In ancient times, during the process of struggling with diseases, Chinese people invented a diagnosis theory called traditional Chinese medicine (TCM) from practices and experiences. TCM contains four diagnostic methods—inspection, listening and smelling examination, inquiry, and pulse taking and palpation [1
]. Pulse taking and palpation is the most unique and key diagnostic method. Pulse refers to the beating of the arteries caused by the cardiac output when the heart contracts. According to TCM, pulse condition refers to the speed, intensity, and depth of the pulse. There are three pulse regions of the radial artery in the wrist called Cun, Guan, and Chi, respectively, as is shown in Figure 1
. The part slightly below the styloid process of the radius bone is the Guan, one finger anterior the Guan is the Cun, and one finger posterior the Guan is the Chi [2
]. The change of pulse condition in each region corresponds to the change of function and state of different organs of the body [3
]. For instance, right Cun reflects the functions of the lungs and the large intestine, while left Cun reflects the states of the heart and the small intestine, etc. [4
]. TCM physicians use three fingers to locate Cun, Guan and Chi in a patient’s wrist radial artery, adjust the intensity of finger compression, and gain the pulse condition. Based on the results of pulse-taking and the other three diagnostic methods, TCM physicians make diagnoses and treatment plans.
In recent years, as a non-invasive and painless means of detection, pulse-taking has been appreciated and welcomed by people at home and abroad. However, there are some limitations in pulse diagnosis. Firstly, judging pulse by the sensation under the finger is subjective and it is easy to deviate from the actual pulse condition [5
]. Secondly, even it takes a great deal of human and material resources to cultivate talents in pulse diagnosis of TCM, the pulse-taking skills are difficult to inherit, which is not conducive to the development and promotion of traditional Chinese medicine. As a result, various research institutes and scholars have investigated the objectivity of pulse-taking [6
]. The research usually includes pulse signal acquisition, processing, and classification.
The design of pulse measurement devices is a hot spot of the objectivity research. The key points of the designed device are simulating the modes of pulse diagnosis in TCM and obtaining important information for distinguishing pulse types. There are two modes of pulse measurement. One is to measure the pulse of a single position (usually Guan), and the other is to measure three positions with the theory of Three Positions and Nine Indicators (TPNI). The first is simple and fast while the second is more precise and effective. TPNI plays an important role in pulse diagnosis, and relevant studies have proven the validity of this theory [4
]. According to TPNI, TCM physicians put three fingers (index, middle, and ring) at Cun, Guan, and Chi, respectively, and apply three different pressures to the three regions, i.e., pressing with light pressure (Fu), pressing with medium pressure (Zhong), and pressing with heavy pressure (Chen). Thus, pulse taking at three positions under three levels of pressure make up nine indicators. Both methods can obtain important information for judging pulse types. There are 28 pulse types in TCM pulse diagnosis. The composition of the pulse condition has four elements—depth, quantity, shape, and strength. Depth refers to the level of pulse depth, quantity refers to the pulse rate, shape refers to the shape of the pulse, and strength refers to the energy of the pulse. Based on these four elements, pulse types can be distinguished. During the process of pulse palpation, physicians find the positions of the region to be measured. Then, they feel the pulse condition and distinguish the pulse type by applying varying static contact pressure (called pulse-taking pressure in TCM) between the fingers and wrist surface. Pulse amplitudes of wrist pulse are different under different pulse-taking pressures. The pulse-taking pressure under which the amplitude of pulse is at maximum is called the best pulse-taking pressure. The best pulse-taking pressure and the trend of pulse amplitude with pulse-taking pressure can be used to judge the depth of pulse, and the pulse waveform can show the quantity, shape, and strength of pulse. Pulse-taking pressure and pulse waveform are significant diagnostic bases. Considering all the factors above, the pulse measurement device should meet the requirement of diagnostic methods in TCM, and the final output of the device should contain pulse-taking pressure and pulse waveform. Therefore, the development of the pulse measurement device should involve sensor selection, pressurizing, position adjusting, overall mechanical structure, and circuit design.
