Silicon micromachined ultrasonic scalpel for the dissection and coagulation of tissue.

This work presents a planar, longitudinal mode ultrasonic scalpel microfabricated from monocrystalline silicon wafers. Silicon was selected as the material for the ultrasonic horn due to its high speed of sound and thermal conductivity as well as its low density compared to commonly used titanium based alloys. Combined with a relatively high Young's modulus, a lighter, more efficient design for the ultrasonic scalpel can be implemented which, due to silicon batch manufacturing, can be fabricated at a lower cost. Transverse displacement of the piezoelectric actuators is coupled into the planar silicon structure and amplified by its horn-like geometry. Using finite element modeling and experimental displacement and velocity data as well as cutting tests, key design parameters have been identified that directly influence the power efficiency and robustness of the device as well as its ease of controllability when driven in resonance. Designs in which the full- and half-wave transverse modes of the transducer are matched or not matched to the natural frequencies of the piezoelectric actuators have been evaluated. The performance of the Si micromachined scalpels has been found to be comparable to existing commercial titanium based ultrasonic scalpels used in surgical operations for efficient dissection of tissue as well as coaptation and coagulation of tissue for hemostasis. Tip displacements (peak-to-peak) of the scalpels in the range of 10-50 µm with velocities ranging from 4 to 11 m/s have been achieved. The frequency of operation is in the range of 50-100 kHz depending on the transverse operating mode and the length of the scalpel. The cutting ability of the micromachined scalpels has been successfully demonstrated on chicken tissue.

Biosensor microprobes with integrated microfluidic channels for bi-directional neurochemical interaction.

This paper reports on silicon-based microprobes, 8 mm long and 250 µm × 250 µm cross-section, comprising four recessed biosensor microelectrodes (50 µm × 150 µm) per probe shank coated with an enzymatic layer for the selective detection of choline at multiple sites in brain tissue. Integrated in the same probe shank are up to two microfluidic channels for controlled local liquid delivery at a defined distance from the biosensor microelectrodes. State-of-the-art silicon micromachining processing was applied for reproducible fabrication of these experiment-tailored multi-functional probe arrays. Reliable electric and fluidic interconnections to the microprobes are guaranteed by a custom-made holder. The reversible packaging method implemented in this holder significantly reduces cost and assembly time and simplifies storage of the biosensor probes between consecutive experiments. The functionalization of the electrodes is carried out using electrochemically aided adsorption. This spatially controlled deposition technique enables a parallel deposition of membranes and is especially useful when working with microelectrode arrays. The achieved biosensors show adequate characteristics to detect choline in physiologically relevant concentrations at sufficient temporal and spatial resolution for brain research. Sensitivity to choline better than 10 pA µm(-1), detection limit below 1 µM and response time of 2 s were obtained. The proposed combination of biosensors and microfluidic injectors on the same microprobe allows simultaneous chemical stimulation and recording as demonstrated in an agarose gel-based brain phantom.

A micro gas preconcentrator with improved performance for pollution monitoring and explosives detection.

This paper presents the optimization of a micro gas preconcentrator based on a micro-channel in porous and non-porous silicon filled with an adequate adsorbent. This micro gas preconcentrator is both applicable in the fields of atmospheric pollution monitoring (Volatil organic compounds--VOCs) and explosives detection (nitroaromatic compounds). Different designs of micro-devices and adsorbent materials have been investigated since these two parameters are of importance in the performances of the micro-device. The optimization of the device and its operation were driven by its future application in outdoor environments. Parameters such as the preconcentration factor, cycle time and the influence of the humidity were considered along the optimization process. As a result of this study, a preconcentrator with a total cycle time of 10 min and the use of single wall carbon nanotubes (SWCNTs) as adsorbent exhibits a good preconcentration factor for VOCs with a limited influence of the humidity. The benefits of using porous silicon to modify the gas desorption kinetics are also investigated.

Enzyme-based choline and L-glutamate biosensor electrodes on silicon microprobe arrays.

