The subject of toxic (or “poisonous”) divisional applications was a popular topic of discussion in the IP world at the end of 2013. Since then, it seems that the discussion has gone quiet. Here we review the concept behind toxic divisional applications and give some further thoughts on whether it is a mere academic theory or whether it has real, practical implications for patent applicants.  

The theory behind a toxic divisional application can be exemplified as follows. Suppose that a first “priority application” is filed relating to a device comprising a spring. Subsequently, a European patent application is filed (which we will refer to as the “completed application”), claiming priority from the first application and relating to the device comprising a spring and, in addition, describing the device more generally as a device comprising a resilient member. Suppose, further, that during prosecution of the completed application, a divisional application is filed with the same original text as the completed application (i.e. a device comprising a spring and a device comprising a resilient member). Also, the completed application is then granted with a claim relating to the device comprising a resilient member.  

The question then arises whether the validity of the completed patent could be challenged (e.g. in an opposition) on the grounds that its divisional application is valid prior art against it. This might initially sound absurd but the logic of the idea is that a divisional application has no special standing under the EPC; it is merely another European patent application which has the effective filing date of its parent application. There is nothing in the EPC per se which says that a divisional application cannot be cited against its parent patent (or, for that matter, vice versa).  

The concept also relies on the EPO’s case law on the consideration of entitlement to priority (principally in EPO Enlarged Board of Appeal Decision G2/98). The analysis goes as follows. The completed patent in our example, above, has a claim to a device comprising a resilient member. Its priority application did not disclose such a broad concept; it only disclosed a device comprising a spring. Therefore the claim in the completed patent is not entitled to the claimed priority date; it is only entitled to the filing date of the completed patent because “a device comprising a resilient member” was first disclosed in the completed application.  

On the other hand, the divisional application contains a disclosure of the device comprising a spring and it claims priority from the priority application which also disclosed the device comprising a spring and therefore the disclosure in the divisional application of “a device comprising a spring” isentitled to the claimed priority date. Furthermore, the disclosure of the device comprising a spring falls within the scope of the claim in the completed patent which relates to a device comprising a resilient member since a spring is a type of resilient member.  

Under the EPC, there are two types of prior art that can be cited against a European patent. Firstly, there is prior art which is subject matter that has been made available to the public prior to the filing date (or priority date if there is one) of the patent. Secondly, there is prior art which is another European patent application that has an effective priority date before that of the patent, even if the other European patent application was not published before the filing date (or priority date) of the European patent. It is worth noting that the second category of prior art can only be cited to show lack of novelty and cannot be cited to show lack of inventive step.  

Thus, according to the theory of toxic divisionals, the divisional application, in our example, can be cited in the second category of prior art because it has a disclosure (of a device comprising a spring) which is entitled to the priority date whereas the completed patent has a claim to broader subject matter which is not entitled to the priority date. Furthermore, the disclosure of a device comprising a spring falls within the scope of the claim to a device comprising a resilient member and therefore a lack of novelty arises.  

Although the example, above, has been given with respect to a situation where the scope of a claim is broadened between a priority application and the completed application, the toxic divisional concept can also arise in other circumstances where there is a claim which does not have entitlement to claim priority from its priority application.  

The natural reaction of many practitioners when first faced with the concept of toxic divisionals was that divisional applications must be a special case and it would be non-sensical that they could be cited as prior art against their parent patent. The counter-argument is that there is no reason for divisional applications to be considered a “special case” because there is nothing in the EPC that indicates that they should be treated differently as prior art. Furthermore, to treat them differently would disadvantage those applicants who initially file separate patent applications instead of relying on the opportunity to file divisional applications from a single application.  

The issue of toxic divisionals has not so far been conclusively put to rest. EPO Board of Appeal Decision T1496/11 is certainly one case where an EPO Board of Appeal has been willing to find lack of novelty in a toxic divisional case. However, it is evident that the patent proprietor in that particular case did not contest the allegation, so it is not entirely clear that the Board of Appeal considered all possible defences against the citing of a toxic divisional application.  

In view of the current uncertainty, various defences against an allegation of lack of novelty due to a toxic divisional have been considered. One particular defence strategy is to argue that it should always be possible to distinguish over the prior art effect of a divisional application by using a disclaimer in the parent patent. In this context, a “disclaimer” is a negative feature of a claim which defines what is outside of the scope of the claim rather than what is inside of the scope of the claim. To continue with our example, if the claim in the completed patent were amended to read “a device comprising a resilient member characterised in that the resilient member is not a spring” then this subject matter, whilst still not entitled to the priority date, would be novel over the divisional application disclosure of a device comprising a spring. We will call this claim containing a disclaimer the “principal claim”. Furthermore, the completed patent could even include a counterpart claim to “a device comprising a spring” which would be entitled to the priority date and therefore could not have the divisional application cited against it. Thus, taken together, the principal claim to “a device comprising a resilient member characterised in that the resilient member is not a spring” and the counterpart claim to “a device comprising a spring” are of the same scope as a claim to “a device comprising a resilient member” but are not anticipated by the divisional application.  

Indeed, taking this argument to its logical conclusion, it is not even necessary to have two separate claims since the case law of the EPO indicates that a single claim can have multiple priorities and therefore it is possible for a single claim to “a device comprising a resilient member” to be mentally separated into a first part relating to “a device comprising a resilient member characterised in that the resilient member is not a spring” which is not entitled to priority and a second part relating to “a device comprising a spring” which is entitled to priority.  

