Ipsofactoj.com: International Cases [2002] Part 12 Case 9 [CAEW]



Alphasense Ltd

- vs -

City Technology Ltd




26 MARCH 2002


Lord Justice Aldous

  1. Alphasense Ltd appeal against the judgment of Mr. David Young QC sitting as a deputy judge of the High Court.  In that judgment he held City Technology Ltd’s patent No. 2094005 valid and infringed.  He went on to grant appropriate relief. 

  2. The appellants submitted that the judge should have held that the patent had not been infringed and was invalid as it was obvious. 


  3. The patent was applied for in 1982 and expired at the end of January 2002.  It is concerned with the use of an electrochemical cell used as a gas sensor.  The basic operation of electrochemical cells is well known.  Such a cell has a minimum of two electrodes (the anode and the cathode) in contact with an electrolyte.  The chemical reaction produces a current.  A simple form is the well-known lead acid battery which uses electrodes made of lead and the electrolyte is a solution of sulphuric acid.  Since the electrodes are of a similar material there is no electromotive force (e.m.f).  Upon charging, oxygen from the electrolyte collects at one electrode and hydrogen at the other.  The oxygen combines with some of the lead to form lead dioxide and the hydrogen is liberated.  There are now two dissimilar electrodes immersed in the sulphuric acid with the result that their joinder produces a current.  During discharge, the lead dioxide of one electrode converts to lead sulphate as does the metallic lead at the other. At the same time the strength of the electrolyte is gradually reduced by the formation of water.  Upon recharging, the lead sulphate of the one electrode is converted back to lead oxide and to metallic lead at the other and the strength of the electrolyte is restored. 

  4. The chemical reaction of such cells produces an electrochemical reaction which is shown by the e.m.f.  This is the result of a potential difference across the interface between an electrode and an electrolyte.  The magnitude of the change in potential difference at each of two electrodes depends upon the current density, the particular reactions at each electrode, the material of the electrodes and the reactor conditions. 

  5. The invention of the patent uses an electrochemical cell as a gas sensor.  Its main application is the detection of carbon monoxide (CO) in coal mines.  As stated on page 3 of the patent, at one electrode (the anode), carbon monoxide is electrochemically oxidised

    CO + H2O = CO2 + 2H+ + 2e

  6. At the cathode a reduction process takes place: e.g.

    ˝ O2 + 2H+ = H2 O

  7. The overall sensor cell reaction is the sum of the two electrode reactions namely:

    CO + ˝ O2 = CO2

  8. By Faraday’s law the flux of carbon monoxide reacting at the anode is proportional to the current.

  9. The patent begins by informing the reader that there is an increasing demand for gas sensing instruments particularly for safety monitoring, pollution control, fire detection and flue gas analysis.  The special demand is for portable, relatively inexpensive instruments which are small, compact, lightweight and robust.  It is stated (at page 1 line 4):

    A particular problem which arises in instruments of this type is to ensure an adequate electrolyte connection between the electrodes, without which the basic operation of the cell is adversely affected.  This connection needs to be stably maintained regardless of attitude changes and of the appreciable volume changes in the electrolyte that can result from changes in ambient humidity. 

    It is an objective of this invention to provide a small compact electro-chemical gas sensor that is simple in construction and has a high degree of integrity, reliability and freedom from the proneness to leakage and the problems mentioned above that have often been found with electro-chemical sensors.

  10. The basic components of the patented sensor can be appreciated from figure 1 of the patent which is an exploded longitudinal section of a first embodiment of the invention:

  11. The sensing electrode 1 is made of porous PTFE support tape to which is bonded a catalyst/PTFE mixture.  The counter electrode 2 is of similar construction, but with a hole 3 through which a wick 4 passes to the chamber 5 which acts as a reservoir for the electrolyte.  The current collectors 6,7 contact the electrodes and lead out to terminal posts.  A top plate 14 has capillary holes 15 which form a diffusion barrier for restricting access of the gas to be detected.  It has a cavity 16 to provide diffusion across the whole surface of the electrode.  The whole assembly is clamped together with nuts and bolts to compress the O ring 12 and to ensure a good seal. 

