WO2018152068A1

WO2018152068A1 - Handheld battery powered cold therapy device

Info

Publication number: WO2018152068A1
Application number: PCT/US2018/017845
Authority: WO
Inventors: Vasily Dronov; Sylvia Helena FLOREZ MARINO
Original assignee: Vasily Dronov; Florez Marino Sylvia Helena
Priority date: 2017-02-15
Filing date: 2018-02-12
Publication date: 2018-08-23

Abstract

A system comprising a handheld device for applying cooling action to the skin and underlying muscles is disclosed. A detailed description and theory of operation of the handheld device are disclosed. The treatment method using such a system is also disclosed. The handheld cooling device uses a Peltier cooler in thermal contact with an application surface and in contact with a phase change material. Thermally conductive bristles are used to transfer heat from the Peltier cooler to the phase change material.

Description

HANDHELD BATTERY POWERED COLD THERAPY

DEVICE

FIELD OF THE INVENTION

[0001] The present invention relates to a medical device, and more particularly, to a handheld therapeutic cooling device.

INTRODUCTION

[0002] Widely documented in the literature, the practice of cold therapy (sometimes referred to as cryotherapy) for soft tissue injury has been utilized for the reduction of pain, swelling, and inflammation. Cold therapy has been shown to decrease both immune cell infiltration and the accumulation of key inflammatory mediators without altering muscle regeneration process. See Vieira Ramos G, Pinheiro CM, Messa SP, Delflno GB, Marqueti R de C, Salvini Tde F, Durigan JL. Cryotherapy reduces inflammatory response without altering muscle regeneration process and extracellular matrix remodeling of rat muscle. See JSci Rep. 2016 Jan 4; 6: 18525. doi: 10.1038/srepl8525. There may also be the potential psychological benefits of cold exposure to reduce the subjective feeling of muscle soreness following exercise. The proposed mechanism of action which contributes to the therapeutic benefit include the slowing of the inflammatory process, reduced nerve conduction velocity resulting in diminished pain, changes in tissue oxygenation, and the induction of vasoconstriction leading to less swelling. Cold therapy may produce a significant degree of vasoconstriction that may persist long after the cessation of active cooling. Lowering the tissue surface temperature only to the mid-twenties Celsius range can induce a vasoconstriction to less than half of the baseline perfusion level. See Mejia N, Dedow K, Nguy L, Sullivan P, Khoshnevis S, Diller KR. An On-Site Thermoelectric Cooling Device for Cryotherapy and Control of Skin Blood Flow. J Med Device. 2015 Dec;9(4):0445021-445026.

[0003] Acne vulgaris is a common skin inflammatory disease that affects greater than

80% of adolescents. Acne is considered a chronic disease often having a negative psychological impact. There are four main factors involved in the acne pathogenesis: sebum production, incomplete desquamation, Propionibacterium acnes proliferation, and inflammation. While highly effective, oral medications are coupled with severe risks, and prolonged antibiotic therapies may be permissive for antibiotic resistance. Thus, additional methodologies are needed to combat this highly prevalent and often debilitating skin disease.

[0004] As early as 1928, Giraudeau reported the use of a carbon dioxide slurry to treat acne. Cold therapy used in a variety of dermatologic applications has been demonstrated to be safe, without reports of local or systemic side effects. Early exploratory experiments using contact probes for a duration of three minutes demonstrated skin samples frozen to -10°C or warmer were all viable See Gage AA, Caruana J A Jr, Monies M. Critical temperature for skin necrosis in experimental cryosurgery, Cryobiology, 1982 Jun; 19(3): 273-82. With its easy superficial application, cold therapy may alleviate acne via sebaceous gland damage resulting in a subsequent decrease in sebum production. Cold therapy has also been utilized for the local reduction of unwanted subcutaneous fat. The putative mechanism involves crystallization of cytoplasmic lipids at temperatures higher than the freezing point of tissue water. Like adipose, sebaceous glands have high lipid content, and thereby may also be susceptible to damage after cold therapy. Reducing inflammation will promote faster resolution of acne lesions, and may also provide therapeutic benefits to a multitude other conditions driven by inflammation.

[0005] Cryotherapy is the use of extreme cold to destroy abnormal or diseased tissue.

