Available online at www.sciencedirect.com
Sensors and Actuators A 143 (2008) 196–203
Characterization and fabrication of wireless flexible
physiological monitor sensor
Wen-Yang Chang
a,b, Te-Hua Fang
c, Yu-Cheng Lin
a,∗aDepartment of Engineering Science, National Cheng Kung University, Tainan 701, Taiwan bMicrosystems Technology Center, Industrial Technology Research Institute, Tainan 709, Taiwan cInstitute of Mechanical and Electromechanical Engineering, National Formosa University, Yunlin 632, Taiwan
Received 19 March 2007; received in revised form 15 October 2007; accepted 21 October 2007 Available online 4 November 2007
Abstract
The study reported a novel design and fabrication process of a wireless flexible physiological monitor module, which is based on a polyimide substrate for a printed circuit and uses on a non-woven material to package the module by a hot-press. The module is sufficiently thin and light to paste on human wrists for monitoring body temperature and heart rate. The advantages of the module include the ability to effectively monitor the physiological signals during the postural change or shaking, a flexible antenna for wireless transmission, and a sensor package made the conductor trace line does not crack after repeated bending. Furthermore, to realize a flexible board, the meander concept was used to improve strength; six thin stainless steel sheets on the polyimide form the banded block, which can withstand repeated bending while worn on the human arm. Experimental results show that the thickness of the flexible physiological sensor is about 2 mm, the minimum radius of curvature is about 2.5 cm, and the specification should be 25–45◦C and 50–200 bpm, respectively. More efficient measurement is achieved with miniaturization and ergonomic design for portable physiological monitor use.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Physiological monitor; Patch-type; Heart rate; Flexible module; Wireless
1. Introduction
As the global population ages and birth rate falls, portable physiological monitoring is being used more frequently for the automated monitoring of physiological parameters[1], per-formed in homes instead of in a hospital. The measurement parameters are mainly body temperature, heart rate, and respira-tory and electrocardiograph (ECG) signals, which are important indicators of health condition [2]. In addition, portable mon-itors are characterized by lightweight, low cost, and comfort for long-term wearing and ambulatory health monitoring, with instantaneous feedback to a user about the current health status
[3]. Therefore, physiological monitor technology has recently focused on integrating a personal digital assistant and miniatur-izing the sensors system, including embedded microcontrollers, wireless networking, and microphysical sensors for portable devices[4,5].
∗Corresponding author. Tel.: +886 6 276 2395; fax: +886 6 276 2329. E-mail address:yuclin@mail.ncku.edu.tw(Y.-C. Lin).
Most researchers have focused on smart detection methods with the transmission of health condition indicators via a wire-less local area network (WLAN)[6,7] or Bluetooth wireless technology[8]. Commercial physiological sensors are usually wristwatch type[9], belt type[10], have many sub-sensing ele-ments attached to human skin[11,12], or are wearable[5]. Most of the devices have a rigid body and considerable weight, which makes them uncomfortable when worn on the human body. The signals are also easily interfered with by shaking.
To resolve these problems, we propose a wireless flexible physiological monitor using a patch type sensor, which is inte-grated with a polyimide (PI) substrate and a thin stainless steel stiffener, and packaged using a non-woven textile. The sensor monitor, without being constrained by a belt, can effectively resolve the previously described problems. The proposed phys-iological sensor uses the meander concept to realize a flexible design, which is based on a double-sided PI with six thin stain-less steel sheets to form the banded block. The module consists of a pair of sensing electrodes, a thermistor sensor, a control electronic unit, and an antenna with a circuit on the body. For a wearable monitor, textiles can provide protective and aesthetic
0924-4247/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2007.10.071
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functions that are compatible with human skin. Furthermore, the non-woven materials can bend when applied to personalized products [3]. Therefore, this study designs and investigates a flexible sensor. It includes the optimal mechanical analysis, a design concept of the flex for improving strength, a control cir-cuit for two electrodes sensing, and using a non-woven material to package the sensor using hot-press.
2. Design and fabrication 2.1. FPC fabrication procedure
In flexible electronics development, flexible printed circuits (FPC) have usually been designed for flexible electronic inter-connectors. PI films have low shrinkage and a coefficient of thermal expansion close to copper over a fairly wide tempera-ture range. In addition, PI is chemically and thermally stable, being able to withstand the harsh chemical environments asso-ciated with circuit board fabrication. In our design, we used a PI film with double-sided adhesive for FPC fabrication. The PI is a Kapton film, model 100 HN, with a thickness of 25m. The copper foil on each side of the film is 35m.
