Page 41 - Tailoring Electrospinning Techniques for Regenerative Medicine - Marc Simonet

2.2 Introduction
Due to its e ectiveness and easiness in fabricating micro and nanofibers with unique properties and controllable structures, electrospinning is a widely used method for carrying out innovative research in various fields, as tissue engineering, drug delivery, textiles, filtration, and electronic devices.1–3
To change and notably to control the final outcome of an electrospinning process, one has to tightly control the polymer solution parameters, process conditions and ambient parameters. While solution properties and processing variables are the most investigated parameters,4–6 growing attention has recently been paid to the role of climate conditions because of their strong influence on bead formation, fiber dimensions and surface textures. Megelski et al. were the first to address pores formation on electrospun fibers to relative humidity.7 Later, many authors agreed that an increase in relative humidity during electrospinning results in an higher surface porosity,8–10 providing evidences related either to solvent volatility,11–14 polymer molecular weight and hydrophobicity,8,15 or thermodynamic phase behavior of the polymer-solvent-non solvent system.11,13,16 Additionally, environmental conditions can a ect the arrangement of macromolecular chains during solidification by changing the solvent evaporation rate, thus influencing thermal and mechanical properties of fibers.10,17,18
Alignment and 3D structuring of fibers are highly
important in tuning performances of electrospun
materials;19 but the e ect of environmental conditions 2 on fiber alignment has not been investigated yet.
Existing studies about electrospinning of oriented
fibers focus mainly on the design of the collecting
setup, such as parallel electrodes,20 rotating discs21–23
and drums,24–26 and the main variables addressed
by authors were the linear velocity of the collector
surface and the e ects on the electric field of
di erent collecting setups. Even though it has been
believed that a simple rotating drum was not the
best approach for collecting highly aligned fibers,27
results published by Sun et al. demonstrated that
electrical properties of the solvent, matched with an
optimal choice of the uptake speed, matter far more
than the collecting target design on the degree of
orientation of fibers.28
As meshes are commonly collected as flat layers of densely packed fibers, their application in specific fields is hindered. Hence, several studies have been published on porosity enhancement of electrospun materials, mostly for tissue engineering, where these three dimensional assemblies allow for a much better cell infiltration within the mesh.29,30 Among all the di erent methods for obtaining 3D porous electrospun materials,31 self-assembling is a very promising one. Bonino et al. reported cone or strands
ELECTROSPINNING POLY(Ε-CAPROLACTONE) UNDER CONTROLLED ENVIRONMENTAL CONDITIONS
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