Page 169 - Biomedical Engineering and Design Handbook Volume 2, Applications

148 MEDICAL DEVICE DESIGN

1.0

Sieving coefficient 0.5 70 A pore radius

50 A pore radius

30 A pore radius
0
10 2 10 3 10 4 10 5
Urea (60) Vitamin B 12 β -microglobulin Albumin
2
Creatinine (113) (1355) (11,600) (67,000)
Solute molecular weight
FIGURE 5.2 The relationship between sieving coefficient and solute molecular weight.
(Reproduced from Ref. 7 with permission.)

2
–7
7
coefficient is 6.3 × 10 cm /s. Moreover, the blood concentration of β -microglobulins is 0.06 g/L,
2
whereas the blood concentration of serum albumin is very high 50 g/L. Therefore, it is difficult to sep-
arate the β -microglobulin from serum albumin using diffusion. The middle molecules are removed
2
by convective transport in high flux dialysis. Although the conventional membranes used in low flux
dialysis with 15- to 30-Å pore size will not leak albumin, the extraction of β -microglobulin is not
2
sufficient. More than 50-Å pore size is required to achieve a 60 percent removal of β -microglobulin.
2
This large pore size leads to significant leakage of albumin. Also, adsorption of blood serum albumin
22
to the dialysis membrane is a major problem. Therefore, the challenge in the design and selection of
the membrane is to achieve an effective removal of middle molecules without the loss of serum albumin.
Extraction (E) is a dimension less parameter that can be used to compare the performance of
23
various dialyzers :
E = (C – C )/(C – C ) (5.2)
Bi Bo Bi Di
Concentration of waste products in the dialysate fluid C is zero as the dialysate fluid enters the
DI
dialyzer. Therefore,
E = (C – C )/C (5.3)
Bi Bo Bi
Although engineers prefer extraction, clinical preference is the use of clearance (K) to compare
various dialyzers:
Clearance K = Q (C – C )/C (5.4)
B Bi Bo Bi
where Q is the blood flow rate to the artificial kidney. For the artificial kidney device, the clearance
B
K = Q E (5.5)
B