Abstract: Applicators for applying an on-skin assembly to skin of a host and methods of their use and/or manufacture are provided. An applicator includes an insertion assembly configured to insert at least a portion of the on-skin assembly into the skin of the host, a housing configured to house the insertion assembly, the housing comprising an aperture through which the on-skin assembly can pass, an actuation member configured to, upon activation, cause the insertion assembly to insert at least the portion of the on-skin assembly into the skin of the host, and a sealing element configured to provide a sterile barrier and a vapor barrier between an internal environment of the housing and an external environment of the housing.