Chemically-modified calcium phosphate coatings via drop-on-demand micro-dispensing technique

Multi-functional, co-substituted hydroxyapatite thin films (Ag,Si-HA) with a composition of ~0.2wt.% silver (Ag) and ~0.7wt.% silicon (Si), were produced on titanium substrates using the drop-on-demand micro-dispensing (DODMD) process. Scanning electron microscopy images revealed that the coating was made up of rod-like apatite crystals measuring ~50nm in length by ~15nm in width, with no change in the morphology after the deposition process. X-ray diffraction analysis confirmed the presence of single-phase Ag,Si-HA with no other secondary phases such as tricalcium phosphate, tetracalcium phosphate or calcium oxide being detected. A study of the Ag,Si-HA thin film by fourier transform infrared spectroscopy indicated the presence of several phosphate bands in the range of 500-700 and 900-1200cm^-1, and a hydroxyl band at 3571cm^-1, which is the characteristic of HA. The surface contact angle measurement for Ag,Si-HA thin film was found to be significantly lower than those of the uncoated substrates, demonstrating the highly hydrophilic nature of these coatings. White light interferometry was used to determine the thickness of the deposited thin film. The thickness was found to be approximately 3.5μm. A scratch test conducted on the coating revealed an adhesion strength of about 160mN. Biological results showed that human mesenchymal stem cells grew generally well on Ag,Si-HA thin films and at the same time, a few bacterial colonies were seen attaching on the surface of Ag,Si-HA. All these results demonstrated that multi-functional, single-layered phase-pure Ag,Si-HA thin films could be deposited using the technique of DODMD.