WO2001013784A1

WO2001013784A1 - Analytical apparatus and systems

Info

Publication number: WO2001013784A1
Application number: PCT/GB2000/003253
Authority: WO
Inventors: Pankaj Madganal Vadgama; Geraldine Patricia Rigby
Original assignee: Iit Limited
Priority date: 1999-08-25
Filing date: 2000-08-23
Publication date: 2001-03-01
Also published as: AU6711600A; GB9920027D0

Abstract

Sensor devices and analytical method using a permeable liquid layer on a sensor surface so that analyte from an external medium can enter the liquid layer and reach the sensor (2) by diffusion. The sensor may be secured within an open-ended tube (1) through which the liquid for the layer is fed in and regulated to maintain required conditions around the sensor. The outward liquid flow (5) maintains sensor activity and allows adjustable control, e.g. through choice of tube configuration and/or sensor position. The liquid for the layer can be fed in at an appropriate part of the tube and may be integral with the main tube. The sensor may be conventional, especially an electrode, and preferably is supported independently within the tube but may be an active sensing area within the tube rim. These arrangements avoid unacceptable dilution of media being examined, especially for monitoring biological media and environments.

Description

ANALYTICAL APPARATUS AND SYSTEMS.

This invention relates to improved analytical apparatus and systems, and more particularly to improvements relating to apparatus for use in systems for monitoring various body parameters, and especially tissue parameters, in vivo by use of electrodes.

It is well known to use electrodes as sensors in systems and apparatus for measuring a variety of parameters in many media by electrochemical analytical methods. This has been applied most commonly to non-biological analyses but has also been used to analyse and study biological media. The study of biological media is especially important as it provides a means for monitoring the conditions or components present in biological products or materials. Increasingly, it is being applied to the study of conditions within tissues, especially living tissues, to allow measurement of parameters which may be critical to life or health - for example as an indication of the need for treatment or other action, possibly with urgency. As several of such parameters can change rapidly, a means for frequent or continual monitoring is desirable, to avoid the delays involved in taking samples and waiting to have them analysed in a laboratory.

To achieve this, various proposals have been made for a sensor device to be implanted within tissue and for the output signals from that sensor device to be fed to measuring apparatus which can then make a record of how selected parameters change with time, and also indicate such changes within a very short time of their occurrence. An especially useful application for such devices is in the monitoring of glucose in body fluids, for example in blood or tissue.

However, though such electrodes usually work well in vitro there is a major problem with the more difficult types of media which need to be examined, especially those encountered in in vivo use which arise from the time required for the stabilisation of the electrode -- i.e. the time required for the signals from the electrode, when placed in vivo, to reach a condition in which the signals cease changing even though the environment around the electrode is not changing. This time delay can be as much as several hours, which effectively prevents use under conditions requiring rapid deployment. Also, in the case of electrodes measuring glucose or other components involving an oxidation/reduction process, there can be a considerable depression of the signal response under in vivo conditions, so that there is poor correlation between the signals and measurements of the same amounts or concentrations of analytes under in vivo and in vitro conditions . It can be seen that such effects are not satisfactory for accurate use in vivo and can restrict severely the usefulness of such electrodes under in vivo conditions despite their otherwise valuable properties when used under in vitro conditions. There is, therefore, a considerable need for some form of improvement for devices and methods for using sensors - and especially using electrodes - which can overcome these disadvantages and allow reliable and convenient use with a variety of media. This relates especially to use in the examination of biological media (whether in vivo or not) and in similar conditions e.g. in organic media.

Any disadvantageous effect on a sensor electrode is inconvenient, as it interrupts the use of the sensor or limits its useful life, and also is not necessarily reversed by cleaning to remove contaminants. So, it is well known to use a variety of sensor devices, usually electrode sensors, in which an active electrode sensing surface is enclosed within a barrier which serves to control and protect the electrode surface from direct contact with surrounding media. In these, the analyte species diffuses through the porous barrier to reach the electrode and be detected there, and reduces the extent to which undesirable components can reach the electrode surface and interfere with operation of the electrode - including the condition which is sometimes described as "fouling of the electrode" - or even de-activate the electrode . Use of a permeable barrier or membrane - and especially a selectively permeable barrier or membrane - has been seen hitherto as the most practical way to maintain the electrode sensor in a satisfactory operative condition. Many of these forms of device have been found to be satisfactory and effective in use, but nevertheless they do have the disadvantages of being more bulky than a bare electrode and requiring great care (often by multistage procedures) in their manufacture and in the choice of the barrier or membrane material .

In such devices, proposals have been made for providing a fluid to assist the action of electrode and barrier systems. This fluid (usually an electrolyte or buffer) may be held trapped within the barrier (or membrane) so that analytes diffuse into it through the barrier while undesirable or interfering components of the surrounding media are excluded by the barrier. Alternatively, a fluid may be provided at the surface of the barrier (or membrane) to prepare or clean the surface where it contacts surrounding media or to set up conditions for calibrating or preparing the electrode system for use. In some instances, the fluid (especially an electrolyte) is provided to serve as a means for establishing or maintaining electrical contact or for completing the electrical circuit required for effective functioning of the electrode sensor system.

For example, in our International Patent Application No. PCT/GB 93/00163 we describe a method for using or installing an electrode in place in vivo which comprises the step of providing, at the site of introduction of the said electrode, a protecting medium which, without injuring the biological environment, suppresses the adverse depressive effect on the electrode's output induced by the hostile biological environment when it has not been modified by the protecting medium. This protecting medium may then be modified or replaced by an aqueous surrounding medium which allows the electrode to become exposed to the bodily biochemical changes of the surrounding environment which is to be monitored. This protecting medium is found helpful by overcoming the fact that tissues themselves can present an environment low in water and high in protein and elastic connective tissue macromolecules (collagen, elastin, glycoproteins) and also surface-active proteins which tend to block or foul electrode surfaces. All of these distort the normal "aqueous solution" conditions used to calibrate the sensor in vitro. The protecting medium is intended to provide a protecting or less hostile environment in which the electrode can function.

