John Naber

Comparison of electrically evoked cortical potential thresholds generated with subretinal or suprachoroidal placement of a microelectrode array in the rabbit

The aim of the study was to directly compare the threshold electrical charge density of the retina (retinal threshold) in rabbits for the generation of electrical evoked potentials (EEP) by delivering electrical stimulation with a custom-made microelectrode array (MEA) implanted into either the subretinal or suprachoroidal space. Nine eyes of seven Dutch-belted rabbits were studied. The electroretinogram (ERG), visual evoked potentials (VEP) and EEP were recorded. Electrodes for the VEP and EEP were placed on the dura mater overlying the visual cortex. The EEP was recorded following electrical stimulation of the MEA placed either subretinally beneath the visual streak of the retina or in the suprachoroidal space in the rabbit eye. An ab externo approach was used for placement of the MEA. Liquid perfluorodecaline (PFCL; 0.4 ml) was placed within the vitreous cavity to flatten the neurosensory retina on the MEA after subretinal implantation. The retinal threshold for generation of an EEP was determined for each MEA placement by three consecutive measurements consisting of 100 computer-averaged recordings. Animals were sacrificed at the conclusion of the experiment and the eyes were enucleated for histological examination. The retinal threshold to generate an EEP was 9 +/- 7 nC (0.023 +/- 0.016 mC cm(-2)) within the subretinal space and 150 +/- 122 nC (0.375 +/- 0.306 mC cm(-2)) within the suprachoroidal space. Histology showed disruption of the outer retina with subretinal but not suprachoroidal placement. The retinal threshold to elicit an EEP is significantly lower with subretinal placement of the MEA compared to suprachoroidal placement (P < 0.05). The retinal threshold charge density with a subretinal MEA is well below the published charge limit of 1 mC cm(-2), which is the level below which chronic stimulation of the retina is considered necessary to avoid tissue damage (Shannon 1992 IEEE Trans. Biomed. Eng. 39 424-6).

Design and development of a MEMS capacitive bending strain sensor

The design, modeling, fabrication and testing of a MEMS-based capacitive bending strain sensor utilizing a comb drive is presented. This sensor is designed to be integrated with a telemetry system that will monitor changes in bending strain to assist with the diagnosis of spinal fusion. ABAQUS/CAE finite-element analysis (FEA) software was used to predict sensor actuation, capacitance output and avoid material failure. Highly doped boron silicon wafers with a low resistivity were fabricated into an interdigitated finger array employing deep reactive ion etching (DRIE) to create 150 ?m sidewalls with 25 ?m spacing between the adjacent fingers. The sensor was adhered to a steel beam and subjected to four-point bending to mechanically change the spacing between the interdigitated fingers as a function of strain. As expected, the capacitance output increased as an inverse function of the spacing between the interdigitated fingers. At the unstrained state, the capacitive output was 7.56 pF and increased inversely to 17.04 pF at 1571 ?? of bending strain. The FEA and analytical models were comparable with the largest differential of 0.65 pF or 6.33% occurring at 1000 ??. Advantages of this design are a dice-free process without the use of expensive silicon-on-insulator (SOI) wafers.

Control of battery electric vehicle charging for commercial time of day demand rate payers

The impact of battery electric vehicle (BEVs) charging on a commercial time of day rate customer is examined. Using the Nissan Leaf and the Chevrolet Volt, a case study of the University of Louisville is used to illustrate the economic and peak demand impacts of uncontrolled BEV charging. A BEV charge control algorithm based on 15-minute-ahead peak kVA forecasting is presented, along with three forecast methods. The algorithms are validated via simulation with the MATLAB software package. Results show uncontrolled charging can cause an increase in the monthly electric bill of up to 22% due to demand charges alone, even at low BEV penetration rates of 10%.

A Power Monitoring and Control System to minimize electricity demand costs associated with Electric Vehicle charging stations

The impact of Battery Electric Vehicle (BEV) charging on the electrical demand of a commercial / industrial energy consumer is examined. A Power Monitoring and Control System (PMCS) is introduced as a method to limit the disincentive to large scale BEV adoption resulting from increased demand costs associated with uncontrolled BEV charging. Reductions in added costs of up to 90% are possible with the incorporation of an intelligent charge control system. Additionally, the PMCS enables transformative charge sharing topologies, including Vehicle to Grid (V2G) and Vehicle to Vehicle (V2V), as solutions for peak shifting and demand response techniques during high demand periods.

