Heating of magnetic material by hysteresis effects

Details Heating of magnetic material by hysteresis effects Heating of magnetic material by hysteresis effects

2001-12-20

2003-07-29

Gilbert, Samuel G. (Department: 3736)

Surgery

Magnetic field applied to body for therapy

Magnetic element placed within body

C607S103000

Reexamination Certificate

active

06599234

ABSTRACT:

FIELD OF INVENTION
The present invention relates to magnetic materials. More particularly, the invention relates to magnetic materials that exhibit high magnetic hysteresis heating in a cyclic magnetic field.
BACKGROUND ART
Each time a ferromagnetic material is exposed to a magnetic field whose amplitude and/or direction varies cyclically in time a small amount of energy is dissipated as heat due to magnetic hysteresis effects. The more rapidly the field is cycled, the greater the rate at which heat is produced by the material. By improving the rate at which a magnetic material heats, it is possible to maximise the potential uses to which this technology may be applied.
One means that has been used to increase the rate at which heat is produced by a magnetic material is to apply a rotating magnetic field to that material rather than the more usual linear alternating field. In our earlier co-owned patent application (“Improved Targeted Hysteresis Hyperthermia for Treating Diseased Tissue”), we have shown that under certain conditions of magnetic field strength and frequency, rotating fields cause far greater heating of magnetic materials compared to a linear alternating field, hence greater heating efficiency is achieved.
Another means to increase the rate at which heat is produced by a material is to improve the magnetic profile of the material and to select materials that display high heating efficiency. Thus, the need to maximise the heating efficiency of the magnetic particle is paramount.
Magnetic materials with an improved magnetic heating efficiency have application in any circumstances where localised heating of unexposed areas is required. For example, the materials may be used in such diverse situations as in rapid heating of cements or epoxies or in the treatment of cancer by hyperthermia therapy.
Where rapid heating of cements or epoxies is required for rapid curing without heating the nearby surfaces or objects, magnetic particles may be dispersed evenly throughout the cement or epoxy (only several parts per thousand would be needed) such that subsequent application of a cyclic magnetic field would cause uniform heating throughout the volume of the cement rather than just heating from the outside in.
In our previous patent applications, “Targeted Hysteresis Hyperthermia as a Method for Treating Diseased Tissue” and “Improved Targeted Hysteresis Hyperthermia for Treating Diseased Tissue”, we disclose techniques for the localised heating of tumours using heat generated by small magnetic particles exposed to a time varying magnetic field. Magnetic particles are incorporated into biocompatible microcapsules that are administered in such a manner that they concentrate in the vascular network surrounding a tumour. A cyclic magnetic field is applied externally and heat from the microcapsules is conducted into the surrounding tumour tissue. Use of appropriately formulated microcapsules, magnetic field conditions and microcapsule dosage ensures that the tumours are heated to lethal temperatures, i.e. above about 42° C., whilst simultaneously sparing healthy tissue.
There are various ways to deliver the magnetic particles to tumours. For example, the magnetic particles can be administered by direct injection into the tumour tissue. In this way it is possible to get large quantities of material into the tumour. Hence, it may be possible to heat tumours to therapeutic levels using magnetic particles with inferior properties.
An alternative route of administration would demand delivery of the magnetic material preferably in microcapsule form via intra-vascular infusion to target the vascular network surrounding the tumour. This technique is preferred since it offers some significant advantages that improve the therapeutic effectiveness compared to direct injection into tumour. These advantages include the following:
(i) The less invasive nature of the delivery technique reduces the likelihood of inadvertent spreading of the cancer;
(ii) Target tumours do not need to be accurately located and exposed to enable injection of the particles;
(iii) A more optimal distribution of heating foci within the tumour will almost certainly obtain using the intra-vascular infusion technique; and
(iv) It will be easier to treat a large number of small nodules such as often occurs in the case of metastastic liver cancer.
A feature of this route of administration is that a smaller number of particles are delivered to the diseased tissue compared to direct injection. Hence, improvements of the heating characteristics of the magnetic particles are extremely important in order to enable treatment of tumours using intra-vascular infusion.
We have previously specified the minimum operating constraints in terms of the strength of the applied field and its frequency assuming whole body exposure to the field. These stipulated field conditions for whole body exposure are that the frequency should be greater than about 10 kHz and the product of frequency and field strength should not exceed 5×10
8
A/m.s. We have also stipulated the minimum Magnetic Heating Efficiency (MHE) which must be achieved by the magnetic microcapsules subject to these conditions.
Certain commercially available materials do perform according to the stipulated conditions, however it is clear that any improvement in the heating efficiency subject to the imposed constraints, would significantly enhance the usefulness of this and other heating techniques.
The present invention seeks to provide a magnetic material with improved magnetic heating characteristics that can be used in diverse methods such as, but not limited to, the heating of cements and in the treatment of diseased tissue.
Throughout the specification, unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.
DISCLOSURE OF THE INVENTION
The present invention consists in a magnetic material having a magnetic heating efficiency of at least 4.5×10
−8
J.m/A.g in a cyclic magnetic field where the product of the amplitude and frequency of the applied field is less than or equal to 5×10
8
A/m.s, and the frequency of the applied field is at least 20 kHz.
The ideal magnetic material is characterised by a perfectly rectangular hysteresis loop, i.e. loop squareness, defined by the ratio of the remanent to saturation magnetisation equal to 1, with coercivity of 25 kA/m or less and high saturation magnetisation. Such a situation is difficult to achieve with an array of randomly aligned particles as is the case when the magnetic materials are dispersed in compositions such as cement or epoxies or in biological tissues.
Preferably, the magnetic material has a predominantly cubic magnetocrystalline anisotropy. Particles with predominantly cubic magnetocrystalline anisotropy come closest to approaching the specified behaviour in a cyclic magnetic field since they can have a hysteresis loop squareness as high as 0.86.
For a random array of particles with other types of anisotropy (e.g. uniaxial anisotropy), loop squareness will not generally exceed 0.5. In considering arrays of particles with the same coercivity but different loop squareness, the maximum value of hysteresis work per cycle (i.e. either W
a
or W
r
) will occur at a higher field for the array with the lower loop squareness. This means the magnetic heating efficiency for these particles will be less than for the particles with higher loop squareness.
Having regard for the field-frequency constraint discussed above, the maximum allowable applied field strength at 20 kHz is 25 kA/m (314 Oe), decreasing proportionately as frequency is increased beyond 20 kHz. Hence, the magnetic material desirably has a coercivity of less than 314 Oe. In addition, the remanence of the material should remain at a level that maximises the MHE.
In one embodiment of the present invention, there is provided a magnetic material ha

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