WO2021058880A1

WO2021058880A1 - Cooling system

Info

Publication number: WO2021058880A1
Application number: PCT/FI2020/050635
Authority: WO
Inventors: Mikael Laine; Jaakko Nieminen; Juuso KORHONEN; Victor Souza; Heikki SINISALO
Original assignee: Aalto University Foundation Sr
Priority date: 2019-09-26
Filing date: 2020-09-28
Publication date: 2021-04-01

Abstract

A cooling system for a transcranial magnetic stimulation device comprising phase change material positioned in thermal connection with one or more coil windings of a heat producing device for the removal of heat from the coil windings to the phase change material.

Description

COOLING SYSTEM

FIELD

[0001] The present invention relates to a transcranial magnetic stimulation device comprising a cooling system.

BACKGROUND

[0002] Transcranial magnetic stimulation (TMS) is a non-invasive tool for stimulating cortical neurons. TMS is used, e.g., for presurgical mapping, for studying cortical effective connectivity, and for the treatment of major depression. TMS has received several regulatory approvals. In TMS, the stimulation is achieved by feeding a brief, strong current pulse (up to several kiloamperes) through the windings of a coil or coils placed over the subject’s scalp. The coils may be used independently from each other or may be used together, e.g. in multi-coil, multi-locus, or a multi-channel TMS system, multiple coils are used together to control the electric field induced in the brain. This causes a time- varying magnetic field that induces an electric field in the cortex, resulting in membrane de- and hyperpolarization in the targeted neurons. In multi-locus TMS (mTMS), the stimulator coil unit (i.e., the transducer) consists of several coils that can be controlled simultaneously and independently (Ilmoniemi and Grandori 1997; Ilmoniemi et al. 2014; Koponen et al. 2018). The electric field (E-field) pattern induced in the brain can be modified by adjusting the current passing through the windings of each individual coil. Thus, mTMS allows controlling the stimulus parameters (i.e., intensity, location, E-field orientation) electronically without a need to move the transducer.

[0003] In TMS, the currents result in heat of the coils that can then be transmitted to other parts of the device. This heating of the transducer (or coil) limits the applicability of the method, as for patient safety and comfort and according to regulations, the surface temperature of the transducer may not rise too high, e.g. exceed 41 °C. Excessive heating of the internals of a transducer may also limit its lifetime. To overcome these limitations, manufacturers have introduced various methods to cool TMS coils. These include methods to circulate air or liquid through the coil with an external cooling system. Coils with integrated air-cooling systems have also been introduced. None of these is truly ideal for TMS. For example, air-cooling methods have limited cooling performance. External cooling systems typically also increase the overall noise level and increase the size and complexity of the stimulation system. The required tubes may also be considered a problem for, e.g., usability and safety of the system. In addition to producing auditory noise, an air-cooling system integrated to the coil may degrade the signal quality of simultaneous electrophysio logical recordings (e.g., electroencephalography or electromyography). There is therefore a need for improvements in cooling systems for TMS and the same needs may exist in other devices, especially related to medical and otherwise sensitive operations.

SUMMARY OF THE INVENTION

[0004] An object of the present invention is to provide simpler and more convenient yet effective way of cooling TMS transducers. By means of embodiments of the invention the device dissipates heat from transducers, to phase change material (PCM) by thermal conduction.

[0005] The invention is defined by the features of the independent claims. Some specific embodiments are defined in the dependent claims.

[0006] According to a first aspect of the present invention, there is provided a cooling system for a heat producing device, in particular a transcranial magnetic stimulation device comprising phase change material positioned in thermal connection with one or more coil windings of a heat producing device, such as the magnetic stimulation device, for the removal of heat from the coil windings to the phase change material. [0007] According to a second aspect of the present invention, there is provided a

TMS device comprising a cooling system having phase change material positioned in thermal connection with one or more coil windings of the transcranial magnetic stimulation device for the removal of heat from the coil windings to the phase change material BRIEF DESCRIPTION OF THE DRAWINGS

[0008] FIGURE 1 shows a cooling unit suitable for integration on TMS coil windings in accordance with at least some embodiments of the present invention;

[0009] FIGURE 2 shows a cross sectional view of a cooling system according to at least some embodiments of the present invention. PCM is stored inside cylinders surrounded by liquid flow.

[0010] FIGURE 3 shows an example of a cooling system according to at least some embodiments of the present invention.

[0011] FIGURE 4 shows an example of a cooling system having axial symmetry in accordance with at least some embodiments of the present invention.

[0012] FIGURE 5 displays simulation results for a possible mTMS cooler in accordance with at least some embodiments of the present invention.

[0013] FIGURE 6 shows a cross sectional view of a cooling system having a carbon- fibre-composite cell structure forming a PCM enclosure in accordance with at least some embodiments of the present invention.

[0014] FIGURE 7 shows a cell structure cooler where heat is conducted to PCM cells through thermal conduction in accordance with at least some embodiments of the present invention

[0015] FIGURE 8 shows a cross sectional view of vertically oriented cells containing PCM in accordance with at least some embodiments of the present invention.

EMBODIMENTS

DEFINITIONS

[0016] For the purposes of the present invention the following definitions are provided. [0017] TMS shall be taken to mean transcranial magnetic stimulation and encompasses the more specific term mTMS meaning multi-locus, multi-coil, or multi channel transcranial magnetic stimulation.

[0018] PCM shall be taken to mean phase change material.

[0019] Fow electrical conductivity and/or electrically insulating shall be taken to mean having an electrical conductivity of less than or equal to 10 ¹⁰ Sm ¹ under operating conditions of the cooling system.

[0020] Operating conditions of the cooling system shall be taken to mean conditions under which heat energy is to be removed from heat producing components of a heat producing device, e.g. when a coil in a TMS device requires cooling.

