# Tiny Capsules for Electronic Paper Displays > MIT's 1999 patent on a special ink made of tiny capsules that can change color when an electric field is applied, forming the basis for early e-readers. - **Patent:** US 5961804 - **Original title:** Microencapsulated electrophoretic display - **Owner:** Massachusetts Institute of Technology - **Granted:** 1999 - **Status:** Public domain (expired) - **Times cited:** 508 - **Field:** consumer_electronics, semiconductors, materials, telecommunications ## What it does This patent describes a material for electronic displays, like early e-readers. It's an 'electrophoretic material' made of a carrier liquid holding many tiny capsules, called microcapsules. Inside each microcapsule are particles that can move when an electric field is applied. Some particles are designed to reflect light better, making them appear bright. Others might be colored or even emit light. The key is that these particles can be moved to show different colors or patterns, and importantly, they stay that way even after the electric field is turned off. This 'bistability' is what makes it work like paper, holding an image without constant power. For example, imagine a microcapsule with black particles and white reflective particles. Applying a field can push the white particles to the front, making the capsule look white, or push the black particles forward, making it look black. ## What it does NOT cover - Materials that require continuous power to maintain their displayed image. - Displays where the particles do not have differential responsiveness to an electric field. - Microcapsules that do not contain multiple phases or particles with contrasting visual properties. - Materials where the visual appearance does not persist after the electric field is removed. - Displays that rely solely on light emission without reflective elements for their appearance. ## The clever bit The innovation lies in encapsulating the moving charged particles within tiny spheres and controlling their movement with an electric field, while also ensuring the image remains visible even when the field is off. This created a stable, low-power display that mimics the appearance of ink on paper. ## Real-world examples 1. Early Amazon Kindle devices 2. E-readers from other manufacturers 3. Electronic shelf labels 4. Some smart cards 5. Digital signage requiring low power ## Why it matters This patent is foundational for the development of electronic paper (e-paper) technology. It describes the core microencapsulated electrophoretic display (EPD) technology that enabled the first generation of e-readers and other bistable displays. The ability to hold an image without power consumption was a significant leap for portable electronic devices. ## Frequently asked questions ### What does Tiny Capsules for Electronic Paper Displays cover? MIT's 1999 patent on a special ink made of tiny capsules that can change color when an electric field is applied, forming the basis for early e-readers. ### Who owns patent US 5961804? Massachusetts Institute of Technology owns this patent, granted in 1999. ### When does this patent expire? This patent has expired and is now in the public domain — anyone can use the invention freely. ### What is patent US 5961804 cited by? This patent has been cited by 508 later patents that build on its ideas. ### What problem does this patent solve? This patent is foundational for the development of electronic paper (e-paper) technology. It describes the core microencapsulated electrophoretic display (EPD) technology that enabled the first generation of e-readers and other bistable displays. The ability to hold an image without power consumption was a significant leap for portable electronic devices. ### What does this patent NOT cover? Materials that require continuous power to maintain their displayed image. **Full plain-English explainer:** https://patentbrief.org/patent/us/5961804/e-ink-electronic-paper-display **Original patent:** https://patents.google.com/patent/US5961804 --- _Source: PatentBrief — https://patentbrief.org. Patent facts are from public records; the plain-English explanation is PatentBrief's._ ## Related patents Semantically similar inventions in the PatentBrief corpus: - [How Liquid Crystal Displays (LCDs) Were Invented](https://patentbrief.org/patent/us/3322485/lcd-liquid-crystal-display) — This 1962 patent describes the first practical way to use organic liquid crystals to create a display that scatters light when an electric current is applied. - [How Organic Diodes Make Light Using Special Molecules](https://patentbrief.org/patent/us/4356429/oled-organic-light-emitting-diode) — Eastman Kodak's 1982 patent on creating light-emitting diodes (OLEDs) using organic materials, specifically a layer of porphyrinic compounds to help inject electrical charges. - [Chester Carlson's Original Xerography Patent](https://patentbrief.org/patent/us/2297691/xerography-electrophotography-photocopier) — Chester Carlson's 1942 patent for xerography, the dry copying process that became the foundation for Xerox machines. - [How Piezoelectric Inkjet Printing Works](https://patentbrief.org/patent/us/3946398/drop-on-demand-inkjet) — A 1970 patent describing how to print images by using electrical pulses to bend a tiny crystal plate, squeezing individual ink drops out of a nozzle on demand. - [Sticky, Tiny Plastic Balls Made from Acrylates](https://patentbrief.org/patent/us/3691140/post-it-repositionable-adhesive) — This 1972 patent describes how to make tiny, sticky, and durable plastic balls (microspheres) using a specific mix of acrylate chemicals and a special water-based process.