WO2019137994A1

WO2019137994A1 - A method of printing an electric heating element, and an electric heating element produced thereby

Info

Publication number: WO2019137994A1
Application number: PCT/EP2019/050533
Authority: WO
Inventors: David Lawson; Mark DIGNUM
Original assignee: Project Paradise Limited
Priority date: 2018-01-12
Filing date: 2019-01-10
Publication date: 2019-07-18
Also published as: GB201800548D0

Abstract

The invention provides a stencilled mesh for use in screen print ing apparatus, a method of screen printing using such a stencilled mesh, and a substrate printed as a result of screen printing using said stencilled mesh. The stencilled mesh is elastically deformable and the stencil is fixedly applied thereto. Ideally, the stencilled mesh is larger in cross-sectional area than a substrate to be printed, and the pattern provided therein defines multiple identical void regions through which a fluent printing composition can be forcibly caused to pass during screen printing onto the substrate upper surface and thus each of said discrete multiple identical void regions results in multiple identical corresponding patterns of said fluent printing composition being applied to said substrate. The invention is characterised in that the identical stencil void regions are continuous and comprise at least two distinct portions, a first portion in which is defined at least two adjacent separate elongate void portions substantially rectangular in shape and orientated with their longer edges parallel with the direction of travel of a squeegee disposed within the printing apparatus and adapted to be caused to move over one of the mesh surfaces, and a second portion in which is defined at least one meandering void portion having an entry point and an exit point at which points said meandering void portion is connected to a respective one of said elongate void portions, and further characterised in that said meandering void portion is serpentine in shape and comprises a plurality of essentially adjacent linear parallel parts, and a corresponding plurality of arcuate or angular parts which connect them, said linear parts being orientated substantially parallel with the longer edges of the elongate void portions and thus also with the direction o f travel of said squeegee, and yet further characterised in that the width of any one of the linear parts of a meandering void portion is at least one order of magnitude smaller than the width of both of the elongate void portions to which it is connected.

Description

A Method of Printing an Electric Heating Element, and an Electric Heating Element produced thereby

Field of the Invention

The present invention relates to a method of printing an electrically energisable heating element, and an electric heating element produced thereby. More specifically, the invention relates to a method of manufacturing an electrical heating element comprising a substrate, which may be flexible or rigid, having essentially parallel planar upper and lower surfaces to at least one of wh ich is applied a conductive composition in a predetermined pattern by a screen printing technique. In this regard at least therefore, the present invention may be regarded in its broadest sense as a method of screen printing, and thus extending to the printed articles produced by conducting that method.

Although the present invention is most usefully concerned with a method of manufacturing an electrically energisable heating element for particular use in what have become known as Electronic Nicotine Delivery Systems (ENDS, herein being both singular and plural as required by context) or (more commonly) "e-cigarettes", in particular "active" ENDS wherein a nicotine-containing formulation is heated or otherwise excited from an ambient state so as to aerosolise it prior to inhalation by a user, it should be understood by the skilled reader that the present invention is not limited to this particular field of application, and indeed the present invention may be used to manufacture any surface mounted electrical heating element being required to exhibit particular predetermined heating characteristics.

Background to the Invention

ENDS have been in widespread use now for some years, and although there has been and continues to be little concrete scientific evidence as to how harmful they are to human health, in particular human lungs, it is largely beyond doubt that the use of any ENDS is significantly less harmful than the smoking of combustible tobacco products, such as cigarettes, cigars, cigarillos, pipes, and hand rolling tobacco. The primary reason for the comparative health benefit of ENDS as compared to conventional combustible tobacco products is that the nicotine-containing smoke inhaled by users of the latter contains significant levels of carcinogens and other toxicant products of combustion, whereas the so-called vapour inhaled by users of ENDS consists primarily only of nicotine, and one or more of: glycerol, polyethylene glycol (PEG), vegetable glycerol (VG), and/or propylene glycol (PG), and derivatives of these compounds, together with natural and/or synthetic flavouring compositions often added to the liquid formulations utilised in ENDS.

Of course, in the case of both ENDS and combustible tobacco products, the chemically active substance is nicotine (C10H14N2), a potent parasympathomimetic stimulant and alkaloid. In essence, nicotine is a drug and like many drugs, it is highly addictive to humans. In sufficient concentrations, nicotine is also highly toxic to humans, and although nicotine only constitutes approximately 0.6- 3.0% of the dry weight of tobacco depending on strain, variety and processing techniques, mere ingestion of only one or two cigarettes, in which there might be as much as 50mg of nicotine and possibly more, can cause quite serious toxic reactions. Those skilled in the art will immediately understand therefore that the dose of nicotine administered by an ENDS is of critical importance - in general, the dose must be sufficient to satisfy the physiological cravings experi enced by users addicted to nicotine, but (arguably) less than that which is typically delivered by a corresponding combustible tobacco product in a similar time scale so that the ENDS can be effective, at least partially, in reducing an addict's dependency on the drug and thus function as a smoking cessation aid.

The majority of currently commonly available ENDS are so-called wick-and-coil devices wherein an electrical heating coil is disposed adjacent, around, within or otherwise proximate a moisture absorbent wick such that a nicotine-containing liquid extant within the wick is heated sufficiently rapidly and to a sufficient degree to cause at least some of that liquid and/or one or more of its constituents to be vapourised, atomized, aerosolized or be otherwise promoted from the wick into the surrounding air in a gaseous or quasi-gaseous phase. The wick-and-coil arrangement may take many different forms, but most commonly both said components will be located within a cartridge or reservoir (a so-called "cartomizer", such term being a conflation of the words "cartridge" and "atomizer") which also contains the nicotine-containing liquid which has been or is to be drawn into the wick. Of course, in order for the coil to be heated, a source of electrical powe r is required, and in this regard, often the most dominant component in any modern ENDS is the rechargeable battery which may be either an integral part of the device as a whole, or a removable and/or detachable component thereof, but in any event, the cartomizer, and thus the heating coil is electrically connected to the battery and a simple switch is provided in a convenient location on the device so that the user can selectively apply and remove electrical current to and from the heating coil and essentially activate the device. Often, the wick-and-coil arrangements of modern ENDS are rather crude and rudimentary devices, and little if any detailed experimental or analytical characterisation of their performance or operating parameters has been conducted, or has previously been regarded as necessary. I n essence, the wick-and-coil heaters commonly employed in cu rrent EN DS have been selected largely on the grounds of their low cost and intrinsic simplicity, and their performance, at least as regards precise temperatu re control, has heretofore not been considered as being particu larly important, provided of course that the heating coil can rel atively quickly (e.g. ~ 1 -2s) achieve a desired operating temperatu re in the region of or above the boiling point of the carrier liquid chemical of which the nicotine-containing formulation is primarily comprised. For example, u ncontaminated PEG, with a molecu lar weig ht of around 4000-6000 has a boiling point in the region of 240-260 deg.C, g lycerol boils at around 290 deg.C, and propylene glycol at around 188 deg.C. The actual boiling points of specific formu lations can vary quite significantly depending on the concentration of nicotine, which boils at 247 deg.C, and the various flavourings and other compositions which may be present in the formulation, but as even the simplest coil heaters can easily and quickly achieve operating temperatu res of at least 200 deg.C, and in some cases in excess of 300 deg.C, with an applied electrical potential in the region of 3- 10 Volts, they have little difficulty in heating the formu lation sufficiently quickly to achieve aerosolisation thereof prior to inhalation.

