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    3D reconstruction of bias effects on porosity, alignment and mesoscale structure in electrospun tubular polycaprolactone

    Year: 2021

    Journal: Polymer, Volume 232, OCT 12

    Authors: Liu, Y.; Chaparro, F. J.; Gray, Z.; Gaumer, J.; Cybyk, D. B.; Ross, L.; Gosser, P.; Tian, Z.; Jia, Y.; Dull, T.; Yarin, A. L.; Lannutti, J. J.

    Organizations: Bureau of Land Management [L14AS00048]

    Keywords: Alignment; Porosity; Electrospinning; Electric field; Bending instability

    Porosity variations in tubular scaffolds are critical to reproducible, sophisticated applications of electrospun fibers in biomedicine. Established laser micrometry techniques produced -14,000 datapoints enabling thickness and porosity plots versus both the azimuthal (phi) and axial (Z) directions following cylindrical mandrel deposition. These 3D datasets could then be 'unrolled' into 'maps' revealing variations in thickness and porosity versus 0, -5, and -15 kV collector biases. As bias increases, thinner, more 'focused' depositions occur. At 0 kV bias, maximum thickness coincides with maximum porosity; at -5 kV bias, maximum thickness coincides with minimum porosity. Porosity maps show that at 0 kV, a concave-down central region of higher (-93-94%) porosity exists bounded on either side by roughly symmetric, parabolic decreases to -87-89% which corresponds to -15v%. At -5 kV, a different concave-up character occurs, showing a central porosity of -82-84% bounded by symmetric, parabolic increases in porosity to -85-86%. At -15 kV, the porosity profile shows either concave-up or linear behavior. Simultaneous decreases in net porosity versus bias (91.1%@0 kV > 83.4%@-5 kV > 80.2%@-15 kV) are sensible, but significant changes in the distribution were unexpected. Surprisingly, at 0 kV, extensive mesoscale surface roughness is evident. Optical profilometry revealed unique features -1600 x 420 mu m in size, standing -210 mu m above the surrounding surface. These shrink to only -440 x 150 mu m in size and -30 mu m higher at -5 kV bias and disappear entirely at -15 kV. Scanning electron microscopy (SEM) resolved these into novel, localized 'domains' containing tightly aligned fibers oriented parallel to the mandrel axis. Observation of 'curly' fibers in the SEMs following -5 and -15 kV indicate buckling instabilities. This agrees with prior observations of residual solvent effects: increased bias causes faster motion toward the mandrel, meaning (1) its solvent content upon arrival is higher, leading to lower viscosities less resistant to buckling/ compaction, (2) higher velocities during deposition cause both decreased porosity/denser packing and increased buckling. Unexpectedly, we also observed substantial orientation along the mandrel axis. By modifying classical bending instability models to incorporate cylindrical electric fields, simulation revealed that horizontal components in the modified electric field alter bending loop shape, causing the observed alignment. This provides a new, easily utilized tool enabling facile, efficient tuning of orientation.
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