HK1147707A - Inductive element for intravascular implantable devices - Google Patents

Description

Inductive element for intravascular implantable devices

Technical Field

The present invention relates generally to electrical components and, more particularly, to inductive components, such as chokes or transformers, having a stenosis forming unit suitable for an implantable medical device, such as an intravascular device.

Background

Implantable medical devices such as pacemakers, defibrillators, and Implantable Cardioverter Defibrillators (ICDs) have been successfully implanted in patients for many years for the treatment of heart rhythm disorders. Pacemakers are implanted for detecting periods of bradycardia and delivering low energy electrical stimulation to improve the heart rhythm. ICDs are implanted in patients for cardioversion or defibrillation that slows the heart rhythm by delivering high-energy electrical stimuli when Ventricular Tachycardia (VT) and Ventricular Fibrillation (VF) are detected. Another implantable device detects Atrial Fibrillation (AF) insertions and delivers electrical stimuli to the atria to restore electrical coordination between the upper and lower chambers of the heart. Yet another type of implantable device stores and delivers drugs and/or gene therapy to treat various conditions, including cardiac arrhythmias. The current generation of all these implanted devices are typically subcutaneous implanted canister-shaped devices that deliver a therapeutic agent through a lead implanted in the heart from the patient's vascular system.

Next generation implantable medical devices may take the form of elongate intravascular devices that can be implanted within the vascular system of a patient, instead of being implanted subcutaneously. Examples of such intravascular implant devices are described, for example, in U.S. patent No. 7,082,336, U.S. published patent applications 2005/0043765a1, 2005/0208471a1, and 2006/0217779a 1. Such devices are about 3-15mm in diameter and about 10-60cm in length to facilitate insertion and implantation of the vessel while allowing a sufficient amount of blood to flow around the device. Such devices include, for example, electronic/electrical components and circuits that perform various functions due to geometric limitations.

The implanted device has a loaded energy storage device, typically a battery, and high voltage conversion circuitry for converting the stored energy into a form suitable for operating the device to deliver electrotherapy therapy. In cardioversion/defibrillation devices, the high voltage switching circuitry typically includes circuitry for generating energy at high voltage (typically at least at 50-800 volts and around 1-40 joules) for use in cardioversion/defibrillation electrotherapy. Because there is only limited energy available in the energy storage device, and because replacing the battery typically requires surgery to remove or contact the implanted device, or a recharging process, which can require extended periods of time to recharge the energy storage device, providing an efficient circuit is important to extend the useful life of the device and also to minimize the size of the device. Therefore, the high voltage switching circuit for the implanted device should be as efficient as possible.

A switch mode power converter is generally used as the most efficient voltage stepping device to step up the voltage from the stored energy to the high voltage required for electrotherapy. This type of converter utilizes the voltage boosting effect caused by the associated time-varying magnetic field generated by an inductive element, such as a choke or transformer, by applying an intermittent current to the inductive element. Many different switching converter topologies and modes of operation are known. Examples include boost converters, flyback converters, single-ended primary inductive converters (SEPIC), and Cuk converters. The boost converter and certain Cuk converter topologies employ one or more inductors, while flyback, SEPIC, and other types of Cuk converters employ a transformer as the primary inductive element for the voltage conversion function. Some particular SEPIC topologies employ both, an inductor and a transformer.

The inductive element (whether an inductor or a set of magnetically coupled inductors) is mainly composed of at least one coil and a core of a material with a high relative permeability, such as a ferromagnetic material. The core acts to define a field region close to the element, thereby increasing its inductance. The core provides a magnetic flux path that directs magnetic flux through the center of the coil and along a return path, which may be contiguous or may have multiple non-contiguous return path portions. There are a number of different known core geometries for the inductive element. Some are formed on ferrite bobbins around enamel-coated wires, which are exposed to the outside, while others completely wrap the wires in ferrite to improve shielding. The core geometry generally includes a circular cylindrical configuration, a C-or E-shaped configuration, a pot-shaped configuration, and a planar configuration.

In the case of a switched mode transformer, a typical winding ratio Np: Ns in a high voltage switching circuit for an implanted device may be a ratio of about 1: 15, where Np is the primary winding and Ns is the secondary winding. Unlike transformers for signal and linear power supplies, transformers for switched mode circuits are designed not only to transfer energy, but also to store the energy for a considerable switching period. For example, one type of power converter switches at about 60kHz (the frequency is selected to keep core eddy current losses low) and has a transformer with a core made of a strong ferrite material with a relative penetration of 2000 to 4000, in which a certain minimum primary inductance is required.

Most of the energy stored in the inductive element is stored in the air gap of the core. A certain amount of air gap capacity is required to store the required energy. However, increasing the slot length decreases the inductance of the transformer or inductor. The coil inductance in the inductive element is directly proportional to the square of the number of coils and to the magnetic cross-sectional area orthogonal to the direction of the magnetic flux generated in space. To supplement the inductance lost by the increased air gap, a greater number of coil turns or a greater cross-sectional area of the flux path is required. More coil windings result in a larger volume and increase the energy loss in the device due to the increased impedance. Increasing the cross-sectional area of the flux path in conventional core geometries involves increasing the size of the core, which in turn reduces the space in the coil or increases the overall size of the device.

In connection with an intravascular implant device, an elongated shape may be used, implanted in a patient's blood vessel, and having a mainly circular cross-section, the magnet cross-sectional area will be limited to be smaller than the cross-sectional area of the implant device if a standard circular pot core is used as the ferrite core of the transformer. Another example of increasing the inductance is increasing the number of coil turns, given this limitation. Unfortunately, this increases the volume of the coil in the transformer, since high voltage transformers require relatively high coil to coil ratios. In addition to increasing the total number of turns resulting in higher total resistance of the coil, this approach requires a longer transformer to house the coil.

Long and narrow pot cores, when used in an implantable intravascular device, have difficulty addressing the coil challenges due to the limited cross-sectional area of the coil across the diameter of the core. In addition, there are practical limits to transformer length in an implantable intravascular device. For example, the housing of the implantable intravascular device must provide a certain amount of flexibility to facilitate passage of the device through the blood vessel. The longer rigid housing element limits the bending radius of the device. In addition, the enclosure that encloses the transformer may require space beyond the transformer end faces to enclose circuitry, input/output hardware, wires, etc.

Other approaches, such as a reduced E-core or derivatives thereof, such as EFD or ER cores, for use within the limited size of an intravascular device may not be feasible in terms of energy storage and inductance requirements for the energy charging circuit. For example, the coil area may not be sufficient to obtain the primary inductance of one transformer target. Even if electrical performance can be achieved at small sizes, the use of a reduced E-shaped inductive element in the housing of the in-tube device wastes housing space because excess space remains in the housing around the inductive element.

Designing an energy converter that can efficiently generate high voltage electrotherapy signals with existing inductive elements presents a significant challenge due to the size limitations of the implantable intravascular devices. Typical core shapes and geometries, such as E-shaped, C-shaped, donut-shaped, and pot-shaped cores, which are generally capable of providing the required functionality and performance requirements of high voltage converters in conventional implanted devices, such as conventional canister-shaped implanted defibrillators, are not well suited for use in the small diameter space of an implantable intravascular device.

