Implantable Bladder Sensors: A Methodological Review

Abstract

The loss of urinary bladder control/sensation, also known as urinary incontinence (UI), is a common clinical problem in autistic children, diabetics, and the elderly. UI not only causes discomfort for patients but may also lead to kidney failure, infections, and even death. The increase of bladder urine volume/pressure above normal ranges without sensation of UI patients necessitates the need for bladder sensors. Currently, a catheter-based sensor is introduced directly through the urethra into the bladder to measure pressure variations. Unfortunately, this method is inaccurate because measurement is affected by disturbances in catheter lines as well as delays in response time owing to the inertia of urine inside the bladder. Moreover, this technique can cause infection during prolonged use; hence, it is only suitable for short-term measurement. Development of discrete wireless implantable sensors to measure bladder volume/pressure would allow for long-term monitoring within the bladder, while maintaining the patient's quality of life. With the recent advances in microfabrication, the size of implantable bladder sensors has been significantly reduced. However, microfabricated sensors face hostility from the bladder environment and require surgical intervention for implantation inside the bladder. Here, we explore the various types of implantable bladder sensors and current efforts to solve issues like hermeticity, biocompatibility, drift, telemetry, power, and compatibility issues with popular imaging tools such as computed tomography and magnetic resonance imaging. We also discuss some possible improvements/emerging trends in the design of an implantable bladder sensor.

Keywords: Biocompatible Materials; Hermetic; Implantable Bladder Sensor; Telemetry; Urinary Incontinence.

Fig. 1.

Bladder volume sensing using a strain sensor (A: adapted from Rajagopalan S, et al. Sensors 2008;8:5081-95 [14], on the basis of Open Access) and a reed switch and magnet (B: adapted from Dreher RD, et al. IEEE Trans Biomed Eng 1972;19:247-8 [6], with permission of the Institute of Electrical and Electronics Engineers).

Fig. 2.

An implantable bladder pressure sensor using PZT as energy source where (A) an acoustic receiver (PZT cantilever) and rectifier circuitry are assembled on a glass substrate; (B) an acrylic holder, proof of mass, and antenna/sensor coil are added; (C) the assembled parts are inserted into a glass tube and an acrylic lid is attached; and (D) the inductive sensor with PDMS membrane and ferrite core are added. Adapted from Kim A, et al. IEEE Trans Biomed Eng 2014;61:2209-17 [61], with permission of the Institute of Electrical and Electronics Engineers. RF, radio frequency; PZT, piezoelectric lead zirconat titanate; PDMS, polydimethylsiloxane.