Development of a centrifugal blood pump with magnetically suspended impeller and the related fluid mechanical problems

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1 Sadhan& Vol. 23, Parts 5 & 6, ct & Dec 1998, pp ndian cademy of Sciences Development of a centrifugal blood pump with magnetically suspended impeller and the related fluid mechanical problems T KMTSU and T TSUKY Department of Mechanical Engineering, Setsunan University, Neyagawa, 572 Japan * Department of Mechanical Engineering, Kyoto University, Kyoto, 66-1 Japan bstract. This paper deals with fluid mechanical problems encountered in eight years of developmental studies on a centrifugal blood pump with magnetically suspended impeller. The main results of the investigation are as follows. The impeller disk friction is dominant anaong all the power losses; the magnetically suspended impeller with radial straight vanes is the most stable. Motor current depends on blood viscosity and flow rate. Consequently, the flow rate and pressure difference can be estimated by self-sensing without any need for flowmeter and pressure transducers. Keywords. Centrifugal pump; magnetically suspended impeller; viscous effect; impeller disk; whirl motion of impeller; flow pattern. 1. ntroduction Fluid mechanics has contributed to solving quite a few problems of biology, of physiology and of artificial organs. s examples of the latter, flows in artificial heart valves and pulsatile ventriculars have been investigated. Recently, non-pulsatile turbo blood pumps have been studied as artificial hearts of the next generation, because these blood pumps are simple, compact, easy to handle and inexpensive, compared to the displacement ventricular type of artificial hearts. t present, commercially available pumps have some drawbacks in the bearing and seal. The force and heat due to friction cause thrombus formation. Consequently, they remain usable only for a few days. To overcome these drawbacks, one of the authors, in collaboration with New Tech Network (NTN) nc., has been developing a centrifugal blood pump with magnetically suspended impeller since 199 (kamatsu et al 1992, 1995). t present, a sheep fitted with this pump continues to survive even after 65 days (1998, January 1). n this paper, we review fluid mechanical problems encountered in developmental studies of the present pump. 597

2 598 T kamatsu and T Tsukiya 2. Structure of the pump The pump is composed of three parts, the magnetic bearing, the impeller and pump housing, and the driving motor. Figure la shows the cross-sectional view of the impeller and pump housing. The impeller is 5 mm in diameter, 3.5 mm in width for flow passage, and 1 mm in total width. s the Z-axial inlet flow turns to radial flow before entering the impeller, the flow in the impeller has no axial component. Since the outlet flow that passes through the impeller is divided into halves by the partition (double volute), the radial reaction forces are balanced. Such a fluid dynamical design reduces the number of control components and control power needed for positioning the impeller. The left end-plate of the impeller faces the outer driving motor across a partition. The right end-plate faces the magnetic bearing (figure 1 b), which is composed of 3 or 4 pieces of electromagnets embedded with samarium permanent magnets, and at the opposite end of the motor rotor, the same number of permanent magnets are embedded. Since the right end-plate of the impeller is made from ferrite stainless steel, which is magnetic, this surface also works as the target for position sensors. The current in the electromagnets is controlled by the proportional, integral, and differential (PD) process of the sensor's signalsl The three componentz Z, x and y of impeller movement shown in figure lb are feedback controlled. The x and y components of the radial movement of the impeller are passively controlled by the restoring forces of Z-directional magnetic coupling, that is, the left-side permanent magnets and the rightside electro- and permanent-magnets. This passive control induces, more or less, a whirl motion in the magnetically suspended impeller. Hence, it is necessary to find appropriate profiles of impeller and double volute for stable rotation of the impeller. 3. Fluid mechanical problems of the pump Generally the blood pump is too small in size and too dominant in viscous effect for application of conventional design methods in fluid engineering. Therefore, it is necessary to find a new design principle for the profiles of vanes and volute from the standpoints of efficiency, hemolysis and stability of whirl motion of the impeller. For three kinds of impellers, the pump characteristics (pressure, efficiency and torque versus flow rate) are shown in figure 2 in terms of non-dimensional quantities of pressure (~p), torque (z), flow rate (4') and efficiency (7). The hemolysis test proved that the hemolysis is so low that there is no difference between the three kinds of impellers. The impeller 7B is most stable for the whirling motion, though the impeller is not recommended fluid dynamically. t present we do not determine the optimal profile of the vanes. The clearance (h) between the impeller end-plate and the wall of the pump housing is about.2 mm. Total input power, output pumping power and the remnant fluid dynamic losses are shown in figure 3. Most of the power losses are caused by the flow in clearance (disk friction losses and leakage losses). For reduction of these friction losses, decrease in size of the impeller is effective, but this contradicts obtaining strong magnetic force for stable rotation. t h =.2 mm, the leak flow rate is about 1.5 L/min for a cardiac output of 5 L/min. This large leakage is quite permissible from its compensating effect of good wash-out for prevention of thrombus

3 y 3 ' ~ i,4 Z X 8y 8X (a) ( Figure 1. Structure of the centrifugal blood pump with a magnetically suspended impeller, (a) cross-sectional view of the pump, (b) components of the magnetic suspension. 1. mpeller passage, 2. electromagnet, 3. permanent magnet, 4. gap sensor, 5. motor, 6. partition, 7. volute, 8. casing.

