Prior art for invalidity is any earlier disclosure that shows the claims of a granted patent were not new or were obvious. The decisive reference is often not a patent but a journal article, a thesis, a product manual or a standards contribution. These documents are called non-patent literature (NPL).
This guide explains where NPL is found as prior art for invalidity and how its date is proved. The full patent invalidity search process is set out in the guide to invalidity search process and cost.
What counts as prior art for invalidity
US law lists the categories of prior art in 35 United States Code (U.S.C.) 102(a)(1). A claimed invention is not new if it was “patented, described in a printed publication, or in public use, on sale, or otherwise available to the public” before the effective filing date.
A printed publication does not need to be printed. Under section 2128 of the Manual of Patent Examining Procedure (MPEP), it qualifies when skilled persons in the field, exercising reasonable diligence, could have located it. An online database, a forum post or a video can therefore count.
Why NPL often holds the killer reference
A killer reference discloses every element of a claim, or comes close enough that one more reference completes an obviousness argument. Such documents often sit outside patent databases:
- Researchers publish before they file, or never file at all.
- Manuals, datasheets and release notes describe product features in working detail.
- In telecoms and computing, standards proposals often circulate years before the related patents issue.
Examination searches tend to focus on patent collections, so the examiner may never have weighed the NPL.
Where the searcher looks
An NPL search uses the same feature list as the patent search, but in the vocabulary of researchers and product teams rather than claims.
| Source | Date evidence |
|---|---|
| Journals and conference proceedings | Publication or presentation date |
| University theses | Library catalogue and shelving records |
| Product manuals and datasheets | Copyright notice, archived web copy |
| Standards documents | Meeting date, upload record |
| Websites and forums | Posting date, archived snapshot |
A non-patent literature search also covers papers in other languages, as a relevant study may not exist in English.
Proving the date and public accessibility
An NPL document helps only if its date can be proved and it was public on that date.
The MPEP treats an electronic publication as available from the date it was posted. Without a date or other evidence of posting, a document cannot be relied on as prior art. In examination, Wayback Machine copies are treated as prima facie accessible at their time stamp. In IPR and litigation they are usually authenticated by an Internet Archive affidavit.
Accessibility can be disputed. In In re Hall, decided by the Federal Circuit in 1986, a doctoral thesis indexed and shelved in a university library was a printed publication. In In re Klopfenstein, decided in 2004, a poster qualified after about 3 days on display to skilled viewers, with no restriction on copying. The searcher therefore records the source, the date evidence and how each document could be found.
Using NPL as prior art for invalidity in each forum
| Forum | Prior art allowed | Threshold or standard of proof |
|---|---|---|
| Inter partes review (IPR) | Patents and printed publications only | Preponderance of the evidence |
| Post-grant review, within 9 months of grant | Any invalidity ground, including public use and sale | Preponderance of the evidence |
| Ex parte reexamination | Patents and printed publications | Substantial new question of patentability to order it |
| District court | All prior art, including public use and sale | Clear and convincing evidence |
Under 35 U.S.C. 311(b), an IPR cannot rely on a product sale, only on printed material such as a dated public manual. In court, the 2011 Supreme Court decision in Microsoft v. i4i requires clear and convincing evidence of invalidity.
Example: a manual that predates the patent
The following example is an illustration and does not describe a real project.
A patent claims a wearable sensor that enters a low-power mode when skin contact is lost. The patent search finds 6 close references, none disclosing that condition. The NPL search finds an earlier fitness tracker manual describing that behaviour, dated 2 years before filing by an archived support page.
The searcher maps the manual in an invalidity claim chart with the archive record. If the manual was publicly accessible, it can support an IPR petition and a court defence.
Commissioning a search for prior art for invalidity
Searches for prior art for invalidity at Synoptic IP cost USD 1,200–10,000 and take 1–3 weeks (checked November 2026). Our patent invalidity search covers patent and non-patent literature, with each reference date-qualified. We quote through our contact page before any work begins.
This guide is general information and is not legal advice.
Frequently Asked Questions
What is non-patent literature in a patent search?
Non-patent literature is any prior art that is not a patent document, such as articles, conference papers, theses, manuals, datasheets, standards contributions, websites and videos. It is searched alongside patents because many inventions were described in papers or products before anyone filed on them.
Can a website be used as prior art to invalidate a patent?
Yes, if its content and date can be proved. The US Patent and Trademark Office treats an internet publication as available from the date it was posted. Where the page shows no date, an archived Wayback Machine snapshot is commonly used, and its authenticity can then be challenged.
Is a PhD thesis a printed publication?
Usually yes, once it has been catalogued and made available in a library, as the court held in In re Hall. A thesis indexed in a way that stops a skilled person from finding it may not qualify, so its catalogue date and indexing are recorded.
Can product sales be used as prior art in an IPR?
No, an IPR can rely only on patents and printed publications. A product sold before the filing date can be raised in district court. In an IPR, the petitioner relies instead on publicly available printed material such as a dated manual or brochure.
How long does an NPL search take in an invalidity project?
Invalidity searches at Synoptic IP take 1–3 weeks in total, depending on the technology, the number of claims and the date evidence each reference needs. In most invalidity projects the NPL search runs alongside the patent search rather than as a separate stage.