Bioinspired non-invasive radial pulse sensor: from biomimetic design, system calibration, to clinic application
© Luo et al.; licensee Springer. 2014
Received: 8 September 2014
Accepted: 24 October 2014
Published: 18 November 2014
The research work aims at developing a real-time non-invasive metabolism and blood circulation surveillance system for monitoring human’s health condition by sensing the various bio-signals on the human body. Our goal is to use the developed system to study the functions and characters of organs and tissues that highly relate with the metabolism and blood circulation system, and also, it is expected to help modeling the entire circulation system. At phase I of the research, in this paper we focus on developing a new low-cost, portable, high-accuracy, non-invasive radial pulse sensor. Inspired by touch capability and related biomechanical advantage of human fingertip, the mechanical design of the sensor mimics the physiological structure of human fingertip. The biomimetic sensor is then well calibrated using a high-accuracy force sensor, and the model is accurately identified by the system identification method. Further the calibrated sensor is applied to diagnose the arterial stiffness by measuring the augmentation index (AI) which is the important biomarker of vascular aging. Preliminary results demonstrate the sensor performance that it is capable of non-invasively, accurately, and reliably measuring radial pulse signals at real time, as well as to quantitatively determine the vessel aging.
KeywordsBiomimetic Non-invasive Radial pulse sensor
Cardiovascular system is built by central and systemic circulation systems, which generates the blood flow by the heart and transmits the blood to the whole body through the vessel pathway . Associated with the blood flow, the pulse is an important signal containing the pathological information like heart function, blood flow resistance, vessel wall elastic, and blood viscosity. These pathological changes are usually expressed on strength, frequency, altitude, and waveform variation of the pulse signal. Instead of invasive thermodilution methods and complicated non-invasive impedance-cardiography, the pulse wave analysis can thus be an alternative way to monitor the cardiovascular system.
One of these pulses is the radial pulse. A diagnosis method relying on the radial pulse signals detected by physician fingers can be traced back to 500 BC in China . The first mechanical device can measure the arterial pressure waveform at radial called sphygmograph which was developed and improved in the nineteenth century . Later, more and more researchers studied into this area, and different kinds of devices to measure the radial pulse wave were investigated. Nowadays, there are various commercial pressure waveform measurement devices developed by different companies and groups. These devices generally use technologies like RF-signal of an ultrasound, applanation tonometry, photoplethysmography, piezoelectric sensor, oscillometric, mechanotransducer, and volume plethysmography . However, most of these sensing devices are only capable of measuring the systolic blood pressure, diastolic blood pressure, and heart rate . The typical calibration for them is only a static process that relies on several discrete reference values from the cuff-based blood pressure sensors ,. This results in most of them could not achieve radial pulse details and dynamic behaviors that contain rich physiological and pathological information reflecting cardiovascular disease and vessel aging. In addition, most of devices are not cost effective and are unwearable, and this limits them to be extensively used for continuously monitoring the pulse signals of users. Without continuously monitoring the signals, the history of health condition related to cardiovascular diseases cannot be recovered for helping the diagnosis process and certain important physiological and pathological characteristics may be generally missed. For example, some characteristics from the pulses are significant during the intense exercise or the sleeping time due to the heavy load of the cardiovascular system and increasing of the blood viscosity; however, they may be lost without wearing a portable pulse sensor that has continuous monitoring capability. Recently, some research work also mentioned that continuous monitoring of the pulses will help to prevent the varied cardiovascular diseases at the early stage as well as to alarm users when the pulse signals become singular so as to avoid the situations like stroke, myocardial infarction, and sudden death ,. In prospect of the increase in demand, developing a low-cost, wearable pulse sensor that can offer real-time continuous monitoring, immunizes mechanical and electronic noises, as well as displays some basic diagnostic results is becoming one of the most popular research topics in this area ,.