One of the fundamental problems of such rudimentary coil heaters however is that the rapid and largely uncontrolled heating of nicotine-containing formulations (commonly called "e-liquids") can cause pyrolysis of one or more of the chemical compositions present in the formulations, again to a largely unknown and uncontrolled extent. I n the same way that the smoking of a conventional tobacco product such as a cigarette involves a degree of pyrolysis in the vicinity of and immediately behind the burning coal of tobacco within the lit cigarette during inhalation, pyrolysis occurring within EN DS in the vicinity of the coil heater can result in sig nificant decomposition of many of the compositions present within the formulation, such that what is u ltimately inhal ed by the user is a n aerosol containing a very complex mixtu re of compou nds, possibly in a variety of different physical states, .i.e as gases, liquids and possibly solids. When it is also considered that EN DS users will often fill their devices with different e-liquids according to preference and taste and entirely without regard for the nature and rating of the heating coil present within their devices, the reader will u nderstand that it is practically impossible to enforce any measure of control on the a erosol produced by the EN DS and its specific composition, except of course that it will contain an amou nt of nicotine. Although the European U nion have recently introduced the Tobacco Products Directive (TPD, Tobacco and Related Products Regu lations 2016) as a means of implementing some form of standardisation on the natu re and compositions of e-liquid formulations, this legal framework is still nevertheless very broad, and there is little if any detailed scientific analysis or experimental data on the actual composition of aerosols produced by ENDS.

It is therefore a first object of the invention to provide a method of manufacturing an electrically energisable heating element whereby the operating characteristics, particularly the operating temperature, of the heating element thus produced are accurately, repeatedly, and precisely determinable, substantially uniform over the entirety of the heating element, and wherein the heat provided thereby is thus highly localised and concentrated in the vicinity of the element itself. In the context of ENDS, such a heating element will naturally markedly improve the repeatability of the aerosolisation of the nicotine-containing formulation such that the general composition of the aerosol produced by successive activations, particularly as regards both the concentration of nicotine and its physical states within the aerosol, will not only be essentially or largely identical, but also capable of being precisely varied as may be required, depending on the specific composition of the formulation with which the heating element comes into contact or very close proximity.

As mentioned previously, the present invention seeks to provide a method of manufacturing such a heating element by means of screen printing.

Screen printing, has been used for centuries as a means of applying a predetermined pattern of printing ink to a substrate, and although there has been significant development of the technique over time, in essence it is still relatively simple, at least conceptually. Traditionally, a silk screen is stretched over and fixed to a rigid rectangular frame, and a blocking pattern or stencil is applied to the silk screen, either before or (more commonly) after mounting in the frame, which defines the pattern which is desired to be printed on the substrate. There are various methods of creating the stencil, but the most common method is to soak or otherwise coat the screen with a photosensitive chemical emulsion and then to expose the coated screen to a predetermined pattern of light (i.e. a negative of the stencil pattern required) whereby the emulsion is caused to harden, solidify or otherwise selectively bind to the mesh in regions where the light is incident, while other un -exposed regions remain liquid and un-bound to the mesh. After exposure, the mesh is simply washed with a suitable solvent, most commonly water, so as to remove any and all liquid emulsion not otherwise bound to the mesh. Once the silk screen mesh with integral stencil pattern has been created, it is disposed a small distance (of the order of mm) above a substrate to be printed and in fixed parallel relationship therewith, and an amount of printing ink or paste is applied to the upper surface of the screen within the bounds thereof defined by the frame. In order to effect the print, a squeegee usually disposed at an acute angle to the screen, is firstly brought into contact with the screen at one end thereof and forced downwardly to cause some corresponding elastic downward deflectio n of the screen until it comes into contact with the substrate. Thereafter, the squeegee is caused to travel along the screen from one side to the other, effectively squeezing the ink in front of it into the mesh in those regions not blocked by the stencil and thus into wetting contact with the substrate. As the squeegee travels over the screen, the natural elasticity of the screen, and the fact that it has been forced into tension by the downward action of the squeegee, causes the screen to "snap off" or spring away from the substrate immediately behind the squeegee edge and in doing so, ink within the mesh apertures is pulled from them at least to some extent due to the tendency of the ink to adhere to the substrate when in contact therewith.

Important variations and/or considerations in the screen printing technique, at least in the context of the present invention, are as follows:

Modern meshes are usually formed of polymer materials such as polyester and nylon, or of metal or alloy wire, such as Aluminium or Steel (the latter being especially useful for high resolution screen printing);

Meshes have various inherent physical characteristics, such thread diameter, aperture size, the overall density of those apertures (usually expressed as a percentage of a unit area of the mesh), the number of threads per inch/cm, and thickness are all important considerations in screen printing; for example the overall thickness of the mesh, the mesh size (usually expressed in threads per inch/cm), and thread diameter will all generally be contributory factors in determining the thickness of the ink layer deposited on the substrate, while the mesh size provides a ready indication of the maximum resolution achievable (i.e. the maximum resolvable width of features in the printed ink pattern); for example, a mesh size of 325 threads per inch (which equates to an aperture size of 44pm) and having a thread diameter of 1.1 mm is capable of printing a layer of ink 16pm thick; Also, the mesh aperture size is critically important where the ink or paste to be printed contains solid particulate matter or is particularly viscous, because obviously such particulate matter cannot pass through the mesh apertures if it is dimensionally larger than them, and a particularly viscous printing ink or paste will be less inclined to pass freely through small mesh apertures;

Two squeegees may be used, a flood squeegee and an inking squeegee, or single squeegee may perform two separate passes over the screen to effect the flood and inking operations; in the flood operation, the relevant squeegee merely passes over the screen exerting only minimal downward pressu re on it so there is no contact with the su bstrate, and is conducted to flood the mesh apertures un blocked by the stencil with printing in k or paste; thereafter, the return or su bsequent pass of the relevant squeegee elastically downwardly deforms the screen into contact with the substrate, printing of which then occurs;

Squeegee desig n a nd orientation are important (knife edge, trailing edge and diamond point are all well known squeegee tip shapes); the ang le of attack of the squeegee is also important to ensu re consistent resu lts - usually this is an acute ang le of 45 deg. With the substrate in the direction of travel of the squeegee over the substrate surface;

Screen Size, Squeegee length/hardness/pressu re and speed of travel can also be important; according to an article entitled "The Basics of Screen Printing Thick Film I n ks" by DuPont®, screen width should be 2-3 times the length of the squeegee, screen length shou ld allow 5-8 cm distance both before and after the squeegee travel, squeegee length shou ld provide a 1 -2 cm excess beyond the stencil patterned area, on each end, and the squeegee shou ld d rop down at least 3 cm prior to the patterned area, and not lift u ntil at least 3 cm past it; in terms of squeegee pressu re, as this is increased the screen m esh is increasingly compressed against the su bstrate and so a thinner layer is deposited on the substrate as the in k or paste may, to a certain extent, be scooped out of the mesh apertures in which it resides; in general, it is essential that the squeegee pressure remains constant through the entire length of travel over the screen; this does of course depend to some extent on the surface of the su bstrate and whether the substrate is level; a value of around 0.2 kg per centimetre width of the squeegee is typical, and good in k/paste layer thickness u niformity on the substrate can be achieved with squeegee pressu res of 0.3 or 0.4 kg per centimetre width;

squeegee travel speeds of in the range 50-250 mm per second cover most scenarios; Snap-off or screen gap (the distance of the un-stressed screen from the substrate): this is a critical parameter in screen printing; As the screen gap increases so more of the available pressu re applied to the squeegee is used to deflect the screen and bring it in contact with the substrate; thus the use of a large screen gap has the same effect on print thickness as the use of low squeegee pressu re;

Classically, the screen gap is set just big enough so that the screen peels away from the substrate immediately behind the squeegee as it travels over the screen surface; If the gap is too small the screen will remain stuck to the substrate surface. I ncreasing the screen gap effectively decreases the distance behind the squeegee at which it will begin to peel away from the su bstrate; althoug h the historic name of "snap off" distance is used, if the elastic recovery of the screen is such that an audible "snapping" sou nd is heard (hence the name), this is highly undesirable, and the screen gap must be increased s lightly to avoid this; A good empirical guide for screen gap setting of a steel screen is (0.004 x screen width), for polyester (0.006 x screen width).

