Patent Publication Number: US-2019192755-A1

Title: Power generator for use in left ventricular assist device (lvad) and total artificial heart (tah) and related methods

Description:
This application is a U.S. National Stage Application of PCT International Application No. PCT/US2017/046835, filed Aug. 14, 2017, which claims the priority benefit to U.S. Provisional Application No. 62/374,799, filed Aug. 13, 2016, and U.S. Provisional Application No. 62/374,832, filed Aug. 13, 2016, the disclosures of which are hereby incorporated by reference in their entirety. 
    
    
     FIELD OF INVENTION 
     The present invention relates generally to medical devices and related methods. More specifically, particular embodiments of the invention relate to implantable power generators for use with, for example, left ventricular assist devices (LVAD) and/or total artificial hearts (TAH). 
     DESCRIPTION OF RELATED ART 
     An LVAD is a surgically implanted mechanical pump that is attached to the heart to assist pumping of blood from the left ventricle to the aorta. An LVAD includes a driveline extending from the pump to a controller positioned outside the patient&#39;s body and a power source connected to the controller to provide power to the pump. The power source usually includes batteries or live electricity. Depending on, for example, the patient condition and/or availability of a heart donor, an LVAD may be a temporary (e.g., weeks to several weeks) or permanent solution to failing heart. While an LVAD works with the heart to help it pump more blood with less work by the heart, a TAH is an artificial heat that completely replaces the failing heart. 
     SynCardia Systems, Inc. is a manufacturer of CardioWest™ Total Artificial Heart (TAH-t), which is an implantable artificial heart intended to keep hospitalized patients alive while they are waiting for a heart transplant. CardioWest™ TAH-t is a pulsating bi-ventricular device that is implanted into the chest to replace the patient&#39;s left and right ventricles (the bottom half of the heart). The device is sewn to the patient&#39;s remaining atria (the top half of the heart). Hospitalized patients are connected by tubes from the heart through their chest wall to a large power-generating console, which operates and monitors the device. 
     AbioCor™ is an implantable, self-contained total artificial heart produced by ABIOMED. AbioCor™ is formed by an implanted pump, an internal rechargeable battery capable of supporting operation for 20 minutes, continuously charged by an external power source, and an electronic package implanted in the patient&#39;s abdominal area. Power to recharge the implanted battery is transferred via transcutaneous energy transmission (TET) system. External battery packs can power AbioCor™ for 4 hours. AbioCor™ was discontinued in 2007. 
     CARMAT is developing an implantable artificial heart equipped with electrical power supply and remote diagnosis systems. The artificial heart consists of two, right and left, ventricular cavities containing two volume spaces each separated by a flexible bio-membrane, one for blood and one for a working fluid. Through hydraulic action via two motorized pump sets, the working fluid displaces the bio-membrane, thus reproducing the movement of the ventricular wall of the human heart. An integrated electronic device regulates how the artificial heart operates according to patients&#39; needs and using information given by sensors and processed by a microprocessor. 
     Both LVADs and TAHs, including the particular devices mentioned above, require a mechanical or electro-mechanical pump that requires a sustained high-density power source external to the patient&#39;s body (e.g., external batteries and power supplied networked with the power grid or other types of electric generators). 
     Thus, there exists a need for an improved power generator that can provide a sustained, high-density power source with long-term energy storage capacity. 
     SUMMARY 
     Therefore, various exemplary embodiments of the invention may provide an improved power generator that overcomes one or more shortcomings and problems of existing LVADs and TAHs. It should be understood that, while the power generator of the present disclosure is described in connection with a LVAD and TAH, the power generator may be applied in many other application that may require power sources with high energy density and long-term energy storage capacity. 
     For example, robotic applications require electrical power normally supplied by cables or tethers connected to stationary or mobile electric power supplies. For robotics applications requiring high power density and low weight, in addition to dimensional constraints as required, for example, by unmanned vehicles, aerial and submergible drones, electric power from portable solar panels or combustion engines can become unpractical or impossible. For example, man and unmanned submergible, non-nuclear electric robots cannot rely on solar or combustion engines. The power generator of the present disclosure may provide an autonomous rotary magnetic drive configured to convert thermal energy from nuclear decay heat can satisfy requirements for robotic applications. 
     In certain exemplary aspects, the rotary magnetic drive of the present disclosure can be totally implanted inside a patient&#39;s body and configured to convert decay heat energy into a rotary magnetic field executing the functions currently executed by the electro-magnetic or permanent magnet motors equipping FDA approved LVADs and TAHs pumping systems. The rotary magnetic drive can also be configured to convert decay heat thermal energy into conditioned electricity, thus replacing the battery and power supply system normally supplying electric power to LVADs and TAH. The rotary magnetic drive of the present disclosure can be scaled and configured to be totally implantable with no need for percutaneous tethers or drivelines to supply electric power to LVADs and TAHs. 
     When the rotary magnetic drive is configured to support medical applications, it represents an implantable energy source based on safely encased alpha-emitting isotopes that release thermal energy as they undergo natural nuclear-decay. In one embodiment, the thermal energy released by the alpha-emitting isotopes is converted into motive power or electricity by a miniaturized thermodynamic engine configured to exchange thermal energy with the environment through the body&#39;s natural heat transfer mechanisms. 
     Alpha-emitting isotopes are often referred to as soft radiation represented by Helium particles ejected by isotopes that undergo natural alpha-decay, and can easily be stopped by thin materials such as a sheet of paper, thus effectively shielding the alpha-emitting isotopes. For these applications, the alpha-emitting isotopes represent the power source of the rotary magnetic drive, and can be produced and manufactured in the form of compact shielded cartridges for simplified installation, removal or replacement at intervals dictated by the LVADs and TAH uninterrupted power generation rate and time duration requirement. The amount of alpha-emitting isotope required to power LVADs and TAHs and the power rating corresponding to the thermal energy released by the alpha-particles depends on the decay rate of the isotopes selected and the isotopes half-life. In other words, the total thermal power produced by the power source is directly proportional to the rate of alpha particles generation, while the duration at which the total thermal power can be produced depends on the isotopes half-life. 
     There are various alpha emitting isotopes that can provide thermal energy and time duration with specifications that satisfy LVADs and TAH application requirements. Most of the available alpha-emitting sources represent adequate power rating and half-life for LVADs applications. However, several of the available alpha-emitting isotopes are not pure alpha-emitters, as the primary alpha-emission may be emitted all together with secondary gamma-ray emissions. In most cases, the gamma-ray emission occurs at a very low rate, relative to the alpha emission, and with energy ranges that can be stopped by adequately designed shields. Shielding requirements for the power source become proportionally more restrictive depending on the type of gamma-rays emitted and their emission frequency. For LVADs and TAH applications, shielding of the power source is necessary to absorb gamma-radiation rather than alpha-particles, and to ensure patients and the public in their surrounding environments are not exposed to harmful radiation. 
     On average, LVADs require approximately 3-10 Watt-electric to electro-magnetically drive the blood pumping LVADs magnetic rotors. This power rating may increase when the LVADs or TAHs are configured to execute blood pumping by positive displacement or pneumatic mechanisms. For configurations involving rotary equipment as part of the blood pumping mechanisms (e.g., impeller rotors), the actual thermal power source rating increases accounting for electric-to-mechanical conversion inefficiencies. 
     In one embodiment of this invention, when the source energy is converted into a rotary magnetic field, thermal energy from the decaying isotopes is directly converted into motive (pumping) power by magnetic coupling with the permanent magnets comprised by the rotary blood pumping impeller. A certain portion of the thermal energy that is not converted into electricity or mechanical power is rejected to the environment by thermally coupling the rotary magnetic drive low temperature heat exchanger to the patient body to execute natural/passive or active convective, conductive and radiative heat transfer mechanisms. 
