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,
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.
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-
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-
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
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
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
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 .
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
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).
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