A common method of freezing lesions is using liquid nitrogen as the cooling solution. For treating benign lesions and warts, there exist a number of over-the-counter products that utilize dimethyl ether propane cryogenic spray (DMEP). Scientific evidence indeed supports the notion that treatment temperatures achieved via use of DMEP are sufficient for reliable cell destruction. See, e.g., Caballero Martinez F, Plaza Nohales C, Perez Canal C, Lucena Martin MJ, Holgado Catalan M, and Olivera Canadas G. Dermatological cryosurgery in primary care with dimethyl ether propane spray in comparison with liquid nitrogen. Translated from: Atencion Primaria, Vol. 18 No. 5 (211, 216), September 30, 1996; see also Gage, et. al. referenced above. The minimum temperature theoretically achievable with DMEP is -57°C which can also be achieved via use of a multistage Peltier cooling element.

[0006] A number of cold therapy devices have been described in prior art, with intended applications covering uses such as cooled garments, cryolipolysis, de-pigmentation of skin, removing facial swelling and others. Most such devices rely on the cooling action produced via a Peltier cooler coupled with a cooling fan (US5800490, US5097828, US 2008/0300529); a Peltier cooler coupled with phase change materials or PCMs (US20070193278), or a cooling pack filled with a phase change material (PCM) (US1301579, WO2007/102362A1). Another example of a system providing a cooling effect relies on evaporation of liquid (US 2014/0081361). A more elaborate cooling system has also been proposed (US9314368).

[0007] The systems described in prior art are either complex and large in size, require preconditioning such as placement into a refrigerated environment prior to use, or when described as "portable" or "handheld", not enough design details are provided to assess the weight and operational time of such device. Additionally, the simplest cooling pack-like devices do not allow for temperature adjustment, and thus cannot easily accommodate for individual user's comfort threshold to cold treatment.

[0008] There is a need for a miniature cold therapy system capable of treating inflammation associated with acne, sports and non-sports related injuries, muscle soreness, swelling in the under-eye or other facial areas, and suitable for broad consumer use. There also exists a need for an electrically powered rechargeable/reusable product which can replace DMEP based over-the-counter products.

SUMMARY

[0009] The present disclosure is directed to a cold therapy system using a Peltier cooler and phase change material (PCM) based applicator capable of applying cold to the skin and underlying muscles in a controlled manner. The system is integrated into a miniature handheld device, and the treatment is self-administered by the patient or user at home or in other settings, such as outdoors. To administer treatment, the applicator is placed on the treated area and the delivery of cold therapy is configured via a user interface (UI).

[00010] Embodiments of the systems disclosed herein meet the following requirements:

(i) it is small (pocket size), (ii) battery powered, (iii) does not require preconditioning, (iv) allows the user to adjust treatment temperature, and, finally, (v) it is optimized to provide cooling action for the longest possible time given a practical device weight.

[00011] Generally, two design implementations are described. In one embodiment, the device achieves optimal performance in a short height/width aspect ratio configuration. Such embodiment represents a cold therapy device with a large applicator area which is best suited, but not limited to, treating sports and non-sports related injuries, and muscle soreness following exercise. In another embodiment, the device achieves optimal performance in a long height/width aspect ratio configuration. Such embodiment represents a cold therapy device with a small applicator area which is best suited, but not limited to, treating acne pimples, under-eye bags and dark circles, and skin lesions and warts. At the center of the disclosed invention, the design and the theory of operation of a Peltier cooler and a PCM based applicator are provided. For a given weight of the device, the device design maximizes its operational time, or the duration of its cooling action.

[00012] An apparatus for providing cold therapy to a user is described herein. The apparatus may comprise a cooling element having a first surface and a second surface, wherein the first surface is configured in thermal contact with an application surface, and the second surface is configured in thermal contact with a phase change material. The cooling element may comprise a Peltier cooler. The phase change material may change phase at a temperature in the range of about 37°C to about 42°C. The phase change material may comprise an organic material selected from the group consisting of paraffins, fatty acids, inorganic salt hydrates, and polyglycols. The apparatus may further comprise a plurality of thermally conductive bristles providing thermal contact between the cooling element and the phase change material. The thermally conductive bristles may comprise wires, strips of foil, or fins. The apparatus may further comprise a heat pipe providing thermal contact between the cooling element and the phase change material. The thermally conductive bristles may be thermally connected with the heat pipe. The phase change material may be contained within a cavity. The cavity may comprise a flexible material. The cavity may include one or more pressure relief components.