  12. Example 1 of the patent used the construction of Figure 1.  The housing components 13, 14 and 17 were of acrylic plastic; the anode was a mixture of platinum black and PTFE particles bonded to porous PTFE tape; the cathode was of similar construction but with a 3 mm diameter hole through its middle.  The electrolyte was IO-N sulphuric acid.  Five capillaries, 1 mm inside diameter and 4 mm length, were in the top plate.  It performed well giving a sensitivity to carbon monoxide of 0.11 uA/ppm and the temperature coefficient of less than 0.05% of signal per oC.

  13. Claim 1, with numerals inserted for convenience, is as follows:

    Claim 1.

    A compact and leak-resistant electrochemical gas sensor in the form of an electro-chemical cell assembly comprising


    a first planar (sensing) electrode


    a second planar (counter) electrode comprising


    a planar hydrophobic porous support


    said support having an opening therein


    a planar hydrophilic non-conducting porous separator interposed between said first and second electrodes and in contact therewith and being in contact with


    a hydrophilic non-conducting porous wick passing through said opening and extending into


    an electrolyte chamber partially filled with an electrolyte contacting said wick

    said assembly permitting access of a gas to be sensed to the sensing electrode and providing an electrolytic connection between the sensing and counter electrodes in all orientations of the assembly.


  14. The appellants’ sensor is diagrammatically shown in the drawing attached to the product description which is reproduced below. 

    FIG 2

  15. The critical elements of the appellants’ sensor are the parts called the wick 19 and separators 16, 14, 12, 11 and 9.  When assembled, the raised portion of the wick 19 touches the separator 16 so that electrolyte is conveyed from the reservoir 20, 21 via the wick to that separator.  From there it is conveyed via separators 14, 12, 11 and 9 to the sensing electrode 8.  The gas to be sensed comes down from the top through a filter membrane to the sensing electrode. 

  16. The appellants submitted before the judge and before us that their sensor did not have features (iii), (iv) and (v) of claim 1.  The essence of their submission was that their wick only extended up to the first separator.  Thereafter the electrolyte was conveyed by separators.  That was not the construction claimed, as the claim required the wick to convey the electrolyte and also differentiated between the wick and the separators. 

  17. The respondents submitted that it was not appropriate to give the claim such a literal construction.  The correct approach was that required by the Protocol on Interpretation of Article 69 EPC.  Thus the claim should be given a meaning which provided fair protection to the patentee and reasonable certainty to third parties.  If that was done, there was infringement. 

  18. Both parties agreed that the claim had to be construed accordingly to the Protocol.  They also agreed that the judge was right to use the Protocol questions, suggested by Hoffmann J in Improver Corporation v Remington Consumer Products [1990] FSR 181 at page 189.  For convenience they are set out below:


    Does the variant have a material effect upon the way the invention works?

    If yes, the variant is outside the claim.

    If no-


    Would this (i.e. that the variant had no material effect) have been obvious at the date of the publication of the patent to a reader skilled in the art. 

    If no, the variant is outside the claim.

    If yes -


    Would the reader skilled in the art nevertheless have understood from the language of the claim  that the patentee intended that strict compliance with the primary meaning was an essential requirement of the invention.

    If yes, the variant is outside the claim.

    On the other hand, a negative answer to the last question would lead to the conclusion that the patentee was intending the word or phrase to have not a literal but a figurative meaning (the figure being a form of synecdoche or metonymy) denoting a class of things which included the variant and the literal meaning, the latter meaning, the latter being perhaps the most perfect, best-known or striking example of the class.

