- Open Access
Pulse wave response characteristics for thickness and hardness of the cover layer in pulse sensors to measure radial artery pulse
© The Author(s) 2018
- Received: 4 June 2018
- Accepted: 30 August 2018
- Published: 4 September 2018
Piezo-resistive pressure sensors are widely used for measuring pulse waves of the radial artery. Pulse sensors are generally fabricated with a cover layer because pressure sensors without a cover layer are fragile when they come into direct contact with the skin near the radial artery. However, no study has evaluated the dynamic pulse wave response of pulse sensors depending on the thickness and hardness of the cover layer. This study analyzed the dynamic pulse wave response according to the thickness and hardness of the cover layer and suggests an appropriate thickness and hardness for the design of pulse sensors with semiconductor device-based pressure sensors.
Pulse sensors with 6 different cover layers with various thicknesses (0.8 mm, 1 mm, 2 mm) and hardnesses (Shore type A; 30, 43, 49, 71) were fabricated. Experiments for evaluating the dynamic pulse responses of the fabricated sensors were performed using a pulse simulator to transmit the same pulse wave to each of the sensors. To evaluate the dynamic responses of the fabricated pulse sensors, experiments with the pulse sensors were conducted using a simulator that artificially generated a constant pulse wave. The pulse wave simulator consisted of a motorized cam device that generated the artificial radial pulse waveform by adjusting the stroke of the cylindrical air pump and an air tube that conveyed the pulse to the artificial wrist.
The amplitude of the measured pulse pressure decreased with increasing thickness and hardness of the cover layer. Normalized waveform analysis showed that the thickness rather than the hardness of the cover layer contributed more to waveform distortion. Analysis of the channel distribution of the pulse sensor with respect to the applied constant dynamic pressure showed that the material of the cover layer had a large effect.
In this study, in-line array pulse sensors with various cover layers were fabricated, the dynamic pulse wave responses according to the thickness and the hardness of the cover layer were analyzed, and an appropriate thickness and hardness for the cover layer were suggested. The dynamic pulse wave responses of pulse sensors revealed in this study will contribute to the fabrication of improved pulse sensors and pulse wave analyses.
- Piezo-resistive pulse sensor
- Thickness and hardness of cover layer
- Artificial pulse wave simulator
- Radial artery pulse
- Pulse wave response
Recently, carotid artery measurement studies for diagnosis of cardiovascular disease have been investigated [1–3], and computational approaches and modeling studies of blood vessels for vascular stenosis and blood flow analysis have been increased [4–7] in western medicine. On the other hand, many studies of eastern medicine have aimed to objectify, quantify, and automate wrist pulse diagnosis by employing modern sensors for data acquisition, data processing, and pattern classification . Precise and accurate pulse wave measurements should be prioritized to diagnose diseases or to determine pulse patterns through pulse wave analysis . Piezo-resistive pressure sensors are commonly used to measure pulse waves of the human radial artery, which represents the most prominent method for precisely measuring radial artery pulse . In-line array pressure sensors are a better alternative to tonometry sensors, which have only one pressure sensor, for conveniently obtaining the spatial information. Jeon et al.  describe a seven-channel pressure sensor array for measuring pulse width. Similarly, Chung et al. , Choi et al. , and Kim et al.  use a two-dimensional pressure sensor array to extract spatial pulse features. Hu et al.  present a sensor probe consisting of a capacitive array sensor with 12 sensing points to determine the optimal pulse-taking position. Peng and Lu  introduce a flexible 5 × 5 capacitive pressure sensor array based on flexible printed circuit boards and integrated CMOS switched capacitor readout circuits for determining pulse patterns. In Chang’s study , a 9-channel sensing probe based on piezoelectric PVDF sensors is described for collecting pulse patterns. Xu et al.  introduce a sensor system with a strain cantilever beam transducer as the main sensor and an array of 7 additional sensors for detecting pulse width.
