Patent Publication Number: US-2022218028-A1

Title: Aerosol-generating system including a cartridge containing a gel and a device for heating the cartridge

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of U.S. application Ser. No. 15/662,472, filed Jul. 28, 2017, which is a continuation of and claims priority to PCT/EP2017/068549, filed on Jul. 21, 2017, and further claims priority to EP 16181956.0, filed on Jul. 29, 2016, the disclosures of each of which are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     Field 
     Example embodiments relate to an aerosol-generating system that heats an aerosol-forming substrate to generate an aerosol, including an aerosol-generating system that heats a gel to form an aerosol. 
     Description of Related Art 
     Aerosol-generating systems operate by heating a liquid formulation to generate an aerosol. Typically, aerosol-generating systems comprise a device portion and a cartridge. In some systems, the device portion contains a power supply and control electronics, and the cartridge contains a liquid reservoir holding the liquid formulation, a heater for vapourising the liquid formulation, and a wick that transports the liquid from the liquid reservoir to the heater. However, there is a potential for leakage of the liquid from the liquid reservoir both during transport and storage, and when the cartridge is connected to the device portion. The use of a wick to transport the liquid from the reservoir to the heater may also add complexity to the system. 
     SUMMARY 
     An aerosol-generating system may include a device body including a power supply and an electrical heater. The electrical heater is electrically connected to the power supply. The system may also include a cartridge containing an aerosol-forming substrate in a form of a thermoreversible gel that is a solid at room temperature. The cartridge is configured to be removably inserted into or connected to the device body. 
     The system is configured such that the electrical heater does not contact the aerosol-forming substrate when the cartridge is inserted into or connected to the device body. 
     The cartridge includes at least one wall between the electrical heater and the aerosol-forming substrate when the cartridge is inserted into or connected to the device body. 
     The electrical heater may include a resistive heating track in or on a substrate material. 
     The cartridge may define a slot, and the electrical heater may be configured to be received in the slot. The slot may be a blind slot. 
     The cartridge includes at least one wall in thermal contact with the electrical heater when the cartridge is inserted into or connected to the device body. 
     The cartridge includes at least one liquid impermeable and vapour impermeable external wall defining a blind cavity, and the aerosol-forming substrate is contained in the blind cavity. 
     The cartridge includes a sealing element sealing the blind cavity. 
     The device body includes a mouthpiece portion separate from the cartridge. 
     The cartridge may define a first chamber and a second chamber separate from the first chamber. 
     At least a portion of the electrical heater may be positioned between the first and second chambers when the cartridge is inserted into or connected to the device body. 
     The thermoreversible gel may include a source of nicotine or a tobacco product. 
     A cartridge (for an aerosol-generating system including a device body and a heater) may include a substrate container containing an aerosol-forming substrate in a form of a thermoreversible gel that is a solid at room temperature. The substrate container is configured to removably connect to or be received in the device body of the aerosol-generating system. The substrate container may define a slot configured to receive the heater. 
     The substrate container includes at least one liquid impermeable and vapour impermeable external wall defining a blind cavity, and the aerosol-forming substrate is contained in the blind cavity. 
     The cartridge may further include a mouthpiece tube holding the substrate container. 
     The mouthpiece tube may include an air flow restrictor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The various features and advantages of the non-limiting embodiments herein may become more apparent upon review of the detailed description in conjunction with the accompanying drawings. The accompanying drawings are merely provided for illustrative purposes and should not be interpreted to limit the scope of the claims. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. For purposes of clarity, various dimensions of the drawings may have been exaggerated. 
         FIG. 1  is a schematic illustration of an aerosol-generating system in accordance with an example embodiment. 
         FIG. 2 a    is a perspective view of a mouthpiece portion in accordance with an example embodiment. 
         FIG. 2 b    is a bottom perspective view of a cartridge in accordance with an example embodiment. 
         FIG. 2 c    is a top perspective view of the cartridge of  FIG. 2   b.    
         FIG. 2 d    is a cross-sectional view of the cartridge of  FIG. 2   b.    
         FIGS. 3 a , 3 b , and 3 c    illustrate a sequence including an insertion of a cartridge into a device body and a piercing of a frangible seal on the cartridge by a mouthpiece portion in accordance with an example embodiment. 
         FIG. 4  is a schematic illustration of another aerosol-generating system in accordance with an example embodiment. 
         FIG. 5 a    is a schematic illustration of a cartridge held within a mouthpiece tube in accordance with an example embodiment. 
         FIG. 5 b    is an exploded view of the elements within the mouthpiece tube of  FIG. 5   a.    
         FIG. 6  is an illustration of the airflow through the mouthpiece tube of  FIG. 5   a.    
         FIG. 7 a    is a schematic illustration of another aerosol-generating device in accordance with an example embodiment. 
         FIG. 7 b    shows the device of  FIG. 7 a    with a cartridge received in a cavity of the device. 
         FIG. 8  shows the cartridge of  FIG. 7 b    in more detail. 
     
    
    
     DETAILED DESCRIPTION 
     It should be understood that when an element or layer is referred to as being “on,” “connected to,” “coupled to,” or “covering” another element or layer, it may be directly on, connected to, coupled to, or covering the other element or layer or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly connected to,” or “directly coupled to” another element or layer, there are no intervening elements or layers present. Like numbers refer to like elements throughout the specification. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     It should be understood that, although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers, and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer, or section from another region, layer, or section. Thus, a first element, component, region, layer, or section discussed below could be termed a second element, component, region, layer, or section without departing from the teachings of example embodiments. 
