Biomedical Engineering, Vol. 44, No. 6, March, 2011, pp. 219221. Translated from Meditsinskaya Tekhnika, Vol. 44, No. 6, Nov.Dec., 2010, pp. 2022. Original article submitted November 2, 2010.

Bearing Units of an Axial Blood Pump: Design and Triboengineering Features V. A. Mal’gichev1*, A. M. Nevzorov1, S. V. Selishchev2, and G. P. Itkin3

An approach to the design of bearing units for an axial blood pump is suggested in this work. The design of the system places a limitation on the time of continuous pump operation. Antifriction materials for triboengineering elements were selected, and aspects of biological compatibility of such materials are discussed. The tribological elements of the system were tested in vivo and in vitro.

Implanted pumps for accessory circulation should meet the requirement of a minimum continuous service life (8 years). The rotor bearing elements are most critical in terms of continuous service life. This is due to mechan ical wear of bearing elements of axial pumps. There are two approaches to construction of wearresistant bearing elements of axial pumps: 1) lubricationfree sliding bear ing elements (Thoratec Corp., Jarvik Heart, Inc.) [1, 2]; 2) contactfree elements (magnetic suspension) (Berlin Heart GmbH) [3]. There are advantages and disadvan tages in both approaches. Sliding bearing elements are relatively simple and inex pensive in comparison with magnetic suspension [4, 5]. The main problem is inability to construct sliding bearings using widely available materials because of biological incompati bility. Therefore, new antifriction materials and friction pairs should be studied for use in implanted mechanisms. These materials should meet the following requirements: low friction coefficient, low wear (assuring requirement of minimum continuous service life), and biocompatibility. The antifriction materials and friction pairs were selected in three stages: laboratory stage, standpoint stage, and experimental stage. The laboratory tests were directed toward increasing wear resistance of biologically compatible materials using weartight coating. The sec ond stage was directed toward design of an experimental setup for testing axial load. The third stage involved test ing axial pumps in animals.

Laboratory Tests of Antifriction Properties of Materials Preliminary tests allowed two materials to be select ed for blood axial pumps: CoCrMo alloy and Al2O3 syn thetic sapphire. Special diamondlike coating (DLC) improved tribo logical parameters of the materials. DLC was applied using reactive ionrate synthesis (method of physical deposition). CoCrMo alloy and Al2O3 synthetic sapphire were coated using Optimol SRV vibrotribometer in dry friction mode (fingerdisk scheme) at load 525 N. CoCrMo samples coated with diamondlike coating, prepared for tests in a vibrotribometer, are shown in Fig. 1. Test condi tions were close to exploitation conditions in terms of pressure, sliding rate, and the absence of lubrication. The laboratory tests determined friction coefficients for the friction pairs of interest. The working chamber of the Optimol SRV vibrotri bometer in tests for tribological parameters of tribological bearing element of the axial pump is shown in Fig. 2.

1

DONAM Company, Moscow, Russia. Moscow State Institute of Electronic Engineering, Zelenograd, Russia; Email: [email protected] 3 Shumakov Federal Scientific Center for Transplantology and Artificial Organs, Moscow, Russia. * To whom correspondence should be addressed. 2

Fig. 1. CoCrMo samples coated with diamondlike coating.

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Fig. 2. Working chamber of Optimol SRV vibrotribometer.

One model is based on a spherical center plate, whereas the second model is a cylindrical sliding bearing with axial point support. The spherical center plate of a blood axial pump made of CoCrMo alloy with DLC is shown in Fig. 3. The cylindrical neck of the axial pump axis made of CoCrMo alloy with DLC is shown in Fig. 4. The load of the axial pump bearings differs as the axial load is applied to only one input bearing unit. Hydraulic tests of blood axial pump were performed during circulation setup and revealed working capacity of the bearings. The spherical center plate is a preferable construction for a loaded bearing unit, whereas an unloaded unit can use any construction. Axial load tests were performed using a special experimental setup based on an integral tensosensor for the loaded pump. The experimental setup for testing the axial load in pump is shown in Fig. 5.

The best friction coefficient (K = 0.2) among uncoated materials is inherent in sapphire. The diamond like coating (thickness 1 μm) decreases the friction coef ficient in the axial pump bearing to K = 0.050.07. This improves tribological parameters of both CoCrMo alloy and sapphire. Biocompatibility of DiamondLike Coating The properties of CoCrMo alloy and synthetic sap phire in living organisms were studied in more detail because such materials have been widely used in orthope dics and ophthalmology. DLC is a relatively new materi al. Therefore, materials with DLC were tested for biolog ical compatibility. DLC was tested for hygienic and toxicological prop erties. Biological compatibility tests included implanta tion into laboratory animals (rats) with further histologi cal assay. The results of the tests revealed biological com patibility of the chosen materials and diamondlike coat ing. Indeed, DLC improved biological compatibility of the materials, especially in histological assay, and provid ed their use in blood axial pumps.

Fig. 3. Spherical center plate of blood axial pump.

Sliding Bearings and Setup for Their Testing at Controlled Axial Load Analysis of a hydrodynamic model of a blood axial pump determined load range in bearings. On the basis of load range data and gap requirements (gap in bearings of blood pumps has to be less than 2 μm), two bearing mod els using DLC were designed.

Fig. 4. Cylindrical neck of axial pump axis.

Triboengineering of Axial Blood Pump Bearing Units

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Conclusions

Fig. 5. Experimental setup for testing axial load in pump: 1) axial pump rotor; 2) axial load stock; 3) tensosensor; 4) micrometric screw.

The tensosensor signal was amplified and measured. Maximal axial load was 500600 Gs at pump pressure 200 mm Hg. This value is consistent with a hydraulic model for friction units. Axial load experimental setup also allows axial pump sliding bearing to be tested.

Axial Pump Sliding Units in Experiments in Animals Axial pump sliding units in experiments on animals supported the suitability of the model. Axial pump bear ings were tested on animals at Shumakov Federal

The concept of blood axial pump suggested in this work allowed axial pump bearing elements to be con structed. These bearings were used in blood axial pumps. Modern coating technology based on diamondlike coatings provides improvement of tribotechnical parame ters of pump bearing and develops wearresistance fric tion pairs. This work was supported by Russian Ministry of Sci ence and Education, project No 02.522.12.2010, 2009. REFERENCES 1. K. Butler, D. Thomas, J. Antaki, H. Borovetz, B. Griffith, M. Kameneva, R. Kormos, and P. Litwak, Artif. Organs, No. 21 (7), 602610 (1997). 2. R. K. Jarvik, Artif. Organs, No. 19 (97), 565570 (1995). 3. C. H. Huber, P. Tozzi, M. Hrni, and L. K. von Segesser, Int. Cardiovasc. Thor. Surg., No. 3, 336340 (2004). 4. S. A. Chernavskii, Friction Bearings [in Russian], Mashgiz, Moscow (1963). 5. B. D. Voronkov, Dry Bearings [in Russian], Mashinostroenie, Leningrad (1979).