Ironburg Inventions Ltd. v. Valve Corporation

United States Court of Appeals
for the Federal Circuit
______________________
IRONBURG INVENTIONS LTD.,
Plaintiff-Appellee
v.
VALVE CORPORATION,
Defendant-Appellant
______________________
2024-2088
______________________
Appeal from the United States District Court for the
Western District of Washington in No. 2:17-cv-01182-TSZ,
Senior Judge Thomas S. Zilly.
______________________
Decided: June 18, 2026
______________________
G REGORY S. TAMKIN , Dorsey & Whitney LLP, Denver,
CO, argued for plaintiff-appellee. Also represented by ERIN
K OLTER, Seattle, WA.
PATRICK A. LUJIN , Shook, Hardy & Bacon, LLP, Kansas
City, MO, argued for defendant-appellant. Also repre-
sented by M ARY PEAL, BASIL TRENT WEBB; TANYA L.
CHANEY, Houston, TX.
______________________
Before CHEN , H UGHES , and STARK , Circuit Judges.
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IRONBURG INVENTIONS LTD . v. VALVE CORPORATION 2
Opinion for the Court filed by Circuit Judge H UGHES.
Concurring opinion filed by Circuit Judge STARK .
H UGHES , Circuit Judge.
Valve Corporation appeals from a supplemental partial
judgment of the United States District Court for the West-
ern District of Washington. The district court ruled that
Valve was estopped under 35 U.S.C. § 315(e)(2) from as-
serting two invalidity grounds, which had not been in-
cluded in a 2016 petition for inter partes review, against
several claims of U.S. Patent No. 8,641,525. Because we
conclude that the district court relied on insufficient evi-
dence to estop one ground, and because the district court
failed to adequately account for hindsight bias in estopping
the other ground, we reverse and remand for further pro-
ceedings not inconsistent with this opinion.
I
A
Ironburg Inventions, Ltd. (Ironburg) owns U.S. Patent
No. 8,641,525, which is directed to a video game controller
with “one or more additional controls located on the back of
the controller in a position to be operated by the user’s
other fingers” while the thumb and forefinger rest on the
top and front of the controller. ’525 Patent, 1:49–58. In one
embodiment, these back controls are described as “inher-
ently resilient” and “elongate in shape.” Id. 3:33, 3:51. An
embodiment of the claimed controller is shown below:
Id. figs. 1, 2.
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IRONBURG INVENTIONS LTD . v. VALVE CORPORATION 3
Claim 2 is illustrative for the purposes of this appeal.
Claim 2, along with cancelled claim 1 from which it de-
pends, is reproduced below:
1. (cancelled) A hand held controller for a game
console comprising:
an outer case comprising a front, a back, a
top edge, and a bottom edge, wherein
the back of the controller is opposite the
front of the controller and the top edge
is opposite the bottom edge; and
a front control located on the front of the
controller;
wherein the controller is shaped to be held
in the hand of a user such that the
user’s thumb is positioned to operate
the front control; and
a first back control and a second back con-
trol, each back control being located on
the back of the controller and each back
control including an elongate member
that extends substantially the full dis-
tance between the top edge and the bot-
tom edge and is inherently resilient
and flexible.
2. The controller of claim 1, further having a top
edge control located on the top edge of the control-
ler and wherein the controller is shaped such that
the user’s index finger is positioned to operate the
top edge control.
Id. 4:41–59.
Ironburg sued Valve Corporation (Valve) in 2015, al-
leging that Valve’s Steam® Controller infringed claims 2,
4, 7, 9–11, and 18 of the ’525 patent. Prior to trial, in
April 2016, Valve filed an IPR petition challenging the
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IRONBURG INVENTIONS LTD . v. VALVE CORPORATION 4
’525 patent, which the Patent Trial and Appeal Board par-
tially instituted prior to the Supreme Court’s decision in
SAS Institute Inc. v. Iancu, 584 U.S. 357 (2018). This IPR,
Number IPR2016-00948 (’948 IPR), culminated in a final
written decision in September 2017.
During the pendency of the ’948 IPR, Valve filed a sec-
ond IPR petition challenging the ’525 patent based on a
prior art reference that Valve contended was newly discov-
ered, Wörn.1 After the Board issued its final written deci-
sion in the ’948 IPR, Ironburg moved to terminate this
second IPR, Number IPR2017-00136 (’136 IPR), based on
estoppel under 35 U.S.C. § 315(e)(1). In January 2018, the
Board granted Ironburg’s motion to terminate, finding that
Ironburg had proven that “a skilled searcher conducting a
diligent search reasonably could have been expected to dis-
cover Wörn.” J.A. 3845. Reviewing the evidence, the Board
specifically found that manual searches of the prior art us-
ing patent classifications and subclassifications,2 as well as
using keywords to further narrow the search results to a
1 U.S. Patent No. 6,362,813.
2 The United States Patent and Trademark Office
classifies patents into categories based on the subject mat-
ter claimed. “There are multiple classification systems
used throughout the international patent system,” includ-
ing “the Cooperative Patent Classification System (CPC),
U.S. Patent Classification System (USPC), [and] the Inter-
national Patent Classification System (IPC).” MPEP § 902
(9th ed. Rev. 01.2024). Within each classification system,
categories—known as classes and subclasses—group
closely related inventions together under alphanumeric
headings and have descriptive titles that are suggestive of
their common subject matter. See MPEP § 902.01. These
classes and subclasses thus serve as an indexing tool often
used early in a patent search.
