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BIOMAT., MED. DEV., ART. ORG., 3(2), 181-191 (1975)
Nuclear Power for the Artificial Heart
WILLIAM E. MOTT, Ph.D.
U.S. Atomic Energy Commission Washington, D.C. 20545
ABSTRACT Studies showed that a totally implantable nuclear-powered artificial heart is practicable, and consequently a prototype system development program was initiated in July 1973. Basic testing of the prototype should be completed in 1977, with extensive studies to qualify the device for clinical use running a t least into the early 1980s.
I.
WHAT IS I T ?
In the nuclear-powered artificial heart, the thermal energy from the decay of a man-made radioisotope, plutonium-238, is converted, through a s e r i e s of thermodynamic and mechanical steps, to blood hydraulic power. The possibility of so utilizing a plutonium-238 source first gave substance to the concept of a completely implantable artificial heart.
*Presented at a symposium on "Current Status in Artificial Organs," October 1973, Cleveland, Ohio. 181 Copyright @ 1975 by Marcel Dekker, Inc All Rights Reserved. Neither this work nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming, and recording, or by any information storage and retrieval system, without permission in writing from the publisher.
MOTT
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182
HEAT SOURCE
MECHANICAL CONVERTER
MECHANISM
VENTRICLES
FIG. 1. Components of nuclear-powered artificial heart. The major components of a nuclear-powered heart a r e illustrated in block form in Fig. 1. Heat generated by the plutonium-238 increases, for example, the temperature, and hence the pressure of the working gas in the energy converter. Expansion of the gas drives a power piston which is coupled through the blood pump drive to the pump ventricles. Connections between the converter, the drive, and the ventricles allow for feedback signals to assure the system is responsive to the needs of the recipients and for the disposition of the waste heat from the conversion process. Thermal insulation is required to isolate the high-temperature side of the thermal converter from the surrounding tissue. 11.
HOW HAS I T C O M E A B O U T ?
At the time when Dr. Kolff, one of the earliest of artificial heart researchers, was beginning to translate h i s thoughts on heart replacement into experimental devices, the use of small radioisotope heat sources to produce unattended power in remote locations was receiving a great deal of attention. The AEC was actively engaged in developing a series of isotopic power units for both manned and unmanned missions in space and for weather stations, navigational beacons, and other special installations in isolated spots around the world. (The first public demonstration of a device for converting isotopic heat to electricity took place in President Eisenhower's office in January 1959.) Thus it is not surprising that the idea of driving an artificial heart with a nuclear power source surfaced when i t did. Particularly in the early 1960s, little escaped the isotope technologists who were always on the alert for new problems to match with their solutions. Officially, the concept of a nuclear-powered artificial heart came
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NUCLEAR POWER FOR THE ARTIFICIAL HEART
183
to light with a proposal submitted simultaneously to the AEC and the National Heart Institute by the Thermo Electron Engineering Corporation (now TECO) in May 1964. This proposal led to a series of discussions between the two agencies on the course to follow. The outcome was that the Heart Institute would take the lead in better defining the total program to develop an artificial heart before either agency took an active role in supporting artificial heart power supply evaluations. In 1965 the Heart Institute initiated its comprehensive study to determine if an artificial heart program was called for, and if so, the direction it should take. The nuclear-powered heart was evaluated, along with biological fuel cells and rechargeable batteries, and was recommended for development. Subsequently (in 1967) the AEC in cooperation with the Heart Institute performed a conceptual design study of implantable nuclear power supplies for circulatory support systems. The study showed that the utilization of radioisotope heat sources for powering implantable heart devices was in principle technically sound. Each of the four thermal converter systems studied had potential for developing into a totally implantable system using state-of- the-art technology. However, although confirming technical feasibility, the studies were not in sufficient depth to establish practicability. That is, the key issue of whether a totally implantable nuclear-powered artificial heart system could be realized under the restraints that must be imposed on weight, volume, shape, performance, isotope inventory, reliability, lifetime, and cost remained an open question. Thus it was until April 1971, when the AEC began studies leading to an evaluation of the practicability of the nuclear approach. The practicability program was conducted in two phases, the first phase lasting for 7 months, the second for 19. The first phase consisted of two parallel analytical studies, one with TRW Systems Group, the other with Westinghouse Electric Corporation. The objective of Phase I was to select from the many alternatives the thermal energy conversion concept having the greatest potential of leading to a practicable fully implantable 10-year device for replacing the total heart. At the end of Phase I each contractor recommended to the AEC one concept for further detailed evaluation during Phase II of the program. Using preestablished evaluation criteria, the Westinghouse concept was selected for the follow-on work. Important factors in the selection were the lower weight, volume, isotope inventory, and heat source temperature and the higher reliability projected for the Westinghouse system. Also, it was felt that the Westinghouse approach had a better understood and developed technological base and, therefore, represented the minimum technological risk.
