Implantable wireless battery recharging system for bladder pressure chronic monitoring.

This paper presents an implantable wireless battery recharging system design for bladder pressure chronic monitoring. The wireless recharging system consists of an external 15 cm-diameter 6-turn powering coil and a silicone-encapsulated implantable rectangular coil with a dimension of 7 mm × 17 mm × 2.5 mm and 18 turns, which further encloses a 3 mm-diameter and 12 mm-long rechargeable battery, two ferrite rods, an ASIC, and a tuning capacitor. For a constant recharging current of 100 µA, an RF power of 700 µW needs to be coupled into the implantable module through the tuned coils. Analyses and experiments confirm that with the two coils aligned coaxially or with a 6 cm axial offset and a tilting angle of 30°, an external power of 3.5 W or 10 W is required, respectively, at an optimal frequency of 3 MHz to cover a large implant depth of 20 cm.

MEMS capacitive accelerometer-based middle ear microphone.

The design, implementation, and characterization of a microelectromechanical systems (MEMS) capacitive accelerometer-based middle ear microphone are presented in this paper. The microphone is intended for middle ear hearing aids as well as future fully implantable cochlear prosthesis. Human temporal bones acoustic response characterization results are used to derive the accelerometer design requirements. The prototype accelerometer is fabricated in a commercial silicon-on-insulator (SOI) MEMS process. The sensor occupies a sensing area of 1 mm × 1 mm with a chip area of 2 mm × 2.4 mm and is interfaced with a custom-designed low-noise electronic IC chip over a flexible substrate. The packaged sensor unit occupies an area of 2.5 mm × 6.2 mm with a weight of 25 mg. The sensor unit attached to umbo can detect a sound pressure level (SPL) of 60 dB at 500 Hz, 35 dB at 2 kHz, and 57 dB at 8 kHz. An improved sound detection limit of 34-dB SPL at 150 Hz and 24-dB SPL at 500 Hz can be expected by employing start-of-the-art MEMS fabrication technology, which results in an articulation index of approximately 0.76. Further micro/nanofabrication technology advancement is needed to enhance the microphone sensitivity for improved understanding of normal conversational speech.

An RF-powered wireless multi-channel implantable bio-sensing microsystem.

An RF-powered wireless three-channel implantable bio-sensing microsystem is developed with blood pressure, EKG, and core body temperature sensing capability for untethered genetically engineered laboratory mice real-time monitoring. A flat silicone blood pressure sensing cuff with a MEMS capacitive pressure sensor is employed to form a novel less-invasive blood pressure sensor, which avoids vessel occlusion, bleeding, and blood clotting associated with the conventional catheter-based sensors. The implantable microsystem can be powered by an adaptively controlled external RF energy source at 4 MHz to ensure a stable on-chip power supply. The overall system dissipates 200 microW and achieves a blood pressure sensing resolution of 1 mmHg within 1 kHz bandwidth, an EKG sensing resolution of 7.4 bits, and a temperature sensitivity of 19 mV/°C measured from 22 °C to 43 °C. A prototype packaged sensor exhibits a weight of 495 mg, which is approximately 2% of a laboratory mouse body mass. On-going research effort is devoted to demonstrate in vivo performance in laboratory animals.

Characterization of ossicular chain vibration at the umbo: implications for a middle ear microelectromechanical system design.

