System Architecture. (a) 3-tiered, 4-coil, 2-sub-system architecture. Each unit is shown in individual boxes. Charging Sub-System (CSS) is illustrated in green and Stimulation Sub-System (SSS) is illustrated in blue. (b) The relation of external, subcutaneous, and pacer units is illustrated in the presence of skin/adipose, tissue/muscle, ribs, and heart.