Patent Publication Number: US-6713671-B1

Title: Magnetically shielded assembly

Description:
REFERENCE TO RELATED PATENT APPLICATION 
     This application is a continuation-in-part of applicant&#39;s copending patent application U.S. Ser. No. 10/260,247, filed on Sep. 30, 2002, which in turn was a continuation-in-part of U.S. Ser. No. 10/054,407, filed on Jan. 22, 2002 now U.S. Pat. No. 6,506,972. 
    
    
     FIELD OF THE INVENTION 
     A shielded assembly comprised of a substrate and, disposed above a substrate, a shield comprising from about 1 to about 99 weight percent of a first nanomagnetic material, and from about 99 to about 1 weight percent of a second material with a resistivity of from about 1 microohm-centimeter to about 1×10 25  microohm centimeters. 
     BACKGROUND OF THE INVENTION 
     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. 
     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. 
     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&#39;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 micro Tesla. 
     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. 
     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. 
     It is an object of this invention to provide a sheath assembly, which is shielded from magnetic fields. 
     SUMMARY OF THE INVENTION 
     In accordance with this invention, there is provided a shielded assembly comprised of a substrate and, disposed above a substrate, a shield comprising from about 1 to about 99 weight percent of a first nanomagnetic material, and from about 99 to about 1 weight percent of a second material with a resistivity of from about 1 microohm-centimeter to about 1×10 25  microohm centimeters. The nanomagnetic material comprises nanomagnetic particles, and these nanomagnetic particles respond to an externally applied magnetic field by realigning to the externally applied field. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described by reference to the following drawings, in which like numerals refer to like elements, and in which: 
     FIG. 1 is a schematic sectional view of a shielded implanted device comprised of one preferred conductor assembly of the invention; 
     FIG. 1A is a flow diagram of a preferred process of the invention; 
     FIG. 2 is an enlarged sectional view of a portion of the conductor assembly of FIG. 1; 
     FIG. 3 is a sectional view of another conductor assembly of this invention; 
     FIG. 4 is a schematic view of the conductor assembly of FIGS. 2; 
     FIG. 5 is a sectional view of the conductor assembly of FIG. 2; 
     FIG. 6 is a schematic of another preferred shielded conductor assembly; 
     FIG. 7 is a schematic of yet another configuration of a shielded conductor assembly; 
     FIGS. 8A,  8 B,  8 C, and  8 D are schematic sectional views of a substrate, such as one of the specific medical devices described in this application, coated with nanomagnetic particulate matter on its exterior surface; 
     FIG. 9 is a schematic sectional view of an elongated cylinder, similar to the specific medical devices described in this application, coated with nanomagnetic particulate (the cylinder encloses a flexible, expandable helical member, which is also coated with nanomagnetic particulate material); 
     FIG. 10 is a flow diagram of a preferred process of the invention; 
     FIG. 11 is a schematic sectional view of a substrate, similar to the specific medical devices described in this application, coated with two different populations of elongated nanomagnetic particulate material; 
     FIG. 12 is a schematic sectional view of an elongated cylinder, similar to the specific medical devices described in this application, coated with nanomagnetic particulate, wherein the cylinder includes a channel for active circulation of a heat dissipation fluid; 
     FIGS. 13A,  13 B, and  13 C are schematic views of an implantable catheter coated with nanomagnetic particulate material; 
     FIGS. 14A through 14G are schematic views of an implantable, steerable catheter coated with nanomagnetic particulate material; 
     FIGS. 15A,  15 B and  15 C are schematic views of an implantable guide wire coated with nanomagnetic particulate material; 
     FIGS. 16A and 16B are schematic views of an implantable stent coated with nanomagnetic particulate material; 
     FIG. 17 is a schematic view of a biopsy probe coated with nanomagnetic particulate material; 
     FIGS. 18A and 18B are schematic views of a tube of an endoscope coated with nanomagnetic particulate material; 
     FIGS. 19A and 19B are schematics f one embodiment of the magnetically shielding assembly of this invention; 
     FIGS. 20A,  20 B,  20 C,  20 D,  20 E, and  20 F are enlarged sectional views of a portion of a shielding assembly illustrating nonaligned and magnetically aligned nanomagnetic liquid crystal materials in different configurations; 
     FIG. 21 is a graph showing the relationship of the alignment of the nanomagnetic liquid crystal material of FIGS. 20A and 20B with magnetic field strength; 
     FIG. 22 is a graph showing the relationship of the attenuation provided by the shielding device of this invention as a function of frequency of the applied magnetic field; 
     FIG. 23 is a flow diagram of one preferred process for preparing the nanomagnetic liquid crystal compositions of this invention; 
     FIG. 24 is a sectional view of a multiplayer structure comprised of different nanomagnetic materials; 
     FIG. 25 is a sectional view of another multilayer structure comprised of different nanomagnetic materials and an electrical insulating layer. 
     FIG. 26 is a schematic view of yet another multilayer structure comprised of nanomagnetic material; 
     FIG. 27 is a schematic of yet another multilayer structure comprised of nanomagnetic material; 
     FIG. 28 is a schematic of yet another multilayer structure comprised of nanomagnetic material; and 
     FIGS. 29,  30 , and  31  are also schematics of other multiplayer structures comprised of nanomagnetic material. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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. 
     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 . 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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 arc 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. 
     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. 
     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. 
     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 . Altematively, 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. 
     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. 
     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 . 
     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. 
     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. 
     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 . 
     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 . 
     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. Nos. 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. 
     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 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 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. 
     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. 
     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. 
     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. 
     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.  1 A). 
     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. 
     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. 
     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. 
     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 . 
     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. 
     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. 
     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. 
     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. 
     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. 
     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 (caloriescentimeters/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). 
     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. 
     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 from 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. 
     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. 
     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. 
     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. 
     In the preferred embodiment of this invention, a film of nanomagnetic is disposed above at feast 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 . 
     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. 
     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. 
     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. 
     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. 
     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. 
     Referring again to FIG. 6, the nanomagnetic material  103  in film  104  preferably has a relative magnctic 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.&#39;s “Handbook of Physics” (McGraw-Hill Book Company, Inc., New York, 1958). 
     Reference also may be had to page 1399 of Sybil P. Parker&#39;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. 
     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. 
     In one embodiment, the nanomagnetic material  103  in film  104  has a relative magnetic permeability of from about 1.5 to about 2,000. 
     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. 
     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. 
     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. 
     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 . 
     In the remainder of this specification, the use of film  104  with various medical devices will be discussed. 
     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. 
     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. 
     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. 
     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. 
     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 &#39;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. 
     As is disclosed in the &#39;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. 
     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. 
     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. 
     FIGS. 8A,  8 B,  8 C, and  8 D are schematic sectional views of a substrate  201 , which is preferably a part of an implantable medical device. 
     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. 
     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 . 
     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 . 
     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 . 
     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 . 