However, the known systems and devices proposed for the examination of a variety of media - including but not necessarily restricted to biological media - are not found to be entirely satisfactory because they cannot maintain a sensor (especially when that is an electrode) in a suitably effective state for sufficiently long periods of time. This can be a disadvantage when monitoring over long periods is wanted and the removal and replacement of an electrode interrupts measurement . To some degree the state of a sensor (e.g. an electrode) can be improved by further addition of the fluid (e.g. buffer or electrolyte), but when this is done it can introduce fresh problems -- especially by impeding the access of analyte to the active surface of the sensor and by the "contaminating" or "diluting" effect that an introduced fluid may have, particularly when introduced in substantial amounts, on the adjacent media under examination and on which measurements are being made and on the measurements themselves. This is especially the case for biological media and in biological tissues, which present components and conditions which can readily interfere with measurements, but can also occur in any media which can contain troublesome components.

Attempts to introduce a flow of fluid can, therefore, have an undesirable effect on the accuracy and reliability of any measurements being made, and therefore need efficient and effective control. These problems occur especially with electrode sensors, but can also occur with other forms of sensors, e.g. optical sensor systems, such as a fiber-optic probe with a sensing interface or window. Confining a layer of liquid over a sensor to protect it from de-activation does not always work well because the means used to confine it can themselves become contaminated and the problem is still there. We have now found that valuable improvements can be obtained using the simple idea of forming a flow of liquid as a layer over the surface of the sensor, so that this acts as the equivalent of a membrane by allowing an analyte to pass through it by diffusion (and so reach the sensor) . Some selectivity can be achieved by choice of the liquid or its composition to reduce access of undesirable competitors of the desired analyte. Also, the flow of the liquid adds a self-cleaning effect. To describe this liquid layer, we use the term "permeable liquid layer." Thus according to our invention we provide an improved device for detecting an analyte in a medium under examination, wherein a permeable liquid layer is formed and maintained over the active surface of a sensor and interposed between the medium under examination and the active surface of the sensor, and so serves to allow a desired analyte to leave the said medium under examination and reach the active surface of the sensor by diffusion through the layer of liquid, and a parameter of the analyte is determined by the response of the sensor to the analyte which reaches it.

In use, the permeable liquid layer flows past the active surface of the sensor at a rate such that the analyte diffuses towards the sensor at a greater rate than the flow of the permeable liquid layer carries it away from the active surface of the sensor. According to a further feature of our invention we also provide a method for detecting an analyte in a medium under examination wherein

(a) the said medium is contacted with a permeable liquid layer interposed between it and a sensor and (b) the permeable liquid layer contacts the active surface of the said sensor and provides a flow of the liquid over the active surface of the sensor sufficient to keep undesirable interfering or fouling components in the said medium away from the active sensor surface while still allowing a desired analyte to reach the sensor surface so that the analyte diffuses from the surrounding medium towards the electrode at a greater rate than the flow of the liquid carries it away from the active sensor surface, and the analyte is detected and measured by the response it produces at the sensor.

In this method, we prefer that the flow of liquid is fed into the medium under examination in a quantity low enough to avoid any unacceptable degree of dilution of the surrounding media being monitored and at a rate such that it can mingle with the surrounding media and form a zone in which the analyte can reach the sensing surface where the rate of flow of the liquid outwards is less than the rate of diffusion of the analyte species inwards towards the sensing surface. The medium under examination may be any of a wide variety, and may be examined for a variety of analyte components. In general, the medium under examination is a liquid or contains a liquid. The liquid may be an individual liquid or a mixture, e.g. of one or more liquids with other compounds - usually in solution. Commonly, for most purposes, the medium will be an aqueous one but non- aqueous or partly aqueous media may be examined quite satisfactorily. The invention is seen as applicable with great advantage to biological media, for example blood or other bodily fluids or tissue, and our method can be carried out on biological media in vivo. The medium under examination is usually static but this is not essential, and the present invention is also applicable to systems in which the medium under examination is moving, e.g. flowing past the sensor device so that streams of both the medium and the liquid forming the permeable liquid layer move alongside each other.

Our invention provides a form of construction and mode of use for a sensor device which is very simple and suitable for use without having to resort to the relatively complex forms of construction described in the art . This is based on the simple concept that a [slow and controlled] flow of a liquid over the electrode sensor surface can be sufficient to keep the undesirable interfering or fouling components away from the electrode, while still allowing the desired analyte to reach the electrode, if the flow of fluid in the zone over the active surface of the electrode is controlled so that the analyte diffuses towards the electrode at a greater rate than the flow of the fluid carries it away from the electrode. To avoid spoiling the measurements by excessive dilution of the surrounding media into which the devices are placed and used, this flow should be kept as low as possible; we refer to such a low flow level as "micro-flow" for ease of description.

The devices and method of our invention may be used in conjunction with a reservoir from which a supply of liquid can be fed to form the permeable liquid layer between the sensor surface and the medium under examination and to maintain it appropriately to perform its desired action. The rate of supply of the liquid to the permeable liquid layer can be controlled by a regulator, which may be situated between the reservoir and the active surface of the sensor if desired, or it may be situated at or after the place where the liquid contacts the active surface of the sensor.

This regulation is not always achieved conveniently or readily by a direct control of the rate of flow of liquid for the permeable liquid layer (or the rate of flow of the permeable liquid layer) from a reservoir or supply, but we have found that it is more suitably done by housing the sensor in the mouth of a tube or sheath provided with means for suitably adjusting the supply of liquid flow in the region of the active surface of the sensor. In this, the liquid flow is usually made to slow down.

Such devices are particularly useful for carrying out analyses of media by insertion into a surrounding medium to be analysed, and therefore not necessarily having to take samples of the media for individual and separate examination .