Ambient temperature, pressure and humidity can also play a role achieving consistent and reliable screen prints.

As the skilled reader may appreciate from the above, screen printing can be a difficult and sometimes largely experimental process to perfect, but nevertheless screen printing machines are commercially available and are capable of achieving reliable and consistent results. Furthermore, although screen printing by its very nature does not lend itself particularly well to the high output speeds achievable by other commercial printing processes such as web-fed flexography and gravure, exceedingly high accuracy and thus quality can be achieved by screen printing, and it is economical at much lower volumes than those required for traditionally higher volume processes. For these reasons, screen printing has been considered, and indeed is currently commercially employed for the so-called "thick film" screen printing of electronic components, where the viscosity of the conductive inks and pastes utilised is such that it would be difficult to employ any other printing technique, and the print runs are l imited to the 10s or low hundreds of units.

Although screen printing has heretofore been considered for the printing of resistive heating elements (see for example components from manufacturers such as Watlow, National Plastic Heater, Sensor & Control Inc., Sensor and Control Inc., Zoppas Industries & Fepa), screen printing conductive inks and pastes on chemically inert substrates such as soda -, silica-, aluminium-, aluminosilicate-, lime- and/or borosilicate-based glasses which exhibit little or no absorbency as regards the printing ink or paste has proven exceedingly difficult to achieve reliably and consistently, especially when the pattern of conductive ink or paste to be printed includes or comprises artefacts, elements or other features of a size < 1 mm.

In the context of ENDS, applicants herefor have realised that the wick-and-coil heaters currently forming and integral and irreplaceable permanent part thereof might usefully be replaced with a disposable, interchangeable resistive heating element which is pre-dosed with an accurately measured amount of a nicotine-containing formulation. This approach is quite radical as regards conventional ENDS design, but does offer a number of important advantages, in particular as regards the dosing precision of nicotine which can be achieved. For example, in conventional ENDS, typical e-liquids contain only relatively low concentrations of nicotine (e.g. 6-20mg/ml), and the vast majority of the heat energy generated by the rudimentary wick-and-coil heaters during activation is devoted to aerosolising, in a rather crude and imprecise fashion, a relatively very large volume of the carrier compound, e.g. PG and or VG. In sharp contrast, a heating element whose temperature characteristics can be much more precisely and repeatably controlled, but which is nevertheless an essentially disposable, low-cost item, can be pre-dosed with a comparatively lesser amount of a nicotine-containing formulation in which the nicotine concentration is proportionately much greater than in conventional e-liquids, so that over each and any heating cycle, the aerosol produced can contain a similar (or, if desired, a slightly greater or lesser) quantity of nicotine to that produced within a conventional ENDS. As the skil led reader will readily understand, the rationale behind providing a disposable, but nevertheless far more precisely controllable heating element which is accurately pre-dosed is that, after 6-8 activations (i.e. inhalations, to mimic the typical number of inhalations performed by a cigarette smoker on each individual cigarette smoked), substantially all the nicotine within the formulation dosed onto the heater will have been aerosolised, and the heater (and any cartridge it is present in) can be considered as spent, and can be replaced afresh. Thus the present invention has as a further object the manufacture of such a heating element.

Summary of the Invention

According to the present invention there is provided a method of screen printing a substantially rigid substrate material with a fluent composition which is or can be rendered electrically conductive, said substrate having essentially planar and parallel upper and lower surfaces, the former of which is to be printed, said method comprising the steps of

Depositing an amount of the fluent composition on an upper surface of an elastically deformable mesh to which a stencil of predetermined pattern has been fixedly applied and within which are defined multiple identical void regions through which said composition can be forcibly caused to pass onto the substrate upper surface during printing,

Disposing the stencilled mesh in close proximate and parallel relationship over and vertically above the substrate so as to define a screen gap of the order of 3mm therebetween,

Bringing a printing squeegee having a resiliently deformable printing edge, being parallel with the plane of the upper surface of the substrate, into contact with the upper surface of the mesh so as to downwardly elastically deform said mesh by a distance greater than the screen gap, and then dragging said squeegee over the mesh and in turn over the substrate, such motion causing both

- the forced filling of the said void regions with said fluent composition such that there is wetting contact with the u nderlying substrate immediately in advance of, or u nderneath the moving squeegee printing edge, and

- the subsequent release of said fluent composition from within said stencil voi d regions onto the upper surface of the su bstrate immediately behind the moving squeegee printing edge,

Characterised in that each identical stencil void region is continuous and comprises at least two distinct parts, a first part in which is defined at least two adjacent separate void portions, and a second part in which is defined at least one patterned void portion into which and away from which a respective one of said adjacent separate void portions extend, the total area of the adjacent separate void portions being at least one order of magnitude g reater than the total area of the patterned void portion, a nd further characterised in that the lateral dimension, being that dimension transverse to the direction of travel of the squeegee, of the adjacent separate void portions is at least one order of mag nitude greater than any corresponding lateral dimension of any element of patterned void portion.

Preferably, the patterned void portion is meandering or spiral in appearance. I n the former case, the meandering void portion is preferably serpentine in natu re and comprises a plurality of essentially adjacent, linear and parallel elements, and a corresponding plurality of arcuate or angular elements which connect them. Most preferably, said linear elements are aligned substantially parallel with the direction of travel of the squeegee. Also preferably, the relatively much large adjacent separate void portions are elongate, and preferably such elongation is in substantially parallel alignment with the direction of travel of the squeegee. Most preferably, the adjacent separate void portions are rectangu lar. I n the case where the patterned void portion is spiral in appearance, the patter is similar to electric heating elements common ly found in domestic appliances such as kettles and electric hobs).

Preferably, the squeegee is provided with a resiliently deformable printing edge, preferably alig ned in parallel relationship with the plane of the upper su rface of the substrate, and retained in such aligned as the squeegee travels over the su bstrate to effect printing thereof.

Most preferably, the arrangement of the mu ltiple identical void regions within the stencilled mesh is such that their first parts are disposed more proximately to the initial posit ion of the in king squeegee before it is caused to travel over the stencilled mesh and effect printing thereof, the second parts of each void region which include the patterned void portions being disposed rearwardly thereof as regards the direction of travel of said inking squeegee such that the inking squeegee forces the fluent composition through the first part voids before the meandering void portions of the second parts, the result being that the elongate portions of the printed substrate are screen printed before the patterned heating element portions. In an alternative preferred embodiment, the stencilled mesh may be rotated through 180°such that the meandering heating element portions are printed before the elongate contact portions.

Thus by providing a stencilled mesh wherein each of the distinct but interconnected parts of the multiple identical void regions are substantially aligned and parallel with the direction of travel of the squeegee, this being the print direction, applicants herefor have found that it is possible not only to achieve reasonably consistent, repeatable and reliable printing of one or more substrates, but also that the resulting printed substrates are provided simultaneously with both (at least) a pair of elongate electrical contact portions as well as a significantly more electrically resistive heating element portion connected between them. Furthermore, and particularly in the case where the patterned void portion meanders, by printing both portions in a single pass, and in the manner described, not only is the layer thickness of both contact and heating element portions substantially uniform, but also it is possible to consistently print the relatively much narrower heating element portion on the substrate because the vast majority of the pattern so printed is completely aligned with the print direction. By printing a heating element in a serpentine pattern which is comprised essentially only of a series of very densely and adjacently disposed narrow linear elements whic h are nevertheless all reliably connected together at their respective ends and also to the contact portions, applicants herefor have also found that the operating characteristics, in particular as regards temperature and rate of change thereof, are extremely predictable and precisely controllable. Yet further, printing such a discrete heating element in the manner described and having an exceedingly densely arranged series of linear parallel conductors also results in extremely localised heating in the region of the element, and directly above and below it, and although there will of course be some conduction of heat through and within the substrate to peripheral regions thereof not directly underneath the heating element, the linear parallel conductors of the heating element still possess a width which is significantly greater than their thickness, and thus heat radiation away from the sides of each of the individual conductors forming the element is very much reduced. Overall, the result is an electrica lly resistive heating element which always performs within very tight tolerances, as desired. Preferably, the length of any adjacent linear parallel part of the meandering void portion is one of: substantially, the same length as the longer edges of the elongate void portions, of the order of two thirds the length of said longer edges, of the order of half the length of said longer edges, of the order of one third the length of said longer edges, and of the order of one quarter t he length of said longer edges. I n a most preferred arrangement, length of any adjacent linear parallel part of the meandering void portion is of the order of one half the length of said longer edges, and of the order of 6-9mm, most preferably 7mm in length, and the length of said longer edges is of the order of 12-18, most preferably 14mm.

