Michael R. Vinciguerra*, Dinesh K. Patel, Wuzhou Zu, Mahmoud Tavakoli*, Carmel Majidi*, and Lining Yao*. ACS Appl. Mater. Interfaces. 2023. PDF | DOI

Abstract: Liquid crystal elastomers (LCEs) have grown in popularity in recent years as a stimuli-responsive material for soft actuators and shape reconfigurable structures. To make these material systems electrically responsive, they must be integrated with soft conductive materials that match the compliance and deformability of the LCE. This study introduces a design and manufacturing methodology for combining direct ink write (DIW) 3D printing of soft, stretchable conductive inks with DIW-based “4D printing” of LCE to create fully integrated, electrically responsive, shape programmable matter. The conductive ink is composed of a soft thermoplastic elastomer, a liquid metal alloy (eutectic gallium indium, EGaIn), and silver flakes, exhibiting both high stretchability and conductivity (order of 105 S m–1). Empirical tuning of the LCE printing parameters gives rise to a smooth surface (<10 μm) for patterning the conductive ink with controlled trace dimensions. This multimaterial printing method is used to create shape reconfigurable LCE devices with on-demand circuit patterning that could otherwise not be easily fabricated through traditional means, such as an LCE bending actuator able to blink a Morse code signal and an LCE crawler with an on/off photoresistor controller. In contrast to existing fabrication methodologies, the inclusion of the conductive ink allows for stable power delivery to surface mount devices and Joule heating traces in a highly dynamic LCE system. This digital fabrication approach can be leveraged to push LCE actuators closer to becoming functional devices, such as shape programmable antennas and actuators with integrated sensing.

Figure 1. Overview of the multimaterial printing process. (a) 4D printed liquid crystal elastomer (LCE) teddy bear with integrated Joule heating trace that was digitally fabricated using the approach outlined in this work. (b) A chiral auxetic-like structure with arbitrarily patterned conductive ink. The actuated state is shown at the bottom of the figure with the conductive ink on the inner side of the actuator. (c) 4D printing of the LCE. As the ink leaves the nozzle, the mesogens and chains are aligned along the printing direction, and the direction of contraction is locked using the ultraviolet (UV) light array. (d) 3D printing of EGaIn-Ag-SIS ink. The conductive ink is patterned directly on top of an LCE substrate. (e) An LCE bending actuator with an infrared LED. Bending is induced by creating a multilayer structure with layers at 0°–90° configurations. The conductive ink can contract with the LCE, providing consistent heating and power to ICs.