As shown in Fig. 1B, the ACH–CCL electrode adopts the form of combining electrode material and hydrogel interface, which consists of CCL and ACH from top to bottom. The CCL consists of a polyimide (PI) substrate layer and a MXene/CNT conductive layer in a laminated structure (Fig. S1). The use of ACH as the electrode interface improves the charge injection effect of the electrodes and the signal fidelity [47]. Based on the enhancement of hydrogel's electrical and mechanical properties by the Hofmeister effect, we introduced aluminum phosphate (AlPO₄), Ti₃C₂T_x, and HMImCl into the PAAm–SA bipolymer network (Fig. 1C and Fig. S2). As a strong Hofmeister effect ion, PO₄³⁻ electrolyzed by AlPO₄ can increase the surface tension of the cavity around the polymer backbone, interfere with the hydrophobic hydration of the macromolecules [48], and induce more Ti₃C₂T_x lamellae to be dispersed and connected to the polymer network through hydrogen bonding interactions, enhancing the CSC and mechanical properties of the hydrogel. PO₄³⁻ is also able to induce entanglement of polymers chains [49], reducing the cavity voids of the hydrogel network [50] and lowering the swelling rate. By using HMImCl to replace water in the hydrogel, the adhesion and dehydration resistance of the hydrogel can be enhanced while Ti₃C₂T_x is less dispersed with water.

Figure 1D outlines the mechanism by which CCL and ACH form tight cross-links through 1,6-diaminohexane (DAH), which is sufficient to support the stable use of hydrogel electrodes (Fig. S3). Among them, the CCL is based on PI, which contains an imide ring on its main chain and can react with DAH in an amine-functionalized imide ring. In ACH, SA containing carboxyl groups, acrylamide (AAm) containing amino groups, and N,N′-methylenebisacrylamide (MBAA) containing imino groups in the hydrogel can form hydrogen bonds with the amino group on DAH. Figure 1E illustrates the adhesion mechanism of the hydrogel layer to the skin, which mainly consists of electrostatic interaction and hydrogen bond. Figure 1F shows a scanning electron microscopy cross-sectional image of the CCL, showing the laminated structure of Ti₃C₂T_x and CNT, which enables the hydrogel electrodes to maintain the conductive stability under bending deformation and have higher CSC and CIC. The ACH–CCL electrodes have good adhesion ability and can be tightly adhered to the surface of human skin without utilizing external assistance (Fig. 1G). The robust adhesion of the hydrogel–skin interface can be observed when it is peeled from a subject's arm (Fig. 1H).

