Mechanically Tough and Highly Stretchable Hydrogels Based on Polyurethane for Sensitive Strain Sensor

Hydrogels with flexible and stretchable properties are ideal for applications in wearable sensors. However, traditional hydrogel-based sensors suffer from high brittleness and low electrical sensitivity. In this case, to solve this dilemma, a macromolecular polyurethane crosslinking agent (PCA) was designed and prepared; after that, PCA and two-dimensional (2D) MXene nanosheets were both introduced into a covalently crosslinked network to enhance the comprehensive mechanical and electrochemical properties of the hydrogels. The macromolecular polyurethane crosslinking agent promotes high-tensile strength and highly stretchable capacity by suitable covalent crosslinking. The optimized hydrogel, which exhibited maximum tensile strength and maximum elongation at break, had results of 1.21 MPa and 644%, respectively. Two-dimensional MXene nanosheets provide hydrogel with high electrical conductivity and strain sensitivity, producing a wearable device for the continuous monitoring of human movements and facial microexpressions. This study demonstrated an efficient structure design strategy for building mechanically tough, highly stretchable, and sensitive dual-mode MXenes-based wearable sensors.


Introduction
Recently, flexible sensors have been widely used in the field of electronic skin [1,2], wearable electronic devices for healthcare monitoring [3,4], and so on.However, commonly used electronic devices, usually made of carbon-based materials [5][6][7] or conductive polymers [8,9], result in flexible sensors suffering from flexibility and brittleness, which limited their application in human monitoring.Hydrogels, which possess mechanical properties similar to skin, offer a promising solution to this challenge.Recently, hydrogels have been considered the next generation of flexible biosensors due to their desirable flexibility and biocompatibility [10].Typically, hydrogels form stable three-dimensional structures with the help of a cross-linker.Wang and co-workers [11] reported a sensor based on a PAM/polyaniline composite hydrogel, which could sense strain against human motion.However, the tensile strength of the hydrogel was only 42 kPa.The hydrogel network was cross-linked through small molecules (MBAA), which are the primary causes of the fragility of hydrogels.The poor mechanical properties of hydrogel limit their applications to flexible sensors.Elasticity is important for hydrogels to adapt to tissue movement and deformation.
To improve the mechanical properties of hydrogels, an effective method is to introduce a multifunctional crosslinker into hydrogels, which could improve the mechanical properties.Polyurethane, which consists of flexible soft segments and hard, rigid segments, exhibited excellent mechanical performances due to the energy dissipation mechanism and intermolecular interactions between hard-hard segments as well as hard-soft segments [12,13].Moreover, polyurethanes, composed of polyisocyanates and macropolyols, are highly versatile because of the range of chemistries that could be employed in their synthesis, resulting in a multitude of structures and properties.Therefore, the introduction of polyurethane into hydrogel could enhance the mechanical properties of flexible sensors.
To further improve the conductivity of flexible sensors, conductive fillers, such as graphene [14,15], metal nanoparticles [16], and conductive polymers, are commonly used in a hydrogel matrix.Among conductive fillers, MXene is considered an attractive candidate for fabricating conductive hydrogels due to its excellent electrical conductivity, and large specific surface area [17][18][19].With excellent electrical conductivity, MXene could form a continuous conductive network in the hydrogels and exhibit resistance change under external force, realizing the improvement in sensitivity of flexible sensors.Moreover, taking advantage of negatively charged hydrophilic surfaces, the hydrogen bonding interaction between MXene and polymer endows the hydrogel with enhanced mechanical strength.Therefore, using MXene as a conductive filler not only endows flexible sensors with conductivity, but also enhances mechanical properties.
In this case, to solve this dilemma, a macromolecular polyurethane crosslinking agent (PCA) was designed and prepared; after that, PCA and two-dimensional (2D) MXene nanosheets were both introduced into a covalently crosslinked network to enhance the comprehensive mechanical and electrochemical properties of the hydrogels.A macromolecular PCA crosslinking agent promotes high-tensile strength and highly stretchable capacity by suitable covalent crosslinking.The optimized hydrogel, which exhibited maximum tensile strength and maximum elongation at break, showed results of 1.21 MPa and 644%, respectively.Two-dimensional MXene nanosheets endowed hydrogel with high electric conductivity and strain sensitivity, producing a wearable device used for continuous monitoring of human motions and facial micro-expressions.This study demonstrated an efficient structure-design strategy for constructing mechanically tough, highly stretchable, and dual-mode sensitive MXenes-based wearable sensors.

