Abstract:
A low cost medical humidifier for adding moisture to a patient air flow is disclosed. The humidifier comprises a water reservoir, a disposable evaporative wick containing a heater, a lid containing electrical contacts to engage the heater in the wick and airway connections. The wick heater comprises a PET film with aluminium tracks and a cover layer of PTC resistive ink. The heater is covered with paper to provide a water pathway via capillary flow. There is a capillary path from the water reservoir to the wick. The wick and heater are manufactured with printing and calendaring processes in reel to reel processes.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to the production of humidified air for use with medical devices. In particular, the present technology relates to the humidification of air intended to be inhaled by a patient. However, it will be appreciated that the invention is not limited to this field of use. 
       BACKGROUND OF THE INVENTION 
       [0002]    The following discussion of the prior art is provided to place the invention in an appropriate technical context and enable the advantages of it to be more fully understood. It will be appreciated, however, that any discussion of the prior art throughout the specification should not be considered as an express or implied admission that such prior art is widely known or forms part of the common general knowledge in the field. 
         [0003]    The respiratory system of a healthy body facilitates gas exchange. The nose and mouth form the entrance to the airways of a patient, with the trachea and bronchi being the conducting airways to take the air to the alveolated region of the lungs where the gas exchange takes place. See for example “Respiratory Physiology”, by John B. West, Lippincott Williams &amp; Wilkins, 9th edition published 2011. 
         [0004]    For example, a respiratory pressure therapy (RPT) device delivers pressurized air to a patient&#39;s airway, acting as a splint to keep the airways open, allowing the patient to breathe normally when asleep. The pressurized air is delivered in quantities beyond that required for respiration, with the excess flow allowed to leak out a mask vent so that therapeutic pressure is maintained. RPT is used to treat sleep apnoea and other respiratory disorders. If this flow of air is not humidified, the patient&#39;s airways can dry out, causing discomfort to the patient. 
         [0005]    The use of a humidifier is intended to produce humidified gas that minimizes drying of the nasal mucosa and increases patient airway comfort. Respiratory humidifiers are available in many forms, and for example may be a standalone device that is coupled to an RPT device via an air conduit; integrated within an RPT device; or be configured so that it is operatively directly coupled to an RPT device. 
         [0006]    Humidifiers typically comprise a water tub having a capacity of several hundred millilitres (ml), a heating element for heating the water in the tub, a control to enable the level of humidification to be varied, a gas inlet to receive gas from the RPT device, and a gas outlet adapted to be connected to an air circuit that delivers the humidified gas to the patient interface. In an RPT application, the water tub should contain more than enough water to last for the sleep duration of the patient. 
         [0007]    Heated pass-over humidification is one common form used with RPT devices. In such humidifiers, the heating element is typically incorporated in a heater plate which sits under, and is in thermal contact with, the water tub. Heat is transferred from the heater plate to the water tub primarily by conduction. The air flow from the RPT device passes over the heated water in the water tub, resulting in water vapour being taken up by the air flow. 
         [0008]    The tub contains the entire volume of water to be used to humidify the flow of air, and receives the flow of air which passes over the water and thereby delivers the humidified flow of air. Such a humidifier configuration presents a number of drawbacks. There is a risk of spillage of the volume of water into the RPT device or to the patient. The entire volume of water must be heated, with a consequent long warm up time and cool down time, of the order of 20 minutes at best. Also, as the tub becomes empty, any contaminants in the water will be adhered as residue in the tub, which will then require cleaning. 
         [0009]    Other humidifiers use wick type arrangements, whereby water is drawn through the wick, and the air flows across the surface of the wick, thereby transferring water to the air flow. Water is generally pumped from the reservoir to the wick. A drawback with such wick systems is that the water reservoir is generally at ambient pressure and the water is fed by pump to the wick which is enclosing the pressurized air flow. Thus the pump or water delivery mechanism must act as a valve to prevent loss of air flow and/or pressure, and the pump must deliver a regulated quantity of water to the wick so that flooding does not occur, but still maintain sufficient water in the wick for adequate humidification. 
