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Page 1: Miniature wireless photoplethysmography devices ... · Miniature wireless photoplethysmography devices: integration in garments and test measurements E. Kviesis-Kipge, V.Me þ¼ ika,

Miniature wireless photoplethysmography devices: integration in garments and test measurements

E. Kviesis-Kipge, V.Mečņika, O. Rubenis

Biophotonics Laboratory, Institute of Atomic Physics and Spectroscopy, University of Latvia, Riga, Latvia

e-mail: [email protected]

ABSTRACT

Wireless PPG devices were developed and embedded in everyday clothes (bandage, scarf, cycling glove and wrist strap) to monitor cardiovascular state of free-moving persons. The corresponding software for measurements also has been developed and tested in laboratory. Real-time measurements of PPG signals were taken in parallel with a professional ECG reference device, and high correlation was demonstrated. Keywords: Photoplethysmography, wireless biomonitoring, PPG sensor, wearable electronics.

1. INTRODUCTION

In the last years wireless technologies have become a self-evident part in many spheres. Medicine is one of the areas where this kind of technology would increase patient’s mobility as the patient’s movement would not be limited due to wires. The goal of this work was to develop and to test a new patient friendly non-invasive wireless measur-ing/monitoring mini-device prototype, which can be used for research purposes, athletes and physiological measure-ments. Photoplethysmography (PPG) is a non-invasive method for studies of the blood volume pulsations by detection and analysis of the tissue back scattered optical radiation. Blood transport dynamics can be monitored at different body sites - fingertip, earlobe, forehead, forearm, etc. – with relatively simple PPG contact sensors. The PPG signal consists of two components - a slowly varying DC offset representing the skin blood volume in the probe-covered area, and a fast, alter-nating AC component that reflects the blood volume pulsations. AC amplitude is directly proportional to the changes in signal during heartbeats.The PPG technique has good potential for express diagnostics and early screening of cardiovas-cular pathologies, as well as for scientific research (physiological measurements) and self-monitoring of vascular condi-tions [1]. Recently various “smart garment” technologies are rapidly developing, and distant PPG monitoring by garment-embedded small optical contact sensors with wireless signal transmitter may find interesting applications in health moni-toring systems, including shape/temporal analysis of human arterial pulse waves and detection of specific vascular mal-functions. In recent decade the field of wearable electronics and smart textiles for healthcare is developing due to textiles with biomedical performance providing more psychophysiological comfort to a wearer than attached medical device during a long term biomonitoring. As ECG method is widely applied for cardiovascular activity assessment, also distant PPG monitoring by garment-embedded miniaturized optical contact sensor may find applications in health monitoring systems.

Biophotonics: Photonic Solutions for Better Health Care III, edited by Jürgen Popp, Wolfgang Drexler, Valery V. Tuchin, Dennis L. Matthews, Proc. of SPIE Vol. 8427, 84273H

© 2012 SPIE · CCC code: 1605-7422/12/$18 · doi: 10.1117/12.922594

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2. METHOD AND EQUIPMENT

The device contains a central processing and control unit - NXP 32 bit ARM7 microcontroller running at 48MHz, single channel reflexion PPG sensor, LED driver, small 240mAh Li-lion accumulator with charger - microUSB connection, two LED for simple using, single push button, along with integrated class 2 Bluetooth transmitter module that provides transmission of the captured biomedical data to host PC or handheld PDA compatible device for online real-time data analysis. The device is designed following our previous research to capture the PPG signal using a 32-bit hardware timer built in the central processor unit and therefore do not require software resources for acquiring high resolution PPG signal [2]. The developed wireless PPG sensor incorporates Si emitting diode and Si photodiode. A silicon PIN photodiode OSRAM - BPW34-FA - with daylight filter and the active surface area of 7mm2 with the peak spectral response wave-length of 880nm was used. A SMD (surface mount device) type of an infrared radiant diode model SIR91-21C/F7 with a peak wavelength of 875 nm, a transmission angle of 20°, and a diameter of 1,9 mm was used. A special screening barrier for the photodiode was made within the sensor to lower the influence of ambient light. A barrier is located between the LED and the photodiode and is 5mm from the edge of sensors (darker vertical line (A) Fig. 1). The sensor dimensions are 10mm x 15mm x 4mm. Complete device is small and lightweight – 11 grams (with accumulator and sensor).

Fig. 1. Device modules before integrating in the garment.

Block-diagram of the prototype device is presented on Figure 2. The central processing unit takes all control of the equipment. Bluetooth transceiver module provides transmission of acquired cardiovascular raw data and commands to the computer. The device uses National Semiconductor's LMX9838 Bluetooth Serial Port class II module, data transmis-sion is possible up to 10 meters. In the module is fully integrated 2,4 GHz antenna. Only a few external components are necessary for Bluetooth to fully operate. Bluetooth module dimensions - 10mm x 17mm x 2.0mm.

Fig. 2. Block diagram of the prototype device.

