Under 35 U.S.C. § 271, anyone who makes, uses, offers to sell, or sells any patented invention domestically, or imports a patented invention into the United States during the term of the patent, is infringing the patent. Anyone who actively induces someone else to infringe the patent is also liable as an infringer. Similarly, anyone who offers to sell, sells, or imports a material component of something that is patented, knowing that the component was especially made for use in an infringement and is not a commodity suitable for a substantial non-infringing use, is also liable as a contributory infringer. The statute enumerates additional specific infringing conduct and conduct that is not infringing.
Generally, to enforce a patent against an infringing product, a patent owner will sue the infringer in a civil lawsuit. Determining whether there was infringement involves a two-step analysis by the court. The first step is claim construction, based on the claim language, the written description of the specification, the patent prosecution history, and extrinsic evidence when necessary to understand the patent. Claim terms are given their ordinary meanings, unless the specification describes a special definition.
Next, the court will look at whether a particular device literally infringes the claim. The elements of each of the patent’s claims will be compared with the invention that is claimed to be infringing. If these elements match the elements of the invention, an infringement will be found.
What if the infringement is not literal, but only some minor aspect of the patent has been tweaked in the accused product? Under the “doctrine of equivalents,” if the individual claim limitations and the infringing invention are sufficiently equivalent both in what they do and how they do it, infringement will also be found. The doctrine of equivalents also applies when two elements are interchangeable and someone with ordinary skill in the art out of which the invention arises would have known the elements were interchangeable at the time of infringement.
The issue is whether any differences are “insubstantial.” There are different ways of analyzing whether a difference is insubstantial. Under the triple identity test, the difference is insubstantial if the feature in the accused product performs substantially the same function, in substantially the same way, and to yield substantially the same outcome as the limitation articulated in the patent claim.
The doctrine of equivalents is substantially limited by several rules, including the all-elements rule, prosecution history estoppel, and the doctrine of vitiation. The all-elements rule provides that the doctrine of equivalents has to be applied to each individual element of a claim, rather than to the whole invention. This means that a substantial equivalent of an element of the patented invention is present in the accused product.
Under the doctrine of vitiation, an element of the accused invention is not equivalent to a limitation of a claimed invention if finding equivalence would completely vitiate or impair the legal meaning of the limitation. Courts look at the totality of the circumstances to determine whether what is alleged to be equivalent can be fairly characterized as an insubstantial change from the claimed subject matter without leaving the limitation meaningless. When there is a clear substantial difference or difference in kind between the accused product and a claim limitation, claim vitiation will apply. For example, finding that all shapes are equivalent structures would vitiate a claim limitation that required a circular shape.
Prosecution history estoppel is a doctrine that stops a patent owner from claiming infringement under the doctrine of equivalents in certain situations, based on statements made during patent prosecution. This means that if you narrowed your claims during the patent prosecution so that the patent would be issued, you cannot broaden the claim with the doctrine of equivalents in order to include what you “surrendered” in order to get the patent issued.