Further preferably the width of any elongate void portion is of the order of 2 -4mm, preferably 3mm, and the width of each and all adjacent linearly (or spirally) parallel parts of the meandering void portion is of the order of 100-300pm, most preferably 200pm. Preferably, the width of the intervening land separating any two adjacent linearly (or spirally) parallel parts of the meandering void portion is of the order of 80-150 pm, most preferably 100 pm. Preferably, the width of the intervening land separating any two adjacent elongate void portions is of the order of 350 -600 pm, most preferably 450 pm.

Preferably, the substrate is substantially rigid and constituted predominantly or exclusively of one or more of the following materials: soda-, silica-, alu miniu m-, aluminosilicate-, lime- and borosilicate-based g lass. In a preferred arrangement, the substrate is a tin float soda l ime glass being 0.5 mm in thickness. I n a most preferred arrangement, the stencilled mesh defines between 20 and 500 multiple identical void regions such that a sing le printing of a substrate (or most preferably a corresponding nu mber of individual appropri ately seized substrates disposed in precise registration with each of said void regions) results in that nu mber of patterns of fluent composition being printed onto said substrate (or substrates).

Preferably the fluent composition is a conductive printin g ink or paste which is in herently conductive, and the method further preferably includes the post -printing steps of drying the substrate (or su bstrates) immediately after the removal from the screen printing apparatus, and then su bsequently firing said printed dried substrate (or substrates) in a kiln. Preferably the drying time is of the order of 10— 15 min at a temperature in the range 100-1 50 deg.C, whereas the kiln firing time is of the order of 1 5 -80min at a temperature in excess of 300 deg.C, and most preferably at a temperature in excess of 500 deg.C, firing being conducted at or above such temperatures for a period of no less than 15min. In the case where the method is used to print multiple individual substrates, each of the substrates is of identical size, most preferably being of the order of 0.35 -0.7mm thick (most preferably 0.5mm), and of the order of 7-15mm wide (most preferably 10mm), and of the order of 15 -30mm long (most preferably 21.5mm). For the avoidance of doubt, and in accordance with the invention, such substrates would all be disposed in identical orientation with a mounting frame or printing bed of a screen printing apparatus such that their longest edges were aligned with the direction of travel which the squeegee is forced to take during the screen printing process. Therefore, the longest edges of such substrates would also be substantially aligned and parallel with the longest edges of substantially elongate void portions and the adjacent linear parallel elements of the meandering void portion of the void regions of the stencil. This of course results in the printed pattern of ink or paste being essentially and substantially completely parallel with the longer edge of said substrate.

Preferably the size of each and substantially a ll the individual apertures within the mesh is selected to be larger than the size of the average size of the particles of any particulate material present within the fluent composition to be printed. It is worth mentioning here that often, conductive inks or pastes comprise particles of a conductive metal, alloy or other material to render them electrically conductive, and it is therefore of course critical that such particles are capable of passing through mesh apertures during printing, otherwise the mesh would act as a filter and the conductive material would be effectively filtered by the screen during printing such that the electrical conductivity of the material ultimately deposited on the substrate would be severely compromised or possibly even eliminated completely, rendering the printed substrate useless as a resistive heating element.

In terms of the mesh used, the following table provides a ready indication of the most common ly available mesh screen sizes and the corresponding dimensions of one edge of the approximately square apertures defined within them:

For the present invention, is preferable that mesh size has a US mesh size in the range 120-450, and most preferably a US mesh size of 325 is employed, as this results in a printed layer of ink or paste having a thickness of the order of 10s of microns, most preferably between 20 -40 and microns thick. Despite these preferences, it is worth mentioning here that the mesh size, and the desired or requisite final thickness of the printed ink or paste is dependent to some degree on (at least) the viscosity and resistivity of the ink or paste used. Thus the particu lar printing ink or paste used is a determinant factor in the manufactu ring process, and can affect the respective dimensions, particularly the thickness dimensions, of the conductive heating element being printed.

I n a preferred arrangement, a first part of each of the multiple identical void regions within the stencilled mesh comprises at least th ree adjacent separate void portions, and in the second part, at least two adjacent patterned (preferably meandering) void portions is provided, a first and a third one of said adjacent separate void portions extend ing into one end of respective first and second patterned void portions, each of which subsequently emerges and extends into a second one of said adjacent separate void portions intervening the first and the third. Such a pattern results in a printed su bstrate in which there is provided two laterally separated heating elements connected between three contact portions.

I n a most preferred arrangement, a first part of each of the multiple identical void regions within the stencilled mesh comprises at least five adjacent separate void portions, and in the second part, at least four patterned (preferably meandering) void portions is provided, preferably arranged in two adjacent pairs, a first and a fifth one of said adjacent separate void portions extending into one end of respective first and fourth patterned void portions, the alternate ends of which emerge and extend into respective second and fourth adjacent separate void portions intervening the first and the fifth adjacent separate void portions, with the final third one of said adjacent separate void portions, preferably intervening one or other or both of the first and fifth, and the second and fou rth, extending into respective ends of the second and third patterned void portions respectively. Such a pattern resu lts in a printed su bstrate on which there are arranged four separate heating elements, preferably arranged in pairs longitudinally (of the substrate) spaced apart and connected between five distinct and separate elongate contact portions.

Most preferably, the total resistance of the conductive material printed on a su bstrate after drying and firing, measured between any two adjacent contact portions from the free ends (remote from the printed heating elements to which they are connected) is in the range 0.8W < R < 15W, most preferably in the range 1 -5W, and most preferably of the order of 4.5W ± 0.5 W. Due to the relative lengths and cross-sectional areas of the printed conductive material in the contact portions and the heating element portions of the printed substrate, the vast majority of the total resistance is attributable to the heating element portion because, althoug h having a much greater overall length (after summing all the adjacent lengths of the serpentine pattern), its width is at least one order of magnitude less than that of the width of a corresponding contact portion, and therefore the resistance provided by the latter is at least one order of magnitude less. Most preferably, the resistance of each contact portion in isolation is of the order of 0.03Q-0.08Q, whereas the resistance offered by the heating element portion in isolation is of the order of 4.07 Q - 4.97 Q. Of course both portions carry the same current during energisation, and because the current density is at least one order of magnitude greater in the heating element portion as compared to the contact portion, and the resistance offered by the former is so much greater than the latter, the heating element portion is prone to very rapid rise in temperature immediately after becoming energised, whereas there is practically no temperature rise in the contact portion whatsoever. As mentioned, previously, the resistivity of the conductive printing ink or paste used will of course directly influence the abovementioned resistances, and in turn the requisite printing thicknesses, but those skilled in the art will appreciate that appropriate selections can be made according to ultimately application and requirements.