DESCRIPTION

[0021] It is an aim of embodiments of the invention to provide a cooling system for a heat producing device, in particular a transcranial magnetic stimulation (TMS) device that solves at least some of the problems mentioned above. In embodiments of the invention it has surprisingly been found that phase change material (PCM) can remove heat from heat producing components such as coils of a TMS device when placed in thermal contact with the TMS device. In embodiments the PCM removes heat from the heat producing components so that the temperature of outer surfaces of the device comprising heat producing components does not exceed 41 °C. Thus embodiments of the cooling system are particularly suited to use in medical devices that have external parts that make contact with a subject or patient. Embodiments of the invention thus address the need to cool, e.g. TMS transducers.

[0022] Figure 1 shows in accordance with at least some embodiments of the present invention a cooling unit that could be integrated on top of TMS coil windings according to an embodiment of the invention. Liquid path is shaded solid grey and is surrounded by PCM on all sides. This variation works best when the device is used in a fixed angle such that the gravitational force helps to circulate the liquid along the grey path. To achieve good cooling performance at all possible rotations, liquid loops could be added perpendicular to the ones shown. Another method to implement optimized cooling performance for several positions/angles of the transducer unit is to have a rotating liquid unit so that it would always be in an optimal angle. This could be done, e.g., manually with an adjustment screw.

[0023] Figure 2 shows in accordance with at least some embodiments of the present invention a cross-section view of an embodiment of the invention, where PCM is stored inside cylinders surrounded by liquid flow. This type of flow geometry is good when a TMS device is used in a level position, e.g. on top of the head of a subject. This embodiment can be especially well suited to having a possibility to change PCM modules, even when the device is being used.

[0024] Figure 3 shows in accordance with at least some embodiments of the present invention an illustration of the geometry of an embodiment of the invention. Figure shows coil (10) placed in a mould on the bottom of the system. For clarity the figure shows less rounds in the coil than actual. The inside surface of the system is covered entirely in liquid (not shown for clarity). PCM material is stored in pipes (20) with liquid flowing around them. On the top and bottom of the system is a grid (30) that promotes heat transfer by air flow.

[0025] Figure 4 shows in accordance with at least some embodiments of the present invention an illustration of an embodiment of the invention with axial symmetry. This geometry allows usage in all possible angles.

[0026] Figure 5 shows in accordance with at least some embodiments of the present invention simulation results for a possible mTMS cooler. In the simulation, the coil generated heat with constant power of 50 W for 20 minutes. The resulting temperature distribution is shown with a scale on the right-hand side. Parts where liquid is flowing can be seen as white stripes, as they are hotter than their surroundings. PCM is found on all sides of the liquid, except the coil side (in this setting, the coil windings are located on the white inner curved surface at the higher temperature). The temperature right under the coil has reached a stable condition and is not increasing as the time goes on. [0027] Figure 6 shows in accordance with at least some embodiments of the present invention a cross section view of an embodiment of the invention with a carbon-fibre- composite cell structure forming a PCM enclosure. In a preferred embodiment the fibres may be aligned with the temperature gradient. Heat transfer from liquid can be further increased by creating more surface contact area with the carbon fibre composite. In a 3D model, the inner PCM enclosure unit could be free to rotate to account for different possible angles of usage.

[0028] Figure 7 shows in accordance with at least some embodiments of the present invention a cell structured cooler where heat is conducted to PCM cells through thermal conduction instead of convection, according to an embodiment of the invention. Heat produced by TMS coil can be distributed horizontally with heat spreader plate so that the PCM cells melt simultaneously. PCM cell unit can be replaced for example by opening the top plastic cover, removing a cell unit and replacing it with a new one.

[0029] Figure 8 shows in accordance with at least some embodiments of the present invention a cross section view of vertically arranged PCM cells. As the PCM gets warmer from the bottom, it changes phase from solid to liquid. Due to gravitation, solid PCM falls below the liquid PCM so that efficient cooling under the PCM cell is realized.

[0030] As described above, embodiments of the present invention relate to a cooling system. In an embodiment is provided a cooling system for a heat producing device, in particular a transcranial magnetic stimulation device comprising phase change material positioned in thermal connection with one or more coil windings of a heat producing device for the removal of heat from the coil windings to the phase change material. Phase change materials have recently gained interest as heat storage in applications where temperature variations are present. These applications include solar engineering, spacecraft thermal control and housing. Typical PCMs can store and release heat energy in thousands of cycles. PCMs can come in organic or inorganic forms. Examples of these are organic paraffins or inorganic salt hydrates. Salt hydrates typically have better thermal conductivity and larger heat storage capacity but they can lose their effectiveness over cycles and can be corrosive to other materials. Salt hydrates are a mixture of water and salts which makes them less attractive choice for a TMS device used in combination with magnetic resonance imaging because the hydrogen nuclei in water would cause signal interference. Lower phase change temperature also means lower coil temperature but the heat storage capacity grows with the phase change temperature. PCM stores energy in a phase change that happens below the desired temperature of the heat producing device, e.g. transducer. Latent heat of the phase change forms a temperature buffer and limits the temperature of the heat producing device, e.g. transducer.

