4. Experimental Section
Materials: Poly (methylmethacrylate (PMMA, 98.5%),
Polydimethylsiloxane (PDMS, 85%), Trichloromethane (CHCl₃, 99%), Poly
vinylidene fluoride-hexafluoro propylene (PVDF-HFP, 99%),
1-ethyl-3-methylimidazolium dicyanamide (EMIM[DCA], 99%). Taro leaf
(Produced in Yuxi City, Yunnan Province), Sand paper. All chemicals were
of analytical purity and used directly without any processing, except
for taro leaves and sandpaper that have been cleaned.
Preparation of PDMS electrodes: PDMS electrodes were prepared by
plant template method combined with electron beam deposition (Figure
S1). The PMMA particles are mixed with trichloromethane at a mass ratio
of 8:100 and stirred until completely dissolved, and then the prepared
PMMA solution was applied dropwise to the surface of fresh taro leaves
then heating at 60 ℃ for 1 h. The PMMA film was peeled off from the leaf
as a PMMA template with the opposite structure of the taro. Next, the
PDMS precursor solution was mixed with the curing agent (10:1) and
smeared onto the above PMMA template then heating at 60 ℃ for 2 h. Then,
the PDMS film was peeled off from the PMMA template to obtain the PDMS
film with taro leaf papillary microstructure (PM-PDMS), and the PMMA
template was cleaned and dried naturally for recycling and reusing.
Finally, the microstructured gold electrode (with the Au film thickness
of 250 nm) was prepared by electron beam evaporation, which was denoted
as PM-PDMS / Au.
Preparation of ionogel dielectric layers: Ionogel with different
roughness were prepared by sandpaper reverse mold (Figure S3). After
completely dissolving 0.2 g PVDF-HFP in 2.5 ml acetone solution, 0.2 ml,
0.4 ml, 0.6 ml, and 0.8 ml EMIM[DCA] ionic liquids were added to the
same four PVDF-HFP solutions, respectively, which were recorded as
EMIM[DCA]-8 wt%, EMIM[DCA]-16 wt%, EMIM[DCA]-24 wt%, and
EMIM[DCA]-32 wt%, and stirred vigorously to ensure the uniform
mixing of ionic liquid and PVDF-HFP. Then a little of the
above-configured mixed solution was applied onto the sandpaper with
different roughness, heating at 60 °C for 2 h to obtain ionogel film
possessing rough surface. Furthermore, a slide of smooth ionogel film
was selected as a substrate for contrastive analysis.
Assembly of flexible ion pressure sensor: The flexible ion
pressure sensor was designed as sandwich-like structure where the
PM-PDMS / Au film acts as the upper and lower electrodes and the ionogel
was employed as the dielectric layer film, and the copper wire was used
as a test electrode. Further, the electrical double layer capacitive ion
pressure sensor can be encapsulated with PI film (Figure 1).
Characterization and measurements: SEM morphological and
elemental characterization of PDMS and PVDF films was performed by the
Phenom Pro X. Characterization of the mechanical properties of PDMS
films and ionogel was exerted using a universal material testing machine
(UTM 4000, Shenzhen Suns Co. Ltd, China). By controlling the stepper
motor to provide different pressures, the performance of the capacitive
pressure sensor at low frequencies is tested with the Victor 4091C
precision LCR meter, and the capacitive sensing performance at high
frequencies is measured by Keithley 4200 SCS semiconductor parameter
tester. Electrochemical workstation (CHI-660e, Shanghai Chenhua Co. Ltd,
China) was employed to test the electrochemical impedance spectra and
cyclic voltammetric curves of all the samples. The ionic conductivity of
the ionogel can be calculated from obtained electrochemical impedance
spectroscopy data. Thermal conductivity monitoring of ionogel was
measured by Hot Disk TPS 2500 S thermal conductivity meter; The
thermoelectric performance of ionogel was characterized by the
customized thermoelectric test system (refer to the published
work).[58]