Patent Publication Number: US-2021186104-A1

Title: Heater assembly having heater element isolated from liquid supply

Description:
The invention relates to aerosol-generating devices that heat a liquid substrate to form an aerosol. In particular, the invention relates to handheld aerosol generating devices that produce an aerosol for user inhalation. 
     Handheld aerosol-generating systems that produce an aerosol for inhalation from a liquid substrate are becoming more widely used, both in the field of medical inhalers for drug delivery and in the field of smoking products that are alternatives to cigarettes, such as e-cigarettes. 
     In an e-cigarette the aerosol is typically formed by heating a liquid aerosol-forming substrate. The liquid is held in a liquid storage reservoir and delivered to a heating element by a capillary material or wick extending between the reservoir and the heating element. A high retention material (HRM) may be placed in contact with the heating element to retain liquid in proximity with the heating element. 
     In one configuration, a mesh heater is simply placed over an HRM containing a liquid aerosol-forming substrate. The mesh heater forms part of an airflow passage through which a user can draw vapour. The heating element is activated in response to a user puff on the device. When the heating element is activated, liquid in the HRM close to the heating element is vaporised and drawn away from the heating element by the user puff. More liquid is then drawn into the HRM from the liquid storage reservoir. The function of the HRM or capillary wick is to ensure an adequate amount of liquid is close to the heating element, whatever the orientation of the system with respect to gravity. So for each user puff a sufficient amount of liquid is vaporised and subsequently forms an aerosol. The heating element and liquid storage reservoir are typically provided together as a disposable cartridge. This arrangement has the advantage of being simple to manufacture and being robust. An example of this type of arrangement is described in WO2015117700A1. 
     One issue with this type of system is heating efficiency. Heat is transferred not only to the liquid that is desired to be vaporised but also, to a significant degree, to the rest of the liquid in the liquid storage reservoir, which is not required to be vaporized during the user puff. The thermal mass of the rest of e-liquid, which is heated by conduction and convection by the e-liquid to be vaporized, creates heat losses at the heater area and so creates the need for extra power. In handheld devices that are typically battery powered, it is particularly critical to improve heating efficiency and so reduce the need to recharge or replace the batteries frequently and allow small form factor batteries to be used. 
     It would be desirable to solve or to decrease the significance of this issue. 
     In a first aspect, there is provided a vaporiser assembly for an electrically operated aerosol-generating device comprising: 
     a generally planar, fluid permeable heating element having a first side and a second side opposite the first side; 
     a liquid transport medium, the liquid transport medium having a first side in contact with the second side of the heating element and a second side opposite to the first side, the heating element extending over a first area of the first side of the liquid transport medium; and 
     a liquid supply conduit having a first end in contact with the second side of the liquid transport medium and extending over only a second area of the second side of the liquid transport medium, wherein the second area is smaller than the first area; 
     wherein the liquid transport medium is arranged to transport liquid from the liquid supply conduit to the first area of the second side of the heating element. 
     Having the liquid supply conduit extend over a relative small area of the liquid transport medium compared to the heating element has the advantage that only a small proportion of the heat generated by the heater is transferred to the liquid in the liquid supply conduit. This provides good heating efficiency for the vaporiser assembly, in that less heat is transferred away from the liquid transport medium compared to the arrangement of the prior art described above. The second area may be less than 50% of the first area and preferably less than 30% of the first area. 
     The liquid transport medium advantageously covers the entire heating element. This is maximises aerosol-generation for a given input power. It also avoids hot spots at the edge of the transport material. Hot spots could lead to the generation of undesirable chemical compounds. 
     The liquid transport medium may have a capillary structure arranged to transport liquid parallel to the second side of the heating element. This allows the liquid to be effectively transported across the entire heating element. In the systems of the prior art, there is the potential for bubbles to form in the HRM or capillary wick, which affect the correct liquid transfer from the liquid storage reservoir to the heating element. With the arrangement of the present invention, the potential for the formation of bubbles in the liquid supply conduit is reduced. The liquid transport medium may be may relatively thin so that vapour formed in the liquid transport can easily escape, and is unlikely to pass back into the liquid supply conduit. 
     A thickness of the liquid transport medium between the first and second sides of the liquid transport medium may be between 1 mm and 5 mm. The liquid transport medium may have an area of between 50 mm 2  and 500 mm 2 . 
