Heater assembly having an expansion member

Abstract

A heater assembly (10) for an aerosol-generating system, the heater assembly (10) comprising: an electrical heating element (12) for heating a liquid aerosol-forming substrate to form an aerosol; a porous body (14) for supplying the liquid aerosol-forming substrate to the electrical heating element (12), the electrical heating element (12) being arranged on a porous outer surface (14a) of the porous body (14); and an expansion member (16) arranged within an interior of the porous body (14) such that liquid aerosol-forming substrate can be supplied to the expansion member (16), wherein the expansion member (16) is configured to expand upon drying to exert a pressure on the interior of the porous body (14) such that the expansion member (16) fractures the porous body (14) and irreversibly disables the heater assembly (10) if the expansion member (16) is not supplied with liquid aerosol-forming substrate.

Description

[0001] HEATER ASSEMBLY HAVING AN EXPANSION MEMBER

[0002] The present disclosure relates to a heater assembly for an aerosol-generating system. In particular, but not exclusively, the present disclosure relates to a heater assembly for a handheld electrically operated aerosol-generating system for heating an aerosol-forming substrate to generate an aerosol and for delivering the aerosol into the mouth of a user. The present disclosure further relates to a cartridge and an aerosol-generating system comprising the heater assembly and also to a method of manufacturing a heater assembly.

[0003] Aerosol-generating systems that heat a liquid aerosol-forming substrate in order to generate an aerosol for delivery to a user are generally known in the prior art. These systems typically comprise an aerosol-generating device and a replaceable cartridge. The cartridge includes a liquid aerosol-forming substrate that is capable of releasing volatile compounds when heated. The cartridge typically also includes a heater for heating the liquid aerosolforming substrate. In known aerosol-generating systems, the heater comprises a resistive heating element wound around a wick that supplies liquid aerosol-forming substrate to the heating element. The aerosol-generating device or cartridge also comprises a mouthpiece. When a user takes a puff on the mouthpiece, an electric current is passed through the heating element causing it to be heated by resistive or Joule heating, which, in turn, heats the liquid aerosol-forming substrate supplied by the wick. This causes volatile compounds to be released from the liquid aerosol-forming substrate that cool to form an aerosol. The aerosol is then drawn into a user’s mouth via the mouthpiece.

[0004] Such known aerosol-generating systems have a number of drawbacks. For example, they can be difficult to manufacture with consistent manufacturing tolerances which can result in inconsistent vapour production and flavour generation. Inconsistent manufacturing tolerances can also affect the transfer of heat from the heating element to the wick reducing the energy efficiencies of such devices. A further problem encountered by such known aerosol-generating system is “dry heating” or a “dry puff’, which arises when the heating element is heated with insufficient liquid aerosol-forming substrate being supplied to the heating element. This can occur, for example, when a user has consumed all of the liquid aerosol-forming substrate in the cartridge such that the cartridge is depleted of liquid aerosolforming substrate and needs replacing. During operation, it is preferable to maintain a supply of liquid aerosol-forming substrate to the heating element such that the heating element is maintained in a wet state because this helps to ensure that a satisfactory aerosol is produced when a user takes a puff. Dry heating can result in overheating of the heating element and, potentially, thermal decomposition of the liquid aerosol-forming substrate, which can produce undesirable by-products and an unsatisfactory aerosol. Allowing the aerosol-generating system to continue to operate when liquid aerosol-forming substrate is not being supplied to the heating element can result in a poor user experience.

[0005] It would be desirable to provide a more energy efficient heater assembly capable of generating a more consistent aerosol. It would be desirable to provide a heater assembly that reduces the likelihood of a user experiencing dry heating or a dry puff and that restricts a user from being able to continue to use an aerosol-generating system when liquid aerosol-forming substrate is not being supplied to the heating element.

[0006] According to an example of the present disclosure, there is provided a heater assembly for an aerosol-generating system. The heater assembly may comprise an electrical heating element for heating a liquid aerosol-forming substrate to form an aerosol. The heater assembly may comprise a porous body for supplying the liquid aerosol-forming substrate to the electrical heating element. The electrical heating element may be arranged on a porous outer surface of the porous body. The heater assembly may comprise an expansion member arranged within an interior of the porous body such that liquid aerosol-forming substrate can be supplied to the expansion member. The expansion member may be configured to expand upon drying to exert a pressure or force on the interior of the porous body such that the expansion member fractures the porous body and irreversibly disables the heater assembly if the expansion member is not supplied with liquid aerosol-forming substrate.

[0007] According to an example of the present disclosure, there is provided a heater assembly for an aerosol-generating system. The heater assembly comprises an electrical heating element for heating a liquid aerosol-forming substrate to form an aerosol. The heater assembly further comprises a porous body for supplying the liquid aerosol-forming substrate to the electrical heating element. The electrical heating element is arranged on a porous outer surface of the porous body. The heater assembly comprises an expansion member arranged within an interior of the porous body such that liquid aerosol-forming substrate can be supplied to the expansion member. The expansion member is configured to expand upon drying to exert a pressure or force on the interior of the porous body such that the expansion member fractures the porous body and irreversibly disables the heater assembly if the expansion member is not supplied with liquid aerosol-forming substrate.

[0008] The term “porous” is used herein to refer to an element or material that is permeable to the liquid aerosol-forming substrate and allows the liquid aerosol-forming substrate to migrate through it.

[0009] An advantage of providing the electrical heating element on a porous outer surface of the porous body is that it helps to alleviate the problems of manufacturing tolerances encountered with wick and coil heaters. This is because the electrical heating element is fixed to, and in contact with, the porous body, which helps to supply liquid aerosol-forming substrate to the electrical heating element. This also helps to transfer heat from the electrical heating element to the liquid aerosol-forming substrate, which helps to improve energy efficiency. The dimensions and arrangement of the electrical heating element relative to the porous body are also fixed, which helps to produce a more consistent aerosol.

[0010] Advantageously, by providing an expansion member within the porous body that expands upon drying to exert a pressure or force on the interior of the porous body if the expansion member is not supplied with liquid aerosol-forming substrate, the porous body can be deliberately fractured or split in a controlled manner when dry heating is encountered. If the expansion member is not being supplied with liquid aerosol-forming substrate, it is indicative that the electrical heating element is also not being supplied with liquid aerosolforming substrate and that dry heating is occurring. Fracturing or splitting the porous body may also break or sever the heating element disposed along the porous outer surface of the porous body. Therefore, fracturing the porous body irreversibly disables the heater assembly and renders the aerosol-generating system unusable until the cartridge containing the disabled heater assembly is replaced. Furthermore, it can reduce the likelihood of the porous body failing in an uncontrolled manner and cracking into multiple fragments, which can occur due to extreme temperature cycling of the heating element during dry heating. This may help to improve the safety of heater assemblies for aerosol-generating systems. Furthermore, it may provide a more reliable mechanical means of disablement in the event of dry heating occurring compared to known systems which tend to use electronic monitoring and disablement.

[0011] The expansion member may comprise a material that has a negative dry-shrinkage coefficient. Typically, most materials decrease in size upon drying and increase in size upon wetting. Such materials are said to exhibit dry shrinkage and are characterized by a positive dry-shrinkage coefficient. However, some materials decrease in size upon wetting and increase in size upon drying. Such materials are said to exhibit dry expansion and are characterized by a negative dry-shrinkage coefficient. Advantageously, an expansion member made from a material a negative dry-shrinkage coefficient can be used to exert a force on the porous body to fracture the porous body when the expansion member is not supplied with liquid aerosol-forming substrate. Such a material can therefore provide a simple mechanical means for disabling the heater assembly in the event of dry heating occurring.

[0012] The expansion member may comprise a hydrophobic material. The expansion member may comprise hydrophobic fibres. The expansion member may comprise a material having a contact angle of greater than 150 degrees. The expansion member may comprise hydrophobic cellulose or superhydrophobic cellulose. As used herein, the term “hydrophobic cellulose” refers to a cellulosic material that has undergone a chemical or physical treatment to make the surface of the cellulose hydrophobic such that it repels water. For example, this can be achieved by coating the fibres of the cellulosic material with low surface energy materials such as hydrocarbon or fluorine containing compounds. However, any suitable means of achieving hydrophobicity may be used.

