Patent Publication Number: US-2020281269-A1

Title: Cartridge for an aerosol-generating system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of and claims the benefit of priority under 35 U.S.C. § 120 to U.S. application Ser. No. 16/723,790, filed on Dec. 20, 2019, which is a continuation application of and claims the benefit of priority under 35 U.S.C. § 120 to U.S. application Ser. No. 15/117,663, filed on Aug. 9, 2016, which is a U.S. National Stage application of PCT/EP2014/077852, filed on Dec. 15, 2014, and claims benefit of priority under 35 U.S.C. § 119 from European Patent Application Nos. 14154552.5, filed on Feb. 10, 2014; 14154553.3, filed on Feb. 10, 2014; and 14154554.1, filed on Feb. 10, 2014. The entire contents of each of which are incorporated herein by reference. 
    
    
     The present invention relates aerosol-generating systems. In particular, the invention relates to handheld aerosol-generating system, such as electrically operated smoking systems. Aspects of the invention relate to a cartridge for aerosol-generating systems, in particular electrically operated smoking systems. 
     One type of aerosol-generating system is an electrically operated smoking system. Handheld electrically operated smoking systems consisting of a device portion comprising a battery and control electronics, and a cartridge portion comprising a supply of aerosol-forming substrate, and an electrically operated vapouriser, are known. A cartridge comprising both a supply of aerosol-forming substrate and a vapouriser is sometimes referred to as a “cartomiser”. The vapouriser typically comprises a coil of heater wire wound around an elongate wick soaked in liquid aerosol-forming substrate. Capillary material soaked in the aerosol-forming substrate supplies the liquid to the wisk. The cartridge portion typically comprises not only the supply of aerosol-forming substrate and an electrically operated vapouriser, but also a mouthpiece, which the user sucks on in use to draw aerosol into their mouth. 
     In some types of electrically operated aerosol-generating devices, a reservoir of aerosol-forming liquid is provided in a tank. In use in the aerosol-generating system, the liquid is conveyed from the tank by capillary action into the wick of a coil wick heater assembly where the liquid is vaporized. When a user draws at the mouthpiece an airstream flows over the heater assembly and the generated aerosol is inhaled by the user. 
     A problem of such a tank device is, that the system stops generating the aerosol if the device is held at such an angle that the liquid aerosol-generating substrate in the tank is not in contact with the capillary system. Moreover, these systems can be prone to leakage for example where the liquid from the tank floods the core or leaks through the airflow path. 
     In other systems, the liquid storage portion of a cartridge is filled with a capillary medium. The liquid aerosol-generating substrate is held in the capillary material and delivered to the wick. With such system the above mentioned problems of the holding angle and the risk of leakage can be reduced. However, some residual liquid will remain in the capillary material after usage, leading to wastage. Further, there can be an inconsistency in puff deliveries in such systems due to decreasing saturation of the capillary medium during usage, which does not allow for a constant high quality smoking experience. 
     It would be desirable to have a cartridge that avoids one or more of the above mentioned or other disadvantages, for example that avoids waste of aerosol-generating substrate liquid, preferably while maintaining or improving aerosol-generating performance of the aerosol-generating system, in which the cartridge is used. 
     According to a first aspect of the invention there is provided a cartridge for use in an aerosol-generating system, for example an electrically operated aerosol-generating system, comprising a liquid storage portion for holding a liquid aerosol-forming substrate. The liquid storage portion comprises at least two parts in fluid communication with each other. The first part of the liquid storage portion comprises a heater assembly, a first capillary material, provided in contact with the heater assembly, and a second capillary material in contact with the first capillary material and spaced apart from the heater assembly by the first capillary material. The second part of the liquid storage portion comprises a container for holding aerosol-forming substrate in liquid form and preferably arranged for supplying the liquid to the second capillary material. The second part of the liquid storage portion may comprise a tank that is substantially empty and is suitable for holding aerosol-forming substrate in liquid form. 
     The capillary material is preferably designed such that it is capable of holding sufficient liquid substrate for several puffs. As the capillary material is located in contact with the heater, the heater is provided independent of the holding angle of the aerosol-generating liquid with sufficient aerosol-generating liquid. The remaining inner volume of the liquid storage portion comprises no capillary material but represents an empty tank for storing aerosol-generating liquid. Under normal handling conditions an aerosol-generating medium, in particular an aerosol-generating smoking device, is transported between puffs and the capillary material will regularly contact and re-absorb new aerosol-generating liquid. 