     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 . 
     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. 
     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. 
     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. 
     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. 
     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. 
     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 . 
     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. 
     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 . 
     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. 
     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. 
     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 . 
     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 . 
     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. Nos. 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. 
     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. 
     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. 
     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. 
     Referring again to FIG. 10, after the nanomagnetic material is coated in step  254 , the coated substrate may be beat-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. 
     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 . 
     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. 
     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. No. 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. 
     Referring again to FIGS. 8A,  8 B,  8 C, and  8 D, 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. 
     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. 
     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. 
     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). 
     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 arc 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. 
     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. 
     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. 
     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. 
     FIG. 11 is a schematic sectional view of a substrate  401 , which is part of an implantable medical device (not shown). Referring to FIG. 11, and in the preferred embodiment depicted therein, it will be seen that substrate  401  is coated with a layer  404  of nanomagnetic material(s). The layer  404 , in the embodiment depicted, is comprised of nanomagnetic particulate  405  and nanomagnetic particulate  406 . Each of the nanomagnetic particulate  405  and nanomagnetic particulate  406  preferably has an elongated shape, with a length that is greater than its diameter. In one aspect of this embodiment, nanomagnetic particles  405  have a different size than nanomagnetic particles  406 . In another aspect of this embodiment, nanomagnetic particles  405  have different magnetic properties than nanomagnetic particles  406 . Referring again to FIG. 11, and in the preferred embodiment depicted therein, nanomagnetic particulate material  405  and nanomagnetic particulate material  406  are designed to respond to an static or time-varying electromagnetic fields or effects in a manner similar to that of liquid crystal display (LCD) materials. More specifically, these nanomagnetic particulate materials  405  and nanomagnetic particulate materials  406  are designed to shift alignment and to effect switching from a magnetic shielding orientation to a non-magnetic shielding orientation. As will be apparent, the magnetic shield provided by layer  404 , can be turned “ON” and “OFF” upon demand. In yet another embodiment (not shown), the magnetic shield is turned on when heating of the shielded object is detected. 
     FIG. 12 is a schematic sectional view of substrate  501 , which is part of an implantable medical device (not shown). Referring to FIG. 12, and to the embodiment depicted therein, it will be seen that substrate  501  is coated with nanomagnetic particulate material  502  which may differ from particulate material  202  and/or particulate material  302 . In the embodiment depicted in FIG. 12, the substrate  501  may be a cylinder, such as an enclosure for a catheter, medical stent, guide wire, and the like. The assembly depicted in FIG. 12 includes a channel  508  located on the periphery of the medical device. An actively circulating, heat-dissipating fluid (not shown) can be pumped into channel  508  through port  507 , and exit channel  508  through port  509 . The heat-dissipation fluid (not shown) will draw heat to another region of the device, including regions located outside of the body where the heat can be dissipated at a faster rate. In the embodiment depicted, the heat-dissipating flow flows internally to the layer of nanomagnetic particles  502 . 
     In another embodiment, not shown, the heat dissipating fluid flows externally to the layer of nanomagnetic particulate material  502 . 
     In another embodiment (not shown), one or more additional polymer layers (not shown) are coated on top of the layer of nomagnetic particulate  502 . In one aspect of this embodiment, a high thermal conductivity polymer layer is coated immediately over the layer of nanomagnetic particulate  502 ; and a low thermal conductivity polymer layer is coated over the high thermal conductivity polymer layer. It is preferred that neither the high thermal conductivity polymer layer nor the low thermal conductivity polymer layer be electrically or magnetically conductive. In the event of the occurrence of “hot spots” on the surface of the medical device, heat from the localized “hot spots” will be conducted along the entire length of the device before moving radially outward through the insulating outer layer. Thus, heat is distributed more uniformly. 
     Many different devices advantageously incorporate the nanomagnetic film of this invention. In the following section of the specification, various additional devices that incorporate the such film are described. 
     The disclosure in the following section of the specification relates generally to an implantable medical device that is immune or hardened to electromagnetic insult or interference. More particularly, the invention is directed to implantable medical devices that are not part of an electrical circuit, and that utilize shielding to harden or make these devices immune from electromagnetic insult (i.e. minimize or eliminate the amount of electromagnetic energy transferred to the device), namely magnetic resonance imaging (MRI) insult 
     Magnetic resonance imaging (MRI) has been developed as an imaging technique to obtain images of anatomical features of human patients as well as some aspects of the functional activities of biological tissue; reference may be had, e.g., to John D. Enderle&#39;s “Introduction to Biomedical Engineering”, Academic Press, San Diego, Calif., 2000 and, in particular, pages 783-841 thereof. Reference may also be had to Joseph D. Bronzino&#39;s “The Biomedical Engineering Handbook”, CRC Press, Boca Raton, Fla., 1995, and in particular pages 1006-1045 thereof. These images have medical diagnostic value in determining the state of the health of the tissue examined. 
     In an MRI process, a patient is typically aligned to place the portion of the patient&#39;s anatomy to be examined in the imaging volume of the MRI apparatus. Such a MRI apparatus typically comprises a primary magnet for supplying a constant magnetic field, Bo, which is typically of from about 0.5 to about 10.0 Tesla, and by convention, is along the z-axis and is substantially homogenous over the imaging volume, and secondary magnets that can provide magnetic field gradients along each of the three principal Cartesian axis in space (generally x, y, and z or x1, x2, and x3, respectively). A magnetic field gradient refers to the variation of the field along the direction parallel to Bo with respect to each of the three principal Cartesian Axis. The apparatus also comprises one or more radio frequency (RF) coils, which provide excitation for and detection of the MRI signal. The RF excitation signal is an electromagnetic wave with an electrical field E and magnetic field B1, and is typically transmitted at frequencies of 3-100 megahertz. 
     The use of the MRI process with patients who have implanted medical assist devices, such as guide wires, catheters, or stents, often presents problems. These implantable devices are sensitive to a variety of forms of electromagnetic interference (EMI), because the aforementioned devices contain metallic parts that can receive energy from the very intensive EMI fields used in magnetic resonance imaging. The above-mentioned devices may also contain sensing and logic and control systems that respond to low-level electrical signals emanating from the monitored tissue region of the patient. Since these implanted devices are responsive to changes in local electromagnetic fields, the implanted devices are vulnerable to sources of electromagnetic noise. The implanted devices interact with the time-varying radio-frequency (RF) magnetic field (B1), which are emitted during the MRI procedure. This interaction can result in damage to the implantable device, or it can result in heating of the device, which in turn can harm the patient or physician using the device. This interaction can also result in degradation of the quality of the image obtained by the MRI process. 
