When is a word 'public domain'?

I was writing part of Psy-Clones yesterday (oh! I'm doing Camp NaNoWriMo this July! And then hopefully straight-up NaNoWriMo in November!) and I wrote about hyposprays. I wanted something that was futuristic, needles being already a thing of the last century and surely soon to be replaced. It didn't even occur to me that this was a problem until I read that chapter to Sue.
"That's from Star Trek," she said. "I'm not sure you can use it."
She's right; this Wikipedia entry about hyposprays specifically lists it as being a Star Trek invention, developed because NBC would not allow them to show needles being used to inject substances. That's fair enough. It seems I'm not alone in automatically using the term 'hypospray' to refer to this device; these three articles, among others, also do. Apparently the term we should all be using is 'jet injector' which is slightly clumsy to me.
Part of my wish to cling to 'hypospray' is that I feel 90% of the readers of Psy-Clones would immediately understand what I'm talking about. It's part of what made Star Trek so good, the 'by-the-way' science fiction stuff they invented. It wasn't a massive thing; it's not like I'm importing the entire Starship Enterprise, or the Borg. It was one of the little life-changing things that the optimistic future will contain, I'm sure.
At what point does a word like 'hypospray' become public property? Consider 'robotics'. Isaac Asimov often pointed out in his essays that he invented the word without even realising it. No-one who uses the term robotics now uses it and immediately thinks 'Oh, that's an Asimov thing, isn't it?'

NaNo is going well; I'm slightly ahead of the curve and trying to do what I did last November. That is to say, I wrote a little more than I needed to each day and finished a day or two early. However, this time, I'm going to try and continue the pace, writing about a 1000 words a day. I missed Saturday because I was ill, but made it up on Sunday, so it's not un-doable. I'm also going to try and write some short stories specifically to be read out for another project. We'll see if that comes to anything!