4 6 T kamatsu and T Tsukiya 7 7B o o o c o ii m l M mi m mo,o. B.4" ~i.2,,.f ii o ho i m ) i<.4 l.a f" J, " im m N.2 7 7B 16 o Figure 2. Pump characteristics curve for different impeller profiles.

5 Centrifugal blood pump f Jrotal power ~.~..3. j~ ~"" Disk triclion losses ~o 2~ n 1. Leakage losses,i~ "~...~ ~ Hydraulic work Flow rate (/rain) Hydraulic losses (13~o) F~ow ra~,e Q=5.(~/min) dic work (34%) Disk... ) friction losses (43%) Figure 3. Distribution of power losses. 3 Re=2 Re=6 u Re=l [ o Re=4 o Re=8 ].4.2 o w~mm~ P l.2 ~". "~ Figure 4. nfluence of Reynolds number (---- r~o/v) on pump characteristics.

6 62 T kamatsu and T Tsukiya ~.15 L o Re Figure 5. Relationship between modified motor current and Reynolds number. formation. The degree of magnetism of red blood cells depends on the content of 2 or C 2. t is quite interesting to investigate the effect of magnetism of the flow in this narrow clearance as well as the detailed flow pattern there. Figure 4 shows the results of the pump characteristics test. The effect of the viscosity is significant in the relationship of the normalized flow rate (4') to the modified motor current (t) characteristics. t should be noted that the value of t at 4' = (clamping the cannula in extracorporeal assist), as shown in figure 5, is strongly dependent upon the Reynolds number. Consequently the blood viscosity can be identified without viscometer from the observed value of t at 4' =. Subsequently the flow rate 4' can be evaluated from the measured motor current t through the specified relation (t versus 4') without flowmeter. Finally the pressure difference P is also obtained through the specified relation with 4' without pressure transducer. n case of intracutaneous assist, the method of determining viscosity will be reported elsewhere. The.25.o.2.4 ~.,3 "o -a.15.jz- r 6J =~.2 i.1,~. o.1 o.oo5 o.1 ~ Flow Rate ~ Flow Rate :Re=4.93xl 4 (81-1,2rpm) : Re=4.93x14 (Glycerolqueous-solution) : Re=7.2xl 4 (B1-2,2rpm) : Re=7.2xl 4 (Glycerol queous-solution) Figure 6. Comparison of pump characteristics using blood and glycerol aqua solution.

7 Centrifugal blood pump 63 pressure obtained for real blood, as shown in figure 6, is a little higher than that for glycerin aqua solution with the same viscosity as blood. This may indicate the effect of slight non- Newtonian turbulent flow. The radial whirl motion of the impeller was observed by dual laser position sensors. The radial straight vanes (7B) prove to be the most stable. The flow patterns in the impellers are observed by a video camera through an optically rotating coordinate system, which has the same rotational speed as the impeller. n the vane 7, the flow separation is observed even at high efficiency point, and in the vane 7B a pair of large vortices appear. Clinically the pump works in a non-pulsatile parallel mode with a weakly beating heart. n this case the pump is influenced by the beating heart. Sometime pulsatile operation of the pump is preferred according to physiological conditions. This is realized by cyclic change of rotational speed of the motor. Therefore, unsteady characteristics of this low inertia pump must also be investigated (Yoshino & kamatsu 1997). For a clinically acute case using a small diameter tube, the higher pressure must be produced by increase in rotational speed. This induces such severe hemolysis that it necessitates development of a blood pump causing low hemolysis. My colleague Tamagawa has studied the mechanism of hemolysis in turbulent flow (Tamagawa et al 1996). 4. Conclusion To establish the design principle for a small-scaled, viscous-dominant pump such as centrifugal blood pump, an elaborate overall investigation of flows about the impeller, volute and impeller disk is very necessary. The mechanism of hemolysis in turbulent shear flow is an interesting subject. References kamatsu T, Nakazeki T, toh H 1992 Centrifugal blood pump with a magnetically suspended impeller. rtif rgans 16:35-38 kamatsu T, Tsukiya T, Nishimura K, Park C H, Nakazeki T 1995 Recent studies of the centrifugal blood pump with a magnetically suspended impeller. rtif rgans 19: Tamagawa M, kamatsu T, Saitoh K 1996 Prediction of hemolysis in turbulent shear orifice flow. rtif rgans 2: Yoshino Y, kamatsu T 1997 Performance and unsteady characteristics of magnetically suspended centrifugal blood pump. JSME nt. J. B4:114-t2