This work describes the design and testing of a wireless implantable bladder pressure sensor suitable for chronic implantation in humans. The sensor 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 would particularly benefit from a wireless bladder pressure sensor providing real-time pressure feedback to an implanted stimulator, resulting in greater bladder capacity while using less power. The pressure sensing system consists of an implantable microsystem, an external RF receiver, and a 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, protecting both the device and the patient.
The implantable microsystem consists of a custom application-specific integrated circuit (ASIC), pressure transducer, rechargeable battery, and wireless telemetry and recharging antennas. Because the battery capacity is extremely limited, the ASIC was designed using an ultra-low-power methodology in which power is dynamically allocated to instrumentation and telemetry circuits by a power management unit. A low-power regulator and clock oscillator set the minimum current draw at 7.5 µA and instrumentation circuitry is operated at low duty cycles to transmit 100-Hz pressure samples while consuming 74 µA. An adaptive transmission activity detector determines the minimum telemetry rate to limit broadcast of unimportant samples. Measured results indicated that the power management circuits produced an average system current of 16 µA while reducing the number of transmitted samples by more than 95% with typical bladder pressure signals. The wireless telemetry range of the system was measured to be 35 cm with a bit-error-rate of 10-3, and the battery was wirelessly recharged at distances up to 20 cm.
A novel biocompatible packaging method consisting of a silicone-nylon mesh membrane and a compliant silicone gel was developed to protect the sensor from water ingress while only reducing the sensor sensitivity by 5%. Dynamic offset removal circuitry extended the system dynamic range to 2,900 cm H2O but limited the sensor AC accuracy to 3.7 cm H2O over a frequency range of 0.002 – 50 Hz. The DC accuracy of the sensor was measured to be approximately 2.6 cm H2O (0.9% full-scale). Functionality of wired prototypes was confirmed in feline and canine animal models, and wireless prototypes were implanted in a female calf large-animal model. Measured in vivo pressure recordings of bladder contractions correlated well with reference catheters (r =0.893–0.994).