     Signal loss and disruption of a magnetic resonance image can be caused by disruption of the local magnetic field, which perturbs the relationship between position and image, which are crucial for proper image reconstruction. More specifically, the spatial encoding of the MRI signal provided by the linear magnetic field can be disrupted, making image reconstruction difficult or impossible. The relative amount of artifact seen on an MR image due to signal disruption is dependent upon such factors as the magnetic susceptibility of the materials used in the implantable medical device, as well as the shape, orientation, and position of the medical device within the body of the patient, which is very often difficult to control. 
     All non-permanently magnetized materials have non-zero magnetic susceptibilities and are to some extent magnetic. Materials with positive magnetic susceptibilities less than approximately 0.01 arc referred to as paramagnetic and are not overly responsive to an applied magnetic field. They are often considered non-magnetic. Materials with magnetic susceptibilities greater than 0.01 are referred to as ferromagnetic. These materials can respond very strongly to an applied magnetic field and are also referred as soft magnets as their properties do not manifest until exposed to an external magnetic field. 
     Paramagnetic materials (e.g. titanium), are frequently used to encapsulate and shield and protect implantable medical devices due to their low magnetic susceptibilities. These enclosures operate by deflecting electromagnetic fields. However, although paramagnetic materials are less susceptible to magnetization than ferromagnetic materials, they can also produce image artifacts due to eddy currents generated in the implanted medical device by externally applied magnetic fields, such as the radio frequency fields used in the MRI procedures. These eddy currents produce localized magnetic fields, which disrupt and distort the magnetic resonance image. Furthermore, the implanted medical device shape, orientation, and position within the body make it difficult to control image distortion due to eddy currents induced by the RF fields during MRI procedures. Also, since the paramagnetic materials are electrically conductive, the eddy currents produced in them can result in ohmic heating and injury to the patient. The voltages induced in the paramagnetic materials can also damage the medical device, adversely interact with the operation of the device. Typical adverse effects can include improper stimulation of internal tissues and organs, damage to the medical device (melting of implantable catheters while in the MR coil have been reported in the literature), and/or injury to the patient. 
     Thus, it is desirable to provide protection against electromagnetic interference, and to also provide fail-safe protection against radiation produced by magnetic-resonance imaging procedures. Moreover, it is desirable to provide devices that prevent the possible damage that can be done at the tissue interface due to induced electrical signals and due to thermal tissue damage. Furthermore, it is desirable to provide devices that do not interact with RF fields which are emitted during magnetic-resonance imaging procedures and which result in degradation of the quality of the images obtained during the MRI process. 
     In one embodiment, there is provided a coating of nanomagnetic particles that consists of a mixture of aluminum oxide (AlO3), iron, and other particles that have the ability to deflect electromagnetic fields while remaining electrically non-conductive. Preferably the particle size in such a coating is approximately  10  nanometers. Preferably the particle packing density is relatively low so as to minimize electrical conductivity. Such a coating when placed on a fully or partially metallic object (such as a guide wire, catheter, stent, and the like) is capable of deflecting electromagnetic fields, thereby protecting sensitive internal components, while also preventing the formation of eddy currents in the metallic object or coating. The absence of eddy currents in a metallic medical device provides several advantages, to wit: (1) reduction or elimination of heating, (2) reduction or elimination of electrical voltages which can damage the device and/or inappropriately stimulate internal tissues and organs, and (3) reduction or elimination of disruption and distortion of a magnetic-resonance image. 
     FIG. 13 is a schematic view of a catheter assembly  600  similar to the assembly depicted in FIG. 2 of U.S. Pat. No. 3,995,623; the entire disclosure of such patent is hereby incorporated by reference into this specification. Referring to FIG. 6 of such patent, it will be seen that catheter tube  625  contains multiple lumens  603 ,  611 ,  613 , and  615 , which can be used for various functions such as inflating balloons, enabling electrical conductors to communicate with the distal end of the catheter, etc. While four lumens are shown, it is to be understood that this invention applies to a catheter with any number of lumens. 
     The similar catheter disclosed and claimed in U.S. Pat. No. 3,995,623 may be shielded by coating it in whole or in part with a coating of nanomagnetic particulate, in any of the following manners: 
     In FIG. 13A, a nanomagnetic material  650  is applied to either the interior wall  650   a  or exterior wall  650   b  of lumens  603 ,  611 ,  613 , and  615 , or imbibed  650   c  into the walls of these lumens within catheter  625 , or any combination of these locations. 
     In FIG. 13B, a nanomagnetic material  650  is applied to the interior walls  650   d  of multiple lumens within a single catheter  625  or the common exterior wall  650   b  or imbibed  650   c  into the common wall. 
     In FIG. 13C, a nanomagnetic material  650  is applied to the mesh-like material  636  used within the wall of catheter  625  to give it desired mechanical properties. 
     In another embodiment (not shown) a sheath coated with nanomagnetic material on its internal surface, exterior surface, or imbibed into the wall of sheath is placed over the catheter to shield it from electromagnetic interference. In this manner, existing catheters can be made MRI safe and compatible. The modified catheter assembly thus produced is resistant to electromagnetic radiation. 
     FIGS. 14A through 14G are schematic views of a catheter assembly consisting of multiple concentric elements. While two elements are shown;  720  and  722 , it is to be understood that any number of over-lapping elements may be used, either concentrically or planarly positioned with respect to each other. 
     Referring to FIGS. 14A-14G, it will be seen that catheter assembly  700  comprises an elongated tubular construction having a single, central or axial lumen  710 . The exterior catheter body  722  and concentrically positioned internal catheter body  720  with internal lumen  712  are preferably flexible, i.e., bendable, but substantially non-compressible along its length. The catheter bodies  720  and  722  may be made of any suitable material. A presently preferred construction comprises an outer wall  722  and inner wall  720  made of a polyurethane, silicone, or nylon. The outer wall  722  preferably comprises an imbedded braided mesh of stainless steel or the like to increase torsional stiffness of the catheter assembly  700  so that, when a control handle, not shown, is rotated, the tip sectionally of the catheter will rotate in corresponding manner. The catheter assembly  700  may be shielded by coating it in whole or in part with a coating of nanomagnetic particulate, in any one or more of the following manners: 
     Referring to FIG. 14A, a nanomagnetic material may be coated on the outside surface of the inner concentrically positioned catheter body  720 . 
     Referring to FIG. 14B, a nanomagnetic material may be coated on the inside surface of the inner concentrically positioned catheter body  720 . Referring to FIG. 14C, a nanomagnetic material may be imbibed into the walls of the inner concentrically positioned catheter body  720  and externally positioned catheter body  722 . Although not shown, a nanomagnetic material may be imbibed solely into either inner concentrically positioned catheter body  720  or externally positioned catheter body  722 . 
     Referring to FIG. 14D, a nanomagnetic material may be coated onto the exterior wall of the inner concentrically positioned catheter body  720  and external catheter body  722 . 
     Referring to FIG. 14E, a nanomagnetic material may be coated onto the interior wall of the inner concentrically positioned catheter body  720  and externally wall of externally positioned catheter body  722 . 
