Surface-engineered bacterial cellulose as template for crystallization of calcium phosphate

Abstract

Bacterial cellulose (BC), produced by Acetobacter xylinum, and cotton linters as reference were surface modified by ozone-induced graft polymerization of acrylic acid and used as a template for crystallization of calcium phosphate. The grafting was verified using attenuated total reflection-infrared radiation (ATR-IR) and electron spectroscopy for chemical analysis (ESCA). ATR-IR revealed an additional absorption band at 1700 cm−1, corresponding to the carbonyl group in polyacrylic acid. ESCA figures show, apart from the characteristic peaks for cellulose, additional peaks at 285 eV and 289 eV that correspond to groups in acrylic acid. The grafting yield is higher on cotton linters compared with BC, which most likely has to do with differences in crystallinity and reactivity of the different cellulose materials. No morphology difference directly caused by grafting could be seen with scanning electron microscopy (SEM), which might indicate that acrylic acid was grafted as a thin film on the surface of the cellulose micro fibrils. Calcium phosphate was formed on the surface-modified cellulose by first pre-soaking the materials in a saturated Ca(OH)2 and later in simulated body fluid (SBF). The atomic ratio of calcium phosphate was determined by ESCA to be about 1.5 for the different materials. Energy dispersive spectroscopy (EDS) was used to map and verify that the crystals were calcium phosphate. Secondary ion mass spectroscopy (SIMS) was also used to verify the presence of calcium phosphate complex onto BC. SEM images showed the difference in dimension, distribution and morphology of the crystals depending on the materials. Smaller and a greater number of crystals were obtained on the surface-modified BC and larger and fewer crystals on surface-modified cotton linters. Structural and grafting differences between the celluloses may lead to differences in nucleation sites and possibly differences in the morphology of the Ca-P crystals. The BC–calcium phosphate composite is expected to be useful as a scaffold for bone tissue regeneration.