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TABLE OF CONTENTS

STATEMENT OF OBJECTIVES

I. GENERAL DISCUSSION

A. Directly Obtained Electricity .3

1. Neurornuscular Potentials 3

a。 Nerve Potentials 4

b. Muscle Potentials

Cardiac Potentials.

2. Direct Current Systems

a. Physiological Electrode Cell

b. Fuel Cell

B. Thermoelectric Converter .12

C. Mechancelectric Converter: 14

1. Sources 14

2.Transducers 13

a. Piezoelectric Crystals 19

b. Permanent Magnet Generators

3. Previous Work

IV. BIBLIOGRAPHY

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STATEMENT OF OBJEL STATEMENT OF OBJECTIVES I. 1.

The object of this study is to survey past work in the area of body produced electrical power as reported in the literature, develop an urderstanding of the various methods and approaches which have been suggested as possible sources, and present this information in a useful form. This report is directed towards the general problem of providing electrical power to implanted electronic devices, such as artificial cardiac pacemakers, requiring less than 200 microwatis for an indefinite time from a completely self contained internal system.

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SUMMARY

A general literature review was first undertaken to both discover previous experimental work and to gather general impressions of thinking on this subject. The review indicated that there were few well documented concluslons available and that although a fresh start night be duplicative, it is needed for a systematic study. Conceivable sources were listed and each has been briefly considered from the physiological and instrumental viewpoints. In trying to evaluate potential usefulness of a source it was sometimes necessary to form conceptual designs of mechanical systems which could utilize that source. Comments on these designs are included in the discussion as contextual information because they illustrate those aspects of the systems which were considered and the design problems encountered, not because these designs. are feit to be the proper solution. We concur with recent statements by investigators active in experimental work that the physiological Galvanic cell and mechancelectric conversion are the two most prcmising systems. although much work is nècessary before either is useful for practical purposes. Throughout this study we repeatedly encountered unanswered questions of 'ne possible effects of biological adaptation on the total implanted system. These effects need not be entirely negative. Adaption in a direction tending to increase power output is a possibility deserving of serious attention.

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HI. GENERAL DISCUSSION

A.Directly Obtained Electricity

Since our goal is electricity produced from an energy source within the body, the simplest system instrumentally and the first which should be evaluated is the direct electrical tap. The systems under consideration in this section are all those that contain electrodes that are in physical contact with the biological environment and in which electrical potentials between electrodes can be measured. These potentials are due to, or at least intimetely associated with, time and space variations in ionic concentrations and fow. The ultimate descriptions and explanations and definitions of these potentials are within the science of irreversible thermodynamics and the subject of considerable controversy. (19) For the purpose of this discussior we will generally consider these potentials operationally. that is, according to what you do to obtain a measurable potential and the characteristics of that potential.

1.Neuromuscular Potentials

The most studied biological electrochemical phenomena 1 are the neuromuscular action potentials. At the cellular level microelectrodes of several microns diameter inserted through neuron or muscle cell membrane indicate potentials of tens of millivolts between points within the cytoplasm and between cytopiasm and extracellular 1 fuid. The time course of variations in these potentials is from 3 millisecond to hours. These "membrone potentials" in themselves are of no use for our purposes because of the fantastically low currents and short life ofa cell damaged by perforation of its membrane. But

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the gross extracellular phenomena associated with the summated effects of thousands of nearby active cell sectiors in nerve and muscle bundles deserve consideration. When a metal wire or dise is placed near a nerve, "compound action potentials" are observed between that electrode and another placed in the body, and when a muscle is used, the "electromyogram" is observed.

Nerve Potentials2。

The use of neural electricity as an energy
source can be dismissed as not feasible on the grounds that drawing
currents wilì cauce stimulation of the excitabie membrane's and dis
ruption of normal function. This conclusion results from observing
that whatever electrical energy is produced is the result of neural
activity (by assumption) and that that level of electrical current must
of necessity be sufficient to cause stimulation since it was generated in order to produce (natural) stimulation. There is no way to avoid returning current to the electrode which supplied it when the voltages irvolved are less than the .4 volt minimum for germanium diodes, and while the two electrode current distributions will differ, it seems reasonable to expect that at least some units will be stimulated. The loss of even a few units in the PNS is undesirable. Also, since neural potentials are of the order of 100 millivolt, a source resistance of 12 ohm is needed to obtain 200 microwatts, This requirement is not compatible with the source. Consider for example, the input imnerve compound action potentials..

