Modification of hydroxyapatite/gelatin composite by polyvinylalcohol

Abstract

Over the past few years biomimetic synthesis [1–3] of artificial bone materials [4] such as hydroxyapatite [HAp], carbonated hydroxyapatite and fluoroapatite have gained increasing attention and some researchers have tried to prepare the apatite composite using collagen [5, 6], denatured collagen (i.e., gelatin [GEL]) [8–11], or polymer [12] as a template for the biomimetic reaction. For decades double diffusion process [13, 14] has been widely used to study the in vitro formation of biologic apatite phase in a stationary system. Its drawback is the constantly changing environment, i.e., the depletion of the solutions of the constituting ions (Ca2+, PO3− 4 ) due to precipitation, leading to a changing supersaturation and possibly changes in the crystallization mechanism [13]. From several years ago HAp/COL nanocomoposites have been developed through the coprecipitation reaction of HAp nanocrystals in soluble collagen [5, 6] and the characteristic feature of this process is the dynamic reaction using active Ca(OH)2 precursor [5] as a free Ca2+ source instead of CaCl2 or Ca(NO3)2. Recently, we have focused on the development of HAp/GEL nanocomposites [10, 11] using the commercial GEL materials. The development of the apatite phase in GEL matrices is very complicated because the commercial GEL contains a variety of protein species and different stages of degraded products. From our experimental experiences a single morphology of apatite phase could be obtained through the vigorous stirring or the introduction of a fluoride source during the co-precipitation. The evolution of the strength in the compact body of HAp/GEL composite is governed by the morphology development of HAp particles and the chemical coordination of HAp crystals with GEL matrices. We noticed that the strength of the HAP-GEL composite is limited by the gelatin phases. The purpose of this study is to use a hydrophilic polymer, ployvinylalcohol (PVA), to modify the gelatin phase without alteration of the apatite phase. PVA is known to give a good biocompatibility and has a potential to bind with gelatin molecules because of its hydrophilic property. The characteristics of the PVA modified HAp-GEL composites will be reported. Cross-linkage between PVA and GEL will be investigated using FTIR and DTA. GA as cross-linkage agent is known to have cytotoxicity, but the limited amount of GA does not cause a significant toxic problem in animal tests [15]. The preparation details of HAp/GEL nanocomposite were previously described by Chang et al. [10]. The amount of Ca(OH)2 and H3PO4 was calculated to make 10 g of HAp and the input amount of GEL precursor was 3 g. Before the co-precipitation the Ca(OH)2 powders were vigorously stirred in deionized(DI) water at room temperature for 12 hrs and the weighed GEL powders were dissolved in the mixture solution of DI water and phosphoric acid at 37 ◦C for 12 hrs to help the homogenization of less denatured GEL coils such as worm-like chains [11]. During the entire coprecipitation process we applied vigorous stirring and the pH was controlled as 8.0 using a digital pH controller. After the reaction, the obtained slurry in the solution was aged at the same temperature for 24 hrs and the slurries were collected using a vacuum filter. As-received PVA flakes (low molecular weight PVA, Aldrich, USA) were dissolved in DI water at 60 ◦C for days and the HAp/GEL composite slurries collected by vacuum filter were vigorously mixed with the PVA solution for hours using a magnetic stirrer at 37 ◦C. After mixing 0.3 g of GA (85% Aldrich, USA) was added to the above mixed slurries under stirring and after 30 min the slurries were shaped using a glass vacuum filter (diameter 20 mm). The shaped body was dried in an incubator at 37 ◦C. The added amount of PVA was 1 and 2 g, and the sample names are coded as HG3PVA1 and HG3-PVA2, respectively. Before vacuum filtering a part of the slurry was sampled for TEM observation and microstructures were characterized by transmission electron microscopy TEM (JEM-1210, Jeol, Japan). The filtered cake was dried at 37 ◦C in the incubator and apatite phase was confirmed using X-ray Diffraction (XRD D-5005, Siemens, German) via the crushed powders. For the dried material the microstructures were characterized using scanning electron microscopy (SEM, JSM6700F, Jeol, Japan). The chemical interactions among HAp crystals, GEL macromolecules and PVA polymers were analyzed using the diffuse reflectance FTIR (Magna 750-R. Nicolet, USA). Following collection of the raw spectra, the spectral band positions were identified by using GRAMS AI 7.0 software (Thermo Galactic, Salem, USA). Thermal analysis (TG-DTA, MacScience, Japan) was also carried out on the dried samples to evaluate the bond formation among HAp crystals, GEL molecules and PVA polymer. The