05/218909

10/23/1973

01/19/1972

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Commissariat, L'energic Atomique A. (Paris, FR)

623/3.220

60/526, 128/DIG.003, 417/401, 417/395, 417/379

A61M1/10; A61M1/12; A61F1/24; F03G7/06

3/1,DIG.2 128/1R,DIG.3,214R 417/379,395,401 60/23,24

Gaudet, Richard A.

Frinks, Ronald L.

I claim

1. An implantable artificial heart of the relaxation type comprising a blood-pumping sub-assembly constituted by ventrical and auricle cardiac modules, a pump connected to and actuating said modules, a motor connected to said pump for transfer of energy by means of a relaxation process, said motor being of the Stirling-cycle type, a casing for said motor, two cooperating cylinders in line for said motor, pistons in said cylinders, one of said cylinders being a first driving-gas cylinder, a cylinder-head for said first cylinder, a source of energy heating said cylinder head, the other of said cylinders being a second spring-action gas cylinder, a piston block common to said cylinders and forming a cold source, a regenerator having a permeable structure for displacement within said first cylinder between said source of energy on said cylinder-head and said piston in said cylinder, means for locking said piston block in a position of minimum volume of gas within said cylinder-head at the end of the systole phase, means for displacement and coupling of said regenerator with said piston block, means for releasing said piston block at the beginning of the distole phase, stopping means for locking said piston block in inoperative position during the diastole and limiting the pumping volume, and means for returning said regenerator and freeing said piston block with return to the starting position by said spring-action gas cylinder throughout the systole phase.

2. An artificial heart according to claim 1, including a diaphragm buffer volume, a circulation system for said auricle cardiac modules, said buffer volume being connected in said system, the pressure of said buffer volume being maintained by a flexible diaphragm at a reference value in the vicinity of atmospheric pressure and the intermediate pressure fluid in the auricular modules and in the buffer volume having the same density as the blood.

3. An artificial heart according to claim 1, wherein said driving-gas cylinder and said spring-gas cylinder have the same cross-sectional area, said spring-gas cylinder extends into at least part of said casing and said piston block having two sections connected together by a rod.

4. An artificial heart according to claim 3, said piston block having a lateral member, means for momentary locking said block at the end of the diastole phase, said connecting rod of said pistons unlocking said block at the end of its movement of withdrawal for displacement by said lateral member when said piston block moves upwards during the systole phase.

5. An artificial heart according to claim 4, said source of energy surrounding said regenerator in the rest condition at the head of said driving cylinder including a sheath containing a radioelement, a thermal capacity enclosing said sheath of a compound having a high latent heat of fusion, and thermal insulation protecting said capacity.

6. An artificial heart according to claim 5, said regenerator being a capillary network longitudinal to the axis of said motor cylinders and thermal insulation on the inactive portions of said regenerator.

7. An artificial heart according to claim 1 said means for displacement and coupling of said regenerator with said piston block during the diastole phase include permanent magnetic circuit between said piston block and a magnetic pallet rigidly fixed to said regenerator and subjected to attraction by said piston block.

8. An artificial heart according to claim 7, said means for returning said regenerator on completion of the diastole phase include an axial bore extending through said piston block, a plunger having a small cross-sectional area contiguously displaced within one end of said bore by said magnetic pallet, said bore being closed at the other end to cause at the proper time the expulsion of said plunger and the return of said regenerator to its starting position when the pressure force of the gas within said bore becomes greater than the magnetic attraction exerted by said piston block on said pallet plus the pressure forces within said driving cylinder.

9. An artificial heart according to claim 3, said means for locking said piston block and limiting the downward travel of said block including catches engaging step-type stops at the end of a diastolic period, said rod of said pistons having a catch-escapement device for releasing said piston block at the end of withdrawal of said pistons.

10. An artificial heart according to claim 4, said means for momentarily locking of the piston block at the end of the systole include lateral magnetic armatures fixed on said piston block and cooperating with permanent magnets, a support for said permanent magnets surrounding said driving cylinder annularly, said support being an internal extension of said casing.

11. An artificial heart according to claim 10, said pumping cylinders being coaxial with said driving cylinder, said rod of said pumping pistons being slidable between said driving cylinder and said support and an annular flange on said rod abuting beneath said support and defining the top position of said pumping pistons at the end of the systole phase.

