Engineering protein-based fiber-reinforced pneumatic actuators

Abstract

Soft actuators provide a basis for building robots that can operate in unstructured or extreme environments. Typical pneumatic soft actuators are made from silicone, which lacks biocompatibility and environmental sustainability. Many hydrogel materials have demonstrated compatibility for biomedical and agricultural fields, yet their application in soft robotics remains limited. In this work, we set out to develop a fabrication method to replace synthetic materials with bioderived alternatives and establish the mechanical characterization of their performance. Our design comprises an entirely protein-based pneumatic actuator. Our base material, gelatin, is mechanically functionalized through plasticizer crosslinking and reinforced with silk threads to build biomaterial fiber-reinforced elastomeric enclosures (bioFREEs). In this paper, we investigate the properties of the biomaterial based on material composition, dehydration state, and fiber reinforcement, and find compositions comparable to traditional materials used for soft actuator fabrication. We conduct Raman spectroscopy analysis to understand the material composition and homogeneity of the hydrogel matrix, as well as the nature of the bonds between the silk thread and the hydrogel matrix in the bioFREE composite. We also study actuator pressure and load-bearing capabilities based on dehydration state and material composition. We find that the protein-based bioFREEs withstand maximum internal pressures of ∼96.5 kPa, exert contraction ratios of ∼17%, and exert blocked forces of almost 30 N when their diameter is 0.5 cm. They have an upper payload-to-weight ratio of ∼519. Together, this demonstrates bioFREEs as a suitable biomaterial alternative to silicone FREEs. We also find evidence that the passive strain-limiting gelatin material properties may be tuned based on dehydration state to mimic the strain-stiffening seen in biological materials or to adapt in variable agricultural microenvironments. Finally, we test the durability and repeatability of these FREEs through loaded and unloaded dynamic tests for at least 40 000 cycles with 82.7 kPa inflation pressure without failure. The mechanical characterization and new fabrication technique for bioFREEs in this work explores durable pneumatic actuators as a platform to employ new materials in traditional soft robot systems.