Tan et al from the Department of Biomedical Engineering at Michigan Technological University developed implantable biosensors using soft and permanent magnets. When a soft magnet was exposed to a magnetic field from a permanent magnet, secondary magnetic fields were created. Measuring changes in the magnetic fields allowed the researchers to quantify strain and pressure. These types of sensors could be miniaturized and used for in-vivo, long-term strain/pressure monitoring.
See the following for more details: http://www.mdpi.com/1424-8220/8/10/6396