New Materials and Fabrication Strategies for Soft Robotics via Photopolymerization

The majority of soft robotic approaches utilize just a handful of material chemistries, such as silicone rubbers or hydrogels, and often use non-scalable materials processing techniques such as molding. Very few materials chemistries are designed with both a processing method (such as additive manufacturing) and particular soft robotic application in mind. To address these challenges, we looked to develop better materials and manufacturing methods for soft actuators, with a particular interest towards soft, yet tough, robotic devices for burrowing applications. Photopolymerization can be utilized to enable new materials and fabrication methods towards these ends. In one such example, we investigated the 3D printing of an underutilized material in soft robotics, polyurethane elastomers, by employing an ultraviolent light-assisted direct ink writing (UV-DIW) approach. Printing is enabled using a dual curable ink composed of two orthogonal chemistries: a photocurable acrylate and a thermally curable step growth polyurethane. By photopolymerizing the ink during the printing process, the light-induced crosslinking locks in the initial structure of the printed part, allowing high aspect ratio prints by preventing sagging during printing and thermal post-curing. This technique affords high spatial resolution (down to 250 microns) and can produce highly extensible elastomers (ε >100%) with a range of mechanical properties (1<E<20MPa). By tuning this dual cure chemistry, multi-material parts can be fabricated with gradient properties through the addition of a second ink reservoir. Such a materials chemistry offers potential for manufacture of entire soft devices in one single step.