Polymeric Biomaterials - Biomaterials - page 66

catalysts. A highly crystalline, linear polymer with a density ranging from 0.94 to 0.965 g/cm 3 is obtained.
LDPE is derived from a high temperature (150-300°C) and pressures (1000-3000 kg/cm 2 ) using free radi-
cal initiators. A highly branched polymer with lower crystallinity and densities ranging from 0.915 to
0.935 g/cm 3 is obtained. LLDPE (density: 0.91-0.94 g/cm 3 ) and VLDPE (density: 0.88-0.89 g/cm 3 ), which are
linear polymers, are polymerized under low pressures and temperatures using metal catalysts with como-
nomers such as 1-butene, 1-hexene, or 1-octene to obtain the desired physical properties and density ranges.
HDPE is used in pharmaceutical bottles, nonwoven fabrics, and caps. LDPE is found in flexible con-
tainer applications, nonwoven-disposable and laminated (or coextruded with paper) foil, and polymers
for packaging. LLDPE is frequently employed in pouches and bags due to its excellent puncture resis-
tance and VLDPE is used in extruded tubes. UHMWPE (MW > 2 × 10 6 g/mol) has been used for ortho-
pedic implant fabrications, especially for load-bearing applications such as an acetabular cup of total
hip and the tibial plateau and patellar surfaces of knee joints. Biocompatibility tests for PE are given by
ASTM standards in F981, F639, and F755.
3.3.3 Polypropylene
Polypropylene (PP) can be polymerized by a Ziegler-Natta stereospecific catalyst which controls the
isotactic position of the methyl group. Thermal ( T g : −12°C, T m : 125-167°C and density: 0.85-0.98 g/cm 3 )
and physical properties of PP are similar to PE. The average molecular weight of commercial PP ranges
from 2.2 to 7.0 × 10 5 g/mol and has a wide molecular weight distribution (polydispersity) which is from
2.6 to 12. Additives for PP such as antioxidants, light stabilizer, nucleating agents, lubricants, mold
release agents, antiblock, and slip agents are formulated to improve the physical properties and pro-
cessability. PP has an exceptionally high flex life and excellent environment stress-cracking resistance,
hence it had been tried for finger joint prostheses with an integrally molded hinge design (Park, 1984).
The gas and water vapor permeability of PP are in-between those of LDPE and HDPE. PP is used to
make disposable hypothermic syringes, blood oxygenator membrane, packaging for devices, solutions,
and drugs, suture , artificial vascular grats , nonwoven fabrics, and so on.
3.3.4 Polymethylmethacrylate
Commercial polymethylmethacrylate (PMMA) is an amorphous ( T g : 105°C and density: 1.15-1.195 g/
cm 3 ) material with good resistance to dilute alkalis and other inorganic solutions. PMMA is best known
for its exceptional light transparency (92% transmission), high refractive index (1.49), good weathering
properties, and as one of the most biocompatible polymers. PMMA can be easily machined with con-
ventional tools, molded, surface coated, and plasma etched with glow or corona discharge. PMMA is
used broadly in medical applications such as a blood pump and reservoir, an IV system, membranes
for blood dialyzer, and in in vitro diagnostics. It is also found in contact lenses and implantable ocular
lenses due to excellent optical properties, dentures, and maxillofacial prostheses due to good physical
and coloring properties, and bone cement for joint prostheses fixation (ASTM standard F451).
Another acrylic polymer such as polymethylacrylate, polyhydroxyethyl-methacrylate (PHEMA),
and polyacrylamide (PAAm) are also used in medical applications. PHEMA and PAAm are hydro-
gels , lightly cross-linked by ethyleneglycoldimethylacrylate to increase their mechanical strength. The
extended wear soft contact lenses are synthesized from PMMA and N -vinylpyrrolidone or PHEMA
which have high water content (above 70%) and a high oxygen permeability.
3.3.5 PS and Its Copolymers
The PS is polymerized by free radical polymerization and is usually atactic. Three grades are avail-
able: unmodified general purpose PS (general purpose polystyrene (GPPS), T g : 100°C), high impact PS
(HIPS), and PS foam. GPPS has good transparency, lack of color, ease of fabrication, thermal stability,