Introduction

The rapid advancement of flexible and wearable microelectronics opens up the opportunity for realizing novel and multifunctional devices, such as real-time monitoring of body signals and delivering precise treatments. Meanwhile, high-performance, miniaturized, and flexible electrochemical energy storage devices are highly required1,2,3,4,5. At present, various energy storage microdevices are being rapidly developed, tailored in both materials and structures for distinct application scenarios6,7,8. Among them, planar micro-supercapacitors (MSCs), characterized by in-plane interdigital electrode configuration fabricated on one substrate, have garnered widespread attention due to their favorable flexibility, facile integration properties with other components, and long intrinsic cycling stability9,10. Over the past few years, the electrochemical performance, mechanical flexibility, shape diversity, and integration of MSCs have been greatly improved owing to progressive understanding to energy storage mechanism, advances in material science and microfabrication technology, and optimization in device structure11,12,13,14.

However, for wearable and flexible energy supply applications, there are still several key issues to be solved in MSCs. First, the mechanical flexibility of most MSCs is limited to mere bendability or a low extent of stretchability, which falls short of meeting the requirements for skin-adherent electronics, especially in response to sudden and drastic motion changes. This is because during body movements, the skin often undergoes a certain degree of stretching deformation, causing detachment of the affixed interface15,16,17. Second, restricted by low energy density, MSCs need frequent charging to meet the continuous operation requirements of electronic devices. In the conventional contact charging mode based on wire-connection, it is necessary to often move MSCs, which conflicts with the goal of wearable microelectronic devices being convenient, intelligent and comfortable14,18. Third, in actual applications, MSCs are required to combine with other components to form integrated microsystems for specific functions. In such cases, the strain difference between different materials in various components would lead to interface delamination during deformation, which is particularly evident under stretching condition19,20,21.

In face of these challenges, some work has been carried out. Representatively, wave and wrinkle structures were designed to accommodate complex deformation in order to attain stretchable MSCs22,23. Contactless charging or self-powered microdevices were demonstrated by combining wireless coils or energy harvesters (e.g., solar cells, thermoelectric generators) with MSCs on one flexible substrate24,25,26,27. To realize the target function, energy storage-consumption integrated systems were constructed by utilizing the same types of materials (e.g., graphene, MXene) as the electrode of MSCs, conductive circuit and current collector of stimulation-response units, suppressing interfacial delamination28,29,30. Nevertheless, contactless charging MSCs and seamlessly integrated energy harvest-storage-consumption microsystem with ultrahigh stretchability have not been reported yet, requiring comprehensive understanding in materials, interface, structure design, and fabrication strategies.

Herein, we report an ultrastretchable, planar, seamlessly integrated energy collection-storage-application microsystem for skin-adherent wireless microelectronics, by utilizing an all-in-one MXene film simultaneously as MSCs, wireless receiver coils (WRCs) and strain sensors, and constructing crumpled structures to accommodate deformation. Notably, the MSCs and strain sensors are ingeniously arranged within the WRCs, endowing the entire stretchable system unparalleled integrity and ultrasmall footprint of only ≈ 1.4 cm*1.4 cm. Impressively, the crumpled-MXene MSCs succeeds on both electrochemical and mechanical fronts. They achieve high specific capacitance of 76.82 F cm–3, enhanced energy density of 10.28 mWh cm–3, and long-term cyclability, while also demonstrating excellent performance durability after 1000 times of repetitive stretching-releasing from 0% to 500% areal strain, and in even dynamic stretching process. Moreover, the MXene WRCs can charge the MSCs to full capacitance in a short time of about 20 s under varied deformation states (uniaxial/biaxial stretching, bending), demonstrating outstanding contactless charging capability. Finally, our integrated microsystem without any material interface can be conformably adhered to different body parts to achieve specific responses to limb movements, such as finger bending, hand back stretching, and pressing (minimum response time of ~140 ms). This work provides guidance for constructing seamlessly integrated all-in-one contactless microscale systems with ultrahigh stretchability for skin-adherent wireless microelectronics applications.

