Bio-inspired color-changing and self-healing hybrid hydrogels for wearable sensors and adaptive camouflage

Abstract

Hydrogels have been extensively investigated for their unique mechanical and ionic conductive properties. Currently, conventional hydrogels are not sufficiently durable for use and do not respond sensitively to environmental stimuli. Accordingly, the development of a hybrid hydrogel exhibiting self-healing ability and environmental responsiveness is of critical significance in broadening its application in smart wearable devices. For the formation of a color-changing and self-healing hybrid hydrogel, thermochromic dye microcapsules and photochromic dye microcapsules were mixed with a multi-branched polyacrylate and zinc sulphate in this study. This hybrid hydrogel exhibited an excellent sensing property, and can be applied to wearable device for monitoring the actions of human joints or faces. Even though the hybrid hydrogel was damaged during its application, the damaged parts of the hydrogel were self-repaired, which was dependent on the powerful ionic bonds and multi-hydrogen bonds among the polyacrylate, zinc ions, as well as microcapsules. Furthermore, under the synergistic effect of microcapsules and polyacrylate, the rapid change of color and current under thermal or UV stimulus facilitates monitoring human physiological health, as well as achieving adaptive camouflage. This smart hybrid hydrogel exhibiting self-healing, conductive, and color-changing properties has promising applications in human body monitoring, safety warning, adaptive camouflage, and artificial intelligence for wearable electronic devices.

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1. Introduction

Electronic devices exhibiting bio-flexible and stretchable functions have been advancing, which is expediting the advancement towards intelligent human life.^1–5 Rapid progress has been achieved in the fabrication of a wide variety of electronic devices (e.g., wearable sensors, optoelectronic sensing devices, electronic skin, and flexible energy storage devices^6–10). Among them wearable sensors indispensably contribute to several aspects e.g., information collection, security warning, and intelligent human life.^11–16 However, the durability or persistence of electronic devices (in particular wearable sensors) is a critical consideration for their application. Inspired by the special abilities of creatures in nature, the introduction of new design strategies promises to solve these problems and give new meaning to smart wearables. For instance, the skin of a chameleon is highly interesting and sensitive and capable of changing color for adaptation, self-protection, or safety warning by sensing external stimuli (e.g., temperature, sunlight, and pressure).^17–24 Moreover, the skin of a chameleon has a self-healing ability even after being injured. Accordingly, chameleon skin provides inspiration for effective fabrication of durable wearable sensors. Thus, a suitable polymer matrix should be selected for wearable sensors.^25–28 Wearable sensors that are formed based on the freely designed polymeric structures of the conductive polymers are highly desirable compared with conventional wearable sensors that are not resistant to stretching.^29–33 To solve the problems regarding the widespread and sustainable application of wearable sensors, self-healing hydrogels with excellent ionic conductivity are an irreplaceable candidate.^34–38 Flexible hydrogels exhibiting high ionic conductivity are one of the irreplaceable candidates for wearable sensing materials, as compared with conventional wearable sensing materials that are not tensile resistant.^39–41

Hydrogels refer to a type of polymer material with a three-dimensional network structure, and exhibit similar properties to biological tissues, so that they have been extensively employed in intelligent wearable devices.^42–45 Conventional hydrogels are poorly flexible, prone to breakage, and not adapted to complex external stimulus responses.^46–48 Accordingly, numerous studies have been conducted to develop hybrid hydrogels by introducing organic and inorganic hybrid networks. Lin et al.⁴⁹ pre-stretched chemically cross-linked poly(acrylamide-co-acrylic acid) and froze the pre-stretched state by loading Fe³⁺. Thus, maximum chelation and molecular conformation were achieved between the carboxylic acid anion and Fe³⁺, so that the mechanical strength increased significantly. Moreover, with titanium oxide polydopamine-perfluorosilicon carbon dot-conjugated chitosan-polyvinyl alcohol-loaded tannic acid as the raw material, Ryplida et al.⁵⁰ prepared a thin-film hydrogel with controlled hydrophobic–hydrophilic transition under UV-Vis irradiation. The formed intermolecular hydrogen bonds and electrostatic interactions in the hydrogel increased its rigidity and elasticity, thus improving its mechanical property. Although hybridization or recombination is capable of enhancing the mechanical properties of the hybrid hydrogels, durability and stability are confirmed as the critical factors for hydrogels, especially for wearable applications. For instance, Wei et al.⁵¹ chelated polyacrylic acid (PAA) with a high molecular weight with calcium ions (Ca²⁺). Subsequently, PAA was physically cross-linked with modified amorphous calcium phosphate to synthesize a Ca-PAA-ACP mineral hydrogel. To be specific, the prepared hydrogen and ionic bonds endowed the hydrogels with excellent durability and stability, so that they can be useful in the smart wearable electronics field. Moreover, although hydrogel materials have been widely used, numerous hydrogels do not possess specific functional properties, such as environmental adaptability. Thus, the design and preparation of intelligent responsive hydrogels is of great significance.^52–56

