use flex circuits for the switch matrix. Displays LCD displays In LCD fabrication, glass is used as a substrate. If thin flexible plastic or metal foil is used as the substrate instead, the entire system can be flexible, as the film deposited on top of the substrate is usually very thin, on the order of a few micrometres. OLED displays Organic light-emitting diodes (OLEDs) are normally used instead of a back-light for flexible displays, making a flexible organic light-emitting diode display. Flexible batteries Flexible batteries are batteries, both primary and secondary, that are designed to be conformal and flexible, unlike traditional rigid ones. Automotive circuits In the automotive field, flexible circuits are used in instrument panels, under-hood controls, circuits to be concealed within the headliner of the cabin, and in ABS systems. Printers In computer peripherals flexible circuits are used on the moving print head of printers, and to connect signals to the moving arm carrying the read/write heads of disk drives. Solar cells Flexible, thin-film solar solar cells have been developed for powering satellites. These cells are lightweight, can be rolled up for launch, and are easily deployable, making them a good match for the application. They can also be sewn into backpacks or outerwear. Skin-like circuits In December 2021, engineers from Keio University in Tokyo and Stanford University announced the creation of stretchable and skin-like semiconductor circuits. In the future, these wearable electronics may be used to send health data to doctors wirelessly. History Patents issued at the turn of the 20th century show interest in flat electrical conductors sandwiched between layers of insulating material. The resulting electrical circuits were to serve in early telephony switching applications. One of the earliest descriptions of what could be called a flex circuit was unearthed by Dr Ken Gilleo and disclosed in a 1903 English patent by Albert Hansen that described a construction consisting of flat metal conductors on paraffin coated paper. Thomas Edison’s lab books from the period indicate that he was thinking to coat cellulose gum applied to linen paper with graphite powder to create what would have clearly been flexible circuits, though no evidence indicates that it was reduced to practice. The 1947 publication "Printed Circuit Techniques" by Brunetti and Curtis a brief discussion of creating circuits on what would have been flexible insulating materials (e.g. paper). In the 1950s Dahlgren and Sanders made significant strides developing and