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The intricate movements of the human body are made possible through the coordination of various skeletal muscle fibers, which work together to create complex motions. While certain muscles are aligned for straightforward movement, others are structured in sophisticated arrangements that enable diverse and multidirectional actions.
Recent advancements have seen scientists and engineers exploring the possibility of utilizing muscle tissues as actuators for what are termed “biohybrid” robots. These innovative machines harness the power of soft, artificially cultivated muscle fibers, allowing them to navigate spaces that traditional mechanical robots may struggle to access. However, the technology has mostly been limited to artificial muscles that operate in a single direction, constraining the mobility of these robotic systems.
MIT engineers have now introduced a groundbreaking technique for cultivating artificial muscle tissues capable of twitching and flexing in multiple directions, simulating a more natural movement. To showcase their approach, they created a muscle-powered structure that mimics the functionality of the human iris, contracting both concentrically and radially as it dilates and constricts, similar to the way our eyes function.
The innovative method involves a novel “stamping” technique. Initially, the team designed a small 3D-printed stamp featuring microscopic grooves, each capable of accommodating a single cell. This stamp was then pressed into a soft hydrogel, which was subsequently seeded with real muscle cells. As the cells grew into the grooves, fiber-like structures formed. When stimulated, the muscle fibers contracted in a coordinated fashion according to the orientation dictated by the grooves.
“Our iris design represents a pioneering achievement in creating a skeletal muscle-powered robot that generates force across multiple directions, something uniquely made possible by our stamping method,” stated Ritu Raman, who holds the Eugene Bell Career Development Professor of Tissue Engineering position in MIT’s Department of Mechanical Engineering.
The MIT team emphasizes that the stamp can be fabricated using standard desktop 3D printers and can accommodate various patterns of microscopic grooves. This versatility may not only allow for the development of complex muscle structures but could also extend to engineered tissues such as neurons and heart cells, ultimately mirroring their natural counterparts.
“Our goal is to create tissues that emulate the complexity found in biological structures,” Raman explained. “Achieving this level of architectural precision is essential for successful fabrication.” The research findings are detailed in an open-access article published in the journal Biomaterials Science. Co-authors from MIT include Tamara Rossy, Laura Schwendeman, Sonika Kohli, Maheera Bawa, and Pavankumar Umashankar, while collaborators include Roi Habba, Oren Tchaicheeyan, and Ayelet Lesman from Tel Aviv University, Israel.
Innovative Engineering Approaches
Raman’s group at MIT focuses on developing biological materials that emulate the sensing capabilities, movement, and responsiveness of genuine tissues. The team aims to apply these bioengineered materials in diverse fields, from medical applications to robotic constructs. One notable initiative involves creating artificial tissues that could restore function in individuals suffering from neuromuscular injuries. Additionally, the implications for soft robotics are significant, with the potential to design muscle-powered swimmers that exhibit flexibility akin to that of fish.
Prior to this research, Raman’s lab had laid the groundwork for growing muscle cells in ways that promote their expansion into fibers without separating. This involved constructing a hydrogel “mat” that offers support for muscle growth and finding methods to “exercise” the cells using light-sensitive genetic modifications. Despite these advances, orchestrating artificial muscle tissue to exhibit predictable movement in multiple directions posed a significant challenge.
“Natural muscle tissues have diverse orientations that allow for complex movement,” Raman noted. “For example, the circular arrangement in our iris or the angled fibers in our limbs demonstrate the intricacies we have yet to replicate in our engineered solutions.”
The Blueprint for Muscle Growth
To conceptualize how to cultivate multidirectional muscle tissue, the team arrived at a straightforward yet innovative solution: stamps. Drawing inspiration from traditional Jell-O molds, the team designed a stamp with microscopic patterns that could be imprinted onto a hydrogel, akin to the muscle-training mats they previously created. These patterns serve as a guide for muscle cells to grow along defined paths.
However, the challenge lay in creating a stamp with features small enough for single cell manipulation and handling a soft material. “How do you craft such a stamp without causing it to tear? It’s significantly more fragile than Jell-O,” commented Raman.
After experimenting with various designs, they successfully engineered a handheld stamp using high-precision printing techniques available at MIT.nano. This allowed them to create intricate groove patterns at a cellular scale on the stamp’s surface. To ensure successful imprinting in the hydrogel, they applied a protein layer that enabled smooth separation after pressing.
As a practical example, the researchers crafted a stamp featuring a pattern that emulates the intricate muscle structure of the human iris. The iris consists of concentric fibers that constrict the pupil and radial fibers that allow for dilation. Once the stamp created the pattern in the hydrogel, the researchers coated it with engineered cells, prompting them to align within the grooves and fuse into muscle fibers, ultimately resulting in an artificial structure reflective of a real iris.
Upon stimulation with light pulses, the artificial muscle demonstrated multidirectional contractions similar to its biological counterpart. While the artificial iris utilized skeletal muscle cells for its creation—known for voluntary movement—the actual human iris comprises smooth muscle cells, which function involuntarily. Raman emphasized that this approach proves the ability to engineer complex and responsive muscle structures.
“This research illustrates our capacity to harness this stamping technique to develop muscle-powered robots capable of multifunctional tasks that prior models could not achieve,” Raman stated. “While we’ve focused on skeletal muscle, this method can be adapted for various cell types.”
Looking ahead, the team aims to extend this stamping methodology to incorporate other cell types and explore different muscular architectures, as well as mechanisms to activate artificial muscles for practical applications.
“By substituting rigid actuators with soft biological systems, particularly in underwater robotics, we can create more energy-efficient and sustainable solutions that are biodegradable,” Raman concluded. “This is the direction we aspire to head towards.”
This work received funding from various U.S. defense and research agencies, including the Office of Naval Research, the Army Research Office, the National Science Foundation, and the National Institutes of Health.
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