* Corresponding author. Tel.: #1-814-865-5352; fax: #1-814-863- 7040. E-mail address: etx@psu.edu (P.W. Brown). Biomaterials 22 (2001) 807}816 Chemically formed HAp-Ca poly(vinyl phosphonate) composites Y.E. Greish, P.W. Brown* Intercollege Materials Research Laboratory, Pennsylvania State University, Sciences and Engineering, Room 136, University Park, PA 16802, USA Received 2 July 1999; received in revised form 23 June 2000; accepted 26 July 2000 Abstract The formation of biocompatible organic}inorganic composites by reactions between tetracalcium phosphate (Ca  (PO  )  O, TetCP) and the biomedical polymer poly(vinyl phosphonic acid) (PVPA) is described. Composites were prepared by hot pressing mixtures of these powders at 80 kpsi and 3003C for 30 min. Composite formation was investigated depending on the proportions of reactants and the processing route used. Two inorganic phases were produced as a result of the acid}base reaction between TetCP and PVPA: hydroxyapatite (Ca  (PO  )  (OH)  , HAp) and anhydrous dicalcium phosphate (CaHPO  , DCPA). The later phase preferen- tially formed at lower TetCP/PVPA ratios while the amount of HAp increased with increasing TetCP/PVPA ratio. The reactions appear to start with the softening of the polymer when heated to ¹'¹  . The #owing polymer surrounds the TetCP grains permitting the TetCP to initially form DCPA crystallites in a matrix of the Ca salt of the polymer. When H  O is added prior to pressing, the DCPA produced reacts with the remaining TetCP forming HAp.  2001 Elsevier Science Ltd. All rights reserved. Keywords: Tetracalcium phosphate; Hydroxyapatite; Poly(vinyl phosphonic acid); Biomaterials; Hot pressing 1. Introduction The formation of synthetic analogues of bones and teeth is a continuing objective. Bones and teeth are microstructurally and compositionally complex, contain- ing both organic and inorganic constituents. Common to these hard tissues are hydroxyapatite and collagen. Ac- cordingly, the selection of hydroxyapatite as the mineral component of a synthetic composite is desirable. The biocompatibility of synthetic apatite is well documented, making it an attractive candidate as a biomaterial. Be- cause hard tissues are composites, they exhibit physical properties, which cannot be realized by a mineral con- stituent alone. Therefore, the polymeric constituent is required. Composites developed for dental applications involve the reaction of acidic polyelectrolytes with basic oxides, as ZnO, or with basic glasses capable of undergo- ing hydrolysis [1]. However, neither of the constituents in these composites is biologically familiar. Thus, it was an objective of this study to explore the formation of composites containing hydroxyapatite. It has been demonstrated hydroxyapatite (Ca  (PO  )  (OH)  , or HAp) can be formed by an acid}base reaction between tetracalcium phosphate (Ca  (PO  )  O, or TetCP) and an acidic calcium phosphate such as monocalcium phosphate monohydrate (Ca(H  PO  )  ) H  O, or MCPM), anhydrous dicalcium phosphate (CaHPO  , or DCPA), or dicalcium phosphate dihydrate (CaHPO  ) 2H  O, or DCPD) [2]. For example, the reac- tion between DCPA and TetCP is given by Eq. (1) [3] 2 CaHPO  #2 Ca  (PO  )  O   } P Ca  (PO  )  (OH)  . (1) The hydrolysis of TetCP (Ca/P"2) to HAp (Ca/P"1.67) in the presence of acidic polyelectrolytes, such as poly(acrylic acid), has been also demonstrated [4]. If su$cient calcium ions react with the carboxyl groups on these polyacids form salt bridges [4]. The Ca/P ratio in the mineral phase produced is 1.67. Among the polyelectrolytes studied are poly(acrylic acid) (PAA) [5] and its copolymer with itaconic acid [6]. Based on observations that the cement bonds both chemically and mechanically to bone [7,8], it has been shown that mix- tures of TetCP and PAA can be used as bone cements with no adverse tissue reaction. Therefore, formation of 0142-9612/01/$ - see front matter  2001 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 6 1 2 ( 0 0 ) 0 0 2 4 3 - X