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b. Muscle Potentials

With muscles the situation differs in several respects that make power pick up from this system at least conceivable. Namely, the volume of muscle is large compared with neural tissue and stimulation of a few units should not seríously disrupt normal function. The large muuscle areas may allow many electrodes to be used simultaneously in order to provide a lower electrical source resistance. For example, if one pair of electrodes prcvides a source resistance of 500 ohm, 40 pair wüll provide 12 ohm. In order to utilize this low voltaxe ac source a miniature transformer can be used to step up the voltage before rectification. From a power standpoint it shouid te raore efficient to u one transformer and rectifier with each pair or erectrodes and sum . dc outputs. frequencies above 100 cps, subminíature transformers( Sirce skeletal muscle EMG signals contain most of their power in 15 1 az, 1 40 in. 31 can be used, Answers to Important blological questions were not found in the literature. No reports describing investigation of EMG signals as power sources were discovèred, although use of EMG signals for control purposes have been frequently studied. Whether or not a low enough source resistance can be achieved and maintained and what type of electrode and implantation is best are questions which will probably have to be answered by experiment. Careful technique may prevent electrodes embedded in muscles performing large movements from causing irritation and pain but this wüll also require study. Judging from the experience ol clinical workers with implanted cardiac pacemakers, small electrodes can be tolerated but dislodgament and lead breakage may be serious problems.

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c. Cardiac Potentials

The strong rhythmic contractions of the myocardiumattractimmediate attention but the characteristics of its electrical potentials present particular difficulties. Unlike skeletal muscle which maintains contraction with a high frequency train of "spikes", cardiac muscle fibers depolarize and repolarize only once per beartbeat. Thus, the energy is contained mostly in low frequency components (1-20 cps). Since the contraction of all areas of the muscle is synchronized, large electrode areas can be used without the losses which would be associated with large electrodes on skeletal muscle. (Current from active non-synchronized skeletal muscle fibers could pass tiru a large electrode to inactive'tissue without passing to the second electrode ol the pair.) Bùt while this allows only one transformer to be used on the heart with one pair of large electrodes, transformers which are designed for very low frequencies are relatively lar ze. heavy and Inefficient. Regardless of the differences in waveform, the ECG on the heart is less than 100 mV (possibly only 10 mV) and thus a maximum soirce resistance of 12 ohm is again required for 200 uW power yield (àssuming the 100 mV peak figure). Unless a radically improved electrode material is developed this resistance is not likely to be achieved, Effective source resistance figures for common electrode materials directiy on muscle have not been found in the literature, and an experíment appears necessary to determine what resistance can be easily obtained and how this might change with time.

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2. Direct Current Systems

These systems are all those that produce electron flow in one direction only between the two electrodes. This is unlike the neuromuscular potential electrode system in which a capacitor would be placed in the circuit to insure that no net charge flows from one electrode to the other. In the neuromuscular system the electrical energy of interest is " ternating current" in that charge flows from electrode A to electrode B and then, milliseconds later, returns to A. In the neuromuscular electrode system net current flow would provide no useful work and undesired reactions might accumulate products on or near the electrodes that resuit in "polarization" and/or electrodedeteriorationwhichinterferes with the desired action. This type of electrode activity, however, can become the desired activity in direct current systems.

a. Physiological Electrode Cell

The Galvanic cell in its simplest and classical form consists only in two different metals (or other conductors) dipping into a common tonic, solution An electric potential, characteristic of the metals, temperature, ionic species and concentration, can be measured between the non-immarsed portions of the two metal electrodes, Since the fluids existing within the body are ionic solutions inter-electrode potentials can be prodcced by inserting two clssimilar conductors anywhere. Because of the complexity of the body fluid composition, variation in composition at different points, induced effects from the presence of foreign material, presence of many membranes with unknowi properties, active processes, ete., ete., the actual chemical reactions and inter-electrode potential can not be predicted, in fact such potentials are not strictly defined. (Nims, p. 5).

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We can consider that there are two general types of reaction
which occur with implanted electrodes. The first, which we will
call type 1, is like that in the classical Galvanic cell described above
in which one or both electrode materials enters into the reaction and..
becomes irreversibly altered or lost. The second type of· reaction
is possible when membranes are present and the chemical environ
ment differs at the two electrodes. In this type 2 reaction, which in
its simplest form is the classical concentration cell, irreversible change
to the electrode surfaces need not occur and the electrodes need not be
dissimilar. The rate of the chemical reactions may be much improved
by dissimilar surfaces; however, for examplè by catalytic action, in
creased effective area, inducement of Iocal environmental change, etc.
This second type of activity can be considered as a fuel cell with the phystological system main aning all the reactants and removing the end products. A special  p:icationof the type 2 system is when similar
, electrodes of "inert metals" or of "non-polarizable" liquid filled tubes are used in conjunction with very high impedance voltage measuring circuits which insure that the potential chemical reactions do not occur at the electrodes. This arrangement is used for investigating so caned natural dc potentiàl gradients within and on the surface of the body (4). Such measurements necessarily must draw virtually no power fromthe chemical energy sources responsible: for the electrode potentials, for as soon as current is drawn reactions occur and the natural concentrations change. Thus any de current producing electrode generator scheme useful for our purposes will have inter-electrode potentials which are more or less unnatural physiologically. That is, the potential difference between electrodes of any type drawing current will be different from that measured between "non-polarizable" electrodes drawing no current, and the local chemical environment surroundingthe electrodes may be grossly

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different. For these reasons the type 1 system in which electrode material change occurs and the type 2 system in which it need not are considered here as two special cases of the general physiological electrode cell system.

