A filter for a liquid delivery device

Abstract

A filter for use as part of a liquid delivery device comprises a first wafer 100 and a second wafer, the first and second wafers bonded to one another by mutually-adjacent faces; the first wafer 100 e

Description

TECHNICAL FIELD This invention relates to a filter for a liquid delivery device. In particular, though not exclusively, this invention relates to a filter for a nebulizer. This invention also relates to a liquid delivery device comprising a filter. BACKGROUND Liquid delivery devices such as soft mist inhalers (SMIs) can be used to produce an aerosol of droplets for inhalation through the mouth and pharyngeal cavity into the lungs of a patient, for nasal administration, or for spraying the surface of the eye. In a liquid delivery device of this kind, liquid pharmaceutical formulations are typically stored in a reservoir. In use, the liquid is conveyed through a riser tube into a pressure chamber from where it is forced under pressure through a nozzle aperture, and is atomised. In this way, liquid / drug delivery devices such as SMIs are able to nebulise a small amount of a liquid formulation according to the required dosage within a few seconds, to produce an aerosol suitable for therapeutic inhalation. Moreover, this can be achieved without requiring the use of a propellant. In devices of this type, a filter is located in the liquid path from the reservoir, between the pressure chamber and the nozzle aperture. The purpose of the filter is to remove any debris that may have accumulated within the liquid from the reservoir, so as to prevent blocking of the nozzle jets and to avoid delivering unwanted material to the user / patent, before the liquid is atomised and delivered to a user. US20200215276 describes and shows a device that uses multiple separate layers of filters, the filters comprising plates with reducing diameter pores. The gaps between the layers allow debris to accumulate without blocking the flow of liquid. US7,896,264 provides an example of a typical type of known filter structure for use with liquid delivery devices such as a Soft Mist Inhaler. The filter as described and shown in this document is incorporated into the nozzle of the inhaler, and comprises a number of channels produced by microstructuring a plate-shaped base plate member so that multiple projections project from the base plate, arranged side-by-side in rows, with the channels of the filter defined by and running between the projections. The microstructured base plate is covered with a cover plate to form the overall filter / nozzle. The channels are narrowly defined in terms of shape, cross sectional area and length. In this specification where reference has been made to patent specifications, other external documents, or other sources of information, this is generally for the purpose of providing a context for discussing the features of the invention. Unless specifically stated otherwise, reference to such external documents is not to be construed as an admission that such documents, or such sources of information, in any jurisdiction, are prior art, or form part of the common general knowledge in the art. SUMMARY OF THE INVENTION It is an object of the present invention to provide a filter for a liquid delivery device which goes some way to overcoming the abovementioned disadvantages or which at least provides the public or industry with a useful choice. It is a further object of the invention to provide a filter for a nebuliser which goes some way to overcoming the abovementioned disadvantages or which at least provides the public or industry with a useful choice. It is a yet still further object of the invention to provide a liquid delivery device that comprises a filter for a liquid delivery device. Accordingly, in a first aspect the present invention may broadly be said to consist in a filter for use as part of a liquid delivery device, comprising: a first wafer; a second wafer, the first and second wafers bonded to one another by mutually-adjacent faces; the first wafer etched so that multiple projections are formed in the inner planar face, the projections formed so that groups of projections form conduits that run within the filter towards the filter outlet; the projections arranged so that at least part of at least one side of each of the conduits has a sawtooth profile. In an embodiment, the projections are formed so that one side of the sawtooth comprises multiple short projections. In an embodiment, the multiple short projections are arranged in parallel with one another and spaced apart from one another in a row. In an embodiment, the multiple short projections are aligned with one another. In an embodiment, the other side of each sawtooth comprises a long projection formed so as to have an overall elongate or stretched ‘S’ shape. In an embodiment, the elongate ‘S’ shape comprises two end parts connected by a central body portion, the end parts substantially the same length as the short projections, the long projection arranged so that the outer one of its end parts is located in parallel with and aligned with the short projections, at the outer end of the row. In an embodiment, the central body part of the ‘S’ shape is angled inwards and upwards so that the other / inner of the end parts of the long projection is located inside, parallel with, and aligned with, the innermost one of a row of four short projections in a tooth directly above. In an embodiment, the upper side of each tooth comprises a plurality of projections formed and arranged so as to have an overall elongate or stretched ‘S’ shape, at least one gap formed between the projections. In an embodiment, the plurality of projections comprise two end projections separated by a gap, the end projections formed so as to be substantially at least the same length as the short projections. In an embodiment, the projections are formed so that the lower side of the tooth is angled away from the horizontal