[00013] Embodiments of the apparatus are small and meant to be handheld. For example, the apparatus may have a total volume of less than 200 cm³. The apparatus may include one or more temperature sensors. The apparatus may comprise a controller configured to adjust a temperature of the application surface based on a parameter sensed by the one or more temperature sensors. The apparatus may comprise a rechargeable battery. Embodiments of the apparatus may further comprise a controller configured to reverse bias the cooling element during recharging of the rechargeable battery. The apparatus may be configured to cool the application surface to atemperature of -5°C to +15°C, 20°C to -5°C, or 60°C to -20°C, for example. The apparatus may include a user interface. Embodiments of the apparatus may further include a controller configured to adjust the temperature of the application surface to a temperature entered in the user interface. The apparatus may include a temperature sensor configured to sense a temperature of a patient's skin and a controller configured to reduce or cease cooling if the sensed temperature falls below a predetermined value.

[00014] Further advantages of the disclosed embodiments will become evident through the examination of the detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[00015] The accompanying drawings illustrate the various embodiments of the principles described herein. The illustrated embodiments are merely examples and do not limit the scope of the disclosure.

[00016] FIG. 1 A shows an example of a cooling device with a short height/width aspect ratio.

[00017] FIG. IB shows an example of a cooling device with a long height/width aspect ratio.

[00018] FIG. 1C shows an example of a cooling device with a long height/width aspect ratio.

[00019] FIG. 2 shows a detailed diagram of one embodiment of a short height/width aspect ratio cooling device.

[00020] FIG. 3 shows a detailed diagram of one embodiment of a long height/width aspect ratio cooling device.

[00021] FIG. 4 shows an embodiment of a cooling device using a rigid auxiliary shell to house the PCM.

[00022] FIG. 5 shows an embodiment of a cooling device using a flexible auxiliary shell to house the PCM.

[00023] FIG. 6 shows computed efficiency of a short height/width aspect ratio cooling device vs its aspect ratio.

[00024] FIG. 7 shows computed efficiency of a long height/width aspect ratio cooling device vs its aspect ratio.

DESCRIPTION

[00025] Two general design configurations of cold therapy devices are disclosed herein: one that is best suited for applications requiring a device with a short aspect ratio (i.e., the height of the device is close to, or smaller than its width), and one that is best suited for applications requiring a device with a long aspect ratio (i.e., the height of the device is close to, or greater than its width). The short aspect ratio device is conceptually depicted in FIG. 1 A. Such device configuration can be used when a larger skin contact area is required, yet the device needs to remain small and lightweight. Examples of such uses may include, but need not be limited to, the treatment of inflammation associated with acute or chronic injuries and associated pain. According to some embodiments, the short aspect ratio device has a total volume of less than about 200 cm³. For example, a cylindrical device, as depicted in FIG. 1A may have a length of less than 2 cm and a diameter of less than 14 cm. It should be noted that the device may have a different shape than illustrated.

[00026] The long aspect ratio device is conceptually depicted in FIGs. IB and 1C. Such device configuration could be used when a smaller skin contact area is required, yet the device needs to be long enough to fit comfortably within the user's hand. Examples of such uses may include, but need not be limited to, the treatment of acne pimples, and skin lesions and warts (IB), and under-eye bags and dark circles (1C). For all configurations in FIG. 1, 405 denotes skin contact surface (i.e, the application surface) of the device. According to some embodiments, the long aspect ratio device has a total volume of less than about 100 cm³. For example, a cylindrical device, as depicted in FIG. IB or 1C may have a length of less than 15 cm and a diameter of less than 3.5 cm. It should be noted that the device may have a different shape than illustrated.

[00027] A detailed diagram of one embodiment of a short aspect ratio device 100 is shown in FIG. 2. In this embodiment, the handheld device 100 comprises an enclosure 401 filled with a phase change material (PCM) 402, a Peltier cooler 403, a cold side thermally conductive plate 404 (in other art, such part is often being referred to as an applicator) with the skin contact surface 405, a hot side thermally conductive plate 406 thermally linked with a plurality of thermally conductive bristles 407, a plurality of pressure relief bulbs 408 with air escape channels 409, a plurality of user controls or the user interface (UI) 410, a battery pack 411, a controller unit 412, a battery charging port 413, and thermistors 414 and 415. The bristles 407 may be welded to the hot side plate 406, or both the bristles 407 and hot side plate 406 may be cast or machined as a single part.

