Machines made flesh
Robots may one day move with muscular grace, thanks to lab-grown tissue, but will these 'bio-bots' be plagued by cramps?
CYBORGS are a familiar presence in the annals of science fiction, but scientists are working on more than a melding of organism and machine – they're looking to create robots manufactured of flesh.
These 'bio-bots' use living tissue tailor-made as components of their machine bodies. Instead of limbs moved by an array of motors and hydraulic pistons, artificially grown muscles power their motion.
The idea is barely more than a concept, really, but it's one that's being 'fleshed out' for real (pun intended) in the lab by engineers at the Massachusetts Institute of Technology (MIT), in the United States. They're using 3D printing technology combined with artificially grown tissue to explore the adaptability of muscle fibres to robot motion.
As the team explain, it's the coordination of many muscle fibres twitching and pulling in sync that enables us to move our bodies. Some of those muscles align in one direction, others form intricate patterns, helping parts of the body to move in multiple ways.
In recent years, scientists and engineers have looked to muscles as potential actuators for so-called “biohybrid” robots – machines powered by soft, artificially grown muscle fibres.
Picture robots that can squirm and wiggle through spaces where traditional machines cannot; that's the idea. However, for the most part, researchers have only been able to fabricate artificial muscle that pulls in one direction, limiting any robot’s range of motion.
That's where the MIT engineers come in. They've developed a method to grow artificial muscle tissue that twitches and flexes in multiple coordinated directions. The team published their results in the journal, Biomaterials Science.
Getting into the groove
As a demonstration, the team grew an artificial, muscle-powered structure that pulls both concentrically and radially, much like the the way 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.
These cells grew along these grooves within the hydrogel, forming fibres. When the researchers stimulated the fibres, the muscle contracted in multiple directions, following the fibres’ 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, the Eugene Bell career development professor of tissue engineering in MIT’s Department of Mechanical Engineering.
The team say 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 added. “To do that, you really need this kind of precision in your fabrication.”
Engineering biology
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.
“One of the cool things about natural muscle tissues is, they don’t just point in one direction,” she said. “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. Natural muscle has multiple orientations in the tissue, but we haven’t been able to replicate that in our engineered muscles.”
In thinking of ways to grow multidirectional muscle tissue, Raman and her team camp up with the deceptively simple idea of the stamp. Inspired in part by the classic jelly mould, the team looked to design a stamp, with microscopic patterns that could be imprinted into the hydrogel mentioned above. Those patterns serve as a 'roadmap' for the cells to follow and grow.
“The idea is simple,” Raman said. “But how do you make a stamp with features as small as a single cell? And how do you stamp something that’s super soft? This gel is much softer than [jelly], and it’s something that’s really hard to cast, because it could tear really easily.”
The researchers fabricated a small, handheld stamp using high-precision printing facilities, which enabled them to print intricate patterns of grooves, each about as wide as a single muscle cell, onto the bottom of the stamp. Before pressing the stamp into a hydrogel mat, they coated the bottom with a protein that helped the stamp imprint evenly into the gel and peel away without sticking or tearing.
As a demonstration, the researchers printed a stamp with a pattern similar to the microscopic musculature in the human iris. The iris comprises a ring of muscle surrounding the pupil. This ring of muscle is made up of an inner circle of muscle fibres arranged concentrically, following a circular pattern, and an outer circle of fibres that stretch out radially, like the rays of the sun. Together, this complex architecture acts to constrict or dilate the pupil.
Light sensitive
Once Raman and her colleagues pressed the iris pattern into a hydrogel mat, they coated the mat with cells that they genetically engineered to respond to light. Within a day, the cells fell into the microscopic grooves and began to fuse into fibres, following the iris-like patterns and eventually growing into a whole muscle, with an architecture and size similar to a real iris.
When the team stimulated the artificial iris with pulses of light, the muscle contracted in multiple directions, similar to the iris in the human eye.
As Raman noted, the team’s artificial iris is fabricated with skeletal muscle cells, which are involved in voluntary motion, whereas the muscle tissue in the real human iris is made up of smooth muscle cells, which are a type of involuntary muscle tissue.
They chose to pattern skeletal muscle cells in an iris-like pattern to demonstrate the ability to fabricate complex, multidirectional muscle tissue.
“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 can’t do,” Raman said. “We chose to work with skeletal muscle cells. But there’s nothing stopping you from doing this with any other cell type.”
She added: “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’s what we hope to build toward.”
Imagine the future, then, where robots are no longer clunky, but move with muscular grace – until cramp sets in. Ouch!
MC