A High-Sensitivity Fully Passive Neurosensing System for Wireless Brain Signal Monitoring

A high-sensitivity, fully passive neurosensing system is presented for wireless brain signal monitoring. The proposed system is able to detect very low-power brain-like signals, viz. as low as $-$ 82 dBm (50 $\mu\hbox{V}_{\rm pp}$ ) at ${\rm f}_{\rm neuro}>\hbox{ 1 kHz}$ . It is also able to read emulated neural signals as low as $-$ 70 dBm (200 $\mu\hbox{V}_{\rm pp}$ ) at ${\rm f}_{\rm neuro}>\hbox{ 100 Hz}$ . This is an improvement of up to 22 dB in sensitivity as compared with previously reported neural signals. The system is comprised of an implanted neurosensor and an exterior interrogator. The neurosensor receives an external carrier signal and mixes it with the neural signals prior to retransmitting to the interrogator. Of importance is that the implanted neurosensor is fully passive and does not require a battery nor rectifier/regulator but is concurrently wireless for unobtrusive neurosensing with minimal impact to the individual's activity. To achieve this remarkable high sensitivity, the sensing system employed: 1) a subharmonic mixer using an anti-parallel diode pair; 2) a pair of implanted/interrogator antennas with high transmission coefficient $\vert{S}_{21}\vert$ ;and 3) a matching circuit between the implanted antenna and the mixer. This neurosensing system brings forward a new possibility of wireless neural signal detection using passive brain implants.