Nanomagnetic shielding assembly

[0001] This application is a continuation-in-part of applicant's patent application U.S. Ser. No. 10/260,247, filed on Sep. 30, 2002 now U.S. Pat. No. 6,673,999, which in turn was a continuation-in-part of application U.S. Ser. No. 10/054,407, filed on Jan. 22, 2002 now U.S. Pat. No. 6,506,972.

[0002] A magnetically shielded assembly comprised of a substrate, nanomagnetic material and means for controlling the temperature of the substrate.

[0003] Many implanted medical devices that are powered by electrical energy have been developed. Most of these devices comprise a power source, one or more conductors, and a load.

[0004] When a patient with one of these implanted devices is subjected to high intensity magnetic fields, currents are often induced in the implanted conductors. The large current flows so induced often create substantial amounts of heat. Because living organisms can generally only survive within a relatively narrow range of temperatures, these large current flows are dangerous.

[0005] Furthermore, implantable devices, such as implantable pulse generators (IPGs) and cardioverter/defibrillator/pacemaker (CDPs), are sensitive to a variety of forms of electromagnetic interference (EMI). These devices include sensing and logic systems that respond to low-level signals from the heart. Because the sensing systems and conductive elements of these implantable devices are responsive to changes in local electromagnetic fields, they are vulnerable to external sources of severe electromagnetic noise, and in particular to electromagnetic fields emitted during magnetic resonance imaging (MRI) procedures. Therefore, patients with implantable devices are generally advised not to undergo magnetic resonance imaging (MRI) procedures, which often generate static magnetic fields of from between about 0.5 to about 10 Teslas and corresponding time-varying magnetic fields of about 20 megahertz to about 430 megahertz, as dictated by the Lamor frequency (see, e.g., page 1007 of Joseph D. Bronzino's “The Biomedical Engineering Handbook,” CRC Press, Hartford, Conn., 1995). Typically, the strength of the magnetic component of such a time-varying magnetic field is about 1 to about 1,000 microTesla.

[0006] One additional problem with implanted conductors is that, when they are conducting electricity and are simultaneously subjected to large magnetic fields, a Lorentz force is created which often causes the conductor to move. This movement may damage body tissue.

[0007] In U.S. Pat. No. 4,180,600, there is disclosed and claimed a fine magnetically shielded conductor wire consisting of a conductive copper core and a magnetically soft alloy metallic sheath metallurgically secured to the conductive core, wherein the sheath consists essentially of from 2 to 5 weight percent of molybdenum, from about 15 to about 23 weight percent of iron, and from about 75 to about 85 weight percent of nickel. Although the device of this patent does provide magnetic shielding, it still creates heat when it interacts with strong magnetic fields.

[0008] It is an object of this invention to provide an assembly, which is shielded from magnetic fields and, which, additionally, is comprised of means for regulating its temperature.

[0009] In accordance with this invention, there is provided a magnetically shielded assembly comprised of a substrate, nanomagnetic material, and means for controlling the temperature of such substrate. The nanomagnetic material comprises nanomagnetic particles, and these nanomagnetic particles respond to an externally applied magnetic field by realigning to the externally applied field.

[0024] FIG. 1 is a schematic sectional view of one preferred device 10 that, in one embodiment, is implanted in a living organism. Referring to FIG. 1, it will be seen that device 10 is comprised of a power source 12, a first conductor 14, a second conductor 16, a first insulative shield 18 disposed about power source 12, a second insulative shield 20 disposed about a load 22, a third insulative shield 23 disposed about a first conductor 14, and a second conductor 16, and a multiplicity of nanomagentic particles 24 disposed on said first insulative shield, said second insulative shield, and said third insulative shield.

[0025] In the embodiment depicted in FIG. 1, the power source 12 is a battery 12 that is operatively connected to a controller 26. In the embodiment depicted, controller 26 is operatively connected to the load 22 and the switch 28. Depending upon the information furnished to controller 26, it may deliver no current, direct current, and/or current pulses to the load 22.

[0026] In one embodiment, not shown, the controller 26 and/or the wires 30 and 32 are shielded from magnetic radiation. In another embodiment, not shown, one or more connections between the controller 26 and the switch 28 and/or the load 22 are made by wireless means such as, e.g., telemetry means.

[0027] In one embodiment, not shown, the power source 12 provides a source of alternating current. In another embodiment, the power source 12 in conjunction with the controller 26 provides pulsed direct current.

[0028] The load 22 may be any of the implanted devices known to those skilled in the art. Thus, e.g., load 22 may be a pacemaker. Thus, e.g., load 22 may be an artificial heart. Thus, e.g., load 22 may be a heart-massaging device. Thus, e.g., load 22 may be a defibrillator.

[0029] The conductors 14 and 16 may be any conductive material(s) that have a resistivity at 20 degrees Centigrade of from about 1 to about 100 microohm-centimeters. Thus, e.g., the conductive material(s) may be silver, copper, aluminum, alloys thereof, mixtures thereof, and the like.

[0030] In one embodiment, the conductors 14 and 16 consist essentially of such conductive material. Thus, e.g., it is preferred not to use, e.g., copper wire coated with enamel. The use of such typical enamel coating on the conductor does not work well in the instant invention.

[0031] In the first step of the process of this invention, step 40, the conductive wires 14 and 16 are coated with electrically insulative material. Suitable insulative materials include nano-sized silicon dioxide, aluminum oxide, cerium oxide, yttrium-stabilized zirconia, silicon carbide, silicon nitride, aluminum nitride, and the like. In general, these nano-sized particles will have a particle size distribution such that at least about 90 weight percent of the particles have a maximum dimension in the range of from about 10 to about 100 nanometers.