Disclosure of Invention

The present invention is generally directed to an inductive element suitable for use in an Intravascular Implantable Device (IID) having an elongated form factor suitable for implantation within a blood vessel. The inductive element includes a core having an outer surface profile corresponding to an inner surface profile of a form factor of the implantable intravascular device. A set of elongate or elliptical coils whose length direction lies along the major length direction of the inductive element. The coil is also positioned to direct the magnetic field radially relative to a longitudinal axis of the form factor of the intravascular implantable device.

In one aspect of the invention, an implantable intravascular medical device includes a structure defining a form factor having an elongate geometry including a length and a substantially circular cross-section defined perpendicular to the length. One example of such a structure is a housing, or a housing portion providing an enclosing insulating sleeve, and having a substantially cylindrical form factor suitable for implantation into a blood vessel. In this form factor, a circuit is disposed that includes an energy storage device, such as a battery, and a conversion circuit that functions to convert the output of the energy storage device to a relatively high voltage. The conversion circuit includes an inductive element having an outer surface shape corresponding to the form factor. The inductive element has a coil arranged to direct a magnetic field substantially perpendicular to the length.

An implantable intravascular medical device according to another aspect of the present invention includes a structure defining a form factor having an elongate geometry including a form factor length and a substantially circular form factor cross-section perpendicular to the form factor length. A circuit is disposed within the form factor and includes an inductive element including a core of magnetic material having a core length and a core cross-section defined perpendicular to the length, and a coil having a plurality of windings defining an annular area. A portion of the core is located in the loop area such that when the coil is energized, a magnetic flux is generated in the core along a forward path and a return path, a sum of a total cross-sectional area of the magnetic flux in the forward path and a total cross-sectional area of the magnetic flux in the return path being greater than a cross-sectional area of the core.

An inductive element (e.g., an inductor or transformer) according to one aspect of the invention includes a core of magnetic material having a core length and a substantially cylindrical outer boundary, at least one coil having a plurality of windings defining an annular area. A portion of the core is located within the annular area such that the at least one coil, when energized, generates a closed magnetic flux along a magnetic flux path through the core, the magnetic flux path having a length less than the core length.

According to another aspect of the invention, an inductive element for an implantable intravascular device includes a core of magnetic material. The core has a major longitudinal length extending along a first reference axis and a core cross-section having a substantially circular outer boundary, the core cross-section being defined in a first reference plane, the plane being orthogonal to the first reference axis. The core includes a cylinder, and at least one coil disposed around the cylinder such that the coil is positioned to direct a magnetic field perpendicular to the first reference axis when energized.

A method of making an implantable intravascular device according to another aspect of the present invention includes making a substantially sealed insulating sheath for encasing an electrical circuit, the insulating sheath having a substantially cylindrical outer surface and defining an inner form factor. An inductive element assembled as a component to the circuit is positioned in the insulating sleeve such that at least a majority of an outer surface profile of the inductive element corresponds to the inner form factor. To this end, a set of elongated windings is positioned lengthwise within the insulating sleeve to direct the magnetic field in a radial direction relative to the insulating sleeve and to provide the magnetic field with a closed magnetic path substantially through the permeable material.

In one embodiment, the inductive element has a circular outer wall that mates with a circular inner wall of a chamber or other enclosure in which the elements are housed. In another embodiment, the inductive element has a cylindrical outer wall sized within a predetermined limit relative to the size of at least a portion of the outer surface of the IID device formed around the inductive element. The assembly of the inductive element includes disposing a set of elongate windings lengthwise in the chamber to direct a magnetic field to extend in a radial direction relative to the chamber; and provides a closed magnetic path for the magnetic field substantially through the permeable material. The closed magnetic channel may be provided by a magnetic core with or without an air gap.

Embodiments of the present invention provide inductive elements that increase the space available for magnetic material within form factors in IID devices while providing a relatively larger and more useful cross-sectional area for the flux path to increase inductance and reduce AC flux density. Increasing the useful space may be achieved by designing more shapes of the transformer profile to match the substantially cylindrical space associated with the form factor. Imparting a large cross-sectional area substantially orthogonal to the flux path may be accomplished by winding the transformer wires in a plane that is substantially orthogonal to the length of the IID device. The direction of the generated magnetic flux passing through the cross-sectional area formed by the winding extends along the vertical axis thereof. This relatively large magnetic flux is useful for certain power conversion topologies, such as, but not limited to: flyback, SEPIC, or Cuk type converters. The inductive element may also be used in other types of power circuits, such as buck-boost regulators, or other circuits that utilize inductors or transformers.

Form factors according to particular embodiments of the present invention may be defined based on the dimensions of the housing of the IID device and the presence of other components within the housing that house the inductive element. For example, in some embodiments where additional electrical or mechanical components, such as wires, interface hardware, or circuitry are provided in the housing in which the inductive component is housed, the form factor may take into account the volume bounded by these components. In a related type of embodiment, the form factor may include space along the length of the transformer such that a wire or circuit passes lengthwise through the inductive element.

These aspects of the invention allow the IID circuit to achieve performance of the power converter circuit in all other circuits that occupy the limited space of the IID device to levels that are not achievable with conventional power converter components within the same size limits. These advantages may lead to higher performance and smaller design of the intravascular implantable device, easier implantation in a patient, and more effective delivery of electrotherapy over a longer service life than devices based on conventional techniques.

Drawings

The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which:

FIG. 1 is a perspective schematic drawing depicting the human heart anatomy;

fig. 2 is a cross-sectional view of an intravascular implantable pacemaker according to an embodiment of the present invention;

FIG. 2A is a schematic representation of FIG. 2;

fig. 3 is a cross-sectional view of an intravascular implantable pacemaker according to another embodiment of the present invention;

FIG. 3A is a schematic representation of FIG. 3;

FIGS. 4A-4E are circuit diagrams depicting different known types of switching regulator topologies;

FIGS. 5A-5G are perspective views of various known geometries of inductive cores;

FIG. 6 is an exploded perspective view illustrating an inductive element assembly for use with narrow form factors, in accordance with an aspect of the present invention;

FIG. 7A is an exploded perspective view illustrating a narrow form factor inductive element assembly in accordance with another aspect of the present invention;

FIG. 7B is a cross-sectional view of the assembled inductive element of FIG. 7A;

FIG. 8 is a graph depicting simulated magnetic flux density through a core of an exemplary inductive element, such as the inductive element of FIG. 6, according to one embodiment of the present invention;

FIG. 9 is a graph depicting simulated magnetic flux density through a core of an exemplary inductive element according to another embodiment of the present invention, such as the inductive element of FIGS. 7A-7B; and

fig. 10 is a diagram illustrating a power conversion circuit and a portion of a control system of a power converter according to an embodiment of the invention.

While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

Detailed Description

In the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one of ordinary skill in the art that the present invention may be practiced without these specific details. In other instances, certain well-known methods, procedures, and components may not have been described in detail so as not to unnecessarily obscure the true technical features of the invention.

Referring now to FIG. 1, there is depicted a general human heart anatomy, including the heart and major blood vessels. The anatomical locations shown below are identified by the listed reference numerals: a right subclavian artery 102a, a left subclavian artery 102b, a Superior Vena Cava (SVC)103a, an Inferior Vena Cava (IVC)103b, a Right Atrium (RA)104a, a Left Atrium (LA)104b, a right innominate/brachiocephalic vein 105a, a left innominate/brachiocephalic vein 105b, a right medial jugular vein 106a, a left medial jugular vein 106b, a Right Ventricle (RV)107a, a Left Ventricle (LV)107b, an aortic arch 108, a descending aorta 109, a right cephalic vein 109a (not shown in fig. 1), a left cephalic vein 109b, a right axillary vein 110a (not shown in fig. 1), and a left axillary vein 110 b.

