Silicones membrane applications

The use of silicones in membrane applications is relatively new. It is, however, a rapidly growing area as evidenced by a number of original papers and reviews published recently. Pervaporation with the use of polymer membranes has been recognized as a versatile separation process in the chemical industry.458 A study of PDMS as an active layer in the composite pervaporation membranes for separation of alcohols and esters has been reported.459 Two-dimensional... 

Small leakage currents or a transistor-like action of the junction are sufficient to generate a small current that may cause undesired passivation. This can be circumvented by application of an additional potential to the etching layer, shown by the broken line in Fig. 4.16 a. This electrochemical etch-stop technique is favorable compared to the conventional chemical p+ etch stop in alkaline solutions, because it does not require high doping densities. This etch stop has mainly been apphed for manufacturing thin silicon membranes [Ge5, Pa7, Kll] used for example in pressure sensors [Hil]. 

Desai, T.A. Hansford, D. Ferrari, M. Characterization of micromachined silicon membranes for immunoisolation and bioseparation applications. 

The removal of phospholipids has been studied most. The use of UF in this de-gumming process is based on the fact that amphiphilic phospholipids form reversed micelles in apolar solvents. With a MW of 20000, these micelles can easily be isolated. Gupta was the first to describe such an application. Phospholipids were completely removed with a 300-pm thick silicone membrane, and their concentration could be reduced to 16 ppm with a PSf membrane and to 23 ppm with a PAN membrane. Also, the color of the crude soybean oil was reduced and its metal content lowered [23]. Other work of Gupta mentions the addition of a solute. 

W. L. Robb, Thin silicone membranes — their permeation properties and some applications. Annals New York Academy of Sciences 146 (1986) 119. 

The miniaturized capacitive arrays play a critical role in the development of microsystems in biomedical applications since it can provide much higher sensitivity compared to the single-element capacitive sensor. Each capacitor of capacitance-based membrane sensor array is composed of a stretchable electrode. Satyanarayana et al. introduce a 3 x 3 array of individual sensor unit cells with an area of 1 cm [2]. Tsouti et al. reported a capacitive membrane-based sensor array made up of 256 elements with an area 1.44 cm [3]. Despite the large number of elements, only 32 contacting pad are used to address all the elements. The sensing units of the array have already been described in the capacitive membrane sensor section. The stretchable electrodes of the device are deposited on a silicon membrane and the counter electrode is fabricated... 

DRIE is essential in fabrication of small features. DRIE is capable of reproducing accurately practically any features that have been defined by lithography. Fluid dynamics studies of microflows utilize DRIE for making accurately dimensioned microchannels and orifices. Membrane stmctures with small holes are useful in many drug delivery, fuel ceU, and cell probing applications. In a patch clamp application, submicrometer apertures have been etched in 20 pm thick silicon membrane. In catalysis studies a small DRIE fabricated capillary leak into a mass spectrometer enabled increased sensitivity and fast respmise times. Integration of... 

Drug delivery concepts have been presented that are based on microfabrication. Possible applications include micromachined silicon membranes to create implantable biocapsules for the immimoisola-tion of pancreatic islet cells, as a possible treatment for diabetes and sustained release of injectable drugs needed over long time periods. 

Chapter 5 will introduce the reader to the reasons for miniaturizing fuel cells and to the specifications required by this miniaturization. It will then show what kinds of fuel cells can fit to these specifications and which fuels can be employed to supply them. The techniques presently used for the realization of miniature fuel cells will be described, underlining particularly the growing part of the microfabrication techniques inherited from microelectronics. It will present an overview on the applications of these latter techniques on miniature fuel cells by presenting several solutions developed throughout the world. It will finally detail, as an example, the complete fabrication process of a particular microfabricated fuel cell based on a silane-grafted porous silicon membrane as the proton-exchange membrane instead of a common ionomer such as Nafion . 

