Sidney K. Wolfson

Zeolitic Ammonium Ion Exchange for Portable Hemodialysis Dialysate Regeneration

&NA; Ammonia removal from a recirculating dialysate stream is a major challenge in developing a truly portable, regenerable hemodialysis system. Three zeolites, type F, type W, and clinoptilolite, were found to have good ammonia ion exchange capacity with linear equilibrium ion exchange coefficients of 0.908, 0.488, and 0.075 L/g, respectively. The linear equilibrium ion exchange coefficient relates dialysate ammonia concentration (μmol/L) to the amount of ammonia absorbed by zeolite (μmol/g) at equilibrium. Ammonia uptake by zeolite powders was fast, with equilibrium reached within 15 sec. Zeolite ammonia ion exchange and regeneration through multiple cycles was studied using an ion exchange column containing clinoptilolite pellets. Zeolite ion exchange capability was regenerated by flushing the column with 2 mol/L sodium chloride after an ion exchange run. The column maintained ammonia ion exchange capacity through six ion exchange/regeneration cycles, demonstrating multiple dialysis use possibilities. Atomic absorption spectroscopy of the column effluent showed no detectible (<1 part per million) Si or Al leached from the zeolite. ASAIO Journal 1995; 41:221‐226.

A micro carbon electrode for nitric oxide monitoring.

A nitric oxide (NO) probe, consisting of a micro carbon fiber working electrode, 10 microns diameter, a platinum counter electrode, and a silver/silver chloride (Ag/AgCl) reference electrode, has been developed. The carbon fiber working electrode is covered with a Nafion cation exchange membrane. Using differential pulse voltammetry (DPV), we found the NO to N2O reduction current peak at approximately -1.35 V versus Ag/AgCl. This has been reported by others. The DPV current outputs are linearly related to dissolved NO concentrations [NO] in the 2-10 microM range. Catecholamines were found not to interfere with the reduction signal. The Nafion membrane also prevents interference by NO2-, NO3-, and amino acids at normal physiologic pH (pH 7.4). The effects of O2 are accounted for through sampling and subtracting background currents from the peak current. To increase sensitivity and shorten response time, a method of integrated pulse amperometry (IPA) was used for the study. The IPA charge outputs (delta C) are linear to the dissolved [NO] in the 50-350 nM range. The carbon fiber electrode has the potential of being miniaturized to a smaller electrode, allowing detection of NO released from the subendothelial space.

Platinized-titanium electrodes for urea oxidation Part II. Concentric spiral coil geometry

Abstract Highly active urea electro-oxidation catalysts were prepared by electrodeposition of Pt onto Ti in a novel, concentric spiral coil geometry. The concentric spiral coil geometry can be used directly in construction of electrochemical reactors. The Pt deposition from chloroplatinic acid required about 1.5 times the stoichiometric charge. Projection of urea conversion activity from cyclic voltammetry measurements to a clinical-scale, portable hemodialysis system indicates that approximately 10 g of Pt will be required for the clinical-scale system. The specific urea conversion activity, as measured by cyclic voltammetry, is linearly related to the specific Pt surface area of the deposition. The depositional growth morphology is found to be self-similar, with a fractal dimension that lies somewhere between uniform deposition and hemispherical growth.

A nitric oxide sensor using reduction current.

Nitric oxide (NO) has a wide range of biologic activity. Methods commonly used for the detection of biologically derived NO are indirect and measure only the amount of NO released during an interval of time. An electrochemical method available is capable of being direct and continuous but is subject to interference. The recent explosion of scientific research into NO activity requires better methods of NO detection. This article reports a new NO electrochemical sensing method and sensor design. The tip of the sensor is covered with a hydrophobic membrane and contains an internal electrolyte. Platinum is used for the working and counter electrodes and silver/silver bromide (Ag/AgBr) for the reference electrode. The components of the internal electrolyte are potassium bromide and sulfuric acid. The NO that diffuses to the working electrode is first oxidized to NO+; the NO+ is reduced to NO; and the reduction current is determined. An integrated pulsed amperometric method is used to achieve the redox of NO and the measurement and integration of the reduction current. The results show that the NO sensor is sensitive and has a rapid response and less interference.