To obtain PPG signal a digital principle was used developed previously. This method is very simple and is suitable for small, portable, cheap, accumulator or battery-operated, PPG monitoring devices. There is practically no limit to the sensor wiring length and sensor configuration due to digital signal already in the sensor. Thus measurement equipment and interconnections to the sensor does not require specially designed shielded wires. The sensor dimensions and shapes virtually no restrictions, and it is all possible without a standard ADC chip.

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The sensor electronic circuit is shown in Figure 3 (A). It consists of two (FET) Field Effect Transistors, four resistors and one photodiode. Developing and adapting the scheme to different needs, number of photodiodes can be increased to 9 not complicating the scheme. Digital PPG signal acquisition principle works as follows: CPU constantly generates a 30μs long pulses with 1 KHz frequency on a port pin that is connected to the P-channel FET Q2 Fig. 3 (A) (test point A). Digital signal output timing diagram is shown in Fig. 3 (B), captured on test point (B).

A B Fig. 3. Electronic schematic of the PPG sensor (A), output waveform from the sensor at testpoint B (B).

In the stage of development of smart clothing prototypes (head bandage, scarf, cycling glove and wrist strap) is required that they all work stable when transmitting PPG data at the same time. These devices must operate in "network". Exactly this condition caused the most problems. Most part of the latest Bluetooth modules support operation within the network, where one is master and seven slaves, that structure is called a piconet. In the computer side we used a Bluetooth module with USB connection, which acted as the master and all smart clothing devices acts as a slave. SSP (Serial Port Profile) was used for data transmission via Bluetooth. Many controllers supports this profile and it is very simple programmable. Transfer data rate to the Bluetooth was chosen 115,2kbps to ensure transmission of 1000 measurements per second. Tests showed: in that way configured device works stable up to 10 meters (only one device at the same time). When all prototype devices (at the same time) are switched on, and connected in the network - stable transmission distance decreased ~ 5 times i.e. by 2 meters, which is totally unacceptable low. It should be noted that the data stream on a computer screen is displayed in real time, so even minor traffic delays or time-lag is highly visible. Testing equipment for data transfer and stability for maximum distance revealed major weaknesses in the Bluetooth operation. Smart clothing prototype electronics PCB Bluetooth module is SMD type and is not intended to replace. It was therefore decided to replace the master Bluetooth module in the PC side. Were purchased and tested several different manufactur-ers class I and class II USB Bluetooth modules. The best results showed Laird Technologies BRBLU03-010A0 USB Bluetooth module (transmission stability, and a network support (piconet)). Importantly, that the module works well with the standard Microsoft drivers, while other modules requires special software drivers to be installed, which needed a special configuration, took a long time and as a result still did not work steadily. It should be noted that the movement around the room, when all the smart clothing equipment is turned on and active (sending data) are very limited. Even a small movements (up to 1 meter), causing data corruption. When tested, with different data transmission speeds, it was found that the master USB Bluetooth module cannot so quickly switch between four slave device data streams without losing data. Since we cannot change the USB Bluetooth module’s software (which is responsible for receiving and switching data streams), the only thing that remains is to reduce the total streaming data rate. As a result, smart wear PPG signal sample rate remained at 1 KHz, but averaging of samples was implemented. Thus, the output sample rate was able to cut down 10 times i.e. 100Hz. Laboratory test results show that in this case the Bluetooth transmission distance increased 2 times i.e. up to four meters. The conclusion was: the Bluetooth module in this configuration (connected in the network) is not really suitable for rapid, continuous data stream with a number of several Bluetooth slave devices. Also PC software was developed for heart rate parameter screening and calculating in real-time. The parameters that can be measured with this device are the pulse shape of the photoplethysmography signal and the signal delay time.

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A B Fig.4. Wearable PPG sensor body area network (head bandage, scarf, cycling glove and wrist strap) (A); comparison of F-F and R-

R intervals (B).

The developed software performs the real-time changes of the pulse wave and such cardiovascular and haemodynamic parameters like heart rate(n/min) and PPG peak ratio, and calculates the following parameters: HRSD – Heart Rate Stan-dard Deviation (n/min), MMHR – Max - Min Heart Rate (n/min), FF– Pulse Width (ms), RRSD – Pulse Width Standard Deviation (ms), RMSSD – RMS Standard Deviation (ms), NN50 – Interval percentage, where ∆ interval > 50 ms [3]. The PPG pulse shape parameters can be measured with this device, and also the signal delay time (if two or more sensors are applied). Quantitative and qualitative analysis of the identified physiological data gives useful information about the patient’s cardiovascular condition [4]. Thus, foot-to-foot interval is identified, which represents the heart cycle length and is referred with R-R interval acquired by ECG (Fig.4 (B)).

