It is well known that a biomaterial is coated in a layer of adsorbed protein almost immediately upon implantation within the body. In order to develop biomaterials that integrate more readily into the body, work must be done to understand the influence that biomaterials have on the orientation and conformation of adsorbed proteins. This work examines three mechanisms for controlling the orientation of adsorbed proteins based on the ability of the adsorbed protein layer to modulate cellular adhesion in vitro.

The first mechanism for controlling protein orientation, charge driven orientation, relies upon the interplay between charged surfaces and the native protein dipole. Specifically, we examine the influence that positively and negatively charged self-assembled monolayers (SAMs) have on the orientation and conformation of osteopontin (OPN) and bone sialoprotein (BSP) by comparing the cell binding abilities of these proteins. Both of these proteins have a cell binding RGD amino acid sequence, and this work has shown that the RGD moiety has a different orientation and therefore accessibility for cellular binding, depending on the underlying surface charge.

The second mechanism for controlling protein orientation is the use of specific protein-protein binding interactions. For example, both OPN and BSP have been immunolocalized in the collagen matrix of developing bone indicating a specific binding interaction between these proteins. In this study, we examine the orientation of OPN and BSP when they are specifically bound to collagen. This is done by comparing the cell binding abilities of OPN and BSP when specifically bound to collagen, to the cell binding abilities of the individual proteins alone. This work shows that cellular adhesion is promoted when either OPN or BSP is specifically bound to collagen, indicating a preferential orientation for cellular adhesion.

The final mechanism for controlling protein orientation is the use of specific protein-substrate binding interactions. Both OPN and BSP have been shown to have a specific hydroxyapatite (HAP) binding domain and this study examines the orientation effects imparted on OPN and BSP when specifically bound to HAP through this domain. The results of this work indicate that HAP does impart a favorable orientation on OPN and BSP for cellular adhesion, but this positive effect on cell binding is limited by the overall surface roughness of the underlying HAP substrate.