[0032] The coated conductors 14 and 16 may be prepared by conventional means such as, e.g., the process described in U.S. Pat. No. 5,540,959, the entire disclosure of which is hereby incorporated by reference into this specification. This patent describes and claims a process for preparing a coated substrate, comprising the steps of: (a) creating mist particles from a liquid, wherein: 1. said liquid is selected from the group consisting of a solution, a slurry, and mixtures thereof, 2. said liquid is comprised of solvent and from 0.1 to 75 grams of solid material per liter of solvent, 3. at least 95 volume percent of said mist particles have a maximum dimension less than 100 microns, and 4. said mist particles are created from said first liquid at a rate of from 0.1 to 30 milliliters of liquid per minute; (b) contacting said mist particles with a carrier gas at a pressure of from 761 to 810 millimeters of mercury; (c) thereafter contacting said mist particles with alternating current radio frequency energy with a frequency of at least 1 megahertz and a power of at least 3 kilowatts while heating said mist particles to a temperature of at least about 100 degrees centigrade, thereby producing a heated vapor; (d) depositing said heated vapor onto a substrate, thereby producing a coated substrate; and (e) subjecting said coated substrate to a temperature of from about 450 to about 1,400 degrees centigrade for at least about 10 minutes.

[0033] By way of further illustration, one may coat conductors 14 and 16 by means of the processes disclosed in a text by D. Satas on “Coatings Technology Handbook” (Marcel Dekker, Inc., New York, N.Y., 1991). As is disclosed in such text, one may use cathodic arc plasma deposition (see pages 229 et seq.), chemical vapor deposition (see pages 257 et seq.), sol-gel coatings (see pages 655 et seq.), and the like.

[0034] FIG. 2 is a sectional view of the coated conductors 14/16 of the device of FIG. 1. Referring to FIG. 2, it will be seen that conductors 14 and 16 are separated by insulating material 42. In order to obtain the structure depicted in FIG. 2, one may simultaneously coat conductors 14 and 16 with the insulating material so that such insulators both coat the conductors 14 and 16 and fill in the distance between them with insulation.

[0035] The insulating material 42 that is disposed between conductors 14/16, may be the same as the insulating material 44/46 that is disposed above conductor 14 and below conductor 16. Alternatively, and as dictated by the choice of processing steps and materials, the insulating material 42 may be different from the insulating material 44 and/or the insulating material 46. Thus, step 48 of the process describes disposing insulating material between the coated conductors 14 and 16. This step may be done simultaneously with step 40; and it may be done thereafter.

[0036] The insulating material 42, the insulating material 44, and the insulating material 46 each generally has a resistivity of from about 1,000,000,000 to about 10,000,000,000,000 ohm-centimeters.

[0037] After the insulating material 42/44/46 has been deposited, and in one embodiment, the coated conductor assembly is preferably heat treated in step 50. This heat treatment often is used in conjunction with coating processes in which the heat is required to bond the insulative material to the conductors 14/16.

[0038] The heat-treatment step may be conducted after the deposition of the insulating material 42/44/46, or it may be conducted simultaneously therewith. In either event, and when it is used, it is preferred to heat the coated conductors 14/16 to a temperature of from about 200 to about 600 degrees Centigrade for from about 1 minute to about 10 minutes.

[0039] Referring again to FIG. 1A, and in step 52 of the process, after the coated conductors 14/16 have been subjected to heat treatment step 50, they are allowed to cool to a temperature of from about 30 to about 100 degrees Centigrade over a period of time of from about 3 to about 15 minutes.

[0040] One need not invariably heat treat and/or cool. Thus, referring to FIG. 1A, one may immediately coat nanomagnetic particles onto to the coated conductors 14/16 in step 54 either after step 48 and/or after step 50 and/or after step 52.

[0041] In step 54, nanomagnetic materials are coated onto the previously coated conductors 14 and 16. This is best shown in FIG. 2, wherein the nanomagnetic particles are identified as particles 24.

[0042] In general, and as is known to those skilled in the art, nanomagnetic material is magnetic material which has an average particle size less than 100 nanometers and, preferably, in the range of from about 2 to 50 nanometers. Reference may be had, e.g., to U.S. Pat. No. 5,889,091 (rotationally free nanomagnetic material), U.S. Pat. Nos. 5,714,136, 5,667,924, and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification.

[0043] The nanomagnetic materials may be, e.g., nano-sized ferrites such as, e.g., the nanomagnetic ferrites disclosed in U.S. Pat. No. 5,213,851, the entire disclosure of which is hereby incorporated by reference into this specification. This patent claims a process for coating a layer of ferritic material with a thickness of from about 0.1 to about 500 microns onto a substrate at a deposition rate of from about 0.01 to about 10 microns per minute per 35 square centimeters of substrate surface, comprising the steps of: (a) providing a solution comprised of a first compound and a second compound, wherein said first compound is an iron compound and said second compound is selected from the group consisting of compounds of nickel, zinc, magnesium, strontium, barium, manganese, lithium, lanthanum, yttrium, scandium, samarium, europium, terbium, dysprosium, holmium, erbium, ytterbium, lutetium, cerium, praseodymium, thulium, neodymium, gadolinium, aluminum, iridium, lead, chromium, gallium, indium, chromium, samarium, cobalt, titanium, and mixtures thereof, and wherein said solution is comprised of from about 0.01 to about 1,000 grams of a mixture consisting essentially of said compounds per liter of said solution; (b) subjecting said solution to ultrasonic sound waves at a frequency in excess of 20,000 hertz, and to an atmospheric pressure of at least about 600 millimeters of mercury, thereby causing said solution to form into an aerosol; (c) providing a radio frequency plasma reactor comprised of atop section, a bottom section, and a radio-frequency coil; (d) generating a hot plasma gas within said radio frequency plasma reactor, thereby producing a plasma region; (e) providing a flame region disposed above said top section of said radio frequency plasma reactor; (f) contacting said aerosol with said hot plasma gas within said plasma reactor while subjecting said aerosol to an atmospheric pressure of at least about 600 millimeters of mercury and to a radio frequency alternating current at a frequency of from about 100 kilohertz to about 30 megahertz, thereby forming a vapor; (g) providing a substrate disposed above said flame region; and (h) contacting said vapor with said substrate, thereby forming said layer of ferritic material.

[0044] By way of further illustration, one may use the techniques described in an article by M. De Marco, X. W. Wang, et al. on “Mossbauer and magnetization studies of nickel ferrites” published in the Journal of Applied Physics 73(10), May 15, 1993, at pages 6287-6289.

