Patent Description:
Heater elements for use as part of an aerosol-generating device are known in the art. More particularly, electrically-powered heater elements are known which generate heat by resistive heating under the action of an electric current. Such electrically-powered resistive heater elements may take the form of a ceramic substrate on which is arranged a metallic resistive heating track. In use, electricity fed to the resistive heating track would induce heating of the track. An aerosol-generating device incorporating such a known resistive heater element may be used with a known aerosol-generating article containing a plug of aerosol-forming substrate. In use, the resistive heater element is inserted within the aerosol-generating article so that the heater element directly contacts the aerosol-forming substrate. The heat imparted to the aerosol-forming substrate from the resistive heater element vaporises constituent elements of the aerosol-forming substrate. On passage through the aerosol-generating article, the vapours evolved from the substrate cool and condense to form an aerosol for inhalation by a user. However, repeated use of the aerosol-generating device with different aerosol-generating articles can result in a gradual build-up of residue on the heater element. This residue can hinder the ability of the heater element to convey heat to the aerosol-forming substrate, and can also impart undesired flavours to vapours evolved from the aerosol-forming substrate. Therefore, the presence of such residue on the heater element can be detrimental to the user experience for the aerosol-generating device. Cleaning may be performed to remove this residue from the surface of the heater element. However, cleaning can result in the application of one or a combination of tensile, compressive and twisting forces being applied to the heater element. As ceramic substrates are inherently brittle, the forces applied to such known ceramic-based heater elements during cleaning can result in the heater element fracturing.

Replacement of the ceramic substrate with a substrate formed of metallic material can help to reduce the likelihood of the heater element fracturing, with metallic materials generally having greater ductility than ceramic materials. However, metallic materials generally possess greater thermal conductivity compared to ceramic materials. When using a metallic material for the substrate of the heater element in place of a ceramic material, the relatively higher thermal conductivity of the metallic substrate can lead to heat conducted into the substrate from the resistive heating track being rapidly conveyed throughout the metallic substrate. The rapid passage of heat throughout the metallic substrate can result in corresponding rapid increases in temperature of the substrate. This increase in substrate temperature can thereby result in overheating of sensitive electronic components of the aerosol-generating device associated with control and operation of the heater element. For example, control circuitry used to control the supply of electrical power to the heater element is often located in close proximity to where the heater element is positioned within the aerosol-generating device. Excessive temperatures in the metallic substrate can therefore be conducted to and damage the sensitive control circuitry. Further, damage to components of the aerosol-generating device can also occur due to heat being radiated from excessively hot regions of the metallic substrate of the heater element.

It is therefore desired to provide an improved resistive heater element which has enhanced heat flow management and can better withstand forces which may be applied to the heater element during cleaning or operation (such as tensile, compressive or twisting forces) without fracturing.

<CIT> relates to a heatable tobacco product and a tobacco unit for use with the heatable tobacco product.

According to a first aspect of the present disclosure, there is provided an elongate metallic heater element for use with an aerosol-generating device. The heater element extends between a proximal end and a distal end. The proximal end is configured for mounting to an aerosol-generating device for electrical communication with the aerosol-generating device. The heater element comprises either or both of: i) a plurality of surface notches formed on a surface of the heater element; and ii) a plurality of sub-surface cavities defined beneath the surface of the heater element. At least some of the plurality of surface notches and sub-surface cavities of the heater element are provided in an inner region of the heater element. The inner region extends between the proximal end and <NUM>% of the length of the heater element relative to the proximal end. At least <NUM>% by volume, or at least <NUM>% by volume, or at least <NUM>% by volume, or at least <NUM>% by volume, or at least <NUM>% by volume of all of the plurality of surface notches and sub-surface cavities of the heater element are provided in the inner region.

As used herein, the term "metallic" is used to mean formed predominantly or wholly of one or more metals. So, the term "metallic" encompasses specific metal elements or alloys.

As used herein, the term "aerosol-generating device" is used to describe a device that interacts with an aerosol-forming substrate of an aerosol-generating article to generate an aerosol. Preferably, the aerosol-generating device is a smoking device that interacts with an aerosol-forming substrate of an aerosol-generating article to generate an aerosol that is directly inhalable into a user's lungs thorough the user's mouth. The aerosol-generating device may be a holder for a smoking article. Preferably, the aerosol-generating article is a smoking article that generates an aerosol that is directly inhalable into a user's lungs through the user's mouth. More preferably, the aerosol-generating article is a smoking article that generates a nicotine-containing aerosol that is directly inhalable into a user's lungs through the user's mouth.

As used herein, the term "aerosol-forming substrate" denotes a substrate consisting of or comprising an aerosol-forming material that is capable of releasing volatile compounds upon heating to generate an aerosol.