In this study, a color-changing and self-healing TDM/PDM/polyacrylate hybrid hydrogel was designed and prepared through the bionic process. As depicted in Fig. 1, thermochromic dye microcapsules and photochromic dye microcapsules were first synthesized, and then these microcapsules were added to the multi-branched polyacrylate solution to prepare a hybrid hydrogel under the complexation of zinc sulphate. The prepared hybrid hydrogel exhibited excellent wearable tensile sensing properties, and it can change its color at different temperatures and under sunlight conditions. The hydrogel had promising self-healing ability due to the strong coordinate bond and multi-hydrogen bonds between the polyacrylate, zinc ions and color-changing microcapsules, thus maintaining electrical conductivity and mechanical and color changing properties even after being damaged. Moreover, this hybrid hydrogel can change its color and electric current quickly under thermal or UV stimuli because of the synergistic effect of color-changing microcapsules and the polyacrylate. As a result, it can timely warn human physiological health through visualization and data sensing, and can be applied for adaptive camouflage.

Fig. 1 Schematic diagram of the preparation process of the TDM/PDM/polyacrylate hybrid hydrogel.

2. Experimental section

2.1 Materials

2-Ethylhexyl acrylate (EHA, purity ≥99%), methacrylic acid (MAA), hydroxyethyl methylacrylate (HEMA), zinc sulphate hepta (purity ≥99.5%), butyl methacrylate (BMA, purity ≥99%), sodium dodecyl benzene sulfonate (SDBS), dichloromethane (purity ≥99.5%) and ethyl acetate (purity ≥99.5%) were purchased from Sinopharm Chemical Reagent Co. Ltd. Temperature dye (TD) microcapsules were provided by Shenzhen Qianse New Material Technology Co., Ltd. Photochromic dye (PD) microcapsules were purchased from Shanghai Jingyan Chemical Co., Ltd. Azodiisobutylcyanide (AIBN) was purchased from Hubei Chengfeng Chemical Co. Furthermore, other reagents were employed without further purification.

2.2 Synthesis of flexible polyacrylate resin

First, a uniform mixture was obtained by dissolving 1.1 g of AIBN, 25 g of AA, 15 g of HEMA and 5 g of EHA into 60 mL of anhydrous ethanol. Afterward, the mixture was heated to 75 °C by magnetically stirring at 300 rpm. Subsequently, the solution was reacted for 8 h to prepare the polyacrylate. The detailed chemical equation is expressed in Fig. S1 (ESI†).

2.3 Preparation of color-changing microcapsules

First, 5 g AA, 10 g HEMA, and 15 g BMA were dissolved in 50 mL of anhydrous ethanol, and then 0.9 g of AIBN was added. Next, the mixture was heated to 75 °C, and the reaction was continued for 8 h by magnetic stirring at 400 rpm to prepare hydroxyl polyacrylate.

1.5 g of hydroxyl polyacrylate resin was dissolved in 10 g of dichloromethane and 3 g of ethyl acetate, and then 0.6 g of TD was added to the polyacrylate resin solution. Afterwards, 200 g of deionized water containing 0.75 g of SDBS was added to the above solution to form the uniform emulsion under ultrasonic conditions. The emulsion in an open flask was magnetically stirred (300 rpm) for 3 h to obtain TD microcapsules (TDMs). The preparation of PD microcapsules (PDMs) is the same as the above preparation method of TDMs.