[0045] In general, the thickness of the layer of nanomagnetic material deposited onto the coated conductors 14/16 is less than about 5 microns and generally from about 0.1 to about 3 microns.

[0046] After the nanomagnetic material is coated in step 54, the coated assembly may be optionally heat-treated in step 56. In this optional step 56, it is preferred to subject the coated conductors 14/16 to a temperature of from about 200 to about 600 degrees Centigrade for from about 1 to about 10 minutes.

[0047] In one embodiment, illustrated in FIG. 3, one or more additional insulating layers 43 are coated onto the assembly depicted in FIG. 2, by one or more of the processes disclosed hereinabove. This is conducted in optional step 58 (see FIG. 1A).

[0048] FIG. 4 is a partial schematic view of the assembly 11 of FIG. 2, illustrating the current flow in such assembly. Referring go FIG. 4, it will be seen that current flows into conductor 14 in the direction of arrow 60, and it flows out of conductor 16 in the direction of arrow 62. The net current flow through the assembly 11 is zero; and the net Lorentz force in the assembly 11 is thus zero. Consequently, even high current flows in the assembly 11 do not cause such assembly to move.

[0049] In the embodiment depicted in FIG. 4, conductors 14 and 16 are substantially parallel to each other. As will be apparent, without such parallel orientation, there may be some net current and some net Lorentz effect.

[0050] In the embodiment depicted in FIG. 4, and in one preferred aspect thereof, the conductors 14 and 16 preferably have the same diameters and/or the same compositions and/or the same length.

[0051] Referring again to FIG. 4, the nanomagnetic particles 24 are present in a density sufficient so as to provide shielding from magnetic flux lines 64. Without wishing to be bound to any particular theory, applicant believes that the nanomagnetic particles 24 trap and pin the magnetic lines of flux 64.

[0052] In order to function optimally, the nanomagnetic particles 24 have a specified magnetization. As is known to those skilled in the art, magnetization is the magnetic moment per unit volume of a substance. Reference may be had, e.g., to U.S. Pat. Nos. 4,169,998, 4,168,481, 4,166,263, 5,260,132, 4,778,714, and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification.

[0053] Referring again to FIG. 4, the layer of nanomagnetic particles 24 preferably has a saturation magnetization, at 25 degrees Centigrade, of from about 1 to about 36,000 Gauss, or higher. In one embodiment, the saturation magnetization at room temperature of the nanomagentic particles is from about 500 to about 10,000 Gauss. For a discussion of the saturation magnetization of various materials, reference may be had, e.g., to U.S. Pat. Nos. 4,705,613, 4,631,613, 5,543,070, 3,901,741 (cobalt, samarium, and gadolinium alloys), and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification. As will be apparent to those skilled in the art, especially upon studying the aforementioned patents, the saturation magnetization of thin films is often higher than the saturation magnetization of bulk objects.

[0054] In one embodiment, it is preferred to utilize a thin film with a thickness of less than about 2 microns and a saturation magnetization in excess of 20,000 Gauss. The thickness of the layer of nanomagentic material is measured from the bottom surface of the layer that contains such material to the top surface of such layer that contains such material; and such bottom surface and/or such top surface may be contiguous with other layers of material (such as insulating material) that do not contain nanomagnetic particles.

[0055] Thus, e.g., one may make a thin film in accordance with the procedure described at page 156 of Nature, Volume 407, Sep. 14, 2000, that describes a multilayer thin film has a saturation magnetization of 24,000 Gauss.

[0056] By the appropriate selection of nanomagnetic particles, and the thickness of the films deposited, one may obtain saturation magnetizations of as high as at least about 36,000.

[0057] In the preferred embodiment depicted in FIG. 4, the nanomagnetic particles 24 are disposed within an insulating matrix so that any heat produced by such particles will be slowly dispersed within such matrix. Such matrix, as indicated hereinabove, may be made from ceria, calcium oxide, silica, alumina. In general, the insulating material 42 preferably has a thermal conductivity of less than about 20 (calories centimeters/square centimeters−degree second)×10,000. See, e.g., page E-6 of the 63^rd Edition of the “Handbook of Chemistry and Physics” (CRC Press, Inc., Boca Raton, Fla., 1982).

[0058] The nanomagnetic materials 24 typically comprise one or more of iron, cobalt, nickel, gadolinium, and samarium atoms. Thus, e.g., typical nanomagnetic materials include alloys of iron and nickel (permalloy), cobalt, niobium, and zirconium (CNZ), iron, boron, and nitrogen, cobalt, iron, boron, and silica, iron, cobalt, boron, and fluoride, and the like. These and other materials are descried in a book by J. Douglas Adam et al. entitled “Handbook of Thin Film Devices” (Academic Press, San Diego, Calif., 2000). Chapter 5 of this book beginning at page 185, describes “magnetic films for planar inductive components and devices;” and Tables 5.1 and 5.2 in this chapter describe many magnetic materials.

[0059] FIG. 5 is a sectional view of the assembly 11 of FIG. 2. The device of FIG. 5, and of the other Figures of this application, is preferably substantially flexible. As used in this specification, the term flexible refers to an assembly that can be bent to form a circle with a radius of less than 2 centimeters without breaking. Put another way, the bend radius of the coated assembly 11 can be less than 2 centimeters. Reference may be had, e.g., to U.S. Pat. Nos. 4,705,353, 5,946,439, 5,315,365, 4,641,917, 5,913,005, and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification.

[0060] In another embodiment, not shown, the shield is not flexible. Thus, in one aspect of this embodiment, the shield is a rigid, removable sheath that can be placed over an endoscope or a biopsy probe used inter-operatively with magnetic resonance imaging.

[0061] As will be apparent, even when the magnetic insulating properties of the assembly of this invention are not 100 percent effective, the assembly still prevents the rapid dissipation of heat to bodily tissue.