The provision of the metallic heater element with a plurality of surface notches formed on a surface of the heater element reduces the rate of heat transfer through the heater element at the location of the surface notches compared to the same heater element lacking any such notches. The effect of forming notches in the metallic heater element is to remove thermally-conductive metal material from the heater element which would otherwise be present to conduct heat. Therefore, the rate of heat transfer and resulting temperatures in one or more regions of the heater element can be controlled by the presence of such surface notches in the vicinity of those one or more regions.

The provision of the metallic heater element with a plurality of sub-surface cavities may provide the same advantages described above for the surface notches. The use of sub-surface cavities may be particularly beneficial in maintaining flexural rigidity of the heater element, due to the inherent constraining effect of the metallic substrate material which would enclose each sub-surface cavity.

The use of surface notches or sub-surface cavities can be contrasted with the use of through-holes in the metallic heater element. The use of a surface notch or a sub-surface cavity of a given volume may maintain a higher level of flexural rigidity in the heater element compared to a through-hole having the same volume. As used herein, the term "through-hole" refers to where an opening extends through and between opposing surfaces of the heater element.

A surface notch would be an open feature, in that a notch would open onto a surface of the heater element. In contrast, a sub-surface cavity would be a closed feature, in that it would be enclosed and hidden from view beneath a surface of the heater element.

In summary, the use of a metallic heater element provided with a plurality of either or both surface notches and sub-surface cavities provides improved thermal management to the heater element. These advantages co-exist alongside the ductility which results from the heater element being formed of a metallic material. This inherent ductility reduces the likelihood of the heater element fracturing when subjected to tensile, compressive, or twisting forces during operational use or cleaning. Further, the surface notches and sub-surface cavities reduce the mass of the heater element compared to the same heater element lacking any such notches or cavities.

The distribution and size of the surface notches and sub-surface cavities may be selected and arranged to provide heat management and temperature control of one or more specific regions of the metallic heater element, whilst retaining sufficient flexural rigidity in the heater element to enable the element to withstand forces encountered during operational use and cleaning. For example, where the metallic heater element is intended to be inserted within the aerosol-forming substrate of an aerosol-generating article, the heater element should possess sufficient flexural rigidity to withstand insertion without the heater element buckling.

The individual notches and cavities may be defined by one or more curved surfaces. By way of example, the surface of a notch may correspond to part of the surface of a sphere or an ellipsoid. By way of further example, the enclosed surface of a cavity may correspond to the surface of a sphere or an ellipsoid. The use of notches and cavities which consist of curved surfaces also reduces the likelihood of individual notches and cavities acting as mechanical stress-raising features when the heater element is subjected to tensile, compressive or twisting forces. However, other shapes for the notches and cavities may also be employed. By way of example, the surface notches may be circular in plan, each defining a cylindrical bore extending into the surface of the heater element. Alternatively, the surface notches may be hexagonal in plan, defining a hexagonal-shaped bore extending into the surface of the heater element.

The heater element may comprise a metallic substrate and one or more resistive heating tracks, the one or more resistive heating tracks arranged on the metallic substrate. In turn, the plurality of surface notches and sub-surface cavities of the heater element may be formed on or within the metallic substrate. In use, electric current may be fed to the one or more resistive heating tracks of the heater element via the proximal end of the heater element. The flow of electric current along the one or more resistive tracks results in heating of the tracks by resistance heating (also known as Ohmic heating or Joule heating).

Metallic materials of the metallic heater element may include titanium or stainless steel. Further, Inconel ® alloy <NUM> may also be suitable, possessing a combination of high temperature strength and oxidation resistance. For Inconel ® alloy <NUM>, the nickel and chromium content provides oxidation resistance, with the aluminium and nickel content also imparting high temperature oxidation resistance. By way of example, the heater element may take the form of a metallic substrate formed of titanium, stainless steel or Inconel ® alloy <NUM>, with one or more resistive heating tracks arranged on one or more surfaces of the substrate. Other metallic materials may be used for the metallic heater element, with the materials chosen dependent upon factors which can include flexural rigidity, thermal conductivity, oxidation resistance and chemical reactivity.

The plurality of surface notches and sub-surface cavities of the heater element may occupy a cumulative volume of between <NUM>% to <NUM>% of the volume of a corresponding heater element free of any such notches and cavities. As used herein, the term "cumulative volume" refers to the summation of the volume occupied by all of the surface notches and sub-surface cavities formed in the heater element. In general, the greater the cumulative volume of the heater element which is occupied by the surface notches or sub-surface cavities, the greater the corresponding reduction in conductive heat transfer through the metallic heater element during use. However, the greater the cumulative volume occupied by the notches or cavities, the less the flexural rigidity of the heater element. Confining the cumulative volume to between <NUM>% to <NUM>% provides a balance between the two conflicting desires of i) reducing excessive heat flow and resulting high temperatures in the metallic heater element, and ii) retaining sufficient flexural rigidity in the heater element to enable the element to withstand axial compressive forces without simply buckling. Conveniently however, the plurality of surface notches and sub-surface cavities of the heater element may occupy a cumulative volume of between <NUM>% to <NUM>% of the volume of a corresponding heater element free of any such notches and cavities.