Here, the hydrogel containing only the PAAm–SA dual network is referred to as IH. On the basis of IH, hydrogels with only MXene are called IHM, hydrogels with only AlPO₄ are called IHA, and the hydrogels with both MXene and AlPO₄ are called IHMA. The addition of “-H” or “-B” after the name denotes the presence of HMImCl or 1-butyl-3-methylimidazolium iodide (BMImI), respectively. Hydrogels containing both MXene, AlPO_4, and HMImCl are referred to as ACH. With Fourier transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR) spectroscopy and x-ray photoelectron spectroscopy (XPS), we characterized ACH, IHA-H, IHM, and IH (Fig. 2A; XPS is shown in Fig. S4 in supporting information). A detailed discussion in Note S1 verifies the presence of these functional components in ACH. It can be hypothesized that there is a strong interaction between Ti₃C₂T_x and the hydrogel network, and a large number of hydroxyl groups on its surface form hydrogen bonds with the amino groups supplied by HMImCl, which enhances the mechanical properties of ACH. The large absorption peak at 1,658 cm⁻¹ is the stretching vibrational band of the carbonyl group, which is absent in IH and IHM due to the absence of HMImCl, suggesting that after solvent replacement, the ionic liquid enters the hydrogel system at a fairly high level. To further quantitatively evaluate the effects of functional materials on the electrical properties of ACH, we prepared the corresponding hydrogels by adjusting the mass fraction of different functional materials in IH. Figure 2B illustrates that the resistivity of the IHM shows a trend of decreasing from 22.55 to 18.24 Ω·m as the mass fraction of Ti₃C₂T_x increases. The conductivity of IHM treated with HMImCl and BMImI, respectively, shows a decreasing and then increasing trend, and the resistivity is lowest at a Ti₃C₂T_x mass fraction of 10 mg/ml (Fig. 2C). This is because that the effect of Ti₃C₂T_x in enhancing the electrical conductivity of IH is weaker than that of ionic liquids, and when the mass fraction of Ti₃C₂T_x is low, it is able to utilize the hydrogen bonding to enhance the displacement effect of ionic liquids, whereas with the increase of the mass fraction of Ti₃C₂T_x, it excessively occupies the position of ionic liquids in the cavities of the hydrogel network, which results in the diminution of the electrical conductivity of hydrogel. Figure 2D shows that the resistivity of IHA decreases from 23.55 to 22.37 Ω·m with increasing AlPO₄ mass fraction. However, the resistivity of IHA shows a marked decrease after ionic liquid replacement, and the resistivity of IHA treated with HMImCl decreased from 14.2 to 4.77 Ω·m, which was better than that of BMImI. After treatment with ionic liquids, the resistivity of IHA decreases more than that of IH because PO₄³⁻ induces a strong Hofmeister effect, which reduces the swelling rate of the hydrogel (Fig. S5) and enhances the displacement of the ionic liquid (Fig. 2E). Comparison of the resistivities of IH, IHM, IHA, IHA-H, and ACH (Fig. 2F) indicates that the addition of PO₄³⁻ and treatment with HMImCl could enhance the ionic liquid solvent displacement using the Hofmeister effect, while the addition of an appropriate amount of Ti₃C₂T_x could reduce the resistivity of the hydrogel to an even greater extent (3.56 Ω·m). Finally, the rate of resistance change during stretching of IH, IHM, IHA-H, and ACH was characterized to investigate the effect of functional materials on the mechanical properties of hydrogels (Fig. 2G). IHM has a higher rate of resistance change during stretching because Ti₃C₂T_x is connected to the hydrogel network through hydrogen bonding, which will be displaced with the hydrogel deformation. The lower rate of resistance change of IHA-H is partly due to the fact that the Hoffmeister effect tightens up the hydrogel network and partly due to the fact that the electrical conductivity of ionic liquids has a smaller change with deformation.

The rate of change of resistance of IHA-H is only slightly smaller than that of ACH, but the resistivity is noticeably higher than that of ACH (~25%), and taken together, the overall electrical performance of ACH is better. In summary, we chose 10 mg/ml Ti₃C₂T_x and 12 mg/ml AlPO₄ as the conductive functional materials to prepare hydrogels and performed solvent substitution with HMImCl to obtain ACH, thus optimizing the conductivity of hydrogels.

To investigate the effect of functional materials on mechanical properties, we selected IH, IHM, IHMA, IHM-H, IHA-H, and ACH for tensile comparison experiments. As shown in Fig. 2H, the modulus and strain-at-break of IHMA are larger than that of IHM, which indicates that the mechanical properties of the hydrogel subjected to the Hofmeister effect are enhanced. After the addition of ionic liquid, ACH exhibited the maximum tensile limit of 1,650%, while the maximum load was lower than that of IHA-H, which may be due to the multiple anchoring interactions (hydrogen bonding, electrostatic coupling, etc.) of MXene with the polymer long chains and high-valence ions, which weaken the strong electrostatic interactions of the high-valence ions (generated by AlPO₄) and promote the energy dissipation of the polymer network [51], resulting in a lower network fracture strength and weaker loading capacity. This confirms that hydrogel schemes optimized for electrical properties can also contribute to the enhancement of mechanical properties.

