I test my blood glucose levels three or four times each day, as I am diabetic. It's a painless procedure, involving a tiny pinprick on the ball of my thumb or a finger, to get a tiny droplet of blood.
This evening, when I tested, I got a little more blood than I needed, so I dabbed my thumb on a
piece of paper, leaving a partial thumbprint on the paper.
So, on to the idea... a small fingerstall or thumbstall (small rubber cap that fits over the end of the digit), perforated with a pattern of tiny holes in the design of your regular signature, surrounded by a more open grid, to allow the ridges and whorls of your finger, or thumb, print to contact the paper.
On the skin side (inside) an array of very thin needles that draw a miniscule amout of blood when you apply pressure to it.
Simply fit the premade Fingerprint Signature cap over the end of your thumb or finger and press briefly where your signature is required.
Your signature should be sufficient ID. However, a fingerprint should clear up any doubt. In the event of further confusion, a DNA test establishes identity beyond question.