3. RESULTS OF THE MEASUREMENTS

A measurement series has been taken to evaluate the measurement accuracy of the wearable PPG sensor network in laboratory conditions. The measurements have been taken from 10 persons (20-23 years old) in rest condition. All the sensors integrated into garments (a head bandage, a cycling glove, a wrist strap and a scarf) have been attached and lo-cated on the corresponding body sites accordingly: temporal artery, the 1st phalange of the forefinger, radial artery and external carotid artery. The PPG signals have been recorded for 4 minutes simultaneously with ECG signal acquired by TLC5000 12 Channel Holter ECG Monitor System (Contec Medical Systems) as a reference device. The acquired physiological data has been analysed by comparing length of foot-to-foot (F-F) interval and R-R interval (Fig.4 (B)). Measurement artefacts have been taken into consideration as well when statistically analysing the data in order to evaluate not only measurement accuracy, but also usability of the developed prototype for every day application.

A B Fig. 5. Detailed results on the prototype measurement accuracy (A); average correlation of the prototype registered da-

ta and Holter ECG monitoring system (B).

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Figure 5 (A) presents results on the prototypes’ measurement accuracy for each tested person (P1…P2), while figure 5 (B) shows the average correlation values for each device. Thus, the highest measurement accuracy and operation stability has been performed by the head bandage prototype (rglove=0.97±0.02). Less accurate measurement results have been obtained by the cycling glove (rh.bandage=0.94±0.06). The wrist strap and scarf have performed approximately equal re-sults, i.e. rwrist=0.923±0.07 and rscarf=0.919±0.08 (Fig.5).

The highest deviation of measurements has been observed when registering PPG signal with the devices integrated into the hand wrist and scarf. Both prototypes require a very precise attachment of the optical sensor to the reached area (radial and external carotid artery accordingly) in order to get a qualitative PPG signal. Still when the sensors are proper-ly attached both of prototypes perform high measurement accuracy (Fig.6).

Thus, it may be concluded that all prototypes have showed high correlation referring to the professional ECG monitoring system. The experiment results also have been influenced by such factors like accuracy of the sensor attachment to a particular body site and stability of the transmitted PPG signal.

Fig. 6. Correlation sample of F-F and R-R interval length.

The mini - devices successfully were integrated in the garments and showed good usability. High quality photoplethys-mography signals were acquired by using the proposed technique and device. Bluetooth connection uses SPP that can be easily connected with mobile phone or PDA for data transfer. The created device is very easy to use – only one pushbut-ton and two informative LED (running, charging, low battery - three levels, Bluetooth connected/disconnected). ARM

r=0,95

r=0,99 r=0,99

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software is built in such way to switch power off if Bluetooth connection is lost, or not connected. Also accidently pushed button would not destroy (interrupt) the measurements. Wireless photoplethysmography finger sensor probe successfully can be used in physiological measurements and sup-plement ultrasound, ECG, and other methods. Bluetooth generally created major problems with data streaming, perhaps, another transceiver modules can suitable for such smart- wear body network. Like Zigbee, NRF, RFM or others.

4. CONCLUSIONS

A mini-device for photoplethysmography (PPG) signal detection and wireless transmission has been developed and inte-grated in smart-wear prototypes. Recent designs of wireless photoplethysmography monitoring devices embedded in wrist cuff, glove, head bandage and scarf are described. First results of distant monitoring of heart rate and pulse wave transit time using the newly developed devices are presented. The wearable electronic PPG devices were tested in the laboratory. Test measurements of heart rate by comparison with professional ECG device have been performed. Potential future applications can be real time shape analysis of human arterial pulse waves and detection of specific vas-cular malfunctions. Device advantages are the low noise, low power, low weight and high resolution PPG signal (sample rate – 1000 measurements per second, that is two to ten times better than similar products commercially available). The prototypes have been tested in the laboratory environment by taking physiological measurements from 10 healthy volunteers in rest condition. The performance of prototype devices has been evaluated by comparing the length of heart cycle with the data simultaneously registered by the Holter ECG monitoring system. Data acquired by the head bandage and cycling glove performed the highest correlation with reference device (rh.bandage=0.94±0.06 and rglove=0.97±0.02). Correlation of physiological data registered by the wrist strap and the scarf was rwrist=0.923±0.07 and rscarf=0.919±0.08.

ACKNOWLEDGMENTS

This work was supported by the European Social Fund, projects #2009/0138/1DP/1.1.2.1.2/09/IPIA/VIAA/004 and #2009/0211/1DP/1.1.1.2.0/09/APIA/VIAA/077 is highly appreciated.

REFERENCES

[1] J.Spigulis., “Optical non-invasive monitoring of skin blood pulsations”, Appl. Opt., 44, 1850-1857 (2005). [2] E.Kviesis-Kipge, “A new technique for optical detection of biosignals”, Latv. J. Phys. Tehn. Sci., N3, Vol. 46, p. 64 – 69 (2009). [3] User manual, Kubios HRV Analysis Software, Chapter 3. [4] R. Erts, J. Spigulis, I. Kukulis and M. Ozols, “Bilateral photoplethysmography studies of the leg arterial stenosis”, Physiol. Meas.

Vol. 26, p. 865-874 (2005).

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