FPC was created by etching the unnecessary copper foil on both sides. The design and fabrication procedures are shown in
Fig. 1. First, the PI substrate was drilled and the surface was cleaned. Then, the substrate was dipped into an Sn2+solution to activate the sensitivity of the surface for electrochemical plating. The activated surface reacted in an acidic solution of palladium chloride, which was transformed into metallic Pd for copper deposition in the plating process later. Next, the substrate was put into a solution of CuSO4 for electroless plating of Cu to
cover the whole surface, including the holes and areas where the surface was electrically insulated. Subsequently, a cover of dry photoresist (PR) was laminated onto both sides for pattern
definition. The PR was exposed through a positive photographic mask and was developed. The pattern was transferred on the photo-mask and the Cu layer was etched simultaneously on both sides after pattern definition. The minimal line width and spacing were 75 and 100m, respectively. There was a cover layer on top of the printed wiring with window for component placement. After FPC fabrication, the stainless steel stiffeners were glued on the bottom of the board with anisotropic conductive film, and the electrical components were soldered onto the board. Finally, the sensor was packaged using silicone and non-woven material.
2.2. Sensing circuit and antenna design
To obtain clear physiological signals from two electrodes, a differential pre-amplifier with an eliminating dc offset and a comparator with hysteresis were used to extract a reliable pulse signal from the ECG signal. The ECG signal, which consid-ers the heart beat as a pump can be measured by the electrical resistance at the body surface, known as the impedance car-diograph method. For pulse measurement from the ECG, the control circuit of the tag module system includes a pair of sens-ing electrodes (I), an ac couple (II), a differential pre-amplifier (III), a low-pass filter (IV), an offset voltage detector (V), a prac-tical differentiator (VI), a comparator with hysteresis (VII), an ASIC preprocessor, a digital signal processor, a flexible antenna and an RF front-end. A partial schematic circuit diagram of the tag module is shown inFig. 2. There are two-step operational amplifiers with a 60 Hz notch filter and a low-pass filter. The digital signal processor includes an A/D, a signal processor, and logic control. In the RF front-end, a frequency modulator and an oscillator circuit make the transmission for the antenna. The differential amplifier (DA) amplifies a small signal and reduces the rejecting common mode noise. In the DA circuit configu-ration, electrodes on the right hand (RHD) side and left hand
198 W.-Y. Chang et al. / Sensors and Actuators A 143 (2008) 196–203
Fig. 2. The main control circuit diagram of the patch type physiological monitor on the tag module includes a pair of sensing electrodes (I), an ac couple (II), a differential pre-amplifier (III), a low-pass filter (IV), an offset voltage detector (V), a practical differentiator (VI), a comparator with hysteresis (VII). After the analog control circuit, an ASIC deal with the pulse signals.
(LHD) side work in an ac coupled circuit. Cdis 10F and R is
100 k. The input into the impedance of a non-inverting DA is
Rd1= Rd2= Rd3= Rd4= Rd5= 100 k, and Rp= 10 k. The gain
equation of the DA is derived using Eq.(1):
Vdout= A1Vd1+ A2Vd2+ (A1+ A2+ A3)VR (1) where A1= Rd6(2Rd1+ Rd5) Rd3Rd5 , A2= Rd7(2Rd2+ Rd5)(Rd3+ Rd6) Rd3Rd5(Rd4+ Rd7) and A3=Rd4(Rd3+ Rd6) Rd3(Rd4+ Rd7)
The ac power supply seriously interferes with the ECG sig-nal. To reject noise, an adaptive band pass filter module is used to amplify the signal at the expected frequency region and to suppress the noise signals. A second order controlled of band rejection filter was designed to eliminate 60 Hz powder noise by setting Rf= 5 M and Cf= 270 pF. The Cr capacitance, 0.68F,
removes the dc signal. Finally, a differentiator provides a low-pass filer, f0= 1/(2πRfCf), and an amplifier.
The last control circuit transforms an ECG signal into a pulse signal using a comparator with hysteresis. Transfer function is based on previous voltage inputs. The circuit has a positive feed-back loop so the output increases rapidly until it saturates, as shown inFig. 3. The hysteresis is defined as VU–VL. The
rela-tionships of VU and VLare shown in Eq.(2), provided by the
ratio of nRp and Rp. Therefore, the hysteresis remembers the
transition and shifts a threshold voltage for generating the pulse signals that from the ECG wave:
VU= (1+ n)VR+ Vsat
n (2a)
VL= (1+ n)VR− Vsat
n (2b)
For a wireless flexible physiological sensor, an antenna design is very important for human body effects[13,14]. The
Fig. 3. The relationship between the ECG wave and the pulse heart rate is based on the comparator with hysteresis. The output voltage of the heart rate rapidly reaches the maximum voltage.
