Microfabrication Environment

Microfabrication needs to be carried out in an extremely clean environment known as clean room, because, in dusty environment, standard particles of micrometer size tend to absorb on the surface and change the nature of the microstructure being fabricated. A clean room is an environment, that is, regulated for temperature and humidity and is permanently traversed by flux of air. The particle introduced to the workplace by human beings and the chemical processes at work are filtered out. 

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This article highlights the most commonly used methods to perform cell sorting in microfluidic devices. Some of the microfluidic techniques presented here are a miniaturized version of conventional laboratory analysis techniques and devices, and take advantage of the reduced sample volumes and increased speed of analysis. Many other types of devices have successfully exploited the novel effects that arise in a microfabricated environment that are not evident on the macroscale. 

In contrast to other analytical methods, ion-selective electrodes respond to an ion activity, not concentration, which makes them especially attractive for clinical applications as health disorders are usually correlated to ion activity. While most ISEs are used in vitro, the possibility to perform measurements in vivo and continuously with implanted sensors could arm a physician with a valuable diagnostic tool. In-vivo detection is still a challenge, as sensors must meet two strict requirements first, minimally perturb the in-vivo environment, which could be problematic due to injuries and inflammation often created by an implanted sensor and also due to leaching of sensing materials second, the sensor must not be susceptible to this environment, and effects of protein adsorption, cell adhesion, and extraction of lipophilic species on a sensor response must be diminished [13], Nevertheless, direct electrolyte measurements in situ in rabbit muscles and in a porcine beating heart were successfully performed with microfabricated sensor arrays [18],... 

In general, optical-based pH measurement techniques require relatively expensive and cumbersome instruments, and their sophisticated method cannot be easily carried out for routine assay. Interfering contact and reactions of the dye molecules, particularly considering in-vivo measurements, cannot be excluded [34], Some other possible factors, such as a weaker signal at shorter response times, complications in microfabrication, and difficulties in attaching the chemical or biological agents to the small fiber tip, are potential limitations for the application of these optical sensors to in-vivo measurements in micro environments [35]. 

Currently, new advances in material science with application in the fabrication of biologically functional surface demand very sophisticated procedures for both preparation and characterization, including dry and wet chemical layers deposition and nano- and microfabrication [222]. While most biosensors are designed to work in mild conditions, the development of new devices for harsh environment is a necessity. However, whatever the final application of the biosensor device is, the electrode material used for their... 

In vivo, living cells constantly communicate with their surroundings. The interaction between the cells and the extracellular microenvironment regulates cell behavior. With nano and microfabrication techniques, researchers are now able to control cell functions and responses through precise manipulation over the physical and chemical environment around a cell, such as the surface chemical composition and topology of the substrate, the medium composition, and the cellular microenvironment [69],... 

The electrode elements of an electrochemical sensor are often metallic, semiconductive, or inert. For metallic electrode elements, noble metals such as gold, platinum, and silver are often used in conventional electrochemical sensors. Mercury or amalgam as electrode materials for microfabricated electrochemical sensors are seldom used due to the difficulty involved. Because mercury has a relatively high vapor pressure, it does not lend itself well in any fabrication process using a vacuum or low-pressure environment. The formation of amalgam requires the use of mercuric ion containing reagent. This step can be elaborate and complicated. 

Microfabrication techniques can be used effectively in manufacturing thick-and thin-film electrochemical sensors. However, a well planned package procedure for the sensor is essential in rendering the sensor practical. The selection of the packaging material to protect the sensor s integrity in a testing environment is a key to its functioning properly. 

Micromachined and microfabricated electrochemical sensors have been used either per se, or as part of a sensor system, in many practical applications. This includes various biosensors and chemical sensors reported in research literature. An example of a practical electrochemical sensor is the yttria-stabilized zirconium dioxide potentiometric oxygen sensor used for fuel-air control in the automotive industry. Thick-film metallization is used in the manufacture of this sensor. Even though the sensor is not microsize, this solid electrolyte oxygen sensor has proven to be reliable in a relatively hostile environment. It is reasonable to anticipate that a smaller sensor based on the same potentiometric or the voltammetric principle can be developed using advanced microfabrication and micromachining techniques. 

The use of polymer-coated cantilevers such as microfabricated beams of silicon is becoming more popular as the basis of nanomechanical sensors [11]. These devices detect physical and chemical interactions between the reactive layer on the surface and the environment [8]. When the polymer interacts with a gaseous species, it swells and causes the cantilever to bend as a result of surface stresses when used in the static mode. In the dynamic mode, the cantilever acts as a microbalance, which responds to changes in resonance frequency. Savran s group at Purdue University has been researching the micromechanical detection of proteins by use of aptamer-based receptor molecules [12]. 

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