Biomedical Engineering Reference
In-Depth Information
Solidifying
filaments
Stretching
Washing and chemical treatment
Insert
pump
Filter
Advancing
rollers
Wind up
Coagulating
bath
Spinneret
Fig. 3.2.4-2 Wet solution spinning process.
to the filament prior to wind-up. So the diameters of
conventional spun fibers fall in a range from about 10 m m
for multifilament yarns to 500 m m or thicker for mono-
filaments. To obtain finer fiber diameters it is necessary to
employ alternative spinning technologies such as the bi-
component fiber (BCF) approach (see later section enti-
tled ''Hybrid BCFs''), or an electrospinning technique.
This method of manufacturing microfibers and nanofibers
has been known since 1934 when the first patent was filed
( Formhals, 1934 ). Since then Freudenberg Inc. has used
this process for the commercial production of ultrahigh-
efficiency filters ( Groitzsch and Fahrbach, 1986 ).
Electrospinning occurs when a polymer solution or
melt is exposed to an electrostatic field by the application
of a high voltage (5-30 kV), which overcomes the surface
tension of the polymer and accelerates fine jets of the
liquid polymer towards a grounded target ( Reneker et al. ,
2000 ). As the polymer jets cool or lose solvent they are
drawn in a series of unstable loops, solidified, and col-
lected as an interconnected web of fine fibers on
a grounded rotating drum or other specially shaped target
( Fig. 3.2.4-3 ).
The fineness of the fibers produced depends on the
polymer chemistry, its solution or melt viscosity, the
strength and uniformity of the applied electric field, and
the geometry and operating conditions of the spinning
system. Fiber diameters in the range of 1 m m down to
100 nm or less have been reported.
In addition to being used to fabricate ultrathin filtra-
tion membranes, electrospinning techniques have also
been applied to the production of nonwoven mats for
wound dressings ( Martin and Cockshott, 1977 ), and
there is currently much interest in making scaffolds for
tissue engineering applications. Nonwoven scaffolds spun
from Type I collagen and synthetic polymers such as
poly( L -lactide), poly(lactide- co -glycolide), poly(vinyl
alcohol), poly(ethylene- co -vinyl acetate), PEO, poly-
urethanes, and polycarbonates have been reported
( Stitzel et al. , 2001; Matthews et al. , 2002; Kenawy
et al. , 2002; Theron et al. , 2001; Schreuder-Gibson et al. ,
2002 ). In addition genetic engineering has been used
to synthesize an elastin-biomimetic peptide polymer
based on the elastomeric peptide sequence of elastin
and expressed from recombinant plasmid pRAM1 in
Motor
Reservoir
Syringe
Nozzle
High voltage
Source
Polymer jet
Rotating and
reciprocating
drum
Motor
Schematic representation of laboratory electrospinning system
Fig. 3.2.4-3 Electrospinning system.
 
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