blair lab at georgetown

Confocal Rheology of Collagen Gels

I am currently investigating stress transmission inside complex biopolymer networks.   Randomly oriented cross-linked collagen fiber networks in particular exhibit non-uniform deformation under applied shear stress.  Collagen fibers can be found inside cells but function primarily as the structural backbone for cells in mammals due to their tensile strength and flexibility. Using the novel techniques of confocal rheology, I wish to understand how applied stress is distributed among clusters of branched fibers and individual fibers.  Doing so will provide us insight into how cells with diverse morphologies interact with and traverse the extracellular matrix.
The fiber network structure of 0.2% type I rat tail tendon collagen imaged using reflectance and fluorescence confocal microscopy are shown below.  Each movie shows a 25 μm traversal along the optical axis.

The movie below shows fluorescent microspheres embedded in a polyacrylamide gel.  The movie shows a 10 μm traversal along the optical axis.  Polyacrylamide is a clear non-fluorescent gel with tunable rigidity.  The fluorescent microspheres serve as displacement markers.

Figure 1 shows a sketch of our two-layer system. The system consists of a collagen fiber network layered over and adhered to polyacrylamide gel embedded with fluorescent microspheres sandwiched between a rheometer measuring tool and a coverslip.  The confocal-rheometer coupling allows us to image three-dimensional volumes of our system over time while simultaneously applying shear stress.

Movie 4 shows the collagen/polyacrylamide interface under shear stress.  Applied shear stress by the rheometer measuring tool at the top of the collagen network transmits through the cross-linked fibers reaching the interface inducing deformation at fiber/gel contact points.
The microsphere positions are tracked in the three-dimensional volume over time (an example shown in figure a) to obtain the non-uniform deformation field (figure b).  Once these displacements are known we can determine the stress transmission at the interface (figure c) and consequently analyze the corresponding network structure causing the deformation (figure d).

For more information please contact Dan Blair