[00028] The device 100 is powered by a battery pack 411, and is controlled via UI 410 which may include but is not limited to push buttons, switches, potentiometers, LEDs, and an alphanumerical or a graphical display. User may receive visual feedback via light sources integrated into the UI 410, or via audio feedback from an audio source incorporated within the controller unit 412. UI 410 may be used to enable/disable device, and to configure the device temperature. The battery pack 411 is preferably a rechargeable Li-Ion type, although other battery types suitable for high current draw application can also be used.

[00029] Aspects of the device may be controlled by one or more control units 412, also referred to as a controller or microcontroller unit. The control unit 412 may comprise a microcontroller for example such as Part Number ATtinyl04, manufactured by Microchip, which is described in data sheets at http://wwl .microchip.com/downloads/en/DeviceDoc/Atmel-42505-8-bit-AVR- Microcontrollers-ATtinyl02-ATtinyl04_Datasheet.pdf, which is incorporated herein by reference. Other types of control circuitry may be used in lieu of a microcontroller as well, such as microprocessors, FPGAs, DSPs, or combinations of these, etc. Control circuitry 412 may also be formed in whole or in part in one or more Application Specific Integrated Circuits (ASICs).

[00030] Once the device is enabled, the Peltier cooler 403 begins to pump heat from the user's skin via skin contact surface 405 and thermally conductive plate 404, and via thermally conductive plate 406 and bristles 407 into the PCM 402 which acts as a high heat storage capacity component. For the case of a short aspect ratio device, the bristles 407 are parallel to each other and normal to the thermally conductive plate 406. Such bristle configuration ensures that the bristle material is only present along the paths on which heat propagates from the Peltier cooler 403 to the PCM 402. This minimizes amount of bristle material and device's weight. The temperature at the user's skin is read via the thermistor 415 and is used by the controller unit 412 to adjust the intensity of the cooling effect produced by the Peltier cooler 403. The intensity of the cooling effect is controlled by adjusting the output voltage or current of a switching power converter incorporated within the controller unit 412. The thermistor 414 is used to monitor the state of the PCM, so that the device can be switched off in the event the entire volume of PCM has changed its state (phase) and it is no longer able to absorb energy. The plurality of flexible pressure relief bulbs 408 with associated air escape channels 409 are needed to allow PCM material expand almost freely inside the enclosure 401 while absorbing energy during the device's operation. The rechargeable battery 411 is recharged via a charging port 413 which can be an industry standard type connector, such as micro-USB, or any other suitable connector type.

[00031] During the device operation, the PCM near the Peltier undergoes phase transition and stops absorbing heat, and thus the bristles 407 have to conduct heat to the PCM located farther from the Peltier cooler 403. As a result, the temperature difference between the hot and the cold side of the Peltier cooler increases.

[00032] The enclosure 401 can be fabricated using a variety of commonly used materials which may include, but are not limited to, plastics and metals. Thermally conductive plates 404 and 406 are fabricated from a metal with high thermal conductivity, such as copper or aluminum. Pressure relief bulbs 408 are fabricated from rubber, silicone or any other flexible but durable material compatible with the PCM. [00033] A good choice of the PCM 402 will have a phase transition temperature of around 37°C-42°C. This temperature range is high enough to prevent unintended phase change of a PCM in a typical use environment, including being stored in a user's pocket, while low enough to ensure an acceptable Peltier cooler efficiency. Examples of such PCMs are organic materials such as paraffins, fatty acids and polyglycols, and inorganic, such as salt hydrates. Some examples of the commercially available materials are: X40, A37 and A40 from PCM Products Ltd. ; OM-37P from RGEES, LLC; and PureTemp 37 and PureTemp 42 from Entropy Solutions, LLC.

[00034] A detailed diagram of one embodiment of a long aspect ratio the handheld device 200 is shown in FIG. 3. In FIG. 3 (and in the other drawings) element numerals have the same identity as described above, unless otherwise noted. The composition and operation of the handheld device 200 is largely the same as that of the device 100 described above, apart from how heat is transferred from the thermally conductive plate 406 to the PCM 402. In device 200, an additional element - a heatpipe 416 - is thermally connected to the plate 406, and the bristles 407 are thermally connected to the heatpipe 416 in a radial pattern. Such configuration allows heat travel further down into the PCM without loss of thermal efficiency. Additionally, radial bristle configuration ensures that the bristle material is only present along the paths on which heat propagates from the heatpipe 416 to the PCM 402. This minimizes amount of bristle material and device's weight. In one embodiment, the bristles 407 are welded to the heatpipe 416. In another embodiment, both the 406 and 407 can be cast or machined as a single component, in which the heatpipe 416 is then inserted. In the latter case, the thermal contact with the heatpipe can be achieved via application of thermal grease or thermally conductive adhesive.

