Urea Sensor Research Articles

Dialysate-based kinetic modeling.

The focus of this review article is urea kinetic modeling based on the exploitation of concentration measurements in the spent dialysate stream. After a review of blood-based urea kinetic modeling, dialysate-based techniques are considered, beginning with dialysate collection techniques and their associated urea kinetic modeling equations. Partial dialysate collection methods and equations for the determination of protein catabolic rate based on a 7-day mass balance period are explored next. This is followed by a description of urea sensors and their application for dialysate-based modeling including the determination of protein catabolic rate, predialysis blood urea nitrogen (BUN), and KT/V. How the output of a urea sensor may allow the detection of significant changes in patient clearance during the course of dialysis is illustrated, as well as how double-pool urea kinetics may be accounted for in KT/V determination. Routine determination of patient lean body mass using creatinine kinetic modeling based on partial dialysate collection or a dialysate-based creatinine concentration sensor is demonstrated. Finally, the potential for complete automation of urea kinetic modeling in dialysis machines of the future is explored.

Comparison of potentiometric enzyme sensors for urea and penicillin-G: differential thick-film sensors versus classical electrodes

A comparative study of urease and penicillinase biosensors was performed in which different transducers were compared, i.e. pH glass electrodes, macroscopic ion-selective electrodes and thick-film transducers. For enzyme immobilization onto the transducers aqueous dispersion polymers were used in all cases in this work. Since these sensors are based on potentiometric measurements the lack of a classical reference electrode when solid state transducers are employed is a crucial feature. Results obtained in this work revealed that thick-film sensors without a classical reference electrode can compensate this deficit when they operate in a differential mode. This means that it is not the absolute potential change of a single selective electrode, induced by the analyte concentration, that is measured but rather the relative change with reference to a non-selective electrode. Enzyme sensors with macroscopic transducers and classical reference electrodes showed the same basic characteristics compared with thick-film sensors. The long term stability of the sensors is sufficient; urease and penicillinase thick-film sensors showed 50 and 75% of the initial response after two weeks of permanent use at room temperature.

尿素センサによる人工腎臓のモニタリング

尿素センサによる人工腎臓のモニタリング

Potentiometric biosensor in double matrix membrane technology

A throw-away urea sensor is described. A pH-sensitive disposable sensor constructed using double matrix membrane technology was used as a transducer. Double matrix membrane technology describes a process where an ion-sensitive polymer matrix membrane is formed in the presence of an additional inert matrix for to improve the reproducibility of sensor production. In this case one-side silver-coated filter paper was used as the inert matrix which was encapsulated by a heat sealing film. A pH-sensitive membrane was casted into this filter paper and on the surface of this membrane a urease containing layer was deposited. The sensor responded to between 10 −2 mol/l and 10 −4 mol/l urea, the response slope was 32·7 mV/decade. The concentration range in which the sensor responds is important for clinical analysis. The main advantages of this pH-based urea electrode include miniature size as well as simple and low cost fabrication in a batch process.

Urea sensors based on PVC membrane pH electrode

Several procedures of urease immobilization on the surface of the polymeric membrane pH electrode with tri- n-dodecylamine as a neutral carrier were compared. The best results were obtained for the urea sensor with covalently bound urease. The sensor characteristics including the effect of buffer, pH and concentration and the effect of stirring rate are presented. These effects are in good agreement with theoretical expectations.

Performance of a planar differential-conductivity sensor for urea

A detailed study of the performance of a planar thin-film differential-conductivity sensor for urea is described. The basic transducer consists of two identical interdigitated conductivity cells forming a differential pair. Special attention has been paid to the influence of the design parameters on the characteristics of the conductivity cells. The performance of the sensor is characterized by its differential gain, which is proportional to the cell constant of the individual cells. A common-mode rejection ratio (CMRR) is defined to quantify the capability of the differential sensor to suppress the influence of background conductivity changes on the output signal. This common-mode rejection reflects the selectivity of the transducer. CMRR values of over 40 dB can easily be obtained, comparable with a selectivity coefficient of 100 and more. Baseline stability is shown to be closely related to the temperature behaviour of the cells. In combination with the urease-containing protein membrane, a urea-specific sensor is obtained showing a linear response between 0.25 and 10 mM, with a typical sensitivity of 34 mV mM -1 of urea.

Development of an optical fibre sensor for ammonia, urea, urease and IgG

An optical fibre sensor utilizing Brilliant Yellow in a thin cellulose acetate membrane as a pH sensor has been developed. By use of gas permeable membrane, urease-immobilized membrane and IgG immobilized membrane we obtained optical sensors for ammonia, urea, urease, IgG suitable for physiological and clinical applications. The system makes use of commercially available instrumentation for absorbance measurement and flow cell for fast analysis.

