A pen that monitors the writer's emotions

From New Scientist magazine: the Philips company has filed a patent for an 'expressive pen' that monitors how the writer is feeling, and alters ink colour and the shape of the nib accordingly.

Philips says: "Signatures are currently always the same, yet some documents will be signed with enthusiasm, others possibly with hesitation. Having a recording of this could be useful for historical reasons."

It will be a brave politician (for example) who submits to revealing how they feel when signing a treaty. For example, would a signature which expresses doubt or anxiety be viewed as less valid than a determined or happy one?