by substantially 30 degrees. In an embodiment, the projections are formed so that the lower side of the tooth is angled away from the horizontal by substantially 60 degrees. In an embodiment, the conduits are substantially aligned with the overall direction of flow through the filter. In an embodiment, the conduits are substantially straight. In a second aspect, the invention may broadly be said to consist in a liquid delivery device comprising a filter as claimed in any one of the preceding statements. In an embodiment, the liquid delivery device comprises a soft mist inhaler (SMI). This invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more said parts, elements or features, and where specific integers are mentioned herein which have known equivalents in the art to which this invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth. Therefore, the foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a side view of a known type of manually-operated nebuliser / Soft Mist Inhaler that has an upper housing part and a lower housing part, the two housing parts rotated relative to one another in use to pump or prime the device for use, the upper housing part having a lid that can be opened and closed in use to allow a user to access a mouthpiece. Figure 2 shows a perspective view from one side and slightly above of the nebuliser / Soft Mist Inhaler of figure 1, the lid shown slightly open. Figure 3 shows a perspective view of the nebuliser of figures 1 and 2 from a generally similar angle as that of figure 2 but with the lid closed, the outer casing elements of the nebuliser / Soft Mist Inhaler shown semi-transparent so as to show some internal detail of several of the main internal parts of the nebuliser. Figure 4 shows a perspective cutaway side view of the upper end of the upper housing part of the nebuliser / Soft Mist Inhaler of figures 1 to 3, the upper housing part enclosed by a lid (shown in the closed position), the upper housing part comprising a mouthpiece and nozzle assembly, the nozzle assembly located within the mouthpiece. Figure 5 shows a cutaway schematic side view of the nozzle assembly of figure 4, the nozzle assembly comprising a nozzle retainer, upper and lower seals, a nozzle chip / filter, a pre-filter and pre-filter holder, and a top nut that screws onto the upper end of an upper tube housing located within the upper housing so as to enclose these items. Figure 6 shows a stylised cutaway view of the interior of a chip / filter of the known prior art type, showing detail of the interior layout of the filter, the chip / filter comprising projections formed by etching, the projections arranged in columns that extend from the lower end of the chip / filter towards the nozzle end, with open passageways formed between the columns, chip prefilter elements shown arranged across the bottom of the filter below the columns and passageways. Figure 7 shows a stylised perspective view of a chip / filter of the known type similar to that shown in figure 6, the chip / filter formed from a glass wafer and an etched silicon wafer arranged back-to-back, the silicon wafer shown semi-transparent so as to show detail of the layout of the columns, passageways, and prefilter elements etched within the silicon wafer. Figure 8 shows a stylised cutaway detail view of the interior of a chip / filter of the known prior art type from the same angle as figure 6, showing detail of a further form of internal chip / filter layout, the projections in the chip / filter formed / arranged so that the columns narrow from the nozzle end towards the lower end of the filter, and the passages widen from the lower end towards the nozzle end. Figure 9 shows a close-up view of the section marked ‘A’ on figure 8, showing detail of the layout of the projections that form a column, and the spacing between these. Figure 10a shows a cutaway side view of the silicon wafer portion of a first embodiment of chip / filter that is formed from a glass wafer and an etched silicon wafer arranged back-to-back in a similar manner to the filter shown in figure 7, the figure showing detail of the interior of the silicon wafer portion, which is etched so as to comprise projections extending within the wafer so as to form columns having a sawtooth profile. Figure 10b shows a close-up detail side view of one interior section of the wafer of figure 10a, showing detail of the structure of the columns. Figure 10c shows a cutaway detail view of part of the interior of the wafer of figures 10a and 10b, showing detail of part of the inlet end of the wafer, the inlet ends of the columns and prefilter projections located below the projections shown. Figure 11 shows a close-up detail view of the structure of the sawtooth projections of a second embodiment of wafer. Figure 12 shows a close-up detail view of a third embodiment of the structure of the sawtooth projections of the wafer. Figure 13a shows a cutaway detail view of part of the interior of a chip / filter according to a fourth embodiment of the invention, the chip / filter comprising projections extending within the filter so as to form columns having a sawtooth profile. Figure 13b shows a cutaway detail view of part of the interior of the chip / filter of figure 11a, showing detail of part of the lower portion of the chip / filter, showing the columns and prefilter projections located below the projections. Figure 14 shows an example of a known, prior art type of material etching, which causes projections to be formed with curved or radiused walls. DETAILED DESCRIPTION Detailed embodiments of the invention will now be described with reference to the figures. General For all of the embodiments described below, the invention is described as forming part of a liquid delivery device (the invention may also be considered as the liquid delivery device itself). Specifically for the embodiments described, the liquid delivery device is a nebuliser or inhaler (more specifically, a Soft Mist Inhaler). A typical