Abstract:

Objective High molecular weight chitosan (HMCS) has many advantages when used in the field of wound management because of its antibacterial properties as well as the cell and tissue growth capabilities. However, fabricating HMCS nanofiber is challenging since the spinning solution's viscosity is extremely high. In order to solve this problem, solution blow spinning was studied and adjusted to fabricate HMCS nanofibers, and the spinning parameters were identified to fabricate polyethylene oxide (PEO) as shell and HMCS as core nanofibers, which were transformable to physical hydrogel when contacting the wound exudate for wound healing.

Method The spinning solutions containing 1.6% mass fraction HMCS and 2.5%-5.0% mass fraction PEO were prepared by dissolving and mixing these two species in 50% mass fraction aqueous acetic acid solutions, with 200 r/min mixer rotation rate and 10 h mixing time. The well-mixed solutions were degassed for 12 h before solution blow spinning. In the spinning process, PEO-HMCS nanofibers were spun with the parameters ranging from 0.04 MPa to 0.10 MPa air pressures and 21 cm to 33 cm collecting distances. The area of the resulting PEO-HMCS nanofibers was 2 010 cm² and the spinning duration was 45 min for each of the four spinning solutions. The temperature was kept at 24 ℃ with the relative humidity of approximately 20% during the solution blow spinning process.

Results The morphologies of the PEO-HMCS nanofibers were observed by scanning electron microscopy and field emission-scanning electron microscopy. The shapes of the nanofibers were straight lines and the fiber surfaces were not smooth, with some ripple shapes. When the PEO mass fractions in solutions increased from 2.5% to 5.0%, the mean diameters of the nanofibers increased from 133 nm to 210 nm, with the nanofibers porosities anchanged and remaining at 0.69. This study also investigated the influences of changing collecting distances on the resulting nanofibers mean diameters, as well as the influences of changing air pressures on the resulting nanofibers mean diameters. When the collecting distances increased from 21 cm to 27 cm, the PEO-HMCS nanofibers mean diameters decreased first and then increased. As the air pressures increased from 0.04 MPa to 0.05 MPa, the mean diameters increased from 637 nm to 790 nm. After further increasing air pressures to 0.07 MPa, the mean diameters dropped to 375 nm. Continuing increasing the air pressures to 0.10 MPa led to the mean diameters decreasing from 359 nm to 397 nm. The detailed nanofiber core shell structures were observed by the transmission electron microscopy. Before immersed in water, the thickness of the fiber shell was 340 nm and the thickness of the fiber core was 35 nm approximately. After immersed in water, the thickness of the fiber shell significantly decreased. When the PEO mass fraction increased from 2.5% to 5.0%, the mean diameters of the PEO-HMCS nanofibers increased from 161 nm to 211 nm, with conductivities decreasing from 1 760 μS/cm to 1 640 μS/cm and viscosities increasing from 42 082 mPa·s to 91 055 mPa·s. The dynamic viscosities of PEO 2.5% mass fraction solution dropped quickly to 0.11 Pa·s before shear rate reached 1 s^-1, and remained unchanged afterwards. The dynamic viscosities of 1.6% HMCS solution decreased slowly during shear rate sweeping from 0.1-1 000 s^-1, and the values were all higher than those of 2.5% PEO solution. For the dynamic surface tensions, higher PEO mass fractions led to lower dynamic surface tensions. Furthermore, no nitrogen element was detected on the nanofiber surfaces by the X-ray photoelectron spectroscopy. The in vivo animal experiment results showed that the PEO-HMCS nanofibers significantly promoted wound healing.

Conclusion The types as well as the mass fractions of HMCS and PEO were studied for fabricating HMCS nanofibers. The nanofibers showed unique morphological structures, mean diameters, and pore distributions. Several specialized solution blow spinning parameters, including air pressures and collecting distances, could influence the fabrication process. The solution viscosities, conductivities, and surface tensions also had an impact on the HMCS nanofibers formation. No HMCS was found on the nanofibers surfaces and only PEO existed. The resulting PEO-HMCS solution blow spinning nanofibers had core shell structures, with PEO mainly locating at the shell region and HMCS at the core region. The shell of the nanofiber was semi-flexible and the core was stiff with no flexibility. The in vivo animal experiment results showed that the PEO-HMCS core shell nanofibers could be used as the physical hydrogels to promote wound healing.

Key words: medical dressing, solution blow spinning, chitosan, polyethylene oxide, micro-nanofiber, core-shell structure