There’s research on needle mechanics but it tends to focus on microneedling for transdermal drug delivery rather than stimulation alone, with collagen induction being the closest parallel relevant to us, which would have longer needles with a high ratio of tip length to diameter.
The tldr is that conical / tapered cone needles offer smoother insertion and are less likely to cause skin trauma compared to pyramidal needles. I’d stick with round needles in stamping-style, vertically penetrating devices.
There are other variables across needle placement density and materials used, which is usually stainless steel and/or titanium. I’m not sure how relevant the metal is, breakage and infiltration from reusing your devices shouldn’t matter if you’re using disposable microneedle heads. Please don’t try to save 30 cents and reuse your needles, you animals. Should be mentioned that I caught a couple of reports on common nickel allergies[1] from stainless steel (which contains varying amounts of nickel) and microneedling[2], even with titanium-coated tips.
If anyone has information on research specific to stimulation and needle shape, please share.