This paper aims at developing a real-time non-invasive metabolism and blood circulation surveillance system for monitoring human’s health condition by sensing the various bio-signals on the human body; as its phase 1, we firstly address developing a high-accuracy, high-sensitivity, low-cost, non-invasive, and wearable radial pulse sensor for the system. More importantly, inspired by touch capability and associated biomechanical advantage of human fingertip, the mechanical design of the sensor mimics is the human fingertip physiological structure. In addition, to pursue the accuracy, the biomimetic sensor is well calibrated using high-accuracy force sensor and the model is accurately identified by the system identification method. The extended applications of the sensor include monitoring human’s health condition in real time to support medical diagnosis process and physiological and pathological studies. It can be also expected to prevent and alarm the varied cardiovascular diseases at the early stage and boosts related clinical studies. Extensive calibration results show the good performance such as high reliability, high accuracy, and high sensitivity of the developed biomimetic sensor. In the first application, the sensor was successfully used to measure one critical factor called augmentation index (AI) that is related to arterial stiffness and cardiovascular diseases .
The paper is structured as follows. In Section ‘Methods’,Subsection ‘Biomimetic radial pulse sensor design’ reviews the human fingertip physiological structure and describes the mechanical and electrical designs of the biomimetic radial pulse sensor. Subsection ‘Sensor calibration, identification, and verification’ presents the calibration and identification processes of the developed sensor and validates the biomimetic performance of the sensing structure. Section ‘Results and discussion’ depicts the radial pulse measurement performance of the sensor, the clinical application and its preliminary results. Finally, Section ‘Conclusions’ concludes the work.
Biomimetic radial pulse sensor design
Human fingertip physiological structure
Mechanical design of biomimetic sensor
Figure 2 illustrates the mechanical design of the biomimetic radial pulse sensor using SolidWorks. Figure 2a displays the three-view diagram of the sensor structure. The length of the sensor is 50 mm, the width is 30 mm, and the height is 25 mm (without the adjustable screw); two grooves at two sides of the sensor can attach an adjustable belt to help wear it at the radial, or other body locations such as brachial, carotid, popliteal, and even superficial temporal comfortably. Figure 2b details the sensor components that functionally mimic the fingertip bio-components and their functions. All components except the sensing film and polydimethylsiloxane (PDMS) damper were made by the 3D printer using the solid plastics. The diameter of the skin contact ball is 10 mm, back side of the ball is glued with the sensing film, and the open side directly contacts with the radial pulse location of wrist. The rigidity of the ball is carefully chosen so that it can transfer the radial pulse signal to the sensing film without significant mechanical loss. The rigid contact ball design can also help user to quickly find the exact pulse points. The sensing film is one of the key components that mimic the mechanoreceptors of fingertip for the developed pulse sensor. It is an ultrasensitive hybrid carbon/polymer-based piezoresistive (HCP) film with thickness of 0.127 mm, width of 3 mm, and length of 12 mm. The film has high sensitivity, very low thermal drift, and low hysteresis that were reported in our previous study . The sensing film is glued on the elastic PDMS damper, the film will be deformed, and its resistance will change following the strength of transferred radial pulse signals.
In the schematic, R6 is the resistor that represents the value of the resistive layer on the HCP sensing film. Its resistance is changed following the micro deformation caused by the radial pulse. At the top of R6, a potentiometer is used to trim the bridge circuit through removing the zero offset. R1, R2, and R5 are the constant resistors with the same resistance as the undeformed resistive layer. R3 is the resistor used to modify the gain of the amplifier AD620, and the maximum gain of the amplifier can reach 10,000. The circuit output V o is the analog voltage signal that is input to a PC for further display and processing through a 16-bit and 200 kS/s DAQ system PCI-DAS6013 made by Measurement Computing Corp.