One embodiment of the present invention describes an intravascular electrotherapy system for different functions of treating various arrhythmias with electrical stimulation. These functions include defibrillation, pacing, and/or cardioversion. In general, an intravascular implant device element for electrophysiological treatment includes at least one device body and typically, but optionally, at least one lead coupled to the body. Alternatively, the implantable intravascular device may be devoid of a lead, such as for a specific embodiment of an implantable intravascular drug/gene therapy device, a hybrid implantable intravascular device that delivers both electrical therapy and/or delivers drug/gene therapy, or another implantable intravascular device that utilizes a high voltage switching circuit, e.g., to deliver power to a drug/gene delivery device/pump or an electrically powered delivery/therapy device may be devoid of a lead.

Various examples of implantable intravascular electrophysiology treatment devices, such as implantable defibrillation and/or pacing devices 20 and leads 28, are given in the specification, in which examples reference numerals such as 20a, 20b, 20c, etc. are used to describe particular embodiments of intravascular device 20, while reference numeral 20 elsewhere is intended to generally refer to such implantable devices as are used in the present disclosure to provide treatments other than cardiac electrophysiology. Similarly, reference numeral 28 generally designates a conductor employed by an embodiment of such a system. Reference numeral 100 generally designates blood vessels and/or blood vessel walls within the human body.

In one embodiment, the device 20 includes some elements known in the art necessary to perform the functions of an implantable electrophysiology treatment device. For example, the device 20 may include one or more pulse generators, including associated batteries, capacitors, microprocessors, and circuitry for generating electrophysiological pulses for defibrillation, cardioversion, and/or pacing. The apparatus 20 may also include detection circuitry for detecting arrhythmias or other abnormal activity of the heart. The particular components provided in device 20 depend on the application of the device, and in particular whether device 20 is used to perform defibrillation, cardioversion, and/or pacing in conjunction with sensing functions, or whether the device is configured to detect and/or deliver drugs/gene therapy or perform other therapeutic or diagnostic functions.

The device 20 may be proportioned to pass through the vasculature and be positioned in the vasculature of a patient with minimal obstruction to blood flow. Suitable locations for introducing the device 20 into the body may include, but are not limited to, the use of a venous system with access via the right or left femoral vein or the right or left clavicular vein. In an alternative embodiment, the implantable intravascular device may be configured for use in the arterial system.

For purposes of describing the present invention, the various portions of the device 20 will be referenced with respect to the locations of the proximal portion 22, the intermediate portion 24, and the distal portion 26 relative to the location of the introduction point into the femoral vein. It will be appreciated, however, that if another access location is used to introduce the device 20, such as the clavicular vein, the various locations 22, 24 and 26 of the device 20 are referenced relative to the superior/inferior location of the device 20 within the vasculature in the torso of a patient.

In one embodiment, the device 20 may be provided with a streamlined maximum cross-sectional diameter in the range of 3-15mm or less, and in a preferred embodiment 3-8mm or less. The cross-section of the device 20 in the transverse direction (i.e., transverse to the longitudinal axis) may be as small as possible while still being compatible with the desired components. This area may be approximately 79mm according to embodiments and/or applications²Or less, in the range of approximately 40mm²Or less, or 12.5-40mm²Within the range of (1).

In one embodiment, the cross-section of the device 20 (i.e., transverse to the longitudinal axis) may have a circular cross-section, although other cross-sections including crescent, flattened, or elliptical cross-sections may also be used. It is desirable to have a smooth, continuous profile for the device to avoid voids or pockets that may cause thrombus to form on the device. It is also desirable to provide a circular cross-section that aids in the removal or implantation of the device so that the device can be easily twisted or rotated during removal or implantation to break off any thrombus or blood clot that may be present.

In one embodiment, the outer surface of device 20 comprises an electrically insulating material, layer or coating, such as ePTFE. For example, it may be desirable to provide some sort of anti-thrombogenic coating (e.g., a perfluorinated compound coating coated with supercritical carbon dioxide) to avoid thrombogenicity on the device 20. It may also be advantageous for the coating to have anti-proliferative properties in order to minimize endothelialization or cell growth, as minimizing growth on the device 20 helps to reduce arterial injury during device implantation. The coating may thus also elute an anti-thrombotic component (e.g. heparin sulphate) and/or a cell growth inhibiting component and/or an immunosuppressant. If the housing of device 20 is a conductor, this layer or coating may be selectively applied or removed to leave exposed electrode areas where desired on the surface of the housing, as depicted in FIGS. 2A-2D.

In one embodiment, the housing of the device 20, or portions thereof, has a shaped unit designed to meet specific external boundary requirements. For example, an outer boundary requirement may be a particular outer geometry (e.g., cylindrical or other suitable circular) within a particular dimensional tolerance. The housing according to this embodiment may also have a housing thickness specification. For example, a particular cylindrical housing may have a 10mm Outer Diameter (OD) boundary, with a specified tolerance of, for example, +/-5% tolerance, and a minimum wall thickness requirement, such as 1 mm.

The components themselves must be sized to fit within the space defined by the housing, taking into account the minimum space left for the components. With reference to the above example of 10mm external diameter +/-5% wall thickness of 1mm, these elements (including the connecting wires between them) must be housed in shaped units having a transverse dimension of 10-10(0.05) -1 or 8.5 mm. These elements are to be positioned within the device 20 to make efficient use of the available space. The size and dimensions of the inductive elements achievable according to the features of the present invention provide further flexibility in the selection and design of these elements, as the design of the inductive elements can provide the desired performance characteristics in an efficient space, leaving relatively more space available for the elements.

Fig. 2A, 2B and 2C show some examples of devices whose contents are arranged in space effectively. Reference numeral 20a in fig. 2A identifies an example. The example device 20a includes one or more elongate housings or shells 32, as shown in the cross-sectional view of fig. 2A, in which the components contained therein are visible. In one embodiment, the housing 32 is a rigid or semi-rigid shell, optionally made of a conductive, biocompatible, sterilizable material, and capable of hermetically sealing the components contained within the housing 32. One example of such a material is titanium, although other materials may be used.

In the housing 32 are electronic components 34 for operation of the control device 20 a. For example, in the embodiment of FIG. 2A, element 34a is associated with delivering a defibrillation pulse via lead 28, while element 34b is associated with a sensing function performed using a defibrillation lead or a sensing electrode on separate lead 28. Isolation of the component 34a from the component 34b may be desirable if electromagnetic interference (EMI) associated with operation of the high voltage circuit 34a may interfere with the performance of the sensing circuit 34 b. Isolation may be achieved by adding physical isolation between the potentially interfering and susceptible components, electric field shielding, magnetic field shielding, or a combination of the above.

The device 20a also includes a source of energy, such as one or more batteries 36, for powering the device. In some cardioversion or defibrillation device embodiments, one or more high voltage capacitors 38 are provided for storing the electrical charge delivered to the lead 28 and/or one or more electrodes 40 exposed on the outer surface of the housing 32. One or more circuit interconnects 42 may provide electrical connections between electronic component 34, one or more leads 28, electrodes 40, battery 36, and capacitor 38.

