Researchers at MIT have developed artificial tendons that make muscle-powered robots stronger and more efficient, with an 11-fold improvement in power-to-weight ratio over earlier muscle-only systems.
The work replaces the long-standing approach of attaching engineered muscle directly to robot skeletons - a method that wastes much of the tissue’s contractile potential and limits usable force.
Biohybrid robots rely on engineered skeletal muscle to drive movement, but previously, these tissues had to cling directly to soft, compliant skeletons to avoid tearing. This design constraint reduced force transmission and limited practical applications.
The MIT team has adopted a strategy used throughout animal physiology: the muscle-tendon unit. Their synthetic tendon, made from a tough PVA: PAA hydrogel functionalized with NHS esters, forms a strong covalent bond with the engineered muscle.
This connection allows muscles to act entirely as actuators rather than structural anchors, while enabling attachment to much stiffer and far more capable robotic mechanisms.
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Creating Robotic Tendons
The researchers produced the hydrogel tendons from thin, laser-cut strips backed with polyurethane and pre-stretched for strength. Engineered muscle fibres, created from optogenetic C2C12 myoblasts embedded in a fibrin-Matrigel scaffold, were bonded to each end of the tendon to form a single muscle-tendon unit (MTU).
These MTUs were mounted to flexure-based skeletons milled from low-density polyethylene. A built-in cam allowed precise pre-stretching, while blue-light pulses triggered contractions.
The team measured displacement, force transmission, and fatigue using high-frame-rate imaging, tensile testing, and a custom temperature-controlled incubation chamber.
A mathematical model describing the muscle, tendon, and skeleton as springs in series predicted that tendon stiffness must exceed 100 N/m to maximise displacement.
Guided by this, the team fabricated tendons with stiffnesses of 500 N/m and 1260 N/m, both of which outperformed the threshold.
The MTUs worked best when held in a “taut” configuration of about 20 to 30 % pre-stretch, and maintained stable displacements up to 4 Hz stimulation.
Crucially, they outperformed bare muscle tissues by generating force more quickly, improving high-frequency actuation.
Durability and Force Transmission
Across 3,600-7,200 contraction cycle tests, the MTUs showed no signs of delamination at the muscle-tendon interface. Fatigue occurred in the muscle tissue itself, not in the synthetic bond.
The modular tendon design also enabled the use of a stiffer skeleton, about 50 times stiffer than those typically used in biohybrid robots.
On this skeleton, the MTUs achieved a force-transmission ratio of around 37 %, with a specific force approximately 29 times higher than compliant designs. Adding mechanical clamps at the tendon-skeleton interface offered no performance advantage, indicating the tendon held securely under load.
Implications for Future Biohybrid Robotics
By reducing the portion of engineered muscle wasted on structural support, the tendon-based design significantly increases the usable power generated by the tissue.
The system also introduces a standardized, modular architecture: MTUs can be attached to different skeletons without needing to rebuild the muscle actuator each time.
However, limitations remain. Notably, the study tested only one tendon material, and real tendons include graded stiffness zones not yet replicated here. But the results show that artificial tendons can make muscle-powered robots much more capable, creating a route to robots with greater dexterity, higher forces, and multi-degree-of-freedom movement.
Journal Reference
Castro N. et al. (2025). Biohybrid Tendons Enhance the Power-to-Weight Ratio and Modularity of Muscle-Powered Robots. Advanced Science, e12680. DOI:10.1002/advs.202512680 https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202512680
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