To protect the pulse sensor damage, a cover layer on the pulse sensor is required because the pressure sensors of the pulse sensor directly contact the skin near the radial artery with considerable force. However, this cover layer coated on the pulse sensor has an adverse effect on sensor performance. A thick cover layer on the pulse sensor has been reported to affect other channels in the pulse sensor array due to the force distribution, and signal distortion may also occur because of the temperature difference between the skin surface and the pulse sensor [19, 20]. Although some studies on pulse sensor cover layers have reported static characteristics according to thickness or temperature , no study has evaluated the dynamic pulse wave response according to the thickness and hardness of the cover layer.
Dynamic pulse waves in the radial artery have been studied and used in various applications, such as arterial stiffness assessments [22, 23], cardiovascular disease diagnoses , central BP monitoring [25–27], and pulse wave analysis . It is necessary to accurately reflect the dynamic response of the pulse waves to apply pulse sensors with a cover layer for these various uses. The dynamic response of the pulse sensor for monitoring radial pulse waves can be altered by the thickness and hardness of the cover layer. Therefore, the changes in pulse wave dynamics must be studied according to the thickness and hardness of the cover layer of the pulse sensor.
In this study, pulse sensors with a cover layer coated with materials of different thicknesses and hardnesses were fabricated. To evaluate the influence of the thickness and hardness of the cover layer on pulse sensor performance, the pulse wave dynamic responses of the fabricated pulse sensors were analyzed using simulated pulse waves generated by a simulator. Analysis of the results revealed the appropriate thickness and hardness of the cover layer for use in pulse sensors. These findings provide guidance on the thickness and hardness of the cover layer that should be used when fabricating pulse sensors.
Fabrication of the pulse sensor
Experiment with the fabricated pulse sensors
In Fig. 8, PDMS refers to the pulse sensor with a cover layer made of 1-mm-thick PDMS; T1_H43 refers to a cover layer made of 1-mm-thick XE14-C2042 (hardness: 43); T1_H49 refers to a cover layer made of 1-mm-thick IVS4546 (hardness: 49); and T0.8_H71, T1_H71, and T2_H71 refer to 0.8-mm-thick, 1-mm-thick, and 2-mm-thick IVS4742 (hardness: 71) cover layers, respectively.
Averaged peak values and rising time of measured pulse wave
Peak values (V) (% ref)
3.42 ± 0.02 (0)
2.84 ± 0.06 (83.08)
2.58 ± 0.05 (75.59)
2.43 ± 0.08 (71.03)
2.52 ± 0.06 (73.77)
1.50 ± 0.02 (43.85)
Rising time (ms) (% ref)
97.75 ± 0.97 (0)
99.78 ± 1.99 (102.07)
96.11 ± 1.45 (96.33)
99.78 ± 0.92 (103.82)
96.33 ± 1.05 (96.55)
110.56 ± 2.22 (114.76)
Comparison of prd and aix for normalized signals of fabricated pulse sensors
Comparison of average pressure according to different cover layers of pulse sensors
AP/Ref (%) (error (%))
The AP errors for calculating the AP rates of the T1_H71 and T2_H71 sensors are too large for reliable pulse wave measurements because the errors are larger than 10%. In addition, it was difficult to fabricate the pulse sensor with the 0.8-mm-thick cover layer covering the 0.5-mm-thick pressure sensor, wire bonding, and underfilling for protection of the wires on the sensor cells. The 0.8-mm-thick cover layer was fabricated to compare only the pulse sensor dynamic responses of other cover layers with the thinnest possible cover layer that could be implemented via our fabrication process. Although the thin cover layer showed good pressure resolution, high waveform reproducibility, and low pressure transfer losses in the pulse sensor dynamics analysis, fabrication of pulse sensors with a cover layer of 1 mm or greater thickness is suggested due to the difficulty in fabricating thinner layers, mass production considerations, and large thickness errors.
