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Recent Program Highlights

Soil is a highly disordered granular material. Slow soil deformation (creep) controls the shape of hills in the natural landscape, and is a precursor of catastrophic landsliding. Our work demonstrates a surprising observation: an apparently static sandpile, sitting on a table, is actually alive with motion. We study a 3D granular heap, confined by walls and prepared by pouring. Via Diffusive Wave Spectroscopy (DWS), we observe the existence of spatially-heterogeneous micro-deformations that decay in size and frequency as time progresses but persist up to 11 days after the preparation of the system; the heap relaxes. We find that this relaxation can be enhanced  (overaged) or reversed (rejuvenated) by tuning the types of disturbances applied to system.

Linearly elastic elastomers coated with matrix proteins are widely used to assess the role of stiffness. Such experiments are often assumed to reproduce the effect of the mechanical environment experienced by cells in vivo.
However, tissues and the extracellular matrix (ECM) are not linearly elastic materials. They exhibit far more complex mechanical behaviors. These behaviors include viscoelasticity, as well as mechanical plasticity, and nonlinear elasticity.
Our theoretical and experimental work has revealed that matrix viscoelasticity regulates fundamental cell processes and can promote behaviors – such as proliferation, motility and spreading – that are not observed with elastic hydrogels in both two- and three-dimensional culture microenvironments.

Animal tissues are composed of cells attached to either the surface of a fibrous network called a basement membrane or embedded within a 3D extracellular or interstitial matrix. As the disease liver fibrosis progresses, the extracellular fibrous networks become denser and more aligned. These physical changes lead to different mechanical properties and structures to which cells are exquisitely sensitive. To better understand the pathological effects of these changes during fibrosis on cells, we have engineered material platforms that mimic the extracellular matrix in tissue health and disease. As an example, we have fabricated fibrous materials that have varied mechanical properties and fiber densities when mechanically loaded due to the chemical adhesion between fibers, similar to natural extracellular matrix (see Figure).

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