Preparation of MXene Nanosheet
The MXene (Ti 3 C 2 T x ) nanosheets were fabricated by selectively etching out the aluminum layer from the MAX precursor using the LiF/HCl method.Firstly, 40.0 mL HCl (9 M) and 3.2 g LiF powder were added in a Teflon vessel at ambient temperature to prepare the HCl-LiF etchant.Subsequently, 2.0 g Ti 3 AlC 2 powder was added slowly to the above etchant followed by magnetically stirring at 800 rpm under 40 • C for 48 h.The etched mixture was centrifugated (3500 rpm, 5 min) and the precipitate was washed using DI water repeatedly until the pH ≥ 7. To acquire single layer MXene nanosheets, the washed precipitate was further exfoliated in an aqueous solution by ultrasonicating (100 Hz) in a ice water bath under N 2 flow for 2 h.Lastly, 30 min centrifugation (3500 rpm) was performed on the above solution.The target MXene product dissolved in the supernatant Polymers 2023, 15, 3902 3 of 12 was collected and sealed in 4 • C for further use and the Mxene content (25.0 mg/mL) was obtained by the freeze-drying method.

Synthesis of Macromolecular Polyurethane Crosslinking Agent (PCA)
Firstly, PEG was dewatered by stirring at 120 • C under reduced pressure for 2 h.Secondly, a small amount of DMPA powder was spread flat on the bottom of a beaker and then placed in a vacuum furnace to dehydrate at 80 • C for 2 h.After that, the IPDI (24.2 g, 109 mmol) and the dried PEG (60.0 g, 30 mmol), DMPA (8.0 g, 60 mmol), and 5 drops of the DBTDL catalyst were added to a 500 mL three-necked, round-bottomed flask equipped with a mechanical stirrer.The mixture was stirred and reacted at 85 • C until the isocyanate group reached the theoretical value; the NCO-terminated group linear oligomer was obtained.Finally, the reaction temperature was reduced to 50 • C with continued stirring, and then HEA (4.10 g, 36 mmol) was added.The reaction was continued for 4 h to obtain a double-bonded, capped, macromolecular polyurethane crosslinking agent (PCA).

Preparation of MXene-Poly(Urethane-Co-AM-Co-CBMA) Hydrogels (Labeled as MPH Nanocomposite Hydrogels)
The MPH nanocomposite hydrogels were prepared as follows: AM (2.0 g), CBMA (64 mg), and PCA were well dispersed in 8.0 g deionized water under magnetic stirring to obtain a homogeneous mixture.After that, the etched 0.2 g of monolayers of MXene and 63 mg of Irgacure 2959 powder were added to the mixture to continue magnetic stirring for adequate dispersion.The mixture was further dispersed by ultrasonication for 5 min and the air bubbles were removed at the same time.The mixture was transferred to a polytetrafluoroethylene mold with a UV lamp and irradiated for 10 min at room temperature to obtain MPH hydrogels (labeled MxPyH, x is the amount of MXene added and y is the mass ratio of PCA to AM). Figure 1 illustrates the synthetic route and preparation of MPH hydrogels.The specific formulation of MPH hydrogel is shown in Table 1.ACH is poly (AM-co-CBMA) hydrogel (without polyurethane cross-linking agent).

