In considering how to respond to an accusation of patent infringement, defendants will often review how long they have been making the accused products. If a defendant has been selling the accused or similar product for a long time, they might have a prior use defense. A prior use defense is where the defendant argues that the patent is invalid because the defendant was selling a product practicing the invention before the application for the asserted patent was filed. The difficulty of a prior use defense is that a defendant rarely has sample of the original product. Defendants can rely on oral testimony describing the public prior use of the product. Oral testimony of public prior use by an interested party, however, is not enough by itself to prove invalidity. The defendant must corroborate any oral testimony of prior use with other evidence.

In Transweb, LLC v. 3M Innovative Props. Co., (Feb. 10, 2016), the Federal Circuit provided guidance as to how much corroboration is needed to invalidate a patent using prior use testimony. In this case, Transweb argued that its prior use samples would invalidate the patent it was accused of infringing. An employee of Transweb testified that he handed out product samples at a public conference. Transweb corroborated that testimony with correspondence from before and after the conference discussing the product samples, showing that a patent application was filed about the same time as the conference that would have covered the product sample, and submitting evidence that it had provided the product samples to a company after the conference. 3M argued that the corroborating information was insufficient because none of it confirmed that the product samples handed out at the conference included the patent claim element at issue. The Federal Circuit rejected 3M's argument. The corroborating evidence does not have independently confirm each aspect of the oral testimony. Instead, the courts should use a "rule of reason" to determine the sufficiency of the corroboration. Given the corroborating evidence in this case, the Federal Circuit confirmed the district court's holding that the oral testimony of public prior use was sufficiently corroborated. This decision clarifies that defendants do not need to corroborate each aspect of prior use testimony, but instead the courts will review the entire circumstance to determine if there is enough corroboration.