     Referring to FIG. 14F, a nanomagnetic material may be coated on the outside surface of the externally positioned catheter body  722 . 
     Referring to FIG. 14G, a nanomagnetic material may be coated onto the exterior surface of an internally positioned solid element  727 . 
     By way of further illustration, one may apply nanomagnetic particulate material to one or more of the catheter assemblies disclosed and claimed in U.S. Pat. Nos. 5,178,803, 5,041,083, 6,283,959, 6,270,477, 6,258,080, 6,248,092, 6,238,408, 6,208,881, 6,190,379, 6,171,295, 6,117,064, 6,019,736, 6,017,338, 5,964,757, 5,853,394, and 6,235,024, the entire disclosure of which is hereby incorporated by reference into this specification. The catheters assemblies disclosed and claimed in the above-mentioned United States patents may be shielded by coating them in whole or in part with a coating of nanomagmetic particulate. The modified catheter assemblies thus produced are resistant to electromagnetic radiation. 
     FIGS. 15A,  15 B, and  15 C are schematic views of a guide wire assembly  800  for insertion into vascular vessel (not shown), and it is similar to the assembly depicted in U.S. Pat. No. 5,460,187, the entire disclosure of such patent is incorporated by reference into this specification. Referring to FIG. 15A, a coiled guide wire  810  is formed of a proximal section (not shown) and central support wire  820  which terminates in hemispherical shaped tip  815 . The proximal end has a retaining device (not shown) enables the person operating the guide wire to turn an orient the guide wire within the vascular conduit. 
     The guide wire assembly may be shielded by coating it in whole or in part with a coating of nanomagnetic particulate, in any of the following manners: 
     Referring to FIG. 15A; the nanomagnetic material  650  is coated on the exterior surface of the coiled guidewire  810 . 
     Referring to FIG. 15B; the nanomagnetic material  650  is coated on the exterior surface of the central support wire  820 . 
     Referring to FIG. 15C; the nanomagnetic material  650  is coated on all guide wire assembly components including coiled guide wire  810 , tip  815 , and central support wire  820 . 
     The modified guide wire assembly thus produced is resistant to electromagnetic radiation. 
     By way of further illustration, one may coat with nanomagnetic particulate matter the guide wire assemblies disclosed and claimed in U.S. Pat. Nos. 5,211,183, 6,168,604, 6,093,157, 6,019,737, 6,001,068, 5,938,623, 5,797,857, 5,588,443, and 5,452,726 the entire disclosure of which is hereby incorporated by reference into this specification. The modified guide wire assemblies thus produced are resistant to electromagnetic radiation. 
     FIGS. 16A and 16B are schematic views of a medical stent assembly  900  similar to the assembly depicted in FIG. 15 of U.S. Pat. No. 5,443,496; the entire disclosure of such patent is hereby incorporated by reference into this specification. 
     Referring to FIG. 16A, a self-expanding stent  900  comprising joined metal stent elements  962  are shown. The stent  960  also comprises a flexible film  964 . The flexible film  964  can be applied as a sheath to the metal stent elements  962  after which the stent  960  can be compressed, attached to a catheter, and delivered through a body lumen to a desired location. Once in the desired location, the stent  960  can be released from the catheter and expanded into contact with the body lumen, where it can conform to the curvature of the body lumen. The flexible film  964  is able to form folds, which allow the stent elements to readily adapt to the curvature of the body lumen. The medical stent assembly disclosed and claimed in U.S. Pat. No. 5,443,496 may be shielded by coating it in whole or in part with a nanomagnetic coating in any of the following manners: 
     Referring to FIG. 16A, flexible film  964  may be coated with a nanomagnetic coating on its inside or outside surfaces, or within the film itself. 
     In one embodiment, a stent (not shown) is coated with a nanomagnetic material. 
     It is to be understood that any one of the above embodiments may be used independently or in conjunction with one another within a single device. 
     In yet another embodiment (not shown), a sheath (not shown), coated or imbibed with a nanomagnetic material is placed over the stent, particularly the flexible film  964 , to shield it from electromagnetic interference. In this manner, existing stents can be made MRI safe and compatible. 
     By way of illustration and not limitation, one may coat one or more of the medical stent assemblies disclosed and claimed in U.S. Pat. Nos. 6,315,794, 6,190,404, 5,968,091, 4,969,458, 6,342,068, 6,312,460, 6,309,412, and 6,305,436, the entire disclosure of each of which is hereby incorporated by reference into this specification. The medical stent assemblies disclosed and claimed in the above-mentioned United States patents may be shielded by coating them in whole or in part with a coating of nanomagmetic particulate, as described above. The modified medical stent assemblies thus produced are resistant to electromagnetic radiation. 
     FIG. 17 is a schematic view of a biopsy probe assembly  1000  similar to the assembly depicted in FIG. 1 of U.S. Pat. No. 5,005,585 the entire disclosure of such patent is hereby incorporated by reference into this specification. 
     Referring to FIG. 17, the biopsy probe assembly is composed of three separate components, a hollow tubular cannula or needle  1001 , a solid intraluminar rod-like stylus  1002 , and a clearing rod or probe (not shown). 
     The components of the assembly are preferably formed of an alloy, such as stainless steel, which is corrosion resistant and non-toxic. Cannula  1001  has a proximal end (not shown) and a distal end  1005  that is cut at an acute angle with respect to the longitudinal axis of the cannula and provides an annular cutting edge. 
     By way of further illustration, biopsy probe assemblies are disclosed and claimed in U.S. Pat. Nos. 4,671,292, 5,437,283, 5,494,039, 5,398,690, and 5,335,663, the entire disclosure of each of which is hereby incorporated by reference into this specification. The biopsy probe assemblies disclosed and claimed in the above-mentioned United States patents may be shielded by coating them in whole or in part with a coating of nanomagmetic particulate, in any of the following manners: Cannula  1001  may be coated, intraluminar stylus  1002  may be coated, and/or the clearing rod may be coated. 
     In one variation on this design (not shown), a biocompatible sheath is placed over the coated cannula  1001  to protect the nanomagnetic coating from abrasion and from contacting body fluids. 
     In one variation on this design (not shown), the biocompatible sheath has on its interior surface or within its walls a nanomagnetic coating. 
     In yet another embodiment (not shown), a sheath (not shown), coated or imbibed with a nanomagnetic material is placed over the biopsy probe, to shield it from electromagnetic interference. In this manner, existing stents can be made MRI safe and compatible. 
     The modified biopsy probe assemblies thus produced are resistant to electromagnetic radiation. 
     FIGS. 18A and 18B are schematic views of a flexible tube endoscope assembly  1180  similar to the assembly depicted in FIG. 1 of U.S. Pat. No. 5,058,567 the entire disclosure of such patent is hereby incorporated by reference into this specification. 
     MRI is increasingly being used interoperatively to guide the placement of medical devices such as endsocpes which are very good at treating or examining tissues close up, but generally cannot accurately determine where the tissues being examined are located within the body. 