Urea biosensor based on amperometric ammonium ion electrode

AbstractAn amperometric urea biosensor is described: urease is immobilized on the amperometric ammonium ion electrode surface, and the ammonium ion produced by hydrolytic decomposition of urea by urease is determined by the pulse amperometric technique. Theoretical considerations on the steady‐state concentration distribution of the substrate (urea) and the product (sum of ammonia and ammonium ion) in the immobilized‐enzyme layer show that the concentration of the product at the ion‐selective electrode surface is proportional to the concentration of the substrate at the solution side surface of the immobilized‐enzyme layer up to a certain limiting concentration. The limiting concentration depends on the enzyme‐loading factor (Thiele modulus) as well as the Michaelis constant of the enzyme reaction. A practical urea sensor was constructed by immobilizing urease on an amperometric ammonia gas sensor. Preliminary experimental results and discussion are given. With the amperometric urea sensor, the correction for the residual ammonium ion in test solution can be easily achieved with an auxiliary, amperometric urease‐removed urea sensor.

ISFET-based urea biosensor

A new construction method of a PVC-matrix membrane ISFET-based urea sensor is described and compared with a pH ISFET-based urea sensor which uses a classical immobilization procedure on a nylon net. The response slope obtained with data from the pH ISFET-urea sensor is 57 mV/decade over a linear working range of 10 −4−3 × 10 −3 M urea with a lifetime over 20 days. This sensor, applied in a flow-injection system, presents a sensitivity of 60 mV/decade and shows an acceptable sampling rate of 35 samples/h. Although its lifetime is shorter under dynamic conditions the membrane is easily changeable in the flow system. The new sensor reported is based on an ammonium ISFET with the PVC-matrix membrane cast on a graphite/epoxy composite. The enzyme is deposited and covalently fixed onto the PVC membrane by means of a fine spraying of glutaraldehyde. This sensor shows a sensitivity of 42 mV/decade over a linear working range between 4 × 10 −5 and 10 −2 M urea and a long-term stability of one week.

Urea Kinetic Modeling with a Prototype Urea Sensor in the Spent Dialysate Stream

The authors have previously demonstrated the feasibility and accuracy of urea kinetic modeling (UKM) based on monitoring urea concentration in the spent dialysate stream (SDS) throughout the hemodialysis (HD) session. They describe here a prototype urea sensor for this purpose and initial experience with HD patients. The sensor is based on ammonium ion and reference electrodes housed in a cell through which the entire SDS passes. The two electrode tips are bathed in urease solution on one side of a dialysis membrane; the SDS flows along the adjacent side. Urea diffusing across the membrane from the SDS is converted by the urease into ammonium ion, which is measured by the electrode pair. For evaluation, the prototype flowthrough urea sensor was installed in the SDS of a Cobe Centry 3 HD machine for 36 HD sessions. Independent measurement demonstrated a linear relationship between mv output of the sensor and logarithm of SDS urea concentration. The use of SDS urea concentration time profiles obtained with this sensor to obtain accurate values of patient protein catabolic rate (PCR) and KT/V is illustrated. Incorporation of urea sensors such as this prototype into HD machines, will permit complete automation of UKM in the near future.

Urea Kinetic Modeling with a Prototype Urea Sensor in the Spent Dialysate Stream

Urea Kinetic Modeling with a Prototype Urea Sensor in the Spent Dialysate Stream

A Disposable Urea Sensor for Continuous Monitoring of Hemodialysis Efficiency

A Disposable Urea Sensor for Continuous Monitoring of Hemodialysis Efficiency

A Disposable Urea Sensor for Continuous Monitoring of Hemodialysis Efficiency

A new sensor for the on-line measurement of urea in the dialysate output is described. The sensor is based on a differential measurement of conductivity changes induced by the urease-catalyzed hydrolysis of urea. The use of screen printing for the batch-fabrication of the basic transducers results in cheap disposable devices. In addition, the sensor has been designed to fit into a standard male luer-adapter, and can be plugged directly into the dialysate line. The in vitro response is linear to urea concentrations exceeding 6620mM. A resolution of 20020 microM has been achieved with a baseline stability of 50 microM/hr. Interferences caused by fluctuations in the ionic strength and the consequent conductivity changes are effectively suppressed by the differential sensor pair. The efficiency of this suppression is expressed in a common mode rejection ratio of typically 40 to 50. Preliminary ex vivo results show the feasibility of the concept. The sensor principle is not restricted to urea but can be extended to other molecules of interest for hemodialysis monitoring, such as creatinine and L- and D-amino acids.