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IRONBURG INVENTIONS LTD . v. VALVE CORPORATION 5
prior art set small enough to manually review, would have
located Wörn.
A third party, Collective Minds Gaming Co. Ltd.
(CMG), also filed an IPR petition challenging the ’525 pa-
tent in January 2018. CMG’s petition raised two new
grounds of obviousness, based on four references (the CMG
Grounds). One invalidity ground was based on U.S. Pub-
lished Patent Application No. 2010/0298053 (Kotkin). The
other invalidity ground consisted of the combination of U.S.
Patent No. 6,760,013 (Willner); Japanese Patent Applica-
tion No. JP-A H10-020951 (Koji); and U.S. Patent
No. 5,773,769 (Raymond) (collectively, Willner-Koji-Ray-
mond). The Board instituted CMG’s IPR on all grounds, but
the IPR was eventually terminated due to a settlement be-
tween CMG and Ironburg.
After the filing of CMG’s IPR, Valve amended its inva-
lidity contentions in the pending district court litigation to
raise the CMG Grounds. In September 2018, Ironburg filed
a motion for IPR estoppel under § 315(e)(2), arguing that
Valve should be estopped from asserting the CMG
Grounds. In November 2019, the district court granted
Ironburg’s motion, estopping Valve from asserting either
Kotkin or Willner-Koji-Raymond at trial. The district court
found that, since Willner was cited on the face of the
’525 patent and because CMG was able to locate Kotkin,
Koji, and Raymond for its own IPR petition, “a skilled
searcher could have been reasonably expected to find all of
these references.” J.A. 4461–63. The district court action
then proceeded to trial on infringement, and in Febru-
ary 2021, the jury returned a complete verdict for Ironburg,
finding willful infringement of claims 2, 4, 7, 9–11, and 18,
and awarding $4,029,533.93 in damages. An initial appeal
to this court followed. See Ironburg Inventions Ltd. v. Valve
Corp., 64 F.4th 1274 (Fed. Cir. 2023) (Ironburg I).
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IRONBURG INVENTIONS LTD . v. VALVE CORPORATION 6
B
We affirmed-in-part, vacated-in-part, and remanded.
Id. at 1301. As pertinent here, we held that the district
court erred by “improperly plac[ing] the burden of proof on
Valve, to show that it could not ‘reasonably . . . have raised’
the [CMG] Grounds in its petition.” Id. at 1297. Instead, we
held, “the burden of proof rests with Ironburg to prove that
these were grounds Valve ‘reasonably could have raised’
during the IPR.” Id. We further adopted the district court’s
conclusion that “an IPR petition ‘reasonably could have
raised’ any grounds that ‘a skilled searcher conducting a
diligent search reasonably could have been expected to dis-
cover.’” Id. at 1298 (citation omitted). This skilled searcher
standard, we found, was “consistent with the statutory re-
quirement that a petitioner be estopped from asserting
‘any ground that the petitioner . . . reasonably could have
raised during . . . inter partes review.” Id. (alterations in
original) (quoting § 315(e)(2)).
We then turned to the evidence considered by the dis-
trict court and criticized its use of CMG’s petition as proof
against Valve. That was because no evidence had been en-
tered into the record as to the reasonable diligence of
CMG’s pre-petition search. Id. at 1298–99. If, after all,
CMG “employed ‘scorched earth’ tactics to find the refer-
ences making up the [CMG] Grounds, then its experience
may be irrelevant to a determination of what would have
been discovered by an ordinarily skilled searcher acting
with merely reasonable diligence.” Id. at 1299. We there-
fore found this evidence insufficient to support a finding of
estoppel, and since Ironburg offered no other evidence and
bore the burden of proof, we vacated and remanded “for the
district court to determine whether Ironburg [could] meet
its burden.” Id. On remand, we left it to the district court
“to address related issues that may arise, such as whether
to reopen the record and permit discovery” and “the rele-
vance of Valve’s own search efforts.” Id. at 1300.
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IRONBURG INVENTIONS LTD . v. VALVE CORPORATION 7
C
On remand, the district court permitted limited addi-
tional discovery, allowing Ironburg to procure “declara-
tions from individuals with personal knowledge about the
efforts used by [CMG] to search for and locate Kotkin
and/or Willner-Koji-Raymond,” as well as “an expert opin-
ion concerning whether an appropriately skilled person
conducting a diligent search would have reasonably been
expected to discover Kotkin and/or Willner-Koji-Raymond.”
