Polymeric Biomaterials - Biomaterials - page 67

low specific gravity (1.04-1.12 g/cm 3 ), and relatively high modulus. HIPS contains a rubbery modifier
which forms chemical bonding with the growing PS chains. Hence, the ductility and impact strength
are increased and the resistance to environmental stress cracking is also improved. PS is mainly pro-
cessed by injection molding at 180-250°C. To improve processability, additives such as stabilizers, lubri-
cants, and mold releasing agents are formulated. GPPS is commonly used in tissue culture flasks, roller
bottles, vacuum canisters, and filterware.
Acrylonitrile-butadiene-styrene (ABS) copolymers are produced by three monomers: acrylonitrile,
butadiene, and styrene. The desired physical and chemical properties of ABS polymers with a wide
range of functional characteristics can be controlled by changing the ratio of these monomers. They are
resistant to the common inorganic solutions, have good surface properties, and dimensional stability.
ABS is used for IV sets, clamps, blood dialyzers, diagnostic test kits, and so on.
3.3.6 Polyesters
Polyesters such as polyethyleneterephthalate (PET) are frequently found in medical applications due
to their unique chemical and physical properties. PET is so far the most important of this group of
polymers in terms of biomedical applications such as artificial vascular graft, sutures, and meshes. It is
highly crystalline with a high melting temperature ( T m : 265°C), hydrophobic, and resistant to hydrolysis
in dilute acids. In addition, PET can be converted by conventional techniques into molded articles such
as luer filters, check valves, and catheter housings. Polycaprolactone is crystalline and has a low melting
temperature ( T m : 64°C). Its use as a soft matrix or coating for conventional polyester fibers was proposed
by recent investigation (Leininger and Bigg, 1986).
3.3.7 Polyamides (Nylons)
Polyamides are known as Nylons and are designated by the number of carbon atoms in the repeating
units. Nylons can be polymerized by step reaction (or condensation) and ring-scission polymerization.
They have excellent fiber-forming ability due to interchain hydrogen bonding and a high degree of crys-
tallinity, which increases strength in the fiber direction.
The presence of -CONH- groups in polyamides attracts the chains strongly toward one another by
hydrogen bonding. Since the hydrogen bond plays a major role in determining properties, the num-
ber and distribution of -CONH- groups are important factors. For example, T g can be decreased by
decreasing the number of -CONH- groups. On the other hand, an increase in the number of -CONH-
groups improves physical properties such as strength as one can see that Nylon 66 is stronger than
Nylon 610, and Nylon 6 is stronger than Nylon 11.
In addition to the higher Nylons (610 and 11), there are aromatic polyamides named aramids. One of
them is poly( p -phenyleneterephthalate) commonly known as Kevlar ® , made by DuPont. This material
can be made into fibers. The specific strength of such fibers is five times that of steel and, therefore, it is
most suitable for making composites.
Nylons are hygroscopic and lose their strength in vivo when implanted. The water molecules serve as
plasticizers which attack the amorphous region. Proteolytic enzymes also aid in hydrolyzing by attack-
ing the amide group. This is probably due to the fact that the proteins also contain the amide group
along their molecular chains which the proteolytic enzymes could attack.
3.3.8 Fluorocarbon Polymers
The best known fluorocarbon polymer is polytetrafluoroethylene (PTFE), commonly known as Teflon® ®
(DuPont). Other polymers containing fluorine are polytrifluorochloroethylene, polyvinylfluoride, and
fluorinated ethylene propylene. Only PTFE will be discussed here since the others have rather inferior
chemical and physical properties and are rarely used for implant fabrication.