Lars Rosengren

Porous silicon as the carrier matrix in microstructured enzyme reactors yielding high enzyme activities

Miniaturization and silicon integration of micro enzyme reactors for applications in micro total analysis systems (mu TASs) require new methods to achieve structures with a large surface area onto ...

Enhanced enzyme activity in silicon integrated enzyme reactors utilizing porous silicon as the coupling matrix

Abstract The performance of porous silicon as a coupling matrix for silicon integrated enzyme reactors has been investigated. A porous silicon layer in a water is expected to yield an increased catalytic activity when coupling an enzyme to the wafer due to the surface enlargement of the porous structure. The porous silicon layer is obtained by anodizing a 1 cm x 1 cm silicon die in a hydrofluoric acid/ethanol mixture at a constant current density of 10, 50 or 100 mA cm−2 for 50 min. Glucose oxidase is immobilized on the porous matrix of three dice and the enzyme activity of the silicon samples is monitored using a colorimetric assay. As a reference sample a polished die is also submitted to the enzyme immobilization. The three porous dice display higher glucose turn-over rates than the unetched reference die. Samples etched at lower current densities show increased turn-over rates. The maximum rise in turn-over rate is more than 30 times for the 10 mA cm−2 sample compared with the reference die. Porous silicon has the potential of being incorporated in a micro total analysis system (μTAS) to give highly efficient enzyme reactors.

Fabrication of 45/spl deg/ mirrors together with well-defined v-grooves using wet anisotropic etching of silicon

The most commonly used microstructure for passive fiber alignment is the ordinary v-groove, defined by {111} planes on a (100) silicon wafer. The plane at the end of the groove, having a 54.7/spl deg/ angle to the surface, can be used as a reflecting mirror. For single-mode fiber applications, a 45/spl deg/ mirror is advantageous together with high accuracy in the position of the fiber, i.e. a smooth mirror and good control of the groove geometry is needed. Two techniques are presented to form 45/spl deg/ mirrors along with well-defined grooves in silicon, using the wet anisotropic etchants EDP and KOH. These techniques are used: (1) to reveal {110} planes on (100) silicon and (2) to make {111} mirrors on wafers that are cut 9.7/spl deg/ off the [100] axis. On (100) silicon, EDP without pyrazine gave the best result. The best mirror and groove reproducibility was found on off-axis cut silicon, using 36 wt.% KOH, with isopropyl alcohol added.

Silicon wafer integrated enzyme reactors

Abstract Enzyme reactors were fabricated on silicon wafers using microstructuring technologies. The reactors were made of several parallel vertically-cut flow channels. The reactor structure occupied a wafer area of 3 ∗ 15 mm. Reactors with two different channel densities were fabricated: 10 channels/ mm, 165 μm deep; and 25 channels/mm, 235 μm deep. Glucose oxidase was immobilised on the reactors and their corresponding enzyme activities were monitored by a colourimetric assay. It was shown that a reactor surface are increase of 3 times gave rise to a proportional enzyme activity increase in the reactor. The maximum glucose turnover rate for the reactor with 25 channels/ mm was approximately 35 nmol/minute and the corresponding apparent K m was approximately 17 mM. A wafer integrated enzyme reactor was also operaed in a microdialysis-based system for continuous glucose monitoring, showing a linear response up to 4 mM glucose.

Passive silicon transensor intended for biomedical, remote pressure monitoring

Abstract A sensor is suggested for continuous monitoring of intraocular pressure and similar biomedical applications. The system consists of a capacitive pressure sensor determining the resonance frequency of a passive LC resonator. The resonance frequency is detected in a grid-dip configuration. The paper mainly deals with the capacitive sensor, which is fabricated in silicon, using a silicon fusion bond to seal the cavity. This process will minimize internal stress and provide the necessary conditions for a small temperature drift and stability over long periods of time. Experimentally, prototype sensors have been fabricated and evaluated in vitro . The results are promising and in accordance with theoretical expectations.

