Ironburg Inventions Ltd. v. Valve Corporation (Fed. Cir. 2026)

Significance

This Federal Circuit decision clarifies critical standards for IPR estoppel under 35 U.S.C. § 315(e)(2), establishing important requirements that significantly constrain patent owners' ability to estop accused infringers from raising previously unavailable prior art in district court litigation.

Key Holdings

Skilled Searcher Standard

• The court affirmed that estoppel applies only to grounds "reasonably could have raised" during IPR, not merely "possibly" discoverable grounds
• Rejected bare accessibility as sufficient: A prior art reference being located within classification codes searched is not sufficient for "discoverability" — something more is required when classification searches return unreviewable numbers of results
• Evidence must show that a skilled searcher would have actually found and reviewed the reference through narrowing techniques (keyword searches, citation analysis, etc.)

Hindsight Bias Prohibition

• Expert search testimony conducted years after the relevant IPR petition is tainted by hindsight bias if it utilizes information only available post-petition
• Forward-and-backward citation searches using data that post-dates the IPR petition cannot establish discoverability at the time of filing
• A searcher's subjective claim to lack knowledge of references does not suffice to eliminate hindsight bias infecting the search methodology itself

Grounds vs. References

• Under § 315(e)(2), the statutory inquiry focuses on discoverability of invalidity grounds, not individual prior art references
• All components of a multi-reference ground must be discoverable for estoppel to apply; failure to establish discoverability of one reference defeats estoppel for the entire ground

Reversal

The court reversed the district court's estoppel determination on both grounds:

• Kotkin ground: Reversed because the district court improperly relied on classification search results showing 26,333 references without requiring further narrowing analysis
• Willner-Koji-Raymond ground: Reversed because Cardinal IP's expert testimony was systematically infected by hindsight bias from the outset

Procedural Posture

The panel consisted of three judges (Chen, Hughes, and Stark). Circuit Judge Stark filed a concurring opinion articulating a two-step framework: (1) findability of prior art references, and (2) discoverability of invalidity grounds once references are found. Stark emphasized that courts lack comprehensive guidance and must develop these standards contextually.

Broader Impact

This decision does not overturn existing precedent but significantly operationalizes the skilled searcher standard established in Ironburg I (Fed. Cir. 2023). It provides meaningful boundaries preventing patent owners from using post-hoc expert searching to establish estoppel, thereby preserving defendants' ability to raise invalidity defenses based on prior art that was theoretically but not practically discoverable.