         [0010]    US2009/0000620 (granted as U.S. Pat. No. 8,550,075) discloses a humidifier which includes a water reservoir and a semipermeable membrane on top, to allow diffusion of water vapour. This humidifier requires a complex, specific structure. In particular, it requires a water distribution member comprising an envelope formed by a first compartment wall and a second compartment wall joined together, the water distribution member being supported by a base plate, a heater apparatus supported on the base plate, and (in effect) underneath the water distribution member. Thus, the entire volume of water in the reservoir is heated. 
         [0011]    WO2015/135040 discloses a humidifier comprising a reservoir and a pump to provide a flow of water to a humidifier wick, the humidifier wick including a heating element and having a profiled shape so as to enclose at least part of the air flow path through the humidifier. 
         [0012]    It is an object of the present invention to provide a more cost effective humidifier and method of humidification. 
       SUMMARY OF THE INVENTION 
       [0013]    In a first broad form, the present invention provides a wick for a humidifier, which includes an internal heating element and which receives water from below via a wicking action. This enables a simple, cost effective humidifier structure in suitable implementations. 
         [0014]    According to one aspect, the present invention provides a multilayer humidifier wick, including at least a top layer, a bottom layer, and a heating element intermediate the top and bottom layer, the top and bottom layer being hydrophilic and in communication so that water will pass from the bottom layer to the top layer by a wicking action, such that operatively the bottom layer passes water through the heating element to the top layer, and the heating element increases the temperature of the water in the wick. 
         [0015]    According to another aspect, the present invention provides a humidifier for a flow of air, including a reservoir adapted to retain water, a lid, an air inlet and an air outlet, and a wick positioned over the reservoir, the wick including at least a top layer, a bottom layer, and a heating element intermediate the top and bottom layer, the top and bottom layer being hydrophilic and in communication so that water will pass from the bottom layer to the top layer by a wicking action, such that operatively the bottom layer passes water through the heating element to the top layer, and the heating element increases the temperature of the water in the wick, and so that operatively the humidity of a flow of air passing from the inlet to the outlet is increased. 
         [0016]    In one form, the humidifier may incorporate a blower for generating the air flow. 
         [0017]    According to another aspect, the present invention provides a method of humidifying an air flow, including at least the steps of:
   (a) Providing a humidifying chamber with an inlet and an outlet, and a reservoir of water;   (b) Providing a wick positioned above but not in contact with the water, and a transport component extending into the water, so that water travels to the wick using a wicking action;   (c) Heating the wick using an internal heating element, so that the water in the wick is increased in temperature; and   (d) Passing the air flow across an upper surface of the wick, so that the humidity of the air flow is increased.   
 
         [0022]    According to another aspect, the present invention provides a multilayer humidifier wick, including at least a top layer and a heating element below the top layer, the top layer being hydrophilic and in fluid communication with a transport component, the transport component operatively extending into the reservoir, so that in use water travels from the transport component to the top layer using a wicking action, the heating element being operatively adapted to increase the temperature of the water in at least the top layer. 
         [0023]    Implementations of the present invention allow for a simple, relatively inexpensive humidifier to be provided. The simplicity and ready manufacturability of the wick, according to suitable implementations of the present invention, allow for easy and cost effective manufacture, and simple assembly and operation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    Illustrative implementations of the invention will now be described with reference to the accompanying figures, in which: 
           [0025]      FIG. 1  shows an exploded perspective view of a humidifier according to one implementation of the present invention; 
           [0026]      FIG. 2  shows an exploded perspective view of the wick assembly according to one aspect of the present invention; 
           [0027]      FIG. 3  shows the underside of the humidifier lid according to one aspect of the present invention; and 
           [0028]      FIG. 4  shows an exploded perspective view of a humidifier according to one aspect of the current technology with a respiratory pressure therapy device housed in the humidifier lid. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0029]    The present invention will be described in more detail with reference to specific implementations. However, it will be appreciated that these are intended as illustrative of the application of the present invention, and not limitative of the scope. 
