Preparation of Polymer Particles Coated with Hydroxyapatite

Journal of Colloid and Interface Science 212, 585–588 (1999) Article ID jcis.1998.6023, available online at http://www.idealibrary.com on

NOTE Preparation of Polymer Particles Coated with Hydroxyapatite Polymer particles coated with hydroxyapatite were prepared by treating Pd 0 immobilized polystyrene-co-acrylic acid particles in aqueous CaCl 2 and NaH 2PO 2 solutions. Hydroxyapatite coating took place at neutral to alkaline pH conditions, and the homogeneous growth of the hydroxyapatite layer on the surface of polymer particles was observed at relatively low temperature (30 – 50°C). The thickness of the hydroxyapatite layer increased with reaction time. © 1999 Academic Press Key Words: polymer particle; styrene-acrylic acid copolymer; hydroxyapatite; composite particle.

fluid and then soaking in highly supersaturated apatite solution. Okuwaki et al. (6, 7) revealed the coating of hydroxyapatite on various metal plates using hydrothermal reactions in Ca(edta) 22–NaH 2PO 4 solutions. On the other hand, the coating of spherical and monodispersed polymer particles with hydroxyapatite is a useful way for the preparation of easyhandling catalysts and chromatographic packing. Recently, we reported that Pd 0 supported polystyrene-co-acrylic acid particles were uniformly coated with rare earth metal orthophosphates (RPO 4); that is, polymer/RPO 4 core-shell type composite particles were prepared by treating polymer particles with aqueous solutions of rare earth nitrates and NaH 2PO 2 (8). In this work, we investigated the formation of hydroxyapatite coated on polymer particles by treating Pd 0 immobilized polystyrene-co-acrylic acid copolymer particles in aqueous CaCl 2 and NaH 2PO 2 solutions.

INTRODUCTION

EXPERIMENTAL

Hydroxyapatite Ca 5(PO 4) 3(OH), which is the major constituent of bone and teeth, is an attractive material in biomaterials such as artificial bone and teeth, chromatographic packing, and catalyst support. Occasionally, its application is focused on the complexing with other materials rather than the individual utilization of hydroxyapatite because of its mechanical brittleness. Especially, the formation of a hydroxyapatite coating on other substances, e.g., polymers and metals, has received much attention in the application as biocompatible materials. From this point of view, various coating methods, e.g., plasma spray (1), sputtering (2), electrophoretic deposition (3), and biomimetic process (4, 5), have been proposed for hydroxyapatite coating. Kokubo et al. (4, 5) reported that bone-like hydroxyapatite is formed on organic substrates by placing the substrates on CaO–SiO 2 based glass soaked in a simulated body

Styrene (St) and acrylic acid (AA) from Wako Pure Chemical were purified by distillation under an Ar atmosphere. Potassium persulfate from Kanto Chemical was recrystallized from water. Tin(II) chloride (SnCl 2), sodium palladium chloride (Na 2PdCl 4), and calcium chloride (CaCl 2) from Wako Pure Chemical were used without further purification. Sodium phosphinate monohydrate (NaPH 2O 2) from Kanto Chemical was used without further purification. Water was used after distillation and deionization. Poly(St-co-AA) fine particles were prepared by copolymerizing styrene with acrylic acid by emulsifier-free emulsion polymerization using potassium persulfate as an initiator (9). The poly(St-co-AA) particles obtained were spherical and monodispersed. The average particle size was 356 nm by transmission

FIG. 1. Transmission electron micrographs of poly(St-co-AA) particles/hydroxyapatite composites obtained at various pH. CaCl 2, 0.125 mol/l; NaH 2PO 2, 0.125 mol/l; polymer particles, 7.5 g/l; temperature, 90°C; reaction time, 6 h. 585

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NOTE The resulting particles were separated by centrifugation and purified by the repetition of serum replacement. Throughout this procedure, the polymer particles coated with hydroxyapatite or the mixture of polymer particles and hydroxyapatite precipitates were obtained. The observation of a transmission electron microscope (TEM) was conducted with a Topcon EM-002B at 200 kV. X-ray diffraction (XRD) analysis was performed in a Rigaku RDA-IB system using CuK a radiation and the assignment of the resulting hydroxyapatite was made based on the literature (10).

