Medtronic Inc., which has long had an interest in implantable hearing devices that transmit sound through the skin, has acquired a Colorado company that makes and sells the technology in the U.S. and Europe.

Medtronic said Friday that it has closed on the acquisition of Sophono Inc., a small privately held Boulder-based firm that developed a hearing implant that attaches to ­surgically implanted magnets and transmits sound through the skin. The device is unique because it doesn't require a small metal "pole" protruding through the skin.

Terms of the deal were not disclosed. Annualized earnings impact from the deal are not expected to be material to Medtronic, which in January moved its corporate headquarters to Ireland but kept its operating headquarters in Fridley.

More than 4,000 patients in 42 countries worldwide have had Sophono's technology implanted to treat single-sided deafness or conductive, mixed hearing loss. The most current version of the device, the Alpha 2 MPO subcutaneous bone conduction hearing system, is intended for patients ages 5 and up.

The Alpha 2 includes two hermetically sealed rare-earth magnets that are implanted in the skull to hold in place a processor worn behind the ear. The processor transmits audio vibrations into the bone, where sound is sensed by the cochlea in the inner ear.

In the 1990s, Medtronic's Audiant division released the first under-the-skin bone conduction hearing aid, but it failed in the market because of post-surgical complications, according to a timeline on Sophono's website.

Six years ago, Sophono incorporated in Colorado to commercialize the German-invented hearing device that eventually became the Alpha 1. The company made a former Medtronic executive, Dr. Markus Haller, its board chairman. Haller also manages worldwide sales and marketing functions for the company.

The World Health Organization says that 360 million people, or 5 percent of the world's population, have disabling hearing loss from genetic causes, infections and diseases, drug use, exposure to noise and aging. The organization estimates that production of hearing aids currently meets less than 10 percent of the global need.

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