Clinical Key works through any mobile browser, but Elsevier also provides a standalone mobile app for Clinical Key, and a Bookshelf app for Clinical Key Student where you can download and store entire medical texts for offline viewing.
Clinical Key Student Bookshelf:
Android: https://play.google.com/store/apps/details?id=com.elsevier.education.ckmed
iOS: https://apps.apple.com/us/app/clinicalkey-student-bookshelf/id1334330753
Clinical Key:
Android: https://play.google.com/store/apps/details?id=com.elsevier.cs.ck
iOS: https://apps.apple.com/us/app/clinicalkey/id1041998175