If I recall correctly several medical conditions can be detected by retinal scans, which has historically made widespread use of the technology undesirable.

EDIT: Sorry, it seems that it's more that medical conditions make it unreliable

> Certain eye-related medical conditions and diseases, such as cataracts and glaucoma, can render a person unable to use retina-scan technology, as the blood vessels can be obscured.

https://www.biometricupdate.com/201307/explainer-retinal-sca...

SmartPipe already has a lock and key algorithm that employs your topographical anal handshake.¹

https://youtu.be/DJklHwoYgBQ

Haha! Wow, the part where there is a device down the pipe that acts as a mini laboratory, analyzes the fecal matter passing through, and sends you medical alerts on your smartphone… I could totally see this exist in the future. Identification could probably be done by sequencing the gut microbiome to avoid the camera taking pictures of your ass.

In case you missed a few months back, looks like it is becoming a reality :-) https://news.ycombinator.com/item?id=23451530

But are they ISO/IEC 27001 complaint