[0031] In one embodiment, the cooling system further comprises a thermal conductive material forming an enclosure for the phase change material. In an embodiment the thermal conductive material is positioned between the one or more coil windings and the phase change material to form the enclosure. The enclosure may be open ended or closed. In an embodiment the enclosure is closed with a lid, e.g. a screw on lid or a push on lid. In one embodiment the lid is formed of the same material as the enclosure. In a further embodiment the lid is formed from a different material from the enclosure, e.g. a rubber, a plastic, a plastic fibre composite, a carbon fibre composite or mixtures of the above materials. In an embodiment the enclosure forms a component selected from the group consisting of tube and cell. In an embodiment in which the enclosure is in the form of a tube, the tube is formed around the one or more coil windings and the PCM is enclosed in the tube around the one or more coil windings. Thus, in an embodiment PCM is in one or more cells, thermally connected to the one or more coil windings of the heat producing device. In a further embodiment in which the enclosure is in the form of a tube, the tube is formed along the one or more coil windings and the PCM is enclosed in the tube along the one or more coil windings. Thus, in one embodiment the tube has the same shape as the coil windings and is positioned on top of the coil windings or next to the coil windings. The purpose of the cell structure is to transport the heat from a heat spreading element such as a heat spreader plate to the sides of the PCM enclosure. The higher the thermal conductivity of this material the faster PCM sliding will happen. If the sides melt too late, the coil might overheat. This material has the same electromagnetic requirements as all the other parts, although unwanted effects are less significant because this part is further away from the coil. The shape of the cell, in an embodiment, is circular as it provides the least amount of surface area with PCM enclosure leading to earlier sliding. On the other hand, contact area is very important for efficient heat transfer but that can be controlled by the diameter of the cell. The other reason for the circular shape was the ease of manufacturing and therefore attractive pricing. The most efficient way to pack the circles is where the centers form a hexagonal lattice. The packing density is over 90%, and only a little amount of space is wasted. The extra space filled with air also has a benefit of thermally insulating the cell making it hotter leading to earlier sliding. The cell structure would therefore in an embodiment comprise vertical tubes in a hexagonal lattice. The diameter, thickness and height of these tubes affect temperature of the coil before and after the PCM has started to slide, timing of the sliding and heat storage capacity. Varying one of these parameters affects several properties. The phase change material can be directly placed into the enclosure, can be placed into the enclosure in a plastic container for example in one or more open-ended cylinders or open-ended geometric tubes. An air gap of approximately 10 % of the PCM volume may be left in the enclosure or in the plastic container to allow for volumetric expansion of the phase change. In some embodiments the enclosures are filled with liquid PCM and then closed with a top cover, which is optionally threaded to seal the enclosure, e.g. by screwing. By providing a top cover the heat storage capacity of the device can be extended.

[0032] As mentioned above the cooling system is suitable for a TMS device, such as an mTMS device. In such devices coil windings may have fixed geometric positions that must not be disturbed. Thus, it is important that the structure of the coil windings and the position of the coil windings in relation to each other remains unchanged. Thus, in an embodiment the structure of the one or more coil windings is unaffected by the cooling system. In a preferred embodiment the structure of the one or more coils windings is unaffected by the cooling system under operating conditions. In a further embodiment the spatial relationship between coil windings remains undisturbed. It follows, therefore, that in an embodiment the heat producing device is a transcranial magnetic stimulation device, such as a TMS device or an mTMS device.

[0033] As mentioned above the cooling system is suitable for a TMS device, such as an mTMS device. In such devices coil windings may also have adjustable geometric positions, e.g. to place the coils in desired positions above or around the scalp of the subject or patient. Thus, it is important that the structure of the coil windings or the position of the coil windings in relation to each other can be changed. Thus, in an embodiment the structure of the one or more coil windings can be adjusted. In a further embodiment the spatial relationship between coil windings and the cooling unit remains undisturbed. It follows, therefore, that in an embodiment the heat producing device is a transcranial magnetic stimulation device, such as a TMS device or an mTMS device. A coil or coils may be associated with a cooling system in an embodiment and optionally a further coil or coils may be associated with a further cooling system. In such embodiments as the position of the coils is adjusted, the cooling unit moves accordingly, so that rate of cooling is maintained at the required rate or adjusted to the required rate. For example, in a (m)TMS system in which multiple coils/transducers are placed around the head of a subject, the coils/transducers could be brought into contact (or close to being in contact) with the scalp to provide efficient stimulation of the cortex. This allows tailoring the coil/transducer arrangement for an individual head shape and size, increasing the efficiency of the stimulation. Thus, in an embodiment one or more of the adjustable coil/transduccrs has a cooling system.

[0034] In an embodiment the cooling system further comprises one or more heat spreading elements having a first surface configured to be thermally connected to the one or more coil windings and a second surface thermally connected to the phase change material. The hottest part of the coil and therefore the coil windings is sometimes in the center of mass as that is where the coil is most tightly packed. For optimal cooling power, this heat is spread along the whole surface of the coil. This is done by placing a heat spreading element such as a heat spreader plate on top of the coil, being in direct contact with the windings. This element transfers heat between the coil and the cell structure and is highly thermally conductive. Suitable material for this is aluminum nitride which can have thermal conductivity of around 150W/mK - 5 times that of aluminum oxide. Thermal conductivity of aluminum nitride was found to be 110 W/(m*K) with steady-state rod method and 170 W/(m*K) with steady state disk method using beryllium oxide, aluminum oxide and aluminum as standards. (Werdecker and Aldinger 1984)

[0035] Aluminium nitride is very hard and much less common compared to aluminum oxide and this makes it quite expensive.

[0036] As mentioned above the heat producing device may have one or more coils that produce heat. The coils may be oriented in the same direction or same plane or oriented in different directions or different planes. Therefore, in embodiments the cooling system may comprise one or more heat spreading elements. The heat spreading elements are oriented according to the coils from which they collect heat. In an embodiment the one or more heat spreading elements is in thermal contact with one or more heat spreading elements, whereas in an alternative embodiment the one or more heat spreading elements are not in thermal contact with one or more heat spreading elements. [0037] The thickness of this element was determined by simulating the cooling system with different heat spreading element thicknesses and a thickness of 3 mm was chosen. If the element is thicker, the growing distance to the cell structure overcomes the benefit of evening out the temperature distribution. If the element is thinner, the heat is not spread efficiently.

[0038] In an embodiment the one or more heat spreading elements are planar, and/or are curved, and optionally are adapted to fit a heat producing device.