     Alpha-emitting isotopes safely encased within a heat transfer and shielding reinforced housing can produce thermal energy. This thermal energy is then converted into forms that can support robotic actuation and management, as well as LVADs and TAHs devices whose pumping functions are executed by magnetic rotary impellers or linear and positive displacement actuators. The amount of thermal energy produced is proportional to the isotope&#39;s natural decay-rate, while the duration at which thermal energy is released is proportional to the isotope&#39;s half-life. One of the candidate alpha-emitting isotopes include Plutonium-238 with a half-life of approximately 87 years. The main Pu238 nuclear decay mode is the alpha emission followed by a very low-energy secondary gamma ray emission. Therefore, among various isotopes, Plutonium-238 shielded with reasonably compact radiation shields can be utilized as a thermal source for the rotary magnetic drive of the present disclosure. 
     One exemplary aspect of the present disclosure may provide a magnetic drive electric and torque generator configured to convert thermal energy from a heat source into mechanical energy to drive a rotary magnetic field and further convert the rotary magnetic field in mechanical torque through magnetic coupling with a mechanical rotary system and into electric energy through magnetic coupling with stationary electro-magnetic coils. Rotary magnetic drive can be configured to support various applications, such as, for example, to drive the impeller of a pump, the propeller of a submergible vehicle, fans, and other generic actuators supporting robotic propulsion and actuation. Size and power rating of the rotary magnetic drive generator of the present disclosure can be scalable enabling totally implantable applications as required by blood pumping devices represented, for example, by LVADs and TAHs. 
     Further, the rotary magnetic drive generator can be configured as an implantable, autonomous, pumping power-generator to replace external or implantable rechargeable batteries and electro-magnetic motors normally equipping LVADs and TAHs. In one exemplary configuration, the rotary magnetic drive may convert thermal energy generated by a heat source, such as nuclear isotopes undergoing nuclear decay, into mechanical energy that drives a rotary magnetic field that can be coupled to various components to generate torque, propulsion, or electricity. In one another exemplary configuration, the rotary magnetic drive can be configured to drive blood pumping magnetic impellers in LVADs and TAHs to eliminate the need to rely on batteries with limited capacity and access to electric power supplies outside of the patient&#39;s body. As the rotary magnetic drive can be configured to produce mechanical energy at scalable power ratings, it can also be utilized to support electric generation for robotic applications. 
     Another exemplary aspect of the present disclosure may provide a power generator capable of supplying variable power ratings for a prolonged period of time based on generic thermal sources, including thermal sources represented by nuclear decaying isotopes. The power generator of the present disclosure may satisfy one or more of the following conditions: i) light weight and fully contained within dimensions and weight requirements characterizing various robotic and specialized applications, including LVADs and TAHs applications; ii) safe, as alpha radiation and low-energy secondary emission gamma rays are shielded by high density materials and by additional means represented by the shape of the materials forming the thermal-hydraulic heat exchanger, utilized to transfer thermal energy from the decaying isotopes to the working fluid, and the working fluid itself as its composition can comprise gamma-ray shielding materials; iii) does not require refueling or recharging of the power source for extended amounts of time (months to decades, depending on the half-life of the isotopes selected0; iv) contains rotary components that are not in contact with one another, thus ensuring frictionless “no wear and tear” operations; v) compactness, modular for integration with the equipment supporting robotic applications, and implantable for medical applications; vi) self-sustained automatic operations, no need for monitoring of functions; vii) for medical application it can be interfaced directly with FDA approved LVADs and TAHs via magnetic coupling; viii) provides extra shielding capabilities by means of routing the radiation-attenuating working fluid configured to circulate within heat exchangers transferring thermal energy from the decaying isotopes to the working fluid, while forming a “fluid wall thickness” that effectively attenuates alpha, beta and gamma radiation; ix) comprises a thermal power source whose decaying isotopes are fully encapsulated, sealed and inaccessible; x) provides power sources configurations wherein the decaying isotopes are manufactured in sealed cartridges formed by materials that satisfy thermal heat transfer and shielding capabilities; xi) can withstand hostile operations without releasing volatiles forms of the isotopes utilized for the generation of thermal energy, even under design basis and beyond design basis accident scenarios, including maliciously breaching of the fuel cartridge; and xii) complies with regulatory requirements for ionizing radiation. 
     To attain the advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, one aspect of the invention may provide a medical device for displacing a bodily fluid inside a patient&#39;s body. In one exemplary embodiment, the medical device may include a source heat exchanger containing a heat generating source and being configured to transfer heat from the heat generating source to a working fluid. The medical device also includes a hollow shaft comprising a plurality of permanent magnets, an impeller shroud disposed inside the hollow shaft, where the impeller shroud defines an internal passageway through which the bodily fluid passes through. The medical device further includes an impeller disposed inside the internal passageway of the impeller shroud, where the impeller is magnetically coupled to the permanent magnets of the hollow shaft. The medical device includes an expander comprising a rotary component mechanically coupled to the hollow shaft, where the expander being driven by the working fluid flowing from the source heat exchanger to rotate the hollow shaft. Rotation of the hollow shaft generates a rotary magnetic field in the hollow shaft to cause the impeller to rotate and displace the bodily fluid flowing through the internal passageway. 
     Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several exemplary embodiments of the invention and together with the description, serve to explain the principles of the invention. 
         FIG. 1  is a schematic view of a power generator, according to an exemplary embodiment of the present disclosure, illustrating the basic thermal-hydraulic connections among various components forming a closed-loop thermodynamic cycle. 
         FIG. 2  is a perspective, partial cut-away view of a power conversion assembly, according to one exemplary embodiment of the present disclosure. 
         FIG. 3  is a cross-sectional view of the power conversion assembly shown in  FIG. 2 , shown with an expander integrally formed with a hollow shaft. 
         FIG. 4  is a schematic view of a power generator, according to another exemplary embodiment of the present disclosure. 
         FIG. 5  is a schematic diagram of a power generator, according to another exemplary embodiment. 
         FIG. 6  is a schematic view of a power generator, according to another exemplary embodiment. 
         FIG. 7  is a schematic view of a power generator, according to another exemplary embodiment. 
         FIG. 8  is a schematic diagram of a power generator, according to another exemplary embodiment. 
         FIG. 9  is a schematic diagram of a power generator, according to another exemplary embodiment. 
         FIG. 10  is a schematic diagram of a power generator, according to another exemplary embodiment. 
         FIG. 11  is a perspective view of the power generator described by  FIGS. 1-3 , according to one exemplary embodiment. 
         FIG. 12  is a perspective cross-sectional view of the power generator shown in  FIG. 11 , illustrating various internal components. 
         FIG. 13  is an exploded view of the power generator shown in  FIGS. 11 and 12 , illustrating various parts of the power generator. 
         FIG. 14  is a perspective cross-sectional view of a recuperator heat exchanger of the power generator shown in  FIGS. 11-13 . 
         FIG. 15  is a partially exploded perspective view of the power generator of  FIG. 11 . 
         FIG. 16  is a perspective view of the recuperator heat exchanger of the power generator of  FIG. 11 . 
         FIG. 17  is a perspective view of power generator  100  of  FIG. 11 , illustrating a different angle of the extended recuperator. 
         FIG. 18  is a perspective view of the power generator shown in  FIGS. 6-10 . 
         FIG. 19  is a perspective cross-sectional view of the power generator shown in  FIG. 18 . 
         FIG. 20  is a perspective view of the power generator coupled to an extended heat exchanger, according to an exemplary embodiment of the invention. 
         FIG. 21  is a transparent perspective view of the power generator and the extended heat exchanger of  FIG. 20 , illustrating the approximate positions of the power generator and the extended heat exchanger when implanted in a patient body. 
