Journal of Sol-Gel Science and Technology 21, 13–25, 2001 c 2001 Kluwer Academic Publishers. Manufactured in The Netherlands. °
Biomimetic Apatite Deposition on Calcium Silicate Gel Glasses A. J. SALINAS∗, M. VALLET-REGI AND I. IZQUIERDO-BARBA Departamento de Qu´ımica Inorg´anica y Bioinorg´anica, Facultad de Farmacia, Universidad Complutense de Madrid, E-28040 Madrid, Spain [email protected]
Received May 10, 2000; Accepted August 16, 2000
Abstract. In order to understand the Biomimetic apatite formation mechanism on gel glasses, a glass (in mol-%) SiO2 80%—CaO 20% (80S20C) was prepared by the sol-gel method and its behaviour in a simulated body fluid (SBF) was studied. To study the role of phosphorous in the in vitro apatite formation, a gel glass (in mol-%) SiO2 80%—CaO 17%—P2 O5 3% (80S17C3P) was prepared comparing its behaviour in SBF with that of 80S20C. In both studies, a protocol without renovation of SBF (static) was used. To mimic the conditions in the living organisms, an in vitro protocol with continuous renovation of solution (dynamic) was proposed. To check the feasibility of dynamic protocol, 80S20C and 80S17C3P were studied in dynamic and results compared with obtained in static. Static studies of 80S20C allowed us to verify that phosphorus is not essential for bioactivity because the apatite-like layer was formed from the phosphorous in SBF. However, a 3 mol-% of P2 O5 in 80S17C3P gel glass favoured apatite crystallization. In dynamic, complete assays were performed with ionic concentrations and pH in solution almost equal to human plasma. After 7 days in dynamic, apatite crystals and crystalline aggregates were larger than in static. Besides, compositional variations were observed in the newly formed layer as a function of the protocol. In static, the layer formed in both glasses contained calcium and phosphorous, (Ca/P molar ratio = 1.6) and silicon. In dynamic, the layer did not contain silicon and the Ca/P molar ratio was 1.2. Differences in composition and pH of assay solution, 8 in static and 7.3 in dynamic could explain these variations. In static, an apatite close to stoichiometric could be formed. In dynamic, a mixture of calcium deficient apatite and other calcium phosphates could constitute the layer. Keywords: bioactive sol-gel glasses, CaO SiO2 and CaO P2 O5 SiO2 systems, Biomimetic apatite formation, static in vitro assays, dynamic in vitro assays 1.
Glasses are materials widely used technologically due to their many applications as monoliths, fibres or coatings in areas including Optics, Electro-optics and Communication Technologies. In the last three decades a relevant application of glasses is being done in the field of Biomaterials [1, 2]. The study of glasses to use as implants began in 1971 when Hench et al.  prepared, by cooling of a
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melt, a family of glasses in the system Na2 O CaO P2 O5 SiO2 , observing that some compositions strongly bonded with the osseous tissues. These glasses behaved into the human body in a different way to the other materials previously implanted which, in the best cases when behaved as bioinert, were isolated of tissues by a non-adherent fibrous capsule. The formation of this capsule results in relative micromovements in the material to tissue interphase. These relative movements increase with time producing, in many cases, the failure of the prostheses. In the last years other materials including glasses, glass-ceramics and hydroxyapatite, have shown bone-bonding ability .
Salinas, Vallet-Regi and Izquierdo-Barba
Glasses able to bond to living tissues without fibrous capsule formation are usually called bioactive [4, 5]. When bioactive glasses contact to biological fluids, an amorphous calcium phosphate layer that crystallizes in an apatite-like structure is formed [6, 7]. The apatite crystals, reinforced by collagen fibres, form the bonding layer between bioactive glasses and living tissues. The apatite layer is also formed when bioactive glasses are soaked in solutions mimicking the human plasma. Thus, the in vitro tests are widely used in the design of new glasses to use as implants [8–10]. The in vitro studies have shown that the silanol groups (Si OH), formed by ionic interchange with protons of medium on bioactive glasses surface, play a major role in the nucleation mechanism of apatite . In past years most of the studies in bioactive glasses were devoted to obtain new compositions or to develop new methods of synthesis. Regarding compositional variations, to the initial bioactive compositions  new components, such as K2 O, MgO, CaF2 , Al2 O3 , B2 O3 or Fe2 O3 , were added increasing the complexity of the systems studied. Besides, in some cases, the addition of new oxides decreased or eliminated the bioactive behaviour. For instance, a 3% of Al2 O3 added at the initial composition of Hench, to improve its mechanical property , eliminated its bioactivity; or the addition of Fe2 O3 to obtain glass-ceramics for hyperthermia treatment of cancer , that decreased the bioactivity in some cases. In 1997 Brink et al.  studied the in vivo bone bonding-ability of 26 melt glasses in the system Na2 O K2 O CaO MgO B2 O3 P2 O5 SiO2 concluding that compositional limits for bioactivity were: 14–30 mol% of alkali oxides (Na2 O + K2 O), 14–30 mol% of alkaline earth oxides (CaO + MgO), and