Complex sentence: Smaller earbud type earpieces, which plugged into the user's ear canal, were first developed for hearing aids but became widely used with transistor radios, which commercially appeared in 1954, with the introduction of the Regency TR-1.
Split into simpler sentences: Smaller earbud type earpieces, which plugged into the user's ear canal, were first developed for hearing aids. They became widely used with transistor radios, which commercially appeared in 1954 with the introduction of the Regency TR-1.