Fabrication of soft actuators in a single casting step

Molds and sacrificial water-soluble hollow cores are printed using fused filament fabrication. A heated water circuit accelerates the dissolution of the core’s material and guarantees its complete removal from the actuator walls (Fig. 1). This enables the fabrication of actuators with non-uniform cross-sections under minimal supervision, thereby reducing the number of iterations necessary during the design and fabrication processes.

Single-step cast molding proved effective in fabricating soft fluidic actuators. However, due to the need to extract the core after elastomer curing, its applicability is limited to actuators with chambers that have a uniform cross-section. We address this issue by utilizing sacrificial cores made of a water-soluble material, PVA, which can be fabricated using standard FFF technology. The casting process ends with the water-soluble core inside the actuator (Fig. 2). The problem lies in the fact that achieving the complete removal of the water-soluble core can be a time-consuming because the dissolution of the material occurs at a slow pace due to the generally small contact area between the material and the solvent. It is challenging because it is difficult to fully remove the soluble material from the actuator walls, especially in intricate geometries.

We found that the water-soluble core can be completely removed in a short amount of time when submerged in a heated water bath and subjected to a flow of water passing through the core (Fig. 2). The cores were specifically designed and fabricated with a through hole to enable the circulation of heated water. This design choice increased the contact area of the water with the core, thereby accelerating the dissolution of the core’s material. The continuous flow of filtered water was maintained by pumping water through the core hole using a water pump. A resistance heats the water to aid the dissolution. The heated water provides energy to the system, facilitating the process of breaking the intermolecular forces in PVA for a more effective dissolution. The PVA manufacturer recommends temperatures higher than 50 °C. Our system maintains the water temperature at approximately 65 °C to strike a balance between achieving a higher temperature without compromising the operability of elements in contact with the heated water, specifically the pump that is rated to operate at temperatures lower than 70 °C. This method implies that the actuators must be designed with 2 open channels for the water to flow through the hollow core, with one of them being sealed afterward. The sealing can be accomplished in 3 different ways: (i) using an end cap made of non-soft material, (ii) using an end cap made of soft material glued to the actuator, or (iii) employing a casting process.

The versatility of the process was demonstrated by successfully fabricating different soft actuators, namely, a linear actuator and 2 bending actuators (Fig. 2D). The actuator’s internal walls are free of soluble material, without leaks and the need for manual post-treatments (Fig. 2E). On the other hand, 2 primary defects were identified: geometrical inaccuracies manifested through non-uniform wall thickness and surface defects on the chamber walls (Fig. 2E). These defects arise from the high sensitivity of PVA water-soluble material to exposure to moisture and humidity, adding complexity to the FFF-based printing process of the cores. During the printing, problems such as popping, bubbling, poor bonding between layers, and inconsistent extrusion were identified, potentially resulting in rough surface finish and deformations on the printed cores. The core’s rough surface finishing, along with intricate chamber geometries, makes core removal more challenging and time-consuming. Our experiments demonstrated that 20 min is sufficient to completely remove the water-soluble material. The above defects do not compromise the functionality of the actuators.

Overall, our hypothesis was confirmed, demonstrating that an effective process for removing the printed water-soluble cores makes it possible to speed up the fabrication of soft actuators using a single casting step.

Publication: https://spj.science.org/doi/10.34133/cbsystems.0137