As shown in FIG. 2A, the elements of device 20a may be arranged in series with one another, giving device 20a streamlined profile. Because the device 20a is intended to be implanted within a patient's vessel, some flexibility is required so that the elongate device can be easily passed through the vessel. Flexibility may be increased by segmenting the device 20, such as by forming one or more spaces 44 in the housing 30 and one or more hinge regions 46 at each space 44. These hinge regions 46 thus form dynamically bendable regions that bend relative to the longitudinal axis of the device 20a as the device 20a is passed through the curved region of the blood vessel and/or positioned.

The component arrangement of the second example implantable intravascular pacing device shown in fig. 2B is identified by reference numeral 20 a. Many of which are the same as those shown in the embodiment of fig. 2A and will not be discussed again in the description of fig. 2B. This second embodiment differs from the first embodiment mainly in that the electronic components 34 can be contained in a separate area of the housing 32. Device 20b may include one or more spaces 44 and hinge regions 46 depending on the element and its location to be attached in device 20 b. This configuration may be used, for example, when apparatus 20 is used only for pacing functions (and thus without the relatively noisy charging circuitry of the defibrillation circuitry), or when isolation of the type shown in the embodiment of fig. 3A to prevent interference noise from the charging circuitry to the sensor circuitry is not required.

A variation of the embodiment of fig. 2A and 2B is the device 20C shown in fig. 2C and 2D. In the device 20c, each segment is separately encased in its own titanium (or other suitable material) casing in the shape of a container 32a, 32b, 32 c. The elements in the containers 32a, 32b, 32c may be electrically connected by, for example, a flexible circuit connector 42 a. In one embodiment, the containers 32a, 32b, 32c utilize an elastomeric material, such as a silicone rubber filler, to make the hinge region 44 a. FIG. 2D depicts device 20c in a bent condition at one hinge region.

Fig. 3 depicts another embodiment variation of an implantable intravascular device useful in the present invention. A flexible device 20 includes one or more rigid housings or containers 32 for housing electronic components 34 for implantation within a patient's vasculature, with hinge regions 44 forming a bellows arrangement 48. The container 32 may be of any suitable shape, cross-section, and length, but in the example shown is cylindrical in shape having a diameter of about 3-15mm and a length of about 20mm to 75 mm. The receptacle 32 may be used to house electromechanical parts or components to form a very compact implant device such as a defibrillator, pacemaker, drug delivery system. Any suitable number of containers 32 may be combined with interconnecting bellows 48. Interconnected mechanical bellows 48 may be used to connect several rigid containers 32 to form the flexible device 20. For many such devices, an arrangement of at least three containers 32 will be included.

In one embodiment, the bellows 48 may be of any suitable shape, but have a cross-sectional shape that approximates the cross-section of the container to avoid problems with edges or corners, such as blood clot formation in a blood vessel. The bellows may be made of a biocompatible material similar to the container. Any coating that electrically insulates the container and/or makes the container more hemodynamically compatible may also be used for the bellows.

In addition to the ability of the bellows 48 to bend from the central or longitudinal axis of the device 20, the bellows 48 can also bend along the central axis of the device. This ability to bend along the central axis provides shock absorption in the longitudinal direction as well as three-dimensional flexibility. This shock absorption helps protect the device 20 and its internal components during implantation by minimizing the motion of the implanted device. Additionally, the shock absorption may provide a 1: 1 torque ratio for steering during implantation. This shock absorption also contributes to the useful life of the device 20, as the natural motion of the patient's body can place some stress on the device 20.

Referring again to fig. 2A, the electronics 34A associated with delivering the defibrillation pulse includes voltage conversion circuitry for converting the lower battery voltage to a higher electrotherapy voltage. An example of a lower battery voltage is a voltage below about 20 volts. An example of a higher electrotherapy voltage is a voltage of about 50 volts or more. In one embodiment, the battery voltage is on the order of 10 volts and the maximum defibrillation voltage is on the order of 700 volts and 1000 volts. In general, voltage converter embodiments can provide a boost on the order of about 5 to 300 times the input voltage of the converter. For example, in one embodiment, a 3 volt battery is used as the energy storage device for powering the boost circuit to deliver a 1000 volt defibrillation pulse with a boost factor of 333.

A switched mode voltage converter circuit may be used to generate a high voltage output that is then used to charge one or more high voltage capacitors located at the output of the voltage converter circuit. In some embodiments, the voltage conversion circuit is capable of storing at least 5 joules of energy in the high voltage capacitor charge in no more than 30 seconds. For example, in one embodiment, the voltage conversion circuit may store about 30 joules in the high voltage capacitor in 10 seconds. The energy stored in the high voltage capacitor is ultimately released to the patient during electrotherapy therapy.

Fig. 4A-4D and 5A-5G are schematic diagrams depicting different examples of various known power conversion circuit topologies. These topologies are known to those of ordinary skill in the art, however, it is to be understood that these topologies, by themselves, do not achieve the performance and efficiency levels taught by the present invention and that such variations of the topologies made in accordance with the teachings of the present invention may be employed within the spirit and scope of the present invention.

Fig. 4A depicts a basic boost converter topology. The boost converter of fig. 4A utilizes a single inductor labeled L1 to store energy in each cycle of the switch SW. When switch SW is closed, inductor L1 is energized and generates a self-induced magnetic field. When the switch SW is opened, the voltage at the node L1-SW-D1 increases as the magnetic field of the inductor L1 decays. The related current passes through an isolation diode D1 to store the energy C_outIs charged to a voltage greater than the input voltage Vin.

Fig. 4B depicts a flyback converter topology. The flyback converter utilizes transformer T1 as an energy storage device as well as a step-up transformer. With switch SW closed, the primary winding of transformer T1 is energized in a manner similar to inductor L1 of FIG. 4A. When the switch SW is open, the voltage across the primary coil is reversed and rises due to the decay of the magnetic field in the primary coil. The change in voltage of the primary coil is magnetically coupled to the secondary coil, which has more windings to step the voltage further on the secondary side. A typical coil turns ratio Np: Ns in IID defibrillator applications of certain embodiments is 1: 15, where Np is the primary coil turns and Ns is the secondary coil turns. The high voltage across the secondary is excited by the diode and stored in capacitor Cou τ.

Fig. 4C depicts a single-ended primary inductive converter ("SEPIC") that provides certain other advantages over other power converters. For example, one advantage of a single-ended primary inductive converter is that a large amount of energy does not need to be stored in the transformer. This reduces the gap length requirement for the transformer since most of the energy in the transformer is stored between its gaps. The battery voltage (e.g., from a LiSVO battery) is applied to the VIN voltage input, the switching element is switched at a fixed frequency, and the duty cycle is varied based on feedback of the battery current into the power converter and the output voltage. The voltage from the output of the step transformer (T1) is excited by diode D1 to produce an output voltage at COUT. At C_OUTThe current capacity indicated here represents the high voltage output capacitance.

Fig. 4D depicts a variant of the SEPIC converter of fig. 4C. The SEPIC topology of fig. 4D has an additional inductive element (L1). The additional inductor L1 may be either discontinuous or magnetically coupled to the high voltage transformer in a single magnetic field configuration, as shown in fig. 4D.