Platinized-titanium electrodes for urea oxidation Part I. Demonstration of efficacy

Abstract Electrodes that are highly electrocatalytically active for urea oxidation were made by platinum electrodeposition onto minimally pretreated titanium surfaces. The Pt deposition from chloroplatinic acid required approximately sixteen times the stoichiometric charge requirement. Selected electrodes were evaluated for urea oxidation activity by a cyclic voltammetry method. A maximum in Pt utilization for urea oxidation was observed at a Pt loading of ∼ 1g/100 g Ti. The platinized Ti surfaces were as much as 1900 times more active for urea oxidation than smooth Pt surfaces on a per unit mass of Pt basis. Extrapolation of these results to clinical scale dialysate regeneration systems indicates that about 5 g Pt will be required for a clinical-scale deureation reactor.

Reactor control and reaction kinetics for electrochemical urea oxidation

Abstract Urea is electrolytically oxidized on anodic platinum surfaces to ammonia and carbon dioxide at potentials in the range of 0.5 to 1.1 V relative to Ag/AgCl. The oxidation is a nonsteady-state phenomenon in which individual reactions follow Tafel kinetics and in which the urea reaction products remain on the surface leading to reversible electrode deactivation and concomitant increase in electrode potential at constant current. An optimal control method, voltage polarity relay, accomplishes simultaneous urea oxidation and electrode regeneration. A simple kinetic model is shown to adequately describe the observed phenomena.

Ammonia transport across hydrophobic membranes. Application to dialysate regeneration.

Removal of ammonia from a recirculating dialysate buffer in a portable hemodialysis application can be achieved by countercurrent, gas phase ammonia transfer across a hydrophobic membrane into an acid solution. Ammonia transfer fluxes as high as 0.076 ümol/sec/m2 have been achieved using a Sarns Turbo Membrane Oxygenator (Sarns-3M, Ann Arbor, MI) with a 1.9 m2 membrane surface area (0.145 ümol/sec actual rate). A simple physical model based upon ammonia desorption at the gas-dialysate buffer interface in a membrane pore, ammonia diffusion through the gas filled pore, and subsequent ammonia absorption at the gas/acid interface side of the pore quantitatively describes the experimental data. The ammonia transfer rate is most dependent upon dialysate buffer pH (higher pH promoting transfer rate) and ammonia concentration in the dialysate buffer (higher concentrations promoting transfer rate). A 500 fold improvement in transfer rate, however, will be required for clinical application.

Voltage polarity relay-optimal control of electrochemical urea oxidation (hemodialysis application)

Voltage polarity relay (VPR) is shown to optimize the urea oxidation rate and urea current utilization under constant current conditions in direct electrochemical urea oxidation. Direct electrochemical urea oxidation is characterized by reversible deactivation of the working electrode due to oxidation products remaining on the surface and the requirement that the working electrode potential remain below about 1.1 V relative to Ag/AgCl to prevent undesirable secondary electrochemical oxidations. The VPR method monitors the potential of the working electrode relative to a suitable reference and changes system polarity when the upper potential set limit is reached. Thus. what was the working electrode becomes the counter electrode and vice versa. Since urea oxidation products are desorbed from the counter electrode when its potential drops below about -0.6 V relative to Ag/AgCl, alternating electrode functions between working and counter provides cyclic electrode regeneration and continuous urea oxidation.<<ETX>>

Interference of glucose sensing by amino acids.

Interference by membrane permeable substances on non-specific electrodes is a major problem in glucose sensing. Alanine, lysine, phenylalanine, and cystine were chosen for study to gain insight into this problem. These compounds represent the classes of mono-amino aliphatic, di-amino aliphatic, aromatic, and sulfur containing amino acids, respectively. Cyclic voltammetry experiments were performed using a Pt electrode (1.77 mm2). The reductive current of glucose at −0.750 V versus Ag/AgCl was measure with increasing concentrations of interfering substances in Krebs-Ringer phosphate buffer (pH 7.4) at 37°C. Experimental results have shown that these amino acids have an inhibitory effect on the glucose signal. An important finding was that the interferences from phenylalanine and cystine were more pronounced than those of lysine and alanine. An initial drop in the glucose signal was seen at less than 2.0 mg/dl of alanine or lysine and at less than 0.5 mg/dl of phenylalanine or cystine. Additional increase in the concentrations of interfering substance did not cause further appreciable signal reduction. The results confirm that glucose sensing using a non-specific electrode is possible in fluids containing interfering substances such as amino acids.