One particular and surprising advantage of the invention not immediately apparent from the above is that the printed resistive element can function both as a heating element and also inherently as a fuse capable of blowing if the current flowing through it exceeds some predetermined threshold value, which is determined by: the specific dimensions of the respective elements within the patterned printed conductor, the printing ink/paste of which it is co nstituted, and (albeit to a much lesser extent) the heat dissipation and insulating characteristics of the substrate material on which it is printed. Thus in some way the heating element can be envisioned and being self-regulating to some extent. For example , if excess energy is applied to the printed conductor, which results in a fusing of the resistive heating element (and thus an electrical discontinuity therein), utilisation of any ENDS device in which the conductor is installed is immediately and perm anently interrupted thus preventing any further aerosolisation of any nicotine-containing formulation extant on the surface of the heating element.

In different and further aspects of the present invention, there is also provided a stencilled mesh for screen printing as described above, and also a printed substrate resulting from the carrying out of the screen printing process described.

A specific embodiment of the invention is now described by way of example and with reference to the accompanying drawings wherein.

Brief Description of the Drawings Figures 1 -3 schematically depict the fundamental components used in conventional (prior art) screen printing, and how they combine to effect the printing of a substrate,

Figure 4 shows a greatly magnified plan view of a substrate printed according to the invention showing a printed conductor pattern having two distinct portions,

Figures 5 & 5A provide a schematic perspective view of the printed substrate of Figure 4, and an enlarged perspective view of one of the distinct portions thereof respectively and showing preferred dimensions of both substrate and the conductor pattern printed thereon,

Figure 5B provides a representative electrical circuit for the conductive pattern printed on the substrate,

Figure 6 provides a microscopic photograph of the resistive heating element distinct portion of the printed substrate, enlarged by a factor of approximately x20,

Figure 7 shows one possible pattern of comprising 20 identical void regions arranged in 4 rows of 5, this being the pattern of identical void regions appearing in or being formed within a stencil according to the present invention and which is employed in the screen printing method according to the present invention, and

Figure 8 shows a (much enlarged) single row of the identical void regions of Figure 7, depicted orthogonally to the orientation thereof in Figure 7.

Detailed Description

Referring firstly to Figures 1 -3, the essential components required for conventional screen printing are general indicated at 2 and comprise a usually square or rectangular screen frame 4 to an underside of which is affixed a screen mesh 6 under slight tension and in such a manner as to provide an effectively planar mesh upper surface which elastically reacts to any slight transverse pressure applied to - thus the screen can be considered, at least to some extent, to be slightly springy (in the z-direction, as defined by axes 20). The mesh itself will consist of a plurality of interwoven threads of a particular polymer, metal or alloy, the warp and weft being usually perpendicularly orientated with respect to one another so as to define essentially square apertures throughout the mesh. In some screen printing arrangements, the thread warp and weft directions are parallel and perpendicular to respective frame edges, and in other (more preferred) arrangements, the thread warp and weft lie at an angle to respective frame edges, for exam pie each being inclined by 45 degrees, or one being inclined at 30 degrees and the other at 60 degrees.

Although a steel mesh is most preferred for the present invention, the use of alternate material meshes is also contemplated, in particular the use of so-called "V-mesh" or "V-screen", being a mesh woven with a thermotropic liquid crystal polyarylate thread available under the trade name "VECRY". Monofilament VECRY thread has been found to be exceptionally useful in very high resolution screen printing applications.

Once a particular mesh orientation has been determined, a stencil pattern is applied to the mesh. This is most commonly achieved by uniformly coating one side (usually the side opposite to that which will ultimately receive ink or paste) with a photochemically active liquid emulsion, and then exposing the completely coated mesh to light of suitable power and wavelength and in a pattern corresponding to the inverse of the pattern ultimately desired to be printed such that areas of emulsion exposed to such light harden and become essentially firmly bonded to the mesh. Thereafter, the mesh is washed with a suitable solvent and unbonded emulsion is simply washed from the mesh to provide a stencil thereon and therein and in which desired void regions 7 are defined. In Figures 1 -3, this stencil is depicted at 8 as a separate layer on the underside of the mesh 6, but practically the hardened emulsion of which it is formed is much more an integral part of the mesh in that, when liquid, the emulsion wi ll permeate into the apertures of the mesh and will surround and engulf all the individual threads thereof. Therefore in practice, the stencil is more formed within the mesh than to one or other side of it, but creating the stencil in the manner described will nevertheless slightly increase the base thickness of the mesh as compared to its thickness before the stencil is applied.

Beneath the mesh is disposed a substrate 10 to be printed which is most commonly secured in a substrate holder, workpiece, or nest 12, often in assisted fashion by means of a vacuum (not shown) and/or rigid but movable frame members (not shown) which serve to clamp the substrate firmly in place with respect to the nest, and furthermore prevent the substrate (or multiple substrates, if many are to be printed simultaneously) from being displaced over the surface of the nest during printing. In the context of the present invention, and for screen printing generally, it is very important that the upper surface(s) of the substrate(s) 10 and the stencilled mesh be both parallel and horizontal, and be separated, as most clearly shown in Figure 1 , by a screen gap 14 which should of course be identical over the entire surface of the substrate prior to printing. As mentioned previously, the screen gap distance is an important parameter in screen prin t, especially when printing very small features of the order of 10s or the low 100s of microns in width, as is the case in the present invention.

Immediately prior to printing, an amount of a printing ink or paste 16 is deposited over the upper surface of the stencilled mesh. In Figure 1, this amount is shown as having bee n applied over an area of the mesh which is greater than the area of the substrate(s) to be printed. In practice, this application is often achieved by first depositing an approximately linear slug of ink 16A parallel to and proximate one side of the mesh frame 4, and allowing this slug of ink to flow under it's own weight and thus settle in place on the upper surface of the stencilled mesh (many viscous printing inks and pastes are often specified with a levelling time of the order of a 1 -10mins). Thereafter, in order that a substantially uniform thickness of printing ink or paste is achieved over the upper surface of the mesh, a flood squeegee shown dotted at 18A is caused to barely kiss the upper surface of the mesh immediately behind the slug 16A and the n drawn over the surface of the mesh towards the opposite side thereof as shown at 20A. This action causes the slug of ink or paste 16A to be spread over the upper surface of the mesh so that it assumes, possibly after being again allowed to settle, the layer-like profile shown at 16, which is of substantially uniform thickness over at least an area corresponding to that of the underlying substrate(s) to be printed. Although the flood squeegee and the spreading function it performs are not essential for screen printing low resolution patterns, in the present invention such flooding is considerably more important (although it is considered that it may be possible to omit the flooding step) because first providing a substantially uniform thickness of printing ink or paste contributes significantly to the overall reliability and consistency of the screen process.

After application of an amount of ink or paste to the upper surface of the mesh, a print squeegee 18 is moved downwardly from the position shown in Figure 1 in which it is removed slightly from the upper surface of the mesh into contacting relationship with the mesh and to an extent where the mesh is slightly elastically deformed downwardly by an distance which is of the order of 10s or 100s of microns greater than the screen gap at the point of contact of the squeegee edge with the upper surface of the mesh, as shown in Figure 1 by dotted lines representing the squeegee and mesh 16B and 6B respectively.

After this, the squeegee is then drawn (most p referably by a pulling or dragging force) over the upper surface of the mesh and thus over the entirety of the upper surface of the substrate(s) being printed. An example positon of the squeegee during its travel is shown in Figure 2. As previously noted above, there are various factors which are important in the screen printing process, many of which are interrelated, but the essence of the screen printing process is that the squeegee, and more particularly its resiliently deformable printing edge effectively both depresses the stencilled mesh into contact with the substrate upper surface(s) and also performs a squeezing function on the amount of ink or paste disposed immediately in front of it as regards its direction of travel (an amount which continuously increases as the squeegee travels) such that the ink or paste is forced into the voids in the stencil pattern such that when the ink/paste filled voids pass underneath the printing edge of the squeegee, ink/paste within them contacts the upper surface of the substrate and is pulled from within said voids by virtue of both

- the tendency of the ink/paste to adhere more to the substrate upper surface than to itself and/or the hardened emulision which defines the stencil voids, and

- the tendency of the mesh to spring away from the upper surface of the substrate immediately behind the moving squeegee as a result of the elastic tension being applied by said squeegee printing edge.

