**1. Introduction**

#### **1.1. Sensor definition and classification**

Sensors are very critical components in all devices and measurement systems. They have been widely used in a lot of fields such as science, medicine, automated manufacturing, environ‐ mental monitoring, and so on. Some cheap sensors are finding their ways applying into all sorts of consumer products, from children's toys, dishwashers to automobiles. To some extent, sensors are multidisciplinary and interdisciplinary field of endeavor. This chapter introduces sensor's basic definition and features, biomedical sensors, equivalent components in circuit, signal filters and amplifiers, biomeasurement systems and design.

There are a lot of terms which are often used for sensors including transducer, meter, detector, and gage. Defining the term sensor is a very difficult task. At present, there is not a uniform definition which is agreed by all of us. The most widely used definition is that which has been applied to electrical transducer by the Instrument Society of America(ANSI MC 1, 1975): "Transducer —A device which provides a usable output is in response to a specified measur‐ and." Furthermore, national standard of China points out that sensors consist of sensing component, converting device and electronic circuit. A transducer is more generally defined as a device which converts energy from one form to another. Output of sensor can be an optical, electrical, chemical, or mechanical signal. In the field of electrical engineering, the measurand is physical, chemical, or biological property or condition measured; hence output of biological signal should be an electrical signal, too.

The words sensor and transducer are both commonly used in the context of measurement systems, and often in an interchangeable manner. Transducer is used more in the United States, and sensor has great popularity in Europe and China. The blurring of lines between the exact meaning of sensors and transducers leads to a degree of confusion. Most but not all sensors are transducers, employing one or more transduction mechanisms to produce an electrical

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output signal. According to the basic sensing principle, sensors are classified into mechanical sensors, electrochemical sensors, biosensors, optical sensors, semiconductor sensors, magnetic sensors, and thermal sensors. From different viewpoints, there are different classifying methods. According to the physical parameters measured by sensors, sensors are classified into resistance displacement sensor, inductive displacement sensor, capacitive displacement sensor, piezoelectric pressure sensor, laser interferometer displacement sensor, bore gagging displacement sensor, ultrasonic displacement sensor, optical encoder displacement sensor, optical fiber displacement sensor, optical beam deformation sensor, flow sensor, imaging sensor, temperature sensor, intelligent sensor and chemical ingredient sensor. Biomedical sensors are used to gain the information on body and pathology, which is a branch of bio‐ medical engineering. Biomedical sensors are classified into physical sensor, chemical sensor and biosensor. Physical sensor could be employed to measure blood pressure, body temper‐ ature, blood flux, blood viscosity, biological magnetic field, etc. Chemical sensor is utilized to detect the ingredient and concentration of body liquid such as PH value, Ca+ concentration, glucose concentration, etc. Biosensor is used to sense enzyme, antigen, antibody, hormone, DNA, RNA and microbe. In nature, biosensor is a kind of chemical sensor, which is mainly used to detect biological signals.

#### **1.2. Sensor package and specifications**

Packaging of certain biomedical sensor is an important consideration during the design, fabrication, and use of the device. Obviously, the biomedical sensor has to be safe, soft, and reliable for biomedical sensors often touch the body skin or inner organs of patients. In the development of implantable biosensor, an additional key issue is to consider the biocompati‐ bility of sensor and operational lifetime in body. When a biomedical sensor is implanted into the body, it inevitably contacts with body fluids. Then body will affect the function of bio‐ medical sensor, or sensor will affect the site that it is implanted. For example, protein absorp‐ tion and cellular deposition can alter the permeability of sensor packaging that is designed to both protect sensor and allow free chemical diffusion of certain analytics between body fluids and the biosensor. Unsuitable packaging of implantable biomedical sensor could lead to drift and a gradual loss of sensor sensitivity and stability overtime. Furthermore, inflammation of tissue, infection, or clotting in a vascular site could produce some harmful or adverse effects on biomedical sensor. Hence, the material used in the construction of sensor's outer body must be biocompatible because they play a crucial pole in determining an overall performance and longevity of implantable biomedical sensor. One convenient method is to utilize various polymer covering material and barrier layers to prevent the toxic sensor components from coming into body. It's very important that packaging material of biomedical sensor must prevent the chemical diffusion of harmful ingredient between biomedical sensor and outer body.

Accurate medical diagnostic procedures require the stringent specifications on the design and use of biomedical sensor. Depending on the intended applications, the performance specifi‐ cations of biomedical sensor may be evaluated to ensure that the measurement meets the design specifications.

In order to understand sensor's performance characteristics, it is very important to learn some of the common terminology associated with sensor specifications. The following definitions are commonly used to describe sensor characteristics and select sensor for particular applica‐ tions.

