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Four U.S. Utility Patent Infringement Defenses the Average Bear Overlooks

John D. Vandenberg

Editor: Shelby A. Stepper


What’s better than winning a patent lawsuit for an accused infringer? Winning on a theory the average bear patent litigator would have missed![1] Here are four underutilized defenses to a charge of U.S. utility patent infringement:

  1. Steps without Acts
  2. Indefiniteness Stemming from Unclear Application of 112(f)
  3. Failure to Mitigate Damages
  4. Expiring Patents Add to the Public Domain
Photo By: Shelby Stepper

Photo by Shelby Stepper

I. Steps Without Acts

Our average bear assumes “step” and “act” are the same in patent law. But they’re not and we know they’re not because the Patent Act says so:

An element in a claim for a combination may be expressed as a means or step for performing a specified function without the recital of structure, material, or acts in support thereof, and such claim shall be construed to cover the corresponding structure, material, or acts described in the specification and equivalents thereof.[2]

Judge Rader explained this critical distinction well on April Fool’s day, 1999:

In general terms, the “underlying function” of a method claim element corresponds to what that element ultimately accomplishes in relationship to what the other elements of the claim and the claim as a whole accomplish. “Acts,” on the other hand, correspond to how the function is accomplished. Therefore, claim interpretation focuses on what the claim limitation accomplishes, i.e., its underlying function, in relation to what is accomplished by the other limitations and the claim as a whole. If a claim element recites only an underlying function without acts for performing it, then § 112, ¶ 6 applies even without express step-plus-function language. [3]

A few have successfully argued that method claims were invalid because they recited steps without supporting acts and the specification failed to disclose and link supporting acts.[4] But our average bear does not argue this so there are few Federal Circuit decisions directly on point. Instead, most patent litigators, and therefore most district court judges with a significant patent docket, associate the “means plus function” treatment of Section 112(f) with apparatus claim elements only, not method steps.

But despair not. Several Federal Circuit decisions distinguish steps from acts in a closely related context: Section 101 patent eligibility.[5]

Takeaway: For each method step in a patent claim ask: does it recite acts that perform the function or only the function/result? If the latter, study the specification to see if it describes acts sufficient to perform the function and whether the specification links those acts to that function.

II. Indefiniteness stemming from unclear application of 112(f)

While on the topic of Section 112(f), let us not forget that it invokes a special rule for interpreting certain types of patent claim language. Whether claim language invokes this special rule typically affects how broadly the claim is interpreted—how much territory it covers. So, if it is unclear whether certain patent claim language invokes this special rule or not, then the scope of the claim is also unclear. This sounds obvious, right? Well, apparently not to our average bear. This is virtually never argued in patent litigation.

But the Patent Office gets it. It instructs its Examiners to consider rejecting claims as being indefinite when it is unclear whether claim language triggers this special rule of interpretation.[6]

Takeaway: Whenever arguing that a claim limitation triggers Section 112(f), consider arguing, in the alternative, that uncertainty on that question renders the claim invalid under Section 112(b).

III. Failure to Mitigate Damages

Patent infringement is a tort. Several legal doctrines apply across all torts. For example, damages for a federal statutory tort are presumptively governed by common law tort damages principles.[7] One tort damages principle is failure to mitigate damages. But, alas, our average bear does not consider whether the patent owner failed to mitigate damages.

Part of the “general theory of damages,” is that “a victim has a duty ‘to use such means as are reasonable under the circumstances to avoid or minimize the damages’ that result from violations of the statute. … If the victim could have avoided harm, no liability should be found against the employer who had taken reasonable care, and if damages could reasonably have been mitigated no award against a liable employer should reward a plaintiff for what her own efforts could have avoided.”[8] Patent owners, like other plaintiffs, have a duty to mitigate damages and cannot recover losses that they could have avoided through reasonable efforts.[9]

Several common actions by patent owners might be considered as failure to mitigate damages for patent infringement, including failure to mark the patent number on patented products, lying in wait while damages accrue, failing to take a clear position on the scope of the patent’s claims or changing one’s position, keeping secret an already licensed supplier of the patented technology, failing to offer a FRAND license when obligated to do so, etc.

Takeaway: Remember patent infringement is a tort subject to common law tort defenses including the failure to mitigate damages.

IV. Expiring Patents Add to the Public Domain

The average bear defending against a patent infringement charge looks at other patents to see if they are relevant prior art, or perhaps for a double patenting defense, but nothing more. The expiration of a non-asserted patent gets no attention.