     Spatially relative terms (e.g., “beneath,” “below,” “lower,” “above,” “upper,” and the like) may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It should be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the term “below” may encompass both an orientation of above and below. The device may be otherwise oriented (rotated  90  degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
     The terminology used herein is for the purpose of describing various embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “includes,” “including,” “comprises,” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     Example embodiments are described herein with reference to cross-sectional illustrations that are schematic illustrations of idealized embodiments (and intermediate structures) of example embodiments. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, example embodiments should not be construed as limited to the shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which example embodiments belong. It will be further understood that terms, including those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     Unless specifically stated otherwise, or as is apparent from the discussion, terms such as “processing” or “computing” or “calculating” or “determining” or “displaying” or the like, refer to the action and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical, electronic quantities within the computer system&#39;s registers and memories into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices. 
     In the following description, illustrative embodiments may be described with reference to acts and symbolic representations of operations (e.g., in the form of flow charts, flow diagrams, data flow diagrams, structure diagrams, block diagrams, etc.) that may be implemented as program modules or functional processes including routines, programs, objects, components, data structures, etc., that perform particular tasks or implement particular abstract data types. The operations be implemented using existing hardware in existing electronic systems, such as one or more microprocessors, Central Processing Units (CPUs), digital signal processors (DSPs), application-specific-integrated-circuits (ASICs), SoCs, field programmable gate arrays (FPGAs), computers, or the like. 
     One or more example embodiments may be (or include) hardware, firmware, hardware executing software, or any combination thereof. Such hardware may include one or more microprocessors, CPUs, SoCs, DSPs, ASICs, FPGAs, computers, or the like, configured as special purpose machines to perform the functions described herein as well as any other well-known functions of these elements. In at least some cases, CPUs, SoCs, DSPs, ASICs and FPGAs may generally be referred to as processing circuits, processors and/or microprocessors. 
     Although processes may be described with regard to sequential operations, many of the operations may be performed in parallel, concurrently or simultaneously. In addition, the order of the operations may be re-arranged. A process may be terminated when its operations are completed, but may also have additional steps not included in the figure. A process may correspond to a method, function, procedure, subroutine, subprogram, etc. When a process corresponds to a function, its termination may correspond to a return of the function to the calling function or the main function. 
     As disclosed herein, the term “storage medium”, “computer readable storage medium” or “non-transitory computer readable storage medium,” may represent one or more devices for storing data, including read only memory (ROM), random access memory (RAM), magnetic RAM, core memory, magnetic disk storage mediums, optical storage mediums, flash memory devices and/or other tangible machine readable mediums for storing information. The term “computer-readable medium” may include, but is not limited to, portable or fixed storage devices, optical storage devices, and various other mediums capable of storing, containing or carrying instruction(s) and/or data. 
     Furthermore, at least some portions of example embodiments may be implemented by hardware, software, firmware, middleware, microcode, hardware description languages, or any combination thereof. When implemented in software, firmware, middleware or microcode, the program code or code segments to perform the necessary tasks may be stored in a machine or computer readable medium such as a computer readable storage medium. When implemented in software, processor(s), processing circuit(s), or processing unit(s) may be programmed to perform the necessary tasks, thereby being transformed into special purpose processor(s) or computer(s). 
     A code segment may represent a procedure, function, subprogram, program, routine, subroutine, module, software package, class, or any combination of instructions, data structures or program statements. A code segment may be coupled to another code segment or a hardware circuit by passing and/or receiving information, data, arguments, parameters or memory contents. Information, arguments, parameters, data, etc. may be passed, forwarded, or transmitted via any suitable means including memory sharing, message passing, token passing, network transmission, etc. 
     According to some example embodiments, there is provided an aerosol-generating system comprising a device comprising a power supply and an electrical heater connected to the power supply; and a substrate cartridge containing an aerosol-forming substrate in the form of a thermoreversible gel that is solid at room temperature; wherein the substrate cartridge is configured to be inserted into or connected to the device prior to use and removed or disconnected from the device after use. 
     In this context, an aerosol-forming substrate is a material or mixture of materials capable of releasing volatile compounds that can form an aerosol. The provision of the aerosol-forming substrate in the form of a gel may be beneficial for storage and transport, or during the operation of the system. By providing the aerosol-forming substrate in a gel, the risk of leakage from the device may be reduced. Replenishing of the device with aerosol forming substrate when depleted or exhausted may also be improved, for example by reducing the risk of leakage or spillage. 
     The provision of the heater within the device, not in the cartridge, allows for the production of relatively simple cartridges compared with integration of the heater in the cartridge. In an example embodiment, the system does not comprise a transport mechanism for transporting the gel to the electrical heater. The contents of the substrate cartridge may be heated in situ to generate a desired aerosol. In this context, in situ means in the same position within substrate cartridge that the contents are held prior to heating. There is no requirement for a capillary wick or pump. The electrical heater may be configured to heat the cartridge to generate a vapour within the cartridge from the gel. 
     The cartridge can be disposed of and replaced with relative ease when the gel has been consumed. 
     The substrate container may contain other materials in addition to the gel. 
     The gel is solid at room temperature. “Solid” in this context means that the gel has a stable size and shape and does not flow. Room temperature in this context means 25 degrees Celsius. 
     The gel may comprise an aerosol-former. As used herein, the term “aerosol-former” refers to any suitable known compound or mixture of compounds that, in use, facilitates formation of a dense and stable aerosol. An aerosol-former is substantially resistant to thermal degradation at the operating temperature of the cartridge. Suitable aerosol-formers are well known in the art and include, but are not limited to: polyhydric alcohols, such as triethylene glycol, 1,3-butanediol and glycerine; esters of polyhydric alcohols, such as glycerol mono-, di- or triacetate; and aliphatic esters of mono-, di- or polycarboxylic acids, such as dimethyl dodecanedioate and dimethyl tetradecanedioate. In an example embodiment, the aerosol formers are polyhydric alcohols or mixtures thereof, such as triethylene glycol, 1,3-butanediol, and glycerine or polyethylene glycol. 