The general case of which the type 1 and 2 systems are examples, is when dissimilar metals, which produce a potential when dipped into a comon ionic solution, are placed in a nonhomogeneous environment that produces a potential between similar electrodes. This is probably a fair description of the situation prevailing in the electrode materiai, electrode placement combinations with which Dr. John Konikoff and others produce the best results, The Konikofl work is a significant source of experimental data and has stimulated much of the recent interest in thè physiological electrode cell power source. For these reasons a brief summary of the work reported in :- reference 12 is included here. The reader is referred to the original paper for details. 1

John Konikolf and Luther Reynolds were the principal workers at the General Electric Company's Space Sciences Laboratory under a contract with NASA in 1963 - 64 to investigate the use of what is re'erred to here as physiological electrode cell potentials as a biologically derived power source. Many combinations 'of electrode materials in several anatomical locations in several species of laboratory animals were tried. .Their final choice of electrode materials was "high speed steel (75% Fe, 5% Cr, 18% W, .3% V, . 7% C)" and a specially prepared "platinum platinum-black" combination. The final choice of location was as follows, "... the PPb electrode was located in . the abdominal cavity dorsal to the peritoneal membrane : and HSS situated subcutaneously but physiçally adjacent to the abdominal incision." The

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longest continuous implant was'128 days, electrodes and sites were as above, The animal was a. rabbit, and a constant resistive load of 10, Qoo ohra was applied between electrodes. After 15 days the output stabilized and thereafter remained at 24 microwatts and,5 volt, The highest power reported in short term studies was 308 microwatt. No new work from either Konikoff or Reynolds has been published since 1954, Telephone conversations with both men indicate that work is continuing, and that recent improvements in the platinum-tlack electrode material have increased the power output threefold for the same electrode area,. Reynolds who is now at Hahnemann Medical College, Philadelphia reports that 200 micröwatts -hasbeen obtained when each 2
electrode is of 1/2 in" area. Thts electrode power generation scheme has the advantages, according to the originators, of simple surgical procedure and no harmful tissue reaction or loss of output at least łor 4 months in the one long term rabbit experiment.

According to the data in the Konikoff report and especially the recent report of Strohl et, al.' (29), when "blologically inert" metals such as plattnum and type 316: stainless are iinplanted, power Ievels greater than 10 uW have not been obtained and the output drops significantiy below this after a few days. Strohl's comments on the inevitable growth of a fibrous membrane around implanted electrodes suggests that the electrodes become isolatsd from the original, dissimilar fonic environments as this membrane grows. The better power outputs and longevity have been obtained in conjunction with an electrode which actively reacts with species present in the exiracellular fluids. Even when covered with (hypothetical) cells tending to maintain identical fonie concentrations around the two electrodes, a reactive electrode can continue to provide current. In evaluating an electrode cell system contpintig reactive electrodes importánt considerations are toxicity of

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products and deterioratioa of performance with time. As Strohl notes. Faraday's first law predicts the electrode weight loss due to tonic solution when the electrode reactions are known quantitatively. For example, .91 of iron wilì be needed to supply 100 uA for 1 year. But at least as important are the hard to predict effects such as loss of effective surface area "catalyst poisoning", uneven surface deteripration and long term Iocal tissue reaction. In conclusion, it appears that there is a reasonable possibility that physiological electrode cells can provide 200 uw for extended periods, but careful long term studies.and an understanding of the active phenomena, which, hopefully, will provide the basis for optimizing the electrode materials, arenecessary. But the simple surgery in low risk areas which has been used, the mechanics - no moving parts, the nondependence on any bodily motion, the inherent freedom from encapsulation problems, and the short term results already achieved, combine to make this a most promising system at this time.

Fuel Celi

1 "Sophisticated direct current systems have been speculated on for producing relatively large quantities of electrical power for running proposed artificial hearts. These systems are usually referred to as fuel cells and usually are considered in reference to known chemical energy sources such as glucose or ATP. These systems are very appealing, largely because the proposed energy source is faicly well understood. Molecular energy yields, available concentrations and naturally occuring reactions can be stated. The development of physical systems to utilize these sources then appears to be a problem amenable to present technological capability since the avail-. able raw materials and necessary operations are known, at least

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in broad outline. This is in contrast to the simpler Galvanic cell systems discussed earlier in which the present state of the art has been reached largely by trial and error without benefit of thorough understanding of the detailed processes involved. Approaching the problem from basic principles and proceeding in accordance with established theory will no doubt achieve practical success in time. The National Institutes of Health recently circulated a Request For Proposal to undertake feasibility studies of implanted biological fuel cells. When these initial studies are completed we wll have a statement of the problem and outline of needed research. For the immediate future, however, the simpler a proposed system is, the greater appears its chance of success.