Thus, in Figure 2, it can be seen that amounts 22 of ink have been deposited on regions of the substrate upper surface lying behind the squeegee after having travelled beyond such regions.

Referring finally to Figure 3, which depicts the position of the various components after a single complete pass of the print squeegee 18, it can be seen that the upper surface of the substrate 10 has been completely printed with the desired pattern which is a direct replica of the pattern of voids 7 provided in the stencilled mesh. Also in Figure 3, after having completed its single pass over the mesh, the squeegee 18 is moved upwardly and away from the mesh, which can naturally , elastically and thus completely return (importantly, if the mesh is to be re-used, without any residual plastic deformation) to its original planar shape and orientation.

Referring now to Figure 4, there is shown a plan view of a single substrate indicated generally at 40 and having been printed in accordance with the present invention with a single continuous pattern (shown hatched) of an electrically conductive material and indicated generally at 42 and which is usefully separated into two distinct portions 44, 46 lying on either side of a notional dividing line shown in dotted at 45. The first portion 44 comprises three separate contact portions 44A, 44B, 44C, whereas the second portion 46 comprises two separate areas 46A, 46B each consisting of a plurality of adjacent substantially linear parallel parts, some of which are referenced at 46C. In order to print such a pattern, or most preferably, multiple substrates with identical repeating patterns, an appropriately stencilled mesh is provided in which a multitude of such patterns are provided.

As can be seen in Figure 4, each of the contact portions 44A, B, C is elongate in that their dimension in the direction of travel of the squeegee is much greater than their width dimension in a direction lateral to that of the squeegee direction of travel. Furthermore, the length of such portio ns occupies a majority of the overall length of the printed substrate 40, most preferably two thirds of that length, whereas the width dimension of each of said contact portions is approximately equal and is of the order of one third of the overall width of the substrate. In terms of the second distinct printed portion 46, the width dimension of each and every one of the adjacent parallel linear parts 46C is equal and at least one order of magnitude less than the width dimension of any respective contact portion 44A, 44B, 44C with which it is directly or indirectly (for example by means of one or more adjacent parallel linear parts) connected. As can also be seen from the Figure, areas 46A, 46B each comprises 10 individual linear parallel parts 46C which are each connected to respective adjacent linear parallel parts at their distal ends so that said adjacent parallel linear parts together form an essentially meandering serpentine pattern of individual conductors in each of areas 46A, 46B, whereas area 44 comprises only three single very much larger conductors which form electrical contacts and by means of which an electrical current can be easily applied (and importantly without any great lateral precision) to the printed conductive pattern as a whole.

Importantly in the context of the present invention, the orientation of the corresponding patterns of voids in the stencilled mesh is aligned with the direction of travel 20 of the printing squeegee 18, as schematically depicted in Figure 4. Thus the squeegee direction of travel 18 is parallel with the longer edges of the contacts and also with each and all of the adjacent linear parallel parts 46C, which are additionally parallel with each other. For a further enlarged depiction of the adjacent linear parallel parts 46C, the reader should refer to Figure 6. Also, although the single substrate 40 of Figure 4 is shown as having only three contact portions 44A-C and 2 intervening and interconnected patterned areas 46A, 46B, alternative arrangements are possible, i n particular a substrate (not shown) on which is printed 5 contact portions and 4 similarly intervening and interconnected patterned areas, preferably arranged either adjacently in a single row, or yet more preferably, arranged in 2 rows, each row containi ng a pair of adjacent patterned areas.

In Figures 5 8i 5A, the various preferred (approximate) dimensions of a single printed substrate, including the dimensions of the printed areas thereof, are provided. Referring briefly to Figure 5B, it is to be noted that the printed conductive pattern applied to the substrate may usefully be approximated by a representative electrical circuit in which the contact portions 44A-C have electrical resistance Rc and the resistive heating element portions 46A, 46B have electrical resistance RE. In light of the fact that the width of each linear parallel adjacent conductor part in the portions 46A, 46B are at least one order of magnitude smaller than the contact portions, it follows immediately that the electrical resistance R_E will be of the order of one order of magnitude greater than that of Rc , from the basic formula resistance R = rί/A where R is resistance in Ohms, ί is the length of a conductor, A is the cross-sectional area of that conductor in m², and p is the resistivity in Q.m. Of course, one of the important benefits and advantages of screen printing conductive patterns as described is that the thickness of the printed ink or paste is essentially uniform over the entire printed area, and where an appropriate mesh dimension is used, a printed layer thickness of the order of 15 -30 pm can be achieved.

Assuming a resistivity value for the conductive printing ink/paste p of 4 x 10 ⁷ Q.m, and a printed layer thickness of 20 x 10 ⁶m (20 microns), and an approximate total length (include one end arcuate portion) of one of the 10 adjacent linear parallel parts that exist within each patterned area 46A of 6.4mm, this gives:

RE = (2 x TO ⁷ x 10 x (6.4 x W^~3))

(20 x W^~6 x WO x W ⁶)

Or

RE =3.2 W

Performing a similar calculation for Rc gives Rc = 0.05 W.

This theoretical calculation matches well with observed experimental data on physical samples, where 2.5 Q < (RE &2R_C) < 10 Q. As previously mentioned, resistance of up to 20 Ohms are contemplated, but in the context of ENDS, a greater overall resistance of the heater element would generally necessitate a larger more powerful battery to achieve the same heating characteristics, and therefore it generally desired to keep the resistance of heating elements manufactured according to the present invention quite low, for example less than 10 Q, and most preferably 4.5Q ±0.5 Q. At this level, a battery capable of delivering in the region of 7.2Volts is most preferred. In an alternative embodiment, where the R_E is reduced to approx.. 1 Ohm, then a battery capable of delivering only 3.7V may be required. Thus the total electrical resistance provided by the conductive pattern between the free ends of any two adjacent contact portions 44A-44C is approximately 3.3 Q, but of course the contact portions themselves offer practically no resistance whatsoever, at least compared that offered by the meandering serpentine patterned conductor. Fu rthermore, critically, because the contact portions 44A, 44B 44C and the intervening portions 46A, 46B are effectively connected in series, the same electrical cu rrent flows throug h each portion when a source of electrical power is con nected to any two adjacent contact portions, but the current density flowing within portions 46A, 46B is at least one order of mag nitude greater than that flowing within the corresponding contact portion, and this is immediately manifested as heat in that the portions 46A, 46 B immediately heat up to a working temperature, which with careful design, can be selected as desired and as required to cause aerosolisation of any modern e -liquid.

I n Figu re 7, there is shown one possible stencil pattern 70 which may be provided within the mesh of a suitable screen, said pattern comprising multiple individual, distinct and separate void patterns 72, 5 of those patterns being referenced in a rearmost row 74 of the overall pattern, as defined according to the inking squeegee 18 and its in itial in king or printing direction of travel 2. Other rows 76, 78, 80 of similarly identical void patterns are also referenced. Single row 74 of said void patterns is shown en large and rotated in Figure 8, in which it can be seen that the perimeter of a first (or last, depending on perspective) pattern 74 can be notionally separated into two distinct regions 72A, 72B, the first 72A of which consists of th ree relatively broad elongate void regions 72A' which serve to form the printed contact portions on the substrate underlying the stencilled mesh after printing, and two meandering void portions 72B' which, being connected to the former voids 72A' at their respective entry and exit points, are of course merely extensions of said portions 72A', and which serve to form the meandering resistive heating element when printed. Thus, any single void 72 is essentially continuous and u ninterrupted throug hout its entire length, such that when a fluent printing composition is caused to pass th rough it du ring printing, th e resu lting printed pattern of that composition is similarly continuous and thus electrically conductive throug hout its entire length, i.e. from the extremity of one contact portion to the extremity of both the other two contact portions thus printed.