12. An artificial heart according to claim 1, including intermediate liquid collectors at pressures including the reference pressure forming with said pumping cylinders a jacket around said driving cylinder.

13. An artificial heart according to claim 1, the heart including a circulation system for recovery of gas leakage resulting from variations in pressure between the volumes of driving gas, spring gas and casing gas.

This invention relates to an implantable artificial heart having wholly independent operation.

It is known to produce artificial hearts for temporary extra-corporal circulations at the time of surgical operations; but the total replacement of a failing heart by an implantable artificial heart has raised and continues to raise a large number of difficulties by reason of the fact that the motor, the cardiac modules and the regulation of the complete unit must be capable of operating in a wholly independent manner for many years.

Up to the present time, it has been mainly endeavoured to solve the problem of an artificial heart for a man in the state of rest or of moderate effort. An artificial heart of this type comprising:

A HYDRAULIC DEVICE FOR PRODUCING THE BLOOD-MOVING PRESSURE WHICH IS DRIVEN BY A RAPID-EXPANSION MOTOR AND A LINEAR-DISPLACEMENT PISTON,

AND A RELAXATION MECHANISM WHICH IS INTERPOSED BETWEEN THE DRIVING PISTON AND THE HYDRAULIC DEVICE AND COMPRISES AN ELASTIC MEMBER SUCH AS A SPRING ENCLOSED BETWEEN TWO COMPONENTS WHICH ARE RIGIDLY FIXED IN ONE CASE TO THE DRIVING PISTON AND IN THE OTHER CASE TO THE DRIVING MEMBER OF THE HYDRAULIC DEVICE, ABUTMENT MEANS FOR LIMITING THE SPACING OF SAID COMPONENTS TO A VALUE AT WHICH THE COMPRESSION OF THE SPRING CORRESPONDS TO MAINTENANCE OF THE END-OF-SYSTOLE PRESSURE BY MEANS OF THE PUMP, MEANS FOR TEMPORARILY LOCKING THE COMPONENT WHICH IS COUPLED WITH THE DRIVING PISTON IN THE STATE OF MAXIMUM EXTENSION OF SAID PISTON DURING ITS WORKING STROKE AND A MEMBER WHICH INITIATES UNLOCKING OF SAID MEANS AT THE END OF EXPANSION OF THE SPRING.

The heart as thus defined employs an alpha radio-element as energy source and the motor employed is of the Rankine or Hirn-cycle type, for example.

The present invention relates to an implantable artificial heart which, in the same manner as the foregoing, utilizes the principle of a complete relaxation assembly comprising the motor, the transmission with elastic system and relaxation mechanism, the pumping sub-assemblies, the cardiac modules, the human-blood circulation systems, and having periods corresponding to those of a normal heart.

This device makes use of the same source of energy and similar cardiac modules but differs essentially in the type of motor as in a part of the relaxation process and therefore of the transmission system. The motor which is employed is of the Stirling-cycle type and this has the advantage of higher efficiency, the possibility of producing a motor which avoids the need for any component involving wear and delicate control such as check-valves and inlet valves and any rubbing of surfaces which are heated to high temperatures, and the obtaining in the cycle of a ratio of maximum and minimum pressures which is substantially lower than that of the Rankine cycle.

More precisely, the invention is concerned with an implantable artificial heart of the relaxation type comprising a blood-pumping sub-assembly constituted by cardiac modules and a pump for actuating said modules, a motor integrated with a mechanism permitting transfer of energy by means of a relaxation process, characterized in that the motor, of the Stirling-cycle type, is essentially constituted by two cooperating cylinders in line, a first driving-gas cylinder with a cylinder-head heated by the source of energy and a second spring-action gas cylinder, a piston block which is common to both cylinders and forms a cold source, a regenerator having a permeable structure which is capable of displacement within the first cylinder between the cylinder-head and the piston and conversely, means for locking the piston block onto the pumping system in the position of minimum volume of gas within the first cylinder with regenerator at the cylinder-head at the end of the systole phase, means for displacement and coupling of said regenerator with the piston block, with means for releasing said piston block at the beginning of the diastole phase, stopping means with locking of said piston block enabling this latter to remain inoperative during the diastole and permitting a limitation of the pumping volume, and means for returning the regenerator and freeing said piston block, thereby displacing the pumping system to the starting position by means of the spring-action gas throughout the systole phase.