Although it has been experimentally shown that the dynamic response of pulse sensors for monitoring pulse waves depends on the cover layer, there are limits to determining the optimal thickness and hardness of the cover layer. In addition, further studies of the cover layer using materials with a wide variety of mechanical properties are needed to determine the optimal cover layer in the pulse sensor. Additionally, in order to analyze a dynamic frequency response for fabricated pulse sensors, it is necessary to trace the frequency response of the pulse sensor as the input frequencies were swept. However, the frequency range of the pulse wave simulator driven by cam is not wide enough to analyze the frequency response (from 50 to 90 bpm). We will develop a pulse generator to analyze the dynamic frequency response of the pulse sensors, and analyze the dynamic response as a further study.
To precisely evaluate the force transfer rate of the pulse sensor, the sensor output with respect to the contact area and the pulse wave can be analyzed through theoretical calculations of the stress and the strain due to the pressure between a cylinder and a flat plate of elastic bodies [40, 41]. However, it is difficult to calculate the pressure on the contact area because of the unknown mechanical properties. Therefore, the force transfer rates of the pulse sensors were evaluated by the AP values, which represent the entire pressure output divided into the number of sensor channels. In addition, it is necessary to understand the forced vibration system of periodic excitation to clarify the damping effect of the cover layers.
A good way to evaluate the reproducibility of the pulse wave by pulse sensors with respect to the external pressure is to compare the output signals of the fabricated pulse sensors with the external pressure as the input signal. However, a reference pulse sensor was used because it would be difficult to perform a direct comparison with the external pressure using our system. A more accurate external pressure on the artificial radial artery can be calculated by the modeling of the waveform transmission considering various properties of the artificial tube, such as elasticity, diameter, length, and thickness. More research on modeling of the artificial radial artery and waveform transmission are required to develop precise sensors for measuring the pulse wave. In addition, the lifetime of the pulse sensors was not verified because durability tests of the pulse sensors with the H43, H49, or H71 cover layers have not yet been conducted. It is also necessary to study whether the silicones used are the most suitable cover layer materials for pulse sensors.
In this study, in-line array pulse sensors with various cover layers were fabricated, the dynamic pulse wave responses according to the thickness and the hardness of the cover layer were analyzed, and an appropriate thickness and hardness for the cover layer were suggested. Pulse sensors with cover layers of 3 different thicknesses (0.8 mm, 1 mm, 2 mm) and 4 different hardnesses (Shore type A; 30, 43, 49, 71) were fabricated. Experiments to evaluate the dynamic responses of the fabricated sensors were performed using a pulse simulator, and 3 repeated measurements were made for each sensor. The averaged amplitudes of the measured pulse pressure were 3.42 V for the PDMS sensor, 2.84 V for the T1_H43 sensor, 2.58 V for the T1_H49 sensor, 2.43 V for the T1_H71 sensor, 2.52 V for the T0.8_H71 sensor, and 1.50 V for the T2_H71 sensor. The normalized waveform analysis using PRD showed that the waveform errors were 1.99% for T1_H43, 0.62% for T1_H49, 1.99% for T1_H71, 0.77% for T0.8_H71, and 11.02% for T2_H71 in relation to the sensor with the PDMS cover layer. AP analysis of the pulse sensors showed that the errors were 4.10% for T1_H43, 8.24% for T1_H49, 13.49% for T1_H71, 8.79% for T0.8_H71, and 27.63% for T2_H71. This study suggests that pulse sensors with a 1-mm-thick H43 cover layer or a 1-mm-thick H49 cover layer are suitable for measuring radial pulse waves, as fabricating pulse sensors with a 0.8-mm-thick cover layer is difficult. This study of dynamic pulse wave response will contribute to accurate and precise pulse wave measurements and analysis.
M-HJ wrote the manuscript and designed and conducted the experiments of the sensor characteristics with Y-MK, Y-JJ designed the pulse sensor and the electrical circuits for the experiments, J-HC installed the experimental setup and helped with the experiments, and Y-MK managed the experiments of the pulse sensor characteristics and contributed to writing and revising the manuscript. All authors read and approved the final manuscript.