  19. The judge reminded himself that his task was to apply the Protocol.  He went on to quote the Protocol questions which he answered in this way:


    Assuming in the Defendants favour that on a literal construction of the word wick there must be a unitary tape or bundle of fibres which extends from the reservoir through the opening in the support, I have no doubt that both the requirements of the Improver questions and the Protocol to Article 69 are satisfied.  Thus assuming the variant in this case is the combined use of a single wick (19) and the centre of the separator (16) which allows the electrolyte to be transported from the reservoir to the separators sandwiched between the electrodes, such arrangement clearly has no material effect upon the way the invention works as it will allow electrolyte to be maintained within the separators at all orientations of the sensor (Question 1 of Improver).  As to whether such a fact [would] have been obvious at the date of publication of a patent to the reader skilled in the art.  I have no doubt that the close contact of the hydrophilic wick (19) with the hydrophilic separator (16) when assembled will allow electrolyte to be so transported to the separators just as a single wick would do so as described in the patent (Question 2 of Improver).  Finally as to whether from the language of the claim it was to be understood that the patentee intended that it was an essential requirement that the wick should be a unitary piece extending from the reserving through the support opening.  I consider there is no good reason why the claim should be so narrowly construed particularly in the light of the teaching that the wick may be made integral with the separator.  (Question 3) The essence of the invention seems to be how the electrolyte is conveyed to the separators sandwiched between the electrodes, namely by means of a hydrophilic wick arrangement which transports the electrolyte from the reservoir to the separators through an opening in the counter electrode support.  This seems to me precisely how the Defendants wick (19) and separator (16) function.  Applying the Protocol, such an interpretation combines a fair protection for the patentee with a  reasonable degree of certainty for third parties - a narrowed interpretation would be unfair to the patentee.

  20. The appellants accepted that the judge was right to hold that the variant, present in their device, had no material effect on the way the invention worked and that that would have been obvious.  However they submitted that he was wrong to conclude that the skilled person would not have understood that the patentee intended strict compliance.  They drew attention to the opening paragraphs of the specification which set out the problem and the object (see paragraph 9 above).  They submitted that in those passages the patentee had made it clear that the invention resided in the particular construction which gave the advantages asserted.  That was emphasised by the claims which set out the essential constructional features which when assembled in the claimed way, solved the problem.  It followed that the patentee had indicated that three features, without variation, constituted the invention. 

  21. I reject that submission. Nowhere in the specification or the claims does the patentee indicate that strict compliance was a requirement.  The specification follows a familiar pattern for mechanical inventions, namely a statement of the problem, followed by the objective and then the consistory clauses.  After that comes the specific description and the claims.  The claims also follow a normal pattern.  There is nothing in the specification or claims which indicates that the fair protection envisaged by the patentee should be confined to a literal interpretation. 

  22. The claim is not concerned with whether the wick is of unitary construction nor with any difference between a separator and a wick.  There is every reason to believe that fair protection for the respondents requires the claim to be construed as including the appellants’ sensors.  Further, I do not believe that third parties would be surprised by that conclusion. 

  23. I conclude that the judge was right to hold that the appellants’ sensors infringed claim 1. 


  24. The sole attack upon the validity of the patent was that it was obvious (sections 72(1)(a) and 1(1)(b) of the Patents Act 1977).  The state of the art relied on was US patent 3 755 125 (Shaw) and also common general knowledge. 

  25. The question of obviousness is best examined using the approach advocated by Oliver LJ when giving the judgment of the court in Windsurfing International Inc v Tabur Marine (G-B) Ltd [1985] RPC 59.  The court has to assume that the notional skilled person is sufficiently interested to address his mind to the subject and to consider the practical application of the information which he is deemed to have.  As Oliver LJ said, the court should then take the following four steps when answering the jury question:

    The first is to identify the inventive concept embodied in the patent in suit. Thereafter, the court has to assume the mantle of the normally skilled but unimaginative addressee in the art at the priority date and to impute to him what was, at that date, common general knowledge in the art in question. The third step is to identify what, if any, differences exist between the matter cited as [forming part of the state of the art] and the alleged invention. Finally, the court has to ask itself whether, viewed without any knowledge of the alleged invention, those differences constitute steps which would have been obvious to the skilled man or whether they require any degree of invention.

  26. The Shaw Patent was applied for in 1971 and was published in 1973.  It discloses an electrochemical gas analyzer.  Its construction can be derived from figures 3 and 4:


    FIG 3





















    FIG 4

  27. The Shaw device has wire mesh electrodes 41, 46 supported by conducting rings 40, 45.  Between the electrodes are non-conducting hydrophilic separators 42, 44.  Depending from the separators 42 are four wicks 43 positioned at 90o to each adjacent wick.  They depend around the electrode 43 into the electrolyte reservoir 11 which is filled with an absorbent sponge-like material.  The gas to be analyzed enters from the top of the stack through a PTFE membrane 47 which prevents egress of the electrolyte.

