Readily prepared and processed multifunctional MXene nanocomposite hydrogels for smart electronics

Booming sophisticated robotics and prosthetics put forward high requirements on soft conductive materials that can bridge electronics and biology, those soft conductive materials should imitate the mechanical properties of biological tissues and build information transmission networks. Until now, it remains a great challenge to handle the trade‐off among ease of preparation, high conductivity, processability, mechanical adaptability, and external stimuli responsiveness. Herein, a kind of readily prepared and processed multifunctional MXene nanocomposite hydrogel is reported, which is prepared via the fast gelation of cationic monomer initiated by delaminated MXene sheets. The gelation time can be adjusted (several seconds to minutes) based on the MXene loadings. By adjusting the MXene ratio, the resulting nanocomposites are ultrastretchable (>5000%), three‐dimensional (3D) printable, and show outstanding mechanical and electrical self‐healing. As expected, the integration of multifunctional systems onto various substrates (e.g., gloves and masks) is further demonstrated via 3D printing and could achieve diverse sensory capabilities toward strain, pressure, and temperature, showing great prospects as smart flexible electronics.


| INTRODUCTION
At the tide of the Internet of Things, booming sophisticated robotics and prosthetics stimulate the development of flexible electronics, which mimic the integrated mechanical properties of human tissues and appending skin-like perceptions (e.g., mechano-and thermosensation). 1,2Among them, flexibility and ideal stretchability that enable mechanical robustness and skin conformability are essential.7][8][9] However, for the majority of current conductive hydrogels, the compatibility between conductive additives and neighboring polymer networks remains a problem and may lead to a reduction of mechanical and conductive stability.On the other hand, high design flexibility and integration of multiple sensors hold great promise for future large-scale applications, whereas most are one-off prototypes and are often limited by difficulties in processability. 10Recently developed printed electronics based on metal 11,12 or carbon nanomaterials 13 combining printing technologies have demonstrated the viability for low-cost and highthroughput manufacturing of ever-smaller integrated electronic devices, while most of them require a postprinting treatment, usually thermal sintering, to obtain highly conductive patterns. 10Therefore, to fully seize the opportunities of these soft electronics, there is a need, yet challenging, to achieve devices with appropriate electromechanical properties, reconfigurability to integrate into a multifunctional electronic system, and large-area utilization.
8][19] These exceptional properties have fueled remarkable advancements in versatile fields such as energy storage, [20][21][22] electromagnetic shielding, 23 and sensing. 17,24,25It is anticipated that by taking full advantage of surface functional groups and negatively charged surfaces of MXene, more interesting properties and functions in MXene-based nanocomposites are worth exploring.
In this study, we report a kind of readily prepared and processed multifunctional MXene-based nanocomposite hydrogels fabricated via rapid self-initiated polymerization (several seconds to minutes) simply by mixing a small amount of delaminated MXene sheets with cationic monomers (methyl chloride quarternized N,Ndimethylamino ethylacrylate, DMAEA-Q), benefiting from surface interaction of monomers and MXene through both hydrogen bonds and electrostatic interaction.Different from the previously reported MXene nanocomposites, MXene flakes act in multiple roles here including initiator, crosslinker, and conductive filler.Moreover, multifunctional integrated systems onto various substrates (e.g., gloves and facemasks) are further realized via three-dimensional (3D) printing of its precursor and could achieve diverse sensory capabilities toward strain, pressure, and temperature.

| Materials
Hydrofluoric and hydrochloric acid were purchased from Shanghai Chemical Corp.Ti 3 AlC 2 (MAX) powder was purchased from Jilin 11 Technology Co., Ltd; the average particle size was 200 meshes.Lithium fluoride was purchased from Shanghai Macklin Bio-Chem Technology Co., Ltd.Cationic monomer DMAEA-Q (80 wt%) was purchased from J&K Scientific Ltd.Ammonium persulfate (APS) was purchased from Aladdin Chemical Co.

| Preparation of exfoliated-Ti 3 C 2 T x (MXene) dispersion
According to our previously reported method, the freezeand-thaw (FAT) method can assist in the exfoliation of the MXene. 26After four cycles of FAT, the thaw dispersion was centrifuged at 3500 r/min for 1 h.The concentration of supernatant MXene dispersion is about 19 mg/mL, which was calculated by measuring the final mass of the known volume of dispersion on a watch glass after drying.

