Steve Majerus

Low-Power Wireless Micromanometer System for Acute and Chronic Bladder-Pressure Monitoring

This letter describes the design, fabrication, and testing of a wireless bladder-pressure-sensing system for chronic, point-of-care applications, such as urodynamics or closed-loop neuromodulation. The system consists of a miniature implantable device and an external RF receiver and wireless battery charger. The implant is small enough to be cystoscopically implanted within the bladder wall, where it is securely held and shielded from the urine stream. The implant consists of a custom application-specific integrated circuit (ASIC), a pressure transducer, a rechargeable battery, and wireless telemetry and recharging antennas. The ASIC includes instrumentation, wireless transmission, and power-management circuitry, and on an average draws less than 9 μA from the 3.6-V battery. The battery charge can be wirelessly replenished with daily 6-h recharge periods that can occur during the periods of sleep. Acute in vivo evaluation of the pressure-sensing system in canine models has demonstrated that the system can accurately capture lumen pressure from a submucosal implant location.

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

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.

Wireless micromanometer system for chronic bladder pressure monitoring

This paper describes a wireless system to monitor urinary bladder pressure comprising an implantable device with an external receiver and wireless battery charger. The device is intended to be implanted within the bladder wall, sealed behind the urothelial lining. This location is protected from the urine stream, thus avoiding mineral encrustation and stone formation, and is suitable to measure intravesical pressure in chronic applications. The implant is dimensionally designed to gain access to the bladder using conventional urological tools, e.g. a cystoscope. The active circuit implant features a custom application-specific integrated circuit (ASIC), rechargeable battery and wireless telemetry. Inductive charging, novel power management schemes and innovative packaging allow this device to be inserted through the urethra, implanted within the bladder wall, and operate for a lifetime of up to 10 years.

Telemetry platform for deeply implanted biomedical sensors

This paper describes the design of low-power platform circuits for deeply implanted biomedical sensors, a bidirectional transceiver with Manchester decoder and a successive-approximation ADC. Potential operating frequencies are analyzed with respect to system limitations of power, size and complexity. It is argued that operation using low carrier frequencies provides high power efficiency while slightly increasing the size of off-chip antennas, and 125-kHz OOK and 27.12-MHz FSK are chosen for forward- and reverse-telemetry, respectively. The 8-bit SAR ADC achieves a balance between power, resolution, and sampling rate, and the scalable architecture increases its versatility. The platform circuits were fabricated in the AMI 0.5-mum process and test measurements confirm the transceiver consumes 1.05 mW while the ADC consumes 50 muW.

Real-Time Classification of Bladder Events for Effective Diagnosis and Treatment of Urinary Incontinence

Diagnosis of lower urinary tract dysfunction with urodynamics has historically relied on data acquired from multiple sensors using nonphysiologically fast cystometric filling. In addition, state-of-the-art neuromodulation approaches to restore bladder function could benefit from a bladder sensor for closed-loop control, but a practical sensor and automated data analysis are not available. We have developed an algorithm for real-time bladder event detection based on a single in situ sensor, making it attractive for both extended ambulatory bladder monitoring and closed-loop control of stimulation systems for diagnosis and treatment of bladder overactivity. Using bladder pressure data acquired from 14 human subjects with neurogenic bladder, we developed context-aware thresholding, a novel, parameterized, user-tunable algorithmic framework capable of real-time classification of bladder events, such as detrusor contractions, from single-sensor bladder pressure data. We compare six event detection algorithms with both single-sensor and two-sensor systems using a metric termed Conditional Stimulation Score, which ranks algorithms based on projected stimulation efficacy and efficiency. We demonstrate that adaptive methods are more robust against day-to-day variations than static thresholding, improving sensitivity and specificity without parameter modifications. Relative to other methods, context-aware thresholding is fast, robust, highly accurate, noise-tolerant, and amenable to energy-efficient hardware implementation, which is important for mapping to an implant device.

Wireless implantable pressure monitor for conditional bladder neuromodulation

Conditional neuromodulation in which neurostimulation is applied or modified based on feedback is a viable approach for enhanced bladder functional stimulation. Current methods for measuring bladder pressure rely exclusively on external catheters placed in the bladder lumen. This approach has limited utility in ambulatory use as required for chronic neuromodulation therapy. We have developed a wireless bladder pressure monitor to provide real-time, catheter-free measurements of bladder pressure to support conditional neuromodulation. The device is sized for submucosal cystoscopic implantation into the bladder. The implantable microsystem consists of an ultra-low-power application specific integrated circuit (ASIC), micro-electro-mechanical (MEMS) pressure sensor, RF antennas, and a miniature rechargeable battery. A strategic approach to power management miniaturizes the implant by reducing the battery capacity requirement. Here we describe two approaches to reduce the average microsystem current draw: switched-bias power control and adaptive rate transmission. Measurements on human cystometric tracings show that adaptive transmission rate can save an average of 96% power compared to full-rate transmission, while adding 1.6% RMS error. We have chronically implanted the wireless pressure monitor for up to 4 weeks in large animals. To the best of our knowledge these findings represent the first examples of catheter-free, realtime bladder pressure sensing from a pressure monitor chronically implanted within the bladder detrusor.

