N-hydroxysuccinimide is a synthetic ingredient used in cosmetics as an ester to soften and condition skin. It’s also a reagent, a substance used to trigger a reaction that leads to a new substance, such as peptides and polymers.

References for this information:

Journal of the American Chemical Society, August 2018, pages 9,374-9,378
Journal of Biomedical Materials Research, July 2009, pages 70-81
Bioconjugate Chemistry, November-December 2001, pages 1,085-1,088