We move thanks to coordination among many skeletal muscle fibers, all twitching and pulling in sync. While some muscles align in one direction, others form intricate patterns, helping parts of the body move in multiple ways.
In recent years, scientists and engineers have looked to muscles as potential actuators for “biohybrid” robots — machines powered by soft, artificially grown muscle fibers. Such bio-bots could squirm and wiggle through spaces where traditional machines cannot. For the most part, however, researchers have only been able to fabricate artificial muscle that pulls in one direction, limiting any robot’s range of motion.
Now, MIT engineers have developed a method to grow artificial muscle tissue that twitches and flexes in multiple coordinated directions. As a demonstration, they grew an artificial, muscle-powered structure that pulls both concentrically and radially, much like how the iris in the human eye acts to dilate and constrict the pupil.
The researchers fabricated the artificial iris using a new “stamping” approach they developed. First, they 3D-printed a small, handheld stamp patterned with microscopic grooves, each as small as a single cell. Then they pressed the stamp into a soft hydrogel and seeded the resulting grooves with real muscle cells. The cells grew along these grooves within the hydrogel, forming fibers. When the researchers stimulated the fibers, the muscle contracted in multiple directions, following the fibers’ orientation.
“With the iris design, we believe we have demonstrated the first skeletal muscle-powered robot that generates force in more than one direction. That was uniquely enabled by this stamp approach,” said Ritu Raman, Eugene Bell Career Development Professor of Tissue Engineering in MIT’s Department of Mechanical Engineering.
The team says the stamp can be printed using tabletop 3D printers and fitted with different patterns of microscopic grooves. The stamp can be used to grow complex patterns of muscle — and potentially other types of biological tissues, such as neurons and heart cells — that look and act like their natural counterparts.
“We want to make tissues that replicate the architectural complexity of real tissues,” Raman said. “To do that, you really need this kind of precision in your fabrication.”
She and her colleagues published their open-access results in the journal Biomaterials Science. Her MIT co-authors include First Author Tamara Rossy, Laura Schwendeman, Sonika Kohli, Maheera Bawa, and Pavankumar Umashankar, along with Roi Habba, Oren Tchaicheeyan, and Ayelet Lesman of Tel Aviv University in Israel.
Raman’s lab at MIT aims to engineer biological materials that mimic the sensing, activity, and responsiveness of real tissues in the body. Broadly, her group seeks to apply these bioengineered materials in areas from medicine to machines. For instance, she is looking to fabricate artificial tissue that can restore function to people with neuromuscular injury. She is also exploring artificial muscles for use in soft robotics, such as muscle-powered swimmers that move through the water with fish-like flexibility.
Raman has previously developed what could be seen as gym platforms and workout routines for lab-grown muscle cells. She and her colleagues designed a hydrogel “mat” that encourages muscle cells to grow and fuse into fibers without peeling away. She also derived a way to “exercise” the cells by genetically engineering them to twitch in response to pulses of light. And, her group has come up with ways to direct muscle cells to grow in long, parallel lines, similar to natural, striated muscles. However, it’s been a challenge, for her group and others, to design artificial muscle tissue that moves in multiple, predictable directions.
“One of the cool things about natural muscle tissues is, they don’t just point in one direction. Take for instance, the circular musculature in our iris and around our trachea. And even within our arms and legs, muscle cells don’t point straight, but at an angle,” Raman noted. “Natural muscle has multiple orientations in the tissue, but we haven’t been able to replicate that in our engineered muscles.”
Here is an exclusive Tech Briefs interview, edited for length and clarity, with Raman.
Tech Briefs: What was the biggest technical challenge you faced while developing this artificial muscle?
Raman: Cells are constantly sensing and responding to their environment, so it can be hard to predict how a muscle cell will react in a specific situation. One of the biggest technical challenges we faced was figuring out what size of grooves could be sensed by the muscle and be used to efficiently pattern their alignment.
Tech Briefs: Can you please explain in simple terms how A) you created the muscle and B) how it works?
Raman: To make our muscle tissues, we start with a frozen vial of mouse muscle cells, thaw them, grow them in a petri dish, and then put the cells on top of the "stamped" jello-like material that we made in this study. The Jello-like material contained grooves, so the cells fell into the grooves and aligned themselves along them, making patterned muscle. The muscle works just like real muscle in our bodies — it contracts/moves when stimulated with an electrical pulse.
Tech Briefs: Do you have any set plans for further research/work/etc.? If not, what are your next steps?
Raman: All research in our lab is used to either: 1) create artificial muscle tissues that can be used to understand and treat muscle diseases that impact healthy human mobility; 2) make safe muscle-powered robots that can perform complex tasks in dangerous environments that are not suitable for humans.
Tech Briefs: Do you have any advice for researchers aiming to bring their ideas to fruition (broadly speaking)?
When faced with a surprising result, it is important to consider every possible theory that explains your observations — keep an open mind! I've been working with muscle for over a decade, and I'm still learning something new and exciting every day.
