Page 355 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

332 BIOMATERIALS

Polyvinyl chloride (PVC) is a rigid glassy polymer that is not used in vivo because it causes a large
inflammatory response probably due to metal stabilizers and residual catalysts. However, PVC soft-
ened with plasticizers such as dioctyl phthalate is used for medical tubing and blood bags. PVC is a
thermoplastic and can be melt processed.

Polypropylene. Degradation: bioinert.

CH 3

CH 2 CH
n

Commercial polypropylene (PP) is isotactic since atactic PP has poor mechanical properties and iso-
tactic is difficult to synthesize. PP has similar structure and properties as HDPE, except that it has
superior flex fatigue properties and a higher melting point. Polypropylene is commonly used for
nondegradable sutures. Like PE, polypropylene can be melt processed. 26

Polyethylene. Degradation: bioinert.

CH 2 CH 2
n

Polyethylene (PE) is a flexible polymer with a T of around –125°C. It is available in three
g
different forms: low density (LDPE), linear low density (LLDPE), and high density (HDPE). LDPE
and LLDPE are typically not used in vivo since they cannot be autoclaved. HDPE can be autoclaved
and is used in tubing for drains and as a catheter material. Ultrahigh-molecular-weight polyethylene
6
(UHMWPE) has a high molecular weight (≈ 2 × 10 g/mol) and is used for the articulating surfaces
of knee and hip prosthesis. UHMWPE, like all PE, has a low coefficient of friction, but is very hard
and abrasion resistant. 30
With the exception of UHMWPE, PE can be melt processed. UHMWPE has a high melting point
and, like PTFE, is formed by pressing and sintering of powders.

Polytetrafluoroethylene. Degradation: very bioinert.

CF 2 CF 2
n

Polytetrafluoroethylene (PTFE: Teflon TM , Fluorel TM ) is best known for its excellent chemical stability
and low coefficient of friction. Expanded PTFE (ePTFE) contains microporosity created by stretching
PTFE film and is used in small-diameter vascular grafts and for artificial heart valve sewing rings
®
(e.g., Gore-tex ).
PTFE is highly crystalline (92 to 98 percent) and degrades near its melting temperature of 327°C;
therefore, it cannot be melt processed even though it is a thermoplastic. Due to the inability to melt
process, PTFE is formed by pressing PTFE powder followed by heating to sinter the powder, or it is
heated and pressed simultaneously (pressure sintered). 31