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The hypospray was actually a real technology predating Star Trek: https://en.wikipedia.org/wiki/Jet_injector

That Wikipedia page goes into detail on why jet injectors are no longer used, but there is no mention of the scarring. It used to be common to see people with a circular scar on their upper arm where they got the injector shot as a child.

Are you thinking of smallpox vaccination scars?

https://www.verywellhealth.com/smallpox-vaccine-scar-7096798

That's something different. The scars I'm referring to were shaped like two half-disks facing each other.

Edit: I asked around and nobody seems to know what I'm talking about. Seems either it was a local peculiarity or a misremembered childhood.

No, I remember getting a vaccine that way, and it was different from the smallpox vaccine which was a bunch of little needles that left a scar. I got it in elementary school -- I want to say 2nd grade. I don't think the spray thing left a scar.

I have one of those, actually. Looks like someone put a cigarette out on my arm there.

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