-responsive materials/properties into gel structures enables to combine each other’s structural and functional advantages for developing next-generation smart stimuli-responsive hydrogels for biosensors,[7] actuators,[8] controlled drug delivery carriers,[9] tissue scaffolds,[10] regenerative medicine,[11] and bioimaging.[12] But, conventional stimuli-responsive hydrogels often suffer from two major limits. Most stimuli-responsive hydrogels only respond to single stimuli and almost all of them are mechanically weak, both of which greatly limit their uses to complex systems. It appears that use of single polymer materials or simple polymer structures prevents multi-stimuli-responsive and mechanically strong properties from coexisting within a single hydrogel. While multi-stimuli-responsive hydrogels is still in its infant state, a proper design of stimuli-responsive

H. Chen, F. Yang, Prof. J. Zheng Department of Chemical and Biomolecular Engineering The University of Akron Akron, OH 44325, USA E-mail: [email protected] Prof. Q. Chen School of Material Science and Engineering Henan Polytechnic University Jiaozuo 454003, China E-mail: [email protected]

DOI: 10.1002/adma.201606900

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A Novel Design of Multi-Mechanoresponsive and Mechanically Strong Hydrogels

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Figure 1.  Synthesis procedure of mechanoresponsive poly(AM-co-MA/SP) hydrogel via micellar copolymerization of acrylamide and methyl acrylate/ spiropyran. The hydrogel exhibits color change from yellow to purple upon force, UV light, and heat stimuli and color reversion from purple to yellow upon white light. Both color change and reversion are triggered by a reversible structural transformation between spiropyran (SP, a ring-close, yellow) and merocyanine (MC, a ring-open, purple) states.

During this gelation process, SP mechanophores were covalently crosslinked with hydrophobic MA, where SP-crosslinked PMA microspheres were stabilized by TWEEN 80 micelles. Meanwhile, hydrophilic polyacrylamide (PAM) chains were also connected to SP-crosslinked PMA microspheres via the copolymerization of AM and MA and/or the coupling termination between radicals of PAM and PMA to form the network of poly(AM-co-MA/SP) hydrogel. To our best knowledge, this is the first report on SP-incorporated mechanoresponsive hydrogels in aqueous solution. We hypothesize that upon successful covalent incorporation of SP into the hydrogel network, SP-crosslinked hydrogels can reversibly alternate between force-induced network damage state (MC state) and light-induced network recovery state (SP state). As a result, we present proof-of-concept experiments to demonstrate that our poly(AM-co-MA/SP) gels can achieve multi-stimuli-responsive property (color and fluorescence changes under the stimuli of external force, UV light, and heat), light-induced self-recovery capacity (≈75% toughness recovery and ≈74% stiffness recovery after the first loading), and strong mechanical properties (tensile stress of 1.45 MPa, tensile strain of 570%, fracture energy of 7300 J m−2) at optimal conditions. Figure 2 shows the optical property change of SP-crosslinked hydrogels under different force, UV, and heat stimuli. As a control, the as-prepared poly(AM-co-MA/SP) hydrogels showed pale yellow color at the resting state. We first monitored the thermoinduced color change of poly(AM-co-MA/SP) hydrogels 1606900  (2 of 8)

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at different temperatures. We observed that the threshold temperature to make gel color change was ≈50 °C (Figure S2, Supporting Information), i.e., hydrogels exhibited pale yellow color at temperatures of 50 oC (Figure 2a). More interestingly, after ≈30 min of exposure to white light, the hydrogel reversed its color from deep purple to original pale yellow, showing a lightinduced recoverable property. In Figure 2b, the hydrogel also demonstrated a reversible photochromic response by reversibly switching the color between pale yellow under white light and purple under UV light (365 nm wavelength). Beyond the thermo- and photoinduced color changes, the hydrogel also exhibited mechanically induced changes in color and fluorescence. Upon uniaxial stretching (Figure 2c), a deep purple coloration appeared along the entire central region of the gel, while no detectable color change was observed at the two ends of the sample, demonstrating that large force-induced deformation occurs in the thinner central region of the sample, consistent with our expectation. Moreover, when the hydrogel was imprinted by a “UAKRON” mold, only the compression region in contact with the “UAKRON” word exhibited a deep purple color, while the rest of uncompressed region remained its original color, resulting in a clear imprint of the “UAKRON” word on the hydrogel (Figure 2d). Such imprint with deep purple color can be easily erased by white light. So, our hydrogels could be used as reusable and erasable materials for color printing/writing. In parallel to visible color change, the hydrogels also showed the mechanically induced fluorescence