[00035] Another embodiment applicable to both the device 100 and the device 200 configurations, is shown in FIG. 4. In this drawing some design details, such as the user interface, batteries, bristles, etc. have been omitted for clarity. Element numerals have the same elements as in earlier drawings, unless otherwise noted. In this embodiment, the PCM 402 is housed in a can 20 formed by a rigid auxiliary shell 418, and the thermally conductive plate 406. The can 20 is formed by inserting the auxiliary shell 418 into the crease of the fold 419 present in the thermally conductive plate 406. The fold is subsequently flattened to yield a hermetic seal between the two components comprising a can - a process widely used in canning foods and beverages. The can 20 is not fully filled with the PCM 402, but instead a small volume 420 of the can 20 is left PCM free and may be evacuated to allow the PCM to expand into the volume 420 during device's operation. In this embodiment, the auxiliary shell 418 can be fabricated from a commonly used metal such as aluminum or steel. The plurality of pressure relief bulbs 408 are not used in this embodiment.

[00036] Yet another embodiment applicable to both the device 100 and the device 200 configurations, is shown in FIG. 5. In this drawing some design details such as the user interface, batteries, bristles, etc. have also been omitted for clarity. Element numerals have the same elements as in earlier drawings, unless otherwise noted. In this embodiment, the PCM 402 is housed in the volume of a can 20 formed by a flexible auxiliary shell 418, and the thermally conductive plate 406. The can 20 is formed by placing the flexible auxiliary shell 418 over the thermally conductive plate 406, and securing it in place using a clamp 421. In this case, the can 20 may be fully filled with the PCM 402, and since the auxiliary shell 418 is flexible and can expand along with the PCM during device's operation. The auxiliary shell 418 can be fabricated from rubber, silicone or any other flexible but durable material. The plurality of pressure relief bulbs 408 are not used in this embodiment.

[00037] The bristles 407 used with the any of the device configurations can have cross- section of any shape so long as they are numerous enough to ensure that the structure formed by the bristles 407 and the PCM 402 behaves as a homogeneous composite material from the heat transfer prospective. Note that such material is anisotropic by design as it conducts heat much more effectively along the direct paths from the heat source (the applicator) to the heat sink (the PCM). In a practical device, bristles 407 can, for instance, be a plurality of thin wires or thin strips of foil, or plurality of fins. Bristles 407 can be fabricated from a metal with high thermal conductivity (copper, aluminum, etc.), or another material such as graphite.

[00038] In devices intended for skin lesion and wart treatments, or other cryosurgical- like applications where treatment temperatures are considerably below freezing, the Peltier cooler 403 will preferably have a multistage configuration.

[00039] Examples of suitable single stage Peltier coolers include CP30238 available from CUI, Inc., NL1025T available from Marlow Industries Inc., or, for example, TEFC1- 03520PM3 available from Thermonamic. A simple and economic multistage Peltier cooler solution can be achieved by stacking two, three or four parts described above or similar ones, and powering them in such way that the thermal efficiency of the resulting Peltier element stack is maximized. It is within the ability of a person skilled in the art to determine an appropriate Peltier cooler based on their specific application. Embodiments of the cooling devices described herein can cool the application surface to temperature such as from about - 5°C to +15°C, or about -20°C to -5°C, or about -60°C to -20°C, or to any temperature within those ranges for example. [00040] The systems described herein do not require preconditioning and thus, can be enabled on demand if, for instance, an unexpected injury requiring cold treatment, has occurred. To use the device, the user would select or set the desired applicator temperature, and enable the device utilizing device's UI 410. If prolonged device placement in a single treatment area is desired, an optional strap can be used to secure the device on the user's body.

[00041] Once the device's battery pack if depleted, it needs to be replaced or recharged. A typical charging time for a Li-Ion battery is around one hour and is sufficiently long to allow the PCM encapsulated in a small device to release the stored heat completely. The process of heat release can be accelerated by reverse biasing the Peltier cooler during device charging, and thus allowing the heat to flow back from the PCM to the applicator of the device.

[00042] The two essential parameters pertinent to a hand-held cold therapy device, or any battery powered device for that matter, are its weight and the operating time (battery run time). For a well-designed device the ratio of its operating time to its weight should be maximized. Below we describe a methodology for optimizing cold therapy's device efficiency, for the cases when either a short or a long aspect ratio device is desired.