| Preparation of MXene-based hydrogel
The MXene-based hydrogel was prepared by mixing MXene dispersion and DMAEA-Q monomer.A detailed composition is provided in Supporting Information: Table S1.After different volumes of MXene dispersion (0.1, 0.25, 0.5, and 1 mL) being added to 1 mL DMAEA-Q solution (80 wt%) in an airtight vessel, the mixture was put on a vortex mixer for about several seconds for uniform mixing.Then, the mixture was placed stationarily for about several minutes for the polymerization of monomers.For the sake of distinction, four samples were marked as M-n (n refers to the mass fraction of MXene to the monomer, and was calculated to be 0.24, 0.60, 1.2, and 2.4).After being left at ambient conditions (25 °C, 50% relative humidity) for at least a week, the equilibrated hydrogels were ready for mechanical and sensory tests.For comparison, pristine poly-DMAEA-Q (PDMAEA-Q) was prepared with the initiator of APS (0.2 mol% of the monomer amount) and proceeded at 70 °C for 6 h.

| 3D printing of the intrinsically stretchable hydrogel
The precursors of the intrinsically stretchable conductor (M-1.2 was chosen) could be used for 3D printing by a 3D BioArchitect workstation (Regenovo).Models for 3D printing were modeled in advance in the accompanying software.During 3D printing, the chosen tip diameter was 0.26 mm and the extrusion speed was 1 mm/s at 25 °C.After being dried for at least a week, the materials were ready for testing.

| Characterizations
Rheological measurements were performed on a HAAKE MARS modular advanced rheometer and on a parallel plate with a 25 mm diameter (PP25).The gelation process with different content of MXene was tracked by oscillation time sweep measurement at a constant strain of 0.1% and a frequency of 1 Hz after a mixture of MXene dispersion and DMAEA-Q was loaded onto the parallel plate rheometer.Viscometry was performed at shear rates ranging from 0.01 to 1000 s −1 at 25 °C.The rise in temperature of the MXene-DMAEA-Q mixture during the polymerization was detected by an infrared (IR) thermal probe (FLIR ONE@PRO) with an accuracy of ±1 °C.Electron paramagnetic resonance (EPR; Bruker A300) spectra were collected to testify the species of radical produced by MXene dispersion (19 mg/mL), in which 5,5dimethyl-1-pyrroline-N-oxide (DMPO) was used as the spin-trapping agent; experiments were conducted in the dark and at room temperature.Scanning electron microscopy (SEM) images were taken on a Zeiss Gemini SEM500 microscope with an energy-dispersive X-ray detector.Transmission electron microscopy images were taken by JEOL JEM2011 F Microscope operated at 200 kV.X-ray powder diffraction (XRD; X'pert PRO PANalytical) with Ni-filtered Cu Kα radiation (40 kV, 40 mA) was used.Fourier transform IR (FTIR) spectra were collected on a Nicolet 6700 spectrometer using the attenuated total reflectance method.Tensile tests were performed on a universal mechanical test machine (Instron 5966) at room temperature (at a deformation rate of 300 mm/min).Notably, since the samples had very large deformation, here the true stress (σ) instead of the nominal stress (σ 0 ) was used, which was calculated from the expression σ = σ 0 λ (λ is the stretch factor) based on the assumption that the samples were incompressible.Young's modulus was calculated by the average slope of 0%−30% of strain from the stress-strain curve.Cyclic tensile tests were carried out at a velocity of 100 mm/mim.The same sample before and after selfhealing was scanned using an X-Ray Computed tomography System (CT, Bruker, SkyScan 1176) at 18 µm spatial resolution (45 kV, no filter).DataViewer and CTVol were used to analyze the data and reconstructed to create a 3D rendering of the interior morphology of the samples.
For conductivity measurement, a cylinder-like conductor was first prepared.Then the resistance (R) was measured by an LCR meter (TH2830) with two Cu plates linked on both sides.The electrical conductivity (σ) is calculated from the following formula: σ L R r = / π 2 , where L and r represent the length radius, respectively.The real-time resistance measurements under various stimuli were obtained using the LCR meter at an AC voltage of 1 V and a sweeping frequency of 1 kHz.AFM images of MXene flakes were collected in a Bruker Multimode 8 instrument using the tapping mode.Zeta potential analysis of MXene dispersion was performed on a Zetasizer (Nano-ZS90).X-ray photoelectron spectra (XPS) of MXene and nanocomposite hydrogel were detected by a photoelectron spectrometer (KRATOS Axis Ultra Dld) with an Al Kα X-ray source (hv = 1486.6eV).Thermogravimetric analysis (TGA, PE Pyris 1) was carried out by heating the sample at 20 °C/min, under purging N 2 at 40 mL/min.