A specific experimental protocol is now provided which provides some further details of the current manner in which su bstrates are screen printed. It shou ld be noted that this experimental protocol is directed towards the printing of 20 separate identical an d individual g lass substrates in a sing le pass of a print squeegee within the screen printing machine. However, it should be mentioned that a modified production protocol mig ht just as easily (and with potentially better resu lts) print a single large g lass substrate with a mu ltitude of patterns, and then afterwards that large glass substrate may be cut into much smaller sections, each having a printed thereon the desired electrical conductor pattern.

There are several steps which must be followed to ensure successful manufacturing of printed glass substrates. The most important aspects of the production process are:

1. Substrate Preparation - Maximising the surface energy and u niformity of the g lass substrates while removing contaminates which reduce screen printing yield

2. Printer Setup - Ensuring that the screen printer has been set up with the appropriate settings to ensure a level and even application of ink, and an appropriate level of snap -off to improve print clarity.

3. Production - The process by which in k is applied using a hig h level of precision to the glass substrate, followed by a drying and firing process to cure and harden the ink print.

4. I nspection - The post-hoc inspection process to document the success of the consumable production, verify and record resistance values, and perform a visual inspection to enable future process optimisation.

SU BSTRATE PREPARATION: It is essential to clean the su bstrate to remove any and all contaminants from the surface of the glass to ensu ring uniform su rface energy to improve ink adhesion and clarity of screen print.

PRI NTER SETU P: A printing frame is loaded in the printing machine and is levelled according to known machine protocols. A pressu rised air su pply (5 bar) is connected to a printer air inlet and a vacuu m line then connects the printer to substrate jig. I PA and a lint free cloth are used to wipe down both sides of the screen, both sides of the Mylar Frame, and Jig, until the cloth comes away clean.

Once the screen printing machine is tu rned on, it can be provided with the following settings: Squeegee pressure Front (print) Squeegee = 05.0 Kg

Rear (flood) Squeegee = 00.0 Kg

Squeegee speed Print carriage fwd = 70 mm/sec

Print carriage rev = 20 mm/sec

Print gap 1 25mm

I NSTALLI NG SQU EEG EES

Squeegeees shou ld be thoroug hly cleaned and dry before installation in the squeegee brackets within the printing machine; the squeegee printing edge shou ld be completely parallel with the bracket u pper edge, but the machine allows calibration to ensure correct alig nment & orientation; CALI BRATION

To achieve consistent, reliable and accurate printing of substrates, various calibration steps are required, and such calibrations will ensu re, among other things, a consistent position and orientation at which the workpiece is in contact with the screen when the substrate holder rises into its print position. For reasons already mentioned, it is critical that the substrate holder (nest) is correctly positioned and orienatated with respect to both the mesh screen itself, the squeegee printing edges. Also, the squeegees must be similarly calibrated to ensure correct squeegee flood and printing pressures are achieved.

PRODUCTION PRINTI NG:

20 clean, sterile d ry glass substrates are removed from a storage container, lig htly polished, and placed into the jig on the screen printer.

Screen printing in k (many conductive screen -printing inks and pastes are commercially available, ideally with their conductivity being provided by particu late silver dispersed th roughout the in k/paste) is then stirred and a slug thereof applied to an uppe rmost surface of the screen (at a position closest to squeegees) and spread over the screen to an extent such that the slug of applied liquid is wider than the squeegees; it can also help to apply some ink to the leading edges of the squeegees, one or both of print and flood. Thereafter, the ink is allowed to settle for a period of time, e.g. 1 - 10minutes.

Either immediately after or before the previous step, the vacuu m pressure may be activated so that the substrates are firmly clamped in position on the su bstrate holder and within the jig, which is then caused to travel completely into the screen printer machine and into position, after which the jig is broug ht into the correct position with regard to the squeegee printing edge, and printing commences. Du ring printing, ideally the ink is evenly spread over the screen, which peels away (or "snaps off" evenly after the squeegee passes over it.

Once the print is complete, the jig returns to its original position and can be inspected. If the majority of substrates (or the su bstrate, where only a single su bstrate is printed) appear correctly printed with the various patterns of conductive in k or paste, they can then be dried and fired in a kiln to permanently affix the printed conductor patterns to the substrate su rface. Ideally, the substrate should be dried on a ceramic plate in an oven set to 125degC for 15 minutes drying time, and then subsequently transferred from the ceramic plate to a steel mesh and then fired in a Carbolite Furnace set to ramp at maximum rate up to 625degC, hold for 10 minutes, then cool as rapidly as possible. The ink guidelines (available from Electroscience Limited) recommend that the total cycle take 1 hour, so after the heater switches off when the hold tim e has been reached, it is possible to accelerate the cool-down by opening the door of the furnace occasionally after the furnace has cooled past 400degC (which should prevent thermal shock).

After heating & firing, the substrates can be then be appropriately and precisely dosed with an amount of a nicotine-containing formulation as required.

Claims

1. A method of screen printing a substantially rigid substrate material with a fluent composition which is or can be rendered electrically conductive, said substrate having essentially planar and parallel upper and lower surfaces, the former of which is to be printed, said method comprising the steps of

Depositing an amount of the fluent composition on an upper surface of an elastically deformable mesh to which a stencil of predetermined pattern has been fixedly applied and within which are defined multiple identical void regions through which said compos ition can be forcibly caused to pass onto the substrate upper surface during printing,

- the forced filling of the said void regions with said fluent composition such that there is wetting contact with the underlying substrate immediately in advance of, or underneath the moving squeegee printing edge, and

- the subsequent release of said fluent composition from within said stencil void regions onto the upper surface of the substrate immediately behi nd the moving squeegee printing edge,

Characterised in that each identical stencil void region is continuous and comprises at least two distinct parts, a first part in which is defined at least two adjacent separate void portions, and a second part in which is defined at least one patterned void portion into which and away from which a respective one of said adjacent separate void portions extend, the total area of the adjacent separate void portions being at least one order of magnitude greater than the total area of the patterned void portion, and further characterised in that the lateral dimension, being that dimension transverse to the direction of travel of the squeegee, of the adjacent separate void portions is at least one order of magnitude greater than any corresponding lateral dimension of any element of patterned void portion.

2. A method according to claim 1 wherein the patterned void portion is one of: meandering and spiral in appearance.

3. A method according to claim 2 wherein patterned void portion meanders and is serpentine in nature and comprises a plurality of essentially adjacent, linear and parallel elements, and a corresponding plurality of arcuate or angular elements which con nect them, said linear elements being aligned substantia lly parallel with the direction of travel of the squeegee.

4. A method according to any preceding claim wherein the adjacent separate void portions are elongate, such elongation being in a direction substantially parallel with the direction of travel of the squeegee.

5. A method according to claim 3 wherein the length of any adjacent linear parallel element of the meandering void portion is one of:

- substantially, the same length as the longer edges of the elongate void portions,

- of the order of two thirds the length of said longer edges,

- of the order of half the length of said longer edges,

- of the order of one third the length of said longer edges, and

- of the order of one quarter the length of said longer edges.

6. A method according to claim 4 when dependent on claim 3, or claim 5 wherein the length of any adjacent linear parallel element of a meandering void portion is of the o rder of one half the length of the longer edges of the elongate void portions.

7. A method according to claim 4 when dependent on claim 3, or any claim dependent thereon wherein the length of any adjacent linear parallel part of the meandering void portion is between 6-9mm, and the length of said longer edges of the elongate void portions is of the order of 12- 18mm.

