Foreseeability Does Not Bar The Doctrine Of Equivalents, Including For Means-Plus-Function Limitations

In the recent Ring & Pinion Service Inc. v. ARB Corp. decision, the U.S. Court of Appeals for the Federal Circuit held that the foreseeability of an equivalent at the time of filing does not, in itself, create a bar to reliance on the doctrine of equivalents (DOE).1 The unanimous Federal Circuit panel confirmed that infringement can indeed be found under the DOE, notwithstanding that, at the time of the application, the equivalent limitation in question was foreseeable to one of ordinary skill.2 Further, Ring & Pinion clarifies how the DOE applies to claims written with functional language, and dispels the notion that prior case law ever precluded the application of the DOE to foreseeable equivalents of means-plus-function claim limitations.3

Under the DOE, "a product or process that does not literally infringe upon the express terms of a patent claim may nonetheless be found to infringe if there is 'equivalence' between the elements of the accused product or process and the claimed elements of the patented invention."4 In another context, notions of "equivalence" are also analyzed when claim terms are drafted in "means-plus-function" form,5 as permitted by 35 U.S.C. § 112(f). When a means-plus-function limitation appears in a claim, it strictly covers only the structures "described in the specification and equivalents thereof."6 There has thus been considerable debate over the last two decades on the application of the DOE to means-plus function limitations. On the one hand, it is widely recognized that the DOE allows enforceable equivalents to read on insubstantial variations in after-arising technology, in effect compensating for the patent drafter's inability to claim unforeseeable matter.7 On the other hand, it has been suggested that if alternative structures were foreseeable at the time of patenting, then means-plus-function claiming required their disclosure in the originally filed specification in the first place, and should bar reliance on the DOE. The apparent tension between these "equivalence" concepts was again on display in Ring & Pinion.

Declaratory judgment plaintiff Ring & Pinion (R&P) claimed before the trial court that its product did not infringe ARB's patent directed to an improved automobile locking differential.8 Claim 1 was deemed representative:

A locking differential comprising a differential carrier . . . , a locking means . . . , [and] cylinder means formed in said differential carrier and housing an actuator position[ed] to cause movement of said locking means relative to said carrier . . . .9

There was no dispute that all limitations were literally met in R&P's "Ziplocker" product except for onethe "cylinder means formed" element. The parties agreed, however, that the "Ziplocker" had an equivalent to the cylinder, albeit one that would have been foreseeable to a person having ordinary skill in the art at the time the patent application was filed. Accordingly, the...