It is critical that in the application of the doctrine of equivalents, even to an individual element, such element is not eliminated in its entirety.Thus, for a patentee who has claimed an invention narrowly, there may not be infringement under the doctrine of equivalents in many cases, even though the patentee might have been able to claim more broadly. If it were otherwise, then claims would be reduced to functional abstracts, devoid of meaningful structural limitations on which the public relies.⁷

A finding of equivalence is a determination of fact. Proof can be made in any form: through testimony of experts or others versed in the technology; by documents, including texts and treatises; and, of course, by the disclosures of the prior art. As with any other issue of fact, final determination requires a balancing of credibility, persuasiveness, and weight of evidence. In addition, once it is determined the doctrine of equivalents is applicable, a finding of infringement does not necessarily follow. The doctrine of equivalents is subject to several types of limitations that may prevent a finding of infringement, preventing the patentee from recapturing through equivalence certain coverage given up during prosecution; estopel, based on the prosecution history of the patent; and invalidity of the claims according to the proposed construction in view of the prior art.

For purposes of this article, let's assume that none of the aforementioned limitations apply. That leaves the critical question of whether the substitution of a methyl group for a hydrogen atom is an equivalent. If the answer is "yes," and no estoppel arguments can be advanced, and the patent is still in effect (which we are assuming for our purposes here), then you as inventor would be required to take a license from the patent owner, namely Bayer, in order to practice your new, improved invention. However, if the answer is "no," then you are free to practice your new invention without the need of a license.

The issue of whether the patent protects you really depends on whether your patent attorney wrote and negotiated a "good" patent, i.e., did the attorney do it correctly? This question typically arises when a competitor starts selling a variation on your new product, such as phenyl aspirin. Let's explore your options in such a situation.

First, what do you compare? Is it an issue of your commercial product against the competitor's? That comparison does not look promising, even under the doctrine of equivalents, as pretty good arguments can be advanced that saturated alkyl groups are not the equivalent of unsaturated phenyl rings. However, you are, at least initially, in luck because that is not the comparison that is made. Instead, you must compare the claims of your issued patent against the allegedly infringing product. For purposes of this article, let's assume that at least one claim in your issued patent has the characteristics that are listed here, with quite a few liberties taken in the syntax (as will be quickly noted by any patent attorney).

Your first thoughts may include an observation that your patent attorney made a mistake, or worse, went off the deep end, because you can recall no atoms from chemistry class called R1 or R2. The credit or the blame for this dates back to 1925 when a chemical applicant named Markush sought to protect an entire class of compounds rather than just the present commercial embodiment.⁸ Therefore, the key is to determine if within one of the permissible classes of chemical moieties, the phenyl group can be found within the definition of R1 and the methyl group can be found within the definition of R2.

Fortunately, it can. The aromatic phenyl ring is a six-membered ring that is considered to be a C6 aryl compound. Within the definition of R1 are C5-18 aryls, of which C6 is a member. Performing the same analysis for the methyl group — a C1 alkyl moiety — it can be seen that the methyl group falls at the beginning of the C1-6 alkyl class of compounds that are covered under the definition of R2. Therefore, your patent attorney has covered not only your commercial methyl aspirin, but a good number of additional compounds, as well. One of these, phenyl aspirin, is protected under your patent.