B. Thermoelectric Converter

Temperature gradients within the body theoretically can be expioited as a source of electrical energy. In recent years considerable research on thermoelectric compositions for use with nuclear reactor heat sources has produced materials with thermcelectric properties much improved over those of conventional thermocouples. For example, a conventional copper-constantin couple will produce 23 microvolt per fahrenheit degree temperature difference while a material 0 of Bismuth-Antimony-Telluride composition produces 77 microvolt/F (l1, 8). Simple calculations using this second figure indicate that with a 5FO temperature difference and 1 ohm resistance for every element 2500 elements connected in series will yield 200 uW at .5 volt. A Japanese group (32) has published a report of a 150 element thermoelectric generator for use on the external body surface. Their device used the Bi-Sb-Te material and the size of the thermoelectric array appearsto be about 2.5 cm x1cmx5cm. The data presented in their report are tot

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clear and well organized and therefore the following calculations based on that report may not be completely correct. A maximum voltage af about 4S0 millivolt (open circuit?) is reported. A series $\overrightarrow { a } \overrightarrow { v } / \overrightarrow { b } ^ { \circ } .$ will produce array of 150 elements of a material producing 77
450 mV at a temperature difference of $3 9 8 ^ { 0 . }$ Since some of their $1 0 ^ { 0 } C ( 5 3 ^ { 0 } E )$ air temperature wíth evaporating alcohol on work was at the cold junction, this temperature difference is possible. The only power output figure mentioned is 20 $\operatorname { u w } / \ c m ^ { 2 }$ . If this was obtained under conditions which produced a .45 volt open circuit voltage and uf $\cos ^ { 2 }$ then the indicated internal their device contacted a skin area of 2. 5 resistance of their device is 2000 ohms, or roughly 13 ohm per element. If this resistance figure is realistic for thermopüles composed of ele ments of 2mm x 1mm.x 5mm size then the 2500 element array mentioned above would produce only 1/3 of the assumed 200 uW or only 15 uw. A total resistance of 13 chm per element appears unnecessarily high, how ever, according to the following calculation, -The resistivity of Bi-Sb-$7 \times 1 0 ^ { 4 }$ ohm - cm (8). Hence an element of the above dimension Te is only should have only 17 mllohm ¿iternal resistaņce. The refore the actual electrical resistance is almost entirely contributed by the contact between the thermoelement and the heat sink conductor and is largely a problem in technique. According to reference 26, contact resistivity in eleraents used in thermoelectric power gènerators may vary between
3 and 4500 microohm $\cdot \sin ^ { 2 } .$ The higher figure indicates a contact resistance of , 23 ohm for an area of.02 cm and, since there are two contacts per element, a total contact resistance of .S ohm for elements the size of those in the Japanese device. This last calculation was the basis for our original assumption of 1 ohm per element.

The conclusion we rcach is that a 2s00 element array operating $5 F ^ { 0 }$

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at each heat sink and a depth of.5cm will produce 200 uW at.5 3
volt, By comparison, 25 cm of medical grade mercury cells
(8 Mallory RM CC- 1W) (20) has a capaeity sf 8 AH which,
n.eglecting age derating, wilt supply 200 uW for 5 years at 1.4
to 10 volt. Since the failure of any one of 5000 contact points in
the series connected array wll cause system failure, and since a 2
5 Fº temperature difference between two 42 em areas., 5 cm apart
does not naturally and rellably exist within the body, the thermocouple system is considered to be not competitive:with conventional
batteries for an Implanted power source.

C. Mechanoelectric Converter è

In this section possible mechanical energy sources will be considered together with mechanical couplirg schemes. Aa arbitrary eriterion of 1 milliwatt net mechanical work in the coupling system was chosea as a practical minimum power level for a final electrical output of 200 microwatt. Brief consideration;of actual mechanical to electrical transducers. namely, piezoelectric erystals and permanent magnet generators, is included. We make the provisional assumption In this section that if a mechanical system can be implanted which wül perform 1 mw work, for example in winding a spring, for over a year, then a transducer can be designed to utilize this energy. Other than work based on electrode cell potentials all known experimental implant power generation has been with piezoelectric crystals.

1. Sources

The obvious mechanical sources are:

Voluntary muscle, joint and limb movements

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Peristalsis (dismissable on grounds of insufficient power)

Respíravory system - rib cage motion. diaphragm muscle, thoracic and abdominal "pressure variation"

Cardiovascular system - heart motion, aorta and large artery pulse expansion, blood flow

Gross body acceleration (self winding watch principle, "random motion power")

Movements associated with voluntary activity in some cases offer Iarge quantities of mechanical power.,"The intermittant character of this activity means, however, that an energy storage system must be included in the design to supply power during periods of inactivity. Rechargeable batteries are the obvious storage device, especially since they are desigmed for and requi.e relatively hign current, short duty cycle charging. These batteries require 50 to 100% more charge current than they return, however. Therefore, any intermittant generator will have to supply 300-400 microwatt average electrical charging power if the tattery undergoes 200 microwatt constant drain. It may not be unreasonable to depend upon or require some particular voluntary movement being performed at some minimum rate for many months, but unless a particular application requires power only during a certain type of activity, it seems more straightforward to couple a motion generator to a continuous activity, such as respiration and blood fow, in which the rate and other operational norms and limits are predictable and unavoidable,

