Patent Infringement Defenses


Patent Infringement Defenses

When an individual or organization is accused of misusing another’s invention, they have several patent infringement defenses at their disposal. Each of these defenses can limit or eliminate liability related to alleged patent infringement. In fact, some of these arguments could invalidate a patent in its entirety. Along with non-infringement, patent invalidation is one of the most common goals of defendants.

Each of these justifications against alleged infringement have nuances. Violating a patent with prior intent of using certain defenses, for instance, could be viewed as evidence of willful infringement.  Typically patent infringement issues begin with the sending of a patent cease and desist letter.  In a reply to such a letter, some of these defenses may be cited.


When a defendant argues that the product or process they’re using doesn’t conform to the claims of a patent registration, they’re utilizing a defense of non-infringement.  Of course, any legitimate defense will assert that the accused’s actions didn’t qualify as a patent infringement.  Arguing non-infringement focuses on the claims of the patent. These claims specify the technical scope that a registration covers. In many cases, showing that a process or product doesn’t meet all of the elements of any claim of the patent is enough to avoid liability.

Courts may consider the Doctrine of Equivalents if literal infringement hasn’t occurred. They’ll look at the following issues:

  • Is substantially the same function performed by the alleged infringing device?
  • Is the function in question performed in substantially the same way?
  • Is the same result achieved by this function?

If all three of these criteria are met, an alleged infringing product or process doesn’t always have to conform to the literal claim.  The Doctrine of Equivalents also applies if two elements are essentially interchangeable and a knowledgeable person in the field would recognize this fact.

If multiple entities are involved in patent misuse – yet no single actor engages in every necessary step to meet the claim requirements – then divided infringement (also known as joint infringement) may have occurred.

This can be used as a patent infringement defense, but courts will consider the following in deciding whether one party can be held accountable for the misuse of others:

  1. Did the entity control or direct another’s performance?
  2. Was there a joint enterprise formed by those involved?

This could cover actions of inducement or patent contributory infringement. Even if an infringer keeps the steps involved in misuse separate from each other, they can be held liable if they control the system (even without controlling each element) and obtain benefit from it.

Invalidity Based on Prior Art

When the U.S. Patent and Trademark Office (USPTO) approves a patent registration, there is a legal presumption that the claims are valid. If an alleged infringer can prove invalidity based on prior art, then the patent becomes unenforceable.

When an individual applies for federal protection of their invention, they’re stating that they know of no prior art which would invalidate their creation. This art constitutes any information in the public domain that shows the patent claims lack originality. Even if the original applicant was unaware of the prior art, its mere existence can nullify the registration.

There are two forms of defense based on prior art:

  • Anticipation defense: One of the basic requirements for patent approval is novelty. The process or product must be completely unknown. An anticipation defense asserts that the property in question doesn’t meet this condition and isn’t novel.
  • Obviousness defense: Even if an invention hasn’t been published in the public domain, it may still be rendered invalid due to prior art. If a skilled person in the specified field considers a creation an obvious extension of the art currently available to the public, an obviousness defense can be used.


Once a patent owner becomes aware of infringing actions, they must file a lawsuit within a reasonable amount of time.  If action is not taken in a reasonable amount of time, the Doctrine of Laches may bar the claim.

The legal system recognizes that evidence becomes scarcer over time. Proving you didn’t do something is far simpler, for instance, if you’re accused of it the next day rather than 10 years down the line. If the courts feel that a patent owner knew of infringement and engaged in an (1) unreasonable delay that was (2) directly prejudicial to the defendant, they may choose to find the accused infringer not liable.

There are two forms of prejudice that apply for this patent infringement defense:

  • Evidentiary prejudice: This can occur if relevant documents are lost over time, memories become less reliable or important witnesses pass away before trial.
  • Economic prejudice: If a defendant isn’t accused of patent infringement, they’ll likely work to grow their business without recognizing their actions are illegal. If investment and expansion occurred after infringement was discovered and continued due to delay, the plaintiff’s actions were prejudicial.

Laches doesn’t claim that infringement never occurred. It’s known as an equitable defense – which essentially asks the courts to excuse a specific behavior due to the plaintiff’s prejudicial actions. If this patent infringement defense is successful, any potential damages related to misuse prior to the lawsuit are eliminated.  However, it does not necessary grant the defendant rights moving forward.

Inequitable Conduct

During the patent registration process, the applicant has a duty of good faith and candor with the USPTO. This means they must be completely truthful with the examining patent attorney and not withhold any information (e.g. existing prior art). If they violate this duty, the patent may become invalid and unenforceable.

Since claims of inequitable conduct are essentially asserting that fraud took place, the defendant must identify specific actions which constitute misconduct. Two elements must be proven for this patent infringement defense to be valid:

  • Deceptive intent: Negligently leaving out information – or even omissions that seem suspicious – aren’t enough to meet the definition of inequitable conduct. It must be proven that the applicant’s intent was to deceive the USPTO through deliberately withholding information it knew to be material to their application.
  • Information materiality: The information withheld must be material to the application. A “but-for” test is used to distinguish whether or not the examiner would have refused a claim assuming they had the information.

In some cases, materiality doesn’t need to be shown. If the court believes that egregious misconduct occurred during the registration process, they may accept inequitable conduct as a patent infringement defense.

Repair Doctrine

The Doctrine of Repair and Reconstruction dictates that an individual who’s authorized to use, distribute or produce a patented item has a lawful right to repair or replace components that are not patented. This typically applies when a patented item has components with a useful life shorter than that of the invention itself. Essentially, owners are legally allowed to repair their property.

The issue at hand is that the patent holder has no rights over unpatented components used in the making of their property. Owners of such items may also be able to permissibly modify them to improve the overall functionality. Once the useful life of the product has ended, though, the purchaser cannot reconstruct the protected invention utilizing unlicensed components.

Patent Misuse

An individual accused of violating the exclusive rights of an inventor can also claim patent misuse as an infringement defense. This occurs when the owner engages in behavior that expands the term or scope of protection granted by law. While a limited monopoly is granted by patent registration, a patent owner cannot assert ungranted rights in order to restrain trade.

The following are examples of patent misuse:

  • Requiring royalties on invalid patents.
  • Engaging in price fixing.
  • Purchasing all patents in the relevant market to hinder competition.
  • Requiring buyers to not do business with competitors as a condition of purchase.
  • Violating any antitrust laws.

If a patent owner engages in these actions, they could lose the right to enforce their rights. In some instances, though, the owner may be allowed to end their abusive practices and validate their rights again.


Alleged infringers can also claim that an inventor or owner is precluded (i.e. estopped) from suing based on their prior behavior. Equitable estoppel occurs when a party believes the owner had no intent of enforcing their patent because of (1) prior assurances or (2) a lack of enforcement activity. For this defense to be valid, the accused must have engaged in actions – to their own detriment – based on this belief.

Estoppel may also serve as a patent infringement defense if prior assertions of the owner are contrary to the rights they’re currently claiming. If their application included limitations of rights or they previously made statements of limitation during litigation or PTAB hearings, they cannot later claim to have these rights. These actions preclude the filing of an infringement action.

Estoppel – along with inequitable conduct and laches – are forms of equitable defense. Although these can be used to avoid liability, courts have found that a belief in invalidity is not an acceptable patent infringement defense.

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If you are involved in a patent infringement matter and have questions about patent infringement defenses, please contact us today.


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