Brain-implantable microprobe arrays, 6.5 mm shaft-length, incorporating several recessed Pt microelectrodes (50 µm×150 µm) and an integrated Ag/AgCl reference electrode fabricated by silicon micromachining dry etching techniques (DRIE) are described. The microelectrodes are coated by an enzyme membrane and a semi-permeable m-phenylenediamine layer for the selective detection of the neurotransmitters choline and L-glutamate at physiologically relevant concentrations. The functionalisation is based on electrochemically aided adsorption (EAA) combined with chemical co-cross-linking using glutaraldehyde and electrochemical polymerisation, respectively. These deposition methods are fully compatible with the fabricated microprobe arrays for the simultaneous detection of several analytes in different brain target areas. They are spatially controlled and allow fabricating biosensors on several microelectrodes in parallel or providing a cross-talk-free coating of closely spaced microelectrodes with different enzyme membranes. A sensitivity of 132±20 µA mM(-1) cm(-2) for choline and 95±20 µA mM(-1) cm(-2) for L-glutamate with limits of detections below 0.5 µM was obtained. The results of in vitro and in vivo experiments confirm the functional viability of the choline and l-glutamate biosensors.

Theoretical Treatment and Numerical Simulation of Potential and Concentration Profiles in Extremely Thin Non-Electroneutral Membranes Used for Ion-Selective Electrodes.

The applicability of extremely thin non-electroneutral membranes for ion-selective electrodes (ISEs) is investigated. A theoretical treatment of potential and concentration profiles in space-charge membranes of << 1 µm thickness is presented. The theory is based on the Nernst-Planck equation for ion fluxes, which reduces to Boltzmann's formula at equilibrium, and on the Poisson relationship between space-charge density and electric field gradient. A general solution in integral form is obtained for the potential function and the corresponding ion profiles at equilibrium. A series of explicit sub-solutions is derived for particular cases. Membrane systems with up to three different ion species are discussed, including trapped ionic sites and co-extracted ions. Solid-contacted thin membranes (without formation of aqueous films at the inner interface) are shown to exhibit a sub-Nernstian response. The theoretical results are confirmed by numerical simulations using a simplified finite-difference procedure based on the Nernst-Planck-Poisson model, which are shown to be in excellent agreement.

Theory and Computer Simulation of the Time-Dependent Selectivity Behavior of Polymeric Membrane Ion-Selective Electrodes.

A theoretical treatment of the time-dependent potential response of ion-selective electrodes to sample solutions containing primary and interfering ions is presented. The theory accounts for the influence of ion fluxes in the electrode membrane and the contacting aqueous sample layer and describes the variations in the apparent selectivity behavior as a function of the measuring time. The applicability of the theory is demonstrated by comparing predicted response curves with results of virtual experiments based on computer simulation. A close and convincing agreement was achieved for a large series of different examples, which confirms that the new theory can be successfully applied for general cases.

Advanced temperature compensation for piezoresistive sensors based on crystallographic orientation.

We describe a highly effective method of reducing thermal sensitivity in piezoresistive sensors, in particular silicon cantilevers, by taking advantage of the dependence of the piezoresistive coefficient of silicon on crystallographic orientation. Two similar strain-sensing elements are used, positioned at 45 degrees to each other: One is set along a crystalline axis associated with a maximum piezoresistive coefficient to produce the displacement signal, while the other is set along an axis of the vanishing coefficient to produce the reference signal. Unlike other approaches, both sensing elements are coupled to the same cantilever body, maximizing thermal equilibration. Measurements show at least one order of magnitude improvement in thermal disturbance rejection over conventional approaches using uncoupled resistors.

Computer Simulation of Ion-Selective Membrane Electrodes and Related Systems by Finite-Element Procedures.

A simple but powerful numerical simulation for analyzing the electrochemical behavior of ion-selective membranes and liquid junctions is presented. The computer modeling makes use of a finite-element procedure in the space and time domains, which can be easily processed (e. g., with MS Excel software) without the need for complex mathematical evaluations. It leads to convincing results on the dynamic evolution of concentration profiles, potentials, and fluxes in the studied systems. The treatment accounts for influences of convection, flow, or stirring in the sample solution that act on the boundary diffusion layer and it is even capable of including the effects of an electrolyte flow through the whole system. To minimize the number of arbitrary parameters, interfacial reactions are assumed to be near local equilibrium, and space-charge influences are considered via phase-boundary potential differences. The applicability of the computer simulation is demonstrated for different ion-selective membranes as well as for liquid junctions. The numerical results are in excellent agreement with experimental data.

High-density electrode array for imaging in vitro electrophysiological activity.