antenna performance needs to be evaluated on a human body and the rate of the RF energy deposit needs to be measured in the biological tissue. The miniaturization of antennas for wristwatch applications has developed rapidly. A study of the literature[15]
show that a monopole antenna is one of the most widely used ele-ments for personal wristwatches. An alternative of monopole is the loop antenna, whose immunity to noise makes it more attrac-tive in a noisy environment. In addition, the loop antenna can be placed either horizontally or vertically on flexible modules. Therefore, in this studying, we chose the loop antenna for RF radiation. To obtain a 433 MHz antenna frequency, optimization parameters were set up to find the best feed point and the side lengths were calculated for resonance and matching frequency. The antenna length, width, and thickness were considered in the simulation. The PI dielectric constant is about 3.2 and the cover layer thickness is 10m. The effective dielectric constant is given as follows[16]: εeff =εPI+ 1 2 + εPI− 1 2 1+ 12h w (3)
where εPI is the dielectric constant of PI, and h and w are a thickness and a width of antenna, respectively. After the simulation, case 6 was chosen for the physiological sensor’s antenna, as shown inFig. 4. Simulation results show that a larger width can obtain good frequency radiation efficiency, a greater length decreases the frequency, and a higher thickness reduces the absorption rate for human. The antenna length, width, and thickness are 60 mm, 1.6 mm, and 28m, respectively. The return loss characteristic value of the S11 is−23.65 dB, IMP
is 52.33 + j6.349, and the SWR is 1.142 at 433.92 MHz.
2.3. Flexible substrate design and mechanics analysis
In general, the electronic component placement on the flex-ible module plays an important role when bending the module repeatedly. Almost all FPC failure modes include the copper foil fatigue and eventual cracking of the conductor due to the stress and strain of the tensile or compressible force. The strain source is mainly an initial crack on the surface of the copper foil,
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movement was easily interfered because the sudden changed on the skin electrode impedances induce a sharp baseline transient signal, which decay exponentially to the baseline value.
4. Conclusion
A flexible and thin patch physiological sensor for human health monitoring and management, with wireless RF trans-mission and non-woven packaging, has been successfully developed. This module is flexible enough to be applied to the body using patch adhesion without being damaged, with a min-imum radius of curvature of about 2.5 cm. Furthermore, it is comfortable to wear, because the tag module is 2 mm thin and weights 4.2 gm. The heart rate sensor is low cost, only a pair of electrodes and a reader can monitor the multi-tag physiological module. We believed that most common physiology parameters, e.g. blood pressure, respiration, and oxygen saturation, could be incorporated simultaneously if these sensing devices are minia-turized and added. This study may provide useful information for designing wireless physiological sensors.
Acknowledgments
The test facilities and resources for this study are supported from the Ministry of Economic Affairs to fund environment construction project of Taiwan government.
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Wen-Yang Chang received the MS degree in Department of Mechanical
Engi-neering from National Cheng Kung University, Tainan, Taiwan, 2001. He is currently working toward the PhD degree in Department of Engineering Sci-ence at National Cheng Kung University. He is also currently with Microsystems Technology Center, Industrial Technology Research Institute, Tainan, Taiwan for studying the flexible smart sensor. His current research involves development of flexible sensor, MEMS design and fabrication, and mechanics simulation.
Te-Hua Fang received the MS and PhD degrees in Department of
Mechan-ical Engineering from National Cheng Kung University, Tainan, Taiwan, in 1995 and 2000, respectively. He is currently with National Formosa University, Taiwan, as an Associate Professor of Institute of Mechanical and Electromechan-ical Engineering, Institute of Electro-optElectromechan-ical and Materials Science, and as the Director of the nanotechnology laboratory. His research interests in nanotechnol-ogy, scanning probe microscopy, optoelectronics, microsensor, and molecular dynamics. He has published over 100 journal papers in the areas of nanoinden-tation, scanning probe microscopy, advanced optoelectronics, and molecular dynamics.
Yu-Cheng Lin is a faculty member in the Department of Engineering Science
at National Cheng Kung University, Tainan, Taiwan. He received his BS and MS in Mechanical Engineering from National Cheng Kung University in 1985 and 1987, respectively. He also received his MS in 1994 and his PhD in 1996 in electrical engineering from the University of Illinois at Chicago. His main research interests include Bio-MEMS, microfluidic systems and nanotechnology in biomedical applications. He currently serves as a member in the international advisory board of the international journal Lab on a Chip.