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184
MOTT
The first 8 months of the Phase I1 program was devoted primarily to a parametric analysis of the Westinghouse Stirling-mechanical converter and its associated blood pump drive, the next 9 months primarily to component and subsystem testing and to the fabrication and testing of a fully instrumented, nonimplantable but realistically sized, bench model of the Stirling-mechanical system, and the final 2 months to data analysis, to preparing an assessment of thermal converter practicability against preestablished criteria, and to report writing. The Phase I1 effort began in December 1971 and was concluded in June 1973. Although the major thrust of the practicability evaluation was at the means of converting the thermal energy from the decay of plutonium-238 to a form suitable for driving the ventricles of a blood pump, considerable attention was given to other issues. Basic to the conclusions were studies on plutonium-238 encapsulation and containment, on thermal insulation for the converter, and on the effects of radiation from nuclear-powered artificial hearts on the recipient, his immediate family, and the general public. To a s s u r e that firsthand information, guidance, and expertise on the biomedical engineering aspects of the artificial heart were available on a day-to-day basis to the AEC and its contractors, a contract was let in June 1971 with the University of Utah (Institute for Biomedical Engineering). During Phase I1 the Utah group under Dr. Kolff's leadership played an important role in establishing practicability criteria, particularly those relating to power and control requirements and to implantation factors, and in designing, fabricating, and evaluating the blood handling components of the bench model system. In February 1972 a program was initiated at the Cleveland Clinic Foundation (Division of Artificial Organs) to define the limits on the volumes and envelope dimensions of the major artificial heart components (thermal converter and blood pump) within which the components would be anatomically acceptable in a larg: percentage of patients. The Cleveland Clinic team lead by Dr. Nose was also deeply involved in developing and refining the practicability evaluation criteria. This overall program led to the following conclusions about the practicability of the nuclear-powered artificial heart when assessed against isotope inventory, weight, volume, shape, performance, reliability, lifetime, environmental impact, and cost.
1. A nuclear-powered artificial heart utilizing the Stirling-mechanical converter and blood pump drive system originated and evaluated by the Westinghouse Electric Corporation was practicable. In arriving at this conclusion the most important considerations were: a. Adequate blood pumping power can probably be obtained with a plutonium-238 inventory under 25 W without resorting to
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NUCLEAR POWER FOR THE ARTIFICIAL HEART
185
thermal energy storage so that neither future plutonium-238 availability nor the risks associated with the nuclear-powered artificial heart, whether to the recipient from the combined effects and radiation or to the members of the recipient's immediate family and to the general population from radiation, should be an overriding factor to limit the development, experimental evaluation, and clinical use of the plutonium-238 heart. b. Projected weights, volumes, and shapes for the system components not only were within the preestablished practicability limits, but the potential for making desired improvements existed. c. Conservatism in design largely compensated for the lack of direct experimental data to support the judgment that the system will function as required over a long period of time. That is, weight, and to some extent volume, were traded off against reliability primarily through the use of rugged components with demonstrated long life. d. Short- and long-term containment of plutonium-238 would be possible under all credible accident conditions when a vented capsule is used. e. Neither criticality nor neutron multiplication would pose problems in the unrestricted use of plutonium-238 powered artificial hearts. f. Plutonium-238 heat sources removed from artificial hearts would have no potential military significance. g. Extrapolations from first-of-a-kind models to mass-produced items suggested that the Stirling-mechanical artificial heart system can be made available in large numbers a t a cost of about $3000 per year, which is about one-half that associated with the home use of an artificial kidney. 2. Although no major technological breakthroughs are required to develop the Stirling- mechanical system into the prototype of a system for long-term animal studies and eventual clinical evaluation, advancements in the technology will have to be made in a number of areas. To perform the necessary research and development on components and subsystems and to convert the resulting technology into prototype hardware will require 3 to 4 years. 111.
