17 Stirling Engines For Artificial Hearts

introduction

The Artificial Heart Program in the United States was established in 1964 by the National Heart Institute (Department of Health, Education and Welfare). The objectives of the program were to develop devices that could assist or totally replace the heart. Some devices were intended for temporary circulatory assistance for patients in hospitals and confined to bed. Other devices were intended for permanent implantation in the body to assist or totally replace the heart in pumping blood and to allow the recipient complete freedom of movement.

Harmison and Hastings (1969) justified the program on the grounds that in Ihe United States, heart disease caused about a million deaths per year and was by far the leading cause of death. 54 per cent, compared with 16 per cent for cancer, the second most common cause. Moreover, data compiled in the early 1960s indicated that about a quarter of the adult population had definite or suspected heart disease. About two million of these were seriously handicapped by the disease. An annual expenditure by the U.S. Federal Government of over 300 million dollars was estimated for welfare benefits to individuals under the age of 65 permanently and totally disabled by heart disease. By way of comparison the total annual budget for the artificial heart program was eight million dollars in 1969.

Wiggers (1957) has provided an excellent summary about the heart and its functions. A more extensive discussion has been given by Longmore (1971). Both these sources are written for Ihe lay person and provide a good foundation for understanding the magnitude of the task facing those seeking to replace the natural heart.

The U.S. artificial heart program was extraordinarily broad in scope. At the First Artificial Heart Conference, held in 1969. ninety-two technical papers were presented by the sixty-three separate contractors on a very wide range of topics. Of this total, sixteen papers were concerned with implantable energy sources and, within this group, only two were devoted to Stirling engines. A third paper discussed a fiuidic control device for coupling Stirling engine gas compressors to the blood pump. It is clear, therefore, that work on Stirling engines was but a small part of a large program. However, because of the specific nature of this book the discussion here will be confined to matters related to Stirling engines.

general considerations

Two research and development programs on Stirling engines have been sponsored by the Artificial Heart Program of the National Institute of

Health from ils beginning in the mid-1960s to the present time. The two research contractors have been the Aerojet Liquid Rocket Company at Sacramento, California and the McDonnel-Douglas Corporation at Richland, Washington. A third program, begun somewhat later at the Thcr-moelectron Corporation, started with a Rankine-cycle steam engine. Resulting from progressive development, the engine became a 'tidal regenerator engine' and changed its character to a closed Stirling-cycle regenerative engine with a condensing/evaporating working fluid. Finally a fourth program for circulatory assist devices, commencing in ihe early '70s was separately funded by the U.S. Atomic Energy Commission with the Westinghouse Electric Co.. Pittsburgh, Pennsylvania. This started off as a rotary steam engine project but changed to a Stirling engine project with North American Philips Inc., New York, as the principal subcontractor.

All these programs for engine and system development have been regularly reported at the Intersociety Energy Conversion Engineering Conferences held annually in the United States and, more fully, in the quarterly progress and annual or semi-annual contractor reports. Progress with blood pumps and surgical techniques are reported ai meetings, and in the Journal of rhe American Society of Artificial Internal Organs.

All the four machines are superficially similar in that they seek to provide the means to convert thermal energy to some form of mechanical work to drive a blood pump. A typical schematic system is shown by the block diagram in Fig. 17.1. (reproduced from Johnston et al. 1977). For many years the source of the thermal energy was intended to be radioisotope, with plutonium 238 as the preferred material (Sandqvist el al. 1975). Its high specific power, moderate radiation levels, and long half

Fig. 17 i Block diagram tor artificial heart (after Johnston er al. 19771

life (89 years) provide lhe prospect of an essentially permanent heat source (lasting more than 10 years). However, an alternative has been proposed because of the large numbers involved (Harmison and Hastings. 1969. estimate a demand of 200 000 units per year). Other factors are the high cost of the isotope and increasing public concern and government restriction against the use of plutonium in such large amounts. Hie alternative (Martini 1977b) is an electrically-heated thermal-storage capsule that would give four to eight hours of artificial heart power before recharge would be necessary. This approach demands the development of satisfactory long-term percutaneous (through the skin) power leads (Lee 1969) or the alternative ol a transcutaneous (across the skin) power transformer (Newgard and Eilers 1969; Thumin 1969).

For the thermal engine systems the power input levels range from 3(1 to 50 watts to develop the 3 to 5 watts of actual hydraulic power required for the blood pump. This whole 30 to 50 watt power input must eventually be dissipated from the system to the body and hence to lhe atmosphere. In all cases the blood is used as the system coolant. For comparison an average man asleep and at rest dissipates about 100 watts of heat. When doing heavy manual work the power level increases to as much as 500 watts. An athlete may be able briefly to exceed a I kW power rate.

The dissipation of an extra 30 to 50 watts of heat from the body is not seen as an insuperable problem and this has been confirmed in many animal experiments. It is imperative to avoid hot spots at any external part of the system in contact with the body for blood cells and other living tissues are highly susceptible to temperature damage.

