A robotic arm made from human tissue: Science advances toward living muscle prosthetics

The goal sounds like science fiction: to create a prosthetic that moves like a natural part of the body, can regenerate, and is built using the patient’s own cells. But it isn’t fiction — it’s a developing line of research in bioengineering. Ambitious and still years away from full realization, it is nonetheless tangible and making rapid progress. In February, the University of Tokyo, in collaboration with Waseda University, created an 18-centimeter arm from human muscle tissue, capable of moving its fingers. Meanwhile, in Barcelona, the Institute for Bioengineering of Catalonia (IBEC) has successfully emulated the internal structure of muscle using 3D bioprinters, allowing not only for reproduction of its natural form but also for functionality.

“In the future, this technology could be useful for the development of prosthetics, drug testing targeting muscle tissue, or research into muscle physiology,” explains Professor Masaharu Takeuchi, from University of Tokyo’s Graduate School of Science and Technology in an email. “It could also be applied to soft robotics systems that require smooth, user-friendly movements,”

Takeuchi notes that the hand developed by his team is the largest of its kind ever built in a lab. It’s composed of fine muscle tissues, grown in a nutrient-rich solution, while the mechanical components are made from biocompatible polymers — materials that can interact with the body without causing adverse reactions, such as allergies or toxicity — and thin support cables.

The professor explains that the greatest challenge was generating enough force from the muscles to move the hand. That requires more muscle mass, but if the tissue is too thick, nutrients and oxygen can’t reach all the cells, causing them to die. The solution was to roll thin sheets of muscle tissue “like sushi rolls” and assemble them into a more complex, multi-joint structure the researchers have named MuMuTAs—short for Multi-Muscle Tissue Assemblies. These MuMuTAs were then integrated into a cable-driven robotic structure.

“The arm is electrically stimulated to stay active. When electricity is applied to the muscles, they contract and pull on the cables connected to the fingers, allowing them to bend, move, and grasp objects, similar to real tendons. These are movements that were not possible until now with biohybrid robots,” explains the Japanese scientist.

At Spain’s IBEC, researchers are exploring alternatives to this external electrical stimulation for the biological robots — or biobots — they are developing. Using sensors and electrodes integrated directly into the muscle, they have created more localized and controlled stimulation systems.

“Until now, like Takeuchi’s group, we stimulated the entire culture medium [a liquid that mimics the body’s internal conditions and which the biobots use to stay alive] with large electrodes, like two pens. Now we’re developing much smaller, more flexible systems integrated into the structure,” explains IBEC director Samuel Sánchez via video call.

This approach not only allows for better emulation of how real muscles work, but also makes it possible to study muscle responses to drugs and other active compounds — like those they applied to aging cells in a project for a cosmetics company.

Functional muscles

The engineering of living systems at the Spanish institute is driven by their use of 3D bioprinters, which work with bioink — a blend of biocompatible materials and living cells. They started seven years ago printing simple shapes like circles and rectangles, and have since advanced to complex forms, including multiple types of human muscle. This has led to IBEC’s most recent breakthrough: reproducing the organized internal architectures that make organs functional.

“Using a programming code, we were able to precisely control all aspects of the bioprinter,” says IBEC researcher Florencia Lezcano. “This allowed us to orient the fibers and obtain muscle structures very similar to those we have in our bodies. Until now, with bioprinting, you could reproduce the silhouette of a muscle, but you couldn’t guarantee that it would work. You could print a specific shape, but the internal fibers weren’t arranged like those in humans.”

The rapid progress has fueled researchers’ hopes. They’ve gone from printing shapes that looked more like worms than biceps to creating muscles that not only resemble their human molds but are also functional and can be locally stimulated.

“Imagine being able to contract your biceps wherever we want, as well as being able to train it, make it stronger. Putting your own cells in it and rebuilding your muscles,” says Sánchez.

What does it take to make this a reality? From Tokyo, Takeuchi says the key is developing a way to control muscle tissue using neural signals — like those coming from the brain or peripheral nerves. “Furthermore, we need to ensure the long-term viability of the tissues outside the laboratory environment,” he adds.

At IBEC, researchers say the main challenge is vascularization — the process of forming blood vessels within tissue. “If we could conceive of a prevascularized organ, what would have to be done is connect it directly to the recipient body, as if it were a transplant. But we are still far from that technology,” says Lezcano.

Sánchez identifies two other major challenges. One is designing a casing that replicates the conditions in which a real hand operates: a stable temperature around 37°C (98.6ºF) and a system to continuously replenish nutrients as cells consume them. The second is scalability — developing a system that can support an organ or muscle at its natural size. “We are going in the right direction. The fact that we can print functional human cells and study their response to certain stimuli is a revolution in itself,” he says.

Beyond the dream of a bionic prosthesis, bioengineering promises to revolutionize how medical and pharmaceutical testing is conducted. Instead of using animals, labs could rely on lab-printed human biological models, accelerating results and improving precision. In the U.S., the FDA already allows, in some cases, skipping animal testing in favor of evaluating drugs with bioprinted tissue. In Europe, the process is moving more slowly.

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