A. van den Berg

Technologies for nanofluidic systems:top-down vs. bottom-up - a review

This paper gives an overview of the most commonly used techniques for nanostructuring and nanochannel fabrication employed in nanofluidics. They are divided into two large categories: top-down and bottom-up methods. Top-down methods are based on patterning on large scale while reducing the lateral dimensions to the nanoscale. Bottom-up methods arrange atoms and molecules in nanostructures. Here, we review the advantages and disadvantages of those methods and give some future perspectives. It is concluded that technology in the region of 1-10 nm is lacking and potentially can be covered by using the pulsed-laser deposition method as a controlled way for thin film deposition (thickness of a few nanometers) and further structuring by the top-down method.

Capillary filling speed of water in nanochannels

The capillary filling speed of water in nanochannels with a rectangular cross section and a height on the order of 100 nm has been measured over a length of 1 cm. The measured position of the meniscus as a function of time qualitatively follows the Washburn model. Quantitatively, however, there is a lower than expected filling speed, which we attribute to the electro-viscous effect. For demineralized water in equilibrium with air the elevation of the apparent viscosity amounts up to 24±11% in the smallest channels (53 nm height). When using a 0.1 M NaCl (aq) solution the elevation of the apparent viscosity is significantly reduced.

Improved piercing of microneedle arrays in dermatomed human skin by an impact insertion method

An electrical applicator was designed, which can pierce short microneedles into the skin with a predefined velocity. Three different shapes of microneedles were used, namely 300 mum assembled hollow metal microneedle arrays, 300 mum solid metal microneedle arrays and 245 mum hollow silicon microneedle arrays. The latter are available as 4x4, 6x6 and 9x9 arrays. When using a velocity of 1 or 3 m/s reproducible piercing of dermatomed and full thickness human skin was evident from the appearance of blue spots on the dermal side of the skin after Trypan Blue treatment and the presence of fluorescently labeled particles in dermatomed skin. Manual piercing did not result in the appearance of blue spots. Transport studies revealed that i) piercing of microneedles with a predefined velocity into human skin resulted in a drastic enhancement of the Cascade Blue (CB, Mw 538) transport, ii) A higher piercing velocity resulted in a higher CB transport rate, iii) The CB transport rate was also dependent on the shape of the microneedles and iv) no difference in transport rate was observed between 4x4, 6x6 and 9x9 hollow silicon microneedle arrays.

Sensitivity control of ISFETs by chemical surface modification

The response of ISFETs (ion-sensitive field-effect transistors) to concentrations of ions, especially H+ ions, is determined by the type of gate surface. Both the number of active surface sites and (proton) association and dissociation constants influence the sensitivity. In the case of a chemically-modified gate surface, a new surface is formed, which generally has a different sensitivity. It is shown that the original pH response of the gate oxide can be either lowered or increased, depending on the reactivity of the added groups. In general, coverage with apolar groups and reduction of the number of sites result in a lower pH response, while addition of basic or acidic groups as well as an increase of active sites give a higher pH response. Using the extended site-dissociation model, which describes the behaviour of a surface composed of two types of sites, theoretical curves for surface potential versus pH are calculated. Measurements with chemically-treated siO2 and Ta2O5 ISFETs confirm the theoretical expectations. The conclusion has been drawn that by a proper choice of chemical treatment, both the point of zero charge (pzc) and the pH-insensitive rage can be changed.

Characterization method for a new diffusion mixer applicable in micro flow injection analysis systems

A new mixer is designed for mixing a phenolic solution into water. The mixer design is such that it can be easily adjusted for the controlled mixing of a specific compound within a certain time. This paper describes the working principle of the mixer as well as a suitable characterization method for the mixer. Measurement results are presented which show the correct working of the mixer. A quantitative measure is introduced to express the extent of mixing performed by the mixer. The characterization method allows the measurement of the flow rate, pressure drop and extent of mixing.

