Use Google Scholar in addition to the other databases you've searched already.
This is the European Patent Office's patent search tool which has access to information about inventions and technical developments. Search for worldwide patents by keyword, inventor, patent number, country.
United States Patent and Trademark Office (USPTO)
Search full text US patents by keyword, name or the cooperative patent classification (CPC) number. Please note, the images need a special TIFF viewer to display. Go to http://www.uspto.gov/patft/help/helpfaq.htm , or http://www.uspto.gov/patft/help/images.htm for more information. The special viewer can be downloaded at http://www.alternatiff.com/.
With Google Patent Search, you can now search the full text of the U.S. patent corpus and find patents that interest you. Search for patents via Google Scholar by entering the keyword, patent number, or citation search. Has a comprehensive list of citations to.
This database lets you access over 75 years of patent descriptions and images. You can search, retrieve and study more than 1,500,000 patent documents.
FreePatentsOnline.com provides fast, easy-to-use access to millions of patents and patent applications. With the most powerful patent search engine on the web, and an ever-expanding data collection, FreePatentsOnline is rapidly becoming the premiere way to search patents. And it's free!"
Derwent Innovations Index (part of Web of Science)
Patent information indexed from over 50 patent issuing authorities from 1963-present with patent citations indexed from the Derwent Patents Citation Index (1973-present).
Patent Scope (part of World Intellectual Property Organization)
Search intellectual property Data contained in the World Intellectual Property Organization's (WIPO) products and services; provides both national and international patent search
Intellectual Property Databases Search from Patex (formerly PATSCAN)
Links to major patent databases (Canada, U.S. and Europe) and to patent classification schemes
SciFinder provides desktop access to Chemical Abstracts and related databases from CAS. Search more than 8000 journals and patent references (from over 32 patent-issuing organizations). It also includes more than 26 million substance records and CAS Registry Numbers.
Date coverage: 1907-present (articles and patents); 1955-present (substances).