     Because of the reduced amount of the capillary material used, the amount of residual liquid remaining in the capillary material after usage of the cartridge is smaller than in conventional cartridges in which the whole liquid storage portion is filled with capillary material. Moreover, performance tests have shown that the TPM (total particulate matter) yield of aerosol-generating smoking devices equipped with cartridges of the present invention is in many examples at least comparable to the performance of aerosol-generating smoking devices equipped with currently available cartridges. 
     Preferably the liquid capacity of the capillary material is such that it can hold enough liquid for 30 to 40 puffs or more. A 3 second puff may include about 1 mg to 4 mg of liquid, for example 3 mg to 4 mg of liquid. Preferably the capacity of the capillary material is between about 30 mg to about 160 mg, preferably 90 mg to about 160 mg or more, preferably 100 mg to 150 mg, for example 130 mg. Where there are two layers making up the capillary material, the capacity of the first and second layers may be such that about 10 to 20% by weight of the liquid capacity is in the first layer. For example, where the capacity of the capillary material is 30 puffs, the capacity of the first layer may be about 5 puffs, and the capacity of the second layer may be about 25 puffs. 
     Without wishing to be bound by any particular theory, it is thought that by the capillary material having capacity for several puffs, for example 30 or more puffs, the risk of leakage from the device is reduced. It is thought that if the capillary material is too small, then on puffing, liquid may be drawn from the reservoir straight through the capillary material and heater without being vapourised, leading to leakage. Also by having a capacity of 90 mg and more, a number of puffs can be taken from the device, even when the liquid in the reservoir is not in direct contact with the capillary material. 
     The heater assembly may be substantially flat and may comprise electrically conductive filaments, without the need for any winding of a heater wire around a capillary wick. 
     The electrically conductive filaments may lie in a single plane. A planar heater assembly can be easily handled during manufacture and provides for a robust construction. 
     The electrically conductive filaments may define interstices between the filaments and the interstices may have a width of between 10 μm and 100 μm. The filaments may 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 heater assembly and the liquid. 
     The electrically conductive 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 ration 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 electrically conductive filaments consist of an array of filaments arranged parallel to one another. 
     The electrically conductive filaments may have a diameter of between 10 μm and 100 μm, preferably between 8 μm and 50 μm, and more preferably between 8 μm and 39 μm. The filaments may have a round cross section or may have a flattened cross-section. The heater 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 heater assembly comprises a mesh or fabric of filaments, the filaments may be individually formed and knitted together. 
     As described in relation to the first aspect, the heater assembly may comprise at least one filament made from a first material and at least one filament made from a second material different from the first material. 
     The heater assembly may comprise an electrically insulating substrate on which the filaments are supported, the filaments extending across an aperture formed in the substrate. The electrically insulating substrate may comprise any suitable material, and is preferably a material that is able to tolerate high temperatures (in excess of 300 degree Celsius) and rapid temperature changes. An example of a suitable material is a polyimide film, such as Kapton®. 
     The heater assembly may comprise an electrically conductive contact in contact with a plurality of the filaments. The electrically conductive contact may be provided between the housing of the liquid storage portion and the electrically insulating substrate. The electrically conductive contact may be provided between the filaments and the electrically insulating substrate. An aperture may be formed in the electrically insulating layer, and the cartridge may comprise two electrically conductive contacts positioned on opposite sides on the aperture to one another. 
     A capillary material is preferably a material that actively conveys liquid from one end of the material to another. The capillary material is advantageously oriented in the housing to convey liquid to the heater assembly. 
     The second capillary material may comprise a fibrous structure wherein the fibres are generally oriented in the direction of movement in the liquid to the heater. The first capillary material may have less oriented fibres. For example the first capillary material may have the structure of a felt. 
     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 to the heater. 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 can be transported by capillary action. The capillary material 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 cellulose acetate, polyester, or bonded polyolefin, polyethylene, terylene or polypropylene fibres, nylon fibres or ceramic. The capillary material may have any suitable capillarity and porosity so as to be used with different liquid physical properties. The liquid has physical properties, including but not limited to viscosity, surface tension, density, thermal conductivity, boiling point and vapour pressure, which allow the liquid to be transported through the capillary device by capillary action. 
     The capillary material may be in contact with the heater, for example with the electrically conductive filaments. The capillary material may extend into interstices between the filaments. The heater assembly may draw liquid aerosol-forming substrate into the interstices by capillary action. The capillary material may be in contact with the electrically conductive filaments over substantially the entire extent of the aperture. 
