Multilayered porous hydroxyapatite coating on Ti6Al4V implant with enhanced drug delivery and antimicrobial properties

Failure of the orthopedic implant due to bacterial infections and poor osseointegration is a common problem in total hip arthroplasty. The challenge here is improving the antibacterial properties of implant surface by loading the drug, without compromising the bioactivity and its performance. Plasma sprayed porous hydroxyapatite (HA) coatings on metallic implant surface, impregnated with antibiotic is found to be a solution. In this context, sustained delivery needs more drug being incorporated in the coating, for which the coating requires to be more porous. However, increased porosity makes the brittle HA coating weaker and unsuitable for intended application. The present study introduces the interesting concept of a multilayer HA coating, with a porosity gradient from bottom to top in different layers. Thin dense HA layer, adjacent to Ti alloy substrate offers strong adhesion with the implant surface, while the gradient increase in porosity on the upper layers maintain the integrity of the coating, while allowing the loading of drug in higher quantity. Developed coating is filled with drug containing biodegradable polymer through vacuum impregnation method. This structure offered an improvement in drug loading by 145% (16 μg/mm²) and increased the drug release duration (in days) by almost 4 times. In addition to these, effective filling of the pores with polymer improved the fracture toughness of the HA coating significantly, by restricting the crack growth. Improved tribological behavior was also noted, along with excellent bioactivity and osseointegration.