         [0030]    In particular, the present invention will be described primarily in the context of an RPT or CPAP machine, intended to deliver a flow of air to a patient in the context of sleep apnoea and similar conditions. However, the present invention is applicable wherever a flow of air, optionally including medical gases or other materials, is to be delivered to a patient, and humidification is desired. For simplicity, the term ‘air’ is used in the specification and claims as a general term to include air alone, oxygen, and other medical gases admixed with air or otherwise. The invention should not be considered as limited to any particular field of use, and can be used to humidify an air flow of any desired type. 
         [0031]    The present invention may be used in conjunction with a conventional CPAP or RPT apparatus, for examples those commercially available from Resmed Limited, and as described in the references noted above, the contents of which are hereby incorporated by reference. 
         [0032]    The present invention may be implemented as a stand-alone system for connection to another apparatus, or as an integrated component within another such apparatus. 
         [0033]    One implementation of the present invention is shown in  FIG. 1 . This shows a humidifier  100  for increasing the moisture content, or absolute humidity, of a flow of air in relation to the ambient air (air surrounding the patient), before the flow of air is delivered to the entrance of the patient&#39;s airways. The humidifier  100  is configured to be coupled directly or indirectly, via an air circuit to a RPT device for receiving the flow of air. The humidifier  100  may be placed upstream or downstream of the RPT device. In one example, the humidifier  100  may deliver a flow of humidified air at approximately 70%-90% relative humidity such as 80% relative humidity and a temperature of approximately 25° C.-30° C. such as 27° C. 
         [0034]    The humidifier  100  includes an air inlet  320  to receive a flow of air, and an air outlet  310  to deliver the flow of air with added humidity. 
         [0035]    A reservoir  120  is configured to hold a predetermined, maximum volume of water  150  (or other suitable liquids, such as medications, scenting agents or a mixture containing such additives). In one form, reservoir  120  may be configured to hold several hundred millilitres of water  150 , for use during at least the length of the patient&#39;s sleep in a day. 
         [0036]    Reservoir  120  may be formed from any suitable material. It is preferably fabricated from transparent material so that the level of the water  150  is easily observed. In one form reservoir  120  may be made from glass so that it provides a substantial base for the humidifier  100  and is dishwasher safe for easy cleaning. In a preferred form reservoir  120  may be made from borosilicate glass to provide heat resistance to thermal shocks. Of course, in alternative implementations polymers or other suitable materials may be used for the reservoir. 
         [0037]    According to this implementation, and as best seen in  FIG. 2 , humidifier  100  includes an evaporative wick  200  configured to draw a flow of water  150  from reservoir  120  through wick  200  to the top surface  295 . The water feeder loop  210  in this implementation is formed from a high wet strength paper strip adhered by fusible web strips  220  to the bottom surface layer  230  of wick  200 . 
         [0038]    Water feeder loop  210  is operatively immersed in the water  150  contained in reservoir  120 , so that water is draw by capillary action to lower surface  230  of the wick  200  to deliver the flow of water to the top surface  295 . Preferably at least the lowest section of the water feeder loop  210  will touch the bottom of the reservoir  120  so that all the water  150  may be drawn to the evaporative wick  200  and evaporated. It will be appreciated that as air is humidified, the level of water  150  in reservoir  120  will reduce, and the water feeder loop  210  needs to remain in contact with water  150 . 
         [0039]    According to this implementation, the water feeder loop  210  is adhered to the bottom surface of the evaporative wick by the use of a fusible web  220 . The fusible web  220  is preferably an open, non-woven web of fine filaments of hot melt adhesive that will adhere both paper elements together but still allow water to pass across the adhesive joint. It is activated by the application of heat to melt the filament and a holding pressure until the filament is below its melting temperature. The melting temperature for the fusible web is preferable above 90° C. and below 150° C. 
