Hypo-allergenic hearing aid. Close-up of a hypo- allergenic hearing aid. This hearing aid is made of a special plastic designed not to trigger any allergic reaction (such as inflammation) of the skin in the ear. The aid fits almost invisibly in- side the ear canal. The small cord sticks out of the ear canal, allowing the aid to be pulled out. The lid of the battery case is slightly open, showing the battery. Hearing aids improve hearing in people with partial deafness. Sound is picked up by a tiny microphone, amplified, and then tran- smitted into the ear by a tiny speaker. Each hear- ing aid is fine-tuned to amplify the range of frequencies most needed by the patient.