Achieving near-infrared-light-mediated switchable friction regulation on MXene-based double network hydrogels

MXene possesses great potential in enriching the functionalities of hydrogels due to its unique metallic conductivity, high aspect ratio, near-infrared light (NIR light) responsiveness, and wide tunability, however, the poor compatibility of MXene with hydrogels limits further applications. In this work, we report a uniformly dispersed MXene-functionalized poly-N-isopropylacrylamide (PNIPAM)/poly-2-acrylamido-2-methyl-1-propanesulfonic acid (PAMPS) double network hydrogel (M—DN hydrogel) that can achieve switchable friction regulation by using the NIR light. The dispersity of MXene in hydrogels was significantly improved by incorporating the chitosan (CS) polymer. This M—DN hydrogel showed much low coefficient of friction (COF) at 25 °C due to the presence of hydration layer on hydrogel surface. After illuminating with the NIR light, M—DN hydrogel with good photothermal effect rapidly raised the temperature to above the lower critical solution temperature (LCST), which led to an obvious increase of surface COF owing to the destruction of the hydration layer. In addition, M—DN friction control hydrogel showed good recyclability and controllability by tuning “on-off” of the NIR light. This work highlights the construction of functional MXene hydrogels for intelligent lubrication, which provides insight for interface sensing, controlled transmission, and flexible robotic arms.


Introduction
In modern society, with increasing environmental complexity, the research and development of artificial intelligence materials is on the agenda.Stimuliresponsive interface materials can rapidly achieve reversible physical or chemical property transformations when stimulated by temperature, pH value, magnetic field, light, and so on [1][2][3][4][5].These materials are mainly based on stimuli-responsive polymers [1] and phase-change materials [6].On the basis of the original response, adding nanoparticles to enrich the versatility of materials has become the main research direction [1,7,8].Meanwhile, compared with passive stimulation, active regulation is more controllable.
Poly-N-isopropylacrylamide (PNIPAM)-based hydrogels have become representative stimuliresponsive hydrogels.When below the lower critical solution temperature (LCST), the PNIPAM molecular chain stretches.Hydrogen bonds are formed, which causes the appearance of a hydrated layer.In contrast, when above the LCST, the PNIPAM molecular chain shrinks.The hydrogen bonds are broken, which causes the disappearance of the hydrated layer [9].Taking advantage of this property, it has been widely used in many situations, such as drug delivery [10], temperature sensors [11], smart actuators [12], friction regulation [13][14][15], and so on.Among them, the 40 Friction 12(1): 39-51 (2024) | https://mc03.manuscriptcentral.com/frictionapplication of friction regulation has gradually attracted attention.For example, Zhu et al. [16] combined PNIPAM as a coating on the surface of polydimethylsiloxane (PDMS) to achieve mutual conversion between hydrophilic lubrication and antibacterial properties.To achieve active regulation, incorporating materials that respond to stimuliresponsive heating inside PNIPAM hydrogels becomes particularly important.Chen et al. [13] fixed Fe 3 O 4 inside PNIPAM microgels and realized the purpose of actively utilizing the stimulus response of near-infrared (NIR) light to control the COF of water lubrication.
MXene is a kind of two-dimensional lamellar material composed of transition metal carbides, nitrides, or carbonitrides with unique metallic conductivity, high aspect ratio, near-infrared photoresponsivity, and widely tunable properties [17].In addition, Ti 3 C 2 T x is one of the most widely used MXene materials, where T x represents different surface termination groups (e.g., OH, O, and F) [17][18][19].Because of its excellent properties, MXene is widely used to realize the functionalization of hydrogels.For example, the excellent electrical conductivity and two-dimensional sheet structure is conducive to the preparation of motion sensors, electrical conductivity and NIR light stimuli-responsiveness benefit the preparation of programmable stimuli-responsive hydrogels and the electromagnetic shielding ability helps prepare electromagnetic shielding materials [12,20,21].However, the applications of MXenes for switchable friction regulation are deficient.In previous reports, MXene easily exhibits agglomeration behavior.This is because various interactions between the MXene surface groups and the polarized groups of various molecules can form in the hydrogel prepolymerization solution [22,23].It is particularly important to make MXene disperse uniformly in the hydrogel, which can increase the utilization of MXene and prevent the hydrogel from bending due to uneven distribution.
In this work, we utilized hydrogen bonding between CS and MXene nanosheets, which greatly reduced the possibility of contact between the surface groups of MXene nanosheets and various molecules in the prepolymerization solution.Moreover, the larger viscosity of the CS solution slowed the settling rate of the inner nanosheets.The treated CS-MXene was then combined with the traditional double-network hydrogel [24], in which 2-acrylamido-2-methyl-1propanesulfonic acid (AMPS) was used as the first-layer network and NIPAM was used as the second-layer network.M-DN hydrogels that can realize the regulation of the COF at the interface by NIR light were prepared.The large amount of free-flowing water inside the M-DN hydrogel provides the possibility for hydration lubrication.During the friction process, the free-flowing water was affected by the amide groups on PNIPAM and formed a hydrated layer, which can reduce the COF between the interfaces.When irradiated with NIR light, the MXene nanosheets reacted rapidly and generated a large amount of heat.As the temperature increased, PNIPAM underwent a phase transition.The hydrogen bonds between PNIPAM chains and water molecules were broken.Thus, the hydrated layer was destroyed, and the COF was increased on the interface [25][26][27].
Through this work, MXene (Ti 3 C 2 T x ) nanosheets were treated through hydrogen bonding interactions between CS and MXene, which caused MXene to be coated with chitosan to weaken the interaction between AMPS and MXene.In this way, a dispersion method of MXene nanosheets in hydrogels was proposed.Thus, the friction regulation of the hydrogel surface through the photothermal effect of MXene nanosheets was realized, which further expanded the application of MXene hydrogels.This friction-tunable hydrogel has great potential in the fields of frictioninterface sensing, intelligent manipulators, and controlled transportation.