  28. The first Windsurfing step is to identify the inventive concept.  It is the combination in claim 1, but the parties agreed that the essence of the invention and its novelty lay in the use of a counter electrode comprising a hydrophobic support having an opening in it and having a wick passing through the opening. 

  29. Before the judge there was a dispute as to what was common general knowledge.  He reminded himself of the difference between what was known or even well-known and common general knowledge (see Wheatley v Drillsafe [2001] RPC 133 at 146).  He went on to hold that the following matters were accepted by the parties to be common general knowledge. 


    .... The following matters were accepted by the parties to be part of the common general knowledge, namely:


    that electrochemical cells may be constructed having planar parallel electrodes


    that by 1981 platinum black PTFE electrodes supported on a porous PTFE support had been accepted in fuel cell technology and would be considered a suitable electrode for a gas sensor


    hydrophilic separators between electrodes were an accepted way of maintaining the electrolyte between the electrodes


    that some form of wick could be used to supply electrolyte to separators between the electrodes.

  30. Those conclusions have to be read against the background evidence of Dr Wykes, the expert witness called by the respondents.  He joined the National Coal Board (later British Coal Corporation) in 1974 and by 1981 had become head of a group responsible amongst other things for instrument research.  After evaluation of commercial and near-commercial sensors he concluded that the NCB should seek to produce an electrochemical sensor.  They chose the City University, from which the respondents were formed, to do the research.  He estimated that his knowledge of electrochemical sensing of toxic gases at that time was somewhere between that of “the informed user and that of a cell designer.”  That being so, Dr Wykes was close to being the skilled person at the time and therefore I believe it is fair to take from his evidence the general background. 

  31. From Dr Wykes’ evidence it is clear that the common general knowledge would have included a working understanding of how electrochemical sensors worked and must have included information on one or more sensors that had been produced.  That would appear to be implicit from the conclusion of the judge.  But the evidence does not establish that the common general knowledge included information as to the details of their construction.  However there can be no doubt that none of them had the construction claimed in claim 1. 

  32. Mr. Platts-Mills QC, who appeared for the appellants, accepted in this Court the conclusion reached by the judge that the appellants had not established that the use of electrodes with holes passing through them was part of the common general knowledge.  However he submitted that the judge should have found that the skilled person would have known that holes could be cut in electrodes used in sensors without impairing their usefulness.  That, he submitted, could be derived from the evidence of Professor Williams, the expert called by the appellants, who established that proposals for electrodes with holes had been published. 

  33. Professor Williams was, at the time he was first engaged to give his opinion on the matters that arose in the case, and still is an eminent scientist.  He accepted in cross-examination, that he was of inventive mind.  He was an expert in electrochemistry and in sensors, but had never worked on electrochemical gas sensors of the type described in the patent.  As he accepted in cross-examination his knowledge of such gas sensors was limited to what he had found in the open literature.  To obtain that knowledge, he first read the patent and then he searched the literature to find out what was known.  He looked for holes in electrodes and found literature which supported his assumption that they were well-known. 

  34. I need not repeat the settled law, set out in Wheatley at pages 145 and 146, as to what constitutes common general knowledge.  The fact that something is well-known does not mean it formed part of the common general knowledge of the notional skilled person.  At best the evidence of Professor Williams showed that holes in electrodes were well-known, but there was no evidence which established that such holes formed part of the common general knowledge nor that the skilled person would know that holes could be cut in them without impairing their usefulness.  

  35. The third step is to identify the difference between Shaw and the invention.  The parties accept that was correctly done by the judge.  He said:


    It is common ground that the novelty of the construction of the sensor the subject of claim 1 of the patent in suit resides in two features namely:


    that the counter electrode comprises a hydrophobic support having an opening therein and


    that a wick passes through said opening.

  36. The fourth question requires the court to decide whether, when viewed without knowledge of the invention, the differences were obvious. 