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Communication Figure 2.  Multi-stimuli-responsive poly(AM-co-MA/SP) hydrogel exhibits reversible color changes between purple color under the stimuli of a) heat, b) UV light (365 nm), c) stretching, d) compression and pale yellow after white light exposure. e) The hydrogel also shows the fluorescent responses to compression, as indicated by black “X” versus green background using the GFP filter and red “X” versus black background using the Cy5 filter. f) SEM image of poly(AM-co-MA/SP) hydrogel with microsphere structures (scale bar = 10 um), and g) in situ confocal image of poly(AM-co-MA/SP) hydrogel with strong color contrast between green fluorescence of microspheres and almost invisible background of PAM chains (scale bar = 50 µm).

change (Figure 2e). As monitored by a fluorescent microscope with different filters, the hydrogel exhibited green (green fluorescent protein (GFP) filter) and black (Cyanine5 (Cy5) filter) fluorescence in the virgin region, but black (GFP filter) and red (Cy5 filter) “X” in the imprint region. Consistently, Figure S3 (Supporting Information) showed that different from the virgin hydrogels with fluorescent emission peak at ≈545 nm, the stretched and heated gels showed similar fluorescent emission peak at ≈635 nm. Scanning electron microscope (SEM) image of poly(AM-co-MA/SP) hydrogels clearly revealed many microspheres with averaged size of 1–30 µm (Figure 2f). In parallel, confocal image under the fluorescein isothiocyanate (FTIC) filter showed a strong color contrast between green fluorescence of microspheres and almost invisible background of PAM chains outside microspheres, indicating the successful encapsulation of SP covalently crosslinked PMA inside the microspheres (Figure 2g). Taken together, the poly(AM-co-MA/ SP) hydrogel demonstrates its unique, reversible, multi-stimuliresponsive color change property. The gels possess thermo-, photo-, and mechano-induced color changes. Such color changes not only provide direct and visible detection of gel deformation, but also can be readily reversed after exposure to white light. Our proof-of-concept experiments demonstrate that when the color-generating SP is covalently incorporated into hydrogel network via micellar polymerization, any local network deformation caused by external stimuli would cause the deformation-induced SP bond breaking. Reversible conversion can also be realized between force-induced network deformable conversion from SP to MC and white light-induced network

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recoverable conversion from MC to SP. Thus, poly(AM-co-MA/ SP) hydrogel has great potential used as smart sensors for damage detection and failure prevention. To examine the effect of SP crosslinker on the mechanical properties of poly(AM-co-MA/SP) hydrogels, Figure 3a shows the typical stress–strain curves of poly(AM-co-MA/SP) hydrogel with SP crosslinkers, poly(AM-co-MA/EGDMA) hydrogel with ethylene glycol dimethylacrylate (EGDMA) crosslinkers, and poly(AM-co-MA) hydrogel without crosslinkers. SP-crosslinked poly(AM-co-MA/SP) hydrogels exhibited excellent mechanical properties and achieved the tensile strength of 1.45 MPa at the fracture strain of 570%. Both tensile values are much higher than those of noncrosslinked hydrogels (tensile strength of 0.13 MPa at the fracture strain of 74%) and EGDMA-crosslinked hydrogels (tensile strength of 0.47 MPa at the fracture strain of 180%). Figure 3b consistently showed that the fracture energy of SP-crosslinked gel was ≈7300 J m−2, which is four times higher than EGDMA-crosslinked gel (≈1700 J m−2). The noncrosslinked gel was too brittle to be tested. In Figure 3c, SP-crosslinked gels showed a large hysteresis loop and dissipated energy of 0.606 MJ m−3 in the first loading–unloading cycle at a strain of 300%, as compared to a much smaller hysteresis loop and energy dissipation (0.081 MJ m−3) of EGDMA-crosslinked gels. The results indicate the force-induced conversion from SP to MC in the PMA microspheres, i.e., breakage of covalent bond (ring-opening reaction) of SP contributes largely to high dissipated energies and high toughness of our SP-conjugated hydrogels. In the immediate second cycle, hysteresis loops became much smaller for both SP-crosslinked gels and