Results

Fabrication of a seamlessly integrated all-in-one microsystem

To avoid the strain mismatch between different materials and maintain the structural integrity during deformation, we chose the two-dimensional (2D) Ti3C2Tx MXene as wireless coils, electrode materials, sensing materials, and interconnectors simultaneously to reduce interface, because of its outstanding overall characteristics including high conductivity, large specific surface area, ease of processing and excellent sensing ability21,29,31,32,33,34,35. The detailed process for integrating multi-tasking MXene into a unified all-in-one system is depicted in Fig. 1a. First, X-ray diffraction (XRD, Fig. 1b) patterns confirmed the impurity-free Ti3C2Tx MXene obtained through in-situ selective etching of the Al atomic layers from the Ti3AlC2 precursor. The MXene possessed a lateral size of ~1 μm and a thickness of merely 2 nm (Fig. 1c and Supplementary Fig. 1), corresponding to mono-layer or few-layer nanosheets, which implies high quality of MXene. Next, the MXene nanosheet powder was homogeneously dispersed into deionized water, followed by vacuum filtration on a polyvinylidene fluoride membrane and vacuum drying at 50 °C to obtain MXene films. Scanning electron microscope (SEM) image revealed the random stacking structure of MXene nanosheets (Fig. 1d), which led to conductivity loss of the MXene film due to the relatively large space between the adjacent nanosheets36. To address this issue, 100 nm thick Au was evaporated on the film surface to work as current collectors. Subsequently, the above film was transferred onto an elastomer substrate pre-stretched up to 600% (areal strain εA = 500%), and after releasing it, the MXene film formed a crumpled structure (Supplementary Fig. 2) to withstand deformation. Further, through rational design, a seamlessly integrated pattern of WRCs (external) and parallel MSCs (internal) with clearly defined electrode edges (Fig. 1e) was achieved via laser-etching with controllable parameters. It should be emphasized that the partial central coil was used as a shared electrode to connect the parallel MSCs, so that the energy collected by the WRCs from varying magnetic field could be directly transmited to the MSCs, reducing the energy loss induced by contact resistance. To further improve the integrity of the microdevice, an ultrathin MXene film without Au was transferred inside the above pattern as a strain sensor without enlarging the size (Fig. 1a). Finally, by drop-coating a safe aqueous gel electrolyte (Supplementary Fig. 3a) composed of concentrated LiCl and polyvinyl alcohol on the microelectrodes of MSCs and subsequently sealed by another elastomer, all-in-one integrated MXene microsystems capable of harvesting-storing-consuming energy within a controllable area were achieved (Supplementary Fig. 3b, c).

Fig. 1: Fabrication and characterization of a MXene integrated microsystem.
Fig. 1: Fabrication and characterization of a MXene integrated microsystem.The alternative text for this image may have been generated using AI.
Full size image

a Schematic of the fabrication process and conceptual application. b XRD patterns of Ti3C2Tx MXene and Ti3AlC2 precursor. c Atomic force microscope (AFM) image of MXene. The inset corresponds the height profile. d SEM image of the filtered MXene film. e Optical microscope image of laser-etched MXene electrodes. Each experiment in (c–e) was repeated three times with similar results.

Stretchability optimization of the integrated microsystem

According to relevant research, the elastic deformation of films generally decreases as their thickness increases, indicating that a thinner electrode possesses greater flexibility37. Nevertheless, the damage to the electrode structure caused by transfer process during preparation needs to be taken into account38, which is more obvious in thin films. Moreover, lower MXene thickness also implies reduction in active materials loading, which is unfavorable for storing more energy. Therefore, a systematic exploration of the film thickness is crucial to optimize the electrodes pursuing both good durability and high energy storage.

As depicted in Fig. 2a, the as-prepared MXene film exhibited flat 2D surface and uniform thickness. Subsequently, the mechanical stability of MXene electrodes within a thickness range of 0.56–3.39 μm was evaluated through biaxial strain fatigue testing. Assisted by liquid metal connection, the resistance evolution of the aforementioned films was detected during 1000 stretch-release cycles over an areal strain range from 0% to 500%. Figure 2b illustrated that the normalized resistance (R/R0) first decreased and then increased rapidly with increasing electrode thickness in the course of repetitive stretches, reaching a minimum around 2 μm. Correspondingly, a nearly identical trend was observed in electrochemical performance (Fig. 2c and Supplementary Figs. 4, 5), indicating that a moderate thickness is optimal. To explicate this point, SEM analysis was conducted on MXene films with different thicknesses. It is observed that thinner films were more susceptible to structural damage during transfer, with cracks gradually spreading within sequential stretch-release processes (Fig. 2d, e). Besides, the size of the wrinkled structure increased with the film thickness (Fig. 2d, e), signifying a rise in flexural stiffness. Naturally, thick films subjected to high stress were more prone to fracture in the period of long-term fatigue testing. As a result, the film electrode with a moderate thickness of 2.26 μm represented the optimal performance in terms of coordination between durability and energy density, which was selected in the following studies.