The development of a high-density active microelectrode array for in vitro electrophysiology is reported. Based on the Active Pixel Sensor (APS) concept, the array integrates 4096 gold microelectrodes (electrode separation 20 microm) on a surface of 2.5 mmx2.5 mm as well as a high-speed random addressing logic allowing the sequential selection of the measuring pixels. Following the electrical characterization in a phosphate solution, the functional evaluation has been carried out by recording the spontaneous electrical activity of neonatal rat cardiomyocytes. Signals with amplitudes from 130 microVp-p to 300 microVp-p could be recorded from different pixels. The results demonstrate the suitability of the APS concept for developing a new generation of high-resolution extracellular recording devices for in vitro electrophysiology.

Development of a multifunctional platform for extra- and intracellular ionic activities recording.

We present the development of a multifunctional platform equipped with an array of silicon nitride micropipettes with dimensions allowing the implementation of extra- and intracellular operations. Micropipettes with outer diameter that ranges from 6 mum down to 300 nm and with walls thicknesses of 500 down to 150 nm are presented. The generic technology developed to fabricate these micropipettes has a number of advantages, including the ability to be implemented as ion-selective electrodes for (A) intracellular and (B) extracellular recordings and as (C) local drug microdispensers.

Planar microcoil-based microfluidic NMR probes.

Microfabricated small-volume NMR probes consisting of electroplated planar microcoils integrated on a glass substrate with etched microfluidic channels are fabricated and tested. 1H NMR spectra are acquired at 300 MHz with three different probes having observed sample volumes of respectively 30, 120, and 470 nL. The achieved sensitivity enables acquisition of an 1H spectrum of 160 microg sucrose in D2O, corresponding to a proof-of-concept for on-chip NMR spectroscopy. Increase of mass-sensitivity with coil diameter reduction is demonstrated experimentally for planar microcoils. Models that enable quantitative prediction of the signal-to-noise ratio and of the influence of microfluidic channel geometry on spectral resolution are presented and successfully compared to the experimental data. The main factor presently limiting sensitivity for high-resolution applications is identified as being probe-induced static magnetic field distortions. Finally, based on the presented model and measured data, future performance of planar microcoil-based microfluidic NMR probes is extrapolated and discussed.

Characterization and fabrication of fully metal-coated scanning near-field optical microscopy SiO2 tips.

The fabrication of silicon cantilever-based scanning near-field optical microscope probes with fully aluminium-coated quartz tips was optimized to increase production yield. Different cantilever designs for dynamic- and contact-mode force feedback were implemented. Light transmission through the tips was investigated experimentally in terms of the metal coating and the tip cone-angle. We found that transmittance varies with the skin depth of the metal coating and is inverse to the cone angle, meaning that slender tips showed higher transmission. Near-field optical images of individual fluorescing molecules showed a resolution < 100 nm. Scanning electron microscopy images of tips before and after scanning near-field optical microscope imaging, and transmission electron microscopy analysis of tips before and after illumination, together with measurements performed with a miniaturized thermocouple showed no evidence of mechanical defect or orifice formation by thermal effects.

Multi-layer microfluidic glass chips for microanalytical applications.

A new, versatile architecture is presented for microfluidic devices made entirely from glass, for use with reagents which would prove highly corrosive for silicon. Chips consist of three layers of glass wafers bonded together by fusion bonding. On the inside wafer faces a network of microfluidic channels is created by photolithography and wet chemical etching. Low dead-volume fluidic connections between the layers are fabricated by spark-assisted etching (SAE), a computer numerical controlled (CNC)-like machining technique new to microfluidic system fabrication. This method is also used to form a vertical, long path-length, optical cuvette through the middle wafer for optical absorbance detection of low-concentration compounds. Advantages of this technique compared with other, more standard, methods are discussed. When the new glass-based device for flow-injection analysis of ammonia was compared with our first-generation chips based on silicon micromachining, concentration sensitivity was higher, because of the longer path-length of the optical cuvette. The dependence of dispersion on velocity profile and on channel cross-sectional geometry is discussed. The rapid implementation of the devices for an organic synthesis reaction, the Wittig reaction, is also briefly described.

Surface biopassivation of replicated poly(dimethylsiloxane) microfluidic channels and application to heterogeneous immunoreaction with on-chip fluorescence detection.