RESULTS AND DISCUSSION

FIG. 2. XRD patterns of poly(St-co-AA) particles/hydroxyapatite composites, CaCl 2, 0.125 mol/l; NaH 2PO 2, 0.125 mol/l; polymer particles, 7.5 g/l; temperature, 90°C; reaction time, 6 h.

electron microscopy. The particles were purified by repetition of serum replacement. The coating of poly(St-co-AA) particles with hydroxyapatite were performed as follows: To 160 ml of aqueous dispersion of poly(St-co-AA) particles (20g) was added 200 ml of 0.2 M aqueous SnCl 2 solution. The mixture was stirred for 1 h at room temperature. The resultant Sn 21 ionimmobilized poly(St-co-AA) particles were purified by serum replacement. This purification was repeated five times, and then 200 ml of 10 22 M aqueous Na 2PdCl 4 solution was added to 200 ml of aqueous dispersion of Sn 21 immobilized polymer particles. The mixture was stirred for 3 h at room temperature. The dispersion was purified by the repetition of serum replacement. As a consequence, Pd 0 metal immobilized polymer particles were obtained after reduction of Na 2PdCl 4 by reducing the power of Sn 21. To the aqueous dispersion of Pd 0 immobilized particles was added an aqueous NaH 2PO 2 and CaCl 2 solution. The mixture was stirred for 3– 6 h at 30 –90°C.

Hydroxyapatite is readily formed by direct reaction of a Ca 21 ion with a orthophosphonic ion generated from orthophosphonium compounds such as NaH 2PO 4. However, these hydroxyapatites were as a whole amorphous and it is not necessarily easy to control the size and its distribution of hydroxyapatite particles. On the other hand, the formation of hydroxyapatite by direct reaction of Ca 21 with PO 432 ions varies depending on the pH of solutions. Therefore, the influence of pH on the formation of hydroxyapatite from Ca 21 ions and PO 232 ions with catalytic amounts of Pd 0 supported on the surface of poly(St-co-AA) particles was investigated. The pH was changed using a NaOH solution. Figure 1 shows TEM photographs of the polymer particles obtained at various pH at 90°C. As shown in the TEM photograph of Ca-1 which was prepared without the addition of NaOH solution, coating or complexing of polymer particles with Ca 5(PO 4) 3(OH) scarcely took place at acidic pH. This is attributable to no formation of crystals of hydroxyapatite at acidic pH. With increasing pH by the addition of NaOH solution, the generation of needle-like crystals on the surface of polymer particles was observed as shown in Fig. 1, Ca-2–Ca-5. XRD analysis showed that the crystals formed on the surface of polymer particles at pH 5.09 (Ca-2) were CaHPO 4 z 2H 2O (Brushite). On the other hand, as shown in Fig. 2, XRD patterns of composite particles formed in more alkaline solutions (Ca-3–Ca-5 in Fig. 1) indicated that these crystals were hydroxyapatite. These results suggested that hydroxyapatite coating particles could be prepared in alkaline solutions by this procedure. However, the crystal sizes of hydroxyapatite were widely distributed and larger than the sizes of original

FIG. 3. Transmission electron micrographs of poly(St-co-AA) particles/hydroxyapatite composites obtained at various temperatures. CaCl 2, 0.125 mol/l; NaH 2PO 2, 0.125 mol/l; NaOH, 0.25 mol/l; polymer particles, 7.5 g/l; reaction time, 6 h.