Fig. 2: Relationship between MXene film thickness and the electrochemical and mechanical performances of MSCs.
Fig. 2: Relationship between MXene film thickness and the electrochemical and mechanical performances of MSCs.The alternative text for this image may have been generated using AI.
Full size image

a 3D morphology of a MXene electrode. b Resistance evolution of the MXene films with varied thickness during fatigue test of 1000 stretch-release times up to 500% areal strain. c Capacitance retention of MSCs versus thickness after the fatigue test. d Mechanism diagram for the impact of MXene film thickness on performance. e SEM images showing the crumple change of MXene films with different thicknesses before (above panel) and after (below panel) the fatigue tests. Scale bars: 50 μm. Each experiment in (e) was repeated three times with similar results.

Electrochemical and mechanical properties of MSCs

Typically, a single MXene MSC in the integrated device with optimal thickness (Supplementary Fig. 6) was selected for subsequent electrochemical performance assessment in a safe and eco-friendly water-in-salt LiCl aqueous gel electrolyte. Attributed to a significant portion of coordination between Li+ with water and weakened intermolecular hydrogen bonding, a high voltage of 1 V was achieved in the MXene MSC (Supplementary Fig. 7).

As shown in Fig. 3a, b, the cyclic voltammetry (CV) curves and galvanostatic charge-discharge (GCD) profiles of the MSC presented standard rectangular and isosceles triangle shapes respectively, revealing its high Coulombic efficiency as well as typical capacitive behavior. The maximum volumetric capacitance of 76.82 F cm–3 was achieved at 10 mV s–1, and it retained 66.78 F cm–3 even when the scan rate was increased to 100 mV s–1, demonstrative of outstanding rate capability (Supplementary Fig. 8). Consequently, the MSC offered a high energy density of 10.28 mWh cm–3 with a corresponding 766.5 mW cm–3 power density, and still maintained 9.07 mWh cm–3 at a superb power density of 4599 mW cm–3 (Supplementary Fig. 9). Moreover, the MSC displayed impressive long-term stability, with a barely noticeable voltage drop and a capacitance retention of 86.4% after 10000 GCD cycles (Fig. 3c).

Fig. 3: Electrochemical and mechanical characterizations of a single stretchable MSC.
Fig. 3: Electrochemical and mechanical characterizations of a single stretchable MSC.The alternative text for this image may have been generated using AI.
Full size image

a CV curves of the MXene MSC acquired from 10 – 100 mV s–1. b GCD profiles measured at various current densities. c Cycling stability tested by GCD at 3.1 A cm–3. The inset shows the initial and last three GCD cycles. d Capacitance retention of the MSC under different areal strains. The inset compares CV curves at 0%, 200% and 500%. e EIS plots under varied areal strains. f Capacitance retention during stretching cycles up to 500% areal strain. The inset shows the photographs of MSC at 0% and 500% after 1000 stretching times. Scale bars: 1 cm. g Comparison of the main properties between this work and recently reported stretchable MSCs12,39,40,41,42,43,44. h GCD profiles obtained under the states of released and dynamically stretching at varying strain rates.