         [0040]    According to other implementations, the water feeder loop  210  may comprise threaded loops of fibrous twist or braid passing through the wick assembly to contact the water  150  in the reservoir  120 . The loop may have a folded, cylindrical, rectangular, conical, or curved shape. It could be formed from, for example, string, rope, braid, woven or non-woven cloth. It could be formed from, for example, paper, cotton, wool, sponge and cellulose fibres, woven, non-woven, twisted or braided natural or synthetic fibres. It could have a profile which is, for example, corrugated, dimpled, perforated, porous, woven, knitted, textured or sintered. 
         [0041]    It will be understood that water feeder loop  210  may be formed in alternative implementations from other suitable porous, hydrophilic material, which will provide the desired wicking action. However, paper is presently preferred on the basis of cost and ease of manufacture. Similarly, while the loop is shown as a simple rectangle, other shapes or proportions could be used as desired, consistent with the requiring wicking function, and with specifically the uptake of sufficient water to the wick  200 . It may, not be formed as a loop, but for example as a simple tail structure or be formed from several structures. 
         [0042]    Wick  200  contains a heater element, formed by a PET film  250  with a patterned layer  260  of conductive material on its top surface, covered in turn by a layer of positive temperature coefficient (PTC) resistive material  270 . The thickness of the patterned layer  260  is preferably from 0.2 μm to 20 μm, preferably 1 μm. Preferably, this heater element is fabricated by the high speed printing of a metal precursor onto PET film  250  such as described in US Patent application No. 20100022078, wherein the final metal track is preferably aluminium but may be silver, copper, carbon or other highly conductive material. 
         [0043]    The patterned layer  260  forms a set of electrodes for the layer of PTC material  270 , and it is the current flowing from one electrode to the other through the PTC layer  270  that provides the heating. As the PTC layer  270  increases in temperature, the resistance increases, thus limiting the heat to a relatively stable temperature. Thus, there is no need for an external or electronic temperature control system: any increase in temperature inherently increases resistance, limiting current in the simple circuit, and so reducing the heat produced in the PTC material. It will be appreciated that this provides a very cost effective and reliable control mechanism for the humidifier. 
         [0044]    The preferred controlled temperature value is preferably about 60° C., but may be designed to be limited to any temperature from 40° C. to 80° C. The PTC layer  270  is applied in a uniform film over the entire surface of the patterned layer  260  on PET film  250  with a thickness from 10 μm to 100 μm, preferably 25 μm. The PTC layer  270  in this implementation consists of a polymeric matrix with embedded carbon particles. The polymeric matrix also forms a protective and waterproof cover to the aluminium tracks, thus protecting the tracks from the corrosive environment which may otherwise be found within the evaporative wick  200  from the presence of water and electric potential. By changing the layout of the tracks  260  and thickness of the PTC layer  270 , different specific temperatures may be obtained. 
         [0045]    It will be appreciated that an implementation of the present invention would be possible, using an electronic control with a sealed conductive heating element. 
         [0046]    A suitable PTC material is the commercially available DuPont  7292  carbon based resistor paste, described at http://www.dupont.com/content/dam/assets/products-and-services/electronic-electrical-materials/assets/datasheets/prodlib/7292.pdf, the contents of which are hereby incorporated by reference. It will also be understood that whilst a particular PTC material is described, any other suitable PTC material could be used. As will be apparent to those skilled in the art, the details of design will be particular to the materials selected, as their resistance, variation of resistance with heat, and other characteristics will need to be considered in finalising the design in a particular implementation. 
         [0047]    The process of forming the metal tracks  260  on the PET layer  250  and covering with PTC  270  material may be performed in a reel to reel (R2R) process. After printing and curing the heater layers, holes  255  are punched in the film through all layers  250 ,  260  and  270 . These holes  255  are designed to allow the flow of water from the bottom layer  230  of the evaporative wick to the top cover  280  of the evaporative wick. 
