Paul Janmey, University of Pennsylvania
Biopolymer-Particle Composites Designed to Match the Mechanical Properties of Soft Biological Tissues
Written by Jessalyn Low
Tissues stiffen in compression but not in extension. However, interestingly, the opposite is observed in polymer networks like fibrin and collagen, which soften in compression and stiffen in extension. This behavior of softening upon compression is observed not only extracellularly but also intracellularly, in particular the actin filaments and microtubules. The difference in response to strain between tissues and polymer networks can be attributed to the fact that most tissues are densely packed cells within a matrix. As such, the mechanical properties of the matrix dominate the properties of the tissue, but presence of cellular inclusions alter the mechanical properties of the network. To test this, fibrin networks were polymerized around dextran beads and it was found that the inclusion of packed beads converted fibrin gel from a compression-softening to compression-stiffening behavior. These results imply that polymer network rheology converts to tissue rheology when volume-conserving inclusions become dense enough to limit network strand configurations. Moreover, it was found that single cells also stiffen in compression, which is postulated to be attributed to the intermediate filaments. Unlike other components of the cytoskeleton, namely actin filaments and microtubules which soften upon compression, intermediate filaments stiffen upon compression. This is thought to be because of their higher flexibility, which reduces their ability to buckle.