[0062] In another embodiment of the invention, there is provided a magnetically shielded conductor assembly comprised of a conductor and a film of nanomagnetic material disposed above said conductor. In this embodiment, the conductor has a resistivity at 20 degrees Centigrade of from about 1 to about 2,000 micro ohm-centimeters and is comprised of a first surface exposed to electromagnetic radiation. In this embodiment, the film of nanomagnetic material has a thickness of from about 100 nanometers to about 10 micrometers and a mass density of at least about about 1 gram per cubic centimeter, wherein the film of nanomagnetic material is disposed above at least about 50 percent of said first surface exposed to electromagnetic radiation, and the film of nanomagnetic material has a saturation magnetization of from about 1 to about 36,000 Gauss, a coercive force of from about 0.01 to about 5,000 Oersteds, a relative magnetic permeability of from about 1 to about 500,000, and a magnetic shielding factor of at least about 0.5. In this embodiment, the nanomagnetic material has an average particle size of less than about 100 nanometers.

[0063] In the preferred embodiment of this invention, a film of nanomagnetic is disposed above at least one surface of a conductor. Referring to FIG. 6, and in the schematic diagram depicted therein, a source of electromagnetic radiation 100 emits radiation 102 in the direction of film 104. Film 104 is disposed above conductor 106, i.e., it is disposed between conductor 106 of the electromagnetic radiation 102.

[0064] The film 104 is adapted to reduce the magnetic field strength at point 108 (which is disposed less than 1 centimeter above film 104) by at least about 50 percent. Thus, if one were to measure the magnetic field strength at point 108, and thereafter measure the magnetic field strength at point 110 (which is disposed less than 1 centimeter below film 104), the latter magnetic field strength would be no more than about 50 percent of the former magnetic field strength. Put another way, the film 104 has a magnetic shielding factor of at least about 0.5.

[0065] In one embodiment, the film 104 has a magnetic shielding factor of at least about 0.9, i.e., the magnetic field strength at point 110 is no greater than about 10 percent of the magnetic field strength at point 108. Thus, e.g., the static magnetic field strength at point 108 can be, e.g., one Tesla, whereas the static magnetic field strength at point 110 can be, e.g., 0.1 Tesla. Furthermore, the time-varying magnetic field strength of a 100 milliTesla would be reduced to about 10 milliTesla of the time-varying field.

[0066] Referring again to FIG. 6, the nanomagnetic material 103 in film 104 has a saturation magnetization of form about 1 to about 36,000 Gauss. This property has been discussed elsewhere in this specification. In one embodiment, the nanomagnetic material 103 a saturation magnetization of from about 200 to about 26,000 Gauss.

[0067] The nanomagnetic material 103 in film 104 also has a coercive force of from about 0.01 to about 5,000 Oersteds. The term coercive force refers to the magnetic field, H, which must be applied to a magnetic material in a symmetrical, cyclicly magnetized fashion, to make the magnetic induction, B, vanish; this term often is referred to as magnetic coercive force. Reference may be had, e.g., to U.S. Pat. Nos. 4,061,824, 6,257,512, 5,967,223, 4,939,610, 4,741,953, and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification.

[0068] In one embodiment, the nanomagnetic material 103 has a coercive force of from about 0.01 to about 3,000 Oersteds. In yet another embodiment, the nanomagnetic material 103 has a coercive force of from about 0.1 to about 10.

[0069] Referring again to FIG. 6, the nanomagnetic material 103 in film 104 preferably has a relative magnetic permeability of from about 1 to about 500,000; in one embodiment, such material 103 has a relative magnetic permeability of from about 1.5 to about 260,000. As used in this specification, the term relative magnetic permeability is equal to B/H, and is also equal to the slope of a section of the magnetization curve of the film. Reference may be had, e.g., to page 4-28 of E. U. Condon et al.'s “Handbook of Physics” (McGraw-Hill Book Company, Inc., New York, 1958).

[0070] Reference also may be had to page 1399 of Sybil P. Parker's “McGraw-Hill Dictionrary of Scientific and Technical Terms,” Fourth Edition (McGraw Hill Book Company, New York, 1989). As is disclosed on this page 1399, permeability is “ . . . a factor, characteristic of a material, that is proportional to the magnetic induction produced in a material divided by the magnetic field strength; it is a tensor when these quantities are not parallel.

[0071] Reference also may be had, e.g., to U.S. Pat. Nos. 6,181,232, 5,581,224, 5,506,559, 4,246,586, 6,390,443, and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification.

[0072] In one embodiment, the nanomagnetic material 103 in film 104 has a relative magnetic permeability of from about 1.5 to about 2,000.

[0073] Referring again to FIG. 6, the nanomagnetic material 103 in film 104 preferably has a mass density of at least about 0.001 grams per cubic centimeter; in one embodiment, such mass density is at least about 1 gram per cubic centimeter. As used in this specification, the term mass density refers to the mass of a give substance per unit volume. See, e.g., page 510 of the aforementioned “McGraw-Hill Dictionary of Scientific and Technical Terms.” In one embodiment, the film 104 has a mass density of at least about 3 grams per cubic centimeter. In another embodiment, the nanomagnetic material 103 has a mass density of at least about 4 grams per cubic centimeter.

[0074] In the embodiment depicted in FIG. 6, the film 104 is disposed above 100 percent of the surfaces 112, 114, 116, and 118 of the conductor 106. In the embodiment depicted in FIG. 2, by comparison, the nanomagnetic film is disposed around the conductor.

[0075] Yet another embodiment is depicted in FIG. 7. In the embodiment depicted in FIG. 7, the film 104 is not disposed in front of either surface 114, or 116, or 118 of the conductor 106. Inasmuch as radiation is not directed towards these surfaces, this is possible.

[0076] What is essential, however, is that the film 104 be interposed between the radiation 102 and surface 112. It is preferred that film 104 be disposed above at least about 50 percent of surface 112. In one embodiment, film 104 is disposed above at least about 90 percent of surface 112.

[0077] In the remainder of this specification, the use of film 104 with various medical devices will be discussed.