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FIG. 4. Transmission electron micrographs of poly(St-co-AA) particles/hydroxyapatite composites obtained as a function of time. Temperature, 30°C. CaCl 2, 0.125 mol/l; NaH 2PO 2, 0.125 mol/l; NaOH, 0.25 mol/l; polymer particles, 7.5 g/l.

polymer particles at alkaline pH, and thus the coating was heterogeneous or the mixtures of polymer particles and hydroxyapatite crystals were formed simultaneously. This behavior is remarkable at high alkaline pH. On the other hand, the generation of large crystals of hydroxyapatite may come from higher reaction temperatures (70 –90°C). When the aqueous dispersion of poly(St-coAA) fine particles without Pd was added to the mixed solutions of NaPH 2O 2 and CaCl 2, no precipitate was formed even at alkaline pH. The crystals of Ca compounds are generated by the function of Pd 0 immobilized on polymer particles. Here we investigated the effect of temperature on the formation of hydroxyapatite. Figure 3 shows TEM photographs of the composites of polymer

FIG. 5. XRD patterns of poly(St-co-AA) particles coated with hydroxyapatite. CaCl 2, 0.125 mol/l; NaH 2PO 2, 0.125 mol/l; NaOH, 0.25 mol/l; polymer particles, 7.5 g/l; reaction time, 6 h.

particles and hydroxyapatite at 30 –90°C. The homogeneous coating of hydroxyapatite was performed at low reaction temperature (30 –50°C) (Ca-11 and Ca-12 in Fig. 3), whereas crystal sizes increased with increasing temperature and the hydroxyapatite layer became heterogeneous (Ca-13 and Ca-14 in Fig. 3). The growth rate of hydroxyapatite from Ca 21 and PO 232 ions by catalytic Pd 0 seems to be relatively high at 70 –90°C. From the facts that a hydroxyapatite layer grows on the surface of polymer particles at neutral to alkaline pH conditions and the homogeneous growth of hydroxyapatite layer takes place at relatively low temperature (30 –50°C), the growth of a hydroxyapatite layer with reaction time at low temperature (30°C) was investigated. Figure 4 shows TEM photographs of the polymer particles as a function of reaction time. The complexation progressed with increasing time. The XRD patterns of resulting particles are shown in Fig. 5. Figure 5 indicates the formation of fine crystals of hydroxyapatite. Ca. 100-nm thickness of a hydroxyapatite layer was homogeneously formed after 4 h (Ca-24 in Fig. 4). This result indicates that the slow growth of hydroxyapatite at low temperature is suitable for the formation of excellent coating of hydroxyapatite on Pd 0 immobilized polymer particles in aqueous solutions of NaH 2PO 2 and CaCl 2. The increase in NaH 2PO 2 concentration was found to suppress the growth of hydroxyapatite layers, because the pH of high NaH 2PO 2 concentration solution shifted to a high acidic pH region. The formation of hydroxyapatite layers in high NaH 2PO 2 concentration solutions is very low. It is known that transitional metals such as Cu and Ag are plated chemically on the polymer surfaces by catalytic action of Pd 0 using solutions of NaH 2PO 2 and metal chlorides. In this process, PO 232 ions are easily oxidized to PO 432 simultaneously with the reduction of transitional metal ions to metal. Therefore, it is thought that the formation of hydroxyapatite from Ca 21 ions and PO 432 which generated throughout the reaction of PO 232 to PO 432 takes place in the vicinity of Pd metal on the surface of polymer particles. As a result, Pd 0 immobilized polymer particles are coated smoothly with hydroxyapatite. The rapid growth of hydroxyapatite is depressed by slow formation of PO 432 at low temperatures below 50°C. It is concluded that the smooth coating of hydroxyapatite from Ca 21 and PO 232 ions on Pd 0 immobilized polymer particles is achieved at low reaction temperatures.

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9. Ceska, G. W., J. Appl. Polym. Sci. 18, 427 (1974). 10. Mclune, W. F. (Ed.), “Powder Diffraction File, Inorganic Volume,” JCPDS International Center for Diffraction, Pennsylvania, 1996. Hisashi Tamai 1 Hajime Yasuda 1 Department of Applied Chemistry Faculty of Engineering Hiroshima University Kagamiyami 1-4-1 Higashi-Hiroshima, 739-8527 Japan Received August 7, 1998; accepted November 30, 1998 1

To whom correspondence should be addressed.