Manipulation of Nanoneedle and Nanosphere Apatite/Poly(Acrylic Acid) Nanocomposites Sz-Chian Liou, 1 San-Yuan Chen, 1 Dean-Mo Liu 2 1 Department of Materials Science and Engineering, National Chiao Tung University, 1001 Ta-hsueh Road, Hsinchu, Taiwan 300, Republic of China 2 ApaMatrix Technologies, Incorporated, 58-7151 Moffatt Road, Richmond, British Columbia, Canada V6Y3G9 Received 18 February 2004; revised 18 August 2004; accepted 24 August 2004 Published online 25 January 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jbm.b.30193 Abstract: Colloidal apatitic nanosphere of 2–5 nm in diameter was synthesized in the presence of poly(acrylic acid), PAA. PAA, which has long been recognized as an inhibitor in the synthesis of hydroxyapatite, is used as a structure-directing agent for the synthesis of calcium-deficient apatite (CDHA) in this study. Experimental observation suggests a critical amount of the low-molecular-weight PAA, above which morphological evolution of CDHA nanoparticles from needle to sphere was observed. This reveals that the PAA acts as an inhibitor for the growth of CDHA crystals. Further incorporation of PAA of high molecular weight formed a highly optically transparent nanocomposite, even with the nanospherical apatite loading up to 35 wt %, suggesting no agglomeration. This was further justified through transmission electron microscopy (TEM), where the CDHA nanospheres were uniformly distributed in the PAA-CDHA nanocomposites. No interfacial crevices were visually observed, indicating a highly compatible interface between the inorganic CDHA and organic PAA phase. © 2005 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 73B: 117–122, 2005 Keywords: calcium-deficient apatite (CDHA); poly(acrylic acid); nanocomposites; nano- sphere CDHA INTRODUCTION A great advantage of using synthetic apatite is to enhance physiological affinity to the host tissues, or increase biolog- ical activity and compatible materials with less biological compatibility due to its chemical and structural similarity to the natural apatite crystals. 1 For most synthetic apatite crys- tals, the resulting Ca/P ratio is controlled at 1.67, this is called stoichiometric apatite. However, in comparison to natural apatite, which has a Ca/P ratio about 1.5, the Ca/P ratio in stoichiometric apatite is higher. The nanometric apatite crys- tals in the mineral tissues offer superior biological properties to surrounding physiological environment by giving higher metabolic activity than that of synthetic apatite. 1 Synthesis of apatite crystals have been well documented for decades through a wide variety of methods, including solid-state chemical reaction, 2–4 coprecipitation, 5,6 and sol– gel. 7 Wet chemistry is the most widely used synthetic method to produce micro- to nanocrystalline apatite particles. 8,9 Re- cently, a number of reports have addressed the in situ syn- thesis of apatite-containing nanocomposites aimed at produc- ing chemically and biologically improved synthetic bones. 10,11 The primary concept is adapted from a naturally occurring process termed biomimetic process, where the nat- ural apatite is synthesized in a physiological environment associated with the presence of natural polymer, such as collagen, and ions, which is essentially an in situ synthetic process. The prime advantage of the in situ synthesis is to produce apatite-based nanocomposites with outstanding chemical and physical homogeneity compared to those de- rived via conventional mechanical mixing. However, few reports address the occurrence of agglomeration of the nano- apatite particles in the matrix, even through the use of an in situ mrthod. 11 Such an agglomeration, although not as exten- sive as those prepared via mechanical blending, can still adversely affect final properties such as mechanical strength and moduli. 11 Although no further clarification has been made to account for the cause of agglomeration, it is possible to assume that it results from a strong interparticle attraction between the nanoparticles after in situ formation. This inves- tigation is an attempt to prepare a CDHA-based nanocom- posite with superior homogeneity by using a two-step syn- thetic process. Poly(acrylic acid), PAA, has been well documented as a suppressant for the nucleation and growth of apatite crystals Correspondence to: San-Yuan Chen (e-mail: sychen@cc.nctu.edu.tw) Contract grant sponsor: National Science Council of the Republic of China; contract grant number: NSC-92-2216-E-009-025 © 2005 Wiley Periodicals, Inc. 117