In Star Trek, the hypo-spray is the replacement of the syringe in our time frame. But does the hypo-spray actually pierce the skin?
I've seen times when they are used on direct skin contact, through clothes, into a bio-bag-thingy (Voyager), and even into open air like an aerosol can.
Is the medication merely absorbed through the skin? Does it pierce the skin? Do the patients feel pain that they are bravely hiding?
As Wikis pointed out, the standard hypospray does not pierce the skin. Their standard mode of operation is similar to the jet injector he mentioned.
They do, however, have other modes of operation - they can dispense aerosol medications. They also have the ability to collect atmospheric samples or store compressed gasses. They can also inject through clothing.
No, it does not pierce the skin. I understand it works in a similar way to the (real world) jet injector. This "uses a high-pressure narrow jet of the injection liquid instead of a hypodermic needle to penetrate the epidermis."
The way I always understood it, the hypo-spray basically atomizes the medication into particles small enough to slip between the cells of the epidermis, and this mist is forced into the skin under pressure. it works like a LOT of very tiny jet injectors.
This is discussed (in considerable detail) in the Star Trek: The Next Generation - Technical Manual, considered to be a canon source of info about the Star Trek universe.
Subcutaneous and intravenous administration of many types of medication is accomplished with the hypospray. This device employs a pinpoint high-pressure low-volume microscopic aerosuspension stream, which permits low-viscosity medication to be administered through the epidermis without mechanical penetration. Certain types of medications can be formulated for a somewhat wider spray pattern, resulting in lesser penetration into the epidermis, but yielding a higher rate of absorption due to the greater skin area exposed to the drug.
Standard hyposprays are designed to accept a standard medication vial, which can be changed as required. Field hyposprays are normally loaded with an inert saline solution that serves as a vehicle fluid for any of five user-selectable concentrated emergency medication ampules.
The Star Trek: Encyclopedia also offers the following info, basically reiterating the text above.
Hypospray.
Medical instrument used by Starfleet medical personnel for subcutaneous and intramuscular administration of medication for many humanoid patients. The hypospray uses an extremely fine, high-pressure aerosuspension delivery system, eliminating the need for a needle to physically penetrate the skin.
Moving down the canon scale, the Star Fleet Medical Reference Manual has an entire panel showing the internal working of the "Air Spray Hypodermic Syringe".
