Fully Aerosol-Jet Printed Trilayer Electrochemical Actuator

Abstract

Electrically driven soft actuators are essential components for numerous emerging technologies that require controlled mobility, including biomedical devices capable of navigating the human body, miniature robots for environmental monitoring, and haptic interfaces. Electrochemical actuators (ECAs) are a promising candidate for these applications due to their capability for large deformations at low actuation voltages (<1 V) and their potential to be scaled down to very small sizes (~μm or below). A typical trilayer ECA that operates in air comprises a solid polymer electrolyte layer sandwiched between two electrode layers, and achieves bending through electric-field-driven migration of ions which causes expansion of one side and contraction of the other. Despite the theoretical possibility of miniaturisation, microfabrication and micro-patterning of trilayer ECAs have remained difficult due to the need to pattern and align electrodes on both sides of the polymer electrolyte while avoiding short circuits between adjacent and opposing electrodes. Existing photolithography-based methods also face challenges in maintaining pattern integrity during the development process, and are wasteful, costly, and time-consuming. This thesis presents a novel microfabrication procedure that produces trilayer ECAs entirely through aerosol jet printing (AJP), enabling low-cost rapid prototyping of micro-ECAs, investigation of actuators with different dimensions for performance optimisation, and creation of precise micropatterns. The first phase of this research focused on developing aerosol-jet-printed ECAs by exploring different electrode materials, including Ag, Ag nanowires, Au, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), and their composites. The first successful fully aerosol-jet-printed ECAs were developed with Au electrodes on Nafion electrolytes. The printed Au electrodes exhibited a microcrack morphology, allowing them to accommodate mechanical stress and maintain conductivity during stretching and after repeated cycles, resulting in reliable, cyclable actuation. However, the Au-based actuators displayed a nonlinear deflection-voltage relationship and exhibited back relaxation behaviour due to the complex electrochemical processes involved. Conductive polymer electrodes were subsequently investigated, such as PEDOT:PSS, where actuation is driven by the intercalation and deintercalation of ions into and out of the electrodes. It is found that fully printed, 1 mm by 5 mm, 12 μm-thin trilayer ECAs, with PEDOT:PSS electrodes and Nafion electrolytes, demonstrated cyclable actuation without back relaxation and displayed linear deflection-voltage relationship during AC actuation tests with voltage amplitudes from 0.1 V to 0.8 V. In frequency sweep tests, noticeable deflections were observed at frequencies up to 10 Hz, beyond which the oscillations became barely visible. DC actuation tests revealed a response time on the order of seconds, a relatively fast response for ECAs with pristine PEDOT:PSS electrodes owing to the small size and thinness of the AJP ECAs. Preliminary experiments with PEDOT:PSS-based composite electrodes showed limited success, but with further optimisation, their performance could be improved, making them a potential area for future investigation. The next stage of this research continued on further miniaturisation and geometrical optimisation of AJP ECAs with PEDOT:PSS electrodes. Design modifications were made, such as the addition of thin Nafion encapsulation layers to prevent layer delamination, and the integration of Au contact electrodes to improve connectivity. Micro-ECAs were fabricated with a cantilever width of 0.4 mm and an electrode width of 0.2 mm. By adjusting the number of printing passes, actuators with varying electrode and electrolyte thicknesses were produced, ranging from 0.36 μm to 1.9 μm for electrodes and 3.5 μm to 12 μm for electrolytes. The DC actuation test results indicated an increase in deflection with a thinner electrolyte layer, verifying the predictions of a theoretical model. Due to the high aspect ratio of the micro-ECAs, the actuation time scale was found to be dominated by electronic conduction along the length of the conductive polymer electrodes. The findings suggest that future improvements for micro-ECAs should focus on increasing the volumetric capacitance of the electrodes, utilising larger effective ion sizes, and enhancing electrode conductivity. The thesis culminates in the multimodal actuation of a fully printed actuator with two individually controlled segments, showcasing the potential of AJP ECAs for more sophisticated and multifunctional devices. Possible future designs are discussed, including the integration of micro-ECAs with proximity sensors consisting of interdigitated electrodes, to construct multi-segment actuating devices capable of navigating obstacle courses – potentially guiding cochlear implants and similar applications.