certain sponges. [4] In this respect the most important minerals are probably those related to calcium ions (HAp) and silicon ions (amorphous silica). Although many other biominerals are known, such as calcium carbonate and magnetite (iron), these are not encountered with collagen in biological systems. Moreover, even though there is no record of collagen-based calcium carbonate skeletons, the first successful experiments for intrafibrillar mineralization of collagen were achieved for this mineral. [5] The mechanism of formation of calcium carbonate inside collagen, however, may have illuminated that of HAp and silica. The formation of minerals, whether crystalline or amorphous, in or around a collagen template is generally termed collagen mineralization (a misnomer as another compound crystallizes within collagen).The reasoning behind collagen mineralization in vitro is two-fold: 1) understanding the processes behind bone mineralization and 2) mimicking bone tissue via synthetic procedures. Although these are two very different aspects, these often go hand-in-hand. Hence, numerous studies, both in living and synthetic systems, were performed to investigate the processes involved in collagen mineralization. [6] In this paper, we deal with the mineralization of collagen with minerals of calcium and silicon and, to a much lesser extent, other minerals, like yttria-stabilized zirconia and combinations of silica and HAp. In Section 2, we first provide a detailed summary of the molecular structure of collagen type I, the collagen type of interest for mineralization purposes. We provide an overview of collagen-based hybrid materials in nature in Section 3. In Section 4, we first cover the in vitro efforts toward mineralization with calcium carbonate and calcium phosphate (Section 4.1), followed by in vitro silicification (Section 4.2). This section is concluded with mineralization with multiphase minerals and other minerals (Section 4.3). In Section 5, the work reviewed here is placed into perspective. We provide some comments on the mechanisms involved in collagen mineralization and end this section with some concluding remarks and prospects for further research.
Collagen Type I
General Introduction about CollagenCollagen is a ubiquitous protein in animals. Only among the vertebrates, already 28 different types of collagen can be found. [7] All collagen-containing supramolecular structures in an organism are formed by different types of collagen and Collagen mineralization is a biological process in many skeletal elements in the animal kingdom. Examples of these collagen-based skeletons are the siliceous spicules of glass sponges or the intrafibrillar hydroxyapatite platelets in vertebrates. The mineralization of collagen in vitro has gained interest for two reasons: understanding the processes behind bone formation and the synthesis of scaffolds for tissue engineering. In this paper, the efforts toward collagen mineralization in vitro are reviewed. First, general introduction toward collagen type I, the main compone...