J.A. 5945. Ironburg then filed a renewed motion for IPR es-
toppel, submitting evidence that it had asked Cardinal IP,
a search company, to perform searches for Kotkin and Will-
ner-Koji-Raymond and contending that these grounds
would have been discoverable to a skilled searcher exercis-
ing reasonable diligence. Valve opposed, relying in part on
evidence of the searches that its own search agent, Landon
IP, had conducted before the filing of the ’948 IPR petition.
On May 3, 2024, the district court again granted Iron-
burg’s motion for IPR estoppel. Ironburg Inventions Ltd. v.
Valve Corp., No. C17-1182, 2024 WL 2091177, at *12
(W.D. Wash. May 3, 2024) (IPR Estoppel Order). Regarding
Valve’s own search efforts, the district court determined
the evidence showed that Kotkin and Koji were reasonably
discoverable. The district court noted that “eleven classifi-
cation searches and 45 keyword searches” were conducted
by the Landon IP team in 2014 prior to the filing of the
’948 IPR petition. Id. at *5. This evidence showed that in
the classification searches run by the Landon IP team, the
exact codes under which Kotkin and Koji were indexed
were included. Thus, because Kotkin would have been in
the readouts returned from searching these classifications,
the court concluded it was reasonably discoverable. For
Koji, the district court went a step further, discussing in a
lengthy footnote how the application of date restrictions
and keyword search terms within Landon IP’s searches
would have returned only 137 search results, within which
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IRONBURG INVENTIONS LTD . v. VALVE CORPORATION 8
Koji would have been reasonably discoverable. See id. at *6
& n.8.
The district court also considered evidence offered by
Ironburg and its Cardinal IP searchers. As regards Ray-
mond, the court considered two sets of searches run by Car-
dinal IP, one of which included forward-and-backward
citation queries,3 to locate the reference. The court struck
one of the forward-and-backward citation searches because
it found that certain forward references this search would
have returned were created after Valve’s ’948 IPR petition,
therefore implicating a hindsight concern. As to the other
search, however, the court found that Cardinal IP’s tech-
niques were conventional, culminating in 368 search re-
sults within which Raymond appeared. The court therefore
concluded that Raymond was discoverable without “ex-
traordinary measures” or “scorched earth” tactics. Id. at *9.
3 Citation searching is another tool used in patent
searching, which relies on checking one or both of two sets
of documents—backwards citations and forward citations.
Per a patent-searching treatise considered by the district
court, backwards citations are “patent documents and pub-
lications that are cited as references on the face of the sub-
ject patent,” and searching backwards citations is often
used to “find the first fundamental discovery of a technol-
ogy.” J.A. 4314–15. Forward citations are “patent docu-
ments and publications that were subsequently cited by
other granted patents,” and a search of forward citations
can help identify “improvement[s] of [a] particular technol-
ogy.” J.A. 4314–15. Thus, running a backwards citation
search on a patent will return references cited by the pa-
tent that issued before the patent’s priority date; running
a forward citation search will return references that cite to
the subject patent and that issued after the patent’s prior-
ity date.
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IRONBURG INVENTIONS LTD . v. VALVE CORPORATION 9
The district court then entered a supplemental partial
judgment containing its estoppel findings against Valve
pursuant to Fed. R. Civ. P. 54(b). Valve appeals. We have
jurisdiction pursuant to 28 U.S.C. § 1295(a)(1).
II
On appeal, Valve primarily argues: (1) that its pre-pe-
tition Landon IP search evidence does not demonstrate
that Kotkin and Koji were discoverable under a proper ar-
ticulation of the skilled searcher standard, and (2) that the
Cardinal IP evidence offered by Ironburg is irrelevant to
the skilled searcher inquiry. We address each argument in
turn.
A
We first consider Valve’s argument that the district
court misapplied the skilled searcher standard to the Lan-
don IP evidence to conclude that Kotkin and Koji were rea-
sonably discoverable.
“Interpretation of the IPR estoppel statute, an issue
unique to patent law, is a question of law we review de novo
applying Federal Circuit law.” Ironburg I, 64 F.4th at 1296
(citation omitted). The estoppel statute provides, in rele-
vant part, that a petitioner in an IPR that results in a final
written decision cannot later assert in a civil action an in-
validity theory based on “any ground that the petitioner
raised or reasonably could have raised during . . . inter
partes review.” 35 U.S.C. § 315(e)(2) (emphasis added).
