A gas-impermeable sealer element (24, 124) for a detonator or other explosive initiation device includes a non-reactive sleeve (26, 126) having a channel (28, 128) formed therein. A reactive material strip (30, 130) is sealed within the channel for transmission of an explosive's initiation signal through the sealer element (24, 124), either alone or in cooperation with transfer charges located at the input and/or output end of the non-reactive sleeve (26, 126). The reactive material strip (30, 130) comprises a reactive metal wire or other substrate (34) having on one or both sides thereof a layer of reactive material (30, 130, 36), either reactive metal foils which react exothermically when ignited, or a deposited fuel-oxidizer reactive material. The reactive materials, upon being energized, react exothermically in the absence of atmospheric oxygen or other extraneous oxidizer and so may be encapsulated, sealed or otherwise isolated from the atmosphere in use.