Sensor fabrication

Many complex systems have been spread on liquid interfaces for a variety of reasons. We begin this chapter with a discussion of the behavior of synthetic polymers at the liquid-air interface. Most of these systems are linear macromolecules however, rigid-rod polymers and more complex structures are of interest for potential optoelectronic applications. Biological macromolecules are spread at the liquid-vapor interface to fabricate sensors and other biomedical devices. In addition, the study of proteins at the air-water interface yields important information on enzymatic recognition, and membrane protein behavior. We touch on other biological systems, namely, phospholipids and cholesterol monolayers. These systems are so widely and routinely studied these days that they were also mentioned in some detail in Chapter IV. The closely related matter of bilayers and vesicles is also briefly addressed. 

Even the tight controls in siUcon integrated circuit manufacturing are not yet sufficient to produce absolutely identical sensors on a single wafer. Cahbration of the final product is usually necessary, often by adjusting the value of a circuit element on the IC such as a resistor. The caUbration process can be automated, but it stiU adds to the cost of batch-fabricated sensors. Clever means of self-caUbration, particularly in field use, are constantiy being sought. 

Fig. 7.6 (a) Microscopic image of the micromachined hole introduced on the fiber cross section, (b) Microscopic image of the fabricated sensor head. Reprinted from Ref. 12 with permission. 2008 Optical Society of America... 

Figure 1. Smart dust mote and its components Micro-fabricated sensors, optical receiver, signal-processing and control circuitry the power source consists of a solar cell and a thick-film battery. (Derived with permission from ref 16. Copyright 2001 IEEE)...

An integrated multi-sensor system requires precise control of sensor characteristics, and may require 10 or more sensors in close proximity. To achieve this, we have to rely on microelectronic processes in order to fabricate sensors with small and precisely controlled feature sizes on silicon. 

Semiconductor fabrication techniques permit the feature size of Si-based devices to reach into the deep submicron regime [i]. Additionally, Si can be anodized electrochemically or chemically (e.g., in an HF-containing electrolyte) to produce a sponge-like porous layer of silicon, with pore dimensions that range from several microns in width to only a few nanometers [ii]. These properties of Si make it a useful substrate for fabricating sensor platforms, photonic devices and fuel cell electrodes [iii]. 

Figures 1IC-E show SEM images of test patterns of silver that were fabricated using pCP with hexadecanethiol, followed by selective chemical etching [102], The SAMs protect the underlying substrates from dissolving by blocking the dilSisional access of etchants. The ability to generate arrays of microstructures of coinage metals with controlled shapes and dimensions is directly useful in fabricating sensors and arrays of microelectrodes.

E691 Wilding, P. and Erickson, K.A. (1990). Evaluation of a hand-held micro-fabricated sensor system, the i-STAT 100, for rapid assay of electrolytes, glucose, urea nitrogen and hematocrit. Clin. Chem. 36, 1206, Abstr. 1185. 

The fabricated sensor films were exposed at room temperature to methanol and toluene vapors at the vapor pressure of 46 and 11 Torr, respectively. This corresponded to concentrations of 61,000ppm and 14,000ppm of methanol and toluene, respectively. 

Molecularly imprinted polymer recognition units are based upon template polymerization techniques (Haupt and Mosbach 2000). The MIP recognition units are formed in the presence of a template molecule that is later leached out or extracted, thus leaving complementary cavities embedded in the Ii nal structure of the polymer. These polymers display high chemical-binding affinity for molecules with structural similarities to the template molecule. Hence, MIPs can be used to fabricate sensors... 

Swager el al. [140] fabricated sensors for alkali ions and electron deficient organic molecules from polypolythiophene based materials X, XI and XII. High chemical reversibility, good steady-state response independent of exposure time and the regaining of its original state in the absence of a signal, were the characteristics of the device. 

Catrysse, M., Puers, R., Herdeer, C., Van Langenhove, L., Egmond, Van, et al., 2003. Fabric sensors for the measurement of physiological parameters. In Transducers 03, 12th International Conference on Solid-state Sensors, Actuators and Microsystems, Boston, MA. 

It has been known since the early stage of conducting polymer research that polyandine fibrils of 100 nm in diameter can naturally form on the surface of an electrode [4,40-45] with a compact microspheriod underlayer. Some recent work demonstrates that pure polyaniline nanofibers can be obtained without the need for any template by controlling the polymerization rate [46—48]. Although this process is not readily scalable from a materials point of view, such work could be very important for making functional devices, since nanofiber-coated electrodes can be used as a platform to fabricate sensors and transistors. Interconnected network-like structures with polyaniline nanoKnkers 10-50 nm wide have also been identified in polymer blends [49-51]. 

PIM-l thin films exhibit excellent optical sensing properties by changing their color on adsorbing organic vapor, even at very low concentration (Figure 10.12). Although the performance of a fiber-optic spectrometer is better than this, for PIM-l the hindrance effect due to humidity is less due to the hydrophobicity. The microporous nature, as well as the behavior towards solvent, is the key to fabricate sensor-based devices with PIMs. ... 

A review (Castano and Platan, 2014) on smart fabric sensors and E-textile technologies summarizes the basic principles and approaches employed when building fabric sensors as well as the most commonly used materials and techniques used in... 

Castano, L.M., Flatau, A.B., 2014. Smart fabric sensors and e-textile technologies a review. Smart Mater. Struct. 23 (5), 27 pages. 

Biomedical sensors imbedded in a WBSN may need to be modified to accommodate the user comfort and rehability. In addition the integrated fabric sensors should be small in size, flexible, and adaptable to E-Tex connectivity. 

Respiratory rate is measured by techniques based on either measuring thoracic expansion or based on measuring changes in skin impedance. For the former technique, most systems use strain gauges made from piezoresistive material combined with textile structures. Hertleer et al. reported a fabric sensor made of SS yam knitted in spandex belt [18]. For the latter technique, noninvasive skin electrodes are placed on the thorax, and the variation of the electrical impedance can be detected during respiration cycles. 

Trifonov T, Rodriguez A, Marsal LF, Pallares J, Alcubilla R (2008) Macroporous silicon a versatile material for 3D stracture fabrication. Sensors Actuators A 141 662-669 Uematsu M, Kageshima H, Shiraishi K (2002) Microscopic mechanism of thermal silicon oxide growth. Comput Mater Sci 24 229-234... 

Control of composition during deposition is more difficult difficult to fabricate sensors teproducibly with consistent characteristics... 

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