     The vaporiser assembly may be used to generate vapour or aerosol for inhalation by a user, for example in an electric smoking system. The construction and operation of the vaporiser assembly may be such that all of the liquid that is held in the liquid transport medium may be vaporised in a single user puff. Liquid that is subsequently drawn into the liquid transport medium to replace the vaporised liquid is vaporised in a subsequent puff. By appropriate selection of the dimensions of the liquid transport medium, a desired and consistent amount of vapour can be produced during each user puff. 
     The vaporiser assembly may comprise a housing, the heating element and the liquid transport medium being retained in the housing, wherein the housing engages or is integral with the liquid supply conduit. With this arrangement the heating element and liquid transport medium may be held together and aligned with one another. 
     In order to allow vapour to escape from the vaporiser assembly, the heating element is fluid permeable. Fluid permeable in this context means that vapour can escape from the liquid transport medium through the plane of the heating element. To allow this, the heating element may comprise apertures or pores though which vapour can pass. For example, the heating element may comprise a mesh or fabric of electrically resistive filaments. Alternatively, or in addition, the heating element may comprise a sheet with holes or slots in it. 
     The heating element may be a resistive heating element, which is supplied directly with an electrical current in use. 
     The resistive heating element may comprise a plurality of interstices or apertures extending from the second side to the first side and through which fluid may pass. 
     The resistive heating element may comprise a plurality of electrically conductive filaments. The term “filament” is used throughout the specification to refer to an electrical path arranged between two electrical contacts. A filament may arbitrarily branch off and diverge into several paths or filaments, respectively, or may converge from several electrical paths into one path. A filament may have a round, square, flat or any other form of cross-section. A filament may be arranged in a straight or curved manner. 
     The resistive heating element may be an array of filaments, for example arranged parallel to each other. Preferably, the filaments may form a mesh. The mesh may be woven or non-woven. The mesh may be formed using different types of weave or lattice structures. Alternatively, the resistive heating element consists of an array of filaments or a fabric of filaments. 
     The filaments may define interstices between the filaments and the interstices may have a width of between 10 micrometres and 100 micrometres. Preferably, the filaments give rise to capillary action in the interstices, so that in use, liquid to be vaporized is drawn into the interstices, increasing the contact area between the heating element and the liquid aerosol-forming substrate. 
     The filaments may form a mesh of size between 60 and 240 filaments per centimetre (+/−10 percent). Preferably, the mesh density is between 100 and 140 filaments per centimetres (+/−10 percent). More preferably, the mesh density is approximately 115 filaments per centimetre. The width of the interstices may be between 100 micrometres and 25 micrometres, preferably between 80 micrometres and 70 micrometres, more preferably approximately 74 micrometres. The percentage of open area of the mesh, which is the ratio of the area of the interstices to the total area of the mesh may be between 40 percent and 90 percent, preferably between 85 percent and 80 percent, more preferably approximately 82 percent. 
     The filaments may have a diameter of between 8 micrometres and 100 micrometres, preferably between 10 micrometres and 50 micrometres, more preferably between 12 micrometres and 25 micrometres, and most preferably approximately 16 micrometres. The filaments may have a round cross section or may have a flattened cross-section. 
     The area of the filaments may be small, for example less than or equal to 50 square millimetres, less than or equal to 25 square millimetres, more preferably approximately 15 square millimetres. The size is chosen such to incorporate the heating element into a handheld system. The heating element may, for example, be rectangular and have a length between 2 millimetres to 10 millimetres and a width between 2 millimetres and 10 millimetres. 
     The filaments of the heating element may be formed from any material with suitable electrical properties. 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, Timetal®, iron-aluminum based alloys and iron-manganese-aluminum based alloys. Timetal® is a registered trade mark of Titanium Metals Corporation. The filaments may be coated with one or more insulators. Preferred materials for the electrically conductive filaments are stainless steel and graphite, more preferably 300 series stainless steel like AISI 304, 316, 304L, 316L. Additionally, the electrically conductive 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 substantially flat 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, a substantially flat filament arrangement with increased resistance reduces parasitic losses. Advantageously, high resistivity heaters allow more efficient use of battery energy. 
     Preferably, the filaments are made of wire. More preferably, the wire is made of metal, most preferably made of stainless steel. 
