Magnetic Toroids for the Stimulation of Biological Tissue Inventor: Kent R. Davey Related Applications The present application claims all rights of priority to U. S. Provisional Application Serial No. 60/145,062 filed July 22,1999, whose contents are hereby fully incorporated herein by reference.

Field of the Invention The present invention relates to the direction of magnetic flux into biological tissue using magnetic stimulators in the form of closed magnetic circuits. The closed magnetic circuits are capable of generating large rates of change of flux with respect to time with little current. The secondary induced current path is completed by the use of a conducting medium surrounding the closed magnetic structure.

Background of the Invention Currently, magnetic stimulation devices exist which use either an open air or cut ferromagnetic core to drive a rapidly changing magnetic field into a patient. The changing magnetic field induces a changing electric field, causing ions to flow. When positive ions are driven into a nerve cell, they raise the internal potential of the nerve cell and initiate an action potential.

The action of such a device, initiating both an afferent and an efferent action potential, is shown in Figure 1. All of the current that flows is ionic and internal. The price paid for this internal current is a large magnetic field generated by a large time changing current. This is usually accomplished through a circuit that transforms the voltage up, rectifies it to DC, charges a capacitor, and fires the thyrister into the coil.

Summary of the Invention It is an object of the present invention is to provide a simple inexpensive means for stimulating tissue with magnetic fields, which, as opposed to electrical stimulation, is noninvasive and is less irritating to the skin.

It is a further object of the invention to provide a device wherein a closed magnetic circuit is used to induce a changing electric field in the medium enclosing the magnetic circuit.

It is a further object to provide a medical magnetic stimulator for therapeutic use.

It is a further object to provide a system for enclosing a closed magnetic toroid wherein a saline tank or other conducting medium comprises the outer conducting path for the toroid, with the inner conducting path being the biological tissue to be stimulated.

It is a further object to provide a closed magnetic circuit in the form of a toroidal bracelet for providing deep penetration of magnetic flux into biological tissue, particularly for therapeutic use, including the stimulation of nerves, muscles, wound generation, and other applications.

It is a further object of the invention to provide at least two toroids for directing magnetic flux into biological tissue. It is a further object to provide two toroids carrying flux in opposite directions for realizing such localized tissue excitation.

It is a further object to provide a magnetic circuit which offers flexibility of meshing with various body parts, particularly wherein the circuit is a folded toroid.

It is a further object of the present invention to provide a medical magnetic stimulator which uses a saw tooth current waveform. It is a further object to use a medical stimulator in the form of a toroid, wherein a saw tooth current waveform is used to excite the toroid and induce a monotonic E field.

It is a further object of the invention to provide a medical magnetic circuit with material having a high permeability-saturation product, particularly a toroid stimulator including a core of nanocrystalline material.

It is a further object of the invention to provide medical magnetic stimulators for treating female and male sexual dysfunction, and a method for treating the same.

It is a further object of the invention to provide a thermal ablation device for excising biological tissue, particularly wherein the device comprises a small toroidal core placed at the end of a laparoscope.

Further objects of the invention will be apparent in conjunction with the disclosure herein.

In accordance with the present invention, a closed magnetic circuit is excited with a nominal amp-turn product to generate a large magnetic flux. An electric field is induced on all paths that enclose that flux. As long as a closed current path exists around that magnetic flux, current will flow. Tissue stimulation is ensured by guaranteeing that a portion of the path inclues the target tissue. By so doing, the invention avoids a half-cell reaction at an electrode interface. Because the current is never concentrated into one spot, less pain is experienced by the patient and the risk of irritation due to high local current density is minimal.

Other advantages provided by the system are flexibility of waveform choice. A saw tooth current waveform is very difficult to achieve when large currents are involved, but relatively easy when the current is minimal. The sawtooth current induces a monodirectional electric field, interrupted by a very short field reversal. The monodirectional E field is useful for electro-osmotic movement of interstitial fluid within the charged Debye layer near collagen tissue. Such movement is potentially useful for wound treatment and nerve regeneration.

Various shapes of toroid type magnetic circuits can be employed to concentrate current in closer confines. Elongated toroids can be used to drive current into odd shaped regions.

The current is always concentrated through the orifice of the toroid. Two bracelet toroids arranged in opposition can be used to achieve a large electric field spatial gradient within a small space. Two toroids placed alongside one another generate a tighter current return path.

Brief Description of the Drawings Figure 1 is a perspective view of a magnetic stimulation device which employs a cut ferromagnetic core to generate a magnetic field, the device being shown stimulating the nerves of the arm.

