Formation of core-shell structured calcium silicate fiber via sol-gel electrospinning and controlled calcination

Abstract

Ceramic nanofibers prepared via precursor sol-gel electrospinning and calcination are widely reported, but few reports address the possible formation of core-shell structured ceramic nanofibers. In this study, core-shell structured calcium silicate (CS) nanofibers were obtained by changing the compositions of the precursor sol-gel solutions and the calcination temperatures to determine the mechanism leading to the core-shell structure during preparation. The sol-gel solutions used to produce CS nanofibers were prepared from calcium nitrate and tetraethyl orthosilicate, and it was found that CS nanofibers prepared from sol-gel solutions with a Ca/Si molar ratio of less than 1:1 displayed a distinguishable core-shell structure with calcination temperatures higher than 1000 °C. The fiber shell was identified as mainly silica with sparse calcium, and the fiber core was composed predominantly of calcium silicate. A tubular morphology was clearly observed with preferential degradation of the fiber core. The fiber shell became thinner or it disappeared if the precursor sol-gel solutions had Ca/Si molar ratios higher than 1:1 or if the applied calcination temperature was less than 1000 °C. After doping the CS fibers with zinc, it was confirmed that the zinc was also predominantly located in the fiber core, similar to calcium. It was proposed that the formation of CS nanofibers with a surface silica layer resulted from the migration of Si-O-Si precursor gel particles toward the fiber surface with solvent evaporation during electrospinning and transformed into a silica shell during calcination. Thus, the calcium (or zinc) elements were enriched inside the fiber to obtain a type of heterogeneous CS nanofiber. These materials are expected to serve as good biomaterials for bone regeneration by providing sustained release of bioactive ions.