#### **(1) Measurement range**

The range of sensor corresponds to the minimum and maximum operation limits that sensor is expected to measure accurately. For example, a pressure sensor may have a nominal performance over the operating range from 0 Pa to 10MPa. **(1)Measurement range**  The range of sensor corresponds to the minimum and maximum operation limits that sensor is expected to measure accurately. For example, a pressure sensor may have a nominal

#### **(2) Sensitivity** performance over the operating range from 0 Pa to 10MPa.

output signal. According to the basic sensing principle, sensors are classified into mechanical sensors, electrochemical sensors, biosensors, optical sensors, semiconductor sensors, magnetic sensors, and thermal sensors. From different viewpoints, there are different classifying methods. According to the physical parameters measured by sensors, sensors are classified into resistance displacement sensor, inductive displacement sensor, capacitive displacement sensor, piezoelectric pressure sensor, laser interferometer displacement sensor, bore gagging displacement sensor, ultrasonic displacement sensor, optical encoder displacement sensor, optical fiber displacement sensor, optical beam deformation sensor, flow sensor, imaging sensor, temperature sensor, intelligent sensor and chemical ingredient sensor. Biomedical sensors are used to gain the information on body and pathology, which is a branch of bio‐ medical engineering. Biomedical sensors are classified into physical sensor, chemical sensor and biosensor. Physical sensor could be employed to measure blood pressure, body temper‐ ature, blood flux, blood viscosity, biological magnetic field, etc. Chemical sensor is utilized to detect the ingredient and concentration of body liquid such as PH value, Ca+ concentration, glucose concentration, etc. Biosensor is used to sense enzyme, antigen, antibody, hormone, DNA, RNA and microbe. In nature, biosensor is a kind of chemical sensor, which is mainly

Packaging of certain biomedical sensor is an important consideration during the design, fabrication, and use of the device. Obviously, the biomedical sensor has to be safe, soft, and reliable for biomedical sensors often touch the body skin or inner organs of patients. In the development of implantable biosensor, an additional key issue is to consider the biocompati‐ bility of sensor and operational lifetime in body. When a biomedical sensor is implanted into the body, it inevitably contacts with body fluids. Then body will affect the function of bio‐ medical sensor, or sensor will affect the site that it is implanted. For example, protein absorp‐ tion and cellular deposition can alter the permeability of sensor packaging that is designed to both protect sensor and allow free chemical diffusion of certain analytics between body fluids and the biosensor. Unsuitable packaging of implantable biomedical sensor could lead to drift and a gradual loss of sensor sensitivity and stability overtime. Furthermore, inflammation of tissue, infection, or clotting in a vascular site could produce some harmful or adverse effects on biomedical sensor. Hence, the material used in the construction of sensor's outer body must be biocompatible because they play a crucial pole in determining an overall performance and longevity of implantable biomedical sensor. One convenient method is to utilize various polymer covering material and barrier layers to prevent the toxic sensor components from coming into body. It's very important that packaging material of biomedical sensor must prevent the chemical diffusion of harmful ingredient between biomedical sensor and outer

Accurate medical diagnostic procedures require the stringent specifications on the design and use of biomedical sensor. Depending on the intended applications, the performance specifi‐ cations of biomedical sensor may be evaluated to ensure that the measurement meets the

used to detect biological signals.

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body.

design specifications.

**1.2. Sensor package and specifications**

Sensitivity refers to the ratio of output change for a given input change. Another way to define sensitivity is to find the slope of calibration line relating the input to the output, as illustrated in figure 1.A high sensitivity implies that a small change in input causes a large change in output. **(2)Sensitivity**  Sensitivity refers to the ratio of output change for a given input change. Another way to define sensitivity is to find the slope of calibration line relating the input to the output, as illustrated in figure6.1.A high sensitivity implies that a small change in input causes a large

For example, a pressure sensor may have a sensitivity of 0.4*V* / *Pa* ; that's to say, the output of this sensor will change 0.4V for 1Pa change in input pressure. If the calibration curve is linear seen in figure 1 (a), then sensitivity of sensor will be constant, whereas the sensitivity of sensor will vary with the input when the calibration is nonlinear, as in figure 1 (b). Alternatively, sensitivity can be defined as the smallest change in input that will result in a detectable change in sensor output. change in output. For example, a pressure sensor may have a sensitivity of /4.0 *PaV* ; that's to say, the output of this sensor will change 0.4V for 1Pa change in input pressure. If the calibration curve is linear seen in figure6.1 (a), then sensitivity of sensor will be constant, whereas the sensitivity of sensor will vary with the input when the calibration is nonlinear, as in figure6.1 (b). Alternatively, sensitivity can be defined as the smallest change in input that will result in a detectable change in sensor output.