[0013] Advantageously, hydrophobic cellulose is a material that exhibits dry expansion and is characterised by having a negative dry-shrinkage coefficient. It has been surprisingly found that an expansion member comprising hydrophobic cellulose can, upon drying, exert sufficient pressure on a porous body to fracture the porous body and disable a heater assembly. That is, when the expansion member is not being supplied with liquid aerosol-forming substrate, the hydrophobic cellulose expands to fracture the porous body. However, during normal operation or storage, the hydrophobic cellulose is maintained in a saturated wet state by liquid aerosol-forming substrate being supplied by the porous body. The expansion member has a significantly reduced size in its wet state such that it does not exert pressure on the porous body or otherwise interfere with the operation of the heater assembly.

[0014] The expansion member may be substantially planar. Geometrically, the term “planar” is used to refer to an expansion member that extends along a surface in two dimensions more than in a third dimension. Advantageously, by being substantially planar, the expansion member can exert pressure both out of the plane and along the plane to help fracture the porous body. Such a configuration may help the porous body to fail in a controlled manner along the plane of the expansion member when the expansion member is not being supplied with liquid aerosol-forming substrate.

[0015] The expansion member may pass through at least 50 percent of a dimension of the porous body in any one plane. Preferably, the expansion member may pass through at least 60 percent of a dimension the porous body in any one plane. More preferably, the expansion member may pass through at least 70 percent of a dimension the porous body in any one plane. Yet more preferably, the expansion member may pass through at least 80 percent of a dimension the porous body in any one plane.

[0016] The expansion member may pass through between 50 percent and 80 percent of a dimension of the porous body in any one plane. Preferably, the expansion member may pass through between 60 percent and 80 percent of a dimension of the porous body in any one plane. More preferably, the expansion member may pass through between 70 percent and 80 percent of a dimension of the porous body in any one plane. Yet more preferably, the expansion member may pass through between 75 percent and 80 percent of a dimension of the porous body in any one plane. Advantageously, passing through at least 50 percent of a dimension of the porous body in any one plane reduces the amount of remaining material of the porous body in that plane for maintaining the structural integrity of the porous body. Tensile stress from the pressure exerted by the expansion member is concentrated in a reduced cross-sectional area to the point where the tensile strength of the material of the porous body is exceeded, thereby fracturing the porous body.

[0017] The expansion member may pass through a length of the porous body in any one plane. The expansion member may pass through a width of the porous body in any one plane. The expansion member may pass through a thickness of the porous body in any one plane.

[0018] The expansion member may be configured to expand to at least 3 times its size, preferably to at least 4 times its size, and more preferably to at least 5 times its size, in its dry state compared to its saturated state. The term “dry” or “dry state” is used herein to refer to an expansion member that has a moisture content of less than 20 percent relative to the dry weight of the material from which the expansion member is made. Advantageously, expanding to at least 3 times its size helps the expansion member to exert a pressure or force on the porous body when the expansion member is not being supplied with liquid aerosolforming substrate to fracture the porous body.

[0019] The expansion member may be configured to exert a pressure of at least 0.3 Newtons per square millimetre, preferably at least 0.4 Newtons per square millimetre, and more preferably at least 0.5 Newtons per square millimetre, on the porous body when the expansion member is in its expanded state. The expansion member may be configured to exert a pressure of between approximately 0.3 and 0.8 Newtons per square millimetre, preferably between approximately 0.4 and 0.7 Newtons per square millimetre, and more preferably between approximately 0.5 and 0.6 Newtons per square millimetre, on the porous body in its expanded state. It will be appreciated that the expansion member is in an expanded state when it is in a dry state, that is, when the expansion member is not being supplied with liquid aerosol-forming substrate. These pressures have been found sufficient to fracture a porous body according to the present disclosure.

[0020] The expansion member may have a first major surface. The expansion member may have a second major surface. The second major surface may oppose the first major surface. The second major surface may be separated from the first major surface by a thickness of the expansion member. A major surface of the expansion member may have a surface area of between 6 and 16 square millimetres, preferably between 7 and 14 square millimetres and more preferably between 8 and 12 square millimetres. These surface areas have been found sufficient to fracture a porous body according to the present disclosure.

[0021] The expansion member may be arranged perpendicular to the porous outer surface on which the electrical heating element is located. Optionally, the expansion member may be arranged parallel to the porous outer surface on which the electrical heating element is located. Both these configurations of expansion member have been found sufficient to fracture a porous body according to the present disclosure.

[0022] The porous outer surface on which the electrical heating element is arranged may comprise a first porous outer surface of the porous body. The first porous outer surface therefore comprises a heating or aerosolization surface of the heater assembly. The porous body may have a second porous outer surface configured to receive liquid aerosol-forming substrate. The second porous outer surface therefore comprises a liquid absorption surface of the heater assembly. The second porous outer surface may be opposite the first porous outer surface. The porous body is configured to supply liquid aerosol-forming substrate from the liquid absorption side to the aerosolization side of the porous body.

[0023] The porous body may comprise a heat resistant material. The porous body may comprise a material having a thermal decomposition temperature of at least 250 degrees Celsius.

[0024] The porous body may be formed from a brittle material. As used herein, the term “brittle” refers to a material that fractures without undergoing substantial elastic or plastic deformation. The porous body may be formed from a material that has a tensile strength for a given cross-sectional area that is less than a pressure or force that can be exerted by the expansion member. However, it will be appreciated that brittle materials such as ceramics typically fail at a tensile stress below their theoretical tensile strength due to inherent weaknesses or flaws in their structure.

[0025] The porous body may comprise a ceramic. The porous body may comprise any suitable inert ceramic or bio-compatible ceramic. Examples of suitable ceramics are ceramics comprising aluminium oxides, zirconium oxides, silicon oxides, calcium silicates and calcium phosphate including hydroxyapatite. The porous body may comprise a ceramic comprising one or more of AI2O3, ZrC>2, SiC>2 and Ca2SiOs. In a preferred example, the porous body comprises a ceramic comprising one or both of SiC>2 and Ca2SiOs. An advantage of using ceramic materials is that they are thermal stable at the temperature at which the heater assembly typically operates and generally have a thermal decomposition temperature that is significantly higher than that of a conventional wick. This may help to reduce the risk of unwanted by-products being produced during dry heating. The porous body may comprise a plurality of interconnected open cell pores.

[0026] The porous body may comprise a capillary material that conveys a liquid through the material by capillary action. The porous body may have a fibrous or porous structure. The porous body may comprise a bundle of capillaries. For example, the porous body may comprise a plurality of fibres or threads or other fine bore tubes. The porous body may comprise fibres or threads of cotton or treated cotton, for example, acetylated cotton. Other suitable materials could also be used, for example, ceramic- or graphite based fibrous materials or materials made from spun, drawn or extruded fibres, such as fiberglass, cellulose acetate or any suitable heat resistant polymer.

[0027] The porous body may comprise a slot for receiving the expansion member. The slot may be located in the porous outer surface of the porous body. The slot may be located in a second porous outer surface of the porous body. The second porous outer surface may be opposite the first porous outer surface. The slot may be located in a porous side surface of the porous body. The slot may extend into an interior of the porous body.

[0028] The porous body may comprise a line of weakness or a fault line or a weak point. The line of weakness, fault line or weak point may comprise a feature that causes the porous body to break or fracture at a stress or force value below the expected or theoretical tensile strength of the material of the porous body. The expansion member may be arranged to cause the porous body to fracture along the line of weakness. The line of weakness, fault line or weak point may comprise a region of reduced cross-sectional area that causes stress to be concentrated in the region of reduced cross-sectional area when the expansion member exerts a force on the porous body. The expansion member may be aligned with the line of weakness, fault line or weak point. The line of weakness, fault line or weak point may comprise a notch. Advantageously, the line of weakness or fault line or weak point helps to reduce the tensile stress at which the material of the porous body fails. This means that the expansion member has to exert less pressure or force to fracture the porous body when the expansion member is not being supplied with liquid aerosol-forming substrate.

[0029] The electrical heating element may comprise a discrete, solid, pre-formed component. The electrical heating element may have any suitable shape or form. Examples of suitable shapes and forms include but are not limited to a band, a strip, a filament, a wire, a mesh, a flat spiral coil, fibres or a fabric. The heating element may be fluid permeable.

[0030] In some examples, the electrical heating element may be at least partially embedded in the porous outer surface of the porous body. In other words, at least a portion of the electrical heating element may extend into the porous body. This arrangement may help to secure the electrical heating element to the porous body and increase contact between the electrical heating element and the porous body to improve heating of the liquid aerosol-forming substrate and aerosol delivery.

[0031] In some preferred examples, the heating element is planar. The planar heating element may extend substantially in a plane.

[0032] In some preferred examples, the heating element comprises a mesh. The heating element may comprise an array of filaments forming a mesh. As used herein the term "mesh" encompasses grids and arrays of filaments having spaces therebetween. The term mesh also includes woven and non-woven fabrics.