| EMI shielding performance measurements
The EMI shielding effectiveness was measured in an Agilent PNAN5244A vector network analyzer in the X-band frequency range (8.2-12.4GHz).All the hydrogel samples were cut into a rectangle shape with lateral sizes of 22.9 mm × 10.2 mm and placed on the opening of the sample holder for measurements.The total EMI shielding effectiveness (SE T ) is calculated as follows: where SE A , SE R , and SE M are the microwave absorption, microwave reflection, and microwave multiple internal reflection, respectively.When SE T > 15 dB, the SE M can be negligible.

| Fast self-initiation and polymerization process
Our material consists of only three components: water, exfoliated MXene nanosheets, and DMAEA-Q.Upon mixing and vigorous stirring, the cation monomers tend to reside on the surface of MXene via both hydrogen bonds and electrostatic interaction, which is more accessible to hydroxyl radicals (•OH) generated by MXene.The schematic diagram of preparation in Figure 1A and Supporting Information: Movie S1 shows that when the MXene content was only 0.6 wt%, the polymerization of cationic monomers occurred rapidly, which was confirmed by the vial inversion method.This is because the viscosity of the system increased significantly during the polymerization process.The rapid self-initiated polymerization process was also recorded by the pseudocolor images using an IR camera, in which the color changed uniformly as the surface temperature rose to approximately 45 °C, as the polymerization was an exothermic process (Figure 1B,C and Supporting Information: Movie S2).Interestingly, we found that the gelation rate could be adjusted by the MXene loadings, ranging from several seconds to minutes.To investigate this, we mixed the same amount of monomer and different content of MXene dispersion and then loaded the mixture onto a parallel plate rheometer for oscillation time sweep measurement.The obtained materials are termed as M-n, where n refers to the mass fraction of MXene to the monomer (from 0.24 wt% to 2.4 wt%); details of the process are summarized in Supporting Information: Table S1.As shown in Figure 1D, mixing monomers with more than 0.60 wt% MXene resulted in nearly immediate gelation: by the start of the rheology measurement, storage modulus (G′) already exceeded loss modulus (G″), exhibiting solid-like behavior.Moreover, G′ and G″ soon reach the stationary value, indicating the end of the polymerization.In contrast, when the MXene content is as little as 0.24 wt %, a relatively long time for gelation has been taken (Supporting Information: Figure S1).Since the added MXene was the aqueous dispersion, the initial water content and MXene content together affected the modulus G′ of as-prepared samples.The modulus G′ of the hydrogels with higher water content would be lower.But it is also considered that MXene also plays an enhancing role in the hydrogel.The M-0.60 had a lower initial water content (35%) in the prepared samples, and a higher MXene content compared to M-0.24, thus showing the highest initial G′.Compared with conventional freeradical polymerizations that take place under thermal conditions, photoradition or high-energy radiation (e.g., the use of a thermal initiator such as APS often requires more than 6 h of heating at the polymerization temperature, and the use of photoinitiator such as Irgacure 2959 often need more than 30 min under UV radiation), [27][28][29] our method is more efficient and energy-saving (Supporting Information: Table S2).
In Figure 2A,B and Supporting Information: Figure S2, MXene flakes with a lateral size of 5-10 μm and thickness of 1.8 nm were prepared via our previously reported FAT method, 26  have been reported to be able to initiate or catalyze the radical polymerization of acrylic monomers via the longtime sonication, 30 which inevitably undermines the integrity and stability of MXene flakes to some extent, and often at the expense of electrical properties.
EPR is a convincing tool for the detection and identification of paramagnetic species such as free radicals.To the best of our knowledge, some studies have investigated the EPR spectra of pristine MXene, but they did not obtain the signals of reactive radicals, 30,31 possibly because the concentration of their test samples is too low (e.g., 0.35 mg/mL).Surprisingly, MXene aqueous dispersion with a relatively high concentration (19 mg/mL), even without the peroxide decoration, displays an obvious hydroxyl radical (•OH) signal in several seconds using DMPO as the spin trapping agent, and the ratio of signal peaks is 1:2:2:1 (Figure 2C).Considering that illumination or heating has been reported to motivate MXene to produce radicals, 32,33 our EPR tests were carried out in a dark environment at room temperature.It is probable that hydroxyl radicals are derived from the defects in monolayers (Supporting Information: Figure S3), similar to what we found in the multilayer MXene, 34 which is highly pertinent to the defect concentration on the surface of MXene. 35,36Upon elevating the concentration of MXene dispersion, DMPO can easily capture short-lived radicals to form relatively long-lived radical-DMPO adducts.
Compared to MXene, the new peaks of F and Ti and the increased intensity of O element in the XPS spectrum of our material confirm the composite of MXene with the polymer (Supporting Information: Figure S4).The internal morphology of the resultant nanocomposite was shown in Figure 2D,E, where continuous and large flexible MXene flakes were embedded and closely integrated with the polymer matrix, demonstrating the construction of the conductive pathway.The element mapping results bear out the homogeneous distribution of MXene flakes (Supporting Information: Figure S5).Besides, from the superficial SEM images (Supporting Information: Figure S6), more MXene flakes were exposed on the surface of the hydrogel with the increased amount of MXene, indicating increased conductive paths constructed by MXene flakes. 37,38he zeta potential of electronegative MXene was measured to be −43.2mV (Supporting Information: Figure S7), which confirmed the electrostatic interactions with a positively charged matrix.The interaction between the MXene flakes and polymer matrix was further studied using FTIR spectroscopy (Figure 2F and Supporting Information: Figure S8).MXene flakes show characteristic peaks located at 3426, 1087, and 566 cm −1 , corresponding to −OH, C-F, and Ti-O, respectively.The stretching band of carbonyl groups (C = O) in the pristine PDMAEA-Q is located at 1728 cm −1 , which shifts to a lower wavenumber at 1724 cm −1 in our material, indicating the formation of hydrogen bonding interaction among the oxygencontaining functional groups, thereby enabling excellent interfacial compatibility.Besides, XRD was used to characterize the interlayer spacing between neighboring MXene flakes (Figure 2G).The existence of PDMAEA-Q on the MXene can induce expansion of the space between neighboring MXene flakes, 39,40 leading to the shift of (002) characteristic diffraction peak from 6.6°to 6.2°after polymerization.Meanwhile, there is no apparent characteristic diffraction peak of TiO 2 (Supporting Information: Figure S9), which also manifests that MXene did not suffer from oxidation after polymerization.