This work was supported by a Grant (K18022) from the Korea Institute of Oriental Medicine (KIOM), funded by the Korean government.
The authors declare that they have no competing interests.
Availability of data and materials
The datasets generated and/or analysed during the current study are available from the corresponding author on reasonable request.
Consent for publication
Ethics approval and consent to participate
This work was supported by a Grant (K18022) from the Korea Institute of Oriental Medicine (KIOM), funded by the Korean government.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.
- Gao Z, Li Y, Sun Y, Yang J, Xiong H, Zhang H, Liu X, Wu W, Liang D, Li S. Motion tracking of the carotid artery wall from ultrasound image sequences: a nonlinear state-space approach. IEEE Trans Med Imaging. 2018;37(1):273–83.View ArticleGoogle Scholar
- Zhao S, Gao Z, Zhang H, Xie Y, Luo J, Ghista D, Wei Z, Bi X, Xiong H, Xu C. Robust segmentation of intima-media borders with different morphologies and dynamics during the cardiac cycle. IEEE J Biomed Health. 2017. https://doi.org/10.1109/JBHI.2017.2776246.Google Scholar
- Gao ZF, Xiong HH, Liu X, Zhang HY, Ghista DJ, Wu WQ, Li S. Robust estimation of carotid artery wall motion using the elasticity-based state-space approach. Med Image Anal. 2017;37:1–21.View ArticleGoogle Scholar
- Liu X, Gao Z, Xiong H, Ghista D, Ren L, Zhang H, Wu W, Huang W, Hau WK. Three-dimensional hemodynamics analysis of the circle of Willis in the patient-specific nonintegral arterial structures. Biomech Model Mechanobiol. 2016;15(6):1439–56.View ArticleGoogle Scholar
- Liu GY, Wu JH, Huang WH, Wu WD, Zhang HN, Wong KKL, Ghista DN. Numerical simulation of flow in curved coronary arteries with progressive amounts of stenosis using fluid-structure interaction modelling. J Med Imaging Health Inform. 2014;4(4):605–11.View ArticleGoogle Scholar
- Wong KKL, Tu J, Mazumdar J, Abbott D. Modelling of blood flow resistance for an atherosclerotic artery with multiple stenoses and poststenotic dilatations. ANZIAM J. 2010;51:66–82.MathSciNetView ArticleMATHGoogle Scholar
- Wong K, Mazumdar J, Pincombe B, Worthley SG, Sanders P, Abbott D. Theoretical modeling of micro-scale biological phenomena in human coronary arteries. Med Biol Eng Comput. 2006;44(11):971–82.View ArticleGoogle Scholar
- Matskiv AS, Kravets SL, Tsemekhin BD. Treatment of purulent and necrotic lesions of the lower extremities in patients with diabetes mellitus. Klin Khir. 1993;9–10:37–40.Google Scholar
- Liu SH, Tyan CC. Quantitative analysis of sensor for pressure waveform measurement. Biomed Eng Online. 2010;9:6.View ArticleGoogle Scholar
- Jun MH, Kim YM, Bae JH, Jung CJ, Cho JH, Jeon YJ. Development of a tonometric sensor with a decoupled circular array for precisely measuring radial artery pulse. Sensors (Basel). 2016;16(6):768. https://doi.org/10.3390/s16060768 View ArticleGoogle Scholar
- Jeon Y-J, Kim JU, Kim Y-M, Bae J-H, Kim J-Y. Development of an array sensor for measuring radial pulse wave. In: 11th International conference on wearable and implantable body sensor networks, 16–19 June 2014, Zurich, Switzerland.Google Scholar
- Chung C-Y, Chung Y-F, Chu Y-W, Luo C-H. Spatial feature extraction from wrist pulse signals. In: International conference on Orange Technologies (ICOT), 2013. New York: IEEE; 2013. p. 1–4.Google Scholar
