Biomedical Engineering Reference
In-Depth Information
Cotton was and still is commonly being used for
bandages, surgical sponges, drapes, and surgical apparel,
and in surgical gowns. In current practice, cotton has
been replaced in many applications by coated nonwoven
disposable fabrics, especially in cases when nonabsorb-
ency is critical.
It is important to note that most synthetic polymers
currently used in medicine were originally developed as
commercial polymers for nonmedical applications and
usually contain additives such as dyes, delustrants, sta-
bilizers, antioxidants, and antistatic agents. Some of
these chemicals may not be desirable for medical appli-
cations, and so must be removed prior to use. To illus-
trate this point, PET, formerly Dacron, which at present
is the material of choice for most large-caliber textile
vascular grafts, was originally developed for apparel use.
A complex cleaning process is required before the ma-
terial can be used in an implant application. Additional
reading relating to this point can be found in
Goswami
et al.
(1977)
and
Piller (1973)
.
Table 3.2.4-1 Textile structures and applications (
Ko, 1990)
Application Material
Yarn
structure
Fabric
structure
Arteries
Dacron T56
Teflon
Textured
Multifilament
Weft/warp knit
Straight/
bifurcations
Woven/non-woven
Tendons
Dacron T56
Dacron T55
Kevlar
Low-twist
filament
Multifilament
Coated woven tape
Hernia
repair
Polypropylene
Monofilament
Tricot knit
Esophagus
Regenerated
collagen
Monofilament
Plain weave Knit
Patches
Dacron T56
Monofilament
Multifilament
Woven
Knit/knit velour
Sutures
Polyester
Nylon
Regenerated
collagen
Silk
Monofilament
Multifilament
Braid
Woven tapes
Synthetic fibers
Various synthetic fibers have been used to fabricate
medical devices over the past 25 years. Starting in the
1950s, various materials were evaluated for use in vas-
cular grafts, such as Vinyon (PVC copolymer), acrylic
polymers, poly(vinyl alcohol), nylon, PTFE, and poly-
ester (PET) (
King
et al.
, 1983
). Today, only PTFE and
PET are still used for vascular graft applications since
they are reasonably inert, flexible, resilient, durable, and
resistant to biological degradation. They have withstood
the test of time, whereas other materials have not proven
to be durable when used in an implant application.
Table
3.2.4-2
shows a partial list of synthetic polymers that
have been prepared as fibers, their method of fabrication,
and how they are used in the medical field.
Most synthetic fibers are formed either by a melt
spinning or a wet spinning process.
Ligaments
Polyester
Teflon
Polyethylene
Monofilament
Multifilament
Braid
Bones and
joints
Carbon in
thermoset or
thermoplastic
Matrix
Monofilament
Woven tapes
Knits/braids
consideration must be given to the device design re-
quirements and the manner in which the fiber is to be
used. For example, collagen-based implantable hemo-
static wound dressings are available in multiple configu-
rations including loose powder (Avitine), nonwoven mats
(Helistat and Surgicel Fibrillar Hemostat), and knitted
collagen fibrils (Surgicel Nu-Knit). In addition, other
materials are also available for the same purpose (e.g.,
Surgicel Absorbable Hemostat is knitted from regen-
erated cellulose). Fibers can be fabricated from non-
absorbable synthetic polymers such as PET or polyester
(e.g., Dacron) and PTFE (e.g., Teflon), or absorbable
synthetic materials such as polylactide (PLA) and poly-
glycolide (PGA) (
Hoffman, 1977
). Natural materials
(biopolymers), such as collagen or polysaccharides like
alginates, have also been used to fabricate medical de-
vices (
Keys, 1996
). And there are recent reports that
biomimetic polymers have been synthesized in experi-
mental quantities by genetic engineering of peptide se-
quences from elastin, collagen, and spider dragline silk
protein, and expressed in
Escherichia coli
and yeast using
plasmid vectors (
Huang, 2000; Teule
et al.
, 2003
).
Melt spinning
With melt spinning the polymer resin is heated above its
melting temperature and extruded through a spinneret.
The number of holes in the spinneret defines the number
of filaments in the fiber being produced. For example,
a spinneret for a monofilament fiber contains one
hole, whereas 54 holes are required to produce the
54-multifilament yarn that is commonly used in vascular
graft construction. Once the monofilament or multifila-
ment yarn is extruded, it is then drawn and cooled prior
to being wound onto spools. The yarn can also be further
processed to form the final configuration. For example,
most yarns used for application in vascular grafts are
texturized to improve the handling characterizes of the
