C.J.M. van Rijn

Development and applications of very high flux microfiltration membranes

Inorganic microfiltration membranes with a pore size down to 0.1 ?m have been made using laser interference lithography and silicon micro machining technology. The membranes have an extremely small flow resistance due to a thickness smaller than the pore size, a high porosity and a very narrow pore size distribution. They are relatively insensible to fouling, because they have a smooth surface, short pore channels and because they can be operated in cross flow configuration at very low transmembrane pressures. Experiments with yeast cell filtration of beer show a minimal fouling tendency and a flux that is about 40 times higher than in conventional diatomaceous earth filtration. The uniform pore distribution makes the membranes suitable for many other applications like critical cell to cell separation, particle analysis systems, absolute filtrations and model experiments.

Determination of particle-release conditions in microfiltration: a simple single-particle model tested on a model membrane

A simple single-particle model was developed for cross-flow microfiltration with microsieves. The model describes the cross-flow conditions required to release a trapped spherical particle from a circular pore. All equations are derived in a fully analytical way without any fitting parameters. For experimental verification of the model ultra-thin microsieves of uniform pore size and distribution were used. The release of trapped particles (polystyrene spheres and yeast cells) was determined by flux measurements as well as by in-line observation through a microscope. The results show that the model gives a fairly good indication of what cross-flow should be applied to keep the pores free for the conditions specified in this paper. In addition it provides us with a simple rule of thumb for the design of cross-flow modules for microsieves. It describes which geometrical demands have to be met to enable filtration without pore blocking, again for the conditions specified in this paper.

Microfiltration membrane sieve with silicon micromachining for industrial and biomedical applications

With the use of silicon micromachining an inorganic membrane sieve for microfiltration is constructed, having a siliconnitride membrane layer with thickness typically 1 pm and perforations typically between 0.5 pm and 10 pm in diameter. As a support a -silicon wafer with openings of loo0 pm in diameter is used. The thin siliconnitride layer is deposited on an initial dense support by means of a suitable Chemical Vapour Deposition method (LPCVD). Perforations in the membrane layer are obtained through the use of standard microlithography and reactive ion etching (RIE). The flow rate behaviour and the pressure strength of the membrane sieve are calculated in a first approximation. A process for manufacturing is presented and some industrial and biomedical applications are discussed. Introduction Sieve Filters Sieve filters are characterized by thin membrane layers with uniformly sized pores and for most applications the membrane layer is sustained by a support. Until now lithographic techniques have not been used for the construction of micro filtration membrane layers made of inorganic materials as siliconnitride and silicon1. Inorganic membrane and in particular ceramic membranes* have a number of advantages above polymeric membranes like high temperature stability, relative inert to chemicals, applicable at high pressures, easy to sterilize and recyclable. However they have not been used extensively because of their high costs and relatively poor control in pore size distribution. Also the effective membrane layer is very thick in comparison to the mean pore size (typically 50 -loo0 times), which resiluts in a reduced flow rate. A composite filtrationmembrane having a relatively thin filtration or sieving layer with a high pore density and a narrow pore size distribution on a macroporous support will show good separation behaviour and a high flow rate. The support contributes to the mechanical strength of the total composite membrane. The openings in the support should be made as large and numerous as possible in Handbook of Industrial Membrane Technology, LPorter, C.Mark and H.Strathmann, 1990 Ceramic Membranes Growth Prospects and Opportunities, K.K. Chan and A.M. Brownstein, Ceramic Bulletin, vol70, 703-707, 1991 order to maintain the flow rate of the membrane layer and to reduce the interaction of the support with the fluid. An established use of inorganic membranes with very thin membrane layers, in particular microsieves with high flow rates, will result in an energyand cost-saving separation technology for present and future innovative applications, like micro liquid handling, modular fluidic systems or micro total analysis systems3. Track etched membrane t 1 0 Tortuous path membrane .. .:. . Pore size Density Log scale 1.0 2.0 5.0 10 2 0 . Pore size in micrometer Figure I , Pore size distribution of various membrane f i l t ers . New Membrane Materials and Processes: A Survey of Work in The Netherlands, C.A. Smolders in Membranes Oxford & IBH Publishing, 1992, ISBN 81-204-0686-9 83 0-7803-2503-6

A hydrogen separation module based on wafer-scale micromachined palladium-silver alloy membranes

A thin but strong and defect free palladium-silver (Pd-Ag) alloy membrane is fabricated with a sequence of well-known thin film and micromachining techniques. A microfabrication process also creates a robust wafer-scale membrane module, which is easy to be integrated into a membrane holder to have gastight connections to the outer world. The microfabricated membranes have been tested to determine the mechanical strength, hydrogen permeability and hydrogen selectivity. The membranes have high mechanical strength and can withstand pressures up to 3 bars at room temperature. High flow rates of up to 3.6 mol H/sub 2//m/sup 2/.s have been measured with a minimal selectivity of 1500 for H/sub 2//He. The membranes survived operation at 450/spl deg/C, which is a temperature relevant for practical application in industry, for more than 1000 hours.

Fabrication and characterization of MEMS based wafer-scale palladium-silver alloy membranes for hydrogen separation and hydrogenation/dehydrogenation reactions

In this paper, a MEMS based wafer-scale palladium-silver alloy membrane (MWSPdAgM) is presented. This membrane has the potential to be used for hydrogen purification and other applications. A palladium-silver alloy layer (Pd-Ag) was prepared by co-sputtering. The thin Pd-Ag alloy has high hydrogen selectivity, high permeation rate as well as high mechanical and chemical stability. Typical flow rates of 0.5 mol H/sub 2//m/sup 2/.s have been measured with a minimal selectivity of 550 for H/sub 2//N/sub 2/. Anodic bonding of thick glass to silicon was used to package the membrane and create a very robust module. The membrane has high mechanical strength and can withstand pressures up to 4 bars at room temperature. The presented fabrication method allows the development of a module for industrial applications that consists of a stack with a large number of glass/membrane plates.

A microsieve for leukocyte depletion of erythrocyte concentrates

A new ultra thin filtration membrane has been used for leukocyte removal from erythrocyte concentrates. This filtration membrane, an Aquamarijn Microsieve(R), has a high pore density and a narrow pore size distribution and shows good separation behaviour. The low surface roughness of the microsieve will contribute to the biocompatibility and will reduce cell rupture, in particular hemolysis, during filtration. In this paper a brief overview of the effects that occur during filtration will be given. Also the results of the experiments of leukocyte removal from erythrocyte concentrates will be discussed.

Ultra-low-power hydrogen sensing with palladium nanowires

Palladium nanowires (50 nm diameter, ~5 mum length) have been fabricated on silicon substrates using conventional techniques. The room temperature hydrogen sensing behavior of individual nanowires was investigated. The nanowires show a reversible response to hydrogen concentrations as low as 2.5 ppm with an ultra low power consumption of less than 10 nW. The electrical resistance increases nonlinearly with the hydrogen concentration. Response times (t90) vary from 5 seconds (H2 concentrations > 20%) to 30 seconds (H2 concentrations < 100 ppm). The response times can be decreased by increasing the applied bias at the cost of sensitivity.