[00043] To simplify the analysis of the cold therapy device's performance, some approximations can be made:

• The device geometries have axial symmetry.

• Thermally conductive bristles 407 are thin and numerous and this makes PCM

402 undergo phase transition in such way that the boundary separating the two phases is a smooth surface normal to the bristle direction. For the case of the short aspect ratio device, such boundary would be a plane moving away from the Peltier cooler; for the case of the long aspect ratio device employing a heatpipe 416, such boundary would be a cylinder expanding away from the heatpipe.

• Thermally conductive bristles 407 conduct heat much more effectively than the PCM. As a result, the thermal conductivity of the PCM 402 becomes irrelevant.

• Because PCM takes a very long time to undergo phase transition, only the thermal conductivity of bristles 407, and not their heat capacity, is relevant for the analysis.

• Because the heat of fusion of the PCM 402 is much greater than its heat

capacity, the latter can be ignored. [00044] Additionally, in order to describe the analysis of the proposed designs, certain quantities are defined below:

• TPT - phase transition temperature of the PCM.

• ΔΤ - temperature elevation of a hot side of the Peltier cooler over the phase transition temperature TPT of the PCM.

• T^tis - target tissue temperature.

• AT^Pelt- temperature difference between hot and cold side of the Peltier cooler.

A_Tp_eit = _TpT _ _Ttis + _Δχ

• D^PCM - density of the PCM.

• HfP™ - heat of fusion of the PCM.

• S^cond - for the short aspect ratio device: the relative area crossed by the

conductive bristles in the plane normal to the whisker direction; for the long aspect ratio device: the total surface area of a cylinder of unit height which is coaxial with a heatpipe, crossed by the conductive bristles.

• K^cond - thermal conductivity of the conductive bristles.

• H - height of the internal device area filled with PCM (the overall device height will be somewhat larger).

• R - radius of the internal device area filled with PCM (the overall device radius will be somewhat larger).

• Rt - thermal resistance of the bristle set.

• x - effective bristle length, or the length of bristle segment conducting heat to the PCM. x is also the distance between the Peltier or the heatpipe and the boundary separating two phases of the PCM.

• Qabs - the rate of heat absorption from the treated tissue.

• Q - the rate of heat generation on the hot side of the Peltier cooler.

[00045] As mentioned above, the thermal efficiency of a Peltier cooler (the ratio of heat pumped to heat generated) decreases markedly as AT^Pelt increases. This implies that as the PCM undergoes phase transition, and the boundary separating the two phases moves farther away from the Peltier cooler, the thermal resistance between the Peltier cooler and the PCM increases and thus causes cooling efficiency to drop.

[00046] In our analysis we assume that the value of T^tls is determined by the therapeutic goals and is maintained constant during treatment. The value of Qabs is dependent on V^1S, tissue properties such as thermal conductivity and blood perfusion rate, treatment tip area, and the use model. For a treatment procedure lasting minutes, Qabs can be assumed to have reached steady state and thus be assigned a constant value. For a given device configuration, Qabs can be estimated via thermal simulations using commercial software such as Cortisol, CST or other.

[00047] For a typical Peltier cooler, data relating the rate of heat generation by the Peltier Q with AT^Pelt and Qabs is readily available. Since by definition AT^Pelt = TPT - T^tls + ΔΤ, Q can also be expressed in terms of ΔΤ, Qabs, TPT and T^tis, where only ΔΤ is assumed to vary during treatment. Due to finite efficiency of the Peltier cooler, Q has an offset greater than Qabs for ΔΤ = 0 and increases monotonically as ΔΤ increases. On the other hand, the rate of heat transfer Q* from the Peltier cooler via thermally conductive bristles to the PCM is expressed via ΔΤ and the thermal resistance of bristles Rt as Q* = ΔΤ7 Rt ~ ΔΤ/χ, where x is the effective bristle length. By equating Q and Q*, ΔΤ is calculated for a given value of x. Since Q* is a monotonic function of both ΔΤ and x, the heat generation rate Q can be uniquely expressed as a function of x, or Q(x).

[00048] Short aspect ratio device. Thermal resistance of bristles Rt is calculated as

D _ Ϊ Ι Λ

t _j 2scond] Cond ' ' "

Using datasheet information for the Q(AT^Pelt, Qabs) of the Peltier cooler, Q is recalculated as a function of x, or Q(x). This step has been described above.