     The housing may contain two or more different capillary materials, wherein a first capillary material, in contact with the heater element, has a higher thermal decomposition temperature and a second capillary material, in contact with the first capillary material but not in contact with the heater element has a lower thermal decomposition temperature. The first capillary material effectively acts as a spacer separating the heater element from the second capillary material so that the second capillary material is not exposed to temperatures above its thermal decomposition temperature. As used herein, “thermal decomposition temperature” means the temperature at which a material begins to decompose and lose mass by generation of gaseous by products. The second capillary material may advantageously occupy a greater volume than the first capillary material and may hold more aerosol-forming substrate that the first capillary material. The second capillary material may have superior wicking performance to the first capillary material. The second capillary material may be cheaper than the first capillary material. The second capillary material may be polypropylene. 
     The first capillary material can be selected from the group of kevlar felt, ceramic paper, ceramic felt, carbon felt, cellulose acetate, hemp felt, PET/PBT sheet, cotton pad, porous ceramic disc or porous metal disc. 
     Preferred materials include Kevlar felt, ceramic paper, ceramic felt, porous ceramic disc or porous metal disc. The first capillary material may comprise glass fibre paper or felt. Preferably the first capillary material includes substantially no organic matter. 
     Preferably the porosity of the first capillary material is less than that of the second capillary material. Preferably the pore size of the first capillary material is less than that of the second capillary material. The pore size may be for example measured as being an average pore size for a region of the capillary material. In this way it can be seen that the aerosol-generating substrate moves more efficiently to the heater. In a broad aspect of the invention, a cartridge is provided which includes a heater and a capillary material in contact with the heater for supplying aerosol-generating substrate to the heater, wherein the porosity or pore size of a region of the capillary material adjacent the heater is less than the porosity or pore size of a region of the capillary material remote from the heater. Thus a single material may be used, for example which has a gradient of pore size in one or more of its dimensions. 
     The first capillary material may have a fiber size/pore size of between 0.1 to 50 μm, preferably of between 0.5 to 10 μm and most preferably of about 4 μm. The first capillary material has a density of below 2 g/ml, and preferably of about 0.5 g/ml. 
     The second capillary material can be a so-called high retention material (HRM) material, selected from the group of polypropylene (PP), polyethylene (PE), polyphenylene sulphide (PPS), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), rolled non-woven material or rolled felts. Preferably the second capillary material comprises a polymeric material. The material may include a coating, for example to reduce hydrophobicity. 
     The second capillary material may have a fiber size/pore size of between 1 to 100 μm, preferably of between 15 to 40 μm and most preferably of about 25 μm. The second capillary material may have a density of below 1 g/ml, and preferably of between 0.1 and 0.3 g/ml. 
     The first capillary material may separate the heater assembly from the second capillary material by a distance of at least 0.8 mm, for example at least 1.5 mm, and preferably between 0.8 mm and 2 mm in order to provide a sufficient temperature drop across the first capillary material. 
     The first and second capillary material can also be made from the same material, and can only be distinguished from each other in that they exhibit different porosities or different capillarity. For example the first capillary material can be compressed such that its pore size or porotisy is reduced and its capillarity is increased compared to the second capillary material which may be used in an uncompressed or at least in a less compressed state. 
     In a preferred embodiment the first and the second material are made from a single continuous element of the same base material. More preferably the material is treated such that a gradient of pore size or porosity is obtained in a direction towards the heater element or the opening, such that the pore size or porosity decreases, for example continuously decreases, within the capillary material towards the heater element. 
     Preferably at least the first capillary material is compressed upon insertion into the first part of the housing of the liquid storage portion such that its effective pore size or porosity is reduced. For example the single continuous element may have the shape of a truncated cone, wherein the diameter of the circular base of the truncated cone is larger than the inner diameter of the cylindrical housing of the liquid storage portion, while the diameter of the truncated apex of the cone substantially corresponds to the inner diameter of the cylindrical housing of the liquid storage portion. Upon insertion the capillary material at the base of the cone of capillary material is more compressed than at the area of the truncated apex. The more compressed material represents the first capillary material and the less compressed material represents the second capillary material. The skilled artisan will readily appreciate that the resulting gradient of compression depends on the relative shape chosen for the capillary element and the housing of the liquid storage portion. 
     In a particularly preferred embodiment, the capillary element has a regular cylindrical shape with circular cross-section and a predefined diameter. The inner surface of the housing comprises a tapered portion at the opening end, such that the capillary material is compressed by this tapered portion upon insertion of the capillary material into the housing. Preferably the inner surface of the housing has a conical shape such that the inner diameter continuously increases from the open end towards the closed end of the cartridge. 