         [0048]    A bottom layer  230  of high wet strength paper is adhered to the bottom surface of the heater assembly  250 ,  260  and  270  using a fusible web  240 . This fusible web  240  is an open, non-woven web of fine filaments of hot melt adhesive that will adhere the paper bottom layer  230  to the heater PET layer  250 . It is activated by the application of heat to melt the filament and a holding pressure until the filament is below its melting temperature. The melting temperature for the fusible web is preferable above 90° C. and below 150° C. The size of the bottom layer  230  is larger all round than the heater element, so that there is a width of paper  230  with fusible web  240  protruding beyond the edge of the heater assembly. This protrusion may be from 0.5 mm to 10 mm, preferably from 1 mm to 3 mm, more preferably 2 mm. The bottom layer paper  230  with fusible web  240  will also be exposed through the holes  255  punched into the layers of the heater assembly. 
         [0049]    It will be appreciated that although top layer  280  and bottom layer  230  are preferably formed from high wet strength paper, other suitable hydrophilic and water absorbent materials could be used. It is critical that the material exhibit an appropriate level of wicking action in order for the wick  200  to be effective. 
         [0050]    A cover layer  280  of high wet strength paper is placed onto the top surface of the heater assembly and bonded to the bottom layer  230  of the evaporative wick  200  through the holes  255  by the exposed fusible web  240  and around the periphery of the evaporative wick by the exposed edge of fusible web  240 . The cover layer  280  has cut-outs  290  located in opposing corners to allow contact to be made to the metal tracks  260  through the PTC layer  270 . 
         [0051]    While the top and bottom layers are shown as generally flat, it will be appreciated that the surface could have an alternative texture, for example corrugated, dimpled, perforated, porous, woven, knitted, or textured. 
         [0052]    An illustrative fabrication process for wick  200  will now be described. It will be appreciated that alternative fabrication processes may be employed, and that this represents only one alternative. 
         [0053]    The preferred sequence of processing is to start with the PET film  250  and apply by printing the patterned layer  260  as described above. The illustrated implementation preferably uses the disposable heater technology available from Heatron in their DPH Disposable Heater, described at http://www.heatron.com/products/details/dph-disposable-heater/. the contents of which are hereby incorporated by reference. Of course, it will be appreciated that suitable alternative heating technologies may be employed. 
         [0054]    The PTC layer  270  may then be applied in a uniform thickness over the metal tracks  260  and PET film  250 . Holes may then be punched through this subassembly. It will be appreciated that the holes must be positioned to avoid perforating or contacting the conductive tracks  260 . It is important in this implementation, as described above, that the PTC layer encases the conductive tracks, to prevent issues with corrosion. 
         [0055]    Of course, it will be understood that the holes in the subassembly need not be punched, and could be formed in any suitable way in one or other of the layers, for example integrally formed in the substrate. The holes may be irregular or regular in pattern and shape, and may be of any suitable size, positioning or shape consistent with proper operation and construction of the subassembly. 
         [0056]    These processing stages may be carried out using R2R technology which is particularly suitable for high volume processing. The fusible web  240  may be produced by forming the hot melt adhesive filaments directly onto the surface of the bottom layer  230  in a R2R process forming an adhesive coated high wet strength paper. The bonding of the bottom layer  230  to the PET layer  250  and to the top layer  280  may be done using a calendaring process which is also R2R. After the bonding of these layers, small holes  235  are punched through the assembly. These holes have diameters from 0.5 mm to 5 mm, preferably 1 mm to 3 mm, more preferably 1.5 mm and are placed approximately central to the holes  255  punched in the heater subassembly  250 ,  260  and  270 . 
         [0057]    Humidifier  100  includes a humidification chamber  360 , through which air flows in order to increase the humidity of the flow of air, prior to the air being delivered to the patient. As can be seen from  FIG. 1 , humidification chamber  360  is formed between the humidifier lid  300  and wick  200 . Wick  200  is seated between lid  300  and the reservoir  120 . 
