Prior art search for researchers: a free, step-by-step patent search

This guide is for academic researchers, PhD students and lab heads who have a result that might be an invention, and for anyone asked by a technology transfer office to "do a quick search first." It is part of our guide to research methods by field.

This is general information, not legal advice. Patent law differs by country and every case turns on its facts. Have a registered patent attorney or agent, or your institution's technology transfer office, review your results before you decide to file, publish or launch a product.

What counts as prior art?

Prior art is everything made available to the public, anywhere and in any language, before the effective filing date of a patent application. The European Patent Convention defines the state of the art as everything made available to the public "by means of a written or oral description, by use, or in any other way" before the filing date [1]. US law covers inventions that were patented, described in a printed publication, in public use, on sale or otherwise available to the public before the effective filing date [2].

For researchers, that includes:

• Patents and published patent applications from any office.
• Journal articles, preprints, theses and dissertations.
• Conference talks, posters and slides, if the public could see them.
• Product manuals, datasheets, websites and videos.
• Public demonstrations, use or sale.

Non-patent literature (NPL) is prior art that is not a patent document: papers, theses, books, standards, manuals and web pages. In many academic fields, the closest prior art is a paper, not a patent.

Your own paper can be prior art

This catches researchers most often. In Europe, your own publication before the filing date generally counts against you; the exceptions are narrow, such as evident abuse or certain recognized exhibitions [3]. The United States gives a one-year grace period for an inventor's own disclosure [2][4], but that protection does not travel to most other countries. The practical rule: if a result might be an invention, talk to your technology transfer office before you submit the paper, poster or thesis.

Where can you do a prior art search for free?

Four free public databases cover most patent documents worldwide. Use at least two, then add the scholarly literature.

Patent Public Search replaced the USPTO's older PatFT and AppFT databases in 2022 [7], so ignore older guides that send you there.

How to do a patent search, step by step

This procedure adapts the USPTO's seven-step strategy for preliminary searches [9] to the free tools above and adds a non-patent literature step.

1. Describe the invention in features. Write one sentence for what it does and a list of its essential features: components, materials, steps, the problem it solves.
2. List keywords and synonyms for each feature. Patents use their own vocabulary: "fastening means," not "screw." Include broader and narrower terms.
3. Run keyword searches in Google Patents. Combine features with AND, synonyms with OR. Read the top 20 to 30 results and mark the closest few.
4. Collect classification codes from the closest hits. Each patent lists its CPC codes on the front page. Note the codes that recur.
5. Check each code's definition. Look up the code in the CPC scheme to confirm it fits your invention, and look at its neighbors for better fits.
6. Search by code plus keywords. In Espacenet and Patent Public Search, combine the CPC code with two or three of your keywords. This finds documents that describe the idea in different words.
7. Follow citations and families. For each close hit, check the documents it cites, the later documents that cite it, and its family members in other countries.
8. Search the non-patent literature. Search papers, theses and conference proceedings for the same features. Dates matter: note when each was published.
9. Record everything. Keep a search log of database, date, query, number of results and documents kept, so a patent attorney can see what you covered.

A worked example (invented)

Example: a postdoc has built a ring that estimates heart rate during sleep from a light sensor.

• Keyword search in Google Patents ("ring" AND "heart rate" AND sleep AND (photoplethysmography OR "optical sensor")): she reads the top 30 and marks 6 as close.
• 5 of the 6 carry CPC code A61B 5/024 (measuring pulse rate or heart rate). She confirms its definition in the scheme.
• Code plus keywords in Espacenet returns 212 documents. Screening titles and abstracts leaves 31.
• Grouping the 31 by patent family leaves 22 inventions, because 9 documents were the same inventions filed in other countries.
• She reads the claims and description of the 22 and keeps 8 as closest prior art.
• A non-patent literature search turns up 3 papers on optical heart-rate rings, one from 2019.
• She sends the 8 patents, 3 papers and her search log to the technology transfer office.

What is patent classification (CPC)?

The Cooperative Patent Classification (CPC) is a system that sorts patent documents into about 250,000 technical categories, managed jointly by the European Patent Office and the USPTO since 2013 [10][11]. Examiners assign codes based on what a document discloses, not the words it uses, so a code search finds documents your keywords miss.

CPC codes are hierarchical. Using the example above:

Search too high in the tree and you get thousands of results; too low and you miss close neighbors. Most searches combine two or three subgroups with keywords. Espacenet's classification browser lets you click through the tree and add codes to a search.

How do you read a patent?

Read the claims first to see what is protected, but judge prior art on the whole document. A patent has five main parts:

1. Front page: title, applicant, inventors, dates (filing, priority, publication), classification codes and cited documents.
2. Abstract: a short summary, often with one drawing.
3. Drawings: numbered figures referred to in the text.
4. Description: the background, the problem and detailed ways of carrying out the invention.
5. Claims: numbered sentences that define the legal scope. Claim 1 is usually the broadest.

A kind code is the letter and number after a patent number that tells you what kind of document it is. In the US, A1 is a published application, B1 is a granted patent with no earlier published application, and B2 is a granted patent that was published as an application first [12]. A published application is prior art from its publication date even if it never becomes a patent.

What is a patent family?

A patent family is a group of patent documents in different countries that share one or more priority applications, so they describe essentially the same invention. The EPO uses two definitions [13]:

• Simple family: documents with exactly the same priority claims, covering the same technical content.
• Extended (INPADOC) family: documents linked directly or indirectly through any shared priority, which can group related but different inventions.

Families matter for three reasons. They let you read the English member of a Japanese or German filing. They stop you counting one invention five times. And they show where protection was sought, though legal status in each country needs checking, ideally by a professional.