[0039] Enclosed PCM has limited heat storage capacity. If the usage is temporary, the cooling system is fully passive and requires no supervision. If the usage is continuous or the heat output is high, cooling power can be increased by PCM replacement. In contrast to state-of-art cooling systems, the present invention provides a simpler, less expensive, quieter, and, compared to air cooling, more effective cooling method. In a preferred embodiment the PCM enclosure is designed to allow replacing PCM cells to replenish heat storage capacity. Thus, in an embodiment the phase change material can be removed from the enclosure. In a further embodiment the phase change material can flow through the enclosure. In a still further embodiment, the enclosure is removable and replaceable. In further embodiments the thermoconductive cell comprising an enclosure containing PCM is also removable and replaceable.

[0040] It has been described above that in embodiments the orientation, size and position of the coil windings may be crucial to the function of the heat producing device. In such embodiments a coil former adapted for purpose can provide stability and continuity to the said orientation, size and position of the coil windings. Thus, in an embodiment the surface of the coil windings to which the heat spreading element is thermally connected is formed of one or more coil formers each coil former comprising means for holding one or more coils windings, preferably grooves for holding one or more coil windings. In a particular embodiment in place of or in addition to grooves the coil former comprises a guiding structure around which the coil is wound to provide coil windings. The coil windings can then in an embodiment be glued into place, e.g. with epoxy or other suitable adhesive. In one embodiment the coil windings can be almost totally submerged or optionally totally submerged in glue so that PCM can be placed above the coil windings and be in good thermal contact with the coil windings. In an alternative embodiment coils are cut from metal sheets or machined from bulk material and the gaps are filled with glue whereby the glue takes the place of the coil former and no additional coil former material is required. The most critical part of the cooling system is the core of the heat generation - the coil. Heat transfer from the coil must be efficient so that the coil does not overheat. This coil former material should be electrically insulating, as there is high voltage between the coil windings. In some embodiments the coil former material is magnetic, e.g. in examples in which a coil or coils, e.g TMS coils have iron cores to enhance stimulation strength. The material should be nonmagnetic so that it does not affect the generated magnetic field, and it needs to be able to withstand the forces that the coil exerts. In addition, this material needs to be machined to make grooves for the windings. Aluminum oxide fits these specifications but because of its hardness the machining is very costly. Other materials such as Macor do not provide enough thermal conductivity to overcome the inexpensive choice of plastics. Therefore, in an embodiment the coil former is mechanically strong 3D-printable polyamide. In a preferred embodiment the heat spreading element itself is a coil former, i.e. the coil windings of the heat producing device are formed about the heat spreading element.

[0041] In an alternative embodiment, a thermally conductive liquid takes the place of a heat spreading element and optionally takes the place of a coil former for the transfer of heat from the coil windings to the PCM. The liquid may be selected form options that do not conduct electricity and suitable liquids such as mineral oils can be used. Thus in an embodiment space between the coil windings and the enclosure is occupied by a liquid. In addition to conduction, heat transfer takes place by natural convention in some embodiments. When using natural convection as a method of heat transfer it is crucial to consider the direction of the gravitational force, as a heat producing device such as a TMS device can be used in multiple positions and angles around the head of a subject. Different geometries have better cooling capabilities in different angles. For an mTMS system with a somewhat fixed transducer placement, it may not be necessary to consider different transducer placements. This, in addition to the size of an mTMS system, makes the cooling solution easier to implement.

[0042] In one embodiment the coil former comprises an electrically insulating material having a conductivity of less than or equal to 10 ¹⁰ Sm ¹ under operating conditions of the cooling system, such as aluminium oxide, preferably a plastic, suitably polyamide. [0043] Heat spreading elements should also be electrically insulating. Therefore, in an embodiment the heat spreading element comprises aluminium nitride.

[0044] It has been found that the optimal thickness for the heat spreading element is 3 mm. In an embodiment the heat spreading element has a thickness in the range of 0.2 to 5 mm, suitably 1 to 3 mm, preferably 3 mm. As the thickness increases above 3 mm, the benefits of evening out the temperature distribution begin to become undone due to the growing distance between the heat producing device and the cell structure.

[0045] PCM moves in cells and enclosures by gravity and or convection, depending on the orientation of the cells, for the removal of heat from the heat producing device. In an embodiment the cells are oriented with respect to the heat producing device to cause flow of phase change material by gravity or by convection or by a combination of gravity and convection.

[0046] In embodiments the cooling system has a cover, preferably a plastic cover. In one embodiment the cooling system comprises a plastic cover whereby a first portion of the plastic cover is positioned between the coil former and the heat producing device and a second portion of the plastic cover encloses the thermally conductive cells. In a further embodiment the cover is formed of a material selected from rubber, plastic fibre composite and carbon fibre composite.

[0047] In an embodiment of the invention the thermally conductive materials could be ceramics such as aluminum nitride or aluminum oxide because of their excellent electric insulation properties but in other embodiments metallic conductors can be used. In an alternative embodiment, however, the cooling system is free of metals. This is of particular importance when the cooling system is to be applied to TMS. In embodiments when the cooling system is applied to TMS devices, such as mTMS devices components of the cooling system should be highly thermally conductive. In an embodiment components of the cooling system are non-magnetic, while in a particular embodiment components of the cooling system are magnetic. In a preferred embodiment components of the cooling system are electrical insulators, which for the purposes of embodiments of the present invention means that components should have an electrical conductivity of less than or equal to 10 ¹⁰ Srn ¹ under operating conditions of the cooling system. [0048] The heat spreading element or elements may be made of various materials. In an embodiment the heat spreading element comprises thermally conductive material selected from the group consisting of aluminium nitride, boron nitride, aluminium oxide, diamond and a composite with high thermal conductivity and low electrical conductivity, preferably the heat spreading element comprises aluminium nitride.

