Wireless, Ultra-Low-Power Implantable Sensor for Chronic Bladder Pressure Monitoring

Abstract

The wireless implantable/intracavity micromanometer (WIMM) system was designed to fulfill the unmet need for a chronic bladder pressure sensing device in urological fields such as urodynamics for diagnosis and neuromodulation for bladder control. Neuromodulation in particular would benefit from a wireless bladder pressure sensor which could provide real-time pressure feedback to an implanted stimulator, resulting in greater bladder capacity while using less power. The WIMM uses custom integrated circuitry, a MEMS transducer, and a wireless antenna to transmit pressure telemetry at a rate of 10 Hz. Aggressive power management techniques yield an average current draw of 9 μA from a 3.6-Volt micro-battery, which minimizes the implant size. Automatic pressure offset cancellation circuits maximize the sensing dynamic range to account for drifting pressure offset due to environmental factors, and a custom telemetry protocol allows transmission with minimum overhead. Wireless operation of the WIMM has demonstrated that the external receiver can receive the telemetry packets, and the low power consumption allows for at least 24 hours of operation with a 4-hour wireless recharge session.

Keywords: ASIC; Design; FSK transmitter; Measurement; ULP; bladder pressure; implant; low-power; neuromodulation; offset cancellation; urodynamics; wireless recharge; wireless sensor.

Fig. 1

Conceptual views of the WIMM a) implanted within a bladder and providing telemetry to an external receiver as well as an implanted neuromodulation device and b) schematic view of the WIMM highlighting the various components.

Fig. 2

The WIMM ASIC integrates instrumentation, telemetry and power management circuitry and achieves ultra-low-power consumption. An external RF receiver/recharger receives telemetry and recharges the implanted micro-battery.

Fig. 3

The PCU schematic and current usage is shown in (a). The 50-kHz clock oscillator in (b) provides the time base for the power control signal generator.

Fig. 4

Timing diagram for some of the power control signals generated by the PCU for the acquisition and transmission of one sample. The signal timing accounts for varying warmup or synchronization periods as required by each circuit, and circuits are turned off to conserve power after they have performed their function.

Fig. 5

Block diagram of the WIMM auto-offset cancellation loop. The accumulator, IDAC, and coarse offset removal current sources comprise the offset cancellation loop, while the amplifier and ADC are part of the pressure sensing instrumentation.

Fig. 6

The WIMM telemetry protocol uses a 680-μs synchronization phase, followed by a 14-bit data packet, including a start pattern. A pseudo-random bit pattern and the offset IDAC value are interleaved throughout successive packets to enable synchronization with minimal overhead.

Fig. 7

The WIMM ASIC was fabricated in the OnSemi 0.5-μm process, and an annotated die photo is shown in (a). Wireless bench tests use a test board with the same components as the implantable WIMM, but in a larger form factor (b). A saline bath is used to simulate transmission through tissue (not shown).

Fig. 8

The WIMM transmits wireless telemetry in bursts, as the RSSI indicates in (a). A detailed view of the received data shown in (b), highlighting the 14-bit packet format. The receiver analyzes the packets and indicates the start of the 8-bit ADC sample with a synchronization pulse.

Fig. 9

Wireless testing of the WIMM demonstrated that 4-hour recharge restores more charge to the battery than is used in 20 hours of operation in (a). The WIMM ASIC drew power from the micro-battery in pulses, as detailed in (b).

Fig. 10

A surgeon holds the packaged prototype in (a), and prepares to implant the device for in vivo study. Pressure recordings from the WIMM implanted within an anesthetized feline closely match those from a reference catheter in (b), after normalizing to correct for DAQ recording discrepancies.