     Referring to FIG. 18A, the endoscope  1100  employs a flexible tube  1110  with a distally positioned objective lens  1120 . Flexible tube  1110  is preferably formed in such manner that the outer side of a spiral tube is closely covered with a braided-wire tube (not shown) formed by weaving fine metal wires into a braid. The spiral tube is formed using a precipitation hardening alloy material, for example, beryllium bronze (copper-beryllium alloy). 
     By way of further illustration, endoscope tube assemblies are disclosed and claimed in U.S. Pat. Nos. 4,868,015, 4,646,723, 3,739,770, 4,327,711, and 3,946,727, the entire disclosure of each of which is hereby incorporated by reference into this specification. The endoscope tube assemblies disclosed and claimed in the above-mentioned United States patents may be shielded by coating them in whole or in part with a coating of nanomagmetic particulate, in any of the following manners: 
     Referring to FIG. 18A; sheath  1180  is a sheath coated with nanomagnetic material on its inside surface  650   a , exterior surface  650   b , or imbibed into its structure  650   c ; and such sheath is placed over the endoscope, particularly the flexible tube  1110 , to shield it from electromagnetic interference. 
     In yet another embodiment (not shown), flexible tube  1110  is coated with nanomagnetic materials on its internal surface, or imbibed with nanomagnetic materials within its wall. 
     In another embodiment (not shown), the braided-wire element within flexible tube  1110  is coated with a nanomagnetic material. 
     In this manner, existing endoscopes can be made MRI safe and compatible. The modified endoscope tube assemblies thus produced are resistant to electromagnetic radiation. 
     FIG. 19 is a schematic illustration of a sheath assembly  2000  comprised of a sheath  2002  whose surface  2004  is comprised of a multiplicity of nanomagnetic material  2006 ,  2008 , and  2010 . In one embodiment, the nanomagnetic material consists of or comprises nanomagnetic liquid crystal material. Additionally, nanomagnetic materials  2006 ,  2008 , and  2010  may be placed on the inside surface of sheath  2002 , imbibed into the wall of sheath  2002 , or any combination of these locations. 
     The sheath  2002  may be formed from electrically conductive materials that include metals, carbon composites, carbon nanotubes, metal-coated carbon filaments (wherein the metal may be either a ferromagnetic material such as nickel, cobalt, or magnetic or non-magnetic stainless steel; a paramagnetic material such as titanium, aluminum, magnesium, copper, silver, gold, tin, or zinc; a diamagnetic material such as bismuth, or well known superconductor materials), metal-coated ceramic filaments (wherein the metal may be one of the following metals: nickel, cobalt, magnetic or non-magnetic stainless steel, titanium, aluminum, magnesium, copper, silver, gold, tin, zinc, bismuth, or well known superconductor materials, a composite of metal-coated carbon filaments and a polymer (wherein the polymer may be one of the following: polyether sulfone, silicone, polymide, polyvinylidene fluoride, epoxy, or urethane), a composite of metal-coated ceramic filaments and a polymer (wherein the polymer may be one of the following: polyether sulfone, silicone, polymide, polyvinylidene fluoride, epoxy, or urethane), a composite of metal-coated carbon filaments and a ceramic (wherein the ceramic may be one of the following: cement, silicates, phosphates, silicon carbide, silicon nitride, aluminum nitride, or titanium diboride), a composite of metal-coated ceramic filaments and a ceramic (wherein the ceramic may be one of the following: cement, silicates, phosphates, silicon carbide, silicon nitride, aluminum nitride, or titanium diboride), or a composite of metal-coated (carbon or ceramic) filaments (wherein the metal may be one of the following metals: nickel, cobalt, magnetic or non-magnetic stainless steel, titanium, aluminum, magnesium, copper, silver, gold, tin, zinc, bismuth, or well known superconductor materials), and a polymer/ceramic combination (wherein the polymer may be one of the following: polyether sulfone, silicone, polymide, polyvinylidene fluoride, or epoxy and the ceramic may be one of the following: cement, silicates, phosphates, silicon carbide, silicon nitride, aluminum nitride, or titanium diboride). 
     In one preferred embodiment, the sheath  2002  is comprised of at least about 50 volume percent of the nanomagnetic material described elsewhere in this specification. 
     As is known to those skilled in the art, liquid crystals are nonisotrpic materials (that are neither crystalline nor liquid) composed of long molecules that, when aligned, are parallel to each other in long clusters. These materials have properties intermediate those of crystalline solids and liquids. See, e.g., page 479 of George S. Brady et al.&#39;s “Materials Handbook,” Thirteenth Edition (McGraw-Hill, Inc., New York, 1991). 
     Ferromagnetic liquid crystals are known to those in the art, and they are often referred to as FMLC. Reference may be had, e.g., to U.S. Pat. Nos. 4,241,521, 6,451,207, 5,161,030, 6,375,330, 6,130,220, and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification. 
     Reference also may be had to U.S. Pat. No. 5,825,448, which describes a reflective liquid crystalline diffractive light valve. The figures of this patent illustrate how the orientations of the magnetic liquid crystal particles align in response to an applied magnetic field. Referring again to FIG. 19A, and in the embodiment depicted therein, it will be seen that sheath  2002  may be disposed in whole or in part over medical device  2012 . In the embodiment depicted, the sheath  2002  is shown as being bigger that the medical device  2012 . It will be apparent that such sheath  2002  may be smaller than the medical device  2012 , may be the same size as the medical device  2012 , may have a different cross-sectional shape than the medical  2012 , and the like. 
     In one preferred embodiment, the sheath  2002  is disposed over the medical device  2012  and caused to adhere closely thereto. One may create this adhesion either by use of adhesive(s) and/or by mechanical shrinkage. 
     In one embodiment, shrinkage of the sheath  2012  is caused by heat, utilizing well known shrink tube technology. Reference may be had, e.g., to U.S. Pat. Nos. 6,438,229, 6,245,053, 6,082,760, 6,055,714, 5,903,693, and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification. 
     In another embodiment of the invention, the sheath  2002  is a rigid or flexible tube formed from polytetrafluoroethylene that is heat shrunk into resilient engagement with the implantable medical device. The sheath can also be formed from heat shrinkable polymer materials e.g., low density polyethylene (LDPE), linear low-density polyethylene (LLDPE), ethylene vinyl acrylate (EVA), ethylene methacrylate (EMA), ethylene methacrylate acid (EMAA) and ethyl glycol methacrylic acid (EGMA). The polymer material of the heat shrinkable sheath should have a Vicat softening point less than 50 degrees Centigrade and a melt index less than 25. A particularly suitable polymer material for the sheath of the invention is a copolymer of ethylene and methyl acrylate which is available under the trademark Lotryl 24MA005 from Elf Atochem. This copolymer contains 25% methyl acrylate, has a Vicat softening point of about 43 degree centigrade and a melt index of about 0.5. 