Preparation of MXene nanosheets
MXene nanosheets were prepared in two main steps.First, Ti 3 AlC 2 (MAX) was etched to obtain few-layered Ti 3 C 2 T x MXene flakes.3.2 g of LiF, 10 mL of pure water, and 30 mL of concentrated hydrochloric acid were added to a 50 mL polytetrafluoroethylene beaker.The mixed solution was placed in a water bath with a stirrer at 40 °C.Subsequently, added 2 g of MAX into the beaker in several portions.The reaction lasted for 24 h.Then, the solution was centrifuged many times www.Springer.com/journal/40544| Friction (3,500 rpm, 5 min) using pure water until the liquid in the centrifuge tube was no longer translucent.The suspension containing few-layered MXene flakes was collected.The collected suspension was centrifuged (10,000 rpm, 10 min) to remove water.Thus, few-layered MXene flakes could be obtained after the freezedrying treatment of the sedimentation.Second, 200 mg of freeze-dried MXene was added to a 100 mL jacketed reactor with 50 mL of pure water.The above dispersion was sonicated for 2 h at 10 °C with 30% power using a probe sonicator.The treated solution was directly lyophilized to obtain dry MXene nanosheets.

Preparation of homogeneously dispersed M-SN hydrogels
2 mg (4, 6, and 8 mg) of freeze-dried MXene nanosheets was dispersed into 5 mL of pure water through water bath ultrasonic.Then, added 0.1 g of CS into the solution.Two drops of glacial acetic acid were added to help dissolve the CS.The above solution was stirred at high speed for 1 h.The subsequent solution was labelled as solution A. Next, 4 g of 2-acrylamido-2-methylpropanesulfonic acid (AMPS), 90 mg of MBAA crosslinker and 10 mg of 2-oxoglutaric acid initiator were added to 5 mL of pure water to obtain homogeneous solution B. Solutions A and B were uniformly mixed, and the air bubbles in the mixed solution were removed through water bath ultrasonic.Then the mixture was photoinitiated under UV irradiation for 30 min to form a covalent cross-linked hydrogel with MXene intercalation.Finally, the prepared hydrogel was immersed in pure water for 24 h to obtain the M-SN hydrogel.

