Biomedical subjects

Wouter Olthuis

Publications and source records attributed to Wouter Olthuis.

10 recordsLinked to original sources

PubMed2007

pH sensitivity of Si--C linked organic monolayers on crystalline silicon surfaces.

The electrochemical behavior of Si--C linked organic monolayers is studied in electrolyte-insulator-Si devices, under conditions normally encountered in potentiometric biosensors, to gain fundamental knowledge on the behavior of such Si electrodes under practical conditions. This is done via titration experiments, Mott-Schottky data analysis, and data fitting using a site-binding model. The results are compared with those of native SiO(2) layers and native SiO(2) layers modified with hexamethyldisilazane. All samples display pH sensitivity. The number of Si--OH groups on the alkylated samples is calculated to be less than 0.7 % of that of a pure SiO(2) insulator, which still causes a pH sensitivity of approximately 25 mV per pH unit in the pH range: 4-7. The alkylated samples hardly suffer from response changes during up- and down-going titrations, which indicates that very little oxide is additionally formed during the measurements. The pK(a) values of all samples with monolayers (4.0-4.4) are lower than that of native SiO(2) (6.0). The long-term drift (of approximately 1 mV h(-1)) is moderate. The results indicate that biosensors composed of alkylated Si substrates are feasible if a cross-sensitivity towards pH in the sensor signal is taken into account.

Journal Article↗

PubMed2006

Assessment of a new prototype hydrogel CO( 2 ) sensor; comparison with air tonometry.

OBJECTIVE: Gastrointestinal ischemia is always accompanied by an increased luminal CO(2). Currently, air tonometry is used to measure luminal CO(2). To improve the response time a new sensor was developed, enabling continuous CO(2) measurement. It consists of a pH-sensitive hydrogel which swells and shrinks in response to luminal CO(2), which is measured by the pressure sensor. We evaluated the potential clinical value of the sensor during an in vitro and in vivo study. METHODS: The response time to immediate, and stepwise change in pCO(2) was determined between 5 and 15 kPa, as well as temperature sensitivity between 25 and 40 degrees C at two pCO(2) levels. Three sensors were compared to air tonometry (Tonocap) in healthy volunteers using a stepwise incremental exercise test, followed by a period of hyperventilation and an artificial CO(2)-peak. RESULTS: The in vitro response time to CO(2) increase and decrease was mean 5.9 and 6.6 min. The bias, precision and reproducibility were +5%, 3% and 2%, resp. Increase of 1 degrees C at constant pCO(2) decreased sensor signal by 8%. In vivo tests: The relation with the Tonocap was poor during the exercise test. The response time of the sensor was 3 min during hyperventilation and the CO(2) peak. CONCLUSION: The hydrogel carbon dioxide sensor enabled fast and accurate pCO(2) measurement in a controlled environment but is very temperature dependent. The current prototype hydrogel sensor is still too unstable for clinical use, and should therefore be improved.

Air↗

PubMed2005

Rapid inorganic ion analysis using quantitative microchip capillary electrophoresis.

Rapid quantitative microchip capillary electrophoresis (CE) for online monitoring of drinking water enabling inorganic ion separation in less than 15 s is presented. Comparing cationic and anionic standards at different concentrations the analysis of cationic species resulted in non-linear calibration curves. We interpret this effect as a variation in the volume of the injected sample plug caused by changes of the electroosmotic flow (EOF) due to the strong interaction of bivalent cations with the glass surface. This explanation is supported by the observation of severe peak tailing. Conducting microchip CE analysis in a glass microchannel, optimized conditions are received for the cationic species K+, Na+, Ca2+, Mg2+ using a background electrolyte consisting of 30 mmol/L histidine and 2-(N-morpholino)ethanesulfonic acid, containing 0.5 mmol/L potassium chloride to reduce surface interaction and 4 mmol/L tartaric acid as a complexing agent resulting in a pH-value of 5.8. Applying reversed EOF co-migration for the anionic species Cl-, SO42- and HCO3- optimized separation occurs in a background electrolyte consisting of 10 mmol/L 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) and 10 mmol/L HEPES sodium salt, containing 0.05 mmol/L CTAB (cetyltrimethylammonium bromide) resulting in a pH-value of 7.5. The detection limits are 20 micromol/L for the monovalent cationic and anionic species and 10 micromol/L for the divalent species. These values make the method very suitable for many applications including the analysis of abundant ions in tap water as demonstrated in this paper.

Electrophoresis, Microchip↗

PubMed2005

Si-C linked organic monolayers on crystalline silicon surfaces as alternative gate insulators.

Herein, the influence of silicon surface modification via Si-C(n)H(2n+1) (n=10,12,16,22) monolayer-based devices on p-type 100 and n-type 100 silicon is studied by forming MIS (metal-insulator-semiconductor) diodes using a mercury probe. From current density-voltage (J-V) and capacitance-voltage (C-V) measurements, the relevant parameters describing the electrical behavior of these diodes are derived, such as the diode ideality factor, the effective barrier height, the flatband voltage, the barrier height, the monolayer dielectric constant, the tunneling attenuation factor, and the fixed charge density (Nf). It is shown that the J-V behavior of our MIS structures could be precisely tuned via the monolayer thickness. The use of n-type silicon resulted in lower diode ideality factors as compared to p-type silicon. A similar flatband voltage, independent of monolayer thickness, was found, indicating similar properties for all silicon-monolayer interfaces. An exception was the C10-based monolayer device on p-type silicon. Furthermore, low values of N(f) were found for monolayers on p-type silicon (approximately 6 x 10(11) cm(-2)). These results suggest that Si--C linked monolayers on flat silicon may be a viable material for future electronic devices.