It is an advantage to feed the supply of liquid required to form the permeable liquid layer through a tube or channel which delivers it at a zone close to the active surface of the sensor. This can be done by making the device of our invention in a form wherein the sensor is part of an assembly comprising a tube or channel through which liquid for the permeable liquid layer is fed and the active surface of the sensor is within this liquid, The end of the tube or channel feeding the liquid can then have a size and shape at the part which is close to the sensor which allows the liquid fed to have the desired access to the sensor surface and flow past it as the permeable liquid layer.

Thus according to our invention we provide an improved sensor device for determining an analyte in a medium, which comprises : - (a) a tubular member having an open end to provide an interface with the medium to be examined, (b) a sensing electrode situated within the tubular member (a) so that its active surface is adjacent to a space at the open end of the said tubular member, and

(c) a channel though which liquid can flow to a zone at the end of the tubular member (a) to form and maintain the permeable liquid layer on the active surface of the electrode . This can be used in conjunction with:-

(d) a regulator to control the rate of supply of the liquid to the said space at the end of the tubular member (a) so as to and maintain the liquid zone over the active surface of the sensing electrode (b) . This regulator, or means for controlling or regulating the flow of liquid, may be made part of the assembly of our device .

An advantageous and simple form of regulation can be achieved when the rate of supply of the liquid to the permeable liquid layer is regulated by the rate at which the layer passes (or is able to pass) to waste or disperse after contacting the active surface of the sensor. This may be for example by its diffusion into the medium under examination, but can also occur by being absorbed into the surroundings of the medium under examination, for example by the negative pressure which the medium (especially a biological medium) can generate within itself. When the medium under examination is not static but is moving past the sensor, the spent permeable liquid layer can be carried away with it .

A further feature of our invention is a method for the detection, measurement and/or monitoring of the presence of an analyte in a medium under examination which comprises inserting into the said medium a device as described herein and measuring the signal output from the sensor.

The invention may be applied to a variety of analytes, as is well known in the analytical techniques of the art, but we find that the invention provides an especially useful application to glucose as the analyte, and to making determinations of glucose.

The tubular member (a) having an open end to provide an interface with surroundings to be monitored, may be made of any conventional material which is suitable compatible with the surroundings in which it is to be used, and preferable is also of dimensions which allow for convenient insertion into the medium in which measurements are to be made (e.g. tissue) while also allowing the electrode (b) to be fitted inside it together with a channel (c) .

Suitable materials for the fabrication of the tubular member (a) include any materials compatible with the media with which they will be used, especially organic polymeric materials, e.g. polyethylene, polypropylene, polyvinyl chloride and the like, and compositions or mixtures based on such materials. Other suitable materials include metals, for example stainless steel, silver, and other conventionally used metals or alloys thereof. Combinations of different materials may be used if desired. The choice of material depends upon such factors as the degree of flexibility or stiffness that is desired, and whether or not the tubular member should be of electrically conducting material which could either interfere with the measurements or assist them, e.g. as a counter-electrode. Another factor of importance for use in tissue, especially in vivo, is that the material should be bio-compatible and acceptable to any biological environment in which it is to be used.

The sensing electrode (b) situated within the tubular member (a) may be any of the various forms of sensing electrode known in the art but is preferably a bare electrode, especially a material comprising a active electrode material ( usually a noble metal, and especially platinum or gold, or an alloy of one or more such metals) . If desired, the active electrode may be covered by a membrane or barrier which is selectively permeable either by its controlled porosity or the nature of the material of which it is made, as well known in the art. The present invention has the advantage, however, that it makes the use of a bare metal electrode much easier and reliable. The sensor used is preferably an electrode sensor and the consequent mode of analysis then will be electrolytic - and most desirably made amperometrically. However, other types of sensor my be used if desired, for example those based on optical detection and measurements . Thus, examples of electrodes which may be used include most of the conventional electrodes and electrode systems, for example : -

(1) any of the metals (in the form of the element or a compound) used for the study of electrochemically active species (e.g. silver, platinum, or any other base metals which are useful for the study of active species, for example ascorbate, paracetamol, etc.), membrane-covered electrodes using such metals and membranes of such materials as ion-exchange polymers or materials of controlled porosity. These include materials such as the polyether sulphone (PES) , polyvinyl chloride (PVC) and commercially available products such as Nafion; these can be used in conjunction with neuro- transmitters (e.g. noradrenalin, dopamine) ; (2) oxidase-based enzyme electrodes, for which the relevant oxidisable species may be for example glucose, lactate, etc . ) and for which the appropriately matched enzyme system can be used;

(3) de-hydrogenase-based enzyme electrodes (For these, the liquid supply to them may be a source of the co- factor

(reagent) to facilitate the functioning of the electrode) ;

(4) oxygen electrodes; these are similar to the enzyme electrodes but can be operated at higher voltages, e.g. approximately +0.6v against a silver/silver chloride (Ag/AgCl) reference electrode. In general, the invention is especially effective and applicable to bare electrodes, for example those made of a material comprising an active electrode material (usually a noble metal, and especially platinum, gold or silver, or an alloy containing one or more such metals.

The channel (c) though which liquid can flow to the said space at the end of the tubular member from a supply reservoir may be f bricated in a variety of ways . The more important part is that which is nearest to the electrode, and in this part the channel should be of a shape and size which allows the desired access to the electrode surface and flow past it .

The parts of the channel (c) more remote from the electrode (b) may be fabricated with more possibility for choice to satisfy manufacturing needs. For example, the channel (c) may be made as an integral part of the main tubular member (a) or separate from it and arrange to feed into the tubular member (a) at a point before the position occupied by the electrode. A very convenient form is for tubular member (a) to accommodate the electrode and its supporting and electrically connecting means and for the channel (c) to be provided within the tubular member (a) by the space around these means . The overall construction may have any convenient configuration provided that the channel (c) meets the electrode (b) in a configuration which allows the desired access to the electrode surface and flow of the liquid for the permeable liquid layer past it. Thus tubular member (a) , electrode (b) and channel (c) may be fitted together in a manner which is symmetrical or asymmetrical, though an approximately symmetrical form - with the electrode and its connection and support within the tubular member (a) - is usually to be preferred.

