Ultra-low-power wireless implantable blood flow sensing microsystem for vascular graft applications

Flow rate monitoring provides an indication for early intervention of vascular graft degradation or failure used in lower limb bypasses and renal haemodialysis. This paper presents an inductively powered implantable blood flow sensing microsystem with bidirectional telemetry capability, which fully integrates the silicon nanowire (SiNW) sensor with tunable giant piezoresistivity, the ultra-low-power ASIC and the high-efficiency transcutaneous coupling coils. Operating at 13.56 MHz carrier frequency, the micro-fabricated coils transfer the power and command forward and backscatter the processed sensor readout information to an external device. The ASIC fabricated in 0.18 μm CMOS process occupies an active area of 1.5×1.78 mm
²
and consumes 21.6 μW totally. The SiNW diaphragm-based sensor provides the gauge factor higher than 300 with tuning voltage below 0.5 V. The proposed solution has demonstrated the 0.176 mmHg/√Hz sensing resolution with small device dimension and the lowest power consumption to the authors' knowledge.