Development and clinical assessment of a heat and moisture exchanger with a multi-magnet automatic tracheostoma valve (Provox FreeHands HME) for vocal and pulmonary rehabilitation after total laryngectomy - Hilgers, F J; Ackerstaff, A H; Van As, C J; Balm, A J; Van den Brekel, M W; Tan, I B (Acta Otolaryngol, 2003)