3D printing and Heat Treatment of Dual-origami Soft Fluidic Bending Actuator
Soft fluidic robots are principally fabricated through molding and casting of elastomers.\cite{Polygerinos2017a,Ilievski2011,Marchese2015} However, the inevitable process of detaching cured elastomer from the molds has led to difficulties in designing complex fluidic networks because the molds are usually buried within the elastomeric structure. As a solution, using sacrificial molds, made of soluble or low melting materials such as polyvinyl alcohol (PVA) and wax, were proposed.\cite{Koivikko2019} In our previous study,\cite{Kim2019} we have fabricated zigzag folded shaped origami architectures by the layer stacking method using PVA molds. However, these fabrication processes are complicated and time-consuming, while requiring experienced hands due to low material stiffnesses of the sacrificial molds. Meanwhile, 3D printing has been recently studied as an automotive fabrication method for soft robots.\cite{Polygerinos2017a,Yap2016,Low2020,SachyaniKeneth2021,Wallin2018} In particular, FDM is a prominent and popular open-source technology with economic merits of low initial and maintenance costs. The final products can be built without supports by bridging gaps within a few centimeters, and in recent studies, researchers have directly fabricated conventional soft bending actuators made with commercially available TPU filaments.\cite{Low2020,Yap2016} However, because FDM printers directly add molten material line by line, defects and gaps often occur at the boundary lines, leading to failure or fluid leakage at high fluid pressure.
We fabricated a dual-origami soft fluidic bending actuator using an FDM 3 printer as shown in Figure 16A. The printed actuator can be actuated immediately after gluing a urethane tubing with an adhesive, but failure or leakage occasionally happened when high pressure (>200 kPa) was applied. For robust fabrication, we attempted a heat treatment of the fluidic soft actuator in an oven, expecting leakage prevention and enhancement of pressure holding. It was important to heat using the appropriate temperature and time; i) when either of them was insufficient, the post-processing was of little avail, and ii) when the conditions were excessive, the processed product was deformed or even melt and collapsed (Figure 17). During the process, the soft bending actuator was placed between aluminum blocks to prevent possible deformation of modules at both ends. The post-processing was successful at 172\(\degree\)C for 2 hours (for reference, melting temperature of TPUs, \(T_m\)>190\(\degree\)C), and the white or transparent TPUs turned yellowish after post-processing. As seen in micrographs in Figure 16B, it was observed that the boundaries between printed lines became blurred and were filled with adhered materials regardless to build direction. We verified the effectiveness by printing small test pouches and comparing maximum sealing pressure for raw and heat-treated pouches. As a result, the raw pouches could withstand 129 kPa on average, while the heat-treated pouches could withstand 756 kPa on average, which is an increase of 5.58 times (Figure 17).