     The gel may comprise a gelling agent. In an example embodiment, the gel comprises agar or agarose or sodium alginate. The gel may comprise Gellan gum. 
     The gel comprises a thermoreversible gel. This means that the gel will become fluid when heated to a melting temperature and will set into a gel again at a gelation temperature. The gelation temperature may be at or above room temperature and atmospheric pressure. Atmospheric pressure means a pressure of 1 atmosphere. The melting temperature is higher than the gelation temperature. The melting temperature of the gel may be above 50 degrees Celsius (e.g., above 60 degrees Celsius, above 70 degrees Celsius, above 80 degrees Celsius). The melting temperature in this context means the temperature at which the gel is no longer solid and begins to flow. The gel may comprise a gelling agent. The gel may comprise agar or agarose or sodium alginate. The gel may comprise Gellan gum. The gel may comprise a mixture of materials. The gel may comprise water. 
     The gel may be provided as a single block or may be provided as a plurality of gel elements, for example beads or capsules. The use of beads or capsules may allow for simple refilling of the first (or second) chamber. The use of capsules or beads may also provide a visual indication as to when a cartridge has already been used, because gel will not form the same capsules or beads on gelation after heating and subsequent cooling. 
     The gel may comprise nicotine or a tobacco product or another target compound for delivery. When the resulting aerosol is to contain nicotine, the nicotine may be contained in the gel or in another solid form in the substrate container rather than in a liquid. The nicotine can be included in the gel with an aerosol-former. Nicotine is irritating to the skin and can be toxic. Preventing any possible leakage of nicotine by locking the nicotine into a gel at room temperature is therefore desirable. 
     Flavour compounds may be contained in the second chamber in a gel. Alternatively or in addition, flavour compound may be provided in another form. For example, the second chamber may contain a solid tobacco material that releases flavour compounds when heated. The second chamber may contain, for example, one or more of: powder, granules, pellets, shreds, spaghettis, strips or sheets containing one or more of: herb leaf, tobacco leaf, fragments of tobacco ribs, reconstituted tobacco, homogenised tobacco, extruded tobacco and expanded tobacco. The solid tobacco material in the second chamber may be in loose form. The tobacco may be contained in a gel or liquid. The second chamber may contain additional tobacco or non-tobacco volatile flavour compounds, to be released upon heating. 
     When agar is used as the gelling agent, the gel may comprise between 0.5 and 5% by weight (e.g., between 0.8 and 1% by weight) agar. The gel may further comprise between 0.1 and 2% by weight nicotine. The gel may further comprise between 30% and 90% by weight (e.g., between 70 and 90% by weight) glycerin. A remainder of the gel may comprise water and any flavourings. 
     When Gellan gum is used as the gelling agent, the gel may comprise between 0.5 and 5% by weight Gellan gum. The gel may further comprise between 0.1 and 2% by weight nicotine. The gel may further comprise between 30% and 99.4% by weight glycerin. A remainder of the gel may comprise water and any flavourings. 
     In one embodiment, the gel comprises 2% by weight nicotine, 70% by weight glycerol, 27% by weight water and 1% by weight agar. In another embodiment, the gel comprises 65% by weight glycerol, 20% by weight water, 14.3% by weight tobacco and 0.7% by weight agar. 
     In an example embodiment, the cartridge does not comprise a transport element or mechanism for transporting the aerosol-former to a heat source or heater. The gel may be heated in situ to generate a desired aerosol. In this context, in situ means in the same position within the cartridge. There is no requirement for a capillary wick or pump. Also, the system does not comprise an additional non-volatile structure within the substrate cartridge for holding or retaining a liquid or gel in proximity to the heater. 
     The device may comprise a device housing having a cavity for receiving the cartridge. The cavity of the device may be substantially cylindrical. The cavity may have a diameter substantially equal to or slightly greater than the diameter of the cartridge. 
     The device may comprise a device body holding the power supply and the heater. The aerosol-generating device may further comprise a mouthpiece separate to the device body. The mouthpiece may be configured for engagement with the device body. The device body may be configured to receive the cartridge in a cavity of the device body. By providing a reusable mouthpiece, separate to the consumable portion, the construction of the consumable portion can be simple. 
     At least one wall of the substrate cartridge may be in thermal contact with the heater. The at least one wall of the substrate cartridge may be positioned between the heater and the aerosol-forming substrate. The at least one wall of the substrate cartridge may be in direct contact with the heater. The gel within the substrate cartridge can then be heated by conduction through the external wall. The substrate cartridge may comprise at least one liquid impermeable and vapour impermeable external wall defining a blind cavity, wherein the aerosol-forming substrate is held in the device body. 
     The cartridge may have any suitable shape. 
     The cartridge may be substantially cylindrical. As used herein, the terms “cylinder” and “cylindrical” refer to a substantially right circular cylinder with a pair of opposed substantially planar end faces. 
     The cartridge may have any suitable size. 
     The cartridge may have a length of, for example, between about 5 mm and about 30 mm. In certain embodiments the cartridge may have a length of about 12 mm. 
     The cartridge may have a diameter of, for example, between about 4 mm and about 10 mm. In certain embodiments the cartridge may have a diameter of about 7 mm. 
     The substrate cartridge or cartridge may comprise a housing. The housing of the cartridge may be formed from one or more materials. Suitable materials include, but are not limited to: metal, aluminium, polymer, polyether ether ketone (PEEK), polyimides, such as Kapton®, polyethylene terephthalate (PET), polyethylene (PE), polypropylene (PP), polystyrene (PS), fluorinated ethylene propylene (FEP), polytetrafluoroethylene (PTFE), epoxy resins, polyurethane resins and vinyl resins. 
