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Patents Court considers 'sequential unmasking' approach to expert evidence

Posted on 9 March 2021

The Patents Court has found that Fisher & Paykel (F&P)'s European Patent (UK) 2023359 concerning the expiratory limbs of breathing circuits and in particular the use of breathable materials to reduce or eliminate condensation (as unconditionally proposed to be amended) was valid. The Court rejected invalidity attacks based on anticipation and obviousness in relation to the prior art (as well as squeeze arguments based on obviousness for lack of a technical contribution or insufficiency for lack of enablement). Flexicare had admitted that its products would infringe if the patent was found valid.

A key point of interest in the case was the approach of the experts when preparing their respective expert reports, which had an impact in particular on the Court's assessment of the obviousness attack. The Court concluded that Fleixcare's expert had let in hindsight when preparing his written evidence, as well as when giving his oral evidence, and this meant that his evidence on obviousness had to be treated with a 'good deal of scepticism'.

The correct approach to instructing experts, specifically the sequence in which they are shown documents, has been considered in a number of cases. The concern is that, if an expert is shown the patent before they have assessed what the common general knowledge is, or said what they consider to be obvious over the prior art, there is a risk of hindsight creeping into their assessment.

In Medimmune v Novartis, Arnold J (as he then was) said that it was preferable for the expert to be asked about the common general knowledge, then shown the prior art, and only then shown the patent – so-called 'sequential unmasking'. However, as the Judge in this case (Meade J) noted, sequential unmasking can be a counsel of perfection. In particular, the expert may already be well aware what the invention of the patent is, e.g., from any commercial embodiment of the patentee. This means therefore that they will not be able to give a completely untainted view of the prior art, though the Court recognised there may still be value in sequencing the documents to aid the expert in disciplining themselves carefully to avoid hindsight. In such cases, the Court stressed the expert must identify how they knew about the invention and when, and reflect carefully on how that might influence them. 

In this case, the expert had prior knowledge of the patent's invention – he had acknowledged this in his report, but the Court concluded that he should have thought carefully about this previous knowledge in order to avoid significant hindsight in his approach to a number of key questions. Hindsight can especially be an issue in relation to simple inventions, such as that in this case, and the Court felt this was a particular concern.

It is worth noting that, in his more recent decision in Akebia v Fibrogen, Arnold J had noted that there was no rule or principle requiring sequential unmasking, and he also acknowledged the practical problems that can arise, and also the extra burdens it can cause e.g., in relation to cross-examination of experts. Meanwhile, Birss J (as he then was, having recently joined Arnold LJ in the Court of Appeal)) in his 2013 decision in HTC v Gemalto, had stressed the need to ensure that approaches like sequential unmasking "are servants and should not become elevated into masters". He noted in particular the potential unfairness of placing this burden on experts, and the risk that the approach may not in fact eliminate hindsight but be capable of giving a false impression.  

The judgment in this case was given by Meade J, recently appointed to the bench (although now Head of the Patents Court following the elevation of Birss J to the court of Appeal). The judgment demonstrates his support of sequential unmasking but also that he is alive to the potential issues it may give rise to. His guidance on how the expert should in particular deal with the situation where they knew of the invention before being instructed should be reviewed carefully by those instructing experts in future cases. 

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