Preparation of homogeneously dispersed M-DN hydrogels
4 g of NIPAM, 5.4 mg of MBAA crosslinker, and 10 mg of 2-oxoglutaric acid initiator were added to 20 mL of pure water to obtain a homogeneous PNIPAM prepolymerization solution.Then, the M-SN hydrogel was placed in a petri dish to fully soak in the prepolymerization solution at 20 °C for 48 h.The treated SN hydrogels were removed.The upper and lower surfaces of the hydrogels were adequately covered by glass plates.Double network hydrogels with uniformly dispersed MXene were obtained by UV light initiation at 20 °C for 30 min.Finally, the M-DN hydrogels were immersed in pure water for 24 h.

Characterization
An X-ray diffractometer (Bruker D8 X-ray, Germany) was used for X-ray diffraction (XRD) measurements.
Transmission electron microscopy (TEM) (FEI, Talos F200X, and USA) was used to characterize the morphology of MXene nanosheets.Porous morphology of lyophilised hydrogel sections obtained by scanning electron microscopy (SEM) (FEI, Helios G4 CX).Characterization of CS and MXene interactions was performed using a TENSOR II Fourier transform infrared (FTIR) spectrometer (Bruker, Germany).The size of the MXene nanosheets was measured using a particle size analyser (Malvern Instruments, UK).X-ray photoelectron spectroscopy (XPS) (PHI 5000 VersaProbe III) with an Al Kα X-ray source was used for elemental analysis.For the photothermal performance study, M-DN hydrogels were placed in a petri dish with appropriate pure water.Then, they were irradiated with an NIR laser (BST808-5-F, Xi'an Best Laser Optronics Co., Ltd.) at a wavelength of 808 nm.The irradiation diameter and distance were 1 mm and 5 cm, respectively.The temperature was measured by infrared thermography (FLIR, E8-XT, and USA).

Mechanical performance tests
The tensile strength at break of the M-SN hydrogel and M-DN hydrogel was tested using a universal mechanical testing machine (INSTRON 5982).For the test at 50 °C, a layer of silicone oil was applied to the surface of the hydrogel to prevent water loss.The constant temperature of 50 °C in the chamber was remained for 5 min before the test of tensile strength at break.The sample was processed into I-beam shape.Their width and length were 4 and 10 mm, respectively.The stretching speed was 50 mm/min.

Rheology testing
The G' and G'' moduli of the M-SN hydrogels and M-DN hydrogels were tested with frequency and temperature as variables using a rotational rheometer (HAAKE MARS III, USA).Tests were carried out on circular hydrogels with a diameter of 25 mm.Frequency change tests were carried out from an angular frequency of 100 rad/s to an angular frequency of 0.1 rad/s.Before the test, the sample was kept at a constant temperature of 25 °C for 5 min before being heated to 50 °C at a rate of 2.0 °C/min.The load was maintained at 0.1 N and the angular frequency at 10 rad/s.

Tribological test
The tribological properties of the M-DN hydrogels were tested using a ball-disk contact reciprocating friction tester (UMT-3, Bruker, Germany).In all tests, the lower surface was wiped to dry and fixed in the sink using double-sided tape to prevent slippage when rubbing.After fixing the M-DN hydrogel in the water bath, the entire surface of the sample was kept flat.Deionized water was added to make water surface parallel to the upper surface of the sample.An amount of water was dripped between the upper friction pair and the sample to maintain the hydrated layer during the whole friction when the NIR light was switched off.The upper friction pair was made of glass ball with 5 mm in diameter to maximize the transmission of NIR light.The reciprocation stroke was 5.0 mm.The COF was derived with the help of software by dividing the frictional force by the normal load.

