Also, piezoelectric technologies are better suited than electromagnetic ones for MEMS implementation, because of the limitations in magnets miniaturization with current state-of-the-art microfabrication processes [21].2.?Transduction PrincipleThe piezoelectric effect converts mechanical strain into electric current or voltage. It is based on the fundamental structure of a crystal lattice. Certain crystalline structures have a charge balance with negative and positive polarization, which neutralize along the imaginary polar axis. When this charge balance is perturbed with external stress onto the crystal mesh, the energy is transferred by electric charge carriers creating a current in the crystal. Conversely, with the piezoelectric effect an external charge input will create an unbalance in the neutral charge state causing mechanical stress.

The connection between piezoelectricity and crystal symmetry are closely established. The piezoelectric effect is observed in crystals without center of symmetry, and the relationship can be explained with monocrystal and polycrystalline structures.In a monocrystal (Figure 3) the polar axes of all of the charge carriers exhibit one-way directional characteristics. These crystals demonstrate symmetry, where the polar axes throughout the crystal would lie unidirectional even if it was split into pieces.Figure 3.Monocrystal.Instead, a polycrystal (Figure 4) is characterized by different regions within the material with different polar axes. It is asymmetrical because there is no point at which the crystal could be cut that would leave the two remaining pieces with the same resultant polar axes.

Figure 4.Polycrystal.In order to attain the piezoelectric effect, the polycrystal is heated to the Curie point along with strong electric field. The heat allows the molecules to move more freely and the electric field forces the dipoles to rearrange in accordance with the external field (Figure 5).Figure 5.(a) Polarizations; (b) Surviving Polarity.As a result, the material possesses piezoelectric effect: a voltage of the same polarity as of the poling voltage appears between electrodes when the material is compressed; and opposite polarity appears when stretched. Material deformation takes place when a voltage Dacomitinib difference is applied, and if an AC signal is applied the material will vibrate at the same frequency as the signal [23�C25].Piezoelectricity is governed by the following constitutive equations, which link the stress T, the strain S, the electric field E and the electrical induction D:{Tp=cpqESq?ekpEkDi=eiqSq+?ikSEk(1)where cpqE is the Young’s modulus, ekp is the piezoelectric coefficient and ?ikS is the clamped permittivity.

The global selectivity of this sensing method is based on the common characteristics of each basic taste substance, for example, bitterness: high hydrophobicity, sourness: proton donors, saltiness: metal cations. The taste sensor system is used in the food, beverage and pharmaceutical industries. Some products developed by these industries using the taste sensor system are now in common use [30].The taste sensor system can quantify the intensities of each basic taste by the membrane potential measurement. Because of the measurement principle, it is difficult to evaluate sweetness using only one sensor electrode. Since sweet substances consist of nonelectrolytes (sugars), positively charged electrolytes (peptides) and negatively charged electrolytes (sulfonyl amides) under acidic conditions (most food environments), three types of sweetness sensor membrane are required for each electric charge type of sweetener.

The sensor in the taste sensing system for nonelectrolytes (sugars and sugar alcohols) has already been developed and commercialized as a sweetness sensor [31,32]. The commercially available sweetness sensor is used in the food, beverage and pharmaceutical industries to estimate the sweet taste intensity of sugars and sugar alcohols. As mentioned above, in principle, it is difficult to develop a sweetness sensor for all sweet substances. Hence, we decided to develop two additional types of sweetness sensor, that is, for positively charged sweeteners (peptides) and for negatively charged sweeteners (sulfonyl amides).

Both positively and negatively charged electrolyte sweeteners are mainly included in high-p
Nanomechanical sensors have attracted considerable interest, as they are a promising tool for real-time and label-free detection of chemical gases and biomolecules [1�C7]. These molecular adsorbates introduce surface stresses upon the detective surface layer and sequentially produce measurable displacement and stress fields in the sensors [8,9]. For cantilever-shaped nanomechanical AV-951 sensors, the output signals are often measured as tip deflections using a position-sensitive photodetector [6] or as strain/stress changes near clamping regions using a Wheatstone bridge [6,10].The sensitivity of the induced surface stress dominates the performance of cantilever-shaped nanomechanical sensors. Understanding the physical mechanisms of adsorption-induced surface stress and their influences on the overall displacement and stress fields are the key to designing next-generation nanomechanical sensors. Surface stresses due to molecular adsorption often arise from two main sources: weak inter-adsorbate interactions and strong adsorbate�Csubstrate interactions [1,9,11].

Figure 2.The gas chamber, pump and sensor array in the e-nose.Table 1.Sensors used for the e-nose prototype.The signal processing and wireless communication unit is shown in Figure 3, which is the brain of the e-nose prototype. The MicaZ node (from Crossbow Technology Inc., USA) is used and a voltage following circuit is situated on the data acquisition board. This unit is in charge of data acquisition, data processing and data transfer. The interface circuit uses only a voltage follower as a buffer between the sensor output and the A/D converter, which makes the system less sensitive to external disturbance. The MicaZ has advantages of the small physical size, low cost and low power consumption, making it ideal for this odor monitoring application. The MicaZ includes a processor and radio.

The processor on the MicaZ primary consists of Atmegal-128L, which is in charge of data acquisition control, data processor control and data transfer control. The radio on the MicaZ primarily consists of a Chipcon CC2420, a basic 2,400 MHz ISM band transceiver compliant to IEEE 802.15.4/ZigBee protocol. Therefore, this unit of data acquisition, data processing, and data transfer ensures continuous data measurement.Figure 3.The interface board and the MicaZ for the e-nose.2.2. Circuit design for the gas sensorsA MOS gas sensor circuit and its interface diagram are shown in Figure 4, where RH is the gas sensor heater; Rs is the output resistance of the gas sensor, which changes with the variation of odor strength due to the presence of detectable odors.

The voltage Vout on the resistor RL will be changed as RS changes, the voltage Vout can be measured, and then output resistance Rs can be calculated as:Rs=Vc��RLVout?RL(1)Figure 4.Block diagram of the data acquisition circuit.The odor strength can be obtained from the table of sensor sensitivity characteristics curve by using the calculated Rs value.3.?MOS gas sensor noise analysisNoise unavoidably appears at all times in an odor sensing system. The two most common forms of noise are the circuit factor noise and environmental factor noise (see Figure 5).Figure 5.Block diagram of the inputs and outputs of an MOS gas sensor.3.1. Circuit factors noiseCircuit noise appears in the odor strength measurement process because the MOS gas sensors must work at the temperature of about 300��C, resulting in high resistor thermoelectric noise. Every semiconductor component of the interface Cilengitide circuit, such as voltage follower and regulated resistor, has its own circuit noise. Random movement of electrons and other charge carriers in resistors and semiconductors variation at random speed will result in random noise. Some noise also comes from factors related to the MOS gas sensors themselves.