In addition, ionic liquids play a major role in influencing the adhesion properties of hydrogels, and we prepared the samples accordingly by adjusting the type and presence of ionic liquids. As shown in Fig. 2I, the tensile load-distance curves of the hydrogels were obtained by the standard shear test method (ASTM F22505), the test substrate was freshly dehairing pig skin (Fig. S6), and the maximum shear strength of the hydrogels can be calculated by Eq. 1:

Figure 2J demonstrates that the shear strength of ACH is the greatest, with an enhancement of about 59.5% (up to 3,360 ± 205 Pa) over the IH, whereas the BMImI-treated hydrogel (IHMA-B) has an enhancement of only 15.8% (up to 2,440 ± 93 Pa). The peeling energy of the hydrogels can be calculated by Eq. 2:

The peeling energy of ACH is about 265.4% higher (0.28 J) than that of IH (Fig. 2K). Comparative experiments show that the hydrogels treated with ionic liquids have better adhesion properties. This is due to the ability of ionic liquids to bind to carboxyl groups on the skin through electrostatic interaction, achieving adhesion enhancement. Figure 2L shows the changes in the relative mass of ACH and IH in the experimental environment at 15°C and 40% relative humidity, and it can be found that ACH is more resistant to dehydration. After 72 h, the relative mass of IH decreases to 57.3%, whereas the relative mass of ACH is 70.8%. In addition, there is no residue or irritation on human skin even after 20 peeling cycles or 1 h of ACH attachment, demonstrating the great skin friendliness of ACH (Fig. S7).

In neurodiagnostics, on-skin electrodes with both collecting and stimulating capabilities are in urgent demand. The ACH is applied as the functional interface of the on-skin hydrogel electrodes, while CCL is employed as the conductor of the electrodes, which is able to enhance the stimulation performance of the electrodes. The 2D conductive material MXene, especially Ti₃C₂T_x, has high conductivity, large CIC, and rich surface functional groups, which is convenient for integration with other materials. At the same time, the use of CNT with high CSC as an additive material is combined with Ti₃C₂T_x, so that the conductive layer has good electrical conductivity and mechanical properties. The laminated structure design can comprehensively improve the electrical properties and bending resistance of the conductive layer.

To investigate the effect of the number of MXene/CNT layers on the electrical properties, we conducted comparative experiments with different numbers of conductive layers. With the area, thickness, and total amount of CNT and Ti₃C₂T_x same for each set of conductive layers, by only changing the number of layers of the laminated structure to 2, 4, and 8, the conductivity of the conductive layers is shown in Fig. 3A. It can be noticed that the conductivity of the CCL shows a fluctuation of increasing and then decreasing with the increase of the number of layers. The voltage–current density variation curve of the CCL is shown in Fig. 3B, and the CSC is obtained by calculating the integral of the current over time (Fig. 3C). It can be found that the number of layers has little effect on the CSC of the conductive layers but shows a trend of slightly decreasing charge capacity with increasing number of layers. Although the number of layers of the laminated structure is different, the total amount of CNT and Ti₃C₂T_x is identical, and the difference in the conductive properties may be attributed to the effect of the air gaps between the CNT and Ti₃C₂T_x layers, and either too large or too small interlayer gaps may affect the electron mobility efficiency and lead to the performance changes. When the number of Ti₃C₂T_x/CNT conducting layers is 4, the CCL has the highest conductivity and higher CSC, indicating an optimal overall performances. To further demonstrate the enhancement of the electrical properties of the CCL by the laminated structure, we compared the electrical properties of the pure Ti₃C₂T_x conductive layer (PM), the Ti₃C₂T_x and CNT mixed-structure conductive layer (MS), and the CCL, while maintaining equal total mass or ratio. Both PM and MS have much lower CSC than CCL, suggesting that the laminated structure creates a larger charge storage space (Fig. 3D). Consistent with expectations, the electrical conductivity of CCL is lower than that of PM but higher than that of MS (Fig. S8). The incorporation of CNT or the design of the laminated structure are both able to reduce the resistivity change rate when the conductive layer was bent, which results in the strongest bending resistance of CCL (Fig. 3E). By combining those effects together, the CCL has the best electrical performance.

