Bioengineers have pulled together to get artificial muscles pulling in multiple directions, an important step towards using them in medical treatments and robots.
Many muscles work in aligned pairs: To straighten an arm, you contract your triceps as your biceps relaxes. Other muscles are arrayed in more intricate patterns, such as the circular bands of sphincter muscle in the iris that work with the radial dilator muscles to change the size of the pupil.
Complex combos of muscles like those in the iris are thought to be what will turn lab-grown tissue from a promising idea into a tool with practical applications.
A team led by MIT's tissue engineering professor Ritu Raman thinks it’s found a way to build those muscles by growing a thin, single-layer sheet of skeletal muscle cells designed to mimic the muscle arrangement of the iris.
Like a natural iris, the lab-made version responds to light by contracting in multiple directions, according to the team's recently published paper.
Unlike a natural iris, which relies on involuntary smooth muscle – which humans can’t control voluntarily – Raman's artificial muscle is built from skeletal muscle cells genetically tweaked to flex under light stimulation.
"With the iris design, we believe we have demonstrated the first skeletal muscle-powered robot that generates force in more than one direction," Raman told MIT News.
Even more impressive than getting the artificial muscle to flex in multiple directions is how the team did it: Using a precisely patterned, handheld stamp fabricated with a high-resolution 3D printer rather than expensive microfabrication facilities or complex multi-step handling.
"This size is well within the print resolution of commercial stereolithographic 3D printers (SLA)," the team noted, adding that different stamp patterns could be swapped out to control the shape and alignment of muscle fibers – even using commercially available printers, provided one has the materials and lab conditions to grow the tissue.
Named STAMP, for "Simple Templating of Actuators via Micro-topographical Patterning," the technique involves pressing a resin-printed stamp etched with microscopic grooves into a soft hydrogel substrate designed to mimic the extracellular environment of real muscle tissue. The grooves are seeded with cultured mouse and human skeletal muscle cells, which grow along the patterns into aligned fibers. These muscle monolayers are genetically engineered to contract in response to light.
"We observed slight shrinkage of the pupil diameter in response to contraction of the multi-oriented actuator mimicking the function of native iris musculature," the team said. In short, it actually worked.
The artificial iris itself isn't tiny - as shown in a diagram in the research paper, the stamp surface was patterned at centimeter scale. Raman told us that there's nothing about her team's results that would preclude larger artificial muscles from being developed using the technology, either.
"There is nothing restricting the size in the planar dimension – you would just need more cells," Raman told The Register in an email. And they could be thicker, too. "We only looked at monolayers in this study but are looking at multilayered constructs in future."
While the paper mentions the use of such tissues for the development of soft, flexible robots with multiple degrees of freedom enabled by artificial muscle cells, Raman said her lab's research isn't restricted to the development of cyborgs.
"Building centimeter-scale muscles, as well as understanding how they connect to tendons and bones, is a broader part of what we study in our lab," Raman explained. "Our goal is not just to make better robots, but to help develop therapies that restore mobility to patients who suffer from muscle diseases or injuries."
Her team has already had success doing that, too: In 2023, Raman and colleagues published a study showing that engineered skeletal muscle tissue grafted into mice with volumetric muscle loss in their hind limbs helped restore mobility. The muscle grafts were designed to contract when exposed to light.
"We then 'exercise' the implant daily by noninvasively shining light on the mouse's leg through the skin," Raman said in 2023. "The approach keeps muscle implants active while they are engrafting with the surrounding host tissue."
Within two weeks, the test subjects had completely recovered functional mobility, according to the team.
Raman and her fellow researchers intend to continue their STAMP work by building larger artificial muscles, as well as experimenting to see whether their method could be used to create other tissues as well.
"There's nothing stopping you from doing this with any other cell type," Raman said. ®