Apart from remarkable electrochemical performance, the MSC also possessed excellent mechanical stability. As observed from Fig. 3d and Supplementary Fig. 10, the CV curves and GCD profiles of the MSC almost completely overlapped during biaxial stretching up to 500% areal strain, with a slight decrease in capacitance of only 5.2%. This minor loss may arise from the escalated interfacial contact resistance between the adjacent MXene layers, and between the MXene with the electrolyte over the course of stretching, consistent with the results of electrochemical impedance spectroscopy (EIS) plots in Fig. 3e. To further validate the stretchability of the MSC, biaxial tensile fatigue tests were conducted from 0% to 500% areal strain. After 1000 stretch-release cycles, the device remained structurally intact and maintained stably overlapping CV curves and GCD profiles at 0% and 500% areal strain (Fig. 3f and Supplementary Fig. 11). The minor increase in capacitance within the initial cycling test could be attributed to more accessible infiltration between the electrode and the electrolyte, after which the capacitance slightly decreased owing to electrode crack extension, eventually reaching an impressive 98.5% retention. As summarized in Fig. 3g, our comprehensive performance surpasses most of the reported stretchable MSCs in both electrochemical and mechanical aspects12,39,40,41,42,43,44, showcasing the exceptional superiority. Impressively, the MSC realized exactly consistent charging-discharging behavior even under dynamic tensile tests (εA from 0% to 500%) at different strain rates compared to released conditions (Fig. 3h and Supplementary Movie 1), which showed that the device could continue to operate normally in the face of sudden and violent tension. In addition, our device delivered excellent capacitance retention characteristics under other static and dynamic mechanical deformations, including uniaxial stretching up to 145% and bending up to 180° (Supplementary Figs. 12 and 13). These findings suggest that the as-fabricated MXene MSCs managed to withstand various external impacts and supply stable energy for wearable electronics.

Evaluation and refinement of wireless charging capacity

As illustrated in Fig. 4a, the seamless integration of WRCs and MSCs (WRC-MSCs) not only allowed continuous harvest and storage of energy through contactless charging, but also minimized the footprint and circuit loss. In general, wireless charging platforms typically consist of a transmitting port (Fig. 4b, left) and a receiving port (Fig. 4b, right). The former converts the input direct current (DC) to high-frequency alternating current (AC) through an oscillator circuit and outputs it to Cu transmitter coils (Supplementary Fig. 14a), which creates a rapidly changing magnetic field within a certain range based on the principle of electromagnetic induction. At this point, an induced electromotive force would be generated in the MXene WRCs placed above Cu coils, due to the change in magnetic flux, thus producing AC (Supplementary Fig. 14b). In order to achieve steady charging, the WRC-MSCs were connected at both terminals with rectifying diodes (red line in Fig. 4a, point C in Fig. 4b), ensuring that positive and negative charges are respectively accumulated in the cathode and anode of MSCs. When point D is attached, the power stored in the MSCs will be released to support the normal operation of the load.

Fig. 4: Wireless charging mechanism and performance of the MXene seamlessly integrated WRC-MSCs.
Fig. 4: Wireless charging mechanism and performance of the MXene seamlessly integrated WRC-MSCs.The alternative text for this image may have been generated using AI.
Full size image

a Schematic for the wireless charging process of WRC-MSCs. b Circuit diagram of the wireless charging platform. c Parameter optimization for the WRCs. d Wireless charge curve of MSCs with the support of WRCs. e Galvanostatic discharge profiles of MSCs at various current densities after wireless charging for around 15 s. f Infrared image of WRC-MSCs after 20 min of continuous wireless charging. g Cycling stability of WRC-MSCs by wireless charge and galvanostatic discharge.

To further enhance the wireless charging effect, we precisely adjusted the key structural parameters of the WRCs and the operating distance (Fig. 4c). According to the principle of electromagnetic induction, the induced electromotive force is proportional to the change rate of magnetic flux. Therefore, as the number of turns increases, the voltage sensed in the WRCs grows (Supplementary Fig. 15a). However, considering the device area and the voltage window of the MSCs, a 4-turn coil was sufficient. Similarly, higher voltage signals were detected in case of smaller spaces between adjacent coils while avoiding short circuits (Supplementary Fig. 15b). Lastly, the distance between the transmitter and the receiver coils was optimized. The results showcased a trend of initially rising and then falling voltage, because the magnetic density attenuates with the distance, and yet alternating field generated in the receiver coils may interfere with the transmitter coils at a very close distance (Supplementary Fig. 15c). Consequently, the final parameters of the WRCs were fixed at 4 turns of the receiver coil, 600 μm of the coil width, 400 μm of the adjacent spacing, and 5 mm of the transmission distance. According to the calculation, the transferring efficiency of the wireless charging process was 54.3% (Supplementary Fig. 16).

