Engineering Biomolecular Actuators from Calcium-Responsive Repeat Proteins

Protein-based hydrogels provide a platform for functional, biocompatible materials in soft robotics. Stimuli-responsive protein hydrogels can function as soft actuators by transforming environmental signals into mechanical motions without the need for an external power source. For soft robotics systems to interface or integrate with biological systems, they must respond to physiological stimuli. Calcium ions act as critical signaling molecules for a wide variety of biological functions, including muscle cell contraction, cell replication, and neurotransmitter release. Calcium-responsive actuators would enable broad biomedical applications, such as artificial muscles and implantable, responsive materials.<br/><br/>The diverse functionality of naturally occurring proteins offers bioinspiration to develop engineered proteins that respond to specific stimuli. Calcium responsiveness emerges in a class of “Repeats-in-Toxin” (RTX) protein domains that undergo reversible conformation changes from random coils to β-roll structures upon binding to calcium. We characterize the changes in the overall size and tertiary structure of RTX domains in the presence and absence of calcium using small angle x-ray scattering (SAXS). The addition of calcium results in a 70% decrease in volume of an RTX domain, representing a large contractile range. To harness this mechanical response, we genetically fused RTX domains between hydrogel-forming associative domains. The mechanical properties of the resulting hydrogels were characterized using shear rheology, which revealed calcium-dependent increases in hydrogel stiffness. By incorporating RTX domains in hydrogel systems, we demonstrate a tunable platform for calcium-responsive biomolecular actuators. RTX-based biomolecular actuators are poised to enable the development artificial muscles to aid in the repair or replacement of injured muscle.