One of the principal attractions of the alternative electrically driven blood pumps is the low power input, for it would reduce the amount of heat that must eventually be dissipated in the body. Electrical systems have paid the Carnot penalty at the power station where the electricity was generated. Therefore, the power input may be as little as 10 watts to develop the 3 to 5 watts power for the blood pump.

The physical structure ami arrangement of the circulatory system and the susceptibility of the blood to mechanical damage require that the blood pump, even for total heart replacement, be a pulsing system, operating at a frequency close to the normal rate (about 50 to 100 beats a minute). For a heart assist system, of course, the pump must be exactly synchronized with the operation ol the natural heart.

In some of the systems tried the engine operated at a high frequency to make the unit small and compact. Others operated al heart-beat rate but in no case was the engine coupled directly to the blood pump. Instead, the different systems were packaged in two units as shown in Fig. 17.2—an engine module and pump module. A variety of arrangements

Fig. 17.2. Heart-assist thermal-energy system concept (aller Johnston ci «Î. 1977).

were used to provide connections between the two modules through which power was transmitted from the engine to drive the blood pump. The McDonnel-Douglas and Thermoelectron systems used hydraulic connections with a hydraulic actuator to drive the blood pump. The Aerojet-General system used a pneumatic connection and pneumatic actuator to drive the pump. The Westinghouse/Philips unit used a flexible rotating shaft with a crank driven blood pump with intermediate speed change gears.

l or all systems a control unit was a necessary component and much engineering ingenuity has been exercised in the development of these electronic or fluidic control units.

A brief description of the four systems is given below although space precludes a complete discussion of details for all systems. The general approach followed is briefly to review published accounts of the most recent versions of the four systems, and provide sources for more complete reference. All the systems have experienced successive generations of development; some are now at the fifth and sixth stages. Development has proceeded in parallel with related development of blood pumps, actuators, control systems, operating experience with laboratory systems and with actual installation m animals—usually calves but sometimes pigs have been used. Space precludes a discussion of all this. Further, the ethical, societal, and legal questions concerning artificial hearts have not been addressed.

There is concern about the use of radioisotope heat sources in artificial hearts. A heat source of 30 watts would require 54 grams (0.121bm) of Pu238. Now the critical mass of a fast unmoderated Pu238 system is (Sandqvist et a!. 1975) 5.2 kg (11.5 lb j with a steel reflector. One nightmarish scenario has a convention of 1000 'Arty-Heartys' providing a determined terrorist organization with all the plutonium necessary for a bomb. There is more legitimate concern about the dangers of inadvertent cremation of artificial heart power sources in hotel or residential fires, in automobile accidents or even in routine cremations following death by natural causes, even perhaps mechanical failure of the blood pump. It would be exceedingly difficult to keep track of all the isotope sources if anything other than a few experimental units were involved.

The progress achieved in fifteen years of development of the artificial heart has been remarkable. Complete, operating, heart-assist systems have been installed in calves free to stand and walk about, fhc longest time of survival to date was about eight months. No attempt has been made here to judge the competing systems for this would require a more intimate knowledge and understanding than can be gleaned from the published reports. Enough progress has been made to demonstrate the technical feasibility of relatively long-term (say live years) mechanical heart-assist systems. There is every reason to believe that total heart replacements are also possible. When and. indeed, if such developments will occur are not certain for there are important issues to resolve.

MCDONNEL-DOUGLAS ARTIFICIA! HEAR! ENGINE (NOW UNIVERSITY OF WASHINGTON!

Johnston et al (1977) have given a good overall summary of progress achieved with McDonnel-Douglas Stirling-engine/hydraulic-pump artificial hearts. A schematic diagram of the system is shown in Fig. 17.3. The engine module contained the thermal energy source. Stirling engine and the hydraulic converter/accumulator. The pump module contained the blood pump and the hydraulic actuator/controller.

Details of the engine module are shown in Fig. 17.4 and some of the performance characteristics are given in Table 17.1 (Johnston et al. 1977). The Stirling engine was basically a piston/displacer in separate cylinder units. In one cylinder the displacer oscillated with a short stroke and was caused to move by the balance of pressure forces acting upon it. These were made up of the helium gas pressure acting on the displacer over the whole area at the top and over part oí the area at the bottom. This pressure changes during the cycle. The displacer piston moves the displacer by operating between the variable engine pressure and the constant pressure of the buffer space. Variation in engine speed was accomplished by a power control valve V, (Fig. 17.3) in the hydraulic

Experimen Stirling Engine

Nuclear heal .source

Thermal storage

High pressure accumulator

Outlet check valve

Inlet check \ valve

Converter piston

Power control valve

Reference accumulator

Delivery line 200 PS1A

1'UMJ1

AcnuTon

Overflow relief valve*

Control lopic

I I Hydraulic tluid 123- 185 PS!A V77771 | lydraulic tluid 200 PS1A 1 I Hydraulic fluid atmospheric ■ - ffl-frl He working gas SS5S3 Pneumatic charge pressure

Engine / heal exchanger

Displacer drive piston

185/123 PSI A

Diaphragm

Pumping chamber

. Flexure ' displacer support

Displacer 185/123 PSIA

Solar Stirling Engine Basics Explained

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The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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