CEs including commercial stimulation electrodes (CSEs) and commercial acquisition electrodes (CAEs) were used as comparison with the ACH–CCL electrodes. As shown in Fig. 3F, the voltage–current density variation curves of the CSE, CCL electrode, and ACH–CCL electrode were measured, and the CSC of each electrode was calculated to be 0.43, 1.92, and 3.325 mC/cm², respectively. This is due to the fact that the conductive functional materials in the ACH provide more space for the charge to attach. The time–charge density curves of the 3 electrodes were measured (Fig. 3G), and the CIC of each electrode was calculated to be 0.134, 0.358, and 0.522 mC/cm², respectively, which indicates that the ACH–CCL electrodes have enhanced CIC. At the same time, the ACH–CCL electrode has a flatter CIC curve, meaning a milder stimulation effect. In addition, due to the good compliance of the ACH interface, the contact impedance of the ACH–CCL electrode (22.82 kΩ·cm², 1,080 Hz) is lower than that of the CAE (52.43 kΩ·cm², 1,080 Hz), which also contributes to the improvement of the SNR of the acquired signal (Fig. 3H). Both CAE and ACH–CCL electrodes were used to acquire EMG signals from the lateral muscle groups of the human forearm (Fig. 3I). It is evident that the ACH–CCL electrodes acquired better signal quality, which is attributed to the ability of ACH to act as an ion channel to enhance charge transfer. Meanwhile, the ACH interfacial layer has stronger conformal adhesion and better noise suppression ability under wet adhesion.

Peripheral nerve conduction study, including examination of motor and sensory nerve conduction, can assist the clinician in the diagnosis of neurologic disorders. In this work, we conducted the medical examination of carpal tunnel syndrome and elbow tunnel syndrome based on the as-fabricated ACH–CCL electrodes.

Carpal tunnel syndrome is one of the most common peripheral nerve entrapment disorders. The motor branch of the median nerve was examined using ACH–CCL electrodes and CEs, respectively, which entailed placing the acquisition electrodes in the center of the thumb abductor digitorum brevis muscle belly and distal to the thumb, the proximal stimulating electrodes just above the brachial artery at the elbow, and the distal stimulating electrodes at the wrist with the carpal tunnel off the radial side of the carpal tunnel (Fig. 4A), and the data from the electromyograph were used for comparison. The waveforms of the motor branch of the median nerve (distal stimulation and proximal stimulation) acquired by the ACH–CCL electrodes are shown in Fig. 4B, and the waveforms acquired by the CAE are shown in Fig. 4C. ACH–CCL electrodes capture smoother signals with lower noise. Comparing the electromyograph results (Table S1), the 4 tested parameters (distal amplitude, proximal amplitude, conduction velocity, and distal motor latency) of the motor branch of the median nerve (Fig. 4D and E) indicate that the subjects were within the normal range. Ensuring that all other conditions were the same, the SNR value of the acquired signal from the ACH–CCL electrodes (35.85 dB) is much higher than that of the CAE (22.86 dB). To achieve the same hyperstimulation effect, we applied to the ACH–CCL electrodes a voltage (110 V proximal, 40 V distal) lower than that of the CSE (135 V proximal, 55 V distal), which provides a prominent enhancement in both safety and comfort for the subjects (Fig. 4F). For the examination of the sensory branch of the median nerve, the acquisition electrodes were placed at the middle finger joint, and the stimulation electrodes were placed at the distal end of the motor branch (Fig. 4G); the waveforms acquired by the ACH–CCL electrodes are shown in Fig. 4H. Comparing the waveforms acquired by the ACH–CCL electrodes with those acquired by the CAE (Fig. S9), the waveforms acquired by the ACH–CCL electrodes are smoother under the same processing, indicating that our electrodes have better acquisition capability and lower noise, which is attributed to fact that the CCL has good bending stability, while the ACH has excellent adhesion and low modulus, and the use of the ACH as the electrode–skin interface enables stable conformal contact with the skin, decreases the contact impedance, and reduces the interference of the skin micromotion on signal acquisition [52]. At the same time, a large number of ions (generated by AlPO₄ and HMImCl) and conductive substances (MXene) act as intermediate channels to achieve capacitive coupling between the CCL and the skin, which further reduces the contact impedance and attenuates the noise generation [47,53]. Parameters such as amplitude, sensory latency, and conduction velocity acquired by both electrodes (Fig. 4I) are consistent with those obtained by the electromyograph (Table S2).