Very conveniently, liquid for the permeable liquid layer is supplied through a tube or channel combined with the sensor and the combination supports the sensor in position in the medium under examination. The means (d) for regulating the rate of supply of the liquid to the said space at the end of the tubular member so as to form a liquid zone may comprise either a configuration or shaping of the tubular member (a) itself (usually at or near the end of the tubular member) or the addition of some regulating means to the tubular member (a) , either in the region of the active sensing electrode (b) or at the exit end of the tubular member (a) , where the device interfaces with the medium under examination, for measurement or monitoring.

Thus, the regulator means (d) may be a porous membrane barrier of sufficient porosity to regulate the required flow of liquid though it out to the surroundings; this is usually situated at the exit end of tubular member (a) but possibly within it. Alternatively, it may comprise a spacing of the sensing electrode (b) back from the exit of the tubular member (a) by a distance that allows the rate of flow of the fluid feed past the electrode (b) to slow down (or otherwise adjust its flow) within the tip of the tubular member to the desired rate for operability before meeting the surrounding medium under examination.

Another form of construction which we have found to be advantageously used in the present invention comprises the arrangement wherein the active sensing area of the electrode (b) is within the rim of the tubular member (a) , either on the inner surface of the tubular member itself or on an inert spacer member within the tubular member (a) . This departs from the more usual construction in which a standard electrode is held inside a tube, but nevertheless provides an active electrode surface within the tube, and appropriate electrical connection can be made to it. Of course, this format will require a more careful choice of the material of which the tubular member (a) is made, so that the desired electrical circuitry can be attained and maintained without being short-circuited. Even so, the optimum positioning of the electrode material and the size of the tubular member can be readily determined by simple trial. An added advantage of this form is that the active electrode material can be spread over an area which can be accurately localised with respect to the tube exit, and the flow of fluid within the tubular member (a) can be regulated by simple means such as an adjustable plug within the end of tubular member (a) - for example approximately concentrically with the tube end.

The permeable liquid layer may be made up of any liquid or mixture of liquids, optionally with additional ingredients (preferably in solution) which has the required properties of permeability to the chosen analyte to which it is to permeable. It also should be compatible with the sensor system used and the surrounding media, equipment or other materials with which it may come into contact. As a particular requirement, it is preferred that it is readily miscible with the medium under examination, so that there is the minimum chance of any difficulty arising from spent liquid from that amount of the permeable liquid layer which has passed the sensor and so has flow away or mix with the surrounding media. This is one for preferring a liquid which is aqueous or, at least, compatible with an aqueous system. Thus it may be, for example :-

(i) an isotonic solution, substantially isotonic with the medium to be monitored (for example having an mOsm of 275-295) , and/or (ii) a buffer solution, compatible with the medium to be monitored, to avoid large pH changes which in turn may affect the electrode signal size. A further useful ingredient is : -

(iii) an anti-coagulant , which serves to reduce the chances of coagulation over the surface of the electrode or any part of its structure (e.g. over its membrane) . (iv) an ingredient for affecting (e.g. enhancing) viscosity of the liquid, for example a reagent or a viscosity- enhancing polymer or monomer, e.g. glycerol . These factors are especially important for use in biological media, and especially for any in vivo use.

The isotonic solution (i) may be a simple aqueous saline solution (sodium chloride solution) , or it may contain additives, as many of the available pharmaceutical media well-known in the art do. Such additives may be any conventional pharmaceutically acceptable additives compatible with saline solutions and useful for modifying or improving them, for example to preserve or sterilise them. Simple examples include potassium ion, lactate (as in Hartmann's Solution) and bicarbonate. Further examples include agents to reduce inflammation (e.g. cortisol) , analgesics or other agents able to reduce pain (e.g. procainamide) , agents which render tissues more permeable to liquids or electrolytes (e.g. hyaluronidase) and so can "open up tight tissues" to provide freer permeability, agents to reduce bacterial growth (e.g. antibiotics), and mixtures or combinations thereof .

Other components which may be used, in solution or suspension, include compounds (for example perfluorocarbons and/or lipids) which dissolve oxygen and so can serve as reservoirs for oxygen to prevent oxygen starvation and/or the oxygen and/or co-factors necessary for the functioning of an oxidase-based enzyme electrode; anti-oxidants , surfactants and many others, provided always that they are pharmaceutically acceptable and do not - by their nature or their concentration - cause any unacceptable, undesirable or adverse effect.

Similarly, the flow may be aqueous and used in a bulk organic solvent sample, or the flow may be an ionic organic solvent used in an aqueous solution, or may even contain a lipophile to aid dissolving a lipid into the flow stream.

Any "active" components used [i.e. those intended to produce effects, and especially effects other than as components of (i) , (ii) ir (iii) above] may if desired be in any convenient or conventional form, for example a "slow-release" or other form which can reduce re-absorption by the circulation. An example of such a "slow release" form is that of liposomeε enclosing the relevant active ingredient or ingredients . The buffer solution may contain any pharmaceutically acceptable buffering components but especially may be based on phosphates in known manner (e.g. a mixture of mono- sodium and di-sodium phosphates) and aimed at producing or maintaining a pH of about 7.0 to 7.8. Preferably, however, an isotonic buffer solution is used.

When an anti-coagulant is used this may be a natural product (for example heparin, hirudine, prostaglandin) or a synthetic product (for example ethylenediamine tetracetic acid -- commonly referred to as "EDTA" -- or an analogue or derivative thereof; such compounds may be conveniently referred to as carboxylated amine compounds) . Mixtures of such materials may be used if desired.

