On the shoulders of giants... or something... (Score 1)

You don't have to put bumps etc on the notes. In Australia each note is 3-4mm longer than another of lower denomination, but they are all the same width. That way, it's easy to handle with vending machines (width) and easy for blind to sort out (length. And they stay nicely. They are also polymer-based, almost indestructible and carry vastly more electronic counter-measures than the old US currency. Don't re-invent the wheel, just check out what other countries have come up with (after countless iterations).