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.