Sensor calibration, identification, and verification
Sensor calibration setup
Two high-performance sensors, a Shimpo FGV-1XY digital force gauge (Shimpo Instruments, Itasca, IL, USA) (capacity: 5 N, resolution: 1 mN, high accuracy: ± 0.2% FS) and a Baumer OADM 20I6X41/S14F laser distance sensor (Baumer Electric, Southington, CT, USA) (range: 70 mm, resolution: 20 µm), were employed in the calibration process. The laser distance sensor OADM 20I6X41/S14F was set up on the vibration-isolated workstation and its laser beam perpendicularly shoot on the moving stage of the micromanipulator for tracking the displacement change of the skin contact ball in the developed radial pulse sensor, when the digital force gauge is controlled to push/release the contact ball during the calibration. The pushing/releasing process was controlled by a Sutter MPC 385 3-D micromanipulator (Sutter Instruments, Novato, CA, USA) (range: 25,000 µm, resolution: 1 µm) where the digital force gauge is attached on the micromanipulator. The process can generate 150-µm height square wave pushing/releasing movement by the program-controlled micromanipulator. Meanwhile, the force information was recorded by the force gauge. In the calibration process, the developed pulse sensor was fixed on the vibration-isolated workstation through a clamping stander. Note that, to meet the real condition that the preload force is applied to the developed sensor during wearing it, an average 1.6 N preload force to the contact ball was initially set by the force gauge FGV-1XY before and during the calibration. In addition, the designed electronic circuit with the gain of 50 times was interfaced with the developed sensor. All signals were recorded to a PC through the DAQ board PCI-DAS6013 (Measurement Computing Corporation, Norton, MA, USA) (resolution: 16 bits, range: ± 10 V, number of channels: 16, accuracy: ± 8.984 mV, speed: 200 kS/s) with sampling rate of 1 kS/s.
Identification and calibration
Biomimetic performance verification of sensing structure
The calibration results of the sensor have demonstrated the good mechanical to electronic conversion performance of the developed sensor. To further verify the biomimetic performance of the sensing structure that is close to the structure advantage of the human fingertip, an experiment was conducted to investigate the force-displacement relationship of serially connected components in the sensing structure consisting of a rigid skin contact ball, a sensing film, a silicone elastic damper, and a ball-shaped bone structure. In this experiment, the setup is similar to the calibration one described in Subsection ‘Sensor calibration, identification, and verification’. Instead of using 150-µm height square wave motion, 150-µm height triangle wave movement was generated by the computer program to push and release the sensing structure through the contact ball. The laser distance sensor OADM 20I6X41/S14F was set to a higher resolution but with a short range (range: 30 mm, resolution: 4 µm) to accurately detect displacement/deformation of the sensing structure caused by pushing and releasing operations.
Results and discussion
Radial pulse measurement and results
Clinic application and results
Next to the achieved accurate radial pulse signals using the sensor, in this section, one clinical application based on the radial pulse measurement is extensionally presented. The application is related to determining the arterial stiffness by computing the AI using the measured radial pulse signals of the sensor.
Recent research shows that the central augmentation index cAI can be estimated by applying a transfer function to the radial pulse augmentation index (rAI) ,, or can be obtained directly from the radial pulses  due to high correlation. This indicates rAI can be also used as the evaluation factor like cAI. In this paper, based on the radial pulse measurement using the developed sensor, we preliminarily start to investigate radial augmentation index rAI as well to correlate it into the cAI for determining the arterial stiffness.
Note that, in this clinic test, two non-smoking health subjects without cardiovascular disease history, subjects A and B, with the same height and similar weight were chosen to measure after 30-min rest in a quiet room with more than 3-h fasted, also abstained from caffeine and alcohol. The developed pulse sensor was worn at the radial on the left arm of each subject. The PPG sensor was clamped at the index finger of the right hand of each subject. Both signals from our pulse sensor and the PPG sensor were collected by the DAQ board PCI-DAS6013 (sample rate: 1 kS/s) in the same time.
Physiological characteristics of the subjects and measured AI and SI indexes
Body mass index
Cardiovascular disease history
AI (our pulse sensor)
In this paper, inspired by touch capability and biomechanical advantage of human fingertip, we developed a high-performance radial pulse sensor with the highly sensitive, durable, low-cost HCP film, and biomimetic sensing structure. Calibration and verification prove the sensor performance that it could extract the accurate and reliable pulse signals non-invasively. One clinic application of the developed sensor is to determine the arterial stiffness based on the calculated augmentation index at radial. The results indicate that the developed sensor can be used in clinic applications and can be potentially extended to use in our real-time non-invasive metabolism and blood circulation surveillance system in the future.
This research work is partially supported under the NSF Career Award CBET 1352006 and the Research Enhancement Grants (REG) from UNR office of the vice president for research and innovation.
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