We propose the use of a microelectromechanical system (MEMS) accelerometer as a middle ear microphone for future totally implantable cochlear prostheses. The MEMS accelerometer would be attached to the umbo to detect and convert the natural bone vibration that occurs in response to external sounds into an electrical signal that represents the acoustic information. The signal could be further processed to stimulate cochlear implant electrodes. To determine the feasibility of our proposal, we conducted a study to investigate whether the characteristics of umbo vibration along two orthogonal axes-one axis being perpendicular to the tympanic membrane and the other axis being parallel to the tympanic membrane plane but orthogonal to the long process of the malleus-differ significantly enough to compromise the sensing performance of the proposed accelerometer should a position misalignment occur during the implant procedure. We used laser Doppler vibrometry to measure the displacement of the umbo along the two orthogonal axes in 4 cadaveric temporal bones at multiple frequencies within the audible spectrum. We found that the peak-to-peak displacement frequency response along both axes was nearly flat from 250 Hz to 1 kHz, and it gradually rolled off with a slope of approximately -20 dB and -40 dB per decade above 1 kHz and 4 kHz, respectively. At each frequency, the displacement exhibited a linear function of the input sound level with a slope of 20 dB per decade. A comparison of measurements along the two axes indicated a similar frequency response, with an average amplitude difference of 20%. The characterization data suggest that the performance of a miniature ossicular vibration-sensing device attached on the umbo would not be degraded in the event of a position misalignment. The data also indicate that a MEMS accelerometer needs to achieve a resolution of 35 i g/sqrt[Hz] to detect normal conversation.

A wireless batteryless in vivo EKG and core body temperature sensing microsystem with 60 Hz suppression technique for untethered genetically engineered mice real-time monitoring.

A wireless, batteryless, and implantable EKG and core body temperature sensing microsystem with adaptive RF powering for untethered genetically engineered mice real-time monitoring is designed, implemented, and in vivo characterized. A packaged microsystem, exhibiting a total size of 9 mm x 7 mm x 3 mm with a weight of 400 mg including a pair of stainless-steel EKG electrodes, is implanted in a mouse abdomen for real-time monitoring. A low power 2 mm x 2 mm ASIC, consisting of an EKG amplifier, a proportional-to-absolute-temperature (PTAT)-based temperature sensor, an RF power sensing circuit, an RF-DC power converter, an 8-bit ADC, digital control circuitry, and a 433 MHz FSK transmitter, is powered by an adaptively controlled external RF energy source at 4 MHz to ensure a stable 2V supply with 156microA current driving capability for the overall microsystem. An electrical model for analyzing 60 Hz interference based on 2-electrode and 3-electrode configurations is proposed and compared with in vivo evaluation results. Due to the small laboratory animal chest area, a 60 Hz suppression technique by employing input termination resistors is chosen for two-EKG-electrode implant configuration.

Wireless powering and data telemetry for biomedical implants.

Wireless powering and data telemetry techniques for two biomedical implant studies based on (1) wireless in vivo EMG sensor for intelligent prosthetic control and (2) adaptively RF powered implantable bio-sensing microsystem for real-time genetically engineered mice monitoring are presented. Inductive-coupling-based RF powering and passive data telemetry is effective for wireless in vivo EMG sensing, where the internal and external RF coils are positioned with a small separation distance and fixed orientation. Adaptively controlled RF powering and active data transmission are critical for mobile implant application such as real-time physiological monitoring of untethered laboratory animals. Animal implant studies have been successfully completed to demonstrate the wireless and batteryless in vivo sensing capabilities.

A laboratory study on a capacitive displacement sensor as an implant microphone in totally implant cochlear hearing aid systems.

A totally implant cochlear hearing aids system, integrating an implant microphone, interface electronics, a speech processor, a stimulator, and cochlear electrodes, can overcome the uncomfortable, inconvenient, and stigma problems associated with the conventional and semi-implantable hearing aids. This paper presents a laboratory feasibility study on the use of an electret condenser microphone (ECM) displacement sensor, serving as an implant microphone, and combined with a spring coupler to directly sense the umbo acoustic vibration. The umbo vibration characteristics were extracted from literature to determine the coupler and sensor requirements. A laboratory model was built to simulate the vibration source and experimentally study the transmission coefficient. Experimental data demonstrate that by using a 5 N/m stiffness spring, the umbo vibration amplitude as high as 67% can be transmitted to the sensor. Measurement of the sensor system on the temporal bone was also made. The minimum detectable sound pressure level (SPL) at 1 kHz is 41 and 67 dB for laboratory and 38 and 64 dB for temporal bone measurement for 1 and 388 Hz bandwidth, respectively. Better performance was achieved in a higher frequency. Results and analysis of this study can be used as a guideline for the future design of displacement sensors as implant microphones.