     In another embodiment of the invention, the sheath  2002  is a collapsible tube that can be extended over the implantable medical device such as by unrolling or stretching. 
     In yet another embodiment of the invention, the sheath  2002  contains a tearable seam along its axial length, to enable the sheath to be withdrawn and removed from the implantable device without explanting the device or disconnecting the device from any attachments to its proximal end, thereby enabling the electromagnetic shield to be removed after the device is implanted in a patient. This is a preferable feature of the sheath, since it eliminates the need to disconnect any devices connected to the proximal (external) end of the device, which could interrupt the function of the implanted medical device. This feature is particularly critical if the shield is being applied to a life-sustaining device, such as a temporary implantable cardiac pacemaker. 
     The ability of the sheath  1180  or  2002  to be easily removed, and therefore easily disposed, without disposing of the typically much more expensive medical device being shielded, is a preferred feature since it prevents cross-contamination between patients using the same medical device. 
     In still another embodiment of the invention, an actively circulating, heat-dissipating fluid can be pumped into one or more internal channels within the sheath. The heat-dissipation fluid will draw heat to another region of the device, including regions located outside of the body where the heat can be dissipated at a faster rate. The heat-dissipating flow may flow internally to the layer of nanomagnetic particles, or external to the layer of nanomagnetic particulate material. 
     FIG. 19B illustrates a process  2001  in which heat  2030  is applied to a shrink tube assembly  2003  to produce the final product  2005 . For the sake of simplicity of representation, the controller  2007  has been omitted from FIG.  19 B. 
     Referring again to FIG. 19A, and in the preferred embodiment depicted therein, it will be seen that a controller  2007  is connected by switch  2009  to the sheath  2002 . The controller  2007 . A multiplicity of sensors  2014  and  2016 , e.g., can detect the effectiveness of sheath  2002  by measuring, e.g., the temperature and/or the electromagnetic field strength within the shield  2012 . One or more other sensors  2018  are adapted to measure the properties of sheath  2012  at its exterior surface  2004 . 
     For the particular sheath embodiment utilizing a liquid crystal nanomagnetic particle construction, and depending upon the data received by controller  2007 , the controller  2007  may change the shielding properties of shield  2012  by delivering electrical and/or magnetic energy to locations  2020 ,  2022 ,  2024 , etc. The choice of the energy to be delivered, and its intensity, and its location, and its duration, will vary depending upon the status of the sheath  2012 . 
     In the embodiment depicted in FIG. 19, the medical device may be moved in the direction of arrow  2026 , while sheath may be moved in the direction of arrow  2028 , to produce the assembly  2001  depicted in FIG.  19 B. Thereafter, heat may be applied to this assembly to produce the assembly  2005  depicted in FIG.  19 B. 
     In one embodiment, not shown, the sheath  2002  is comprised of an elongated element consisting of a proximal end and a distal end, containing one or more internal hollow lumens, whereby the lumens at said distal end may be open or closed, is used to temporarily or permanently encase an implantable medical device. 
     In this embodiment, the elongated hollow element is similar to the sheath disclosed and claimed in U.S. Pat. No. 5,964,730; the entire disclosure of which is hereby incorporated by reference into this specification. 
     Referring again to FIG. 19A, and in the embodiment depicted therein, the sheath  2002  is preferably coated and/or impregnated with nanomagnetic shielding material  2006 / 2008 / 2010  that comprises at least 50 percent of its external surface, and/or comprises at least 50 percent of one or more lumen internal surfaces, or imbibed within the wall  2015  of sheath  2002 , thereby protecting at least fifty percent of the surface area of one or more of its lumens, or any combination of these surfaces or areas, thus forming a shield against electromagnetic interference for the encased medical device. 
     FIG. 20A is a schematic of a multiplicity of liquid crystals  2034 ,  2036 ,  2038 ,  2040 , and  2042  disposed within a matrix  2032 . As will be apparent, each of these liquid crystals is comprised of nanomagnetic material  2006  . In the configuration illustrated in FIG. 20A, the liquid crystals  2034  et seq. are not aligned. 
     By comparison, in the configuration depicted in FIG. 20B, such liquid crystals  2034  are aligned. Such alignment is caused by the application of an external energy field (not shown). 
     The liquid crystals disposed within the matrix  2032  may have different concentrations and/or compositions of nanomagnetic particles  2006 ,  2009 , and/or  2010 ; see FIG.  20 C and liquid crystals  2044 ,  2046 ,  2048 ,  2050 , and  2052 . Alternatively, or additionally, the liquid crystals may have different shapes; see FIGS. 20D,  20 E, and  20 F and liquid crystals  2054  and  2056 ,  2058 ,  2060 ,  2062 ,  2064 , and  2066  . As will be apparent, by varying the size, shape, number, location, and/or composition of such liquid crystals, one may custom design any desired response. 
     FIG. 21 is a graph of the response of a typical matrix  2032  comprised of nanomagnetic liquid crystals. Three different curves, curves  2068 ,  2070 , and  2072 , are depicted, and they correspond to the responses of three different nanomagnetic liquid crystal materials have different shapes and/or sizes and/or compositions. 
     Referring to FIG. 21, and for each of curves  2068  through  2072 , it will be seen that there is often a threshold point  2074  below which no meaningful response to the applied magnetic field is seen; see, e.g., the response for curve  2070 . 
     It should be noted, however, that some materials have a low threshold before they start to exhibit response to the applied magnetic field; see, e.g., curve  2068 . On the other hand, some materials have a very large threshold; sec, e.g., threshold  2076  for curve  2072 . 
     One may produce any desired response curve by the proper combination of nanomagnetic material composition, concentration, and location as well as liquid crystal geometries, materials, and sizes. Other such variables will be apparent to those skilled in the art. 
     Referring again to FIG. 21, it will be seen that there often is a monotonic region  2078  in which the increase of alignment of the nanomagnetic material is monotonic and often directly proportional; see, e.g., curve  2070 . 
     There also is often a saturation point  2080  beyond which an increase in the applied magnetic field does not substantially increase the alignment. 
     As will be seen from the curves in FIG. 21, the process often is reversible. One may go from a higher level of alignment to a lower level by reducing the magnetic field applied. 
     The frequency of the magnetic field applied also influences the degree of alignment. As is illustrated in FIG. 22, for one nanomagnetic liquid crystal material (curve  2082 ), the response is at a maximum at an initial frequency  2086  but then decreases to a minimum at frequency  2088 . By comparison, for another such curve (curve  2090 ), the response is minimum at frequency  1086 , increases to a maximum at point  2098 , and then decreases to a minimum at point  2092 . 
