Page 77 - Tailoring Electrospinning Techniques for Regenerative Medicine - Marc Simonet

4.2 Introduction
Electrospun polymer meshes have received significant attention in the last decade due to, e.g. the versatility they o er in the design of sca old architectures. Key areas of interest for their use include regenerative medicine,1,2 wound dressing,3 and drug delivery systems.4 For these applications, gaining control of various sca old characteristics, most prominently the fiber geometry, is essential. This need for control of sca old properties has led to significant e orts towards improving current electrospinning methods. Studies have focused on elucidating the influence of certain processing parameters on sca old properties, such as fiber diameter and inter-fiber spacing (i.e., the distance between two adjacent fibers in one plane, o en referred to as pore size). Currently, porosity can be controlled in-plane by changing, for example, the nature of solvent from which the sca olds are spun, and/or the voltage applied during the electrospinning procedure.5–8 Realizing and controlling a high porosity in the third dimension has, however, proven to be more di icult. A theoretical model proposed by Eichhorn and Sampson illustrated the direct dependency of the mean pore size on the fiber diameter.9 This model implies that conventional electrospinning allows fabrication of only “quasi- 3D” architectures, based on densely packed stacks of two-dimensional fibrous layers. Each layer in
itself has large pores, but due to the tight assembly they cannot continue throughout the cross section of the sca old.10–12 As an example, spinning a sca old with fibers of 100 nm will result in a mean pore radius of ~10 nm and less.9 So fabricating truly three-dimensional porous structures, seems to be more challenging, at least when using conventional electrospinning methods.
Gaining control of sca old porosity in three
dimensions is highly desirable for tissue engineering 4 applications. Previous studies demonstrated that conventionally electrospun sca olds may not allow
for su icient cellular ingrowth, resulting in a non-
uniform tissue formation.13,14 Therefore, porosity
control in all three dimensions represents a critical
step towards fabrication of more versatile tissue
engineering sca olds. In addition, pore sizes should
ideally be adjustable from 5 to 500 μm depending on
the cell type that is used, since each cell type requires
a specific pore size for optimal cell attachment, proliferation, and migration.15,16
There have been several studies dedicated towards adjusting void spaces in electrospun sca olds based on spinning bimodal fibers,17,18 simultaneous or sequential spinning of di erent materials followed by a selectively dissolving step11,19 or application of the so-called salt leaching method.20 Disadvantages of these processes are that they
TAILORING THE VOID SPACE AND MECHANICAL PROPERTIES IN ELECTROSPUN SCAFFOLDS
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