First, the CV curves and GCD profiles of the parallelly-connected MSCs inside the WRCs were tested, which provided nearly ideal capacitive behaviors (Supplementary Fig. 17). Based on this, the energy collection and storage characteristics of the WRC-MSCs in the flat state were evaluated. As seen in Fig. 4d, with the help of WRCs, the voltage of MSCs rapidly boosted and gradually leveled off, and finally stabilized at 1.0 V in <20 s. Afterward, the device was able to discharge properly at various current densities with negligible voltage drop, without obvious difference from the GCD profiles (Fig. 4e and Supplementary Fig. 17b), demonstrative of wireless charging reliability of the WRC-MSCs. Moreover, the infrared image in Fig. 4f indicated that the peak temperature of the WRC-MSCs was only 24.1 °C (20.0 °C of background) after up to 20 min of continuous wireless charging, which implied its safety and prevention of overcharging. Encouragingly, no capacitance decay was observed in 50 cycles of wireless-charging/galvanostatic-discharging after placing the device for 2 months under ambient conditions (Fig. 4g), suggesting the long-term practicability of the contactless charging devices.

All-in-one seamlessly integrated microsystem

To further extend the application scenarios of the wireless charging devices, the integration of the seamless system was upgraded by incorporating a piece of thin MXene film as a resistive strain sensor without further enlarging the footprint (≈ 1.4 cm*1.4 cm). In this all-in-one MXene integrated microsystem (Fig. 5a), the MSCs acted as a bridge between the energy harvesting unit and the sensing unit, storing the energy converted by the WRCs and supplying it to the strain sensor when needed, which could independently realize the long-term sensitive monitoring of human body movement without requiring cumbersome contact charging. Moreover, it merits special attention that additive-free MXene was used to fabricate the wireless coils, electrode materials, sensing materials, and interconnectors simultaneously, which effectively reduced the interface between different components and greatly contributed to significant structural stability of the stretchable system.

Fig. 5: Collaboration and sensing capability of a wearable MXene integrated microsystem.
Fig. 5: Collaboration and sensing capability of a wearable MXene integrated microsystem.The alternative text for this image may have been generated using AI.
Full size image

a Schematic for the wireless charging and sensing processes of the microsystem. b Wireless charge curve of MSCs based on WRCs at 500% areal strain. c Galvanostatic discharge profiles of MSCs at 500% areal strain after wireless charge. d Plots of simultaneous wireless charge and galvanostatic discharge for the MSCs with different initial states of charge. e CV curves of the MSCs in microsystem at 500% areal strain. f Photographs of the device mounted on finger joint and hand back. g–i, Current signals from the MXene strain sensors powered by WRC-MSCs as a response to the actions of finger bending (g), hand back stretching (h), and pressing (i).

To simulate the scenarios for wearable application, we first measured the power supply capability of the microsystem under various deformations. It can be seen that the device was able to be wirelessly charged to full capacitance in about 20 s and then discharged at different rates on demand with the areal strain from 100% to 500% (Fig. 5b,c and Supplementary Fig. 18). The voltage drop may be attributed to a slight loss of conductivity under strains, which did not influence the system applicability in any obvious manner. Moreover, the performance was maintained with a high degree of consistency under other deformations like uniaxial stretching to 145% and bending to 180° (Supplementary Figs. 19 and 20). Furthermore, the integrated microsystem could continue to operate smoothly even after 1000 cycles of biaxial stretching to 500% areal strain (Supplementary Figs. 21 and 22), all of which prove the reliability of our microsystem while facing various deformations in actual scenarios. In addition, it is necessary to consider the situation of concurrent charging and discharging encountered in everyday life. As revealed in Fig. 5d, regardless of whether starting at high or low states of charge, upon connection to the fast wireless charging and high-rate (3.1 A cm–3) discharging at the same time, the device rapidly reached a stable high voltage plateau indefinitely without any damage, once again verifying the excellent practicality of our integrated microsystem. Undoubtedly, the device can also be charged and discharged in a conventional manner, providing more options for daily usage (Fig. 5e and Supplementary Fig. 23).

