Experimental Soft Condensed Matter Group Harvard University, Prof. D. A. Weitz Surface chemistry effects for microrheology

Microrheology methods allow the precise characterization of the mechanical and microstructural properties of soft materials. We measure the thermal motion of embedded tracer particles, from which we determine the local mechanical response. This interpretation depends sensitively on the interactions between the particles and surrounding medium. In biomaterials, tracer movements are strongly affected by protein adsorption, so the ability to characterize and control the amount of protein adsorbed onto colloids is critical.

Nonspecific protein adsorption onto colloids can be driven by electrostatic or hydrophobic interactions. One common scheme used to reduce protein adsorption onto surfaces is to pre-incubate with a blocking protein, such as BSA or casein, which is known to be "sticky." The blocking proteins will adsorb strongly to the surface, and should occupy all available binding sites, preventing any other protein from docking on the surface. In practice surfaces covered by blocking proteins are ill-characterized and patchy; often additional can adsorb.

Poly(ethylene glycol)-coated surfaces also resist protein adsorption. The detailed microscopic mechanism of the resistance remains unknown, but the conformational entropy and solvation of the chains are thought to play a role. We have developed a robust protocol for binding short poly(ethylene glycol) (PEG) chains to the surfaces of colloidal particles using only commercial reagents. The PEG-coated beads are remarkably better at resisting the adsorption of proteins!

Here is schematic of a colloidal particle in a complex biomaterial. If sticky, the particle will recruit filaments and globular proteins to its surface, which may cause the sphere to attach to a part of a heterogeneous network or locally deplete a strongly binding protein from the bulk solution. Grafting short polymer chains to the surfaces reduces the amount of protein adsorbed.

In microrheology measurements, the thermal movements of embedded tracers are used to measure the local viscoelastic response. Conventionally, we assume that if the particles move a lot, the viscosity and elastic modulus must be small. If the particles move slowly or are constrained, then the viscous or elastic modulus must be larger. This interpretation relies on assumptions about the particle-material interface, and uncontrolled protein adsorption can also affect particle mobility -- leading to errant interpretations of local microenvironments. Look at the example below...

Here, the graphs on the left show the trajectories of "sticky" carboxylated beads in fibrin, a blood protein involved in clotting. At these concentrations, the fibrin forms a heterogeneous network with a mesh size of roughly 5 microns, much larger than the bead diameters of 200 or 500 nm. The graphs on the right show "unsticky" PEG-coated beads in the same material. The amount of particle movement is clearly correlated with the ability of proteins to bind to the surface of the colloids. In this case, the carboxylated beads attach to the polymer strands, and their movements reflect the thermal undulations of the network. The PEG beads move through the large mesh, and their movements reflect the background viscosity of the buffer solution. Thus, by tuning the protein-binding capacity of the spheres, we can tune their sensitivity to different mechanical parameters.

This page is maintained by:

Megan Valentine
Department of Physics
Division of Engineering and Applied Science
Harvard University
9 & 15 Oxford Street, McKay Laboratory
Cambridge, MA 02138
617-495-3705

valentin@fas.harvard.edu