- Choi SD, Kim SW, Kim GW, Ahn MC, Kim MS, Hwang DG, Lee SS. Development of spatial pulse diagnostic apparatus with magnetic sensor array. J Magn Magn Mater. 2007;310(2):E983–5.View ArticleGoogle Scholar
- Kim SW, Hwang DG, Choi YK, Lee HS, Park DH, Lee SS, Kim GW, Lee SG, Lee SJ. Improvement of pulse diagnostic apparatus with array sensor of magnetic tunneling junctions. J Appl Phys. 2006;99(8):08R908.View ArticleGoogle Scholar
- Hu CS, Chung YF, Yeh CC, Luo CH. Temporal and spatial properties of arterial pulsation measurement using pressure sensor array. Evid Based Complement Altern Med. 2012;2012:745127. https://doi.org/10.1155/2012/745127.Google Scholar
- Peng JY, Lu MSC. A flexible capacitive tactile sensor array with CMOS readout circuits for pulse diagnosis. IEEE Sens J. 2015;15(2):1170–7.MathSciNetView ArticleGoogle Scholar
- Chang H, Chen J-X. Piezoelectric pulse diagnosis transducer of 9×9 sensing arrays and pulse signal processing. In: International conference on applied informatics and communication. New York: Springer; 2011. p. 541–8.Google Scholar
- Xu L, Meng MQ-H, Shi C, Wang K, Li N. Quantitative analyses of pulse images in traditional Chinese medicine. Med Acupunct. 2008;20(3):175–89.View ArticleGoogle Scholar
- Jun M-H, Jeon YJ, Kim Y-M. Interference effects on the thickness of a pulse pressure sensor array coated with silicone. J Sens Sci Technol. 2016;25(1):35–40.View ArticleGoogle Scholar
- Jun M-H, Jeon YJ, Kim Y-M. Signal change and compensation of pulse pressure sensor array due to wrist surface temperature. J Sens Sci Technol. 2017;26(2):141–7.View ArticleGoogle Scholar
- Yoo SK, Shin KY, Lee TB, Jin SO, Kim JU. Development of a radial pulse tonometric (RPT) sensor with a temperature compensation mechanism. Sensors (Basel). 2013;13(1):611–25.View ArticleGoogle Scholar
- Zhang YL, Zheng YY, Ma ZC, Sun YN. Radial pulse transit time is an index of arterial stiffness. Hypertens Res. 2011;34(7):884–7.View ArticleGoogle Scholar
- Filipovsky J, Ticha M, Cifkova R, Lanska V, Stastna V, Roucka P. Large artery stiffness and pulse wave reflection: results of a population-based study. Blood Press. 2005;14(1):45–52.View ArticleGoogle Scholar
- Chrysohoou C, Angelis A, Tsitsinakis G, Spetsioti S, Nasis I, Tsiachris D, Rapakoulias P, Pitsavos C, Koulouris NG, Vogiatzis I, Dimitris T. Cardiovascular effects of high-intensity interval aerobic training combined with strength exercise in patients with chronic heart failure. A randomized phase III clinical trial. Int J Cardiol. 2015;179:269–74.View ArticleGoogle Scholar
- Nelson MR, Stepanek J, Cevette M, Covalciuc M, Hurst RT, Tajik AJ. Noninvasive measurement of central vascular pressures with arterial tonometry: clinical revival of the pulse pressure waveform? Mayo Clin Proc. 2010;85(5):460–72.View ArticleGoogle Scholar
- Townsend RR. Analyzing the radial pulse waveform: narrowing the gap between blood pressure and outcomes. Curr Opin Nephrol Hypertens. 2007;16(3):261–6.View ArticleGoogle Scholar
- Takazawa K, Kobayashi H, Shindo N, Tanaka N, Yamashina A. Relationship between radial and central arterial pulse wave and evaluation of central aortic pressure using the radial arterial pulse wave. Hypertens Res. 2007;30(3):219–28.View ArticleGoogle Scholar