     Thus, one may influence the response of a particular nanomagnetic liquid crystal material by varying its type of nanomagnetic material, and/or its concentration, and/or its shape, and/or the frequency to which it is subjected. Referring again to FIG. 19 A, one may affect the shielding effectiveness of shield  2002  by supplying a secondary magnetic field (from controller  2007 ) at the secondary frequencies which will elicit the desired shielding effect. 
     FIG. 23 is a flow diagram illustrating a preferred process  2094  for making nanomagnetic liquid crystal material. 
     Referring to FIG. 23, and in step  2100 , the nanomagnetic material of this invention is charged to a mixer  2102  via line  2104 . Thereafter, suspending medium is also charged to the mixer  2102  via line  2106 . 
     The suspending medium may be any medium in which the nanomagnetic material is dispersible. Thus, e.g., the suspending medium may be a gel, it may be an aqueous solution, it may be an organic solvent, and the like. In one embodiment, the nanomagnetic material is not soluble in the suspending medium; in this embodiment, a slurry is produced. For the sake of simplicity of description, the use of a polymer will be described in the rest of the process. 
     Referring again to FIG. 23, the slurry from mixer  2102  is charged via line  2108  to mixer  2110 . Thereafter, or simultaneously, polymeric precursor of liquid crystal material is also charged to mixer  2108  via line  2112 . 
     As is known to those skilled in the art, aromatic polyesters (liquid crystals) may b used as such polymeric precursor. These aromatic polyesters are commercially available as, e.g., Vectra (sold by Hoechst Celanese Engineering Plastic), Xydur (sold by Amoco Performance Plastics), Granlar (sold by Granmont), and the like. Reference may be had, e.g., to pages 649-650 of the aforementioned “Materials Handbook.” Reference also may be had, e.g., to U.S. Pat. Nos. 4,738,880, 5,142,017, 5,006,402, 4,935,833, and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification. 
     Referring again to FIG. 23, the liquid crystal polymer is mixed with the nanomagnetic particles for a time sufficient to produce a substantially homogeneous mixture. Typically, mixing occurs from about 5 to about 60 minutes. 
     The polymeric material formed in mixer  2110  then is formed into a desired shape in former  2112 . Thus, and referring to Joel Frados&#39; “Plastics Engineering Handbook,” Fourth Edition (Van Nostrand Reinhold Company, New York, N.Y., 1976), one may form the desired shape by injection molding, extrusion, compression and transfer molding, cold molding, blow molding, rotational molding, casting, machining, joining, and the like. Other such forming procedures are well known to those skilled in the art. 
     One may prepare several different nanomagnetic structures and join them together to form a composite structure. One such composite structure is illustrated in FIG.  24 . 
     Referring to FIG. 24, assembly  2120  is comprised of nanomagnetic particles  2006  ,  2010 , and  2008  disposed in layers  2122 ,  2124 , and  2126 , respectively. In the embodiment depicted, the layers  2122 ,  2124 , and  2126  are contiguous with each, thereby forming a continuous assembly of nanomagnetic material, with different concentrations and compositions thereof at different points. The response of assembly  2120  to any particular magnetic field will vary depending upon the location at which such response is measured. 
     FIG. 25 illustrates an assembly  2130  that is similar to assembly  2120  but that contains an insulating layer  2132  disposed between nanomagnetic layers  2134  and  2136 . The assembly  2130   
     The insulating layer  2132  may be either electrically insulative and/or thermally insulative. 
     FIG. 26 illustrates an assembly  2140  in which the response of nanomagnetic material  2142  is sensed by sensor  2144  that, in the embodiment depicted, is a pickup coil  2144 . Data from sensor  2144  is transmitted to controller  2146 . When and as appropriate, controller  2146  may introduce electrical and/or magnetic energy into shielding material  2142  in order to modify its response. 
     FIG. 27 is a schematic illustration of an assembly  2150 . In the embodiment depicted, concentric insulating layers  2152  and  2154  preferably have substantially different thermal conductivities. Layer  2152  preferably has a thermal conductivity that is in the range of from about 10 to about 2000 calories per hour per square centimeter per centimeter per degree Celsius. Layer  2154  has a thermal conductivity that is in the range of from about 0.2 to about 10 calories per hour per square centimeter per centimeter per degree Celsius. Layers  2152  and  2154  are designed by choice of thermal conductivity and of layer thickness such that heat is conducted axially along, and circumferentially around, layer  2152  at a rate that is between 10 times and 1000 times higher than in layer  2154 . Thus, in this embodiment, any heat that is generated at any particular site or sites in one or more nanomagnetic shielding layers will be distributed axially along the shielded element, and circumferentially around it, before being conducted radially to adjoining tissues. This will serve to further protect these adjoining tissues from thermogenic damage even if there are minor local flaws in the nanomagnetic shield. Thus, in one embodiment of the invention, there is described a magnetically shielded conductor assembly, that contains a conductor, at least one layer of nanomagnetic material, a first thermally insulating layer, and a second thermally insulating layer. The first thermal insulating layer resides radially inward from said second thermally insulating layer, and it has a thermal conductivity from about 10 to about 2000 calories-centimeter per hour per square centimeter per degree Celsius The second thermal insulating layer has a thermal conductivity from about 0.2 to about 10 calories per hour per square centimeter per degree Celsius, and the axial and circumferential heat conductance of the first thermal insulating layer is at least about 10 to about 1000 times higher than it is for said second thermal insulating layer. 
     In another embodiment of the invention, there is provided a magnetically shielded conductor assembly as discussed hereinabove, in which the first thermally insulating layer is disposed between said conductor and said layer of nanomagnetic material, and the second thermally insulating layer is disposed outside said layer of nanomagnetic material. 
     In another embodiment, there is provided a magnetically shielded conductor assembly as discussed hereinabove wherein the first thermally insulating layer is disposed outside the layer of nanomagnetic material, and wherein the second thermally insulating layer is disposed outside said first layer of thermally insulating material. 
     In another embodiment, the shield is comprised of a abrasion-resistant coating comprised of nanomagnetic material. Referring to FIG. 28, it will be seen that shield  2170  is comprised of abrasion resistant coating  2172  and nanomagnetic layer  2174 . 
     A Composite Shield 
     In this portion of the specification, applicants will describe one embodiment of a composite shield of their invention This embodiment involves a shielded assembly comprised of a substrate and, disposed above a substrate, a shield comprising from about 1 to about 99 weight percent of a first nanomagnetic material, and from about 99 to about 1 weight percent of a second material with a resistivity of from about 1 microohm-centimeter to about 1×10 25  microohm centimeters. 
     FIG. 29 is a schematic of a preferred shielded assembly  3000  that is comprised of a substrate  3002 . The substrate  3002  may be any one of the substrates illustrated hereinabove. Alternatively, or additionally, it may be any receiving surface which it is desired to shield from magnetic and/or electrical fields. Thus, e.g., the substrate can be substantially any size, any shape, any material, or any combination of materials. The shielding material(s) disposed on and/or in such substrate may be disposed on and/or in some or all of such substrate. 