Characterization
Fourier Transform Infrared (FT-IR) spectroscopy was performed using a Nicolet 560 FT-IR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) with a test wavelength range of 4000 to 700 cm −1 at a resolution of 4 cm −1 .Scanning electron microscopy (SEM) analysis was performed using Quanta 250 scanning electron microscopy (FEI, Hillsboro, OH, USA) to analyze the morphology of freeze-dried hydrogel samples.The samples were electrically conductive by spraying gold.The operating voltage of the MXene, ACH, PCA, and MPH hydrogels by scanning electron microscopy was 5 kV.

Characterization
Fourier Transform Infrared (FT-IR) spectroscopy was performed using a Nicolet 560 FT-IR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) with a test wavelength range of 4000 to 700 cm −1 at a resolution of 4 cm −1 .Scanning electron microscopy (SEM) analysis was performed using Quanta 250 scanning electron microscopy (FEI, Hillsboro, OH, USA) to analyze the morphology of freeze-dried hydrogel samples.The samples were electrically conductive by spraying gold.The operating voltage of the MXene, ACH, PCA, and MPH hydrogels by scanning electron microscopy was 5 kV.
The electrochemical performance of the assembled hydrogel sensor was analyzed using the ZAHNER ENNIUM electrochemical workstation, and calculate the sensitivity (S) of the sensor according to the formula (1) where ∆R = R t − R 0 .R t and R 0 are the real-time resistance and the initial resistance of the hydrogel, respectively.P is the real-time stress or strain of the hydrogel.The Instron 5967 tensile testing machine (Instron Electron Instrument Co., Ltd., Norwood, MA, USA) was used for the mechanical property testing of hydrogels.Mechanical measurements of hydrogels are performed at a rate of 50 mm/min.The samples are dumbbell-shaped (30 mm length × 4 mm width × 3 mm thickness).In addition, compression tests are performed at 80% using a cylindrical MPH hydrogel with a diameter of 20 mm at a rate of 10 mm/min.To prevent the evaporation of water from the hydrogels, glycerin was used during the tensile test.

Structural Characterization
The FT-IR curves in Figure 2a record the characteristic peaks of the PCA, ACH, and MPH hydrogels to characterize their chemical structures.The peaks at 1247 cm −1 , 1541 cm −1 , 1695 cm −1 , and 3325 cm −1 were attributed to C-O stretching vibration, C-N bending vibration, C=O stretching vibration, and N-H stretching vibration.The appearance of these characteristic peaks is associated with urethane (-NHCOO-) groups.In addition, the spike at 1109 cm −1 was derived from C-O-C stretching vibration.Characteristic peaks at 2275 cm −1 to 2240 cm −1 are associated with isocyanate groups, and their disappearance indicates that isocyanate groups are fully reacted.These results indicate the successful synthesis of PCA.The peaks at 1668 cm −1 , 3429 cm −1 , and 1120 cm −1 were related to C=O, N-H, -CO-O-stretching vibrations, respectively [20,21].These peaks appeared in both ACH and MPH.The TEM image depicted the successful preparation of the MXene sheet.In addition, the characteristic peaks of the urethane groups also appeared in MPH, indicating that PCA was successfully introduced into hydrogel.These FT-IR results indicate the successful synthesis of MPH hydrogels.
ing the mechanical properties of MPH.As a result, MPH hydrogels can resist extern stretching and compression, solving part of the problem of traditional hydrogels that a easily damaged by external forces [26][27][28].Figure 3c shows a monolayer of MXene u formly dispersed in MPH hydrogels.MXene did not show significant aggregation MPH, indicating that its introduction did not cause serious degradation of hydrogel pro erties.Cross-sectional microscopic morphologies of lyophilized ACH, PCA, and MPH hydrogels were studied by SEM [22][23][24].The dense and porous cross-sectional structure of ACH is presented in Figure 3a.This resulted in unsatisfactory mechanical properties of ACH.As shown in Figure S1, it can be observed by the SEM image that the cross-section of PCA shows many folds [25].As shown in Figure 3b-f, the introduction of PCA into MPH does not disrupt its porous structure.The pore size of MPH is smaller than that of the pure ACH hydrogel, which substantially improves the mechanical properties of MPH compared to pure ACH hydrogel.In addition, a large number of groups on the surface of MXene nanosheets were able to form many hydrogen bonds with MPH, further improving the mechanical properties of MPH.As a result, MPH hydrogels can resist external stretching and compression, solving part of the problem of traditional hydrogels that are easily damaged by external forces [26][27][28].Figure 3c shows a monolayer of MXene uniformly dispersed in MPH hydrogels.MXene did not show significant aggregation in MPH, indicating that its introduction did not cause serious degradation of hydrogel properties.

