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Biomedical subjects

M A Arnold

Publications and source records attributed to M A Arnold.

At least 19 recordsLinked to original sources
PubMed1992

Fiber-optic ammonia sensor for measuring synaptic glutamate and extracellular ammonia.

A fiber-optic ammonia gas sensor designed for neurochemical applications is presented. Parameters evaluated in terms of effect on the steady-state and dynamic response of this sensor include the indicator dye, concentrations of indicator and total ammonia nitrogen in the internal solution, volume of the internal solution, structure of the gas-permeable membrane, and temperature. The final ammonia sensor responds over the concentration range from 7 to 3000 nM with a limit of detection of 7 nM and response times ranging from 2 to 5 min. Glutamate oxidase is immobilized at the tip of this ammonia sensor to provide a glutamate biosensor with a detection limit of 0.1 microM when operated at pH 7.8. In addition, this ammonia sensor is used to measure extracellular ammonia levels in perfused retinal and eye-cup tissue preparations. These measurements indicate a calcium-dependent, potassium-evoked release of ammonia during these depolarization conditions.

Ammonia↗
PubMed1992

Dual-enzyme fiber-optic biosensor for glutamate based on reduced nicotinamide adenine dinucleotide luminescence.

Response characteristics are presented for a dual-enzyme fiber-optic biosensor for glutamate. An enzyme layer composed of glutamate dehydrogenase (GDH) and glutamate-pyruvate transaminase (GPT) is used to produce reduced nicotinamide adenine dinucleotide (NADH) at the tip of a fiber-optic probe. NADH luminescence is monitored through this probe and the measured fluorescence intensity is related to the concentration of glutamate. GDH catalyzes the formation of NADH, and GPT drives the GDH reaction by removing a reaction product and regenerating glutamate. Optimal response is obtained in a pH 7.4 Tris-HCl buffer maintained at 25 degrees C in the presence of 4 mM NAD+ and 10 mM L-alanine. The temperature profile reveals a strong negative temperature effect which is attributed to the temperature dependency of NADH luminescence. Under optimal conditions, the sensor sensitivity is 0.127 nA/microM over the 1-10 microM concentration range, the detection limit is 0.13 microM, and response times range from 4 to 8 min. The sensor response is stable for 12 days when stored at 4 degrees C. Selectivity for glutamate is excellent over most of the common amino acids as well as ascorbic acid, uric acid, taurine, and GABA. Only slight responses were observed for glutamine and lysine. The effect of ammonia on the glutamate response was found to be minimal at total ammonia nitrogen concentrations as high as 200 microM.

Biosensing Techniques↗
PubMed1992

Measurement of water sorption by resin composite adhesives with near-infrared spectroscopy.

Spectroscopic methodology was used to follow the water uptake of disks of two resin composite luting agents during long-term storage. Fourier-transform near-infrared (FT-NIR) spectroscopy was used for collection of spectra over the 4200-6500 cm-1 spectral region. The 5200-cm-1 absorption band of water was monitored initially, at 24 h, seven days, two weeks, and then monthly over a period of 12 months. The disks were stored in water, air, and desiccated conditions. Disks were also subjected to two alternate cycles (two weeks each) of hydration and desiccation. A spectral manipulation program was used for quantitation of the area under the water absorbance band. Area ratios demonstrated similar water-uptake patterns for the two adhesives. Rapid water uptake was seen within the first two weeks of storage. Alternate hydration and desiccation of the samples showed that water uptake by the filler/polymer network was partially reversible. The desiccant was unable to remove all the water from the samples, which indicated that some water was tightly held within the sample matrix. FT-NIR spectroscopy is recommended as an effective method for study of the equilibration of resin composites in water.

Absorption↗
PubMed1991

Selectivity of membrane electrodes based on derivatives of dibenzyltin dichloride.

Selectivity properties are established for membrane electrodes prepared by incorporating bis(p-methylbenzyl)tin dichloride, dibenzyltin dichloride, and bis(p-chlorobenzyl)tin dichloride in plasticized polymer membranes. These electrodes display an unusually high level of selectivity for dibasic phosphate over many common anions. Electrodes prepared with the p-chloro derivative possess the best detection limit and the highest degree of selectivity for phosphate. Selectivity coefficients are calculated for phosphate relative to the following groups of anions: salicylate, benzoate, thiocyanate, iodide, nitrate, bromide, chloride, acetate, fluoride, pyrophosphate, arsenate, adenosine 5'-cyclic monophosphate, adenosine 5'-monophosphate, adenosine 5'-diphosphate, and adenosine 5'-triphosphate. Tin-carbon hyperconjugation within the organotin compound is hypothesized to be critically important in the selective response to phosphate. Enhancement of tin-carbon hyperconjugation by increasing the electron-withdrawing power of substituents on the benzyl ring is predicted to provide even higher levels of selectivity for phosphate.