     By way of illustration and not limitation, the substrate  3002  may be, e.g., a foil comprised of metallic material and/or polymeric material. The substrate  3002  may, e.g., comprise ceramic material, glass material, composites, etc. The substrate  3002  may be in the shape of a cylinder, a sphere, a wire, a rectilinear shaped device (such as a box), an irregularly shaped device, etc. 
     In one embodiment, the substrate  3002  preferably a thickness of from about 100 nanometers to about 2 centimeters. In one aspect of this embodiment, the substrate  3002  preferably is flexible. 
     Referring again to FIG. 29, and in the preferred embodiment depicted therein, it will be seen that a shield  3004  is disposed above the substrate  3002 . As used herein, the term “above” refers to a shield that is disposed between a source  3006  of electromagnetic radiation and the substrate  3002 . 
     The shield  3004  is comprised of from about 1 to about 99 weight percent of nanomagnetic material  3008 ; such nanomagnetic material, and its properties, are described elsewhere in this specification. In one embodiment, the shield  3004  is comprised of at least about 40 weight percent of such nanomagnetic material  3008 . In another embodiment, the shield  3004  is comprised of at least about 50 weight percent of such nanomagnetic material  3008 . 
     Referring again to FIG. 29, and in the preferred embodiment depicted therein, it will be seen that the shield  3004  is also comprised of another material  3010  that preferably has an electrical resistivity of from about about 1 microohm-centimeter to about 1×10 25  microohm-centimeters. This material  3010  is preferably present in the shield at a concentration of from about 1 to about 1 to about 99 weight percent and, more preferably, from about 40 to about 60 weight percent. 
     In one embodiment, the material  3010  has a dielectric constant of from about 1 to about 50 and, more preferably, from about 1.1 to about 10. In another embodiment, the material  3010  has resistivity of from about 3 to about 20 microohm-centimeters. 
     In one embodiment, the material  3010  preferably is a nanoelectrical material with a particle size of from about 5 nanometers to about 100 nanometers. 
     In another embodiment, the material  3010  has an elongated shape with an aspect ratio (its length divided by its width) of at least about 10. In one aspect of this embodiment, the material  3010  is comprised of a multiplicity of aligned filaments. 
     In one embodiment, the material  3010  is comprised of one or more of the compositions of U.S. Pat. No. 5,827,997 and 5,643,670. 
     Thus, e.g., the material  3010  may comprise filaments, wherein each filament comprises a metal and an essentially coaxial core, each filament having a diameter less than about 6 microns, each core comprising essentially carbon, such that the incorporation of 7 percent volume of this material in a matrix that is incapable of electromagnetic interference shielding results in a composite that is substantially equal to copper in electromagnetic interference shielding effectives at 1-2 gigahertz. Reference may be had, e.g., to U.S. Pat. No. 5,827,997, the entire disclosure of which is hereby incorporated by reference into this specification. 
     In another embodiment, the material  3010  is a particulate carbon complex comprising: a carbon black substrate, and a plurality of carbon filaments each having a first end attached to said carbon black substrate and a second end distal from said carbon black substrate, wherein said particulate carbon complex transfers electrical current at a density of 7000 to 8000 milliamperes per square centimeter for a Fe +2 /Fe +3  oxidation/reduction electrochemical reaction couple carried out in an aqueous electrolyte solution containing 6 millmoles of potassium ferrocyanide and one mole of aqueous potassium nitrate. 
     In another embodiment, the material  3010  may be a diamond-like carbon material. As is known to those skilled in the art, this diamond-like carbon material has a Mohs hardness of from about 2 to about 15 and, preferably, from about 5 to about 15. Reference may be had, e.g., to U.S. Pat. No. 5,098,737 (amorphic diamond material), U.S. Pat. No. 5,658,470 (diamond-like carbon for ion milling magnetic material), U.S. Pat. No. 5,731,045 (application of diamond-like carbon coatings to tungsten carbide components), U.S. Pat. No. 6,037,016 (capacitively coupled radio frequency diamond-like carbon reactor), U.S. Pat. No. 6,087,025 (application of diamond like material to cutting surfaces), and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification. 
     In another embodiment, material  3010  is a carbon nanotube material. These carbon nanotubes generally have a cylindrical shape with a diameter of from about 2 nanometers to about 100 nanometers, and length of from about 1 micron to about 100 microns. 
     These carbon nanotubes are well known to those skilled in the art. Reference may be had, e.g., to U.S. Pat. No. 6,203,864 (heterojunction comprised of a carbon nanotube), U.S. Pat. No. 6,361,861 (carbon nanotubes on a substrate), U.S. Pat. No. 6,445,006 (microelectronic device comprising carbon nanotube components), U.S. Pat. No. 6,457,350 (carbon nanotube probe tip), and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification. 
     In one embodiment, material  3010  is silicon dioxide particulate matter with a particle size of from about 10 nanometers to about 100 nanometers. 
     In another embodiment, the material  3010  is particulate alumina, with a particle size of from about 10 to about 100 nanometers. Alternatively, or additionally, one may use aluminum nitride particles, cerium oxide particles, yttrium oxide particles, combinations thereof, and the like; regardless of the particle(s) used, it is preferred that its particle size be from about 10 to about 100 nanometers. 
     In the embodiment depicted in FIG. 29, the shield  3004  is in the form of a layer of material that has a thickness of from about 100 nanometers to about 10 microns. In this embodiment, both the nanomagnentic particles  3008  and the electrical particles  3010  are present in the same layer. 
     In the embodiment depicted in FIG. 30, by comparison, the shield  3012  is comprised of layers  3014  and  3016 . The layer  3014  is comprised of at least about 50 weight percent of nanomagnetic material  3008  and, preferably, at least about 90 weight percent of such nanomagnetic material  3008 . The layer  3016  is comprised of at least about 50 weight percent of electrical material  3010  and, preferably, at least about 90 weight percent of such electrical material  3010 . 
     In the embodiment depicted in FIG. 30, the layer  3014  is disposed between the substrate  3002  and the layer  3016 . In the embodiment depicted in FIG. 31, the layer  3016  is disposed between the substrate  3002  and the layer  3014 . 
     Each of the layers  3014  and  3016  preferably has a thickness of from about 10 nanometers to about 5 microns. 
     In one embodiment, the shield  3012  has an electromagnetic shielding factor of at least about 0.9., i.e., the electromagnetic field strength at point  3020  is no greater than about 10 percent of the electromagnetic field strength at point  3022 . 
     In one preferred embodiment, illustrated in fig. 31, the nanomagnetic material preferably has a mass density of at least about 0.01 grams per cubic centimeter, a saturation magnetization of from about 1 to about 36,000 gauss, a coercive force of from about 0.01 to about 5000 Oersteds, a relative magnetic permeability of from about 1 to about 500,000, and an average particle size of less than about 100 nanometers.