known type of nebuliser or inhaler in or with which the nozzle of the present invention can be used is shown in figures 1 to 5. In these figures, a chip / filter forms part of the nebuliser / inhaler. This known type of chip / filter can be changed or swapped for the filter of the present invention. Chips / filters according to embodiments of the invention, and variations of those embodiments, are described below. As shown in figures 1 to 5, the nebuliser 1 comprises an upper housing part 2 and a lower housing part 3. The upper housing part 2 contains a nozzle retaining structure / nozzle assembly (described in detail below) and a mouthpiece 5 within the lid. The lower housing part 3 contains a liquid reservoir (not shown), which typically comprises a cartridge that can be removed and replaced as required. A riser tube or capillary tube 6 extends between the reservoir and the nozzle assembly. A lid 4 is hingedly connected to the upper housing part, the lid configured so that when closed, the lid encloses the nozzle retaining structure / nozzle assembly. In use, a user rotates the upper and lower housing parts 2, 3 relative to one another to pump or prime the device 1 for use. Liquid from the reservoir is sucked up the tube 6 towards the upper end of the tube. When a user then triggers the nebuliser (e.g. by pressing button 7), the tube 6 is forced rapidly upwards with the head of the tube 6 forcing liquid through the nozzle assembly, and then through the mouthpiece 5, for delivery to a user as a spray or aerosol of fine droplets. For the purposes of this specification, references to orientations such as ‘upper, ‘lower’, ‘top’, ‘bottom’, ‘vertical’, ‘horizontal’ and similar or related references should be taken as meaning with respect to an orientation with the nebuliser stood upright with the mouthpiece at the upper end, even if in use the orientation would differ from this. These references to orientation should not be taken as absolute. Nozzle Retaining Structure I Nozzle Assembly Nozzle retaining structures / nozzle assemblies are shown in figures 4 and 5. A typical known type of nozzle assembly comprises the following main parts: a filter holder 9; a pre-filter 10; an upper seal or nozzle seal 11; a lower seal 12; a nozzle chip / filter 13; a top nut 14, and; a nozzle retainer 15. The filter holder 9, pre-filter 10, upper and lower seals 11, 12, nozzle retainer 15 and nozzle chip 13 are contained within the top nut 14. The top nut 14 is screwed to the top of an upper tube housing 8 (that contains the tube 6, with tube 6 moving axially / vertically within the tube housing 8), the top nut 14 and upper tube housing 8 mutually threaded to allow them to be screwed together. As outlined above, the tube 6 moves along a passage within the upper tube housing 8, the passage aligned axially within the upper tube housing 8. The head of the tube 6 fits snugly within the passage. In use, fluid from the reservoir (not shown) travels through the hollow centre of the tube 6 (which forms a capillary tube), and during use is forced under pressure through the pre-filter 10, which is located directly above the top end of the passage. The prefilter 10 is held in place by the filter holder 9, with the filter holder 9 located above and directly adjacent to the top end of the tube housing 8. The lower seal 12 is located between the filter holder 9 and tube housing 8 so as to seal between the two and prevent fluid from leaking out through the seam or gap between the two. The nozzle chip 13 is located directly above the top end of the pre-filter 10, the nozzle chip 13 located within and held in place by the upper seal 11. The upper seal 11 locates into a recess within the nozzle retainer 15, which extends around the sides and top of the chip / filter 13 and upper seal 11. Channels run through the chip / filter 13 to the outlet end, the outlet end of the nozzle chip 13 comprising one or more spray jets. The top nut 14 and tube housing 8 are mutually threaded so that top nut 14 can be screwed onto the tube housing 8, so as to hold the lower seal 12, filter holder 9, filter 10, upper seal 11, chip / filter 13 and nozzle retainer 15 in place within the top nut 14, the nut 14 substantially surrounding and enclosing the other elements of the nozzle retaining structure I nozzle assembly. Filter First Embodiment A first embodiment of the filter of the present invention is shown in figures 10a - 10c. The filter in this embodiment is formed from a glass wafer and an etched silicon wafer arranged back-to-back and bonded at their mutually-adjacent large inner faces, similar to the prior art chip / filter shown in figure 1 (which comprises a glass wafer 16 and silicon wafer 17). Internal detail of the etched silicon wafer 100 is shown in figures 10a - 10c. The etched silicon wafer 100 is etched so as to create passageways 119, leaving multiple projections 120 formed in one of the larger planar surfaces (the inner surface or face, facing towards and in contact with the glass wafer when the filter is assembled). As shown in figure 10a, in this embodiment the projections are arranged in a linear manner, in double or paired rows 118 that extend within the filter - from the inlet end (the lower end in figure 10a), towards the nozzle end. The paired rows 118 are aligned in a similar manner to the columns 18 of the prior art. Open passageways 119 are formed between the paired rows 118 (similar to the passages 19 of the prior art). In use, liquid / gases flow through the paired rows 118 as shown by the arrows 400 on figure 10b, along the paired rows, towards the outlet. It can be seen that the space inside each of the paired rows forms a conduit for fluids / gases. As shown in figure 10c, the wafer 100 also comprises prefilter elements 121 arranged across the bottom of the wafer 100, between the inlet and the columns 118 and the passageways 119. The prefilter elements perform a similar function to the prefilter elements 21 of the prior art. As can be