Abstract:
A moisture sensor for monitoring an access to a patient for a system by which, via a flexible line, a liquid is fed to and/or out from a patient, for monitoring the vascular access in extra-corporeal blood treatment or for monitoring a central venous catheter for acute dialysis, and a method of producing a moisture sensor are described. The moisture sensor has a substrate material with an electrically conductive structure having conductor paths arranged at a distance from one another and connected together across a terminating resistor. The terminating resistor is an electrically conductive film which is applied in a section of the substrate material on which are formed electrical contacting regions for connecting the terminating resistor to the conductor paths, such that an external terminating resistor is not required and the moisture sensor can be produced easily in large numbers complete with the terminating resistor.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to U.S. Provisional Patent Application Ser. No. 61/669,162, filed on Jul. 9, 2012, and Application No. DE 10 2012 013 471.1, filed in the Federal Republic of Germany on Jul. 9, 2012, each of which is expressly incorporated herein in its entirety by reference thereto. 
    
    
     FIELD OF INVENTION 
     The present invention relates to a moisture sensor for monitoring an access to a patient for a system by which, via a flexible line, a liquid is fed to a patient and/or a liquid is fed out from the patient, and in particular for monitoring the vascular access in extra-corporeal blood treatment. As well as this, the present invention also relates to a method of producing a moisture sensor for monitoring an access to a patient. 
     BACKGROUND INFORMATION 
     In the field of medical engineering, there are various known systems with which, via a flexible line, liquids can be withdrawn from patients or liquids can be fed to patients. The access to the patients is generally gained in this case by means of a catheter for insertion in organs of the body or by means of a needle for puncturing vessels. During the examination or treatment, it has to be ensured that proper access exists to the patient. It is therefore necessary for the patient access to be monitored. 
     Proper access to the patient is also a particular prerequisite for the pieces of extra-corporeal blood treatment apparatus which have an extra-corporeal blood circuit. The known pieces of extra-corporeal blood treatment apparatus include for example pieces of dialysis apparatus and cell separators which require access to the patient&#39;s vascular system. In extra-corporeal blood treatment, blood is withdrawn from the patient along a flexible arterial line having an arterial puncturing needle and is fed back to the patient along a flexible venous line having a venous puncturing needle. In acute dialysis at intensive care stations on the other hand, what is used as a patient access is a central venous catheter in the patient&#39;s neck. 
     There are various known arrangements of different forms for monitoring the vascular access. These known monitoring arrangements generally rely on the safety devices which are provided as standard in the pieces of blood treatment apparatus and which trigger an immediate shutoff of the extra-corporeal blood circuit if there is not a proper vascular access. 
     There are known arrangements for monitoring a vascular access which have a device for detecting moisture to enable the escape of blood to be detected at the site of the puncture. These known moisture sensors take the form of a pad which has to be applied to the site of the puncture and which consists of an absorbent material to which an electrically conductive structure is applied or in which an electrically conductive structure is embedded. The electrically conductive structure comprises conductor paths arranged at a distance from one another which are arranged on or in the substrate material. Wetting of the substrate material by a liquid results in a change in the resistance between the conductor paths. 
     The known moisture sensors are connected by a connecting line to an arrangement which senses a change in the resistance between the conductor paths. 
     An electrically conductive structure which comprises two conductor paths open at the ends which are arranged at a distance from one another is a disadvantage in that the sensing arrangement is not able to detect either any damage to the conductor paths or a misconnection of the moisture sensor, because a break in a conductor path or the connecting line or the making of a faulty contact at the connection does not cause a change in resistance. It is therefore known for the ends of the two conductor paths to be connected together across a terminating resistor. If the resistance of the moisture sensor which is measured by the sensing arrangement corresponds to the resistance of the terminating resistor across the conductor paths, it can be assumed that the moisture sensor is intact and is correctly connected to the sensing arrangement. 
     There are known moisture sensors which consist of different substrate materials. Textile materials for example are used as substrate materials. The application of the terminating resistor to the substrate material is found to be problem in production. There are therefore known moisture sensors which do not have a terminating resistor but which are connected to an external terminating resistor. However, it is costly and complicated to connect up external terminating resistors. 
     International Patent Publication No. WO 2011/116943 describes a moisture sensor which takes the form of a woven material comprising non-conductive warp filaments and non-conductive weft filaments and conductive warp filaments and conductive weft filaments, which are so arranged that they form an electrically conductive structure which comprises a first and a second conductor path. The ends of the two conductor paths take the form of connecting contacts which are arranged on a tongue with a lateral offset from one another. The connector of a connecting cable by which the moisture sensor is electrically connected to the sensing arrangement is connected to the tongue. The sensing arrangement measures the electrical resistance between one pair of connecting contacts of the moisture sensor while the other pair of connecting contacts are connected together across a terminating resistor which is incorporated in the connector. 
