Heat generation and hemolysis at the shaft seal in centrifugal blood pumps

Hemolytic Performance of a MagLev Disposable Rotary Blood Pump (MedTech Dispo): Effects of MagLev Gap Clearance and Surface Roughness

Mechanical shaft seal bearing incorporated in the centrifugal blood pumps contributes to hemolysis and thrombus formation. In addition, the problem of durability and corrosion of mechanical shaft seal bearing has been recently reported from the safety point of view. To amend the shortcomings of the blood-immersed mechanical bearings, a magnetic levitated centrifugal rotary blood pump (MedTech Dispo Model 1; Tokyo Medical and Dental University, Tokyo, Japan) has been developed for extracorporeal disposable application. In this study, the hemolytic performance of the MedTech Dispo Model 1 centrifugal blood pump system was evaluated, with special focus on the narrow blood path clearance at the magnetic bearing between rotor and stator, and on the pump housing surface roughness. A pump flow of 5 L/min against the head pressure of 100 mm Hg for 4 h was included in the hemolytic test conditions. Anticoagulated fresh porcine blood was used as a working fluid. The clearance of blood path at the magnetic bearing was in the range of 100-250 micro m. Pump housing surface roughness was controlled to be around Ra = 0.1-1.5 micro m. The lowest hemolytic results were obtained at the clearance of 250 micro m and with the polished surface (Ra = 0.1 micro m) yielding the normalized index of hemolysis (NIH) of less than 0.001 g/100 L, which was 1/5 of the Biopump BP-80 (Medtronic Inc., Minneapolis, MN, USA, and 1/4 of the BPX-80. In spite of rough surface and narrow blood path, NIH levels were less than clinically acceptable level of 0.005 g/100 L. The noncontact, levitated impeller system is useful to improve pump performance in blood environment.

Hemolysis Resulting From Surface Roughness Under Shear Flow Conditions Using a Rotational Shear Stressor

The degree of hemolysis as a function of surface roughness value and roughened area under shear flow conditions was investigated using a rotational shear stressor. The shearing portion of the stressor is cone shaped in its upper and lower positions, with a cylindrical central section. Surface roughness was applied to the cylindrical section. Bovine blood was sheared for 30 min over a set of roughened surfaces of between arithmetic mean roughness (Ra) 0.1 and 0.8 mm covering 10% of the surface area of the cylindrical section (equivalent to 1.8% of the whole blood contact area) at a shear rate of 3750/s. The threshold value thus obtained for rapid increase in hemolysis was between Ra 0.6 and 0.8 mm. When sheared with a roughened surface of Ra 0.8 mm applied to the cylindrical surface at areas between 0 and 100% (equivalent to between 0 and 18% of the whole blood-contacting area), the hemolysis level did not increase from 10 to 100%, but a significant difference was obtained between 0 and 10%. This suggests that red blood cells were destroyed not by fatigue failure caused by rolling on the roughened surface, but due to the high shear stress generated by surface roughness. Moreover, it appears that the shear stress was generated over the entire cylindrical section, regardless of the area of surface roughness.

Hemolysis caused by surface roughness under shear flow

In this study, the relationship between the degree of roughness of blood contact surfaces under laminar shear flow conditions and the level of hemolysis resulting from this roughness was investigated using a rotational shear stressor. Unlike previous in vitro experiments that used a pumped circuit, the level of hemolysis was directly evaluated under a constant shear flow. In total, 1.8% of the blood contact area was roughened to an arithmetic mean roughness (Ra) value of between 0.4 and 9.2 microm by machine processing and a shear load was applied for 30 min at a shear flow rate of 3750 s(-1). As a result, the threshold Ra value for the induction of hemolysis was found to be between 0.4 and 0.8 microm. In addition, the results of this experiment suggested that the high shear stress resulting from surface roughness plays a major role in determining the level of hemolysis caused by surface roughness.

Development of design methods for a centrifugal blood pump with a fluid dynamic approach: results in hemolysis tests

The purpose of this study was to examine the relationship between local flow conditions and the hemolysis level by integrating hemolysis tests, flow visualization, and computational fluid dynamics to establish practical design criteria for centrifugal blood pumps with lower levels of hemolysis. The Nikkiso centrifugal blood pump was used as a standard model, and pumps with different values of 3 geometrical parameters were tested. The studied parameters were the radial gap between the outer edge of the impeller vane and the casing wall, the position of the outlet port, and the discharge angle of the impeller vane. The effect of a narrow radial gap on hemolysis was consistent with no evidence that the outlet port position or the vane discharge angle affected blood trauma in so far as the Nikkiso centrifugal blood pump was concerned. The radial gap should be considered as a design parameter of a centrifugal blood pump to reduce blood trauma.

Shaft/shaft-seal interface characteristics of a multiple disk centrifugal blood pump

A multiple disk centrifugal pump (MDCP) is under investigation as a potential left ventricular assist device. As is the case with most shaft driven pumps, leakage problems around the shaft/shaft seal interface are of major interest. If leakage were to occur during or after implantation, potential events such as blood loss, clotting, blood damage, and/or infections might result in adverse effects for the patient. Because these effects could be quite disastrous, potential shaft and shaft seal materials have been investigated to determine the most appropriate course to limit these effects. Teflon and nylon shaft seals were analyzed as potential candidates along with a stainless steel shaft and a Melonite coated shaft. The materials and shafts were evaluated under various time durations (15, 30, 45, and 60 min), motor speeds (800, 1,000, 1,200, and 1,400 rpm), and outer diameters (1/2 and 3/4 inches). The motor speed and geometrical configurations were typical for the MDCP under normal physiologic conditions. An air and water study was conducted to analyze the inner diameter wear, the inner temperature values, and the outer temperature values. Statistical comparisons were computed for the shaft seal materials, the shafts, and the outer diameters along with the inner and outer temperatures. The conclusions made from the results indicate that both the tested shaft seal materials and shaft materials are not ideal candidates to be used for the MDCP. Teflon experienced a significant amount of wear in air and water studies. Nylon did experience little wear, but heat generation was an evident problem. A water study on nylon was not conducted because of its molecular structure.

Mapping of pump efficiency on the pressure-flow curve of a centrifugal blood pump

Because pump efficiency is closely related to heat generation and blood trauma in a centrifugal blood pump, it is quite important to study pump efficiencies in a variety of conditions. In the present study, pump efficiencies were mapped on the pressure head-flow rate curves of 4 different pumps; BioMedicus BioPump (BP-80), Nikkiso (NK), Gyro C1E3, and Gyro PI601 (diameter of the impeller, NK: 50 mm, C1Ee3: 65 mm, and PI601: 50 mm). The mapping of pump efficiency revealed the following findings. First, the cone type (BP-80) has less pump efficiency than the impeller type (NK and C1E3); second, the miniaturization of the C1E3 to the PI601 has resulted in an increase in pump efficiency; and third, the diameter of the impeller may contribute to the pump efficiency of an im peller type pump. The mapping of the pump efficiency, as demonstrated in this study, is useful for the analysis of hydraulic pump performance in a wide range of clinically applied conditions.