Unlocking the (Super) Human Potential of Artificial Muscle

We need more muscle.

That’s not a vanity statement for those who want to look good or a performance issue for those who want to be better, stronger, and faster.

It’s a medical issue and has been for a long time. And if some people don’t have their own muscle or can’t strengthen what they have…well, we’ll have to make it.

For decades, scientists have been working toward creating artificial muscle that could help mobilize prosthetics, power robots, or even be implanted within the body to help with paralysis or other mobility issues.

But that simple thought…creating artificial muscle…makes you think about just how unique a tissue it is and how difficult it is to mimic the powerful mechanics inside our bodies.

“Muscle is incredibly efficient. It’s 50% of the human body on average, at least for a male,” said Ryan Truby, PhD, an engineering professor at Northwestern University. “It’s this incredibly important thing that material scientists and roboticists still struggle to replicate. But if you could make these artificial muscles, everything from advanced prosthetics and replacement of lost muscle function, all the way to things like robotics would be enormously benefitted.”

A lot of people are trying. One research group has made progress.

The Raw Materials of Artificial Muscle

Cheng-Hui Li, PhD, professor of chemical engineering at Nanjing University in Nanjing, China, led a study of a new polymer with muscle-like qualities. 

Li hopes the polymer could someday work with conditions like volumetric muscle loss — a substantial loss of skeletal muscle tissue, typically resulting from traumatic injuries, surgical procedures (such as tumor removal or amputation), congenital defects, or nerve damage.

Li and colleagues call the artificial muscle polymer they created PFPE–PCL because it’s made of perfluoropolyether and polycaprolactone diol. Their research showed that it’s very strong — it can lift objects weighing more than 5,000 times its own weight — and can reliably perform reversible contraction and extension motions with heat activation.

Using heat is a common strategy to get materials to expand and contract, according to Truby (who was not involved in the study). He noted that thermally powered muscles can create 98 times more power per muscle mass than natural muscle. The major downside is that the heat is incompatible with medical applications. “You don’t want to burn somebody if you’re trying to figure out how to implant this,” he said.

Another challenge in the artificial muscle field is getting the materials to work with something like bone. “Muscle is meant to pull on tendons, which allow muscle to transmit the force that it produces, onto bones…It’s hard to take a soft material and have it attached to a hard one,” Truby pointed out.

One thing researchers can do right now: Attach artificial muscle to existing muscle.

In a case like volumetric muscle loss, there is usually some muscle present to connect an artificial muscle to, which is what Li and colleagues did with their polymer in rats. They removed about 30% of the tibialis anterior (calf) muscle, then sutured a small piece of the polymer to the ends of the remaining muscle.

The goal is to have the artificial muscle help improve the function of the existing tissue and make it stronger.

The polymer “replicates the mechanical characteristics of natural muscle,” Li said, noting that like your muscles, their research showed that it’s easy to bend and stretch, it can handle a lot of force, it reacts quickly to changes, and it’s tough.

Though the polymer doesn’t have contractile function inside the rats (because it can’t be heated), their research found that it did seem to help with muscle growth and performance. They analyzed the rats’ tibialis anterior muscle 4 weeks after they removed some and stitched in the polymer and compared it with normal muscle or muscle that had 30% removed but did not have the polymer added. The polymer seemed to enhance contraction forces and promote muscle tissue growth.

Li said that their research indicated that the polymer “facilitates tissue regeneration by enhancing cell alignment, stimulating muscle fiber growth, and promoting vascularization.”

The Big Picture is Focused, but Incomplete

Regarding Li’s study overall, Truby said, “this is a very common example of efforts to take soft materials and figure out how to get them to change shape somehow, that’s ultimately the goal.” But he reiterated that heat isn’t the best way to power an artificial muscle for medical applications.

Ideally, Truby said, artificial muscle would be powered with electric currents, more like our own muscles. His own lab is working on electronic-driven muscles that are well suited to robotics applications.

In a work, published last year, Truby and colleagues developed a new soft, flexible device that can expand and contract to make robots move. The device has a special three-dimensional–printed part shaped like a cylinder that’s designed to expand outward when you twist it. By turning a motor one way or the other, the whole device extends or contracts, like a muscle.

“We have figured out some cool ways of making shape changing structures that behave like muscle, by just simply turning a servo motor,” Truby said.

However, Truby said there’s a long way to go before we see artificial muscles in medical applications. “In some ways, we have made strides, but in many others, we just have this glaringly huge, long way to go,” he noted.

Li said they are going to keep working toward taking their polymer toward human clinical trials. However, for medical applications, it’s more like a muscular scaffold than a working muscle.

But could heat-activated muscle polymers like Li’s be applied to an exosuit to enhance human’s abilities? “There’s definitely opportunities for augmenting human performance with these types of technologies. And if you look, yes, progress has been made, but it’s still so hard to replicate just what muscle is capable of.”

Truby believes it’s going to have to be a multidisciplinary effort in chemistry, robotics, materials science, and biomedical engineering to make big strides toward artificial muscles. “This is one of those grand engineering challenges where it’s going to take a few different perspectives really coming together to knock down some of those barriers that we keep being held back by.”