TODAY
We thus arrive a t where the AEC is today (Table 1). Following the successful conclusion of the practicability study, a prototype system
186
MOTT
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TABLE 1. AEC Artificial Heart Program Milestones Proposal to AEC and NHI to develop nuclear-powered heart
May 1964
AECfNHI conceptual design study on nuclear power sources
June 1967 to December 1967
AEC practicability study on nuclearpowered hearts
April 1971 to June 1973
Initiation of prototype system development program
July 1973
development program was started in July 1973. A s part of this program, valuable information is being accumulated with the bench units built last year for the practicability study, and detailed design will soon begin on the system shown in Fig. 2. This system has an energy conversion unit, a blood pumping unit, and an interconnecting flexible shaft for transmitting and controlling the pumping power, Figure 3 pictures the Stirling-mechanical converter which provides rotary power to the flexible shaft of the blood pump drive. A s now
Scale-lnches
FIG. 2. Nuclear-powered Stirling-mechanical artificial heart.
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NUCLEAR POWER FOR THE ARTIFICIAL HEART
THERMAL INSULATIO
187
PU-238 HEAT SOURCE
AT EXCHANGER
OLANT LINES POWER PISTON
SHAFT iCTlON
OUTBOARD SEAL
Scale-Inches 0
~
I
1
~
2
I
~
l
~
I
~
l
~
FIG. 3. Stirling-mechanical converter.
conceived, the estimated weight and volume of the converter is 1.9 kg and 0.9 liters. The blood pump drive mechanism (Fig. 4) takes the rotating shaft output of 1800 rpm from the Stirling-mechanical converter and through reduction gearing and a scotch yoke mechanism actuates the pump diaphragms at 120 beats/min. The maximum design output of the pump is 12 liter/min against normal circulatory resistance. The calculated performance of the f i r s t implantable model of the Stirling-mechanical system is given in Table 2.
188
MOTT
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DIAPHRAGM DISPLACER CUP
/VENTRICLE
\
DOME
FIG. 4. Blood pump for Stirling-mechanical system. TABLE 2. Calculated Performance of First Implantable Model of the Stirling Mechanical System Initially
After 10 Years
Thermal input (W)
33
31
Converter efficiency (%)
14
17
Converter output (W) Pump drive efficiency (%) Power to pump diaphragms (W) Ventricular efficiency (%) Available power to blood (W)
5.6 85 4.8
70 3.4
5.2
85 4.4
70 3.1
NUCLEAR POWER FOR THE ARTIFICIAL HEART
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IV.
189
THE FUTURE
The objective of the AECs 3-1/2-year prototype system development program is to advance the technology leading to a fully implantable nuclear-powered artificial heart that can be qualified for clinical use in the 1980s. Under present plans the fabrication and testing of the prototype system should be completed by early 1977. In this program the first implantation in an animal of a total system (implantable version of the bench model developed during Phase I1 of our practicability program) will not take place before early 1975. Along the way to the prototype, advancements (other than those required on biomaterials) w i l l have to be made to: 1. Reduce the weight and volume of both the thermal conversion and blood pumping units. 2. Improve the efficiency and reliability of the Stirling converter, the power transmission mechanism, and the power coupling to the blood pump diaphragms. 3. Improve the system's responsiveness to the needs of the body a s directed by other developments. 4. Lessen the influence of thermal insulation and excess heat dissipation requirements on the overall design of the system. 5. Make the system implantable with minimum surgical risk without sacrificing reliability and life time.