Finally, we investigated the detection of motion signals by the integrated microsystem while being adhered on body (Fig. 5f). After wirelessly charging the MSCs by virtue of the WRCs, the strain sensors were plugged to operate, probing current variations in response to external signals. Throughout repeated movements of the finger joint and the back of the hand, corresponding to typical unidirectional and bidirectional strains respectively, the MXene strain sensor gave out clear and uniform current fluctuations (Fig. 5g, h). Moreover, when the sensor was touched by a fingertip, it achieved a short response time of ~140 ms in the relaxed state (Fig. 5i) and demonstrated accurate responses even in the stretched state (Supplementary Fig. 24). The preceding tests have proven that there were no current fluctuations from the discharge behavior of MSCs during dynamical strain stimulation, further signifying the high stability and sensitivity of the integrated microsystem. Besides, distinguishable responses to different movements were consistent with the results from individual sensor testing, providing a foundation for selectively monitoring body signals from various parts (Supplementary Fig. 25). Therefore, it is confirmed that our MXene-based ultrastretchable microsystem possesses a very high level of integration and compatibility, showing immense potential in the field of wearable electronics.

In addition, we believe that the pre-stretching method and all-in-one strategy presented in this study could be also applicable to other materials, provided they meet the following criteria: (1) they should possess excellent electrical conductivity, high specific capacitance, and good sensing properties in order to be applied as WRCs, MSCs, and sensors, respectively; (2) they should exhibit strong interactions at the microscopic level to avoid the detachment from the substrate. To validate generality of the two principles, graphene meeting these criteria was selected as another typical example. As shown in Supplementary Fig. 26, the graphene-based seamless integration system also demonstrated ultrahigh stretchability, prolonged wireless charging capability, and outstanding sensing performance.

Discussion

In summary, we have presented a seamlessly integrated, contactless-charging MXene microsystem consisting of WRCs, MSCs, and strain sensors with ultrahigh stretchability, owing to the collaborative optimization in materials, interface, structural design, and fabrication strategies. To minimize the interface, multi-tasking MXene was selected simultaneously as an energy collection-storage-application material to form an all-in-one integrated microsystem, thereby enhancing the durability and robustness in actual scenarios. Structurally, we developed a wrinkled electrode and optimized its thickness, which was responsible for the excellent stretchability and durability. In the device design level, MSCs and strain sensors were ingeniously arranged inside WRCs and shared electrodes between MSCs and WRCs maximized space utilization, meanwhile realizing the capability of contactless charging and motion detection for the microsystem. As a result, our devices offered superb stretchability, prolonged contactless charging capability, exceptional deformation compatibility, and excellent integration. Therefore, we believe that the design and optimization strategies proposed in this work can be extended to fabricate various ultrastretchable and seamlessly integrated microsystems, customizable to meet diverse requirements. Furthermore, incorporating microelectronics to enable the coils to simultaneously perform non-contact energy harvesting and wireless data transmission represents a future research direction, opening new avenues for the development of skin-attachable wireless electronics and miniature soft robots.

Methods

Preparation of MXene nanosheets

First, 2 g LiF (AR, Aladdin Co., Ltd.) and 40 mL HCl (AR, Knowles Co., Ltd.) with a concentration of 9 mol L–1 were mixed in a centrifuge tube, followed by stirring for 30 min. Next, 2 g Ti3AlC2 (99.9%, Xinxi technology Co., Ltd.) was added in the mixture slowly, which was then stirred at 350 rpm in a 35 °C constant thermostatic water bath for 24 h. The resulting suspension was added with an appropriate amount of deionized water, and centrifuged repeatedly (3500 rpm (1027 x g), 10 min) until the pH ≥ 5. Successively, 40 mL of C2H5OH (AR, Fuyu fine chemical Co., Ltd.) was mixed with the retained precipitate, which was sonicated for 20 min to redisperse and then centrifuged (10000 rpm (~8385 x g), 20 min) to obtain the delaminated Ti3C2Tx paste. Similarly, the previous treatment process was repeated again with water to produce the dispersion of MXene. Finally, the MXene nanosheet powder was acquired with the aid of freeze-dryer for 3–5 days.

Preparation of LiCl aqueous electrolytes

1 m LiCl electrolyte and 15 m LiCl water-in-salt electrolyte were prepared by dissolving separately 0.005 mol and 0.075 mol of LiCl (AR, Aladdin Co., Ltd.) into 5 g of deionized water. Later, 0.5 g polyvinyl alcohol with the molecular weight of ~205000 (AR, Macklin Co., Ltd.) was added into the 15 m LiCl electrolyte, which was continuously stirred at 90 °C for 2 h to acquire a transparent gel electrolyte.