         [0058]    Humidification chamber  360  preferably contains structures to channel the airflow and increase the turbulence of air flow within the chamber, so that the air entering from inlet  320  does not simply flow in a smooth or laminar way to outlet  310 . Turbulent or disrupted airflow will result in an increased rate of evaporation from the wick, and hence more effective humidification of the air flow. 
         [0059]    Referring to  FIG. 3 , in this implementation the lid  300  has surface projections in the form of ribs  340 , extending from the inner surface of the lid  300  to abut to the surface of the evaporation wick  295 . In a preferred form, the ribs  340  may be spaced 10 mm to 30 mm apart, preferably 15 mm to 25 mm apart and more preferably 20 mm apart and the ribs  340  may have a height of 10 mm to 30 mm, preferably 15 mm to 25 mm, more preferably 20 mm. 
         [0060]    Lid  300  may also contain turbulators  350  that encourage turbulence of the airflow along the channels formed by the ribs  340 . In some implementations, these turbulators  350  may be at an angle of 25° to 65° to the air flow direction, preferably 35° to 55°, and more preferably at 45° to the air flow direction. The height of the turbulators  350  may be 5% to 45% of the height of the ribs  340 , preferably 15% to 35% of the height of the ribs  340 , more preferably 25% of the height of the ribs  340 . 
         [0061]    Lid  300  incorporates cavities  330  that locate and hold spring loaded contacts  380 . Contacts  380  have flat pads to press onto and provide effective electrical contact with the PTC layer  270 . As described above, contact cut-out areas  290  provide a recess to allow for direct electrical contact through the upper layer  280  to the PTC layer  270  for contacts  380 . Contacts  380  also connect to the power lead  390  that is connected into the lid  300 , preferably by a removable plug arrangement. Thus, contacts  380  facilitate supply power to the heater layers  250 ,  260 ,  270 . 
         [0062]    Ribs  340  also act as stiffening members to reinforce the lid so that pressure may be contained within the sealed assembly without excessive deformation. The ribs  340  also direct the air flow evenly across the evaporative surface  295  to maximize the humidification added to the air flow. 
         [0063]    The operation of the illustrative humidifier will now be described. Initially, reservoir  120  is filled with water  150 . The wick  200  is placed on top of the reservoir  120  and the lid  300  is placed onto the wick  200  and clamped into place using simple clamps (not shown) to form an airtight seal between the reservoir  120 , wick  200  and lid  300 . Thus, the risk of accidental water spillage from the reservoir is minimised. 
         [0064]    Water  150  is drawn from reservoir  120  by capillary action along water feeder loop  210  through fusible web  220  to the bottom layer  230  of wick  200 . The water then spreads out over the entire bottom layer  230  by capillary action. At the holes  255  in the PET film  250  and PTC layer  270 , water is drawn by capillary action from bottom layer  220  through fusible web  240  to the top layer  290 . There the water spreads out by further capillary action across the entire surface of the top layer  295 . 
         [0065]    When power is applied to the patterned layer  260 , the water contained in wick  200  becomes heated, encouraging evaporation from the surface  295  of top layer  290 . Operatively, air is flowing across surface  295 , and the air flow takes up evaporated water as it passes through. At the bottom layer  230 , the air adjacent quickly reaches a relative humidity of 100% and no further evaporation occurs, because there is no air flow over this surface in the reservoir below the evaporative wick  200 . There is no heat directed to the water  150  in the reservoir  120  so it does not become hot. Thus the heat application is efficient with little or no heat being lost. There is no attempt to heat or keep heated the entire reservoir, only the portion in wick  200 . 
         [0066]    Furthermore, as the water is drawn into the feeder wick  210 , residues are not left in the reservoir  120  as a result of heat forced evaporation, and so the reservoir remains clean of such residues. Any impurities in the water  150  are left within the evaporative wick  200  as the water is evaporated. 
