Micro.

"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."