     A moisture sensor which comprises an arrangement of conductor paths arranged on a substrate layer is described in German Application No. DE 10 2010 023 132. The two conductor paths of the moisture sensor are connected together by a contact layer whose resistance changes in the wetted state. 
     International Patent Publication No. WO 2010/091852 described a moisture sensor in which the conductor path structure and the terminating resistor are applied to a substrate material by screen printing. A printing paste containing silver is used for the conductor path structure and a printing paste containing carbon for the resistor structure. Although it is fundamentally possible for the terminating resistor to be applied by printing even in the case of a moisture sensor made of a textile material, it is found to be a disadvantage because of the additional steps in the procedure which are called for by the printing process. 
     SUMMARY 
     An object underlying the present invention is to provide a moisture sensor for monitoring a patient access which can be inexpensively produced in large numbers. It is also an object of the present invention to specify a method of producing the moisture sensor inexpensively in large numbers. 
     The moisture sensor according to the present invention has a substrate material to be applied to the patient&#39;s skin which has an electrically conductive structure which comprises conductor paths arranged at a distance from one another which are connected together across a terminating resistor. The substrate material of the moisture sensor according to the present invention may be different kinds. It may be a textile material or a non-textile material. However, the substrate material should preferably be absorbent. 
     The moisture sensor according to the present invention is characterised in that the resistor is an electrically conductive film which is applied in a section of the substrate material on which are formed electrical contacting regions for connecting the terminating resistor to the conductor paths. The electrical contacting regions for connecting the terminating resistor may be provided on the underside of the moisture sensor adjacent the patient or on the upper side thereof remote from the patient. The contacting regions do however have to be provided on the same side to enable the electrically conductive film to be applied. 
     The moisture sensor according to the present invention has the advantage that an external terminating resistor is not required. This simplifies the connecting up of the moisture sensor. The moisture sensor according to the present invention can be easily produced in large numbers complete with the terminating resistor by applying the conductive film as a web of film during the production process. The web of film having been applied, the moisture sensors can be separated out, complete with the terminating resistor, by cutting, punching or stamping the moisture sensors, complete with the terminating resistor, out of the web of material they form together. 
     The advantages of the moisture sensor according to the present invention become particularly apparent when the substrate material is a textile material, and in particular a woven material, to which it is difficult to apply pasty substances to form the terminating resistor. 
     In a preferred exemplary embodiment which allows the method of production to be carried out particularly easily, the electrically conductive film and the substrate material are adhesive bonded together by an electrically conductive adhesive. Electrically conductive films and electrically conductive adhesives are part of the prior art. 
     Rather than their being adhesive bonded by a conductive adhesive, an alternative exemplary embodiment envisages the electrically conductive film and the substrate material being connected by a covering film which extends over the electrically conductive film and which is applied to the substrate material. In this alternative exemplary embodiment the connection of the covering film and the substrate material does not call for the use of an electrically conductive adhesive. 
     The electrically conductive structure may take different forms. In a preferred exemplary embodiment, the electrically conductive structure has a first conductor path and a second conductor path, with the electrical contacting portions for the connection of the terminating resistor being formed at one end of the first conductor path and one end of the second conductor path. The other end of the first conductor path and the other end of the second conductor path preferably take the form of electrical connecting contacts for an electrical connector for connecting the moisture sensor to the sensing arrangement. The electrical contacting portions for the terminating resistor are preferably arranged on one side of the substrate material and the electrical connecting contacts for the connector are preferably arranged on the other side thereof. This makes it possible for the terminating resistor to be produced by applying the electrically conductive film to the side of the moisture sensor which has the contacting portions. 
     The substrate material of the moisture sensor preferably takes the form of a planar material, with the electrical contacting portions and the electrical connecting contacts being arranged on a tongue belonging to the planar material. The tongue on the one hand allows the connector to be connected easily and on the other hand allows the electrically conductive film to be applied because the tongue is physically separated from the electrically conductive structure. 
