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Patents are usually filed first in their country of origin, and then filed in other countries as needed. For this reason, you may need to look in several search engines to ensure you've done a thorough search. However, many countries' patents are indexed through the World Intellectual Property Organization's PatentScope database, so you should definitely include this tool in your search strategy.
[UPDATE March 23, 2021: The Fresh Patents website is currently not working. We have contacted the service provider and we thank you for your patience]. Covers US patent applications. Interesting to monitor for new and emerging technologies.
Google Patents covers the entire collection of granted patents and published patent applications from the USPTO, European Patent Office, and World Intellectual Property Organization. US patent documents from 1790, EPO and WIPO from 1978.
The USPTO houses full text for patents issued from 1976 to the present and PDF images for all patents from 1790 to the present. Search the PatFT database to see if a patent has already been filed or granted that is similar to your patent. HINT: Once you have the patent number for a document you want to see, use the Pat2PDF (http://www.pat2pdf.org/) site to retrieve the document quickly.
The PatentScope database provides access to international Patent Cooperation Treaty (PCT) applications in full text format on the day of publication, as well as to patent documents of participating national and regional patent offices.
The issue date is searchable from 1869 to the last updated date. The filing date is not available for patents registered prior to August 15, 1978. The priority date and the national entry date are not available for patents registered prior to October 1, 1989.