forming a solid fiber. The polymer fiber is then
pulled from the coagulation bath and taken
through a number of draw stations where the
fiber is stretched. These draw stations typically
include ovens to heat the fiber during the pulling (drawing) process. However, wet spinning
uses heating as low as body temperature. In
wet spinning, the residual solvents (and non-solvents from the coagulating bath) provide the
molecular mobility required to allow the polymer chains to align and create entanglement
sites, resulting in high mechanical properties.
While the solvents aid the processing of the
fibers to allow the process to occur anywhere
from room to body temperature, exposure to
the solvents and non-solvents during extrusion
may destroy incorporated drugs or biological
agents. However, it is possible to protect the
drug from the solvent by enveloping the drug
in an emulsion or a nanoparticle, or trapping it
within a hydrogel or other types of excipients.
Prior to incorporation in medical applications,
however, the solvents must be completely
cleaned from the fibers. Several processes can
be used to effectively remove residual solvent
to levels of as low as 1/10 of the allowable
limit set by FDA guidelines without exposing
the loaded drugs or biologics to temperatures
higher than body temperature, and thus preserving their viability.
An additional and significant advantage of
wet spinning processing is the broad range of
polymers that may be processed, including
both synthetic and biopolymers. For example,
core-sheath format fibers can be made with
a carbohydrate-based hydrogel interior and a
hydrophobic synthetic (i.e., PLLA or PLGA)
sheath. An advantage of fibers produced
through wet spinning processing
that are inherently phase-sepa-
rated (whether core-sheath or
other formats) is the resulting
control over release kinetics of
the drug of interest, as well as in-
creased protection of hydrophil-
ic drugs and biological agents.
Advantages of Wet Spinning
Fibers for Implantable
Engineering techniques such as altering the
porosity of the fiber can dramatically change
the release kinetics of the fiber. For example, by
appropriate choices of solvent and non-solvent
systems, the fibers can have inherent porous
or solid internal morphology. The method of
protecting the drug also changes the release
kinetics. The shape of the release kinetics
profile can also be tailored by extruding blends
of polymers with different degradation rates.
All of these control parameters result in an
unparalleled ability to control drug delivery.
The controlled, localized drug delivery
capability offered by wet-spinning fibers enables
medical device designers to locally affect the
body’s response to the device. Depending on the
choice of drug, it is possible to mitigate unwanted responses and promote desired responses.
Since all drug interaction occurs at the surface
of the device, rather than through systemic
distribution, drugs may be administered locally
with very little exposure to the rest of the body,
as typically only cells within a few millimeters of
the device are impacted by the drug.
Beyond use in medical devices, drug-loaded
fibers provide excellent drug delivery depots
where precise placement within the body is
desired, such as within a solid tumor. In these
cases, drug-loaded fibers may deliver a range of
drugs from small pharmaceuticals to viruses for
periods up to six months.
The medical device industry has experienced
significant evolution in recent years and drug
delivery technology promises to accelerate development. Similar to the revolutionary impact
drug eluting stents had in the field of cardiovascular medicine, a wide range of medical applications stand to significantly benefit from the
incorporation of wet-spun drug-loaded fibers.
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