Medical sensor is the key apparatus or device that can perceive non-electric biological information. Medical sensor extends doctor’s sense organ to collect vital sign information and extends the qualitative sense to quantitative examination.

1. Purpose

In medical field, there are many parameters to indicate vital sign. See table below.

Most of the parameter information belongs to non electricity, which is hard to magnify and process. Medical sensor is the kind of device that can transform non-electricity into electricity. The biological electricity in Table 1 itself also belongs to electricity, but in ionic conduction status inside the organism, which needs medical electrode to transform the ionic conduction into electronic conduction and then magnify and process. Therefore, medical electrode can be regarded as a special type of medical sensor.

Main purpose of medical sensor:

1. Providing information: heart beat sound, blood pressure, pulse, body temperature, blood flow and etc. can act as important physiological parameters for clinical diagnosis and basic study.
2. Monitoring: long-time continuous monitoring some physiological parameters to see if they exceed normal range in order to monitor patient status and send alarm for abnormal status.
3. Biochemical examination: exam the component and content of all kinds of body fluids and solution by using molecule recognition capability of medical sensor.
4. Auto control: according to the physiological information provided by medial sensor, adjust the response of implement unit in order to realize auto control. For example, injection pump adjust its quantity of boost according to flow sensor, which realizes the auto control on dosing per unit time.
5. Treatment: medical electrode is often used in signal detection and treatment. For example, the electrode of demand-controlled internal pacemaker can not only act as the detection electrode for autonomous cardiac electricity, but also act as the stimulate electrode for pacemaker sending pulse in no autonomous cardiac electricity case, where the sensor provides the treatment function.

2. Types

There are many types of medical sensor, but no unified classification method. Medical sensors can be classified into different types according to working principle, detection types, function corresponding to human sense organs and etc.

a. Classified by working principle

1. Physical sensor: the sensor made according to physical nature and effect. The amount of this kind of sensor is the most, such as metal resistance strain sensor, semiconductor piezoresistive sensor, piezoelectric sensor, photoelectric sensor and etc.
   
2. Chemical sensor: the sensor made according to chemical nature and effect. This kind of sensor usually use ion selective sensitive film to transform non electricity such as chemical component, content, density and etc. to related electric quantity, such as various ion sensitive electrode, ion sensitive tube, humidity sensor and etc.
   
3. Biological sensor: the sensor using biological active material as molecule recognition system. This kind of sensor usually uses enzyme to catalyze some biochemical reaction or exam the type and content of large molecule organic substance through some specific combination. It is the new developed sensor in the near half of century, such as enzyme sensor, microorganism sensor, immunity sensor, tissue sensor, DNA sensor and etc.

b. Classified by detection type

As shown in Table 1, there are displacement sensor, flow sensor, temperature sensor, speed sensor, pressure sensor and etc. As for pressure sensor, there are metal strain foil pressure sensor, semiconductor pressure sensor, capacity pressure sensor and other sensor that can detect pressure. As for temperature sensor, it includes thermal resistance, thermocouple, PN junction temperature sensor and other sensor that can detect temperature.

c. Classified by sensor function corresponding to human sense organs

This is the method that classify sensors according to the human sense organ that sensor can replace, such as vision sensor, including various optical sensor and other sensor that can replace visual function; hearing sensor, including sound pick-up, piezoelectric sensor, capacity sensor and other sensor that can replace hearing function; olfaction sensor, including various gas sensor and other sensor that can replace smelling function. This kind of classification is good for the development of simulation sensor.

Besides the above classification, the sensor can be classified according to sensor material, structure, energy conversion mode. They all have their own advantages and disadvantages.

3. Features and Requirement of Medical Sensor

Medical sensor is mainly used to detect human biological signal. It needs to have special characteristics for biological signal to meet medical requirement. Hence, medical sensor has the following characteristics:

1. High sensitivity, able to detect weak biological signal.
2. High signal-to-noise rate to pick up useful information in the interference and noise background.
3. Good accuracy to ensure accuracy and reliability of detected information.
4. Fast response speed, able to change according to biological information.
5. Good stability to keep low shifting for long-time detection and stable output.
6. Good interchangeability, debug and maintenance.

In addition, medical sensor is mainly used in human body. Compared with ordinary sensor, medical sensor has the following specific requirement:

1. The material of sensor that contact with human body, especially implanted in human body must be nontoxic and has good compatibility with biological tissue. And long-time contact will not cause rejection reaction or inflammation.
   
2. Medical sensor will not or minimally affect normal physiological activity during detection; otherwise the detected information will be inaccurate.
   
3. Medical sensor should have good electric safety, especially the sensor that contacts human body should have good insulation feature, complying with the electric safety standard to prevent micro electroshock.
   
4. Medical sensor should have suitable structure and performance for easy clean and sterilization, preventing cross infection of hazardous substances.

