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Layer-by-layer assembly of novel bone-mimetic hydroxyapatite-fibrous clay-biopolymer hybrid membranes

Final Report Summary - CLAY BIOMIMETICS (Layer-by-layer assembly of novel bone-mimetic hydroxyapatite-fibrous clay-biopolymer hybrid membranes)

Bone repair and regeneration is considered an important area of research concerning human health. This project targets at fabrication of novel biomimetic hydroxyapatite (HAP)-fibrous claybiopolymer hybrid membranes by layer-by-layer assembly (LBL) for potential uses in this field. To achieve the project goal, we have carried out a series of experiments and obtained the following main results.
First, we chose natural fibrous nanoclay, namely sepiolite, as the template for synthesis of hydroxyapatite (HAP) nanocrystals using a chemical co-precipitation method, due to its porous structure and negative surface charge. The dimensions and morphology of HAP nanocrystals grown on the fibre surfaces can be tailored by adjusting the surface chemistry of sepiolite and synthesis conditions such as the pH value of the solution, temperature and aging time. It was found that carbonated HAP nanorods were successfully grown on the sepiolite surface with a preferred orientation to the c-axis. Strong acid-activation increased the specific surface area of the sepiolite by 205% and also transformed the sepiolite to silica fibers with an elastic modulus being 395% of the original value. The novel HAP/acid-activated sepiolite biocomposite has a specific surface area of 182 m2 /g and an elastic modulus of over 20 GPa, considerably higher than those of the HAP synthesized without sepiolite. Such hierarchically assembled HAP/sepiolite biocomposites with the controlled size, improved modulus and similar biological functions to HAP are promising in tissue engineering and biological load-bearing devices. The research results are published in Nanoscale (2011, 3, 693-700) (Objective 1).

Next, we fabricated sepiolite-chitosan hybrid membranes by a layer-by-layer assembly method. Due to the negative charged surface of sepiolite and positive charged properties of chitosan, the sepiolite nanofibers and chitosan macromolecular chains acting as polyelectrolytes were alternatively adsorbed onto a glass substrate layer by layer during a dip-coating process. As shown in Figure 1a, the sepiolite nanofibers were aligned to one direction and a novel hybrid membrane was formed on the glass slide. But, such membrane was too thin and fragile to be peeled off for property assessment, which limits its further application as tissue scaffolds. As a feasible and versatile technique, electrospinning can produce two or three dimensional nanofibrous scaffolds, demonstrating its high potential in tissue engineering. To solve the problem associated with LBL, we set up an electrospinning facility to fabricate polymer or nanofiller-reinforced polymer nanofibers, and obtained nanofibrous membranes with randomly dispersed or unidirectionally oriented fibres (Figure 1b), depending on the design and type of collectors, the electrospinning conditions and the materials in use (Objective 2).

In order to improve the mechanical properties of the polymer nanofibers, we have compared the reinforcement effects of sepiolite and graphene oxide (GO) on a biopolymer, i.e. gelatin. Under the same processing and testing conditions, sepiolite exhibited lower reinforcing efficiency than GO due to its lower modulus (ca. 10 GPa versus ~217 GPa), aspect ratio (ca.20 versus ~1000) and specific surface area (148 m2 /g versus ~460 m2 /g) as well as less oxygen-containing functional groups on the surface. For example, the Young’s modulus of gelatin was improved by 10% with the addition of 1 wt% sepiolite, while it was increased by 65% in the presence of the same amount of GO. The research results of the reinforcement effects of fibrous sepiolite on cellular and non-cellular gelatin, and of GO on gelatin films have been published in J Mater Chem (2011, 21, 9103-9111) and Soft Matter (2011, 7, 6159-6166), respectively (Objectives 2 and 3).