The projected characteristics of the prototype system a r e given in Tables 3 and 4. Looking to the long term, the logical follow-on, upon the completion of the basic developmental work on the prototype, would seem to be an extensive program of bench and animal studies to qualify the device for clinical use and to obtain an understanding of the implications of replacing a natural heart by an artificial heart. Such a program could well run into the early 1980s. It appears now, a s it has in the past, that a major technical b a r r i e r to be overcome before a clinically acceptable heart replacement system can be realized is associated with the materials that interface directly with the blood. Needed is a material that will be blood compatible, impervious to the bioenvironment, and capable of flexing a few hundred million times. Another observation is that if there is to be a completely implantable artificial heart in the 1980s, it will be powered by plutonium-238; and without question a plutonium238 powered heart, regardless of i t s technological assets, will s t i r many more emotions and evoke much stronger criticism than would
MOTT
190 TABLE 3. Calculated Performance of Prototype System
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Thermal input (W) Converter efficiency (%)
Initially
After 10 Years
24 21
22 21
Converter output (W)
5.0
4.6
Pump drive efficiency (%)
90
90
Power to pump diaphragm (W)
4.5 75
4.1 75
3.4
3.1
Ventricular efficiency (%) Available power to blood (W)
TABLE 4. Calculated Prototype System Characteristics Converter weight (kg)
1.5
Converter volume (liter)
0.7
System weight (kg)
2.2
System volume (liters)
1.4
System efficiency (%)
14
a heart powered by any other means. However, looking forward to 20 to 25 years of research and development, even with technology advancing a t a much slower rate than it is today, it is conceivable that breakthroughs, for example in battery technology, could lead to the early phasing out of nuclear-powered devices. We should look upon the nuclear heart, therefore, as the workhorse of the 1980s and 199Os, and certainly as the device, if there is to be a device, to establish the efficacy and the contraindications of the permanent assist and total heart replacement procedures. In closing I want to emphasize that the progress that has been made by the AJ3C toward the nuclear-powered artificial heart has only occurred as a consequence of a closely coordinated and cooperative effort by a rather diverse group of organizations and individuals. For the thermal conversion and blood pumping units the participants have been the Westinghouse Electric Corporation Astronuclear Laboratory
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NUCLEAR POWER FOR THE ARTIFICIAL HEART
19 1
with i t s subcontractor the Philips Laboratories of the North American Philips Corporation, the University of Utah, the Cleveland Clinic Foundation, and the Universities Center- Jackson. Development of the source of thermal energy to drive the system required the cooperation of the Savannah River Laboratory where the plutonium-238 was produced, the Los Alamos Scientific Laboratory where the powder from Savannah River was purified and formed into a solid cylinder, the Mound Laboratory where the cylinder was encapsulated, and the TRW Systems Group where the source was designed and the encapsulating materials fabricated. Information on thermal and radiation effects has come from programs at the Pacific Northwest Laboratory and the University of Cornell, and assisting in the attack on the thermal insulation problem has been the Westinghouse Research Laboratories. My sincere thanks to all who have contributed to our past successes. I look forward to a continuing association as we proceed with the prototype development program. Dr. Kolff spoke yesterday of the voyages of Henry Hudson. When Hudson sailed west in search of China he had little to support him but his great conviction that he would succeed. As we proceed on the journey for an artificial heart we have an advantage over Henry as we can see a route, long and difficult as i t may be. But as surely as with Henry Hudson, forces may cause u s to sail up a shallowing river, into the depths of an endless bay, and finally to be landed on a rocky and barren shore from which no new journey can begin. Received by editor May 24, 1974
BIOMAT., MED. DEV., ART. ORG., 3(2), 181-191 (1975)
Nuclear Power for the Artificial Heart
WILLIAM E. MOTT, Ph.D.