         FIG. 22  is a perspective view of a power generator coupled to an extended heat exchanger, according to another exemplary embodiment. 
         FIG. 23  is a functional schematic diagram of the power generator and extended heat exchanger of  FIG. 22 , illustrating the flow patterns of the working fluid in and out of the power generator  100 . 
         FIG. 24  is a transparent perspective view of the power generator and the extended heat exchanger of  FIG. 22 , illustrating the approximate positions of the power generator and the extended heat exchanger when implanted in a patient body. 
         FIG. 25  is a perspective view of a power generator coupled to an extended heat exchanger, according to another exemplary embodiment. 
         FIG. 26  is a transparent perspective view of the power generator and the extended heat exchanger of  FIG. 25 , illustrating the approximate positions of the power generator and the extended heat exchanger when implanted in a patient body. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Reference will now be made in detail to the exemplary embodiments consistent with the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
       FIG. 1  schematically illustrates various components constituting a power generator  100  incorporating a power conversion assembly  150  for use in, for example, a LVAD or TAH, according to one exemplary embodiment of the present disclosure. While the present invention will be described in connection with a particular type of a LVAD or TAH, various aspects of the present disclosure may be used with any other types of LVADs and/or TAHs. Moreover, certain aspects of the inventions may be applied to, or used in connection with, any other device or machine that may need an uninterrupted, long-term power supply, such as, for example, robotics, propulsion devices, and actuators, some of which will be described throughout the disclosure. 
     As shown in  FIG. 1 , various components of power generator  100  are thermal-hydraulically interconnected to operate in a closed-loop Rankine thermodynamic cycle with a working fluid  104 . Working fluid  104  may comprise any fluid that exhibits adequate thermal-physical properties to execute thermodynamic power cycles. In some exemplary embodiments, working fluid  104  may be an organic fluid. Working fluid  104  may also contain high-density materials, such as, for example, lead- or tungsten-based material, to function as radiation shielding. 
     Power generator  100  may include a housing  101  containing a source heat exchanger  102 , a power conversion assembly  150 , a recuperator heat exchanger  120 , and a heat sink interface  160  for thermally communicating with an ultimate heat sink  127 . 
     Housing  101  may be a sealed containment enclosing source heat exchanger  102  therein and having an inlet  114  and an outlet  115 . Source heat exchanger  102  may include a heat generating source and one or more heat transfer channels and surfaces coupled to the heat generating source to transfer heat from the heat generating source to working fluid  104 . As will be described in more detail later, in some exemplary embodiments, the heat generating source may include a nuclear material that releases decay heat. For example, the nuclear material that releases decay heat may include nuclear isotopes emitting alpha particles, such as, for example, Pu 238 . In alternative embodiments, source heat exchanger  102  may include or coupled to other types of thermal energy source, such as, for example, combustion products, solar cells, and geothermal source, depending on the type of application for which the power generator of the present disclosure may be used. 
     Housing  101  may be configured to thermally insulate source heat exchanger  102  from the environment surrounding housing  101 . Housing  101  may also include a radiation shield  103  that substantially surrounds source heat exchanger  102  to protect the surrounding from radiation emitted by the nuclear material. In some exemplary embodiments, housing  101  may be sufficiently large to contain an inventory of working fluid  104 . The structural configuration of housing  101  and source heat exchanger  102  will be described in detail later. 
     Power conversion assembly  150  may include a hollow shaft  107 , an expander  106  having single- or multi-stage power turbine rotors mechanically coupled to hollow shaft  107 , a pump  134  having one- or multi-stage turbine rotors mechanically coupled to hollow shaft  107 . 
     As will be described in more detail later, the turbine rotors of pump  134  may be mechanically coupled to a proximal portion of hollow shaft  107 , and the turbine rotors of expander  106  may be mechanically coupled to a distal portion of hollow shaft  107 . To minimize axial shifts of hollow shaft  107  due to the thrust effects of working fluid  104  when compressed by pump  134  and expanded in expander  106 , the turbine rotors of pump  134  and the turbine rotors of expander  106  can be arranged in a way that the directions of pump thrust  206  and expander thrust  207  are opposed against one another to minimize or nullify the thrust effects. 
       FIG. 2  is a perspective view of an exemplary power conversion assembly  150  with its top portion and expander  106  (see  FIG. 3 ) removed to better illustrate the internal components therein.  FIG. 3  is a cross-sectional view of power conversion assembly  150 , illustrating its various rotary and stationary components. As shown in  FIG. 3 , expander  106  may be fixed to or integrally formed with hollow shaft  107 . In this embodiment, expander  106  includes an expander casing  209  concentrically disposed over hollow shaft  107  and a plurality of fins or blades extending from one or both of an interior surface of expander casing  209  and an exterior surface of hollow shaft  107 . If expander  106  is integrally formed with hollow shaft  107 , expander casing  209  may represent an outer wall of hollow shaft  107 , and the plurality of fins or blades may extend from the interior surface of expander casing  209 . 
     As shown in  FIGS. 2 and 3 , power conversion assembly  150  may also include an impeller shroud  112  disposed inside hollow shaft  107  and an impeller  109  disposed inside impeller shroud  112 . Hollow shaft  107  and impeller shroud  112  are concentrically arranged with respect to the rotational axis of impeller  109 . Impeller shroud  112  defines an internal passageway through which a fluid to be pumped  111  (i.e., blood of a patent in case of an LVAD or TAH) can pass through. 
     Impeller shroud  112  may be stationary, and impeller  109  may be magnetically suspended inside impeller shroud  112 . For example, on the interior wall or surface of hollow shaft  107 , a plurality of permanent magnets  108  are radially disposed (e.g., embedded with or fixed to hollow shaft  107 ) about the rotating axis of impeller  109  to magnetically couple impeller  109  to permanent magnets  108 . When hollow shaft  107  rotates as a result of an expansion by a working fluid  104  inside expander  106 , permanent magnets  108  generate rotary magnetic fields that magnetically couple impeller  109  and exerts rotational forces on impeller  109  (e.g., similar to that generated by coils with a stator and/or rotor of an electrical motor), thereby exerting rotational forces on impeller  109 . 
     In some exemplary embodiments, magnetic coupling between permanent magnets  108  and impeller  109  can be enhanced by magnetizing impeller blades  109   a . Alternatively, magnetic coupling between permanent magnets  108  and impeller  109  can be enhanced by attaching permanent magnets to tips  110  of blades  109   a , as shown in  FIGS. 2 and 3 . As a result, the rotary magnetic fields generated by permanent magnets  108  is converted into mechanical pumping power exerted onto the fluid  111  (e.g., blood) passing through the internal passageway defined by shroud  112 . 
     In addition or as an alternative to the magnetic coupling between permanent magnets  108  and impeller  109 , impeller  109  may be mechanically supported via bearings structurally coupled to impeller shroud  112  without significantly obstructing the flow of fluid  111  in the internal passageway defined by impeller shroud  112 . 
     In some exemplary embodiments, hollow shaft  107  may be configured to float over impeller shroud  112  via working fluid  104 . For example, hollow shaft  107  and impeller shroud  112  may be configured in a way that working fluid  104  can form hydrodynamic films in an annular gap  202  between the inner surface of hollow shaft  107  and the outer surface of impeller shroud  112 , as shown in  FIG. 2 . Accordingly, working fluid  104  provides a low-friction, non-contact interface between impeller shroud  112  and hollow shaft  107  without requiring any additional a lubricant or friction-reducing material. 
     In some exemplary embodiments, to ensure concentricity of impeller  109  when fluid  111  passing through impeller shroud  112  exerts loading forces on impeller  109 , the tips  110  of blades  109   a  of impeller  109  may be shaped to cause fluid  111  to form hydrodynamic films in the gap between the tips  110  of blades  109   a  and the inner surface of impeller shroud  112 . The hydrodynamic films may allow impeller  109  to remain in a concentric position, thus creating low-friction, hydrostatic and hydrodynamic bearings. 