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Figure 3.  Mechanical properties of poly(AM-co-MA/SP) hydrogel. a) Tensile stress–strain curves of poly(AM-co-MA/SP) hydrogel with SP crosslinkers, poly(AM-co-MA/EGDMA) hydrogel with EGDMA crosslinkers, and poly(AM-co-MA) hydrogel without crosslinkers. b) Fracture energy of poly(AM-coMA/SP) and poly(AM-co-MA/EGDMA) hydrogels. Poly(AM-co-MA) hydrogel is too brittle to be tested for its fracture energy. c) Hysteresis loading– unloading tests for poly(AM-co-MA/SP) and poly(AM-co-MA/EGDMA) hydrogels. The dissipated energies are 606 and 190 kJ m−3 for SP-crosslinked gel and 81 and 34 kJ m−3 for EGDMA-crosslinked gel in the first and second cycles, respectively. d) Dependence of SP concentrations on the mechanical strength of SP-crosslinked hydrogels.

EGDMA-crosslinked gels, indicating a deformation of the gel network causes the softening of the gels that cannot be recovered immediately. It should be noted that our SP-crosslinked hydrogels can achieve high mechanical properties comparable to double-network hydrogel that is considered as the toughest hydrogel (failure tensile stress of 1–10 MPa, tensile strain of 1000%–2000%, and fracture energy of 103–104 J m−2). Clearly, incorporation of SP crosslinkers into PMA network can significantly improve the mechanical strength and toughness of poly(AM-co-MA/SP) hydrogels. The strong mechanical properties of SP-crosslinked hydrogels enable to quantitatively visualize color change from pale yellow to purple as increase of tensile strains. Gradual color change toward purple during stretching is linked to the deformation extent of the SP-linked PMA chains, i.e., as stretching force acting on the hydrogel increases to a certain level, it would be transferred from polymer network to SP molecules, resulting in the mechanically induced ring-opening conversion from SP to MC, in accompany with large energy dissipation. As expected, no color change was observed for poly(AM-co-MA) gels (containing no crosslinkers) and poly(AM-co-MA/EGDMA) gels (containing no SP mechanophore) upon stretching (Figure 3a, inset images). To our best knowledge, SP-conjugated hydrogel is the first gel toughened by the SP→MC transition. 1606900  (4 of 8)

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We further conducted a series of tensile tests to examine the dependence of mechanical properties of SP-crosslinked hydrogels on the concentrations of the two networks (MA and AM) and SP crosslinker. In Figure 3d and Table S1 (Supporting Information), with the increase of SP concentrations from 0.09 to 0.36 mol% of MA at the fixed AM and MA concentration of (≈25 wt%), the SP-crosslinked gels monotonically increased their tensile strength (σf) from 0.29 to 1.45 MPa, elastic modulus (E) from 262 to 989 KPa, and deformation energy (W) from 1.1 to 4.42 MJ m−3, while retaining similar ruptured strains (εf) at ≈600%. SP-crosslinked hydrogels also exhibited the dependence of SP concentrations on the color change. It can be seen in Figure S5 (Supporting Information) that upon stretching different SP-crosslinked gels to the same strain of 400%, the deformed gels containing the higher SP concentrations showed the darker purple color, indicating that more SP→MC conversions occur in the gels with the higher SP concentrations. The mechanical properties of the SP-crosslinked gels also depended on the concentrations of hydrophobic MA, hydrophilic AM monomers, and PBPO initiator. As hydrophobic MA monomer increased its concentrations from 20 to 30 wt% (at the fixed AM of 25 wt% and SP of 0.18 mol% of MA, Table S1, Supporting Information), σf, E, and W increased from 0.42 to 0.93 MPa, 320 to 405 KPa, and 1.86 to 2.63 MJ m−3, respectively, but εf