     The first capillary material and the second capillary material may comprise different regions of the same capillary material element. Compression of the capillary material when laced in the housing may be such that the pore size or porosity of the capillary material reduced or continuously reduces towards the heater assembly. 
     In a further embodiment the first and the second capillary material again are formed from a single continuous piece of the same material. The capillary material may be rectangular web of capillary material having a thickness that amounts to below 50%, preferably to about 25% of the inner diameter of the cylindrical housing of the cartridge. The width of the web of capillary material corresponds to the peripheral circumference of the housing. The web of capillary material can have any desired length and preferably is about half the length of the housing of the cartridge. The web of capillary material is rolled up to form a cylindrical shape. By rolling up, the central portion of the web is compressed to a higher degree than the outer portions of the web such that a gradient of pore size or porosity is obtained in radial direction of the rolled-up web of capillary material. In the middle the rolled-up capillary material an air channel is formed. A tube shaped fluid permeable heater element is provided in the air channel such that the heater is in direct contact with the inner surface of the rolled-up capillary material. Upon rolling up the capillary material the material portion that is closer to the center axis of the cylinder is more compressed than the material in the radial outward located portion of the capillary material. Thus, again a gradient of pore size is obtained, wherein the pore size of the capillary material is continuously decreased within the capillary material in the direction to the heater element. The capillary material is in fluid connection with a liquid reservoir, wherein the liquid reservoir is provided in the part of the housing that is not occupied by the capillary material. A partition is provided within the housing to ensure that the liquid substrate is not in direct communication with the air flow channel. 
     The web of capillary material may also comprise a plurality of layers of capillary material, such that the liquid retention properties of the capillary material can be designed in any desired way that is most suitable for the given aerosol-generating system. 
     In a particular preferred embodiment the heater element is rolled-up together with the capillary material, such that in only one manufacturing step a combined capillary material with radial gradient and included heating element is obtained. 
     The liquid storage portion may be positioned on a first side of the electrically conductive filaments and an airflow channel positioned on an opposite side of the electrically conductive filaments to the liquid storage portion, such that air flow past the electrically conductive filaments entrains vapourised liquid aerosol-forming substrate. 
     Preferably, the aerosol generating system comprises a housing. Preferably, the housing is 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. Preferably, the material is light and non-brittle. The material may include PET, PBT or PPS. 
     Preferably, the aerosol-generating system is portable. The aerosol-generating system may have a size comparable to a conventional cigar or cigarette. The smoking system may have a total length between approximately 30 mm and approximately 150 mm. The smoking system may have an external diameter between approximately 5 mm and approximately 30 mm. 
     The aerosol-forming substrate is a substrate capable of releasing volatile compounds that can form an aerosol. The volatile compounds may be released by heating the aerosol-forming substrate. 
     The aerosol-forming substrate may comprise plant-based material. The aerosol-forming substrate may comprise tobacco. The 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 aerosol-forming substrate may alternatively comprise a non-tobacco-containing material. The aerosol-forming substrate may comprise homogenised plant-based material. The aerosol-forming substrate may comprise homogenised tobacco material. The aerosol-forming substrate may comprise at least one aerosol-former. The aerosol-forming substrate may comprise other additives and ingredients, such as flavourants. 
     Preferably the liquid storage portion comprises an opening and the heater assembly extends across the opening of the housing. The heater assembly may comprise an electrically insulating substrate on which the heater element is supported. The electrically insulating substrate may comprise any suitable material, and is preferably a material that is able to tolerate high temperatures (in excess of 300 degree Celsius) and rapid temperature changes. An example of a suitable material is a polyimide film, such as Kapton®. The electrically insulating substrate may have an aperture formed in it, with the heater element extending across the aperture. The heater assembly may comprise electrical contacts connected to the electrically conductive filaments. 
     According to a second aspect of the invention there is provided a cartridge for use in an aerosol-generating system, for example an electrically operated aerosol-generating system, with a liquid storage portion comprising a housing for holding a liquid aerosol-forming substrate, wherein the liquid storage portion comprises at least two parts in fluid communication to each other. The first part of the liquid storage portion comprises a first capillary material, provided at the vicinity of the opening of the housing, and a second capillary material in contact with the first capillary material and spaced apart from the opening by the first capillary material. The second part of the liquid storage portion may be substantially empty and being suitable for holding aerosol-forming substrate in liquid form. 
     The cartridge preferably further comprises a fluid permeable heater assembly extending across the opening of the housing. 
     In the embodiments of the present invention wherein the first part of the liquid storage portion occupies below 50%, preferably between 10% and 30%, more preferably between 15% and 25% and most preferably about 20% of the volume of the liquid storage portion. 