Some dilution of the media under examination by the incoming fluid supply is unavoidable, but should be kept to a minimum, i.e. as much as is necessary to ensure that the sensing surface is kept clean and efficient without undue disturbance of the surrounding media being measured or monitored or the conditions of the subject, especially when the use in in vivo.

The central concept is that a flow of fluid is fed into the medium under examination in a quantity low enough to avoid any unacceptable degree of dilution of the surrounding media being monitored and at a rate such that it can mingle with the surrounding media and form a zone in which the analyte can reach the sensing surface where the rate of flow of the fluid outwards is less than the rate of diffusion of the analyte species inwards towards the sensing surface. This is best attained by a construction which allows the outward flow of liquid (for the permeable liquid layer) to reach, at the sensing surface, a zone in which it is changed to adjust the liquid flow and assist it to interface with the surrounding medium under examination and circulate in a manner which carries it over a sensing area of the sensor electrode at which the desired analyte can reach the active electrode surface by diffusion and there cause the electrode to produce an output signal indicative of its presence and amount. For example, this can be done by making the liquid flow slow down - e.g. by a change, usually an increase, in the breadth of the flowing stream of fluid. This increase in breadth of the stream, by making the fluid flow more slowly, allows it to mingle with the surrounding media that is under examination and form a mixture which can then circulate in a manner which carries it over a sensing area of the sensor electrode at which the desired analyte can reach the active electrode surface by diffusion and there cause the electrode to produce an output signal indicative of its presence and amount, or any other appropriate parameter. In use, the electrode sensor devices of our invention may be immersed in a sample of the medium to be examined and then linked with a suitable cathode in conventional manner. Measurement of the voltage, current and the like may be taken and the measurements taken and recorded as desired, intermittently or continuously, using conventional apparatus. Electrolytic operation in the amperometric mode is preferred, but other modes may be used if desired. By selecting an appropriate sensor electrode, the devices and method of the present invention may be used for detecting by electrolytic analysis the amount of analyte present as such or indirectly formed (e.g. from an enzymic reaction) .

The medium to be examined, analysed and selected components analysed and monitored may be any medium but the invention is especially valuable when applied to biological media. Such media may be a fluid (e.g. blood) or a partly solid material (e.g. tissue). Thus the medium may be a live animal or person, - where necessary - non-living tissue. The invention can therefore be applicable to the study of such subjects as meat, fruit, fish, and other organic products, especially when the product has any tendency to change in a manner which may be critical to its value or usefulness .

The sensor devices of our invention have the advantages of allowing measurements and monitoring of changes in the measurements with time in a wide variety of subject materials, especially in the biological and medical field, and overcoming much of the disadvantage of short working life previously encountered with earlier known forms of electrode sensor devices. Particularly, they enable a significant reduction to be made in the problems of introducing fluids to assist the electrode to be maintained in a satisfactory working condition in conditions which tend to foul or de-activate the electrode ' s working surface .

The results obtained from the examination of a medium using our present invention can be used in a variety of ways, as is conventional. They may be just made for recording or evaluating as a routine procedure (e.g. for record purposes) , or they may be used to derive other information or make decisions, for example to study conditions in media under examination as a method for knowing whether they remain steady or how they may change . In cases in which a change - or rate of change - of a condition or parameter of a medium is important, the invention provides a very convenient way for maintaining a continuing watch on this and, if a significant change is observed, for taking appropriate action - for example to counteract or note the consequences of the change. In the examination of biological media, the invention may be used for routine analysis (e.g. of meat, fish and other forms of organic materials, as for example in making assessments of their condition, quality, or fitness for sale or for human or animal consumption) . For medical purposes, the results may be used for the purposes of diagnosis or as a guide for decision-making on treatment and the like and the invention can also provide an apparatus in which a device as described herein is used in combination with a device for administering a drug or treatment based on the results of our method. However, the invention also may be used for determinations of parameters which are of value for other purposes which are not connected with any diagnosis or treatment as might be carried out by doctors but only for making observations which are of interest but upon which no actions of a diagnostic or treatment nature are made .

The invention is illustrated but not limited by the accompanying drawings, which are schematic and not drawn to scale.

In the drawings, Figures 1, 2 and 3 all represent views of various forms of construction for sensor devices according to the present invention, shown in cross-section in a lengthways direction. In Figure 1, a main outer tube (1) houses within its internal cavity a sensor electrode (2) so that a space (3) remains between its surface and the inner surface of tube (1) and surrounds the electrode (2) , and the end of the tube (1) is closed by a porous barrier (4) which is in contact with the end of the sensor electrode (2) . The tube (1) is adapted to take in and conduct a supply of fluid from a reservoir or supply source (not shown) so that the fluid flows through the space (3), which serves as a channel for it, and the porous barrier (4) and then out into the surrounding medium as a flow as marked by the arrows (5) .

In operation, the return diffusion of an analyte species present in the surrounding medium under examination in the stream of fluid through out from the space or channel (3) is limited by the porous barrier (4) , for example a membrane, and outside that barrier (4) the flow of fluid slows down and mingles with the surrounding medium under examination, and the resulting mixture moves on, flowing as indicated by arrows (5) . Through natural circulatory motion, some of it moves back towards the central area of the face of the barrier (4) which is against the end face of the sensing electrode (2) . This return flow, indicated by arrows (5A) , continues and passes the central area of the face of barrier (4) and then away, and this is continually renewed. In meeting the central area of the face of barrier (4) in the zone (6) , the analyte diffuses in through the barrier (4) and reaches the end face of the active electrode (2) - where its presence is detected. The output signal from the sensing electrode (2) as a result of the analyte reaching it is fed to conventional measuring apparatus (not shown) and thereby the concentration of analyte is measured and can be recorded.

