Jan Lichtenberg

Sample pretreatment on microfabricated devices

The integration of sample pretreatment into microfluidic devices represents one of the remaining hurdles towards achieving true miniaturized total analysis systems (muTAS). The challenge is made more complex by the enormous variation in samples to be analyzed. Moreover, the pretreatment technique has to be compatible with the analysis device to which it is coupled in terms of time, reagent and power consumption, as well as sample volume. This review provides a thorough overview of the developments in this field to date.

A microchip electrophoresis system with integrated in-plane electrodes for contactless conductivity detection

We present a new approach for contactless conductivity detection for microchip‐based capillary electrophoresis (CE). The detector integrates easily with well‐known microfabrication techniques for glass‐based microfluidic devices. Platinum electrodes are structured in recesses in‐plane with the microchannel network after glass etching, which allows precise positioning and batch fabrication of the electrodes. A thin glass wall of 10–15 νm separates the electrodes and the buffer electrolyte in the separation channel to achieve the electrical insulation necessary for contactless operation. The effective separation length is 34 mm, with a channel width of 50 νm and depth of 12 νm. Microchip CE devices with conductivity detection were characterized in terms of sensitivity and linearity of response, and were tested using samples containing up to three small cations. The limit of detection for K+ (18 νM) is good, though an order of magnitude higher than for comparable capillary‐based systems and one recently reported example of contactless conductivity on chip. However, an integrated field‐amplified stacking step could be employed prior to CE to preconcentrate the sample ions by a factor of four.

Sample preconcentration by field amplification stacking for microchip-based capillary electrophoresis.

A microchip structure for field amplification stacking (FAS) was developed, which allowed the formation of comparatively long, volumetrically defined sample plugs with a minimal electrophoretic bias. Up to 20‐fold signal gains were achieved by injection and separation of 400u2005μm long plugs in a 7.5 cm long channel. We studied fluidic effects arising when solutions with mismatched ionic strengths are electrokinetically handled on microchips. In particular, the generation of pressure‐driven Poiseuille flow effects in the capillary system due to different electroosmotic flow velocities in adjacent solution zones could clearly be observed by video imaging. The formation of a sample plug, stacking of the analyte and subsequent release into the separation column showed that careful control of electric fields in the side channels of the injection element is essential. To further improve the signal gain, a new chip layout was developed for full‐column stacking with subsequent sample matrix removal by polarity switching. The design features a coupled‐column structure with separate stacking and capillary electrophoresis (CE) channels, showing signal enhancements of up to 65‐fold for a 69u2005mm long stacking channel.

Chemical sensing using an integrated microfluidic system based on the Berthelot reaction

Abstract The Berthelot reaction is a well-established colorimetric method for the determination of ammonia. It has been investigated with the particular aim of incorporating it into a simple, reliable analytical microfluidic sensing system. Absorbance measurements for the complex formed when this reaction is performed in microfluidic chips compare very well to those obtained in a spectrophotometric system. The very high reproducibility and efficiency of mixing by diffusion in the microfluidic chip make it a useful tool for future studies of other chemical methods where kinetics are a limiting factor for the response time.

Progress in the realisation of an autonomous environmental monitoring device for ammonia

Progress in the development of a micro-fluidic system for colorimetric monitoring of ammonia in drinking and wastewater is described. The ultimate goal is to have a miniaturised instrument that can produce accurate, reliable measurements, is easy to operate, has minimal power consumption, and can operate autonomously for a year. In this study, the indophenol reaction is incorporated into a simple, reliable analytical micro-fluidic system. Absorbance measurements for the blue ammonia-indophenol complex formed in the micro-fluidic system are shown. A key issue is the limiting stability of hypochlorite, a reagent used in the assay. The effects of hypochlorite concentration and impurities on the stability of hypochlorite are investigated and discussed. Decomposition is shown to be very dependent on the presence of heavy-metal impurities. With low levels of these catalytic metals and careful storage, hypochlorite has been shown to be stable for over a year.