U.S. Atomic Energy Commission Washington, D.C. 20545
ABSTRACT Studies showed that a totally implantable nuclear-powered artificial heart is practicable, and consequently a prototype system development program was initiated in July 1973. Basic testing of the prototype should be completed in 1977, with extensive studies to qualify the device for clinical use running a t least into the early 1980s.
I.
WHAT IS I T ?
In the nuclear-powered artificial heart, the thermal energy from the decay of a man-made radioisotope, plutonium-238, is converted, through a s e r i e s of thermodynamic and mechanical steps, to blood hydraulic power. The possibility of so utilizing a plutonium-238 source first gave substance to the concept of a completely implantable artificial heart.
*Presented at a symposium on "Current Status in Artificial Organs," October 1973, Cleveland, Ohio. 181 Copyright @ 1975 by Marcel Dekker, Inc All Rights Reserved. Neither this work nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming, and recording, or by any information storage and retrieval system, without permission in writing from the publisher.
MOTT
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182
HEAT SOURCE
MECHANICAL CONVERTER
MECHANISM
VENTRICLES
FIG. 1. Components of nuclear-powered artificial heart. The major components of a nuclear-powered heart a r e illustrated in block form in Fig. 1. Heat generated by the plutonium-238 increases, for example, the temperature, and hence the pressure of the working gas in the energy converter. Expansion of the gas drives a power piston which is coupled through the blood pump drive to the pump ventricles. Connections between the converter, the drive, and the ventricles allow for feedback signals to assure the system is responsive to the needs of the recipients and for the disposition of the waste heat from the conversion process. Thermal insulation is required to isolate the high-temperature side of the thermal converter from the surrounding tissue. 11.
HOW HAS I T C O M E A B O U T ?
At the time when Dr. Kolff, one of the earliest of artificial heart researchers, was beginning to translate h i s thoughts on heart replacement into experimental devices, the use of small radioisotope heat sources to produce unattended power in remote locations was receiving a great deal of attention. The AEC was actively engaged in developing a series of isotopic power units for both manned and unmanned missions in space and for weather stations, navigational beacons, and other special installations in isolated spots around the world. (The first public demonstration of a device for converting isotopic heat to electricity took place in President Eisenhower's office in January 1959.) Thus it is not surprising that the idea of driving an artificial heart with a nuclear power source surfaced when i t did. Particularly in the early 1960s, little escaped the isotope technologists who were always on the alert for new problems to match with their solutions. Officially, the concept of a nuclear-powered artificial heart came
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NUCLEAR POWER FOR THE ARTIFICIAL HEART
183
to light with a proposal submitted simultaneously to the AEC and the National Heart Institute by the Thermo Electron Engineering Corporation (now TECO) in May 1964. This proposal led to a series of discussions between the two agencies on the course to follow. The outcome was that the Heart Institute would take the lead in better defining the total program to develop an artificial heart before either agency took an active role in supporting artificial heart power supply evaluations. In 1965 the Heart Institute initiated its comprehensive study to determine if an artificial heart program was called for, and if so, the direction it should take. The nuclear-powered heart was evaluated, along with biological fuel cells and rechargeable batteries, and was recommended for development. Subsequently (in 1967) the AEC in cooperation with the Heart Institute performed a conceptual design study of implantable nuclear power supplies for circulatory support systems. The study showed that the utilization of radioisotope heat sources for powering implantable heart devices was in principle technically sound. Each of the four thermal converter systems studied had potential for developing into a totally implantable system using state-of- the-art technology. However, although confirming technical feasibility, the studies were not in sufficient depth to establish practicability. That is, the key issue of whether a totally implantable nuclear-powered artificial heart system could be realized under the restraints that must be imposed on weight, volume, shape, performance, isotope inventory, reliability, lifetime, and cost remained an open question. Thus it was until April 1971, when the AEC began studies leading to an evaluation of the practicability of the nuclear approach. The practicability program was conducted in two phases, the first phase lasting for 7 months, the second for 19. The first phase consisted of two parallel analytical studies, one with TRW Systems Group, the other with Westinghouse Electric Corporation. The objective of Phase I was to select from the many alternatives the thermal energy conversion concept having the greatest potential of leading to a practicable fully implantable 10-year device for replacing the total heart. At the end of Phase I each contractor recommended to the AEC one concept for further detailed evaluation during Phase II of the program. Using preestablished evaluation criteria, the Westinghouse concept was selected for the follow-on work. Important factors in the selection were the lower weight, volume, isotope inventory, and heat source temperature and the higher reliability projected for the Westinghouse system. Also, it was felt that the Westinghouse approach had a better understood and developed technological base and, therefore, represented the minimum technological risk.