The motor as thus designed makes it possible, as will become apparent hereinafter, to achieve self-regulation of the systole phase as a result of a suitable choice of the volumes of gas enclosed within the cylinders by the piston block, as a function of the operating pressures.

Further properties and advantages of the invention will be brought out by the following description which gives one non-limitative example of construction of an implantable artificial heart in accordance with the invention, reference being made to the accompanying drawings, in which :

FIG. 1 is a general arrangement diagram of a blood-pumping assembly with its driving system ;

FIG. 2 is a diagram showing on the one hand the different phases 2a, 2b, 2c and 2d of one operating cycle of the Stirling-cycle motor for driving the blood-pumping assembly and on the other hand, at the bottom portion thereof, the diagram of pressures within the driving and spring-action cylinders and of the resultant force on the blood-circulating pump ;

FIG. 3 is a sectional diagram on a diametrical plane showing one example of construction of an artificial heart which operates physiologically in accordance with the schematic data of FIGS. 1 and 2.

As shown diagrammatically in FIG. 1, the driving system of the pumping assembly for the two blood circulation systems comprises in known manner a motor M controlling by means of a common rod T the displacement of two coupled pistons PG and PD respectively within the pumping cylinders CG and CD. The motor M is an assembly which constitutes a source of energy (radioelement) and has the functions of heat storage, of energy conversion and of regulation of the relaxation cycle in conjunction with the complete assembly comprising the artificial heart and human blood circulation systems.

In a manner which is also known, the pumping assembly comprises four cardiac modules each having a flexible diaphragm, namely VG and VD corresponding respectively to the left and right ventricles, OG and OD to the left and right auricles.

The circulation of the blood between the venae cavae and the pulmonary artery takes place in the direction of the arrows F ₄ and F ₁ , and between the pulmonary vein and the aorta in the direction of the arrows F ₃ and F ₂ ; in the same manner as in a natural heart, the cardiac modules are fitted with artificial valvules B ₁ B ₂ B ₃ B ₄ .

Provision is also made in accordance with the invention for a buffer volume VT which is mounted on the auricular module circuit and the pressure of which is maintained by means of a flexible diaphragm at a reference value in the vicinity of atmospheric pressure. Each flexible diaphragm is shown diagrammatically in the figure by a continuous wavy line.

In accordance with physiological requirements, the two ventricles and the two auricles act respectively in a synchronous manner, with the result that the two ventricles are in the maximum blood-filling phase whilst the two auricles are in the minimum filling phase ; but, by virtue of the buffer or compensation volume, the blood volumes which pass into the auricles at each pulsation can be smaller than those which pass into the ventricles.

In order to fill the ducts C ₁ , C ₂ , an incompressible fluid, namely a liquid, is employed to fill the ducts C ₁ and C ₂ from the pistons PD and PG up to the flexible ventricle diaphragms. In order to fill the ducts C ₃ , C ₄ , C ₅ from the rear faces of the pistons PD and PG up to the diaphragms of the auricles and of the buffer volume, any fluid can be employed although a liquid having the same density as the blood is preferable. This makes it possible to balance all the hydrostatic pressures and to make the cardiac beat independent of the position adopted by the patient who carries the artificial heart.

Within the driving and pumping assembly of FIG. 1 which has just been described, the equilibrium of the pistons PD and PG is practically indeterminate when no effort is produced on the rod T in order to bring this latter downwards (diastole phase) ; in consequence, in order to adjust the diastole period, it is only necessary to produce a small variation in the restoring force of the two pistons or, at a constant value of restoring force, to cause a variation in a throttling action in one of the ducts.

This assembly as described corresponds to the most complete pumping system ; reasons of medical practice can make it necessary :

either to dispense with the auricles

or to dispense with the ducts C ₄ and C ₅ , the auricles being intended to operate simply as flexible modules.

There will be described hereinafter the general structure of the motor M of the Stirling-cycle type which is coupled with the pistons PG and PD by means of the rod T, reference being made to FIG. 2.

This motor is made-up of two essential parts :

a driving cylinder CM filled with driving gas, a driving piston PM being intended to move within said cylinder and also to perform the function of a cold source (SF) and a regenerator R essentially composed of a capillary network. This cylinder receives heat from the hot source (SC) which is mounted on the head of said cylinder ;

a volume V filled with spring-action gas which performs the function of a fluid spring, and comprising a spring-action cylinder which is aligned with the cylinder CM and in which moves a spring-action piston PR ; this piston is coupled with the piston PM and may also form only one unit with this latter.

