Statements made by patent practitioners to the patent office during patent prosecution can come back to bite you in related applications. Prosecution disclaimer allows a court to limit the literal scope of the claims in an infringement action. For prosecution disclaimer to attach, the accused infringer bears a heavy burden in asserting it: “the disavowal must be both clear and unmistakable.”

The patents at issue in MIT v. Shire cover methods for growing organs in vivo as an alternative to organ transplantation. Prior art methods for making thin organs such as skin were not useful for making thicker organs because cells at the center of the developing organ tended to die due to decreased diffusion of oxygen and nutrients to those cells. MIT scientists developed a “biodegradable, synthetic matrix that provides[s] support for cell growth applicable to tissue engineering for vascularized organs,” the subject of U.S. Patents 5,770,193 and 5,759,830.

MIT sued Shire for patent infringement, alleging that Shire’s Dermagraft® scaffold infringed the ‘193 and ‘830 patents. Shire asserted the doctrine of prosecution disclaimer as to three terms, but the opinion focused primarily on “vascularized organ tissue.” If vascularized organ tissue was construed to include skin, then Shire could be liable for infringement. It was undisputed that the ordinary meaning of vascularized organ tissue included skin and the ‘193 and ‘830 disclosures support that. However, Shire asserted that under the doctrine of prosecution disclaimer, vascularized organ tissue should be construed to exclude skin.

During the prosecution of the parent application 06/933,018, MIT attempted to overcome a prior art rejection by arguing that the prior art was limited to use for skin and could not be used for other organ equivalents. The examiner maintained the rejection over several office actions, eventually forcing MIT to limit the claims in the parent application to “scaffolds for growing ‘non-skin organ cells,’” that the examiner rejected as constituting new matter. MIT abandoned the ‘018 application and continued to prosecute the related ‘193 and ‘830 applications. Both included the same non-skin limitation. MIT eventually replaced the non-skin limitation with a limitation directed to the thickness of the claimed cell mass, which earned MIT another new matter rejection under § 112. In the end, MIT removed the thickness limitation and added the limitation “to produce functionalized organ tissue in vivo.”

Shire offers several statements made by MIT as evidence of prosecution disclaimer. The opinion discusses each in turn, but the most unambiguous statements were related to the “non-skin” claim limitation and the inventor’s declaration.

Shire points to the non-skin claim amendment as evidence of disclaimer, but the court found that this amendment did not overcome the presumption that vascularized organ tissue had an ordinary and customary meaning. Although the claim amendment clearly disavowed claim scope that would include skin, the court found that because the amendment was made in connection with different claims—the claims at the time did not recite vascularized organ tissue— then disclaimer did not attach.

Shire also pointed to a declaration, where the inventor stated that prior art methods were “limited to a very thin layer of cells” and that “the claimed method is not a method for making very thin structures.” The court was not persuaded that these statements constituted disclaimer because the statements were offered in support of the thickness limitation. The thickness limitation does not recite vascularized organ tissue, so disclaimer did not attach.

It is a rarity to prevail on a prosecution disclaimer argument in the courts. Practically speaking, the “clear and unmistakable” disavowal means that the statements by the patentee were made to overcome an examiner’s rejection and the PTO was persuaded to allow the claims. If it’s a losing argument—the examiner maintains the rejection or issues a new rejection in response—then there is no harm, no foul. Most importantly, the statements must be made in connection with the exact disputed claim terms for disclaimer to attach.