Page 350 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

BIOPOLYMERS 327

Oxidized cellulose is used as a degradable hemostatic agent. The acid groups promote clotting when
placed in wounds. Furthermore, oxidized cellulose swells with fluid to mechanically close damaged
vessels. Oxidized cellulose sheets are placed between damaged tissues following surgery to prevent
postsurgical adhesions. The sheets separate tissue during healing and dissolve in a few weeks after
healing occurs. 24

13.3.4 Elastomers
Silicones and polyurethanes are the two classes of elastomers used for in vivo applications. Both are
versatile polymers with a wide range of mechanical properties. Polyurethanes tend to be stiffer and
stronger than silicones, while silicones are more inert and have the advantage of being oxygen per-
meable. Polyurethanes are more versatile from a processing standpoint since many polyurethanes are
thermoplastics, while silicones rely on covalent cross-linking and are therefore thermosets.

Polyurethane Elastomers. Degradation: bioinert or slow bioerosion.

O H H O
O C N R' N C OR"
n

The above repeat unit can describe most polyurethanes. Polyurethanes are a versatile class of block
copolymers consisting of a “hard block” (R′) and a “soft block” (R′′). The hard block is a glassy
polymer (T above room temperature) often synthesized by polymerizing diisocyanates with glycols.
g
R′′ is a low T (T << room temperature) polyester or polyether. Polyurethanes with polyester soft
g g
blocks are degradable, while those with polyether blocks degrade very slowly. Polyurethanes are
usually elastomers since hard and soft blocks are present. Rubbers of different hardness or durometer
can be prepared by varying the ratio of R′ to R′′. Covalently cross-linked polymers can be prepared
by using monomers with functionalities greater than 2. But the most useful polyurethanes for medical
applications are the thermoplastic elastomers since these can be melt processed or solution cast.
Polyurethanes have good fatigue strength and blood compatibility and are used for pacemaker lead
25
insulation, vascular grafts, and ventricular assist device/artificial heart membranes. Table 13.4 shows
TABLE 13.4 Properties of Chronoflex Thermoplastic Polyurethanes Available from CardioTech
Property ASTM procedure Typical values
Hardness (shore durometer) ASTM D-2240 80A 55D 75D
Ultimate tensile strength (psi) ASTM D-638 5500–6500 6000–7500 7000–8000
Elongation at break (%) ASTM D-638 400–490 365–440 255–320
100% secant modulus (psi) ASTM D-638 770–1250 1850–2200 5300–5700
300% secant Modulus (psi) ASTM D-638 700–1400 1700–2000 2700–3200
Flexural strength (psi) ASTM-D790 350 550 10,000
Flexural modulus (psi) ASTM D-790 5500 9300 300,000
Melt index (g/10 min) 210°C ASM D-1238 8 5 3
2.17 kg
Vicat softening point (F/C) ASTM D-1525 160/70 180/80 –
Water absorption (%) ASTM D-5170 1.2 1 0.8
Specific gravity ASTM D-792 1.2 1.2 1.2
Coefficient of friction ASTM D-1894 1.5 0.8 0.64
Abrasion resistance ASTM D-1044 0.008 0.035 0.053
(% loss at 1000 cycles)
Melt processing temp. (°F/°C) 375–430/190–220
Recommended sterilization Gamma; E-beam; ethylene oxide
Class VI biocompatability test U.S.P. XXII Pass Pass Pass