Synthesis and characterization of M-DN hydrogels
Figure 1 shows the dispersion principle of MXene (Ti 3 C 2 T x ) and the preparation process of the M-DN hydrogel.There were a large number of groups (such as -OH, -O, and -F) on the surface of the MXene sheet, as shown in Fig. 1(a).These groups can form strong hydrogen bonds with the molecular chains in the hydrogel network.The MXene surface groups interact easily with the polar groups of various molecules in the hydrogel prepolymerization solution [22,23], which results in the agglomeration of MXene sheets and prevents the MXene sheet from being uniformly dispersed in the hydrogel.In addition, this phenomenon was more apparent as the concentration of MXene sheets increased.To solve this problem, a small amount of CS was introduced to promote the dispersion of MXene, as shown in Fig. 1(b).CS played a dual role in the MXene dispersion problem.First, the strong hydrogen bonds between the hydroxyl as well as amino groups of CS and the hydroxyl groups on the MXene surface can weaken the interaction between the MXene surface groups and molecules of the prepolymerization solution.Second, CS triggered the transformation of the solution into a sol state because of its feature of a short-chain polymer, which could slow down the sedimentation rate of MXene.On this basis, a method for friction-controlled M-DN hydrogels was devised [24], which is shown in Fig. 1(c).The mass of this hydrogel before and after lyophilization changed from 0.3251 to 0.0512 g, possessing a water content of up to 84.3 wt%.As shown in Fig. 1(d), suitable initiators were tested to verify the effect of CS on the MXene dispersion.The thermal initiator (potassium persulfate, KPS) and the photoinitiator (2-oxoglutaric acid) were added to the same concentration of MXene nanosheet dispersion.The results showed that KPS caused the agglomeration of MXene nanosheets quickly, while 2-oxoglutaric acid did not.Therefore, it was more suitable to use photoinitiator to prepare hydrogels.In addition, the monomer also had an effect on the MXene nanosheets.As shown in Fig. S1(a) in the Electronic Supplementary Material (ESM), the surface in the etched Ti 3 C 2 T x contained a large number of hydroxyl groups that could interact with water molecules, resulting in excellent dispersion.However, agglomeration rapidly occurred after a certain amount of AMPS was dissolved.This was attributed to the phase separation caused by the interaction between ionized AMPS and MXene.It was interesting to note that the solution in which the MXene nanosheets and AMPS were comingled gradually gelled with time (Fig. S1(b) in the ESM).This was enhanced with increasing concentrations of MXene nanosheets.The phenomenon also poses difficulties for the preparation of MXene hydrogels.The modification of MXene with chitosan significantly addressed these problems.When equal concentrations of MXene and CS-MXene were added to the AMPS prepolymerization solution, the unmodified MXene agglomerated in 10 s, while CS-MXene still had good dispersibility after 24 h.The two solutions were further initiated to form a hydrogel.As shown in Fig. 1(e), CS-MXene was dispersed uniformly, and the whole material was not bent.This highly dispersed method was beneficial to M-DN hydrogels with a larger receptive specific surface area when irradiated by NIR light, and the high transparency could promote fully exothermic irradiation parts.
To uniformly disperse MXene inside the hydrogel, further reduction of MXene size was considered based on the previously reported Ti 3 AlC 2 (MAX) etching method [17,18].Probe sonicator was used to obtain smaller MXene nanosheets (Fig. S2 in the ESM).The XRD was performed to compare MAX with MXene.aluminium (Al) was etched completely, and the (002) peak in the MAX phase was shifted from 9.5° to 7.0°.Compared with the previously reported multilayer MXene sheet (002) peak shifts (9.5° to 9.0°), the shift amplitude of the MAX phase is larger due to the greater distance between the layers of the sheet [18].It indicates that few-layer or single-layer MXene nanosheets were obtained (Fig. 2(a)).The morphology of MXene nanosheets was characterized by TEM.It was observed that the MXene nanosheets exhibited a nanoscale oligomeric state, which was consistent with the particle size scale tested in Fig. 2(b).Energy dispersive spectrometry (EDS) analysis showed that a large number of oxygen (O) and fluorine (F) groups were distributed on the MXene nanosheet surface, which demonstrated that the probe sonication approach did not destroy the various groups on the MXene surface (Fig. S3 in the ESM).The composition and changes in the elements of MXene were investigated by X-ray photoelectron spectroscopy (XPS).The full spectrum showed distinct peaks of O and F on the surface of the MXene nanosheets.The high-resolution XPS spectra of Ti 2p, fitted by Ti 2p3/2 to Ti 2p1/2, were consistent with other previous reports [28,29], which confirmed that MXene nanosheets were successfully prepared (Figs.2(c) and 2(d)).
After modification by CS, the elements on the CS-MXene surface were analysed by XPS.It was evident that the peaks of titanium (Ti) and F did not appear in the full spectra.Instead, the nitrogen (N) peak of CS appeared, indicating that CS was encapsulated on the MXene surface (Fig. 2(c)).The | https://mc03.manuscriptcentral.com/frictionbinding mode between CS and MXene was recognized by FTIR spectra.As shown in Fig. 2(e), the shift in the -OH characteristic peak from 3,434 cm -1 (red line) to 3,424 cm -1 (blue line) suggests the formation of strong hydrogen bonding interactions between CS and MXene [21].Comparing the MXene sheet modified with CS to the unmodified MXene after lyophilization, CS did not affect the overall black color.This means that CS had essentially no effect on the NIR light response of the MXene.In addition, the CS-modified MXene evidently became more compact and difficult to crush due to the excellent fixation effect of CS on MXene, which made it difficult to peel (Fig. 2(f)).After mixing CS-MXene with the prepolymerization solution, the hydrogel was prepared.The large pores of several hundred microns inside the single network hydrogel and double network hydrogel were observed by SEM.The messy microporous structure of the single network hydrogel contrasts sharply with the uniform regular pore structure of the dual network hydrogel, as shown in Figs.2(g)-2(i) and 2(g)-2(ii).This regular pore structure was more conducive to improve the hydrogel strength and form more hydrated layers.The EDS analysis showed that the basic carbon (C) and O elemental distribution was consistent with the holes.In addition, the Ti elements did not show large-scale agglomerations, confirming that the modified MXene was more uniformly dispersed.