[0049] The cells, tubes and enclosures of the cooling system are formed of thermally conductive material in embodiments. In one embodiment the cells and/or tubes comprise thermally conductive material selected from the group consisting of thermally conductive plastics and carbon fibre composites. In a further embodiment the enclosures comprise thermally conductive material selected from the group consisting of thermally conductive plastics and carbon fibre composites.

[0050] In an embodiment the one or more enclosures are further enclosed in a container having one or more inlets and one or more outlets, said container providing one or more channels for the flow of a liquid from the one or more inlets about the one or more cells or tubes to the one or more outlets. This allows heat to be delivered from the heat producing device to enclosures containing PCM by a thermally conductive liquid.

[0051] In a preferred embodiment the liquid is selected from the group consisting of liquids that have an electrical conductivity of less than or equal to 10 ¹⁰ Sm ¹ under operating conditions of the cooling system.

[0052] Since in embodiments the enclosures are in thermal contact with one or more elements of the cooling system, enclosures of certain embodiments are formed from a material selected from the group consisting of thermally conductive plastics and carbon fiber composites, preferably polyester, for the conduction of heat between the heat producing device and the PCM.

[0053] In a further embodiment the cooling system comprising one or more temperature sensors. A temperature sensor may provide an operator with a signal e.g. that PCM should be replaced when a certain temperature is reached.

[0054] A phase change detection mechanism can provide similar benefits. Thus in an embodiment the cooling system comprises one or more phase change detection mechanisms. [0055] Further embodiments relate to the phase change material. In one embodiment the phase change material is organic or inorganic. PCMs exist in organic or inorganic forms. Organic forms include paraffins and lipids. Inorganic PCMs are salt hydrates which typically have better thermal conductivity and larger heat storage capacity but they may lose their effectiveness faster over multiple phase change cycles and may be corrosive to other materials. Salt hydrates are a mixture of water and salts which makes them less attractive a choice for a TMS device because, if used inside a magnetic resonance imaging scanner, the hydrogen nuclei in water may cause signal interference.

[0056] Solid PCM is usually encapsulated inside a constrained volume however in an embodiment in which PCM may be circulated through tubes micro-encapsulated PCM slurry is used to mix PCM into the circulating fluid.

[0057] In an embodiment the phase change material is selected from the group consisting of organic paraffins. In a further embodiment the phase change material is selected from the group consisting of organic lipids and salt hydrates.

[0058] The present invention addresses the need to cool TMS transducers. Thus in an embodiment is provided a TMS device comprising a cooling system according to any of the embodiments described herein. In a further embodiment, the TMS device is an mTMS device.

[0059] According to embodiments liquid and/or solid heat transfer medium and phase-change material (PCM) are used to store heat energy. PCMs have recently gained interest as a heat storage. So far, they have been used in buildings and also in space and solar applications (Souayfane et al. 2016; Helm et al. 2014; Barzin et al. 2015). According to one embodiment of the invention, the heat generating coil windings of a TMS device are submerged in a highly electrically insulating liquid (e.g., https://www.paratherm.com/heat- transfer-fluids/paratherm-cr-heat-transfer-fluid/). Heat gets then transferred to PCM directly from this liquid or in combination with a solid thermal conductor (e.g., carbon fiber composite). The liquid is circulated around the device in order to transfer the heat efficiently. By natural convection, the warm liquid gains momentum in the direction opposite to the gravitational force. The liquid is rotated around the device in the following manner: The liquid is guided along a slim path that follows the shape of the coil, making its way towards the apex of the device. From here the liquid path is curled backwards making its way down to the bottom of the device, completing the rotation. [0060] PCM is stored inside this rotatory path so that the liquid is in close contact with it. PCM can also be stored outside this path, where possible. Heat transfer can be increased by embedding a structure with high heat capacity inside the PCM. Most of the generated heat energy is stored in PCM during the phase change from solid to liquid. The system can be implemented with replaceable PCM modules so that the heat storage can be extended. A system with replaceable PCM modules may include means to determine when a change of modules is needed. This may be based on the current temperature of the coil unit or, e.g. , its temperature history, or it may be derived from the history of applied TMS pulses. With a transparent window this could be also done visually. Another way would be to observe light scattering within the PCM (LED and a photodetector). For efficient cooling, the melting point of the PCM needs to be higher than the ambient temperature but lower than the maximum allowed surface temperature of 41 °C for a TMS coil. In some applications, the preferable temperature range may be different. Suitable PCM could be a paraffin wax with desired melting temperature (e.g., https://www.rubitherm.eu/en/index.php/productcategory/organische-pcm-rt).

[0061] The cooling of the coil bottom (the part of TMS device that touches patients or subjects head) may be improved by leaving a 1-mm or so gap between the bottom of the coil windings and the inner bottom surface of the coil former; air can then be forced through this gap to keep the bottom surface of the device within the allowed temperature range. Another PCM with higher melting temperature under the coil could be also considered. For maximum effectiveness the coil bottom material can be polyurethane with a low heat conductivity.

[0062] When using natural convection as a method of heat transfer it is crucial to consider the direction of the gravitational force, as a TMS device can be used in multiple positions and angles around the head. Different geometries have better cooling capabilities in different angles. Therefore, in the following, different variations for different situations are presented. For an mTMS system with a somewhat fixed transducer placement, it may not be necessary to consider different transducer placements. This, in addition to the size of an mTMS system, makes the cooling solution easier to implement. Different geometry options and simulation results for an mTMS solution are also presented.

[0063] It should thus be understandable that the described cooling system can be implemented in various ways, including the following approaches: • One or several slim paths, various geometry options

• Combination with an external air/liquid-cooling system

• Enhanced cooling of the coil bottom by forced air or liquid flow through a gap between the windings and the coil former

• Combination with an air/liquid-cooling system integrated to the transducer

• Replaceable PCM modules

• Possibility to change these during a stimulation session. [0064] To minimize the induction of eddy currents in a system containing, e.g., carbon-fiber-based materials, the geometry of such material can be designed so that the amount and/or size of closed relatively well electrically conductive loops is minimized.

