Body powers brain monitor

IMEC (Interuniversity Microelectronics Centre) has developed a battery-free wireless two-channel EEG system powered by a hybrid power supply using body heat and ambient light.

The supply combines a thermoelectric generator that uses the heat dissipated from a person’s temples and silicon photovoltaic cells.

The entire system is wearable and integrated into a device resembling headphones. The system can provide more than 1 mW indoors, which is more than enough for the targeted application.

Thermoelectric generators using body heat typically show a drop in generated power when the ambient temperature is in range of the body temperature.

Especially outside, the photovoltaic cells in the hybrid system counter this energy drop and ensure a continuous power generation.

Moreover, they serve as part of the radiators for the thermoelectric generator, which are required to obtain high efficiency.

Compared with a previous EEG demonstrator also developed within the framework of the Holst Centre, the system is smaller and lighter. Combined with full autonomous operation, no maintenance and an acceptable low heat flow from the head, it further increases the patient’s autonomy and quality of life.

Potential applications are detection of imbalance between the two halves of the brain, detection of certain kinds of brain trauma and monitoring brain activity.

The generator comprises six thermoelectric units made up from miniature commercial thermopiles. Each of the two radiators has an external area of 4 × 8 cm² that is made of Si photovoltaic cells. Thermally conductive comb-type structures (so-called thermal shunts) have been used to eliminate the thermal barrier between the skin and the thermopiles that is caused by the person’s hair on the generator.

The system uses an ultra-low-power biopotential readout application-specific integrated circuit to extract high-quality EEG signals with micro-power consumption.

A low-power digital-signal processing block encodes the extracted EEG data, which are sent to a PC via a 2.4 GHz radio link.

The whole system consumes 0.8 mW, well below the power produced to provide full autonomy.