Fabrication of stretchable MSC, WRC-MSCs, and integrated microsystem

MXene films with various thicknesses were prepared through vacuum-assisted filtration of uniform MXene re-dispersions at different concentrations on polyvinylidene fluoride membrane (0.22 μm, 5 cm), followed by drying in a vacuum oven at 50 °C for 2 h. To improve the conductivity, 100 nm thick Au was thermally evaporated on the surface of MXene film under 4 × 10–4 Pa. Subsequently, the above film was gently pressed onto a pre-stretched elastic tape (VHB 4910, 3 M Co., Ltd.) with an areal strain of 500%. After peeling off the membrane and releasing the strain, a crumpled MXene film was formed. Finally, ultrastretchable MSC and WRC-MSCs were obtained by ultraviolet laser-etching (TR-W-UV05, Weilanjiguang Co., Ltd.) according to computer-designed patterns and then drop-coating LiCl aqueous gel electrolyte. As for the seamlessly integrated microsystem, it necessitated transferring a piece of previously prepared ultrathin MXene film as a strain sensor to the interior of the WRC-MSCs stretched to 500% areal strain. The areas of WRCs, MSCs, and strain sensors were 0.8184, 0.1512, and 0.015 cm–2, respectively (Supplementary Fig. 27). In addition, all devices were sealed by a thinner elastic tape (VHB 4905, 3 M Co., Ltd.).

Materials characterization

The morphology and structure of MXene were investigated by XRD (5–85°, SmartLab), AFM (NanoWizard), and SEM (S-5500) techniques. The 2D and 3D structure of electrodes were characterized by digital optical microscope, SEM (Quanta 200 F), and step profiler (Alpha step D-600). The properties of 15 m LiCl electrolyte were examined by Fourier transform infrared spectroscopy tests (400–4000 cm–1, Nicolet iS50). The infrared images of the device were recorded by an infrared imager (TiS75 + ).

Mechanical and electrochemical measurement

Mechanical fatigue testing was carried out with a custom digital stepper, which could precisely move according to the set parameters. For the electrochemical performances of WRCs, MSCs and strain sensors, commercially available GaInSn liquid metal was used to link the device with an electrochemical workstation (CHI760E) or a battery tester (LANHE M340A). The EIS was conducted in the frequency range from 1 Hz – 100 kHz with an amplitude of 5 mV. The electrochemical stability window of 15 m LiCl electrolyte was evaluated by linear sweep voltammetry using glassy carbon as a working electrode, Pt as a counter electrode and Ag/AgCl as a reference electrode, respectively. In wireless charging tests, the input voltage was controlled through a DC power supply (Victor).

The volumetric capacitance CV (F cm–3) of MSCs was calculated from the CV curves and GCD profiles according to the following Eqs. (1) and (2):

$${C}_{V}=\frac{{\int }_{{U}_{l}}^{{U}_{h}}I\left(U\right){dU}}{2v\cdot \left({U}_{h}-{U}_{l}\right)\cdot V}$$
(1)
$${C}_{V}=\frac{I\cdot \Delta t}{\left({U}_{h}-{U}_{l}\right)}$$
(2)

where Uh and Ul (V) are the limits of the test potential, I(U) (A) is the corresponding current, v (V s–1) is the scan rate, V (cm3) is the volume of electrodes, I (A cm–3) is the volumetric current density, and Δt (s) is the discharge time.

The volumetric energy density EV (mWh cm–3) and power density PV (mW cm–3) of MSCs were obtained according to the following Eqs. (3) and (4):

$${E}_{V}=\frac{1}{2}\cdot {C}_{V}\cdot \frac{{\left({U}_{h}-{U}_{l}\right)}^{2}}{3.6}$$
(3)
$${P}_{V}=\frac{{E}_{V}}{\Delta t}\cdot 3600$$
(4)

Ethical statement

All human participants were fully informed about the purpose, risks, and benefits of this study, and provided informed consent for the data included in this article. The experiment of sensing tests was conducted with the approval of the Institutional Review Board of Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences (approval number: SIAT-IRB-240915-H0914).

Reporting summary

Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.