A contact-force regulated photoplethysmography (PPG) platform

Photoplethysmography (PPG)¹ is an optical measurement technique that can be used to monitor volumetric changes in the microvascular bed of tissue. The intensity of light reflected from or transmitted to the tissue is proportional or inversely proportional to the microvascular volume. PPG is generally used for real-time physiological status monitoring¹ such as blood oxygen level, heart rate, blood pressure and cardiac output. Because the PPG signal contains information related to the peripheral blood circulation, including skin vasomotion, which is controlled by the sympathetic nervous system,² behavior of blood vessels are closely related to the autonomous nervous system (ANS).^3,4 Therefore, a PPG signal that includes the amplitude (PPGA) and the interval (PPGI) can also be used to analyze and estimate nociception,^2,5,6 sleep apnea syndrome,⁷ and psychological stress.^8,9 For example, nociception stimulates the ANS, causing higher heat rate and peripheral vasoconstriction.⁶ Psychological stress also induces the increases in ANS-related cardiovascular responses such heart rate and blood pressure.¹⁰ Such PPG signals, however, are affected by a number of factors,^11–13 such as optical absorption and reflection of the skin tissue, motion artifacts, respiratory status, environmental light, etc.

Over the other factors, the contact-force between the PPG probe and the tissue greatly influences the PPG signals.^14–17 In particular, an external force with a contact-force range of 0.2 ∼ 1.8 N applied by the PPG probe deforms the blood vessels causing a change in the PPGA in the amount of about 70 %.¹⁸ Although conventional PPG sensors are pressed against the skin with a spring-chip^19,20 or a stretchable band,^21–23 the contact-force still changes due to the subject’s slight posture changes²⁴ such as wrist extensions or flexions. Because of this vulnerability from the contact-force, an ANS monitor based on PPG has been mainly applicable to patients under general anesthesia who are motionless.^2,5 To increase the reliability of the PPG signals, effective methods that reduce the dependency of the contact-force are required.

In previous studies,^25–28 contact-force compensation for a PPG signal using correction factors had been proposed. However, the correction factors have wide variations among individuals due to the differences in the elastic characteristics of the skin tissue and the blood vessel compliance, etc. In this paper, we propose a PPG platform integrated with a thermo-pneumatic type regulator to regulate the contact-force during the measurement. The platform monitors the contact-force between the PPG probe and the skin and adjusts the displacement of the probe to maintain a constant contact-force, thus providing reliable PPG signals even during changes in body posture.

Figure 1 shows the overall structure of the force-regulator integrated PPG platform. The present PPG platform, designed to measure PPG signal on the wrist, consists of a force-regulator, a main body, a force sensor, and a PPG probe (see Fig. 1(a)). A wrist is the most common site offering high comfortability for patients. We chose a radial artery showing highest PPG measurement sensitivity among the measuring sites on wrist. A thermo-pneumatic actuator^29–32 for which thermal energy is converted to mechanical energy by an expansion fluid is chosen as a force-regulator. It has the advantages of being able to generate a force up to 2 N and is easily integrated into the portable PPG platform. The force-regulator consists of an expansion cavity, a heater layer, and a deformable membrane. The fluid expansion in the cavity is controlled by the electrical power at the heater. The phase transition of the fluid between liquid and gas causes a significant change in the pressure in the cavity and consequently, the force at the deformable membrane.^30,31 The expansion fluid is a performance fluid (PF-5060, 3M, USA), which is a clear, colorless, and non-toxic liquid with a low boiling temperature of 56 °C. The micromachined gold electrode on a glass substrate with a resistance of 30 Ω serves as a heater with an effective heating size of 5 mm × 22 mm. The deformable membrane is a stretchable latex rubber (G5, Kimberly-Clark, USA) with a thickness of 0.2 mm. The contact-force is measured by the conductance change at a flexible sensor (FlexiForce A201, Tekscan, USA) with a thickness of 0.2 mm. The conductance of the force sensor was measured as proportional to the applied force in the range of 0.2 ∼ 4.0 N. For the measurement of the PPG signal, a reflective-type module (RP520, LAXTHA Inc., Korea) was placed at the bottom of the platform.

Figure 1(b) shows the operation procedure of the PPG platform. Initially, the PPG probe is above the measuring site. As the gold electrode is heated, deformation of the membrane begins, and the PPG probe moves down. During the PPG measurement, the force sensor continuously monitors the contact-force, and the electric power supplied to the heater is controlled to maintain a constant force. Figure 2 shows the fabricated force-regulator integrated PPG platform. The total size of the current PPG platform is 35 mm in diameter and 19 mm in thickness. The PPG platform can be worn on the wrist like a watch (Fig. 2(b)). A programmable sourcemeter (2459, Keithley Instruments, USA) was connected to the electrode and supplied electrical power up to 3 W. A data acquisition (DAQ) board (NI-USB-6212, National Instruments, USA) was connected to the force sensor and the PPG modules. A customized Labview (National Instruments, USA) based program was developed to control the contact-force and signal analysis.