         [0067]    It is envisaged that wick  200  is disposable and replaced at a regular frequency. The frequency of replacement will vary depending on the quality of water  150  used. Pure water such as distilled water will enable the evaporative wick  200  to last for several months, whereas tap water  150  may require replacement of the evaporative wick  200  within weeks or even days depending on the quality of the tap water. However, it would be possible to construct a wick which was not disposable, from more durable materials, if this was desired. 
         [0068]    It will be appreciated that the construction and form of the wick as described in this implementation are such that it is relatively inexpensive to manufacture, and hence having a disposable wick is economical. The design illustrated makes it very easy for a user to replace wick  200 , as well as to refill the reservoir as required. 
         [0069]    It will be appreciated that a particular size of reservoir with a capacity of a few hundred millilitres is illustrated. It will be appreciated that the principle of the present invention is applicable to any desired size or shape of reservoir. The reservoir could include, for example, openings for filling, or connections to other water storage devices. 
         [0070]    In the illustrative example, as the air flow starts, pressure builds up in the humidification chamber  360 . Small holes  235  punched through the top layer  280  of wick  200  allow this pressure to pass into the reservoir filled with water and equalize the pressure across the evaporative wick  200 . These small holes  235  also prevent splashes from the water surface passing across the evaporative wick  200  into the air stream, so preventing any biohazards in the water  150  passing into the air stream. Furthermore, these small holes  235  act as a deterrent to spill if the humidifier is accidentally tipped over, as the water flow through these holes  235  will be very small, thus minimizing the amount of water lost or spilt to the inlet  320  or outlet  310 . 
         [0071]    An advantage of implementations of the present invention is that the volume of water held within the evaporative wick  200  is controlled by gravity and surface tension, so it will never become “over full” but will always be saturated while there is water  150  present in the reservoir  120 . 
         [0072]    Another advantage of implementations of the present invention is that the temperature of the evaporative wick  200  will be controlled by the PTC layer  270 , such that it never goes above its specific design temperature. The changing resistance of the PTC layer  270  with temperature controls the power that is consumed by the evaporation of the water from the wick  200 . Furthermore, the humidity added to the airflow is controlled by the inability of the air to exceed 100% relative humidity at a specific temperature, so as the air reaches 100% relative humidity at the evaporative wick  200  surface, no more moisture will be added to the airflow. 
         [0073]    The evaporative wick may be supplied in various design configurations so that a lower specified temperature may be used for a patient in a warm humid environment with low flow therapy, or a higher specified temperature evaporative wick  200  may be supplied to a patient in a dry cool environment with high flow therapy. As the evaporative wicks  200  are consumable and preferably replaced regularly, an available selection of different specific temperature wicks  200  will allow a patient to achieve a maximum level of comfort in their specific environment. 
         [0074]    It will be appreciated that a different sequence of layers in wick  200 , or additional layers, may be used to achieve the same function. Further, whilst specific materials are described, the invention may be implemented using alternative materials with appropriate characteristics. The materials may include additional additives or functions. For example, anti-fungal or anti-microbial additives could be used, for example nano-silver, to increase the working life of wick  200 . 
         [0075]      FIG. 4  illustrates an alternative implementation, in which the lid contains a blower. In this form, there is no inlet from an RTP device, the unit has an integrated blower and takes in ambient air, which is humidified using similar components to the previous example. 
         [0076]    The lid  300  in this implementation includes a blower housing  410  that, in this form, includes an inlet filter  470 , blower  430 , muffler  440 , PCB assembly  420 . The PCB assembly may include sensors, communications, data storage, display and controls (not illustrated). The form factor of this combination is small as there is only an increase in height of the lid to accommodate blower  430 , muffler  440  and PCB assembly  420  containing controls, sensors and communications. 
         [0077]    The blower may be a suitably sized electric motor and associated fan. In general, the blower, muffler, and associated electronics may be as used in conventional RPT devices. 
         [0078]    The data storage, display and controls may be suitably used with a smart device (such as a smart phone, tablet or the like) connected using wireless communications, for example near field communications or Bluetooth. The smart device may send the data to the cloud, display the current functioning of the RPT device and allow control of the RPT device. 
