Page 78 - Tailoring Electrospinning Techniques for Regenerative Medicine - Marc Simonet

CHAPTER 4
o en lead to heterogeneous sca old morphology and the increase in pore size is still predominately determined and limited by the selection of the used fiber diameter due to the layer-by-layer spinning methodology. Moreover, extra processing steps and additional chemicals are required to realize the desired architectures, rendering these procedures more complex. This may lead, among other things, to an increase in legislator a airs for future possible application.
The focused, low density, uncompressed nanofiber (FLUF) method21,22 and low-temperature electrospinning (LTE)23 are two fiber diameter independent techniques which do not require additional chemicals, and can drastically and homogenously increase the void space in sca olds. The principle of the FLUF method is based on a specifically designed target, featuring needles that are radially aligned to the center of a concave target dish. Electrospun fibers are randomly deposited in- between these arrays of grounded needles, hence do not deposit any longer in a layer-by-layer fashion, thus leading to higher porosities. A disadvantage of the FLUF method is that it can only be used with a specific target, limiting the method to cotton ball shaped sca olds, whereas with the LTE technique the target shape can be freely chosen. This allows the direct spinning of three-dimensional structures such as blood vessels or heart valves. In the LTE technique, polymer fibers are deposited onto growing ice
crystals that form on the cooled (-70 °C) spinning target from the surrounding humid environment. The ice crystals act as void spacers because they are intimately embedded within the polymer fibers. This results in sca olds with large void space and a porosity that is several times higher than sca olds produced by conventional electrospinning methods. On the downside, so far LTE is reported elusive as an on/o process, with no control over the gain in void space. Nevertheless studies with LTE spun sca olds show that this process is capable to significantly reduce the problem of limited cell infiltration in electrospun sca olds.24–27
In addition to allowing cell infiltration, increasing the void space has a large and direct influence on the mechanical properties of the electrospun sca old. Electrospun sca olds of materials such as poly(lactic acids) (PLA) and poly(ε-caprolactone) (PCL) have Young’s moduli of a few MPa and higher in the low, for tissue physiological, strain range. The values found in literature for the Young’s moduli of electrospun PCL sca olds are in between 1 and 8 MPa,28 while the elastic moduli of many native tissues in their natural loading environment are in the range of kPa.29,30 Since there are many studies suggesting that the correct mechanical cues of the used sca olds are crucial for the cell functionality, viability and tissue development,31,32 using sca olds in the sti ness range of the targeted native tissues may be beneficial. Therefore, e orts should focus on
76