The heater element may be entirely free of any through-holes extending through a thickness of the heater element. In contrast with the surface notches and sub-surface cavities, a "through-hole" would extend all the way through and between opposing surfaces of the heater element.

Alternatively however, the heater element may further comprise a plurality of through-holes extending through a thickness of the heater element. In this manner, the heater element may include a combination of different categories of features which correspond to removal of metallic material from the metallic heater element to provide improved thermal management of the heater element in use. For example, the heater element may include a plurality of through-holes alongside either a plurality of surface notches, or a plurality of sub-surface cavities. Alternatively, the heater element may include all of a plurality of through-holes, a plurality of surface notches and a plurality of sub-surface cavities. The plurality of through-holes may be distributed along the length of the heater element. Individual through-holes may allow for the flow of air across the heater element during use.

The plurality of surface notches and sub-surface cavities of the heater element may be formed to define one or more honeycomb arrangements. As used herein, the term "honeycomb arrangement" refers to a repeating pattern of the notches or cavities. The honeycomb arrangement may be two-dimensional; for example, the honeycomb arrangement may be defined by a discrete layer of surface notches or sub-surface cavities extending along two axes orthogonal to each other. Alternatively, the honeycomb arrangement may be three-dimensional. The use of such a honeycomb arrangement ensures that the region of the heater element in which the honeycomb arrangement is situated has near uniform thermal and structural properties.

The heater element may comprise the plurality of surface notches and be entirely free of any sub-surface cavities. Alternatively, the heater element may comprise the plurality of sub-surface cavities and be entirely free of any surface notches.

At least some of the plurality of surface notches and sub-surface cavities of the heater element are provided in an inner region of the heater element, the inner region extending between the proximal end and <NUM>% of the length of the heater element relative to the proximal end. As the proximal end is configured for mounting to an aerosol-generating device, ensuring that the inner region of the heater element contains at least some of the surface notches and sub-surface cavities may help to avoid overheating of electronic components of the aerosol-generating device located adjacent to the proximal end. As control electronics of the aerosol-generating device will often be located at or close to the proximal end of the heater element, providing notches or cavities in the inner region of the heater element can avoid excessive temperatures developing in this region of the heater element and in neighbouring components of the aerosol-generating device. At least <NUM>% by volume, or at least <NUM>% by volume, or at least <NUM>% by volume, or at least <NUM>% by volume, or at least <NUM>% by volume of all of the plurality of surface notches and sub-surface cavities of the heater element are provided in the inner region. The provision of such a high volumetric proportion of surface notches and/or surface cavities provides a barrier to conductive heat transfer through the metallic material of the heater element in the inner region and thereby enhances thermal protection to components of the aerosol-generating device in close proximity to the inner region of the heater element. In one example, all of the plurality of surface notches and sub-surface cavities of the heater element may be provided in the inner region of the heater element.

Where at least some of the plurality of surface notches and sub-surface cavities of the heater element are provided in the inner region of the heater element (as described in the preceding paragraph), the surface notches and sub-surface cavities of the inner region may extend laterally across at least <NUM>%, or at least <NUM>% of the lateral width of the heater element. Such an arrangement of notches or cavities helps to provide a barrier to conductive heat transfer through the metallic substrate close to the proximal end, with the barrier extending across nearly all of the lateral width of the heater element.

Where some of the plurality of surface notches and sub-surface cavities of the heater element are provided in the inner region of the heater element (as described in the preceding paragraphs), some of the plurality of surface notches and sub-surface cavities of the heater element may also be provided in a middle region of the heater element, the middle region extending between <NUM>% and <NUM>% of the length of the elongate heater element relative to the proximal end. Including surface notches or sub-surface cavities in the middle region of the heater element in addition to those in the inner region may provide a more gradual change in heat transfer rate through the heater element when progressing from one region of the heater element to another. Ensuring such gradual changes in heat transfer rate through the heater element may be beneficial in avoiding excessive thermal stresses being developed in the metallic material of the heater element.

The heater element may extend along a longitudinal axis and laterally outwards from the longitudinal axis to define a blade having opposed first and second elongate surfaces. Providing the heater element in the form of a blade makes the element particularly suitable for insertion through and within the aerosol-forming substrate of an aerosol-generating article. The heater element may further comprise a resistive heating track arranged on the first elongate surface, the heater element comprising a plurality of the surface notches on the second elongate surface, in which the surface notches on the second elongate surface form at least <NUM>% by volume of all of the plurality of surface notches of the heater element. The second elongate surface may be free of any resistive heating track. As discussed in preceding paragraphs, the passage of electric current along the resistive heating track would result in resistive heating of the track. The first elongate surface may be substantially free of surface notches. By way of example, any surface notches of the heater element may instead be provided on the second elongate surface.