Consistent with our holding in Ironburg I, we agree with
the parties that this statutory language requires a focus on
what reasonably, rather than possibly, could have been
raised prior to the filing of a relevant IPR petition. See
Ironburg I, 64 F.4th at 1297–98. This timing point has a
necessary corollary—when evidence is introduced that
sheds light on what a skilled searcher may have discovered
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IRONBURG INVENTIONS LTD . v. VALVE CORPORATION 10
only after the time of the relevant IPR petition, such evi-
dence is likely irrelevant to the skilled searcher inquiry.4
Still, there are two open disputes regarding the stand-
ard. First, we have not fully addressed whether the skilled
searcher inquiry poses a question of law, reviewable de
novo, or a question of fact, reviewable for clear error. We do
not opine on this issue here because we conclude that the
district court’s error regarding the Landon IP evidence is
apparent under either standard of review.
Separately, we have not yet had the opportunity to take
a conclusive stance on what “discover[y]” within the skilled
searcher inquiry entails. Ironburg I, 64 F.4th at 1298.
Valve argues that a searcher must read and understand a
reference to “discover” it, raising the District of Massachu-
setts’s decision in Palomar: “[I]t is not enough simply to lo-
cate a reference; that reference must be read, and
interpreted, and understood in the context of the patent.”
2020 WL 2115625, at *14. Ironburg argues in response
that requiring the proponent of estoppel to prove “discov-
ery” of the basis for a petitioner’s invalidity combination
places an impossible burden on that proponent—i.e., re-
quiring them to show that a skilled searcher would recog-
nize the invalidity of their own patent. Ultimately,
however, because we conclude that Kotkin was not discov-
erable on this evidence under any conception of what the
4 Many district courts evaluating IPR estoppel is-
sues have agreed on this point, disregarding evidence of a
skilled searcher’s efforts that are found to be “tainted by
hindsight.” Palomar Techs., Inc. v. MRSI Sys., LLC,
No. 18-10236, 2020 WL 2115625, at *15 (D. Mass. May 4,
2020); see GeigTech E. Bay LLC v. Lutron Elecs. Co.,
No. 18-cv-05290, 2023 WL 8827572, at *7 (S.D.N.Y.
Dec. 21, 2023); EIS, Inc. v. IntiHealth Ger GmbH,
No. 19-1227, 2023 WL 6797905, at *4–5 (D. Del. Aug. 30,
2023).
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IRONBURG INVENTIONS LTD . v. VALVE CORPORATION 11
skilled searcher inquiry requires, we also find it unneces-
sary to resolve this question.
We therefore turn directly to the evidence before us of
the Landon IP search.
i
On remand, evidence of Valve’s pre-petition search ef-
forts involving Landon IP was limited. The district court
primarily considered a declaration from Jamila Williams,
a prior art searching professional with Landon, who had
assisted with Valve’s 2014 prior art searches leading up to
the ’948 IPR. This declaration, which had been initially of-
fered prior to Ironburg I and was reoffered on remand, in-
cluded two appendices of search queries as exhibits. The
first list of queries had a cut-off date of June 17, 2011, and
it included 76 searches within a U.S. patent database.
J.A. 7613 ¶ 9; J.A. 7616–18. This list of search queries was
not provided with the results. The second list of search que-
ries consisted of 11 searches within a Japanese patent da-
tabase, completed with the same cutoff date, which
included the numbers of results alongside the search que-
ries. J.A. 7613 ¶ 9; J.A. 7620–21 (Japanese Language
Search); J.A. 7622–24 (Translated Search). The search ev-
idence considered on remand by the district court therefore
consisted of classification searches and the number of re-
sults returned from one of these searches.
ii
Valve argues that the district court erred when it con-
cluded that Kotkin and Koji were discoverable merely be-
cause they are located in “classifications that Valve’s own
consultant believed were relevant.” IPR Estoppel Order,
2024 WL 2091177, at *6. Specifically, Valve contends that
such an understanding of discovery completely ignores the
number of references returned by the classification search
and, as a result, whether they were understood by the
searcher.
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IRONBURG INVENTIONS LTD . v. VALVE CORPORATION 12
Regarding Kotkin, the district court’s analysis ended
after simply noting that the exact IPC and CPC classifica-
tions containing Kotkin and Koji were searched. This is
notwithstanding that Landon IP’s search strings reveal
that 26,333 references were returned by the Japanese-lan-
guage database when searched for classifications contain-
ing both Kotkin and Koji. The district court purported to
rely on classification searches based on the Board’s appli-
cation of § 315(e)(1) estoppel in the ’136 IPR, which primar-
ily looked to classification searches in identifying the
reasonable discoverability of Wörn. The district court also
found that classification searching satisfied the standard
of reasonableness set forth in certain district court cases to
consider the issue of IPR estoppel: “One way to show what
a skilled search would have found would be (1) to identify
the search string and search source that would identify the
allegedly unavailable prior art and (2) present evidence,
likely expert testimony, why such a criterion would be part
of a skilled searcher’s diligent search.” IPR Estoppel Order,
2024 WL 2091177, at *6 (citation omitted); see Clearlamp,
LLC v. LKQ Corp., No 12 C 2533, 2016 WL 4734389, at *9
(N.D. Ill. Mar. 18, 2016). Thus, no other analysis was con-
ducted by the district court regarding Kotkin.