seen in figures 10a and 10b, the projections are arranged so that the sides of each of the columns / passageways has a sawtooth or Christmas tree profile. In this first embodiment, each sector - that is, each of the ‘teeth’ or ‘branches’ that forms the side of the column / passageway - comprises a first side formed from a number of short projections (sides 120a) and a second side formed from a longer projection (sides 120b), with the short projections forming one side of a tooth, and the longer projection forming the other side. The shorter projections 120a of each ‘tooth’ are arranged in a substantially straight line. The longer projection 12b is also substantially straight. As shown in figures 10a - 10c, each side of each tooth is angled away from a plane perpendicular to the direction of flow (shown by arrows 400). The overall internal angle between the sides of a tooth is angle ‘A’, which in this embodiment is substantially 60 degrees, with each side angled equally away from the plane perpendicular to the direction of flow (each side angled at approximately 30 degrees away from this plane). Second Embodiment A second embodiment of the filter of the present invention is shown in figure 11. In this embodiment, the etched silicon wafer is etched so that multiple projections 220 (220a, 220b) are formed in one of the larger planar surfaces (the inner surface or face, facing towards and in contact with the glass wafer when the filter is assembled). The projections are arranged in paired rows 218 that extend within the filter from the inlet end (the lower end in figure 11), towards the nozzle end. The columns 218 are aligned in a similar manner to the columns 118 of the first embodiment. Open passageways 219 are formed between the columns 218 (similar to the passages 119 of the first embodiment). As for the first embodiment, the projections are arranged so that the outer side of each of the paired rows has a sawtooth or Christmas tree profile. Each tooth comprises four short projections 220a and a longer projection 220b, with the short projections forming one side of a tooth, and the longer projection forming the other side. The four shorter projections 220a of each ‘tooth’ are arranged in parallel with one another and spaced apart from one another. The shorter projections 220a are also aligned with one another (that is, the ends form points on a straight line, and are not offset from one another). Each longer projection 220b in the ‘tooth’ is formed so as to have an elongate or stretched ‘S’ shape, with two end parts connected by a central body portion. The end parts of the ‘S’ are substantially the same length as the shorter projections 220a. In each tooth, the longer projection 220b is arranged so that the outer one of its end parts is located in parallel with and aligned with the shorter projections, at the outer end of the row of four. The central body part of the ‘S’ shape is angled inwards and upwards so that the other / inner of the end parts of the longer projection 220b is located inside, parallel with, and aligned with, the innermost one of a row of four shorter projections 220a in the tooth directly above. The projections are formed so that that side of each tooth formed mostly by the shorter projections 220a is angled away from a line perpendicular to the direction of flow shown by arrows 400, with that side of the tooth formed by the longer projection 220b parallel to the line perpendicular to the direction of flow. The angle between the two sides of the tooth is substantially 30 degrees (angle ‘B’ as shown in figure 11). Third Embodiment A third embodiment which is a variation of the second embodiment is shown in figure 12. In this embodiment, each tooth has four short projections 320a substantially the same as the projections 220a described above for the second embodiment. However, in place of the solid / continuous ‘S’-shaped longer projections 220b of the first embodiment, the inlet-facing side of the tooth in this embodiment comprises three separate projections that together form an arrangement equivalent to the longer projection of the first embodiment. That is, the ‘longer projection’ of this third embodiment is similar to the longer projection 220b of the second embodiment, but divided into three separate parts: a central projection 320c and two end parts 320d, one at each end of the central projection 320c. The projections 320c and 320d are located in the same position as the centre part and end part of the continuous projection 320b of the first embodiment, but with gaps or breaks located at the inner ends of the end projections 320d so that the projections 320c and 320d form a ‘broken’ ‘S’ shape (the ‘S’ formed by central projection 320c and two end projections 320d, one at each end of the central projection 320c). This ‘broken S’ is located in the same position as for the longer projection 220b of the second embodiment shown in figure 11, with the two shorter projections 320c of this second embodiment located so they are equivalent to the ends of the ‘S’ of the first embodiment, but with a gap at each of their inner ends. The gaps between the end projections 320d and the central projection 320c are sized and located so the end parts / end projections 320d are the same size / length as the four short projections 220a of the first embodiment, and these end projections 320d are arranged in parallel with, and aligned with, the four short projections 320a. As for the second embodiment, the projections are formed so that that side of each tooth formed mostly by the shorter projections 320a is angled away from a line perpendicular to the direction of flow, with that side of the tooth formed by the longer projection 320c parallel to the line perpendicular to the direction of flow. The angle between the two sides of the tooth is substantially 30 degrees (that is, the arrangement is substantially the same as described above for the second embodiment, and as shown by angle ‘B’ in figure 11 for the second