Use the 1mm Dermaroller. Here’s the extract from the study on needle length:
After evaluation of all of the biopsies in Dr Schwarz’s study, new collagen fibers were only found close to the corium and not deeper than 0.5 to 0.6 mm. Although the Dermaroller model MF8 has 1.5 mm long needles, no significant new collagen and elastin formation could be found in the sub-dermal tissue.
6 weeks after micro-needling, collagen and elastin fibers are increased by 1000% and to a depth of 0.5 to 0.6 mm. Although we do not know much about the mechanisms of the Dermaroller and how its needles induce such a dramatic amount of collagen in a short session… …we have scientific proof that the needling procedure triggers new collagen fibers, new melanozyts, stimulates revascularization, hair growth, evening in scars (Camirand), re-pigments stretch marks, fills depressed acne scars, softens scars and so on. Quality needles do not damage the skin in the sense of a classical injury. The pricking channels close within 60 to 90 minutes and infections, therefore, are unlikely.
Since the above research was released, a company called Follica (pioneers in hair follicle neogenesis) studied the specific needle sizes that are most effective for hair follicle neogenesis. They agree that 1mm is the optimal length.