     The internal passageway defined by impeller shroud  112  is isolated from the closed-loop circuit of working fluid  104  to prevent mixing of working fluid  104  and fluid  111  passing through the internal passageway of impeller shroud  112 . In addition, impeller shroud  112  may be made of a thermal insulating material to inhibit heat transfer between working fluid  104  and fluid  111 . 
     In an alternative embodiment, where heat transfer between working fluid  104  and fluid  111  is desired, impeller should  112  may be made of a material exhibiting high thermal conductivity to enhance heat transfer between working fluid  104  and fluid  111 . 
     As mentioned above, power generator  100  consistent with the present disclosure may be used to support various applications. For example, power generator  100  of the present disclosure may be used to actuate various types of actuators (e.g., linear or rotary actuators), and fluid  111  in communication with the internal passageway of impeller shroud  112  may be hydraulic oil used to pressurize the actuators. When power generator  100  of the present disclosure is applied to support propulsion, impeller  109  can be retrofitted with a propeller for submerged applications, where fluid  111  in the internal passageway of impeller shroud  112  can be a liquid (e.g., water or liquid metal) or gas (e.g., air). 
     With reference to  FIG. 1 , the thermodynamic cycle of power generator  100  will be explained. Working fluid  104  is pressurized by a single- or multi-stage turbine rotors of pump  134 . Pressurized working fluid  104  exits an outlet  130  of pump  134  and enters a low-temperature portion  120   a  of recuperator heat exchanger  120  via a high-temperature channel  131 . Working fluid  104  exiting an outlet  116  of expander  106  enters a high-temperature portion  120   b  of recuperator heat exchanger  120  via a low-temperature channel  118 . Low-temperature portion  120   a  and high-temperature portion  120   b  of recuperator heat exchanger  120  are configured to exchange heat with one another. Accordingly, as pressurized working fluid  104  from pump  134  passes through recuperator heat exchanger  120 , working fluid  104  is pre-heated to increase its energy content by heat transfer from working fluid  104  flowing from expander  106  and through high-temperature portion  120   b.    
     Pressurized and pre-heated working fluid  104  exits recuperator heat exchanger  120 , passes through a high-pressure channel  132 , and enters a housing  101  via an inlet  114 . Inside housing  101 , working fluid  104  flows through source heat exchanger  102  and is further heated to increase its energy content by heat transfer from the heat generating source (e.g., decay heat from alpha-emitting nuclear isotopes). 
     With increased energy content, working fluid  104  exits source heat exchanger  102  of housing  101  and flows into expander  106  via one or more high-temperature channels  117 . In one exemplary embodiment, high-temperature channel  117  may be configured to support the functions of recuperator heat exchanger  120 . In another exemplary configuration, high-temperature channel  117  may be configured to thermally insulate working fluid  104  from the environment surrounding high-temperature channel  117 . 
     As working fluid  104  enters expander  106  via an inlet  105 , it expands and rotates the turbine rotors of expander  106  coupled to hollow shaft  107  (see also  FIG. 3 ), thereby converting the thermal energy of working fluid  104  into mechanical energy in the form of torque applied to hollow shaft  107 . 
     Torque applied to hollow shaft  107 , in turn, rotates the turbine rotors of pump  134  to pressurize working fluid  104 . Further, as described above, rotating hollow shaft  107  creates rotary magnetic fields by permanent magnets  108  mechanically coupled to or embedded in hollow shaft  107 . Since permanent magnets  108  are magnetically coupled to impeller  109 , the rotary magnetic fields generated by rotating hollow shaft  107  exert rotational forces on impeller  109 . 
     When working fluid  104  is discharged from outlet  116  of expander  106  to low-temperature channel  118 , its energy content is relatively low (e.g., proportional to the efficiency of expander  106 ). Low-temperature channel  118  may be configured to insulate working fluid  104  from the surrounding. In one exemplary embodiment, low-temperature channel  118  may constitute a portion of recuperator heat exchanger  120 . 
     After exchanging thermal energy in recuperator heat exchanger  120 , working fluid  104  flows into heat sink interface  160  via a channel  121  and an interface inlet  122  for thermally communicating with ultimate heat sink  127 . When power generator  100  of the present disclosure is used in a LVAD or TAH, heat sink interface  160  may be implanted inside a patient&#39;s body along with power generator  100 , where heat sink interface  160  exchanges heat energy with a patient&#39;s body portion (e.g., tissues, bones, body fluids, skin surface) via various heat transfer mechanisms (e.g., conductive, convective, and radiative) to reject thermal energy to ultimate heat sink  127  (e.g. air surrounding the patient). 
     In an exemplary embodiment, as shown in  FIG. 1 , heat sink interface  160  may include an extended heat exchanger  124  having heat transfer surfaces that, depending on the type of LVAD or TAH (or other application), allow heat transfer between working fluid  104  and a first thermal interface  125 . First thermal interface  125  may be a sealed tank enclosing extended heat exchanger  124  with a cooling fluid. For example, first thermal interface  125  may be a pool of bodily fluid (e.g., urine inside a patient&#39;s bladder), and extended heat exchanger  124  can be submerged in the pool of bodily fluid. In an alternative embodiment, extended heat exchanger  124  may include a solid thermal interface  125  with a high thermal conductivity, such as a metallic element implanted in a patient&#39;s body. 
     Heat sink interface  160  may further include a second thermal interface  126  for allowing further heat transfer between working fluid  104  and ultimate heat sink  127 . For example, second thermal interface  126  may include a pass-through mesh thermally coupled to ultimate heat sink  127 . In another exemplary embodiment, second thermal interface  126  may be configured to enable a fluid of ultimate heat sink  127  to mix with the fluid of first interface  125 . 
     The configuration of extended heat exchanger  124  in relation to first thermal interface  125  and second thermal interface  126  may vary significantly depending on the type of LVAD or TAH (or other applications) and the patient conditions. For example, for non-medical applications, such as, for example, propulsion, actuation, or robotics, extended heat exchanger  124  can be configured to transfer thermal energy from working fluid  104  directly to the ultimate heat sink  127  via finned radiators thermally coupling working fluid  104  with the air and/or water environments. 
     After being cooled down by extended heat exchanger  124  and with its temperature at its lowest value with respect to the thermodynamic Rankine cycle, working fluid  104  exits extended heat exchanger  124  via an outlet  123 . Working fluid  104  then flows into an inlet of pump  134  via a cold channel  128 , thus resetting the thermodynamic cycle of working fluid  104 . In one exemplary embodiment, cold channel  128  can be thermally coupled to extended heat exchanger  124  to further extend its heat transfer surfaces and further increase condensing effectiveness of working fluid  104  prior to entering pump  134 . 
       FIG. 4  is a schematic view of a power generator  100 ′, according to another exemplary embodiment of the present disclosure. One of the main differences between power generator  100 ′ shown in  FIG. 4  and power generator  100  described above with reference to  FIG. 1  is that power generator  100 ′ includes radial magnets  401  and an electromagnetic stator  400  to produce electricity and mechanical torque. Radial magnets  401  are mechanically coupled to hollow shaft  107 , and a variable magnetic field is generated by radial magnets  401  when hollow shaft  107  rotates. Electromagnetic stator  400  comprising integrated electric coils is configured to convert the variable magnetic field into electricity conditioned by a controller  212 . 
     The generated electricity in the form of AC or DC is then transmitted through integrated leads  213  to controller  212 . Controller  212  is configured to condition the AC or DC electricity produced by electromagnetic stator  400  to supply power to various instrumentation and/or processing systems, such as, for example, sensors and data acquisition and processing systems that may provide information indicative of the performance of power generator  100 . Controller  212  may also be configured to transmit the information wirelessly to an external device via an antenna  208 . 