2.4 Preparation of the TDM/PDM/polyacrylate hybrid hydrogel

20 g of zinc sulfate was mixed with 80 mL of deionized water by ultrasound treatment for 15 min to obtain a homogeneous solution. After that, 0.3 g of TDMs and 0.3 g of PDMs were added to the previous aqueous solution of zinc sulfate to obtain a zinc sulfate/TDM/PDM mixture. Finally, 30 mL of zinc sulfate/TDM/PDM solutions was mixed with 30 g of prepared polyacrylate by mechanical stirring for 10 min to obtain a TDM/PDM/polyacrylate hybrid hydrogel. In addition, 20 g of zinc sulfate was mixed with 80 mL of deionized water by ultrasound treatment for 15 min to obtain a homogeneous solution. Subsequently, 30 mL of zinc sulfate solution was mixed with 30 g of prepared polyacrylate by mechanically stirring for 10 min to obtain the control hybrid hydrogel.

2.5 Characterization

Scanning electron microscopy (SEM, Regulus810) was used to observe the surface morphology of TD, PD, TDM/PDM and the hybrid hydrogel. An energy dispersive spectrometer (EDS) was used to analyze the surface elements of the hybrid hydrogel. The chemical structures of the TD, PD, polyacrylate and TDM/PDM were characterized using a Fourier transform infrared (FT-IR) spectrometer (Nicolet Nexus 470, ThermoFisher Corp. USA) in the wavenumber range from 500 to 3800 cm⁻¹. The self-healing process of the TDM/PDM/polyacrylate hybrid hydrogel sample with a size of 10 mm × 10 mm × 2 mm was examined using an optical microscope (KEYENCE, VHX-7000). A precision digital multimeter (Keithley 2400, SMU) was employed to examine the resistance of the TDM/PDM/polyacrylate hybrid hydrogel. The I–T curves at different voltages were measured using an electrochemical workstation (GAMRY, Interface 1010B). The stress–strain curve of the TDM/PDM/polyacrylate hybrid hydrogel with a sample size of 30 mm × 10 mm × 2 mm was measured using an electronic universal testing machine (EMS303, Mark-10). The temperature of the hybrid hydrogel was measured using an infrared thermal camera (FLIRT440). The sample size is 30 mm × 20 mm × 2 mm, and the stretching rate is 100 mm min⁻¹. The rheological properties of the hydrogels were examined using a rotational rheometer (Physica MCR 301, Anton Paar), and a gap of 2.5 mm was maintained. The pore size of the hybrid hydrogel was analyzed using a fully automated piezometric pore size analyzer (MicroActive AutoPore V 9600 Version, Micromeritics Instrument Corporation). The particle size of TDMs and PDMs was tested using a wet and dry laser particle size meter (Winner2309, Jinan Micro-Nano Particle Instrument Co.).

3. Results and discussion

3.1 Preparation and characterization of colored self-healing hybrid hydrogels

The prepared polyacrylate resin was transparent and capable of flowing (Fig. 2a and b). Subsequently, the prepared TDMs and PDMs and zinc sulphate were introduced into the polyacrylate resin to prepare colored hybrid hydrogels (Fig. 2c). In addition, rheological tests were performed on the fully gelatinized hydrogel (TDM/PDM/polyacrylate hybrid hydrogel) and the incompletely gelatinized hydrogel (TDMs/PDMs, zinc ions, and polyacrylate not fully crosslinked), respectively. As depicted in Fig. S2 (ESI†), the energy storage modulus and loss modulus in both hydrogel systems increased with the increase of frequency. The increase in the fully gelatinized hydrogel was nearly linear, and the energy storage modulus was constantly higher than the loss modulus, thus suggesting the better stability and solid state behavior of the hydrogel. However, the energy storage modulus and loss modulus of incompletely gelatinized hydrogel were significantly smaller than those of the fully gelatinized hydrogel. No significant difference was identified between the values of energy storage modulus and loss modulus, and the loss modulus was kept to be higher larger than the energy storage modulus. The above result suggested that the incompletely gelatinized hydrogel was not fully cross-linked, and it was in a semi-solid state. As depicted in Fig. 2d and e, the TD and PD microcapsules were all round in shape with a size of 2–10 μm, and their surfaces were covered with a layer of polymers. In addition, as indicated by the result of the particle size distribution test, the particle size of the PDM primarily ranged from 1 to 10 μm (Fig. S3a, ESI†), consistent with the particle size of TDMs (Fig. S3b, ESI†). Besides, the particle size distribution of both was consistent with that in the SEM images. Fig. 2f presents the FT-IR spectra of the dyes, polyacrylates, TDMs, and PDMs. A significant absorption peak appearing at around 3400 cm⁻¹ is attributed to the –OH group, which originated from the polyacrylate. At 1720 cm⁻¹, a new peak was identified, attributed to the C=O group, thus further confirming the surface coating of TD and PD microcapsules with polyacrylate. As depicted in Fig. 2g through Fig. 2i, the prepared hybrid hydrogel had a rough surface; notably, there were many holes or humps in the cross-section of the hybrid hydrogel (Fig. 2h and i), due to the addition of microcapsules. As depicted in Fig. S4a (ESI†), the process of mercury pressure and mercury withdrawal of the hydrogel after swelling indicated the different sizes of pore structures covered in the hydrogel. Fig. S4b (ESI†) presents the distribution of the pore structure of the hybrid hydrogel, with the peak of the respective stage as the maximum amount of mercury feeding the pore size. The pore size of the hydrogel macromolecular network chain after swelling and drying was primarily distributed at 10 000 nm, consistent with the pore size identified using the SEM several times before swelling. Furthermore, based on the EDS mapping of the hybrid hydrogel surface elements (Fig. S5, ESI†), it comprised C, O and Zn elements, thus suggesting that zinc ions were hybridized into the polyacrylate hydrogel.