     The housing of the cartridge may be formed from one or more thermally conductive materials. The interior of the cartridge may be coated or treated to comprise one or more thermally conductive materials. Use of one or more thermally conductive materials to form the cartridge or coat the interior of the cartridge can increase heat transfer from the heater to the gel. Suitable thermally conductive materials include, but are not limited to, metals such as, for example, aluminium, chromium, copper, gold, iron, nickel and silver, alloys, such as brass and steel and ceramics, or combinations thereof. At least one wall of the housing may have a thermal conductivity greater than  10  Watts per metre per Kelvin at room temperature. In an example embodiment, the housing comprises a least one wall formed from aluminium. 
     In embodiments in which the cartridge is configured to be heated inductively, the housing of the cartridge may comprise a susceptor, for example a susceptor layer. The susceptor layer may for example form a wall of the housing or may be a coating applied to the interior or exterior of the housing. A susceptor may be located within a chamber in the cartridge. For example, the gel may comprise a susceptor material. 
     Cartridges for use in aerosol-generating systems may be formed by any suitable method. Suitable methods include, but are not limited to, deep drawing, injection moulding, blistering, blow forming, and extrusion. 
     The cartridge may comprise a mouthpiece configured to allow an application of a negative pressure on the mouthpiece to draw the aerosol from the system. Where the cartridge comprises a mouthpiece, the mouthpiece may comprise a filter. The filter may have a low particulate filtration efficiency or very low particulate filtration efficiency. Alternatively, the mouthpiece may comprise a hollow tube. The mouthpiece may comprise an airflow modifier, for example a restrictor. 
     The cartridge may be provided within a mouthpiece tube. The mouthpiece tube may comprise an aerosol-forming chamber. The mouthpiece tube may comprise an airflow restrictor. The mouthpiece tube may comprise a filter. The mouthpiece tube may comprise a cardboard housing. The mouthpiece tube may comprise one or more vapour impermeable elements within the cardboard tube. The mouthpiece tube may have a diameter similar to a conventional cigarette, for example around 7 mm. The mouthpiece tube may have a mouth end configured for the application of a negative pressure to draw the aerosol therethrough. The cartridge may be held in the mouthpiece tube, for example at an opposite end to the mouth end. 
     An open end of the substrate cartridge may be sealed by one or more frangible sealing elements. 
     The one or more frangible barriers may be formed from any suitable material. For example, the one or more frangible barriers may be formed from a foil or film, for example comprising a metal. Where the cartridge comprises one or more frangible barriers sealing one or both of the first chamber and the second chamber, the device body may further comprise a piercing member configured to rupture the one or more frangible barriers. 
     Alternatively or in addition, the substrate container may be sealed by one or more removable barriers. For example, the substrate container may be sealed by one or more peel-off seals. 
     The one or more removable barriers may be formed from any suitable material. For example, the one or more removable barriers may be formed from a foil or film. for example comprising a metal. 
     An open end of the substrate container may be sealed by a vapour permeable element, for example a membrane or mesh configured to allow the escape of vapour from the substrate container through the membrane or mesh. Alternatively, the substrate container may be sealed by a pressure activated valve that allows for the release of vapour through the valve when a pressure difference across the valve exceeds a threshold pressure difference. 
     The substrate container may comprise a first chamber, containing the gel and a second chamber separate to the first chamber. The second chamber may contain the same gel as the first chamber or may contain a different gel or different material to the first chamber. 
     The first and second chambers may be fixed together permanently or they may be separable from one another. The first and second chambers may be provided separately and fixed together using a suitable mechanical interlock, such as a snap fitting or a screw fitting. Alternatively, the first and second chambers may remain separate during use. 
     By providing the first and second chambers separately, a “mix and match” type set of choices may be made available. The contents of the first chamber may provide a particular dosage of a target compound for delivery, such as nicotine, and may provide a particular density of aerosol, and a range of options may be made available. The contents of the second chamber may primarily provide flavour compounds, and a range of options for the second chamber may be available. An adult vaper can choose one chamber from the range of first chambers and one chamber from the range of second chambers and may fit them together to form a complete cartridge. 
     Even when the first and second chambers are provided together and permanently fixed to one another, the same mix and match approach may be taken by a manufacturer to provide a range of different cartridges. 
     The first and second chambers may be of the same size and shape as one another or they may have a different size or shape to one another. The size and shape of the first and second chamber may be chosen to suit their contents, and to provide for a particular heating rate in use. 
     It is also possible to have more than two chambers. It may be desirable to have three or more chambers in the cartridge, with at least two of the chamber having different contents. 
     The first and second chambers may contain different compositions. Both the first and second chambers may contain a gel. In an example embodiment, neither the first chamber nor the second chamber contains a liquid at room temperature. Also, neither the first chamber nor the second chamber comprises a liquid retention material or a wicking material. 
     The first and second chambers may be positioned side by side or one within the other or may be arranged in series such that an air flow can pass first through one chamber and then through the other. 
     The cartridge may comprise a slot between the first and second chambers. The slot may be configured to receive a heating element. The heating element may be received in the slot for example when the cartridge is installed in an aerosol-forming device. The provision of a slot into which a heating element is received may provide for efficient heating by facilitating that heat energy from the heating element is passed directly to the interior of the substrate container rather than for example heating other elements of the system or the ambient air. The slot may be a blind slot. Blind in this context means closed at one end. The provision of a blind slot allows the heating element to be shielded from the vapour or aerosol generated by the system and can help to prevent the build-up of condensates on the heater. 
     Where the substrate comprises first and second chambers, the slot may be provided between the first the second chambers. For example, the slot may be provided within a wall separating the first and second chambers. 
     The electrical heater may comprise a resistive heater. The electrical heater may comprise one or more heating elements. 
     The electric heating element may comprise one or more external heating elements, one or more internal heating elements, or one or more external heating elements and one or more internal heating elements. In this context, external means outside of the cavity and internal means inside of the cavity of the device body. 
