In the past 2 decades, the ventricular assist devices (VADs) have emerged as a widely used modality for the treatment of end-stage heart failure, with significant impact on patient survival.1–3 Despite significant advances in pump technology, which have simultaneously decreased blood trauma while increasing reliability and patient comfort, the energy supply of implantable VADs still remains external. A driveline physically connects the VAD to an outside world, thus contributing to significant morbidity and potential mortality from infectious complications.4–6 Additionally, some fatal pump failures have resulted from electronic fraying of the driveline in modern rotary pumps.7 Even when functioning properly, extracorporeal parts of the VADs result in insufficient usability and suboptimal ergonomics.8 Portable batteries used to power the VADs only provide energy for several hours and must be carried by the patient, thus not allowing full mobility.
VAD Power Requirements
Work provided by the native heart varies between 1 to 6 watts (W) depending on age and physiological state; 1.75 W is often used as an average power.9
For modern rotary pumps, manufacturers report nominal power consumption up to 10–14 W.10 This is several thousand times higher than the power consumed by a pacemaker, which uses microwatts during normal operation and milliwatts for occasional wireless communication.11 Battery- or capacitor-based energy storage technologies are simply not capable of powering blood pumps for more than a few hours and should not be considered as permanent power sources. In contrast, even non-nuclear pacemakers are completely intracorporeal and do not require a driveline with an extracorporeal power source (pacing the heart to pump instead of pumping for the heart).
There are fundamentally three potential solutions to achieve powering an intracorporeal device with these large requirements. The first is a chemical process that uses ingested material to convert chemical energy into electrical energy within the body. Glucose fuel cells have been proposed9,12 and recently successfully implanted in an animal model,13 but the power requirements are very high and the associated technology is only conceptual.
The second potential solution, which has been pursued since the late 1960s14 but is recently being reintroduced,15,16 is percutaneous energy transfer via radiofrequency inductance. Although significant advancement has occurred, this technology has associated problems including the potential to overheat and difficulties maintaining alignment between the transmitting and receiving coils. Percutaneous energy transfer only affects the way the energy is transferred to the VADs (eliminating the need for a driveline), and does not address the insufficient energy density of the batteries. Unfortunately, it may take several years for advances in battery and percutaneous energy transfer technologies to actually affect the VAD patients.
The third potential solution to high-power requirements of implanted blood pumps would be in using a nuclear radioisotope as a power source, namely Plutonium-238 (238Pu). 238Pu is a manmade alpha-emitting radioisotope with a half-life of 87.7 years. Its radioactive decay emits alpha and gamma waves and produces thermal energy that can be converted into useful mechanical or electrical energy using either a mechanical engine or a thermoelectric generator. One milliliter of 238Pu can generate approximately 3.5 W of thermal power. Given this high energy density and long half-life, 238Pu could eventually allow a totally intracorporeal VAD that lasts the duration of the patient’s lifetime. A rigorous selection of this fuel as the ideal choice for this application is included in report by Huffman et al.17
As early as the 1960s, 238Pu was used to power implantable heart pacemakers as well as certain prototypes of VADs and total artificial hearts. A PubMed search of the term “plutonium powered” yields 27 results, mostly published in the 1970s as visualized on the histogram (Figure 1). Although use of 238Pu for mechanical circulatory support never passed the stage of experimentation18–20 and prototypes,21 the pacemakers powered by 238Pu were implanted in hundreds of patients who over the decades developed comparable morbidity when compared with conventional pacemakers, without side effects attributed to the radioisotope.22,23
Design of Plutonium-238 Powered Circulatory Support Systems
Between 1964 and 1973, the National Heart, Lung, and Blood Institute sponsored a well-coordinated development effort of fully implantable nuclear-powered circulatory support systems, including Total Artificial Hearts and VADs. Development efforts of an implantable nuclear source converged on a cylindrical fuel container approximately 1.25 inches in diameter and 2 inches long. This container was capable of generating 50 W of thermal energy (heat), which was converted to useful energy with fairly low (4–7.5%) efficiency engines.24 The temperature of capsules reached as high as 1000 degrees Fahrenheit, so thermal insulation, in addition to the radiation shielding, was clearly required. Researchers identified three primary technical hurdles that had to be overcome, and studies that were fairly convincing suggested that many of the risks were surmountable.
1. Rejection of the waste heat: Any heat emitted from the nuclear source, which is not converted to useful energy, because of either engine inefficiency or an excess supply of heat, must be rejected from the body. Several groups conducted experiments demonstrating the ability of animals to reject this waste heat.25
2. Controlling the emitted radiation to levels safe for the local tissue: Extensive modeling,26 benchtop,24 and animal implant experiments were done to predict the dosage of radiation surrounding the capsule as well as the safety of these levels of radiation. Animal models using both unshielded27 and shielded28 sources resulted in apparently normal histology.
3. A very substantial part of the original design effort decades ago (Figure 2) was on the mechanical engine necessary to convert the heat provided by the 238Pu capsule into pneumatic or hydraulic energy required to drive the pump (Thermo Electron), or the development of a mechanical scotch-yoke system to drive the pulsing ventricle (Atomic Energy Commission pump). The complexity and unreliability of these mechanical engines were perhaps the most difficult engineering hurdles to overcome.25,29 Nowadays, because nearly all modern rotary VADs are electrically powered, the transfer of energy to the pump could conceivably be done with thermoelectric solid-state devices that convert thermal energy to electrical energy based on the Seebeck effect.30 This technology could be remarkably less complex and more robust as compared with the mechanically complex Stirling29 and Rankine31 cycle-based engines of the past.
Of course, 238Pu is a very dangerous biohazard and is not free of risks. If used in implantable medical devices, its leakage could pose significant risks to patients and others around them.32 Its ubiquitous presence in assist devices could create a risk of theft and inappropriate use such as nuclear arms.33
In 1978 a Stanford cardiothoracic surgeon, Dr. Eugene Dong, published a science-fiction book called Heart Beat, which tells the story of a dying man who receives a 238Pu-powered total artificial heart, subsequently getting kidnapped by a terrorist who threatens the entire Bay Area with nuclear contamination.34 Of note, the book begins with a quote from Bertolt Brecht’s Galileo: “I submit that as scientists we have no business asking what the truth may lead to.” The book became so popular that it was again published in 1982, clearly showing public concern about this topic.
Given the rising concerns,35 the use of 238Pu was discontinued in implantable medical devices before 238Pu-powered VADs reached the clinical stage. Currently, 238Pu cannot be even used experimentally for this purpose.
Current Applications of Plutonium-238
Notably, more recently 238Pu found its role outside our planet, becoming a preferable nonsolar source of energy in space technology. 238Pu-powered spacecrafts like Gallileo, Cassini, and New Horizons have proven to function very reliably far away from the sun. Most recently we witnessed the launch of Mars Science Laboratory, the most advanced Mars rover to date that has a 238Pu-powered thermoelectric generator (Figure 3).
In summary, 238Pu is an excellent energy source; however, the risks of leakage and theft discouraged the community from pursuing this technology and from approving its use many decades ago. If acceptable system controls could be shown as having overcome these safety risks, 238Pu would be an ideal energy source for this purpose.
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Ventricular assist device; Plutonium-238