In the extracellular matrix chemical cues are present that control all aspects of cell biology. These surface-bound and soluble factors provide the necessary adhesion and signaling for normal cellular activity, and without such matrix support, the cells will apoptose. Implanted medical device surfaces lack the molecular features that provide guidance to the surrounding cells to afford optimal in vivo integration and function. Controlling the interface between the material and biological realms that occurs at the surface of these devices would have important implications for a range of medical technologies ranging from sensors to orthopaedic implants. To achieve these goals, we have developed a non-covalent coating, which we have termed an Interfacial Biomaterial (IFBM) that can direct biological processes at the interface between the surfaces of synthetic and biological materials. The approach entails identifying specific and high affinity adhesion peptides via phage display technology, and then, via chemical synthesis, assembling two or more peptides with known adhesion domains to create a multi-functional interfacial biomaterial (see Figure). Consequently, these coatings are highly modular and adaptable, as the surface adhesion peptide can be interchanged with other unique adhesion peptides specific for a discrete metal, ceramic, or plastic surface. These multi-functional materials are amenable to coating and patterning techniques suggesting their use for applications ranging from proteomics to tissue engineering (See Figure).
In one application we designed, synthesised, and evaluated a non-fouling PEGylated IFBM as a cytophobic coating to prevent cell attachment. Application of this coating to a polystyrene surface reduced both mammalian and bacterial cell adhesion in vitro (see Figure). Both the PEG and PS-binding peptide domains were required in the macromolecule, as neither the peptide nor the PEG alone inhibit cell binding when coated on the surface.
In a second application we focused on cell binding to metal stents. Metal stents are commonly used in cardiovascular therapy and are favored for their inertness and mechanical strength; however, negative consequences can arise from suboptimal tissue integration. If the artery re-occludes due to smooth muscle cell proliferation, in a process called restenosis, a second procedure is required to re-establish blood flow. Restenosis used to occur in about 25% of patients, but with the introduction of stents that elute a mitotic inhibitor, such as paclitaxel, this number has been reduced significantly. An alternative approach to the use of drugs would be to develop materials that better integrate with the surrounding endothelial cells which line the artery to afford a pro-healing response. Ideally, such materials would provide cues to appropriately direct cellular activity, for example, through integrin interactions which control an assortment of biological processes. As part of our first foray into this area, we have synthesized a pro-cellular IFBM coating that adheres to a titanium surface under flow. Specifically, Ti disks were coated with an IFBM that binds Ti and HUVEC cells and incubated in a chamber under flow in the presence of serum (see Figure). Cells were retained on the surface. We next confirmed that integrin-RGD binding was responsible for the attachment using a competition experiment with RGDS and RGES tetrapeptides.
It should be noted that there is also a clear difference between surface adhesion and biologically appropriate adherence. In particular, apoptosis has been correlated with the lack of strong mechanical ECM ligation or soluble antagonist ligation of the αvβ3 and α5β1 integrins, both known RGD interactors. It follows that the induction of integrin binding by an RGD-terminated IFBM should similarly promote resistance to cell death induced by caspase-8 activators such as tumor necrosis factors (TNF) if we are able to ligate critical integrins in sufficient number. As shown in the Figure, cell attachment is observed, even at the maximum cocktail concentration of 100 ng/mL. Even though these cells are apoptosed, the coating provides sufficient integrin binding to retain them on the surface. Importantly, in the absence of any surface IFBMs, apoptosis occurs at TNF-α concentrations as low as 0.1 pg/mL, demonstrating the ability of the IFBM coating to increase cellular apoptotic resistance.