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Communication Figure 4. White light-induced color and mechanical recovery of poly(AM-co-MA/SP) hydrogels using cyclic loading–unloading tests, including a) color recovery, b) hysteresis loop recovery, and c) stiffness (elastic modulus) and toughness (energy loss) recoveries (n = 3). The first and second loading–unloading tests (namely, 1st-original and 2nd-no recovery) were run after each other immediately without giving any resting time and white light treatment, while the 3rd–5th tests (namely 3rd, 4th, and 5th-recovery) were performed on the gels with ≈30 min of exposure to white light after every unloading.

decreased from 710% to 540%. Similar trends were observed as hydrophilic AM monomer increased from 25 to 40 wt% (at the fixed MA of 20 wt% and SP of 0.18 mol% of MA, Table S1, Supporting Information). The effect of the initiator PBPO concentration on the mechanical strength of poly(AM-co-MA/SP) hydrogels was also studied by tuning PBPO concentration from 0.04–0.4 mol% of MA. As shown in Figure S4 (Supporting Information), the SP-crosslinked gels at a PBPO concentration of 0.04 mol% exhibited much lower tensile stress than those gels prepared at 0.1–0.4 mol% of PBPO, but tensile strains of the gels prepared at different PBPO concentrations were similar to each other. Among three factors examined, the change of crosslinker SP concentration appears to be the most effective way to improve the mechanical strength of SP-crosslinked gels. Different from conventional crosslinkers that usually increase gel strength but decrease its toughness, the increase of SP content introduces more energy dissipation via dynamic conversion from SP→MC under stretching, which actually improves mechanical strength and stiffness, but does not compromise hydrogel’s toughness. Considering the mechanical-induced network deformation and color change can be reversed by white light via MC→SP conversion, we examined the self-recovery and energy dissipation of poly(AM-co-MA/SP) hydrogels using the five consecutive loading–unloading tests on the same gel specimen but without

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or with white-light treatment. The first two loading–unloading tests (namely, 1st-original and 2nd-no recovery) were run after each other immediately without giving any resting time and white light treatment, while the 3rd–5th tests (namely 3rd, 4th, and 5th -recovery) were performed on the gels with ≈30 min of exposure to white light after every unloading. At the first glance in Figure 4a, visual inspection showed that when the 1st-original gel was stretched to four times its original length, its color was changed from pale yellow to purple. After the first unloading, the gel at the relaxed state still kept dark purple that served as a starting point for the 2nd-no recovery loading. But, on the subsequent 3rd–5th loading–unloading-recovery tests, after every unloading the relaxed gel was exposed to white light for ≈30 min and the gel actually reversed its color back to pale yellow before the next loading. This loading–unloadingrecovery process can be repeated on the same gel for multiple times, and the gel always exhibits the same light-induced color change behaviors. Consistently, Figure 4b showed that during the 1st-original cycle, poly(AM-co-MA/SP) hydrogels showed the largest hysteresis loop and dissipated the highest energy of 0.64 MJ m−3 at λ = 3. After the immediate 2nd-no recovery cycle, the hysteresis loop became much smaller. But, after exposure of the gel to white light in the 3rd–5th recovery cycles, all gels exhibited large hysteresis loops. To be more quantitative, we define both stiffness and toughness recovery

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Figure 5.  Strain-induced dynamic color change of poly(AM-co-MA/SP) hydrogels. a) Optical images of the hydrogels at both stretched state (stretch ratios of 0%–400%) and the corresponding relaxed state. b) The x,y chromaticity diagram for the color change pathways of both stretched gels at loading state and relaxed gels at unloading state (standard CIE 1931).

