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  • 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: cobba@inbox.lv

    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

    Proc. of SPIE Vol. 8427 84273H-1

    Downloaded from SPIE Digital Library on 22 Jun 2012 to 85.254.232.1. Terms of Use: http://spiedl.org/terms

  • 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.

    Proc. of SPIE Vol. 8427 84273H-2

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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

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