In vivo RF powering for advanced biological research.

An optimized remote powering architecture with a miniature and implantable RF power converter for an untethered small laboratory animal inside a cage is proposed. The proposed implantable device exhibits dimensions less than 6 mmx6 mmx1 mm, and a mass of 100 mg including a medical-grade silicon coating. The external system consists of a Class-E power amplifier driving a tuned 15 cmx25 cm external coil placed underneath the cage. The implant device is located in the animal's abdomen in a plane parallel to the external coil and utilizes inductive coupling to receive power from the external system. A half-wave rectifier rectifies the received AC voltage and passes the resulting DC current to a 2.5 kOmega resistor, which represents the loading of an implantable microsystem. An optimal operating point with respect to operating frequency and number of turns in each coil inductor was determined by analyzing the system efficiency. The determined optimal operating condition is based on a 4-turn external coil and a 20-turn internal coil operating at 4 MHz. With the Class-E amplifier consuming a constant power of 25 W, this operating condition is sufficient to supply a desired 3.2 V with 1.3 mA to the load over a cage size of 10 cmx20 cm with an animal tilting angle of up to 60 degrees, which is the worst case considered for the prototype design. A voltage regulator can be designed to regulate the received DC power to a stable supply for the bio-implant microsystem.

Effect of incus removal on middle ear acoustic sensor for a fully implantable cochlear prosthesis.

System miniaturization and steady progress towards a totally implantable prosthetic system is the current trend in cochlear implant technology. To achieve this objective, the external microphone of present implants needs to be implantable. This goal can be accomplished by placing a miniature accelerometer on the ossicular chain in the middle ear to detect and convert bone vibrations into an electrical signal for further processing and stimulating cochlear electrodes. This paper describes the characterization of the umbo of a human temporal bone before and after the removal of the incus to determine the impact of the resulting change in umbo mechanics and attached accelerometer performance. With the removal of the incus, the umbo vibration acceleration frequency response in the direction perpendicular to the tympanic membrane increases by 5 dB below 2 kHz. Above 2 kHz the response diverges due to the change of ossicular chain resonant frequency caused by the removal of the incus. However, at each frequency the umbo vibration acceleration exhibits a linear function of the input sound pressure level (SPL) with a slope of 20 dB per decade before and after removal of the incus. A commercial accelerometer attached to the umbo shows similar characteristics. From the measurement results of umbo characterization, a miniaturized implantable accelerometer with a packaged mass below 20 milligrams, a sensing resolution of 35 microg rms/square root Hz, and a bandwidth of 10 kHz would be required to detect normal conversation.

Novel long-term implantable blood pressure monitoring system with reduced baseline drift.

A novel long-term less-invasive blood pressure monitoring system with fluid-filled cuff is proposed for advanced biological research. The system employs an instrumented elastic cuff attached with a rigid isolation ring on the outside wall of the cuff. The cuff is wrapped around a blood vessel for real-time blood pressure monitoring. The elastic cuff is made of bio-compatible soft silicone material and is filled with bio-compatible insulating silicone oil with an immersed MEMS pressure sensor. This technique avoids vessel penetration and substantially minimizes vessel restriction due to the soft cuff elasticity, thus attractive for long-term monitoring. A rigid isolation ring is used to isolate the cuff from environmental variations to suppress baseline drift in the measured waveform inside the monitoring cuff. The prototype monitoring cuff is wrapped around the right carotid artery of a laboratory rat to measure real-time blood pressure waveform. The measured in vivo blood waveform is compared with a reference waveform recorded simultaneously by using a commercial catheter-tip transducer inserted into the left carotid artery, showing matched waveforms with a scaling factor about 0.03 and a baseline drift of 0.6 mm Hg. The measured baseline drift is three times smaller compared to using a cuff without a rigid isolation ring.