2.1. Electrical and mechanical properties of electrodes
We first assessed the electrical and mechanical properties of the electrode material to identify a composition suitable for the actuators. As mentioned previously, gelatin and glycerol were used as the base material of the electrodes. After curing, the mixture of gelatin and glycerol exhibited elastomer-like characteristics, such as high stretchability (up to 209.6% of strain) and durability (multiple operations).[18,21] Its electrical resistivity was ~3500 Ω·m according to our preliminary experiments, which limits the actuation of the biodegradable electrohydraulic soft actuators. Hence, we employed NaCl as an additive for the gelatin-glycerol mixture to enhance the conductivity and investigate its influence on the electrical and mechanical properties. Specifically, we measured the sheet resistance and Young’s modulus of the electrode material for different amounts of NaCl (see Experimental Section for the details of the fabrication process of the electrodes and the measurements). The amounts of NaCl were 0, 1.25, 2.5, 3.75, and 5 g, which correspond to the mass fractions relative to the amount of gelatin and glycerol of 0, 2.7, 5.3, 7.7, and 10 wt%, respectively. Note that the mass fraction presented here does not include the presence of distilled water (initial mass 120 g) used in the mixing process of the electrode material. After curing, the water mostly evaporated and had an equilibrium state.
As shown in Figure 3a, the sheet resistance decreases with increasing fraction of NaCl. This indicates that the conductivity of the electrode material can be controlled by adjusting the amount of additive. For 10 wt%, the resistivity is 258 Ω·m, which is lesser than that of silicon (640 Ω·m[39]). This suggests that our material is conductive and can provide electric actuation in the devices developed in this study. Figure 3b shows the plot of the measured Young’s modulus of the electrode material. The modulus takes the lowest value of 0.04 MPa for 0 wt% and increases up to 0.07 MPa until 5.3 wt%. Interestingly, from 5.3 wt% to 10 wt%, the modulus decreases from 0.07 MPa down to 0.06 MPa. The reason is as follows. Originally, gelatin binds to water molecules and forms a gel. Concurrently, sodium and chloride ions also bind to water molecules. This binding force is stronger in hydrated NaCl than in gelatin gel. Specifically, the binding force (van der Waals force) of gelatin molecules and water molecules is 0.4–4.0 kJ/mol.[40] On the contrary, the ionic binding force is 20 kJ/mol.[40] Therefore, hydrated NaCl binds the excess water in the sample (0–5.3 wt%) and hardens the electrodes, leading to a greater Young’s modulus. However, from 5.3 wt% to 10 wt%, the water molecules originally bonded to gelatin now bond to NaCl, resulting in a lower Young’s modulus.
The results show that there is a trade-off relationship between conductivity and compliance of the material. Depending on the application, softer and more conductive electrodes can be used, for instance, in soft electrically driven actuators. In this case, the presence of mechanically passive electrodes minimized the effect on actuation performance, and the electric current went through within a short time, thus ensuring a fast response. On the contrary, in some cases, it is difficult to pattern electrodes if they are too soft, making the fabrication complex.