       FIG. 5  is a schematic, functional diagram of power generator  100  with enhanced structural details, according to various exemplary embodiments of the present disclosure. In this embodiment, source heat exchanger  102  is integrally formed with power conversion assembly  150 , and power generator  100  includes a shield  200  substantially surrounding source heat exchanger  102 . Shield  200  may be provided in addition to or in alternative to radiation shield  103  shown in  FIGS. 1 and 4 . 
     As shown in  FIG. 5 , source heat exchanger  102  may be formed of a conically- or cylindrically-shaped annular heat exchanger and configured to contain a heat generating source (e.g., alpha-emitting isotopes). Pump  134  may include a pump shroud  210  to which a plurality of pump stators  205  are attached. Source heat exchanger  102  may substantially surround pump shroud  210 . 
     Starting from extended heat exchanger  124 , condensed working fluid  104  flows through cold channel  128  and enters recuperator heat exchanger  120 . Low-temperature portion  120   a  and high-temperature portion  120   b  of recuperator heat exchanger  120  may be formed of two concentric annular channels with a wall separating the annular channels serving as the heat transfer surfaces. Working fluid  104  then enters inlet  129  of pump  134  to be pressurized through multi-stage turbine rotors  134  and pump stators  205 . 
     At outlet  130  of pump  134 , working fluid  104  is pressurized and enters source heat exchanger  102  to increase its energy content via thermal exchange with the heat generating source contained in source heat exchanger  102 . After flowing circumferentially and axially through source heat exchanger  102 , working fluid  105  flows through a hydraulic coupler of inlet channel  105   a  that directs working fluid  105  from pump  134  to inlet  105  of expander  106 . 
     Working fluid  104  then enters inlet  105  of expander  106  and starts expanding through multi-stage expander stator  600  and multi-stage turbine rotors of expander  106 , thereby converting a portion of thermal energy of working fluid  104  into torque energy to rotate hollow shaft  107 . Rotating hollow shaft  107  drives pump  134  because hollow shaft  107  is mechanically coupled to turbine rotors of pump  134 . After exiting expander  106 , working fluid  104  flows circumferentially and axially through discharge chamber  304  and enters recuperator heat exchanger  120  to release another portion of its thermal energy to working fluid  104  flowing through high-temperature channel  118  in opposite direction. Working fluid  104  then enters extended heat exchanger and is condensed to reset the Rankine thermodynamic cycle. 
     Power generator  100  shown in  FIG. 5  may be configured to separate working fluid  104  at inlet  129  of pump  134  from working fluid  104  at inlet  105  of expander  106  by a seal  204 . Seal  204  may sealingly surround the outer surface of hollow shaft  107 . In one embodiment, seal  204  may be a non-contact seal. In another embodiment, seal  204  may be a contact seal designed to be lubricated with working fluid  104 . 
     As best shown in  FIGS. 2 and 3 , hollow shaft  107  is mechanically coupled to permanent magnets  108 . In one embodiment, permanent magnets  108  may be configured to provide radial load bearing surfaces for hollow shaft  107  to rotate over hydrodynamic films of working fluid  104  that wet the outer surfaces of impeller shroud  112 . Hollow shaft  107  rotates concentrically with respect to impeller shroud  112  as hydrodynamic films of working fluid  104  are formed throughout annular gap  202 . As hollow shaft  107  rotates and its inner surfaces are wetted by working fluid  104 , hydrodynamic pressure develops within annular gap  202 , effectively maintaining hollow shaft  107  levitated and concentric with respect to impeller shroud  112 . 
     Additional radial and axial loads, exerted on hollow shaft  107  by the operations of pump  134 , expander  106 , and impeller  109 , may be supported by tapered surfaces  203 . Tapered surfaces  203  can be polished and lobed bearing surfaces extended from and mechanically coupled as part of power conversion assembly  150 . For example, tapered surfaces  203  can be integral parts of hollow shaft  107 . Tapered surfaces  203  can be configured to perform thrust and radial load bearing functions as working fluid  104  trapped within annular gap  202  forms hydrodynamic films between tapered surfaces  203  and correspondingly tapered portions of impeller shroud  112 . In one exemplary embodiment, tapered surfaces  203  can be magnetized to perform magnetic thrust bearing functions with respect to impeller  109 . In another exemplary embodiment, tapered surfaces  203  can be formed by permanent magnets oriented in a way to magnetically couple with magnetized blades  110   a.    
     To actively control and assist stabilization of impeller  109 , stator permanent magnets  305  can be configured to be part of or embedded with the structures forming shield  200 . Stator permanent magnets  305  can be configured to magnetically provide a constant magnetic field and an active magnetic field through electronically controlled coils forming the stator components of stator permanent magnets  305 . Electronic control of stator permanent magnets  305  can be executed through controller  212 . Stator permanent magnets  305  can be further configured to produce electric power at rating sufficient to supply power to controller  212  and wireless data transmission via antenna  208  as described above with reference to  FIG. 1 . 
       FIG. 6  is a schematic view of a power generator  100 , according to another exemplary embodiment consistent with the present disclosure. Power generator  100  of  FIG. 6  differs from power generators  100  and  100 ′ described above with reference to  FIGS. 1-5  in that power generator  100  of  FIG. 6  is configured to produce electricity only, whereas power generators  100  and  100 ′ of  FIGS. 1-5  are configured to generate both electricity and torque. 
     More specifically, power generator  100  shown in  FIG. 6  replaces impeller  109  with a magnetic stator  135  having stator poles  136 . As a result, permanent magnets  108  can generate a rotary magnetic field as a result of expansion of working fluid  104  in expander  106 , where the rotary magnetic field couples permanent magnets  108  with stator poles  136 . Stator poles  136  may include electric coils for the purposes of converting the rotary magnetic field into electricity using a method known in the electric AC or DC generator art. 
     The rest of the components of power generator  100  in  FIG. 6  are substantially similar to those of power generator  100  described above with reference to  FIG. 1  and, therefore, the detailed descriptions of the remaining components are omitted herein. 
       FIG. 7  is a schematic view of a power generator  100 , according to another exemplary embodiment of the invention. Power generator  100  shown in  FIG. 7  differs from power generator  100  shown in  FIG. 6  in that recuperator heat exchanger  120  is configured to pre-heat working fluid  104  prior to entering pump  134 . In this configuration, working fluid  104 , with an increased energy content via thermal exchange through recuperator heat exchanger  120 , is pressurized by pump  134  and flown into source heat exchanger  102  via high-temperature channel  131 . After passing through source heat exchanger  102 , working fluid  104  enters expander  106  via high-temperature channel  132  to expand. Like power generator  100  of  FIG. 6 , the rest of the components of power generator  100  of  FIG. 7  are substantially similar to those of power generator  100  described above with reference to  FIG. 1  and, therefore, the detailed descriptions of the remaining components are omitted herein. 
       FIG. 8  is a functional, schematic diagram of a power generator  100 , according to the features shown and described in  FIGS. 6 and 7 .  FIG. 8  illustrates an exemplary configuration of power generator  100  showing in greater detail the components within housing  101  that contains, shields and thermally couples source heat exchanger  102 . When source heat exchanger  102  represents thermal energy produced as a result of decaying isotopes, it can be configured to form a shielded radial thermal source embedded with heat exchanger surfaces of housing  101 . In one configuration, source heat exchanger  102  can be configured to form a substantially cylindrical structure surrounding the turbo-machinery components (rotary and stationary) forming pump  134 . In another configuration, source heat exchanger  102  can be configured to be further extended and surround the turbomachinery components forming expander  106 . 
