Active superelasticity of epithelial tissues

Hello fellow Mechanicians,

I am delighted to share our recent article in Nature (https://www.nature.com/articles/s41586-018-0671-4 , read-only link: https://rdcu.be/batkj) that uncovers a surprising aspect of the mechanics of epithelial tissues, which are cohesive cellular sheets lining internal and external surfaces in our body. We probed the constitutive behavior of epithelia using a novel method to create pressurized domes of controlled size and shape. This microscopic bulge test (fig. a) revealed a tensional plateau while the tissue deformed reversibly reaching up to 300% areal strains. Strikingly, barely stretched and superstretched cells (up to 1000% areal strain) coexisted within the tissue with uniform tension. These features are defining hallmarks of superelasticity, a mechanical behavior of some high-tech metal alloys such as nitinol, which are capable of undergoing large and reversible deformations thanks to a microscopic material instability based on a phase transformation. We showed that in epithelial cells, such a softening instability is triggered by stretch-induced dilution of the actin cortex, while excessive cellular deformations are arrested by a network of intermediate filaments. This strain softening followed by a re-stiffening at large strains results in a ‘bi-stable effective energy landscape of active origin’ (fig. b), which explains the tensional plateau and extreme heterogeneity in cellular strain at nearly constant tension. We term this behavior ‘active superelasticity’ which may explain how mammalian embryos are able to accommodate extreme strains and this novel epithelial function may lead to potential bioengineering applications.

Fig. (a) Microscopic bulge test of epithelial domes shown schematically (top panel) and an experimental image of a dome forming a spherical cap with measured tractions on the substrate shown as blue arrows. (b) Coexistence of barely stretched (green, blue) and superstretched (orange, magenta) cells on top of a deflating dome (top panel). A schematic explanation of the phase transition based on the effective two-well energy landscape of active origin.