rates by calculating the ratios of elastic modulus (E, represents a stiffness recovery) and energy loss (Uhys, represents a toughness recovery) at different loading cycles to those values at the first one, respectively. It can be seen in Figure 4c that after the first recovery in the 3rd cycle (i.e., 3rd-recovery cycle), poly(AM-co-MA/SP) hydrogels can recover its toughness by ≈75% and stiffness by ≈74%. Even after 4th- and 5th-recovery cycles, the toughness/stiffness recovery rates can still maintain at ≈66%/72% and ≈65%/68%, respectively, which are much higher than ≈29% of toughness recovery and ≈26% of stiffness recovery for the untreated gel in the 2nd no recovery cycle. To better link the mechanical force to mechanochemistryinduced color change of SP, we conducted additional loading– unloading tests on poly(AM-co-MA/SP) hydrogels at different strains. Figure 5a showed optical images of the same SPcrosslinked hydrogels at both loading and unloading states. The corresponding stress–strain curves were shown in Figure S6 1606900  (6 of 8)

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(Supporting Information). When the stretching ratio was less than 50% (i.e., the gel was stretched to 1.5 time its original length), the gels did not show obvious color changes between loading and unloading states. This indicates that the stretching force is not large enough to transfer to SP, so that conversion from SP to MC is not mechanically triggered. However, as gels were stretched to two or more times their original lengths (i.e., stretching strain > 100%), they showed dynamic color changes with increasingly purple as stretching force (i.e., stretching strain), indicating that stretching force indeed correlates to the conversion of SP to MC. Visual inspection also showed that even after unloading, the relax gels still retained intense pinklike color throughout the entire gauge section and the gels after relaxing from higher stretching forces hold noticeably darker pink, indicating that the relax gels cannot immediately recover their network deformation. The lack of MC→SP conversion in the relax gels also provides strong evidence that the color

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cycle (Figure 4b). But, once SP-crosslinked PAM chains are deformed, they cannot reform immediately, so on subsequent loadings the energy dissipation of the gels is much reduced. As compared to mechanically strong SP-crosslinked hydrogels, the same poly(AM-co-MA) gel systems without SP crosslinkers and with EGDMA crosslinked gels show very poor mechanical properties (Figure 3a). Lack of strong mechanical properties in both gels without SP crosslinkers provides strong evidence that introduction of SP-crosslinking structures in the gels can greatly improve mechanical properties. Due to a reversible SP↔MC isomerization reaction, the ring-opened MC structure can be reversibly transformed to the ring-close SP form under white light stimuli, leading to the reconstruction of crosslinks with PMA chains and the partial recovery of mechanical properties. In summary, we developed a new method of incorporating SP mechanophore into the hydrogels via copolymerization of micelles with hydrophobic MA and hydrophilic AM, producing a novel multi-stimuli-responsive, tough poly(AM-co-MA/SP) hydrogels. The resulting hydrogels not only exhibited remarkable optical response from pale yellow to purple color and from green to red fluorescence under external stimuli of force, heat, and UV light, but also simply reversed its color back to the original one by white light. Moreover, our SP-crosslinked mechanoresponsive hydrogel also demonstrated its excellent mechanical properties (tensile strength of 1.45 MPa, tensile strain of ≈600%, and fracture energy of ≈7300 J m−2), comparable to those well-known tough DN hydrogels. The high strength and toughness of our SP-crosslinked hydrogels is mainly contributed by the covalent bond breakage of SP mechanophore via SP to MC conversion. Reversible conversion in both color change and mechanical properties can be realized multiple times between force-induced network deformable conversion from SP to MC and white light-induced network recoverable conversion from MC to SP. We hope that SPcrosslinked hydrogels can function as a heat/light/mechanoresponsive sensor for direct, easy, visible detection for material damage/sensing/imaging, and the method we developed could serve as a general strategy for the design of new smart, strong mechanophore materials.