embodiment). Fourth Embodiment A fourth embodiment is shown in figures 13a and 13b. In this embodiment, the structure of the filter / chip is substantially similar to that of the first and second embodiments, with paired rows 418, passageways 419, and prefilter elements 421. In this embodiment, the projections 420a, 420b are formed so that that side of the tooth formed by the shorter projections (side 425) is angled away from a line perpendicular to the direction of flow by substantially 60 degrees (angle ‘C’ as shown in figure 13a), with the upper or outer-facing side of the tooth (side 426) arranged substantially horizontally. Upper End The filters as shown in the prior art devices of figures 6 and 8 contain an upper section that narrows towards the outlet end, so that fluids / gases within the filter are funnelled towards the outlet. As shown in the prior art devices in figure 6 or figure 8, the columns within the filter are formed so that they do not extend into this narrowing section. In embodiments of the filter of the present invention (as shown for example in figure 10a), the filter also narrows towards the outlet end, so that fluids / gases within the filter are funnelled towards the outlet - e.g. outlet 500 in figure 10a. However, in contrast with the prior art, the columns (e.g. columns 118, 218) are formed so that their upper ends (those ends closest to the filter outlet) extend into the narrowing upper section or funnelling section of the filter. The tops of the columns are therefore arranged at different or staggered intervals or different ‘heights’. This arrangement provides the advantages of maximising the total filtering area and maximising the bonding area between the wafers in the highest pressure region (near to the nozzle jets) whilst still allowing an area between the filter and nozzle jets for flow smoothing, balancing, and equalisation prior to the jets. Use In use, for all of the embodiments and variations described above and as shown in figures 10a -10c, 11, and 12, the direction of flow along the paired rows 119, 219 is ‘upwards’ - that is, as shown in e.g. figure 10a or figure 10b, from the bottom of the figure towards the top, as shown by arrows 400 in figure 10b. As fluid / gas is forced through the filter, it passes from the paired rows (e.g. 119, 219) and into the narrow gaps between each of the shorter projections (e.g. the projections 120a, 220a), passing up the inside of each paired row (e.g. the paired rows 118 in figure 10a for the first embodiment). The fluid / gas then passes into the upper part of the filter. The fluid / gas is than forced out of the nozzle as an aerosolised mist. Variations of this structure are described below. Variations For all of the embodiments of filter described above and shown in the figures, a silicon substrate is etched, and then bonded to a plain or un-etched glass substrate to create the microfluidic filtering structure. In a variation of the structure described above, the filter / chip can be constructed by etching into two or both layers and sandwiching / bonding those together to complete the filtering structure. The layers could for example be glass-silicon, silicon-silicon, or glass-glass. When etching a silicon substrate, the process used is typically a DRIE technique, which is an anisotropic etching process. Anisotropic etching removes material at different rates in different directions. Typically, this process etches material more quickly in one direction (e.g. into the face of a wafer) than in other directions (e.g. spreading sideways into the wafer). This results in a more directional and controlled etch profile. In contrast, glass etching is carried out with chemical etching which is isotropic in nature. Isotropic etching removes material uniformly in all directions. This means that the etching rate is the same in every direction, leading to a rounded etch profile (such as for example that shown in figure 14). When using an isotropic etching process, the wall formed by the isotropic etching process is not planar - it is curved or ‘radiused’ as widely as its depth, as shown for example in figure 14. Isotropic and anisotropic etching are two fundamental techniques used in microfabrication and semiconductor manufacturing processes. Both methods are used to remove material from a substrate. However, as outlined above, the two processes differ significantly in terms of the directionality and uniformity of the etching process. It should be noted that the isotropic etching process can be used on silicon, and the anisotropic process can be used on glass. As noted above, for all of the specific embodiments described and shown, the substrates are formed from glass or silicon. Other materials can also be used as appropriate, and the isotropic or anisotropic etching process can be used on these other materials. The use of stacked filtering branches in this way increases the filtering capacity over a filter where only one of the two substrate layers is etched. Advantages Filtering structures such as are described above, with branched filter elements, assist with 5 reducing the space requirement for the filtering structures. This helps to improve the filtering capacity, via an increased total filtering area. This is an advantage over the staggered structures which extend parallel to the direction of flow such as are already known in the art. Furthermore, wide flow channels (such as for example the passageways 119, 219) are created between the filter structures (the columns 118, 218). This allows fluid to flow past blocked 10 elements and on to areas of non-blocked filters. This reduces the likelihood of the liquid delivery device becoming substantially or completely blocked, and / or increasing the resistance to flow through the device. Another benefit over the prior art is that the denser structure eliminates the need for supporting pillars between the filters due to the increased supported area and bonding area between the two substrates.