     The electrical resistance of the filaments of the heating element may be between 0.3 Ohms and 4 Ohms. Preferably, the electrical resistance is equal or greater than 0.5 Ohms. More preferably, the electrical resistance the heating element is between 0.6 Ohms and 0.8 Ohms, and most preferably about 0.68 Ohms. 
     Alternatively, the heating element may comprise a heating plate in which an array of apertures is formed. The apertures may be formed by etching or machining, for example. The plate may be formed from any material with suitable electrical properties, such as the materials described above in relation to filaments of a heating element. 
     The heating element may be a susceptor element. As used herein, a “susceptor element” means a conductive element that heats up when subjected to a changing magnetic field. This may be the result of eddy currents induced in the susceptor element and/or hysteresis losses. Advantageously the susceptor element is a ferrite element. The material and the geometry for the susceptor element can be chosen to provide a desired electrical resistance and heat generation. 
     The susceptor element may be a ferrite mesh susceptor element. Alternatively, the susceptor element may be a ferrous susceptor element. 
     The susceptor element may comprise a mesh. As used herein the term “mesh” encompasses grids and arrays of filaments having spaces therebetween, and may include woven and non-woven fabrics. 
     The mesh may comprise a plurality of ferrite or ferrous filaments. The filaments may define interstices between the filaments and the interstices may have a width of between 10 μm and 100 μm. Preferably the filaments give rise to capillary action in the interstices, so that in use, liquid to be vapourised is drawn into the interstices, increasing the contact area between the susceptor element and the liquid. 
     The filaments may form a mesh of size between 160 and 600 Mesh US (+/−10%) (i.e. between 160 and 600 filaments per inch (+/−10%)). The width of the interstices is preferably between 75 μm and 25 μm. The percentage of open area of the mesh, which is the ratio of the area of the interstices to the total area of the mesh is preferably between 25 and 56%. The mesh may be formed using different types of weave or lattice structures. Alternatively, the filaments consist of an array of filaments arranged parallel to one another. 
     The filaments may have a diameter of between 8 μm and 100 μm, preferably between 8 μm and 50 μm, and more preferably between 8 μm and 40 μm. 
     The area of the mesh may be small, preferably less than or equal to 500 mm2, allowing it to be incorporated in to a handheld system. The mesh may, for example, be rectangular and have dimensions of 15 mm by 20 mm. 
     Advantageously, the susceptor element has a relative permeability between 1 and 40000. When a reliance on eddy currents for a majority of the heating is desirable, a lower permeability material may be used, and when hysteresis effects are desired then a higher permeability material may be used. Preferably, the material has a relative permeability between 500 and 40000. This provides for efficient heating. 
     The housing may also be vapour permeable to allow for the escape of vapour. The housing may be vapour permeable adjacent to the second side of the liquid transport medium. This allows vapour to escape from opposite sides of the fluid transport material, further reducing the likelihood of bubbles being trapped that interfere with liquid transport. 
     The vaporiser assembly may comprise a liquid retention material in the liquid supply conduit. This may ensure the supply of liquid to the liquid transport medium whatever the orientation of the vaporiser assembly with respect to gravity. The liquid retention material is preferably different to the liquid transport medium. The liquid supply conduit may comprise one or more capillary tubes. 
     The liquid supply conduit may extend generally orthogonal to the first side of the heating element. This maximises the distance between the heating element and a second end of the liquid supply conduit. In use, the second end of the liquid supply conduit may be adjacent to a main liquid reservoir. 
     When viewed in a direction orthogonal to the first side of the heating element, the first area may not completely cover the second area. This reduces the transfer of heat from the heating element to the liquid supply conduit. When viewed in a direction orthogonal to the first side of the heating element, the heating element may not overlap the second area. This further increases a distance between the heating element and the first end of the liquid supply conduit and so reduces the transfer of heat from the heating element to the liquid supply conduit. The liquid supply conduit may have a cross sectional area of around 25% of the area of the liquid transport medium. The liquid supply conduit may have a diameter of between 2 mm and 5 mm. 
     In a second aspect, there is provided a cartridge for an aerosol-generating system, the cartridge comprising a vaporiser assembly in accordance with the first aspect and a liquid reservoir, the liquid supply conduit having a second end opposite the first end in communication with the liquid supply reservoir. 