[0078] Many implanted medical devices have been developed to help medical practitioners treat a variety of medical conditions by introducing an implantable medical device, partly or completely, temporarily or permanently, into the esophagus, trachea, colon, biliary tract, urinary tract, vascular system or other location within a human or veterinary patient. For example, many treatments of the vascular system entail the introduction of a device such as a guidewire, catheter, stent, arteriovenous shunt, angioplasty balloon, a cannula or the like. Other examples of implantable medical devices include, e.g., endoscopes, biopsy probes, wound drains, laparoscopic equipment, urethral inserts, and implants. Most such implantable medical devices are made in whole or in part of metal, and are not part of an electrical circuit.

[0079] When a patient with one of these implanted devices is subjected to high intensity magnetic fields, such as during magnetic resonance imaging (MRI), electrical currents are induced in the metallic portions of the implanted devices. The electrical currents so induced often create substantial amounts of heat. The heat can cause extensive damage to the tissue surrounding the implantable medical device.

[0080] Furthermore, when a patient with one of these implanted devices undergoes MRI, signal loss and disruption the diagnostic image often occur as a result of the presence of a metallic object, which causes a disruption of the local magnetic field. This disruption of the local magnetic field alters the relationship between position and frequency, which are crucial for proper image reconstruction. Therefore, patients with implantable medical devices are generally advised not to undergo MRI procedures. In many cases, the presence of such a device is a strict contraindication for MRI (See Shellock, F. G., Magnetic Resonance Procedures: health Effects and Safety, 2001 Edition, CRC Press, Boca Raton, Fla., and Food and Drug Administration, Magnetic Resonance Diagnostic Device: Panel Recommendation and Report on Petitions for MR Reclassification, Federal register, 1988, 53, 7575-7579). Any contraindication such as this, whether a strict or relative contraindication, is serious problem since it deprives the patient from undergoing an MRI examination, or even using MRI to guide other therapies, such as proper placement of diagnostic and/or therapeutics devices including angioplasty balloons, RF ablation catheters for treatment of cardiac arrythmias, sensors to assess the status of pharmacological treatment of tumors, or verification of proper placement of other permanently implanted medical devices. The rapidly growing capabilities and use of MRI in these and other areas prevent an increasingly large group of patients from benefiting from this powerful diagnostic and intra-operative tool.

[0081] The use of implantable medical devices is well known in the prior art. Thus, e.g., U.S. Pat. No. 4,180,600 discloses and claims an implantable medical device comprising a shielded conductor wire consisting of a conductive copper core and a magnetically soft alloy metallic sheath metallurgically secured to the conductive core, wherein the sheath consists essentially of from 2 to 5 weight percent of molybdenum, from about 15 to about 23 weight percent of iron, and from about 75 to about 85 weight percent of nickel. Although the device of this patent does provide magnetic shielding, it still creates heat when it interacts with strong magnetic fields, and it can still disrupt and distort magnetic resonance images.

[0082] Thus, e.g., U.S. Pat. No. 5,817,017 discloses and claims an implantable medical device having enhanced magnetic image visibility. The magnetic images are produced by known magnetic imaging techniques, such as MRI. The invention disclosed in the '017 patent is useful for modifying conventional catheters, stents, guide wires and other implantable devices, as well as interventional devices, such as for suturing, biopsy, which devices may be temporarily inserted into the body lumen or tissue; and it is also useful for permanently implantable devices.

[0083] As is disclosed in the '017 patent, paramagnetic ionic particles are fixedly incorporated and dispersed in selective portions of an implantable medical device such as, e.g., a catheter. When the catheter coated with paramagnetic ionic particles is inserted into a patient undergoing magnetic resonance imaging, the image signal produced by the catheter is of higher intensity. However, paramagnetic implants, although less susceptible to magnetization than ferromagnetic implants, can produce image artifacts in the presence of a strong magnetic field, such as that of a magnetic resonant imaging coil, due to eddy currents generated in the implants by time-varying electromagnetic fields that, in turn, disrupt the local magnetic field and disrupt the image.

[0084] Any electrically conductive material, even a non-metallic material, and even if not in an electrical circuit, will develop eddy currents and thus produce electrical potential and thermal heating in the presence of a time-varying electromagnetic field or a radio frequency field.

[0085] Thus, there is a need to provide an implantable medical device, which is shielded from strong electromagnetic fields, which does not create large amounts of heat in the presence of such fields, and which does not produce image artifacts when subjected to such fields. It is one object of the present invention to provide such a device, including a shielding device that can be reversibly attached to an implantable medical device.

[0086] FIGS. 8A, 8B, 8C, and 8D are schematic sectional views of a substrate 201, which is preferably a part of an implantable medical device.

[0087] Referring to FIG. 8A, it will be seen that substrate 201 is coated with nanomagnetic particles 202 on the exterior surface 203 of the substrate.

[0088] Referring to FIG. 8B, and in the embodiment depicted therein, the substrate 201 is coated with nanomagnetic particulate 202 on both the exterior surface 203 and the interior surface 204.

[0089] Referring to FIG. 8C, and in the preferred embodiment depicted therein, a layer of insulating material 205 separates substrate 201 and the layer of nanomagnetic coating 202.

[0090] Referring to FIG. 8D, it will be seen that one or more layers of insulating material 205 separate the inside and outside surfaces of substrate 201 from respective layers of nanomagnetic coating 202.

[0091] FIG. 9 is a schematic sectional view of a substrate 301 which is part of an implantable medical device (not shown). Referring to FIG. 9, and in the embodiment depicted therein, it will be seen that substrate 301 is coated with nanomagnetic material 302, which may differ from nanomagnetic material 202.

[0092] In one embodiment, the substrate 301 is in the shape of a cylinder, such as an enclosure for a medical catheter, stent, guide wire, and the like. In one aspect of this embodiment, the cylindrical substrate 301 encloses a helical member 303, which is also coated with nanomagnetic particulate material 302.

[0093] In another embodiment (not shown), the cylindrical substrate 301 depicted in FIG. 9 is coated with multiple layers of nanomagnetic materials. In one aspect of this embodiment, the multiple layers of nanomagnetic particulate are insulated from each other. In another aspect of this embodiment, each of such multiple layers is comprised of nanomagnetic particles of different sizes and/or densities and/or chemical densities. In one aspect of this embodiment, not shown, each of such multiple layers may have different thickness. In another aspect of this embodiment, the frequency response and degree of shielding of each such layer differ from that of one or more of the other such layers.

