The best mode requirement is one of the three separate description requirements found in 35 U.S.C. 112(a); the other two are the enablement requirement and the written description requirement. In a nutshell, the best mode requirement says that the inventor must disclose any preferences relating to the claimed invention.
The best mode requirement still exists, although the America Invents Act (AIA) has largely removed any consequences for failing to disclose the inventor’s best mode, which means the current state of the law is at best a bit murky. But why would you ever want to file a patent application that does not disclose something that you prefer or regard as better? The goal of filing a patent application has to be to completely disclose your invention with as much detail and description as possible, paying particular attention to alternatives and variations. See also Tips & Tricks to Describe Your Invention in a Patent Application. So while you may be able to get away with not disclosing any preferences doing so would likely be a tragic mistake.
The best mode requirement historically created a statutory bargained-for-exchange between the patentee and the people. In exchange for the patent, which gives the patentee the right to exclude others from practicing the claimed invention for a certain time period, the public receives detailed knowledge of the invention. The written description requirement essentially requires the patent to define the metes and bounds of the invention, and the enablement requirement mandates that the patent explain how to both make and use the invention so that one of relevant skill in the art would be able to make and use the invention without undue experimentation. Best mode goes one step further. In addition to defining the metes and bounds in a way that explain how to make and use the invention it is necessary for the inventor to also disclose any preferences that pertain to the claimed invention.
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In 1995 the United States Court of Appeals for the Federal Circuit succinctly explained the basics of the best mode requirement:
The sole purpose of the best mode requirement is to restrain inventors from applying for patents while at the same time concealing from the public preferred embodiments of their inventions, which they have in fact conceived. The best mode inquiry focuses on the inventor’s state of mind at the time he filed his application, raising a subjective factual question. The specificity of disclosure required to comply with the best mode requirement must be determined by the knowledge of facts within the possession of the inventor at the time of filing the application.
See Glaxo, Inc. v. Novopharm LTD., 52 F.3d 1043, 1050 (Fed. Cir. 1995).
In Glaxo the Federal Circuit went on to compare and distinguish the best mode requirement and the enablement requirement as follows:
Enablement looks to placing the subject matter of the claims generally in the possession of the public. Best mode looks to whether specific instrumentalities and techniques have been developed by the inventor and known to him at the time of filing as the best way of carrying out the invention. The enablement requirement, thus, looks to the objective knowledge of one of ordinary skill in the art, while the best mode inquiry is a subjective, factual one, looking to the state of the mind of the inventor.
Thus, the best mode requirement was a safeguard against the desire on the part of some inventors (or their employers) to obtain patent protection without making as full a disclosure as required by the statute. Failure to disclose the best mode of the claimed invention was grounds for finding a claim at issue invalid.
With the enactment of the AIA the best mode requirement has largely been gutted, although it still remains a part of 35 U.S.C. 112(a). Ironically, although the best mode requirement remains in the statute and needs to be satisfied at the time the patent application is filed, the failure to disclose the best mode is no longer a basis on which any claim of an issued patent may be invalidated or otherwise unenforceable. This applies to all litigation proceedings commenced on or after September 16, 2011. Moreover, a previously filed application no longer needs to disclose the best mode in order to be entitled to claim priority. See 35 U.S.C. 119(e)(1) and 35 U.S.C. 120.
The question must then become – why would Congress keep the best mode requirement if they were going to render it useless insofar as determining validity and priority claims? Why not eliminate the best mode requirement from 35 USC 112? That is a good question for which there is no satisfactory answer.
The United States Patent and Trademark Office (USPTO) has noticed that while the lack of a best mode disclosure cannot be used to invalidate a claim or to prevent a claim of priority, the disclosure of the best mode remains one of the patentability requirements pursuant to 35 U.S.C. 112(a). For example, on September 20, 2011, just days after President Obama signed the AIA into law, a memorandum authored by Robert W. Bahr, who was then Senior Patent Counsel and Acting Associate Commissioner for Patent Examination Policy, was circulated to the Patent Examining Corps. In this memorandum Bahr explained:
Section 15 of the Leahy-Smith America Invents Act does not eliminate the requirement in 35 U.S.C. 112, first paragraph, for a disclosure of the best mode, but does amend 35 U.S.C. 282 (the provisions that sets forth defenses in a patent validity or infringement proceeding) to provide that the failure to disclose the best mode shall not be a basis on which any claim of a patent may be canceled or held invalid or otherwise unenforceable. As this change is applicable only in patent validity or infringement proceedings, it does not alter current patent examining practices set forth in MPEP 2165 for evaluation of an application for compliance with the best mode requirement of 35 U.S.C. 112.
Thus, 35 U.S.C. 112 still requires the disclosure of the best mode known to the inventor, so legally applicants are still required to disclose the best mode even though failure to do so will not invalidate an otherwise valid claim. Of course, one could legitimately ask the viability of the USPTO continuing to consider best mode given that it is a purely subjective test in the first part, which seeks to determine what the inventor actually preferred. The patent examiner would only in the most extremely rare cases ever be privy to information relating to the subjective preferences of inventors. Still, it is theoretically possible in that rare case, and if the patent examiner is made aware of preferences held by the inventor the claims implicated should not be granted. Thus, there is a distinction between issued patent claims and claims not yet issued when it comes to best mode.
Of course, much of this is really making a mountain out of a molehill. Playing fast and loose with the best mode requirement does not make a lot of sense, at least not for most inventors. Perhaps some inventors believe it to be advantageous to withhold information about preferences, but if you withhold information and do not include it in your patent application then it will not be a part of your invention. In other words, withholding the best mode not only calls into question whether you have satisfied the best mode requirement but will call into question whether your written description has fully described the complete invention.
For example, if you have a preference that relates to something that you are claiming and you do not disclose that preference you have violated the best mode requirement. Furthermore, your written description will not provide support for that which you did not include, so that preference is not a part of your invention, which means that you will not own the exclusive rights to that which you prefer.
Ask yourself this: If you have a preference why would you now want to own that preference? Most inventors certainly should want to disclose the best mode of their invention so that they can own the rights associated with what they prefer. Ordinarily it doesn’t make good sense to seek a patent on a version of the invention that is not optimal and allow the optimal version of the invention to remain unprotected. If anything you probably want to do it exactly the other way around.
Generally speaking, disclosing the best mode makes all the sense in the world for a variety of reasons. Being cute and trying to hide key preferences will ordinarily do nothing other than come back and bite you even today when there are really no consequences for violating the best mode requirement once you have an issued patent.