Minerva Surgical, Inc. v. Hologic, Inc.
The Court upheld the century-old rule that an inventor who sells a patent cannot turn around and claim that same patent is invalid in court — but it narrowed the rule, holding it cannot apply when the buyer later expanded the patent's scope beyond what the inventor originally assigned.
The decision sends the case back to the lower court to determine whether the patent at issue was in fact materially broadened after the sale, which would free the inventor's company to challenge the expanded claims.
“By saying one thing and then saying another, the assignor wants to profit doubly—by gaining both the price of assigning the patent and the continued right to use the invention it covers.”
The majority explains why allowing a patent seller to later challenge the patent's validity is unfair dealing.
How it got here: The federal district court barred Minerva's invalidity defense, found infringement, and a jury awarded Hologic about $5 million; the Federal Circuit affirmed; Minerva asked the Supreme Court to step in and the Court agreed to hear it.
The Case in Depth
What happened
Csaba Truckai invented a device to treat abnormal uterine bleeding and assigned his patent application to his company, Novacept, which was eventually acquired by Hologic. Truckai then founded a competing company, Minerva Surgical, and developed a similar device. Hologic later returned to the patent office and expanded the patent's claims to cover applicator heads broadly, then sued Minerva for infringement. Minerva tried to defend itself by arguing the expanded patent was invalid.
The question before the Court
Can an inventor who sells a patent later argue in court that the patent is invalid — and does it matter if the buyer expanded the patent's claims beyond what the inventor originally sold?
The Court's answer
No, not in general — the Court reaffirmed that an inventor who sells or assigns a patent cannot later claim the patent is invalid, because selling the patent carries an implicit promise that it is valid. Allowing the inventor to profit from the sale and then escape liability for using the same invention would be unfair dealing. The Court rejected arguments that the Patent Act of 1952 or two prior decisions had already abolished this rule.
But the rule has firm limits. If the buyer later expanded the patent's claims beyond what the inventor originally assigned, the inventor never made any representation about the validity of those new, broader claims. With no prior inconsistent representation, there is nothing to estop — and the inventor can challenge those new claims. Because the Federal Circuit wrongly refused to consider whether the patent claims were materially broadened after the assignment, the case was sent back for that determination.
Curious how the Court got there? See the step-by-step legal reasoning →
Why it matters
Inventors who assign patents — including employees required to sign over future inventions — need to know that buyers cannot use the assignment to immunize later, broader patent claims from legal challenge. Businesses that acquire and then expand patents get clarity that their windfall expansions may not receive the same protection as the original claims the inventor actually sold.
What changes now
The case returns to the Federal Circuit, which must now decide whether Hologic's post-assignment patent claim — covering all applicator heads regardless of moisture permeability — is materially broader than the claims Truckai originally assigned. If it is, Minerva can challenge the new claim's validity; if it is not, assignor estoppel stands and the invalidity defense remains blocked. The underlying damages award of roughly $5 million remains subject to that outcome.
What this does not decide
The Court does not decide whether Hologic's specific claim is actually materially broader than what was assigned — that factual question goes back to the Federal Circuit. The opinion also leaves open other edge questions about assignor estoppel's reach, such as how it applies to nominal-consideration assignments or precise privity rules.
Concurrences and dissents
Dissent — Justice Alito
Justice Alito argued that the question the Court agreed to decide — whether assignor estoppel bars Minerva's defense — cannot be answered without first deciding whether the 1924 Westinghouse precedent that created the doctrine should be overruled. Because neither the majority nor the principal dissent squarely confronts that stare decisis question, Justice Alito would have dismissed the case as improvidently granted. He criticized the majority for reading a judge-made rule into a statute that contains no textual support for it.
Dissent — Justice Barrett
Justice Barrett, joined by Justices Thomas and Gorsuch, argued that the Patent Act of 1952 does not incorporate assignor estoppel. Congress did not ratify the Westinghouse decision when it reenacted the patent assignment provision, because the doctrine was unsettled by 1952 — Scott Paper had cast serious doubt on it — and because the 1952 Act added language specifying that patents have 'the attributes of personal property,' which undermined Westinghouse's foundational analogy between patent assignments and real-property deeds. Without textual or well-settled common-law support, the doctrine has no place in current law.
How the Court got there
The legal reasoning, step by step
- The Court reaffirmed the doctrine of assignor estoppel — a long-standing rule rooted in fairness that prevents a patent seller from later claiming the patent is invalid. When someone assigns (sells) a patent, they make at least an implicit representation to the buyer that the patent is valid. To then turn around and say the patent is worthless would be to profit twice — once from the sale and again by escaping liability — which courts have treated as unfair dealing since the 1800s.
- The Court rejected the argument that the Patent Act of 1952 abolished assignor estoppel by providing that invalidity 'shall be a defense in any action' involving infringement. Similar language existed in the patent statute when the Court approved the doctrine in 1924 in Westinghouse. More broadly, that reading would also wipe out other well-established common-law preclusion rules like res judicata and equitable estoppel — a result Congress never intended, since it legislated against a backdrop that included those principles.
- The Court also rejected the claim that two later cases had quietly buried the doctrine. Scott Paper (1945) merely declined to apply assignor estoppel in an unusual situation — where the patent had already expired and the device was in the public domain — without questioning the doctrine's core fairness principle. Lear (1969) eliminated a different rule called licensee estoppel, while expressly distinguishing it from assignor estoppel and noting that an assignor's equities are 'far more compelling' than a licensee's.
- The Court clarified that assignor estoppel is bounded by the fairness principle that justifies it: it applies only when the inventor's invalidity claim contradicts an explicit or implicit representation made in the assignment. Three situations fall outside the doctrine's reach — (1) assignments made before specific patent claims exist, such as blanket employment agreements to assign future inventions; (2) cases where a later change in law makes previously valid patents invalid; and (3) situations where the patent's claims were materially broadened after the assignment.
- The third limit was dispositive here. Because Hologic went back to the patent office after acquiring the rights and drafted a new, broader claim covering all applicator heads — not just the moisture-permeable ones Truckai originally described — Truckai could not have warranted the validity of those new claims when he assigned his application. Without that prior representation, there is no inconsistency and no basis for estoppel on the new claims.
- The Federal Circuit erred by treating it as 'irrelevant' whether Hologic had expanded the assigned claims. Whether the new claim is materially broader than what Truckai assigned is the central question that determines whether estoppel applies — and the case was sent back for that determination.
Doctrinal impact
Cases affected by this decision
Reaffirms Westinghouse Elec. & Mfg. Co. v. Formica Insulation Co. (266 U.S. 342)
The century-old decision approving assignor estoppel is upheld and its fairness rationale endorsed, though the doctrine is clarified and narrowed.
Distinguishes Scott Paper Co. v. Marcalus Mfg. Co. (326 U.S. 249)
Scott Paper only declined to apply assignor estoppel in an unusual public-domain situation; it did not abolish the doctrine.
Distinguishes Lear, Inc. v. Adkins (395 U.S. 653)
Lear eliminated licensee estoppel, a different doctrine, and expressly preserved the stronger equities behind assignor estoppel.