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184
MOTT
The first 8 months of the Phase I1 program was devoted primarily to a parametric analysis of the Westinghouse Stirling-mechanical converter and its associated blood pump drive, the next 9 months primarily to component and subsystem testing and to the fabrication and testing of a fully instrumented, nonimplantable but realistically sized, bench model of the Stirling-mechanical system, and the final 2 months to data analysis, to preparing an assessment of thermal converter practicability against preestablished criteria, and to report writing. The Phase I1 effort began in December 1971 and was concluded in June 1973. Although the major thrust of the practicability evaluation was at the means of converting the thermal energy from the decay of plutonium-238 to a form suitable for driving the ventricles of a blood pump, considerable attention was given to other issues. Basic to the conclusions were studies on plutonium-238 encapsulation and containment, on thermal insulation for the converter, and on the effects of radiation from nuclear-powered artificial hearts on the recipient, his immediate family, and the general public. To a s s u r e that firsthand information, guidance, and expertise on the biomedical engineering aspects of the artificial heart were available on a day-to-day basis to the AEC and its contractors, a contract was let in June 1971 with the University of Utah (Institute for Biomedical Engineering). During Phase I1 the Utah group under Dr. Kolff's leadership played an important role in establishing practicability criteria, particularly those relating to power and control requirements and to implantation factors, and in designing, fabricating, and evaluating the blood handling components of the bench model system. In February 1972 a program was initiated at the Cleveland Clinic Foundation (Division of Artificial Organs) to define the limits on the volumes and envelope dimensions of the major artificial heart components (thermal converter and blood pump) within which the components would be anatomically acceptable in a larg: percentage of patients. The Cleveland Clinic team lead by Dr. Nose was also deeply involved in developing and refining the practicability evaluation criteria. This overall program led to the following conclusions about the practicability of the nuclear-powered artificial heart when assessed against isotope inventory, weight, volume, shape, performance, reliability, lifetime, environmental impact, and cost.
1. A nuclear-powered artificial heart utilizing the Stirling-mechanical converter and blood pump drive system originated and evaluated by the Westinghouse Electric Corporation was practicable. In arriving at this conclusion the most important considerations were: a. Adequate blood pumping power can probably be obtained with a plutonium-238 inventory under 25 W without resorting to
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NUCLEAR POWER FOR THE ARTIFICIAL HEART
185
thermal energy storage so that neither future plutonium-238 availability nor the risks associated with the nuclear-powered artificial heart, whether to the recipient from the combined effects and radiation or to the members of the recipient's immediate family and to the general population from radiation, should be an overriding factor to limit the development, experimental evaluation, and clinical use of the plutonium-238 heart. b. Projected weights, volumes, and shapes for the system components not only were within the preestablished practicability limits, but the potential for making desired improvements existed. c. Conservatism in design largely compensated for the lack of direct experimental data to support the judgment that the system will function as required over a long period of time. That is, weight, and to some extent volume, were traded off against reliability primarily through the use of rugged components with demonstrated long life. d. Short- and long-term containment of plutonium-238 would be possible under all credible accident conditions when a vented capsule is used. e. Neither criticality nor neutron multiplication would pose problems in the unrestricted use of plutonium-238 powered artificial hearts. f. Plutonium-238 heat sources removed from artificial hearts would have no potential military significance. g. Extrapolations from first-of-a-kind models to mass-produced items suggested that the Stirling-mechanical artificial heart system can be made available in large numbers a t a cost of about $3000 per year, which is about one-half that associated with the home use of an artificial kidney. 2. Although no major technological breakthroughs are required to develop the Stirling- mechanical system into the prototype of a system for long-term animal studies and eventual clinical evaluation, advancements in the technology will have to be made in a number of areas. To perform the necessary research and development on components and subsystems and to convert the resulting technology into prototype hardware will require 3 to 4 years. 111.