Fig. 2 (a) Photograph of the polyacrylate in a bottle. (b) Photograph of the polyacrylate in an inverted bottle. (c) Photograph of the TDM/PDM/polyacrylate hybrid hydrogel at the bottom of an inverted bottle. (d) SEM image of the TDM. (e) SEM image of the PDM. (f) FT-IR spectra of the PD, TD, polyacrylate, TDM and PDM. (g) SEM image of the TDM/PDM/polyacrylate hybrid hydrogel. (h) Cross-section SEM image of the TDM/PDM/polyacrylate hybrid hydrogel. (i) Amplified SEM image of (h).

The prepared TDM/PDM/polyacrylate hybrid hydrogel was easy to bend and twist without leaving any marks (Fig. 3a and b), thus illustrating an excellent flexible property. Even under 200 g of weight load, the hybrid hydrogel was not pulled off (Fig. 3c), and the TDM/PDM/polyacrylate hybrid hydrogel stretched from 20 mm to 2380 mm (Fig. 3d). As depicted in Fig. 3e, the hybrid hydrogel easily formed a considerable number of interesting shapes through the different moulds, thus having a promising application in the personalization based on people's requirements. Next, the mechanical property of the hybrid hydrogel was investigated. As depicted in Fig. 3f, the tensile property of the control hydrogel without the TDM/PDM was significantly poorer than that of the TDM/PDM/polyacrylate hybrid hydrogel. The maximum stress of the TDM/PDM/polyacrylate hybrid hydrogel reached 0.015 MPa, significantly higher than that of the control hybrid hydrogel (0.0042 MPa), and the corresponding strain of the TDM/PDM/polyacrylate hybrid hydrogel with 1180% was twice as much as that of the control hybrid hydrogel (590%). Notably, the enhanced mechanical properties of the hydrogel formed after cross-linking were primarily dependent on the coordination bonding formed by the zinc ion and the polyacrylate and the multiple hydrogen bonding formed by the TDM and the PDM with the polyacrylate. Furthermore, the mechanical properties of some of the hydrogels that were reported were compared (Table S1, ESI†). The stress and strain of the hydrogels were closely correlated with the sample size, and the hydrogel prepared in this study exhibited a large strain at a smaller fracture stress, thus suggesting that the hydrogel exhibited better flexibility.

Fig. 3 (a and b) Photographs of the TDM/PDM/polyacrylate hybrid hydrogel during the bending and twisting processes. (c) Photograph of the TDM/PDM/polyacrylate hybrid hydrogel able to withstand a 200 g mass. (d) Photograph of the TDM/PDM/polyacrylate hybrid hydrogel showing stretched flexibility. (e) Photograph of varying shapes of the TDM/PDM/polyacrylate hybrid hydrogel. (f) Stress–strain curves of the control hybrid hydrogel and the TDM/PDM/polyacrylate hybrid hydrogel.

