A Kirigami Approach of Patterning Membrane Actuators

Abstract

Ionic electroactive polymer actuators are typically implemented as bending trilayer laminates. While showing high displacements, such designs are not straightforward to implement for useful applications. To enable practical uses in actuators with ionic electroactive polymers, membrane-type film designs can be considered. The significantly lower displacement of the membrane actuators due to the lack of freedom of motion has been the main limiting factor for their application, resulting in just a few works considering such devices. However, bioinspired patterning designs have been shown to significantly increase the freedom of motion of such membranes. In this work, we apply computer simulations to design cutting patterns for increasing the performance of membrane actuators based on polypyrrole doped with dodecylbenzenesulfonate (PPy/DBS) in trilayer arrangements with a polyvinylidene fluoride membrane as the separator. A dedicated custom-designed device was built to consistently measure the response of the membrane actuators, demonstrating significant and pattern-specific enhancements of the response in terms of displacement, exchanged charge and force.

Keywords: PPy/DBS trilayer; bioinspired; membrane actuators; patterning; simulation.

Scheme 1

Membrane designs. No patterning: M1; patterned: M2 (middle hole, diameter 4 mm), M3 (a hole and four straight cuts with a 2 mm space between the hole and the cuttings), M4 (a hole and 8 straight cuts), M5 (a hole and spiral cuts).

Scheme 2

In-house data acquisition system. (A) computer with LabView, (B) data acquisition system (SC-2345), (C) current amplifier, (D) stand for the distance sensor, (E) measurement device and (F) distance sensor.

Figure 1

Assembly steps of the measurement device (scale bar 6 cm) (a–d) with an M2 membrane: the lower electrode as the counter-electrode and the upper electrode as the working electrode. A schematic view of the device with the electrodes, the membrane actuator and the connections in place (e).

Figure 2

(a) The galvanostatic polymerization curve (two-electrode cell) of polypyrrole (PPy) on a chemically PPy-coated polyvinylidene fluoride (PVdF) membrane at −5 °C, (0.1 mA cm⁻², 4000 s); (b) SEM micrographs (scale bar 50 μm) of the surface of an M1 actuator with the cross section in the inset showing arrows of PVdF and the deposited conducting polymer. (c) EDX surface spectra of M1 after actuation, oxidized at +0.7 V (black curve) and reduced at −0.7 V (red curve).

Figure 3

Finite element simulations of deflections (extant as color scales) resulting from a uniform constant pressure of 0.7 kPa on the membranes (a) M1, (b) M2, (c) M3, (d) M4 and (e) M5. The arrows on the membrane symbolize the vertical pressure on each point of the membrane. The green dotted points refer to the part of the membrane being fixed (1 cm from the edge), as under the experimental conditions.

Figure 4

Square wave potential step (16.7 mHz, ± 0.7 V, dotted line) measurements in bis(trifluoromethane)sulfonimide lithium salt (LiTFSI-aq) electrolyte. (a) Two subsequent cycles (4th to 5th) of displacement δ response of membrane actuators M1 (black line) and patterned M2 (red), M3 (green), M4 (blue) and M5 (pink). (b) Displacements against log frequency of M1 (■), M2 (●), M3 (♦), M4 (★) and M5 (◄); and (c) displacements against charge density upon reduction. (d) The actuation speed ν of M1–M5-type membrane actuators against log frequency. The dashed line in (c) represents the linear fit and is shown here only for orientations.

Figure 5

Diffusion coefficients obtained from Equations (2) and (3), from square wave potential step measurements in LiTFSI-aq electrolyte in a potential range of ± 0.7 V. From the membrane actuators M1 (■), M2 (●), M3 (♦), M4 (★) and M5 (◄), the diffusion coefficients upon reduction D_red are shown in (a) and those upon oxidation D_ox are presented in (b) against the applied frequency f.

Figure 6

Displacements of membrane actuators M1 (■), M2 (●), M3 (♦), M4 (★) and M5 (◄) with different loads (counter-forces) driven at 8.33 mHz at ± 0.7 V in aqueous LiTFSI electrolyte.