The district court’s conclusion regarding Kotkin was
error. First, as noted above, the Board in the ’136 IPR did
not consider mere classification searches, without more, to
be the endpoint of its analysis; rather, it went on to con-
sider that these classifications would be manually
searched, or that keyword searching could have been used
to narrow the classification search results down further to
an amount capable of manual review. Similarly, we con-
clude that the Clearlamp standard relied upon by the dis-
trict court does not go far enough when an unreviewably
large number of references are returned by a skilled
searcher’s diligent search. We do not hold that manual re-
view of search results will in all instances be necessary to
establish discovery. We also do not go so far as to endorse
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IRONBURG INVENTIONS LTD . v. VALVE CORPORATION 13
Valve’s vision of the discoverability standard, though we
note the normative force of Valve’s argument that thou-
sands of sources, not further narrowed in any way, likely
cannot be considered discovered for estoppel purposes un-
der § 315(e)(2). We hold only that something more is re-
quired for estoppel when a classification search alone
returns an unreviewable number of search results—a de-
mand not met here.
This conclusion is in line with preponderant evidence
of record in this case, which indicates that keyword
searches or other narrowing techniques would be used af-
ter an initial classification search. See IPR Estoppel Order,
2024 WL 2091177, at *5 (district court acknowledging but
not considering in its analysis that “45 keyword searches”
were performed by Landon IP); J.A. 6179–81 (Cardinal IP
searcher mentioning both keyword and citation searches as
complementary to classification searches); J.A. 7545–46
(Landon IP professional declaring that a skilled searcher
would have performed sequential citation-based searches
and classification-based searches until a “search query re-
sult list small enough to manageably review” is returned).
Because the district court was confronted with thousands
of references in the Landon IP Kotkin search results yet
still found that no narrowing was required for Kotkin to be
deemed discovered, we conclude that the district court
erred.
Further, we determine this error warrants reversal on
the issue of estoppel. As noted, Landon IP’s search efforts
were not meaningfully developed further on remand to in-
clude more than Ms. Williams’s classification search re-
sults already in evidence. Since we have found this
evidentiary basis insufficient to support a finding of discov-
ery under the skilled searcher test, “nothing of record war-
rants a further exercise of the fact-finding function or
indicates any possibility that the appealed judgment might
be sustained by such exercise.” Panduit Corp. v. Dennison
Mfg. Co., 810 F.2d 1561, 1565–66 (Fed Cir. 1987). We
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IRONBURG INVENTIONS LTD . v. VALVE CORPORATION 14
therefore reverse the judgment of the district court insofar
as it estopped Valve from raising the Kotkin ground of in-
validity based on Landon IP’s search evidence.5 On re-
mand, the district court shall consider the invalidity of the
’525 patent in light of Kotkin.
B
We next consider Valve’s argument that the Cardinal
IP searches offered on remand by Ironburg are irrelevant
to the skilled searcher inquiry as tainted by hindsight bias.
i
The district court considered extensive evidence from
Cardinal IP on remand. Among this evidence was testi-
mony from Seth Greenia, a search professional at Cardi-
nal, and testimony from Brian Hameder, a project
manager with Cardinal.
In his declaration, Mr. Greenia reported being able to
locate Kotkin, Koji, and Raymond across two phases of
search conducted in 2023: an initial search—inclusive of
both classification and citation searches—and a
5 The Landon IP evidence was also used by the dis-
trict court to purportedly demonstrate the discoverability
of Koji. IPR Estoppel Order, 2024 WL 2091177, at *5–6 &
n.8. However, we need not pass on the district court’s treat-
ment of Koji given our conclusion below that Raymond was
not discoverable by a skilled searcher at the time of Valve’s
’948 IPR petition. Section 315(e)(2) speaks in terms of
“ground[s],” not prior art references, see Ingenico Inc. v.
IOENGINE, LLC, 136 F.4th 1354, 1365 (Fed. Cir. 2025),
and Raymond and Koji are both part of the Willner-Koji-
Raymond invalidity ground. A finding that one element of
the ground was not discoverable necessarily means that
ground could not have reasonably been raised at the time
of Valve’s IPR petition and estoppel does not apply.
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IRONBURG INVENTIONS LTD . v. VALVE CORPORATION 15
supplemental classification search focused on “the use of
an inherently resilient and flexible paddle or elongated
member.” J.A. 6178 ¶¶ 8–11; J.A. 6180–82 ¶¶ 20–28. The
district court noted that Mr. Greenia conducted this sup-
plemental search because Raymond, which arises not from
the video game context but in the amateur radio context,
was located but considered only of “peripheral relevance”
after the initial search. IPR Estoppel Order, 2024 WL
2091177, at *8 (quoting J.A. 6182 ¶ 27). Mr. Greenia also
represented that the references in question were not made
available to him prior to his search, which the district court
credited as evidence of a lack of hindsight bias, see IPR Es-
toppel Order, 2024 WL 2091177, at *10 n.17.