In Figure 2, a sensor electrode (2) is set back from the open end of a main outer tube (1) so that (as in Figure 1) a space (3) remains between its surface and the inner surface of tube (1) and surrounds the electrode (2) . As the tip of the electrode (2) is set back from the open end at the tip of the tube (1) , the greater space (7) within the tube (1) between the end of the electrode (2) and the end of the tube (1) causes the flow of fluid past the electrode (2) to slow down in the wider open zone (7) within the tip of the tube (1) and, as in Figure 1, the analyte diffuses into this zone (7) and is detected and measured at the tip of the sensor electrode, and the mixture of fed fluid and the surrounding medium flows away and is replenished.

In Figure 3, a main outer tube (1) houses within its internal cavity a core of inert material (8) so that, as an inner electrode (2) served in Figure 1, a space (3) remains between its surface and the inner surface of tube (1) and surrounds the inert core (8) . At the rim of the tube (1) , on its inner face, is an active sensing surface (9, 10) which acts as the sensing electrode and is connected electrically by conducting means (not shown) to conventional measuring apparatus (not shown) . In operation, a feed flow of fluid from a reservoir or supply (not shown) moves along space (3) and passes the inert core (8) and then the active sensing surface at its end rim (9, 10) , and then slows down as it escapes as indicated by arrows (5) into the wider, open zone of surrounding medium under examination and mingles with it so that sufficient analyte can diffuse back slowly against this slowing flow of liquid and reach the active sensing surfaces (9, 10) and be detected and measured there.

As an alternative to the active sensing surfaces (9, 10) being on the inner rim of the end of tube (1) , active sensing surfaces (11, 12) can be situated on the rim of the inert inner core (8) , similarly connected to conventional measuring apparatus, and used as the source of the output signal for this purpose. Although the active sensing surfaces (9, 10) and (11, 12) are mentioned above as alternatives, both may be used if desired and appropriate connection means (not shown) used to connect them to the measuring apparatus .

As an alternative to having the sensor electrode housed in an internal cavity and set back from the open end of a main outer tube (as in Figure 2) , the tip of the electrode (2) can be advanced and positioned so as to protrude or project beyond the end of the surrounding tube. This can have advantages in getting the sensor closer to the media under examination and so may be able to achieve, for example, good proximity to tissue (effectively, though not strictly literally, we describe this as "tissue contact") and minimal diluent effects. The extent to which an electrode protrudes can be adjusted to suit particular conditions and materials used; a convenient distance for a "needle" electrode is about 2.5 mm beyond the tube end. A further variant is that in which the flow of the permeable liquid layer is over (e.g. parallel to) the surface of a flat electrode surface. In this variant, the combination of a flat electrode surface with a parallel flow of the permeable liquid layer and a parallel flow of the medium under examination can form a stable protective film.

The materials and dimensions are not specifically given here, as they can vary according to the particular factors of the use and the media in which the device is used, but the materials are conventional. Likewise, the fluid fed through the channels (3) are conventional, for example an isotonic aqueous saline solution, optionally containing one or more additives such as buffers and/or pharmaceutically acceptable adjuvants.

The mode of use for the devices in general is for the assembly of the tube (1) with the inner electrode (2) or inert core (8) , as the case may be, to be connected to the supply of fluid to be used for feeding over the active sensing surfaces, and inserting this into the medium to be studied or monitored. When measurements are to be made in tissue, the tube can be inserted into the mass of tissue to reach the site to be studied and then the rate of flow from the reservoir adjusted to secure the condition that the rate of flow slows down on leaving the initial supply channel (3) to a rate which is less than the rate at which the desired analyte can diffuse back towards the active sensing surface, e.g. of the sensor electrode (2) or the sensing surfaces (9, 10, 11, 12). The detection, measurement and/or monitoring of the presence of an analyte in the medium under examination may be carried out by measuring the signal output from the sensor. This can be done by conventional means (not shown) and a parameter of an analyte can be derived from from such a measurement. The dimensions of the components and the rates of flow of the liquid can be varied considerably, and the main requirement is that these parameters are matched so as to form and maintain the desired liquid layer over the electrode surface. The size can be (and usually are) very small, which is especially apt for in vivo use and for use with other biological media or in situations which really need small apparatus. As an example of size and flow we have found suitable for this, we mention an electrode of diameter in the range 0.014 to 0.5 mm set within a cannula made of an insulating plastic material and of internal diameter approximately l mm, with a liquid flow through the cannula and around the electrode of at least 60 - and preferably at least 100 - microlitres per hour. This rate of flow suited the case in which the cannula was inserted into biological tissue which readily accepted the flow, but may need to be increased when the device is set into less receptive media. This can be done, for example, by raising the position of the reservoir supplying liquid to increase the hydrostatic pressure of the supply to the cannula.

These figures can be scaled up, provided the size does not become large enough to let turbulent flow spoil the formation of the permeable liquid layer on the electrode. The optimum figures for any particular circumstances can therefore be determined by reasonable and simple trial.

As indicated above, the usefulness of the devices and methods described herein is not restricted to use in vivo but can be applied with great advantage to practically any media - and especially to any which are not conveniently accessible by simple conventional sampling procedures for some form of individual examination in a laboratory. Thus, they are well suited to use for examination of a wide variety of media which require more or less continual monitoring and/or are most conveniently examined in situ.

Claims

WHAT WE CLAIM IS:-

1. A device for detecting an analyte in a medium under examination, wherein a permeable liquid layer is formed and maintained over the active surface of a sensor and interposed between the medium under examination and the active surface of the sensor, and so serves to allow a desired analyte to leave the said medium under examination and reach the active surface of the sensor by diffusion through the layer of liquid, and a parameter of the analyte is determined by the response of the sensor to the analyte which reaches it .

2. A device as claimed in Claim 1 wherein the permeable liquid layer flows past the active surface of the sensor at a rate such that the analyte diffuses towards the sensor at a greater rate than the flow of the permeable liquid layer carries it away from the active surface of the sensor.