[0094] FIG. 10 is a flow diagram of a preferred process of the invention. In FIG. 2, reference is made to one or more conductors as being the substrate(s); it is to be understood, however, that other substrate(s) material(s) and/or configurations also may be used.

[0095] In the first step of this process depicted in FIG. 10, step 240, the substrate 201 (see FIG. 8A) is coated with electrical insulative material. Suitable insulative materials include nano-sized silicon dioxide, aluminum oxide, cerium oxide, yttrium-stabilized zirconium, silicon carbide, silicon nitride, aluminum nitride, and the like. In general, these nano-sized particles will have a particle distribution such that at least 90 weight percent of the particles have a dimension in the range of from about 10 to about 100 nanometers.

[0096] The coated substrate 201 may be prepared by conventional means such as, e.g., the process described in U.S. Pat. No. 5,540,959, the entire disclosure of which is incorporated by reference into this specification. This patent describes and claims a process for preparing a coated substrate, comprising the steps of: (a) creating mist particles from a liquid, wherein: 1. said liquid is selected from the group consisting of a solution, a slurry, and mixtures thereof, 2. said liquid is comprised of solvent and from 0.1 to 75 grams of solid material per liter of solvent, 3. at least 95 volume percent of said mist particles have a maximum dimension less than 100 microns, and 4. said mist particles are created from said first liquid at a rate of from 0.1 to 30 milliliters of liquid per minute; (b) contacting said mist particles with a carrier gas at a pressure of from 761 to 810 millimeters of mercury; (c) thereafter contacting said mist particles with alternating current radio frequency energy with a frequency of at least 1 megahertz and a power of at least 3 kilowatts while heating said mist to a temperature of at least 100 degree centigrade, thereby producing a heated vapor; (d) depositing said heated vapor onto a substrate, thereby producing a coated substrate; and (e) subjecting said coated substrate to a temperature of from about 450 to about 1,400 degree centigrade for at least 10 minutes.

[0097] By way of further illustration, one may coat substrate 201 by means of the process disclosed in a text by D. Satas on “Coatings Technology Handbook” (Marcel Dekker, Inc., New York, N.Y., 1991). As is disclosed in such text, one may use cathodic arc plasma deposition (see pages 229 et seq.), chemical vapor deposition (see pages 257 et seq.), sol-gel coatings (see pages 655 et seq.), and the like.

[0098] Referring again to FIGS. 8C and 8D, and by way of illustration and not limitation, these Figures are sectional views of the coated substrate 201. It will be seen that, in the embodiments depicted, insulating material 205 separates the substrate and the layer of nanomagnetic material 202. In order to obtain the structure depicted in FIGS. 8C and 8D, one may first coat the substrate with insulating material 205, and then apply a coat of nanomagnetic material 202 on top of the insulating material 205; see, e.g., step 248 of FIG. 10.

[0099] The insulating material 205 that is disposed between substrate 201 and the layer of nanomagnetic coating 202 preferably has an electrical resistivity of from about 1,000,000,000 to about 10,000,000,000,000 ohm-centimeter.

[0100] After the insulating material 205 has been deposited, and in one preferred embodiment, the coated substrate is heat-treated in step 250 of FIG. 10. The heat treatment often is preferably used in conjunction with coating processes in which heat is required to bond the insulative material to the substrate 201.

[0101] The heat-treatment step 250 may be conducted after the deposition of the insulating material 205, or it may be conducted simultaneously therewith. In either event, and when it is used, it is preferred to heat the coated substrate 201 to a temperature of from about 200 to about 600 degree Centigrade for about 1 minute to about 10 minutes.

[0102] Referring again to FIG. 10, and in step 252 of the process, after the coated substrate 201 has been subjected to heat treatment step 250, the substrate is allowed to cool to a temperature of from about 30 to about 100 degree Centigrade over a period of time of from about 3 to about 15 minutes.

[0103] One need not invariably heat-treat and/or cool. Thus, referring to FIG. 10, one may immediately coat nanomagnetic particulate onto the coated substrate in step 254, after step 248 and/or after step 250 and/or after step 252.

[0104] In step 254, nanomagnetic material(s) are coated onto the previously coated substrate 201. This is best shown in FIGS. 8C and 8D, wherein the nanomagnetic materials are identified as 202.

[0105] Nanomagnetic material is magnetic material which has an average particle size less than 100 nanometers and, preferably, in the range of from about 2 to about 50 nanometers. Reference may be had, e.g., to U.S. Pat. No. 5,889,091 (Rotationally Free Nanomagnetic Material), U.S. Pat. Nos. 5,714,136, 5,667,924, and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification.

[0106] The nanomagnetic material may be, e.g., nano-sized ferrites such as, e.g., the nanomagnetic ferrites disclosed in U.S. Pat. No. 5,213,851, the entire disclosure of which is hereby incorporated by reference into this specification. This patent discloses and claim a process for coating a layer of ferrite material with a thickness of from about 0.1 to about 500 microns onto a substrate at a deposition rate of from about 0.01 to about 10 microns per minute per 35 square centimeters of substrate surface, comprising the steps of: (a) providing a solution comprised of a first compound and a second compound, wherein said first compound is an iron compound and said second compound is selected from the group consisting of compound of nickel, zinc, magnesium, strontium, barium, manganese, lithium, lanthanum, yttrium, scandium, samarium, europium, terbium, dysprosium, holmium, erbium, ytterbium, lutetium, cerium, praseodymium, thulium, neodymium, gadolinium, aluminum, iridium, lead, chromium, gallium, indium, cobalt, titanium, and mixtures thereof, and wherein said solution is comprised of from about 0.01 to about 1 kilogram of a mixture consisting essentially of said compounds per liter of said solution; (b) subjecting said solution to ultrasonic sound waves at a frequency in excess of 20 kilohertz, and to an atmospheric pressure of at least about 600 millimeters of mercury, thereby causing said solution to form into an aerosol; (c) providing a radio frequency plasma reactor comprised of a top section, a bottom section, and a radio frequency coil; (d) generating a hot plasma gas within said radio frequency plasma reactor, thereby producing a plasma region; (e) providing a flame region disposed above said top section of said radio frequency plasma reactor; (f) contacting said aerosol with said hot plasma gas within said plasma reactor while subjecting said aerosol to an atmospheric pressure of at least 600 millimeters of mercury, and to a radio frequency alternating current at a frequency of from about 100 kilohertz to about 30 megahertz, thereby forming a vapor; (g) providing a substrate disposed above said flame region; and (h) contacting said vapor with said substrate, thereby forming said layer of ferrite material.