Figure 2 is a perspective view of a toroid placed external to a stimulation site on the human anatomy.

Figure 3 is an cross-sectional view of a saline tank with a toroid placed therein, illustrating the field lines of the induced current from the toroid.

Figure 4 is an cross-sectional view of a toroid placed on the skin, the toroid and skin being immersed in a saline tank, illustrating the field lines of the induced current from the toroid.

Figure 5 is a perspective view of a toroid constructed as a"bracelet"and placed around an arm or a leg.

Figure 6 is a perspective view of a double toroid assembly placed around an arm or a leg, which forces a tighter gradient in the induced electric field.

Figure 7 is a perspective view of a folded toroid, which allows for deep penetration into the anatomy with large E field gradients.

Figure 8 is a graph illustrating a typical current and the commensurate E field used to excite the toroid in wound healing devices.

Figure 9 is a perspective view of internal toroids suspended in an agar gel and fired in a 3 phase delayed pattern to accomplish vaginal floor stimulation.

Figure 10 is a perspective view of a toroid laparoscope with a toroid advanced past the end for ablative RF heating.

Detailed Description of the Invention and the Preferred Embodiments In accordance with the present invention, closed magnetic circuits are provided for inducing a changing electric field in the medium surrounding the magnetic circuit, and particularly in biological tissue. Further in accordance wih the invention, methods are provided wherein the circuits are used for inducing action potentials for nerve and muscle stimulation; for fostering wound healing; for stimulating bone and soft tissue regeneration; for treatment of male and female sexual dysfunction; for application of heat to biological tissue, including tissue ablation; or so forth.

Figure 1 is an illustration of a magnetic stimulation device, in accordance with previous work by the present inventor, which employs a cut ferromagnetic core to generate a magnetic field. The device is shown with the cut core placed over the arm, to stimulate the nerves leading to the arm muscles. A large electric current is pulsed through the core winding.

Commensurate with this current is a magnetic field that penetrates into the arm, as shown by the dotted lines. Since the magnetic field (B) is changing with time, it induces an electric field within the polyelectrolytic solution of the body, resulting in ion flow. When the ions enter a nerve cell, they raise or lower the intracellular potential. An increase of the intracellular potential initiates an action potential which propagates away from the stimulation site. The object of the present invention is to induce the currents within the body with a smaller magnetic field driving current.

As shown in Figure 2, a ferromagnetic toroid 1 is provided in accordance with the present invention, the toroid being shown placed on top of the skin of a person's hand. If desired, the toroid can be placed on any other area of human or animal anatomy, or it can be inserted into an appropriate bodily orifice or cavity. The apparatus can also be implante inside the anatomy if necessary, e. g. under the skin or in any other suitable location.

The toroid 1 is wound with copper wire 2 so that the magnetic field is azimuthal. A time varying current is injected through the winding of copper wire 2. When the toroid and the arm are immersed in a saline tank 3, a return path becomes available for ions to flow outside the hand. Ions flow instantaneously opposite to the current on the winding of the toroid. If the current at time to is entering through the hole of the toroid, positive ions will flow out of the hole, and negative ions will flow in. All that is required for proper function is for a complete conducting path to link the magnetic toroid.

Instead of saline, a suitable conducting path or secondary conducting medium 3 is an agar gel. Alternatively, the conductive medium can be another electrolytic or electrically conductive material (whether a paste, gel, or solution), including, for example, EKG paste or so forth. Folding the skin (when the skin is coated with the conductive material) around a portion of a simple toroid or elongated toroid, whether around an edge and/or into the center, etc., can also be used to provide a simple closure path for current.

Figure 3 illustrates the current which flows when the toroid is placed within a saline tank, with a time changing current being injected through the toroid. The current contour lines 4 will flow in a circular path around the toroid. If the toroid is coated with a nonconductive coating, ions are prevented from entering the toroid and are forced to flow around the perimeter.

In accordance with the method of the present invention, in actual usage the toroid is placed on the anatomy (e. g. on top of the skin), or within the anatomy (e. g. within the vaginal canal). Figure 4 illustrates the placement of the toroid on the skin, with the toroid and skin being immersed in a saline tank. The figure depicts an axi-symmetric analysis. Thus, eventhough the conductive media shown on the top right side and on the bottom right side are not explicitly shown on the left half, they are assumed to be present there as well.

