There are different models. Some with caps, some with auto closing valves like the one I posted. The "valve" closes flush with the top when you remove the syringe, so you just swipe with a swab to sanitize.
All have a bacteria proof filter for the vent air it allows in the vial to equalized pressure, preventing vacuum from forming like you've likely experienced withdrawing with a syringe.
Some have a 5um fluid particle filter:
Yes, you leave it in the vial for the life of the vial.
I use a new one for each vial. The ones without a particle filter should easily last for several vials. The ones with a particle filter should be changed with each vial.
You do have to careful not to crank the syringe down hard, it's not necessary and you'll eventually crack the plastic.
So it just depends on what you're looking for.
There's also a model with a smaller spike suitable for peptide vials.
The cheapest are around $1.50 (cap, no particle filter).
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