What is the specific absorption rate limit for wireless medical devices and how is it tested?

The specific absorption rate (SAR) is the measure of RF energy absorbed by body tissue when exposed to electromagnetic fields from wireless devices, expressed in watts per kilogram (W/kg). For wireless medical devices, the SAR limits are: FCC (United States): 1.6 W/kg averaged over 1 gram of tissue for general population exposure, and 8 W/kg for occupational exposure in specific controlled conditions; ICNIRP/EU: 2 W/kg averaged over 10 grams of tissue for general population, 10 W/kg for occupational. Medical implants operating at MICS power levels (25 microwatts EIRP) produce SAR values far below these limits (typically 0.001-0.01 W/kg). However, higher-power medical devices (RF ablation systems, diathermy equipment, and MRI systems) can produce significant SAR and require careful design and testing. SAR testing is performed using a tissue-simulating phantom (a shell shaped like a human body section, filled with tissue-simulating liquid). The device under test (DUT) is placed at the appropriate position relative to the phantom. A robotic probe system measures the electric field distribution inside the phantom liquid, and SAR is calculated from: SAR = sigma x |E|^2 / rho, where sigma is the tissue conductivity, |E| is the electric field magnitude, and rho is the tissue density.