Research Focus

The mission of the Cell Biomechanics Lab is to investigate the role that mechanics has in cellular biology. We are interested in how mechanical factors affect the structure-function relationship in cells and tissue. To pursue these goals, we are developing new tools - biosensors, medical devices, and computational models - that provide discovery and understanding into the underpinnings of mechanobiology. The greater impact of our work is to advance new strategies for circumventing obstacles in developmental biology, cardiovascular disease, and cancer and by conducting transformative research in regenerative medicine, tissue engineering, and biomedical implants.

We are interested in how biophysical forces, adhesivity, spatial organization, and material properties affect the regulation of cells. We seek to differentiate how these mechanical factors regulate cells and tissue separately from biochemical ones like hormones, growth factors, and cytokines. Since cells are the basic building blocks of organisms, it is critical to identify how they transduce mechanical interactions at the micro- and nano-scale (mechanotransduction). Mechanical factors can act as signals that cells receive from their microenvironment. For example, fluid shear stress can affect endothelial permeability and gene expression, tensile strain can drive them to reorient, and matrix elasticity can dictate how much adhesion area cells cover. Cells also use actin-myosin forces to transmit these signals to each other (cell-cell) or to the surrounding matrix (cell-matrix) during tissue remodeling events such as angiogenesis, wound healing, scaffold implantation, and inflammation.

Our current research focuses in the Cell Biomechanics Laboratory can be grouped into three broad categories, each of which is briefly described below.

Cell Mechanics Measurements

By their dynamic nature, cells are active materials that are difficult to measure with traditional characterization techniques. Instead, innovative tools are needed that are appropriately-sized, biocompatible, and biofunctional. One tool we use to measure cell mechanics is an array of microfabricated silicone microposts. Cells plated onto the array attach to the tips of the microposts and generate traction forces to deflect the individual microposts (Sniadecki, et al. 2007). Cells generate traction forces through the ratcheting of myosin proteins on actin filaments of the cytoskeleton. They use these forces to migrate from one location to the next or to develop internal contractility or pre-stress that defines the shape of cells and tissue. Nearly all cell types generate traction forces but the degree of pre-stress differs between cell types (Lemmon, et al. 2007). Cells are responsive to mechanical cues and modulate their pre-stress with respect to the elasticity and adhesivity of the underlying matrix. To measure traction forces, the microposts behave like simple cantilever beams, in which the deflection of the tips directly reports the traction forces of the adherent cell on the array. Since each force sensor is mechanically decoupled, this system can generate maps of subcellular traction forces. With this tool, we can measure how forces are generated and transmitted within the cytoskeleton and by observing cells on the microposts over time, we can also capture how these forces are dynamically coordinated during migration or contraction.

In addition to studying single-cell mechanics, we are investigating the mechanical interactions that cells have with neighboring cells in a monolayer. We can spatially control the space that cells occupy with micro-contact printing. This technique controls the placement of matrix proteins and blocks cellular adhesion from all other locations. We can stamp matrix areas of different shapes, e.g. squares, rectangle, circles, etc., and thereby control the spatial interactions that cells have with their neighbors. We have used this in conjunction with the microposts arrays to measure how traction forces and the adhesivity of adherens junctions that connect cells are coordinated within different shapes of monolayers (Nelson, et al. 2005). We see that cells have different levels of participation in monolayer mechanics and can vary their traction forces and cell-cell contacts to collectively regulate the tissue structure and function.

Mechanotransduction

Mechanical forces that act on cells are important regulators in the health and development of human tissue. These physical cues affect cellular growth, differentiation, migration, gene expression, protein synthesis, and apoptosis. Yet, an understanding of how mechanical forces lead to the activation of biochemical signaling pathways that direct these functions remains elusive. Evidence points to focal adhesions as the central regulators of mechanotransduction, for they are both signaling and anchoring structures that connect a cell's actin-myosin motors to the underlying matrix. We are interested in ways to apply mechanical force to cells and their mechanosensitive structures.

One technique we have developed uses nanofabricated magnetic nanowires in the micropost array to create a sensor-actuator system for cell mechanics. The nanowires are embedded into the silicone microposts to create a robust tool that can apply piezomagnetic forces and simultaneously measure traction forces at the focal adhesions of cells. Focal adhesions are "spot-weld" structures (100-2000 nm) that transduce internal and external forces into biochemical responses. Focal adhesion growth and signaling increases with the application of external force and we found that a local force leads to an increase in local FA protein accumulation at magnetic microposts, but not at nearby, nonmagnetic microposts. We have also seen that cells respond to external force at focal adhesions through the dynamic regulation of their own internal traction forces showing that the mechanics of cells is coordinated with respect to mechanical factors outside the cell (Sniadecki, et al. 2007).

We are also developing microfluidic approaches to apply shear stress to cells to mimic the fluid mechanics of the cardiovascular system. We use the same microfabrication techniques of the micropost arrays to build microchannels in which cells are cultured. Using various pumping schemes we can stimulate cells with the identical fluid forces that they would encounter in vivo. We can then assay their response to various biochemical factors and fluid shear stress levels in order to screen therapeutic agents to cardiovascular disease.

Cell Mechanics Models

We seek to develop mathematical models of the coordination of cell mechanics within single cells and monolayers of cells. A fundamental description of mechanobiology and cellular organization across multiple length scales, from the structure of the cytoskeleton to the shape of collective cells within tissue, can provide needed information for biomedical implants and tissue engineering. We are also interested in mathematically expressing the dynamic events of migration and contraction and how the many small parts come together (myosin, actin, focal adhesions, adherens junctions, etc.) and spatiotemporally coordinate their activity to produce cellular function.

We have used finite element analysis to model the generation and transmission of stress within monolayers of cells. This approach is matching to cellular mechanics because individual cells in a monolayer act like finite elements within a sheet. We have found that the traction forces observed within monolayers matches well with the predicted stresses (Nelson, et al. 2005). We seek to further develop these models from a "bottom-up" approach that utilizes what we know about mechanotransduction and cell mechanics at the single cell level to articulate what is observed during morphogenesis and adult tissue structures.

Summary

Cells use mechanical factors from the outside and inside to guide their collective function, but there is a lack of appropriate tool-sets with which to study these phenomena. Our micro- and nanofabrication techniques and modeling approaches can allow us to measure these mechanical factors, manipulate the physical interactions, and verify our mathematical predictions. By controlling mechanical interactions at the small scale, we strive to build up a working knowledge of biomechanics that guides new approaches in treatment and prevention of diseases.