The phrase "study at skin" can be substituted with a variety of synonyms, depending on the context in which the phrase is being used. Some alternatives for "study at skin" could include analyzing the surface tissue, examining the outer layer, investigating the epidermis, or scrutinizing the dermal layer. In medical contexts, synonyms such as dermatological research or skin-focused inquiry might be appropriate. Similarly, in cosmetics or skincare contexts, other options such as skin exploration, complexion inspection, or surface analysis may be more fitting. Ultimately, the choice of synonym will depend on the specific circumstances and intended meaning of the phrase.