This riposte seemed to settle the matter until it was pointed out that the EPO case law on multiple priorities in a claim does come with some strings attached. More specifically, in EPO Enlarged Board of Appeal Decision G2/98, it was decided that a claim could relate to multiple priorities even where a generic term or formula in a claim was used, provided that the generic term or formula gives rise “to the claiming of a limited number of clearly defined alternative subject matters”. Therefore, in a case where a priority application contains a single example but the priority claiming patent instead uses a generic term which covers both the example and other examples, entitlement to priority would be seen to depend on whether that general term does give rise to the claiming of a limited number of clearly defined alternative subject matters. Returning to our example, the term “resilient mechanism” covers “a spring” but also potentially covers an unlimited number of alternative subject matters which are arguably not clearly defined. Therefore, in cases where the alternative subject matters are not clearly defined or are not of limited number, it is not possible for a claim to contain multiple-priorities and for this reason, the concept of mentally separating the claim between subject matter that is not entitled to the claimed priority date (but which is novel over the divisional application) and subject matter that is entitled to the priority date (and which therefore could not have the divisional application cited against it) would not apply.  

You may be thinking that, while the “mental separation” approach to priority entitlement might not work, surely it would still be possible to have two separate claims, one disclaiming the subject matter in the divisional application and one (the counterpart claim) entitled to the priority date. As demonstrated above, this should also avoid the issue of “a toxic divisional” application and considerations of claims containing multiple priority dates would not apply.  

The difficulty, here, is that it was stated in the leading decision on disclaimers in claims (Enlarged Board of Appeal Decision G1/03) that a claim containing a disclaimer must meet the requirements of clarity. Therefore, a disclaimer which, itself is not clear in the sense that it is not clear what does and does not fall within the scope of the disclaimer, renders the claim as a whole unclear.  

In the previous example, splitting the claim into two and having one claim relating to “a spring” and one relating to “resilient mechanism characterised wherein the resilient mechanism is not a spring” probably would be permissible.  

However, if a priority application were to contain a disclosure that is not “clear” in the sense of the requirements for defining the scope of a patent claim then it might not be possible to obtain an allowable claim which disclaims that subject matter. An example of this would be if a priority application referred to “a composition having a pH of between about 8 and about 9” whereas the completed application referred to the composition without specifying the pH. The word “about” in relation to a parameter in the priority application is generally regarded as being unclear in a claim under European patent practice. It would probably be accepted to have a priority-entitled, counterpart claim that related to the pH being “between 8 and 9” (i.e. omitting the word “about”) but the problem arises for the principal claim that would read, “a composition characterised in that the pH is not between 8 and 9”. Arguably, the divisional application’s priority-entitled disclosure of “between about 8 and about 9” would still fall within the scope of the claim since “about 8” could be slightly less than 8. (Although one strategy would be to argue that it is not a direct and unambiguous disclosure which is required to show lack of novelty.) Therefore, does this mean that where there is a disclosure in the priority application that does not meet the standards of clarity for a claim, the toxic divisional can be a genuine problem?  

One item of case law which does not so far seem to have been taken into account in consideration of the issue of toxic divisionals is Board of Appeal Decision T10/01 in which it was stated obiterthat, “a disclaimer being broader than strictly necessary to restore novelty may, depending on the circumstances of the case, be allowed, if this turns out to be necessary to avoid an otherwise resulting unclarity of the claim”. In other words, this decision indicates that it may be acceptable for a disclaimer to disclaim more than is disclosed by a prior art document if the prior art document makes a disclosure which is unclear. Let’s return to the situation where the priority application states “between about 8 and about 9”. To take the concept espoused in Decision T10/01 to its logical conclusion, it might be considered acceptable for the completed patent to disclaim “between 7.5 and 9.5” on the basis that a skilled person would not, in the context, consider “about 8” to be any less than 7.5 or “about 9” to be any more than 9.5. Applying this concept to the discussion on toxic divisional applications, one reaches the conclusion that even if the disclosure in the priority application does not meet the requirements for clarity of a claim, it would still be possible to carve out a disclaimer which was clear, even if that disclaimer was broader in scope than the original disclosure. Does this therefore close the argument for toxic divisionals down after all?  

Not completely. The eagle-eyed reader will have noted that following on with our example, if a priority application contains the feature “between about 8 and about 9” and the counterpart claim in the completed patent reads, “between 8 and 9”, the disclaimer in the principal claim would read “characterised in that the pH is not between 7.5 and 9.5”. There is a gap of unclaimed subject matter where the pH is between 7.5 and 8 and between 9 and 9.5. The gap equates, if you like, to the “grey area” caused by the ambiguous word “about”. However, that grey area is no longer covered by either claim. As such, this strategy provides a solution of sorts, and would certainly be preferable to losing a patent altogether, but could still leave some undesirable gaps in protection.  

In conclusion, the issue of toxic divisional applications remains uncertain. It is believed that there are several oppositions and appeals before the European Patent Office that currently raise allegations of lack of novelty due to a toxic divisional application. It appears that a referral to the EPO Enlarged Board of Appeal may be imminent (see the minutes of hearing of appeal T557/13) although the exact questions that will be asked are not yet clear. Until we see the outcomes of these cases, it will be difficult to know for certain whether toxic divisionals are a mere academic thought experiment or are a genuine problem to patent applicants and proprietors. As and when such challenges are raised, the strategy outlined above might be helpful in overcoming an invalidity challenge based on a “toxic divisional”. In the meantime, it is important to assess the risks whenever a priority-claiming application is filed which contains different subject matter from its priority application and whenever a divisional application is filed from such a priority-claiming application.