What if living muscle could power machines with the same force and precision as engineered actuators? Engineers at MIT have made an advance toward that dream reality by fabricating artificial tendons from tough hydrogel, which allowed biohybrid robots to run three times faster and exert thirty times more force than muscle alone.
Till now, biohybrid robots-machines that combine living muscle tissue with a synthetic skeleton-have been limited by the mechanical mismatch between the soft biological actuators and rigid structural elements. In the body of naturally occurring organisms, it’s the tendons that bridge that gap, efficiently transmitting force while preventing damage. The MIT team, led by Ritu Raman, assistant professor of mechanical engineering, mimicked this geometry in the lab. The researchers designed modular “muscle-tendon units” that attach lab-grown skeletal muscle to robotic skeletons.
Tendons consisted of a polyvinyl alcohol–polyacrylic acid hydrogel, functionalized with NHS esters to form strong covalent bonds to muscle tissue. The material, developed in the lab of Xuanhe Zhao, is highly stretchable and mechanically robust, able to adhere to biological and synthetic surfaces. Laser-cut into thin strips and backed with polyurethane for reinforcement, each tendon was pre-stretched to optimize its stiffness prior to bonding to engineered muscle fibers embedded in a fibrin–Matrigel scaffold.
The team modeled the system as three springs in series-muscle, tendon, and skeleton-to find the optimal stiffness of the tendon that allowed maximum displacement and transmission of forces. The threshold stiffness from simulations was 100 N/m; fabricated tendons at 500 N/m and 1260 N/m were above this value and delivered high performance. Milled from low-density polyethylene, the flexure-based skeletons with the mounted muscle-tendon units could sustain displacements upon stimulation up to 4 Hz, not showing the collapse observed at bare muscle actuators in high-frequency actuation.
In controlled tests, the tendon-enhanced gripper’s speed tripled while its force output increased thirtyfold compared to muscle-only designs. The power-to-weight ratio rose elevenfold, which means far less muscle mass was required for the same mechanical output. Durability tests revealed no delamination at the muscle-tendon interface after 7,000 contraction cycles; fatigue occurred only in the muscle tissue itself. On stiffer skeletons-about fifty times more rigid than typical biohybrid frames-the units attained a force-transmission ratio of 37%, with a specific force roughly twenty-nine times higher than compliant designs.
These hydrogel tendons also overcame one of the perennial problems of biohybrid robotics: the tearing of muscles under load. Providing that intervening level of stiffness between soft tissue and rigid parts enabled secure attachment without sacrificing actuation efficiency. “You just need a small piece of actuator that’s smartly connected to the skeleton,” Raman pointed out, underscoring another aspect of the modularity in the design. Each muscle–tendon unit can be integrated into various robotic architectures without rebuilding the actuator, opening pathways for scalable manufacturing.
That advance is built on certain core principles of the design of biohybrid actuators, in which engineered skeletal muscle would provide externally controllable, high-force contractions and regenerative capacity. As demonstrated in previous work, these muscle actuators could be electrically or optogenetically stimulated to achieve precise motion control. In the MIT system, optogenetic modification allowed contractions triggered specifically by blue light, with no physical contact during activation-a key advantage for applications at very small scales or involving delicate systems.
It is only that hydrogels are especially tailored for use in such tendons because of their tunable mechanical properties, biocompatibility, and the ability to mimic most extracellular matrix environments, as much of soft robotics research points out. Precise engineering of the material stiffness and adhesion for optimal force transmission in such applications can be achieved by tuning polymer composition, cross-linking density, and incorporating nanomaterials.
The implications go far beyond laboratory demonstrations: modular hydrogel tendons have the potential to allow multi-degree-of-freedom biohybrid robots that possess higher dexterity and versatility for varied tasks ranging from microsurgical tools to autonomous exploratory machines operating in hazardous environments. Already, Raman’s group is working on skin-like protective casings to shield the muscle actuators against such environmental stability issues as dehydration and contamination.
Merging biological adaptability with engineered precision, MIT’s artificial tendon system redefines the performance envelope of muscle-powered robotics, providing an effective, repeatable, and scalable interface between living tissue and mechanical systems.
