In biological systems, biomineralization produces hierarchical architectures where the orientation, texture, morphology and crystal size are controlled [1]. The key point lies in the fact that a surface with an organized arrangement of functional groups (± SO3H, ±OH, ±OPO2H, ±NH2, etc.) resulting from organization of organic matrix (collagen, polysaccharide, etc.) acts as the template for the nucleation and growth of minerals from a supersaturated solution [1, 2]. Four stages are involved in biomineralization: supramolecular preorganization, interfacial molecular recognition, vectorial regulation and cellular processing [1]. With these four stages as archetypes for the design of materials synthesis, a new process termed biomimetic synthesis or template synthesis was born [1]. A variety of inorganic materials have been synthesized by this new process, such as nanoparticles [3], thin ®lms [4], coatings [4], porous materials [5] and materials with complex forms [6]. The key point still lies in the design and preparation of an organized functionalized surface. In biomineralization, an organized functionalized surface results from the pre-organization of an organic matrix through a self-assembly process [1, 2]. In biomimetic synthesis, the organization of functional groups can be realized either through a self-assembly process of amphiphile surfactant (lipid, stearic acid, etc.) in liquid-state systems such as langmuir monolayers [7], microemulsions [8], liquid crystals [9] and so on, or through grafting or adsorption of functional groups or organic moleculars on solid substrates [4, 10, 11]. The former process is suitable for the biomimetic synthesis of nanoparticles, porous materials and materials with complex forms [3, 5, 6]. The latter process is suitable for the biomimetic synthesis of ®lms or coatings on solid substrates [4, 10]. However, unlike the self-assembly process of surfactants in liquidstate systems [7±9] or chemisorption of functional moleculars on a surface of single-crystal substrate [11], which can result in an organized functionalized surface, the grafting on of polymers or chemisorption on polycrystalline inorganic substrates cannot get organized surfaces where the functional groups form an nearly uniform ` lattice'' across the whole substrate surfaces. Recently, we developed a new method to obtain an organized functionalized surface on an inorganic solid substrate for biomimetic synthesis of coatings. The basic consideration can be described as follows. First, a highly textured substrate is prepared. Second, the textured substrate is chemically treated to introduce chemisorption of functional groups or moleculars and thereby covered with a nearly organized functionalized surface. Third, the functionalized substrate is put into a supersaturated solution to induce heterogeneous nucleation and growth on the substrate to form coating through biomimetic molecular recognition. In this process, the texturing of the substrate before chemisorption of functional groups or moleculars makes that a major part of crystals with same planes exposed, that is, a major part of domains with the same atomic arrangement on the surface and thus a major part of domains with the same arrangement of chemisorbed functional groups on co-ordinatively unsaturated surface atoms. Such a functionalized surface is much more organized and more suitable to act as a template for the biomimetic growth of ®lms or coatings than a functionalized surface derived from the chemisorption on a poorly textured or non-textured substrate. In this letter, we use biomimetic synthesis of biocompatible calcium phosphates coating on titanium as an example to demonstrate the above considerations. Such coating has been widely investigated recently for its biomedical application as bone implants [12]. A 2 mm thick titanium plate was prepared by rolling deformation to introduce (0 0 0 1) texture, a typical kind of rolling texture for the hexagonal metals [13], and cut into small square pieces (1 cm 3 1 cm). To remove oxide surface layer, the plates were polished to be about 1.5 mm thick, and put into 31% (in mass) solution of hydrogen peroxide (H2O2), then aged at room temperature for one day and rinsed in deionized water for 1 min. Subsequently, one plate was placed in a 50-ml supersaturated solution of hydroxyapatite (Ca10 (PO4)6(OH)2, HA)([Ca 2] 4 mM) that was prepared following the procedure described in [7] to induce coating formation. For the sake of comparison, a control titanium plate with no strong (0 0 0 1) texture was prepared by rolling using a different procedure and subjected to the same subsequent experiment as above. The X-ray diffraction (XRD) spectra of textured and control plates before immersion into HA solution are shown in Fig. 1. It was interesting to see that a white coating appeared on the textured plate after two days and grew thicker until aging for about one week. No
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Jan 17, 2002
Claudio Morterra + 2
Open Access