     The capillary material extends across the complete cross-section of the first part of the liquid storage portion, such that it is not possible for the liquid aerosol-generating substrate to flow directly to the heater assembly or the opening of the cartridge. 
     According to a further aspect of the invention there is provided an aerosol-generating system comprising a cartridge according to the present invention. 
     The system may further comprise electric circuitry connected to the heater assembly and to an electrical power source, the electric circuitry configured to monitor the electrical resistance of the heater assembly or of one or more filaments of the heater assembly, and to control the supply of power to the heater assembly dependent on the electrical resistance of the heater assembly or the one or more filaments. 
     The electric circuitry may comprise a microprocessor, which may be a programmable microprocessor. The electric circuitry may comprise further electronic components. The electric circuitry may be configured to regulate a supply of power to the heater assembly. Power may be supplied to the heater assembly 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 heater assembly in the form of pulses of electrical current. 
     The system advantageously comprises a power supply, typically a battery, within the main body of the housing. As an alternative, the power supply may be another form of charge storage device such as a capacitor. The power supply may require recharging and may have a capacity that allows for the storage of enough energy for one or more smoking 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 heater assembly. 
     Preferably, the aerosol generating system comprises a housing. Preferably, the housing is 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. Preferably, the material is light and non-brittle. 
     Preferably, the aerosol-generating system is portable. The aerosol-generating system may have a size comparable to a conventional cigar or cigarette. The smoking system may have a total length between approximately 30 mm and approximately 150 mm. The smoking system may have an external diameter between approximately 5 mm and approximately 30 mm. 
     The aerosol-forming substrate is a substrate capable of releasing volatile compounds that can form an aerosol. The volatile compounds may be released by heating the aerosol-forming substrate. 
     The aerosol-forming substrate may comprise plant-based material. The aerosol-forming substrate may comprise tobacco. The 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 aerosol-forming substrate may alternatively comprise a non-tobacco-containing material. The aerosol-forming substrate may comprise homogenised plant-based material. The aerosol-forming substrate may comprise homogenised tobacco material. The aerosol-forming substrate may comprise at least one aerosol-former. The aerosol-forming substrate may comprise other additives and ingredients, such as flavourants. 
     Preferably the system comprises a mouthpiece, wherein the cartridge is inserted into the system in an orientation that the opening of the cartridge points away from the mouthpiece. 
     In another preferred embodiment the cartridge is inserted into the system in an orientation that the opening of the cartridge points towards the mouthpiece. Depending on the circumstances of usage one of these orientations of the cartridge may provide for superior performance compared to the other. 
     According to a further aspect of the invention there is provided a cartridge for use in an aerosol-generating system, comprising: a liquid storage portion comprising a housing for holding a liquid aerosol-forming substrate, the liquid storage portion comprising a heater assembly, and a capillary material, provided in contact with the heater assembly, wherein the average porosity or pore size of a region of the capillary material adjacent the heater assembly is smaller than the average porosity or pore size of a region of the capillary material remote from the heater assembly. A portion of the capillary material in the region may be compressed to reduce its porosity or pore size. The liquid storage portion may comprise at least two parts in fluid communication with each other, the first part of the liquid storage portion comprising the capillary material, and the second part of the liquid storage portion comprising a container for holding aerosol-forming substrate in liquid form and supplying the liquid to the higher porosity or pore size region of the capillary material. 
     The present invention is also directed to a method for manufacturing a cartridge for use in an electrically operated aerosol-generating system, comprising the steps of providing a liquid storage portion comprising a housing with a first part and a second part, providing heater assembly, placing a first capillary material in the first part of the housing of the liquid storage portion, such that first capillary material is provided in direct contact with the heater assembly, placing a second capillary material in the first part of the housing of the liquid storage portion, such that the second capillary material is in contact with the first capillary material and is spaced apart from the heater assembly by the first capillary material. The second part of the liquid storage portion is substantially empty and is suitable for holding aerosol-forming substrate in liquid form. 
     Preferably the first capillary material is compressed during or before insertion into the housing such that its pore size or porosity is reduced compared to the pore size or porosity in the relaxed state. 
     An aspect of the invention further provides a method of manufacture of a cartridge for use in an aerosol-generating system, comprising: providing a liquid storage portion comprising a housing, providing a heater assembly, placing a capillary material in the housing of the liquid storage portion, such that the capillary material is provided in direct contact with the heater assembly, wherein the method includes the step of compressing a portion of the capillary material during or before placing in the housing such that the porosity or pore size of the portion of the capillary material is reduced. 