     In the exemplary embodiment shown in  FIG. 8 , working fluid  104  enters low-temperature channels  118  arranged to form the low- and high-temperature portions  120   a  and  120   b  of recuperator heat exchanger  120 , respectively defined by substantially cylindrical thermal-hydraulic channels with heat transfer surfaces (as shown in  FIG. 19 ) to enhance thermal energy transfer. Working fluid  104  flows from extended heat exchanger  124  into inlet  129  of pump  134  formed by one or multiple pump stators  205  arranged to be mechanically coupled to pump shroud  210 . 
     Working fluid  104  increasingly pressurizes through the stages of pump  134  and as it pressurizes working fluid  104 , it generates a pump thrust in direction  206 . To mitigate or neutralize the pump thrust, the components forming expander  106  are configured to generate an expander thrust in a direction  207  opposite with respect to pump thrust direction  206 . As pressurized working fluid  104  flows at the last stage of outlet expander  134 , it enters source heat exchanger  102  via source inlet  114 . Decay heat induced radiation is attenuated by the shields represented by the materials of source heat exchanger  102  and housing  101 . In this configuration, housing  101  comprises first shield  103  and first shield front and back caps  103   a  and  103   b.    
     Shield  200  further contributes to attenuating radiation. First shield front cap  103   a  can be configured to seal the assembly, via O-rings or other suitable seals  301 , from the front portions of power generator  100 . The assembly coupling to hollow shaft  107  rotates concentrically to the central portions of magnetic stator  135  by floating over hydrodynamic annular gap  202  (as shown in  FIG. 5 ), filled by working fluid  104  forming films between the outer surface of magnetic shroud  211  and the inner surfaces (hollow portions) of hollow shaft  107 . Counter opposing axial thrust and radial loads are induced by tapered surfaces  203  to ensure that hollow shaft  107  and the turbomachinery components coupled to hollow shaft  107  remain centered and concentric and maintain clearances between the stationary and rotary components. In agreement with the thermal-hydraulic schematic shown in  FIG. 7 , pressurized hot working fluid  104  flows out of source heat exchanger  102  and through inlet channel  105   a  to inlet the first stage of expander  106  through inlet  105  for expansion of working fluid  104 . 
     As described in  FIG. 5 , to prevent back flow of the hot working fluid  104  back into the low-pressure channels represented by the first stages of pump  134 , one or multiple seals are positioned between hollow shaft  107  and the stationary assembly mechanically coupled to the stators of pump  134  and expander  106 . As for the power generator  100  configurations shown in  FIGS. 1-5 , hollow shaft  107  comprises rotary permanent magnets  108  configured to generate a rotary magnetic field as they are mechanically coupled to or embedded with the hollow portions of shaft  107 . 
     Annular gap  202  is filled with working fluid  104  to form hydrodynamic regions with pressurized working fluid  104 . Supply of working fluid  104  within annular gap  202  is assisted by inlets  500  of working fluid  104  (shown in  FIG. 23 ), where working fluid  104  is pressurized by pump  134 . Pressurized working fluid  104  is also supplied to the clearance formed by tapered surfaces  203  and the outer surfaces of magnetic shroud  211 . In one configuration, the first gap  201  formed by the inner surfaces of magnetic shroud  211  (hollow portions), and the outer surfaces of stator poles  136  can be configured to be filled with air or an inert gas. In another configuration, the first gap  201  formed by the inner surfaces of magnetic shroud  211  (hollow portions) and the outer surfaces of stator poles  136  can be configured to be filled with a fluid to enhance thermal transfer and cool down stator poles  136  and magnetic stator  135 . As the magnetic field rotates due to the expander  106  driven rotary permanent magnets  108 , the stator poles  136  magnetically couple to the rotary magnetic field and convert the magnetic energy into electricity through coils comprised by the stator poles  136 . 
     Electricity produced by expander  106  through the magnetic stator  135  is conditioned and controlled by controller  212  so as to provide conditioned electric power outside of power generator  100  through electric line  113 . In one configuration, wireless data transfer and control communications with external controllers and data acquisition can occur via antenna  208 . In another configuration, data transfer and control communications with external controllers and data acquisition can occur via electric line  113  configured to carry conditioned electric power and data. 
       FIG. 9  is a cross-sectional view and functional schematic illustrating another exemplary embodiment of power generator  100 , where source heat exchanger  102  is positioned substantially within a central location and includes the assembly forming shaft  107 . In this embodiment, magnetic coupling between the rotary permanent magnets  108  and stationary stator poles  136  occurs as described in  FIG. 8 . In the configuration shown in  FIG. 9 , magnetic stator  135  comprises and shields source heat exchanger  102 . 
     Accordingly, hollow shaft  107  is mechanically coupled to the rotary turbo-machinery components forming expander  106 , pump  134  and rotary permanent magnets  108 , while stationary stator poles  136  are integrated with stator  135  and source heat exchanger  102 . As shown in this figure, working fluid  104  pressurized by the last stage of pump  134  enters source heat exchanger  102  through source inlet  114  (left of  FIG. 9 ), which can be configured to allow working fluid  104  to flow across shaft  107  through a clearance or outlet formed at the edge of at least one of the tapered surfaces  203 . As working fluid  104  flows through inlet  114 , it enters source heat exchanger  102  forming, in this configuration, a portion of magnetic stator  135 . 
     As working fluid  104  increases its energy content via thermal energy exchange with source heat exchanger  102 , it flows out of source outlet  115  and enters high-temperature channel  117  formed by a substantially annular chamber comprised by the inner walls of magnetic shroud  211  and the outer walls of stator poles  136 . Hot and pressurized working fluid  104  then flows into expander inlet  105  to expand through expander  106  by expanding through one or multiple expander stators  600  and proportional number of turbine rotors forming expander  106 . Hot and pressurized working fluid  104  flows through rotary channels  300  (shown with more clarity in  FIG. 10 ). As for the generator configurations described in  FIGS. 5 and 8 , to prevent back flow of working fluid  104  through high-temperature channels  117 , first seal  204  and second seal  204 A mitigate or prevent working fluid  104  leakages between the outlet of pump  134  and inlet  105  of expander  106 . In this configuration, electricity produced by the coils of stator poles  136  is conditioned by controller  212  as described in  FIG. 8 . 
       FIG. 10  is a cross-sectional view and functional schematic illustrating another exemplary embodiment of power generator  100 , where source heat exchanger  102  is positioned substantially within a central location as part of an assembly forming hollow shaft  107 . The magnetic coupling between rotary permanent magnets and stationary electro-magnetic stators occurs through radial permanent magnets mechanically coupled to shaft  107  (hereinafter referred to as radial permanent magnets  410 ) and first stator  400 . First stator  400  comprises electromagnetic coils and leads  213  electrically connecting to controller  212 . Accordingly, radial permanent magnets  401  can be configured to be part of the thrust and radial load bearings represented by tapered surfaces  203 , and bearing journal represented by magnetic shroud  211 . 
     As for the power generator  100  described in  FIG. 9 , working fluid  104  executes a thermodynamic cycle as it circulates through the various components within housing  101  thermal-hydraulically coupled to extended heat exchanger  124 . In this configuration, working fluid  104  enters the central portions of power generator  100  to circulate through source heat exchanger  102 , crossing shaft  107  via fluid channels  402  through tapered surfaces  203  so as to also provide lubrication to these surfaces. To further control axial movement of shaft  107 , radial permanent magnets  401  can be configured to provide counter-opposing magnetic forces by regulating radial first stator  400  and radial second stator  400   a , both controlled by controller  212 . In this configuration, radial permanent magnets  401  are coupled at both ends of shaft  107  to produce electric power by radially coupling with radial first and second stators  400  and  400   a  respectively. 