     The one or more external heating elements may comprise an array of external heating elements arranged around the inner surface of the cavity. In certain examples, the external heating elements extend along the longitudinal direction of the cavity. With this arrangement, the heating elements may extend along the same direction in which the cartridge is inserted into and removed from the cavity. This may reduce interference between the heating elements and the cartridge. In some embodiments, the external heating elements extend along the length direction of the cavity and are spaced apart in the circumferential direction. Where the heating element comprises one or more internal heating elements, the one or more internal heating elements may comprise any suitable number of heating elements. For example, the heating element may comprise a single internal heating element. The single internal heating element may extend along the longitudinal direction of the cavity. 
     The electric heating element may comprise an electrically resistive material. Suitable electrically resistive materials include but are not limited to: semiconductors such as doped ceramics, electrically “conductive” ceramics (such as, for example, molybdenum disilicide), carbon, graphite, metals, metal alloys and composite materials made of a ceramic material and a metallic material. Such composite materials may comprise doped or undoped ceramics. Examples of suitable doped ceramics include doped silicon carbides. Examples of suitable metals include titanium, zirconium, tantalum and metals from the platinum group. Examples of suitable metal alloys include stainless steel, Constantan, nickel-, cobalt-, chromium-, aluminium- titanium- zirconium-, hafnium-, niobium-, molybdenum-, tantalum-, tungsten-, tin-, gallium-, manganese- and iron-containing alloys, and super-alloys based on nickel, iron, cobalt, stainless steel, Timetal®, iron-aluminium based alloys and iron-manganese-aluminium based alloys. Timetal® is a registered trade mark of Titanium Metals Corporation, 1999 Broadway Suite 4300, Denver Colo. In composite materials, the electrically resistive material may optionally be embedded in, encapsulated or coated with an insulating material or vice-versa, depending on the kinetics of energy transfer and the external physicochemical properties required. The heating element may comprise a metallic etched foil insulated between two layers of an inert material. In that case, the inert material may comprise Kapton®, all-polyimide or mica foil. Kapton® is a registered trade mark of E.I. du Pont de Nemours and Company, 1007 Market Street, Wilmington, Del. 19898, United States of America. A flexible heating element of this type may be conformed to the shape of the cavity and may extend around the periphery of the cavity. 
     The electric heating element may be formed using a metal having a defined relationship between temperature and resistivity. In such embodiments, the metal may be formed as a track between two layers of suitable insulating materials. An electric heating element formed in this manner may be used both as a heater and a temperature sensor. 
     Where the electric heating element comprises a susceptor, the aerosol-generating device body may comprise an inductor arranged to generate a fluctuating electromagnetic field within the cavity and an electrical power supply connected to the inductor. The inductor may comprise one or more coils that generate a fluctuating electromagnetic field. The coil or coils may surround the cavity. 
     The device body may be capable of generating a fluctuating electromagnetic field of between 1 and 30 MHz, for example, between 2 and 10 MHz, for example between 5 and 7 MHz. In addition, the device body may be capable of generating a fluctuating electromagnetic field having a field strength (H-field) of between 1 and 5 kA/m, for example between 2 and 3 kA/m, for example about 2.5 kA/m. 
     The aerosol-generating system may comprise a single heater to provide for a simpler device construction. The single heater may be configured as an external heater that in use is positioned externally to the cavity. Alternatively, the single heater may be configured as an internal heater that in use is positioned internally to the cavity and received in a slot in the cartridge. In an example embodiment, the single heater is configured as an internal heater. 
     Where the single heater is configured as an internal heater, the aerosol-generating device may comprise guide means to facilitate proper alignment of the internal heater with the cartridge. 
     The single heater may be an electric heating element comprising an electrically resistive material. The electric heating element may comprise a non-elastic material, for example a ceramic sintered material, such as glass, alumina (Al 2 O 3 ) and silicon nitride (Si 3 N 4 ), or printed circuit board or silicon rubber. Alternatively, the electric heating element may comprise an elastic, metallic material, for example an iron alloy or a nickel-chromium alloy. 
     The single heater may have any shape suitable to heat the cartridge. The electrical heater may be positioned between first and second chambers of the cartridge when the cartridge is connected to or received in the device body. In an example embodiment, the heater does not project from the aerosol-generating device. 
     The electrical heater may surround the substrate cartridge. The electrical heater may comprise one or more electrically resistive tracks on a flexible substrate. The electrical heater may comprise one or more electrically resistive tracks on a rigid substrate material. The electrical heater may project into the cavity of the device. 
     The aerosol-generating system may further comprise one or more temperature sensors configured to sense the temperature of at least one of the electrical heater elements. In such example embodiments, the system may comprise a controller and the controller may be configured to control a supply of power to the electrical heater based on the sensed temperature. The controller may be configured to supply power to the heater continuously after activation of the system rather than in response to detected puffs. 
     The system may comprise electronic circuitry to control the supply of power to the electrical heater. The electronic circuitry may be a simple switch. Alternatively the electronic circuitry may comprise one or more microprocessors or microcontrollers. The electronic circuitry may be programmable. 
     The electrical power supply may be a DC voltage source. In an example embodiment, the power supply is a battery. For example, the power supply may be a Nickel-metal hydride battery, a Nickel cadmium battery, or a Lithium based battery, for example a Lithium-Cobalt, a Lithium-Iron-Phosphate or a Lithium-Polymer battery. The power supply may alternatively be another form of charge storage device such as a capacitor. The power supply may require recharging and may have a capacity that allows for the storage of enough energy for use of the aerosol-generating device with one or more aerosol-generating articles. 
     The aerosol-generating system is configured to generate an aerosol. The aerosol-generating system may be a handheld system and may comprise a mouthpiece on which a negative pressure can be applied. 
     In an example embodiment, the system does not comprise a transport mechanism for transporting the aerosol-former to the heater. The contents of the cartridge may be heated in situ to generate a desired aerosol. In this context, in situ means in the same position within the first and second chambers that the contents are held prior to heating. There is no requirement for a capillary wick or pump. 
