Implantable Device Functions as Soft Robot for Treating Muscle Atrophy

Muscles waste as a result of not being exercised enough, as happens quickly with a broken limb that has been immobilized in a cast, and more slowly in people reaching an advanced age. Muscle atrophy, how clinicians refer to the phenomenon, is also a debilitating symptom in patients suffering from neurological disorders, such as amyotrophic lateral sclerosis (ALS) and multiple sclerosis (MS), and can be a systemic response to various other diseases, including cancer and diabetes. Mechanotherapy, a form of therapy given by manual or mechanical means, is thought to have broad potential for tissue repair. The best-known example is massage, which applies compressive stimulation to muscles for their relaxation. However, it has been much less clear whether stretching and contracting muscles by external means can also be a treatment. So far, two major challenges have prevented such studies: limited mechanical systems capable of evenly generating stretching and contraction forces along the length of muscles, and inefficient delivery of these mechanical stimuli to the surface and into the deeper layers of muscle tissue.

Now, bioengineers at the Wyss Institute at Harvard University (Boston, MA, USA) have developed a mechanically active adhesive named MAGENTA, which functions as a soft robotic device and solves this two-fold problem. In an animal model, MAGENTA successfully prevented and supported the recovery from muscle atrophy. One of MAGENTA’s major components is an engineered spring made from nitinol, a type of metal known as “shape memory alloy” (SMA) that enables MAGENTA’s rapid actuation when heated to a certain temperature. The researchers actuated the spring by electrically wiring it to a microprocessor unit that allows the frequency and duration of the stretching and contraction cycles to be programmed.

The other components of MAGENTA are an elastomer matrix that forms the body of the device and insulates the heated SMA, and a “tough adhesive” that enables the device to be firmly adhered to muscle tissue. In this way, the device is aligned with the natural axis of muscle movement, transmitting the mechanical force generated by SMA deep into the muscle. The researchers are advancing MAGENTA, which stands for “mechanically active gel-elastomer-nitinol tissue adhesive,” as one of several Tough Gel Adhesives with functionalities tailored to various regenerative applications across multiple tissues.

After designing and assembling the MAGENTA device, the team tested its muscle deforming potential, first in isolated muscles ex vivo and then by implanting it on one of the major calf muscles of mice. The device did not induce any serious signs of tissue inflammation and damage, and exhibited a mechanical strain of about 15% on muscles, which matches their natural deformation during exercise. Next, to evaluate its therapeutic efficacy, the researchers used an in vivo model of muscle atrophy by immobilizing a mouse’s hind limb in a tiny cast-like enclosure for up to two weeks after implanting the MAGENTA device on it.

“While untreated muscles and muscles treated with the device but not stimulated significantly wasted away during this period, the actively stimulated muscles showed reduced muscle wasting,” said first-author and Wyss Technology Development Fellow Sungmin Nam, Ph.D. “Our approach could also promote the recovery of muscle mass that already had been lost over a three-week period of immobilization, and induce the activation of the major biochemical mechanotransduction pathways known to elicit protein synthesis and muscle growth.”

“With MAGENTA, we developed a new integrated multi-component system for the mechanostimulation of muscle that can be directly placed on muscle tissue to trigger key molecular pathways for growth,” said senior author and Wyss Founding Core Faculty member David Mooney, Ph.D. “While the study provides first proof-of-concept that externally provided stretching and contraction movements can prevent atrophy in an animal model, we think that the device’s core design can be broadly adapted to various disease settings where atrophy is a major issue.”

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Wyss Institute at Harvard University