The University of Tokyo has developed a thin flexible polymer Braille display.
First announced earlier this year (EW 06/09/05), the device was revealed in full last week by its inventor Dr Takayo Someya at the International Electron Devices Meeting in Washington.
“The Braille display is a thin, flexible sheet of plastic containing soft actuators and organic transistors,” Someya told EW. “It is driven by Braille input, similar to an e-book. The sheet is bendable and carried in a pocket.”
Only 1mm thick including actuators and its active matrix, the display works by pushing hemispheres up though a 10µm PDMS (polydimethylsilane) membrane to a height of 0.25mm above the display surface.
Each hemisphere is at the end of a 1x4mm cantilever machined from a 300µm sheet of Nafion: an insulating perfluorinated polymer electrolyte membrane which can hold ions — in this case Li+ ions.
The other end of the cantilever is anchored and gold conductors on its top and bottom surfaces allow a field to be imposed through the cantilever.
3V is sufficient to cause electro-osmosis which bends the cantilever by differential osmotic pressure. When the voltage is removed the cantilever straightens and this can be done at up to 2Hz.
Cantilevers are arranged to position their dots in the traditional 2x3 Braille character array and the 4cm prototype has four rows of six characters.
Beneath the cantilever layer are the active matrix transistors required to operate the dots. These are top-contact (up-side down) pentacene Fets built on a PEN (poly(ethylene naphthalate)) film substrate.
A 50nm gold layer is deposited and patterned to form the gates then polyimide precursors are spin-coated and cured to form 240nm gate dielectric layers.
The channel is 50nm of pentacene and the source and drain are, like the gate, 50nm gold. 8µm of parylene (poly-chloro-para-xylylene) passivates the structure. Connections between cantilever and transistor layers are through silver paste.
When combined, the transistors cannot deliver enough current to operate the dots at 2Hz. The best speed so far obtained is 0.9s for 200µm lift with a VGS of -30V. A bias of -10V give multi-second response times and +8v is required to turn the depletion-mode transistors off completely.
Speed could be improved, said Someya, by better transistors with either higher carrier mobility or devices with a higher channel length/width ratio. Existing channel length is 20µm with 1cm2/Vs mobility.
Polymer cantilevers push up 0.9mm radius dots through a membrane to form a Braille display. Response time is 0.9s. The elastomer surface is coated with a lubricating fluoride to allow fingers to slide. Cutaways show cantilever and transistor layers.