The plurality of surface notches and sub-surface cavities of the heater element may be arranged in one or more laterally-symmetric groups. The one or more laterally-symmetric groups may comprise a first group, a second group and a third group. The first group may be symmetrically disposed about the longitudinal axis and extend laterally across at least <NUM>%, or at least <NUM>% of the lateral width of the heater element in an inner region of the heater element. The inner region may extend between the proximal end and <NUM>% of the length of the heater element relative to the proximal end. The second and third groups may be symmetrically disposed from each other on either side of the longitudinal axis in a middle region of the heater element, the middle region extending between <NUM>% and <NUM>% of the length of the heater element relative to the proximal end. The second and third groups may be axially spaced from the first group. The second and third groups may join each other at the longitudinal axis.

Preferably, the heater element is configured to be removably mountable to the aerosol-generating device. In this way, the heater element can be removed for cleaning before being remounted. Further, the heater element may be removed and swapped with a replacement heater element.

In a second aspect of the present disclosure, there is provided an aerosol-generating device configured to receive an aerosol-forming substrate. The aerosol-generating device comprises an elongate metallic heater element as described in relation to the first aspect of the present disclosure. The aerosol-generating device also comprises a power source. The proximal end of the heater element is mounted to a mounting location of the aerosol-generating device. The power source is in electrical communication with the heater element so as to, in use, resistively heat the heater element.

The power source is preferably in the form of a battery, thereby providing a source of electrical power to the aerosol-generating device and helping to make the device portable. Conveniently, the battery is rechargeable; by way of example, a lithium ion battery may be used as the power source.

The aerosol-generating device may be configured such that, in use with an aerosol-forming substrate received in the device, the heater element extends within the aerosol-forming substrate to heat the aerosol-forming substrate and generate an inhalable aerosol therefrom. By way of example, the heater element may be formed as a blade as described in the preceding paragraphs. The distal end of the blade may terminate in a point, thereby assisting in entry of the blade into the aerosol-generating substrate. One or more longitudinal edges of the blade may also be sharpened.

Preferably, the aerosol-forming substrate is a solid aerosol-forming substrate. However, the aerosol-forming substrate may comprise both solid and liquid components. Alternatively, the aerosol-forming substrate may be a liquid aerosol-forming substrate.

Preferably, the aerosol-forming substrate comprises nicotine. More preferably, the aerosol-forming substrate comprises tobacco. Alternatively or in addition, the aerosol-forming substrate may comprise a non-tobacco containing aerosol-forming material.

If the aerosol-forming substrate is a solid aerosol-forming substrate, the solid aerosol-forming substrate may comprise, for example, one or more of: powder, granules, pellets, shreds, strands, strips or sheets containing one or more of: herb leaf, tobacco leaf, tobacco ribs, expanded tobacco and homogenised tobacco.

Optionally, the solid aerosol-forming substrate may contain tobacco or non-tobacco volatile flavour compounds, which are released upon heating of the solid aerosol-forming substrate. The solid aerosol-forming substrate may also contain one or more capsules that, for example, include additional tobacco volatile flavour compounds or non-tobacco volatile flavour compounds and such capsules may melt during heating of the solid aerosol-forming substrate.

Optionally, the solid aerosol-forming substrate may be provided on or embedded in a thermally stable carrier. The carrier may take the form of powder, granules, pellets, shreds, strands, strips or sheets. The solid aerosol-forming substrate may be deposited on the surface of the carrier in the form of, for example, a sheet, foam, gel or slurry. The solid aerosol-forming substrate may be deposited on the entire surface of the carrier, or alternatively, may be deposited in a pattern in order to provide a non-uniform flavour delivery during use.

In a preferred embodiment, the aerosol-forming substrate comprises homogenised tobacco material. As used herein, the term "homogenised tobacco material" refers to a material formed by agglomerating particulate tobacco.

Preferably, the aerosol-forming substrate comprises a gathered sheet of homogenised tobacco material. As used herein, the term "sheet" refers to a laminar element having a width and length substantially greater than the thickness thereof. As used herein, the term "gathered" is used to describe a sheet that is convoluted, folded, or otherwise compressed or constricted substantially transversely to the longitudinal axis of the aerosol-generating article.

Preferably, the aerosol-forming substrate comprises an aerosol former. As used herein, the term "aerosol former" is used to describe any suitable known compound or mixture of compounds that, in use, facilitates formation of an aerosol and that is substantially resistant to thermal degradation at the operating temperature of the aerosol-generating article.

Suitable aerosol-formers are known in the art and include, but are not limited to: polyhydric alcohols, such as propylene glycol, triethylene glycol, <NUM>,<NUM>-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. Preferred aerosol formers are polyhydric alcohols or mixtures thereof, such as propylene glycol, triethylene glycol, <NUM>,<NUM>-butanediol and, most preferred, glycerine.

The aerosol-forming substrate may comprise a single aerosol former. Alternatively, the aerosol-forming substrate may comprise a combination of two or more aerosol formers.