     The heating element and liquid transport medium may be separable from the liquid supply reservoir. The liquid supply conduit may be fixed to the heating element and liquid supply conduit, or may be fixed to the liquid supply reservoir, or may be fixed to both. The liquid supply conduit may take the form of a bottleneck of the liquid supply reservoir. The liquid supply reservoir may comprise a reservoir housing. The reservoir housing may be integral with the liquid supply conduit. 
     In a third aspect, there is provided an aerosol-generating system comprising a vaporiser assembly in accordance with the first aspect, a liquid reservoir, the liquid supply conduit having a second end opposite the first end in communication with the liquid supply reservoir, a power supply, and control circuitry configured to control a supply of power from the power supply to the vaporiser assembly. 
     The aerosol-generating system may be a handheld system. The aerosol-generating system may comprise a mouthpiece through which a user can inhale aerosol generated by the aerosol-generating system. 
     The aerosol-generating system may comprise a main unit and a cartridge that engages with the main unit in use. The main unit may comprise a housing. The housing may hold the power supply and the control circuitry. The vaporiser assembly and liquid reservoir may be provided in the cartridge. The vaporiser assembly may be part of the main unit and the liquid reservoir provided in the cartridge. The housing may receive at least a portion of the cartridge. The mouthpiece may be part of the main unit or part of the cartridge. 
     The aerosol-generating system may comprise an air flow passage extending from an air inlet, past the vaporiser assembly to an outlet. The outlet may be in a mouthpiece. 
     The aerosol-generating system may have a size comparable to a conventional cigar or cigarette. The aerosol-generating system may have a total length between about 30 mm and about 150 mm. The aerosol-generating system may have an external diameter between about 5 mm and about 30 mm. 
     The power supply may be 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-Iron-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 require recharging 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; 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 atomiser assembly. 
     The control circuitry may comprise a microcontroller. The microcontroller is preferably a programmable microcontroller. The control circuitry may comprise further electronic components. The control circuitry may be configured to regulate a supply of power to the heating element. Power may be supplied to the heating element continuously following activation of the system or may be supplied intermittently, such as on a puff-by-puff basis. The power may be supplied to the aerosol-generating element in the form of pulses of electrical current. The control circuitry may include an airflow sensor and the control circuitry may supply electrical power to the heating element when user puffs are detected by the airflow sensor. 
     In operation, a user may activate the system by puffing on a mouthpiece or providing some other user input, for example by pressing a button on the system. The control circuitry then supplies power to the heating element power may be supplied to the heating element for a predetermined time period or for the duration of a user puff. The heating element then heats the liquid in the liquid transport medium to form a vapour that escapes from the vaporiser assembly into an air flow passage through the system. The vapour cools and condenses for form an aerosol that is then drawn into the user&#39;s mouth. 
     In all aspects of the invention, the liquid may be a liquid aerosol-forming substrate. As used herein with reference to the present invention, an aerosol-forming substrate is a substrate capable of releasing volatile compounds that can form an aerosol. Volatile compounds may be released by heating the aerosol-forming substrate. 
     The liquid aerosol-forming substrate may be liquid at room temperature. 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 aerosol-forming 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. 
     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. 
     The liquid aerosol-forming substrate may comprise nicotine and at least one aerosol former. 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%. 
     In all aspects, the liquid transport medium is a material that a conveys liquid from one end of the material to another. The liquid transport medium may be a capillary material. The capillary material may have a fibrous or spongy structure. The capillary material preferably comprises a bundle of capillaries. For example, the capillary material may comprise a plurality of fibres or threads or other fine bore tubes. The fibres or threads may be generally aligned to convey liquid aerosol-forming substrate towards the heating element. Alternatively, the capillary material may comprise sponge-like or foam-like material. The structure of the capillary material forms a plurality of small bores or tubes, through which the liquid aerosol-forming substrate can be transported by capillary action. The liquid transport medium is exposed to the high temperature of the heating element and so must be stable at those temperatures. 
     The liquid transport medium may comprise any suitable material or combination of materials. Examples of suitable materials are a sponge or foam material, ceramic- or graphite-based materials in the form of fibres or sintered powders, foamed metal or plastics material, a fibrous material, for example made of spun or extruded fibres, such as glass fibre, cellulose acetate, polyester, or bonded polyolefin, polyethylene, terylene or polypropylene fibres, nylon fibres or ceramic. The fibres may be woven or may form an amorphous structure. The liquid transport medium may have any suitable capillarity and porosity so as to be used with different liquid physical properties. The liquid aerosol-forming substrate has physical properties, including but not limited to viscosity, surface tension, density, thermal conductivity, boiling point and vapour pressure, which allow the liquid aerosol-forming substrate to be transported through the liquid transport medium by capillary action. 
