Integrated Biomaterials Science

964 Gerardo Catapano
polymer-poor zones thicken up till they break the polymer coating and yield
an interconnected porous structure (i.e., the sponge layer). Slow, uniform
solvent extraction generally yields symmetric membranes.
Different techniques for solvent extraction characterize different phase
inversion techniques and yield different membrane wall structure. Those
mostly used for the preparation of medical membranes extract the solvent
by thermal precipitation and by immersion precipitation. In the thermal
precipitation technique, the polymer is dissolved in a solvent at high (vs.
low) temperature and is spun in a spinneret or is extruded. In the preparation
of hollow fiber membranes, the fiber lumen is generally filled with a
gas through the inner concentric needle to prevent its mechanical collapse.
Phase separation occurs by suddenly cooling down (vs. heating up) the
dope. Membrane formation is generally followed by a thorough extraction
of the remaining solvent and other extractables. Symmetric and asymmetric
membranes can be prepared according to this technique. Microporous
polypropylene (PP) membranes for TP and blood oxygenation are obtained
by cooling down the polymeric dope. The same technique can also be used
to prepare membranes of polyethylene (PE), polyvinylidenfluoride (PVDF),
polyacrylonitrile (PAN), and polysulfone (PS). Polyamide (PA) and cellulosic
membranes are precipitated by heat evaporation of the solvent. Most
commercial membranes are prepared according to the immersion precipitation
technique. Also in this case, the polymer is dissolved in a solvent (vs. a
solvent/nonsolvent mixture) and is spun in a spinneret (vs. extruded) to
shape the dope. Phase separation occurs by immersing the dope in a
coagulation bath containing large amounts of nonsolvent for the polymer.
As a result of solvent diffusion out of the dope and nonsolvent diffusion into
the dope, the dope separates into a polymer-rich and a polymer-poor phase.
Polymer precipitation occurs as its concentration exceeds the solubility limit
according to the mechanism previously described. Symmetric and asymmetric
membranes made of a number of different polymers (e.g., cuprammonium
(Cu), cellulose acetate (CA), PAN, PS) are prepared according to this
technique.
32.3.5. Membrane Materials
The first membranes used in hemodialysis were made of cellophane, a
type of regenerated cellulose. At the beginning of the 1970s, Cuprohan
(Cu), another type of regenerated cellulose, was successfully used in
hemodialysis and until recently has remained the membrane material most
used in medical applications. Membranes were prepared according to
the cuprammonium process by dissolving cellulose in an ammoniacal
solution of copper hydroxide to form a copper–ammonia–cellulose