TODAY
We thus arrive a t where the AEC is today (Table 1). Following the successful conclusion of the practicability study, a prototype system
186
MOTT
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TABLE 1. AEC Artificial Heart Program Milestones Proposal to AEC and NHI to develop nuclear-powered heart
May 1964
AECfNHI conceptual design study on nuclear power sources
June 1967 to December 1967
AEC practicability study on nuclearpowered hearts
April 1971 to June 1973
Initiation of prototype system development program
July 1973
development program was started in July 1973. A s part of this program, valuable information is being accumulated with the bench units built last year for the practicability study, and detailed design will soon begin on the system shown in Fig. 2. This system has an energy conversion unit, a blood pumping unit, and an interconnecting flexible shaft for transmitting and controlling the pumping power, Figure 3 pictures the Stirling-mechanical converter which provides rotary power to the flexible shaft of the blood pump drive. A s now
Scale-lnches
FIG. 2. Nuclear-powered Stirling-mechanical artificial heart.
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NUCLEAR POWER FOR THE ARTIFICIAL HEART
THERMAL INSULATIO
187
PU-238 HEAT SOURCE
AT EXCHANGER
OLANT LINES POWER PISTON
SHAFT iCTlON
OUTBOARD SEAL
Scale-Inches 0
~
I
1
~
2
I
~
l
~
I
~
l
~
FIG. 3. Stirling-mechanical converter.
conceived, the estimated weight and volume of the converter is 1.9 kg and 0.9 liters. The blood pump drive mechanism (Fig. 4) takes the rotating shaft output of 1800 rpm from the Stirling-mechanical converter and through reduction gearing and a scotch yoke mechanism actuates the pump diaphragms at 120 beats/min. The maximum design output of the pump is 12 liter/min against normal circulatory resistance. The calculated performance of the f i r s t implantable model of the Stirling-mechanical system is given in Table 2.
188
MOTT
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DIAPHRAGM DISPLACER CUP
/VENTRICLE
\
DOME
FIG. 4. Blood pump for Stirling-mechanical system. TABLE 2. Calculated Performance of First Implantable Model of the Stirling Mechanical System Initially
After 10 Years
Thermal input (W)
33
31
Converter efficiency (%)
14
17
Converter output (W) Pump drive efficiency (%) Power to pump diaphragms (W) Ventricular efficiency (%) Available power to blood (W)
5.6 85 4.8
70 3.4
5.2
85 4.4
70 3.1
NUCLEAR POWER FOR THE ARTIFICIAL HEART
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IV.
189
THE FUTURE
The objective of the AECs 3-1/2-year prototype system development program is to advance the technology leading to a fully implantable nuclear-powered artificial heart that can be qualified for clinical use in the 1980s. Under present plans the fabrication and testing of the prototype system should be completed by early 1977. In this program the first implantation in an animal of a total system (implantable version of the bench model developed during Phase I1 of our practicability program) will not take place before early 1975. Along the way to the prototype, advancements (other than those required on biomaterials) w i l l have to be made to: 1. Reduce the weight and volume of both the thermal conversion and blood pumping units. 2. Improve the efficiency and reliability of the Stirling converter, the power transmission mechanism, and the power coupling to the blood pump diaphragms. 3. Improve the system's responsiveness to the needs of the body a s directed by other developments. 4. Lessen the influence of thermal insulation and excess heat dissipation requirements on the overall design of the system. 5. Make the system implantable with minimum surgical risk without sacrificing reliability and life time.