Power management circuits for a 15-μA, implantable pressure sensor

A custom IC for wireless bladder pressure sensing incorporates power-management circuitry to limit the active time of the instrumentation circuitry and to minimize telemetry rate. Instrumentation circuits are operated with low duty factors in a pipelined manner to generate 100-Hz pressure samples. Telemetry rate is adapted according to sample activity, which is measured using a 2nd-order FIR filter. Measured results indicate that the number of transmitted samples is less than 5% of those acquired, resulting in an average telemetry rate of 1.5 Hz and corresponding current of 0.6 μA. A low-power regulator and clock oscillator set a baseline current draw of 7.5 μA while instrumentation and digital circuits consume an average of 4.7 μA, for a total average consumption of just 12.8 μA.

Wireless battery charge management for implantable pressure sensor

Implantable medical devices intended for chronic application in deep bodily organs must balance small size with battery capacity. Wireless battery recharge of implanted sensors is a viable option to reduce implant size while removing the physical and regulatory hindrance of continuous RF powering. This paper presents wireless battery recharge circuitry developed for an implantable pressure sensor. The circuits include an RF/DC rectifier, voltage limiter, and constant-current battery charger with 150-mV end-of-charge hysteresis. An AM demodulator drawing zero DC current allows for transmission of commands on the recharge carrier. Reception of a time- and value-coded shutdown command places the implantable system into a 15 nanoampere standby mode. The system can be wirelessly activated from standby by reactivating the external wireless recharge carrier. Test results of the wireless system showed a standby current of 15-nA such that the implant standby time is limited by battery self-discharge. Wireless recharge tests confirmed that a constant recharge rate of 200 μA could be sustained at implant depths up to 20 cm, but with low power transfer efficiency <; 0.1% due to small implant coil size. Battery charge measurements confirmed that daily 4-hour recharge periods maintained the implant state of charge and this recharging could occur during periods of natural patient rest.

Long-term evaluation of a non-hermetic micropackage technology for MEMS-based, implantable pressure sensors

This paper reports long-term evaluation of a micropackage technology for an implantable MEMS pressure sensor. The all-polymer micropackage survived > 160 days when subjected to accelerated lifetime testing at 85°C in a 1% wt. saline solution. The package exhibited minimum impact on sensitivity and linearity, which deviated by less than 5% and 0.3%, respectively. A 6-month in vivo evaluation of 16 MEMS-based pressure sensors showed that the micropackage exhibits good biocompatibility and provides excellent protection. To the best of our knowledge, these results establish new lifetime records for devices packaged using an all-polymer micropackaging approach.

Real-time, autonomous bladder event classification and closed-loop control from single-channel pressure data

Urinary incontinence, or the loss of bladder control, is a debilitating condition affecting millions worldwide, which significantly reduces quality of life. Neuromodulation of lower urinary tract nerves can be used to treat sensations of urgency in many subjects, including those with Spinal Cord Injury (SCI). Event driven, or conditional stimulation has been investigated as a possible improvement to the state-of-the-art open-loop stimulation systems available today. However, this requires a robust, adaptive, and noise-tolerant method of classifying bladder function from real-time bladder pressure measurements. Context-Aware Thresholding (CAT) has been previously shown to work well on prerecorded single contraction urodynamic data. In this work, for the first time, we present real-time detection of multiple serial bladder contractions using urodynamic recordings from human subjects with SCI and Neurogenic Detrusor Overactivity (NDO). CAT demonstrated a high degree of accuracy and noise tolerance on prerecorded data from 15 human subjects, with a mean accuracy of 92% and average false positive rate of 0.3 false positives per contraction. Analysis of event detection latencies showed that CAT identified and responded to events 1.4 seconds faster than the original human experimenter. Finally, we present a case study in which CAT was used live for real-time autonomous, closed-loop bladder control in a single human subject with SCI and NDO, successfully inhibiting four consecutive unwanted bladder contractions and increasing bladder capacity by 40%.