Douglas J. Jackson

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).

Development of Ultra-Miniaturized Piezoresistive Pressure Sensors for Biomedical Applications

Ultra miniaturized 0.69-French piezoresistive pressure transducers are designed and fabricated for biomedical applications. Silicon on insulator (SOI) and deep reactive ion etching (DRIE) technologies are used for the fabrication of the pressure sensors. A combination of SOI and DRIE technologies eliminates the dicing step and results in uniform diaphragm thickness. The dimensions of the final fabricated sensor die are 650 mum times 230 mum times 150 mum (length, width, thickness) with 2.5 mum thick diaphragms. Sensitivity of the sensors with half Wheatstone bridge configuration is determined to be 27-31 muV/V/mmHg.

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.

Alternative fabrication methods for capillary electrophoretic device manufacturing

This work represents research that explores the development of novel manufacturing methods to create microcapillary electrophoretic (CE) devices. Nontraditional substrates that were investigated include polymers such as SU-8, poly dimethylsiloxane (PDMS), acetate, Riston, Kapton, polyimide, and polyester. Hot embossing, chemical etching, micro-molding, wafer level bonding, chemical treatment, and lamination techniques were developed for these substrates. The purpose of this paper is to explore the feasibility of micromachining a select group of alternative materials.

Analysis of frequency lock-in using integrated colpitts and multivibrator oscillators for RFID-based telemetry

This paper discusses the design and development of a multi-vibrator oscillator, the design and development of an integrated circuit (IC) Colpitts oscillator and performance comparison of the two oscillators regarding frequency lock-in behavior. Experiments were conducted to determine frequencies at which lock-in is most likely to occur and recommendations are made to prevent locking of oscillators that are used in RFID telemetry systems.

A frequency counter based analog-to-digital converter for a RFID telemetry system

This paper describes a method for determining a digital representation of a value for a remote sensing element using a novel and lower power method of analog to digital conversion. This conversion process is most effective for low-frequency, radio frequency identification (RFID) sensing systems where the sensing element tags are powered by an inductively coupled carrier signal of fixed frequency. The method uses a specially gated frequency counter to create a digital value that is related to the systems carrier frequency and the frequency of variable oscillator used with the sensing element. This conversion method is highly scalable while maintaining a low current consumption. One configuration yields four bits of resolution with a conversion time of 1.34 ms and a current consumption of 30 muA. The ASIC was fabricated on a 1.5 mum CMOS process from AMI Semiconductor.

Digital and mixed-signal integrated circuits for an RFID telemetry system

Digital and mixed-signal circuits have been developed for use in an inductively-coupled RFID telemetry system that can interface with a sensor. The cells developed include a turn-on/brown-out detector, clock recovery circuit, a cyclic-redundancy code (CRC) generator, and a frequency-shift keying (FSK) modulator. These cells were designed for use in an RFID tag that also uses a novel approach to performing analog-to-digital conversion. The circuits were fabricated using the AMI 1.5 um CMOS process and tested using LabVIEWtrade. A key feature of these cells is their low current consumption of only 1-2 uA for the CRC generator and 5 uA for the clock recovery circuit.

Fabrication of a Glass Capillary Electrophoresis Microchip With Integrated Electrodes

In this chapter, a detailed outline delineating the processing steps for microfabricating capillary electrophoresis (CE) with integrated electrochemical detection (ECD) platforms for performing analyte separation and detection is presented to enable persons familiar with microfabrication to enter a cleanroom and fabricate a fully functional Lab-on-a-Chip (LOC) microdevice. The processing steps outlined are appropriate for the production of LOC prototypes using easily obtained glass substrates and common microfabrication techniques. Microfabrication provides a major advantage over existing macro-scale systems by enabling precise control over electrode placement, and integration of all required CE and ECD electrodes directly onto a single substrate with a small footprint. In the processing sequences presented, top and bottom glass substrates are photolithographically patterned and etched using wet chemical processing techniques. The bottom substrate contains seven electrodes required for CE/ECD operation, whereas the top substrate contains the microchannel network. The flush planar electrodes are created using sputter deposition and lift-off processing techniques. Finally, the two glass substrates are thermally bonded to create the final LOC device.

Dual capillary electrophoresis devices with electrochemical detection on a single platform

The purpose of this paper is to demonstrate the feasibility of developing a single lab-on-a-chip (LOC) platform capable of performing dual, simultaneous separation and detection of multiple analytes. Computational modeling was performed to determine optimum device geometry and performance. The soda-lime glass-based device was fabricated using traditional microtechnology processes, including UV photolithography, buffered oxide etch (BOE), electrode deposition and compression thermal bonding. The device was characterized with a mixture of dopamine (2mM) and catechol (2mM) in a phosphate buffer (20mM, 6.5 pH). Modeling results yielded migration velocities of 0.6 mm/s and 0.42 mm/s for dopamine (electrokinetic (EK) mobility=60,000 /spl mu/m/sup 2//V/spl middot/s) and catechol (EK mobility=42,000 /spl mu/m/sup 2//V/spl middot/s), respectively. Experimental results obtained from microchips exhibiting the same EK mobilities demonstrated identical electropherograms in both detection channels with migration velocities of 0.58 mm/s for dopamine and 0.41 mm/s for catechol.

Smart brief monitoring system for assisted living

A system for monitoring the moisture level and temperature of an adult disposable brief has been designed and tested. The system uses an inexpensive (<;