     A particularly preferred exemplary embodiment makes provision for a substrate material of non-conductive warp filaments and non-conductive weft filaments and conductive warp filaments and conductive weft filaments, the electrically conductive structure being formed by conductive warp filaments and/or weft filaments. The advantages of the moisture sensor according to the present invention are particularly apparent in this exemplary embodiment because the web of conductive film can easily be connected to the web of woven material. The electrically conductive film may be any film which has a suitable resistance. What are suitable as electrically conductive films are for example the known carbon films or polymer films. 
     The value of the terminating resistor should be more than the resistance of the wetted moisture sensor and less than the value of the internal resistance of the sensing arrangement including the resistance of the connecting cable. If the sensor is to respond to single drops of liquid regardless of the relative humidity of the surroundings, a value of 100 kohms to 1000 kohms has been found to be advantageous for the terminating resistor in practice. 
     The method of producing the moisture sensor is characterised in that, in the first step of the method, a web of woven material is woven from non-conductive warp filaments and non-conductive weft filaments and conductive warp filaments and conductive weft filaments, the conductive and non-conductive warp and weft filaments being so arranged that the said conductive and non-conductive warp and weft filaments form an electrically conductive structure in the woven material. A method of weaving of this kind is part of the prior art. This method of weaving is described in International Patent Publication No. WO 2011/116943. 
     Only one additional step of the method, to produce the terminating resistor, follows the weaving process, this step being easily able to be incorporated into the production process. In this step of the method, a web of an electrically conductive film, which is preferably a self-adhesive film, is applied to the web of woven material. The width of the web of film is smaller in this case than the width of the web of woven material, the web of film being applied to that section of the substrate material on which the electrical contacting portions for the terminating resistor are formed. The individual moisture sensors are then separated out by cutting, punching or stamping the sensors out from the web of material to the preset outline. The method according to the present invention may also comprise further steps which include for example washing, impregnation, application of a layer for adhesion, and covering with a backing layer, such as for example siliconised paper, and the like. 
     Exemplary embodiments of the present invention are explained in detail below with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows the principal components of a blood treatment apparatus which has an arrangement for monitoring a vascular access. 
         FIG. 2  is a schematic view of the moisture sensor having the electrically conductive structure. 
         FIG. 3A  is a plan view of a first exemplary embodiment of the connecting tongue of the moisture sensor. 
         FIG. 3B  is a view from below of the connecting tongue shown in  FIG. 3A . 
         FIG. 4A  is a plan view of a second exemplary embodiment of connecting tongue. 
         FIG. 4B  is a view from below of the connecting tongue shown in  FIG. 4A . 
         FIG. 5  is a plan view of a third exemplary embodiment of the connecting tongue. 
         FIG. 6  shows an exemplary embodiment of the connecting tongue of the moisture sensor together with a first exemplary embodiment of the terminating resistor. 
         FIG. 7  shows the connecting tongue shown in  FIG. 6  together with a second exemplary embodiment of the terminating resistor. 
         FIG. 8  is a view of the principal steps in the method of producing the moisture sensor according to the present invention. 
         FIG. 9  is a view of the step of the method which comprises the application of the terminating resistor to the moisture sensor. 
         FIG. 10  shows an alternative exemplary embodiment of the connecting tongue of the moisture sensor. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows the principal components of a blood treatment apparatus, and in particular a haemodialysis apparatus A for acute dialysis, which has an arrangement B for monitoring a vascular access and in particular a vascular access which has a central venous catheter. The monitoring arrangement B is part of the haemodialysis apparatus A in the present exemplary embodiment. The dialysis apparatus will first be described with reference to  FIG. 1 . 
     The haemodialysis apparatus A has a dialyser  1  which is divided into a blood chamber  3  and a dialysis-fluid chamber  4  by a semi-permeable membrane  2 . The vascular access to the patient is obtained by means of a central venous catheter  5  which is connected to the patient&#39;s neck. The central venous catheter  5  is part of the extra-corporeal blood circuit I, which is merely indicated and which includes the blood chamber  3  of the dialyser  1  and comprises flexible lines  6 ,  7 . A blood pump  8  is provided to pump the blood in the extra-corporeal circuit. 
     The dialysis-fluid circuit II of the dialysis apparatus A comprises a dialysis-fluid source  10  to which a dialysis-fluid infeed line  11 , which runs to the inlet of the dialysis-fluid chamber  4  of the dialyser  1 , is connected. Running off from the outlet of the dialysis-fluid chamber  4  of the dialyser  1  is a dialysis-fluid outfeed line  12  which runs to an outlet  13 . A dialysis-fluid pump  14  is connected into the dialysis-fluid outfeed line  12 . 
