Espacenet - Espacenet offers free access to more than 70 million patent documents worldwide, containing information about inventions and technical developments from 1836 to today.
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Patent Lens is a worldwide, open-access, free full-text patent informatics resource. Search and retrieve the full-text of more than 9 million patent documents from US, Europe, Australia and WIPO, their status and counterparts in up to 70 countries.
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USPTO - United States Patent and Trademark Office provides full text of granted patents since 1976 and full-page images since 1790.