Young Sik Seo

Field Distribution Models of Spiral Coil for Misalignment Analysis in Wireless Power Transfer Systems

This paper presents design and optimization methods for spiral coils utilized for wireless power transfer in wireless medical implant applications. A theoretical model was examined for near-field distributions of spiral-type transmitter antennas in both orthogonal components. Finite-element simulations were performed to verify the theoretical radiation patterns. Receiver antenna voltages were measured at planes of interest as a means to map field distributions. Theoretical, simulation, and experimental results were conducted in free space and they agreed well. Understanding the orthogonal field components and their distributions in various distances between the worn transmitter coil outside the body and the receiver coil of implant that has a much smaller size provides a means to find the optimal location and angle to harvest maximum energy. The analysis method for near-field wireless power transmission can be utilized to determine design strategies of the transmitter spiral coil with considerations also in the amplifier circuit and physical constraints in practical scenarios to obtain maximum power and link efficiency for the implant devices. The method can be extended to investigate field distributions affected by human tissues, which construct a much more complex environment, and will be conducted in future works.

A Passive Radio-Frequency pH-Sensing Tag for Wireless Food-Quality Monitoring

We present a new method, suitable for food quality management by wirelessly monitoring pH level changes in food with a flexible pH sensor embedded in a batteryless radio-frequency (RF) transponder. The wireless sensor tag includes a flexible pH sensor based on miniature iridium oxide (IrOx) and silver chloride (AgCl) sensing electrodes integrated on a deformable substrate, and batteryless wireless communication circuitry. The sensor tag and reader system is designed to achieve convenient, long-term, and on-demand wireless in situ monitoring of food quality, especially for large-quantity applications and continuous monitoring from place of production to retail stores. Low-cost IrOx sol-gel fabrication process was applied on polymeric substrates to form the flexible sensing films, and a sensitivity of -49.7 mV/pH was achieved. Inducting coupling provides electromagnetic energy from the reader to drive the transponder circuits that retransmit the sensor-data modulated signals back to the reader. The electrochemical potential created by the IrOx/AgCl sensing electrodes is converted to a modulated frequency and the system achieves a sensitivity of 633 Hz/pH. The wireless pH sensing system was tested for in situ monitoring of the spoilage processes in fish meats continuously for over 18 h. The feasibility of wirelessly monitoring pH values in fish meats that could be used to identify spoilage remotely has been demonstrated.

Investigation of wireless power transfer in through-wall applications

In this work, we proposed a through-wall wireless power transfer system and investigated effects of various wall materials. The power transfer system was based on inductive coupling of metal coils at 1.3-MHz resonance. Softwood lumber, concrete brick and drywall with insulation filling were tested at two different thicknesses. Two sets of coils, each set consisting of two coils with identical dimensions, having radii of 5 and 15 cm were utilized. Each experiment was conducted with sequential tuning in receiver circuit, operating frequency and in transmitter circuit to reach maximum output power or maximum power transfer efficiency. The output power and transfer efficiency as well as their changes were obtained before and after tuning for different media and thicknesses. It is concluded that the power attenuation with spacing distance dominates the output power and transfer efficiency, while tuning could counteract the parasitic effects in the material and recover the power lost in deviation from resonance. The power attenuation with distance requires design considerations in coil dimensions. With larger coils, more power can be collected through thicker walls and the system tolerates more variation in wall thickness. Tests on randomly chosen walls in our laboratory building were conducted to validate the system performance.

A wireless strain sensor system for bladder volume monitoring

A wireless strain sensor system has been designed to monitor the bladder volume in patients suffering from urinary incontinence. An interdigitated capacitive (IDC) strain sensor was micro-machined from a 127-µm thick brass shim with a laser system, followed by an encapsulation process to package the sensor in elastic polydimethylsiloxane (PDMS). A proof-of-concept passive telemetry platform was developed to employ the sensor in vivo and a commercial wireless module was utilized for networking and data recording. The system includes a transducer implant, an external wearable unit and a base station. The implant harvests electromagnetic energy from the external unit, supplies power to operate the sensor and transduces the sensor data back. The wearable unit processes and transfers the signals to the base station connected to a computer which continuously displays and records the strain sensor data on the bladder. The sensor was calibrated and the entire system was tested with a bladder phantom model. Results were promising and demonstrated the feasibility of our passive wireless strain sensing system for urinary incontinence management.

An implantable batteryless wireless impedance sensor for gastroesophageal reflux diagnosis

A new method for long term monitoring of gastroesophageal reflux is presented. The impedance of the reflux in the esophagus can be determined remotely without the need of a battery in the sensor implant. The implant includes an energy harvesting circuit, sensing electrodes, an antenna and an impedance to frequency converter. An external reader provides power to the implant and measures the impedance values simultaneously. A prototype with an overall size of 0.5×1×3.1cm3 was made with a printed circuit board and discrete components, and packaged in polydimethylsiloxane. In vivo experiments were conducted in pig cadavers. The results show good correlation between impedance and pH values of the acid solutions flushed into the esophagus, and good signal-to-noise ratios with the transducer inside the body. The impedance sensor can detect nonacid materials due to the frequency shift differences between air and solutions. The batteryless wireless impedance sensor is able to detect every reflux episode, either acid or non-acid, which provides more accurate diagnosis for the gastroesophageal reflux disease.