Implantable wireless microsystems for the measurement of intraocular pressure

Wireless microelectronic systems designed for implantation within the eye are being developed to enable non-contact, high-accuracy and continuous recording of intraocular pressure (IOP). These microsystems will revolutionize the treatment and understanding of glaucoma by increasing the frequency and accuracy of IOP measurements. Past development of such systems has been focused on Radio Frequency (RF) based, transistor-less, resonant LC-tank systems, utilizing capacitive pressure sensors. However, complex external-wireless interfaces have limited implementation. To address this, RFID-based passive radio architectures incorporating DAQ systems and MEMS pressure sensors have been developed. One such system, using a load-modulated bi-directional RFID reader combined with a custom RFID transponder composed of a MEMS pressure sensor, FRAM memory chip, and mixed-signal CMOS, has been demonstrated, but is limited to 2.5 cm read range. A new 915 MHz system under development extends operating range closer to 100 cm. Power is derived from an energy harvesting system containing a RF-to-DC converter and a voltage multiplier circuit connected to a voltage regulator. An oscillator tied to the pressure sensor performs pressure sensing in real time. The electrically small antenna fits into an area less than 10mm by 10mm. (Wireless implant, intraocular pressure sensor, glaucoma)

Design, modeling, fabrication and testing of a MEMS capacitive bending strain sensor

Presented herein are the design, modelling, fabrication and testing of a MEMSbased capacitive bending strain sensor utilizing a comb drive. This sensor is designed to be integrated with a telemetry system that will monitor changes in bending strain to assist orthopaedic surgeons with the diagnosis of spinal fusion. ABAQUS/CAE version 6.5 finite element analysis (FEA) modelling software was used to predict sensor actuation, capacitance output and the avoidance of material failure. Highly doped boron silicon wafers with a low resistivity were fabricated into an interdigitated finger array employing deep reactive ion etching (DRIE) to create 150 µm sidewalls with 25 µm spacing between the adjacent fingers. For testing, the sensor was adhered to a steel beam, which was subjected to four-point bending. This mechanically changed the spacing between the interdigitated fingers as a function of strain. As expected, the capacitance output increased as an inverse function of the spacing between the interdigitated fingers, beginning with an initial capacitance of 7.56 pF at the unstrained state and increasing inversely to 17.04 pF at 1571 µe of bending strain. The FEA and analytical models were comparable with experimental data. The largest differential of 0.65 pF or 6.33% occurred at 1000 µe.

The design of a bi-directional, RFID-based ASIC for interfacing with SPI bus peripherals

The design of a mixed-signal Integrated Circuit (IC) for data logging and wireless telemetry applications is presented. The 3.3mm × 3.3mm IC is integrated into a RFID transponder along with a MEMS pressure sensor and FRAM memory. The sensor and memory ICs are independently addressable via the serial peripheral interface (SPI) standard. The IC was designed for fabrication using a 0.5-µm CMOS process and implements a RF front-end, SPI Master and digital Finite State Machine (FSM). The wireless front-end operates via a 13.56MHz inductive coupled system for power transfer and bi-directional data transmission to a reader and has on-chip voltage-regulation, data recovery, and load modulation circuits. The digital logic implements a custom developed wireless SPI protocol, allowing for direct communication with SPI peripherals via the RF front-end. Additionally, a digital FSM is responsible for data logging operation, in which pressure data is autonomously measured and stored in memory over constant intervals. Test results are presented for an independently fabricated digital circuit consisting of 4,014 equivalent 2-input NOR gates with a measured current consumption of 12µA at Vdd=2.8V and FCLK=140kHz.

Analysis of forecasting algorithms for minimization of electric demand costs for electric vehicle charging in commercial and industrial environments

The large scale deployment of Electric Vehicle (EV) charging infrastructure can result in high added utility costs due to the peak demand cost structure in utility bills for commercial and industrial users. A method to minimize this disincentive to EV adoption is proposed that relies on forecasting demand so that EV charging activity can be intelligently controlled. This study examines multiple forecasting models and techniques to determine the optimal algorithm for use in the proposed control system. Simulation results are presented for each of the forecasting algorithms with the best mean absolute percent error of 1.26% using a neural network with averaging. This results in a reduction in the peak demand electric costs of approximately 95%.

The Development of a RFID Based Mixed Signal ASIC for the Wireless Measurement of Intraocular Pressure

The initial development of a wireless transponder for an IOP monitor capable of performing uninterrupted measurement for 24 hours a day is presented. This work eliminates the need for a permanent power and data link between an external reader and the ophthalmic implant, thereby greatly reducing the required patient interaction. The transponder consists of a custom developed Application Specific Integrated Circuit (ASIC), pressure sensor, FRAM memory, and a super capacitor based power module. The ASIC was fabricated using a 0.5-µm CMOS process and implements a RFID-based inductively coupled front-end operating at 13.56 MHz. The data logging circuit is designed to sample pressure once every 10 minutes. Power consumption of the transponder operating at VDD=2.8 V and FCLK=140 kHz is estimated at 30 µA during data logging read/write phases and 12 µA for timing operations.

A High Gauge Factor Capacitive Strain Sensor and its Telemetry Application in Biomechanics

A highly sensitive strain sensing system has been developed using a capacitive MEMS bending strain sensor for telemetry application in biomechanics such as spinal fusion monitoring. This telemetry sensor system is capable of detection with a linear gauge factor as high as 249 in frequency domain. The task is accomplished by converting the capacitive strain to frequency using a low power capacitance-frequency converter circuit that modulates the 125 kHz magnetic carrier source from the interrogating reader. The reader demodulates the 125 kHz signal and recovers the strain information from the sensor. Experimentally, various situation tests were performed with loads on a material test system (MTS) machine up to 1000 micro- strains to simulate corpectomy model on a stainless rod. Strain measurements were proved reliable within 10 cm range.