3.2 Self-healing property of the TDM/PDM/polyacrylate hybrid hydrogel

Fig. 4a illustrates the mechanism of self-healing of the TDM/PDM/polyacrylate hybrid hydrogel. The cut hybrid hydrogel was repaired in 3 min. The major reason for this result is that the hydroxyl and carboxyl groups of the polyacrylate were prone to form multi-hydrogen bonds with TDMs and PDMs. Besides, the hydroxyl or carboxyl groups of polyacrylates and TDMs/PDMs formed coordination bonds with zinc ions. It is noteworthy that the rapid formation of multiple hydrogen bonds and coordination bonds led to the self-healing property of the TDM/PDM/polyacrylate hybrid hydrogel. The effect of different proportions of zinc ions on the self-healing rate of the TDM/PDM/polyacrylate hybrid hydrogel was further characterized. The self-healing efficiency is written as follows:where L represents the healed length of the hybrid hydrogel, L₀ denotes the crack length of the hybrid hydrogel, and S expresses the self-healing rate. As depicted in Fig. S6 (ESI†), the self-healing rate of the TDM/PDM/polyacrylate hybrid hydrogel containing 0.69 mol L⁻¹ was higher than that of several other groups of hydrogels. In a certain concentration range, the self-healing rate of the hybrid hydrogel was increased with the concentration of zinc ions, primarily due to the multiple coordination bonding bonds that were formed by the zinc ions with the groups on the polyacrylates. Based on that, TDMs and PDMs significantly facilitated the self-healing of the hybrid hydrogel. The repaired hybrid hydrogel lighted up an LED lamp again because of its self-healing ability though it was cut before (Fig. 4b–d). In the self-repairing process of the damaged hybrid hydrogel, its crack was narrowed by degrees, and the crack was not identified on the repaired hybrid hydrogel surface (Fig. 4e–g), thus suggesting that the TDM/PDM/polyacrylate hybrid hydrogel completed the whole self-healing. Furthermore, the repaired hybrid hydrogel was easily bent and twisted (Fig. 4h and i). Besides, it even lifted a weight of 200 g with ease (Fig. 4j), and it stretched to 6 times its original length without any cracks (Fig. 4k). The above results reveal that the repaired TDM/PDM/polyacrylate hybrid hydrogel maintains its original mechanical properties even after being damaged. As depicted in Fig. 4l and m, the stress/strain of the hybrid hydrogel declined after 5 times cutting/healing cycles, and its corresponding resistivity was slightly enhanced. Fortunately, the strain of the final hybrid hydrogel reached 800%, thus revealing that the repaired hybrid hydrogel also has flexible sensing applications.

Fig. 4 (a) The self-healing mechanism of the TDM/PDM/polyacrylate hybrid hydrogel. (b–d) Photographs of the conductivity of the TDM/PDM/polyacrylate hybrid hydrogel when lighting an LED lamp during self-healing. (e–g) Optical microscopy photographs of the TDM/PDM/polyacrylate hybrid hydrogel during the self-healing process (origin, cut, and healed). (h–i) Photographs of the bending and twisting of the TDM/PDM/polyacrylate hybrid hydrogel skinsuit after self-healing. (j) Photograph of the repaired TDM/PDM/polyacrylate hybrid hydrogel being loaded a weight of 200 g. (k) Photograph of the repaired TDM/PDM/polyacrylate hybrid hydrogel being stretched. (l) Stress–strain diagram of the self-repairing process (before, after) of the TDM/PDM/polyacrylate hybrid hydrogel. (j) Variations in the resistivity of the TDM/PDM/polyacrylate hybrid hydrogel around self-healing.

