Patent Intelligence · Public Domain IP
World-class patents, now free
These are the most-cited patents that have entered the public domain. Once a patent expires, the invention is free to use, build on, manufacture, and sell — no license required. Here are the ones that mattered most.
With 15+ citations
220 expired patents
Combined citations
58,937
Most represented domain
Biotechnology
By Technology Domain
Top 75 Most-Cited Expired Patents
Ranked by forward citation count. All are confirmed expired — no license needed.
21
How a Modern Camera Sensor Captures Light and Converts It to Data
25
Tiny Capsules for Electronic Paper Displays
62
How Pulse Code Modulation Digitizes Analog Signals
67
How Hopfield Networks Use Resistors to Mimic Brain-Like Memory
73
How IBM's Storage Controllers Keep Data Backups in the Right Order
What "expired" actually means
Once a patent expires — whether by reaching the 20-year term, lapsing for unpaid maintenance fees, or being abandoned — the invention permanently enters the public domain. Anyone can make, use, sell, or import the invention. No license. No royalties. Forever.
High citations = real impact
Forward citation count measures how many later patents cited this one. High-citation patents built foundations that others built upon — these weren't trivial inventions. Finding a highly-cited expired patent in your domain is finding a blueprint the best engineers in the world have already validated.
What you can do with them
You can manufacture the exact invention. You can reverse-engineer it. You can build products that incorporate it without modification. You can publish how it works. The only restrictions: don't claim you own the patent, and note that later improvements may still be protected.