Nope, and there's an stereotype on medical doctors that their signature is just a garbled or gibberish, which very often is true
these days we also have "digital signatures" you can include in an email, which are just encrypted material verifyable by some encryption keys, it varies but the leading ones are PGP and/or GPG types
so valid is one that's consistent, could be your fingerprint if you wish, and due to its consistency also verifyable.