       FIG. 11  illustrates an exemplary perspective view of power generator  100  described with reference to  FIGS. 1-5 , according to an exemplary embodiment of the invention. In this embodiment, power generator  100  is configured to convert thermal energy to pump fluid  111  by magnetically driving impeller  109 . Accordingly, one end of power generator  100  is equipped with inlet  803  for fluid  111  to circulate via hydraulic channels or tubing coupled to power generator  100 . 
     At the opposite end of power generator  100 , outlet  804  provides hydraulic coupling for a hydraulic channel to enable fluid  111  to circulate out of power generator  100 . Depending on the applications of power generator  100  and the physical thermal- and chemical-properties of fluid  111 , inlet  803  and outlet  804  can be configured to utilize seals  805  formed by sealing materials compatible with fluid  111 . When power generator  100  is configured to be implantable, for example, to support or replace LVADs or TAH applications, the hydraulic channels are represented by arteries and fluid  111  is blood. For applications employing power generator  100  as a submergible propeller, outlet  804  can be shaped as a nozzle to obtain thrust. At one end of power generator  100 , working fluid  104  is configured to flow through cold inlet  128   a , connected to cold channel  128  (see for example  FIGS. 1-5 ), while hot outlet  121   a  provides hydraulic coupling with hot channel  121  ( FIGS. 1-5 ). Overall, inlet and outlet  128   a  and  121   a , respectively, provide hydraulic coupling for thermal-hydraulic channels coupled to extended heat exchanger  124  shown in  FIGS. 1-10 and 24 . 
       FIG. 12  is an exemplary perspective cross-sectional view of power generator  100  shown in  FIG. 11 , illustrating in greater details the generator internals. As also shown in  FIGS. 14 and 15 , recuperator heat exchanger  120  comprises multilayered channels (see the dashed area) defined by a plurality of layers  906  and a plurality of fins  603  extruding across layers  906  to provide extended heat transfer surface for working fluid  104  to exchange thermal energy when circulating through recuperator heat exchanger  120 . In one configuration, layers  906  are configured to induce working fluid  104  to circulate in one direction, for example, toward the inlet of pump  134 , while working fluid  104  discharged at the outlet of expander  106  and flowing in another layer  906  circulates in the opposite direction, so as to obtain a counter-flow heat exchanging mechanisms across multiple layers  906 , thus enabling a higher heat exchanger effectiveness and integration within power generator  100 . Therefore, working fluid  104  flows in both direction across multiple layers  906  of recuperator heat exchanger  120  throughout the circumference of power generator  100 . 
     Given the high number of elements forming power generator  100 ,  FIG. 12  illustrates the position of various internal components of power generator  100  with respect to one another while the exploded assembly view shown in  FIG. 13  shows individual components all concentrically positioned with respect to the center line of impeller  109 . To further increase the heat transfer surface areas within the power generator  100 , extended recuperator  800  surrounds a rectangular and radial configuration of source  702 , generically indicated as source heat exchanger  102  in  FIGS. 1-10 , and is configured to accommodate and shield source  702  ( 102 ). 
       FIG. 13  is an exemplary exploded view of power generator  100  shown in  FIGS. 11 and 12 , illustrating the order in which the components are assembled with respect to rotary and stationary parts of the assembly all together with recuperator heat exchanger  120 , source  702  and extended recuperator  800 . The configuration of the components of power generator  100  and their assembly sequence as shown in  FIG. 13  reflects the schematic and functioning principles shown in  FIGS. 1-5 . 
       FIG. 14  is an exemplary perspective cross-sectional view of recuperator heat exchanger  120 , showing its internal components within power generator  100  of  FIG. 11  and illustrating in greater detail the extended surfaces thermally coupled across different layers  906  (see also  FIG. 15 ) of the heat exchanger. Each layer  906  is structurally coupled to helical fins  603  to increase heat transfer surface area and working fluid  104  turbulence as it flows through annular turning channels formed by combining fins  603  with the walls forming layers  906 . Each two layers  906  represent the inner and outer walls of an annular channel. Furthermore, as fins  603  extrude across multiple layers, each annular channel can be configured to represent hot- or cold-fluid channels  121 ,  128  and low-temperature channels  118 , where working fluid  104  is cooled prior to exiting the generator and pre-heated prior to entering source heat exchanger  102  or  702 , as described by the schematic and functioning diagram shown in  FIGS. 1-5 . Therefore, a minimum of two layers  906  define a heat transfer annular turning channel, where working fluid  104  circulates and transfers across different layers by flowing through hydraulic radial channels  904 , disposed substantially radially with respect to the centerline of recuperator heat exchanger  120 . 
       FIG. 15  illustrates a three-dimensional cut-away view of an end portion of power generator  100 , showing in greater detail multiple layers  906  forming multiple annular channels A, B and C. In one configuration, working fluid  104  enters power generator  100  at inlet  128   a  and flows through annular channel A to transfer thermal energy with working fluid  104  circulating in counter- or parallel-flow within channels B and C. As working fluid  104  flows through the various components forming the thermodynamic cycle, it can be configured to flow back toward the portion of power generator  100  shown in this figure, and into annular channel C. This is the case, for example, in which working fluid  104  flows through extended recuperator  800 , from right to left of power generator  100 . Once flowing toward the end of annular channel C, working fluid  104  can cross through annular channels B and A and be hydraulically coupled to pump  134  through multiple radial channels  904 . Multiple radial channel  904  are positioned throughout the circumference of recuperator heat exchanger  120  to reduce back pressure of working fluid  104  as it circulates through the internal components of power generator  100 . Each radial channel can be configured to form an hydraulic passage formed by walls  905 , extruding across multiple layers  906 , to enable working fluid  104  circulating in one annular channel (e.g., channel A) and flow into another annular channel (e.g., channel C) without physically mixing with warmer or cooler working fluid  104  circulating in annular channel (e.g., channel B). 
       FIG. 16  is an exemplary partially exploded perspective view of the power generator  100  of  FIG. 11 , illustrating the shape of heat transfer surfaces further extending the total heat transfer surface area of the recuperator (hereinafter referred to as extended recuperator  800 ) with a substantially zig-zagged geometry so as to inhibit radiation from source  702  (or  102 ) out of source housing  703  (equivalent to housing  101  shown in  FIGS. 1-5 ), thus executing dual functions: extending the surface areas of recuperator heat exchanger  120  to increase heat transfer with working fluid  104  and shielding radiation potentially emitted by source  702  (equivalent to source heat exchanger  102  in  FIGS. 1-5 ). 
       FIG. 17  is an exemplary perspective view with a different angle of the extended recuperator  800  of power generator  100  shown in  FIG. 11 , illustrating high-temperature channels  132 . In this configuration, the heat source (e.g., alpha emitting source) is embedded with the source housing  703  ( FIG. 16 ), and working fluid  104  is pressurized through high-temperature channels  132  through radial inlet/outlet channels  704  to execute energy exchange between source  702  and working fluid  104  circulating through source housing  703 . 
       FIG. 18  is a perspective view of power generator  100  described with reference to  FIGS. 6-10 , illustrating power generator  100  configured to convert thermal energy into electricity. Working fluid  104  enters power generator  100  at the cold inlet  128   a  and exits at hot outlet  121   a . Depending on applications, cold inlet  128   a  and hot outlet  121   a  can be reversed (e.g., working fluid  104  flowing hot out of outlet  128   a  and cold into inlet  121   a ), and power generator  100  converts thermal energy into conditioned electricity distributed by electric line  113 . 
       FIG. 19  is a perspective cross-sectional view of power generator  100  described above with reference to  FIG. 18 , illustrating the generator internal components configured to substantially surround and shield the thermal source. Power generator  100  shown in this figure is configured to solely produce electricity, however the rotary and stationary turbomachinery components described for power generator  100  shown in  FIGS. 11-19  are substantially similar. 