- Larsson M, Bjallmark A, Lind B, Balzano R, Peolsson M, Winter R, Brodin LA. Wave intensity wall analysis: a novel noninvasive method to measure wave intensity. Heart Vessels. 2009;24(5):357–65.View ArticleGoogle Scholar
- Vlachopoulos C, O’Rourke M, Nichols WW. McDonald’s blood flow in arteries: theoretical, experimental and clinical principles. Boca Raton: CRC Press; 2011.Google Scholar
- Choudhury MI, Singh P, Juneja R, Tuli S, Deepak KK, Prasad A, Roy S. A novel modular tonometry-based device to measure pulse pressure waveforms in radial artery. J Med Devices. 2018;12(1):011011. https://doi.org/10.1115/1.4039010 View ArticleGoogle Scholar
- Jeon YJ, Kim JU, Lee HJ, Lee J, Ryu HH, Lee YJ, Kim JY. A clinical study of the pulse wave characteristics at the three pulse diagnosis positions of Chon, Gwan and Cheok. Evid Based Complement Altern Med. 2011. https://doi.org/10.1093/ecam/nep150.Google Scholar
- Momentive Performance materials Inc., Silicone Material Solutions for LED Packages and Assemblies. https://www.momentive.com/en-us/search-results/?q=xe14-c2042. Accessed 7 Dec 2017.
- Kim TH, Ku B, Bae JH, Shin JY, Jun MH, Kang JW, Kim J, Lee JH, Kim JU. Hemodynamic changes caused by acupuncture in healthy volunteers: a prospective, single-arm exploratory clinical study. BMC Complement Altern Med. 2017;17(1):274.View ArticleGoogle Scholar
- Bae JH, Ku B, Jeon YJ, Kim H, Kim J, Lee H, Kim JY, Kim JU. Radial pulse and electrocardiography modulation by mild thermal stresses applied to feet: an exploratory study with randomized, crossover design. Chin J Integr Med. 2017. https://doi.org/10.1007/s11655-017-2972-0.Google Scholar
- Bae J-H, Jeon YJ, Lee S, Kim JU. A feasibility study on age-related factors of wrist pulse using principal component analysis. In: 2016 IEEE 38th annual international conference of the IEEE engineering in medicine and biology society (EMBC). New York: IEEE; 2016. p. 6202–5.Google Scholar
- Baek JY, An JH, Choi JM, Park KS, Lee SH. Flexible polymeric dry electrodes for the long-term monitoring of ECG. Sens Actuators A Phys. 2008;143(2):423–9.View ArticleGoogle Scholar
- Chen CY, Chang CL, Chang CW, Lai SC, Chien TF, Huang HY, Chiou JC, Luo CH. A low-power bio-potential acquisition system with flexible PDMS dry electrodes for portable ubiquitous healthcare applications. Senors (Basel). 2013;13(3):3077–91.View ArticleGoogle Scholar
- Xu L, Yao Y, Wang H, He D, Wang L, Jiang Y. Morphology variability of radial pulse wave during exercise. Biomed Mater Eng. 2014;24(6):3605–11.Google Scholar
- Duprez DA, Kaiser DR, Whitwam W, Finkelstein S, Belalcazar A, Patterson R, Glasser S, Cohn JN. Determinants of radial artery pulse wave analysis in asymptomatic individuals. Am J Hypertens. 2004;17(8):647–53.View ArticleGoogle Scholar
- Norden BN, Norden BN. On the compression of a cylinder in contact with a plane surface. Gaithersburg: U.S. Dept. of Commerce, National Institute of Standards and Technology; 1973.View ArticleMATHGoogle Scholar
- Puttock M, Thwaite E. Elastic compression of spheres and cylinders at point and line contact. Australia: Commonwealth Scientific and Industrial Research Organization; 1969.Google Scholar