But what about if a patent has already issued? Is there any way to invalidate it or at least let the public know that it may be invalid without filing a lawsuit?

Yes, there is.  You have a different, and better, set of options once a patent has issued. You can cite prior art against the patent for inclusion in its public file, or you can directly challenge the patent in a reexamination proceeding.

In this post, I will explain the cheaper, easier option of the two, leaving reexamination proceedings for a future post.

Anonymous Submission of Prior Art For the Record

Requirements

Your first option has the advantage of being free.  Any member of the public may anonymously submit prior art they believe to be relevant to the validity of a patent as provided by 37 CFR §1.501. This may be done at any time before the patent has expired.

You must cite the prior art in writing and explain its pertinence and applicability to the patent.  You must also state that you believe the prior art to bear on the patentability of at least one claim of the patent.

Include with your submission a certification of service indicating that a copy of the citation was delivered to the patent owner at the correspondence address of record. In the event that delivery is not possible, you should include a duplicate copy of your citation.

Benefits

If these requirements are met, and if the patent is not currently involved in a reexamination proceeding, your citation will be included in the patent file and will be a matter of public record.  Anyone who comes upon the patent in a prior art or clearance search, who is being threatened with an infringement action, or who is considering licensing the patent will likely review the patent file and discover the cited art.

Citing prior art against an issued patent is a good solution if you do not have the time or money to directly challenge the patent yourself.  The public availability of this prior art will make it easier for another interested party to challenge the patent if it becomes important to do so.

A patent owner who discovers prior art that someone could believe is invalidating  can also make use of this procedure.  A patent owner is allowed to include with the citations an explanation of how the claims differ from the prior art.

This could be useful in heading off an invalidity challenge, but it also has substantial downsides.  First, you make easily available a document that might otherwise not be found that could be used against your patent.  Second, you make public statements that you may later wish you could take back. For at least those reasons I believe this use of the procedure is very rare.

Anonymity

To maintain your anonymity, you should not identify yourself in the citation or any accompanying papers.  Otherwise, your identity may become a part of the public record patent file.

Reexamination will be discussed in a future post.  Have any questions about the submission of prior art against an issued patent?  Let me know in the comments.