The hearing-impaired need two sets of ears, one for face-to-face conversations and another for use with cell phones and certain other electronic equipment. The reason is that hearing aids must switch input modes, either by manual intervention or automatically. Devices capable of self-adjustment often get mixed reviews, but a nanosensor based on giant magnetoresistance (GMR) is about to change this picture.
GMR thin films exhibit giant magnetoresistance (10%–20%) in response to a magnetic field. The films have two or more magnetic layers separated by a nonmagnetic layer. Because of spin-dependent scattering of the conduction electrons, the resistance is maximum when the magnetic moments of the layers are antiparallel and minimum when they are parallel. The structures used in GMR sensors are unpinned sandwiches, antiferromagnetic pinned spin valves, and antiferromagnetic multilayers. One industrial application of the technology is read heads in hard disks.
At present, the only manufacturer of GMR medical sensors is NVE Corp. Starkey Laboratories (www.starkey.com) is the world's leading manufacturer of custom hearing instruments. The two companies have combined their know-how to create a hearing aid that is both extremely tiny and capable of automatic switching that is transparent to the wearer.
The new sensors use GMR elements embedded in a conventional IC. Because the GMR effect requires conduction layers thinner than the mean free path of conduction electrons, the critical conduction layers in the hearing aids are only ~3 nm thick. The sensor element is combined with conventional electronics on a single chip measuring 1.2 × 1.3 × 0.2 mm (0.05 × 0.05 × 0.008 in.), a small fraction of the size of electromechanical sensors.
GMR technology is also used in pacemakers and implantable defibrillators, where they allow high-speed communication between a practitioner and the implant.