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REFERENCES
Photoplethysmography (PPG) is a simple and low-cost optical technique that can be used
to detect blood volume changes in the micro vascular bed of the tissue. It is often used
for non-invasive measurements at skin surface.1
PPG has two types: transmitted and reflected. For the transmitted type, an infrared light
source (wavelength 600 – 1300 mm) is generally used. This wavelength range has a
large penetration depth in the tissues and the arterial pulsations can be recorded with
light transmitted through fingertip, earlobe or other relatively thin organs.2
Reflection photoplethysmography detects the tissue back-scattered radiation with time
resolution3. The PPG signal consists of AC and DC components. The AC component
reflects the vascular pulsations, and the DC component represents the light scattered
from relatively steady blood volume and tissue layers, which are the components without
a pulsatile signal4.
Multi-spectral photoplethysmography (MS-PPG) biosensor is intended for analysis of
peripheral blood volume pulsations at different vascular depths. The light penetration
depth in skin varies depending on wavelength - for example, green light penetrates only
Stratum corneum and epidermal layer (till 0,2 mm), but red and infrared radiation
penetrates also in dermal layer (till 2 – 3 mm). Consequently, parallel analysis of PPG
signals at different wavelengths might help to assess skin damages and pathologies at
various tissue depths.
This study continues our previous research5 with the aim to understand more deeply
pulse shape changes at different tissue depths.
INTRODUCTION
The newly developed biosensor confirmed its ability to detect PPG signals at three laser
wavelengths simultaneously and to detect temporal differences in the signal shapes at these
wavelengths that correspond to different penetration depth in skin.
Our results suggest that further tests are necessary to understand the different PPG signal
shape at wavelength 405 nm and the systolic rising time shift. The time interval from foot to
incisura (the notch between systole and diastole) was the same independently on the PPG
pulse duration.
Analysis of the MS-PPG signal shapes and baseline variations at three wavelengths
provides information on haemodynamic parameters at different vascular depths and we may
conclude that newly developed method could be useful in dermatology for skin assessment.
CONCLUSIONS
To illustrate the results after analysis with both programs, Figure 2. and Figure 3.
demonstrates that normalized mean PPG signal shape from all 5 measurements at
wavelength 405 nm was different to signals at red and NIR wavelengths, and also systolic
rising time had a shift (Figure 2.) if we compare wavelengths 405 nm, 660 nm and 780 nm.
In 70% cases we noticed systolic rising time shift of 660nm and 780nm wavelengths pulses
relatively to 405nm (Figure 2a) but in 30% cases opposite effect took place (Figure 2b).
This phenomenon needs further studies to be properly explained.
EXPERIMENTAL RESULTS
Multi-spectral photoplethysmography biosensor
Instutute of Atomic Physics and Spectroscopya, University of Latvia,
Raina Blvd 19, Riga, LV-1586, Latvia
Financial support from European Social Fund (grant #2009/0211/1DP/1.1.1.2.0/09/APIA/VIAA/077)
is highly appreciated.
ACKNOWLEDGEMENTS
1. Allen, J. “Photoplethysmography and its application in clinical physiological
measurement”, Physiol. Meas. Vol. 28, R1-R39 (2007).
2. Maeda, Y., Sekine, M., Tamura, T., “The Advantages of Wearable Green Reflected
Photoplethysmography”, J.Med.Syst. (2010), online 10.1007/s10916-010-9506-z
Springer Science+Business Media, LLC 201 or
http://www.springerlink.com/content/2274316hu8724050/fulltext.html
3. Ugnell, H., Öberg, P.Ǻ., “Time variable photoplethysmographic signal: its dependence
on light wavelength and sample volume,” Proc. SPIE 2331, 89-97 (1995).
4. Asada, H., H., Shaltis, P., Reisner, A., Rhee, S., and Hutchinson, R., C., “Mobile
monitoring with wearable photoplethysmographic biosensors,” IEEE Eng. Med. Biol. Mag.
22, 28-40 (2003).
5. Gailite, L., Spigulis, J., Lihachev, A., “Multilaser photoplethysmography technique,”
Lasers Med. Sci. 23, 189-193 (2008). Figure 3. Mean PPG signal shapes for each wavelength with standard deviation (SD).
b a
Figure 2. Examples of PPG signal time shifts: a) 660nm and 780nm delayed relatively to 405 nm,
b) 405nm delayed relatively to 660nm and 780nm pulses ahead.
Subjects. The multi-spectral photoplethysmography recordings were obtained from 11
male volunteers. The volunteers were healthy men. The age of volunteers was between
22 and 40 years.
Protocol. Measurements were performed in a laboratory (a well-ventilated room under
reasonable constant temperature which is typically 20°C). Each volunteer was asked to
relax and sit in the chair. Before the measurement each volunteer was asked to calm
down for a 10 minutes after which measurements were performed. The MS-PPG
recording time was between 90 and 120 s. The recordings were taken 5 times with pause
of 2 minutes.
Biosensor system. Set up consists of a PPG sensor, a central system control unit and a
Li-ion accumulator (Figure 4b.). The signals acquired from measuring photodiode
discharge time are inverse to the absorption of the light. The biosensor operates in
contact reflection mode, with simultaneous parallel recording of PPG signals at each
wavelength
Experimental data analysis. Experimental data were analyzed with the PPG-analysis
software which is a specially created for analysis (Figure 1.).
METHODS
Figure 1. PPG-analysis window screenshot during processing of experimental data
Figure 4b. Block diagram of the biosensor device.
b
Figure 4a. View of laser diode from the side.
a
Dimensions of the BRIR laser diode are 5.60 mm x 4.40 mm and weight 0.3 g (without stand) .
Figure 4. The prototype biosensor device.
The newly developed optical fiber biosensor
comprises one 3-wavelengths laser diode BRIR
(Blue, Red and InfraRed) and a single photodiode
with multi-channel signal output processing; special
software was created for visualization and
measurements of the MS-PPG signals. The contact
sensor head is connected to the device by a 1 meter
long cable. The measuring area for sensor head is
5 mm2.
Dimensions of the prototype equipment are
140 mm x 90 mm x 35 mm and weight 250 g; it is
battery-powered and can operate up to 10 hours
without recharging.
Diode Ith Max power
Infra red 780 nm ~21 mA ~15 – 20 mW
Visible Red 660 nm ~21 mA ~15 – 20 mW
Violet 405 nm ~25 mA ~15 – 20 mW
Table 1. Current vs. power for BRIR laser diode
Multi-spectral photoplethysmography biosensor Lasma Asarea, Edgars Kviesis – Kipgea, Andris Grabovskisa, Uldis Rubinsa, Janis Spigulisa, Renars Ertsa