[0033] The filaments may be formed by etching a sheet material, such as a foil. This may be particularly advantageous when the heater assembly comprises an array of parallel filaments. If the heating element comprises a mesh or fabric of filaments, the filaments may be individually formed and knitted together.

[0034] The heating element may comprise an electrically resistive heating element. The heating element may be made from any suitable electrically conductive material. Suitable 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-, aluminum-, titanium-, zirconium-, hafnium-, niobium-, molybdenum-, tantalum-, tungsten-, tin-, gallium-, manganese- and iron-containing alloys, and super-alloys based on nickel, iron, cobalt, stainless steel, Timetai®, iron-aluminum based alloys and iron-manganese-aluminum based alloys. Timetai® is a registered trade mark of Titanium Metals Corporation. The heating element may be made from stainless steel, for example, a 300 series stainless steel such as AISI 304, 316, 304L, 316L. In a preferred example, the electrical heating element may comprise one of more of NiCr and TiZr.

[0035] Additionally, the heating element may comprise combinations of the above materials. A combination of materials may be used to improve the control of the resistance of the heating element. For example, materials with a high intrinsic resistance may be combined with materials with a low intrinsic resistance. This may be advantageous if one of the materials is more beneficial from other perspectives, for example price, machinability or other physical and chemical parameters. Advantageously, high resistivity heating allow more efficient use of battery energy. The electrical heating element may be formed from an electrically conductive material deposited on to the porous outer surface. As used herein, the term “electrically conductive material” denotes a material having a resistivity of 1x10'2Qm, or less. As used herein, the term “deposited” means applied as a layer or coating by a physical or chemical process, for example in the form of a liquid, plasma or vapour which subsequently condenses or aggregates to form the electrical heating element, rather than simply being laid on or fixed to the porous body as a solid, pre-formed component.

[0036] The electrical heating element may be deposited directly on to the porous outer surface. In other words, the electrically conductive material that forms the electrical heating element is deposited onto the porous outer surface of the porous body such that the electrical heating element is in direct contact with the porous outer surface.

[0037] In some examples, the electrically conductive material of the electrical heating element may be at least partially diffused into the porous outer surface of the porous body. As used herein, the term “diffused into the porous outer surface” means that the electrically conductive material is interspersed with the material of the porous outer surface at the interface between the electrically conductive material and the porous body, for example, by extending into the pores of the porous outer surface. This arrangement may help to secure the electrical heating element to the porous body and increase contact between the electrical heating element and the porous body to improve heating of the liquid aerosol-forming substrate and aerosol delivery.

[0038] The electrically conductive material from which the electrical heating element is formed may be deposited onto the porous outer surface in any suitable manner. For example, the electrically conductive material may be deposited onto the porous outer surface of the porous body as a liquid using a dispensing pipette or syringe, or using a fine-tipped transferring device such as a needle.

[0039] In some examples, the at least one heating element comprises a printable electrically conductive material printed on the porous outer surface of the porous body. In such embodiments, any suitable known printing technique may be used. For example, one or more of screen-printing, gravure printing, flex-printing, inkjet printing. Such printing processes may be particularly applicable for high speed production processes.

[0040] Alternatively, the electrically conductive material, from which the electrical heating element is formed, may be deposited onto the porous outer surface of the porous body by one or more vacuum deposition processes, such as evaporation deposition and sputtering.

[0041] The at least one heating element may be formed from any suitable electrically conductive material. In certain preferred embodiments, the electrically conductive material comprises one or more of a metal, an electrically conductive polymer and an electrically conductive ceramic.

[0042] Suitable electrically conductive metals include, but are not limited to, aluminium, silver, nickel, gold, platinum, copper, tungsten, and alloys thereof. In some embodiments, the electrically conductive material comprises a metal powder suspended in a glue, such as an epoxy resin. In one embodiment, the electrically conductive material comprises silver-loaded epoxy.

[0043] Suitable electrically conductive polymers include PEDOT (poly(3,4- ethylenedioxythiophene)), PSS (poly(p-phenylene sulfide)), PEDOT: PSS (mixture of both PEDOT and PSS), PANI (polyanilines), PPY (poly(pyrrole)s), PPV (Poly(p-phenylene vinylene)), or any combination thereof.

[0044] Suitable electrically conductive ceramics include ITO (Indium Tin Oxide), SLT (lanthanum-doped strontium titanate), SYT (yttrium-doped strontium titanate), or any combination thereof.

[0045] The electrically conductive material may further comprise one or more additives selected from a group consisting of: solvents; curing agents; adhesion promoters; surfactants; viscosity reduction agents; and aggregation inhibitors. Such additives may be used, for example, to aid deposition of the electrically conductive material on the porous outer surface of the porous body, to increase the amount by which the electrically conductive material diffuses into the porous outer surface of the porous body, to reduce the time required for the electrically conductive material to set, to increase the level of adhesion between the electrically conductive material and the porous body, or to reduce the amount of aggregation of suspended particles, such as metal particles or powder, in the electrically conductive material prior to application onto the porous outer surface of the porous body.

[0046] The heater assembly may further comprise first and second electrical contacts connected to the electrical heating element. Each electrical contact may be disposed at opposite sides of the porous outer surface. The electrical heating element may extend between the electrical contacts. The electrical heating element may form an electrical connection therebetween.

[0047] The electrical contacts may be formed from any suitable material. Examples of suitable materials for the electrical contacts include but are not limited to copper, zinc and gold.

[0048] In one example, the first and second electrical contacts may be formed from an electrically conductive material deposited directly onto the porous outer surface of the porous body. The electrical heating element may extend between the electrical contacts in a wavelike or serpentine manner. This helps to increase the length of the heating element between the electrical contacts that is in contact with the porous outer surface, which helps to improve heating of the liquid aerosol-forming substrate.

[0049] The heater assembly may comprise a plurality of expansion members. Advantageously, a plurality of expansion members may help to increase the amount of pressure or force exerted on the porous body to help fracture the porous body when the expansion members are not being supplied with liquid aerosol-forming substrate.

[0050] According to another example of the present disclosure, there is provided a cartridge for an aerosol-generating system. The cartridge may comprise any of the heater assemblies described above. The cartridge may comprise a liquid storage portion or reservoir configured to hold a liquid aerosol-forming substrate. The liquid storage portion may be arranged at an opposite side of the heater assembly to the porous outer surface.

[0051] According to another example of the present disclosure, there is provided a cartridge for an aerosol-generating system. The cartridge comprises any of the heater assemblies described above and a liquid storage portion or reservoir configured to hold a liquid aerosolforming substrate. The liquid storage portion is arranged at an opposite side of the heater assembly to the porous outer surface.

[0052] As used herein, the term “aerosol-forming substrate” refers to a substrate capable of releasing volatile compounds that can form an aerosol. Volatile compounds may be released by heating the liquid aerosol-forming substrate.

[0053] The aerosol-forming substrate may be liquid at room temperature. The aerosolforming substrate may comprise both liquid and solid components. The liquid aerosol-forming substrate may comprise nicotine. The nicotine containing liquid aerosol-forming substrate may be a nicotine salt matrix. The liquid aerosol-forming substrate may comprise plant-based material. The liquid aerosol-forming substrate may comprise tobacco. The liquid aerosolforming substrate may comprise a tobacco-containing material containing volatile tobacco flavour compounds, which are released from the aerosol-forming substrate upon heating. The liquid aerosol-forming substrate may comprise homogenised tobacco material. The liquid aerosol-forming substrate may comprise a non-tobacco-containing material. The liquid aerosol-forming substrate may comprise homogenised plant-based material.

[0054] The liquid aerosol-forming substrate may comprise one or more aerosol-formers. An aerosol-former is any suitable known compound or mixture of compounds that, in use, facilitates formation of a dense and stable aerosol and that is substantially resistant to thermal degradation at the temperature of operation of the system. Examples of suitable aerosol formers include glycerine and propylene glycol. 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. The liquid aerosol-forming substrate may comprise water, solvents, ethanol, plant extracts and natural or artificial flavours.

[0055] The liquid aerosol-forming substrate may comprise nicotine and at least one aerosolformer. The aerosol-former may be glycerine or propylene glycol. The aerosol former may comprise both glycerine and propylene glycol. The liquid aerosol-forming substrate may have a nicotine concentration of between about 0.5% and about 10%, for example about 2%.

[0056] The liquid storage portion may be disposed at a first side of the heater assembly. An airflow channel may be disposed at an opposite side of the heater assembly to the first side. The airflow channel may be adjacent to the electrical heating element. An airflow path may extend past the electrical heating element. The airflow path may be configured to convey the aerosol. The cartridge body may be configured such that air flow past the heater assembly entrains vapourised aerosol-forming substrate.

