Civil Engineering Reference
In-Depth Information
the decrease of the complement activation. h e cellulose triacetate is signii cantly more
heat resistant than cellulose acetate. Due to the complete hydroxyl substitution replace-
ment in the production of cellulose triacetate, leukopenic response decreases in which
the white blood cell count decreases from baseline usually in a range of 35-40% [20].
16.3.2.2
Hemophan Membranes
Hemophan dialyzers are the most commonly used products containing modii ed cellu-
losic membrane used along with cellulose acetate dialyzers. For the preparation of this
polymer the chemical structure of ultrapure natural cellulose is altered by etherii cation
of a dei ned part of the free OH-groups in the cellulosic backbone with a diethylami-
noethyl group. h e substitution mechanism for the production of
Hemophan mem-
branes
is dif erent from that used for cellulose acetate. In order to produce hemophan
membranes only a small percentage (less than 5%) of hydroxyl group of cellulose are
substituted by tertiary ammonium groups, as opposed to the preparation of cellulose
acetate membranes. However, the tertiary amine replacement is more signii cant in the
production of hemophan membranes. h e decrease in complement activation and leu-
copenia is approximately the same with cellulose acetate membrane [20].
16.3.3
Synthetic Membranes
In order to solve the problems that occurred with unmodii ed cellulosic membranes,
synthetic membranes were developed. h e i rst synthetic polymeric membrane was
produced in the early 1970s. Since that time, various synthetic polymers such as poly-
sulfone, polyamide, poly(methyl methacrylate), polyethersulfone, polyethersulfone/
polyamide have been used in the production of synthetic hemodialysis membranes
[20,21]. Synthetic membranes have large mean pore size and thick wall structure.
h ese properties provide high ultrai ltration rate, which is necessary for hemodialysis
to be achieved with relatively low transmembrane pressures [20]. h e main dif erence
in synthetic and cellulosic membranes is the chemical composition of the membrane.
Synthetic membranes are made from manufactured thermoplastics, while both modi-
i ed and unmodii ed cellulosic membranes are prepared from natural polymers [20].
Membranes composed of either regenerated cellulose or cellulose esters are man-
ufactured through distinct processes of modifying and crosslinking cellulose i bers
derived from wood pulp or cotton i bers to form i lms with dif ering properties and
pore sizes. Variations in the manufacturing process signii cantly change the properties
and pores sizes of the i lm. While similar in composition, most of the cellulosed-based
membranes manufactured are not necessarily useful for dialysis. For dialysis applica-
tions, regenerated cellulose-based membranes are extruded as tubing or sheets and then
dried. Glycerol is frequently added as a humectant to prevent cracking during drying
and to help maintain the desired pore structure. Regenerated cellulose membranes are
very hydrophilic and hydrate rapidly when introduced to water. Due to their additional
crosslinking, regenerated cellulose membranes have better chemical compatibility and
heat stability than membranes made from cellulose esters. Regenerated cellulose mem-
branes are more resistant to organic solvents and to the weak or dilute acids and bases