Fig. 4E depicts a Cuk converter topology. A Cuk converter includes two inductors, L1 and L2, and two capacitors, C1 and C_outA switch SW and a diode D1. The capacitor C is used to transfer energy and is alternately connected to the input and output of the converter by means of a transistor and a diode commutation. Two inductors L1 and L2 are used to couple an input voltage source (Vi) and a capacitor C, respectively_outThe output voltage of (a) is converted to a current source. Similar to the voltage conversion circuit described above, the output voltage to input voltage ratio is related to the duty cycle of the switch SW. Alternatively, inductors L1 and L2 may be magnetically coupled, such as T1^*As noted. In this arrangement, inductors L1 and L2 may be wound on a single core.

Fig. 5A-5G also depict different core geometries known in the prior art. Fig. 5A-5D depict different E-shaped cores. Figure 5A depicts a classic E-shaped core. The cross-section of the central leg is substantially larger than the two peripheral legs, typically by a factor of two. In this geometry, the flux density through the core is substantially uniform, assuming that the coil or coils are all wound on the center leg.

FIG. 5B depicts an EFD core in which the center leg is narrower in one dimension but wider in the orthogonal direction. This type of geometry is advantageous for thin inductive elements. Fig. 5C depicts an ER core in which the center leg has a circular cross-section.

Fig. 5D depicts an EP core in which a substantially cylindrical center leg is partially surrounded by a magnetic flow loop core material. Between the central leg and the surrounding portion is a space in which a single or multiple coils are disposed. Figure 5E depicts a pot core similar to the EP core with the core legs at least partially surrounded by core material with a space between which the coil is disposed.

Fig. 5F is a schematic diagram depicting an inductor or transformer assembly employing a two-piece core structure. The coil 502 is wound on a bobbin 504, and the bobbin 504 is positioned such that the coil 502 is wound on the center leg of an E-core 506. An I-core 508 is secured to the open end of the E-core 506 with a clip 510. The E-shaped body 506 and I-shaped core 508 are positioned to form a "zig-zag" configuration in which a closed magnetic flux path is directed by the magnetic material through the center leg of the E-shaped core 506 and the center of the coil 502, and back through the peripheral legs of the E-shaped core 506.

Figure 5G depicts another inductive element assembly formed using a two-part core. In fig. 5G, a pair of opposing ER cores 512a and 512b are employed. The coil 502 is wound on a bobbin 504, which is longer than the center leg of either ER core. When assembled, the pair of ER cores come together to form a complete magnetic flux path. Where an air gap is desired, the center leg of one or both of the E-cores may be shorter than either of the peripheral legs. Similar structures may be assembled with pot cores, different types of E-cores, and other variations. As mentioned above, these conventional geometries are not well suited for use in IID (intravascular implantable devices).

FIG. 6 is a schematic diagram depicting an inductive element 600 according to one embodiment of the invention. The narrow shape of sensing element 600 is well suited for use in packaging intravascular implant devices, such as device 20. In an exemplary embodiment, the outer surface of the inductive element 600 substantially conforms to the inner surface of a portion of the housing 32. In this arrangement, a large amount of the interior space of the portion of the housing 32 that houses the inductive element 600 is used to direct the magnetic flux. This provides inductive element 600 with a relatively greater inductance than conventional geometry inductive elements, which do not occupy a significant proportion of the volume of the interior space in a comparable portion of housing 32 as does inductive element 600.

Inductive element 600 is assembled from a substantially cylindrical magnetic core 602 having an outer surface 603, a major length l in the direction of longitudinal reference axis x, and a substantially circular (e.g., circular, elliptical, etc.) cross-section in the transverse y-z plane. The core 602 itself is made up of two halves, a lower half 602a and an upper half 602 b. Located in core halves 602a and 602b are one or more wire coils 604. Although a single coil is shown for simplicity, it will be appreciated that multiple coils may be employed to provide multiple transformer coils coupled to each other.

Each half 602a and 602b has a mating surface 606 and a cut-out portion 607. The cutout portion 607 is defined by a bottom surface 608, opposing sidewalls 609, and a post 610. The post 610 projects from the bottom surface 608 along a reference axis z and has a major length lp along a longitudinal reference axis x, a minor width Wp along a reference axis y, and a projected height hp along the reference axis z. Post 610 also has a top surface 612 that is substantially coplanar with mating surface 606.

In a related embodiment, the top planar surface 612 is not coplanar with the mating surface 606, but is recessed relative to the mating surface 606. With this configuration, when the core halves are assembled, the top plane 612 of core half 602a does not make intimate contact with the top plane of the corresponding core half 602 b. The resulting structure may leave an air gap between the opposing top surfaces 612. The height of either or both legs 610 of core half 602a or 602b may be designed to provide an air gap of a particular size to achieve a desired magnetic field characteristic of inductive element 600. As described above, the length of the gap determines the amount of energy stored by inductive element 600, which also affects the inductance of inductive element 600.

In another embodiment, core halves 602a and 602b are not identical. For example, the bottom core half 602a may have a cylinder while the upper core half 602b has no cylinder. In this example embodiment, the post may have a post height in the z-axis direction that is greater than the height of opposing sidewall 609. In a related embodiment, the column height is about twice the height of the opposing sidewall 609 in the z-axis direction.

The coil 604 has a major length Ic along the reference axis x and a minor width wc along the reference axis y. Thus, coil 604 has elongated or elliptical windings defining a corresponding elongated or elliptical annular region lying longitudinally along the major axis of core 602. In one embodiment, coil 604 is sized such that, when inductive element 600 is assembled, the windings of coil 604 do not extend beyond the cylindrical outer periphery of core 602. As shown in fig. 6, coil 604 is wrapped around or in a surrounding manner on a protruding post 610 in the x-y reference plane. In an exemplary embodiment, the coil 604 is preformed with sufficient tolerance to allow the coil 604 to be slid onto the post 601. In another embodiment, coil 604 is actually wound on post 610.

In operation of this embodiment, a main magnetic flux component is generated by the current in coil 604 and passes through protruding post 610 in a first direction along the z-reference axis. The secondary magnetic flow component (summed to approximate the primary magnetic flow component) returns through the remainder of the core 602 to complete the magnetic circuit (i.e., substantially perpendicular through the mating surface 606 in the opposite direction of the z-reference axis). As can be seen from the geometry of inductive element 600, coil 604 produces a main flux component along an axis that is substantially perpendicular to the length l of the major axis of core 602 (i.e., in the y-z plane).

In general, the inductance of a wire coil is a function of the relative permeability, the number of windings in the coil, the loop area defined by the coil, and the height dimension of the coil structure. The inductance is directly proportional to the loop area and inversely proportional to the height dimension of the coil structure. Thus, qualitatively, a coil with a large loop area and a short coil structure height will produce a large inductance per unit length of wire comprising the coil. Thus, in the constrained elongate shaped unit of the endovascular implant device, the geometry of the coil 604 provides the required inductive characteristics. The elongated or oval windings of coil 604 provide a relatively large loop area and a relatively small coil structure height. For example, in one embodiment, the major elliptical loop dimension lc of the coil 604 is greater than 2.3 times the structural height of the coil 604. In a related embodiment, the square root of the loop area of the coil 604 is greater than 1.7 times the height of the coil 604.

In one embodiment, the loop area of coil 604 is larger than the cross-sectional area of inductive element 600 (the cross-section taken in a plane perpendicular to the major length direction l of inductive element 600, such as the y-z plane). In a related embodiment, the annular area of the coil 604 is greater than the cross-sectional area of the shaped unit encasing the inductive element 600.

