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Helical material enables cell survival during elastic strain

By Stephen Riffle June 6, 2019
Scanning electron micrographs of (b) an unstrained and (c) a strained hierarchical helix nanofiber yarn. Credit: Proceedings of the National Academy of Sciences

There are many ways to tear a muscle, but only a handful of methods for repairing it. The more severe cases that result from traumatic injury or surgery attract the attention of tissue engineers Yiwei Li and Ming Guo at the Massachusetts Institute of Technology’s department of Mechanical Engineering, two researchers whose work aims to make regenerative medicine a viable option for muscle repair. Earlier this year, Li and Guo co-authored an article, published in the Proceedings of the National Academy of Sciences (along with seven other co-authors), demonstrating the potential utility of nanoscale yarn in the regeneration of elastic tissues.

The project began with the realization that cells can only withstand a limited amount of stretching before they break. Yet, somehow, cells within muscles and tendons remain intact despite routinely experiencing an elastic strain—due to limb extension and flexion, for example—that far exceeds what any individual cell can tolerate. This property of elastic tissue is problematic from an engineering perspective. “Cells obviously deform, but from our work and others, we see that they die if you deform them too much. So that’s a limitation to [engineering] repair. You can have a nice material as a basis, but the cells will die if the deformation is too large,” Guo says.

In the pursuit of regenerative medicine, mechanical engineering combines with biology to form biological scaffolds—materials designed to aid tissue growth and regeneration, typically by providing a structured environment with biochemical cues that encourage healthy tissue formation. Many scaffolds are formed using synthetic polymers because they can be easily modified (by changing the types and ratios of polymers used) to adjust a scaffold’s mechanical properties. While researchers have been able to form elastic scaffolds reminiscent of muscles and tendons, no one had figured out how to create a biological scaffold that could undergo large amounts of elastic deformation without deforming the cells contained within it.

Li, a post-doctoral researcher in Guo’s laboratory, and his co-authors turned to nature for a possible solution. Tendons and muscles contain specific types of collagen that are arranged into helical structures (these are found in the matrix of proteins and cells that surround individual muscle cells, for example). The helical structure of these proteins is thought to convey elastic properties to the tissue.

To replicate the helically-structured collagen in the laboratory, Li and his colleagues used electrospinning to form nanofibers using materials common to biological scaffolds, including poly(lactic-co-glycolic acid) or PLGA which has been FDA-approved for certain clinical uses. Collectively, the aligned nanofibers formed a larger, sheet-like structure that could be attached to an electric motor and twisted, giving rise to a primary yarn fiber. Further twisting of the primary fiber resulted in a helical superstructure referred to as nanofiber yarn. When tested, the yarn proved flexible: elongating up to 15× its own length without breaking and withstanding repeated cycles of stretching up to 6× its length without deterioration.

In describing the results, Guo says that he “wasn’t too surprised” because “from the mechanics point of view, we know these types of helical structures pretty well. So, we predicted that this would happen. But being able to repeatedly stretch the materials six times its length over a week, that number was pretty surprising to me.”

The next step was to show how this stretching might affect cells. Mesenchymal stem cells (MSC), a type of stem cell that can differentiate into muscle cells, were cultured atop the yarn and observed during stretching. Rather than stretching with the fiber, the cells simply reoriented themselves, appearing to rotate as the nanofibers uncoiled. This meant that cells were not experiencing the same stretching force that was being exerted on the overall structure and were thus able to survive yarn deformation (a phenomenon known as nonaffine deformation). This was in contrast to cells that were placed on primary, non-helical nanofibers, the majority of which died when stretching. This was shown to be true for other human cell types as well.

Shengqiang Cai, an associate professor in mechanical and aerospace engineering at the University of California, San Diego who was not involved with the study, explains nonaffine deformation as an intuitive concept that can be seen in everyday materials. “The helical structure, or the geometry itself, is not uncommon at all. You can find this structure almost everywhere, like in a spring. We know that metals like copper and steel cannot undergo too much elastic deformation [only about 1% at most]. However, if you turn the metal wire into a spring, we all know the spring will stretch several times its own length without any problem. So, this amplified [elasticity] is through the geometric design,” Cai says.

Li and his colleagues further observed that the nanofiber structure of the yarn could influence the differentiation of MSCs into muscle cells—likely through alterations in cell shape that prompted an increase in pro-myocyte differentiating factors, such as the protein TAZ.

Though we are a long way from seeing nanofiber yarn being used to regenerate torn muscles, it is not a stretch to imagine that one day we might. Guo and Li both anticipate that these findings could aid in the engineering of flexible biological scaffolds for the repair of elastic tissue. Cai found it exciting to envision the potential use of helical nanofiber yarn in biorobotic hybrid technology—machines formed using a combination of biological and synthetic materials.

“Beyond the applications that [the researchers] point out,” Cai can envisage the helical yarn being used as a deformation amplifier. If you take cells that can produce active deformation, like cardiomyocytes, and place them on a helical yarn, “although each individual cell deforms just a little bit, I suspect you’d see large deformation [in the larger helical structure],” Cai says. It is possible then that we may one day see helical nanofibers used in regenerative medicine to shield cells from deformation, and in biohybrid soft robotic technology to amplify it.

Read the abstract in Proceedings of the National Academy of Sciences.