     Responsible for controlling the dialysis apparatus is a central control unit  15  which operates the blood and dialysis-fluid pumps  8 ,  14  via control lines  16 ,  17 . The central control unit  15  is connected by a data line  18  to an alarm unit  19  which gives a visual and/or audio and/or tactile alarm if anything untoward happens. 
     In the present exemplary embodiment, the monitoring arrangement B, which is only shown schematically, is used to monitor a Luer lock connector  9  having parts  9 A and  9 B, for connecting the central venous catheter  5  to a flexible line  9 C belonging to the extra-corporeal blood circuit I. The monitoring arrangement B has a device  100  for detecting moisture which is arranged at the point  21  of the connection to the flexible line. This moisture sensor  100  is only shown in schematic form in  FIG. 1 . As well as this, the monitoring arrangement B also has a sensing arrangement  22  which is electrically connected to the moisture sensor  100  by a connecting line  23 . The connecting line  23  is connected to the moisture sensor  100  by an electrical connector  23 A. 
     The sensing arrangement  22  is connected to the central control unit  15  of the dialysis apparatus A by a data line  24 . In the event of blood escaping from the point of connection  21  to the flexible line and wetting the moisture sensor  100 , the sensing arrangement  22  of the monitoring arrangement B generates a control signal which the central control unit  15  receives via the data line  24 , the central control unit  15  then making an intervention in the blood treatment. The control unit  15  stops the blood pump  8  and generates an alarm signal so that the alarm unit  19  gives an audio and/or visual and/or tactile alarm. 
     The moisture sensor  100  according to the present invention will be described in detail herein. As a substrate, the moisture sensor has a multi-ply woven material which comprises electrically conductive and electrically non-conductive warp and weft filaments (e.g., monofilaments, carbon fibres, silver coated polyamide yarns). The moisture sensor  100  has an underside adjacent the patient&#39;s skin and an upper side remote from the patient&#39;s skin. It has a central region  100 A to which are connected a left-hand and a right-hand arm  100 B,  100 C, respectively, which extend round a circular cut-out  100 D. Connected to the central region  100 A on the opposite side from the arms is a connecting tongue  100 E. 
     The electrically conductive and electrically non-conductive warp and weft filaments are so arranged in the multi-ply woven material that an electrically conductive structure is formed. The electrically conductive structure comprises warp and weft filaments extending orthogonally to one another which are electrically conductive. In  FIG. 2 , the electrically conductive warp and weft filaments are identified as K[1] to K[8] and S[1] to S[12] respectively. The intersections of the warp and weft filaments are indicated in  FIG. 2  by circles. The moisture sensor shown in  FIG. 2  is a preferred exemplary embodiment for monitoring the venous and/or arterial needles in chronic haemodialysis. 
     The electrically conductive structure L 1 , L 2  comprises two conductor paths L 1  and L 2 . The start of each conductor path is identified as “A” and the ends of the conductor paths as “B”. The first conductor path L 1  extends from the connecting tongue  100 E over the central region  100 A to the left-hand arm  100 B, from the left-hand arm over the central region to the right-hand arm  100 C, and from the right-hand arm over the central region back again to the tongue  100 E. The second conductor path L 2  extends from the tongue  100 E over the central region  100 A to the left-hand arm  100 B, from the left-hand arm  100 B over the central region to the right-hand arm  100 C, and from the right-hand arm over the central region to the tongue  100 E. The two conductor paths L 1  and L 2  extend parallel to and at a distance from one another. They each have a plurality of portions which are positioned orthogonally to one another. 
     One end L 1 A and L 2 B of the first and second conductor paths L 1 , L 2  form two outer connecting contacts on the upper side of the tongue  100 E ( FIG. 3A ), whereas the other ends L 1 B and L 2 A of the first and second conductor paths L 1 , L 2  form two inner contacting portions on the underside of the tongue  100 E ( FIG. 3B ). These contacting portions are shown schematically in  FIGS. 3A and 3B . 
     The two inner contacting portions L 1 B and L 2 A are electrically connected to a terminating resistor R which is not shown in the drawings. The terminating resistor R is a conductive film which is applied to the upper side or underside of the tongue  100 E. 
     In the exemplary embodiment, the electrically conductive film R by which the contacting portions L 2 A and L 1 B of the conductor paths L 1 , L 2  are connected together is applied to the underside of the tongue  100 E. This film is indicated by single hatching in  FIG. 3B . 
