Engineers at the Massachusetts Institute of Technology have now created a technique for creating synthetic muscle tissue that can flex and twitch in a variety of coordinated directions. To illustrate this, they created a synthetic, muscle-driven structure that pulls both radially and concentrically, simulating the iris's ability to dilate and contract the pupil in the human eye. The study was published in the journal Biomaterials Science.
MIT engineers 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. Image Credit: Researchers
The human ability to move is a result of the synchronized twitching and pulling of numerous skeletal muscle fibers. While some muscles only line up in one direction, others create complex patterns that enable various body parts to move.
Researchers and engineers have recently examined muscles as possible actuators for “biohybrid” robots, which are devices driven by soft, synthetic muscle fibers. These bio-bots could move and wriggle in areas that are inaccessible to conventional machines. However, the majority of the time, scientists have only been able to create artificial muscle that pulls in a single direction, which restricts the range of motion that any robot can achieve.
The researchers used a novel “stamping” technique they created to create the artificial iris. They started by 3D printing a tiny, portable stamp with tiny grooves, each as tiny as a single cell.
After pressing the stamp into a soft hydrogel, they seeded the resulting grooves with actual muscle cells. These hydrogel grooves allowed the cells to proliferate and form fibers. The muscle contracted in several directions by the fibers' orientation when the researchers stimulated the fibers.
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.
Ritu Raman, Eugene Bell Career Development Professor of Tissue Engineering, Department of Mechanical Engineering, Massachusetts Institute of Technology
According to the team, the stamp can be fitted with various patterns of microscopic grooves and printed using tabletop 3D printers. The stamp can be used to grow intricate muscle patterns that mimic the appearance and behavior of their natural counterparts, as well as possibly other biological tissue types like neurons and heart cells.
Ritu Raman said, “We want to make tissues that replicate the architectural complexity of real tissues. To do that, you really need this kind of precision in your fabrication.”
First author Tamara Rossy, Laura Schwendeman, Sonika Kohli, Maheera Bawa, and Pavankumar Umashankar are among the co-authors at MIT. Roi Habba, Oren Tchaicheeyan, and Ayelet Lesman are from Tel Aviv University in Israel.
Training Space
The goal of Raman's lab at MIT is to create biological materials that replicate the body's actual tissues' ability to sense, move, and respond. Her group's overall goal is to use these bioengineered materials in everything from machines to medicine.
For example, she wants to create artificial tissue that can help people with neuromuscular injuries regain their function. She is also investigating the use of artificial muscles in soft robotics, such as swimmers with muscles that propel them through the water with the flexibility of fish.
For lab-grown muscle cells, Raman has previously created what could be considered gym platforms and exercise regimens. To promote muscle cell growth and fiber fusion without peeling away, she and her colleagues created a hydrogel “mat.”
By genetically modifying the cells to twitch in response to light pulses, she also devised a method to “exercise” the cells. Additionally, her group has developed methods for directing muscle cells to grow in long, parallel lines that resemble the striated muscles found in nature. However, creating artificial muscle tissue that moves in several predictable directions has proven difficult for her group and others.
One of the cool things about natural muscle tissues is, they do not 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 do not point straight, but at an angle. Natural muscle has multiple orientations in the tissue, but we have not been able to replicate that in our engineered muscles.
Ritu Raman, Eugene Bell Career Development Professor of Tissue Engineering, Department of Mechanical Engineering, Massachusetts Institute of Technology
Muscle Blueprint
The team came up with a surprisingly straightforward idea while brainstorming methods to develop multidirectional muscle tissue: stamps. Drawing inspiration from the iconic Jell-O mold, the team set out to create a stamp that would feature microscopic patterns that could be incorporated into a hydrogel, akin to the muscle-training mats they have already created.
The imprinted mat's patterns could then act as a guide for the growth and development of muscle cells.
Raman said, “The idea is simple. But how do you make a stamp with features as small as a single cell? And how do you stamp something that is super soft? This gel is much softer than Jell-O, and it is something that is really hard to cast because it could tear really easily.”
After experimenting with different iterations of the stamp design, the team finally settled on a strategy that proved surprisingly effective. The researchers used MIT.nano's high-precision printing facilities to create a small, portable stamp.
This allowed them to print complex groove patterns onto the stamp's bottom, each of which was roughly the width of a single muscle cell. They applied a protein to the bottom of the stamp before pressing it into a hydrogel mat. This helped the stamp imprint uniformly into the gel and peel off without tearing or sticking.
To illustrate their point, the scientists created a stamp that resembled the tiny muscles found in the human iris. The pupil is surrounded by a ring of muscle that makes up the iris. This ring of muscle is composed of an outer circle of muscle fibers that extend radially, resembling the sun's rays, and an inner circle of muscle fibers arranged concentrically, in a circular pattern. This intricate structure works together to either dilate or constrict the pupil.
After pressing the iris pattern onto a hydrogel mat, Raman and her associates covered the mat with cells that they had genetically modified to react to light. The cells settled into the tiny grooves within a day, fused into fibers, followed the patterns of the iris, and eventually grew into a complete muscle that resembled the size and structure of an actual iris.
Similar to the iris in the human eye, the team's artificial iris muscle contracted in multiple directions when they stimulated it with light pulses.
While the muscle tissue in the real human iris is composed of smooth muscle cells, a type of involuntary muscle tissue, Raman points out that the team's artificial iris is made of skeletal muscle cells, which are involved in voluntary motion.
To show that they could create intricate, multidirectional muscle tissue, they decided to arrange skeletal muscle cells in an iris-like pattern.
In this work, we wanted to show we can use this stamp approach to make a ‘robot’ that can do things that previous muscle-powered robots cannot do. We chose to work with skeletal muscle cells. But there is nothing stopping you from doing this with any other cell type.
Ritu Raman, Eugene Bell Career Development Professor of Tissue Engineering, Department of Mechanical Engineering, Massachusetts Institute of Technology
Although the team employed precision-printing methods, she points out that standard tabletop 3D printers can also be used to create the stamp design. In addition to investigating various muscle architectures and methods for activating artificial, multidirectional muscles to perform beneficial tasks, she and her colleagues intend to apply the stamping method to other cell types in the future.
Raman said, “Instead of using rigid actuators that are typical in underwater robots if we can use soft biological robots, we can navigate and be much more energy-efficient, while also being completely biodegradable and sustainable. That is what we hope to build toward.”
The US Office of Naval Research, US Army Research Office, US National Science Foundation, and US National Institutes of Health provided funding for this work.
Journal Reference:
Rossy, T., et al. (2025) Leveraging microtopography to pattern multi-oriented muscle actuators. Biomaterials Science. doi.org/10.1039/d4bm01017e.