Intermolecular force mapping of platelet surfaces on collagen substrata.

The interactions between plasma proteins and platelets are responsible for surface adsorption and activation of platelets, which leads to initiation of platelet-mediated thrombotic events at biomaterial surfaces. We are seeking to gain a fundamental understanding of these interactions. The atomic force microscope (AFM) has been used to create force maps across platelets adsorbed onto collagen substrata using peptide-modified probes. Combining the imaging and force-measuring capabilities of AFM, the force-mapping mode has been used to measure interactions of peptide-modified AFM probes with the surface. Observed differences in the force of adhesion are clearly evident in the platelet samples fixed in air, proving the ability of the AFM system to map adhesion. When this system is changed to a fluid environment we are no longer able to see such evident adhesion because of the membrane flexibility; instead the deformability of the surface is mapped. The specific interaction between the peptide sequence RGD and platelets was measured in a non-mapping mode of the AFM. Although this does not provide a force map, we can see significant differences between the forces measured on the substrate and those measured with a control hexapeptide.