[0065] Thus, instead of a carbon fiber cell structure, the system may include, e.g., a set of parallel carbon-fiber-based rods to enhance the distribution of heat within PCM. These rods may be kept in place with material having relatively poor electrical conductivity. Aligning the fibers so that the eddy currents form perpendicular to them would also reduce the induced magnetic field. If carbon fiber epoxy composite is used, the electrical conductivity is so small that the induced eddy currents are negligible.

[0066] In the following, embodiments are described in which a liquid medium is not used.

[0067] Heat could be transferred through a liquid medium that would circulate around the device driven by natural convection. However, heat can also be conducted through solid material, for example ceramic pipes and thermal energy can be used to move heat-storage material towards the temperature gradient. In this case, no liquid heat transfer medium is necessary. Phase-change material (PCM) is stored on top of the coil in a vertical cell structure so that the PCM has a degree of freedom towards the gravity gradient. The cell structure is ideally made from a highly thermally conductive but electrically insulating and non-magnetic material, e.g., aluminum oxide or aluminum nitride. The PCM may be stored inside thin enclosures to allow replacement of the PCM cells to increase the heat storage capacity when needed. Heat can be conducted from the coil windings to the cell structure through very highly thermally conductive material like aluminum nitride. A small air gap below the coil will lower the temperature of the bottom cover significantly. The system is able to function passively and no external fan is required.

[0068] Figures 7 and 8 describe embodiments of the invention using PCM cells that are arranged in contact or in close proximity with a heat source such as TMS coils.

[0069] As the PCM gets warmer from the bottom, it changes phase from solid to liquid. Due to gravitation, solid PCM falls below the liquid PCM so that efficient cooling under the PCM cell is realized.

[0070] High cooling power can be achieved when thermal energy is used to move solid PCM towards the coil. This phenomenon is illustrated in Fig. 8. As the PCM gets warmer from the bottom, it changes phase from solid to liquid. This liquid keeps getting warmer without absorbing too much of energy and it slows down the cooling efficiency. The solid form of a material is often more dense than its liquid form; thus, in a mixture the solid falls below the liquid. In a cell, the solid is, however, held up by the friction between the PCM and the enclosure. But, as the cell gets warmer, it heats up the PCM on the sides of the enclosure and the solid PCM becomes free-flowing and falls towards the gravity gradient. If the gravity gradient has a component towards the coil, it means that in the bottom of the cell, the hot liquid PCM gets replaced by solid PCM whose temperature is always smaller than or equal to the phase change temperature. We have now achieved a situation where the bottom of the cell is at a constant temperature defined by the phase- change temperature. By having the bottom of the cell close to the coil a large cooling power can be achieved. Also low thermal conduction of the PCM will not be a problem as the solid material moves towards the coil. The optimal geometric shape of the cell structure could be hexagonal or circular because of their beneficial surface-to -volume ratio, which makes the sliding happen earlier. In an alternative embodiment, e.g. when PCM is not positioned above the device to be cooled, the PCM moves in the cell, thus transferring heat away from the device to be cooled by natural convection only. In one embodiment both natural convection and sliding in combination with gravity provide means for transfer of heat away from the heat producing device. PCM sliding depends on the frictional force between the enclosure and the PCM versus the gravitational pull on the PCM. Static friction turns into viscous friction as the PCM starts turning into liquid near the boundary. The moment at which the PCM starts to slide depends on the angle in which the TMS coil is used because of the changing gravitational force. To account for this, the TMS coil could be momentarily turned upright between TMS pulses or the patients head could be adjusted to have the coil in a more upright angle.

[0071] In contrast to other TMS coil cooling methods, this design uses heat storage to contain the heat. When a material changes phase between solid, liquid or gas an internal energy jump is present. This manifests as temperature buffer as the material stays at a constant temperature until the required energy is absorbed. This energy is called latent energy. Usually the latent energy is greatest when the phase changes from liquid to gas but the change in volume is extreme. In places where volumetric expansion is not a problem, this type of phase change cooler can be used. One example of this is so-called immersion cooling, where servers are immersed in a liquid ( e.g ., mineral oil) that boils when the server heats up storing a large amount of energy. In the case of TMS device, this volumetric change can be problematic so the face change happens from solid to liquid. The phase change material (PCM) needs to change phase between the ambient temperature and the maximum allowed temperature of the device to be effective.

[0072] Heat is conducted from the coil to the PCM through highly thermally conductive material. The larger the surface area between the PCM and the thermal conductor is the faster the heat exchange happens. Also, the larger the surface area perpendicular to the temperature gradient is the faster the heat transfer happens away from the coil. Therefore, the PCM is stored in a vertical cellular structure. This structure has also two other very important properties. Firstly, the PCM inside the cell is free to move vertically allowing replacement of the PCM enclosure extending the heat storage capacity. Secondly, an advantageous phenomenon happening between the solid and the liquid phase is possible to happen. This phenomenon is illustrated in figure 8. As the PCM gets warmer from the bottom it changes phase to liquid. This liquid keeps getting warmer without absorbing too much of energy and it slows down the cooling efficiency. The solid form of a material is often more dense than its liquid form so in a mixture the solid falls below the liquid. In this case, the solid is held up by friction between the PCM and the enclosure. However, as the cell gets warmer it heats up the PCM on the sides of the enclosure and the solid PCM becomes free-flowing and falls towards the gravity gradient. If the gravity gradient has a component towards the coil it means that in the bottom of the cell, the hot liquid PCM gets replaced by solid PCM whose temperature is always smaller or equal to the phase change temperature. We have now achieved a situation where the bottom of the cell is at a constant temperature defined by the phase change temperature. By having the bottom of the cell close to the coil a large cooling power can be achieved. Also low thermal conduction of the PCM will not be a problem as the solid material moves towards the coil. From here on the phenomenon will be referred to as PCM sliding.