It has been assumed in FIG. 2 that said pistons PM and PR were connected together by means of a coupling member fitted with a component A which may or may not, depending on the phases, be in contact with a component B which is rigidly fixed to the rod T.

The cross-sectional areas of the cylinders CM and CR can be different in the case in which it is sought to obtain part of the spring effort by means of the gas contained in the motor casing ; moreover, the volume T can itself be constituted by the volume of said casing. In FIG. 2, it has been sought to remain in a general but simple case, namely in which the cross-sectional areas of the cylinders CM and CR are the same while the volume V is separate and distinct from the volume of the casing ; the simplification lies in the fact that the effects of the pressure within the casing of the moving system constituted by the block of the pistons PM and PR are nullified.

The thermodynamic behaviour of said motor M in conjunction with the performance of the two cardiac phases will be described hereinafter by referring successively to FIGS. 2a, 2b, 2c, 2d and to the diagram of FIG. 2. In this diagram, there have been plotted as abscissae the displacement e of the block of pistons PM and PR or of the rod T, and as ordinates the driving pressures PC _M and spring-action pressures P _V and the resultant force F _T on the rod T which displaces the pumping pistons PG and PD.

FIG. 2a is a dead-point position corresponding to the end of the systole phase.

The regenerator R is at the head of the cylinder CM and its face which is oriented towards the hot source is at the temperature TC of the hot source ; the face of said regenerator which is directed towards the driving piston PM or cold source SF is at the temperature Tf of the cold source; by means which are not illustrated in the drawings, the regenerator of FIG. 2a is rapidly displaced from position 2a to the position of FIG. 2b. The regenerator which has a very small mass is essentially constituted by a longitudinally permeable body, the pressures on each side of said body being practically identical ; the displacement mentioned above therefore does not entail any need to overcome friction forces and inertia.

During this first very short period outside the regenerator, there is no motion of any moving system, but the pressure within the cylinder CM changes from P _CM to P _CM2b and, similarly, the gas contained in the cylinder CM changes from the temperature Tf to the temperature TC at constant volume after passing through the regenerator ; and the following thermodynamic phase which corresponds at the outlet to the diastole phase develops as follows :

since the pressure P _CM2b within the cylinder CM is higher than the pressure P _V2b = P _V2a within the volume V and since the regenerator R remains coupled with the piston PM, the moving system (R - PM - PR - A) moves from the position of FIG. 2b to that of FIG. 2c at the time of fast acceleration and deceleration. At the end of the displacement, the moving system is immobilized in position 2c. Thermodynamically, this phase corresponds to isothermal expansion at hot temperature TC of the Stirling cycle. During this expansion, the pressure changes from P _CM2b to P _CM2c and the work of the driving gas makes it possible to store energy by compression of the spring-action gas which is contained in the volume V and which changes from the pressure P _V2b to P _V2c . At the end of this thermodynamic phase, the regenerator R is returned to its starting position (FIG. 2d) and the pressure of the gas which varies progressively at constant volume within the driving cylinder decreases from P _CM2c to P _CM2d whereas its temperature changes from TC to Tf since it passes through the regenerator.

From the cardiological point of view, the diastole phase takes place as follows :

From the beginning of withdrawal of the moving systems (R - PM - PR - A), the component A releases the component B and therefore, through the intermediary of the rod T, the pumping pistons PD and PG. Since these pumping pistons are in substantially indeterminate equilibrium as indicated earlier, these latter are capable of withdrawing under the action of a small restoring force. This force can be supplied by a spring of any type or simply by adjusting the gas pressure which prevails within the casing, so that a resultant of forces appears on T by virtue of its leak-tight passage ; the means will be chosen exactly as a function of medical requirements in order to ensure compliance with the time-duration set for the diastole phase. During the withdrawal of the pistons PD and PG, therefore of T and of B, filling of the cylinders CD and CG and therefore of the ventricles takes place. At the end of the phase, the component B comes once again into contact with the component A which is coupled with the moving system, this latter having been maintained in the inoperative position (see FIG. 2d) ; the component B is fitted with means not shown in this figure which effect the release of the component A. The following cardiological phase, namely the systole phase, is of major importance. Its thermodynamic and cardiological aspects are synchronous. Parameters must be imposed on this phase in such a manner as to be substantially equal to those of the heart of a man who is either in the state of rest or undergoing moderate exertion.