| Processability and 3D printability
Interestingly, the precursor mixture presents both processability and 3D printability, depending on the different addition of MXene aqueous dispersion.For the precursor mixture of M-0.24, there is a relatively long liquid-like time (G′ and G″ are very close) in the initiation and polymerization process (Figure 1D and Supporting Information: Figure S1), which can be utilized to shape by casting the mixture into different silicone molds as letter "A" and a heart (Figure 3A).Besides, the precursor mixture of M-1.2 displays shearthinning behavior (Figure 3B), which is suitable for direct ink writing.Therefore, as shown in Figure 3C,D, it can be printed on various substrates such as rubber gloves and elastomer tapes, showing a conformal contact with the dynamic curved interfaces.

| Mechanical and electrical properties
Outstanding mechanical performance is crucial for sophisticated soft electronic skins, sensors, and devices.In our work, after evaporating the excess solvent in air, interactions among polymers and MXene flakes are dominant for the reduced distance in the equilibrated hydrogels.In Figure 4A, the tensile stress and strain vary as the content of MXene, demonstrating ultrastretchability (>5000%).Upon increasing the mass ratio of MXene flakes from 0.24 wt% to 2.4 wt%, Young's modulus of the equilibrated hydrogels increases from 70 to 190kPa (Supporting Information: Figure S10), showing compliance with human tissues (e.g., 7 kPa for muscle and 85 kPa for skin 41 ).When the MXene mass ratio is 0.6 wt%, the resultant equilibrated hydrogel can be stretched up to 10,000%, which is greatly superior to the maximum strain of previously reported MXene hydrogel (3400%).Here, the true stress (σ) instead of the nominal stress (σ 0 ) was used.The ultrastretchability is primarily attributed to the robust and adaptive hybrid networks composed of abundant noncovalent interactions between exfoliated MXene sheets and polymer chains, which effectively dissipate energy during the stretching process.MXene flakes can act as high-functionality crosslinkers for their abundant surface functional groups and high specific surface areas. 42However, more MXene content (M-2.4) would impair extensibility as a result of high crosslinking density and internal aggregation of MXene flakes.When the M-1.2 is stretched to a shorter strain, it is also elastic and recoverable, demonstrated by a manual stretch-release process (Movie S3 and Supporting Information: Figure S11).Upon unloading the stress, it could recover to its original shape within 20 s.To further evaluate the resilience of the hybrid networks, the cyclic tensile tests at a strain of 500% were performed (Figure 4B), which exhibits a relatively small hysteresis loop and a low residual strain of 29% in the first loading-unloading cycle.The subsequent hysteresis loops remained almost unchanged, indicating excellent fatigue resistance.This also benefits from the existence of multiple dynamic interactions in the hybrid networks, which can act as temporary crosslinks to maintain the skeleton when subjected to short-term deformation and avoid the free sliding of the polymer chains. 43However, as the tensile strain increases (e.g.,1000%; Supporting Information: Figure S12), the hybrid polymer networks are more inclined to fatigue, which is also widely observed in chemically crosslinked materials. 44,45esides, multiple dynamic interactions also facilitate the formation of well-organized conductive pathways of MXene among the polymer matrix.The conductor can achieve a high conductivity with only a small amount of MXene (e.g., 1.0 S/m for M-0.24 and 7.1 S/m for M-2.4), which exhibits distinct advantages among the currently reported MXene-based conductive composites ( and Supporting Information: Figure S13).Comprehensive consideration of stretchability, conductivity, and processability, we chose M-1.2 for further measurements.
