Alginate-controlled formation of nanoscale calcium carbonate and hydroxyapatite mineral phase within hydrogel networks Minli Xie a,1 , Magnus Ø. Olderøy a,1 , Jens-Petter Andreassen b , Sverre Magnus Selbach c , Berit L. Strand d , Pawel Sikorski a, * a Department of Physics, Norwegian University of Science and Technology, Trondheim NO-7491, Norway b Department of Chemical Engineering, Norwegian University of Science and Technology, Trondheim NO-7491, Norway c Department of Materials Science and Engineering, Norwegian University of Science and Technology, Trondheim NO-7491, Norway d Department of Biotechnology, Norwegian University of Science and Technology, Trondheim NO-7491, Norway article info Article history: Received 13 January 2010 Received in revised form 22 March 2010 Accepted 23 March 2010 Available online 30 March 2010 Keywords: Biocomposite Alginate Hydroxyapatite Calcium carbonate Nanostructure abstract A one-step method was used to make nanostructured composites from alginate and calcium carbonate or calcium phosphate. Nanometer-scale mineral phase was successfully formed within the gel network of alginate gel beads, and the composites were characterized. It was found that calcite was the dominating polymorph in the calcium carbonate mineralized beads, while stoichiometric hydroxyapatite was formed in the calcium phosphate mineralized beads. A combination of electron microscopy, Fourier-transform infrared spectroscopy, thermogravimetric analysis and powder X-ray diffraction showed that alginate played an active role in controlling mineral size, morphology and polymorphy. For the calcium phosphate mineralized beads, alginate was shown to modulate stoichiometric hydroxyapatite with low crystallinity at room temperature, which may have important applications in tissue engineering. The results pre- sented in this work demonstrate important aspects of alginate-controlled crystallization, which contrib- utes to the understanding of composite material design. Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. 1. Introduction In tissue engineering and regenerative medicine, there is a great need for biomaterials which can be used as structural support, as scaffolds for specific cell types or as fillers. These materials must be biocompatible, and biodegradability might be an advantage [1] in providing optimal functionality to the tissue which is to be regenerated. It is also a great advantage for such biomaterials to be lightweight. This is a good incentive to look at biomimetic fab- rication strategies, where the structure is highly controlled and optimized at several length scales. In bone-tissue engineering, the mechanical properties of the biomaterial are vital, and may be satisfied by structurally clever designs inspired by fabrication strategies found in nature. There are numerous examples of organic/inorganic composite materials in nature, where the arrangement of different phases is tightly controlled at several length scales. A good example is bone, in which collagen fibres are mineralized with hydroxyapatite (HA). The HA crystallization is controlled in such a way that the high compression strength of HA and the elasticity of collagen is com- bined to yield a material with both these properties [2]. The key to understand the structure/function relationship in bone is to study the collagen/HA interplay and structural arrangement from the nanometer to the micrometer scale. HA particle size and align- ment in the collagen fibres are vital to the mechanical properties, and a well-defined structure on higher length scales provides mac- roscopically consistent mechanical properties [3]. Recently, the nanostructure of mineralized collagen was mimicked with the aid of poly-L-aspartic acid [4], which was shown to play an impor- tant role in the mineralization process. It is important to realize that well-defined structures at the nano- meter scale in natural materials are a result of interactions between the organic and inorganic phases. For example, Metzler et al. [5] showed, by X-ray adsorption near edge spectromicroscopy, that cal- cite-interacting peptides and proteins (mollusk nacre-associated polypeptides and sea urchin spicule matrix protein) disrupt CAO bonds in calcite. Ordering of the amino acid side chains as a conse- quence of polypeptide association with the mineral phase and car- boxylate binding was also observed [5]. Metzler et al.’s paper clearly showed that carboxyl groups in aspartate and glutamate par- ticipate in polypeptide–mineral associations. Therefore, the biomi- metic fabrication approach should utilize such interactions and not depend on preformed mineral particles. A one-step method, where the inorganic phase is formed within an organic matrix to 1742-7061/$ - see front matter Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actbio.2010.03.034 * Corresponding author. Tel.: +47 73598393. E-mail address: pawel.sikorski@phys.ntnu.no (P. Sikorski). 1 These authors contributed equally to this work. Acta Biomaterialia 6 (2010) 3665–3675 Contents lists available at ScienceDirect Acta Biomaterialia journal homepage: www.elsevier.com/locate/actabiomat