The component A being released and the pressure P _V2d within the volume V being higher than the pressure P _CM2d within the driving cylinder CM, the entire moving system moves from the right towards the left while thrusting forward the pistons PD and PG by means of the components A, B and T. By means of the fluid contained within the ducts C ₁ and C ₂ , the blood is therefore discharged from the ventricles until the components of the motor revert to the position of FIG. 2a. A further cycle can then begin.

During this phase, from the thermodynamic point of view, the pressure within the volume V decreases slightly from P _V2c = P _V2d to P _V2a = P _V2b along the reverse path and the pressure within the driving cylinder increases from P _CM2d to P _CM2a at the time of an isothermal compression. In consequence, the resultant F _T of the gas pressure forces on the moving system is proportional to the difference in pressures (P _V - P _CM ) and it is this force which, by means of different intermediate elements, serves to compress and eject the blood from the ventricles.

The manner in which self-regulation of the systole phase of the artificial heart can be carried out by modifying the resultant force F _T during the thermodynamic period just mentioned will now be shown, assuming once again that the heart under consideration is a human heart in the rest condition or in a state of moderate effort.

It is useful to recall that, in this case :

the two ventricles are synchronous

the pressures within the right ventricle are substantially lower than those in the left ventricle.

These pressures which are well known are a function of the resistances of the two blood-circulation systems and of the time of contraction of the ventricles.

In consequence, if it is assumed that the transmission of pressures to the blood by means of the intermediate fluid and of the diaphragms is perfect :

any force applied to the rod T which is common to the pistons PD and PG gives rise within the ventricles to different pressures corresponding to the resistances of the circulation system on which these ventricles depend ;

any application of a pressure which is variable in a substantially linear manner to an artificial ventricle so that the values of said pressure at the beginning and end of application are substantially equal to that of a natural heart makes it possible to obtain a systole phase having a time-duration which is substantially identical with that of a natural heart ;

and in order to make a synthesis of the two foregoing propositions, any force which is variable in a continuous and substantially linear manner and applied to the rod T so that, at the beginning of its application, said force should be capable of producing within the cylinders CD and CG pressures whose sum is substantially equal to the sum of the pressures within the two ventricles of a normal heart at the beginning of a systolic period and that, at the end of its application, said force should be capable of producing within the cylinders CD and CG pressures whose sum is substantially equal to the sum of the pressures within the two ventricles of a normal heart at the end of the systolic period, makes it possible to reproduce substantially, in pressure and time in the case of both ventricles, the systole phase of a normal human heart.

When the thermodynamic and cardiological aspects are grouped together, it is consequently apparent that, by choosing on the one hand a displacement of the moving system of the motor corresponding to the cross-sectional areas of the pistons PD and PG in order to obtain the systolic volumes of a normal heart and, on the other hand, the volumes of the cylinder CM and of the reservoir V as a function of operating pressures, it is possible to reconstitute a systole phase which is substantially identical to that of the heart of a man at rest or in a state of moderate effort.

Self-regulation of the systole phase of the artificial heart proposed is therefore in fact achieved, all the more so as the time of this phase decreases in the case of an effort accompanied by a reduction in the resistances of the blood-circulation systems.

Since the foregoing considerations may appear to be somewhat theoretical, one example of practical arrangement of a driving system for an implantable artificial heart as intended for the application of the successive processes hereinabove set forth will be finally described both in regard to the structure and operation of the system. This arrangement is illustrated in FIG. 3 in cross-section on a diametral plane. All the elements of FIGS. 1 and 2 are again shown in this figure although with different references and specific mention is made of the means for regulating the relaxation cycle in accordance with thermodynamic and cardiological requirements.

In the following description of the structure, the references of the corresponding elements of FIGS. 1 and 2 have been placed between brackets.