Characterization of the mechanical properties of M-DN hydrogels
The PNIPAM component in the M-DN hydrogel acted as both a ductile substance and a molecular chain in Meanwhile, the molecular chains contracted, resulting in poor ductility.Therefore, the stress and strain of the M-DN hydrogel were reduced but still remained at 0.15 MPa (Fig. 3(a)) [30].The compressive strengths of the M-SN and M-DN hydrogels were tested to reveal the compression condition during the friction process.The results showed that the M-DN hydrogel had a strong compression resistance of 1.0 MPa, which far exceeded the strength of the M-SN hydrogel and was sufficient to guarantee its strength for use at low loads (Fig. 3(b)).To further investigate the strength differences between the single and double network hydrogels, rheological tests were carried out on the M-SN hydrogels and M-DN hydrogels.The energy storage modulus (G') and loss modulus (G") of the double network hydrogels were both an order of magnitude higher than those of the single network, exhibiting greater energy dissipation and use strength (Fig. 3(c)).
To measure the phase change temperature of the M-DN hydrogels, a temperature scan of their mechanical properties was carried out by using a rheometer.The sudden rise in G" at approximately 40 °C confirmed that PNIPAM underwent a phase change and molecular chain contraction.It transformed from the hydrophilic state to hydrophobic state at this point, which provided a reference for the subsequent frictional regulation of the temperature inflection point (Fig. 3(d)).In addition, the hydrogel's resistance to both tensile and compressive processes was accompanied by covalent bond breakage, which was a major challenge for the versatile applications of the hydrogel.Ten loading-unloading tensile tests at 30% strain were carried out on M-DN hydrogels at room temperature.The stresses and strains remained essentially unchanged, indicating good resilience with little breakage of covalent bonds under these tensile strain conditions (Fig. S5(a) in the ESM).Then, loadingunloading compression tests were carried out at  different strains (15%, 18%, and 20%).It was evident that the compression strength decreased from 0.29 to 0.2 MPa (approximately 69.0% of the original value) as the number of cycles increased after 10 cycles of the 15% strain test.As the strain increased, more covalent bonds were broken.The magnitude of this change increased sequentially but remained high compressive strength (Figs.S5(b)-S5(d) in the ESM), which endowed the M-DN hydrogel with high strength to prevent breakage in friction applications.

