Wireless neural probes based on one-port SAW delay line and neural firing-dependent varicap diode

A battery-free, wireless neural probe system was developed for reading neural signals in the brain by using a one-port surface acoustic wave (SAW) reflective delay line, neural firing-dependent varicap diode, two antennas, and a network analyzer as measurement unit. The one-port SAW reflective delay line replaces existing complex wireless transceiver system composed of ∼5000 electronic components and makes battery-free, wireless measurements possible. The varicap diode interconnected with sharp metal shank via operational amplifier (op-amp) was electrically linked to the corresponding split-type reflectors on a one-port SAW reflective delay line. A 4.3 nH inductor was also placed in between the split-type reflector on one-port SAW reflective delay line and varicap diode to obtain a large linearity and high sensitivity through impedance matching effect. As electrical pulses imitating neural signals were applied to the sharp metal shanks, overall impedance perturbations between the split-type reflector and external varicap diode under reverse bias were observed, giving rise to amplitude changes in the reflection peaks in the time domain depending on the magnitude of the electrical pulses. Good linearity and sensitivity were observed at the amplitude variations in terms of electrical pulses. Coupling-of-modes (COM) modeling and impedance matching simulations were also performed to predict device performances and compare experimental results.