[0057] The porous outer surface may comprise a first porous outer surface or aerosolization surface of the porous body. The porous body may comprise a second porous outer surface or a liquid absorption surface. The second porous outer surface or liquid absorption surface may be opposite the first porous outer surface. The liquid storage portion may be arranged on the same side of the heater assembly as the second porous outer surface or liquid absorption surface.

[0058] An aerosolization cavity may be arranged on the same side of the heater assembly as the first porous outer surface or aerosolization surface. The aerosolization cavity may be in fluid communication with the first porous outer surface or aerosolization surface to receive aerosol from the heater assembly. The aerosolization cavity may be in fluid communication with an airflow pathway to entrain aerosol in the airflow.

[0059] The cartridge may have a mouthpiece arranged at a mouth end of the cartridge. The mouth piece may have an aerosol outlet through which generated aerosol may be drawn by a user. The cartridge may have a connection end configured to couple the cartridge to an aerosol-generating device.

[0060] The cartridge may comprise an air inlet. The cartridge may comprise an enclosed airflow passage from the air inlet to the aerosol outlet. The enclosed airflow passage may extend from the air inlet, past the heater assembly, to the aerosol outlet. The enclosed airflow passage may pass around an external surface of the liquid storage portion. Alternatively, the enclosed airflow passage may pass through the liquid storage portion. For example, the liquid storage portion may have an annular cross-section defining an internal passage, and the airflow passage may extend through the internal passage of the liquid storage portion.

[0061] The cartridge may comprise a first airflow pathway that extends from the air inlet towards the heater assembly in a first direction. The cartridge may comprise a second airflow pathway that extends past the electrical heating element and is configured to entrain the aerosol. The cartridge may comprise a third airflow pathway that extends from the heater assembly to an aerosol outlet in a second direction. The second direction may be opposite to the first direction. The second airflow pathway may provide a fluid connection between the first airflow pathway and the third airflow pathway.

[0062] The cartridge may comprise a cartridge housing. The cartridge housing may be formed from a durable material. The cartridge housing may be formed from a liquid impermeable material. The cartridge housing may be formed form a mouldable plastics material, such as polypropylene (PP) or polyethylene terephthalate (PET). The cartridge housing of the cartridge may define a portion of the liquid storage portion or reservoir. The cartridge housing may define the liquid storage portion. The cartridge housing and the liquid storage portion may be integrally formed. Alternatively, the liquid storage portion may be formed separately from the outer housing and arranged in the outer housing.

[0063] According to another example of the present disclosure, there is provided an aerosolgenerating system. The aerosol-generating system may comprise any of the above described cartridges. The aerosol-generating system may comprise an aerosol-generating device. The aerosol-generating device may comprise a power supply for supplying power to the heater assembly. The aerosol-generating device may comprise control circuitry for controlling the supply of power from the power supply to the heater assembly. The cartridge may be removably couplable to the aerosol-generating device.

[0064] According to another example of the present disclosure, there is provided an aerosolgenerating system comprising any of the above described cartridges and an aerosolgenerating device. The aerosol-generating device comprises a power supply for supplying power to the heater assembly and control circuitry for controlling the supply of power from the power supply to the heater assembly. The cartridge is removably couplable to the aerosolgenerating device.

[0065] The aerosol-generating device may comprise a housing. The housing may be elongate. The housing may comprise any suitable material or combination of materials. Examples of suitable materials include metals, alloys, plastics or composite materials containing one or more of those materials, or thermoplastics that are suitable for food or pharmaceutical applications, for example polypropylene, polyetheretherketone (PEEK) and polyethylene. The material is preferably light and non-brittle.

[0066] The aerosol-generating device housing may define a cavity or recess for receiving a portion of a cartridge. The aerosol-generating device may have a connection end configured to removeably connect the aerosol-generating device to a cartridge. The connection end may comprise the cavity or recess for receiving the cartridge.

[0067] The aerosol-generating device may have a distal end, opposite the connection end. The distal end may comprise an electrical connector configured to connect the aerosolgenerating device to an electrical connector of an external power supply, for charging the power supply of the aerosol-generating device.

[0068] The aerosol-generating system may comprise an air inlet. The air inlet may be arranged at an interface between the cartridge and the aerosol-generating device. The aerosol-generating system may comprise an enclosed airflow passage from the air inlet to an aerosol outlet in a mouthpiece. The enclosed airflow passage may extend from the air inlet, past the heater assembly, to the aerosol outlet.

[0069] The aerosol-generating system may comprise a first airflow pathway that extends from the air inlet towards the heater assembly in a first direction. The aerosol-generating system may comprise a second airflow pathway that extends past the electrical heating element and is configured to entrain the aerosol. The aerosol-generating system may comprise a third airflow pathway that extends from the heater assembly to an aerosol outlet in a second direction. The second direction may be opposite to the first direction. The second airflow pathway may provide a fluid connection between the first airflow pathway and the third airflow pathway.

[0070] The power supply may be any suitable power supply. Preferably, the power supply is a DC power supply. The power supply may be a battery. The battery may be a Lithium based battery, for example a Lithium-Cobalt, a Lithium-lron-Phosphate, a Lithium Titanate or a Lithium-Polymer battery. The battery may be a Nickel-metal hydride battery or a Nickel cadmium battery. The power supply may be another form of charge storage device such as a capacitor. The power supply may be rechargeable and be configured for many cycles of charge and discharge. The power supply may have a capacity that allows for the storage of enough energy for one or more user experiences of the aerosol-generating system; for example, the power supply may have sufficient capacity to allow for the continuous generation of aerosol for a period of around six minutes, corresponding to the typical time taken to smoke a conventional cigarette, or for a period that is a multiple of six minutes. In another example, the power supply may have sufficient capacity to allow for a predetermined number of puffs or discrete activations of the aerosol-generating system.

[0071] The control circuitry may comprise any suitable controller or electrical components. The controller may comprise a memory. Information for performing the above-described method may be stored in the memory. The control circuitry may comprise a microprocessor. The microprocessor may be a programmable microprocessor, a microcontroller, or an application specific integrated chip (ASIC) or other electronic circuitry capable of providing control. The control circuitry may be configured to supply power to the heating element continuously following activation of the device, or may be configured to supply power intermittently, such as on a puff-by-puff basis. The power may be supplied to the heating element in the form of pulses of electrical current, for example, by means of pulse width modulation (PWM).

[0072] The control circuitry may comprise further electronic components. For example, in some embodiments, the control circuitry may comprise any of: sensors, switches, display elements.

[0073] The aerosol-generating system may comprise a puff detector. The puff detector may be configured to detect when a user draws on the aerosol-generating system. The puff detector may be any suitable sensor that is capable of detecting when a user draws on the aerosol-generating device. For example, the puff detector may be an airflow sensor. The control circuitry may be configured to supply power to the heating element when the puff detector detects a user drawing on the aerosol-generating system.

[0074] According to another example of the present disclosure, there is provided a method of manufacturing a heater assembly for an aerosol-generating system. The method may comprise compressing an expansion member. The method may comprise maintaining the expansion member in a compressed state during manufacture of the heater assembly. The method may comprise arranging the compressed expansion member within a porous body. The porous body may be configured to supply a liquid aerosol-forming substrate to a porous outer surface of the porous body. The expansion member may be arranged within the porous body. Liquid aerosol-forming substrate may be supplied to the expansion member. The expansion member may be configured, in use, to expand upon drying. The expansion member may be configured, in use, to exert a pressure on the interior of the porous body upon drying. The method may comprise providing an electrical heating element for heating the liquid aerosol-forming substrate. The electrical heating element may be provided on the porous outer surface. According to another example of the present disclosure, there is provided a method of manufacturing a heater assembly for an aerosol-generating system. The method comprises: compressing an expansion member and maintaining the expansion member in a compressed state during manufacture of the heater assembly; arranging the compressed expansion member within a porous body, the porous body being configured to supply a liquid aerosolforming substrate to a porous outer surface of the porous body, the expansion member being arranged within the porous body such that liquid aerosol-forming substrate can be supplied to the expansion member, the expansion member being configured, in use, to expand upon drying to exert a pressure on the interior of the porous body; and providing an electrical heating element for heating the liquid aerosol-forming substrate, the electrical heating element being provided on the porous outer surface.