3. A device as claimed in Claim 1 or Claim 2 wherein the sensor is an electrode and the measurement of the analyte is made electrolytically, especially amperometrically .

4. A device as claimed in Claim 3 wherein the electrode is a bare electrode, especially a material comprising an active electrode material (usually a noble metal, and especially platinum, gold or silver, or an alloy of one or more such metals) .

5. A device as claimed in Claim 3 wherein the electrode is an oxidase-based enzyme electrode, a de-hydrogenase-based enzyme electrode, or an oxygen electrode

6. A device as claimed in Claims 1 to 5 which includes a reservoir from which a supply of liquid can be fed to form the permeable liquid layer.

7. A device as claimed in Claims 1 to 6 which incorporates a regulator which controls the supply of liquid so that the permeable liquid layer between the sensor surface and the medium under examination is formed and maintained appropriately to perform its desired action.

8. A device as claimed in Claims 1 to 7 in which the supply of liquid fed to form the permeable liquid layer is conveyed through a tube or channel which delivers the liquid at a zone close to the active surface of the sensor.

9. A device as claimed in Claims 1 to 8 wherein the rate of supply of the liquid to the permeable liquid layer is controlled by a regulator between the reservoir and the active surface of the sensor.

10. A device as claimed in Claims 1 to 8 wherein the rate of supply of the liquid to the permeable liquid layer is controlled by a regulator at or after the place where the liquid contacts the active surface of the sensor.

11. A device as claimed in Claims 1 to 10 wherein the rate of supply of the liquid to the permeable liquid layer is regulated by the rate at which the layer passes to waste after contacting the active surface of the sensor, for example by diffusion into the medium under examination or by being absorbed into the surroundings of the medium under examination, e.g. by a negative pressure therein.

12. A device as claimed in Claims 1 to 11 wherein the sensor is part of an assembly comprising a tube or channel through which liquid for the permeable liquid layer is fed and the active surface of the sensor is within this liquid, and the end of the tube or channel feeing the liquid has a size and shape at the part which is close to the sensor which allows the liquid fed to have the desired access to the sensor surface and flow past it as the permeable liquid layer.

13. A device as claimed in any of Claims 1 to 12 which comprises : -

(a) a tubular member having an open end to provide an interface with the medium to be examined,

(b) a sensing electrode situated within the tubular member (a) so that its active surface is adjacent to a space at the open end of the said tubular member, and (c) a channel though which liquid can flow to a zone at the end of the tubular member (a) to form and maintain the permeable liquid layer on the active surface of the electrode .

14. A device as claimed in Claim 13 wherein the channel (c) through which liquid can flow to the said zone at the end of the tubular member (a) is an integral part of the tubular member (a) .

15. A device as claimed in Claim 13 or Claim 14 wherein the channel (c) through which liquid can flow to the said zone at the end of the tubular member (a) is made separate from the main tubular member (a) but is arranged to feed into it at a point before the position occupied by the electrode.

16. A device as claimed in Claims 13 to 15 wherein the sensing electrode (b) situated within the tubular member (a) is within the open end of the said tubular member and does not protrude from it, and there is a space at the open end of the tubular member adjacent to the said electrode.

17. A device as claimed in Claims 13 to 15 wherein the sensing electrode (b) situated within the tubular member (a) protrudes beyond the open end thereof.

18. A device as claimed in Claims 13 to 17 which includes means for regulating the rate of supply of the liquid to the said space at the end of the tubular member so as to and maintain the liquid zone over the active surface of the sensing electrode.

19. A device as claimed in Claim 18 wherein the supply of liquid is regulated by the configuration of the end of the tubular member (a) .

20. A device as claimed in Claims 13 to 19 wherein the outward flow of liquid reaches, at the sensing surface, a zone in which the shape of the end of the tubular member (a) changes the breadth of the flowing stream of liquid (usually by increasing in size and so slowing down the liquid flow) to adjust the liquid flow and assist it to interface with the surrounding medium under examination and circulate in a manner which carries it over a sensing area of the sensor electrode at which the desired analyte can reach the active electrode surface by diffusion and there cause the electrode to produce an output signal indicative of its presence and amount.

21. A device as claimed in Claims 13 to 20 wherein the sensing electrode (b) situated within the tubular member (a) is covered by a membrane or barrier which is selectively permeable, either by its controlled porosity or the nature of the material of which it is made .

22. A device as claimed in Claims 13 to 21 wherein the tubular member (a) having an open end to provide an interface with surroundings to be monitored, is made of a material which is compatible with the medium under examination, preferably an organic polymeric material

(e.g. polyethylene, polypropylene, polyvinyl chloride and compositions or mixtures based on such materials) or a metal (e.g. stainless steel, silver, and other conventionally used metals or alloys thereof), or combinations thereof, and especially is bio- compatible to make it acceptable for use in a biological environment .

23. A device as claimed in any of Claims 13 to 22 wherein the the tubular member (a) accommodates the electrode and its supporting and electrically connecting means and the channel (c) is provided by the space around these means.

24. A device as claimed in Claims 13 to 23 wherein the channel (c) meets the electrode (b) in a configuration which allows the desired access to the electrode surface and flow of liquid past it, especially in an approximately symmetrical form, preferably in a form in which the electrode and its connection and support are within the tubular member (a) .

25. A device as claimed in Claims 13 to 24 wherein the rate of supply of the liquid to the said zone at the end of the tubular member is regulated by the configuration or shaping of the tubular member (a) itself either in the region of the active sensing electrode (b) or at the exit end of the tube (a) , where the device interfaces with a medium to be measured or monitored.

26. A device as claimed in Claims 13 to 25 wherein a porous membrane barrier of sufficient porosity is incorporated to regulate the flow of liquid though it out to the surroundings, e.g. situated at the exit end of tubular member (a) or within the tubular member (a) .