     The invention also provides an aerosol-generating system as described herein being an electrically operated smoking system. 
     The term “substantially flat” filament arrangement preferably refers to a filament arrangement that is in the form of a substantially two dimensional topological manifold. Thus, the substantially flat filament arrangement extends in two dimensions along a surface substantially more than in a third dimension. In particular, the dimensions of the substantially flat filament arrangement in the two dimensions within the surface is at least 5 times larger than in the third dimension, normal to the surface. An example of a substantially flat filament arrangement is a structure between two substantially parallel surfaces, wherein the distance between these two surfaces is substantially smaller than the extension within the surfaces. In some embodiments, the substantially flat filament arrangement is planar. In other embodiments, the substantially flat filament arrangement is curved along one or more dimensions, for example forming a dome shape or bridge shape. 
     The term “filament” preferably refers 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 term “filament arrangement” preferably refers to an arrangement of one or preferably a plurality of filaments. The filament arrangement 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. 
     It will be understood that, where appropriate, features of one aspect of the invention may be provided in relation to another aspect of the invention, in any appropriate combination. 
    
    
     
       The invention will be further described, by way of example only, with reference to the accompanying drawings in which: 
         FIGS. 1 a  to 1 d    are schematic illustrations of a system, incorporating a cartridge, in accordance with an embodiment of the invention 
         FIG. 2  shows a cartridge with porous medium according to a first aspect of the present invention; 
         FIG. 3  shows an exploded view of a similar cartridge as shown in  FIG. 2 ; 
         FIG. 4  shows a cartridge with a single porous medium compressed by the shape of porous material upon insertion into the housing; 
         FIG. 5  shows a cartridge with a single porous medium compressed by the shape of inner surface of the housing upon insertion into the housing; 
         FIG. 6  shows a capillary material rolled-up into a cylindrical shape and being provided with a tube-shaped heater in the centre. 
     
    
    
       FIGS. 1 a  to 1 d    are schematic illustrations of an aerosol-generating system, including a cartridge in accordance with an embodiment of the invention.  FIG. 1 a    is a schematic view of an aerosol-generating device  10  and a separate cartridge  20 , which together form the aerosol-generating system. In this example, the aerosol-generating system is an electrically operated smoking system. 
     The cartridge  20  contains an aerosol-forming substrate and is configured to be received in a cavity  18  within the device. Cartridge  20  should be replaceable by a user when the aerosol-forming substrate provided in the cartridge is depleted.  FIG. 1 a    shows the cartridge  20  just prior to insertion into the device, with the arrow  1  in  FIG. 1 a    indicating the direction of insertion of the cartridge. 
     The aerosol-generating device  10  is portable and has a size comparable to a conventional cigar or cigarette. The device  10  comprises a main body  11  and a mouthpiece portion  12 . The main body  11  contains a battery  14 , such as a lithium iron phosphate battery, control electronics  16  and a cavity  18 . The mouthpiece portion  12  is connected to the main body  11  by a hinged connection  21  and can move between an open position as shown in  FIGS. 1 a  to 1 c    and a closed position as shown in  FIG. 1 d   . The mouthpiece portion  12  is placed in the open position to allow for insertion and removal of cartridges  20  and is placed in the closed position when the system is to be used to generate aerosol, as will be described. The mouthpiece portion comprises a plurality of air inlets  13  and an outlet  15 . In use, a user sucks or puffs on the outlet to draw air from the air inlets  13 , through the mouthpiece portion to the outlet  15 , and thereafter into the mouth or lungs of the user. Internal baffles  17  are provided to force the air flowing through the mouthpiece portion  12  past the cartridge, as will be described. 
     The cavity  18  has a circular cross-section and is sized to receive a housing  24  of the cartridge  20 . Electrical connectors  19  are provided at the sides of the cavity  18  to provide an electrical connection between the control electronics  16  and battery  14  and corresponding electrical contacts on the cartridge  20 . 
       FIG. 1 b    shows the system of  FIG. 1 a    with the cartridge inserted into the cavity  118 , and the cover  26  being removed. In this position, the electrical connectors rest against the electrical contacts on the cartridge, as will be described. 
       FIG. 1 c    shows the system of  FIG. 1 b    with the cover  26  fully removed and the mouthpiece portion  12  being moved to a closed position. 
       FIG. 1 d    shows the system of  FIG. 1 c    with the mouthpiece portion  12  in the closed position. The mouthpiece portion  12  is retained in the closed position by a clasp mechanism, The mouthpiece portion  12  in a closed position retains the cartridge in electrical contact with the electrical connectors  19  so that a good electrical connection is maintained in use, whatever the orientation of the system is. The mouthpiece portion  12  may include an annular elastomeric element that engages a surface of the cartridge and is compressed between a rigid mouthpiece housing element and the cartridge when the mouthpiece portion  12  is in the closed position. This ensures that a good electrical connection is maintained despite manufacturing tolerances. 