         [0079]    The size of the lid  300  with blower housing  410  as described is such that is easily handled by the patient to remove it from the reservoir  120  to allow removal of the wick  200 , filling the reservoir  120  with water  150 , and replacement of the wick  200 . Wick  200  is as described in relation to the other implementation. 
         [0080]    Sensors (not illustrated) located within the blower housing  410  may allow a more sophisticated control of the humidification of the air flow, by sensing the ambient temperature and humidity, and the air flow. On this basis a calculation of the amount of moisture to be added to the air flow is possible, either through direct calculation or by look up tables. The amount of moisture to be added to the air flow is directly related to the amount of energy delivered to the evaporator wick  200 , so a target level of humidity may be easily achieved by controlling the delivered energy to the wick  200 . 
         [0081]    It is also contemplated that an implementation of the present invention could use a heater layer, and a top layer, with the top layer being connected to the water feeder loop (or other wicking structure) either directly or through the heater layer. Thus, the water feeder loop would more directly feed the top layer. If the connection was direct from the loop to the top layer, then the heater layer may not need to be porous. Similarly, suitable constructions of a multilayer wick could allow for passage of water to the top layer without passing through the heater layer. 
         [0082]    The heating layer has been described in the context of a planar conductive electrode pattern on a suitable substrate. The substrate has perforations to allow for the wicking passage of water through the substrate. It will be appreciated that heater layer could be made somewhat permeable in other ways, for example including shaped openings, or mesh sections within the substrate. A heater of different construction which allowed wicking passage of water could also be used. The use of alternative heating, for example a coated wire material in which the conductor is sealed from the water, or in which the conductive material will not degrade if exposed to the water, could allow for a porous webbing or similar material to be used for the substrate. 
         [0083]    One implementation of the present invention includes the feature that the temperature of the wick may be directly estimated by measuring the resistance of the wick heater  265  when the power is off, and using look up tables to determine the temperature. There is generally an established relationship for the temperature dependant resistance function for the PTC material, made available by the manufacturer in the case of the preferred material noted above. This may be used to determine if water is present in the wick  200  or reservoir  120  as the wick will reach maximum temperature when the water has all evaporated. 
         [0084]    Although the illustrated implementations use layers of generally equal size and shape (other than as discussed in relation to the fusible web), it will be appreciated that in alternative implementations different layers could be differently shaped from each other. The holes and orifices need not be circular. The overall shape of the wick  200  is preferably so as to fit neatly and seal within the humidifier as described, but in alternative implementations this may not be the case. 
         [0085]    Although the present invention has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the technology. In some instances, the terminology and symbols may imply specific details that are not required to practice the technology. For example, although the terms “first” and “second” may be used, unless otherwise specified, they are not intended to indicate any order but may be utilised to distinguish between distinct elements. Furthermore, although process steps in the methodologies may be described or illustrated in an order, such an ordering is not required. Those skilled in the art will recognize that such ordering may be modified and/or aspects thereof may be conducted concurrently or even synchronously. 
         [0086]    All references, websites and documents referenced in the specification are hereby incorporated by reference into this disclosure. 
         [0087]    It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the technology. 
       FIGURE REFERENCE LIST 
       [0000]    
       
         Air bleed hole  235   
         Air inlet  460   
         Blower  430   
         Blower housing  410   
         Blower outlet  450   
         Contact cut-out  290   
         Evaporative surface  295   
         Fusible web  240   
         Humidification chamber  360   
         Humidifier  100   
         Inlet  320   
         Lid  300   
         Metal tracks  260   
         Muffler  440   
         Outlet  310   
         PCB Assembly  420   
         PET layer  250   
         PTC layer  270   
         Power lead  390   
         Reservoir  120   
         Rib  340   
         RPT device  400   
         Spring contact  380   
         Spring contact recess  330   
         Turbulator  350   
         Water  150   
         Water via  255   
         Wick  200   
         Wick base  230   
         Wick cover  280