• FreePatentsOnline (FPO) - http://www.freepatentsonline.com/

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• Google Patents - https://patents.google.com/

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• Indian Patent Search (IPAIRS) - https://ipindiaservices.gov.in/publicsearch

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• SureChEMBL - https://www.surechembl.org/search/
  
  • SureChEMBL is a publicly available large-scale resource containing compounds extracted from the full text, images and attachments of patent documents. The data are extracted from the patent literature according to an automated text and image-mining pipeline on a daily basis.

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• Espacenet Patent Search - https://worldwide.espacenet.com/patent/

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• US Patent & Trademark Office (PTO) - https://www.uspto.gov/

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• WIPO - Search International and National Patent Collections : (https://patentscope.wipo.int/search/en/search.jsf)
  • This patent search tool allows you not only to search the PCT database of about 2 million International Applications but also the worldwide patent collections.