EXAMPLES

[0073] The following describes examples of a cooling system according to embodiments of the invention. [0074] First, the desired volume of PCM was calculated so that the volume inside the cell could be determined. The enclosure was designed to store two thirds of the volume needed for one session. As the enclosure is modular, switching the enclosure once would be sufficient for the whole session. The extra volume was to account for the case that PCM closer to the center of the coil might melt faster and reduce cooling power before all PCM has melted.

[0075] Next, the diameter of the pipe was determined. Diameter of the pipe determines the number of pipes. Because the temperature is not evenly distributed in the heat spreader plate, the diameter needs to be small enough that the sides of the pipe heat up at roughly the same rate. The maximum diameter was therefore determined to be 5 cm. As the diameter gets smaller, the relative amount of PCM enclosure compared to the PCM will grow and PCM volume is reduced. The pipes also need to be packed somewhat efficiently over the heat spreader plate. Hexagonal circle lattice was fit over the heat spreader plate area and best fitting configurations were noted while lowering the diameter from earlier defined maximum. Configurations with 7, 10 or 19 pipes were found to be the best fitting. Any amount higher than that would be troublesome when building the device and too expensive to manufacture. The diameters were around 5 cm, 4 cm and 3 cm respectively. Increased number of pipes indicates better fitting to the area, leading to more storage volume.

[0076] After that, the thickness or inner diameter of the pipe was determined. Thickness affects the surface area between the cell structure and the heat spreader pad. An optimal surface area was determined first and then it could be applied to all configurations to determine the needed thickness. This was done iteratively by simulating the cooling device with different thicknesses and finding a thickness that keeps the coil in desired temperature until the sliding happens and the cooling power increases significantly. Heat was generated with pulse frequency of 5 Hz. Thickening the pipe reduces storage volume of the PCM which is unwanted. After the optimal area was found different thicknesses for each configuration was determined.

[0077] The final step was to find the height of the pipe. This was determined by the PCM volume needed in the system. Configurations with fewer pipes needed longer pipes partly because of the less efficient fitting and partly because of increased thickness. Cooling in different angles

[0078] Sliding direction of the PCM depends on the angle between the gravitational force and the cell axis. PCM cell cooling power in different angles were tested to evaluate this angle dependence. Testing was done with a single cell attached to the center of the heat spreader pad. Temperature of the cell was measured at upper edge where liquid phase PCM should float. Hypothesis was that the beginning of the sliding would be seen as sudden increase in temperature at the specified location when the warm liquid PCM floats to the top. The results revealed that this hypothesis was not correct.

[0079] Instead of having the PCM cells as, e.g., separate cylinders that are assembled together, the PCM cells may be of the form that results from, e.g., drilling cylindrical holes into a suitable solid material that conducts heat well.

[0080] For different uses of a device, the system may include sets of cooling units

(e.g., cells) that are of different size, shape, of arrangement. For example, for uses requiring only a moderate degree of cooling performance, a compact and lightweight cell set can be used; for uses requiring higher cooling performance, a more powerful cooling unit can be used.

[0081] The system may contain one or more spreading plates. For example, such a plate may be placed above the coil windings, or such a plate may be situated between two or more layers of coil windings. In one embodiment, at least part of the coil former is made of material that conducts heat well. [0082] In one embodiment of the invention, the cell structure can be rotated or moved within the system. This may be beneficial when PCM cells are exposed to varying degrees of heat. By circulating the cells so that those that have experienced the least amount of heat exposure are replaced with those that have experienced the highest amount of heat exposure, the cooling performance may be improved. This kind of a process may be automatic (e.g., motor controlled) or initiated by an operator.

[0083] It is to be understood that the embodiments of the invention disclosed are not limited to the particular structures, process steps, or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting.

[0084] Reference throughout this specification to one embodiment or an embodiment means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Where reference is made to a numerical value using a term such as, for example, about or substantially, the exact numerical value is also disclosed.

[0085] As used herein, a plurality of items, structural elements, compositional elements, and/or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary. In addition, various embodiments and example of the present invention may be referred to herein along with alternatives for the various components thereof. It is understood that such embodiments, examples, and alternatives are not to be construed as de facto equivalents of one another, but are to be considered as separate and autonomous representations of the present invention.

[0086] Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided, such as examples of lengths, widths, shapes, etc., to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.

[0087] While the forgoing examples are illustrative of the principles of the present invention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the claims set forth below.

[0088] The verbs “to comprise” and “to include” are used in this document as open limitations that neither exclude nor require the existence of also un-recited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of "a" or "an", that is, a singular form, throughout this document does not exclude a plurality.

INDUSTRIAL APPLICABILITY

[0089] At least some embodiments of the present invention find industrial application inheat producing devices, in particular in heat producing devices that require cooling, e.g. in medical devices such as TMS devices, including mTMS devices, multi coil, multi-locus, or multi-channel TMS devices.