In a third aspect of the present disclosure, there is provided a method of manufacturing a heater element. The method comprises providing a metallic substrate and forming either or both of i) a plurality of surface notches on a surface of the metallic substrate; and ii) a plurality of sub-surface cavities defined beneath the surface of the metallic substrate.

A plurality of metallic blanks may first be cut from a sheet of the metallic material, with each blank forming the "metallic substrate" for a given heater element. Conveniently, the surface notches and sub-surface cavities may be formed in the sheet of metallic material prior to cutting the sheet into individual blanks. Alternatively, the surface notches and sub-surface cavities may be formed in each blank after being cut from the sheet of metallic material.

Advantageously, the forming step may comprise etching of the metallic substrate to form the plurality of surface notches. By way of example, a chemical or mechanical etching process may be used to form the notches in the surface of the substrate. Chlorine etching is an example of a suitable chemical etching process. Mechanical etching may take the form of the notches being machined into the surface of the substrate.

Preferably, the manufactured heater element resulting from the method is in accordance with the metallic heater element described in the preceding paragraphs in relation to the first aspect of the present disclosure.

In a fourth aspect of the present disclosure, there is provided a method of manufacturing a heater element. The method comprises providing a supply of metallic material; additively manufacturing the heater element from the supply of metallic material to progressively form the heater element to comprise either or both of: i) a plurality of surface notches on a surface of the heater element; and ii) a plurality of sub-surface cavities defined beneath the surface of the heater element.

The use of additive manufacturing to form the heater element is particularly beneficial in enabling the formation of hidden structural features, such as the sub-surface cavities. By way of example, the metallic material may be provided in powder form, or alternatively as a slurry of powder and liquid. Conveniently, the additive manufacturing step may comprise three-dimensional screen printing.

Examples will now be further described with reference to the figures, in which:.

<FIG> is a schematic view of an aerosol-delivery system <NUM>. The aerosol-generating system <NUM> is a smoking system for generating an inhalable aerosol. The system <NUM> is formed of a combination of an aerosol-generating device <NUM> and an aerosol-generating article <NUM>.

The aerosol-generating device <NUM> has an elongate housing <NUM> formed of a polymer material. The elongate housing <NUM> contains a power source <NUM>, a controller <NUM> and a mounting location <NUM>. A metallic heater element <NUM> is detachably mounted to the mounting location <NUM> by use of a push-fit connection. The heater element <NUM> is formed of a metallic substrate <NUM> and has a resistive heating track <NUM> overlaid on a surface of the substrate (see, for example, <FIG>). The structure of the heater element <NUM> is described in more detail in subsequent paragraphs. The heater element <NUM> is mounted to the mounting location <NUM> so that the resistive heating track <NUM> is electrically coupled to the mounting location. An access opening <NUM> is provided at one end of the elongate housing <NUM>. A blind cavity <NUM> extends from the access opening <NUM> into the interior of the elongate housing <NUM>. The heater element <NUM> extends from a closed end <NUM> of the blind cavity <NUM> towards the access opening <NUM>.

The aerosol-generating article <NUM> is cylindrical in form and extends between a distal end <NUM> and a mouth end <NUM>. The aerosol-generating article <NUM> has a wrapper <NUM>. The wrapper <NUM> is in the form of a cigarette paper. A plug of aerosol-forming substrate <NUM>, a hollow acetate tube <NUM>, a tubular spacer element <NUM> and a mouthpiece filter <NUM> are co-axially and sequentially arranged within the wrapper <NUM>. The aerosol-generating article <NUM> is received within the blind cavity <NUM> such that the heater element <NUM> inserts within the plug of aerosol-forming substrate <NUM>.

For the aerosol-generating device <NUM>, the power source <NUM> is coupled to the controller <NUM> to provide power thereto. For the example shown, the power source <NUM> is a rechargeable lithium ion battery. The controller <NUM> is coupled to the mounting location <NUM> to provide electric current to the mounting location <NUM> and thereby to the resistive heating track <NUM> of the heater element <NUM>. The controller <NUM> is in the form of control electronics and incorporates a memory module 23a. The memory module 23a contains instructions accessible by a processor (not shown) of the controller <NUM> to control the supply of electric current to the resistive heating track <NUM> of the heater element <NUM>. Electric current fed from the controller <NUM> to the mounting location <NUM> results in resistive heating of the resistive heating track <NUM>. Some of the heat generated by the resistive heating track <NUM> is conducted into the underlying metallic substrate <NUM> of the heater element <NUM>.