3.3 Sensing ability of the TDM/PDM/polyacrylate hybrid hydrogel

The TDM/PDM/polyacrylate hybrid hydrogel may have promising applications in sensing for its superior conductivity and flexibility. Based on this, the human activity sensing of the hydrogel was investigated (ΔR/R₀, where R₀ denotes the original resistance value of the hydrogel; ΔR represents the absolute value of the change in resistance produced). The sensing performance of the hybrid hydrogel was first examined under the bending of 45° and 90°, respectively. Notably, the ΔR/R₀ values increased with the increase of the angle of bending, and the hybrid hydrogel rapidly changed with the changing angle (Fig. 5a). When this hybrid hydrogel was attached to the wrist of a wearer with repeated bending, the current signal was easily detected by the expression of ΔR/R₀ (Fig. 5b). Moreover, the ΔR/R₀ values of the hybrid hydrogel attached to the cheek of a wearer was changing with the blowing movement of the mouth of the wearer (Fig. 5c); its ΔR/R₀ values changed at different frequencies (Fig. 5d), such that the facial movement can be quickly and accurately detected. Furthermore, the hybrid hydrogel attached to the larynx of a wearer even detected the change in resistance as swallowing food (Fig. 5e). The facial micro-movement of the wearer was further examined. As depicted in Fig. 5f and g, the hybrid hydrogel attached to the wearer's glabellum or jaw expressed the change in signal accurately when smiling (Fig. 5g). All the above results suggest that the TDM/PDM/polyacrylate hybrid hydrogel can be applied to the throat or face to collect the change sensing signal even under minor changes. The transient response of the damaged hybrid hydrogel was recovered again immediately after the repair though the transient response corresponding to the cut TDM/PDM/polyacrylate hybrid hydrogel disappeared when the hybrid hydrogel was stretched (Fig. 5h). Besides, the TDM/PDM/polyacrylate hybrid hydrogel maintained the high response state even after 500 bending/recovering cycles (Fig. 5i). In addition, this TDM/PDM/polyacrylate hybrid hydrogel attached to a wearer's finger wrote a word “Sensor” on the screen, and it was sufficiently responsive and accurate to serve as an electronic skin (Fig. 5j–l). Accordingly, the above beneficial effects reveal that the TDM/PDM/polyacrylate hybrid hydrogel has promising sensing applications in movement monitoring, body information capture, smart screen writing, and so forth.

Fig. 5 (a and b) The response tests of the TDM/PDM/polyacrylate hybrid hydrogel at different finger and wrist bending angles. (c and d) The response tests of the TDM/PDM/polyacrylate hybrid hydrogel attached to the laryngeal and cheek of a wearer. (e) The TDM/PDM/polyacrylate hybrid hydrogel attached to the cheek of a wearer to detect the blowing posture at different frequencies. (f) The TDM/PDM/polyacrylate hybrid hydrogel attached to the glabellum for monitoring human frowning behavior at different voltages or jaw. (g) The TDM/PDM/polyacrylate hybrid hydrogel attached to the jaw for monitoring the behavior of the human facial smile. (h) Variation of the relative resistance of the TDM/PDM/polyacrylate hybrid hydrogel during stretching (original, cut, and healed). (i) Reproducibility of sensing property of the TDM/PDM/polyacrylate hybrid hydrogel under stretching/healing cycles. (j–l) The screenwriting performance of TDM/PDM/polyacrylate hybrid hydrogels.

