The interface lollipop needs an array of contacts that will stimulate the surface of the tongue. How will this be implemented? I plan to use standard PCB features without any exotic small dimensions, to allow for cheap and easy manufacturing.
To avoid having an enormous number of lines to control each "pixel" individually, I will make an array of rows and columns, where each row and each column has a separate driver. Only one row and one column will be driven at a time. The intersection of the active row and active column will switch on a particular "pixel".
I plan to implement this using a 2-sided PCB with vias for each "pixel". Each column of vias will be connected vertically on the non-contact side of the board. On the contact side, horizontal rows of copper will surround each row of vias, but the vias will be isolated from the copper by a thin annular gap. By selecting the appropriate diameter for the soldermask removal around each via, a copper ring will be exposed around each via. The soldermask will cover the rest of the contact side of the board.
The vias (each column) will be driven high when selected, and the copper rings (each row) will be pulled low. The voltage across the gap from via to ring will stimulate the tongue (the voltage will be quite low!).
When a particular row and column are driven, there will also be a voltage drop between all the vias in the column, and all the rings in the row. But the distance between them will be much greater than the small separation on the intersecting "pixel". My hope is that the surrounding "low level" pixel stimulation will be negligible, but it remains to be seen how effective this method is. There is plenty of scope for adjusting the voltage and the pulse duration to an appropriate level to get a good result at the target pixel, while limiting the spurious effect along the associated row and column.