     Of course other mechanisms for maintaining a good electrical connection between the cartridge and the device may, alternatively or in addition, be employed. For example, the housing  24  of the cartridge  20  may be provided with a thread or groove (not illustrated) that engages a corresponding groove or thread (not illustrated) formed in the wall of the cavity  18 . A threaded engagement between the cartridge and device can be used to ensure the correct rotational alignment as well as retaining the cartridge in the cavity and ensuring a good electrical connection. The threaded connection may extend for only half a turn or less of the cartridge, or may extend for several turns. Alternatively, or in addition, the electrical connectors  19  may be biased into contact with the contacts on the cartridge. 
     Other cartridge designs incorporating a capillary material arrangement in accordance with this disclosure can now be conceived by one of ordinary skill in the art. For example, the cartridge may include a mouthpiece portion, may include more than one heater assembly and may have any desired shape. Furthermore, a capillary assembly in accordance with the disclosure may be used in systems of other types to those already described, such as humidifiers, air fresheners, and other aerosol-generating systems 
     The exemplary embodiments described above illustrate but are not limiting. In view of the above discussed exemplary embodiments, other embodiments consistent with the above exemplary embodiments will now be apparent to one of ordinary skill in the art. 
     The cartridge shown in  FIG. 2  comprises a housing  24  made from polypropylene, with a two-part liquid storage portion. The first part  32  of the liquid storage portion comprises a first capillary material  36  and a second capillary material  38 . The second part  34  of the liquid storage portion is an empty tank that can be filled or partly filled with liquid aerosol-generating substrate. 
     At the upper end of the cartridge a ceramic substrate  42  is provided. The substrate  24  defines an opening  44  and has electric contacts (not shown) at opposite sides thereof. A heater element  46  is connected to the electrical contacts of the substrate  32  and extends over the opening  44  defined by the substrate. 
     Both the first capillary material  36  and the second capillary material  38  retain liquid aerosol-forming substrate. The first capillary material  16 , which is in direct contact with the heater element  46 , has a higher thermal decomposition temperature (at least 160 degree Celsius or higher such as approximately 250 degree Celsius) than the second capillary material  38 . The first capillary material  36  effectively acts as a spacer separating the heater element  46  from the second capillary material  38  so that the second capillary material  38  is not exposed to temperatures above its thermal decomposition temperature. The thermal gradient across the first capillary material  36  is such that the second capillary material  38  is exposed to temperatures below its thermal decomposition temperature. The second capillary material  38  may be chosen to have superior wicking performance to the first capillary material  36 , may retain more liquid per unit volume than the first capillary material  36  and may be less expensive than the first capillary material  36 . In this example the first capillary material  36  is a heat resistant material, such as a fiberglass or fiberglass containing material and the second capillary material  38  is a polymer such as high density polyethylene (HDPE), or polyethylene terephthalate (PET). 
       FIG. 3  is an exploded view of a cartridge similar to the cartridge of  FIG. 2 . The cartridge comprises a generally circular cylindrical housing  24  comprising a first part  32  and a second part  34 . The first part of the housing  24  contains a first and a second capillary material  36 ,  38  that are soaked in a liquid aerosol-forming substrate. In this example the aerosol-forming substrate comprises 39% by weight glycerine, 39% by weight propylene glycol, 20% by weight water and flavourings, and 2% by weight nicotine. A capillary material here is a material that actively conveys liquid from one end to another, and may be made from any suitable material. In this example the capillary material is formed from polyester. 
     The housing  24  has an open end to which a heater assembly is fixed. The heater assembly comprises a substrate  42  having an aperture  44  formed in it, a pair of electrical contacts  48  fixed to the substrate  42  and separated from each other by a gap  40 , and an resistive heater element  46  extending over the aperture  44  and being fixed to the electrical contacts  48  on opposing sides of the aperture  44 . 
     The heater assembly is covered by a removable cover  26 . The cover  26  comprises a liquid impermeable plastic sheet that is glued to the heater assembly but which can be easily peeled off. A tab is provided on the side of the cover to allow a user to grasp the cover when peeling it off. It will now be apparent to one of ordinary skill in the art that although gluing is described as the method to a secure the impermeable plastic sheet to the heater assembly, other methods familiar to those in the art may also be used including heat sealing or ultrasonic welding, so long as the cover may easily be removed by a consumer. 