A micromachined pressure/flow-sensor

The micromechanical equivalent of a differential pressure flow-sensor, well known in macro mechanics, is discussed. Two separate pressure sensors are used for the device, enabling to measure both, pressure as well as volume flow-rate. An integrated sensor with capacitive read-out as well as a hybrid, piezo-resistive variant is made. The fabrication processes are described, using silicon and glass processing techniques. Based on the sensor layout, equations are derived to describe the sensor behavior both statically as well as dynamically. With the derived equations, the working range of the sensor and the thermal and time stability is estimated. The computed results of the stationary behavior are verified with the measured data. A good similarity in linearity of the pressure/flow relation is found. The computed hydraulic resistance, however, differs from the measured value for water with 21%. This difference can be explained by the high sensitivity of the resistance to the resistor channel cross-section parameter in combination with the difference between the rounded etched shape and the rectangular approximation. From fluid dynamics simulations, a working range bandwidth of about 1 kHz is expected. Thermal influences on the sensor signal due to viscosity changes are in the order of 2% flow signal variation per Kelvin. From these results, it can be concluded that the sensor can be used as a low cost, low power consuming flow and pressure-sensing device, for clean fluids without particles and without the tendency to coat the channel walls. If a high accuracy is wanted, an accurate temperature sensing or controlling system is needed.

How electrical and chemical requirements for refets may coincide

After discussing the features of a differential ISFET/REFET measuring concept, the published attempts to construct a proper REFET are summarized. It is concluded that the present REFETs are based upon the addition of a blocking polymeric layer to the gate surface of an ISFET, but that this approach fails with respect to the required insensitivity to ionic strength variations as well as with respect to the electrical stability. As a solution to these problems, this paper describes the development of a REFET concept that is based on the chemical attachment of a non-blocking polymeric layer. Characterization methods of these layers with respect to the electrical as well as the chemical behaviour are given and discussed. Finally, the experimental results of an acrylate/polyHEMA-REFET are shown in a differential ISFET/REFET system.

Porous silicon bulk micromachining for thermally isolated membrane formation

A novel low thermal budget technique is proposed for the preparation of thermally isolated silicon membranes. The selective formation of porous silicon in a p-type silicon wafer results in an undercut profile below the implanted n-type silicon regions. The sacrificial porous layer is subsequently removed in a dilute KOH solution. A non-stoichiometric LPCVD nitride layer combination forms the suspension of the single-crystalline silicon membranes. This technique eliminates the need for epitaxial substrates and backside alignment, and proves to be very efficient in the realization of a high-temperature micro-hotplate operating with minimum power consumption for the purpose of integrated gas sensors.

Heat and mass transfer in a square microchannel with asymmetric heating

This paper describes the heat and mass transfer in a square microchannel that is heated from one side. This microchannel represents a reaction channel in a microreactor that is used to study the kinetics of the catalytic partial oxidation of methane. The microchannel is contained in a silicon wafer and is covered by a thin silicon sheet. At the top side of this sheet, heating elements are present which mimic the heat that is produced as a result of the exothermic chemical reaction. Correlations for Nusselt and Sherwood numbers as a function of the Graetz number are derived for laminar and plug flow conditions. These correlations describe the heat and mass transport at the covering top sheet of the microchannel as well as at its side and bottom walls. By means of computational fluid dynamic simulations, the laminar flow is studied. To determine an approximate laminar flow Nusselt correlation, the heat transport was solved analytically for plug flow conditions to describe the influence of changes in the thermal boundaries of the system. The laminar flow case is experimentally validated by measuring the actual temperature distribution in a 500 μm square, 3 cm long, microchannel that is covered by a 1 μm and by a 1.9 μm thick silicon sheet with heating elements and temperature sensors on top. The Nusselt and Sherwood correlations can be used to readily quantify the heat and mass transport to support kinetic studies of catalytic reactions in this type of microreactor.

Electrophoretic separation of DNA in gels and nanostructures.

The development of nanostructure devices has opened the door to new DNA separation techniques and fundamental investigations. With advanced nanotechnologies, artificial gels (e.g. nanopillar arrays, nanofilters) can be manufactured with controlled and ordered geometries. This contrast with gels, where the pores are disordered and the range of available pore sizes is limited by the level of cross-linking and the mechanical properties of the gel. In this review, we recall the theories developed for free-solution and gel electrophoresis (extended Ogston model, biased reptation and entropic trapping) and from this perspective, suggestions for future concepts for fast DNA separation using nanostructures will be given.