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Respiration

The first continuous motion source which we will consider is respiration, Since the object of respiratory mechanical motion is to pump air; .a fluid flow system operating oa the pressure volurmne changes found in the thoracic and abdominal cavities during the respiratory cycle is an obvious possibility. During conditions of quiet rest the variation in pressure within the human adult thorax is approximately 3mm Hg millijoule per breath must be done for an average mechanical power of 1 milliwatt. If we approximate the phase lag to be expected between pressure and volume by assuming no phase lag but with only one half the pressure variation-(i, e.'.02 Nt/cm), then the volume of fluid (silicon ou, gas, isotonic saline, ete.) which must be pumped each breath according to the relation PV  2 x 10ª joule is 10 em³. Since the volume calculated in this manner is inversely proportional to breath rate and intrathoracic pressure, the 'volume required in most experimental animals will be less.

An elementary,non-differential system responsive to respiratory 1 pressure variations of 3mm Hg would probably be disabled by normal atmospheric pressure variations of one or two inches of mercury. ,Insensitivity to ambient "dc" pressure is inherent in a differential system, however, and because the intra-abdominal respiratory pressure variation is out of phase with the intrathoracic, two bellows, one in each cavity. connected by a tube would comprise such a system. It appears that this system can provide the necessary mechanical energy without obvious size and weight objections. A simple implantation procedure with a subcuianeous tube tunnel is conceivable, although all surgical questions as well as those on materials, size, shape and irritation require extensive

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design and experimentation. In summary, a respiratory fluid pumping system is recommended as deserving of further attention.

Direct mechanical coupling to respiratory motion remains as another possibility. The change in dimension of the rib cage and diaphragm are attractive. The method of coupling might be something working on the principle found in retracting tape measures. A cable is wound on a drum and a spring tends to keep the cable wound up. If. the drum package is firmly attached in some convenient location and the cable held against the under side of the diaphragm or in a subcutaneous tunnel around the chest with the far end of the cable attached, then the drum would rotate back and forth dùring each'breath. A ratchet drive to wind a second spring would allow for any "zero position" of the cable. extension with the second spring drivinigthe actual transducer. Perhaps placing the cable inside a silicon rubber tube filled with silicon grease, the tube being of the bellows type to'allow it to lengthen easily. would improve the sealing and tissue irritation situation. While quantitative data on the diaphragm has not. been sought, it certainly appears that sufficient power is ayaÜable from dlaphragm mnotion and also from chest expansion, The main problems are expected to be in materials, packaging and surgical technique, Apart from material fatigue and sealing, tissue erosion and cell destruction from too great applied pressures must be avoided, for even living tissue appiying pressure unnaturally (e. g, an aneurysm) can erode its way through other tissue. The experience with bone plates, wires and other prostheses which have been used for many years showsthat direct mechanical attachments to internal structures can be accomplished, however.

Cardiovascular System

The other continucus mechanical source is the cardiovascular system.

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The work of Doctors Parsonnet and.Kennedy demonstrates, at least for short perfods. that the expansion of the great arteries and the movement of the heart can be tapped for mechanical power. Specfic conuments on these sources are included in the discussions of their experiments. In general, however, a significant design problem is concerned with sccomodating long and short term variations in the properties of the system being coupled to. With the arteries some of these variables are changes, whether natural or induced, in the artery cross section, arterlal wall elasticity, average blcod pressure, systolicdiastolic differential pressure, postural configuration and relative direction of gravity.

Random Motion

Gross body acceleration operating on a mechanical system similar in principle to the self winding watch refers to voluntary motion, espécially walking, and the criticism of non-continuous sources applies. The only detailed consideration of this system is found in Dr. Long's article (1t). The advantage of this system is'that all the operating parts can be enclosed in a hermetically sealed rig.d box. Except for the weight involved this box could be attached to the diaphragm or heart to take advantage of the continuous motion in these locations. But in order to demonstrate that the welght is prohibitive, consider à mass of M kilograms being forced to move back and forth over a distance of lcm according to the sine law at a frequency of 1 per second, The maximum velocity achieved will be $\pi = 1 0 ^ { 2 }$ meters/second. The kinetic energy at this point is $\frac { 1 } { 2 } N V ^ { 2 } = ( \frac { \pi ^ { 2 } M } { 2 } N ^ { 4 } ) j$ joule. If we could somehow utilize all this energy each cycle,the mass for 1 millijoule is approximately 2kg. While the force necessary to accelerate this mass is only .2 Nt, which

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could probably be provided by the diaphragm, the force necessary to support this mass against gravity is 20 Nt, which the diaphragm could not support. The artificlal heart discussants are seriously considering weights of this magnitude for long term implantation; thus we cannot a priori dismiss a random motion system as unworkable, but it does not appear to be competitive with conventional mercury bateries in power per pound.