Mr. Hameder submitted two declarations on remand.
In the first, Mr. Hameder testified to the cost of
Mr. Greenia’s initial and supplemental searches. See
J.A. 7370–71 ¶¶ 8–10. In the second, submitted after his
deposition in the matter, Mr. Hameder purported to ad-
dress a potential issue in Mr. Greenia’s initial search by
applying date filters to exclude any results from after the
time of Valve’s ’948 IPR petition. See J.A. 8247–48 ¶¶ 3–6.
Mr. Hameder’s bottom-line conclusion in his second decla-
ration is that Kotkin, Koji, and Raymond would have all
been discoverable from Mr. Greenia’s initial search, re-
gardless of whether proper date filtering had been applied.
See J.A. 8248 ¶ 7.
ii
Valve’s major arguments against the relevance of Car-
dinal IP’s evidence are twofold: (1) that Mr. Greenia’s ini-
tial search, including later corrections by Mr. Hameder,
incorporates hindsight information; and (2) that the sup-
plemental classification search conducted by Mr. Greenia
was motivated entirely by hindsight.
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IRONBURG INVENTIONS LTD . v. VALVE CORPORATION 16
a
We first address Mr. Greenia’s initial search. As noted,
insofar as it was initially offered to show the discoverability
of Raymond by a skilled searcher, the district court struck
a certain search string, number 34, from this search be-
cause it was a forward-and-backward citation search,
which would have revealed patents and other publications
that arose after Valve’s 2016 petition in the ’948 IPR. See
IPR Estoppel Order, 2024 WL 2091177, at *8. These in-
clude, for instance, a design patent, U.S. Patent
No. D983,269 S, issued to Ironburg in 2023, which cites
Raymond—likely because Raymond was raised as part of
the CMG Grounds in CMG’s IPR. At the same time, the
district court did not consider Mr. Greenia’s evidence that
his initial search uncovered Kotkin and Koji on remand
considering its conclusions regarding the Landon IP evi-
dence. Ironburg offers this evidence on appeal, however, as
an alternative ground for affirmance given Mr. Hameder’s
corrections.
Valve argues that Mr. Hameder’s second declaration
was a belated attempt to control adequately for hindsight
bias in Mr. Greenia’s search results. Valve also argues that
the district court failed to contend with the ways that hind-
sight bias infected Mr. Greenia’s initial search methodol-
ogy from the very beginning. We agree on both points.
Mr. Hameder’s second declaration fails to adequately
account for the hindsight bias in Mr. Greenia’s initial
searches and is therefore not probative regarding the dis-
coverability of Kotkin, Koji, or Raymond to a skilled
searcher exercising reasonable diligence at the time of
Valve’s ’948 IPR petition. This, as Valve points out, is be-
cause it was not just the search string number 34 that
made use of forward-and-backward citation searching
without date restrictions: one of Mr. Greenia’s early
searches, number 3, also made use of forward-and-back-
ward citation searching that would have pulled in later-
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IRONBURG INVENTIONS LTD . v. VALVE CORPORATION 17
arising patents and publications that cite Kotkin, Koji, and
Raymond. J.A. 6204 (showing that search number 3 is a
“cta” search like number 34). Based on the sources re-
turned by this early citation search, Mr. Greenia then
crafted keywords and selected additional classifications to
guide his subsequent iterative searches. J.A. 7645,
7658–59. We therefore hold that the district court’s finding
that Mr. Greenia’s searches were not infected with hind-
sight bias—based on his mere testimony that he had not
been shown the references before searching—was error.
Given that Mr. Greenia’s first search was affected by
hindsight bias from an early point, we conclude that
Mr. Hameder’s corrections in his second declara-
tion—namely, applying date filters to only the citation
search struck by the district court—do not go far enough to
render Mr. Greenia’s initial set of classification and cita-
tion searches probative to the skilled searcher inquiry at
the relevant time. Instead, Mr. Hameder’s corrections,
which were revealed five months after Mr. Greenia com-
pleted his initial search, are “too little too late.” Appellant
Br. 56. Mr. Greenia’s initial classification and citation
search is therefore not probative regarding a skilled
searcher’s discovery of Kotkin, Koji, or Raymond at the
time of Valve’s IPR petition.
b
Finally, we turn to Valve’s argument that
Mr. Greenia’s supplemental search, which also purports to
have located Raymond, was motivated solely by hindsight
bias. Valve argues that this supplemental search epito-
mizes hindsight bias because of deposition testimony from
Mr. Hameder suggesting that the search was only con-
ducted “because there was another reference that had not
yet been found.” J.A. 7665. Ironburg responds, citing
Mr. Greenia’s declaration testimony, that supplemental
searches are “common practice,” and that nothing in the
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IRONBURG INVENTIONS LTD . v. VALVE CORPORATION 18
record suggests that this supplemental search was tainted
by hindsight. Appellee Br. 52 (citing J.A. 6178 ¶ 8).
