Our muscles are nature’s actuators. Tendon tissue generates the forces that make our bodies move. In recent years, engineers have used real muscle tissue to launch “biohybrid robots” made of both living tissue and synthetic parts. By combining lab-grown muscles with synthetic skeletons, researchers are creating a menagerie of muscle-powered crawlers, walkers, swimmers and grabbers.
However, for the most part, these designs are confined in the amount of traffic and power they can produce. Now MIT engineers intend to raise the power of biobots using artificial tendons.
In the study will appear in the magazine today researchers developed artificial tendons made of robust and versatile hydrogel. They attached rubber-band-like tendons to both ends of a compact piece of lab-grown muscle, creating a “muscle-tendon unit.” They then connected the ends of each artificial tendon to the fingers of the robotic gripper.
When they stimulated the central muscle to contract, the tendons pulled the gripper fingers together. The robot brought its fingers together three times faster and with 30 times more force compared to the same structure without connecting tendons.
The researchers anticipate that the novel musculotendinous module can be adapted to a wide range of biohybrid robot designs, much like a universal engineering element.
“We introduce artificial tendons as interchangeable connectors between muscle actuators and robotic skeletons,” says lead author Ritu Raman, an assistant professor of mechanical engineering (MechE) at MIT. “Such modularity can facilitate the design of a wide range of robotic applications, from microscale surgical tools to adaptive, autonomous research machines.”
MIT co-authors of the study include graduate students Nicolas Castro, Maheera Bawa, Bastien Aymon, Sonika Kohli and Angel Bu; BA Annika Marschner; Ronald Heisser, Ph.D.; graduates Sarah J. Wu ’19, SM ’21, PhD ’24, and Laura Rosado ’22, SM ’25; and MechE professors Martin Culpepper and Xuanhe Zhao.
Muscle gains
Raman and her colleagues at MIT are at the forefront of biohybrid robotics, a relatively novel field that has emerged in the last decade. They focus on combining synthetic, structural robotic parts with living muscle tissue as natural actuators.
“Most of the actuators that engineers typically work with are really difficult to make in small sizes,” Raman says. “Above a certain size, the basic physics don’t work. The cool thing about muscles is that each cell is an independent actuator that generates force and creates movement. So basically you can make really small robots.”
Muscle actuators also have other benefits that Raman’s team has already demonstrated: tissue can grow stronger with exercise and heal naturally when injured. For these reasons, Raman and others predict that muscular droids could one day be sent to explore environments too remote or hazardous for humans. Such muscular bots could build up strength for unforeseen traverses or heal themselves when facilitate is unavailable. Biohybrid bots can also serve as compact surgical assistants performing fragile micro-scale procedures inside the body.
All these future scenarios motivate Raman and others to find ways to combine living muscles with synthetic skeletons. Projects so far have involved growing a strand of muscle and attaching both ends to a synthetic skeleton, much like wrapping a rubber band around two posts. When a muscle is stimulated to contract, it can connect parts of the skeleton to generate the desired movement.
However, Raman says this method creates a lot of wasted muscle that is used to attach tissue to the skeleton rather than to make it move. And this connection is not always secure. Muscles are quite pliable compared to skeletal structures, and the difference can cause the muscle to tear or detach. Moreover, often only the contractions in the middle part of the muscle do any work – a relatively compact amount and generate little force.
“We were wondering how do we stop wasting muscle material, make it more modular so it can be attached to anything, and make it work more efficiently?” – says Ramana. “The body’s solution is to create tendons that are half the stiffness of muscles and bones, bridging the mechanical mismatch between soft muscles and the stiff skeleton. They are like thin cables that effectively wrap around joints.”
“Smart Connection”
In their novel work, Raman and her colleagues designed artificial tendons to connect natural muscle tissue to a synthetic gripper skeleton. They chose hydrogel – a pliable but robust polymer-based gel. Raman received hydrogel samples from her colleague and co-author Xuanhe Zhao, who is a pioneer in hydrogel development at MIT. Zhao’s group has developed recipes for hydrogels with varying strengths and extensibility that can stick to many surfaces, including synthetic and biological materials.
To find out how forceful and stretchy the artificial tendons needed to be to be suitable for the gripper design, Raman’s team first modeled the design as a straightforward arrangement of three types of springs, each representing a central muscle, two connecting tendons, and the gripper skeleton. They assigned a certain stiffness to the muscles and skeleton that were known previously, and from this they calculated the stiffness of the connecting tendons that would be required to move the gripper by the desired amount.
Based on this modeling, the team developed a recipe for a hydrogel with a specific stiffness. Once the gel was made, the researchers carefully etched it into lean cables, creating artificial tendons. They attached two tendons to either end of a compact sample of muscle tissue that they had grown using laboratory techniques. They then wrapped each tendon around a compact post at the end of each finger of the gripping robot – the framework was developed by MechE professor Martin Culpepper, an expert in designing and building precision machines.
When the team stimulated the muscle to contract, the tendons in turn pulled on the gripper to bring his fingers together. Through numerous experiments, scientists found that the musculotendinous gripper worked three times faster and produced 30 times more force compared to when the gripper is actuated by a band of muscle tissue alone (without any artificial tendons). The novel tendon-based design was able to maintain this performance for over 7,000 cycles, or muscle contractions.
Overall, Raman noted that adding artificial tendons increased the robot’s power-to-weight ratio by a factor of 11, meaning the system needed significantly fewer muscles to do the same work.
“You just need a small actuator that is intelligently connected to the framework,” says Raman. “Normally, if a muscle is really soft and attached to something that has a lot of resistance, it will just tear before it moves anything. But if you attach it to something like a tendon that is resistant to tearing, it can actually transfer its force through the tendon and can move a skeleton that it otherwise wouldn’t be able to move.”
The novel muscle and tendon assembly design successfully combines biology with robotics, says biomedical engineer Simone Schürle-Finke, associate professor of health sciences and technology at ETH Zürich.
“Hard hydrogel tendons create a more physiological architecture of muscles, tendons and bones, which significantly improves force transmission, durability and modularity,” says Schürle-Finke, who was not involved in the study. “This moves the field toward biohybrid systems that can operate reproducibly and ultimately function outside the laboratory.”
With the novel artificial tendons installed, Raman’s group continues to work on other features, such as leather-like protective covers, to enable muscle-powered robots to work in practical, real-world conditions.
This research was supported in part by the U.S. Department of Defense Office of Army Research, the MIT Research Support Committee, and the National Science Foundation.