[0107] By way of further illustration, one may use the techniques described in an article by M. De Marco, X. W. Wang, et al. on “Mossbauer and Magnetization Studies of Nickel Ferrites”, published in the Journal of Applied Physics 73(10), May 15, 1993, at pages 6287-6289.

[0108] In general, the thickness of the layer of nanomagnetic material deposited onto the coated substrate 201 is from about 100 nanometers to about 10 micrometers and, more preferably, from about 0.1 to 3 microns.

[0109] Referring again to FIG. 10, after the nanomagnetic material is coated in step 254, the coated substrate may be heat-treated in step 256. In this optional step 256, it is preferred to subject the coated substrate 201 to a temperature of from about 200 to about 600 degree Centigrade for from about 1 to about 10 minutes.

[0110] In one embodiment (not shown) additional insulating layers may be coated onto the substrate 201, by one or more of the processes disclosed hereinabove; see, e.g., optional step 258 of FIG. 10.

[0111] Without wishing to be bound to any particular theory, the applicants believe that the nanomagnetic particles 202 trap and pin magnetic lines of flux impinging on substrate 201, while at the same time minimizing or eliminating the flow of electrical currents through the coating and/or substrate.

[0112] In order to function optimally, the nanomagnetic material(s) 202 preferably have a specified magnetization. As is know to those skilled in the art, magnetization is the magnetic moment per unit volume of a substance. Reference may be had, e.g., to U.S. Pat. Nos. 4,169,998, 4,168,481, 4,166,263, 5,260,132, 4,778,714, and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification.

[0113] Referring again to FIGS. 8A, 8B, 8C, and 8D, the layer of nanomagnetic particles 202 preferably has a saturation magnetization, at 25 degree Centigrade, of from about 1 to about 36,000 Gauss, and preferably from about 1 to about 26,000 Gauss. In one embodiment, the saturation magnetization at room temperature of the nanomagnetic particles is from about 500 to about 10,000 Gauss. For a discussion of the saturation magnetization of various materials, reference may be had, e.g., to U.S. Pat. Nos. 4,705,613, 4,631,613, 5,543,070, 4,901,741 (cobalt, samarium, and gadolinium alloys), and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification. As will be apparent to those skilled in the art, especially upon studying the aforementioned patents, the saturation magnetization of thin films is often higher than the saturation magnetization of bulk objects.

[0114] In one embodiment, it is preferred to utilize a thin film with a thickness of less than about 2 microns and a saturation magnetization in excess of 20,000 Gauss. The thickness of the layer of nanomagnetic material is measured from the bottom surface of such layer that contains such material to the top surface of such layer that contain such material; and such bottom surface and/or such top surface may be contiguous with other layers of material (such as insulating material) that do not contain nanomagnetic particles. Thus, e.g., one may make a thin film in accordance with the procedure described at page 156 of Nature, Volume 407, Sep. 14, 2000, that describes a multiplayer thin film that has a saturation magnetization of 24,000 Gauss.

[0115] By the appropriate selection of nanomagnetic particles, and the thickness of the film deposited, one may obtain saturation magnetizations of as high as at least about 36,000 Gauss.

[0116] In the preferred embodiment depicted in FIG. 8A, the nanomagnetic material 202 may be disposed within an insulating matrix (not shown) so that any heat produced by such particles will be slowly dispersed within such matrix. Such matrix, as indicated hereinabove, may be made from ceria, calcium oxide, silica, alumina, and the like. In general, the insulating material 202 preferably has a thermal conductivity of less than about 20 (calories centimeters/square centimeters−degree second)×10,000. See, e.g., page E-6 of the 63^rd Edition of the “Handbook of Chemistry and Physics” (CRC Press, Inc. Boca Raton, Fla., 1982).

[0117] The nanomagnetic material 202 typically comprises one or more of iron, cobalt, nickel, gadolinium, and samarium atoms. Thus, e.g., typical nanomagnetic materials include alloys of iron, and nickel (permalloy), cobalt, niobium and zirconium (CNZ), iron, boron, and nitrogen, cobalt, iron, boron and silica, iron, cobalt, boron, and fluoride, and the like. These and other materials are described in a book by J. Douglass Adam et al. entitled “Handbook of Thin Film Devices” (Academic Press, San Diego, Calif., 2000). Chapter 5 of this book beginning at page 185 describes “magnetic films for planar inductive components and devices;” and Tables 5.1 and 5.2 in this chapter describes many magnetic materials.

[0118] Some of the devices described in this application are substantially flexible. As used in this specification, the term flexible refers to an assembly that can be bent to form a circle with a radius of less than 2 centimeters without braking. Put another way, the bend radius of the coated assembly can be less than 2 centimeters. Reference may be had, e.g., to U.S. Pat. Nos. 4,705,353, 5,946,439, 5,315,365, 4,641,917, 5,913,005, and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification.

[0119] Some of the devices described in this specification are substantially rigid. One such device is a rigid sheath that is adapted to be placed over an endoscope or biopsy probe used inter-operatively with magnetic resonance imaging.

[0120] As will be apparent, even when the magnetic insulating properties of the assembly of this invention are not absolutely effective, the assembly still reduces the amount of electromagnetic energy that is transferred to the coated substrate, prevents the rapid dissipation of heat to bodily tissue, and minimization of disruption to the magnetic resonance image.

A Self-Regulating Shielded Substrate

[0121] In one embodiment of the invention, described in this section of the specification, a shielded substrate that comprises means to regulate its temperature is described. In this embodiment, the substrate can be substantially any size, any shape, any material, or any combination of materials. The shielding material disposed on and/or in such substrate may be disposed on and/or in some or all of such substrate.

