Ralph Wilke

Multiple internal reflection poly(dimethylsiloxane) systems for optical sensing

Compact poly(dimethylsiloxane)-based (PDMS) multiple internal reflection systems which comprise self-alignment systems, lenses, microfluidic channels and mirrors have been developed for highly sensitive absorbance measurements. With the proper definition of air mirrors at both sides of the sensing region, the optical path of the light from the LED has been meaningfully lengthened without a dramatic increase of the mean flow cell volume. By recursive positioning of such air mirrors, propagating multiple internal reflection (PMIR) systems have been designed, simulated and characterized. Experimental results confirm the ray-tracing predictions and allow the determining that there are some regions of the mean flow cell volume that do not contribute to the increase of the sensitivity. The tailoring of the sensing region, following the optical path, results in a similar limit of detection (110 nM) for fluorescein diluted in phosphate buffer. Finally, a ring configuration, labelled RMIR, has also been developed. With the addition of a third air mirror, the LOD can be decreased to 41 nM with the additional advantage of a substantial decrease of the length of the sensing region. These results confirm the validity of the proposed systems for high sensitivity measurements.

Poly(dimethylsiloxane) hollow Abbe prism with microlenses for detection based on absorption and refractive index shift.

In this paper we report on an optical detection method that utilizes two physical effects for signal transduction, namely absorption and shift of refractive index. The device consists of a hollow prism and was fabricated by means of soft-lithography. It exhibits a high degree of monolithic integration. In order to keep down the amount of external equipment that is necessary to run the device, we were able to integrate several functions, such as focussing of light and alignment of optical fibres. Since all components are fabricated in the same material and in the same process, compatibility with other microfluidic devices or components can be achieved easily. The functional efficiency and the performance of the detector were tested by investigating solutions containing fluorescein, with concentrations between 5 and 1000 microM. The results clearly show the two regions in which the two physical effects are effective.

A micromachined capillary electrophoresis chip with fully integrated electrodes for separation and electrochemical detection

A novel analytical microsystem with fully integrated electrodes for electrophoresis and amperometrical detection is described. With respect to the lab-on-a-chip concept a capillary electrophoresis (CE) microsystem has been fabricated with a total of six gold electrodes for sample injection, separation and electrochemical detection using standard microfabrication technologies. The device is a ready-to-use system that does not need any extra mechanical apparatus for electrode insertion. The CE-chip has successfully been tested by measuring hydrogen peroxide, ascorbic acid and uric acid simultaneously. All three oxidizable species could be detected in less than 70 s. Glucose was detected by performing an enzymatic reaction along the separation channel. The microsystem showed a very good reproducibility.

Enhancement of the response of poly(dimethylsiloxane) hollow prisms through air mirrors for absorbance-based sensing

The hollow prisms are photonic lab-on-a-chip systems with a high degree of monolithic integration that consist of micro-optical (prism and microlenses), microfluidics and structural elements (self-alignment systems) obtained in PDMS by soft lithography. Despite their interesting optical and sensing properties, their working principle, based on the absorption of the working wavelength (lambda=460 nm) by the different substances that can fill the hollow prisms, always involves at least one reflection at the walls of the hollow prism. Due to the low refractive index contrast between the PDMS and the phosphate buffer that fills the hollow prism, the reflectivity at this interface is very low, requiring long integration times. In this paper, we tackle this severe limitation with the definition of an air mirror, which solves the low reflectivity problems: with the appropriate design, the working wavelength matches with the condition of total internal reflection (TIR) only at the air mirror and is reflected back to the hollow prism. Experimental results have shown that the use of air mirrors enhances the sensing properties of the hollow prisms due to several reasons: first, the integration time is strongly reduced, from 2.5s to 80 ms. Second, although the integration time is reduced, the signal-to-noise ratio (SNR) is increased from 12 dB to 19.5 dB. Third, an important improvement of the LOD (with values close to 1 microM and 400 nM for fluorescein and methyl orange diluted in phosphate buffer, respectively) has been experimentally measured. Finally, as compared to the system without the air mirror, the sensitivity is increased by a factor between 1.32 and 2.49 (depending on the geometry used), respectively when this simple, however effective element is included into the system.