Benzyl Compounds↗
PubMed1991

Paraterminal ligaments of the distal phalanx.

The paraterminal ligaments of the distal phalanges have been studied by dissection. They are a normal feature of all distal phalanges in both the hand and foot, and connect the paraterminal spines and paraterminal tubercles of the distal phalanx on both sides. Branches of the proper palmar digital artery and nerve pass under the ligament to reach the matrix of the nail, which they supply.

Arteries↗
PubMed1990

Determination of physiological levels of glucose in an aqueous matrix with digitally filtered Fourier transform near-infrared spectra.

A procedure is described for the measurement of clinically relevant concentrations of glucose in aqueous solutions with near-infrared (NIR) absorbance spectroscopy. A glucose band centered at 4400 cm-1 is used for this analysis. NIR spectra are collected over the frequency range 5000-4000 cm-1 with a Fourier transform spectrometer. A narrow-band-pass optical interference filter is placed in the optical path of the spectrometer to eliminate light outside this restricted range. This configuration provides a 2.9-fold reduction in spectral noise by utilizing the dynamic range of the detector solely for light transmitted through the filter. In addition, a novel spectral processing scheme is described for extracting glucose concentration information from the resulting absorbance spectra. The key component of this scheme is a digital Fourier filter that removes both high-frequency noise and low-frequency base-line variations from the spectra. Numerical optimization procedures are used to identify the best location and width of a Gaussian-shaped frequency response function for this Fourier filter. A dynamic area calculation, coupled with a simple linear base-line correction, provides an integrated area from the processed spectra that is linearly related to glucose concentrations over the range 1-20 mM. The linear calibration model accurately predicted glucose levels in a series of test solutions with an overall mean percent error of 2.5%. Based on the uncertainty in the parameters defining the calibration model and the variability of the magnitudes of the integrated areas, an overall uncertainty of 7.8% was estimated for predicted glucose concentrations.

Calibration↗
PubMed1990

The relationship between ventricular fluid pressure and body position in normal subjects and subjects with shunts: a telemetric study.

Using a chronically implanted telemetric pressure sensor, we have determined the quantitative relationship between changes in body position and ventricular fluid pressure in normal subjects and subjects with shunts. The method allows accurate, reliable measurement of negative as well as positive pressures. We describe the derangement of postural intraventricular pressure regulation caused by placement of a shunt, as well as the influence of various shunt systems and the antisiphon device on this problem. Ventriculoatrial, ventriculoperitoneal, and ventriculopleural shunts all caused similar severely abnormal postural intracranial pressure relationships. The antisiphon device was generally effective in restoring normal pressures in patients in the upright position. We discuss the implications of our findings for understanding the mechanisms of postural intracranial pressure regulation in patients without hydrocephalus.

Adolescent↗
PubMed1988

Feasibility of continuous glutamate monitoring in perfused retinal tissue with a potentiometric biosensing probe.

A potentiometric biosensing probe for glutamate has been evaluated as a possible tool to measure the release of glutamate from the isolated retina of Bufo marinus. This probe is based on carbon dioxide detection, following enzymatic conversion of glutamate to gamma-aminobutyric acid (GABA) via glutamic acid decarboxylase (GAD). Probe response characteristics of dynamic range, limit of detection, pH dependency, and selectivity are described. Probe modifications were required for sensor operation in an upside down configuration which was demanded by the need to mount and perfuse the retinal tissue directly at the sensor tip. Overall, these results indicate that this particular potentiometric biosensor is not well suited for direct glutamate measurements in retinal tissue because of pH incompatibility between the sensor and the tissue, and because of high background carbon dioxide levels released from the retina at the pH optimum of the probe. Despite this drawback, the sensor could be utilized to provide a continuous "downstream" monitor of glutamate levels during the course of an experiment, after pH buffering of tissue perfusate. Alternative approaches to probes more compatible with direct tissue measurements are discussed.

Animals↗
PubMed1979

A telemetric pressure sensor for ventricular shunt systems.

A fully implantable telemetric differential pressure sensor has been developed which is incorporated with a shunt valve system. It is a unique single diaphragm device that allows in vivo confirmation of zero-point and pressure calibrations. The telemetry is implemented by a resonant circuit in the sensor and a radio frequency detector outside the body, and the in vivo pressure calibration is derived from a pressure-balancing method. The pressure sensor has been implanted with shunts in several patients, and is shown to be valuable both for measurements of intraventricular pressure and for determining proper function of hydrocephalus shunt valves and catheters.

Catheterization↗