Large Solid state dot matrix rewritable display

The blow-moulded lid on the McDonald’s coffee cup I was given included seven little circular features on top of it as part of a decorative relief. These button-shaped features were dispersed in a symmetrical pattern, possibly intended to echo the McDonald’s 'M' trademark symbol. The coffee itself was highly palatable but I didn't require the lid, nor had I requested its provision since I was dining in, so I imagined that perhaps the intention could be for me to determine some other application for the lid other than to prevent the coffee from cooling down at a reasonably acceptable rate. This purpose was not immediately obvious so I obligingly commenced a study.

Well, the little domes were circular, maybe 2-3mm deep and about 4mm in radius. Closer investigation revealed that rather than being hemispherical they each had a flat top. They also had three interesting properties. They were mechanically bistable. When pressed, they would invert and latch into a concave shape. Pressing the rear of the button would restore the button back to its original convex shape. The mechanical state-change could be repeated indefinitely. Another intriguing property was that in the process of mechanically toggling the feature, the dark brown colour of the top of the button would change as a result of the strain to a pale creamy colour closer to that of 'café-au-lait'. Then, most unexpectedly, I noted that by rubbing the top of the feature using a fingernail, or using the little plastic stick with which I had also been provided on the basis of no request, perhaps to act as a stirrer should I have been so inclined to use it, this pale colour could be restored to its previous darker appearance. I repeated the process several times on one of the buttons, each time the behaviour including the colour reversion occurred reliably and repeatably.

As another one of my gifts to the world I therefore propose a large, low-cost portable light-weight dot -matrix panel including an array of such blow-moulded buttons. This panel would not require any form of power supply for its operation. Selected button dots would be individually depressed and released in order to toggle their colour from dark to pale. In this manner, the dot-array could for example be used to display numerals, symbols or simple icons. To erase the display and to reset it prior to re-use, the paler dots would simply be polished to restore each of them rapidly to their original darker state.