Optimization of poly(dimethylsiloxane) hollow prisms for optical sensing

A new generation of simple, robust and compact microfluidic systems with optical readout is presented. The devices consist of hollow prisms fabricated by soft-lithography, together with microlenses and self-aligned channels for fibre optic positioning, conferring the system with a high degree of monolithic integration. Its working principle is based on the absorption of the working wavelength (lambda = 460 nm) by the different substances that can fill the hollow prisms. By modifying the volumes and geometries, optimization of the presented systems has been achieved. Results show how the limit of detection (LOD) for fluorescein and methylorange diluted in phosphate buffer can be significantly lowered, by increasing the size of the prism or increasing the total deviation angle. For our investigations we used concentrations for which the Beer-Lambert law is fulfilled and the measurements showed a LOD in the microM range for both species. Finally, since the change in the fractions of the methylorange as a function of the pH causes a variation of the total absorption of the solution, the hollow prisms have also been used for pH measurements.

Novel three-axis silicon probe with integrated circuit on chip for microsystem components

A three-axis micro probe with wiring of the piezoresistors to Wheatstone bridges on the silicon membrane was currently developed for application in high precision coordinate measuring machines (CMM). The methods and results of the experimental investigation of the main properties of the 3d micro probe are presented. The influence of temperature and light onto the probe performance is described. Finally a brief outlook on the optimization of the threshold sensitivity is given.

Silicon three-axial tactile probe for the mechanical characterization of microgrippers

A three-axial tactile micro probe for the investigation of the mechanical behavior of micro grippers and other micro assembly equipment has been developed using silicon micromachining technology. The sensor has been used to measure the restoring forces of flexural hinges in a micro gripper gear, to calibrate an integrated gripping force sensor, and to measure the generated forces of actuators used in micro grippers. The tactile micro probe is an advancement of a 3-D force sensor presented earlier.

Mikrotaster für Anwendungen in der taktilen Wegmesstechnik

Abstract Das weiterentwickelte und optimierte Design eines auf Bulk-Silizium-Technologie basierenden 3-D-Kraftsensors wird beschrieben. In vorangegangenen Untersuchungen konnte gezeigt werden, dass sich dieser Kraftsensor zum Einsatz in der taktilen Wegmesstechnik als Antastsensor eignet. Das Sensorelement besteht aus einer mit mikrotechnischen Herstellungsverfahren gefertigten Silizium-Bossmembran. In diesem Bericht wird ein neues Design und eine neue Auswertungsstrategie für diesen Sensortyp beschrieben.

Three-dimensional measurement system with microtactile sensor

With the development of micro-technology new measurement methods for microsystem components are increasingly required. Many Institutes have developed new methods for these requirements recently. In the Physikalisch-Technische Bundesanstalt (PTB) a precise three dimensional measurement system is currently under development. The probing process of a 3d tactile sensor which has been developed in cooperation with the Institute for Microtechnology of the Technical University of Braunschweig, will be investigated using this precise 3d measurement system. The 3d precision measuring machine consists of a coarse and a fine stage. The coarse stage has a travel range of 25 mm ×25 mm ×13 mm with a resolution of 50 nm. The fine stage mounted on top of the coarse stage and driven by piezoelectric transducers in three axes has a travel of 80 μm and is controlled by integrated capacitive gauges with a resolution of 1.22 nm. A metrology frame has been added on the fine stage, consisting of a three axis laser interferometer for simultaneous measurement of the displacement in all axes. The specimens to be measured are set in the measurement frame and the probing ball of the tactile sensor is centered at the cross point of the three laser beams (Abbé measurement principle). The measurement system is aimed at a 3d uncertainly < 100 nm for the investigation of tactile sensors in a measuring range of 25 mm ×25 mm ×13 mm.

Multiple internal reflection photonic lab on a chip

In this work it is shown how the inclusion of the air mirrors allows the implementation of a highly sensitive absorbance based biosensor based on Multiple Internal Reflection (MIR). They have been fabricated in a single-mask step and replicated in Poly(dimethylsiloxane) (PDMS) using soft lithographic techniques. Experimental results confirm the ray-tracing predictions: if the air mirror is accurately defined, the propagating light matches the conditions of Total Internal Reflection (TIR) at the PDMS-air interface, which results in a complete reflection of the light towards the system. Additionally, the shape of the air mirrors can be modified so as to allow the light focusing at concrete places. With the recursive positioning of air mirrors, the optical path length has been meaningfully lengthened without a dramatic increase of the volume. The experimental results also matches with the ray tracing predictions and allows obtaining a LOD of 110 nM. The results are auspicious for the implementation of the absorbance-based photonic lab on a chip systems.