Mechanical Properties
The comprehensive mechanical properties of hydrogels are key factors in the preparation of wearable sensors.To expand the application fields of hydrogels and solve the problems of traditional hydrogels, sensors need to have excellent elasticity, mechanical strength, and durability [29,30].To investigate the effect of different PCA contents on the mechanical properties of MPH hydrogels, a universal material testing machine is carried out to investigate the mechanical properties of hydrogels.Figure 4a exhibited the gradual stretching of MPH from its original length to 300% of its original length.No damage, such as tearing, occurred in the hydrogel during this process.When the external force was removed, it could immediately revert to its initial state, demonstrating that the MPH hydrogel had outstanding self-recovery capabilities and good mechanical properties [31][32][33].
Polymers 2023, 15, x FOR PEER REVIEW 7 of 12 According to Figure 5b, MPH hydrogels have an excellent linear relationship between stress and strain.The introduction of a PCA crosslinking agent could form covalent crosslinking with the ACH hydrogel.In addition, the large number of polar groups on PCA can form abundant hydrogen bonds with the MPH molecular chain, thus greatly improving the stability of the hydrogel's 3D-crosslinked network [39].In summary, the dual crosslinking formed by PCA can significantly improve the comprehensive mechanical properties of MPH hydrogels.As shown in Figure 4b, the tensile strength of MPH hydrogels increased gradually as PCA content increased from 0 wt% to 0.5 wt%.The introduction of double-bond capped polyurethane as a crosslinking agent in MPH hydrogels markedly improved the tensile strength of the hydrogels [34].As the content of PCA increased to 1 wt%, the crosslink density increased.However, MPH's tensile strength began to decline.In addition, there was a substantial decrease in strength and elongation at break when PCA content increased to 2 wt%.It can be explained by the fact that excessive cross-linking causes difficulties in the movement of molecular chains, and smaller forces cause them to break.This also explains the gradual decrease in elongation at break due to the decrease in chain movement with increasing PCA content in Figure 4c.As a result, the high-PCA content caused excessive Polymers 2023, 15, 3902 7 of 12 cross-linking, which rendered the MPH hydrogel brittle and showed a substantial decrease in both elongation at break and strength [35][36][37].The Young's modulus of ACH, M 0.2 P 2 H, M 0.2 P 1 H, M 0.2 P 0.5 H, M 0.2 P 0.25 H, and M 0.2 P 0 H were 0.35, 1.19, 1.82, 1.85, 1.75, and 0.98 MPa, respectively.With the addition of MXene, the Young's modulus of the hydrogel was significantly improved.With the addition of PCA, the Young's modulus of the hydrogel first rose to a maximum of 1.85 MPa and then decreased, which proved that the increase in crosslinking degree would significantly improve the hydrogel's strength, but the excess degree of crosslinking would hinder the movement of the polymer chain and reduce the hydrogel's stiffness.Therefore, there is an optimal choice for the introduced content of PCA to balance the flexibility and toughness of the MPH hydrogel.Through this series of mechanical property tests, when the PCA content is 0.5 wt%, the comprehensive mechanical properties of the MPH hydrogel are the best, so the M 0.2 P 0.5 H hydrogel is used for the next research test.
The use of hydrogels is inevitably confronted with repetitive deformation processes over a long period, so excellent fatigue resistance and recoverability are essential [38].During cyclic stretches, the strain gradually increased from 100% to 400%, and the M 0.2 P 0.5 H hydrogel showed a small hysteresis (Figure 5a) with excellent stability.According to Figure 5b, MPH hydrogels have an excellent linear relationship between stress and strain.The introduction of a PCA crosslinking agent could form covalent crosslinking with the ACH hydrogel.In addition, the large number of polar groups on PCA can form abundant hydrogen bonds with the MPH molecular chain, thus greatly improving the stability of the hydrogel's 3D-crosslinked network [39].In summary, the dual crosslinking formed by PCA can significantly improve the comprehensive mechanical properties of MPH hydrogels.

