Searching for Patents can be tricky because of language or descriptions used. Keyword searching may not be comprehensive so you may need to try different keywords and databases to search for patents. Two resources we could suggest for in-depth patent search analysis, see: Patent Information Users Group (PIUG)  and Current Challenges in Patent Information Retrieval, ed. by Mihai Lupu, et al.

United States Patent and Trademark Office Patent Full-text DatabasesSearches full-text American patents from 1976 to the present and also searches patent applications since March 2001.
The European Patent OfficeEspacenet offers free access to more than 70 million patent documents worldwide, containing information about inventions and technical developments from 1836 to today.
Canadian Patents DatabaseThe Canadian Patent Office lets you access 93 years of Canadian patent descriptions and images. You can search, retrieve and study more than 2,130,000 patent documents.
ScopusThe citation index from Elsevier also searches for web and patent documents. To see these additional results, click Web or Patents in the Go to results line above your search results.
Free Patents OnlineA free database not connected to any government, offering international patents as well as other features.
Patents.comAnother patent search not connected to a government, but includes international patents and patent applications.
Patent LensA free public resource for patent system navigation worldwide.
Google Patent SearchGoogle Search specific to patents.