4. Development of Medical Sensor

In recent years, according to the characteristics of clinical medicine and requirement of clinical application, medical sensor technology has changed dramatically from traditional larger-size and poor-performance medical sensors to new medical sensor technology. New medical sensor technology can be included in the five aspects below.

1. Smart sensor

Smart Sensor (Intelligent Sensor) first appears at 1980s and is the combination of computer technology, micro-electronic technology and sensor. It usually has the following capabilities:

1. Able to judge and decide strategy according to detected signal;
2. Able to implement related operation via software control;
3. Have input and output interface, able to communicate with outside;
4. Self-detecting, self-correcting and self protection.

The application of smart sensor technology improves the performance of medical devices, which reflects in the following aspects:

1. Auto data processing
   Smart sensor changes the traditional passive parameter measurement into active measurement. It can not only calculate the unknown parameter according to measured parameter, but also judge via calculation and actively select to-be-measured parameter to realize AZS, auto balance, auto compensation, auto measuring range selection and etc.
2. Self detection, diagnosis and alarm
   Smart sensor can monitor the working status according to pre-set algorithm and diagnose the fault reason and display it in fault code format. Currently most of modern medical devices such as CT, hemodialyzer, ultrasonic instrument and auto biochemistry analyzer have self diagnosis function, which are all based on smart sensor technology. The combination of smart sensor and monitor can select to display or periodical circular display various measured parameters and compare with set value for alarm when exceeding upper or lower limit.
3. Interface function
   Smart sensor technology use standard digital output, easy to realize data exchange and share between sensor and system, sensor and network via interface technology, forming networked intelligent sensor system. For example, using smart sensor to measure the content of food of a diabetes patient, the measured data is transferred to hospitalÕs information center via network and is analyzed according to the patientÕs specific situation, and calculate the exact dose of insulin that the patient need to be injected, and then control the injection pump of this patient via network to complete auto on-demand drug delivery.

2. Micro-sensor

Modern sensor has changed from traditional structure design and manufacturing technique to micromation. Micro-sensor is made by micro-mechanic technology, including photoengraving and corrosion. Its sensitive component is micron-level small.

Micro-sensor can enter the part of human body such as the inside of viscera and disease focus that is unreachable for traditional sensor to get information. In addition, since the micro-sensor is very small, it largely reduce the impedance and effect to normal physiological activity, which makes the measured value more genuine and reliable.

3. Multi-parameter sensor

In clinical medicine, it usually needs several physiological parameters and multiple sensors. Multi-parameter sensor is a detection system with small dimension and multiple functions, using single sensor system to measure multiple parameters simultaneously to realize the functions of multiple sensors. Multi-parameter sensor integrates various sensitive components on one chip. Since the working condition is same, it’s easy to compensate and correct the system difference. Compared with using multiple sensors, it has high accuracy, good stability, small dimension, less weight and lower cost. The current developed "electronic nose" of simulation sensor is a kind of multi-parameter sensor that can identify various gases.

4. Remote-control sensor

In clinical medicine, in most cases, patients need to be implanted or swallow some sensors or definite-time release drug device for measuring parameters in their bodies. These sensors or devices need to be remote controlled outside of patient bodies. Remote-control sensor is a kind of new sensor that combines remote-control technology and sensor technology. For example, Swallow Òelectronic pill?micro remote-control sensor to measure pH value of gastric juice, gastric pressure, digestive juice content and etc., and send the result to the outside receiver via micro wireless transmitter.

5. Noninvasive detecting sensor

With the living standard improved, people’s health awareness is being enhanced continuously. People will not be satisfied by only disease treatment, but propose more requirements for disease prevention and breaking away the subhealthy status. In addition, with the aging of population, community medical treatment and self health care have become more and more important. All of the above requires the convenient examination of physiological parameters, which should easy to operate and acceptable and can be completed in community or even at home. Hence, noninvasive detection is an important direction for sensor research. Noninvasive detection means no or nearly no invasion during detection. Noninvasive detection is not only more acceptable by receivers, but also has little effect on human body, more reliable, easy-to-operate, easy for sterilization and less infection possibility. Noninvasive detecting sensor should have higher sensibility, accuracy, anti-interference and signal-to-noise rate. For example, finger-clip opto-electrical capacity blood flow pulse wave sensor is used to measure cardiac output per pulse, peripheral resistance, vessel elasticity, blood viscosity and other blood parameters, but the accuracy still needs to be improved.

The development of medical sensor has basically changed traditional mode, forming the development trend of smart, micro, multi-parameter, remote-control and noninvasive, and achieved some technical breakthrough. Other new types of sensor such as DNA sensor, fiber sensor, and biological tissue sensor are also being developed. The revolution of medical sensor technology will promote the development of modern medicine.