(201a) Triaxial Electrospun Fibers: Fabrication and Characterization

Abdurizzzgh Khalf, Sundar Madihally Chemical Eng, Oklahoma State

University, Stillwater, OK, United States.

Triaxial electrospinning provides an effective method for fabrication of desirable biocomposite for particular applications. Modified process (based on core-sheath fluid flow where the enforced sheath polymer encapsulates the core liquid) can enable different features such as hydrophilicity, hydrophobicity, and mechanical strength.  Selection of solvents with appropriate boiling points for each layer is critical as rapid solvent evaporation from the jet surfaces could cause instabilities in fiber formation.  However, the effect of solvent volatility and relative polymer molecular weights on uniform encapsulation of the core polymer in triaxial electrospinning process is not well understood.  In this study, we investigated the effect of solvent volatilities and MW on uniform fiber formation.  Formed fibers were characterized for distribution of component via hydrophilicity, mechanical, rheology and biology properties.  We explored combinations of polycaprolactone (PCL), cellulose acetate (CA), and polyvinyl alcohol (PVA) and mineral oil (as the inner core) of various molecular weights.  Hollow fibers were developed by selectively removing the mineral oil.  Different solvent mixtures were tested based on their boiling points, determined using a process simulator ChemCAD.  Solution viscosities were evaluated at various shear rates.  Obtained fibers were analyzed by scanning electron microscopy and fiber sizes were also characterized using digital micrographs.  Differential scanning calorimetry (DSC) and FTIR were performed to characterize various components in the triaxial fibers.  Tensile tests (both wet and dry) were also measured to assess the uniform distribution of polymers.  24-h viability of human umbilical vein endothelial cells was also evaluated.  The hydrophilicity of electrospun fiber was also measured by contact angle.  Micrographs indicated the formation of triaxial structured fiber of outer hydrophobic PCL/CA/hollow, PCL/PVA/hollow and outer hydrophilic CA/PCL/hollow fibers.  Fiber sizes were uniform and ranged in micrometers size in all the formed configurations.  While switching the configuration of polymers, rapid solvent evaporation (volatility) of the inner core caused phase separation, leading to failure in obtaining fibers. Solvents mixture (chloroform and methanol) of outer sheath solution PCL with low boiling point of 54 C^o caused the polymer molecules to evaporate faster from the charged jet surface and diffuse easily from the inside of the charged jets to the outer surface, which significantly enhanced the outer sheath solution wrapping ability and gave the jet the capability to maintain the core-sheath jet fabrication. However, fibers did not form using chloroform and methanol as solvent mixture for intermediate polymer (PCL).   The instability region (stretching and elongation) of the core-sheath liquid jets occurred during electrospinning process that prohibited the formation of the fibers was due to the interactions between the viscoelastic forces and surface tension that governor the solution evaporation rate.  The high volatility of the intermediate solution (PCL) weakens the outer sheath wrapping ability and results wet fibers formation.  It was required to regulate its solvent mixture boiling point by adding acetic acid to moderate the spinning rate of PCL by lowering the solvent evaporation rate.  This significantly strengthened the sheath jet stability allowing full control of CA polymer to encapsulate the intermediate polymer (PCL) and raising the wrapping ability of CA spinning solution.  80 kDa and 45 kDa PCL solutions molecular weight were tested while keeping all other parameters constant.  Solution of 45 kDa MW showed better electrospinning performance compared to 80 kDa solution.  At high molecular weight (80 kDa), the strength of sheath solution increased eventually making it tough for electrical force to drug and enforce the sheath polymer to encapsulate the core liquid.  The jet appeared to be broken along the length of spinning distance indicating an inability of sheath polymer to wrap the core polymer.  Moreover, better electrospinning performance was obtained when the inner core MW was smaller or equal to that of the outer sheath.  Stripping outer CA in CA/PCL/hollow fibers confirmed the distribution of inner PCL.  Endothelial Cells attached and spread on these fibers suggesting no toxicity from the solvents.  Tensile tests (both wet and dry) indicated that CA hollow fibers with the inner PCL showed significant improvement in load carrying capacity and stiffness in hydrated conditions. DSC thermographs showed presence of both PCL and CA components. The rheology results suggest that the fibers could be formed when the viscosity of the core is less than that of sheath. Fibers produced from CA/PCL/hollow and PCL/CA/hollow had an average diameter of 11.6 (±3.9) and 9.5 (±1.5 µm) respectively. In summary, the triaxial structured fiber fabricated in this work offers new features, such as hydrophobicity, hydrophobicity and mechanical strength. Enhance the structural morphology by increasing the fiber thickness (breaking the limitation in fiber size produced by traditional electrospinning process).  Upon formation of the fiber, outer sheath can be extracted, exposing the desired inner sheath the feature serve polymers that cannot electrospun in their own. Solvents properties study showed that outer sheath should have a boiling point less than that in the inner cores.  Better control in the structure and wall thickness of formed fibers could be obtained when the inner core MW is smaller or comparable to that of the outer sheath.