The reference numeral 1 designates the heat source together with its sheaths (the present Applicant recommends the use of Pu 238 as radioelement) ; the reference numeral 2 designates a thermal capacity constituted by a compound having a high latent heat of fusion which is capable as a result of fusion and solidification at the temperature chosen for the hot source of permitting variations in power of the motor ; the reference numeral 3 designates the thermal insulation of this capacity which covers the heat source ; the numeral 5 refers to the driving cylinder (CM) extended by a jacket with a cylinder-head 4 which performs the function of hot source for the driving gas following the Stirling cycle within the cylinder 5. In short, the hot source (SC) is constituted by the assembly 1, 2, 3 and 4.

The reference 6 designates a single-unit piston of the plunger type which forms both the driving piston (PM) and the spring-action piston (PR) ; this piston 6 comprises an open permanent magnetic circuit, the function of which will be explained hereinafter ; the numeral 7 refers to the spring-action cylinder (CR) which is extended by the spring volume 8 (V), the complete assembly being intended to contain the spring-action gas.

The numeral 9 designates the regenerator (R) which comprises a longitudinal capillary network with a longitudinal and transverse thermal insulation ; the regenerator is provided axially and at the lower end with a magnetic pallet 10 and this latter serves to couple the regenerator with a plunger 11 which is capable of displacement within an axial bore of the piston 6. Said bore is closed-off at the lower end by a stationary plunger 12 which is rigidly fixed to the casing, with the result that a variable volume of gas which therefore has a variable pressure can be enclosed between the plungers 11 and 12.

The reference numeral 13 designates magnetic armatures (A) which are rigidly fixed to the piston 6, said armatures being intended to cooperate with magnets 14 which are stationary with respect to the casing and also with an annular flange 19' (B). It has been assumed that the magnets 14 are fixed on a support which forms an internal extension of the casing and surrounds annularly the driving cylinder 5.

The reference 15 designates the left-hand pumping cylinder (CG) which is concentric with the driving cylinder and the reference 16 designates the right-hand pumping cylinder (CD) which is concentric with the preceding ; there is shown at 17 the left-hand pumping piston (PG) and at 18 the right-hand pumping piston (PD) which is rigidly fixed to the preceding ; the reference 19 represents the common cylindrical rod (T) for controlling the pumping pistons ; said rod is integral with the piston 17 and therefore also with the piston 18 and terminates at the lower end in the annular flange 19' (B) which was mentioned earlier ; the rod 19 slides along the driving cylinder and its annular flange is capable of coming into abutment beneath the support of the magnets 14.

The reference numeral 20 designates a left-hand intermediate liquid collector and 21 designates a right-hand intermediate liquid collector ; 22 represents the intermediate liquid duct (C ₂ ) which is connected to the collector 20 and the reference 23 designates the corresponding duct (C ₁ ) of the collector 21. The reference pressure (namely of VT) is given by the fluid collector 24 with a duct 25. The complete assembly of collectors and pumping chambers 15 and 16 forms a kind of annular casing around the driving cylinder 5.

It is further apparent from FIG. 3 that a catch system 26 which is secured to the piston 6 is capable of cooperating at the end of downward travel with stops 27 which are rigidly fixed to the casing whilst an escapement system 28 supported by the rod 19 is capable of releasing the catches from their stops.

The casing is designated by the reference 29 and is intended to ensure leak-tightness of the assembly.

The operation of the driving assembly of FIG. 3 is parallel to that of the assembly of FIGS. 1 and 2 and will therefore be described more briefly ; however, the auxiliary processes which ensure that the assembly is fully self-contained will be clearly brought out.

The driving device is shown in FIG. 3 in the starting position of FIG. 2a, that is to say at the end of the systole phase :

since the piston 6 is motionless by virtue of the armatures 13 and the magnets 14, the magnetic circuit of the piston 6 attracts the pallet 10 and the regenerator 9 which is applied against the piston 6 with the assistance of an increase in pressure within the driving cylinder whereas the temperature of the "driving" gas within the cylinder 5 increases from the low temperature Tf to the high temperature T _C and the pressure of this same gas changes from P _CM2 to P _CM2b . During this phase, the plunger 11 which is thrust-back by the pallet 10 compresses the gas contained in the cylindrical bore which is internal to the piston 6. The following thermodynamic phase which corresponds to the beginning of the diastole phase takes place as follows :

the pressure rise within the cylinder 5 is such that the force of immobilization of the piston 6 as supplied by the assembly consisting of armatures 13 and magnets 14 is no longer sufficient and decreases very rapidly as the piston 6 accelerates and decelerates, then comes to a standstill by virtue of the system of catches 26 and stops 27. During this rapid phase, the driving gas contained in the cylinder 5 expands isothermally from P _CM2b to P _CM2c while absorbing heat from the hot source and the "spring-action" gas contained in the volume 8 is compressed from P _V2b to P _V2c .