In use, heat generated by the heater element <NUM> is conveyed to the plug of aerosol-forming substrate <NUM>. The heating of the aerosol-forming substrate <NUM> results in vapours being evolved from the aerosol-forming substrate. In response to a user drawing on the mouth end <NUM> of the article <NUM>, a flow of ambient air (as indicated by arrows in <FIG>) is sucked into an air passageway <NUM> circumferentially arranged between the elongate housing <NUM> and the blind cavity <NUM>. The flow of air then enters the distal end <NUM> of the aerosol-generating article <NUM> and passes through the plug of aerosol-forming substrate <NUM> to mix with the vapours evolved from the aerosol-forming substrate. The vapour mixture then passes downstream through the interior of the aerosol-generating article <NUM> towards the mouth end <NUM>, during which time the vapour condenses to form aerosol. The aerosol passes through the mouthpiece filter <NUM>, from where it is inhaled into the lungs of the user.

<FIG> and <FIG> show perspective views from of the upper and lower surfaces respectively of the heater element <NUM> used in the aerosol-delivery system <NUM> of <FIG>. The terms "upper" and "lower" are used in a relative sense only. As stated above, the heater element <NUM> is formed of a metallic substrate <NUM>. The metallic substrate <NUM> is titanium or stainless steel. However, in alternative examples, the metallic substrate <NUM> may be formed of other metals or alloys. The resistive heating track <NUM> is overlaid on a surface of the metallic substrate <NUM> (see <FIG>). For the example shown in <FIG> and <FIG>, the resistive heating track <NUM> is in the form of fine metal wire deformed into a coil shape. However, in alternative examples (not shown), the resistive heating track <NUM> may take other forms, such as being a metal sheet which has been stamped or otherwise formed into a coil-shaped pattern. The metallic substrate <NUM> of the heater element <NUM> extends longitudinally along axis <NUM> between proximal end <NUM> and distal end <NUM>, plus laterally outwards from the axis to define a blade-shaped profile for the heater element <NUM>. The heater element <NUM> is detachably mounted to the mounting location <NUM> at proximal end <NUM>. First and second planar surfaces <NUM>, <NUM> define respective lower and upper surfaces of the metallic substrate <NUM>. The resistive heating track <NUM> is arranged on the lower surface <NUM> (see <FIG>). A plurality of surface notches <NUM> are formed on the upper surface <NUM>. The surface notches <NUM> extend partially through the thickness, t, of the metallic substrate <NUM>. The plurality of surface notches <NUM> are arranged in three groups 49a,b,c. Group 49a of surface notches <NUM> are provided in an inner region <NUM> of the heater element <NUM>, the inner region extending between the proximal end <NUM> and approximately <NUM>% of the length, L, of the heater element relative to the proximal end. For the example shown in <FIG>, approximately <NUM>% of all of the surface notches <NUM> formed on the heater element <NUM> are located in this inner region <NUM> in group 49a. Groups 49b and 49c contain the remaining <NUM>% of the surface notches <NUM> formed on the heater element <NUM>, with both groups being laterally symmetric about axis <NUM> in a middle region <NUM> of the heater element. The middle region <NUM> extends between <NUM>% and <NUM>% of the length, L, of the heater element <NUM> relative to the proximal end <NUM>. These two laterally symmetric groups 49b and 49c of surface notches <NUM> join at axis <NUM> to define an arrowhead shape when viewed in plan, i.e. in the direction of arrow A. The surface notches <NUM> of group 49a extend across in excess of <NUM>% of the lateral width, W, of the heater element <NUM>. For the heater element <NUM> of <FIG> and <FIG>, no surface notches <NUM> are defined on the lower surface <NUM> of the metallic substrate <NUM>. However, in alternative examples (such as the exemplary heater element of <FIG>, discussed in subsequent paragraphs), a plurality of surface notches <NUM> is also provided on the lower surface <NUM> of the metallic substrate <NUM>.

For the heater element of <FIG> and <FIG>, the surface notches <NUM> defined on the heater element <NUM> occupy a cumulative volume of ~<NUM>% of a corresponding heater element free of any such notches. This provides ~<NUM>% mass reduction relative to such a corresponding heater element free of any such notches. In alternative examples, the surface notches <NUM> occupy a larger or smaller cumulative volume according to the degree of heat flow management desired. Further, in alternative examples (not shown), the distribution of surface notches <NUM> along the length, L, and across the width, W, of the heater element <NUM>, and the proportion of surface notches <NUM> in the inner and middle regions <NUM>, <NUM> may differ from that shown and discussed for the heater element of <FIG> and <FIG>.

<FIG> show views of portions of various exemplary heater elements <NUM> having differing arrangements of surface notches or sub-surface cavities to the heater element illustrated in <FIG>,b. For convenience, features common to the various exemplary heater elements <NUM> are referred to using like reference signs.