[0075] The expansion member may be compressed in a dry state. The step of compressing the expansion member may comprise mechanically compressing the expansion member. The expansion member may be compressed by a press. The expansion member may be maintained in the compressed state in the press until inserted into the porous body of the heater assembly. The compressed expansion member may be maintained in a compressed state by impregnating it with a resin whilst in the compressed state such that the expansion member is held in the compressed state once the resin has cured. The resin may be soluble in the liquid aerosol-forming substrate. The resin may be configured to dissolve on contact with the liquid aerosol-forming substrate once the heater assembly is installed within a liquid filled cartridge.

[0076] The expansion member may be compressed by wetting the expansion member. The expansion member may be maintained in a compressed state by keeping the expansion member wet during manufacture.

[0077] The step of arranging the compressed expansion member within the porous body of the heater assembly may comprise moulding the porous body around the expansion member. The expansion member may comprise a liquid soluble thermal protection coating to protect it when the ceramic porous body is cured or fired. Alternatively, the expansion member may be inserted in a wet state into a slot in the porous body..

[0078] The step of providing an electrical heating element for heating the liquid aerosolforming substrate on a porous outer surface of the porous body may comprise depositing an electrically conductive material on the porous outer surface using any suitable physical or chemical deposition process.

[0079] Features described in relation to one of the above examples may equally be applied to other examples of the present disclosure. The invention is defined in the claims. However, below there is provided a non- exhaustive list of non-limiting examples. Any one or more of the features of these examples may be combined with any one or more features of another example, embodiment, or aspect described herein.

[0080] Example Ex1 : A heater assembly for an aerosol-generating system, the heater assembly comprising: an electrical heating element for heating a liquid aerosol-forming substrate to form an aerosol; and a porous body for supplying the liquid aerosol-forming substrate to the electrical heating element.

[0081] Example Ex2: A heater assembly according to Example Ex1 , wherein the electrical heating element is arranged on a porous outer surface of the porous body.

[0082] Example Ex3: A heater assembly according to Example Ex1 or Ex2, further comprising an expansion member arranged within an interior of the porous body such that liquid aerosolforming substrate can be supplied to the expansion member, wherein the expansion member is configured to expand upon drying to exert a pressure on the interior of the porous body such that the expansion member fractures the porous body and irreversibly disables the heater assembly if the expansion member is not supplied with liquid aerosol-forming substrate.

[0083] Example Ex4: A heater assembly according to Example Ex3, wherein the expansion member comprises a material that having a negative dry shrinkage coefficient.

[0084] Example Ex5: A heater assembly according to Example Ex3 or Ex4, wherein the expansion member comprises a hydrophobic material.

[0085] Example Ex6: A heater assembly according to Example Ex4 or Ex5, wherein the expansion member comprises hydrophobic cellulose.

[0086] Example Ex7: A heater assembly according to any of Examples Ex3 to Ex6, wherein the expansion member is substantially planar.

[0087] Example Ex8: A heater assembly according to any of Examples Ex3 to Ex7, wherein the expansion member passes through at least 50 percent of a dimension of the porous body in any one plane.

[0088] Example Ex9: A heater assembly according to Example Ex8, wherein the expansion member passes through at least 60 percent of a dimension the porous body in any one plane.

[0089] Example Ex10: A heater assembly according to Example Ex9, wherein the expansion member passes through at least 70 percent of a dimension the porous body in any one plane.

[0090] Example Ex11 : A heater assembly according to Example Ex10, wherein the expansion member passes through at least 80 percent of a dimension the porous body in any one plane. Example Ex12: A heater assembly according to any of Examples Ex3 to Ex7, wherein the expansion member passes through between 50 percent and 80 percent of a dimension the porous body in any one plane.

[0091] Example Ex13: A heater assembly according to Example Ex12, wherein the expansion member passes through between 60 percent and 80 percent of a dimension of the porous body in any one plane.

[0092] Example Ex14: A heater assembly according to Example Ex13, wherein the expansion member passes through between 70 percent and 80 percent of a dimension of the porous body in any one plane.

[0093] Example Ex15: A heater assembly according to Example Ex13, wherein the expansion member passes through between 75 percent and 80 percent of a dimension of the porous body in any one plane.

[0094] Example Ex16: A heater assembly according to any of Examples Ex3 to Ex15, wherein the expansion member is configured to expand to at least 3 times its size in its dry state compared to its saturated state.

[0095] Example Ex17: A heater assembly according to Example Ex16, wherein the expansion member is configured to expand to at least 4 times its size in its dry state compared to its saturated state.

[0096] Example Ex18: A heater assembly according to Example Ex16, wherein the expansion member is configured to expand to at least 5 times its size in its dry state compared to its saturated state.

[0097] Example Ex19: A heater assembly according to any of Examples Ex3 to Ex18, wherein the expansion member is configured to exert a pressure of at least 0.3 Newtons per square millimetre on the porous body when the expansion member is in its expanded state.

[0098] Example Ex20: A heater assembly according to Example Ex19, wherein the expansion member is configured to exert a pressure of at least 0.4 Newtons per square millimetre on the porous body when the expansion member is in its expanded state.

[0099] Example Ex21 : A heater assembly according to Example Ex20, wherein the expansion member is configured to exert a pressure of at least 0.5 Newtons per square millimetre on the porous body when the expansion member is in its expanded state

[0100] Example Ex22: A heater assembly according to any of Examples Ex3 to Ex21 , wherein the expansion member is configured to exert between approximately 0.3 and 0.8 Newtons per square millimetre of pressure on the porous body in its expanded state. Example Ex23: A heater assembly according to Example Ex22, wherein the expansion member is configured to exert between approximately 0.4 and 0.7 Newtons per square millimetre of pressure on the porous body in its expanded state.

[0101] Example Ex24: A heater assembly according to Example Ex22, wherein the expansion member is configured to exert between approximately 0.5 and 0.6 Newtons per square millimetre of pressure on the porous body in its expanded state.

[0102] Example Ex25: A heater assembly according to any of Examples Ex3 to Ex24, wherein a major surface of the expansion member has a surface area of between 6 and 16 square millimetres.

[0103] Example Ex26: A heater assembly according to Example Ex25, wherein a major surface of the expansion member has a surface area of between 7 and 14 square millimetres.

[0104] Example Ex27: A heater assembly according to Example Ex26, wherein a major surface of the expansion member has a surface area of between 8 and 12 square millimetres.

[0105] Example Ex28: A heater assembly according to any of Examples Ex3 to Ex27, wherein the expansion member is arranged perpendicular to the porous outer surface.

[0106] Example Ex29: A heater assembly according to any of Examples Ex3 to Ex27, wherein the expansion member is arranged parallel to the porous outer surface.

[0107] Example Ex30: A heater assembly according to any of Examples Ex1 to Ex29, wherein the porous body comprises a material having a thermal decomposition temperature of at least 250 degrees Celsius.

[0108] Example Ex31 : A heater assembly according to any of Examples Ex1 to Ex30, wherein the porous body comprises a ceramic.

[0109] Example Ex32: A heater assembly according to Example Ex31 , wherein the porous body comprises a ceramic comprising one or both of SiC>2 and Ca2SiOs.

[0110] Example Ex33: A heater assembly according to any of Examples Ex1 to Ex32, wherein the porous body comprises a slot for receiving the expansion member.

[0111] Example Ex34: A heater assembly according to Example Ex33, wherein the slot is located in the porous outer surface of the porous body.

[0112] Example Ex35: A heater assembly according to Example Ex33, wherein the porous outer surface comprises a first porous outer surface and the slot is located in a second porous outer surface of the porous body, the second porous outer surface being opposite the first porous outer surface.

[0113] Example Ex36: A heater assembly according to Example Ex33, wherein the slot is located in a porous side surface of the porous body. Example Ex37: A heater assembly according to any of Examples Ex3 to Ex36, wherein the porous body comprises a line of weakness and the expansion member is arranged to cause the porous body to fracture along the line of weakness.

[0114] Example Ex38: A heater assembly according to Example Ex37, wherein the line of weakness comprises a notch.

[0115] Example Ex39: A heater assembly according to any of Examples Ex1 to Ex38, wherein the electrical heating element is formed from an electrically conductive material deposited on to the porous outer surface.

[0116] Example Ex40: A heater assembly according to any of Examples Ex1 to Ex38, further comprising electrical contacts connected to the electrical heating element, each electrical contact being disposed at opposite sides of the porous outer surface such that the electrical heating element extends between the electrical contacts and forms an electrical connection therebetween.

[0117] Example Ex41 : A heater assembly according to Example Ex40, wherein the electrical heating element extends between the electrical contacts in a wave-like or serpentine manner.

[0118] Example Ex42: A heater assembly according to any of Examples Ex3 to Ex42, wherein the heater assembly comprises a plurality of expansion members.

