Bioinspired Deposition-Conversion Synthesis of Tunable Calcium Phosphate Coatings on Polymeric Hydrogels

Inspired by natural mineralization processes, here we present the stepwise mineralization of hydrogels with synthetic control over the amount of deposited CaP_i and selective tuning of the coating composition. Alternate immersion of the hydrogel at 3 min intervals in calcium salt (Ca²⁺) and inorganic phosphate (P_i) solutions under mild aqueous conditions results in the layer-by-layer deposition of a precursor CaP_i polymorph, dicalcium phosphate dihydrate (DCPD, Ca/P_i 1.12 ± 0.07), as a surface coating. Successive immersion cycles were shown to linearly increase the amount of deposited Ca²⁺ and P_i ions over 20 cycles enabling direct control over the mineral coating density and crystal morphology. Conversion of the DCPD coating to apatite (CaP_i 1.61 ± 0.02) is induced by aqueous hydrolysis at physiological temperature and pH (7.4, 37 °C, 5 days). After conversion, the apatite coating density was found to correlate with the amount of mineral initially deposited as DCPD, indicating this approach to mineralization imparts simultaneous synthetic control over the coating composition and density on the hydrogel substrate. Mineralized coatings were characterized by XRD, ATR-IR spectroscopy, SEM-EDX, and quantitative analysis of Ca²⁺ and P_i ions. Supplementation of the conversion solution with Ca²⁺, P_i, SBF, F^-, or citrate ions results in apatite coatings exhibiting variations in chemical composition and morphology. In the presence of added Ca²⁺ ions and SBF, an increase in Ca²⁺ content of the coating is observed, and the resulting particles exhibit growth as plates and petal like clusters, respectively. Conversion with F^- ions results in the formation of spherical F-apatite particles that exhibit clearly resolved peaks in the XRD pattern. Citrate ions were found to restrict the growth of apatite particles. The described deposition-conversion approach overcomes longstanding limitations in CaP_i-based biomaterials as a versatile method for the predictable and tunable synthesis of CaP_i coatings of preformed biopolymer substrates.