Smart micro/nano engineered implant surfaces – A platform for osteogenesis

G. Manivasagam

Keywords: medical implant, osteogenesis, nanosurfaces


Inspite of the fact that research towards materials development for medical implants has attained saturation, still there is a dire need to fine tune their mechanical and surface properties to prevent failure. As the world is witnessing a dramatic growth in the young and ageing population owing to advancement in health care sectors a parallel rise for replacement of damaged and diseased tissue is also obvious. Focus towards enhancement of biocompatibility by refining the grain size towards nano regime gained attention in early nineties. A substantial increase in osseointegration was witnessed with increase in surface energy, topography and geometric cues of nanograined material. This motivated the researchers to work towards development of nanosurfaces by modifying it through mechanical means or chemical processes or physical means like laser patterning. This talk will cover the various techniques adopted by us to tailor the bulk and surface of the implants to enhance biocompatibility, antibacterial activity, corrosion and wear resistance. Anodization is one of the techniques that lead to the formation of oxide nanotubes on the surface of an implant that efficiently influence the cellular differentiation into bone cells by certain epigenetic mechanisms. Optimization of the tube diameter plays a major role in this process. There has always been a challenge to have optimum bone attachment as well as getting rid of bacterial adherence. Hydrothermal treatment on pure titanium has demonstrated enhanced calcium deposition as well as higher bactericidal activity compared to untreated titanium. The bactericidal nature of these nanoflowers can be justified by the ability of their sharp edges to puncture the cell wall of motile bacteria. The hydrothermal treatment implies that sharp nanotextured surfaces such as nanoflowers, wires, spears can be well explored for the inhibition of bacterial growth. Apart from chemical modifications, mechanical surface treatment on implants has been utilized for many applications especially for dental implants as it does not lead to any adhesion issues as faced with the coating methods. Nanocrystalline (NC) surface can be achieved through several processes and one such simple and mechanical means of obtaining it is through surface mechanical attrition treatment (SMAT). This process deals with the interaction of metallic balls of uniform diameter on the implant surface at controlled pressure and duration to attain NC structure. This nanocrystallization increases the charge carrier density of the passive oxide layer formed on the implant surface which in turn prevents the denaturation of the adsorbed cell-adhesive proteins such as fibronectin. A net positive charge is developed on the otherwise neutral oxide layer, which favors cell adhesion. Other physical means of treating the surface to improve the osseointegration is through laser patterning.It is considered to be a suitable method to create nano/micron topography that enhances osteogenic efficiency of the medical implants. The shape of topography is obtained from texturing is generally limited to grooves and ridges. Cellular and sub-cellular scale topography affects cell adhesion on implant substrates and influence depends on the shape of pattern (eg. grooves and pits). Proliferation and cell differentiation is only sensitive to nanometer to several micron scale topography, but not topographies in ten- to hundred micron scales as mentioned in previous section. However, not all cell types are insensitive to ten- to hundred-micron scale topography; thus, the modulation of cell phenotype by surface topography is usually considered to be cell type specific. Wear and wear accelerated corrosion of titanium and cobalt based alloys have led to failure and recall of metallic implants. Engineering the bulk properties to improve the wear and corrosion resistance has not led to the substantial changes and coating of ceramics has been a suitable solution and this has been commercialized. Thinning of coatings and poor adhesion are the challenges encountered with this technology. Nanoceramic coatings developed using plasma spray process exhibits high resistance to wear and corrosion as well as to tribocorrosion. The formation of melted and unmelted nanoparticles acts as a source that inhibits crack propagation and prevents the failure of the coatings. Thus this talk will throw light on various surface modifications attempted on orthopedic implants to achieve micron/nanotopgraphy to induce the required osteogenesis .