<FIG> is a plan view (in the direction of arrow A of <FIG>) of the upper surface <NUM> of a portion of an exemplary heater element <NUM>. A plurality of surface notches <NUM> are formed on the upper surface <NUM> of the metallic substrate <NUM>. The surface notches <NUM> are circular in plan, being of radius, r. The surface notches <NUM> are formed in a repeating honeycomb-type pattern, in which adjacent rows of the notches are offset from each other. As shown in the sectional view of <FIG>, each of the surface notches <NUM> extends into the upper surface <NUM> of the metallic substrate <NUM> to provide a notch surface profile which is part-spherical. Each notch <NUM> extends part way through the thickness, t, of the substrate <NUM> for a depth, d (see <FIG>). As the notch surface profile is part-spherical, the depth d equates to radius r. In alternative examples (not shown), the surface profile of the notches <NUM> is ellipsoidal. In further alternative examples (not shown), the dimensions of the notches <NUM> are varied in different regions of the heater element <NUM>. Further, the spacing between adjacent notches <NUM> may be varied in different regions of the heater element <NUM>. By way of example, with reference to <FIG>, the notches <NUM> in inner region <NUM> may be larger or spaced closer together than those in middle region <NUM>. Such variation in notch dimensions and spacing between adjacent notches <NUM> in different regions of the heater element <NUM> can be used to provide different levels of thermal conductivity in those different regions.

<FIG> is a plan view (in the direction of arrow A of <FIG>) of the upper surface of a portion of another exemplary heater element <NUM>. A plurality of surface notches <NUM> are formed on the upper surface <NUM> of the metallic substrate <NUM>. In common with the example of <FIG>,<FIG>, the surface notches <NUM> are circular in plan, being of radius, r, and formed in a repeating honeycomb-type pattern, in which adjacent rows of the notches are offset from each other. However, as shown in the sectional view of <FIG>, each of the surface notches <NUM> has a cylindrical bore which extends into the upper surface <NUM> of the metallic substrate <NUM> to provide a notch surface profile which is cylindrical. Each notch <NUM> extends part way through the thickness, t, of the substrate <NUM> for a depth, d (see <FIG>). In alternative examples (not shown), the dimensions of the notches <NUM> are varied in different regions of the heater element <NUM>. Further, the spacing between adjacent notches <NUM> may be varied in different regions of the heater element <NUM>. By way of example, with reference to <FIG>, the notches <NUM> in inner region <NUM> may be larger or spaced closer together than those in middle region <NUM>. Such variation in notch dimensions and spacing between adjacent notches <NUM> in different regions of the heater element <NUM> can be used to provide different levels of thermal conductivity in those different regions.

<FIG> is a plan view (in the direction of arrow A of <FIG>) of the upper surface <NUM> of a portion of an exemplary heater element <NUM>. A plurality of surface notches <NUM> are formed on the upper surface <NUM> of the metallic substrate <NUM>. The surface notches <NUM> are hexagonal in plan. The surface notches <NUM> are formed in a repeating honeycomb-type pattern, in which adjacent rows of the notches are offset from each other. As shown in the sectional view of <FIG>, each of the surface notches <NUM> has a hexagonal bore which extends into the upper surface <NUM> of the metallic substrate <NUM>. Each notch <NUM> extends part way through the thickness, t, of the substrate <NUM> for a depth, d (see <FIG>). In alternative examples (not shown), the dimensions of the notches <NUM> are varied in different regions of the heater element <NUM>. Further, the spacing between adjacent notches <NUM> may be varied in different regions of the heater element <NUM>. By way of example, with reference to <FIG>, the notches <NUM> in inner region <NUM> may be larger or spaced closer together than those in middle region <NUM>. Such variation in notch dimensions and spacing between adjacent notches <NUM> in different regions of the heater element <NUM> can be used to provide different levels of thermal conductivity in those different regions.

<FIG> is a plan view (in the direction of arrow A of <FIG>) of the upper surface <NUM> of a portion of an exemplary heater element <NUM>. The notch arrangement on the upper surface <NUM> is identical to that of the heater element <NUM> of <FIG>, with a plurality of surface notches <NUM> formed on the upper surface of the metallic substrate <NUM>. The surface notches <NUM> are hexagonal in plan and formed in a repeating honeycomb-type pattern, in which adjacent rows of the notches are offset from each other. However, the heater element of this example differs from that of <FIG>,<FIG> in that an arrangement of surface notches <NUM> are also provided on the lower surface <NUM> of the metallic substrate <NUM>. As for the notches on the upper surface <NUM>, the notches on the lower surface <NUM> are hexagonal in plan. However, the notches <NUM> on the lower surface <NUM> are fewer in number than those on the upper surface <NUM>. As can be seen in <FIG>, the notches <NUM> on the lower surface <NUM> are positioned on either side of the resistive heating track <NUM>. As shown in the sectional view of <FIG>, each notch <NUM> extends part way through the thickness, t, of the substrate <NUM> for a depth, d. In alternative examples (not shown), the dimensions of the notches <NUM> are varied in different regions of the heater element <NUM>. Further, the spacing between adjacent notches <NUM> may be varied in different regions of the heater element <NUM>. By way of example, with reference to <FIG>, the notches <NUM> in inner region <NUM> may be larger or spaced closer together than those in middle region <NUM>. Such variation in notch dimensions and spacing between adjacent notches <NUM> in different regions of the heater element <NUM> can be used to provide different levels of thermal conductivity in those different regions.