Without permanently crosslinked networks, the conductor possesses self-healing performance.According to the 3D reconstructed images from micro-CT, the fractured sample displays no apparent cracks after the 48 h selfhealing process without additional auxiliary means (Figure 4E).Moreover, the broken conductive network can also follow the dynamic reconstruction of the selfhealing polymer and autonomously heal to recover not only its mechanical properties but also its high conductivity. 46,47e monitored the resistance change of the conductor before and after damage, which was able to return to its normal baseline within seconds after the damage occurred (Figure 4F).A slight fluctuation in resistance could be observed in the case of slightly mismatched edges.
Moreover, benefitting from the ingenious combination of a highly conductive architecture, hydrophilic polymer chains, and an internal water-rich environment, 19,48,49 the assembled hydrogel may present surprising electromagnetic interference (EMI) shielding performance (Supporting Information: Figure S14).Taking M-0.6 and M-1.2 as examples, Supporting Information: Figure S15A shows the variation in the total shielding effectiveness (SE T ) at a thickness of 2 mm over a frequency range of 8.2-12.4GHz (X-band).The EMI shielding performances of reflection (SE R ) and absorption (SE A ) are also plotted in Supporting Information: Figure S15B.The average total EMI SE (SE T ) of M-0.6 and M-1.2 was determined to be 36.1 and 42.0 dB, respectively.With the increase of the MXene content, the EMI SE shows an obvious improvement.Such EMI SE T values over 20 dB are sufficient for many commercial applications.
Overall, the combination of properties in terms of the cost-saving manufacturing method, ultrastretchability, adaptability, high conductivity, autonomous selfhealability, and 3D printability makes the conductor promising for integration into wearable devices.

| Integrated conformal sensory systems
Based on the resistance circuit design, the conductor can report electronic responses to external strain, pressure, and temperature stimuli, imitating the mechanoreceptor and thermoreceptor of human skin.As shown in Figure 5A,B, the sensitive resistance response could be attributed to the contact or separation between MXene flakes caused by the deformation of this conductor under external mechanical forces.Specifically, upon the tensile deformation of 100%, the conductor shows increased resistance due to the expanded spacing between the MXene nanosheets. 50In comparison, the compressioninduced decreased distance and better contact between adjacent MXene nanosheets resulted in the declined resistance of the conductor (Figure 5B).Owing to the recoverability of the conductor along with robust interface interactions with MXene nanosheets, these electromechanical signals show good reversibility and reliability, even subjected to 100 consecutive cycles (Figure 5D,E).Moreover, the MXene networks with high thermal conductivity also ensure a fast temperature response by boosting the electron transmission efficiency with an increase in temperature. 25As a result, the resistance of our conductor decreases as the ambient temperature increases and remains stable among a relatively wide temperature range of 10-50 °C (Figure 5C,F), making it advantageous in complex environments.
We further show a flexible device with various function units integrated by 3D printing onto a rubber glove, which is mechanically compliant and self-adhesive (Figure 5G).It can not only act like a mechanoreceptor to detect finger movements, showing increasing resistance when the finger bends (Figure 5H and Supporting Information: Movie S4), but also serve as a thermoreceptor to sense environmental variations caused by a thermal lance, showing decreasing resistive signals (Figure 5I and Supporting Information: Movie S5).Moreover, respiration, another important signal, can be used as an early warning system for respiratory diseases such as sudden infant death syndrome or asthma. 51,52As shown in Figure 5J,K, the stretchable conductor can also be 3D printed onto a face mask and worn by a human; periodic respiration of a human produces reliable and stable resistance changes, allowing for real-time monitoring of the respiratory rate and depth.Respiration under different breathing patterns (normal or deep breathing) is consistent with resistance changes caused by changes in temperature and pressure of the nasal airflow. 53