2. Transducers

à. Piezoelectric Crystals

The mechanical energy to electrical energy transducer most often considered for úse in biological power applications is the piezoelectric crystal. Manufactured crystals of lead zirconate, lead titanate (PZT) composition have far superior properties for power transduction than do natural crystals such as quartz and rochelle salt.. These manufactured crystals are produced in a form known as "ceramic bimorphs". Quantitative data on the relevant characteristics of these peizoelectric ceramics have been developed as part of this study from information available from the Clevite Corporation (9, 10, 22). Data of this sort are necessary for evaluating the practicablity and design requirements of this method of power generation.

Efficiency

One of the most appealing characteristics of these transducers is an attainable conversion efficiency greater than 50%. This efficiency refers to the ratio of net mechanical energy' supplied to the crystal to electrical energy supplied by the crystal under optimum conditions of mounting and matching. The simplest system for driving a crystal is to have the mechanical source directly coupled, that is, when the source (e. g. expanding aorta) moves,the erystal is deformed. proportionately.

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/With this type of mechanical coupling a differently defined "efficfency" is significant. Net mechanical work means total work done on the crystal to deform it minus the total work done by the crystal as it relaxes to its unstressed state, $[ \int \limits _ { 0 } ^ { \infty } F _ { i } ^ { ( x ) } v ( t ) d t ]$ . Since with direct coupling the source has to be capable or supplying enough work to deform the crystal, and since as far as the transducer is concerned any work it does on the source (during the relaxation phase) is lost forever, the ratio of total work done on the crystal

$$
[ \frac { 1 } { 2 } \int \limits _ { 0 } ^ { \infty } [ | \mathbf { F } ( t ) \mathbf { v } ( t ) + \mathbf { F } ( t ) \mathbf { v } ( t ) ] d t ] ^ { - 1 }
$$

to total electrical work done by the crystal is a meaningful figure, The usual model for these $\cos y \sin z$ is:

Electrical

Mechanical

The Clevite literature contains tables for calculating: Ce, an electrical capacitance; $\mathbb { M } ,$ the transducer ratio in volts per Newton; $\mathbf { C } m _ { \theta }$ the mechanical compliance; and $\mathbb { M } _ { \ell }$ the mass, in terms of: the dimensions, $t , \tilde { w } , \tilde { \mathrm { T } } ;$ specific crystal type, PZT-SB, PZT-5H, , etc. Ltype of connection within the bimorph, series or parallel; method of mouriting and drive, e. g. cantilever mounting with driving force at the free end. With the above model the internal stored electrical energy under short circuit conditions is $w \equiv \frac { 1 } { 2 } \cos ^ { 2 }$ where V = NF with F the force in Newtoas. If an external capacitance C is connected and the crystal deformed by a force $\mathbf { r _ { \star } }$ external work will be done in charging this capacitor. It can be shown that for the greatest external work, the external capacitor must be equal in value to Ce in which case $\frac { 2 } { 4 } \pi _ { \theta }$ joules are supplied, Since in actual use a bridge rectifier would be used which allows an equal quantity of electrical work to be done as the crystal relaxes, $\frac { 1 } { 2 } W e$ is the theorctical maximum electrical energy avalable. The mochanfcal work done on the crystal in deformation is approximately $\frac { 1 } { 2 } F D _ { i }$

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where d. the deformation, ts CmF, thus $v _ { \sqrt { 1 } } = \frac { 1 } { 2 } c m ^ { 2 } .$ The ratio $i s : \frac { \frac { 1 } { 2 } \pi e } { \sqrt { 2 } \pi } = \frac { - \frac { c _ { e } \pi ^ { 2 } - 2 } { 2 } } { 2 c _ { e } \pi ^ { 3 } }$ which for a PZT-5B parallel bimorph

2 x1σ9 LW 3 L 2 T WTI cantilever is which reduces to (2) (2.8 × 15) C3 WT 3

,032 or 3.2%. Of great significance is that all dimensions and the magnitude of force drop out! Also, a series connected cantilever bimorph, and an end - supported - center - driven mounting of either series or parallel connection can be shown to have the same efficiency, and the PZT - 5H material differs.only slightly. Thus. while choosing dimensions, mounting, ete, will certainly effect the quantity of electrical energy produced in a direct-coupled system, this ent-gy can never be greater than 3. 2%of the mechanical work supplied (as defined above). Mustrations of the significance of these figures will be found in the discussion of previous experimental work.