[0119] Example Ex43: A cartridge for an aerosol-generating system, the cartridge comprising: a heater assembly according to any of Examples Ex1 to Ex42; and a liquid storage portion configured to hold a liquid aerosol-forming substrate; wherein the liquid storage portion is arranged at an opposite side of the heater assembly to the porous outer surface.

[0120] Example Ex44: An aerosol-generating system comprising: a cartridge according to Example Ex43; and an aerosol-generating device comprising a power supply for supplying power to the heater assembly and control circuitry for controlling the supply of power from the power supply to the heater assembly; wherein the cartridge is removably couplable to the aerosol-generating device.

[0121] Example Ex45: A aerosol-generating system according to Example Ex44, further comprising an air inlet.

[0122] Example Ex46: A aerosol-generating system according to Example Ex45, wherein the air inlet is arranged at an interface between the cartridge and the aerosol-generating device.

[0123] Example Ex47: A aerosol-generating system according to Example Ex45 or Ex46, wherein the aerosol-generating system comprises a first airflow pathway that extends from the air inlet towards the heater assembly in a first direction; a second airflow pathway that extends past the electrical heating element and is configured to entrain an aerosol; and a third airflow pathway that extends from the heater assembly to an aerosol outlet in a second direction, the second direction being opposite to the first direction; and wherein the second airflow pathway provides a fluid connection between the first airflow pathway and the third airflow pathway.

[0124] Example Ex48: A method of manufacturing a heater assembly for an aerosolgenerating system, the method comprising: compressing an expansion member and maintaining the expansion member in a compressed state during manufacture of the heater assembly; arranging the compressed expansion member within a porous body, the porous body being configured to supply a liquid aerosol-forming substrate to a porous outer surface of the porous body, the expansion member being arranged within the porous body such that liquid aerosol-forming substrate can be supplied to the expansion member, the expansion member being configured, in use, to expand upon drying to exert a pressure on the interior of the porous body; and providing an electrical heating element for heating the liquid aerosolforming substrate, the electrical heating element being provided on the porous outer surface.

[0125] Example Ex49: A method according to Example Ex48, wherein the expansion member is compressed in a dry state.

[0126] Example Ex50: A method according to Example Ex49, wherein the compressed expansion member is maintained in a compressed state by impregnating it with a resin whilst in the compressed state.

[0127] Example Ex51 : A method according to Example Ex50, wherein the resin is soluble in the liquid aerosol-forming substrate.

[0128] Example Ex52: A method according to Example Ex48, wherein the expansion member is compressed by wetting the expansion member.

[0129] Examples will now be further described with reference to the figures in which:

[0130] Figure 1 is a schematic perspective view of a heater assembly according to an example of the present disclosure;

[0131] Figures 2A and 2B are a schematic side and plan view respectively of the heater assembly of Figure 1 ;

[0132] Figure 3 is a schematic perspective view of the heater assembly of Figure 1 in which the porous body has been fractured and showing the forces that were exerted by the expansion member to cause the porous body to fracture;

[0133] Figures 4A to 4C are schematic side views of heater assemblies according to further examples of the present disclosure;

[0134] Figure 4D is a schematic plan view of a heater assembly according to another example of the present disclosure; and Figure 5 is a schematic cross-sectional view of an aerosol-generating system according to an example of the present disclosure.

[0135] Figure 6 is a flowchart of a method of manufacturing a heater assembly for an aerosolgenerating system according to an example of the present disclosure.

[0136] It will be appreciated that the figures in the present application are schematic and have been simplified for the purposes of clarity. Consequently, some features may have been omitted and the features are not necessarily drawn to scale.

[0137] References to orientations such as vertical, horizontal, above, below, upper and lower, etc. when describing the features of the present disclosure are not intended to imply any limitation on the orientation of those features but are merely intended to show the relative spatial arrangement of features, particularly with reference to the figures or in normal use. It will be appreciated that the features of the present disclosure may have different orientations in use.

[0138] Referring to Figure 1 , there is shown a heater assembly 10 comprising a heating element 12 for heating a liquid aerosol-forming substrate to form an aerosol and a cuboidshaped porous body 14 for supplying the liquid aerosol-forming substrate from a reservoir or liquid storage portion (not shown) to the electrical heating element 12. The electrical heating element 12 is arranged on a first porous outer surface 14a of the porous body 14. In the example heater assembly 10 of Figure 1 , the electrical heating element 12 has been deposited on the first porous outer surface 14a of the porous body 14 by a suitable physical or vapour deposition process or printing process. The electrical heating element 12 comprises a nickelchromium (NiCr) alloy, although it will be appreciated that other suitable electrically conductive materials suitable for resistive heating may be used. The porous body comprises a ceramic formed from silicon dioxide (SiC>2) or calcium silicate (Ca2SiOs), although other suitable ceramics may be used.

[0139] A substantially planar expansion member 16 is arranged within an interior of the porous body 14. This arrangement means that, when liquid aerosol-forming substrate is available to be supplied to the electrical heating element 12, it is also supplied to the expansion member 16. Conversely, if liquid aerosol-forming substrate is not available to be supplied to the electrical heating element 12, the expansion member 16 will also not be supplied with liquid aerosol-forming substrate. The expansion member 16 is made from hydrophobic cellulose, which exhibits dry expansion, that is, the hydrophobic cellulose has a smaller size when wet and expands upon drying. Hydrophobic cellulose is therefore characterised by having a negative dry-shrinkage coefficient. If the expansion member 16 is not supplied with liquid aerosol-forming substrate, for example, if dry heating is occurring due to the liquid aerosol- forming substrate in the reservoir or liquid storage portion being depleted, then the expansion member 16 will expand. This exerts a pressure or force on the interior of the porous body 14 such that the expansion member 16 deliberately fractures the porous body 14 and irreversibly disables the heater assembly 10 in a controlled manner.

[0140] As can be seen in Figure 1 , the expansion member 16 passes through more than 50 percent of the height and width of the porous body 14 in the vertical plane. Consequently, less than 50 percent of the material of the porous body 14 remains in the vertical plane in which the expansion member 16 is located. Therefore, the stress in the porous body 14 that results from the pressure or force exerted by the expansion member 16 is concentrated in the remaining material of the porous body 14, which helps to fracture the porous body 14 in a controlled and specific fashion, that is, along the vertical plane in which the expansion member 16 is located.

[0141] Hydrophobic cellulose expands to at least five times its size when dry compared to its size when it is saturated with liquid aerosol-forming substrate. In the example heater assembly 10 of Figure 1 , the dimension of the porous body are 9.2 x 4.1 x4.6 millimetres. The dimension of the expansion member 16 when wet are 3 x 3 x 1.5 millimetres. When dry, it was found that the expansion member 16 had expanded to 15 x 15 x 7.5 millimetres, that is, the expansion member 16 was five times its size in its dry state compared to its wet state. The expansion of the expansion member 16 was found to exert a pressure of 0.5 Newtons per square millimetre or a tensile force of 4.5 Newtons, which was sufficient to fracture the porous body 14. It is possible to use expansion members of other sizes in the heater assembly 10. For example, an expansion member with major surfaces having a surface area of 10.2 square millimetres was found to exert a tensile force of 5.1 Newtons upon drying, which was also sufficient to fracture the porous body 14.

[0142] The heater assembly 10 of Figure 1 further comprises electrical contacts 18 that are electrically connected to the electrical heating element 12. The electrical contacts 18 are disposed on the same porous outer surface 14a as the electrical heating element 12 and at opposite sides of the porous outer surface 14a. The electrical heating element 12 extends between the electrical contacts 18 in a serpentine or wave-like manner to increase the length of the electrical heating element 12 between the electrical contacts 18 and thus the amount of heating of the porous outer surface 14 during operation of the heater assembly 10. In the example heater assembly 10 of Figure 1 , the electrical contacts comprise one or more of copper, zinc or gold, although other suitable materials may be used.