Near-infrared photoresponse properties of M-DN hydrogels
The MXene nanosheets in the M-DN hydrogel conferred unique NIR photoresponsiveness to the hydrogel.As shown in Fig. 4(a), the hydrogel was placed in a petri dish with the addition of an appropriate amount of water.It was found that the response temperature showed a gradient increase for different concentrations of MXene nanosheets doped into the hydrogel.M-DN hydrogels were stimulated by NIR light before and after the response.It could be visually observed that the M-DN hydrogel became whitish from a completely transparent state, and the water spilled around it.This phenomenon occurred because the PNIPAM molecular chain became hydrophobic and contracted, destroying the surface as well as the internal hydration layer of the hydrogel (Fig. 4(b)).
The effects of the MXene nanosheet concentration and NIR light intensity on the M-DN hydrogels were further evaluated.As shown in Fig. 4(c), the temperature of five different concentrations of MXene nanosheets under NIR light irradiation was tested.The results showed that the changing rate of temperature and final value increased with concentration of MXene nanosheets.Then, the effects of different irradiation powers on the M-DN hydrogel (0.8 mg/mL) were explored.Figure 4(d) shows that both the changing rate of temperature and the final value increased with power, even reaching over 80 °C at 2.0 W/cm 2 .Combined with the previous rheological tests, it www.Springer.com/journal/40544| Friction was concluded that 40 °C was the phase transition temperature for this system.Thus, an irradiation power of 1 W/cm 2 was selected to modulate the 0.8 mg/mL hydrogel to achieve a fast response and a low upper limit temperature.Under the above conditions, the M-DN hydrogel of 0.8 mg/mL was tested for cycling at NIR light-regulated temperature with the boundary line of 40 °C.The M-DN hydrogel could quickly rise above that temperature under NIR light stimulation, while it quickly cooled to below 40 °C with the help of the surrounding water when the NIR light was switched off.The phenomenon remained repetitive under three cycles (Fig. 4(e)), which achieved a stable and rapid NIR light-stimulated phase transition.

Friction-modulated properties of M-DN hydrogels
To investigate the effect of NIR light on the friction modulation of M-DN hydrogels, the COF of hydrogels was obtained by a universal mechanical tester (UMT-3) in reciprocating mode.The glass ball and the hydrogel were used as the upper and the lower friction pair, respectively.As shown in Fig. S6(a   | https://mc03.manuscriptcentral.com/frictionlayer on the surface of the hydrogel was the key to achieve low friction.When the NIR light was turned on, MXene rapidly responded and generated a large amount of heat, which raised the temperature of the hydrogel above the LCST.At this point, PNIPAM underwent a phase transition into the hydrophobic state.Consequently, the surface hydration layer was destroyed rapidly.At the same time, the internal hydration layer also decreased.These aspects decreased the friction reduction capacity and thus increased the COF [27,31,32]. Initial tests of the loads and frequencies required for hydrogel testing were carried out prior to modulation.The results showed that the COF continued to increase as the load increased.Even though the double network hydrogel was still a flexible material, the friction pair caught in the hydrogel under higher load conditions, increasing the resistance during friction.The friction pairs didn't completely contact at lower load conditions.Therefore, 1 N was chosen as the load pressure for all subsequent tests.Friction was unstable both at high and low frequency conditions.Thus, the tests were carried out at 1 Hz (Fig. S7 in the ESM).As shown in Fig. 5(b), the power of the NIR light was adjusted after the COF was stable for 300 s.The COF of the hydrogel changed from ~0.02 to ~0.15 with increasing power.This change rate increased with power, which was consistent with the previous NIR light response rate.In Fig. 5(c), the comparison on the response rates of hydrogels with different MXene nanosheet concentrations under NIR light (1 W/cm 2 ) were conducted.It took approximately 100 s to reach the highest COF for the sample with a 0.8 mg/mL MXene nanosheet in M-DN hydrogel.It was evident that the COF of the hydrogel changed more dramatically under NIR light irradiation with the increase of MXene nanosheets concentration.Three successive modulated cycle tests were performed on 0.8 mg/mL M-DN hydrogels under NIR light with a power of 1 W/cm 2 .Repeated switching of the NIR light stimulated the MXene nanosheet response.It enabled the M-DN hydrogel with the fast transition between low and high COF.The low and high COF value always remained approximately 0.02 and 0.135, respectively.This is because the uniformly dispersed MXene nanosheets enhanced their utilization, thus prompting a rapid increase in temperature.The surface and a certain amount of internal hydration layer could be quickly destroyed by the phase transition of PNIPAM.When the ambient temperature returned to room temperature after the water loss, PAMPS had strong water absorption capacity as well as the recovery of hydrophilicity.The rapid absorption of pure water in the tank increased the amount of hydrated layer.As a result, the COF returned to the original value (Fig. 5(d)).To further verify the modulation stability of the M-DN hydrogels under 1 N loading condition, three samples were taken for ten modulation cycles.As shown in Fig. 5(e), the mean COF value remained at approximately 0.02 for low values and 0.135 for high values.It indicated that the M-DN hydrogel was more stable under 1 N loading condition for modulation.This confirmed the potential of M-DN hydrogels as controlled drives, flexible robotic arms, and gripping soft materials [33].

