Page 669 - Carrahers_Polymer_Chemistry,_Eighth_Edition

632 Carraher’s Polymer Chemistry

Polymers are also used as sutures. Fighters and other athletics have used poly(alpha-cyanoacry-
lates), super glues, to quickly stop blood fl ow in surface cuts. Today, super glue is also used for, in
place of, or along with more traditional polymeric suture threads for selected surface wounds, inter-
nal surgery, and in retinal and corneal surgery. The alpha-cyanoacrylates monomers (19.31) undergo
anionic polymerization in the presence of water forming polycyanoacrylates. More about sutures is
detailed in Section 19.4.

N
N
R 1
HO + HC R 1
2
2
O O (19.31)
O O
R R
Poly(alpha-cyanoarylate)

Siloxanes are the most extensively used synthetic biomaterial. They are used for a number of rea-
sons, including flexibility, chemical and biological inertness, low capacity to bring about blood clot-

ting, overall low degree of biological toxicity, and good stability within biological environments.

Artificial skin had been made from a bilayer fabricated from a cross-linked mixture of bovine
hide, collagen, and chondroitin-b-sulfate derived from shark cartilage with a thin top layer of silox-
ane. The siloxane layer acts as a moisture- and oxygen-permeable support and to protect the lower
layer from the “outer world” allowing skin formation to occur in conjunction with the lower layer.
Poly(amino acid) films have also been used as an “artificial” skin. Research continues in search of a

skin that can be effectively used to cover extensive wounds and for burn patients.
Elastomeric siloxanes have also been used in encapsulating drugs, implant devices, and in maxillofa-
cial applications to replace facial portions lost through surgery or trauma. Transcutaneous nerve simula-
tors are made from “conductive” siloxanes. These are employed in the treatment of chronic severe pain
through application of low-level voltage to the nerves disrupting transmission of pain impulses to the
brain. Siloxanes are also used in extracorporeal blood oxygenation employed in the heart–lung assist
machine that is routinely used in open heart surgery. The “heart” of the apparatus is the membrane that
must allow ready transport of oxygen and carbon dioxide yet retain moisture and the blood cells. The
siloxane membranes can be made using a PE or PTFE screen in an organic dispersion of silicon rubber.
When dried, thin films are obtained that are used in the heart–lung assist device.

Siloxane-containing devices have been also used as contact lenses, tracheostomy vents, tracheal
stents, antireflux cuffs, extracorporeal dialysis, ureteral stents, tibial cups, synovial fluids, toe joints,

testes penile prosthesis, gluteal pads, hip implants, pacemakers, intraaortic balloon pumps, heart

valves, eustachian tubes, wrist joints, ear frames, finger joints, and in construction of brain mem-
branes. Almost all of the siloxane polymers are based on various polydimethylsiloxanes.
The kidney removes waste material from our blood. Because of partial or total kidney failure,
many persons are on hematolysis. The first hemadialysis units were large and by today’s standards

not very effective and the semipermeable tubes, made mainly of cellophane, had very limited life-
times. Initially, dialysis treatment was expected to be for only terminal patients but as the life expec-
tancy of dialysis patients increased as did the demand for dialysis, smaller, more effi cient dialysis
machines have emerged. At the heart of these advances is the fi ltering material. Today bundles of

microhollow fibers are used in the construction of hemadialysis cells. The fibers are “heparinized”
to discourage blood clotting. The fibers are mainly made of polyacrylonitrile. Polycarbonate, cellu-

lose acetate, and rayon fibers are also being used.

The most widely used hip joint replacement is largely a polished cobalt–chromium alloy that
moves against a specially designed ultrahigh molecular weight polyethylene (UHMWPE). This

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