Page 80 - Tailoring Electrospinning Techniques for Regenerative Medicine - Marc Simonet

CHAPTER 4
Electrospun fibers were collected over a maximum period of 60 minutes onto the rotating cylindrical, hollow collection drum of 90 mm diameter and a wall thickness of 5 mm. The drum’s length was varied from 30, 50 to 100 mm in order to change the ratio between fiber and ice crystal deposition. As the only di erence to conventional electrospinning, for the LTE process the drum was loaded with dry ice (frozen CO2, technical grade, average temperature -70 °C; BOC, London, United Kingdom). During all experiments, the relative humidity (measurement accuracy: ~2.5 %) and air temperature were recorded with a Testo 625 thermohygrometer (Testo AG, Mönchaltorf, Switzerland). The air temperature was kept above 21 °C to allow the fibers to dry in between electrospinning needle and target. The relative humidity in the surroundings of the collector drum was kept above 40 % by placing a hot water bath into the electrospinning setup. A er spinning, the electrospun sca olds were dried overnight under vacuum at room temperature.
4.3.3 Materials characterization
Fiber morphology
The electrospun sca olds were characterized using scanning electron microscopy (SEM; Inspect F, FEI, Eindhoven, The Netherlands) at a voltage of 3 kV. For this purpose, the sca olds were coated with a 5 nm thin gold layer using a CED 030 (Balzers, Elland, United Kingdom). The fiber diameters of the resulting
sca olds were determined from scanning electron micrographs, measuring a minimum of 30 fibers.
3D Sca old morphology
The three-dimensional geometry of the electrospun sca olds was analyzed using a nanofocus computed tomography (CT) system (Nanotom®, Phoenix|X- ray, GE Sensing & Inspection Technologies GmbH, Wunstorf, Germany), operated at 50 kV and 450 μA allowing a 3.75 μm resolution. Due to the equipment resolution of 3.75 μm, we were limited to analyze only the sca olds with ≥9 μm fiber diameter.
The spatial fiber distances (SFD) on these 3D CT scans were calculated using the distance transform method implemented within the Scanco Medical MicroCT80 so ware (Scanco Medical, Bassersdorf, Switzerland).38 The term spatial inter fiber distance is used here rather than the o en used pore size, because the microstructure of the electrospun sca old does not well resemble that of a material with spherical pores.
Porosity (ε) of the sca olds was determined according to Equation:
(1)
with ρ0 the bulk polymer density and ρ the density of the electrospun polymer sca olds. The latter was determined gravimetrically from the weight and thickness of sca old samples over a defined area.
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