Patents and File Wrapper Estoppel

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By Rich Stim

The official file in which a patent is contained at the U.S. Patent and Trademark Office is known as a “file wrapper.” All statements, admissions, correspondence, or documentation relating to the invention are placed in the file wrapper. If, during the patent application process, the inventor admits limitations to the invention or disclaims certain rights, those admissions or disclaimers will become part of the file wrapper and the patent owner cannot later sue for infringement over any rights that were disclaimed in the file wrapper. This defense is known as file wrapper estoppel (or prosecution history estoppel). Estoppel means that a party is prevented from contradicting a former statement or action.

Rule of Court

File wrapper estoppel is a rule of court under which patent applicants are bound by the statements they make in their patent application and subsequent correspondence and documents filed with the U.S. Patent and Trademark Office (USPTO) in the course of prosecuting the application. 

The file wrapper estoppel rule becomes pertinent if the patent owner should ever seek enforcement of his or her patent in court. In that event, the inventor would be prevented from describing the scope of the invention differently from how it was described in earlier documents in the USPTO patent file (again, colloquially referred to as the file wrapper). In short, an inventor is stuck with what’s already been told to the USPTO—and cannot later try to broaden a patent’s coverage that has been surrendered by original claim language, or in an amendment or a written argument during the earlier prosecution stage.

Avoiding File Wrapper Estoppel

Because of the file wrapper estoppel rule, it is good practice to follow these two rules:

  • Do not say anything negative about an invention in a patent application (your words may come back to haunt you).
  • Draft patent claims as broadly as possible in light of the pertinent prior art.

Example: Otto invents a medical tool that uses a fiber optic strand, laser light, specially cloned antibodies, and certain chemicals to detect the presence of various substances in human tissue. In his patent application, Otto drafts his claims very broadly, without a limiting reference to his use of fiber optics. After the USPTO rejects the initial claims, Otto amends his application so that the claims now specify fiber optics as the method of transmitting the laser light. In his cover letter transmitting the amendment to the USPTO, Otto admits that his original claim was wildly overbroad and thanks the examiner for helping him pare his claim down to the appropriate scope. The patent is then granted. Ten years later, Lewis, a famous heart surgeon, makes and uses a similar diagnostic tool, using a magnetic rather than fiber optic means for transmitting the laser light waves. If Otto brings an infringement action against Lewis, alleging that his patent is sufficiently broad in scope to preclude Lewis’s device, the court will hold Otto to his admission about the overly broad scope of his original claim, as found in the file wrapper, which clearly limits his patent to a fiber optic means.