in the selected embodiment shown in fig1 and 2 the tracheotomy filter 1 comprises a circular elastomer flange 10 and a rigid metal or molded polymer casing 20 . the flange 10 comprises an annular flange 11 around an annular rim 12 . using double - sided adhesive tape 15 which provides a fluid - tight join , the flange 11 is fixed to the skin at the base of the neck 2 of a tracheotomy patient around a surgical opening 13 which joins the trachea to the skin . a sleeve 14 more or less aligned with the rim 12 projects around the central apperture in the flange 11 and is inserted in the surgical opening 3 to centre the tracheotomy filter 1 relative to the surgical opening . taking the tracheotomy patient as the orientation reference point , the rim 12 occupies an anterior position and the sleeve 14 a posterior position relative to the flange 11 . in what follows these terms &# 34 ; anterior &# 34 ; and &# 34 ; posterior &# 34 ; will be used in relation to the patient wearing the tracheotomy filter . the rim 12 is a section of tube ; at its anterior end it is fitted with a ring 13 projecting from the periphery of the tube section . the generally disk - shaped casing 20 includes a tubular rim 21 in the bore of which is formed a groove 22 complementary to the ring 13 whereby the casing 20 is removably fixed to the flange 10 . the anterior side of the casing 20 includes a circular window 23 aligned with the central aperture in the flange 10 and provided with a grill 24 . the casing 20 houses a disk - shaped filter mass 40 which is made from an open pore polyurethane foam with hydrophilic properties . a barrier grill 30 is disposed between the filter mass 40 and the surgical opening 3 . the grill 30 has a convex posterior component 31 and a plane anterior component 32 . these component are joined by their circular edges in a ring which abuts the anterior periphery of the central opening in the flange 10 . the convex posterior component 31 and in this embodiment the anterior component 32 are in the form of a grill with a square mesh of approximately 4 mm . apart from the fact that it facilitates centring of the tracheotomy filter in the surgical opening , the sleeve 14 prevents the posterior component 31 of the grill 30 bearing directly against the edge of the surgical opening 3 , despite the fact that by virtue of its convex shape it enters this opening . in the known manner , the filter disk 40 is cooled by the ambient air and causes some of the moisture contained in the exhaled air to condense , the moisture content of the exhaled air being the saturation moisture content at the temperature of the lungs . most of the condensed moisture remains in the filter mass , because of its hydrophilic properties . the condensation warms the filter mass by releasing the latent heat of evaporation . during the next indrawn breath , the inhaled air takes up moisture by evaporation of some of the moisture retained by the filter mass and is warmed through contact with the moisture that is not taken up . after a few respiratory cycles ( exhalation and inhalation ) steady state conditions are obtained similar to those which apply in the case of natural respiration through the upper airway . the function of the blocking grill is to block mucus expelled from the trachea . the mucus , formed by organic defense reaction against attack by dust , microorganisms and detached dead cells in particular , is in the form of an aqueous gel often of high viscosity and with sufficient surface tension to form autonomous globules . if the mucus reaches the moist filter mass 40 it tends to spread thereon and then to dry in contact with the inhaled air as this takes up the water constituting the gel . this clogs the filter mass . however , the relatively large pieces of mucus which would cause most of the clogging of the filter mass strike the convex posterior component 31 of the grill 30 and stick to it . their viscosity and surface tension prevent them being fragmented by entrainment in the exhaled air with the result that they are drawn out in the direction of this air current in the space between the posterior component 31 and the anterior component 32 , but without reaching the filter mass 40 , whilst continuing to adhere to the posterior component of the grill . it will be understood that if the mesh size of the posterior component 31 of the grill 30 is too small the mucus will clog the grill . if the mesh size is too large , on the other hand , the probability that the posterior side 31 of the grill 30 will block the mucus becomes low and the grill becomes less effective . experiments have shown that a 4 mm square mesh with a wire diameter of around 1 mm works well . it will have been understood that the plane anterior side 32 of the grill 30 serves only to hold the filter mass 40 in position so that it is not drawn against the posterior component 31 by the inhaled air . the presence of the double - sided adhesive tape 15 , which is of course of a physiologically acceptable kind , enables re - use of the tracheotomy filter after it has