Muscle atrophy is a serious complication of many medical conditions. Any issue that requires bed rest or restricted activity leads to muscles that don't get used routinely, which causes them to lose mass and waste away. Physical therapy exercises can combat this, but they aren't always an option for people with severely disabling conditions. Researchers have developed a way to help people mitigate muscle atrophy, even when they can't be physically active: an implantable device that triggers muscle contractions.
In a lab setting, scientists have demonstrated that stretching bits of cultured muscle tissue helped stimulate the growth of new cells. Unfortunately, there's a big problem with this. External manipulation of the body can only do so much and isn't able to really reach deeper tissues effectively.
This raised two key challenges that researchers faced when it came to creating a device to prevent muscle atrophy. For one, the device needed to be able to generate force along the surface of a muscle. Secondly, it needed to actually be able to effectively attach to the muscle so that it could generate that force.
In order to overcome the challenge of generating force, scientists turned to a group of interesting materials called “shape-memory alloys.” These are also known as memory alloys, memory metals, or smart alloys. They're unique in that they can be stretched, bent, or otherwise manipulated into a shape, hold that shape, and then return to their original form when heated. These materials have proven useful for other medical devices, and were a natural choice for this application.
In this case, researchers used an alloy of titanium and nickel known as nitinol. They formed this into a spring, which is able to stretch at room temperature, remain stretched, and then return to a tight coil once its warmed. The scientists used an electrical current to provide the heat to retract the spring and were able to stretch it out again once it cooled back to room temperature.
The next challenge was adhering it to muscle tissue. Without the ability to do this, it wouldn't matter how well the nitinol spring could stretch and retract—it wouldn't effectively move muscles well enough to exercise them. Researchers first embedded the spring into a rubbery polymer, then added a sticky substance called a hydrogel.
The completed device is named MAGENTA, an acronym for “mechanically active gel-elastomer-nitinol tissue adhesive.”
Once the device was made, researchers tested it on a piece of engineered tissue in vitro. They adhered MAGENTA to the tissue using the hydrogel and ran an electric current to the wire. The spring and elastic polymer retracted, which caused the tissue to retract as well. Once the current was turned off, the tissue and spring relaxed again.
They then repeated this experiment using a small piece of naturally grown tissue removed from a mouse in order to confirm that it was able to function the same way as it did with the engineered tissue.
The next phase involved implanting the device in a living mouse. Researchers placed MAGENTA in the hind legs of a group of mice, then applied electricity. The skin over the device visibly contracted, and ultrasound imagery confirmed that everything was working as intended. The springs coiled tightly when electrically heated, causing the mice's muscles to contract. Once the electricity was withdrawn, the springs and muscles relaxed again. MAGENTA also produced only minor localized inflammation, and no severe adverse effects.
While MAGENTA certainly seemed to work the way it was intended, scientists still needed to prove that it was effective at mitigating muscle atrophy. For this, they induced muscle wasting in two groups mice by immobilizing one of their hind legs. One group received MAGENTA implants, while the other did not.
After two weeks of regular use, the mice with MAGENTA implants had larger, stronger muscles than those without.
MAGENTA has been adapted for human use by swapping electricity for lasers. Instead of relying on wires to provide the spring with heat, the springs are warmed by shining a small laser through the skin. This makes the device wireless and less obtrusive.
The need to stimulate the contraction and relaxation of tissue isn't limited to treating muscle atrophy. MAGENTA could be used to help muscles move to stimulate blood flow and remove excess fluid, exercise heart muscle to improve its ability to contract, or stretch skin. The nitinol spring also isn't limited to just stretching and retracting, either—it could be trained into other configurations, depending on what the situation calls for, and still return to its original shape when heated.
Muscle atrophy is a major complication of disabling conditions that can make it very difficult, if not impossible, for patients to preserve or improve function. MAGENTA may provide a safe, effective way to combat muscle atrophy. In the future, this technology could also be adapted for other situations that call for the stretching or relaxing of tissue.