[0143] Figures 2A and 2B are a schematic side and plan view respectively of the heater assembly of Figure 1. As described above, the electrical heating element 12 and electrical contacts 18 are arranged on the first porous outer surface 14a of the porous body 14, which first porous outer surface 14a is shown uppermost in Figure 2A. The first porous outer surface 14a therefore represents a heating or aerosolization surface of the heater assembly 10. The expansion member 16 is arranged within an interior of the porous body 14 and extends vertically upwards from a lower or second porous outer surface 14b for greater than 50 percent of the height of the porous body 14 to leave only a portion X of the material of the porous body 14 above the expansion member 16. The second porous outer surface 14b receives liquid aerosol-forming substrate from a reservoir or liquid storage portion (not shown) and therefore represents a liquid absorption surface of the heater assembly 10. Extending down to the second porous outer surface 14b helps to ensure that the expansion member 16 is supplied with liquid aerosol-forming substrate, when liquid aerosol-forming substrate is available, because the expansion member 16 will be in contact with the liquid aerosol-forming substrate. When liquid aerosol-forming substrate is not supplied to the expansion member 16 such that the expansion member 16 expands, stress is concentrated in the portion X of the material of the porous body 14. The expansion member 16 may be arranged within the porous body 14 by moulding the porous body 14 around the expansion member 16. Alternatively, a slot may be formed in the second porous outer surface 14b and the expansion member 16 may be inserted into the slot in a compressed or wet state.

[0144] As can be seen in Figure 2B, the expansion member 16 extends across greater than 50 percent of the width of the porous body 14 to leave only portions Y of the material of the porous body 14 on either side of the expansion member 16. When liquid aerosol-forming substrate is not supplied to the expansion member 16 such that the expansion member 16 expands, stress is also concentrated in the portions Y of the material of the porous body 14.

[0145] Figure 3 is a schematic perspective view of the heater assembly 10 of Figure 1 in which the porous body 14 has been fractured by the expansion member 16. Figure 3 shows the forces that were exerted by the expansion member 16 to cause the porous body 14 to fracture. The expansion member 16 has two major surfaces, that is, the surfaces of the expansion member 16 with the greatest surface area. The major surfaces of the expansion member 16 exert a pressure or force out of the plane of the expansion member 16, that is, in a direction orthogonal to the major surfaces of the expansion member 16 when the expansion member 16 expands, as denoted by arrows A in Figure 3. The pressure or forces denoted by arrows A act to push the portions of the porous body 14 either side to the expansion member 14 apart to fracture the porous body 14. The expansion member 16 also exerts pressure or force in it own plane, as denoted by arrows B. The pressure or forces denoted by arrows B act on any material in their path and further help to fracture the porous body 14. As discussed above, the electrical heating element 12 is deposited or printed on to the first porous outer surface 14a of the heater assembly 10 of Figure 1. Such methods for forming an electrical heating element result in an electrical heating element that is relatively mechanically weak. As can be seen in Figure 2A, the fracturing of the porous body 14 has resulted in the electrical heating element 12 being broken. This breaks the electrical circuit between the electrical contacts 18 such that the electrical heating element 12 can no longer be operated and the heater assembly 10 is irreversibly disabled.

[0146] Figures 4A to 4D are schematic views of heater assemblies according to further examples of the present disclosure. The heater assemblies of Figures 4A to 4D all have a similar construction to that of Figure 1 and like reference numerals have been used to refer to like features.

[0147] Figure 4A shows a schematic side view of a heater assembly 11 comprising a porous body 14 having an electrical heating element 12 and electrical contacts 18 arranged on a first porous outer surface 14a of the porous body 14. The heater assembly 11 of Figure 4A differs from that of Figure 1 in that the expansion member 17 is arranged within the porous body 14 parallel to the first porous outer surface 14a at substantially the mid-point of the height of the porous body 14. The expansion member 17 passes through more than 50 percent of the length of the porous body 14 in the horizontal plane and also passes through more than 50 percent of the width (not shown) of the porous body 14 in the horizontal plane. Similar to the heater assembly 10 of Figure 1 , forces act both in and out of the plane of the expansion member 17 to fracture the porous body 14 when the expansion member is not supplied with liquid aerosol-forming substrate.

[0148] Figure 4B shows a schematic side view of a heater assembly 13 comprising a porous body 14 having an electrical heating element 12 and electrical contacts 18 arranged on a first porous outer surface 14a of the porous body 14. The heater assembly 13 of Figure 4B differs from that of Figure 1 in that the porous body 14 has a line of weakness in the form of a notch 20 on the second porous outer surface 14b, shown lowermost in Figure 4B. Although it cannot be seen in Figure 4B, the notch 20 extends across the entire width of the porous body 14. The notch 20 provides a region of reduced material thickness above the notch 20. An expansion member 19 is located directly above the notch 20 and extends vertically up to the first porous outer surface 14a. Similar to the heater assembly 10 of Figure 1 , forces act both in and out of the plane of the expansion member 19 to fracture the porous body 14 when the expansion member is not supplied with liquid aerosol-forming substrate. Due to the presence of the notch 20, stress is concentrated in the region of reduce material thickness above the notch 20 and the porous body fractures in a controlled manner along the line of the notch 20. By extending up to the first porous outer surface 14a, the expansion member 19 may help to lift the electrical heating element 12 off the porous outer surface 14a such that the electrical heating element 12 breaks.

[0149] Figure 4C shows a schematic side view of a heater assembly 15 comprising a porous body 14 having an electrical heating element 12 and electrical contacts 18 arranged on a first porous outer surface 14a of the porous body 14. The heater assembly 13 of Figure 4C differs from that of Figure 1 in that the porous body 14 has a plurality of expansion members 21 arranged within an interior of the porous body 14. Similar to the heater assembly 10 of Figure 1 , forces act both in and out of the plane of both of the expansion members 21 to fracture the porous body 14 when the expansion members 21 are not supplied with liquid aerosol-forming substrate.

[0150] Figure 4D is a schematic plan view of a heater assembly 23 comprising a porous body 14 having an electrical heating element 12 and electrical contacts 18 arranged on the porous body 14. The heater assembly 23 of Figure 4D differs from that of Figure 1 in that the expansion member 25 is arranged asymmetrically within the porous body 14. The expansion member 25 extends from one side or edge 14c of the porous body and passes through more than 50 percent of the width of the porous body 14 to leave a portion Y of the material of the porous body 14 on the other side or edge 14d of the porous body 14 to the side of the expansion member 25. The expansion member 25 also extends from the lower to the upper porous outer surface of the porous body, that is, it passes through the entire height of the porous body 14. When liquid aerosol-forming substrate is not supplied to the expansion member 16 such that the expansion member 16 expands, stress is concentrated in the portion Y of the material of the porous body 14 to fracture the porous body.

[0151] The expansion members 17, 19, 21 and 25 of Figures 4A to 4D respectively may be arranged within the porous body 14 by moulding the porous body 14 around the expansion member 16. Alternatively, the porous bodies 14 of the heater assemblies 13 and 23 of Figures 4B and 4D respectively may have a slot arranged in a porous outer surface or side of the porous body 14 and the expansion members 19 and 25 may be inserted into the slot in a compressed or wet state.

[0152] Figure 5 is a schematic cross-sectional view of an aerosol-generating system 50 according to an example of the present disclosure. The aerosol-generating system 50 comprises two main components, a cartridge 100 and a main body part or aerosol-generating device 200. The aerosol-generating device 200 comprises a recess 202 for receiving a connection end 102 of the cartridge 100. The cartridge 100 is removably connectable to the aerosol-generating device 200 by inserting the connection end 102 of the cartridge 100 into the recess 202. The connection end 102 of the cartridge 100 and the recess 202 of the aerosol-generating device 200 each have electrical contacts or connections (not shown) which are arranged to cooperate to provide an electrical connection between the cartridge 100 and the aerosol-generating device 200. The aerosol-generating device 200 contains a power source in the form of a battery 204, which in this example is a rechargeable lithium ion battery, and control circuitry 206. The aerosol-generating system 50 is portable and has a size comparable to a conventional cigar or cigarette.

[0153] The cartridge 100 comprises a cartridge housing 104 and a mouthpiece 106. The cartridge housing 104 contains a reservoir or liquid storage portion 108 for holding liquid aerosol-forming substrate 110. The liquid storage portion 108 has an opening in its lower end or base and a heater assembly 10 is arranged in the opening. The heater assembly 10 corresponds to the heater assembly 10 in Figure 1 but is inverted in the cartridge 100 of Figure 5 such that the first porous outer surface 14a of the porous body 14 comprising the electrical heating element (not shown) is lowermost in Figure 5 and the second porous outer surface 14b of the porous body 14 is uppermost. As discussed above, the second porous outer surface 14b receives liquid aerosol-forming substrate 110 from the liquid storage portion 108 and represents a liquid absorption surface of the heater assembly 10. The porous body 14 supplies liquid aerosol-forming substrate by conveying the liquid aerosol-forming substrate 110 through the thickness of the porous body 14 to the electrical heating element, where, in use, it is heated by the electrical heating element to generate an aerosol. The first porous outer surface 14a therefore represents a heating or aerosolization surface of the heater assembly 10. The generated aerosol passes into a aerosolization cavity 112 between the first porous outer surface 14a of the heater assembly 10 and an interior surface of the base of the cartridge housing 104. The electrical contacts (not shown) on the first porous outer surface 14a of the heater assembly 10 are electrically connected to the electrical connections (not shown) on the connection end 102 of the cartridge to allow electrical power to be supplied to the heater assembly 10.

