Working Content> MacroModels > Solids 

Prerequisites:

• Soft matter

Biological matter often behaves both as an elastic solid and as a viscous fluid, and is therefore considered to be viscoelastic. Living cells and tissues, in spite of great biological complexity, can be characterized as viscoelastic matter. Many materials in our body e.g. the extracellular matrix which surrounds cells and tissues and with which cells interact, such as collagen, are also viscoelastic in nature. Cells behave in an elastic manner over short time scales in order to withstand sudden forces from surrounding cells, while over longer time scales they behave in a viscous manner. This property allows cells, for example, to squeeze inside narrow blood vessels or between other cells by undergoing large deformations in response to forces applied over long time scales.

Cellular viscoelasticity arises due to the co-existence of solid and liquid phases. Cells and tissues have high water content as well as a structural matrix consisting of polymers. These biopolymers can support cell shape and provide cells with a structural rigidity. However, they are also highly dynamic and can undergo large-scale rearrangements. A living cell is a complex dynamical system, which constantly undergoes remodeling to adapt to changing environmental conditions. Cells adapt their mechanical properties in order to match that of their surroundings. The mechanical changes in cells under normal conditions and in response to external forces may be highly complex and difficult to measure. However, recent advances in rheological techniques have enabled the measurement of the mechanical properties of living matter. Cellular mechanical properties can be measured by several advanced techniques such as Atomic Force Microscopy, compression between parallel plates, magnetic tweezers and optical cell stretching. 

The mechanical properties of cells and their surroundings are important for regulating many biological functions such as cell growth, cell movement, wound healing, cancer metastases, and cell differentiation or the determination of cell fate. In a landmark experiment a few years ago, it was discovered that stem cells (cells that have not specialized into particular types) grown on soft matrices differentiate into different cell types depending on the elastic material of the matrix. For example, stem cells grown on soft surfaces with low values of elastic modulus become brain cells, while cells grown on stiff surfaces with high elastic modulus become bone cells. These findings showed that cellular biochemical and genetic response are linked to the physical properties of cells and their surroundings. How this happens is a subject of active research.

Arpita Upadhyaya 10/22/13