Dec 14 2018
The basic building block of tissues, muscles, ligaments, and tendons in mammals is collagen. It is also extensively used in cosmetic and reconstructive surgery.
A collagen fibril mounted on a MEMS mechanical testing device. At the bottom is a single human hair for size comparison. (Image credit: University of Illinois Department of Aerospace Engineering)
Scientists have a good insight into how it behaves at the tissue level; however, some key mechanical properties of collagen at the nanoscale still remain obscure. A recent experimental study carried out by researchers at the University of Illinois at Urbana-Champaign, Washington University in St. Louis, and Columbia University on nanoscale collagen fibrils reported on, previously unexpected, reasons for collagen being such a resilient material.
Since the size of one collagen fibril is around one-millionth of that of the cross-section of a strand of human hair, equally small equipment is required to study it. The team from the Department of Aerospace Engineering at the University of Illinois at Urbana-Champaign developed tiny devices—Micro-Electro-Mechanical Systems—smaller than 1 mm in size, to test the collagen fibrils.
“Using MEMS-type devices to grip the collagen fibrils under a high magnification optical microscope, we stretched individual fibrils to learn how they deform and the point at which they break,” stated Debashish Das, a postdoctoral scholar at Illinois who worked on the study. “We also repeatedly stretched and released the fibrils to measure their elastic and inelastic properties and how they respond to repeated loading.”
Unlike a rubber band, if you stretch human or animal tissue and then release it, the tissue doesn’t spring back to its original shape immediately. Some of the energy expended in pulling it is dissipated and lost. Our tissues are good at dissipating energy–when pulled and pushed, they dissipate a lot of energy without failing. This behavior has been known and understood at the tissue-level and attributed to either nanofibrillar sliding or to the gel-like hydrophilic substance between collagen fibrils. The individual collagen fibrils were not considered as major contributors to the overall viscoelastic behavior. But now we have shown that dissipative tissue mechanisms are active even at the scale of a single collagen fibril.
Debashish Das, Postdoctoral Scholar, University of Illinois at Urbana-Champaign.
A very fascinating and unforeseen discovery of the study is that upon repeated stretching and relaxing, collagen fibrils can become tougher and stronger.
If we repeatedly stretch and relax a common engineering structure, it is more likely to become weaker due to fatigue. While our body tissues don’t experience anywhere near the amount of stress we applied to individual collagen fibrils in our lab experiments, we found that after crossing a threshold strain in our cyclic loading experiments, there was a clear increase in fibril strength, by as much as 70 percent.
Ioannis Chasiotis, Professor, University of Illinois at Urbana-Champaign.
According to Das, the collagen fibrils themselves considerably provide the toughness and energy dissipation observed in tissues.
What we found is that individual collagen fibrils are highly dissipative biopolymer structures. From this study, we now know that our body dissipates energy at all levels, down to the smallest building blocks. And properties such as strength and toughness are not static, they can increase as the collagen fibrils are exercised.
Debashish Das, Postdoctoral Scholar, University of Illinois at Urbana-Champaign.
What’s the next move? Das stated that this new insight into the properties of single collagen fibrils may enable scientists to design better dissipative synthetic biopolymer networks for tissue growth and wound healing, for example, which would be biodegradable as well as biocompatible.
The co-authors of the study titled “Energy dissipation in mammalian collagen fibrils: Cyclic strain-induced damping, toughening, and strengthening” were Julia Liu, Debashish Das, Fan Yang, Andrea G. Schwartz, Guy M. Genin, Stavros Thomopoulos, and Ioannis Chasiotis. It is published in Acta Biomaterialia.
The research was supported by the National Science Foundation and National Institutes of Health and by the National Science Foundation Science and Technology Center for Engineering MechanoBiology. Das’ effort was supported by a grant from the National Science Foundation.