Electrical and Sensing Properties
A dual-mode sensor based on the M 0.2 P 0.5 H hydrogel was assembled and data collected as shown in Figure 6a.When the M 0.2 P 0.5 H hydrogel was employed as a stress sensor, sensitivity S 1 of the sensor was calculated as 10.09 kPa −1 in the stress range of 0-6 kPa.However, when the stress was in the range of 6 kPa to 15 kPa, the sensitivity S 2 was calculated as 1.76 kPa −1 .Although its sensitivity decreases at higher pressures, the signal remains stable.As a motion monitoring sensor, its strain sensing capability is also essential.As shown in Figure 6b, its tensile sensitivity was investigated in the strain range of 0-200%.Based on the relative resistance change curve of the M 0.2 P 0.5 H hydrogel as a strain sensor, its sensitivity S 1 was calculated to be 1.17 at 0-100% strain.As the strain continued to increase, the sensitivity of the M 0.2 P 0.5 H hydrogel at 100-200% strain increased substantially, and the sensitivity of S 2 reached 2.62.
resistance change curves for the first, middle, and last 10 cycles of the hydrogel ten cycles.The results showed that the relative resistance change of the hydrogel's sensor cyclic tensile tests was almost unchanged from cycle to cycle [40,41].The above res suggest that the M0.2P0.5Hhydrogel sensor has excellent electrochemical stability and g fatigue resistance.It is important to study the stability of the relative resistance change signal of hydrogels as sensors during their long-term use.The strain sensor was tested for 1000 tensile cycles.In the experimental durability test shown in Figure 7a, the M 0.2 P 0.5 H hydrogel was subjected to 1000 tensile cycles at 50% strain.Figure 7b-d  sensor, its sensitivity S1 was calculated to be 1.17 at 0-100% strain.As the strain continued to increase, the sensitivity of the M0.2P0.5Hhydrogel at 100-200% strain increased substantially, and the sensitivity of S2 reached 2.62.
It is important to study the stability of the relative resistance change signal of hydrogels as sensors during their long-term use.The strain sensor was tested for 1000 tensile cycles.In the experimental durability test shown in Figure 7a, the M0.2P0.5Hhydrogel was subjected to 1000 tensile cycles at 50% strain.Figure 7b-d

Human Motion Monitoring Application
As shown in Figure 8a, the M 0.2 P 0.5 H hydrogel was assembled as a strain sensor using copper wires as conductive wires to monitor various behaviors of the human body.Additionally, connecting sensors and phones via Bluetooth for data collection and processing.The MPH hydrogel undergoes a corresponding change in strain with body movement, resulting in a change in electrical resistance.At the same time, a Bluetooth data transmitter collects the signal of the resistance change of the M 0.2 P 0.5 H and transmits it to the smartphone [42].Finally, the smartphone processes the resistance change data to draw a relative resistance change curve, thus achieving the purpose of real-time monitoring of human movement.