Wireless power transfer by inductive coupling for implantable batteryless stimulators

This study investigated wireless power transfer with inductive coupling at a distance addressing the power requirement for chronic gastrostimulator implants. The energy harvesting system was designed to collect 3 to 20 mW power to operate an implantable stimulator to deliver 1 to 6 mA electric current into stomach tissues. The power transfer system efficiencies were investigated with different dimensions and turn numbers in coil antennas, distances between the two antennas, and variable loads. Clinical practicality and patient comforts were considered for implanting in the stomach through endoscopic procedures. Thus, the antenna size of the transmitter was configured to be between 4 and 6 cm in diameter, to increase portability while the implant coil was fixed at 10×35 mm2. The distance between the two antennas varied from 4 to 10 cm in air. The system efficiency measured as the ratio of output power to input power included tuning a class-E amplifier in the transmitter at 1.3 MHz carrier frequency. A maximum efficiency was achieved at 9.59%. At all distances measured, the delivered power to the implant was more than 3 mW which was the minimal requirement for the operation of the implant.

Micro pH Sensors Based on Iridium Oxide Nanotubes

Iridium oxide (IrOx) nanotubes have been grown on microelectrodes utilizing a patterned nanoporous aluminum oxide (AAO) template to form a micro pH sensor. A layer of 900-nm thick aluminum sputtered on a silicon wafer was locally anodized at various voltages to investigate the morphology of the patterned AAO templates. The electrode position of IrOx inside the supportive AAO template was carried out to yield IrOx nanotubes with a diameter ranging from 80 to 110 nm, corresponding to the AAO pore sizes. The fabrication processes produced free-standing IrOx nanotubes on top of the 50 × 100 μm2 micro-scale electrodes. Material characterization along with fabrication parameters was investigated. Potentiometric responses of the miniature sensors to hydrogen ions in terms of sensitivity, repeatability, response time, and reliability were performed. Super-Nernstian response was achieved. pH measurements for higher resolution targeting biological applications and sensor temperature dependence were also conducted.

Position and angular misalignment analysis for a wirelessly powered stimulator

This study investigated the effects in inductive coupling of wireless power due to the contribution of both positional and angular misalignment between the transmitter and implanted stimulator antennas. The implant antenna radius was limited to 1.5 cm for endoscopic implantation while the transmitter coil radius was kept at 21 cm in order to transfer sufficient power for tissue stimulation. The power transfer distances between the two antennas examined were 5, 7 and 10 cm, typical for stimulator implant applications. Theoretical radiation patterns were examined with magnetic field distribution and compared with measured output voltages. Experimental results agreed well with the theoretical models. The optimal location for maximal output power in the receiver was identified with respect to different angular misalignments and distances. The method can guide the re-adjustment strategy of the transmitter coil worn by the patient after implantation and during therapy.

A wireless system for monitoring transcranial motor evoked potentials.

Intraoperative neurophysiological monitoring (IONM) is commonly used as an attempt to minimize neurological morbidity from operative manipulations. The goal of IONM is to identify changes in the central and peripheral nervous system function prior to irreversible damage. Intraoperative monitoring also has been effective in localizing anatomical structures, including peripheral nerves and sensorimotor cortex, which helps guide the surgeon during dissection. As part of IONM, transcranial motor evoked potentials (TcMEPs), and somatosensory evoked potentials (SSEPs) are routinely monitored. However, current wired systems are cumbersome as the wires contribute to the crowded conditions in the operating room and in doing so not only it limits the maneuverability of the surgeon and assistants, but also places certain demand in the total anesthesia required during surgery, due to setup preoperative time needed for proper electrode placement, due to the number and length of the wires, and critical identification of the lead wires needed for stimulation and recording. To address these limitations, we have developed a wireless TcMEP IONM system as a first step toward a multimodality IONM system. Bench-top and animal experiments in rodents demonstrated that the wireless method reproduced with high fidelity, and even increased the frequency bandwidth of the TcMEP signals, compared to wired systems. This wireless system will reduce the preoperative time required for IONM setup, add convenience for surgical staff, and reduce wire-related risks for patients during the operation.

Wireless implants for in vivo diagnosis and closed-loop treatment

In this presentation, we will review and discuss recent advances in the research of wireless telemetry for medical applications in our group at UT-Arlington, particularly those based on a similar platform for in vivo monitoring of physiological parameters. System for recording ECoG signals in brain, recording in vivo gastric myoelectric activities in stomach, sensing in vivo strain variations in bladder, and detecting reflux episodes in esophagus have been demonstrated. These systems consist of passive transducers for physiological signal transduction and an active transceiver for signal relay and recording. The real-time in vivo physiological signal acquisition and related neuro-/gastro-stimulation form a closed loop between the human body and control electronics. Continuous feedback mechanisms that could be implemented in the closed loop provide treatment strategies for optimization to reach a desired comfort level for individual patient. The wireless systems quantitatively document symptoms and associated physiological signals over a long term while allowing the patients to resume regular daily activities. This will enable more precise diagnosis and prognosis of diseases for the caregivers.