Cubital tunnel syndrome is the second most prevalent clinical condition after carpal tunnel syndrome, which is caused by progressive damage to the ulnar nerve due to compression at the elbow. In the examination of the ulnar nerve, the signal acquisition point of the motor branch is on the spreading muscles of the little finger; the proximal stimulation point is below the elbow, along the ulnar nerve trunk and about 5 cm above the medial-superior humeral ankle; and the distal stimulation location is on the ulnar side of the wrist, close to the parallel cross-section of the wrist (Fig. 5A). Again, using the data measured by the electromyograph for comparison, we completed the examination of the ulnar nerve with the ACH–CCL electrodes and CE. The waveforms of the motor branch of the median nerve (distal stimulation and proximal stimulation) acquired by the ACH–CCL electrodes are shown in Fig. 5B, and the waveforms acquired by the CAE are shown in Fig. 5C. The results demonstrate that the ACH–CCL electrodes are more capable of acquiring high-quality signals. Comparing the results measured by the electromyograph (Table S3), the results of the 4 tested parameters (distal amplitude, proximal amplitude, conduction velocity, and distal latency) (Fig. 5D and E) of the subjects showed that all the indexes of the subjects were within the normal range. With other conditions being equal, the SNR of the acquired signals from the ACH–CCL electrodes (35.58 dB) was much higher than that of the CAE (18.92 dB). To achieve the same stimulation effect, we applied to the ACH–CCL electrodes a voltage (90 V proximal, 38 V distal) lower than that of the CSE (120 V proximal, 45 V distal), which proves the improved safety and comfort of the ACH–CCL electrodes (Fig. 5F). For the examination of the sensory branch of the ulnar nerve, the stimulation location of the sensory branch was the same as the distal stimulation location of the motor branch, and the acquisition electrode was located at the little finger joint (Fig. 5G); the waveforms acquired by the ACH–CCL electrodes are shown in Fig. 5H. The amplitude, sensory latency, and conduction velocity results (Fig. 5I) are all within 10% of the electromyograph results (Table S4). From the comparison of the waveforms acquired by the ACH–CCL electrodes with those acquired by the CAE (Fig. S10), it is demonstrated that the ACH–CCL electrodes have greater acquisition capability.

CNTs were purchased from Xianfeng Nanomaterials Technology Co., Ltd. (Jiangsu, China). Ti₃C₂T_x was purchased from Xinen Technology Co., Ltd. (Foshan, China). Polyvinyl alcohol, MBAA, SA, tetramethylethylenediamine, and calcium sulfate (CaSO₄) were purchased from Aladdin Biochemistry Technology Co. (Shanghai, China). PI was purchased from Bomi Technology Co. AAm was purchased from McLean Biochemistry Technology Co. (Beijing, China). AlPO₄ was purchased from Alfa Aesar Chemical Co. (Shanghai, China). Ammonium persulfate was purchased from Sinopharm Chemical Reagent Co. (Shanghai, China). HMImCl was purchased from Biotech Pharmaceutical Technology Co. (Shanghai, China).