The projected characteristics of the prototype system a r e given in Tables 3 and 4. Looking to the long term, the logical follow-on, upon the completion of the basic developmental work on the prototype, would seem to be an extensive program of bench and animal studies to qualify the device for clinical use and to obtain an understanding of the implications of replacing a natural heart by an artificial heart. Such a program could well run into the early 1980s. It appears now, a s it has in the past, that a major technical b a r r i e r to be overcome before a clinically acceptable heart replacement system can be realized is associated with the materials that interface directly with the blood. Needed is a material that will be blood compatible, impervious to the bioenvironment, and capable of flexing a few hundred million times. Another observation is that if there is to be a completely implantable artificial heart in the 1980s, it will be powered by plutonium-238; and without question a plutonium238 powered heart, regardless of i t s technological assets, will s t i r many more emotions and evoke much stronger criticism than would
MOTT
190 TABLE 3. Calculated Performance of Prototype System
Artif Cells Blood Substit Immobil Biotechnol Downloaded from informahealthcare.com by UB Kiel on 10/25/14 For personal use only.
Thermal input (W) Converter efficiency (%)
Initially
After 10 Years
24 21
22 21
Converter output (W)
5.0
4.6
Pump drive efficiency (%)
90
90
Power to pump diaphragm (W)
4.5 75
4.1 75
3.4
3.1
Ventricular efficiency (%) Available power to blood (W)
TABLE 4. Calculated Prototype System Characteristics Converter weight (kg)
1.5
Converter volume (liter)
0.7
System weight (kg)
2.2
System volume (liters)
1.4
System efficiency (%)
14
a heart powered by any other means. However, looking forward to 20 to 25 years of research and development, even with technology advancing a t a much slower rate than it is today, it is conceivable that breakthroughs, for example in battery technology, could lead to the early phasing out of nuclear-powered devices. We should look upon the nuclear heart, therefore, as the workhorse of the 1980s and 199Os, and certainly as the device, if there is to be a device, to establish the efficacy and the contraindications of the permanent assist and total heart replacement procedures. In closing I want to emphasize that the progress that has been made by the AJ3C toward the nuclear-powered artificial heart has only occurred as a consequence of a closely coordinated and cooperative effort by a rather diverse group of organizations and individuals. For the thermal conversion and blood pumping units the participants have been the Westinghouse Electric Corporation Astronuclear Laboratory
Artif Cells Blood Substit Immobil Biotechnol Downloaded from informahealthcare.com by UB Kiel on 10/25/14 For personal use only.
NUCLEAR POWER FOR THE ARTIFICIAL HEART
19 1
with i t s subcontractor the Philips Laboratories of the North American Philips Corporation, the University of Utah, the Cleveland Clinic Foundation, and the Universities Center- Jackson. Development of the source of thermal energy to drive the system required the cooperation of the Savannah River Laboratory where the plutonium-238 was produced, the Los Alamos Scientific Laboratory where the powder from Savannah River was purified and formed into a solid cylinder, the Mound Laboratory where the cylinder was encapsulated, and the TRW Systems Group where the source was designed and the encapsulating materials fabricated. Information on thermal and radiation effects has come from programs at the Pacific Northwest Laboratory and the University of Cornell, and assisting in the attack on the thermal insulation problem has been the Westinghouse Research Laboratories. My sincere thanks to all who have contributed to our past successes. I look forward to a continuing association as we proceed with the prototype development program. Dr. Kolff spoke yesterday of the voyages of Henry Hudson. When Hudson sailed west in search of China he had little to support him but his great conviction that he would succeed. As we proceed on the journey for an artificial heart we have an advantage over Henry as we can see a route, long and difficult as i t may be. But as surely as with Henry Hudson, forces may cause u s to sail up a shallowing river, into the depths of an endless bay, and finally to be landed on a rocky and barren shore from which no new journey can begin. Received by editor May 24, 1974