When do you need a patent attorney?

As soon as a decision depends on the answer. Your search finds documents; a professional interprets them. Bring in a registered patent attorney or agent, usually through your technology transfer office, for:

• Patentability: whether your claims are new and inventive over the prior art you found.
• Freedom to operate: whether making or selling a product would infringe someone else's patent in force.
• Validity: whether an existing patent could be challenged.
• Timing: when to file and what you can publish meanwhile.

Do not treat an AI chatbot's view on any of these as an answer. Language models can misstate what a patent claims, and none of them can give you a legal opinion.

Using AI for Research for the non-patent literature

AI for Research searches US patents through USPTO PatentsView in Official Sources, at no credit cost. It does not cover other offices, so use the free databases above (Google Patents, Espacenet and WIPO Patentscope) for patents filed elsewhere, and use AI for Research for the scholarly side and for keeping the work in order. Nothing it returns is a patentability, validity or freedom-to-operate opinion. More on the patent search tool page.

• Search papers as prior art. Find Papers searches 250M+ scholarly works, including preprints, with a year filter so you can see what was public before a date. 0 credits.
• Trace a key paper's citations. Citation Map shows the references and citing papers of a seed paper, 0 credits.
• Ask questions of PDFs. Save papers and downloaded patent PDFs to My Library and ask "Does this document describe an optical sensor worn on a finger?" Each answer cites the page. Storage is free; each question costs 1 credit. Many patent PDFs are scanned images without a text layer, which the Library cannot read, so use the text version on the patent office site for those.
• Share with your technology transfer office. Put the papers, notes and search log in a Project and share it by email as "can view" or "can comment."

Treat everything here as preparation for a professional review, not a substitute for one. See pricing for plan details.

FAQ

How do I search for prior art for free?

Start with keywords in Google Patents to find a few close documents, then note their CPC classification codes. Search those codes with your keywords in Espacenet, the USPTO's Patent Public Search and WIPO Patentscope. Follow the citations and family members of the closest hits. Then search the scholarly literature, theses and conference papers, because non-patent literature counts as prior art too. Record every query and result.

What counts as prior art?

Prior art is anything made available to the public before your filing date that describes the invention: patents and applications, journal articles, theses, conference posters and talks, preprints, product manuals, websites and public use or sale. It can be in any language and from any country. Your own earlier publications can count against you too, depending on the country's rules. A patent professional decides what matters for a specific application.

Can I publish a paper before filing a patent?

It is risky. In Europe, a publication before the filing date generally destroys novelty, with only narrow exceptions. The United States gives inventors a one-year grace period for their own disclosures, but relying on it limits where you can patent. If an invention might be patentable, talk to your technology transfer office or a patent attorney before you submit a paper, poster or thesis.

What is the difference between Google Patents and Espacenet?

Both are free and search patents from many countries. Google Patents is quick for plain-language keyword searches, machine-translates non-English documents and includes some non-patent literature. Espacenet, run by the European Patent Office, is built around classification searching, patent families and legal status data from many offices. Most searchers use both: Google Patents to get started, Espacenet to search by CPC code and check families.

Is a prior art search the same as a patentability opinion?

No. A prior art search finds documents. A patentability opinion is a legal judgment about whether your claims are new and inventive in light of those documents, and only a registered patent attorney or agent should give one. The same applies to freedom-to-operate questions about whether you can sell a product without infringing someone else's patent. Use your search to prepare for that conversation.

Next steps

• Searching the scholarly side in depth? Learn citation snowballing to follow a key paper's references and citations.
• Running a lab that produces inventions? Set shared rules with AI tools for research labs.
• Back to the series guide: AI for qualitative research and research by field.

Sources

1. European Patent Office. European Patent Convention, Article 54: Novelty. https://www.epo.org/en/legal/epc/2020/a54.html
2. 35 U.S.C. § 102: Conditions for patentability; novelty. Legal Information Institute, Cornell Law School. https://www.law.cornell.edu/uscode/text/35/102
3. European Patent Office. European Patent Convention, Article 55: Non-prejudicial disclosures. https://www.epo.org/en/legal/epc/2020/a55.html
4. USPTO. Manual of Patent Examining Procedure, 2153: Prior art exceptions under 35 U.S.C. 102(b)(1). https://www.uspto.gov/web/offices/pac/mpep/s2153.html
5. Google. Google Patents help: coverage. https://support.google.com/faqs/answer/7049585
6. European Patent Office. 160 million reasons to search Espacenet. https://www.epo.org/en/searching-for-patents/helpful-resources/patent-knowledge-news/160-million-reasons-search-espacenet
7. USPTO. Patent Public Search. https://www.uspto.gov/patents/search/patent-public-search
8. WIPO. Milestone celebration: over 100 million patent documents in PATENTSCOPE (2022). https://www.wipo.int/en/web/patentscope/w/news/2022/news_0001
9. USPTO. 7 Step U.S. Patent Search Strategy Guide (2016). https://www.uspto.gov/sites/default/files/documents/7%20Step%20US%20Patent%20Search%20Strategy%20Guide%20(2016)%20Long%20Version.pdf
10. Cooperative Patent Classification. https://www.cooperativepatentclassification.org/
11. European Patent Office. Annual Report 2012: Cooperative Patent Classification. https://report-archive.epo.org/about-us/annual-reports-statistics/annual-report/2012/highlights/cpc.html
12. USPTO. Kind codes included on USPTO patent documents. https://www.uspto.gov/learning-and-resources/support-centers/electronic-business-center/kind-codes-included-uspto-patent
13. European Patent Office. Patent families at the EPO. https://link.epo.org/web/Patent_Families_at_the_EPO_en.pdf