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