[00049] Suppose now that at a given time x, the boundary separating the two phases of the PCM is located at the distance χ from the heat source. After time dx, the boundary separating the two phases of the PCM moves by distance άχ. The relationship between άχ and dx is determined by equating the amount of total heat generated by the Peltier cooler during time interval dx, to the total heat needed to melt the PCM confined to the volume between the two planes crossing bristles normally at the distances χ and χ+άχ from the Peltier cooler:

(l-S^cond) nR² d_X D^PCM H_f ^PCM = Q(_X) dx, or

dx dx

(2) .

Q(X) nR²(l-S^cond)D^PCMH ^CM

Integrating Eq. (2), the following result is obtained:

[00050] Once left-hand side of Eq. (3) is integrated numerically (analytical solutions may also exist for some functional forms of Q(x)), the relationship between x and time t is determined, and therefore, Q(x) can be expressed as a function of t, or Q(t). Note that the performance of the short aspect ratio device will not notably degrade if the device's cross section of not strictly circular. If for a given application a non-circular device shape is preferred, the quantity nR² in Eqs. (1-3) can be substituted with the base area of such non- circular shape.

[00051] For a given energy capacity of the device battery E^bat, the device operating time tMAx is determined from the following relationship:

I₀ ^tMAX(m - Cla_bs} dt = E^bat (4).

The value of x corresponding to the value ΪΜΑΧ gives the internal device cavity height H for which the entire volume of PCM material is usable for a given battery energy capacity E^bat.

[00052] Note that the weight of such a device depends strongly on the aspect ratio of the device and the density of the thermally conductive bristles. As the bristle density increases, the thermal efficiency of the Peltier cooler increases due to the decrease in the thermal resistance Rt, however the device weight also increases. Decreasing aspect ratio of the device without changing its volume decrease Rt, however the surface area of the device increases relative to its volume, and so does the weight of the enclosure relative to that of the PCM. Thus, Eqs. (3) and (4) need to be solved for a range of parameter values R and S^cond until the ratio of ΪΜΑΧ to the device's weight is maximized.

[00053] In FIG. 6, computed device efficiency vs its aspect ratio is shown. For each of the points on the graph, the value of parameter S^∞nd was optimized to yield the highest device efficiency. For the purpose of this simulation, the battery capacity was assumed to be 3.7W/h, battery weight was 25g, the bristle material was copper, the PCM was a wax based material with Hf^PCM = 218J/g, D^PCM = 0.9g/cm³. The enclosure was made of ABS plastic with wall thickness of 1.5mm. The Peltier device data used in the simulation was for part CP30238 from CUI, Inc., with two Peltier devices being used to produce the needed cooling effect. Qabs and V^1S were assumed to be 10W and +10°C respectively. Under these assumptions, the calculated optimal value of R is 6.0cm, optimal value of H is 1.1cm, optimal weight of the device is 205g, and the total operating time of the device is 18 minutes. Note that the final dimensions of the device will be somewhat larger in order to accommodate the battery.

[00054] Long aspect ratio device. Thermal resistance of bristles Rt is calculated as

D _ Ϊ l c \

t Condj Condf] ' ' "

Using the datasheet information for the Q(AT^Pelt, Qabs) of the Peltier cooler, Q is recalculated as a function of x, or Q(x). This step has been described in more detail earlier.

[00055] Suppose now that at a given time x, the cylindrical boundary separating two phases of the PCM is coaxial with the heatpipe and has radius χ. After a time dx, the boundary's radius increases by άχ. The relationship between άχ and dx is determined by equating the amount of total heat generated by the Peltier cooler during time interval dx, to the total heat needed to melt the PCM confined to the volume between the two cylinders with radii χ and

(2πχΗ -S^condH) άχ D^PCM Hf^CM = Q(y) dx, or

Integrating Eq. (6), the following result is obtained:

t

[00056] Once the left-hand side of Eq. (7) is integrated numerically (analytical solutions may also exist for some functional forms of Q(x)) the relationship between x and t is determined, and therefore, Q(x) can be expressed as a function of time t, or Q(t), and for a given energy capacity of the device battery E^bat, the device operating time ΪΜΑΧ is determined from Eq. (4). The value of x corresponding to the value ΪΜΑΧ gives the internal device cavity radius R for which the entire volume of the PCM material is usable for a given battery energy capacity E^bat.