In the last few years, many scholars and research institutes have done research on pulse measurement devices. In terms of sensor usage, most studies use pressure sensors or optical sensors. In general, TCM doctors take the pulse with fingers. The pressure exerted by the fingers and the force of the pulse beat are key factors for judging pulse types. However, pulse measurement devices using optical sensors measure the change of blood flow in the artery, which is different from TCM in measurement principle. The content of the pulse diagrams obtained by the two measurement methods is different. Therefore, the measurement results of optical sensors need to be converted and interpreted before corresponding to the TCM theory. The relationship between the two measurement results requires much research. It is indirect to use optical sensors for measurement. In addition, most current research using optical sensors without pressurization structure renders it impossible to simulate the measurement method of TCM physicians and obtain pulse-taking pressure [2
]. Consequently, the difference in measurement principle and the lack of pulse information limit the usage of optical sensors in the development of pulse measurement devices. In the aspect of the sensors’ positioning, most existing pulse pressure acquiring devices only contain one channel, resulting in the measurement of one region at a time. When it comes to the devices with multi-channels, because of the complex adjustment mechanism, it takes a long time to locate three regions, and the operation must be guided by a professional. As for the pressurizing mechanism, there are several methods, such as mechanical, pneumatic, and hydraulic. Among these, the mechanical pressurization is the most common one. Mechanical pressurizing devices are usually large in size and heavy in weight, thus it is inconvenient to carry them [12
]. Finally, most existing wearable devices can only measure heart rate. There are no pressurization parts in these types of devices, thus the static pressure at three regions cannot be adjusted, which leads to the best pulse-taking pressure being unavailable. Thus, the pulse information obtained is insufficient [17
]. To conclude, the study of the pulse measurement device is of great significance, and there are some problems and defects in the existing equipment that is used to measure pulse that remain to be solved.
This study proposes a wearable combined wrist pulse measurement system that consists of combined wristbands, pressure sensors, pumps and airbags, signal acquisition circuit, signal processing software, a microcontroller unit (MCU), and other parts. The sensors can be easily located to different regions by the mechanism on the wristband. The pressurizing is pneumatic, which reduces the weight and size to make it portable and comfortable to use. It supports two modes; one is measuring the pulse of a single region with a single wristband, and the other is measuring the pulse of three regions simultaneously with combined wristbands. Meanwhile, it can apply various pressures on measuring regions.
This study proposed a wearable combined wrist pulse measurement system that integrated a pressurization mechanism. The average error of heart rate measurement is less than 2%. The measurement error is within the range of ±3.804 gf. The RE of system measurement is less than 2.5%. In addition, this system supports single-region measurement and three-region measurement, which are both consistent with the finger palpation technique in TCM. Additionally, positions of wristbands can be adjusted to correspond to the regions of different people. These advantages make the proposed device more suitable and reliable for TCM experiments and research. A series of experiments were done to verify the feasibility of the device. The results show that this system can measure wrist pulse at Cun, Guan and Chi regions and obtain the basic pulse information, including pulse rate, pulse waveform, and the best pulse-taking pressure. The trend of pulse amplitude with pulse-taking pressure is consistent with the description of TCM theories. We also found that there is an increasing trend of the best pulse-taking pressure at three regions with the increase in BMI.
The proposed system and other pulse measurement devices presented in recent studies are summarized in Table 3
. Peng Wang proposed a multi-channel pulse signal acquiring system [14
]. In Zhou et al.’s study, they proposed a novel three-channel self-pressurized wrist pulse system [12
]. Jessica E. T. Kabigting proposed a wearable pulse-taking device [22
In conclusion, the system is feasible to collect wrist pulse and helpful for TCM diagnosis. Combining the advantages—small volume, light weight, easy to operate, and convenient to carry—it can meet the needs of experimental research and home healthcare. Besides, because of the flexible usage modes of combined wristbands, a variety of combinations are available for the user’s experimental design. In the future, we will improve the function of the system and expand the pressurization range to 200 gf to meet the requirements of different people. In addition, we will study the calibration of the system in the long-term use for users. Furthermore, in order to make the system more accurate and standardized, we will conduct more experiments on the base of TCM. This is of great importance to the objectivity research of pulse diagnosis in TCM.