Again, we agree with Valve. Ironburg does not provide
sufficient reasoning that excuses Mr. Hameder’s unequiv-
ocal admission that the supplemental search was only un-
dertaken in the first instance because of the need to locate
Raymond—clear evidence of hindsight bias. As with
Mr. Greenia’s initial searches, this hindsight-driven sup-
plemental search cannot be considered probative regarding
the discovery of Raymond at the relevant time for the
skilled searcher inquiry.
Thus, as a whole, the Cardinal IP evidence fails to es-
tablish the discoverability of Kotkin, Koji, and Raymond to
a skilled searcher at the time of Valve’s IPR petition be-
cause it is tainted by hindsight. To the extent the district
court relied on the Cardinal IP evidence to estop the Will-
ner-Koji-Raymond ground, its judgment must also be re-
versed. On remand, the district court shall also consider
the invalidity of the ’525 patent in light of Willner-Koji-
Raymond.
III
We have considered the parties’ remaining arguments
and find them unpersuasive. The judgment of the district
court is reversed. The district court’s estoppel of the Kotkin
ground was based on undeveloped, overgeneralized evi-
dence, and its estoppel of the Willner-Koji-Raymond
ground failed to adequately account for hindsight concerns.
We therefore remand for further proceedings not incon-
sistent with this opinion.
REVERSED AND REMANDED
COSTS
Costs to Appellant.
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United States Court of Appeals
for the Federal Circuit
______________________
IRONBURG INVENTIONS LTD.,
Plaintiff-Appellee
v.
VALVE CORPORATION,
Defendant-Appellant
______________________
2024-2088
______________________
Appeal from the United States District Court for the
Western District of Washington in No. 2:17-cv-01182-TSZ,
Senior Judge Thomas S. Zilly.
______________________
STARK , Circuit Judge, concurring.
As district courts have noted, we have not provided
them, nor the Patent and Trademark Office’s Patent Trial
and Appeal Board (“Board”), much guidance on how the
skilled searcher test is to be applied. See, e.g., GeigTech E.
Bay LLC v. Lutron Elecs. Co., 2023 WL 8827572, at *5
(S.D.N.Y. Dec. 21, 2023) (“[T]he Federal Circuit has not re-
fined exactly what facts or circumstances qualify as a
‘skilled searcher conducting a diligent search.’”); EIS, Inc.
v. IntiHealth Ger GmbH, 2023 WL 6797905, at *3 (D. Del.
Aug. 30, 2023) (“[T]he Federal Circuit has not expanded on
the facts or circumstances that constitute a ‘skilled
searcher conducting a diligent search.’”). As an appellate
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IRONBURG INVENTIONS LTD . v. VALVE CORPORATION 2
court, with no factfinding role, we necessarily limit our de-
cisions (with rare exceptions not applicable here) to the is-
sues fairly presented to and crystallized by the tribunals
we review. See generally Ironburg Inventions Ltd. v. Valve
Corp., 64 F.4th 1274, 1299 (Fed. Cir. 2023) (“Ironburg I”)
(deciding only that the burden to prove what a skilled
searcher would have reasonably discovered rests with the
party asserting estoppel). And, to be sure, the skilled
searcher issue has arisen so far in only a handful of ap-
peals. See, e.g., Ingenico Inc. v. IOENGINE, LLC,
136 F.4th 1354, 1365 (Fed. Cir. 2025); Ironburg I, 64 F.4th
1274; Intuitive Surgical, Inc v. Ethicon LLC, 25 F.4th 1035
(Fed. Cir. 2022). These realities exacerbate the “challeng-
ing” task courts and the Board confront when asked to ad-
dress IPR estoppel disputes governed by 35 U.S.C. § 315(e).
Ironburg I, at 1298.
Our opinion resolving the case before us today, which I
join in full, does not, of course, fill in all the details of IPR
estoppel. See Op. at 10-11 (“[W]e have not yet had the op-
portunity to take a conclusive stance on what ‘discover[y]’
within the skilled searcher inquiry entails . . . [and] find it
unnecessary to resolve this question [here].”) (internal quo-
tation marks omitted; first, third, and fourth bracketed al-
terations added). We do, however, make several important
contributions to the ongoing development of the skilled
searcher test. I write separately to highlight them.
I
Prior to our opinion in Ingenico, 136 F.4th at 1365, dis-
trict courts had sometimes focused the skilled searcher in-
quiry on whether “a skilled searcher would have been able
to find a particular reference.” EIS, 2023 WL 6797905, at
*3 (emphasis added); see also Clearlamp, LLC v. LKQ
Corp., 2016 WL 4734389, at *9 (N.D. Ill. Mar. 18, 2016) (“In
accordance with the § 315(e)(2) standard, the datasheet
can be used in civil litigation only if it could not have been
found by a skilled searcher performing a diligent search.”)