Experimental Section Hydrogel Synthesis: Mechanophore spiropyran crosslinker (10 mg, 0.36 mol% of MA) and the PBPO (IAGURE 819, 5 mg, 0.2 mol% of MA) were dissolved in methyl acrylate (0.5 mL, ≈25 wt%). The solution was then dissolved in 1% TWEEN 80 aqueous solution (1 mL, ≈50 wt%) and vortexed for 5 min to form a uniform emulsion. Then acrylamide (0.5 g, 25 wt%) was added to the emulsion, followed by vortex mixing for another 5 min. The mixture was injected into a glass mold with a 1 mm thick Teflon spacer and exposed to white light (Ultra-Violet Products Ltd (UVP), 8 W). The gel will be formed after ≈2 h photopolymerization. More detailed descriptions of mechanical tests and characterizations are provided in the Supporting Information.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

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change is induced solely by mechanical force, rather than by temperature change or photolytic activation. Quantitatively, the color changes in the gauge section of digital images of the gels were analyzed using the RGB (red, green, blue) color channels (Figure S9, Supporting Information). Figure 5b showed the x,y chromaticity diagram for the color change pathways of both stretched gels at loading state and relaxed gels at unloading state. Upon loading, the stretched gels showed a linear pathway from pale yellow to purple with increasing stretching strain from 0% to 400%. After unloading, the relaxed gels showed a distinct pathway toward pink color. Different color change pathways during the loading and unloading processes indicate a secondary color transition due to the isomerization of MC .[17,23,24] In parallel, we also conducted the UV–vis spectrum to quantitatively monitor the color change of the SP-crosslinked gels at different strains of 0%–400%. In Figure S7 (Supporting Information), the hydrogel at a low strain of 50% showed almost identical UV–vis spectrum to the virgin gel at 0% strain, indicating no detectable color change for the gels at low strains of 0%–50%. But, when the gels were stretched to 100% or above, UV–vis spectrum showed a broaden peak at ≈550 nm and its peak intensity increased as strains, with a high R2 value of 0.91. This indicates that the SP→MC conversion as indicated by color change is correlated with the higher strains. Additionally, UV–vis spectrum and photo visual inspection were also used to examine the resting time effect on the color change of the SPcrosslinked hydrogels, where the gels were stretched to 400% and rested for 0, 5, and 30 min before analysis. As shown in Figure S8 (Supporting Information), the SP-crosslinked hydrogels that rested for different times did not show observable difference in both UV–vis spectrum and photo images, indicating that the resting time has no influence on the color change of the gels. We have shown that the sufficient deformation of SPcrosslinked hydrogels caused by heat, UV light, and mechanical force can trigger a ring-opening SP→MC conversion, leading to visible color change. Particularly, such SP→MCinduced color change can be readily controlled by mechanical forces/strains due to the linear relationship between color change and stretching ratio. Moreover, any stimuli-induced color change and network deformation in our hydrogels can be reversed after exposure to white light. From a viewpoint of network structure, we propose a toughening and color change mechanism for poly(AM-co-MA/SP) hydrogels. Considering that hydrophilic PAM chains outside the microspheres are more soft and ductile than hydrophobic PMA/SP ones inside the microspheres, when mechanical force is applied to the gels, PAM chains outside the microspheres are first stretched, but they cannot bear large stress due to soft nature. So dominant force will be transferred from PAM chains to the microspheres and the associated PMA/SP chains, induces the deformation and dissociation of SP-crosslinked PMA chains, and probably break the linkages of SP. All effects will activate mechanophore conversion from SP to MC and result in color change from yellow to purple. So, deformation energy is mainly dissipated by the deformation of the SP-linked PMA chains, where SP-linked microspheres likely serve as sacrificial architecture to protect further network fracture, as supported by a large hysteresis of the gels in the first loading–unloading

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Acknowledgements J.Z. thanks financial support from NSF (Grant No. DMR-1607475) and in part from NSF (Grant No. CBET-1510099). Q.C. thanks for financial support, in part, from National Nature Science Foundation of China (Grant No. 21504022), Henan Province (Grant Nos. 12B430007, 13A430015, 16IRTSTHN005, and 17HASTIT006), and Henan Polytechnic University (Grant No. 72105/001). Received: December 21, 2016 Revised: February 7, 2017 Published online:

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Adv. Mater. 2017, 1606900