  37. From what I have already said about him, it is clear that Professor Williams did not represent the notional skilled person.  He was too inventive and was too expert in his field and also had not been involved in the particular field with which the invention was concerned.  It followed that his views as to what was obvious could not carry much, if any, weight. 

  38. Professor Williams was cross-examined by Mr. Purvis, the respondent’s counsel, as to the steps necessary to redesign Shaw so that it would fall within claim 1.  He agreed that it would have been necessary to change the mesh electrode used in Shaw to the sort of electrode used in the patented constructions. That would not be just substitution, as the new electrode would need to be supported on a shelf.  Next, the current collectors would have had to be re-routed.  This could be done by getting rid of the gold collector rings and routing the collectors through the support.  It would also be necessary to remove the four wicks and their corresponding notches and replace them with one wick passing down a hole cut in the counter electrode and its shelf support. 

  39. Despite the evidence of Professor Williams, I am unable to find any reason why in 1981 the skilled person would want to make the changes to Shaw that would be necessary to convert it into a claim 1 sensor.  To the contrary, as Professor Williams accepted, Shaw had used four wicks for a purpose and the change from four wicks to one would not materially make any difference to the size or cost. 

  40. That was, I believe, accepted by Professor Williams. Mr. Purvis asked him (Evidence 3 page 305):


    What I want to put to you, Prof. Williams, is that the skilled man might think of making some trivial alterations to Shaw, like the material out of which he makes the housing, the precise material out of which he makes his electrodes, the particular electrolyte that he is using, but he is highly unlikely to think (without ingenuity) of abandoning Shaw’s carefully provided slots and his carefully provided four wicks round the side in favour of something which, on your own evidence, he has never seen before in the field of gas sensors, namely a wick cut through the middle of the counter electrode.


    If you started rigidly from Shaw then you are correct.

  41. Despite the submissions of Mr. Platts-Mills, I have no doubt that the judge was right to categorise the evidence of Professor Williams on this issue as an ex post facto analysis.  Once Professor Williams knew the destination (claim 1), he was able to derive a route from Shaw to claim 1 with the aid of selected reference to documents thrown up by a literature search designed to illustrate the route.

  42. Dr Wykes considered what changes he would have made in 1981 without knowledge of the patent.  He envisaged that he might have wanted to adopt a design which allowed for use of a fuel-cell type sensing electrode and simpler gas input control.  He said that he would have been surprised if it would have occurred to a skilled person to make any major alterations to the wicking route.  He was of the opinion that the idea of cutting holes in the Shaw electrodes would not have occurred to him. He said:

    To change to the central wick would have required a clear analysis of the benefits of not taking the electrolyte by the obviously offered routes and this would have had to have been followed with an intellectual leap to the creation of a new route involving an electrode-threading topology, coupled with the appreciation that that could be achieved without compromising the electrode.  In my opinion an unimaginative designer with the common general knowledge of the subject in 1981 would not have derived the inventive concepts of the patent from Shaw.

    Dr Wykes did not resile from that view in cross-examination. 

  43. The judge was right to reject the attack upon claim 1 based upon Shaw.  It is only with the benefit of knowledge of the patent that it is possible without the exercise of invention to postulate the combination of steps necessary to arrive at the invention claimed in claim 1.  Further the only relevant evidence, that of Dr Wykes, was to the contrary. 

  44. Common general knowledge per se was also said to be a relevant starting point for a successful attack on the validity of claim 1. Mr. Platts-Mills submitted that claim 1 was obvious in the light of the common general knowledge even if it was not obvious from Shaw.  His case started from the evidence of Dr Wykes that a suitable cell “would be likely to adopt planar electrodes, hydrophilic separators and to employ a reservoir.”  That being so, he submitted, there were only two routes for the electrolyte from the reservoir to the sensing electrodes, namely from the side or through the counter electrode.  Both routes were obvious.  It followed that there could be nothing inventive in arranging for the wick to pass through the counter electrode to the reservoir.  Claim 1 was obvious. 

