The opto-mechanical integration of light-emitting and light-sensing elements into bio-sensing wrist wearables is a fundamental step in the wearable design process. The quality of the signal can be greatly affected by choosing components and geometries that minimize crosstalk and maximize signal to noise.
This article discusses the optical and mechanical aspects to be considered for optimal performance. The optical portion focuses on the interaction of light with the skin and blood as well as selection of LEDs and photodetectors. The mechanical portion provides suggestions that increase coupling between the optics and the skin.
Introduction
Wrist-based wearables are gaining prominence with customers who want to track their physiological parameters during fitness, daily activity, and sleep. These physiological parameters can be obtained non-invasively using optical sensors to detect the heart-rate signal. This technology has been established in the medical sector and can now be transferred to wearable wrist applications.
Theory of Operation for Optical Sensors
The principle on which optical sensors measure heart rate is called photoplethysmography (PPG). As the heart pumps, the volume of blood transported in the arteries changes. More blood flows through the arteries when the heart expels blood (systolic phase) and less blood flows when the heart draws blood in (diastolic phase). When the blood volume changes between systolic and diastolic heart beats, it results in a change in the optical absorption coefficient of the arterial layer. By optically illuminating the tissue and measuring the transmitted light, the absorption change due to the blood volume change can be determined and the heart-rate pulsatile signal can be recovered.
On certain regions of the body such as the wrist, transmissive heart-rate measurements are logistically difficult, so reflective measurements are used. Reflective heart-rate monitors consist of a light source and a detector arranged in the same plane (Figure 1). The light emitted penetrates the skin, tissue, and blood vessels and is either absorbed, scattered, or reflected. A small portion of the emitted light eventually reaches the photodetector. As the volume of blood in the arteries changes with every heartbeat, the fraction of light absorbed and, subsequently, the strength of the detector signal change.
Interaction of Light with the Skin
Skin consists of three main layers from the surface: the blood-free epidermis layer (100 μm thick), vascularized dermis layer (1-2 mm thick), and subcutaneous adipose tissue (1-10 mm thick, depending on the body site). Typically, the optical properties of these layers are characterized by absorption µa, scattering μs coefficients, and the anisotropy factor g.
The absorption coefficient characterizes the average number of absorption events per unit path length of photons traveling in the tissue. The main absorbers in the visible spectral range are melanin: blood composed of oxyhemoglobin (Hb), deoxyhemoglobin (HbO2), and lipids. In the IR spectral range, absorption properties of skin dermis are dominated by absorption of water.
Figure 2 presents a planar seven-layer optical model of human skin. The layers included in this model are the stratum corneum, the living epidermis (the two layers of dermis each has been divided into two layers, first papillary dermis and superior blood net dermis and second reticular dermis and inferior blood net dermis), and, finally, the subcutaneous adipose tissue layer. Table 1 presents thickness of the layers as well as typical ranges of blood, water, and melanin contents, and refractive indices of the layers.
corneum and the innermost layer is the subcutaneous adipose tissue, or fat layer.
The heart-rate pulsatile signal originates from the arterial bed lying down in the inferior blood net dermis layer highlighted as the sixth layer in the seven-layer tissue model in Table 1. The absorption spectrum of this layer can be calculated using the absorption spectra of tissue constituents and its corresponding volume fractions by:
Where 
are the absorption coefficients and volume fractions of melanin, water, and lipids, respectively. 
and 
are the absorption coefficients of oxyhemoglobin and deoxyhemoglobin, and 
is the volume fraction of blood. 
is the blood oxygen saturation coefficient, typically around 95% in healthy individuals. Using Equation 1 and measured absorption spectra for the tissue constituents [1] the absorption coefficient in the inferior blood net dermis can be calculated as a function of wavelength. Figure 3 plots the results.
As can be seen from Figure 3, the peak absorption coefficients correspond to wavelengths around 540 nm and 570 nm. At these wavelengths the absorption change due to the blood volume change will be greatest, and the photodiode will measure the strongest pulsate signal.
Component Selection
LED Wavelengths and Efficiency
For acquiring the best PPG signal, i.e., largest AC heart-rate signal, the LED illumination wavelength should be as close as possible to the absorption peaks of the blood HbO2 at around 540 nm and 570 nm (Figure 3). However, due to the known "green gap" range in the LEDs’ luminous efficacy around 560 nm, commercially available LEDs are very dim at these two desired wavelengths and, hence, not very useful for practical applications where high signal-to-noise ratio (SNR) is required. Therefore, green LEDs emitting around 530 nm are utilized in most commercial PPG sensors available on the market.
