Page 351 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

328 BIOMATERIALS

properties of thermoplastic polyurethane elastomers available from CardioTech International Inc.
These values are for the Chronoflex C series of polymers.
Silicone Elastomers (polysiloxanes). Degradation: bioinert.

CH 3
Si O
CH 3
n
Poly(dimethyl siloxane)

Silicone elastomers are cross-linked derivatives of poly(dimethyl siloxane) (PDMS). Polysiloxane
liquids with functional endgroups such as OH or sidegroups such as CH=CH can be molded at room
2
temperature and cross-linked to form elastomers using various cross-linking agents. Silicone elas-
tomer kits consisting of the polysiloxane precursor liquids and cross-linking agents are commercially
available from corporations such as GE Bayer Silicones (Table 13.5). Silicones cross-linked at room
temperature are called room-temperature vulcanized (RTV) elastomers and those requiring elevated
temperatures are called heat-cured silicone elastomers.

TABLE 13.5 Mechanical Properties for Cured Silicone

Shore Tensile Elongation Tear strength,
Product hardness strength, psi at break, % lb/in
Silopren HV 3/322 30A 800 600 86
Silopren HV 3/822 80A 1300 400 142
Silopren HV 4/311 35A 1000 600 57
Silopren HV 4/811 80A 1450 400 114
Silopren LSR 4020 22A 940 1000 86
Silopren LSR 4070 70A 1300 400 114
Source: Available from GE Bayer Silicones

Silicones are more flexible and of lower strength than polyurethanes. But they are more chemically
stable and are used for artificial finger joints, blood vessels, heart valves, breast implants, outer ears,
and chin and nose implants. Silicones have high oxygen permeability and are used for membrane
oxygenators and soft contact lenses. 26

13.3.5 Rigid Polymers
Most bioinert, rigid polymers are commodity plastics developed for nonmedical applications. Due to
their chemical stability and nontoxic nature, many commodity plastics have been used for implantable
materials. The following section on rigid polymers is separated into bioinert and bioerodable mate-
rials. Table 13.6 is at the end of the section on bioinert materials and contains mechanical property
data for these polymers. Table 13.6 is roughly ordered by elastic modulus. Polymers such as the
nylons and poly(ethylene terephthalate) slowly degrade by hydrolysis of the polymer backbone. But
they are considered bioinert since a significant decrease in properties takes years.
Most rigid degradable polymers degrade without the aid of enzymes and are therefore bioerod-
able. Table 13.7 is at the end of the section on bioerodable polymers and shows mechanical property
data for these polymers.