  45. That submission was rightly rejected by the judge.  He said:


    It follows that when designing an electrochemical gas sensor cell at the priority date of the patent in suit (3rd February 1981) the assumed notional skilled person or persons would have no particular reason to consider constructing a cell having an opening in the counter electrode support and/or the counter electrode itself yet alone to provide such an opening to allow a wick to pass through the opening in order to transport electrolyte from a reservoir to the hydrophilic separator located between the electrodes.


    I consider the Defendants approach based on common general knowledge is one which  the courts have consistently condemned. Starting with the well known fuel cell electrodes and hydrophilic separators sandwiched between them it is contended that there is only one way to construct the cell if it is to be compact, namely to locate the reservoir underneath the counter electrode and provide a wick which can only be located either to pass around the counter electrode and its support or through an opening in it.  As can be seen from the prior art referred to, electrochemical cells may have electrodes which are cylindrical (either hollow or solid with a hole in the centre) or planar - hence a sensor could be constructed using electrodes having any one of a number of configurations and the question of whether or where a reservoir for the electrolyte will be required will depend on the choice of configuration of the electrode.  Likewise the question of whether or where a wick will be required will depend on (1) the electrode configuration (2) the requirement and location of the reservoir.

  46. The fallacy of Mr. Platts-Mills’s submission begins to appear upon appreciation that the starting point, the common general knowledge, is further away from claim 1 than the selected and nearest piece of prior art (Shaw).  Even assuming that the skilled person started with planar electrodes, hydrophilic separators and a reservoir, there is no evidence which suggests that the electrodes would be placed one above the other.  As Dr Wykes pointed out, other persons skilled in the art had adopted a variety of arrangements. 

    The general approach was present in the original Hersch cell (Figure 1) and is shown also in the US patents cited by the defendants.  The Hersch cell had a vertical electrode geometry with a hydrophilic separator contacting a reservoir below, the electrolyte being transported to the interelectrode body of the cell by entry between the electrodes.  In some later cells, gas handing compactness .... tended to favour planar electrode geometry, but the electrolyte transport to the reservoir was essentially similar to the route adopted by Hersch.  I only knew of cells which, if adopting hydrophilic contact with a reservoir external to the inter electrode space, did so in this manner, i.e. through the natural gap caused by the electrode separator.

    A good example of such a construction is disclosed in US Patent 3 399 267 (Oswin) which was applied for in 1975.

  47. The fallacy of Mr. Platts-Mills’ argument becomes completely exposed, when it is appreciated that despite the searches of Professor Williams and the fact that PTFE electrodes were said in 1975 to be well-known, the nearest that anybody came to the invention was Shaw.  If all that was necessary was a selection from two obvious alternatives, then it is surprising that nobody got closer than Shaw. 

  48. The judge was right to reject this argument of the appellants.  It was based upon hindsight reasoning: it was inconsistent with what happened: it was not supported by cogent evidence and such evidence as there was pointed to the contrary conclusion.  Dr Wykes said:


    This invention allowed designs, such as those given in the Patent’s examples, that produced a smaller, lighter, more instrinically robust, low cost of ownership, power-sparing, long-life sensor, than I had seen before in low concentration sensors.  I was impressed with the simplicity and elegance of the solution and felt that, for the first time, there was a design solution that might allow relatively trouble free, hand-held, trace CO monitoring and at a reasonable price and cost of ownership.  My recollection is that the general reaction of my colleagues was that the Claimant’s invention was a very neat solution with a high potential for successful industrial completion.  This assessment was borne out in practice.  The technical success of sensors based on these designs transformed the attitude of industries such as coal-mining to the use of electrochemical cells in such applications.


  49. The judge was right.  The patent was valid and infringed.  I would dismiss the appeal. 

    Lord Justice Mance

  50. I agree. 

    Lord Justice Latham

  51. I also agree


Improver Corporation v Remington Consumer Products [1990] FSR 181; Windsurfing International Inc v Tabur Marine (G-B) Ltd [1985] RPC 59; Wheatley v Drillsafe [2001] RPC 133


Mr. Mark Platts-Mills QC and Mr. Robert Onslow (instructed by Dorsey and Whitney) for the Appellants

Mr. Iain Purvis (instructed by Walker Morris) for the Respondents

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