Output

It was shown above that the maximum electrical energy which can be produced in one deforrnation cycle is $\frac { 1 } { 4 } \leq e ^ { 2 } N ^ { 2 } F ^ { 2 }$ The value of F, of course, can not exceed the force necessary to fracture the crystal. For a cantilcver beam or length L and thickness T, the strain is

$$
S = \frac { 3 \pi } { 2 i ^ { 2 } } D
$$

where D is the distance the free end is displaced. Maximum strain before fracture is an intrinsic material parameter and one value holds (approximately) for any configuration. Mr, Carmen Ceraano of Clevite

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has recommended $5 \times 1 5 ^ { 4 }$ (5o% of the fracturing strain)as the maximum strain to apply in a real system. We can assume a maximum deñection, then, of Dmax $= \frac { 2 I ^ { 2 } } { 3 I } \cdot 5 \times 1 0 ^ { 4 }$ for a cantilever mounted crystal and a maximum force of Fmax $= \frac { D \ + \sin x } { \cos x }$ (D in inches implies Cm in in/Nt). Electrical output per deflection is, therefore, for a PZT-5B parallel bimorph cantilever:

$$
\begin{array} { r l } { \frac { \varsigma } { 4 \mathrm { \ c m } ^ { 2 } \bar { D } \mathrm { m a x } ^ { 2 } } } & { = \frac { \big ( 2 \times 1 0 ^ { 3 } \frac { \mathrm { L W } } { \bar { \Upsilon } } \big ) \quad \left( , 3 \frac { \mathrm { L } } { \mathrm { W T } } \right) ^ { 2 } \left( \frac { 1 } { 9 } \frac { \bar { \Upsilon } } { 1 0 ^ { 3 } } - \frac { \mathrm { L } ^ { 2 } } { \bar { \Upsilon } } \right) ^ { 2 } } { \big ( 4 \big ) \ \left( 1 , 1 \times 1 0 ^ { 7 } \frac { \mathrm { L } ^ { 3 } } { \mathrm { W T } ^ { 3 } } \right) } , } \\ & { = 4 2 0 \ \tilde { \mathrm { L W T } } \mathrm { m i c r o j o u l e } , } \end{array}
$$

with L, W, T in inches. Since the bimorphs are only available in thicknesses close to 20 mil, the maxîmum output per deflection is 8.4 microjoule per square inch of area. Again, it can be shown that this figure is obtained for series as well as parallel connection, for center drive as well as cantilever.

The unic area output per deflection can be increased four times by allowing a "denlection"to be from - Dmax to + Dmax and back again, thàt is, by Including a spring so that with no "external force" applied the crystal is defected in the opposite direction from that which the external force produces. This variation in mounting does not effect efficiency since four times as much mechanical work is required. At the maximum strain of $5 \times 1 0 ^ { 4 }$ with a spring loaded crystal, the absolute minimum surface area necessary to produce 200 uW is $\frac { 6 } { N }$ square inches, where N is the number of deflections per second, and, according to our previous definition of 3, 2% efficiency, at least S250 uW mechanical power to deform the crystal(s) is required. In order to extend the lifetime of the crystal by reducing fatigue it will probably prove

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necessary to use smaller deflections than were assumed in the output calculations above. The output varies as the square of the strain however, so that at a maximum strain of 25% of the fracturing point $\frac { 2 4 } { 1 5 }$ square inches of area are required.

If added mechanicat complexity is introduced (e.g. springs. ratchets, escapements, etc.) all of the work during the active phase (e. g, systole) can be retained in the crystal transducer package. While extra moving parts will cause losses themselves, some improvement in efficlency can be expected and the mechanisms can allow other important features to be incorporated, such as a improved crystal mounting and drive with a fuid-free eîvironment. The diagram below will illustrate the type of system we had in mind.

A box of suitable material is fitted with a fexible metal "window" of the corrugated metal bellows type seen in wall barometers, which allows both mechanical motion transmission and the possibility of maintaining a fluid barrier. The bellows' movement winds a spring through a ratchet, which prevents auy loss of energy back through the bellows. The spring drives a wheel with fingers that deflect the end of a cantülever mounted crystal. The alignment is such that when the crystal is deformed a presclected distance, the finger slides off and the crystal is set in oscillation at its natural frequency. All of the work done in the original deformation thus must be dissipated within the crystal, and electrical energy is available during each cycie of oscillation. The escapement prevents the next finger from engaging the crystal unti sufficient time has passed for the oscillation to damp out. Since several

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finger-pushes per second are possible, a relatively small area or crystal can be used. This arrangement also provides the desirable feature of allowing input energy to accumulate in the spring until sufficient force is developed to drive a finger over the crystal in the event that the, input falls below normal, If excess input energy is available the crystal is driven more often, but, since its maxinum deflection is always the same, it can not be broken. E. Van Haaften of Bulova Watch has designed a system similar in some respects to the above. (18)

Permanent Magnet Generators

. A transducer not generally considered in

discussions ot impianted power is the permanent magnet generator. Because of the intrinsic weight of màgnet and core materials, and because the usual mechanical input is rotational, this type of generator has little Immediate appeal. However, if as has been suggested above, crystals require sealing in a total enclosure and drive through spring and gear mechanisnis for optimum results then rotational Input is not a relative disadyantage. If animplanted systém must function for many years a p. m. generator should be satisfactory, while the fatigue lifetime of a crystal is not well understood. That weight is not an impossible obstacle is demonstrated by a generator manufactured by Rotating Components, Inc. and advertised in the 1966-67 Electrical Engineers Master catalog. This unit is 1.31" long by .95" diameter, weighs 2.5 oz. and, we calculate, can produce 100 milliwatt at 60 revolutions per second. A device especially designed for 200 uW output and low rotational speeds, therefore, should not be objectionably large, heavy or inefficient. We recommend that a p. m. generator not be dismissed until further data on the practical requírementa and limitations of crystals ,become available, If the physiological and instrumentation problems of

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obtaining mechanical work in the 10 milliwatt range can be solved, a p. m. generator may well be the better suited transducer.

