A holder apparatus (10) of a bio-signal device comprises a holder (100), which comprises a pocket (100A) for a bio-signal processing device (200), an extension (100B) with a hollow (100C), the hollow (100C) and the pocket (100A) forming a continuous cavity (101) through the holder (100), a connector (106), and an elastic seal (108) with a hole (108A). A shape of the elastic seal (108) is matched with a shape of the hollow (100C) of the extension (100B) at an interface of the pocket (100A), and the hollow (100C) and a shape of the hole (108A) is matched with a shape of the connector (106) for sealing an interface between the connector (106) and the holder (100) while the connector (106) and the elastic seal (108) are within the hollow (100C) and the connector (106) is in contact with the elastic seal (108). Sealant filler (204) fills the hollow (100C) of the extension (100B) and is in physical contact with the connector (106), which is in the hollow (100C) against the seal (108), the elastic seal (108) and the sealant filler (204) allowing the connector (106) to mate electrically with a counter-connector (202) moved within the cavity (101) in a direction from the pocket (100A) toward the hollow (100C).