Core-Sheath Differentially Biodegradable Nanofiber Structures for Tissue Engineering

In recent years, it has been shown that the nanofiber structures prepared using electrospinning can serve as near ideal substrates for engineering tissues. Various biodegradable polymers of natural and synthetic origins have been used to construct the nanofiber scaffolds. The use of natural polymers is important in that they contain specific cell recognition sites that are capable of binding cells. Synthetic biodegradable polymers, on the other hand, can provide the necessary mechanical properties and their degradation rate can be controlled positively. When used alone, however, neither can provide an ideal structure for long-term development of tissues. This is because the regenerated natural polymers, although greatly biocompatible, are weak and degrade rapidly and uncontrollably, while the synthetic polymers, although mechanically more stable, are not as biocompatible. The focus of the current investigation was, therefore, to combine natural and synthetic polymers and to produce materials that have novel hybrid properties at the nano level. An optimum structure proposed was a differentially biodegradable bicomponent nanofiber with the sheath of natural and the core of synthetic polymers. Co-axial electrospinning was used to prepare the proposed core-sheath nanofibers. A major objective of the current research was to develop and optimize the technology to produce uniform bicomponent nanofibers of predictable morphologies by understanding the effects of various material and process variables such as solution concentration, solvent type, solution flow rate, and applied voltage. Two natural polymers (collagen and gelatin) and one synthetic biodegradable polymer (PCL) were used to develop the proposed structures. The factors that affected the bicomponent fiber formation were: interfacial tension between sheath and core solutions, volatility of the solvent, and applied voltage. By minimizing the interfacial tension, selecting the solvents with low vapor pressure, and adjusting the voltage to a value lying within a particularly narrow range, uniform bicomponent fibers were obtained. Other factors such as polymer concentrations and flow rates were shown to directly affect the dimensions of the sheath and the core. It was hypothesized that the bicomponent structures produced would be differentially biodegradable in that the natural polymer sheath would degrade relatively faster, after initiating the cell activity, and the synthetic polymer core would degrade at a much slower rate and continue to support the cell growth over the required longer period of time. Collagen-PCL sheath-core bicomponent structure was visualized as the most suitable combination for tissue engineering applications and hence, the degradation behavior of this particular structure was investigated. The results showed that for the observed thirty day period of degradation, only collagen from the sheath degraded. The degradation rate of the polymer was highest within the first hour and decreased continuously with time. The degradation within the first hour was high enough to cause the polymer to lose about 50% of the weight. Degradation in PCL was too small to be effectively measured. The degradation behavior of collagen was modeled using a mathematical approach. The model suggested that the mass loss of the polymer due to degradation was proportional to the natural logarithm of time.