     As shown in  FIG. 19 , hydraulic channels  500  are more clearly visible. In one exemplary configuration of power generator  100 , hydraulic channels  500  represent a series of radially distributed flow channels on hollow shaft  107  assembly (also generically shown in the schematic of  FIG. 10  under fluid channels  402  and rotary channels  300 ). Hydraulic channels  500  enable working fluid  104  to flow across hollow shaft  107  to supply working fluid  104  to tapered surfaces  203  or provide flow paths for working fluid  104  to inlet/outlet stationary source assembly  700 . 
     Additionally, the multi-stage rotary components of pump  134  and expander  106  are shown along with multi-stage pump stators  205  and expander stator  600 . As source  702  (equivalent to  102 ) is positioned concentrically, substantially in the central portions of power generator  100 , inside source assembly  700 , working fluid  104  flows through the high-temperature channel  132  (source heat exchanger and shield) through hydraulic channels  500 . More generally, working fluid  104  flows through the various components forming power generator  100  to execute energy exchange starting with recuperator heat exchanger  120  (shown within dashed areas). Working fluid  104  is then pressurized by pump  134  prior to entering source assembly  700 , where working fluid  104  increases its energy content. Working fluid  104  flows through source assembly  700  and expands through rotary components of expander  106  to convert the energy of working fluid  104  into mechanical energy in the form of torque at shaft  107 . 
     Sets of rotary permanent magnets  108  (or  401  for power generator  100  configured as shown in  FIG. 10 ) are mechanically coupled to shaft  107  to generate a rotary magnetic field, further coupled to axial or radial electro-magnetic coils (not shown in this figure but designated with reference number  136  in  FIG. 8 , and reference number  400  in  FIG. 10 ) to produce electricity. 
     The electricity produced by thermal conversion of working fluid  104  into electric power is controlled and conditioned by controller  212 , shown embedded with thermal and radiation shield  502  and/or embedded with shield  501 . As working fluid  104  discharges at outlet  116  of expander  106 , it enters the central annular channel of recuperator heat exchanger  120  to transfer thermal energy to working fluid  104  that is flowing in counter-flow configuration and is thermally coupled by the annular channels comprised by recuperator heat exchanger  120 . In some configurations, working fluid  104  further circulates through internal flow pathways (not shown) into the extended heat exchanger comprised by source assembly  700 . As working fluid  104  flows toward the hot outlet  121   a  of power generator  100 , it provides thermal and radiation shield through a jacket  503  configured to substantially surround radial shield  501   a , wherein radial shield  501   a  comprises the expander shroud  209 . 
       FIG. 20  is a perspective view of power generator  100  described in  FIGS. 8-19  and configured as shown in  FIG. 11 , which is coupled to extended heat exchanger  124  by hot and cold channels  121  and  128 , respectively, for use in a LVAD or TAH, according to an exemplary embodiment of the present disclosure. Hot and cold channels  128  and  121  are configured to extend the heat transfer surfaces from recuperator heat exchanger  120 , comprised by power generator  100  housing, to further extended heat transfer surfaces wetted by working fluid  104  as it flows through these hot and cold thermal-hydraulic channels coupling power generator  100  to the extended heat exchanger  124 . In this configuration, hot and cold channels  121  and  128  form a heat exchanger thermally coupled with the ultimate heat sink  127  through the patient body  901 , shown in  FIG. 21  and represented by tissues, body fluids, bones, skin, inhaled and exhaled air, sweat, etc. 
       FIG. 21  is a transparent perspective view of the power generator  100  and extended heat exchanger  124  of  FIG. 20 , illustrating the approximate position of power generator  100  and extended heat exchanger  124  when implanted in a patient body  901 . In this configuration, fluid  111  is blood flowing from/to arteries or from/to heart ventricles in/out of power generator  100  via LVAD hydraulic coupling  903  (e.g., aorta) and  902  (e.g., ventricle). Hot and cold channels  121  and  128  and extended heat exchanger  124  are thermally coupled with body  901  internals to transfer thermal energy rejected by the closed-loop Rankine cycle actuated by power generator  100 . In this configuration, thermal energy rejected by the Rankine cycle is mainly transferred from the extend heat exchanger  124  to the body  901  internals via second thermal interface  126 . 
       FIG. 22  is a perspective view of power generator  100  of  FIG. 11 , coupled to a variation of extended heat exchanger  124  as the heat transfer surfaces characterizing hot and cold channels  121  and  128  are further extended to define the entirety of extended heat exchanger  124  heat transfer surfaces, according to another exemplary embodiment of the present disclosure. In this configuration, the length of hot and cold channels  121  and  124  can be configured to be extended to further increase the surface area exposed to body  901  internal tissues, fluids, bones etc., to further rejecting thermal energy discharged by the Rankine cycle to the ultimate heat sink  127  (e.g. air surrounding body  901 ). In this configuration, hot and cold channels  121  and  128  further distribute temperature through body  901  as working fluid  104  condenses through thermal transfer with the body  901  and the ultimate heat sink  127 . The extended hot and cold channels  121  and  128  can be configured to be comprised by the second thermal interface  126  described in  FIG. 1 . 
       FIG. 23  is a functional schematic diagram of power generator  100  and extended heat exchanger  126   a  of  FIG. 22 , illustrating the flow patterns of working fluid  104  as working fluid circulates in and out of power generator  100  and through the hot and cold channels  121  and  128 , respectively. Hot working fluid  104  discharged by expander  106  and exiting power generator  100  after energy exchange with recuperator heat exchanger  120  flows internally through a flexible heat exchanger  126   a  comprising hot and cold channels  121  and  128 , respectively, and second thermal interface  126  so as to enable positioning within body  901  as shown in  FIG. 24 . To protect body  901  internals from the highest temperature represented by working fluid  104  as it cools down through energy exchange with body  901 , the hot channel  121  is positioned substantially centrally with respect to cold channel  128 , where cold channel  121  can be configured to substantially surround hot channel  121 . 
       FIG. 24  is a transparent perspective view of power generator  100  and extended heat exchanger  126   a  of  FIGS. 22 and 23 , according to another exemplary embodiment of the present disclosure. In this illustration, the approximate positions of power generator  100  is shown along with flexible extended heat exchanger  126   a  which can be configured for positioning in, for example, the abdominal regions of body  901  to enhance energy exchange with body  901  while minimizing hot temperature spots as working fluid  104  cools down while flowing throughout the flexible heat exchanger. 
       FIG. 25  illustrates an application of power generator  100  when configured to supply electric power via electric line  113  to a FDA-approved LVAD  900 . In this configuration, power generator  100  may include extended heat exchanger  124  and/or flexible heat exchanger  126   a  shown in  FIGS. 22-24 . This configuration of power generation  100  is described with reference to  FIGS. 6-10, 18, and 19 . In this configuration, power generator  100  converts thermal energy from source heat exchanger  102  or  702  into conditioned electricity, distributed outside of power generator  100  by electric line  113 . 
       FIG. 26  is a transparent perspective view of power generator  100  and extended heat exchanger  124  of  FIG. 25 , illustrating exemplary positions of power generator  100  and extended heat exchanger  124  when implanted in a patient body. Power generator  100  comprises all the components described, for example, in  FIGS. 18, 20, 22, and 23  so as to provide an electric generator fully encapsulated within the second thermal interface  126 . 
     For all non-implantable applications (e.g., robotics), power generator  100  can be configured to include the heat exchangers configured to transferring thermal energy to the ultimate heat sink  127 , namely, extended heat exchanger  124 , flexible heat exchanger  126   a  and the heat exchanger represented by the hot and cold channels  121  and  128 , respectively. Alternatively, depending on the application, for non-implantable applications, power generator  100  can be positioned at a distance from the extended heat exchanger  124 , which can be represented by a finned radiator configured to condense working fluid  104 . 
     Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.