Integrated Biomaterials Science

Assist Devices 967
b. Synthetic Polymers. The better comprehension of the mechanisms
of membrane formation gathered in the last twenty years made it possible
to prepare membranes from synthetic engineering thermoplastics. A variety
of hydrophobic materials have been used, such as acrylic (co-)polymers,
polysulfone, polyamide, polycarbonate, polyolefines, and blends thereof.
Acrylic polymers. Polyacrylonitrile (PAN) used for medical membranes
is a copolymer consisting of a mixture of an acrylic and acrylonitrile
monomer (approximately 15:85, mole:mole) and a copolymer such as
allylsulfate or methyl acrylate. Membranes are mostly produced by phase
separation for immersion precipitation of the polymer from organic solvents
into an aqueous coagulation bath. PAN is rather more hydrophobic than
cellulosic materials. As a result, it retains only small amounts of water and
readily adsorbs globular proteins. The latter feature enhances membrane
apparent rejection toward middle molecular proteins (e.g.,
and endotoxins. These membranes are dried and stored with
glycerine. Details on the preparation of polymethylmethacrylate (PMMA)
and polyethylene-co-vinylalcohol (EVAL)/vinylacetate (PVA) membranes
manufactured for hemodialysis, hemo(dia)filtration, and plasmapheresis are
not available.
Polysulfone. The molecular structure of the polymer features repeating
phenolic rings which give this material high rigidity, resistance to tensile
stress and compression, as well as high thermal and mechanical stability.
Polysulfone (PS) is one of the most hydrophobic materials used in medicine
and equilibrates at room temperature with only 0.85–21% water. The
polymer is often made more hydrophilic by addition of polyvinylpirrolidone
(PVP) to the dope or by chemical insertion of hydrophilic moieties into the
polymeric backbone. As with most glassy polymers, it is possible to prepare
PS asymmetric membranes in a number of configurations and with a broad
range of permeability and sieving spectra (e.g., MWCO from 1,000 to
500,000). Owing to the polymer properties, PS membranes can be heatsterilized
at 121 °C and may be repeatedly exposed to pH from 1 to 13
without significant alteration of membrane properties.
Polyamide. The chemical structure of polyamides features a
amidic bond where R may be either an aliphatic or an
aromatic group. Good blood biocompatibility has recently been exhibited
by membranes made of an aromatic–aliphatic copolyamide. Membranes are
prepared by casting a dope made of copolyamide and PVP in dimethylsulfoxide
(DMSO) in an aqueous bath. The use of DMSO as a solvent is quite
convenient. In fact, in addition to being a good solvent, the polymeric blend
is easily removed with water and has been reported to have low biological
toxicity. The good miscibility of the copolyamide and PVP reportedly
results in the formation of hydrophobic and hydrophilic domains (approx.