Maxim Integrated has explored illumination wavelengths at both sides of the green absorption peak, the widely used true-green LEDs at 530 nm and yellow LEDs at 590 nm. While both wavelengths are available with large luminous efficacy from multiple LED vendors, we have found the OSRAM PointLED [2] product line to offer the most suitable form-factor for building Maxim wrist PPG sensor wearable prototypes.
Photodiode
The photodiode is one of the most critical component selection choices in a wearable heart-rate monitor since it is the first stage in the receiving path of the system. There are many photodiode options available in the broad market, so it is important to choose one with high responsivity at key operating wavelengths or ranges thereof. Responsivity is a measure of the electrical output per optical input and is often expressed in current produced per watt of incident radiant power (A/W).
A high-responsivity device will be able to detect the small heart-rate signals returning from scattering inside the wrist tissue. Si PIN photodiodes have the largest responsivity in the visible/NIR wavelength range and are available from many manufacturers. Vishay and OSRAM Si PIN photodiodes are particularly useful for biosensing wrist wearables due to their small form factor [3,4].
Optomechanical Design Considerations
Overview
Designing a good optical PPG solution is very complex and is often underestimated. The previous section discussed key features and specifications to consider when choosing the optical components. Now let us focus on the integration of these components to make a complete heart-rate monitor for wrist wearables.
Successful integration will maximize both the signal received by the sensor and the signal-to-noise parameter. The latter can be increased by maximizing the signal that has penetrated deep enough into the skin to detect a PPG signal while minimizing crosstalk, which is the signal on the sensor from sources other than the PPG signal. A typical opto-mechanical integration design is shown in Figure 4. Here the LEDs and photodiode are encapsulated in a transparent material to provide a moisture barrier and interface between the optical components and the wrist. Barriers between the LEDs and photodiode provide optical isolation, ensuring only light that has traversed through skin tissue reaches and is detected by the photodiode. The entire assembly protrudes from the bottom of the wrist band to ensure tight contact with the skin. These features are discussed in detail below.
Encapsulation
As with any wrist wearable design, customers need some sort of sealant for the optical design. This sealant provides waterproofing properties and increases the signal received by the sensor. By choosing a sealant that has an index of refraction close to that of human skin (~1.4), transmission losses due to Fresnel reflections can be minimized. Additionally, a sealant that provides some “give” can increase contact area and pressure with the skin. Silicones are commonly used sealants. Table 2 provides good silicone candidates for an encapsulation material and their characteristics.
Crosstalk-Suppressing Features - Light Barriers
Crosstalk consists of signal incident on the photodiode that has not traversed through any skin layers. High levels of crosstalk will drown out the pulsating heart-rate signal, rendering the wearable monitor incapable of effectively measuring PPG. Thus, crosstalk between the LED emission and the photodetector should be minimized for optimal performance. To maintain a low level of crosstalk, physical absorbing light barriers can be used. An example barrier is shown in the detail view in Figure 4.
Mesa – Increasing Contact with Skin
A raised mesa is a commonly used technique to help mitigate motion artifacts by ensuring proper coupling between the device and the skin. Figure 4 shows the mesa concept which helps to ensure the proper skin contact required for heart-rate detection.
Spacing Between LEDs and Sensor
One of the major design considerations in building a reflectance heart-rate monitor is determining the optimum separation distance between the LEDs and the photodiode. This distance should be selected such that PPG signals with both maximum and minimum pulsatile components can be detected. These pulsatile components depend not only on the amount of arterial blood in the illuminated tissue, but also on the systolic blood pulse strength in the peripheral vascular bed.
There are two techniques that can enhance the quality of the plethysmogram signal. One technique is to use a large LED driving current, which increases the effective penetration depth of the incident light via the higher, light intensity. For a given LED - photodiode separation, using higher levels of incident light results in illuminating a larger pulsatile vascular bed. As a result, the reflected plethysmogram will contain a larger pulsate signal component. However, in practice the LED driving current is limited by the manufacturer to a specified maximum power dissipation. The alternate method is to place the photodiode close to the LEDs. However, if the photodiode is too close to the LEDs, the photodiode will be saturated by the large non-pulsatile component obtained by multiple scattering of incident photons by the blood-free stratum corneum and epidermal layers in the skin. For a constant LED intensity, the total light detected by the photodiode decreases roughly exponentially as the radial distance between the LEDs and the photodiode is increased. In other words, the effect of LED/photodiode separation on the reflected pulse amplitude of both green and yellow plethysmograms is decreased with the increase in separation. Thus, the selection of a separation distance has a trade-off. A plethysmogram with a larger pulsate signal component can be achieved by placing the PD farther apart from the LEDs, but a higher LED driving current will be needed to overcome absorption due to increased optical path length.