Amperometric ammonium ion sensor and its application to biosensors

A urea biosensor based on the amperometric determination of ammonia produced by hydrolytic decomposition of urea by urease is described. The urease is immobilized on the amperometric ammonium ion-selective electrode sensor. This urea sensor gives the current response proportional to the concentration of urea in the range of a few to 100 ΩM in test solutions. The response can be corrected for the residual ammonium ions. The urea sensor was successfully applied to the analysis of biological fluids. An amperometric creatinine biosensor is also described.

The application of optodes in FIA-based fermentation process control using the software package FIACRE

Optical biosensors for the determination of substrates like ethanol and urea that can be principally used in fermentation process control are described. The ethanol sensor measures the increase of NADH fluorescence during the enzymatic conversion of ethanol using an enyzme system of ADH and AlDH. The urea sensor is based on a pH sensor measuring the change in fluorescnece of the pH indicator fluorescein. Both sensors are suitable for controlling fermentation processes when ethanol and urea are produced or consumed. For this purpose the software package FIACRE has been developed to run three different FIA processes simultaneously by the use of optical biosensors; it is applied to a model fermentation with Enterobacteriaceae Proteus vulgaris controlled by an urea optode. A decrease of urea during two fermentations over 9 and 24 hours, respectively, is shown.

Urea sensor with single poly(propylene) membrane

Urease was immobilized on the plasma-aminated surface of a hyfrophobic poly(propylene) (PP) membrane. This membrane, with urease matrix on one side while maintaining its original hydrophobic property on the other, was used to construct the urea sensor. The new urea sensors had response sensitivities ranged from 19 mV/decade to 30 mV/decade depending on the conditions of the plasma reaction. The enzyme electrode using single membrane gave a shorter response time as compared to the corresponding conventional electrode employing two separate PP membranes. The sensitivity of the enzyme electrode increased with increasing buffer pH and reached a maximal level (40 mV/decade) at pH 7.6. The response sensitivity of the electrode was not affected by the change of buffer strength. Deamination of the plasma-modified hydrophobic PP membrane did not occur in aqueous environment judging from the stability of the urea electrode up to 12 days of operation.

KT/V and protein catabolic rate determination from serial urea measurement in the dialysate effluent stream.

A bloodless technique of evaluating protein catabolic rate (PCR) and KT/V (K, clearance; T, dialysis time; V, urea distribution volume) in hemodialysis patients is presented based on serial measurement of urea in the dialysate effluent stream. PCR follows from equating urea generation and urea removal over a 7 day cycle, changes in body stores being comparatively negligible: PCR = 0.026 [U1 + U2 + U3]/BWdry + 0.17, where U1 is the amount of urea in mmol appearing in the dialysate for each session in the 7 day period. KT/V is obtained from the slope of the natural logarithm of spent dialysate urea concentration-time plot: KT/V = [- slope.T + 3.delta BW/BWdry]/[1 - 0.01786.T(hr], where delta BW = amount ultrafiltered in liters. The dialysate-based approach was validated and compared with conventional urea kinetic modeling (UKM) for 17 patients studied for three consecutive dialyses. The dialysate-based and UKM values of PCR agreed well when in vivo clearance values based on total dialysate collection were used for UKM. KT/V values agreed poorly on a session-by-session basis but were nearly equivalent when averaged for the three dialyses of the week. These findings lay the foundation for UKM automation with a urea sensor in the effluent dialysate stream.

Quantitative characterization of enzymes adsorbed on to Langmuir-Blodgett films and the application to a urea sensor

The amounts of enzymes, urease and glucose oxidase (GOD), adsorbed to stearylamine and cetylamine monolayers at the air-water interface were measured. We made a urea sensor using an ion sensitive field effect transistor (ISFET) as a transducer on which urease was immobilized by the LB technique. It is shown that the characteristics of the urea sensor can be improved by using the LB technique.

Amino acids and urea sensors using parsley seeds as biocatalyst

Minced parsley seeds coupled with a potentiometric ammonia gas sensing electrode showed effective biocatalytic activity, i.e. responded to l-asparagine, l-glutamine, l-serine and urea with a nernstian slope of 43–51 mV per decade of substrate concentration. Very good time stability with no significant loss of biocatalytic activity over 30 days was maintained. A celery seed immobilized ammonia electrode showed similar behaviour.

Amino acids and urea sensors using parsley seeds as biocatalyst

Minced parsley seeds coupled with a potentiometric ammonia gas sensing electrode showed effective biocatalytic activity, i.e. responded to l-asparagine, l-glutamine, l-serine and urea with a nernstian slope of 43–51 mV per decade of substrate concentration. Very good time stability with no significant loss of biocatalytic activity over 30 days was maintained. A celery seed immobilized ammonia electrode showed similar behaviour.