By comparison, the geometry of the coil structure of a toroidal-type coil (in which the coil structure length is similar to or greater than the loop area), such as used in a pot core or in a core assembled with a bobbin, requires significantly longer wires to achieve the same inductance as a similarly sized coil 604. The increased number of wires corresponds to a greater electrical impedance of the inductive element, thereby reducing the efficiency of operation as a stored energy.

The geometry of the core 602, when sized to fit the form factor of the IID, also provides other advantages over conventional pot cores. For example, core 602 provides a shorter magnetic flux path and a larger magnetic flux cross-sectional area than a conventional pot core. In an exemplary embodiment, the total length of the closed magnetic flux path is less than the length/of the core 602. Such magnetic loop geometries of the core 602 advantageously have less reluctance because of higher inductance per unit core volume compared to the geometry of the pot core.

Comparing the core geometry of core 602 with a conventional type-E or type-C core, core 602 performs optimally in this IID form factor. Thus, core 602 has more magnetic material over a larger cross-sectional area of magnetic flux than an approximately sized E-shaped or C-shaped core. In an exemplary embodiment, the sum of the areas of surfaces 606 and 612 is greater than the area defined by the outer boundaries of a cross-section of inductive element 600 (the cross-section taken in a plane perpendicular to the major length l direction of inductive element 600, e.g., in the y-z plane). In a related embodiment, the sum of the areas of surfaces 606 and 612 is greater than the cross-sectional area of the form factor encasing inductive element 600. In another embodiment, the cross-sectional area of post 610, such as the area of surface 612 alone, for example, is greater than the area defined by the outer boundary of the cross-section of inductive element 600.

In another related embodiment, the sum of the annular area of the coil 604 and the partial cross-sectional area of the core 602 that is coplanar with the coil 604 and carries the return magnetic flux exceeds the area defined by the outer boundary of the cross-section of the inductive element 600. In a related embodiment, the total area of the cross-section of the magnetic flux path proceeding and returning through the core 602 is greater than the outer boundary of the cross-section of the inductive element 600.

Fig. 7A is a schematic diagram of an inductive element 700 according to another embodiment. Inductive element 700 includes a substantially cylindrical core 702 having a lower half 702a and an upper half 702b, and a coil 704. Coil 704 is substantially similar to coil 604 of fig. 6, both having a substantially elongated shape, with the major direction of the coil lying in a major direction corresponding to the cylindrical core.

A cross-sectional shape of a portion of each of the core halves 702a and 702b is substantially similar to the cyrillic letter "ethylene oxide" (Unicode characters 0x 042D). Each core half 702a and 702b is provided with a mating surface 706 extending along the major length l of inductive element 700, a cavity 707 defined by a substantially cylindrical inner surface 708, and a post 710 projecting from inner surface 708. When core halves 702a and 702b are joined together to form a core assembly, the joined core halves define a pair of opposed cavities having a D-shaped cross-section and a length extending along the major dimension of core 702.

The geometry of core 702 of inductive element 700 can also be described as a cylindrical shell having a length l, and inner and outer diameters defined by outer cylindrical surface 703 and inner cylindrical surface 708. The post 710 substantially bridges the diameter over a length. In one embodiment, post 710 may completely bridge the diameter without an air gap in the post. Otherwise, where an air gap is desired, the post 710 bridges almost all of the diameter, leaving an air gap.

Coil 704 (optionally, an additional coil) is assembled to fit within cavity 707 and to be located, i.e., in a surrounding manner, around post 710. In one embodiment, the coil 704 does not extend beyond the end of the core 702 (and beyond the l-dimension). The top surface 712 of the post 710, like the top surface 612, may be coplanar with respect to the mating surface 706, or recessed. Thus, the assembled inductive element 700 may or may not have an air gap. In one embodiment, the mating surface 706 has a surface area equal to the surface area of the top surface 712 such that the return path of magnetic flux through the core has the same reluctance as the forward path. In a related embodiment, the sum of the surface areas of surfaces 706 and 712 is greater than the area defined by the outer perimeter of the cross-section (cross-section perpendicular to length l) of inductive element 700. In another related embodiment, the surface area of the mating surface 706 alone is greater than the area defined by the outer perimeter of the cross-section of the inductive element 700.

The cross-sectional view of fig. 7B depicts the assembled inductive element 700. Core halves 702a and 702b are positioned together with mating surfaces 706 in close proximity. The top surface 712 of the post 710', as depicted, is recessed relative to the mating surface 706 creating an air gap 720. Coil 704 is positioned within cavity 707, in a surrounding manner, around post 710'.

The operation of inductive element 700 is similar to inductive element 600. The current in coil 704 generates a magnetic flux in coil 702. The main flux component passes through protruding cylinder 710. The flux return path is distributed throughout the rest of the core 702. The closed geometry of core 702 along the cylindrical wall provides additional magnetic shielding compared to core 602. In a related embodiment, the ends of the core 702 are also closed, thereby substantially eliminating any field edge effects outside the boundaries of the core. This embodiment is to some extent a combination of core 602 (having a closed end) and core 702 (having a closed cylindrical wall).

Fig. 8 and 9 depict the magnetic flux density through each core 602 and 702, respectively, based on computer-aided simulation results. A comparison of the model of fig. 9 to that of fig. 10 suggests that core 702 provides a more uniform magnetic flux density through its volume than core 602. This can be explained from the fact that core 702 provides a larger magnetic flux return path surface area and a shorter total magnetic flux path. In addition, points along the return path in core 702 are substantially equidistant from the forward magnetic flux path as those of core 602. Thus, core 702 provides a magnetic circuit geometry having less reluctance and a greater magnetic flux density than core 602.

For any of the voltage conversion circuits described herein, as well as other power circuits utilizing inductive elements in accordance with the present invention, the inductive elements can maximize conversion circuit performance and efficiency within substantial geometric constraints. More generally, in power converters that utilize mutually coupled coils, such as certain Cuk, SEPIC or flyback topologies, multiple coils may be incorporated into embodiments of the inductive element of the present invention. It will be appreciated by those skilled in the art that the inductive element of the present invention can be constructed using known techniques and materials, such as, for example, from powdered ferrite stock. Different ranges of magnetic permeability of the core material may be employed for different applications.

Embodiments of the present invention can achieve certain performance characteristics in the defined geometry of IID devices that are not available in power conversion circuits using conventional inductive elements. For example, an implantable vascular device defibrillator according to one embodiment, having a diameter less than 15mm, and in one embodiment less than about 8mm, utilizes a Cuk or SEPIC power conversion circuit having an inductive element of the type described above. The power conversion circuit can convert the battery voltage to an electrotherapy voltage at least ten times greater than the battery voltage and output energy at an operating efficiency of at least 60% at a rate voltage of at least 1W with sufficient battery power.

Fig. 10 is a schematic diagram depicting a power converter circuit and a portion of a control system for the power converter according to an example embodiment of the invention. The power conversion topology in this example is a SEPIC power conversion circuit, in which inductor L1 is cross-coupled to transformer T1. Inductor L1 and transformer T1 are multi-coil winding inductive elements having the geometry described in the above embodiments. In this example, all coil windings are wound on the same core.

