Graduate Research Assistant Massachusetts Institute of Technology Cambridge, Massachusetts, United States
Introduction:: Unlike metal and plastic, biological materials can communicate, adapt to stimuli, and repair sustained damage. Incorporating these types of materials into engineered systems could thus foster smarter, more adaptable machines. Our lab has already shown that engineered skeletal muscle coupled to a flexible elastomer ‘skeleton’ can act as a locomotive soft robot that generates force efficiently, strengthens with exercise, and heals from damage [1]. However, its dependence on a muscloskeletal interface driven by contact friction between the muscle and the elastomer may lead to inefficient force transmission through slippage. In addition, this frictional interface also forces the design of the muscle bioactuator to take a ring shape that must be constrained around the skeleton (Fig. 1A). This morphology restricts how the engineered muscle can be used to produce useful work and limits its applicability as a robotic component. So, we have begun the development of a bioinspired tendon to act as a dedicated musculoskeletal interface, enabling efficient force transmission and the modular use of our skeletal muscle bioactuator.
Materials and Methods:: We developed a hydrogel adhesive tendon with our collaborators in Professor Xuanhe Zhao’s lab at MIT. It is composed of a dry poly(acrylic acid) hydrogel functionalized for tissue binding with N-Hydroxysuccinimide (NHS) ester groups. To characterize the properties of the critical interface between the synthetic tendon and the engineered muscle actuator, we performed tensile testing on differentiated and undifferentiated engineered muscle samples (n=3) bound to the synthetic tendon. Muscle samples were manufactured from cultured C2C12 mouse myoblasts. Differentiated and undifferentiated populations were seeded into a fibrin and matrigel matrix, then cast into square polydimethylsiloxane (PDMS) molds. Once prepared, muscle samples of each group were layered between two square sheets of the synthetic tendon. This layered sample was then suspended in a tensile testing fixture by double-sided tape and pulled apart until interface breakage was observed (Fig. 1B).
To understand the effect of the synthetic tendon on short term cell viability, a colorimetric MTS assay was performed, along with a pH exposure test. Absorbance readings were taken at 490nm from collected samples of control and experimental C2C12 myoblast cultures that had been incubated with synthetic tendon overnight (n=3). pH readings were probed at 0 and 30 minutes of control and experimental samples of Dulbecco’s Modified Eagle Medium (DMEM) that had also been exposed to the synthetic tendon (n=3).
Results, Conclusions, and Discussions:: Our trials indicate that although there is no significant difference between the mean force-at-break values for undifferentiated and differentiated muscle, the magnitude of this parameter for any muscle sample type (minimum of 519.8mN) is much larger than any force currently produced by our muscle actuators (mean contractile force of 300uN) (Fig. 1C) [1]. Thus, this interface should remain intact through muscle actuator function. A non-significant difference in mean absorbance and mean pH between experimental and control groups shows there is no significant effect on cellular viability due to synthetic tendon exposure (Fig. 1D). The increase in pH over the experiment time seen in both groups can be attributed to the reaction of the media to an environment that is low in CO2, as it has been designed to be balanced in a CO2 rich environment, like a cell incubator. As such, we have shown that the tendon forms a well-bound interface between itself and the muscle and that there is no significant impact on muscle cultures due to the synthetic tendon.
This hydrogel tendon has been shown to be a suitable biocompatible musculoskeletal interfacial material. With the development of a synthetic tendon, our next steps are to characterize the tendon-skeleton interface. Once established, a muscle actuator will no longer be restricted to a ring shape, allowing for simple muscle strip actuators that can be anchored in any location by the synthetic tendon. No longer in a ring, the muscle fibers will be able to align themselves completely along the muscle body, potentially allowing for increased force generation. Anchored by the synthetic tendon, the muscle actuator will no longer be able to slip against a skeletal structure, providing more efficient force transmission. Finally, its unconstrained nature will enable new designs of machines that can utilize this bioactuator.
Robust and efficient bioactuators promise to have a significant impact on the field of robotics. With the introduction of a synthetic tendon, bioactuators will be capable of driving a variety of systems that can perform higher-level functional behaviors, like walking, swimming, and gripping.
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References (Optional): : [1] R. Raman, C. Cvetkovic, and R. Bashir, “A modular approach to the design, fabrication, and characterization of muscle-powered biological machines,” Nat. Protoc., vol. 12, no. 3, Art. no. 3, Mar. 2017, doi: 10.1038/nprot.2016.185.
