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Research Focus

   Our lab investigates how cells are influenced by mechanical interactions at the micro and nanoscale. To pursue these goals, we are developing new tools - micro- and nano-devices, quantitative image analysis, and computational models - that we use to understand the underpinnings of biomechanics and mechanobiology. The greater impact of our work is to delineate how cell mechanics affects cardiovascular disease and cancer in order to catalyze new strategies for their treatment. By working at the intersection of mechanics and biology, we are also developing an increased understanding on the theories of soft, active, and multifunctional materials.

   Specifically, we are interested in how biophysical forces, adhesivity, spatial organization, internal structure, and material properties affect the behavior of cells. Since cells are the basic building blocks of organisms, if we can better understand cellular mechanotransduction processes, which are how cells detect and recognize these mechanical factors, then it would be possible to integrate, simulate, and study these interactions at larger scales, e.g. tissue or organ systems. For example, fluid shear stress can affect vascular endothelial permeability and gene expression, tensile strain can drive them to reorient, and matrix elasticity can dictate how much adhesion area cells cover. These changes can be detrimental to the function of the walls of the arteries and veins and can cause cardiovascular disease like hypertension and strokes. Cells also use their internal actin-myosin forces to transmit these mechanical 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:

Cell Mechanics Measurements

   Single cell mechanics: By their dynamic nature, cells are active materials that are difficult to measure with traditional characterization techniques. Instead, innovative tools are needed that are biocompatible, biofunctional, and as small as cells, or smaller. One set of tools we use to measure cell mechanics are arrays of micro or nano-posts. Cells plated onto these array attach to the tips of the posts and generate traction forces that deflect the posts like a cantilever beam d = (L3 / 3p E d4) F (Sniadecki, et al. 2007). Cells generate forces through the interaction of myosin motor proteins on actin filaments in 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 and helps to hold them together. 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 posts 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. Current Funding: NSF CAREER, NIH-NHLBI R21, DARPA-YFA.

   Multicellular mechanics: 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 can also measure the retraction forces of many platelets within a clot (Liang, et al. 2010), the tugging force between pairs of cells (Liu, et al. 2010), and the heterotypic interactions between monocytes and monolayers of endothelial cells (Liu, et al. 2010). We have found that cells cling tightly together when subjected to shear flow and this tugging force improves the concentration of adhesive proteins at the cell-cell contact (Ting, et al. 2012). Overall, we see that cells have different levels of participation in multi-cellular mechanics and can vary their traction forces and cell-cell contacts to collectively regulate their structure-function relationships. Current Funding: NIH-NHLBI R21, NIH-NIBIB F32, Past Funding: NIH-NHBLI T32, UW RRF.

Mechanotransduction

   Focal adhesions: 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). Current Funding: NSF CAREER, NIH-NHLBI R21 Past Funding: NIH-NHLBI T32.

   Shear flow: We are developing fluidic approaches to apply shear stress to cells to mimic the fluid mechanics of the cardiovascular system (Ting, et al. 2012). We use the same microfabrication techniques of the micropost arrays to build shear flow bioreactors 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. Current Funding: DARPA-YFA, NIH-NIBIB F32, Past Funding: UW RRF.

   Matrix stiffness: Cells also rely on their cytoskeletal tension to interpreting mechanical factors in the extracellular microenvironment. Matrix stiffness and the spread area of a cell can spread strongly influence how cytoskeletal tension is delivered to the focal adhesions of a cell, but it has been uncertain if these factors were intrinsically linked. To decouple the factors, we used arrays of posts that were fabricated with different heights, diameters, and densities and printed with patterned areas of available matrix ligand. We have found that matrix stiffness and spread area are independent factors of contractility, but the spatial distribution and density of focal adhesions underneath a cell greatly influence cytoskeletal tension. These findings led us to consider the effective shear modulus of the substrate, derived from the density and flexibility of the posts underneath a cell, which we confirmed to be a factor of cytoskeletal tension. Our viewpoint is that cells modulate their traction forces not only in response to the local stiffness at their focal adhesions, but also in response to the effective stiffness of their microenvironment. The effect of substrate stiffness also plays a role in the contractile structure of cardiomyocytes (Rodriguez, et al. 2011). Current Funding: NSF CAREER.

Cell Mechanics Models

   Migration: We are developing mathematical models of the coordination of cell mechanics during migration. Specifically, we are interested in mathematically expressing the dynamic events of migration and contraction and how the many small parts come together (myosin, actin, focal adhesions, etc.) and spatiotemporally coordinate their activity to produce cellular function (Han, et al. 2011). We use multi-physics finite element analysis to model the generation of traction forces during cell migration that incorporates elasticity and biochemical activity within a cell. Current Funding: NSF CAREER.

   Monolayers: We have also 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 have also used a novel, mathematical approach to measure the intercellular forces between cells in a monolayer (Ting, et al. 2012). 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. Past Funding: NIH-NHBLI T32.

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.