D. Mass: “Ensnarement” — a Potent but Often Overlooked Defense to the Doctrine of Equivalents — Need Not Be Raised in the Pleadings

In a recent decision, the U.S. District Court for the District of Massachusetts refused to strike an expert’s report that raised an ensnarement defense that was not raised in the pleadings, finding that neither the court’s local patent rules nor its scheduling order identified when the defense needed to be raised.¹ Moreover, it found that sufficient time existed before trial to cure any potential prejudice to the plaintiff.²

What is ensnarement?

Ensnarement is a counterpoint to the doctrine of equivalents. The doctrine allows a patentee to assert infringement of an accused device or process even though it does not fall within the literal scope of the claims but is essentially equivalent to the claimed invention. For example, what was claimed and what was incorporated in the accused conduct may function in essentially the same way to achieve the same result before making them equivalent.³ Or there may be “insubstantial differences” between the claimed language and the corresponding element in the accused product or process such that both are essentially the same.⁴

Ensnarement is based on the principle that the allowable range of equivalents to a patent claim cannot encompass or “ensnare” the prior art. To establish infringement under the doctrine of equivalents, the patentee — after a successful verdict — can posit a hypothetical claim that literally covers the accused product or process and then prove that the claim is valid over the prior art.⁵ Unlike the clear and convincing evidence standard a defendant must satisfy to establish invalidity, the burden on the patentee is the lesser preponderance of the evidence standard.⁶ The hypothetical claim is “not an opportunity to freely redraft granted claims… Slight broadening is permitted… but not narrowing.”⁷ If the patentee fails “to submit a proper hypothetical claim for consideration, he is unable to meet his burden of proving that his doctrine of equivalents theory did not ensnare the prior art.”⁸

The ensnarement defense took shape in Wilson Sporting Goods Co. v. David Geoffrey & Associates.⁹ After a jury found that the golf balls sold by defendant Dunlop infringed under the doctrine of equivalents, Dunlop appealed. On appeal, the Federal Circuit, recognizing that application of the doctrine “can be a difficult question to answer,” announced a useful, although not mandatory, two-step analytical approach to assist lower courts in their analysis of whether the alleged equivalent would have improperly “ensnared” the prior art.¹⁰ Accordingly, the patentee first constructs a hypothetical claim that literally covers the accused conduct, and the court then decides whether the U.S. Patent and Trademark Office (PTO) would have allowed the claim to issue over the prior art:

To simplify analysis and bring the issue onto familiar turf, it may be helpful to conceptualize the limitation on the scope of equivalents by visualizing a hypothetical patent claim, sufficient in scope to literally cover the accused product. The pertinent question then becomes whether that hypothetical claim could have been allowed by the PTO over the prior art. If not, then it would be improper to permit the patentee to obtain that coverage in an infringement suit under the doctrine of equivalents. If the hypothetical claim could have been allowed, then prior art is not a bar to infringement under the doctrine of equivalents.¹¹

This validity analysis is deemed appropriate as the patentee is “seeking patent coverage beyond the limits considered by the PTO examiner.”¹² Because the patentee must once again prove infringement and now establish patentability, ensnarement gives an alleged infringer a second bite at an (enlarged) apple. Although the Federal Circuit in Key Manufacturing Group, Inc. v. Microdot, Inc., observed that an ensnarement proceeding does not envision a “full-blown patentability analysis” of the hypothetical claim¹³, courts have conducted mini-trials on the defense.¹⁴

Factual background

Plaintiff Maquet moved to strike portions of the reports of defendant Abiomed’s expert, Dr. Lakshmi Prasad Dasi.¹⁵ Among other reasons, Maquet argued that Dr. Dasi’s rebuttal report impermissibly introduced the affirmative defense of ensnarement — a defense that defendant Abiomed had never raised.¹⁶ In fact, Abiomed asserted its ensnarement defense just two weeks after the close of fact discovery and one week before opening expert reports were due.¹⁷The day after Abiomed served its rebuttal report, it also moved for partial summary judgment of noninfringement under the doctrine of equivalents, asserting that Maquet failed to meet its burden and prove that its assertion of equivalence did not ensnare the prior art.¹⁸ Maquet responded that it had no obligation to put forward a hypothetical claim until after Abiomed came forward with specific prior art that the claims allegedly ensnared.¹⁹ Abiomed countered that it had identified such art in Dr. Dasi’s opening invalidity report.²⁰

Timeliness of the ensnarement defense

The district court relied on the Federal Circuit’s decision in Jang v. Boston Scientific Corp.²¹ to find that there was nothing improper about raising the defense of ensnarement in a pretrial motion in limine.²² “It appears, therefore, that unlike other affirmative defenses, ensnarement need not even be pleaded.”²³

The court then considered whether the ensnarement defense had been waived.²⁴ Given that neither the local patent rules nor the court’s scheduling order prescribed deadlines for raising the defense, the belated assertion was not untimely.²⁵ The court also found that there was ample time before trial, to cure any prejudice Maquet might suffer.²⁶ Significantly, a trial date had not even been scheduled.

Timing of identification of an ensnarement defense

The court then considered the proper sequence of steps to assert and defend against an ensnarement defense.²⁷ The court found that Abiomed had the initial burden to identify the ensnared prior art before Maquet was required to propose a hypothetical claim.²⁸ In finding so, the court relied on the defendant’s initial burden of persuasion that required it to present evidence establishing the alleged defense.²⁹ Only then does the patentee need to posit a hypothetical claim that does not ensnare the prior art identified by the defendant. The court also noted that the hypothetical claim analysis is just one way Maquet could defend against ensnarement — it could also “directly compare [] the prior art with a single element . . . of the accused device in an attempt to determine the extent to which the prior art limits the application of the doctrine of equivalents.”³⁰

The court denied Abiomed’s motion for partial summary judgment of noninfringement with respect to ensnarement, finding the issue was not yet ripe.³¹ The court also denied Maquet’s motion to strike the portions of Dr. Dasi’s report that addressed ensnarement, finding the defense was neither untimely nor waived.³²

Takeaways

This decision raises an important consideration — who needs to establish ensnarement and when does the defense need to be raised? This decision held that the defendant has the initial burden of identifying prior art that ensnares the asserted claims. Then, the patentee has the burden of drafting the appropriate hypothetical claim and establishing that it did not ensnare the prior art. While this decision found that the defense need not be pled or identified during fact discovery, it is a better practice to identify the defense earlier to avoid any arguments regarding waiver or prejudice to the patentee. Both parties should be prepared to address ensnarement immediately following a decision on validity and infringement.³³