     The aerosol-generating device may be a portable or handheld aerosol-generating device that is comfortable to hold between the fingers of a single hand. 
     The aerosol-generating device may be substantially cylindrical in shape. The aerosol-generating device may have a length of between approximately 70 millimetres and approximately 120 millimetres. 
     In some example embodiments, there is provided a cartridge for an aerosol-generating system, the aerosol-generating system comprising a heater, the cartridge comprising a substrate cartridge containing an aerosol-forming substrate in the form of a thermoreversible gel that is solid at room temperature, wherein the cartridge is configured to removably connect to or be received in a body of the aerosol-generating system and wherein the cartridge comprises a slot configured to receive the heater. 
     Features of the cartridge described in relation to the first aspect of the example embodiments may apply to the cartridge of the second aspect of the example embodiments. In particular, the substrate cartridge may comprise at least one liquid and vapour impermeable external wall defining a blind cavity, wherein the aerosol-forming substrate is contained in the blind cavity. “Blind” in this context means closed at on end. The cartridge may comprise a mouthpiece tube, wherein the substrate cartridge is held in the mouthpiece tube. The mouthpiece tube may have a mouth end for the application of a negative pressure. The mouthpiece tube may comprise an air flow modifier, such as a restrictor. 
       FIG. 1  is a schematic illustration of an aerosol-generating system in accordance with an example embodiment. The system comprises an aerosol-generating device  10  and a cartridge  20  (e.g., replaceable cartridge). The aerosol-generating device comprises a device body  12  and a mouthpiece portion  14 . 
     The device body  12  comprises a power supply, which may be a battery  16  (e.g., lithium ion battery) and electronic control circuitry  18 . The device body also includes heater  22 , which is in the form a blade that projects into a cavity  24  in the housing of the device body. The heater is an electric heater comprising an electrically resistive track on a ceramic substrate material. The control circuitry is configured to control the supply of power from the battery  16  to the heater  22  (e.g., electric heater). 
     The mouthpiece portion  14  engages the device body using a simple push fitting, although any type of connection, such as a snap fitting or screw fitting may be used. The mouthpiece portion in this example embodiment is simply a tapered hollow tube, without any filter elements, and is shown in more detail in  FIG. 2 a   . However, it is possible to include one or more filter elements in the mouthpiece portion. The mouthpiece portion comprises air inlet holes  42  and encloses an aerosol-forming chamber  40  (shown in  FIG. 1 ) in which vapour can condense in an airflow prior to exiting the system. 
     The cartridge  20  comprises a housing defining two blind chambers. The two chambers (first and second chambers  30 ,  32 ) are open at a mouthpiece end. A membrane  37  (shown in  FIG. 1 ) seals the open end of the chambers. A removable seal may be provided over the membrane and may be peeled off before vaping. A blind slot  34  is provided between the two chambers for the heater  22  to be received in. The blind slot  34  is closed at the mouthpiece end. A first chamber  30  holds a first gel, containing nicotine and aerosol-former, and the second chamber  32  holds a second gel, containing shredded tobacco leaves. 
       FIG. 2 b    is a bottom perspective view of the cartridge housing.  FIG. 2 c    is a perspective view of the cartridge housing. The cartridge  20  has a generally cylindrical shape. The first and second chambers are of equal size and shape and are separated by a dividing wall  36 . The blind slot  34  is within the dividing wall  36 . A channel  38  is provided in a wall of the cartridge housing to engage a corresponding rib in the cavity  24 . This ensures that the cartridge can only be inserted into the cavity  24  in one orientation, in which the heater blade is received in the blind slot  34 . 
       FIG. 2 d    is a cross section through the cartridge housing of  FIGS. 2 b  and 2 c    showing the shape of the blind slot  34 . The shape of the slot matches the blade shape of the heater. 
     The first gel in the first chamber  30  comprises one or two aerosol formers such as glycerin and polyethylene glycol. The relative concentration of the aerosol formers can be adapted to the particular requirements of the system. In this example embodiment, the gel in the first chamber  30  comprises (by weight): 2% nicotine, 70% glycerin, 27% water, 1% agar. 
     The gelling agent may be agar, which has the property of melting at temperatures above 85° C. and turning back to gel at around 40° C. This property makes it suitable for relatively hot environments. The gel will not melt at 50° C., which is useful if the system is left in a hot automobile in the sun, for example. A phase transition to liquid at around 85° C. means that the gel only needs to be heated to a relatively low-temperature to induce aerosolization, allowing low energy consumption. It may be beneficial to use only agarose, which is one of the components of agar, instead of agar. 
     The second gel in the second chamber  32  comprises (by weight): 65% glycerin, 20% water, 14.3% solid powdered tobacco, 0.7% agar 
     Further or different flavors, such as menthol, can be added either in water or in propylene glycol or glycerin prior to the formation of the either of the gels. 
     The amount of gel provided in each cartridge can also be chosen to suit particular needs. For instance, each cartridge may contain enough gel to provide a single-occasion quantity for vaping or may contain sufficient gel for a multiple-occasion quantity for vaping. 
     In operation, the system is configured to operate in a continuous heating mode. This means that the heater  22  heats the cartridge throughout an operating session rather than in response to sensed puffs. The system may be turned on using a relatively simple switch (not shown) such that the heater heats the cartridge. A temperature sensor may be included in the system so that an indication can be provided as to when an operating temperature has been reached, at which aerosol is generated. The gels become liquid upon heating above 85° C. Aerosol containing nicotine and glycerin is generated at temperatures between 180° C. to 250° C. During operation, the heater operates at approximately 250° C. The heater may operate for a fixed time period after activation (e.g., 6 minutes) or may operate until the system is switched off. The operating time may depend on the amount of gel contained within the cartridge. 