U.S. patent law gives a right to copy and use certain inventions.[10] Specifically, a device, which was described in an expired patent (even a third-party’s patent), is part of the public domain.[11]

Even if public domain does not provide a complete defense, it may reduce damages. For example, to the extent that the value of the accused feature, or the loss by the patent owner, is attributed to what was described in an expired patent, and thus in the public domain, it should be excluded from the damages calculation.[12]

Takeaway: Identify expired patents, whether or not prior art, that describe part or all of the subject matter claimed and assert that their disclosures are free to all in the public domain.



[1] Of course, all bears are exceptional.

[2] 35 U.S.C. § 112(f) (emphasis added).

[3] Seal-Flex, Inc. v. Athletic Track and Court Construction, 172 F.3d 836, 849–50 (Fed. Cir. 1999) (Rader, J. concurring) (emphasis added). See O.I. Corp. v. Tekmar Co., 115 F.3d 1576, 1582–83 (Fed. Cir. 1997) (“We interpret the term ‘steps’ to refer to the generic description of elements of a process, and the term ‘acts’ to refer to the implementation of such steps.”). See generally

[4] E.g., Dionex Softron GmbH v. Agilent Techs., Inc., 811 F. App’x 630, 631, 632 (Fed. Cir. 2020) (non-precedential) (in interference, affirming method claim step in issued patent (“controlling the valve to switch among predetermined valve positions to transfer the sample loop between a low pressure and a high pressure”) triggers Sec. 112(f) (and is indefinite) based on expert testimony “that the claim term would not connote acts to a person of skill sufficient to perform the recited function.”).

[5] E.g., Internet Patents Corp. v. Active Network, Inc., 790 F.3d 1343, 1345, 1348 (Fed. Cir. 2015) (Newman, J.) (method claim recited “maintaining said state upon the activation of another of said icons, wherein said maintaining allows use of said Back and Forward navigation functionalities without loss of said state;” claim invalid under Sec. 101; “IPC’s proposed interpretation of “maintaining state” describes the effect or result dissociated from any method by which maintaining the state is accomplished upon the activation of an icon.”).

[6] See MPEP § 2181 I. (“In the event that it is unclear whether the claim limitation falls within the scope of 35 U.S.C. 112(f), a rejection under 35 U.S.C. 112(b) may be appropriate.”).

[7] Cf. Staub v. Proctor Hospital, 562 U.S. 411, 417 (2011) (“we start from the premise that when Congress creates a federal tort it adopts the background of general tort law”).

[8] Faragher v. City of Boca Raton, 524 U.S. 755, 806-07 (1998) (creating partial affirmative defense for employer’s vicarious liability for a supervisor harassing an employee).

[9] Cf. Bank of Stockton v. Verizon Commc’ns, Inc., 375 F. App’x 746 (9th Cir. 2010) (“Under California law, ‘[a] plaintiff has a duty to mitigate damages and cannot recover losses it could have avoided through reasonable efforts.’”) (citations omitted).

[10] Bonito Boats, Inc. v. Thunder Craft Boats, Inc., 489 U.S. 141, 165 (1989) (“For almost 100 years it has been well established that in the case of an expired patent, the federal patent laws do create a federal right to ‘copy and to use.’ Sears and Compco extended that rule to potentially patentable ideas which are fully exposed to the public.”).

[11] Scott Paper Co. v. Marcalus Mfg. Co., 326 U.S. 249, 256 (1945) (“the patent laws preclude the patentee of an expired patent and all others … from recapturing any part of the former patent monopoly; for those laws dedicate to all the public the ideas and inventions embodied in an expired patent.”); Singer Mfg. Co. v. June Mfg. Co., 163 U.S. 169, 185 (1896) (“It is self-evident that on the expiration of a patent the monopoly created by it ceases to exist, and the right to make the thing formerly covered by the patent becomes public property.”); Gilead Sci., Inc. v. Natco Pharma Ltd., 753 F.3d 1208, 1214 (Fed. Cir. 2014) (2-1) (“it is a bedrock principle of our patent system that when a patent expires, the public is free to use not only the same invention claimed in the expired patent but also obvious or patentably indistinct modifications of that invention”).

[12] Cf. AstraZeneca AB v. Apotex Corp., 782 F.3d 1324, 1338 (Fed. Cir. 2015) (reasonable royalty sought: “When a patent covers the infringing product as a whole, and the claims recite both conventional elements and unconventional elements, the court must determine how to account for the relative value of the patentee’s invention in comparison to the value of the conventional elements recited in the claim, standing alone.”).


John Vandenberg has been practicing patent litigation for about 40 years, the past 33 at Klarquist Sparkman LLP in Portland, Oregon. He successfully argued for Nautilus at the U.S. Supreme Court to change patent claim “indefiniteness” law. Shelby Stepper is a litigation associate at Klarquist and took the above photo.



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