Biomimetic organic-inorganic composite coatings for metal implants used in orthopedics and dentistry. Preparation and characterization

Biomimetic organic-inorganic composite coatings for metal implants, consisting of an organic matrix interspersed with calcium deficient carbonate apatite crystals have been prepared and characterized. The coatings were synthesized by first depositing the organic matrix in the form of polyelectrolyte multilayers, followed by incubation of the construct into a low supersaturation, metastable calcifying solution (MCS) to induce and sustain calcium phosphate crystallization. Single and/or mixed multilayers were constructed using layer by layer deposition of poly-L-lysine, (PLL) alternating with poly-L-glutamate, (PGA), poly-L- aspartate (PAA), and/or chondroitin sulfate (CS). Coatings were characterized by FTIR, XRD (not shown), SEM + EDX, AFM, CLSM and mechanical testing. The PEMLs had a grainy topography with aggregate sizes increasing in the order: (PLL/PGA)_n < (PLL/PAA)_n < (PLL/CS)_n. Multilayers. of (PLL/PGA)_n and (PLL/PAA)_n promoted, whereas (PLL/CS)_n slightly inhibited nucleation and early crystal growth of calcium phosphate. Generally, platelet-like crystals of calcium deficient carbonate apatite were obtained, their structure and morphology depending on the conditions of the MSC rather than on the composition of the underlying PEML. Force spectroscopy measurements with a colloid probe (AFM) show that calcification significantly increased the mechanical stiffness of the coating as compared to uncalcified PEML films. Calcified PEML coatings typically revealed Young's moduli in the range of 5 - 10 MPa, whereas earlier determined values for uncalcified PEML films were in the range of ~9 kPa [for (PLL/CS)n films] to several hundred kPa [for (PLL/PGA)n films]. After crystallization coatings were strongly attached to the underlying substrate as shown by standardized adhesive tape testing.