     The cartridge housing is formed of aluminium, which is a good thermal conductor. The heater is never in contact with the gel or any generated vapour or aerosol. It is held in the blind slot  34  and so is isolated from the generated aerosol. This ensures that there is no build-up of condensates on the heater, which might lead to the generation of undesirable compounds in operation. 
       FIGS. 3 a , 3 b , and 3 c    illustrate an example embodiment in which the chambers of the cartridge are sealed by a frangible sealing element. The mouthpiece portion is used to pierce the sealing element to allow vapour generated in the chambers to escape from the two chambers. 
       FIG. 3 a    illustrates the insertion of the cartridge  20  into the device body  12 . As in  FIG. 1 , the cartridge comprises first and second chambers  30 ,  32  and a blind slot  34  between the chambers. The chambers are sealed by sealing element  50 . 
       FIG. 3 b    shows the cartridge inserted into the device, with the heater  22  received in the blind slot  34  between the chambers. A mouthpiece portion  14  is then connected to the device body  12 .  FIG. 3 b    illustrates the direction of insertion of the mouthpiece portion. The mouthpiece portion is provided with piercing elements  52  which act to pierce the frangible sealing element and provide an escape passage  54  for vapour generated in the first and second chambers. 
       FIG. 3 c    shows the mouthpiece portion  14  in a fully inserted position, with the piercing elements  52  extending into the first and second chambers and allowing vapour to escape from the first and second chambers  30 ,  32 , into an aerosol-forming chamber in the mouthpiece portion. The vapour cools and is entrained in an airflow in the mouthpiece portion to form an aerosol. As in the example embodiment of  FIG. 1 , the mouthpiece portion may be provided with air inlets. Alternatively or in addition, an airflow path into the mouthpiece portion may be provided through the device. Alternatively or in addition, an airflow path may be provided through the first and second chambers. 
       FIG. 4  is a schematic illustration of another aerosol-generating system in accordance with an example embodiment. The aerosol-generating device  210  of  FIG. 4  operates by using induction heating rather than by using resistive heating. Instead of using a resistive heater either around or inside the cavity in which the cartridge is received, the device body  212  comprises an induction coil  224  surrounding the cavity and a susceptor material  222  is provided in the cavity, in this example as part of the cartridge. 
     The device body  212  comprises a power supply, which may be a battery  216  (e.g., lithium ion battery) and electronic control circuitry  218 . The device body  212  also includes an induction coil  224 , which extends around a cavity in the housing of the device body  212 . The device body  212  also comprises electronic circuitry  220  to generate an AC signal which is provided to the induction coil  224 . 
     The mouthpiece portion  214  is similar to the mouthpiece portion shown in  FIG. 1  and encloses an aerosol-forming chamber  240 . In this example air inlets  242  are provided at the junction of the mouthpiece portion and the device body. 
     The cartridge of  FIG. 4  is similar to the cartridge shown in  FIG. 1 . The composition of the gels in the two chambers of the cartridge may be the same as in the example embodiment of  FIG. 1 . However, rather than having a blind cavity for receiving a heater, the wall of the cartridge separating the two chambers comprises a susceptor material  222 , such as a layer of iron, that heats up in the alternating magnetic field. The susceptor material in this example is provided as part of the cartridge rather than part of the device body, but it is possible for the susceptor material to be provided as part of the device body or both in the cartridge and the device body. The entire cartridge may be formed from a susceptor material, or a susceptor material may be provided as a coating on one of more surfaces of the cartridge. It is also possible to provide susceptor material within the first and second chambers, suspended in the gel or other material contained there. 
     A sealing element is provided to seal the first and second chambers in the same manner as described with reference to  FIG. 1 . A cartridge piercing arrangement similar to that shown in  FIG. 3  may be used to open the cartridge using the mouthpiece portion  214 , with suitable adaptations made for the different airflow path. Alternatively, a simple peelable seal may be used and a vapour permeable membrane provided across the open end of the first and second chambers  230 ,  232 . 
     In operation, the system is configured to operate in a continuous heating mode as in the example embodiment of  FIG. 1 . This means that when the device is switched on, the device supplies an AC signal to the induction coil in order to generate an alternating magnetic field in the cavity. This induces current flow in the susceptor resulting in a heating of the susceptor. If a ferromagnetic material is used as the susceptor, hysteresis losses may also contribute to the heating. The induction coil may be described as an induction heater in this context. By controlling the magnitude and frequency of the AC signal, the temperature within the first and second chambers can be controlled. A temperature sensor may be provided within the cavity and a feedback control loop used. The induction heater may operate for a fixed time period after activation (e.g., 6 minutes) or may operate until the system is switched off. 
       FIG. 5 a    is a schematic illustration of a cartridge held within a mouthpiece tube in accordance with an example embodiment. In  FIG. 5 a   , the cartridge  330  is held within a mouthpiece tube  300 . A flow restrictor  350  and lining tubes  340 ,  360 ,  370  are also held within the mouthpiece tube  300 . The components held within the mouthpiece tube  300  are shown in an exploded view in  FIG. 5   b.    
     The cartridge  330  is similar to the cartridge shown in  FIG. 2 c   . However, the cartridge  330  has no membrane or sealing element but includes airflow channels  335  formed in the walls of the cartridge and air inlets  334  at the top of the airflow channels to allow air into the open ends of the first and second chambers. 
     The mouthpiece tube is formed from cardboard and has a diameter of 6.6 mm and a length of 45 mm. Lining tubes  340  are formed from polyetheretherketone (PEEK) and are provided to prevent the cardboard mouthpiece tube from absorbing moisture from within the mouthpiece tube. The lining tubes can be made relatively thin (e.g., a thickness of 0.3 mm). A flow restrictor  350  is provided to restrict the airflow to ensure mixing of air with vapour from the cartridge and ensure the generation of an aerosol within the space following the restrictor, in lining tube  360 . 