     In all aspects, the liquid retention material in the liquid supply conduit may also be a capillary material. However, it need not withstand temperatures as high as the liquid transport medium. The liquid retention material may be a foam, a sponge or a collection of fibres. The liquid retention material may be formed from a polymer or co-polymer. In one example the liquid retention material is a woven polypropylene and poly(ethylene-terephthalate). 
    
    
     
       Embodiments of the invention will now be described in detail, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic illustration of an aerosol-generating system of a first embodiment of the invention; 
         FIG. 2 a    shows in detail a vaporiser assembly for the embodiment shown in  FIG. 2 ; 
         FIG. 2 b    is an underside view of the vaporiser assembly of  FIG. 2   a;    
         FIG. 3 a    is a schematic cross-section of a vaporiser assembly of a second embodiment of the invention; 
         FIG. 3 b    is a view of the back side of vaporiser assembly of  FIG. 3 a   ; and 
         FIG. 4  is a schematic illustration of an aerosol-generating system of a third embodiment of the invention. 
     
    
    
       FIG. 1  is a schematic illustration of an aerosol-generating system in accordance with a first embodiment of the invention. The system comprises two main components, a cartridge  100  and a main body  200 . A connection end  115  of the cartridge  100  is removably connected to a corresponding connection end  205  of the main body  200 . The main body contains a battery  210 , which in this example is a rechargeable lithium ion battery, and control circuitry  220 . The aerosol-generating device  10  is portable and has a size comparable to a conventional cigar or cigarette. 
     The cartridge  100  comprises a housing  105  containing an atomising assembly  120  and a liquid storage compartment  130  defining a liquid supply reservoir. A liquid aerosol-forming substrate is held in the liquid storage compartment. The atomising assembly is connected to a bottleneck of the liquid storage compartment. The atomising assembly comprises a heating element  135 , in the form of a fluid permeable mesh, on a liquid transport medium  136 . The liquid transport medium  136  covers the entire heating element. A liquid supply conduit  138  extends between the bottle neck of the liquid storage compartment and the liquid transport medium  136 . A high retention material (HRM) or capillary material is placed within the liquid supply conduit  138 . Liquid from the liquid storage compartment is drawn into the liquid supply conduit and from there is spread across the liquid transport medium. This means that there is a particular volume of liquid in the liquid transport medium, adjacent the heating element, which can be readily vaporised by the heating element. 
     An air flow passage  140 ,  145  extends through the system from an air inlet  150  past the heater element  135  and from the heating element to a mouth end opening  110  in the housing  105 . 
     The heating element  135  is a susceptor that is inductively heated when exposed to a high frequency oscillating magnetic field. A inductor coil  225 , which in this example is a pancake coil, is positioned within the main body, adjacent to the heating element  135 . The control circuitry supply a high frequency oscillating current to the coil  225 , which in turn generates a time varying magnetic flux across the heating element. 
     The system is configured so that a user can puff or suck on the mouth end opening of the cartridge to draw aerosol into their mouth. In operation, when a user puffs on the mouth end opening, air is drawn through the airflow passage from the air inlet, past the heating element, to the mouth end opening. The control circuitry controls the supply of electrical power from the battery  210  to the coil  225 . This in turn controls the temperature of the heating element and so amount and properties of the vapour produced by the atomising assembly. The control circuitry may include an airflow sensor and the control circuitry may supply electrical power to the coil when user puffs on the cartridge are detected by the airflow sensor. This type of control arrangement is well established in aerosol-generating systems such as inhalers and e-cigarettes. So when a user sucks on the mouth end opening of the cartridge, the atomising assembly is activated and generates a vapour that is entrained in the air flow passing through the air flow passage  140 . The vapour cools with in the airflow in passage  145  to form an aerosol, which is then drawn into the user&#39;s mouth through the mouth end opening  110 . 
