Optimizing surface characteristics for cell adhesion and proliferation on titanium plasma spray coatings on PEEK

Abstract Background Titanium plasma spray coating on polyetheretherketone (PEEK) is a recent innovation to interbody spacer technology. The inherent hydrophobic properties of standard, uncoated PEEK implants can hamper cell attachment and bone healing during fusion. The addition of Ti coating not only offers initial stability due to increased surface roughness, but long term stability due to bony ongrowth created from osteoconductive microenvironment on the device surface. The previously established hydrophilic and osteophilic properties of commercially pure titanium (CPTi) can potentially provide an ideal environment promoting cell attachment and bony ingrowth when applied at the endplate level of the fusion site. Since the surface material composition and topography is what seems to directly affect cell adhesion, it is important to determine the ideal titanium coating for the highest effectiveness. Purpose The purpose of the study is to determine whether there is an optimal surface roughness for the titanium coatings and whether different polishing methods have a greater effect than roughness/topography in mediating cell adhesion to the surface. Study Design/Setting The study was divided into two phases. In phase one, the effects of varying surface roughnesses on identical polishing method were compared. In phase two the effect of varying polishing methods was compared on identical surface roughnesses. Methods Coating thickness, porosity, and surface roughness were characterized using an optical microscope as per ASTM F 1854 standards. For both phases, PEEK coupons with plasma sprayed CPTi were used and human mesenchymal stem cells (hMSCs) at an initial density of 25,000 cells/cm2 were seeded and cultured for 24 hours before fixation in 10% formalin. The cultured hMSCs were visualized by 4', 6-diamidino-2-phenylindole (DAPI) staining, a fluorescent stain that binds to the DNA of living cells. Samples were imaged using an environmental scanning electron microscope (eSEM) (Carl Zeiss Microscopy, Thornwood, NY) using a backscattered detector (BSD). Results Image analysis of the CPTi coatings showed uniform and rough surfaces. For phase one, roughness was evaluated as fine, medium and coarse. eSEM image analysis and cell counting by DAPI demonstrated that hMSCs have a tendency to form stronger adhesion and greater pseudopodia extensions on fine roughness surfaces. Individual hMSCs were seen forming cytoplasmic processes extending across the width of a pore.