2. Results and discussion

2.1. Preparation and characterizations of MM i-skin
MM i-skin device is designed as a classic sandwich-like structure, mainly composed of two key parts: PDMS thin film with papillary microstructure Au (PM-PDMS / Au) electrode and dielectric layers, which the upper and lower electrode layers are led out by copper wires, and the whole device is encapsulated by PI tape (Figure 2 a). Here, the microstructured PM-PDMS / Au electrode was prepared by a low-cost bio-template method, and the template method can well reproduce the original microstructure of plant leaves from SEM images of the surface of taro leaves (Figure 2b, Figure 2c, and Figure S2). The inset in Figure 2a shows the plan micrograph of the PM-PMDS / Au electrode and the microscopic enlarged image of a randomly selected single mastoid. And the insets in them display its longitudinal (with an average height of 17.01 µm) and transverse (with an average width of 11.8 µm) size distribution. The PVDF-HFP containing [EMIM]DCA ionogel was prepared as the dielectric layer benefiting from its superior chemical stability, mechanical properties, and plasticity due to the existence of crystalline phase PVDF and amorphous HFP in PVDF-HFP,[39,40] the schematic diagram of its synthesis mechanism is shown in Figure 2d. The optical and microscopic images of the ionogel show that it has the typical porous structural characteristics of gel (Figure 2e, and the inset in it displays the groove microstructure of ionogel matching the papillary microstructure). And EDS elemental mapping reveals the exclusive distribution of nitrogen (N), which conforms to the elemental distribution characteristics of [EMIM]DCA ionogel, indicating that the ionic liquid has been successfully loaded and uniformly distributed in PVDF-HFP 3D networks (Figure 2f). The crystal structures and phase of [EMIM]DCA ionogel loaded with different amounts of ionic liquids were further researched by X-ray diffraction (XRD) (Figure 2g). XRD results show that the XRD patterns of pure PVDF-HFP at 17.9° and 19.5° correspond to the (100) and (110) planes of the PVDF-HFP\(\alpha\)-phase.[41] In an ionogel with a [EMIM]DCA loading of 16 wt%, a new peak can be observed at 20.5° due to the (200) plane of the PVDF-HFP\(\beta\)-phase.[42] This is consistent with the FTIR patterns (Figure 2h). When the [EMIM]DCA loading was further increased, the XRD pattern broadened and the intensity of these crystalline peaks decreased significantly, indicating that the crystallinity of PVDF-HFP in the ionogel decreased. While its corresponding crystalline peaks intensity when the loading was 24 wt% is higher, which also shows that the crystallinity of PVDF-HFP in the ionogel is relatively high under this ionic liquid content. This can be attributed to [EMIM]DCA-induced shrinkage of PVDF-HFP polymer chains. The FTIR spectra also reveals that the presence of ionic liquid significantly changes the crystal structure of PVDF-HFP (Figure 2h). The FTIR band of the characteristic peaks at 760 cm-1 and 794 cm-1 could be attributed to the α-phase, while the peak at 838 cm-1 can be attributed to the β-phase of PVDF-HFP.[43-45] While with the addition of different types of ionic liquids ([EMIM]CL, [EMIM]DCA, [EMIM]BF4), the α-phase peak in PVDF-HFP weakened and the β-phase peak became more obvious. Among the three types of ionic liquids, the weakening of the α-phase peak and the enhancement of the β-phase peak in PVDF-HFP added with [EMIM]CL-24 wt% are the most obvious, mainly due to the stronger ion-dipole interactions between Cl- and PVDF-HFP components. In addition, Figure S4 also provides details of the characteristic peaks of crystalline region, non-crystalline region and different ionic liquids in the PVDF-HFP form the FTIR spectra. The Raman spectra further reveals the interaction between anions and PVDF-HFP, among which [EMIM]DCA-24 wt% exhibits the strongest Raman spectra intensity, indicating that there is a strong interaction between [DCA]- ions and PVDF-HFP (Figure 2i). Such a result further revealed that the ionic liquid was successfully loaded into the PVDF-HFP network. Therefore, the PVDF-HFP based ionogel with a loading of 24 wt% [EMIM]DCA ionic liquid was adopted for subsequent studies.