Simulated Comparison of LED – Photodiode Separation
To evaluate the effects of the LED - photodiode separation, let’s define two key figures of merit for PPG measurements: collection efficiency (CE) and prefusion index (PI). Collection efficiency is the fraction of power back on the photodiode for a given LED output. This optical signal incident on the photodiode is converted into current and consists of a large constant DC and a small variable AC component. The DC component contains no heart-rate information, while the AC corresponds to the pulsing arterial blood [5] (Figure 5). The perfusion index, defined as the ratio of AC to DC, is the ratio of the pulsatile blood flow to the non-pulsatile static blood flow in the peripheral tissue. The PI provides an indication of the pulse strength at the sensor site. The higher the PI, the better the performance. The perfusion index depends both on the path length through inferior blood net dermis, l, and change in absorption coefficient, 
, both of which are wavelength dependent:
The perfusion index varies depending on skin type, motion artifacts, ambient light, fitness levels, and fat content in the body. In wrist-based applications, PI values range from 0.02% for a very weak pulse to 2% for an extremely strong pulse.
Since a good PPG signal is a tradeoff between total power and PI, the figure of merit to examine when determining the optimal LED/photodiode spacing is the product of the collection efficiency and perfusion index (CE x PI). This quantity is proportional to the AC signal strength: a higher CE x PI value corresponds to a greater AC signal.
The effects of LED - photodiode separation on perfusion index were determined by ray trace simulation. The simulation used a Monte Carlo method to trace optical rays propagating in complex, inhomogeneous, randomly scattering, and absorbing media. The geometry simulated consisted of 1 mm x 1 mm active area detectors distanced 1-10 mm from an Lambertian emitting LED. The seven-layer model of the skin discussed in Section 3 was placed above the LED and detectors. Figure 6 shows the simulation setup.
The simulation determines the collection efficiency for a given wavelength and LED/photodiode spacing. Post-processing the simulation results yields the corresponding path length in the inferior blood net dermis layer. Knowing the path length and change in the absorption coefficient, , the PI can be calculated by Equation 2. The simulation results are given in Figures 7-9 for 530 nm, 560 nm, 574 nm, and 590 nm. From Figure 9 it is evident that up to 3 mm LED/photodiode separation, 574 nm will yield the highest PPG signal. Above 3 mm separation, the 590 nm PPG signal quality outperforms other wavelengths.
Maxim offers ICs that are suited for wearable, wrist-based heart-rate detection applications. The MAX86140 / MAX86141 devices are complete integrated optical data acquisition systems, ideal for optical pulse oximetry and heart-rate detection applications. Both include high-resolution optical readout, signal-processing channels with ambient-light cancellation, and high-current LED driver DACs to form a complete optical readout signal chain. The MAX86140 consists of a single optical readout channel, while the MAX86141 has two optical readout channels that can operate simultaneously. Both the MAX86140 and MAX86141 have three LED drivers and are well suited for a wide variety of optical-sensing applications.
While the MAX86140 / MAX86141 devices take care of the data acquisition, the customer must decide how to integrate the LEDs and photodetector into their industrial design. This article outlined how reflective heart-rate monitors operate, including the interaction with the skin and the selection of the light emitting and sensing elements. The article also provided practical recommendations on how to integrate the optical components to maximize signal quality.
Summary
Small, powerful analog front-end electronics are facilitating the ability to incorporate biosensing functionalities such as heart-rate monitoring in consumer wrist wearables. The performance of these wearable sensors depends greatly on careful optical component selection and opto-mechanical integration into the end customer’s design. Key parameters to consider when choosing the optical components are the wavelength and luminous efficacy for the LED, wavelength, and responsivity of the photodiode. To ensure the highest quality signal, encapsulation, crosstalk-suppressing barriers, and careful choice of LED – photodiode separation should be considered.
References
[1] T. Lister, P. A. Wright, and P.H. Chappell, “Optical Properties of Human Skin”, Journal of Biomedical Optics 17(9), September 2012.
[2] Osram, PointLED Datasheet Version 1.0, LT PWSG, March 18, 2016.
[3] Vishay Semiconductors, Silicon PIN Photodiode Datasheet, VEMD5010X01, March21, 2016.
[4] Osram, Silicon PIN Photodiode Datasheet Version 1.4, BP104S, March 31, 2016.
[5] J. G. Webster, "Design of Pulse Oximeters", Series in Medical Physics and Biomedical Engineering, Taylor & Francis, New York, USA, 1997.
About the author
Judy Hermann is a Principal Member of the Technical Staff, Advanced Sensors, Industrial & Healthcare Business Unit, Maxim Integrated.