A Wireless Implantable Microsystem for Real-Time Bladder Volume Monitoring

This paper describes an implantable wireless microsystem for making embedded ultrasound distance measurements and its applications in real-time bladder volume (BV) estimation. A tuned inductive link at 125 kHz is used for both power transfer and bidirectional data telemetry with the implanted unit. Implant power consumption is minimized by using a single-bit correlation receiver to estimate ultrasound time-of-flight (ToF) from A-mode data acquired by an on-board piezoelectric transducer. In vitro tests of the device (external diameter =30 mm including coil) in saline solution show a maximum operating depth of 3.2 cm from a 10 cm diameter external coil operated at +/-18 V. Wireless BV measurements using both spherical and non-spherical bladder phantoms show estimation errors < 15\% for volumes in the 50-300 mL range.