Journal Article↗

PubMed2005

Microchip analysis of lithium in blood using moving boundary electrophoresis and zone electrophoresis.

The determination of inorganic cations in blood plasma is demonstrated using a combination of moving boundary electrophoresis (MBE) and zone electrophoresis. The sample loading performed under MBE conditions is studied with the focus on the quantitative analysis of lithium. A concentration adjustment takes place when the sample components migrate into the chip during the sample loading step. Using a heart-cutting method, a diluted sample plug is subsequently separated with capillary zone electrophoresis. The excessive dispersion that is typical of the samples with a high ionic strength is thereby prevented. The method can be easily applied to commercially available capillary electrophoresis microchips under the condition that the electroosmotic flow is suppressed. For the first time the lithium concentration is determined in the blood plasma from a patient on lithium therapy without sample pretreatment. Using a microchip with conductivity detection, a detection limit of 0.1 mmol/L is obtained for lithium in a 140 mmol/L sodium matrix.

Electric Conductivity↗

PubMed2004

An efficient method for the fabrication of temperature-sensitive hydrogel microactuators.

In this article a new method for the photolithographical deposition of temperature-sensitive hydrogels is presented. The method can be used in conjunction with standard 365 nm UV-photolithography to accurately dimension and position temperature-sensitive hydrogel microactuators in a highly parallel fashion. A number of characteristics of the hydrogels were investigated. These include: the photolithographical reproduction quality, the effect of the crosslinking density in the hydrogels on their swelling behavior, the swelling hysteresis behavior, the effect of dimensional constraints on the swelling of the hydrogels and the effect of copolymerization with an ionizable comonomer on the temperature behavior of the hydrogels. The method presents a considerable improvement in the microfabrication of temperature-sensitive hydrogel microactuators and has potential for the mass-fabrication of these interesting microactuators.

Equipment Design↗

PubMed2004

Sampling small volumes of ambient ammonia using a miniaturized gas sampler.

The development of a gas sampler for a miniaturized ambient ammonia detector is described. A micromachined channel system is realized in glass and silicon using powder blasting and anodic bonding. The analyte gas is directly mixed with purified water, dissolving the ammonia that will dissociate into ammonium ions. Carrier gas bubbles are subsequently removed from the liquid stream through a venting hole sealed with a microporous water repellent PTFE membrane. A flow restrictor is placed at the outlet of the sampler to create a small overpressure underneath the membrane, enabling the gas to leave through the membrane. Experiments with a gas flow of 1 ml min(-1), containing ammonia concentrations ranging from 9.4 ppm to 0.6 ppm in a nitrogen carrier flow have been carried out, at a water flow of 20 microl min(-1). The ammonium concentration in the sample solution is measured with an electrolyte conductivity detector. The measured values correspond with the concentration calculated from the initial ammonia concentration in the analyte gas, the fifty times concentration enhancement due to the gas-liquid volume difference and the theoretical dissociation equilibrium as a function of the resulting pH.

Journal Article↗

PubMed2004

Electroosmotic guiding of sample flows in a laminar flow chamber.

The so-called address-flow principle is described: a valveless, electroosmotically driven technology used for controlling the stream profile in a laminar flow chamber. The method is explained, and a theoretical description and experimental verification are presented. Adjustment of the flow of two electroosmotically controlled guiding streams, running parallel to a central sample stream, can be used for positioning the sample stream in the dimension perpendicular to the flow direction. The results presented show that address-flow microfluidics allow easy and accurate control of sample stream position and width. The electroosmotic flow (EOF)-controlled guiding of microfluidic flows described in this paper, is a new unit operation that might aid in separation and collection in microfluidic devices. One possible application of address-flow microfluidics is guiding of capillary electrophoresis-separated components over a multisensor array, in order to perform affinity assays.

Animals↗

PubMed2003

Stimulus-sensitive hydrogels and their applications in chemical (micro)analysis.

In this tutorial review the use of stimulus-sensitive hydrogels as sensors and actuators for (micro)analytical applications is discussed. The first part of the article is aimed at making the reader familiar with stimulus-sensitive hydrogels, their chemical composition and their chemo-physical behavior. The prior art in the field, that comprises a number of sensors ranging from metal ion-sensitive sensors to antigen-sensitive sensors and a few actuators, is also treated in this part. The second part of the article focusses on the use of stimulus-sensitive hydrogels for microsensors and microactuators as well as their application in micro total analysis systems. The benefits of stimulus-sensitive hydrogels, their miniaturisation and the use of 365 nm UV-photolithography as a fast economical manufacturing technique are discussed.

Journal Article↗

PubMed2002

Optimization of an electrolyte conductivity detector for measuring low ion concentrations.

The optimization process of a planar interdigitated conductivity detector for measuring very low electrolyte concentrations for use in a lab-on-chip gas detection system is described. An electrical equivalent of the sensor is given, which includes the double layer capacitance dependency on the electrolyte concentration, resulting in a better description of the impedance of the sensor. The cell constant of the sensor is minimized to reduce the cell resistance in low specific conductivity solutions under the restriction of a small electrode area (> or = 0.1 cm(2)) for fast measurement, prescribed by the ammonia detection system. The small size makes it suitable for integration in micro channels. The developed sensor has a cell constant of 7.9 m(-1) resulting in a maximum resistance for deionized water of 177 k Omega at a frequency of 1 kHz.

Journal Article↗