been removed by replacing the ring 15 of double - sided adhesive tape which will have lost its adhesive properties after being detached . the casing is easy to open so that the spent filter mass 40 can be replaced and the grill 30 cleaned . note that the relatively large size of the window 23 enables medication aerosols to be sprayed onto the filter mass 40 through the grill 24 . in everyday life , however , the fact that the window 23 is open at the front has the drawback of being exposed to direct contact , especially with the clothes , and is not particularly esthetic . fig3 shown to a smaller scale a tracheotomy filter 1 which is fitted with a cap 50 primarily to prevent the wearer &# 39 ; s clothes blocking the window 23 in the cover 20 . the cap 50 comprises a ring 51 which slides with moderate friction on the rim 21 of the casing 20 , a front plate 52 the same outside diameter as the ring 51 and spacers 53 between the ring and the front plate 52 . in addition to its function of protecting the window 23 of the casing , the cap 50 conceals the opening and the grill and can be finished in a way which softens the functional appearance of the casing . the cap 50 as shown in fig3 can additionally be used by persons who wear an implant to re - enable speaking through the mouth , the operation of this implant requiring temporary blocking of the surgical opening . to this end the sliding travel of the ring 51 over the periphery or edge 21 of the casing 20 is sufficient to allow the front plate 52 , which is normally spaced from the window 23 , to be pressed in until it contacts the anterior side of the casing and blocks the window 23 . fig4 a and 4b show a cap specifically designed to protect the tracheotomy filter 1 from running liquid , during a shower or while participating in water sports , for example . a circular ring 61 on the cap 60 fits over the peripheral edge 21 of the casing of the tracheotomy filter 1 with moderate friction . over one half - circumference the ring 61 is extended by an edge 63 which extends as far as a front plate 64 . over the other half - circumference the ring 61 is joined to a plate 65 parallel to the front plate 64 . the plates 64 and 65 are joined together at the sides to form conjointly a flat skirt 62 . it will be understood that by directing the opening in the flat skirt 62 downwards the tracheotomy filter 1 is protected umbrella - fashion so that a shower can be taken with no risk of drowning . the cap shown in fig5 has been designed with the intention of improving the heating of inhaled air , especially in cold weather . this cap 70 comprises two oblong chambers , one chamber 72 which has an annular edge 71 which fits with moderate friction over the peripheral edge 21 of the cover of the tracheotomy filter 1 and another chamber 73 below the ring 71 , the chambers 72 and 73 having a common wall to enable partial overlapping of the chambers . the chamber 72 has a valve 75 facing the grill 24 of the cover of the tracheotomy filter adapted to open from the inside towards the outside in response to the increase in pressure on exhalation and a valve 74 closing an opening in the wall common to the chambers 72 and 73 and adapted to open from the chamber 73 into the chamber 72 in response to a reduction in pressure in the chamber 72 on inhalation . the chamber 73 has a ducting function and includes in its lower part and in the wall facing towards the body of the patient a window protected by a convex grill 76 which bears against the thorax of the patient below the surgical opening . the inhaled air will therefore be taken from below the clothing , in contact with the thorax , through the chamber 73 whose walls are also substantially at body temperature and then , after passing through the valve 74 , through the chamber 72 before reaching the tracheotomy filter 1 . exhaled air , on the other hand , passes directly from the chamber 72 to the exterior through the valve 75 . this prevents the formation of a dead volume of expired vitiated air containing a high percentage of carbon dioxide in the passageway 73 conveying air warmed by contact with the body . the invention is of course not limited to the examples described but encompasses all variant executions thereof within the scope of the claims . note that the arrangement of fig5 could be adapted to breathing from an air supply via a breathable air pipe , for example for scuba diving or snorkelling .
an artificial nose comprising a housing mounted around the surgical opening by a base ring with a two - sided adhesive containing a hydrophilic filtering disc which forms an alternate heat and moisture exchanger between the exhaled and inhaled air . a grid is arranged between the filter and the surgical opening and has a plane front surface contacting the filter , and a convex rear surface with calibrated through - passages . the distance between the front surface and the rear surface ensures , together with the calibre of the through - passages , that mucus ejected from the trachea is essentially unable to reach the filter .