       FIGS. 4A and 4B  show the upper side and underside of a second exemplary embodiment of the tongue  100 E which differs from the first exemplary embodiment in the arrangement of the contacting portions and connecting contacts on the upper side and underside of the tongue, there being an outer and an inner connecting contact L 1 A, L 2 B ( FIG. 4A ) situated on the upper side of the tongue  100 E and an outer and an inner contacting portion L 2 A, L 1 B situated on the underside of the tongue ( FIG. 4B ). In this exemplary embodiment, the conductive film R is applied to the underside of the tongue  100 E, which has the contacting portions L 2 A, L 1 B ( FIG. 4B ). The conductive film R is once again indicated by single hatched lines. 
       FIG. 5  shows a further exemplary embodiment of the tongue  100 E in which the two connecting contacts and the two contacting portions are situated only on the upper side of the tongue  100 E. The connecting contacts L 1 A, L 2 B are for example situated on the inner side adjacent the central portion  100 A and the contacting portions L 2 A, L 1 B on the outer side of the tongue, the connecting contacts and contacting portions being offset from one another laterally. In this exemplary embodiment, the conductive film R is applied to the upper side of the connecting tongue  100 E, the film extending however only over the outer half of the connecting tongue in which the contacting portions L 2 A, L 1 B are situated. The conductive film R is indicated by single hatched lines. 
       FIG. 6  shows the contacting portions L 2 A, L 1 B on the upper side of a tongue  100 E which are connected together electrically by the electrically conductive film R. The electrically conductive film R extends only over the bottom half of the connecting tongue. The distance between the parallel contacting portions L 2 A, L 1 B in line form is identified as L and the width of the conductive film R as D. With a given sheet resistance for the film, the resistance R 0  of the terminating resistor R can be set within certain limits by the distance L between the contacting portions and the width D of the film. With a given sheet resistance for the film, the resistance R 0  is proportional to the distance L and inversely proportional to the width D. The conductive film R is once again indicated by single hatched lines. 
     The resistance R 0  of the terminating resistor should be between 100 kohms and 1000 kohms. This gives a sheet resistance for the electrically conductive film which is between 2×10 4  ohm/sq and 5×10 7  ohm/sq. This value corresponds to the sheet resistance of known electrically conductive polymer films (e.g., dissipative polymers). It is however also possible for known electrically conductive carbon films which are used for protection against electrostatic charges to be used as the terminating resistor. 
     In a preferred exemplary embodiment, the electrically conductive films are adhesive bonded to the connecting tongue  100 E of the moisture sensor  1  by an electrically conductive adhesive, in which case the film may cover a part of the tongue or its entire area.  FIG. 7  shows an exemplary embodiment in which the electrically conductive film R covers the entire area of the tongue  100 E of the moisture sensor. 
       FIG. 8  is a schematic view of the method step comprising the production of the web of woven material  70  from warp and weft filaments  50 ,  60 , on which web of woven material  70  are situated a plurality of moisture sensors laid out in columns and rows which each have an electrically conductive structure. In a step of the method which follows the weaving process, a web  80  made of an electrically conductive film is applied to the upper side or underside of the tongues, the woven material and film being adhesive bonded together by an electrically conductive adhesive. The web of woven material is applied to the upper side or underside of that region of the tongue on which the connecting portions for connecting to the terminating resistor are situated. To apply the web of film  80  to the web of woven material  70 , the film and the web of woven material pass through two pressure-applying rollers  85 ,  90  by which the requisite applied pressure is exerted on the webs. 
       FIG. 9  is a schematic view of that part of the web of woven material  70  in which the web of film  80  is applied to the upper side of the web of woven material. The web of film  80  indicated by the single hatched lines covers the entire area of the upper side of the tongues  100 E which have the contacting portions L 2 A, L 1 B. The direction of revolution of the pressure-applying rollers (not shown) between which the webs run is indicated by an arrow. 
     The webs of film and woven material  70 ,  80  having been bonded together, the moisture sensors are cut, punched or stamped out, in order then to be finished and packed. The method of production comprises other steps which are known to the person skilled in the art but are not shown in  FIG. 8 . 
       FIG. 10  shows an alternative exemplary embodiment of the connecting tongue  100 E of the moisture sensor in which a woven material and a film are adhesive bonded together by an electrically conductive adhesive. In this exemplary embodiment, the electrically conductive film R lies only on the contacting portions L 2 A, L 1 B. The electrically conductive film R is fixed in place by a covering film  95  which extends beyond the edges of the electrically conductive film R. On the underside, the covering film  95  is provided with an adhesive or adhering layer, thus causing the covering film to adhere to the tongue  100 E to fix the electrically conductive film in place.