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IRONBURG INVENTIONS LTD . v. VALVE CORPORATION 3
(emphasis added). Then, in Ingenico, 136 F.4th at 1365, we
made clear that “a[n] [invalidity] ground is not the prior art
asserted during an IPR.” As the court’s opinion reaffirms
today, “Section 315(e)(2) speaks in terms of ‘ground[s],’ not
prior art references.” Op. at 14 n.5. Consequently, for pur-
poses of the skilled searcher test, the patentee’s burden to
show discoverability of invalidity grounds is not automati-
cally satisfied by proving the findability of the prior art ref-
erence(s) on which the invalidity ground is based.
In my view, then, a patentee relying on the skilled
searcher test to estop a challenger, pursuant to § 315(e)(2),
from pressing a particular ground of invalidity in district
court, must prove at least the following: (i) the prior art
reference(s) at issue were findable by a skilled searcher
conducting a diligent search and (ii) the skilled searcher
also reasonably would have been expected to discover,
based on the reference(s) found, the invalidity ground the
challenger seeks to assert in court.
Today’s case centers on the first step of this analysis:
whether the prior art references on which Valve relies were
reasonably findable. Valve does not separately challenge
the second step; that is, Valve does not contend that, even
if all of its references were findable, the grounds for inva-
lidity were somehow not reasonably discoverable. Thus,
today’s opinion, properly, does not evaluate what may be
required to show a ground for invalidity would have been
reasonably discoverable once a skilled searcher is pre-
sented with one or more findable references.1
1 I will not elaborate on this issue either, other than
to suggest that there may be instances in which the ground
of invalidity is so apparent on the face of the prior art ref-
erence that, once the reference is found, the invalidity
ground is necessarily discovered. More generally, it may
be that evidence bearing on the first step – the findability
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IRONBURG INVENTIONS LTD . v. VALVE CORPORATION 4
II
The court’s opinion does have something to say about
the first step I identified above. Specifically, it holds that,
in at least some circumstances, “something more” than
proving the mere accessibility2 of a prior art reference is
required in order to establish a skilled searcher performing
a diligent search would have found the reference.3 Op. at
13. Determining precisely when “something more” must be
demonstrated to meet the burden at step one, and in those
instances discerning what that “something more” must be,
calls for context-dependent analysis. What might suffice to
establish that a skilled searcher reasonably could have
been expected to find a prior art reference will vary based
on factors like the field of the invention, the state of the
prior art, and the search strategies a diligent searcher in
inquiry – is also pertinent to the second step – the discov-
erability assessment. But these are two different require-
ments, the first relating to references and the second
concerning grounds.
2 The Kotkin example from this case represents
what I would view to be a reference that is merely accessi-
ble as opposed to truly findable. A skilled searcher could
access Kotkin by conducting a classification search, but the
mass of other references returned by that same search
makes it unreasonable to assume a skilled search would, in
fact, find the Kotkin reference. In my view, then, a prior
art reference must be accessible in order to be findable, but
accessibility alone does not prove findability of a reference.
3 Today’s opinion, and prior ones, speak in terms of
the “discoverability” of prior art references. I use “findabil-
ity” here to underscore that the ultimate § 315(e)(2) ques-
tion – whether a ground was discoverable – requires two
analytical steps, one relating to the references themselves
and a second relating to the invalidity grounds based upon
them.
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IRONBURG INVENTIONS LTD . v. VALVE CORPORATION 5
the space would have employed. See Ironburg I, 64 F.4th
at 1298. Because this inquiry is fact-specific, we must
largely leave development of the “something more” require-
ment to the district courts and the Board.
Today’s case does, nonetheless, provide helpful exam-
ples of findability, one where “something more” is not pre-
sent and one where it is. As the court holds, “something
more” than accessibility is not demonstrated when, as with
the Kotkin reference, the only evidence in the record is that
a skilled searcher would have undertaken a classification
search that yielded, in addition to the reference now being
asserted by the patent challenger, tens of thousands other
references. Op. at 12-13. In contrast, “something more”
may be satisfied where, as with a different reference Valve
is estopped from asserting invalidity grounds based upon,4
the reference is expressly cited on the face of the patent
being challenged. See J.A. 14-15 (“Indeed, Willner was ac-
tually identified by a skilled searcher, and the reference
appears on the face of the [asserted] patent.”) (emphasis
added); see also J.A. 5964. These examples, while far from
exhaustive, may prove to be useful guideposts as courts
and the Board encounter IPR estoppel issues in an unpre-
dictable array of specific scenarios.
4 The parties do not dispute on appeal, nor did they
below, that a skilled searcher conducting a diligent search
reasonably could have been expected to find U.S. Patent
No. 6,760,013 (“Willner”). See J.A. 14-15.
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