The preparation of core-shell fibers was of particular interest for those core materials that would not form fibers via electrospinning by themselves

and co-electrospinning could also facilitate manufacturing of polymer nanotubes.Furtheremore

coaxial electrospinning would provide an alternative and simple means to encapsulate drugs to deliver a biomolecular drug in a sustained fashion.In this study

core-shell structured chitosan (CS)/poly(vinyl alcohol)(PVA)-poly(propylene carbonate) (PPC) fibers were prepared by coaxial electrospinning.The coaxial electrospinning setup in this study was a compound spinneret

consisting of two concentrically arranged capillaries that were flatted stainless steel needles.A certain amount of the two polymer solutions were contained separately in two plastic syringes connected to the coaxial spinneret

and the flow rate in each capillary was adjusted in a double-way medical syringe pump.The liquid flowing on the outside was PPC solution

and the inner liquid was CS/PVA solution.First

the effects of mixed solvents on electro-spinnability of PPC solutions were investigated to choose an appropriate solution system using a conventional electrospinning setup.The properties of PPC solutions in chloroform/dimethylformamide (DMF) mixed solvents with different volume ratios were tested

and the solution conductivity had a tendency to increase ranged from 0.02 to 13.43 μs·cm-1 when DMF proportion had increased.The morphology and diameter of fibers were determined with scanning electron microscopy (SEM).The results indicated that the use of chloroform/DMF (1/1) mixed solvents could avoid agglomeration of polymer at the capillary tip during coaxial electrospinning process

and contributed to good morphology and most narrow fiber distribution of the fibers (300～600 nm). Secondly

the morphology and structure of the core-shell fibers were characterized by SEM and transmission electron microscopy (TEM).In addition

hydroxyapatite (HAp) was successfully encapsulated into the core of composite fibers

which was characterized by fourier transform infrared spectroscopy (FTIR).The SEM and TEM observation indicated that the total diameter of the CS/PVA(core)-PPC(shell) fibers amounted to about 300 nm

whereas the diameter of the core region was about 60 nm.The core-shell structure could clearly been seen with a sharp interface between the core and shell fiber.In some cases

however

the interface was perturbed

due to an onset of instability.The diameter range of core-shell structured fibers was broader than general single-fluid electrospun fibers

which attributed to the flow instability of the inner dope and the bending instability during the electrospinning process.FTIR results showed the appearance of a peak at 1038 cm-1 due to the PO3-4 wagging

which confirmed the presence of HAp in the final fibers.The present study would provide a basis for further design and optimization of processing conditions to control the structure of core-shell composite fibers

and ultimately obtain novel functional biomimetic fibers applied in drug release and tissue engineering.