prostheticknowledge:

NailO

Proof of concept wearable tech project from MIT Media Lab is a trackpad that can be fitted to the nail of a thumb:

Researchers at the MIT Media Laboratory are developing a new wearable
device that turns the user’s thumbnail into a miniature wireless track
pad.

They envision that the technology could let users control wireless
devices when their hands are full — answering the phone while cooking,
for instance. It could also augment other interfaces, allowing someone
texting on a cellphone, say, to toggle between symbol sets without
interrupting his or her typing. Finally, it could enable subtle
communication in circumstances that require it, such as sending a quick
text to a child while attending an important meeting.

More Here