The CNT dispersion (0.15 wt%, 0.2 ml) was uniformly dropped on the solvent microporous filtration membrane (50 mm in diameter, 0.2 μm in diameter) to cure it into a film by negative pressure extraction filtration. Then, Ti₃C₂T_x dispersion solution (5 mg/ml, 0.2 ml) was uniformly dropped on the CNT film, and after curing, a layered conductive layer was obtained. The number of conductive layers could be adjusted by repeating the above steps. Then, the conductive layer was heated in an oven at a constant temperature (70°C, 5 min). Polyvinyl alcohol (10 wt%, 1 ml) solution was spin-coated on the glass plate to form a sacrifice layer, and the prepared conductive layer was placed on the sacrifice layer. Glass rods were dipped in PI solution (20 wt%, 2 ml) and spread over the surface of the conductive layer, then they were placed in a spin-coating machine for spin coating (350 r/min, 90 s). Next, the conductive layer was vacuumed in the vacuum chamber (10 min) to remove the bubbles in the PI solution and then cured in the oven (200°C, 1.5 h). Next, the sample was placed in a water bath wrap at 80°C for 1 h to remove the sacrificial layer. Finally, the conductive layer was immersed in 1,6-hexanediamine solution (10 wt%, 5 ml) for 4 to 6 h and poured into an acrylic mold to finish the preparation of CCL.

The monomer prepolymerization solution was prepared by adding 2 g of AAm, 1.1 mg of MBAA, and 0.2 g of SA to 15 ml of deionized water and stirred with a magnetic stirrer at 400 r/min for 5 to 6 h. Afterwards, 6.5 mg of tetramethylethylenediamine, 180 mg of AlPO₄, and 150 mg of Ti₃C₂T_x were added, shaken for 5 min, and degassed after ultrasonication for 10 min. The cross-linking promotion solution was prepared by adding 650 mg of ammonium persulfate and 112 mg of CaSO₄ to 10 g of deionized water. Subsequently, 5 ml of monomer prepolymerization solution and 200 μl of cross-linking promotion solution were withdrawn with 2 syringes, connected through luer fittings, mixed homogeneously, and poured into acrylic molds, and reacted for 4 h. After that, the hydrogel was removed from the mold and covered with HMImCl (30 wt%) for solvent replacement for 1.5h to complete the ACH preparation.

Preliminary steps are the same as above, using 2 syringes to extract 5 ml of monomer pre-polymerization solution and 200 μl of cross-linking promoter solution, respectively, connected through the luer connector, mixed homogeneously, and then poured into the acrylic mold with CCL, heated at 60°C for 10 to 15 min. After heating, the 2 parts formed a tight whole, which was separated from the mold, and the solvent replacement was done by covering the hydrogel with HMImCl.

Interfacial images of the MXene/CNT laminated structures were observed using a scanning tunneling electron microscope (MIRA4, TESCAN, Czech Republic).

The square resistance and conductivity of CCL were measured using a 4-probe tester (RTS-8, Tianjin Norexinda, China), and the CIC and CSC of ACH–CCL electrodes and CCL were tested using an electrochemical workstation (Autolab PGSTAT302N, Metrohm, Switzerland).

XPS spectra were recorded by an x-ray photoelectron spectrometer (AXIS-ULTRA DLD-600W, Shimadzu-Kratos, Japan).

Fourier transform infrared spectrograms were recorded by a controlled atmosphere Fourier transform infrared spectrometer (Nicolet iS50, Thermo Fisher, USA).

CEs included CSEs (Dantec Keypoint G4, Denmark) and CAEs (LT-8, Shanghai LITU Medical Appliances Co.).

The contact impedance of the CAEs and ACH–CCL electrodes was measured using a LCR meter (E4980AL, Keysight, USA).

In the testing of the motor branch of the median nerve, a USB-6218 acquisition card was used in conjunction with a SC-2002 low-frequency pulser (Shanghai Xinshang Medical Devices, Shanghai, China) in lieu of an electromyograph, with a stimulation frequency of 2 Hz, a pulse width of 0.2 ms, and an acquisition frequency of 30 kHz.

After an explanation of the nature and possible consequences of the studies, all subjects gave fully voluntary and informed consent to participate in the experiment.