Poly(dimethylsiloxane) (PDMS) appeared recently as a material of choice for rapid and accurate replication of polymer-based microfluidic networks. However, due to its hydrophobicity, the surface strongly interacts with apolar analytes or species containing apolar domains, resulting in significant uncontrolled adsorption on channel walls. This contribution describes the application and characterization of a PDMS surface treatment that considerably decreases adsorption of low and high molecular mass substances to channel walls while maintaining a modest cathodic electroosmotic flow. Channels are modified with a three-layer biotin-neutravidin sandwich coating, made of biotinylated IgG, neutravidin, and biotinylated dextran. By replacing biotinylated dextran with any biotinylated reagent, the modified surface can be readily patterned with biochemical probes, such as antibodies. Combination of probe immobilization chemistry with low nonspecific binding enables affinity binding assays within channel networks. The example of an electrokinetic driven, heterogeneous immunoreaction for human IgG is described.

Electrokinetically driven microfluidic chips with surface-modified chambers for heterogeneous immunoassays.

This article presents the first example of a microfluidic chip for heterogeneous bioassays using a locally immobilized biospecific layer and operated electrokinetically. The reaction chamber has picoliter dimensions and is integrated into a network of microchannels etched in glass. The high affinity of protein A (PA) for rabbit immunoglobulin G (rIgG) was exploited for chip testing, with PA being immobilized on microchannel walls and fluorescently labeled (Cy5) rIgG serving as sample. It was possible to operate the chip in an immunoaffinity chromatographic manner, using electrokinetically pumped solutions. Concentration of antibody from dilute solution onto the solid phase was demonstrated, with signal gains of approximately 30 possible. A dose-response curve for Cy5-rIgG was obtained for concentrations down to 50 nM, for an incubation time of 200 s. The flexibility of chip layout was demonstrated for competitive immunoassay of rIgG, using both a combined sample/tracer incubation and sequential addition of these solutions. With assay times generally below 5 min for this unoptimized device, the microfluidic approach described shows great potential for many high-throughput screening applications.

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 400 microm 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 69 mm long stacking channel.

Near-field optical microscopy based on microfabricated probes.

We demonstrate high resolution imaging with microfabricated, cantilevered probes, consisting of solid quartz tips on silicon levers. The tips are covered by a 60-nm thick layer of aluminium, which appears to be closed at the apex when investigated by transmission electron microscopy. An instrument specifically built for cantilever probes was used to record images of latex bead projection patterns in transmission as well as single molecule fluorescence. All images were recorded in constant height mode and show optical resolutions down to 32 nm.

Fabrication and characterization of a silicon cantilever probe with an integrated quartz-glass (fused-silica) tip for scanning near-field optical microscopy.

A cantilever-based probe is introduced for use in scanning near-field optical microscopy (SNOM) combined with scanning atomic-force microscopy (AFM). The probes consist of silicon cantilevers with integrated 25-mum-high fused-silica tips. The probes are batch fabricated by microfabrication technology. Transmission electron microscopy reveals that the transparent quartz tips are completely covered with an opaque aluminum layer before the SNOM measurement. Static and dynamic AFM imaging was performed. SNOM imaging in transmission mode of single fluorescent molecules shows an optical resolution better than 32 nm.

Towards better scanning near-field optical microscopy probes--progress and new developments.

Several approaches are described with the aim of producing near-field optical probes with improved properties. Focused ion beam milling allows the fabrication of small apertures in a controlled fashion, resulting in probes with excellent polarization properties and increased transmission. Microfabrication processes are described that allow the production of apertures of 30-50 nm, facilitating the mass-fabrication of apertured tip structures that can be used in a combined force/near-field optical microscope. Finally, possible future developments are outlined.

Miniaturized time-scanning Fourier transform spectrometer based on silicon technology.

We present a miniaturized Fourier transform spectrometer (FTS) based on optical microelectromechanical system technology. The FTS is a Michelson interferometer with one scanning mirror. A new type of electrostatic comb drive actuator moves the mirror. We have measured a nonlinearity of the driving system of +/-0.5 mum for a displacement of 38.5 mum . A method is presented to correct the spectrum to get rid of the nonlinearity. The driving reproducibility is +/-25 nm. The measured resolution of the spectrometer after the phase correction is 6 nm at a wavelength of 633 nm.