[00057] Note that the weight of such device also depends strongly on the aspect ratio of the device and the density of the thermally conductive bristles. As the bristle density increases, the thermal efficiency of the Peltier cooler increases due to the decrease in the thermal resistance Rt, however the device weight also increases. Increasing aspect ratio of the device without changing its volume decrease Rt, however the surface area of the device increases relative to its volume, and so does the weight of the enclosure, and that of the heatpipe, relative to that of the PCM. Thus, Eqs. (7) and (4) need to be solved for a range of parameter values H and S^cond until the ratio of tMAx to the device's weight is maximized.

[00058] In FIG. 7, the computed device efficiency vs the device aspect ratio is shown.

For each of the points on the graph, the value of the parameter S^cond was optimized to yield the highest device efficiency. For the purpose of this simulation, the battery capacity was assumed to be 1.85W7h, battery weight was 12.5g, the bristle material was copper, the PCM was a wax based material with Hf^PCM = 218J/g, D^PCM = 0.9g/cm³, the enclosure was made of ABS plastic with wall thickness of 1.5mm. The Peltier device data used in the simulation was for part number CP30238 from CUI, Inc., with a single Peltier device being used to produce the needed cooling effect. Linear weight of the heatpipe was assumed to be 1.2g/cm. Qabs and V^1S were assumed to be 3W and 0°C respectively. Under these assumptions, the calculated optimal value of R is 1.4cm, the optimal value of H is 10cm, optimal weight of the device is 106g, and the total operating time of the device is 19 minutes. Note that the final dimensions of the device will be somewhat larger in order to accommodate the battery.

[00059] Although particular embodiments have been shown and described, the above discussion should not limit the present invention to these embodiments. Various changes and modifications may be made without departing from the spirit and scope of the present invention. Thus, the present invention is intended to cover equivalent embodiments that may fall within the scope of the present invention as defined by the claims.

Claims

WHAT IS CLAIMED IS:

1. An apparatus for providing cold therapy to a user, the apparatus comprising:

a cooling element having a first surface and a second surface, wherein

the first surface is configured in thermal contact with an application surface, and the second surface is configured in thermal contact with a phase change material.

2. The apparatus of claim 1 , wherein the cooling element comprises a Peltier cooler.

3. The apparatus of claim 1, wherein the phase change material changes phase at a temperature in the range of about 37°C to about 42°C.

4. The apparatus of claim 1, wherein the phase change material comprises an organic material selected from the group consisting of paraffins, fatty acids and poly glycols.

5. The apparatus of claim 1, wherein the phase change material is composed of salt hydrates.

6. The apparatus of claim 1 , further comprising a plurality of thermally conductive bristles providing thermal contact between the cooling element and the phase change material.

7. The apparatus of claim 6, wherein the thermally conductive bristles comprise wires, strips of foil, or fins.

8. The apparatus of claim 1 , further comprising a heat pipe providing thermal contact between the cooling element and the phase change material.

9. The apparatus of claim 8, further comprising thermally conductive bristles thermally connected with the heat pipe.

10. The apparatus of claim 1, wherein the phase change material is contained within a cavity.

11. The apparatus of claim 10, wherein the cavity comprises a flexible material.

12. The apparatus of claim 10, wherein the cavity comprises one or more pressure relief components.

13. The apparatus of claim 1 , having a total volume of less than 200 cm³.

14. The apparatus of claim 1 , further comprising one or more temperature sensors.

15. The apparatus of claim 14, further comprising a controller configured to adjust a temperature of the application surface based on a parameter sensed by the one or more temperature sensors.

16. The apparatus of claim 1, further comprising a rechargeable battery.

17. The apparatus of claim 16, further comprising a controller configured to reverse bias the cooling element during recharging of the rechargeable battery.

18. The apparatus of claim 1 , wherein the apparatus is configured to cool the application surface to a temperature of -5°C to +15°C.

19. The apparatus of claim 1 , further comprising a user interface.

20. The apparatus of claim 19, further comprising a controller configured to adjust the temperature of the application surface to a temperature entered in the user interface.

21. The apparatus of claim 21 , further comprising a temperature sensor configured to sense a temperature of a patient's skin and a controller configured to reduce or cease cooling if the sensed temperature falls below a predetermined value.

22. The apparatus of claim 1 , configured to cool the application surface to -20°C to -5°C.

The apparatus of claim 1, configured to cool the application surface to -60°C to -20°C.