Torres N, Duch M, Santander J et al (2009) Porous Silicon Membrane for Micro Fuel Cell Application J. New Mater electrochem Syst 12(2-3) 93-96 Torres N, Duch M, Santander J et al (2009) Si micro-turbine by proton beam writing and porous silicon micromachining. Nucl Instr Meth Phys Res Sect B-Beam Interact Mater Atoms 267(12-13) 2292-2295... 

This chapter focuses on cell immunoisolation and bio-filtration applications of porous silicon membranes. After an introduction on immunoisolation for the treatment of diabetes, the different materials used for that function are reviewed and compared. Applications involving porous silicon are then presented in more detail. Other uses of microfabricated porous silicon membranes in hemofiltration and protein sorting are also discussed. 

The handbook chapter Porous Silicon Membranes reviews the different fabrication routes and applications of mesoporous and macroporous silicon membranes. Among inoiganic membrane materials, porous silicon has been the most studied for immunoisolation (macroencapsulation). Desai and Ferrari developed a micro fabrication technique to produce controlled slit pores in 0.5-5 pm-thick Si membranes (see Fig. 2) (Leoni and Desai 2004 Desai et al. 1998, 2000a, b, 2004). The width of these pores (constant length of 45 pm) can be varied from 7 to 100 nm with less than 5 % of variation. The pores are produced by dissolution of a thermally grown sacrificial silicon oxide layer. These membranes present a porosity of 1 % (Leoni and Desai 2004 Desai et al. 2000b). 

Desai TA, Hansford DJ, Ferrari M (2000a) Micromachined interfaces new approaches in cell immunoisolation and biomolecular separation. Biomol Eng 17(l) 23-36 Desai TA, Hansford DJ, Leoni L, Essenpreis M, Ferrari M (2000b) Nanoporous anti-fouling silicon membranes for biosensor applications. Biosens Bioelectron 15(9-10) 453-462 Desai TA, West T, Cohen M, Boiarski T, Rampersaud A (2004) Nanoporous microsystems for islet cell replacement. Adv Drug Deliv Rev 56(11) 1661-1673 Dunleavy M (1996) Polymeric membranes. A Rev Appl Med Dev Tech 7(4) 18-21... 

Porosified silicon membranes of defined thicknesses were first studied in the 1990s and have now been realized by electrochemical anodization, micromachining techniques, and the annealing of ultrathin deposited films. The three fabrication routes produce very different morphologies and levels of porosity. A variety of applications have been explored for both macroporous and mesoporous membranes and these are also surveyed. Wholly microporous membranes in silicon, where all pores have diameters less than 2 nm, have not been achieved to date. 

Table 1 collates the varied potential applications that have been explored for porous silicon membranes, both mesoporous and macroporous. Membrane thicknesses vary from submicron to tens of microns to the full thicknesses of wafers (hundreds of microns). Most studies have utilized anodization to realize the porosity. Notable exceptions highlighted in Table 1 are the micromachining and deposition/anneal techniques already mentioned. 

Chu KL et al (2006) A nanoporous silicon membrane electrode assembly for on-chip micro fuel cell applications. J Microelectromech Syst 15(3) 671... 

Cruz S et al (2005) F abrication and optimization of porous silicon substrates for diffusion membrane applications. J Electrochem Soc 152(6) C418-C424... 

Desai TA et al (2000) Nanoporous anti-fouling silicon membranes for biosensor applications. Biosens Bioelectr 15 453-462... 

O Halloran GM et al (1998) Porous silicon membrane for humidity sensing applications. Sensor applications paper presented at Eurosensors Xll, 13-12 Sept, pp 901-904 Ohkura Y et al (2013) Flash ignition of freestanding porous silicon films effects of film thickness and porosity. Nano Lett 13(ll) 5528-5533... 

Pagonis DN et al (2006) Free-standing macroporous silicon membranes over a large cavity for filtering and lab-on-chip applications. Microelectron Eng 83 1421 Palavicini A et al (2013) Infrared transmission in porous silicon multilayers. Opt Photonic J 3 20-25... 

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