Phase-Selective Electroprecipitation of Calcium Phosphate Thin Films at Physiological Temperatures Stephanie D. Huelin,†,‡ Holly R. Baker,† Erika F. Merschrod S.,*,‡ and Kristin M. Poduska*,† Department of Physics and Physical Oceanography and Department of Chemistry, Memorial UniVersity of Newfoundland, St. John’s, Newfoundland, A1B 3X7 Canada
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 12 2634-2636
ReceiVed June 29, 2006
ABSTRACT: A protein-compatible combination of temperature and pH conditions allows for phase-selective electrosynthesis of bonelike calcium phosphate materials. The procedure involves an acidic aqueous electrolyte at 20-40 °C for an electroprecipation reaction based on nitrate reduction. X-ray diffraction and Raman spectroscopic data confirm that hydroxyapatite (Ca10(PO4)6(OH)2) is the kinetically preferred phase under galvanostatic electroprecipitation conditions with lower phosphate concentrations (∼0.03 M) in the electrolyte, whereas brushite (CaHPO4‚2H2O) is favored when electrolytes with higher phosphate concentrations are used. Titration data suggest that the origin of this concentration-dependent phase selectivity is related to differences in buffering capacity of the electrolytes in the immediate vicinity of the working electrode surface, which explains the variability in phases in previous reports of calcium phosphate electrosynthesis. Critical parameters for controlling film formation are galvanostatic control of the electroprecipitation (for local pH changes that trigger deposition), phosphate concentration (to regulate buffering effects), and the presence of ammonia and ammonium ions in the bulk electrolyte (for complexing to improve film coverage and uniformity). Calcium phosphates used in biocompatible coatings range from amorphous films to crystalline phases such as brushite (CaHPO4‚ 2H2O), octacalcium phosphate (Ca8H2(PO4)6‚5H2O, OCP) or hydroxyapatite (Ca10(PO4)6(OH)2, HAp), the phase which most resembles the mineral component of bone.1,2 It is not yet understood what impact phase and crystallinity have on the bioactivity of calcium phosphate coatings, but one documented difference among varied calcium phosphates is their relative solubilities.3 For an implant in vivo, solubility differences could lead to a changed chemical environment around an implant, which could affect cell response and other bioactivity. Thus, there are both fundamental and practical medical incentives for synthesis methods that allow phase-selective calcium phosphate formation. Given the biomedical relevance, it is no surprise that the synthesis of thin calcium phosphate films has been widely studied, both alone and with added protein to increase biocompatibility. Recent reviews document a range of techniques that have been utilized to synthesize calcium phosphate thin films, including plasma spraying, pulsed laser ablation, RF sputtering, and sol-gel synthesis.1,2 Many of these techniques require multiple steps and/or heat treatments (801600 °C) at some stage of the processing in order to yield the thermodynamically favored HAp instead of other calcium phosphate phases such as brushite or monetite. Unfortunately, such heating conditions would destroy proteinaceous material, so their use in protein-mineral composites must rely on multistep processing. Electrochemically assisted precipitation from aqueous electrolytes presents a lower-temperature (more protein-friendly) alternative for producing HAp coatings.4 Many studies have reported procedures for using nitrate-containing solutions that enable the electroprecipation of other calcium phosphate phases: brushite, monetite, or OCP. For example, recent studies by Fan et al. focused on the characterization of electrolytically deposited OCP-collagen composite coatings on silicon substrates.5 Harsh heat treatments6,7 and alkaline postdeposition treatments8-11 can also transform other electroprecipitated calcium phosphate phases into HAp. In contrast, there are few reports of electroprecipitation methods to form HAp directly. In some studies, the reaction temperatures used were too high (50-200 °C)12-16 to allow protein incorporation, or the electrolytes themselves (containing additional solvents)17 would be * To whom correspondence should be addressed. E-mail: kris@ physics.mun.ca (K.M.P.);
[email protected]. (E.F.M.S.). Fax: 709.737.8739 (K.M.P.); 709.737.3702 (E.F.M.S.). † Department of Physics and Physical Oceanography, Memorial University of Newfoundland. ‡ Department of Chemistry, Memorial University of Newfoundland.
destructive to protein. Others have used electrochemically assisted precipitation in multiple steps to avoid protein denaturation.7 Herein, we describe a method for selective electrochemically assisted precipitation of HAp or brushite thin films that can be extended to protein-mineral composite materials. Structural data on the deposits and titration data from the electrolytes suggest that this phase selectivity involves changes in electrolyte buffering capacities for different phosphate (PO43- and protonated ions) concentrations in the presence of ammonium ions. Our method uses acidic electrolytes at low temperature (20-40 °C) to produce HAp or brushite directly without the need for postdeposition heat7,11,18 or chemical treatments.7-11 By avoiding high deposition temperatures12,13,15,16,18,19 and denaturing electrolytes,17 we also allow for the option of simultaneous protein incorporation. For collagen, the main protein in bone, cartilage, and other structural tissue, this requires both mild temperatures (