8. A method according to claim 4 when dependent on claim 3, or any claim dependent thereon wherein the width of any elongate void portion is of the order of 2 -4mm, and the width of each and all adjacent linear parallel parts of the meandering void portion is of the order of 100-300 pm.

9. A method according to claim 8 wherein the width of any elongate void portion is of the order of 3mm, and the width of each and all adjacent linear parallel parts of the meandering void portion is of the order of 200pm.

10. A method according to claim 4 when dependent on claim 3, or a ny claim dependent thereon wherein the width of the intervening unprinted land separating any two adjacent linear parallel parts of the meandering void portion is of the order of 80 -150 pm.

1 1. A method according to any preceding claim wherein the substrate is substantially rigid and constituted predominantly or exclusively of one or more of the following materials: soda -, silica-, aluminiu m-, alu minosilicate-, lime- and borosilicate-based g lass.

12. A method according to any preceding claim wherein the substrate is a tin float soda lime g lass 0.5mm in thickness.

13. A method according to any preceding claim wherein the stencilled mesh defines between 20 and 500 multiple identical void regions such that a single printing of one or a corresponding number of individual appropriately sized substrates resu lts in that nu mber of patterns of fluent composition being printed onto the said substrate, or each individual su bstrate receiving an individual printed pattern as the case may be.

14. A method according to any preceding claim wherein the fluent composition is a conductive printing ink or paste which is inherently conductive.

1 5. A method according to any preceding claim further including the post-printing steps of d rying the substrate immediately after the removal from the screen printing apparatus.

16. A method according to claim 1 5 wherein the d rying time is of the order of 10— 15 min at a temperatu re in the range 100-150 deg.C.

17. A method according to any preceding claim wherein the method i ncludes the fu rther step of firing said printed su bstrate in a kiln.

18. A method according to claim 1 7 wherein the kiln firing time is of the order of 1 5 -80min at a temperature in excess of 500 deg.C, firing being conducted at or above such temperatures for a period of at least 15min.

19. A method according to any preceding claim wherein mu ltiple individual substrates are simultaneously printed, each of said substrates being of identical size and of rectangu lar cross- sectional shape, being of 0.35-0.7mm thick, of the order of 7- 15 mm wide, and of the order of 1 5 - 30mm long, each of said substrates being disposed with their longest edges alig ned and parallel with the direction of travel of the printing sq ueegee.

20. A method according to any preceding claim wherein the size of each and substantially all the individual apertures within the mesh is selected to be larger than the size of the average size of the particles of any particulate material present within the fluent composition to be printed.

21. A method according to any preceding claim wherein a first part of each of the multiple identical void regions within the stencilled mesh comprises at least three adjacent separate void portions, and in the second part, at least two adjacent patterned (preferably meandering) void portions is provided, a first and a third one of said adjacent separate void portions extending into one end of respective first and second patterned void portions, each of which s ubsequently emerges and extends into a second one of said adjacent separate void portions intervening the first and the third.

22. A method according to any preceding claim wherein a first part of each of the multiple identical void regions within the stencilled mesh comprises at least five adjacent separate void portions, and in the second part, at least four patterned (preferably meandering) void portions is provided, preferably arranged in two adjacent pairs, a first and a fifth one of said adjacent separate void portions extending into one end of respective first and fou rth patterned void portions, the alternate ends of which emerge and extend into respective second and fourth adjacent separate void portions intervening the first and the fifth adjace nt separate void portions, with the final third one of said adjacent separate void portions, preferably intervening one or other or both of the first and fifth, and the second and fourth, extending into respective ends of the second and third patterned void portions respectively.

23. A substrate printed according to the method of any preceding claim.

24. A substrate according to claim 23 wherein the total resistance of the conductive material printed on a substrate after drying and firing, measured between any two adjacent contact portions from the free ends thereof, said free ends being remote from the printed heating elements to which they are connected, is in the range 0.8W < R < 15W,

25. A substrate according to claim 24 wherein the total resistance is in the range 1 -5W.

26. A stencilled mesh for use in screen printing apparatus wherein said mesh is elastically deformable and the stencil is fixedly applied thereto, said mesh being larger in cross-sectional area than a substrate to be printed, and said pattern defining multiple identical void regions through which a fluent printing composition can be forcibly caused to pass onto the substrate upper surface during printing and which thus define the patterns of said fluent printing composition applied to said substrate,

Characterised in that the identical stencil void regions are continuous and comprise at least two distinct parts, a first part in which is defined at least two adjacent separate void portions, and a second part in which is defined at least one patterned void portion into which and away from which a respective one of said adjacent separate void portions extend, the total area of t he adjacent separate void portions being at least one order of magnitude greater than the total area of the patterned void portion, and further characterised in that the lateral dimension, being that dimension transverse to the direction of travel of the squeegee, of the adjacent separate void portions is at least one order of magnitude greater than any corresponding lateral dimension of any element of patterned void portion.

27. A stencilled mesh according to claim 26 wherein the patterned void portion i s one of: meandering and spiral in appearance.

28. A stencilled mesh according to claim 27 wherein patterned void portion meanders and is serpentine in nature and comprises a plurality of essentially adjacent, linear and parallel elements, and a corresponding plurality of arcuate or angular elements which connect them, said linear elements being aligned substantially parallel with the direction of travel of the squeegee.

29. A stencilled mesh according to any of claims 26-28 wherein the adjacent separate void portions are elongate, such elongation being in a direction substantially parallel with the direction of travel of the squeegee.

30. A stencilled mesh according to claim 28 wherein the length of any adjacent linear parallel part of the meandering void portion is one of:

- of the order of two thirds the length of said longer edges,

- of the order of half the length of said longer edges,

- of the order of one third the length of said longer edges, and

- of the order of one quarter the length of said longer edges.

31. A stencilled mesh according to any of claims 28 or 30 wherein the length of any adjacent linear parallel part of the meanderi ng void portion is of the order of one half the length of the longer edges of the elongate void portions.

32. A stencilled mesh according to claim 31 wherein the length of any adjacent linear parallel part of the meandering void portion is between 6-9mm, and the length of said longer edges of the elongate void portions is of the order of 12 - 18mm.

33. A stencilled mesh according to any of claims 26-32 wherein the lateral dimension of a respective adjacent separate void portion is of the order of 2-4mm, and the width of each and all adjacent linear parallel element of a patterned void portion is of the order of 100-300pm.

34. A stencilled mesh according to any of claims 26-33 wherein the width of the intervening non-void land separating any two adjacent linearly or spirally parallel parts of the patterned void portion is of the order of 80-150 pm.

35. A stencilled mesh according to any of claims 26-34 wherein the stencilled mesh defines between 20 and 500 mu ltiple identical void regions.

36. A stencilled mesh according to any of claims 26-35 wherein the size of each and substantially all the individual apertu res within the mesh is selected to be larger than the size of the average size of the particles of any particulate material present withi n the fluent composition to be printed.

37. A stencilled mesh according to any of claims 26-36 wherein a first part of each of the multiple identical void regions within the stencilled mesh comprises at least th ree adjacent separate void portions, and in the second part, at least two adjacent patterned void portions is provided, a first and a third one of said adjacent separate void portions extending into one end of respective first and second patterned void portions, each of which subsequently eme rges and extends into a second one of said adjacent separate void portions intervening the first and the third.

38. A method according to any of claims 26-36 wherein a first part of each of the multiple identical void regions within the stencilled mesh comprises at least five adjacent separate void portions, and in the second part, at least four patterned void portions is provided, preferably arranged in two adjacent pairs, a first and a fifth one of said adjacent separate void portions extending into one end of respective first and fourth patterned void portions, the alternate ends of which emerge and extend into respective second and fou rth adjacent separate void portions intervening the first and the fifth adjacent separate void portions, with the final third one of said adjacent separate void portions, preferably intervening one or other or both of the first and fifth, and the second and fourth, extending into respective ends of the second and third patterned void portions respectively.