New shapes in (100) Si using KOH and EDP etches

Wet chemical etching in EDP, pure KOH and KOH with isopropyl alcohol (KOH/IPA) are known to etch different crystal planes with different etch rates. It is shown in this work that etching in EDP or KOH/IPA reverses the etch-rate ratios of (100) and (110) planes, as compared to etching in pure KOH. This can be used to reveal other relatively slow etching planes, other than (111), to be used in new structures. V-grooves, retro-reflector ridges and mesa structures were fabricated bounded by (110) planes with 45 degrees angles to the (100) surface. Also, by using a corner-compensated mask aligned in the (100) direction and etched in pure aqueous KOH, free-standing cubic studs bounded by (100) planes were fabricated.

A system for wireless intra-ocular pressure measurements using a silicon micromachined sensor

A system for the continuous monitoring of intra-ocular pressure is described. The sensor is intended for permanent implantation, integrated with an artificial lens. The system consists of a capacitive pressure sensor, determining the resonance frequency of a passive LC network, and a detector, based on the grid-dip technique. The passive pressure sensing element is micromachined from two silicon wafers and connected to a gold-wire coil. Prototype sensors have been evaluated experimentally in vitro. Pressure has been measured remotely, with promising results.

Pore morphology influence on catalytic turn-over for enzyme activated porous silicon matrices

Abstract The enzyme glucose oxidase (GOx) was coupled to porous silicon of different morphologies and the catalytic turn-over of glucose was recorded for the samples. The recorded catalytic turn-over of the samples clearly indicated the influence of morphology, with respect to dopant concentration and current density, of the porous silicon carrier matrix. The highest rise in catalytic turn-over (350 times), when compared to a non-porous surface, was recorded for a sample with an n-type epilayer on an n+-type substrate anodised at 100 mA/cm2. A storage and operational stability measurement was performed on the sample showing the highest catalytic efficiency. After 5 months of refrigerated storage a 2% loss of activity was noted, and after 4 days of constant glucose load (0.5 mM) a 56% loss of activity was recorded. A BET (Brunauer, Emmet, and Teller) nitrogen adsorption analysis was performed on one of the substrate types, p+-type (0.001–0.025 Ω cm). In spite of the fine porous morphology with a high surface area the recorded enzyme activities were moderate. The pore morphology achieved on this substrate most likely comprised too small pores in a too dense porous matrix giving poor diffusion conditions to give efficient access for the enzyme during the coupling procedure and for the reactant transport during operation to fully utilise the surface enlargement of the porous layer.

THE IOP-IOL. A PROBE INTO THE EYE

Abstract. The intraocular lens (IOL) implant can be looked upon as a probe into the space of the eye. Adapting that view, it is logical to furnish the IOL‐probe with biomedical sensors that would explore its environment. A sensor is presented for continuous monitoring of intraocular pressure (IOP), incorporated in the haptics of an IOL. The sensor consists of a capacitative spiral circuit, needing no energy, correlating its resonance frequency to the actual IOP. This resonance frequency is remotely and non‐invasively detected by an external device located in a spectacle frame.

Porous Silicon Carrier Matrices in Micro Enzyme Reactors-Influence of Matrix Depth

Abstract The influence of the carrier matrix depth was investigated for porous silicon enzyme reactors. For the experiments, <110> oriented silicon, p-type (20–70 Ω cm), was used. Porous silicon was generated on planar surfaces and on anisotropically pre-etched high aspect-ratio parallel channel reactors. For each type of sample the porous silicon layer was generated for three depths, controlled by the anodisation time, and two current densities, to yield different morphologies. Glucose oxidase (GOx) was immobilised on the porous matrix by standard procedures for immobilisation of enzymes on silica. The enzyme activity of the samples was monitored by a colorimetric assay. The results clearly display the influence of the matrix depth for both the planar and the reactor structures. A 170-fold increase in catalytic turn-over, in comparison to an identical non-porous reference, was recorded for a reactor with an average pore depth of 10 μm. At depths above 10 μm the increase in catalytic efficiency levelled off. For the planar samples the levelling off occurred at an average pore depth of 20 μm.