The Influence of Polyelectrolytes on the Formation and Phase Transformation of Amorphous Calcium Phosphate

A promising route to the fabrication of biomimetic coatings for artificial implants is the deposition of organic/inorganic composite materials consisting of polyelectrolyte multilayers alternating with layers of "in situ" grown calcium phosphate crystals. To facilitate understanding of the underlying mechanisms, in this paper we discuss the influence of polyelectrolytes (PEs), polystyrene sulfonate (PSS), poly-L-lysine (PLL), and poly-L-glutamic acid (PGA) on the formation and properties of amorphous calcium phosphate (ACP) and on the nucleation and growth morphology of the crystalline phase. pH vs time curves revealed three distinct precipitation events, i.e., (I) precipitation of ACP, (II) secondary precipitation of a crystalline phase upon the amorphous precursor, and (III) solution-mediated phase transformation and crystal growth. Finally, crystalline mixtures with low Ca/P molar ratios (1.39), consisting of octacalcium phosphate crystals and small amounts of apatite, were obtained. From the pH vs time curves, the induction time, t_i, preceding crystal nucleation was determined. All PEs at low concentrations induced and at high concentrations inhibited nucleation. The efficiency of induction increased in the order: LMw PGA ≅ HMw PGA < LMw PLL < HMw PLL < PSS, while the inhibition efficiency increased as LMw PLL ≅ HMw PLL < PSS < LMw PGA < HMw PGA. ACP particles formed in the presence of PE were smaller and less aggregated and had a higher surface charge than in the controls. All investigated PEs also inhibited growth of the crystalline phase in a nonspecific way.