Freedom-to-operate Analysis

A "freedom to operate" study is effectively an infringement study - you search existing patents to determine if there are any viable, enforceable patents which your invention may infringe. By viable/enforceable, I mean those patents which are still in force - they have not expired, and they have not lapsed due to, for example, the non-payment of annuities or maintenance fees which are usually required in most countries. For example, here in the US, a patent may have issued in 1991 but since the normal patent term is 20 years, the patent has probably expired and therefore the patented invention is now in the public domain. Exceptions exist for some pharmaceutical patents, but in general, the lifetime of most current patents is 20 years. In addition, maintenance fees are required to be paid by 3.5/4 years after the patent issues, 7.5/8 years after the patent issues, or 11.5/12 years after the patent issues - if you don't pay the fees by 3.5, 7.5, or 11.5 years, you can still pay the fees up until 4,8, and 12 years, but there is a penalty surcharge. If you do not pay the fees at all, the patent lapses even though it it well short of its normal 20 year term.