Silicone based membrane

Not only hydrocarbon systems, but also silicon rubbers (Lee 1986), can be pyrolyzed to obtain silicon-based membranes. Details of the pyrolysis are mainly reported for nonmembrane applications. A recent example is the paper of Boutique (1986) for the preparation of carbon fibers used in aeronautical or automobile constructions. 

Coming, diameter 1 mm, wall thickness about 0.2 mm) have been used to make pyrolyzed silicon-based membranes. Pyrolysis was carried out in a closed chamber in N2 or He atmosphere at temperatures between 600 and 800°C followed by an oxidation step in air at temperatures of 500-900°C. 

Among the various materials are crosslinked PAN, polyphosphazenes, polyphe-nylenesulfide, polyetheretherketone, and various polymer blends [28-31]. Particularly interesting is the use of zeolites as filler in organic polymers, which aims at improving the performance of (silicone-based) membranes for separations in nonpolar solvents, by adding more cross-links to the membrane material [32, 33]. 

Boock R, Rixman M (2011) Silicone based membranes for use in implantable glucose sensors. US 8,543,184B2... 

Silicones find practical application in different membrane unit operations for treating gaseous and liquid mixtures. This is due to their solubility controlled transport, which allows the selective separation of organics from air or from water. Polymer blending, polymer grafting, addition of different solid fillers or ionic Hquids, are the most effective strategies for improving the stabihty as well as the selective transport of silicones. The industrial applications of silicone-based membrane systems present environmental benefits such as reduced waste and recovered/recycled valuable raw materials that are currently lost to fuel or to the flares. 

However, there seems to be some drawback in the solubility or dispersibility of ion-sensing material in silicone rubber. This is mainly because silicone rubber does not contain a large quantity of plasticizer as the membrane solvent, in which neutral carriers can be dissolved easily, unlike in plasticized-PVC ion-sensing membranes. This issue is serious, especially with silicone-rubber membranes containing neutral carriers that show high crystallinity. Valinomycin, a typical ionophore, seems applicable to silicone-rubber-based K" -selec-tive electrodes [7,8,12-14]. Conventional crown-ether-based neutral carriers are also quite soluble in silicone rubber. 

FIG. 4 Selectivity comparison among silicone-rubber-membrane Na -ISFETs based on calixarene neutral carriers (1), (2), and (5). 

FIG. 6 Time-course changes of potential response for silicone-rubber-membrane Na+-selective electrodes based on neutral carriers (5), (2), and bis(12-crown-4) on changing Na concentration from 1 X 10 to 3 X 10 M. (From Ref. 22.)... 

FIG. 11 Potential response of Na+-selective electrodes based on silicone-rubber membranes modified chemically by triethoxysilylated calix[4]arene (8) (O) without anion excluder ( ) with TFPB ( ) modified chemically by triethoxysilylated tetraphenylborate (9) as well. (From Ref. 44.)... 

Applicability in biological ion assay is an important factor for biocompatible potentio-metric ion sensors. Attempts were made to determine Na" " concentrations in human blood sera by using silicone-rubber membrane Na+-ISFETs based on (5) [Fig. 17(a)] [29]. The found values for Na concentration in undiluted, 10-fold diluted, and 100-fold diluted serum samples are in good agreement with the Na" " calibration plots. Even in the undiluted serum samples, only a slight potential shift was observed from the calibration. This indicates that the calixarene-based silicone-rubber-membrane Na+-ISFETs are reliable on serum Na assay. For comparison with the silicone-rubber membrane, Na -ISFETs with corresponding plasticized-PVC membrane containing (2) or (5) were also tested for the Na assay. The found values of Na" " concentration... 

The Na -selective electrodes based on silicone-rubber membranes modified chemically by (8) and (9), were also investigated for Na assay in control serum and urine [22]. The found values for the Na concentrations in both of the serum and urine samples are in good agreement with their corresponding actual values with a relative standard deviation of about 1%. These results suggest that the Na -selective electrodes based on silicone-rubber membranes modified chemically by calix[4]arene neutral carrier (8) are reliable on assay in human body fluid. 

The design of bioeompatible (blood compatible) potentiometric ion sensors was described in this chapter. Sensing membranes fabricated by crosslinked poly(dimethylsiloxane) (silicone rubber) and sol gel-derived materials are excellent for potentiometric ion sensors. Their sensor membrane properties are comparable to conventional plasticized-PVC membranes, and their thrombogenic properties are superior to the PVC-based membranes. Specifically, membranes modified chemically by neutral carriers and anion excluders are very promising, because the toxicity is alleviated drastically. The sensor properties are still excellent in spite of the chemical bonding of neutral carriers on membranes. 

Silicon-based pressure sensors are amongst the most common devices making use of this process. A thin low-n-doped epitaxial layer on the wafer determines an etch stop depth and thus the thickness of e.g. the pressure sensor membrane. 

J. Pick, K. Toth, E. Pungor, M. Vasak, and W. Simon, A potassium-selective silicone-rubber membrane electrode based on neutral carrier, Anal Chim Acta 64, 477-480 (1973). 

In this last section some recent developments are mentioned in relation to gas separations with inorganic membranes. In porous membranes, the trend is towards smaller pores in order to obtain better selectivities. Lee and Khang (1987) made microporous, hollow silicon-based fibers. The selectivity for Hj over Nj was 5 at room temperature and low pressures, with permeability being 2.6 x 10 Barrer. Hammel et al. 1987 also produced silica-rich fibers with mean pore diameter 0.5-3.0nm (see Chapter 2). The selectivity for helium over methane was excellent (500-1000), but permeabilities were low (of the order of 1-10 Barrer). 

Lee, K. H. and S. J. Khang. 1986. A new silicon-based material formed by pyrolysis of silicon rubber and its properties as a membrane. Chem, Eng. Common. 44 121-32. 

Figure 5.8 — Probe-type sensor based on continuous circulation of a stream containing an acid-base indicator for the batch determination of COj in sea water, (a) Reagent delivery capillary, (d) Reagent exit capillary, (c) Optical fibre from source, (d) Optical fibre to detector, (e) White silicone rubber membrane. (/) White silicone sealant, (g) Epoxy resin, (/i) 0-ring. (/) Sensor housing. (/) Optical cable. (Reproduced from [12] with permission of the American Chemical Society).

---