Conclusions
In summary, we achieved the construction of a well-disperse MXene double-network hydrogel by incorporating the chitosan (CS) polymer, which could rapidly responded to the near-infrared light (NIR light) with good recyclability and controllability.Poly-N-isopropylacrylamide (PNIPAM) polymer was selected as the second layer of the MXene dual-network hydrogel, which could increase both the hydrogel strength and the sensitivity under the external stimuli.The thermo-responsive behavior of PNIPAM polymer endowed the MXene-functionalized PNIPAM/CS double network hydrogel (M-DN hydrogel) with rapid transition between the low friction state (μ~0.021) and the high friction state (μ~0.135)by forming and disrupting the hydration layer below and above the lower critical solution temperature (LCST), respectively.This MXene-based hydrogels show great potential in friction modulation and intelligent lubrication.In addition, the high strength of M-DN enabled the potential application in interfacial sensing, controlled actuation, and flexible robotic arms.

Fig. 1
Fig. 1 Dispersion of MXene improvement and the preparation of M-DN hydrogels.(a) Chemical structure of MXene nanosheets; (b) schematic diagram of the hydrogen bonding interaction between CS and MXene nanosheets; (c) schematic diagram of the preparation of M-DN hydrogels; (d) comparing dispersibility of MXene before and after improvement; and (e) difference between the CS-MXene@PAMPS single network hydrogel (M-SN hydrogel) and MXene@PAMPS hydrogel.

Fig. 3
Fig. 3 Characterization of the mechanical properties of M-DN hydrogels.(a) Results of tensile strength at break of the M-SN hydrogel and M-DN hydrogel at 25 and 50 °C, respectively; (b) comparison of the compressive strength of M-SN hydrogels with M-DN hydrogels; (c) comparison of the energy storage modulus and loss modulus of the M-SN hydrogel and M-DN hydrogel; and (d) variation in the energy storage modulus and loss modulus of the M-DN hydrogel from 25 to 50 °C.

Fig. 4
Fig. 4 NIR photoresponse properties of M-DN hydrogels.(a) Infrared thermal images of M-DN hydrogels with different MXene nanosheet concentrations and different NIR light irradiation times; (b) schematic diagram of the M-DN hydrogel response and optical images before and after the response; (c) temperature curves of M-DN hydrogels with different MXene nanosheet concentrations as a function of irradiation time at NIR light irradiation power of 1 W/cm 2 ; (d) temperature versus time curves for 0.8 mg/mL M-DN hydrogels irradiated with different powers of NIR light; and (e) temperature cycling profile of the 0.8 mg/mL M-DN hydrogel under NIR light irradiation at a power of 1.0 W/cm 2 .
) in the ESM, the downwards pressure volume of the glass sphere was difficult to calculate accurately.The double network hydrogel was stiffer than conventional hydrogels.To test the contact area, dye was applied to the glass spheres.The size of the contact area between the glass sphere and the hydrogel was tested indirectly by pressing the glass sphere down onto the paper on the surface of the hydrogel.As shown in Fig.S6(b) in the ESM, the contact area was approximately 3.14 mm 2 and was fully covered by NIR light.As shown in Fig.S6(c) in the ESM, the NIR light during the experiment was illuminated in a circle with a diameter of 1 cm, which was sufficient to cover the entire test area of 5 mm reciprocating stroke.As shown in Fig.5(a), without the NIR light irradiation, PNIPAM was hydrophilic.The hydrated