To verify the effect of regulating the contact-force on the PPG measurement, we compared two PPG signals from a healthy subject (IRB approval number: KRISS-IRB-2017-04) for two cases: 1) without and 2) with contact-force regulation. The PPG platform was placed on the radial artery wrist (see Fig. 2(b)), and the subject sat on a chair comfortably and placed his arm on a desk at the height of his chest. To change the contact-force condition, the wrist changed its posture according to the measurement stages shown in Figure 3(a). At the same time, the PPG platform measured the contact-force and PPG signal, simultaneously. For the contact force regulation, the target contact force was determined as 0.6 N which showed the highest amplitude for the subject. We analyzed PPGA and PPGI through the experimenter’s eye-inspections based on the general guidelines of PPG waveform configurations.³ As shown in Figure 3(b), the contact-force showed a significant difference depending on the wrist posture without contact-force regulation. Average contact-forces for each measurement stage were measured as 0.45 ± 0.01, 1.01 ± 0.01, 0.40 ± 0.02, 0.56 ± 0.03, and 0.30 ± 0.01 N, respectively. Coefficient of variation (CV), defined as a ratio of the standard deviation to the average, was calculated as 50.9 % for the five stages. Even in each posture, the contact-force showed noticeable variation with a maximum difference of 0.154 N. This amount of difference in the contact-force could result in over a 10 % change in the PPGA.¹⁷ Due to the variation in the contact-force, the PPG signal exhibits an inconsistent profile through the measurement stages (Fig. 3(b)).

The regulation of the contact-force, however, provided a constant contact-force of 0.6 N between the PPG module and the skin shown in Fig. 3(c). The contact-forces for each measurement stage were measured as 0.60 ± 0.01, 0.62 ± 0.03, 0.61 ± 0.01, 0.60 ± 0.02, and 0.62 ± 0.01 N, respectively, with 1.8% of CV. During the regulation of the contact-force, the PPGA level also appeared to be consistent at every stage because the contact-force was consistent despite the changes in the wrist posture. Figure 4 shows a detailed comparison of the PPG signals between the two cases. For each measurement stage, average PPGAs without and the regulation of the contact-force were 43.2 % and 7.2% of the CV, respectively, (Fig. 4(a)). When comparing the PPGI (Fig. 4(b)), regulating the contact-force also reduced the CV from 8.4 % to 1.0 %. In the case of PPGI, no significant effects were observed when regulating the contact-force. Generally, the interval of the PPG (PPGI) was less sensitive to the signal intensity variation the than the amplitude (PPGA), but the variation near the PPG peak point causes an error in the PPGI detection.

In order to confirm the contact force dependency of the PPGI and PPGA, PPG signals were additionally measured for the 4 subjects, where the PPG platform was placed on the radial artery wrist changing its posture according to the measurement stages shown in Figure 3(a). Table I shows the measurement values and coefficient of variation (CV). From the results, CV of PPGA showed significantly high value with 82.6 % ± 67.1 %, meaning that PPGA is highly dependent on the contact force. On the other hand, CV of PPGI showed 5.7 % ± 3.2 %, indicating that PPGI does not critically affected by the contact force. This variation can be because of the subject’s physiological states.

This paper focused on contact force between the probe and the skin when the condition of wrist posture is changed. To obtain the more constant conditions for PPG measurement, anatomical factors such as tissue tension, artery vessel tension, artery vessel diameter, local blood volume/velocity should be further investigated in order to obtain better measurement conditions.

Previous contact force compensation studies corrected the measurement value based on the correction coefficients which are different person to person. Therefore, the compensation methods required individualization process, limiting the measurement accessibility for the broad population of users. Meanwhile, the present method using contact force regulation prevents contact force variation in advance, capable to effectively cancel the contact force effect without any individual specific coefficient factors.

In conclusion, we proposed a force-regulator integrated PPG platform capable of maintaining a consistent contact-force during a measurement. By using contact-force regulation based on a feedback algorithm, the proposed PPG platform shows an improved PPG measurement for which the variation in the PPGA was reduced from 43.2 % to 7.2 % despite the changes in the wrist posture. The proposed PPG platform can be applied to biosignal measurements in various fields such as PPG-based ANS monitoring to estimate nociception, sleep apnea syndrome, and psychological stress.

This work was supported by the Korea Research Institute of Standards and Science under the project “Establishment of measurement standards for medical metrology”, grant 18011083, and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (grant 17442002).