[0122] Referring to FIG. 11, assembly 400 is comprised of a substrate 201 (described elsewhere in this specification) and, disposed over such substrate 201, a layer 202 of nanomagnetic material. In the embodiment depicted, the layer 202 is preferably from about 100 nanometers to about 3 microns thick.

[0123] In the embodiment depicted, and disposed beneath layer 202, is a layer 402 of thermoelectric material. As is known to those skilled in the art, thermoelectric material exhibits a thermoelectric effect. Thermoelectric effects occur when mobile charge carriers in liquids and solids are subjected to the influence of temperature gradients and/or particular electrical potential gradients. See, e.g., an article by C. L. Foiles entitled “Thermoelectric Effects” appearing at pages 1263-1264 of R. G. Lerner et al.'s “Encyclopedia of Physics,” Second Edition (VCH Publishers, Inc., New York, N.Y., 1991).

[0124] In one embodiment, the thermoelectric effect is exhibited by a thermistor. Thermistors are materials whose resistance decreases as temperature increases, and they are often used to control temperature by, removing heat in air conditioning systems.

[0125] By way of illustration, one may use one or more of the thermoelectric materials disclosed in U.S. Pat. No. 6,388,185 (microfabricated thermoelectric power-generation devices), U.S. Pat. No. 6,307,143 (thermoelectric materials and thermoelectric conversion element), U.S. Pat. No. 6,288,321 (thermoelectric power generation), U.S. Pat. No. 6,222,243 (thermoelectric device), U.S. Pat. No. 6,207,886 (Skutterudite thermoelectric material), U.S. Pat. Nos. 6,096,966, 5,982,013, 5,969,290, 5,929,351 (cobalt-antimony based thermoelectric material), U.S. Pat. Nos. 5,912,429, 5,897,330, 5,864,087, 5,824,561, 5,763,293 (thermoelectric module formed of V-VI group compound), U.S. Pat. No. 5,726,381 (amorphous thermoelectric alloys), U.S. Pat. No. 5,712,448 (cooling device featuring thermoelectric and diamond materials for temperature control of heat-dissipating devices), U.S. Pat. No. 5,665,176 (n-type thermoelectric materials), U.S. Pat. No. 5,507,879 (sensor utilizing thermoelectric material), U.S. Pat. No. 5,484,490 (p-type thermoelectric material), and the like. The entire disclosure of each of each of these United States patents is hereby incorporated by reference into this specification.

[0126] Referring again to FIG. 11, and in the preferred embodiment depicted, as energy is absorbed and dissipated in nanomagnetic layer 202, its temperature tends to increase, and this temperature increase is transmitted, at least in part, to thermoelectric layer 402. This temperature increase in layer 402 will tend to produce electricity in such layer 402 which, ultimately, will be used to cool the layer 402 and the substrate 201.

[0127] Referring again to FIG. 12, and in the preferred embodiment depicted therein, below temperature 404, substantially no electricity is produced by the thermoelectric layer 402. From temperature 404 to temperature 406, the layer 404 converts heat to electricity at an efficiency 408. At temperatures above temperature 406, substantially no electricity is produced by the layer 402.

[0128] As will be apparent, by the appropriate choice of thermoelectric material(s), one can custom design the temperature response curve depicted in FIG. 12, or any other desired temperature response curve. Thus, by the judicious use of the proper thermoelectric material(s), one can insure that the substrate 201 also remains within a specified temperature range.

[0129] Referring again to FIG. 11, and in the preferred embodiment depicted therein, a layer 410 of conductive material (such as copper, aluminum, etc.) is disposed beneath layer 402. When the temperature in layer 402 reaches temperature 404, electricity starts to flow through connector 410 and also through connector 412. Such an application of electricity to the thermoelectric layer 414 will cause it to cool.

[0130] In one embodiment, the phenomenon of thermoelectric cooling is utilized. In this embodiment, an electric current is sent through a thermocouple whose cold junction is thermally coupled to the cooled substrate while the hot junction dissipates heat to the heat sink.

[0131] By way of further illustration, reference may be had, e.g., to U.S. Pat. No. 6,351,951 (thermoelectric cooling device), U.S. Pat. Nos. 6,338,570, 6,334,311 (thermoelectric cooling temperature control), U.S. Pat. No. 6,308,519 (thermoelectric cooling system), U.S. Pat. No. 6,282,907 (thermoelectric cooling apparatus), U.S. Pat. No. 6,266,962 (thermoelectric cooling apparatus), U.S. Pat. No. 6,256,996 (nanoscopic thermoelectric coolers), U.S. Pat. No. 6,222,113 (electrically isolated substrates for thermoelectric coolers), U.S. Pat. Nos. 6,000,225, 5,884,485, 5,881,560 (thermoelectric cooling system), U.S. Pat. No. 5,813,233 (thermoelectric cooling device), U.S. Pat. Nos. 5,802,855, 5,724,818 (thermoelectric cooling module), U.S. Pat. No. 5,715,684 (thermoelectric converter), U.S. Pat. No. 5,713,208 (thermoelectric cooling apparatus), U.S. Pat. Nos. 5,609,032, 5,409,547, and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification.

[0132] Referring to FIG. 11, as substrate 201 is cooled, layer 402 of thermoelectric material also will tend to be cooled, especially when the layer 410 also is a good heat conductor (as is, e.g., copper). As layer 402 is cooled to temperature 404, the efficiency of electricity generation decreases to substantially zero.

[0133] As the efficiency of electricity generation decreases, then the cooling effect decreases, and the temperature of substrate 201 and of thermoelectric layer 402 tends to increase. As these temperatures tend to increase, the efficiency of electricity generation tends to increase. Thus, the system is self-regulating, tending to maintain the substrate 201 within a specified temperature range.

[0134] As will be apparent, by the choice of appropriate materials, configurations, and dimensions, one can produce substantially any desired temperature profile (see FIG. 12) and, consequently, any desired control system for a specified temperature range.

[0135] In one embodiment, the assembly 400 is implanted within a living organism.