     The embodiments shown in  FIGS. 1-3  all rely on inductive heating. Inductive heating works by placing an electrically conductive article to be heated in a time varying magnetic field. Eddy currents are induced in the conductive article. If the conductive article is electrically isolated the eddy currents are dissipated by Joule heating of the conductive article. In an aerosol-generating system that operates by heating an aerosol-forming substrate, the aerosol-forming substrate is typically not itself sufficiently electrically conductive to be inductively heated in this way. So in the embodiments shown in  FIGS. 1-3  a susceptor element is used as the conductive article that is heated. The aerosol-forming substrate is then heated by the susceptor element by thermal conduction, convention and/or radiation. Because a ferromagnetic susceptor element is used, heat is also generated by hysteresis losses as the magnetic domains are switched within the susceptor element. 
     The embodiments described in  FIGS. 1-3  use an inductor coil to generate a time varying magnetic field. The inductor coil is designed so that it does not undergo significant Joule heating. In contrast the susceptor element is designed so that there is significant Joule heating of the susceptor. 
     The oscillating magnetic field passes through the susceptor element, inducing eddy currents in the susceptor element. The susceptor element heats up as a result of Joule heating and as a result of hysteresis losses, reaching a temperature sufficient to vapourise the aerosol-forming substrate close to the susceptor element. The vapourised aerosol-forming substrate is entrained in the air flowing from the air inlet to the air outlet, as explained in more detail below and cools to form an aerosol within the mouthpiece portion before entering the user&#39;s mouth. The control electronics supplies the oscillating current to the coil for a predetermined duration, in this example five seconds, after detection of a puff and then switches the current off until a new puff is detected. 
       FIG. 2 a    illustrates the vaporiser assembly of  FIG. 1  in more detail. In the example shown in  FIG. 2  the vaporiser assembly has a housing  137 . The housing  137  is integrally formed with the liquid storage container. The housing  137  holds the mesh susceptor  135 , the liquid transport medium  136  and the capillary material  139  within the liquid supply conduit  138 . 
     The heating element  135  comprises a stainless steel mesh. It is generally planar.  FIG. 2 b    is an underside view of the vaporiser assembly. The mesh is generally rectangular but has a central aperture  131  cut out. The central aperture is such that, when viewed in a direction orthogonal to the plane of the mesh, the aperture covers the liquid supply conduit. The outline of the liquid supply conduit  138  is illustrated in dotted line in  FIG. 2 b   . In this way the heating element is removed from the liquid supply conduit and so there is no significant heat transfer from the heating element to the liquid in the liquid supply conduit. The aperture can be of any shape. For example, it may be circular to match a circular liquid supply conduit. In this example, the aperture is square. 
     In this example the liquid transport medium  136  is formed from a glass fibre material. Glass fibre typically has adequate heat resistance. The glass fibre is woven and provides capillary action to transport liquid in a direction parallel to the surface of the mesh susceptor element. In particular, the liquid transport medium is arranged to transport liquid away from the area in contact with the liquid supply conduit to the periphery of the liquid transport medium. 
     The capillary material  139  in the liquid supply conduit  138  is oriented to convey the liquid to the liquid transport medium  136 . In this example that is orthogonal to the surface of the mesh susceptor element. The capillary material  139  may be comprised of woven polypropylene or poly(ethylene-terephthalate) (PET). 
     It can be seen from  FIG. 2 b    that that the area of the liquid supply conduit in contact with the liquid transport medium is only a fraction of the total area of the liquid transport medium. The smaller the area of liquid supply conduit in contact with the liquid transport medium the lower the heat transfer from the heater back to the liquid in the liquid supply conduit. However, the area of contact needs to be sufficiently large to allow for the replenishment of liquid across the entire liquid transport medium in a short time. This allows a user to take successive puffs within a short time and still receive sufficient and consistent aerosol with each puff. In this example the liquid supply conduit has a diameter of around 5 mm and the liquid transport medium has an area of around 300 mm 2 . The capillary material in the liquid supply conduit may have a similar volume to the liquid transport medium. 
     In use, when the induction coil  225  is activated as a result of a sensed user puff, the heating element heats up to a temperature sufficient to vaporise the liquid held in the liquid transport medium  136 . The heating is maintained for a duration sufficient to vaporise substantially all of the liquid in the liquid transport medium. This is may be fixed time period of two seconds for example. The current through the coil is then stopped and the heating element cools until the next activation of the coil. Following vaporisation of the liquid in the liquid transport medium, more liquid flows from capillary material in the liquid supply conduit into the liquid transport medium. At the same time, liquid from the liquid storage compartment replaces liquid in the liquid supply conduit. In this way another, similar volume of liquid is delivered to the heating element ready for the next user puff. His provides consistent aerosol volume. And the isolation of the heating element from the main part of the liquid storage compartment improves heating efficiency. 
