Three-dimensional printing of bilayer microstructures

Abstract

Bilayer structure is one of the most common forms with simple geometry for asymmetric stimuli-responsive actuation, which is widely found in the plants such as pinecones, orchid tree seedpods, and wheat owns in nature. By mimicking these bilayer structures, researchers have developed plenty of bilayer actuators driven by a variety of external stimuli, including heat, light, chemicals, or electromagnetic fields, and reduced the dimension to micro/nanoscale to apply them to advanced biomedical devices. However, it is still challenging to directly fabricate and place bilayer microstructures with the existing techniques. Therefore, it is necessary to develop a new method for high-precision, low-cost and simple bilayer fabrication. This thesis introduces a strategy to 3D print bilayer multi-stimuli responsive microstructures. First, we developed a one-step, continuous 3D printing approach to fabricate bilayer microwires via a double-barrelled theta-shaped micropipette. In this study, we proposed a 3D printing method based on the guidance of a two-phase meniscus. Polystyrene (PS) and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) were selected as the active and inactive layers respectively, due to their different volume expansion rates when absorbing or desorbing water in the ambient air. The printed structure presented excellent and durable actuation performance under the stimuli of humidity and infrared light. Furthermore, the technique was used to directly print multi-segmented microstructures for programmed actuation and demonstrated a microgripper that could capture microparticles in the air. Second, we developed a versatile protocol to engineer the bilayer’s interface during the two-phase-meniscus-guided 3D printing process to program actuation performance in in-situ. After a thorough characterization, we show that we can control the interfacial bonding between two material layers, PEDOT:PSS and Poly(vinyl alcohol)(PVA) in this study, by varying the speed of meniscus stretching: At a slow stretching, two separated layers are produced, whereas a bonded bilayer with is obtained simply by increasing the stretching speed. This structural difference results in a difference in mechanical actuation under moisture and temperature, as confirmed by a full-scale characterization. We expect this technique to open the possibility to program the actuation in 3D microsystems in a simple, versatile, and cost-effective manner and to reinforce the practicality of 3D printing technology.