Improving Participant Recruitment in Clinical Trials: Comparative Analysis of Innovative Digital Platforms - Alexia Georgia Bikou; Elena Deligianni; Foteini Dermiki-Gkana; Nikolaos Liappas; José Gabriel Teriús-Padrón; Maria Eugenia Beltrán Jaunsarás; Maria Fernanda Cabrera-Umpiérrez; Christos Kontogiorgis (Journal of Medical Internet Research)