3.4 Bio-inspired color-changing and self-healing properties of the TDM/PDM/polyacrylate hybrid hydrogel

The body surface of a chameleon is colorful, thus changes its color under different environmental conditions (Fig. 6a) (e.g., sunlight and temperature). The TDM and the PDM were introduced into the self-healing hydrogel to inspire its color changing property (e.g., chameleon organisms). The TD was formed by coalescing and curing the wall material through physical changes and encapsulating the core material, which was prepared from an electron transfer organic compound system. Besides, triaryl and phthalides were employed as the electron donors to provide the color changing substrate, and phenol was adopted as the electron acceptor to provide the color shade. The relative change in the redox potential between the electron donor and the electron acceptor with similar initial potentials under thermal stimulation at 38 °C was different, thus resulting in electron transfer between the electron donor and the electron acceptor. As a result, the molecular structure of the electron donor and the color changed.^57–59 The PD was reversible for color change under UV radiation, which could activate the diaryl ethylene isomer of the thermodynamically stable organic photochromic compound in it, allowing it to undergo reversible opened-loop and closed-loop transitions and to repeat the color change several times.^60–63 The polyacrylate coating of TD and PD microcapsules did not affect their discoloration properties. The color of TDM aqueous solution could be changed from blue to white when heated to 38 °C (Fig. S7, ESI†), while the color of the PDM aqueous solutions changed from white to deep red under UV light irradiation (Fig. S8, ESI†). After TDM and PDM were added into the polyacrylate, respectively, the formed TDM/polyacrylate hybrid hydrogel and PDM/hybrid hydrogel could also change their color under heating or UV light irradiation (Fig. S9 and S10, ESI†). From this, it is possible to combine different TD and PD microcapsules, thus enabling multiple colors to be switched freely. Accordingly, the color changing property of the TDM/PDM/polyacrylate hybrid hydrogel was further investigated under different environmental conditions. As depicted in Fig. 6b, the blue TDM/PDM/polyacrylate hybrid hydrogel turned green under UV light irradiation. When the UV light was removed, the hybrid hydrogel could return to its original color blue again. Analogously, TDM/PDM/polyacrylate hybrid hydrogel could change its blue to deep red at 38 °C, and its original color blue was restored when the environmental temperature was below 38 °C (Fig. 6c). Interestingly, the color of the TDM/PDM/polyacrylate hybrid hydrogel changed from blue to aurantium under the action of combined heating and UV light irradiation, and the color of it was reverted to its original color after removing the combine action (Fig. 6d). It is noteworthy that this TDM/PDM/polyacrylate hybrid hydrogel can show a wide range of color changing like a chameleon (e.g., deep red, yellow, green, blue, and white) under different external stimuli (e.g., UV light and temperature) (Fig. 5e). Moreover, the dark or light color of the TDM/PDM/polyacrylate hybrid hydrogel was shown by adjusting the amount of TDMs/PDMs in the hydrogel, such that the TDM/PDM/polyacrylate hybrid hydrogel changed to different colors in complex environments. When this TDM/PDM/polyacrylate hybrid hydrogel was placed on the skin of a wearer, its color dramatically changed from blue to white (Fig. 6f), reminding that her or his body temperature may be over 38 °C. Moreover, the TDM/PDM/polyacrylate hybrid hydrogel attached to the cheek of a wearer changed its color rapidly from blue to violet under strong sunlight (Fig. 6g), which indicates that UV light from sunlight at this time is strong and may damage her or his skin. Furthermore, the above color-changing hybrid hydrogels on the surfaces of flowers and leaves changed their color to the corresponding color of flowers or leaves (Fig. 6h). This excellent color changing property (e.g., a chameleon) will provide a potential application in the outdoor stealth garment.

Fig. 6 (a) Pictures of color changing of a chameleon. (b–d) Color changing and recovering of TDM/PDM/polyacrylate hybrid hydrogels under UV light, below 38 °C, under UV light and 38 °C, respectively. (e) Photos of different colors of the hybrid hydrogels. (f) Photos of color changing of the TDM/PDM/polyacrylate hybrid hydrogel on a hand of a wearer at different temperatures. (g) Photos of color changing of the TDM/PDM/polyacrylate hybrid hydrogel on a face of a wearer under different levels of sunlight. (h) Photos of color changing of the TDM/PDM/polyacrylate hybrid hydrogel on the surface of flowers and leaves for adaptive camouflage.

When the skin of a chameleon was injured, the chameleon could repair its skin and the color changing property was also recovered (Fig. 7a). In addition, self-healing of the TDM/PDM/polyacrylate hybrid hydrogel was visualized. The two parts of cut TDM/PDM/polyacrylate hybrid hydrogel came into contact with each other under heating from 25 °C at a rate of 2 °C min⁻¹, and the hybrid hydrogel healed (Fig. 7b). With the rise of the temperature to 38 °C at 2 °C min⁻¹, the TDM/PDM/polyacrylate hybrid hydrogel changed its color from blue to white, while this damaged hybrid hydrogel achieved the self-healing in 3 min. Notably, the occurrence of the discoloration can indicate that the hydrogel has achieves its self-healing. The above visual healing process was even more significant than microscopic microscopy and fluorescent labelling to monitor the self-healing process. After the hybrid hydrogel was placed back in the 25 °C environment, the color of the repaired hybrid hydrogel turned to blue, and the hydrogel sustained the twisting and stretching action without any cracks (Fig. 7c and d). Furthermore, the repaired TDM/PDM/polyacrylate hybrid hydrogel could change color rapidly (Fig. 7e). The discolored TDM/PDM/polyacrylate hybrid hydrogel also exhibited excellent stretching and twisting properties (Fig. 7f). The LED lamp could be lighted again by the repaired hybrid hydrogel (Fig. 7g), suggesting that its electrical conductivity was also recovered.