The switching mode of operation of transistor Q1 is to periodically energize inductor L1 and the primary winding of transformer T1. The current through the primary winding of transformer T1 is sensed and fed to the control circuit as shown. Also, the output voltage HV is detected and fed to the control circuit. The output voltage is controlled by varying the duty cycle of the drive signal to the switching transistor Q1.

By sensing both, i.e., the output voltage and current through the primary winding of transformer T1, the power converter in this example can be dynamically controlled to provide the required output at the best possible efficiency in each case. These conditions may change due to the occurrence of internal and external events. For example, as the battery energy is depleted to its useful life, the battery voltage will tend to drop. In one embodiment, the control circuit regulates the operation of the power converter to accommodate such conditions.

In the embodiment shown in fig. 10, the functional blocks depicted on the left hand side of the chip boundary are implemented as Application Specific Integrated Circuits (ASICs). The circuit portion depicted on the right hand side of the chip boundary is implemented using discrete electronic components. In a related embodiment, the resistor bank, e.g., six resistors for adjusting the current sense signal, is implemented using a resistor network, such as a thin film resistor network on the same substrate. Such an arrangement may advantageously provide well-matched resistances with similar temperature coefficients and similar levels of heating during operation.

The present invention may be embodied in other specific forms without departing from its essential attributes. For example, features of the present invention are not limited to use with implantable defibrillation devices. Other types of devices having very small forming requirements and utilizing inductive elements may also benefit from these features of the invention. Such as implantable drug delivery devices, electrical stimulation devices, patient monitoring and data communication devices, and the like, may utilize one or more sensing elements of the present invention.

In addition, the present invention is not necessarily limited to the power conversion circuit. The inductive element according to the invention may be used in other types of circuits for other types of functions, e.g. for filtering, signal impedance matching, etc. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, reference being made to the appended claims rather than to the foregoing description to indicate the scope of the invention. For purposes of interpreting the claims herein, the specification in section six of U.S. patent law U.S. C.35, section 112, indicates that the contents of this specification need not be incorporated by reference unless the claims recite "means for" or "step for".

Claims (43)

1. An intravascular implantable medical device comprising:

a structure defining a form factor, the form factor having an elongated geometry including a length and a cross-section perpendicular to a direction of the length, the structure being adapted for implantation within a blood vessel of a patient; and is

Within the form factor is disposed an electrical circuit comprising an energy source reservoir, wherein the electrical circuit is operative to convert the stored energy output to a relatively higher voltage, the electrical circuit comprising an inductive element having an outer surface shape corresponding to the form factor, and at least one coil of the inductive element being positioned to direct a magnetic field substantially perpendicular to the length direction.

2. The implantable intravascular medical device of claim 1, wherein the circuit is a power conversion circuit.

3. The implantable intravascular medical device of claim 1, wherein the implantable intravascular medical device comprises a defibrillator, and wherein the relatively higher voltage is a defibrillation therapy voltage.

4. The implantable intravascular medical device of claim 1, wherein the inductive element is a transformer having a plurality of mutually coupled coils.

5. The implantable intravascular medical device of claim 1, wherein the cross-section is substantially circular and the form factor is substantially cylindrical.

6. The implantable intravascular medical device of claim 1, wherein the structure defining the form factor comprises a substantially sealed insulating sleeve.

7. The implantable intravascular medical device of claim 1, wherein the structure defining the form factor includes at least one shell portion having a substantially cylindrical outer surface and an inner surface defining at least a portion of the form factor.

8. The implantable intravascular medical device of claim 1, wherein the structure defining the form factor includes a plurality of housings operably connected to allow bending along a length of the structure.

9. The implantable intravascular medical device of claim 1, wherein the form factor is defined relative to an outer surface of a structure defining the form factor.

10. The implantable intravascular medical device of claim 1, wherein the inductive element comprises at least one coil of wire, wherein the coil is positioned entirely within a boundary defined by an outer surface of the inductive element.

11. The implantable intravascular medical device of claim 1, wherein the inductive element comprises at least one coil of wire, wherein the coil is configured to direct the magnetic field in a direction substantially parallel to a reference axis of the cross-section.

12. The implantable intravascular medical device of claim 1, wherein the inductive element includes a core of magnetic material having a substantially cylindrical outer boundary.

13. The implantable intravascular medical device of claim 12, wherein the core comprises a cylinder having an elliptical cross-sectional boundary with an elliptical major axis oriented along a length of the substantially cylindrical outer boundary, and wherein a cylinder height of the cylinder is oriented in a radial direction relative to the substantially cylindrical outer boundary, and wherein a coil of wire is wrapped around the cylinder.

14. The implantable intravascular medical device of claim 13, wherein the post is formed as a projection from an inner surface of the core.

15. The implantable intravascular medical device of claim 13, wherein the core comprises a pair of core halves, each half having a mating surface, the core halves mating with each other at the mating surfaces when the core is assembled from the pair of core halves.

16. The implantable intravascular medical device of claim 15, wherein the post includes a pair of post portions, each post portion corresponding to one of the core halves.

17. The implantable intravascular medical device of claim 13, wherein the post includes an air gap.

18. The implantable intravascular medical device of claim 1, wherein the implantable intravascular device comprises a defibrillator, the electrical circuit is a power conversion circuit, and the relatively higher voltage is a defibrillation therapy voltage;

said form factor being substantially cylindrical, said structure defining the form factor comprising at least one shell having a substantially cylindrical outer surface and an inner surface defining at least a portion of the form factor, wherein the at least one shell comprises a substantially closed insulating sleeve;

the inductive element is a transformer having a plurality of mutually coupled coils, all arranged within a boundary defined by an outer surface of the inductive element, and arranged to direct a magnetic field in a direction substantially parallel to a reference axis of the cross-section;

said inductive element comprising a core of magnetic material having a substantially cylindrical outer boundary and a cylinder having an elliptical cross-sectional boundary with the direction of the major elliptical axis lying along the length of said substantially cylindrical outer boundary, the height of the cylinder lying in a radial direction relative to said substantially cylindrical outer boundary, and a coil wound around said cylinder;

the core comprises a pair of core halves each having mating surfaces, and the cylinder comprises a pair of cylinder portions such that when the core is assembled from the pair of core halves, the core halves mate with each other at the mating surfaces, and each cylinder portion corresponds to one of the core halves and is arranged such that the cylinder contains an air gap.

19. An intravascular implantable medical device comprising

Defining a form factor having an elongated geometry including a form factor length and a cross-section perpendicular to the length, an

A circuit located within the form factor, the circuit including an inductive element;

wherein the inductive element comprises:

a magnetic material core having a core length and a core cross-section perpendicular to the length; and

a coil having a plurality of winding groups defining an annular area;

wherein a portion of the core is located within the annular area such that when said core is energized, magnetic flux is generated within the core along the forward path and the return path; and is

Wherein a sum of a total cross-sectional area of the magnetic flux of the forward passage and a total cross-sectional area of the magnetic flux of the return passage is larger than a cross-sectional area of the core.

20. The implantable intravascular medical device of claim 19, wherein a sum of a total cross-sectional area of the magnetic flux in the advancement channel and a cross-sectional area of the magnetic flux in the return channel is greater than a cross-sectional area of the form factor.

21. The implantable intravascular medical device of claim 19, wherein the annular area is greater than at least one area selected from the group consisting of the core cross-sectional area, and the cross-sectional area of the form factor.

