Growth kinetics and compositional interaction of electro-deposited calcium phosphate/chitosan coating

Objectives: Electrolytically deposited calcium phosphate/chitosan coating demonstrated good bone marrow stromal cells attachment. This study aimed to understand the coating's growth kinetics as well as the interaction between calcium phosphate and chitosan. Methods: Ti6Al4V plates were placed as cathode and electrolytically deposited in the calcium phosphate solution supplemented with or without chitosan. The plates were taken out at different time points. All coatings were observed with SEM, determined by thin film XRD and FTIR, and measured with magnetic induction probe. The electrolyte pH value was recorded through pH meter. The existence of chitosan was verified with Raman scanning electron microscope. The organic content in the coatings was determined through thermogravimetric analyser, using the weight loss of coating without chitosan as reference. Results: SEM micrographs found that calcium phosphate crystals homogeneously distributed into chitosan aggregates as early as 30 minutes. The chitosan aggregates were seen finally surrounded with calcium phosphate globules and filled in the space between the crystals after 12 hours. XRD patterns and FTIR spectra demonstrated that the coating experienced a compositional conversion from octacalcium phosphate to carbonate apatite. During the whole progress, the electrolyte pH steadily decreased from 6.3 to 5.6, and the coating thickness steadily increased to 33.2 mm. The electrolyte pH decrease and coating thickness increase were linearly correlated with the elapsed deposition time, with coefficients of determination (R²) at 0.99 and 0.98, respectively. Raman spectra could demonstrate clearly the existence of chitosan in the hybrid coating. As chitosan concentration in the electrolyte increased from 0.05 g/l to 0.15 g/l, and then to 0.20 g/l, the chitosan content within the hybrid coatings gradually increased from 2.3 % to 5.1 %, and then fell down to 4.9 %. Conclusion: Chitosan deposited simultaneously and gradually with calcium phosphate. However, this deposition also inhibited calcium phosphate formation.