Claims

1. A filter for use as part of a liquid delivery device, comprising:a first wafer;a second wafer, the first and second wafers bonded to one another by mutually-adjacent faces;the first wafer etched so that multiple projections are formed in the inner planar face, the projections formed so that groups of projections form conduits that run within the filter towards the filter outlet;the projections arranged so that at least part of at least one side of each of the conduits has a sawtooth profile.

2. A filter as claimed in claim 1 wherein the projections are formed so that one side of the sawtooth comprises multiple short projections.

3. A filter as claimed in claim 2 wherein the multiple short projections are arranged in parallel with one another and spaced apart from one another in a row.

4. A filter as claimed in claim 3 wherein the multiple short projections are aligned with one another.

5. A filter as claimed in claim 4 wherein the other side of each sawtooth comprises a long projection formed so as to have an overall elongate or stretched ‘S’ shape.

6. A filter as claimed in claim 5 wherein the elongate ‘S’ shape comprises two end parts connected by a central body portion, the end parts substantially the same length as the short projections, the long projection arranged so that the outer one of its end parts is located in parallel with and aligned with the short projections, at the outer end of the row.

7. A filter as claimed in claim 6 wherein the central body part of the ‘S’ shape is angled inwards and upwards so that the other / inner of the end parts of the long projection is located inside, parallel with, and aligned with, the innermost one of a row of four short projections in a tooth directly above.

8. A filter as claimed in claim 4 wherein the upper side of each tooth comprises a plurality of projections formed and arranged so as to have an overall elongate or stretched ‘S’ shape, at least one gap formed between the projections.

9. A filter as claimed in claim 8 wherein the plurality of projections comprise two end projections separated by a gap, the end projections formed so as to be substantially at least the same length as the short projections.

10. A filter as claimed in any one of claims 1 to 9 wherein the projections are formed so that the lower side of the tooth is angled away from the horizontal by substantially 30 degrees.

11. A filter as claimed in any one of claims 1 to 9 wherein the projections are formed so that the lower side of the tooth is angled away from the horizontal by substantially 60 degrees.5 12. A filter as claimed in any one of claims 1 to 11 wherein the conduits are substantially aligned with the overall direction of flow through the filter.

13. A filter as claimed in any one of claims 1 to 12 wherein the conduits are substantially straight.

13. A liquid delivery device comprising a filter as claimed in any one of claims 1 to 12.10 14. A liquid delivery device as claimed in claim 13 wherein the liquid delivery device comprises a soft mist inhaler (SMI).

Images