27. A device as claimed in Claims 13 to 26 wherein the means (d) comprises a spacing of the sensing electrode (b) is spaced back from the exit of the tubular member

(a) by a distance that allows the rate of flow of the liquid feed past the electrode (b) to slow down to the desired rate within the tip of the tubular member before meeting the surrounding medium under examination.

28. A device as claimed in Claims 13 to 27 wherein the active sensing area of the electrode (b) is within the rim of the tubular member (a) , either on its inner surface itself or on an inert spacer member within the tubular member (a) .

29. A device for determining an analyte in a medium, substantially as described with reference to the accompanying drawings .

30. A method for detecting an analyte in a medium under examination wherein the said medium is contacted with a permeable liquid layer interposed between it and a sensor and the permeable liquid layer contacts the active surface of the said sensor provides a flow of the liquid over the active surface of the sensor sufficient to keep undesirable interfering or fouling components in the said medium away from the active sensor surface while still allowing a desired analyte to reach the sensor surface so that the analyte diffuses from the surrounding medium towards the electrode at a greater rate than the flow of the liquid carries it away from the active sensor surface, and the analyte is detected and measured by the response it produces at the sensor.

31. A method as claimed in Claim 30 wherein a flow of liquid is fed into the medium under examination in a quantity low enough to avoid any unacceptable degree of dilution of the surrounding media being monitored and at a rate such that it can mingle with the surrounding media and form a zone in which the analyte can reach the sensing surface where the rate of flow of the liquid outwards is less than the rate of diffusion of the analyte species inwards towards the sensing surface.

32. A method as claimed in Claim 30 or 31 wherein the sensor is an electrode and the detection and measurement of the analyte is carried out electrolytically , especially amperometrically .

33. A method as claimed in any of Claims 30 to 32 wherein the electrode is

(i) an oxidase-based enzyme electrode, or (ii) a de-hydrogenase-based enzyme electrode, or (iii) an oxygen electrode, or

(iv) a bare electrode, especially a material comprising an active electrode material (usually a noble metal, and especially platinum, gold or silver, or an alloy of one or more such metals) .

34. A method as claimed in any of Claims 30 to 33 wherein the medium under examination is a liquid or contains a liquid.

35. A method as claimed in any of Claims 30 to 34 wherein the permeable liquid layer is chosen to be miscible with the medium under examination, and preferably is an aqueous medium.

36. A method as claimed in any of Claims 30 to 35 wherein the medium under examination is a biological medium, for example blood or other bodily fluids or tissue.

37. A method as claimed in any of Claims 30 to 36 wherein the liquid forming the permeable liquid layer is

(a) a solution substantially isotonic with a medium to be monitored in vivo (e.g. having mOsm of 275-295),

(b) a buffer solution, compatible with a medium to be monitored in vivo, to avoid large pH changes which in turn may affect the electrode signal size, and/or

(c) a solution containing an anti -coagulant, which serves to reduce the chances of coagulation over the surface of the sensor or any relevant part of its structure.

38. A method as claimed in any of Claims 30 to 37 wherein a supply of liquid to form the permeable liquid layer is conveyed through a tube or channel which delivers the liquid at a zone close to the active surface of the sensor.

39. A method as claimed in any of Claims 30 to 38 wherein the permeable liquid layer is supplied through a tube or channel combined with the sensor and the combination supports the sensor in position in the medium under examination .

40. A method as claimed in any of Claims 30 to 39 wherein the permeable liquid layer is formed by a flow of a supply of liquid from a reservoir which is regulated so that the permeable liquid layer between the sensor surface and the medium under examination is formed and maintained appropriately to perform its desired action.

41. A method as claimed in any of Claims 30 to 40 wherein the rate of supply of the liquid to the permeable liquid layer is regulated by the rate at which the layer passes to waste after contacting the active surface of the sensor, for example by diffusion into the medium under examination or by being absorbed (e.g. by a reduced pressure) into the medium under examination.

42. A method as claimed in any of Claims 30 to 41 wherein the outward flow of the supply of liquid reaches, at the sensing surface, a zone in which it is changed to adjust the liquid flow and assist it to interface with the surrounding medium under examination and circulate in a manner which carries it over a sensing area of the sensor electrode at which the desired analyte can reach the active electrode surface by diffusion and there cause the electrode to produce an output signal indicative of its presence and amount .

43. A method as claimed in any of Claims 30 to 42 wherein the flow of liquid for the permeable liquid layer (or the rate of flow of the permeable liquid layer) is regulated by housing the sensor in the mouth of a tube or sheath provided with means for making the supply of liquid flow suitably in the region of the active surface of the sensor (especially when the sensor is an electrode) .

44. A method as claimed in any of Claims 30 to 43 wherein the flow of liquid for the permeable liquid layer (or the rate of flow of the permeable liquid layer) is regulated

(e.g. kept low) enough to avoid spoiling the measurements by excessive dilution of the media under examination.

45. A method for detection, measurement and/or monitoring of the presence of an analyte in a medium, especially a biological medium, which comprises inserting into the said medium a device as claimed in any of Claims 1 to 30, and measuring the signal output from the sensor.

46. A method as claimed in any of Claims 30 to 45 wherein the sensor, with provision for the attendant permeable liquid layer is inserted into the medium under examination and a flow of the liquid over the sensor surface is maintained while the measurements of the response of the sensor are made and a parameter of an analyte is derived from the measurement .

47. A method as claimed in any of Claims 30 to 46 wherein the analyte is glucose.

48. A method as claimed in any of Claims 30 to 47 which is carried out on a biological medium in vivo .

49. Use of a device as claimed in any of Claims 1 to 30 or in a method as claimed in any Claims 30 to 48 for the purpose of making glucose determinations.

50. A method for determining an analyte in a medium substantially as described with reference to the accompanying drawings .