Indirect electro-osmotic pumping

The manipulation of liquids within a microcapillary network nremains a considerable challenge in the development nof miniaturized total chemical analysis systems n(μTAS). Fluid manipulation can be achieved using (micro) nmechanical pumps connected or integrated into the device, and by nusing an electric field (E) for generation of electro-osmotic flow n(EOF). For glass microdevices, electro-osmotic pumping (EOP) is nmost attractive, since no moving parts and/or valves are required. nIn its simplest embodiment, EOP in microfluidic devices ninvolves imposing an E along the full length of the channel by nimmersing electrodes into open solution reservoirs situated at both nends of the channel. Electrolytically generated gases at the electrodes ndrift to the surface of the solution reservoirs and escape into the air. nIn more complex situations, however, EOP in a subsection of a nmicrochannel may be required. For sampling, for example, from nbrain tissue in living organisms, the presence of electrodes in the n‘sample reservoir’ (i.e., the brain), and thus outside the microdevice nis undesirable, since potentials applied to external electrodes interfere nwith the sampling environment. In these cases, electrodes need nto be integrated into the microfluidic device. The use of electrodes nin a microchannel, however, is not trivial. Electrolytic gases get ncaught in the sealed microchannel and hence effectively interrupt nthe electric field, and thus fluid movement. A number of napproaches to avoid bubble formation during spatially localized napplication of voltages in microfluidic networks have been nreported. In one example, a 1-mm-thick poly(dimethylsiloxane) n(PDMS) substrate containing the microchannel was sealed with a nglass cover plate containing the electrodes.1 Electrolytic gases nformed at the electrodes dissipated through the highly gas-permeable nPDMS film into the air. nAn alternative method for application of the electric field is the nuse of a conducting barrier between the electrodes and the channel. nA Nafion membrane has been presented as an interface nbetween an open reservoir containing the electrode and a nmicrochannel.2 Electrolytic gases dissipate into the air via the open nreservoir, while the electrical contact afforded by the membrane nensured that an E was applied to the closed microchannel. A similar napproach involves the use of adjacent side channels, which are nelectrically connected, via porous barriers, but where fluid nexchange is strongly limited.3,4 Either the porous membrane was nformed using a thin layer of potassium silicate, in or the contact nwas directly over the glass wall separating adjacent channels. nThe three approaches mentioned above allow the creation of nfield-free zones in addition to regions where the field is applied. In nthe field-free regions, charge-independent fluid transport can be ncontrolled by EOP elsewhere in the microfluidic system, an effect nwe term “electro-osmotic indirect pumping” (EOIP) to distinguish nbetween EOP in- and outside the electric field. nIn this paper, a glass microdevice for both EOP and EOIP using nelectrically connected side channels is presented. Electrical contact nbetween the main and side channels is achieved by electrical breakdown nof the glass barrier between these channels. Electrical breakdown nfor initiating liquid contact between disconnected channels nhas been demonstrated in PDMS devices.5 To our knowledge, this nis the first time that electrical breakdown for initiation of electrical ncontact between glass microchannels is presented. Cross injection nby a combination of EOP and EOIP is demonstrated.

Consequences of opposing electrokinetically and pressure-induced flows in microchannels of varying geometries

This work confirms experimentally the linear dependence of electro-osmotic velocity on an imposed hydrostatic counter pressure. The effect of hydrostatic pressure-induced flow (PH) becomes significant on electroosmotic flow (EOF) in microchannels with increasing hydraulic diameter (Dh). A detailed investigation of flow streamlines when EOF and PH coexist in opposing directions was carried out, using both simulation tools and microspheres to visualize flows. It was shown that bi-directional flow can be generated. Furthermore, numerical calculations partially explain formation of circulating microsphere clusters at the expansion of narrow channels.

Fabrication of Microfluidic Channels with Symmetric Cross-Sections for Integrated NMR Analysis

We present a glass micromachining technology with improved alignment in order to process channels with symmetric cross-sections. Two well-defined marks deep-etched on glass are superimposed to align the wafers on a standard mask aligner. The best alignment resolution obtained to date was ± 5 μm.

Microfluidic Device for Nucleic Acid Fragmentation by Shear Force

A microfluidic device for random fragmentation of DNA or RNA based on the application of a hydrodynamic shear force to these biomolecules is presented. Different from conventional fragmentation instruments, the microchip-based implementation allows molecules to be repetitively sheared and broken during a single run through the device. It therefore requires comparatively few external components and has a low dead volume in the order of 0.5 μL. 48 kbp λDNA could be fragmented down to an average size of 4 kbp at a flow rate of 1 mL/min.

A miniaturized analysis system based on capillary electrophoresis and contactless conductivity detection

Microchip-based capillary electrophoresis systems for chemical analysis rely in most cases on optical detection methods. Conductivity detection, especially in its contactless, oscillometric form, is a very interesting alternative to build detectors suitable for small ions with good long-term stability. In this paper, a new planar type oscillometric detector for glass-based μTAS is presented, which can be fabricated in a straightforward two-mask process using conventional IC technology. The detector was used for the analysis of water samples containing sodium and potassium with a comparatively low detection limit of 35 µM. The operation frequency range is between 20 and 200 kHz.