CITED DOCUMENTS

Barzin, J., Chen,

Young, B.R., Farid, M.M., “Application of PCM energy storage in combination with night ventilation for space cooling,” in Applied Energy, vol. 158, pp. 412-421, 2015; Helm, M., Hagel, K., Pfeffer, W., Hiebler, S., Schweigler, C., “Solar heating and cooling system with absorption chiller and latent heat storage - a research project summary,” in Energy Procedia. vol. 48, pp. 837-849, 2014;

Ilmoniemi, R., Grandori, F. Programmable applicator of electromagnetic fields, especially for the stimulation of central and peripheral nervous systems and for tissue therapy and hyperthermia applications. Finnish Patent No. 100458 (December 15, 1997);

Ilmoniemi, R., Koponen, F., Nieminen, J., Jamefelt, G., mTMS coil device with overlapping coil windings, US Patent Application US 2014/0357935 A1 (2014);

Koponen, F.M, Nieminen, J.O., Ilmoniemi, R.J., “Multi-locus transcranial magnetic stimulation — theory and implementation,” in Brain Stimulation, vol. 11, no. 4, pp. 849- 855, 2018;

Souayfane, F., Fardoun, F., Biwole, P.-H. “Phase change materials (PCM) for cooling applications in buildings: a review,” in Energy and Buildings, vol. 129, pp. 396-431, 2016;

Werdecker, W., Aldinger, F., “Aluminum nitride-an alternative ceramic substrate for high power applications in microcircuits,” in IEEE Transactions on Components, Hybrids, and Manufacturing Technology, vol. 7, no. 4, pp. 399M04, 1984.

Claims

1. A cooling system for a heat producing device, in particular a transcranial magnetic stimulation device comprising phase change material positioned in thermal connection with one or more coil windings of a heat producing device for the removal of heat from the coil windings to the phase change material.

2. The cooling system according to claim 1, further comprising a thermal conductive material forming an enclosure for the phase change material.

3. The cooling system according to claim 1 or 2, wherein the structure of the one or more coil windings is unaffected by the cooling system

4. The cooling system according to any of claims 1 to 3, wherein the structure of the one or more coil windings is unaffected by the cooling system under operating conditions.

5. The cooling system according to any of claims 1 to 4 wherein the enclosure forms a component selected from the group consisting of tube and cell.

6. The cooling system according to any of the preceding claims, wherein the heat producing device is a transcranial magnetic stimulation device.

7. The cooling system according to any of the preceding claims, further comprising one or more heat spreading elements having a first surface configured to be thermally connected to the one or more coil windings and a second surface thermally connected to the phase change material.

8. The cooling system according to any of the preceding claims, wherein one or more heat spreading elements is in thermal contact or is not in thermal contact with one or more heat spreading elements.

9. The cooling system according to any of the preceding claims, wherein the one or more heat spreading elements are planar, and/or are curved, and optionally are adapted to fit a heat producing device.

10. The cooling system according to any of the preceding claims, wherein the phase change material can be removed from the enclosure.

11. The cooling system according to any of the preceding claims, wherein the phase change material can flow through the enclosure.

12. The cooling system according to any of the preceding claims, wherein the enclosure is removable.

13. The cooling system according to any of the preceding claims, wherein the surface of the coil windings to which the heat spreading element is thermally connected is formed of one or more coil formers each coil former comprising means, for holding one or more coil windings, preferably grooves for holding one or more coil windings.

14. The cooling system according to any of the preceding claims, wherein the heat spreading element is a coil former.

15. The cooling system according to any of the preceding claims, wherein space between the coil windings and the enclosure is occupied by a liquid.

16. The cooling system according to any of the preceding claims, wherein the coil former comprises an electrically insulating material having a conductivity of less than or equal to 10¹⁰ Srn ¹ under operating conditions of the cooling system, such as aluminium oxide, preferably a plastic, suitably polyamide.

17. The cooling system according to any of the preceding claims, wherein the heat spreading element comprises aluminium nitride.

18. The cooling system according to any of the preceding claims, wherein the heat spreading element has a thickness in the range of 0.2 to 5 mm, suitably 1 to 3 mm, preferably 3 mm.

19. The cooling system according to any of the preceding claims, wherein the cells are oriented with respect to the heat producing device to cause flow of phase change material by gravity or by convection or by a combination of gravity and convection.

20. The cooling system according to any of the preceding claims further comprising a plastic cover whereby a first portion of the plastic cover is positioned between the coil former and the heat producing device and a second portion of the plastic cover encloses the thermally conductive cells

21. The cooling system according to any of the preceding claims, wherein the cooling system is free of metals.

22. The cooling system according to any of the preceding claims, wherein the heat spreading element comprises thermally conductive material selected from the group consisting of aluminium nitride, boron nitride, aluminium oxide, diamond and a composite with high thermal conductivity and low electrical conductivity, preferably the heat spreading element comprises aluminium nitride.

23. The cooling system according to any of the preceding claims, wherein the cells or tubes comprise thermally conductive material selected from the group consisting of aluminium nitride, boron nitride, aluminium oxide, diamond and a composite with high thermal conductivity and low electrical conductivity.

24. The cooling system according to any of the preceding claims, wherein the enclosures comprise thermally conductive material selected from the group consisting of aluminium nitride, boron nitride, aluminium oxide, diamond and a composite with high thermal conductivity and low electrical conductivity.

25. The cooling system according to any of the preceding claims, wherein the one or more enclosures are further enclosed in a container having one or more inlets and one or more outlets, said container providing one or more channels for the flow of a liquid from the one or more inlets about the one or more cells or tubes to the one or more outlets.

26. The cooling system according to any of the preceding claims, wherein the liquid is selected from the group consisting of liquids that have an electrical conductivity of less than or equal to 10¹⁰ Srn ¹ under operating conditions of the cooling system.

27. The cooling system according to any of the preceding claims, wherein the enclosures are formed from a material selected from the group consisting of thermally conductive plastics and carbon fiber composites, preferably polyester.

28. The cooling system according to any of the preceding claims further comprising one or more temperature sensors.

29. The cooling system according to any of the preceding claims further comprising one or more phase change detection mechanisms.

30. The cooling system according to any of the preceding claims, wherein the phase change material is organic or inorganic.

31. The cooling system according to any of the preceding claims, wherein the phase change material is selected from the group consisting of organic paraffins.

32. The cooling system according to any of the preceding claims, wherein the phase change material is selected from the group consisting of organic lipids and salt hydrates.

33. A TMS device comprising a cooling system according to any of the preceding claims.

34. The TMS device according to claim 34, wherein the device is an mTMS device.