<FIG> is a plan view (in the direction of arrow A of <FIG>) of the upper surface <NUM> of a portion of an exemplary heater element <NUM>. In contrast to the exemplary heater elements <NUM> of <FIG>, no surface notches <NUM> are provided on the metallic substrate. Rather, a layer of spherical sub-surface cavities <NUM> is embedded within the metallic substrate <NUM>, each cavity having diameter, Φ (see the sectional view of <FIG>). The outline of these sub-surface cavities <NUM> is shown with a dashed line in the plan view of <FIG>. The sub-surface cavities <NUM> are formed in a repeating honeycomb-type pattern, in which adjacent rows of the embedded cavities <NUM> are offset from each other (see <FIG>). In alternative examples (not shown), the sub-surface cavities <NUM> are ellipsoidal in form. In further alternative examples (not shown), the dimensions of the sub-surface cavities <NUM> are varied in different regions of the heater element <NUM>. Further, the spacing between adjacent cavities <NUM> may be varied in different regions of the heater element <NUM>. By way of example, with reference to <FIG>, sub-surface cavities <NUM> in inner region <NUM> may be larger or spaced closer together than those in middle region <NUM>. Such variation in cavity dimensions and spacing between adjacent sub-surface cavities <NUM> in different regions of the heater element <NUM> can be used to provide different levels of thermal conductivity in those different regions.

<FIG> is a sectional view of a heater element <NUM> similar to that shown in <FIG>, but differing in that three layers of sub-surface cavities <NUM> are embedded within the metallic substrate <NUM>. In alternative examples (not shown), where multiple layers of sub-surface cavities <NUM> are embedded within the metallic substrate <NUM>, the layers may differ from each other in terms of the size or spacing of the cavities <NUM> in each layer.

<FIG> is a sectional view of a heater element <NUM> corresponding to a combination of the examples of <FIG>,<FIG> and <FIG>,<FIG>. As can be seen, an arrangement of surface notches <NUM> are formed on the upper surface <NUM> of the metallic substrate <NUM>, the notches being hexagonal in plan (as per <FIG>). However, in addition, a layer of sub-surface cavities <NUM> is embedded in the metallic substrate <NUM> (as per <FIG>).

<FIG> shows a perspective view of five different heater elements <NUM>, each having different arrangements of surface notches <NUM>.

<FIG> is a schematic view of a machining assembly <NUM> for manufacturing a heater element from a sheet <NUM> of the metallic substrate <NUM> material. The machining assembly <NUM> has a tool holder <NUM> holding a machining tool <NUM>. The sheet <NUM> of metallic substrate is provided and secured in position relative to the machining assembly <NUM>. The sheet <NUM> has a thickness corresponding to the desired thickness, t, of the heater elements <NUM> described in the previous paragraphs, but has a width and a length sufficient for multiple heater elements <NUM> to be formed from a single sheet <NUM>. The machining assembly <NUM> operates the machining tool <NUM> to machine an individual surface notch <NUM> in the upper surface of the metal sheet <NUM> and then traverses (see arrows in <FIG>) the tool holder <NUM> and machining tool <NUM> over the surface of the metal sheet <NUM> to repeat the machining operation at multiple desired locations. Once the machining operation is completed, resistive heater tracks <NUM> are arranged at predetermined intervals on the lower surface of the sheet <NUM>. Individual heater elements <NUM> are then cut from the sheet <NUM>, with dashed lines in <FIG> showing the outline of two such heater elements <NUM>. In another example (not shown), the surface notches <NUM> are chemically etched in the surface of the metal sheet <NUM>, rather than being mechanically formed.

Claim 1:
An elongate metallic heater element (<NUM>) for use with an aerosol-generating device (<NUM>), the heater element (<NUM>) extending between a proximal end (<NUM>) and a distal end (<NUM>), the proximal end (<NUM>) configured for mounting to an aerosol-generating device (<NUM>) for electrical communication with the aerosol-generating device (<NUM>);
wherein the heater element (<NUM>) comprises either or both of:
i) a plurality of surface notches (<NUM>) formed on a surface of the heater element; and
ii) a plurality of sub-surface cavities (<NUM>) defined beneath the surface of the heater element;
in which at least some of the plurality of surface notches (<NUM>) and sub-surface cavities (<NUM>) of the heater element are provided in an inner region (<NUM>) of the heater element, the inner region (<NUM>) extending between the proximal end (<NUM>) and <NUM>% of the length of the heater element relative to the proximal end (<NUM>), and in which at least <NUM>% by volume, or at least <NUM>% by volume, or at least <NUM>% by volume, or at least <NUM>% by volume, or at least <NUM>% by volume of all of the plurality of surface notches (<NUM>) and sub-surface cavities (<NUM>) of the heater element are provided in the inner region (<NUM>).