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.