[0154] The connection end 102 of the cartridge 100 and the recess 202 of the aerosolgenerating device 200 each have electrical contacts or connections (not shown) which are arranged to cooperate to provide an electrical connection between the cartridge 100 and the aerosol-generating device 200.

[0155] Air inlets 114 are arranged at an interface between the mouthpiece 106 and the aerosol-generating device 200. In the example of Figure 5, the air inlets 114 are shown as two discrete openings. However, it will be appreciated that an air inlet may extend continuously around the perimeter of the mouthpiece 106 at the interface between the mouthpiece 106 and the aerosol-generating device 200. A first or intake airflow pathway 116 extends from the air inlets 114 into the recess 202 of the aerosol-generating device 200 between an inner wall of the recess 202 and an outer surface of the cartridge housing 104. The air from the first airflow pathway 116 then passes around the base of the cartridge body 104. The base of the cartridge housing 104 has an aperture 118 that passes through the cartridge housing 104 into the aerosolization cavity 112. A second airflow pathway 119 passes through the aperture 118 and impinges on the first porous outer surface 14a comprising the electrical heating element (not shown). Air from the second airflow pathway passes across the first porous outer surface 14a of the heater assembly 10 entraining aerosol generating by the heater assembly. A third or exhaust airflow pathway 120 extends from the aerosolization cavity 112 to an aerosol outlet 122 formed in a proximal end of the mouthpiece 106. The third airflow pathway flows past the liquid storage portion 108 between an interior wall of the cartridge housing 104 or mouthpiece 106 and an exterior wall of the liquid storage portion 108. The airflow pathway through the aerosol-generating system 50 in Figure 5 is denoted by dashed arrows.

[0156] The aerosol-generating system 50 is configured so that a user can puff or draw on the mouthpiece 106 of the cartridge 100 to draw aerosol into their mouth through the aerosol outlet 122. In operation, when a user puffs on the mouthpiece 106, air is drawn through the first and second airflow pathways 116, 119 from the air inlets 114, past the heater assembly 10 and to the aerosol outlet 122 via the third airflow pathway 120. The control circuitry 206 controls the supply of electrical power from the battery 204 to the cartridge 100 when the system is activated. The electrical power supplied to the cartridge 100 controls the amount and properties of the vapour produced by the heater assembly 10. The control circuitry 206 may include an airflow sensor (not shown) and the control circuitry 206 may supply electrical power to the heater assembly 10 when user puffs are detected by the airflow sensor. Alternatively, a user may activate the aerosol-generating system 50 by pressing a button (not shown). When a user puffs on the mouthpiece 106 of the cartridge 100, the heater assembly 10 is activated and generates a vapour that is entrained in the air flow passing across the heater assembly 10. The vapour cools within the aerosolization cavity 112 to form an aerosol, which is then drawn into the user’s mouth via the third airflow pathway 120 and aerosol outlet 122.

[0157] Figure 6 is a flowchart of a method 300 of manufacturing a heater assembly for an aerosol-generating system. The method 300 comprises a step S1 of compressing an expansion member and maintaining the expansion member in a compressed state during manufacture of the heater assembly. The expansion member may be compressed mechanically in a dry state, for example, within a press and maintained in that state until inserted into the porous body of the heater assembly. The compressed expansion member could be maintained in a compressed state by impregnating it with a resin whilst in the compressed state. The resin could be soluble in the liquid aerosol-forming substrate such that the resin dissolves on contact with the liquid aerosol-forming substrate once the heater assembly is installed within a liquid filled cartridge such that the expansion member is then maintained in a compressed state by virtue of being wet. As a further alternative, the expansion member could be compressed by wetting it and maintained in a compressed state by keeping it wet during manufacture.

[0158] The method 300 comprises a step S2 of arranging the compressed expansion member within the porous body of the heater assembly. The compressed expansion member may be arranged within the porous body by moulding the porous body around the expansion member. For example, a compressed expansion member that is being maintained in a compressed state by being impregnated with a liquid soluble resin may be placed within the mould forming the porous body. A liquid soluble thermal protection coating could also be applied to the compressed expansion member to protect it when the ceramic porous body is cured or fired. Alternatively, a slot may be formed in a porous outer surface of the porous body and the expansion member may be inserted into the slot in a compressed or wet state.

[0159] The method 300 comprises a step S3 of providing an electrical heating element for heating the liquid aerosol-forming substrate on a porous outer surface of the porous body, that is, heating or aerosolization surface of the porous body. As discussed above, the electrical heating element could be provided by depositing an electrically conductive material on the porous outer surface using a physical or chemical deposition process.

[0160] The numbering of the steps of the method 300 is not intended to imply any particular order of carrying out the steps of the method. The steps S1 to S3 could be performed in any suitable order and may have further steps included before, after or in between steps S1 to S3.

Claims

Claims1 . A heater assembly for an aerosol-generating system, the heater assembly comprising: an electrical heating element for heating a liquid aerosol-forming substrate to form an aerosol; a porous body for supplying the liquid aerosol-forming substrate to the electrical heating element, the electrical heating element being arranged on a porous outer surface of the porous body; and an expansion member arranged within an interior of the porous body such that liquid aerosol-forming substrate can be supplied to the expansion member, wherein the expansion member is configured to expand upon drying to exert a pressure on the interior of the porous body such that the expansion member fractures the porous body and irreversibly disables the heater assembly if the expansion member is not supplied with liquid aerosol-forming substrate.

2. A heater assembly according to claim 1 , wherein the expansion member comprises a material that having a negative dry shrinkage coefficient.

3. A heater assembly according to claim 2, wherein the expansion member comprises hydrophobic cellulose.

4. A heater assembly according to any of claims 1 to 3, wherein the expansion member is substantially planar.

5. A heater assembly according to any of claims 1 to 4, wherein the expansion member passes through at least 50 percent of a dimension of the porous body in any one plane.

6. A heater assembly according to any preceding claim, wherein the expansion member is configured to expand to at least 3 times its size, preferably to at least 4 times its size, and more preferably to at least 5 times its size, in its dry state compared to its saturated state.

7. A heater assembly according to any preceding claim, wherein the expansion member is configured to exert between approximately 0.3 and 0.8 Newtons per square millimetre of pressure on the porous body in its expanded state.

8. A heater assembly according to any preceding claim, wherein a major surface of the expansion member has a surface area of between 6 and 16 square millimetres.

9. A heater assembly according to any preceding claim, wherein the expansion member is arranged perpendicular to the porous outer surface.

10. A heater assembly according to any of claims 1 to 8, wherein the expansion member is arranged parallel to the porous outer surface.

11. A heater assembly according to any preceding claim, wherein the porous body comprises a line of weakness and the expansion member is arranged to cause the porous body to fracture along the line of weakness.

12. A heater assembly according to any preceding claim, further comprising electrical contacts connected to the electrical heating element, each electrical contact being disposed at opposite sides of the porous outer surface such that the electrical heating element extends between the electrical contacts and forms an electrical connection therebetween.

13. A heater assembly according to any preceding claim, wherein the heater assembly comprises a plurality of expansion members.

14. A cartridge for an aerosol-generating system, the cartridge comprising: a heater assembly according to any of claims 1 to 13; and a liquid storage portion configured to hold a liquid aerosol-forming substrate; wherein the liquid storage portion is arranged at an opposite side of the heater assembly to the porous outer surface.

15. An aerosol-generating system comprising: a cartridge according to claim 14; and an aerosol-generating device comprising a power supply for supplying power to the heater assembly and control circuitry for controlling the supply of power from the power supply to the heater assembly; wherein the cartridge is removably couplable to the aerosol-generating device.

16. A method of manufacturing a heater assembly for an aerosol-generating system, the method comprising: compressing an expansion member and maintaining the expansion member in a compressed state during manufacture of the heater assembly; arranging the compressed expansion member within a porous body, the porous body being configured to supply a liquid aerosol-forming substrate to a porous outer surface of the porous body, the expansion member being arranged within the porous body such that liquid aerosol-forming substrate can be supplied to the expansion member, the expansion member being configured, in use, to expand upon drying to exert a pressure on the interior of the porous body; and providing an electrical heating element for heating the liquid aerosol-forming substrate, the electrical heating element being provided on the porous outer surface.

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