At the end of this phase, the pressure of the gas which is present within the internal cylindrical bore of the piston 6 has increased by the virtue of the fact that the plungers 11 and 12 have come closer together until the value attained is such that the pressure force applied to the plunger 11 becomes larger than the force of attraction of the pallet 10 added to the pressure forces within the cylinder 5. The regenerator is then returned to the head of the cylinder 5. The pressure of the driving gas then changes from P _CM2c to P _CM2d while its temperature changes at the same time from T _C to T _f .

The return of the regenerator by means of a fluid spring is not an exclusive design and could equally well be carried out by means of a mechanical or magnetic spring.

The accompanying diastole phase develops in the following manner :

At the outset of the thermodynamic phase which has just been described, the rod 19 is released. The two pumping pistons 17 and 18 can therefore withdraw under the conditions already explained at the time of operation of the driving assembly of FIG. 2 ; control of the movement of withdrawal can be obtained in this case by self-regulating means of medical type to be defined but in which the pressures within the collectors 20 and 21 (namely those of the ventricles) would be higher than in the collector 24 (reference pressure) ; it is also possible to maintain the pressure within the casing at a value which is lower than the pressure in the collector 24. As is readily apparent, mention can also be made of a mechanical, fluid, or magnetic spring action although this has the disadvantage of entailing the need for additional parts of the appreciable weight.

At the end of the diastole phase, the escapement system 28 which has moved back together with the rod 19 releases the catch system 26. The following synchronous thermodynamic phase of the systole phase then begins :

Under the action of the pressure forces of the gas contained in the volume 18 which are higher than those of the gas contained in the cylinder 5, the piston 6 returns to its starting point and exerts a thrust on the two pumping pistons 17 and 18 by means of the rod 19 ; the blood is discharged from the ventricles. The gas contained in the cylinder 5 is compressed isothermally from the pressure P _CM2d to the pressure P _CM2a while yielding heat which is largely imparted to the fluid of the collectors surrounding the cylinder. Part of this heat can be yielded directly to human tissues by conduction through the different components and the casing.

It is worthy of note that the catch stops 27 can be designed in steps as shown in FIG. 3 in order to permit a number of positions of immobilization of the piston 6 without thereby changing the performance of the phases. By virtue of this precaution, the operation of the heart can be made stable in spite of any slight changes in the parameters of this latter which may possibly occur as a result of wear, variations in heat-transfer processes and so forth. The heart can therefore change slightly in systolic volume and in frequency but does not stop.

The locking and unlocking system with catches and escapements can be replaced by a magnetic system.

As suggested in FIG. 3, the cylinders 5 and 7 could be combined in a single cylinder without any opening towards the exterior ; the efforts of the pressure forces could then be transmitted magnetically, thereby providing the advantage of ensuring perfect leak-tightness between the cylinders 5 and 7 and the surrounding atmosphere beneath the casing. However, it should be noted that this type of transmission system is heavier than a mechanical system.

In the driving arrangement shown in FIG. 3, all the necessary seals are formed by means of joints which are represented diagrammatically by hatched circles.

In the case of seals concerning the intermediate liquids, the use of unrolling seals is to be recommended.

So far as concerns the seals relative to the motor itself, by reason of the excessive pressures of the gas within the cylinders with respect to the pressure which prevails within the casing and which is close in value to atmospheric pressure, use will be made of the usual O-ring seals ; however, a system of recovery of gas leakages by operation of the motor becomes necessary. A system of this type is illustrated in FIG. 3 and it then becomes necessary to make use of a common gas for the driving gas, spring and casing. This system is made up as follows :

A cylinder 30 rigidly fixed to the casing terminates in a non-return valve 33 within the spring-action volume 8 ; a piston 31 which is coupled with the piston 6 is capable of displacement within said cylinder. Moreover, if the pressure within the cylinder 5 drops, this cylinder can be supplied with gas from the volume 8 by opening the calibrated valve 32.

It is readily apparent that the present invention has been described in the foregoing solely by way of explanation without any limitation being implied and that any modifications of detail can be made therein without departing from its scope.