Page 172 - Biomedical Engineering and Design Handbook Volume 2, Applications

DESIGN OF ARTIFICIAL KIDNEYS 151

Essential nutrients

Purified water

Ion exchanger/ Mixing Heater Deaerator Conductivity pH Temp
filter chamber cell/ probe sensor
Heparin concentration Pressure
pump monitor control
Pressure
measurement

Flow meter
Active clamp

Arterial
pressure Roller Dialyzer
measurement pump
Flow meter
Flow meter
Pressure
Pump probe Bypass
Blood leak
To drain detector
FIGURE 5.3 The dialysis system for low flux dialysis.

the dialysis system. The stringent requirements for the water quality demand several levels of water
+
+
–
purification, including ion exchange and distillation. The required nutrients (Na , K , Cl , glucose, etc.)
are proportioned and mixed with water. The mixture is then heated to body temperature to avoid heat
transfer between the dialysate fluid and the blood. The heated dialysate fluid is now deaerated to trap
bubbles. The mixing and heating, etc., are performed in the dialysis machine (Fig. 5.4). The dialysis
fluid usually flows counterclockwise outside the hollow fiber (semipermeable membrane) carrying
the blood. The hollow fibers are preassembled and prepackaged into a cartridge with blood and dial-
ysis fluid inlet and outlet manifolds. These cartridges come in various sizes having different number
of fibers and therefore different surface area (Fig. 5.5). In low flux dialysis, the transmural
(transmembrane) pressure gradient across the hollow fiber is negligible; however, a slight negative
pressure is used to remove some water. On the other hand, for high flux dialysis, the negative pres-
sure has to be precisely controlled. Since there is a significant rate of fluid removal from the blood,
a make-up solution is introduced into the blood stream before it is returned to the vein (Fig. 5.6).
This fluid replacement has to be regulated precisely. Other components of the dialysate circuit
include a blood leak detector in the dialysate output line (Fig. 5.3). The blood leak detector detects
for any blood in the dialysate fluid that might have leaked from the hollow fiber (across the mem-
brane) into the dialysate fluid using a photo detector. While the dialysate fluid does not absorb red
or infrared light, the red blood cell absorbs red and infrared light. The dialysate fluid is then pumped
into the drain. The pressure in the dialysate fluid is precisely regulated. 42
There is a roller blood pump in the arterial line between the arterial access and the blood inflow
manifold of the artificial kidney machine. Blood pressure is measured in the arterial line between the
access and the blood pump. Also, blood pressure is measured in the return venous line. An air leak
detector in the return blood line (downstream of the exhaust blood manifold) detects any air in the
blood line. In addition, there is a heparin pump placed before the blood enters the venous return