Fig. 7 (a) Figures of chameleons repairing their damaged colorful skin. (b) Visual self-healing process of the TDM/PDM/polyacrylate hybrid hydrogel. (c and d) Twisting and stretching properties of the repaired TDM/PDM/polyacrylate hybrid hydrogel. (e) Discoloration properties of the repaired TDM/PDM/polyacrylate hybrid hydrogel. (f) Stretching and twisting properties of the repaired TDM/PDM/polyacrylate hybrid hydrogel after discoloration. (g) Photo of the repaired TCP/PCP/polyacrylate hybrid hydrogel lighting a LED lamp.

Due to the color changing property and conductivity of the TDM/PDM/polyacrylate hybrid hydrogel, it can be applied to the monitoring of body temperature change based on color or resistance changes. As depicted in Fig. 8a, when a short circuit occurred in the TDM/PDM/polyacrylate hybrid hydrogel attached to the body, the monitored signal was immediately transmitted to a computer or mobile phone for early warning, so that the problem could be solved in time. Moreover, when a child had a fever, the TDM/PDM/polyacrylate hybrid hydrogel attached to the forehead was able to show a rapid color change, warning that the child had a temperature well above normal. To prove the above results, the TDM/PDM/polyacrylate hybrid hydrogel was placed on the palm of a hand to detect surface temperature change using a thermal camera, thus suggesting that the TDM/PDM/polyacrylate hybrid hydrogel was more sensitive to temperature responses at around 38 °C (Fig. 8b). Furthermore, the TDM/PDM/polyacrylate hybrid hydrogel exhibited various resistance change ratio (ΔR/R₀) when placed in different temperature environments (e.g., 25 °C, 38 °C, 65 °C, and 95 °C) (Fig. 8c). After 5 heating/cooling cycles between 25 °C and 38 °C, the resistance change ratio (ΔR/R₀) of the TCP/PCP/polyacrylate hybrid hydrogel still varied significantly (Fig. 8d). However, its resistance change ratio (ΔR/R₀) changed slightly at 25 °C. Furthermore, at 38 °C, the hybrid hydrogel still exhibited a sensing property quickly responding to the angle of a finger (Fig. 8e).

Fig. 8 (a) Early warning process for monitoring human fever and short circuit through color changing. (b) Real-time thermal photographs of the TDM/PDM/polyacrylate hybrid hydrogel taken by an infrared thermal imager during the heating process. (c) The resistance change ratio (ΔR/R₀) of the TDM/PDM/polyacrylate hybrid hydrogel at 25 °C, 38 °C, 65 °C and 95 °C. (d) The resistance change ratio (ΔR/R₀) of the TDM/PDM/polyacrylate hybrid hydrogel after heating/cooling cycles. (e) Sensing performance of the TDM/PDM/polyacrylate hybrid hydrogel at 38 °C.

4. Conclusion

In brief, a color-changing and self-healing TDM/PDM/polyacrylate hybrid hydrogel was well prepared by mixing the thermochromic dye microcapsules and photochromic dye microcapsules with the multi-branched polyacrylate. The prepared hybrid hydrogel exhibiting high conductivity and good tensile properties can serve as a sensor to monitor the actions of human joints or faces. Due to the robust ionic and multiple hydrogen bonding among the polyacrylate, zinc ions and TDM/PDM, the hybrid hydrogel exhibited an excellent self-healing capability, and it restores its mechanical, conductive and sensing properties after the self-healing process. Furthermore, the combination of TDM/PDM and polyacrylate synergistically enables the hybrid hydrogel to change color rapidly under heating or UV irradiation along with the variation of electrical resistance, which can be applied to the monitoring of human physiological health and for adaptive camouflage. The TDM/PDM/polyacrylate hybrid hydrogel prepared in this study exhibited a unique combination of self-healing, conductivity and color changing in a balanced way. The prepared self-healing and color-changing TDM/PDM/polyacrylate hybrid hydrogel will have extensive applications in human life activity monitoring, health monitoring, and safety warning in the future.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was financially supported by the Innovation Team and Talents Cultivation Program of National Administration of Traditional Chinese Medicine (ZYYCXTD-D-202206), and the Fundamental Research Funds for the Central Universities (JUSRP11918).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2tc03102g

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