ELSEVIER MaterialsScienceand EngineeringC 4 ( 1996) 133-137 Growth of oxalate crystals on films of acrylate polymers ’ J. Burdon, M. Oner, P. Calvert Arizona Materials Labs., University zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHG of Arizona, Tucson, AZ, USA Abstract Cast films of a series of acr,ylic copolymers and homopolymers were immersed in a crystallizing solutions of calcium oxalate which also contained a growth inhibitor. IHydroxylated polyacrylates, but not carboxylated polyacrylates, were found to form films of calcium oxalate. Microscopic observations of the time sequence for film formation suggested that crystals are nucleated inside the polymer film and then grow up through the surface. Keywords: Oxalate crystals; Acryl ate polymers 1. Introduction Biological mineralization demonstrates the possibility of growing inorganic minerals locally on or in polymer sub- strates [ 11. This ability could usefully be transferred to the formation of patterned films of active inorganic compounds for application as devices, such as chemical, stress or light sensors. Some synthetic ex,amples of such patterned growth of minerals from aqueous solutions have been developed recently [ 21. The prevailing belief is that the growth of inorganic crys- tals on polymer substrates will be determined by epitaxial nucleation, where there is a close match of two-dimensional spacing between ions on the crystal surface and charged groups on the polymer. Given the disordered nature of most polymer surfaces, it is difficult to see how true epitaxy can occur. Mann and co-workers have emphasized the impor- tance of a match in local geometry between binding groups on the polymer and ions in the crystal surface [ 31. A number of other mechanisms ,may play a role in con- trolling the site of mineralization [4]. These include local generation of precipitating ions, adsorption of nuclei from solution onto the polymer surface, exclusion of growth inhib- itors from the surface region and nucleation within a swollen polymer due to reduced surface energies. ’ Paper presentedat SymposiumS: Biomolecular aad Biomimetic Mate- rials MRS Fall Meeting, Boston, IJSA, November 2%December 2,1994. This work relates to Department of Navy Grant NOO014-95-1-0131 issued by the Office of Navel Research. The United States Government has a royalty-free license throughout the world in an copyrightable material con- tained herein. 092%4931/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSDIO928-4931(95)00135-X Even if surface nucleation effects are important, they are not the only component of the problem. Nucleation and growth in the solution must be sufficiently slow so that the surface process dominates. This usually requires low super- saturations and low growth rates, at which point diffusion of ions to the surface can be limiting and mixing processes are important. This paper describes the growth of calcium oxalate crystals on a range of acrylate polymers and copolymers. The min- eralizing solutions were adjusted to give slow growth rates and had polyacrylic acid in solution as a growth inhibitor. The volume of the polymer was very small to avoid producing any significant change in the solution equilibrium. This study builds on previous work on inhibition of crystal growth of calcium oxalates by soluble acrylic polymers [ 51. An earlier paper also reported calcium oxalate mineralization of cross- linked gels of copolymer (butylmethacrylate-methacrylic acid) [6]. 2. Experimental 2.1. Polymer synthesis and$lm preparation Copolymers and homopolymers were synthesized in 100 ml 3-neck round-bottomed flasks. Reactions were performed in cyclohexanone at 65 “C with 1 ml of stock azoisobutyron- itrile (AIBN) initiator solution (0.5 g AIBN in 20 ml cycloh- exanone) . Total monomer volume was 10 ml. The reaction was stirred for 24 h. Polymer films were formed by casting thin films on microscope slides, air drying for 4 h, followed by vacuum-oven dry at 55 “C for 24 h.