Figure6.1 Input versus output calibration curve of a typical sensor **Figure 1.** Input versus output calibration curve of a typical sensor

#### **(3)Accuracy (3) Accuracy**

**(5)Resolution** 

**(6)Reproducibility** 

Accuracy refers to the difference between the true value and the actual value measured by sensor. Classically, accuracy is expressed as a ratio between the preceding difference and the true value; it is specified as a percent of full-scale readings. Here, note that the true value could be traceable to a primary reference standard. **(4)Precision**  Accuracy refers to the difference between the true value and the actual value measured by sensor. Classically, accuracy is expressed as a ratio between the preceding difference and the true value; it is specified as a percent of full-scale readings. Here, note that the true value could be traceable to a primary reference standard.

Precision refers to the degree of measurement reproducibility under the same conditions. Very reproducible readings indicate a high precision. Precision should not be confused with accuracy. For an example, measurement may be very precise but not necessary accurate.

When the input is increased from some arbitrary nonzero value, the output of a sensor will not change until a certain input increment is exceeded. Accordingly, resolution is defined

as the smallest distinguishable input change that can be detected with certainty.

### **(4) Precision**

Precision refers to the degree of measurement reproducibility under the same conditions. Very reproducible readings indicate a high precision. Precision should not be confused with accuracy. For an example, measurement may be very precise but not necessary accurate.

## **(5) Resolution**

When the input is increased from some arbitrary nonzero value, the output of a sensor will not change until a certain input increment is exceeded. Accordingly, resolution is defined as the smallest distinguishable input change that can be detected with certainty.

#### **(6) Reproducibility**

Reproducibility describes how close measurements are when same input is repeatedly exerted into same sensor under same conditions. When the range of measurement is small, the reproducibility is very high. For example, a temperature sensor may have a reproducibility of ±0.1V/℃ for a measurement range from 20℃ to 80℃. Here, what need to be noticed is that reproducibility can vary depending on the measurement range. In other words, readings can be highly reproducible over one range and less reproducible over a different operating range.

#### **(7) Offset**

Offset refers to the output value when input value is zero, seen in figure 1 (a) and (b).

#### **(8) Linearity**

Linearity of sensor also called nonlinear error of sensor's characteristic curve; it is a measure‐ ment of the maximum deviation between calibration curve and fitting curve. Usually, linearity of sensor is expressed as a percent of full-scale readings or a percent of the actual readings. Linearity could be expressed as the following equation:

$$
\sigma\_{L\text{incarity}} = \pm \frac{\Delta L\_{\text{max}}}{Y\_{F.S}} \times 100\% \tag{1}
$$

Here, *σLinearity* ——linearity of sensor; *ΔL* max =max(*Vcal* −*V fit*), *ΔL* max is the maximum error between calibration line and fitting line; *YF* .*<sup>S</sup>* is the full-scale meaning value of sensor, *YF* .*<sup>S</sup>* =*Y*max −*Yo*, *Y*max is the maximum deviation of output, *Y*0 is the deviation without any input value.

#### **(9) Response time**

The response time indicates that the time it takes a sensor to reach a percent of its final steadystate value when input of sensor is changed. For example, it takes 10 seconds for pressure sensor to reach 95 percent of its maximum value when a change in pressure of 1Pa is measured. Ideally, a short response time indicates the ability of a sensor to respond quickly to change in input.

#### **(10) Drift**

**(4) Precision**

180 Advances in Bioengineering

**(5) Resolution**

**(6) Reproducibility**

**(7) Offset**

**(8) Linearity**

value.

input.

**(9) Response time**

Precision refers to the degree of measurement reproducibility under the same conditions. Very reproducible readings indicate a high precision. Precision should not be confused with accuracy. For an example, measurement may be very precise but not necessary accurate.

When the input is increased from some arbitrary nonzero value, the output of a sensor will not change until a certain input increment is exceeded. Accordingly, resolution is defined as

Reproducibility describes how close measurements are when same input is repeatedly exerted into same sensor under same conditions. When the range of measurement is small, the reproducibility is very high. For example, a temperature sensor may have a reproducibility of ±0.1V/℃ for a measurement range from 20℃ to 80℃. Here, what need to be noticed is that reproducibility can vary depending on the measurement range. In other words, readings can be highly reproducible over one range and less reproducible over a different operating range.

Offset refers to the output value when input value is zero, seen in figure 1 (a) and (b).

Linearity could be expressed as the following equation:

s

Linearity of sensor also called nonlinear error of sensor's characteristic curve; it is a measure‐ ment of the maximum deviation between calibration curve and fitting curve. Usually, linearity of sensor is expressed as a percent of full-scale readings or a percent of the actual readings.

> max .

