2.2. Capacitive ion skin with multi-stage microstructure (C-iskin)
It has been found in previous studies that the existence of multi-level three-dimensional micro-nano structures can effectively improve the sensing performance of devices.[46,47] The cross-sectional schematic diagram of the C-iskin structure shows a classic sandwich-like structure (Figure 3 a-i). In the initial state (Figure 3a-ii), the ionogel contains free-moving cations and anions without external pressure and voltage. When a voltage is applied (Figure 3a-iii), opposite charges on the electrodes attract ions in the ionogel layer, forming microscale capacitors with nanometre separation. This leads to ultra-high capacitance per unit area, as electron-ion pairs constitute the capacitor structure.[48] The equivalent circuit diagram indicates that the capacitance CEDL1 and CEDL2 formed between each of the upper, lower electrode layers and the ionogel are connected in series to equivalently constitute the capacitance of the entire device (Figure S5). This kind of electrical double layer capacitor can effectively improve its capacitance performance.[49]
Sensitivity, response time, pressure responsiveness, cyclic stability, and linear response are the most important sensing properties of the pressure sensor.[50] The response and reply curve of the C-iskin when the stepper motor exerts a force of 2 kPa on the sensor exhibiting extremely short response time (~ 46 ms) and reply time (~ 0.42 s) (Figure 3B, and Figure S6 provides an enlarged curve plots of response and recovery time for a single cycle), whose response time is the same as that of human skin (30-50 ms).[51] This extremely short response time can be ascribed to the special double-layered papillary microstructure electrode and the ionogel with suitable concentration and roughness. To elucidate that, the capacitive performance of double-layered mastoid structure and interlocking capacitive sensors were further studied, proving that the interlocking capacitive sensors can effectively improve the capacitive response capability (Figure S7a). In the interlocking configurations consisting of ionogel with concave microstructure and double-layered mastoid microstructured electrodes, the effect of different surface roughness (0, 180, 600, 1000-grit) of ionogel on the capacitive sensing sensitivity was further studied in Figure S7b, the SEM image of which is shown in Figure S8. When the gel is formed on the 600-grit sandpaper, more pits with a diameter of about 20 μm appeared on the surface, which was comparable to the size of the papillary microstructured electrodes, with which an interlocking configuration was well formed. As a result, the capacitive sensitivity of the interlocked configuration with the 600-grit sandpaper substrate inverted with ionogel as the dielectric layer was significantly better than the other three substrates. In addition, different concentrations of ionic liquids (8 wt%, 16 wt%, 24 wt%, and 32 wt%) was studied to explore the relationship between the ionic skin sensitivity and the ionic liquid concentration (Figure S9), which shows that the sensitivity of the double-layered capacitor sensors increases with the increase of ionic liquid concentration, the highest sensitivity was found at an ionic liquid concentration of 24 wt%. Owing to the establishment of the electrical double layer within the gel, the quantity of free [EMIM]- and DCA+ increases as the ionic liquid content rises. When free electrons are present on both sides of the electrode, more anions and cations are attracted, leading to an increase in capacitance and a corresponding increase in sensitivity. However, excessive concentration of the ionic liquid (over 32 wt%) can cause the the adhesion of the dielectric layer to the electrode layer and poor recovery of the sensor, eventually resulting in a slight decrease in the sensitivity of the sensor. Therefore, the C-iskin composed of PVDF-HFP-based ionogel with a loading of 24 wt% [EMIM]DCA ionic liquid and a roughness of 600-grit exhibits high sensitivity of 77.4 kPa-1 below 1 kPa (Figure S9).
Pressure responsiveness is one of the criteria for evaluating sensing performance, the device was able to maintain a stable and reliable electrical signal even when in a pressure of 2, 8, and 30 kPa (Figure 3c, and Figure S10a also explores the minimum detection limit of the C-iskin). The results show that the C-iskin has a stable response, excellent matching between pressure and capacitance changes, high repeatability, and clear signals under various pressures. which is much higher than the conventional response range of human skin (<10 kPa). Furthermore, the deformation state of PM-PDMS films under different pressures (0 to 50 kPa) and maximum pressure detection limit of C-iskin was explored by means of simulation (Figure 3d). As the exerted force was gradually increased from 0 to 50 kPa (in steps of 10 kPa), the PM-PDMS films still maintained their original shape and the microstructures were not destroyed, which indicates the excellent stability of the sensor. And the rest of the deformation under different pressures was further studied (see Figure S11 for more details). In addition, the capacitance variation of the C-iskin shows excellent matching with different loading / unloading times and operating frequencies, ensures stable response and high repeatability of the C-iskin (in Figure S10b-d). In order to investigate the mechanical durability of the C-iskin, it was subjected to 1500 loading / unloading cycles at a pressure of 2 kPa. The results show that the C-iskin doesn’t suffer from fatigue (Figure 3e), suggesting excellent repeatability, stability, and durability, and the two insets show that the waveform remains essentially unchanged at the beginning and the end of the cycle, with a slight increase in amplitude. In fact, the cycling curve amplitude as a whole exhibit a slight upward trend with the increase in the number of cycles, which attributed to the structural damage of the double-layered mastoid-like microstructures after prolonged loading / unloading cycles, resulting in a shortened distance between the two electrodes.
Stretchability and flexibility are the fundamental requirements to fulfill the wearable health monitoring devices. The mechanical properties of PM-PDMS films, ionogel films with different ionic liquid concentrations, and microstructured ionogel films obtained from the above preparations were analyzed (Figure S12). The results indicate the PDMS film can withstand 140% strain with a fracture strength of 1.2MPa, while PM-PDMS exhibits an unprecedented high fracture strength of 2.0MPa at 134% strain, which is increased by 60% (Figure S12a-b). The elongation at fracture of the two is similar, while the Young’s modulus of PM-PDMS is 74% higher than that of PDMS. The results discussed above show that the introduction of microstructure plays an important role in improving the mechanical properties of C-iskin. Similarly, the analysis results of ionic liquid concentration (Figure S12c-d) and gel roughness (Figure S12e-f) show that [EMIM]DCA-24 wt% and 600 mesh have excellent comprehensive mechanical properties.
Thanks to the excellent integrated sensing capabilities of the C-iskin, it has great potential in wearable health monitoring applications. C-iskin was placed on the vocal cords, upper lip and wrist of the volunteers respectively to monitor vocalization, respiration and pulse signals, and its output curve shows that the above movements and signals can be easily and accurately detected (Figure 3f-h). Therefore, C-iskin has important application prospects in health monitoring, robotic motion monitoring and prosthetics.