     In the embodiment shown in  FIGS. 2 a  and 2 b   , the vaporiser housing  137  is not fluid permeable and covers a back face of the liquid transport medium. This means that vapour generated in the liquid transport medium must escape through the susceptor  136  to be entrained in the airflow. 
       FIGS. 3 a  and 3 b    illustrate another embodiment of a vaporiser than can be used in the system shown in  FIG. 1 , in which vapour generated in the liquid transport medium  336  can escape both through a first side of the liquid transport medium adjacent the heating element (in the example of  FIGS. 3 a  and 3 b    again a mesh susceptor) and through a second side, opposite the first side. 
       FIG. 3 a    is a schematic illustration of the vaporiser assembly and a portion of the liquid storage compartment  330 . The basic shape of the vaporiser assembly is the same as in the embodiment of  FIG. 2 . The housing  337  is integrally formed with the liquid storage compartment. The heating element  335  is separated from the main body of the liquid storage compartment by a bottleneck, formed by liquid supply conduit  338 . The housing  337  holds the mesh susceptor  335 , the liquid transport medium  336  and the capillary material  339  within the liquid supply conduit  138 . 
     The heating element  335  comprises a stainless steel mesh and is generally planar. The liquid transport medium  336  is formed from a glass fibre material. The glass fibre is woven and provides capillary action to transport liquid in a direction parallel to the surface of the mesh susceptor element. In particular, the liquid transport medium is arranged to transport liquid away from the area in contact with the liquid supply conduit to the periphery of the liquid transport medium. 
     The capillary material  339  in the liquid supply conduit  338  is oriented to convey the liquid to the liquid transport medium  336 . In this example that is orthogonal to the surface of the mesh susceptor element. The capillary material  339  may be comprised of woven polypropylene or poly(ethylene-terephthalate) (PET). 
     In use, when the induction coil  225  is activated as a result of a sensed user puff, the heating element heats up to a temperature sufficient to vaporise the liquid held in the liquid transport medium  3136 . The heating is maintained for a duration sufficient to vaporise substantially all of the liquid in the liquid transport medium. This is may be fixed time period of two seconds for example. The current through the coil is then stopped and the heating element cools until the next activation of the coil. Following vaporisation of the liquid in the liquid transport medium, more liquid flows from capillary material in the liquid supply conduit into the liquid transport medium. At the same time, liquid from the liquid storage compartment replaces liquid in the liquid supply conduit. In this way another, similar volume of liquid is delivered to the heating element ready for the next user puff. His provides consistent aerosol volume. And the isolation of the heating element from the main part of the liquid storage compartment improves heating efficiency. 
     It can be seen from  FIG. 3 b    that that the housing  337  allows vapour to escape both through the heating element  335  and through a rear face of the liquid transport medium  336 . The passage of the vapour is illustrated by the arrows in  FIG. 3   a.    
     The main airflow past the vaporiser is indicated by dotted arrow  340 . Vapour escaping through the rear face of the liquid transport medium  336  can join the main airflow by passing through apertures  342  formed in the vaporiser housing  337 .  FIG. 3 b    is a view of the back face of the liquid transport medium  336 , which illustrates the housing construction. The rear face of the housing  337  holding the liquid transport medium and the heating element  335  is formed with a central portion  343  that joins, or is integral with, the liquid supply conduit  338  and a peripheral frame  344  which is joined to the central portion by a plurality of ribs  345 . Between the ribs are spaces through which vapour can escape from the liquid transport medium. 
     In this example, the frame  344  has a size and shape that matches the cavity in the cartridge in which it is positioned. This is to confine airflow through the cartridge to the desired air flow passage or passages. So, in order to allow vapour that has escaped into the space  341  behind the rear face of the liquid transport medium  336  to join the main air flow  340 , slots or apertures  342  are formed through the vaporiser housing. Alternatively, the vaporiser assembly may simply be made smaller that the cavity in which it is received so that vapour can move around the periphery of the housing  137  to join the main air flow. 