Here, *σLinearity* ——linearity of sensor; *ΔL* max =max(*Vcal* −*V fit*), *ΔL* max is the maximum error between calibration line and fitting line; *YF* .*<sup>S</sup>* is the full-scale meaning value of sensor, *YF* .*<sup>S</sup>* =*Y*max −*Yo*, *Y*max is the maximum deviation of output, *Y*0 is the deviation without any input

The response time indicates that the time it takes a sensor to reach a percent of its final steadystate value when input of sensor is changed. For example, it takes 10 seconds for pressure sensor to reach 95 percent of its maximum value when a change in pressure of 1Pa is measured. Ideally, a short response time indicates the ability of a sensor to respond quickly to change in

*<sup>Y</sup>* (1)

*F S L*

 100% <sup>D</sup> *Linearity* =± ´

the smallest distinguishable input change that can be detected with certainty.

Drift refers to the change in sensor reading when the input keeps constant. Drift is divided into temperature drift and zero point drift. Zero point drift refers to the output without any input or with a constant input. Zero point drift could be expressed as the following equation:

$$D\_{zoro} = \frac{\Delta Y\_0}{Y\_{F.S}} \times 100\% \tag{2}$$

Temperature drift refers to the change of output with the change of temperature. It means the deviation of sensor output, which could be expressed as the following equation:

$$D\_{Tump} = \frac{\Delta Y\_{\text{max}}}{Y\_{F.S} \times \Delta T} \times 100\% \tag{3}$$

Here, *ΔT* is the change of temperature.

#### **(11) Hysteresis**

In some sensors, the input-output characteristic follows a different nonlinear trend, depending on whether input increase or decrease, as in figure 2. For example, a certain pressure sensor could produce a different output voltage when the input pressure varies from zero to full scale and then back to zero. When the measurement is not perfectly reversible, the sensor will show its hysteresis. If a sensor exhibits hysteresis, the input-output relationship is not unique, but depends on the direction change to the input value of sensor.

**Figure 2.** Input versus output response of a sensor with hysteresis

#### **1.3. Special features of biosensor**

Biosensor is a kind of device which senses biomaterial and its concentration, and which converts the biosignal into electrical signal. Biosensor has the function of acceptor and converter, which configuration is seen in figure 3. In biosensor, the physicochemical change of the biologically active material resulting from the interaction with the analyte must be converted into an electrical output signal by an appropriate converter. Biosensor's sensing components mainly have enzymes, cells, antibodies, DNA (Deoxyribonucleic acid), chemical electrode, microbe and other biologically active agents in analytical devices. In the course of detecting the parameters of analytes, biomaterial should be always immobile. In order to develop biosensor, some biotechnology has to be studied and applied, such as DNA biosensor, PH sensor, microelectrode, and so on.

The special features of biosensor are the following:


According to biological sensing component, biosensor may be divided into five classes: enzyme sensor, microbe sensor, cell sensor, tissue sensor, and immune sensors. According to the signal converter of biosensor, biosensor may be also divided into five classes: bioelectrode sensor, semiconductor biosensor, optical biosensor, piezoelectric biosensor and thermal biosensor. According to the interaction between sensing component and measured material, biosensor can be divided into two classes: affinity biosensor and catalytic biosensor.

#### **1.4. Biomedical sensor's application**

converter, which configuration is seen in figure 3. In biosensor, the physicochemical change of the biologically active material resulting from the interaction with the analyte must be converted into an electrical output signal by an appropriate converter. Biosensor's sensing components mainly have enzymes, cells, antibodies, DNA (Deoxyribonucleic acid), chemical electrode, microbe and other biologically active agents in analytical devices. In the course of detecting the parameters of analytes, biomaterial should be always immobile. In order to develop biosensor, some biotechnology has to be studied and applied, such as DNA biosensor,

**1.** Biological active material immobilized is used as catalyst, and expensive reagents could

**2.** Biosensor has intensive specificity. Biomaterial only senses definitive ingredient and it is

According to biological sensing component, biosensor may be divided into five classes: enzyme sensor, microbe sensor, cell sensor, tissue sensor, and immune sensors. According to the signal converter of biosensor, biosensor may be also divided into five classes: bioelectrode sensor, semiconductor biosensor, optical biosensor, piezoelectric biosensor and thermal biosensor. According to the interaction between sensing component and measured material,

**4.** Biosensor's accuracy is very high, which relative error could reach one percent.

biosensor can be divided into two classes: affinity biosensor and catalytic biosensor.

PH sensor, microelectrode, and so on.

182 Advances in Bioengineering

The special features of biosensor are the following:

**5.** Biosensor's analyzing system is very simple.

**6.** The cost of biosensor is very low.

**Figure 3.** Common configure of a biosensor

be repeatedly used to detect same biological parameters.

**3.** Biosensor could quickly analyze the result of the measurand.

not affected by color and concentration of measured material.

In biomedical field, main applications of biomedical sensor are as follows:


Of course, biomedical sensor such as PH sensor could be also employed to detect our atmos‐ phere and condition to improve our living situation.