       FIG. 6  illustrates the airflow within the mouthpiece tube of  FIG. 5 a    during operation. The mouthpiece tube is shown within the cavity  24  of a device body  12  of the type shown in  FIG. 1 . But the device body  12  of  FIG. 6  does not have a mouthpiece portion  14 .  FIG. 6  illustrates only the end of the device that receives the mouthpiece tube. The battery and control circuitry is not shown. The device includes device air inlets  355  that allow air into an internal airflow passage  365  formed in the device around the periphery of the cavity  24 . A spacer element  352  is positioned in a base of the cavity to allow air to flow from the internal airflow passage  365  into the cavity  24  and then into the airflow channels  335  in the cartridge  330  and through the air inlets  334  into the interior of the mouthpiece tube. 
     The cartridge shown in  FIGS. 5 a  and 5 b    may be heated by heater of the type shown in  FIG. 1  or of the type shown in  FIG. 4 or 7   a  (described below). In operation, the system is configured to operate in a continuous heating mode as in  FIG. 1 . This means that the heater heats the cartridge throughout an operating session rather than in response to sensed puffs. The system may be turned on using a relatively simple switch (not shown) such that the heater heats the cartridge. The gels in the first and second chambers become liquid upon heating and vapour containing nicotine and glycerin is generated at temperatures between 180° C. to 250° C. 
     When the system is at the operating temperature, a negative pressure may be applied to a mouth end of the mouthpiece tube to draw air through the mouthpiece tube. Air is drawn into a distal end of the mouthpiece tube, opposite the mouthpiece end from the internal airflow passage  365 . The air travels up the airflow channels  335  and through air inlets  334  into space  345 . The air mixes in space  345  with vapour from the first and second chambers. The mixed air and vapour then passes through the flow restrictor  350 , after which it cools to form an aerosol. After operation, the mouthpiece tube, including the cartridge, can be withdrawn from the device and disposed of. Mouthpiece tubes of this type may be sold in packs to provide for multiple operations of the system. 
       FIG. 7 a    is a schematic illustration of another aerosol-generating device in accordance with an example embodiment.  FIG. 7 a    shows a cross-sectional view of an aerosol-generating device  400  for use with a container or cartridge  500  as shown in  FIG. 8 . The aerosol-generating device comprises a housing  402  (e.g., outer housing), containing a power supply  404  such as a rechargeable battery and control electronics or control circuitry  406 . The housing  402  further comprises a cavity  408  configured to receive a container or cartridge  500 . A heater  410  extends around the periphery of the cavity  408 . The control circuitry is connected to the heater  410 . The heater is formed from one or more metal heating tracks sandwiched between two layers of flexible, thermal stable substrate material, such as polyimide. The aerosol-generating device  400  further comprises a mouthpiece  412  attachable to a proximal end of the aerosol-generating device housing  402  by a push fitting or screw fitting. The mouthpiece comprises a piercing portion  414 , air inlets  418  and an air outlet  416 . 
     The container or cartridge  500  that is placed in the cavity  408  of the device, is shown in  FIG. 8 . The container has a housing  510  formed from aluminium, which is a good thermal conductor. The housing of the container is in the form of a cup that defines a blind cavity. The housing  510  may be manufactured using suitable known techniques, such as deep drawing. The container contains a gel  515 . In this example embodiment, the gel comprises 2% by weight nicotine, 70% by weight glycerol, 27% by weight water and 1% by weight agar. In another embodiment, the gel comprises 65% by weight glycerol, 20% by weight water, 14.3% by weight tobacco and 0.7% by weight agar. The gel is sealed in the cavity of the container by a frangible sealing foil  514 . The sealing foil is welded, heat sealed or adhered to a lip  512  of the housing  510 . This type of container can be made relatively inexpensively. 
       FIG. 7 b    shows a cross-sectional view of the aerosol-generating device  400  with a container or cartridge  500  received in the cavity  408  of the housing. In use, the container or cartridge  500  is inserted into the cavity  408  of the aerosol-generating device  400 , and the mouthpiece  412  is attached to the housing  402 . By attaching the mouthpiece, the piercing portion  414  pierces the sealing foil  514  of the container, and forms an airflow pathway  415  from the air inlets  418 , through or across the container to the air outlet. A button (not shown) may be pressed to activate the device. After activating the device, the heater is supplied with power by the control electronics or control circuitry  406  from the power supply  404 . The heater then directly heats the external wall of the cartridge. When the temperature of the container or cartridge  500  reaches the operating temperature of about 250 degrees Celsius, an indicator (not shown) may indicate that a negative pressure may be applied to the mouthpiece at the air outlet  416 . When a negative pressure is applied to the mouthpiece, air enters the air inlets  418 , proceeds through the mouthpiece and into the container or cartridge  500 , entrains vapourised gel, and then exits through the air outlet  416  in the mouthpiece. The heater may operate for a fixed time period after activation (e.g., 6 minutes) or may operate until the system is switched off. 
     When the gel in the cartridge has become exhausted, the cartridge can be removed and replaced by a new cartridge. 
     The example embodiments described have each been described as configured to operate a continuous heating scheme, in which the heater is activated for a desired or predetermined time period. However, the systems described may be configured to operate in different ways. For example, power may be provided to the heater or induction coil for only the duration of each puff, based on signals from an airflow sensor within the system. Alternatively, or in addition, power to the heater or induction coil may be switched on and off in response to an actuation of a button or switch. 
     While a number of example embodiments have been disclosed herein, it should be understood that other variations may be possible. Such variations are not to be regarded as a departure from the spirit and scope of the present disclosure, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims. For instance, different arrangements for airflow through the system may be provided and different heating arrangements can be envisaged, such as non-electrical heaters.