3. Previous Work

Dr. Joha H. Kennedy

Dr. John H. Kennedy and Carl C. Enger at the Cleveland Metropolitan General Hospital have published several reports of their work with a self-powered cardiac pacemaker. (6, 7) The devices constructed and implanted by this group consist partially in a piezoelectric ceramic crystal mounted beneath a flexible plastic cover. The package containing the crystal generatoras well as the rectifier and pacemaker electronics is sutured to the rib cage in a position where the beating heart applies pressyire through the cover to the crystal. This system has, for short periods, provided effective pacing via stimulating electrodes in'several experimental trials with dogs. Since our interest-is in the power generating aspects, we will concentrate cur attention on the crystal.

In the most recent paper (7) the crystal dimensions reported are 3. 75 x 1.87 x .05 cm and drive parameters are described as, "... the mechanical energy needed to operate the self-powered pacemaker is 200 newtons or 21, 4Cm." Let us assume that what was meant is a mechanical force of 2.0 newton. Using design ecuations supplied by Clevite, the manufacturer of the ceramic crystals, it can be shown that for an end supported, center driven erystal of the quoted dimensions, the force necessary to produce a strain of 5 x 1ō4 (the recommended maximum) is 2, 3 Nt. According to our calculations a PZT-5B crystal of the quoted dimensions deformed by a 2, 3 Nt force will produce no more than 9.1 microjoule, and only 7 microjoule at 2.0 Nt. With a spring loaded crystal (and 4.0 Nt.) 28 microjoule is possible. Kennedy mentions the figure of

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23 microjoule, but his reference is technically ambiguous. In order to provide a low ripple electrical source, it appears a larger capacitor than the optimum for maximum efficiency,was used, however. An output voltage of 1, 75 volts is mentioned. The maximum energy which can be supplied into a 1. 75 volt source by a PZT-5B crystal with the above dimensions and 2, 0 Nt driving force during a deformation - relaxation cycle depends on whether a series or parallel type of crystal bimorph was used. The best choice is parallel which can provide 5.4 microjoule. With a heart rate of 120/min the maximum power that could be produced is therefore 1l uW.

It is worth noting that the mechanical power necessary to deform a directly coupfed erystal that provides 11 uW of electrical power is at least 340 uW. Therefore, uf the transducer package can be designed to provide 20% efliciency rather than 3.2%, 68 u W could be obtained with no change in the mechanical power input. Ard if the displacement is increased from, 2mm to , 6mm, enough mechanical power is available to produce 200 uW at 20% efficiency. Éven U only 10% efficiency is attainable, a de∩lection of 1. 2mm to increase the power input does not appear unreasonable, Dr. Kennedy reporis that after a one year implantation no damage to the adjacent myocardium was found. Electrical output apparently has not been maintained beyond a few days because of leakage of fluid into the package. Improvements in the crystal mounting and drive, in materials and packaging, and in surgical technique should be undertaken.

Dr. Victor Parsonnet

Dr. Victor Parsonnet and his co-workers at the Newark, New Jersey Beth Israel IHospital have published several papers describing their experiments with ceramic bimorphs mounted on the aorta. (e. g.18, 21)

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Their latest device contains two PZT-5 crystal slabs as the arms of a spring clothespin-like device which clamps onto the aorta. Each slab is $1 \frac { 1 } { 4 }$ by $1 \frac { 1 } { 2 }$ inches, an area which, according to our previous calculations for unidirectional stress, can produce 31 microjoule per deflection. Thts group has chosen a maximum stress of 20% of the modulus of rupture or  or the stress necessary to pro- $\frac { 2 } { 3 }$ duce the 31 microjoule output (and a better choice from the fatigue lifetime viewpoint). At this stress about 5 microjoule can be produced per deflection or about the same output as Kennedy has achieved. As did Dr. Kennedy, Dr. Parsornet has experteaced difficulty with fuids leaking through his silicon rubber encapsulation which has limited the electrical lifetime to a few hours.

For an artery clamp of theclothespín type the mechanical work done by each expansion of the artery is roughiy proportional to: systolicdiastolic pressure differential, normal variation in arteríal diameter in each cycle, arterial diameter,and length of artery used. For the following set of parameters, 20 mm Hg, 2mm, 1cm, 4cm, we calculate an energy yield of 500 microjoulé per beat, which is 1 milliwattat 120 beats/ minute. Aortic expansion does appear to be capable of producing the necessary quantity of mechanical work, but it remains for improved mechanical designs to meet the sealing, efficient drive, -and adaptation requirements which are demanded of a successful long term system.

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V. BIBLIOGRAPHY

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