     The arrangement of  FIGS. 3 a  and 3 b    have the advantage that vapour generated in the liquid transport medium has many exit paths. This reduces the likelihood of bubbles being trapped in the liquid transport medium or migrating to the liquid supply conduit and interfering with efficient liquid transfer to the heating element. 
     The embodiments described so far have included a heating element that is heated by inductive heating. However, it is possible to use a resistive heater instead.  FIG. 4  is a schematic illustration of an aerosol-generating system in accordance with a third embodiment of the invention. The system is similar to the system shown in  FIG. 1  but uses resistive heating rather than inductive heating. 
     The device comprises two main components, a cartridge  400  and a main body  500 . A connection end  415  of the cartridge  400  is removably connected to a corresponding connection end  505  of the main body  500 . The main body contains a battery  510 , which in this example is a rechargeable lithium ion battery, and control circuitry  520 . 
     The cartridge  400  comprises a housing  405  containing an atomising assembly  420  and a liquid storage compartment  430  defining a liquid supply reservoir. A liquid aerosol-forming substrate is held in the liquid storage compartment. The atomising assembly is connected to a bottle neck of the liquid storage compartment. The atomising assembly comprises a heating element  435 , in the form of a fluid permeable mesh, on a liquid transport medium  436 . A liquid supply conduit  438  extends between the bottle neck of the liquid storage compartment and the liquid transport medium  436 . A high retention material (HRM) or capillary material  439  is placed within the liquid supply conduit  438 . Liquid from the liquid storage compartment is drawn into the liquid supply conduit and from there is spread across the liquid transport medium. This means that there is a particular volume of liquid in the liquid transport medium, adjacent the heating element, which can be readily vaporised by the heating element. 
     An air flow passage  440 ,  445  extends through the system from an air inlet  450  past the heater element  435  and from the heating element to a mouth end opening  410  in the housing  405 . 
     As in the previously described embodiment, the heating element  435  comprises a stainless steel mesh and is generally planar. However, the vaporiser assembly also comprises a pair of electrical contact pads  460  positioned on opposite sides of the heating element. The contact pads are formed of electrically conductive material, such as copper, and are electrically connected to one another through the heating element  435 . 
     The contact pads  460  face main body and are contacted by electrical contact pins  560  on the main body. The electrical contact pins are spring loaded to ensure good contact with the contact pads  460  when the cartridge is connected to the main body. The electrical contact pins  560  on the main body are connected to the control circuitry  520 . Electrical power is supplied to the heating element from the battery  510  through the electrical contact pads and electrical contact pins. 
     The liquid transport medium  436  is formed from a glass fibre material. The glass fibre is woven and provides capillary action to transport liquid in a direction parallel to the surface of the mesh susceptor element. In particular, the liquid transport medium is arranged to transport liquid away from the area in contact with the liquid supply conduit to the periphery of the liquid transport medium. 
     The capillary material  439  in the liquid supply conduit  438  is oriented to convey the liquid to the liquid transport medium  436 . In this example that is orthogonal to the surface of the heating element. The capillary material  439  may be comprised of woven polypropylene or polyethylene-terephthalate) (PET). 
     The system is configured so that a user can puff or suck on the mouth end opening of the cartridge to draw aerosol into their mouth. In operation, when a user puffs on the mouth end opening, air is drawn through the airflow passage from the air inlet, past the heating element, to the mouth end opening. The control circuitry controls the supply of electrical power from the battery  410  to the heating element  435 . This in turn controls the temperature of the heating element and so amount and properties of the vapour produced by the atomising assembly. The control circuitry may include an airflow sensor and the control circuitry may supply electrical power to the coil when user puffs on the cartridge are detected by the airflow sensor. This type of control arrangement is well established in aerosol-generating systems such as inhalers and e-cigarettes. So when a user sucks on the mouth end opening of the cartridge, the atomising assembly is activated and generates a vapour that is entrained in the air flow passing through the air flow passage  440 . The vapour cools with in the airflow in passage  445  to form an aerosol, which is then drawn into the user&#39;s mouth through the mouth end opening  410 . 
     The embodiments described all have the advantage of isolating only that volume of liquid that is desired to be heated in each user puff from the remaining liquid in the liquid storage compartment so that that volume of liquid can be quickly and efficiently vaporised with relatively little heat transfer to the remaining liquid.