| CONCLUSIONS
In summary, we report a 3D printable and ultrastretchable MXene nanocomposite hydrogel via the fast gelation of cationic monomer initiated by delaminated MXene sheets.The resulting conductors behave with excellent elastic and self-healing capability by taking advantage of the multiple dynamic interactions, including electrostatic and hydrogen bonding interaction.Meanwhile, large-size and intact MXene flakes evenly dispersed in the matrix endow the resulting conductors with prominent electromechanical performance and great EMI effectiveness over 40 dB at X-band.By varying the MXene ratio, the conductors are customizable either by 3D printing or molding, which also enables us to realize the integration of electronic sensors.Due to the facile one-step synthesis method and versatile performance, the strategies and materials developed in this work can be extended to other intriguing materials and applications.
Schematic illustration and identification of rapid self-initiation and polymerization process.(A) Schematic illustration of rapid polymerization process by simply mixing delaminated MXene with cation monomers.(B) The vial-inversion method to verify the polymerization.(C) Infrared (IR) images upon mixing (left) and during polymerization (right).Scale bars are 1 cm.(D) Gelation process with different content of MXene tracked by measuring storage modulus (G′) and loss modulus (G″) as a function of time at a constant strain of 0.1% and a frequency of 1 Hz.DMAE-Q, methyl chloride quarternized N,N-dimethylamino ethylacrylate; PDMAEA-Q, pristine poly-DMAEA-Q.
which ensures successful implementation of high exfoliation yield for large MXene flakes.To date, peroxide-and TiO 2 -decorated MXene F I G U R E 2 Synthesis and characterization of MXene and our material.(A) Transmission electron microscopy (TEM) and (B) atomic force microscopy (AFM) images of exfoliated MXene flakes.(C) Electron paramagnetic resonance (EPR) spectra of MXene dispersion (19 mg/mL).(D) TEM and (E) scanning electron microscopy (SEM) images of our material.(F) Fourier transform infrared (FTIR) spectra of pristine poly-methyl chloride quarternized N,N-dimethylamino ethylacrylate (PDMAEA-Q), MXene, and our material.Dotted lines are the second derivative curves.(G) X-ray diffraction (XRD) patterns of MXene and our material.

F I G U R E 3
Processability and three-dimensional (3D) printability.(A) The processability of the precursor mixture (M-0.24) and various large shapes can be made by casting the mixture into different silicone molds (scale bars: 1 cm).(B) Shear-thinning behavior of the precursor ink (M-1.2).Photographs showing 3D printed patterns on various substrates, including (C) rubber gloves and (D) an elastomer tape (scale bars: 2 cm).

F I G U R E 4
Mechanical and electrical properties.(A) True tensile stress-strain curves of the equilibrated hydrogels with different contents of MXene.(B) Cyclic stress-strain curves of the stretchable equilibrated hydrogel in the strain range of 500%.(C) The conductivity of the equilibrated hydrogel as the MXene content changes.(D) Schematic illustration of the self-healing process based on dynamic interactions.(E) Micro-CT three-dimensional reconstructed images of the equilibrated hydrogel after fractured (up) and self-healed (down).(F) The electrical self-healing capacity of the equilibrated hydrogel measured by real-time resistance.

F
I G U R E 5 (A) The reversible resistance-strain curves as the strain sensor.(B) The reversible resistance-pressure curves as the pressure sensor.(C) The reversible resistance-temperature curves as the temperature sensor.(D) Resistance-strain cyclic curves in the strain range of 0%-100%.(E) Resistance-pressure cyclic curves in the pressure range of 0-6.9 kPa.(F) Resistance-temperature cyclic curves in the temperature range of 10-50 °C.(G) A photo shows the integrated mechanoreceptor and thermoreceptor on a rubber glove.(H) Mechanoreceptor: Detection of finger movements.Inset photographs are from Supporting Information: Movie S4. (I) Thermoreceptor: Detection of the environmental temperature variations caused by a thermal lance.Inset photographs are from Supporting Information: Movie S5. (J) The wearable mask with a printed conductor, which can be used for (K) respiratory monitoring (scale bars: 5 cm).