22. The implantable intravascular medical device of claim 19, wherein

Said implantable intravascular device comprises a defibrillator and said electrical circuit is a power conversion circuit;

said form element being substantially cylindrical and having a substantially circular cross-section, and said structure defining the form element comprising a shell having a substantially cylindrical outer surface and an inner surface defining at least a portion of said form element, wherein said shell comprises a substantially closed insulating sleeve;

the inductive element is a transformer having a plurality of mutually coupled coils disposed entirely within a boundary defined by an outer surface of the inductive element;

said core having a substantially cylindrical outer boundary and a cylinder, said cylinder having an elliptical cross-sectional boundary with the major axis of the ellipse lying along the length of said substantially cylindrical outer boundary, the cylinder height of the cylinder lying in a radial direction relative to said substantially cylindrical outer boundary, and the coil surrounding said cylinder;

the core comprises a pair of core halves, wherein the core halves are fitted to each other at fitting surfaces when the core body is assembled from the pair of core halves, and the column body comprises a pair of column body portions each corresponding to one of the core halves and arranged such that the column body contains an air gap.

23. An inductive element, comprising:

a core of magnetic material having a core length and a substantially cylindrical outer boundary; and

at least one coil having a plurality of winding groups defining an annular area;

wherein a portion of the core is located within the annular area such that the at least one coil, when energized, generates a closed magnetic flux along a magnetic flux path through the core, wherein the magnetic flux path has a length less than the length of the core.

24. The inductive element of claim 23, wherein:

the induction element is a transformer which is provided with a plurality of mutual coupling coils and is completely arranged inside the outer boundary;

the core comprises a cylinder having a boundary with an elliptical cross-section with a major elliptical axis oriented along a length of the substantially cylindrical outer boundary, a cylinder height oriented in a radial direction relative to the substantially cylindrical outer boundary, and a coil surrounding the cylinder;

the core includes a pair of core halves, wherein the core halves are fitted to each other at fitting surfaces when the core is assembled from the pair of core halves, and the column includes a pair of column portions each corresponding to one of the core halves and arranged such that the column contains an air gap.

25. An implantable medical device comprising:

means for providing a substantially sealed insulating sleeve defining a form factor having an elongated geometry and a substantially circular cross-section;

means for storing energy;

means for converting an output of the means for storing energy to a high voltage signal;

wherein the means for converting the output of the means for storing energy comprises an inductive element comprising means for directing a magnetic flux, the means having a shape corresponding to the form factor.

26. An inductive element for an implantable intravascular device, comprising:

a core of magnetic material having a major longitudinal axis direction extending along a first reference axis and a core cross-section having a substantially circular outer boundary, the core cross-section being defined in a first reference plane orthogonal to the first reference axis, wherein the core comprises a cylinder;

at least one coil disposed about the cylinder, wherein the at least one coil directs a magnetic field perpendicular to the first reference axis when energized.

27. The inductive element of claim 26, wherein the at least one coil does not protrude beyond the outer boundary.

28. The inductive element of claim 26, wherein the outer boundary is substantially elliptical such that the core is substantially cylindrical.

29. The inductive element of claim 26, wherein the core provides a substantially continuous magnetic field shield around the at least one coil.

30. The inductive element of claim 26, wherein the cores are separated by a length such that the first core portion and the second core portion mate with each other at their respective first and second mating surfaces that are parallel to the first reference axis.

31. The inductive element of claim 30, wherein each core half includes a portion of substantially "expanded" cross-section, wherein when the core halves are mated to one another to form a core assembly, the resulting core cross-section defines a pair of opposing "D" shaped voids.

32. The inductive element of claim 31, wherein each core half includes at least another portion having a cross-sectional shape selected from the group consisting of a substantially D-shaped cross-section, a substantially C-shaped cross-section, or any combination thereof.

33. The inductive element of claim 26, wherein the pillar has a height direction and an elliptical cross-section, wherein the height direction extends along a second reference axis parallel to the first reference plane, the elliptical cross-section being defined in a second reference plane orthogonal to the second reference axis, wherein a major elliptical axis of the elliptical cross-section extends along the first reference axis.

34. The inductive element of claim 33, wherein the first and second mating surfaces are perpendicular to the second reference axis.

35. The inductive element of claim 33, wherein the cylinder has an elliptical minor axis direction parallel to the first reference plane, wherein the elliptical minor axis is smaller than a corresponding outer boundary portion coplanar with the elliptical minor axis direction.

36. The inductive element of claim 26, wherein the at least one coil comprises a plurality of mutually coupled coils.

37. The inductive element of claim 26, wherein:

the outer boundary being substantially elliptical such that the core is substantially cylindrical;

the core provides a substantially continuous magnetic field shield around the at least one coil;

said cores being longitudinally spaced apart with the first and second core portions cooperating with each other at their respective first and second cooperating surfaces parallel to said first reference axis and perpendicular to said second reference axis;

each core half includes a portion having an "expanded" shaped cross-section, wherein when the core halves are abutted to one another to form a core assembly, the resulting core cross-section defines a pair of opposed "D" shaped voids;

the cylinder having a height direction defined along a second reference axis parallel to the first reference plane and an elliptical cross-section defined within a second reference plane orthogonal to the second reference axis direction, the elliptical cross-section having an elliptical major axis direction along the first reference axis direction;

the cylinder having an elliptical minor axis direction parallel to the first reference plane and wherein the elliptical minor axis direction is smaller than a corresponding outer boundary portion coplanar with the elliptical minor axis direction; and

at least one coil comprising a plurality of mutually coupled coils and not protruding beyond said outer boundary.

38. A method of making an Implantable Intravascular Device (IID), the method comprising:

forming a substantially sealed insulating sleeve encasing the circuit, wherein the insulating sleeve has a substantially cylindrical outer surface and defines an inner form factor;

assembling an inductive element that is part of the circuit within the insulating sleeve such that at least a majority of an outer surface profile of the inductive element corresponds to the inner form factor; comprises that

A set of elongated windings are positioned lengthwise within the insulating sheath to direct a magnetic field along the opposite direction

Extending in a radial direction of the insulating sleeve; and is

Providing the magnetic field with a closed magnetic channel substantially through the permeable material.

39. The method of claim 38, wherein the step of assembling the inductive element further comprises:

the set of elongated windings is wound around the cylinder.

40. The method of claim 38, wherein the step of assembling the inductive element further comprises:

a plurality of insulated windings are provided to be coupled to each other, thereby forming a transformer apparatus.

41. The method of claim 38, wherein the step of assembling the inductive element further comprises:

a core of magnetic material is provided having a substantially cylindrical shell with a cylinder substantially bridging the diameter of the substantially cylindrical shell over at least a portion of the length of the cylindrical shell.

42. An intravascular implantable medical device comprising:

a structure defining a substantially cylindrical form factor having a length and a diameter of less than 15 mm; and

a power conversion circuit located within the form factor and including an energy storage device, wherein the circuit operates such that:

converting the voltage of the energy storage device to a higher voltage that is at least ten times greater than the voltage of the energy storage device;

and outputting energy at a set power of at least 1W at an operating efficiency of at least 60%.

43. The implantable intravascular medical device of claim 42, wherein the power conversion circuit comprises an inductive element having at least one inductive coil assembled with a magnetic core, wherein the inductive coil has a geometry including a loop area and a coil height along an axis perpendicular to the loop area such that a square root of the loop area is greater than the height of the coil.