3D Printed Soft Composites with Tunable Mechanical Properties

The use of soft engineering materials such as silicones has been gaining more traction due to the emergence of technologies such as soft robotics, pre-surgical organ models, and wearable electronics. where compatibility with soft biological systems and the ability to mimic their functionalities are essential. For the continued advancement and broad utilization of these technologies, there is a need for further development of soft engineering materials with properties required in relevant applications. Particularly, the ability to precisely control the mechanical properties, as well as their spatial distribution towards development of functionally graded biomimetic compliant structures is a significant research interest. Additive manufacturing with multi-polymer systems have proven to be an effective method to achieve this goal, however, the commonly used Polyjet approach is highly expensive and limited in material capabilities. Towards addressing this challenge, this work presents novel soft composites, consisting of a silicone matrix and thermoplastic elastomer reinforcements, fabricated through low-cost extrusion-based additive manufacturing. Mechanical properties of these composites are functions of the reinforcement geometry that can be precisely controlled. We use a customized 3D printer with direct ink write (DIW) and fused filament fabrication (FFF) capabilities to print composites with a sinusoidal reinforcement pattern. We demonstrate that changes in the amplitude and frequency of these sine waves led to significant differences in the hyperelastic behavior of the composites. Specifically, decreases in amplitude and frequency led to an overall stiffening of the composite, while increasing these parameters led to a softer stress-strain response that approached that of a non-reinforced silicone sample. Additionally, changing these parameters independently led to differences in strain-hardening behavior. Finally, we demonstrate the ability of this approach to seamlessly control the spatial compliance distribution of composites, by printing parts with sinusoidal reinforcements of spatially varying amplitude and frequency.