       FIG. 4  shows an embodiment wherein the housing  24  has the shape of a regular cylinder with circular cross-section. The first and the second capillary material are made from the same material and are integrally formed as single continuous piece of capillary material  60  having the shape of a truncated cone. The diameter of the truncated apex of the cone corresponds to the inner diameter of the cylindrical housing. The diameter of the base of the cone is twice as large as the inner diameter of the cylindrical housing. The capillary material  60  is inserted apex first into the cylindrical housing  24  until the surface of the base of the cone lies flush with the front face of the cylindrical housing. Upon insertion the capillary material  40  is compressed, whereby due to the relative shape of the capillary material and the cylindrical housing, compression of the capillary material  60  is increased towards end face of the cylindrical housing. At the same time pore size or porosity of the capillary material is reduced such that the pore size or porosity of the capillary material in the vicinity of the end face of the housing is smaller than the pore size or porosity of the capillary material located in the center of the cylindrical housing. The open end of the cylindrical housing at the right hand side in  FIG. 4  is provided with a closure, such that the inside of the cylindrical housing forms a tank reservoir for holding liquid aerosol-generating substrate. At the other end the heater assembly as illustrated in  FIGS. 2 and 3  can be provided. 
       FIG. 5  shows an alternative embodiment having a similar effect as the embodiment depicted in  FIG. 4 . In this case the inner surface of the housing is provided with a conical shape, such that the interior tapers towards one end of the housing  24 . Here the inner diameter of housing  24  at the left hand side in  FIG. 5  is half the inner diameter of housing  24  at the right hand side. Again the first and the second capillary material are made from the same material and are integrally formed as single continuous piece of capillary material  60 . The piece of capillary material  60  has regular cylindrical form with a circular cross-section. The diameter cylindrical piece of capillary material  60  corresponds to the inner diameter of the housing  24  at the right hand side In  FIG. 5 . The capillary material  60  is inserted into the housing  24  until an end face of the capillary material  60  lies flush with the smaller diameter front face of the cylindrical housing, i.e with the end face at the left hand side of the housing  24 . Again the capillary material  60  is compressed upon insertion, whereby due to the relative shape of the capillary material and the cylindrical housing, compression of the capillary material  60  is increased towards the left hand side end face of the cylindrical housing  24 . At the same time pore size or porosity of the capillary is reduced such that the pore size or porosity of the capillary material  60  in the vicinity of the end face of the housing is smaller than the pore size or porosity of the capillary material  60  located in the center of the cylindrical housing. Again, the open end of the cylindrical housing at the right hand side in  FIG. 5  is provided with a closure, such that the inside of the cylindrical housing forms a tank reservoir for holding liquid aerosol-generating substrate. At the other end face of the housing, the heater assembly as illustrated in  FIGS. 2 and 3  can be provided. 
     In  FIG. 6  a further embodiment is depicted, whereby only the capillary material  50  that is to be used with a cylindrically housing is shown. The first and the second capillary material again are formed from a single continuous piece of the same material  50 . The capillary material is a rectangular piece web of capillary material having a thickness that amounts to about 25% of the inner diameter of the cylindrical housing of the cartridge. The width of the web of capillary material corresponds to the peripheral circumference of the housing. The length of the web of capillary material is about half the length of the housing of the cartridge. The web of capillary material is rolled up to form a cylindrical shape. In the middle of the rolled-up capillary material an air channel  52  is formed. A tube shaped fluid permeable heater element  54  is provided in the air channel  52  such that the heater  54  is in direct contact with the inner surface  56  of the rolled-up capillary material  50 . Upon rolling up the capillary material the material portion  50   a  that is closer to the center axis of the cylinder is more compressed than the material  50   b  in the radial outward located portion of the capillary material. Thus, again a gradient of pore size or porosity is obtained, wherein the pore size or porosity of the capillary material  50  is continuously decreased within the capillary material in the direction towards the heater element  54 . The capillary material is in fluid connection with a liquid reservoir (not shown), wherein the liquid reservoir is provided in the part of the housing that is not occupied by the capillary material. A partition is provided within the housing to ensure that the liquid substrate is not in direct communication with the air flow channel  52 . 
     It will be understood that different methods and configurations are possible to obtain the capillary material having a different pore size or porosity in different regions. In each example, a region of smaller pore size or porosity is located at one end of the capillary material. The region of smaller pore size or porosity is then located at the heater. The gradient in pore size or porosity then enhances the capillary action in the material, to draw aerosol-generating substrate liquid to the heater.