Conclusions
In summary, an MXene nanosheet layer was first obtained by etching Ti3AlC2 through the LiF/HCl etching method.Then, IPDI, PEG, and DMPA are used to synthesize the prepolymer, which is then capped by HEA to obtain a polyurethane crosslinking agent with double-bond capping.After adding MXene and mixing well, PCA was used as a cross-linking reagent, and AM and CBMA were copolymerized as monomers under UV irradiation.An MXene-doped poly(Urethane-co-AM-co-CBMA) copolymerization MPH hydrogel was prepared.The chemical structures of pure ACH, PCA, and MPH hydrogels were characterized by FT-IR.The microscopic morphologies of the ACH, PCA, and MPH hydrogel were observed and analyzed by SEM.The mechanical properties were investigated at different PCA contents, and M0.2P0.5H was selected as the hydrogel with comprehensive performance for further sensing performance tests.Finally, the M0.2P0.5H was combined with a Bluetooth transmission device to form a human motion monitoring sensor that clearly and stably monitors human joint movements at different amplitudes.It has great potential for its application to human motion monitoring.

Polymers 2023 , 12 Figure 1 .
Figure 1.Schematic illustration of the preparation of the MPH nanocomposite hydrogels.(a) Synthesis route of PCA and hydrogel, (b) preparation process of MPH, (c) schematic representation of the interaction of MXene with MPH.

Figure 1 .
Figure 1.Schematic illustration of the preparation of the MPH nanocomposite hydrogels.(a) Synthesis route of PCA and hydrogel, (b) preparation process of MPH, (c) schematic representation of the interaction of MXene with MPH.

Figure 4 .
Figure 4. Stretchability of MPH hydrogels.(a) Photographs of MPH hydrogel at original length, stretched to 300% and relaxed to original length; (b) Typical stress-strain curves of the MPH hydrogel; (c) Elongation at break and (d) tensile strength of MPH hydrogels with different PCA contents.

Figure 4 .
Figure 4. Stretchability of MPH hydrogels.(a) Photographs of MPH hydrogel at original length, stretched to 300% and relaxed to original length; (b) Typical stress-strain curves of the MPH hydrogel; (c) Elongation at break and (d) tensile strength of MPH hydrogels with different PCA contents.

Figure 4 .Figure 5 .
Figure 4. Stretchability of MPH hydrogels.(a) Photographs of MPH hydrogel at original leng stretched to 300% and relaxed to original length; (b) Typical stress-strain curves of the MPH hyd gel; (c) Elongation at break and (d) tensile strength of MPH hydrogels with different PCA conten

Figure 5 .
Figure 5. Mechanical properties of the M 0.2 P 0.5 H hydrogel.(a) The cyclic tensile curve of the M 0.2 P 0.5 H hydrogel from 100% to 400% strain; (b) Tensile stresses corresponding to the M 0.2 P 0.5 H hydrogel at different tensile strains.

Figure 6 .
Figure 6.The relative resistance changes of the M0.2P0.5Hhydrogel as (a) the stress sensor and (b) strain sensor.

Figure 6 .
Figure 6.The relative resistance changes of the M 0.2 P 0.5 H hydrogel as (a) the stress sensor and (b) the strain sensor.
further amplifies the relative resistance change curves for the first, middle, and last 10 cycles of the hydrogel tensile cycles.The results showed that the relative resistance change of the hydrogel's sensors in cyclic tensile tests was almost unchanged from cycle to cycle[40,41].The above results suggest that the M 0.2 P 0.5 H hydrogel sensor has excellent electrochemical stability and good fatigue resistance.
further amplifies the relative resistance change curves for the first, middle, and last 10 cycles of the hydrogel tensile cycles.The results showed that the relative resistance change of the hydrogel's sensors in cyclic tensile tests was almost unchanged from cycle to cycle[40,41].The above results suggest that the M0.2P0.5Hhydrogel sensor has excellent electrochemical stability and good fatigue resistance.

Figure 6 .
Figure 6.The relative resistance changes of the M0.2P0.5Hhydrogel as (a) the stress sensor and (b) the strain sensor.

Figure 7 .
Figure 7.The sensing performance of the M 0.2 P 0.5 H hydrogel.(a) The relative resistance changes of the M 0.2 P 0.5 H hydrogel sensor in 1000 cycles of stretching; (b-d) The relative resistance changes corresponding to each period.

Table 1 .
The chemical composition of MPH hydrogels.