With the outside of the toroid 1 being immersed in saline 3, ions can still flow in a circular path around the toroid. The nerve and body tissue has a conductivity that is generally lower than saturated saline. Thus, in Figure 4, the interstitial fluid 5 of the desired portion of the anatomy, will, in general, have a lower conductivity. A conductivity discontinuity exists at the interface. The normal component of current density J must be continuous through this interface. The tangential components of this E field must also be continuous across the interface. These boundary conditions cause the current to exhibit a bend at the interface. The current always bends towards the normal on the side of the fluid with the lower conductivity as shown in Figure 4.

There are many instances where deeper penetration is required. In accordance with a further embodiment of the invention, shown in Figure 5, a toroid is provided in the form of a"bracelet"placed around an arm or leg appendage 6. A secondary conducting medium 3 is again required to complete the current path. The induced current 4 travels into the center of the appendage. This current will not penetrate bone, but it will make it to the axis of the appendage if allowed. This is a significant difference from conventional cut core stimulators.

It is possible to show mathematically that the cut core or air core stimulator can never induce current on the axis of a cylinder or in the center of a sphere. Such restrictions are circumvented using the toroid and the external conducting medium.

With long axons, it is the gradient of the electric field that initiates an action potential.

It is where the electric field changes that the ions enter or leave the nerve cell. A stronger gradient may be realized by stacking two toroids carrying flux in opposite directions as shown in Figure 6. The current lines 4 change rapidly midway between the two stacked toroids.

Yet another alternative that accomplishes the stronger gradient in the electric field is to fold the toroid as shown, for example, in Figure 7. The fold allows the ferromagnetic carrier to conform around a larger tissue mass and/or to closely mesh with or conform to desired body parts. The real criteria is that the ferromagnetic flux carrier be continuous with no air gaps. In all such cases, the magnetic field will not penetrate the target biological tissue.

But Faraday's law demands that The changing magnetic field requires an electric field. When the path enclosing the B field is conductive throughout, a current will flow. This current, in the form of ion flow, initiates an action potential. The induced E field causes free ions to flow. When the biological tissue is charged (as collagen is in a nonisostatic pH environment), interstitial fluid also moves. This electroosmotic movement, known as a streaming potential, is believed to be instrumental in wound healing. A monotonically directed E field is believed to be instrumental towards that end. The current required to excite the toroid should look like that pictured in Figure 8. This waveform has been shown to be instrumental in external osteogenesis applications.

In accordance with the invention, the toroids should have both a high B field saturation level, and a high permeability. The high saturation insures that a large flux can be forced through a small core cross-section. The high permeability ensures that a small current is required to generate the flux. This translates into stimulation with high efficiency with small winding resistive losses. Preferably, nanocrystalline material is used, as it has a high product of these two quantities. To keep the magnetic field eddy current losses small, the core should be wound in a continuous spool with 1 to 4 mil thick nanocrystalline tape, impregnated with an epoxy paste.

Sexual dysfunction is somewhat common among women, especially those on antidepressants. Direct stimulation of the appropriate muscle groups is one potential means of treating the problem. Figure 9 shows a rod or vaginal probe 12 for insertion into the vaginal canal, with three toroids 7,8 and 9 suspended in an agar gel 3 or simply filled with electrolytic paste. The gel/conducting paste 3 ensures a continuous electrical circuit with the primary electrical stimulation occurring on the vaginal walls. When the toroids are excited in sequence-e. g., 7,8,9, pause, 9,8,7-a wave-like contraction results. This controlled contraction may be instrumental in arousing sexual function. The toroids may also be placed on a line external to the vagina at the top of the vulva. The same sequence may be placed around the penis to likewise create a wave-like contraction for treatment of male sexual dysfunction and/or stimulation in quadriplegic patients.

In a further embodiment, injected RF currents can also be employed for ablation in various applications requiring tissue excision. In this embodiment, a very small toroid is placed at the tip of a toroid, as shown in Figure 10. The induced current is forced through the orifice of the toroid. At high frequencies, the induced current is very effective in heating tissue near this orifice. Body polyelectrolytic fluid provides the closure path around the toroid.

Having described this invention with regard to specific embodiments, it is to be understood that the description is not meant as a limitation since further modifications and variations may be apparent or may suggest themselves. It is intended that the present application cover all such modifications and variations, including those as fall within the scope of the appended claims.

References The following references, which relate to magnetic stimulation, are fully incorporated herein by reference: U. S. Patent Documents 5,725,471 Mar 1998 Davey 4,940,453 Jul. 1990 Cadwell 5,066,272 Nov. 1991 Eaton et al.

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