Micro.

"Topical retinoids are the most important drug class to reverse the signs of aging, also known as all-trans retinoic acid, tretinoin, sold under a variety of brands including Retin-A Micro, a prescription-only topical form of vitamin A that can visibly improve mild to moderate photo damage, including fine and coarse wrinkles, freckles, other pigmentation, and improve overall skin texture after months of daily use. This is accompanied by restoration of collagen formation. One of the advantages we have discovered of using a topical retinoid is that it can improve the appearance of wrinkles because it gets down to the deeper layers of the skin to not only improve collagen production to smooth out wrinkles but also to help actively protect the collagen that you have. A skin biopsy taken before and after ten to twelve months of tretinoin versus placebo found that while skin collagen formation dropped fourteen percent in the control group, it rose eighty percent in the retinoic acid group."

"The micro sponges are basically little delivery systems with hollows that the tretinoin drug is stuck in, and those hollows allow the tretinoin to be protected from exposures like UVA. The micro sponges are a delivery system that helps overcome the photostability issue. In theory, if you are using tretinoin in a micro sponge delivery system, you could in theory use it during the daytime. In a laboratory test after 24 hours of continuous exposure to incandescent bulbs, Retin-A Micro retained 98 percent of the tretinoin, whereas only 31 percent of tretinoin gel remained. The micro sponge delivery system used in Retin-A Micro does protect the drug tretinoin from degradation from fluorescent bulbs or incandescent bulbs. In another study looking at solar simulated light, which is meant to mimic natural sunlight exposure, after six hours of solar simulated light exposure only 16 percent of the tretinoin micro degraded, whereas 90 percent of the tretinoin degraded without the microsphere technology."