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Commercially available wearable devices have been used for fitness and health management and their demand has increased over the last ten years. These “general wellness” and heart-rate monitoring devices have been cleared by the Food and Drug Administration for over-the-counter use, yet anecdotal and more systematic reports seem to indicate that their error is higher when used by individuals with elevated skin tone and high body mass index (BMI). In this work, we used Monte Carlo modeling of a photoplethysmography (PPG) signal to study the theoretical limits of three different wearable devices (Apple Watch series 5, Fitbit Versa 2 and Polar M600) when used by individuals with a BMI range of 20 to 45 and a Fitzpatrick skin scale 1 to 6. Our work shows that increased BMI and skin tone can induce a relative loss of signal of up to 61.2% in Fitbit versa 2, 32% in Apple S5 and 32.9% in Polar M600 when considering the closest source-detector pair configuration in these devices.

Obesity is a worldwide epidemic contributing directly to several cardiovascular risk factors including hypertension and type 2 diabetes. Wearable devices are becoming better at quantifying biomarkers relevant for the management of health and fitness. Unfortunately, both anecdotal evidence and recent studies indicate that some wearables have higher levels of error when utilized by populations with darker skin tones and high body mass index (BMI). There is an urgent need for a better evaluation of the limits of wearable health technologies when used by obese individuals.(1) To review the current know-how on changes due to obesity in the skin epidermis, dermis, and subcutis that could affect the skin optical properties; (2) for the green wavelength range, to evaluate the difference in absorption and scattering coefficients from the abdominal skin between individuals with and without elevated BMI. The changes include alterations in layer thickness and cell size, as well as significant differences in chromophores and scatterer content, e.g., water, hemoglobin, collagen, and lipids.We have summarized literature pertaining to changes in skin and its components in obesity and report the results of our search using articles published between years 1971 and 2020. A linear model was used to demonstrate the absorption and reduced scattering coefficient of the abdominal skin of individuals with and without elevated BMI in the green wavelength range (530 to 550 nm) that is typically found in most wearables.The general trends indicate a decrease in absorption for both dermis and subcutis and an increase in reduced scattering for both epidermis and dermis. At 544-nm wavelength, a typical wavelength used for photoplethysmography (PPG), the absorption coefficient's relative percentage difference between high and low BMI skin, was 49% in the subcutis, 19% in the dermis, and negligible in the epidermis, whereas the reduced scattering coefficient relative difference was 21%, 29%, and 165% respectively.These findings suggest that there could be significant errors in the output of optical devices used for monitoring health and fitness if changes due to obesity are not accounted for in their design.

Wearable devices, with Photoplethysmography (PPG)-based sensors, are helping patients to monitor chronic health conditions outside the clinic. The prime source of PPG signals is the blood volume change in the dermal vasculature. Here, we present a novel approach of using a skin model, containing double vascular layer within the dermis to investigate the pulsatile contribution from the region. Finite Element Method (FEM) is used to design vessels and PPG signals from the wrist are extracted by studying light transport through Monte Carlo simulations. By assessing PPG sensors in common wearables, the influence of obesity on the PPG signals are also investigated.

Obesity leads to a higher risk of diabetes and cardiovascular diseases. Wearable devices can be used to manage and promote the healthy lifestyle among the obese by measuring heart rate, heart rate variability, perfusion, and pressure pulse-wave velocities. While operational challenges are common in wearable devices using electrical or thermal sensors, those with optical sensors are more robust. Current optical sensors rely on fluctuations in light intensity due to spatio-temporal variations in tissue absorption. The thick layer of adipose tissue in high body mass index (BMI) individuals strongly scatters light, reducing the optical contrast and signal to noise ratio. Moreover, higher BMI alters chemical concentrations— like water, oxygenation, and blood volume in the dermal layer— and thus the optical properties (OP). Although OP of the skin exists in literature, no study has strictly recorded the effect and magnitude of a higher BMI on them. In this study, we combine the spatial frequency domain spectroscopy (SFDS) with a multi-sensor blood flow imaging device (PulseCam) to characterize the OPs and monitor the vascularization in the obese. The effects of skin morphology and physiology on the performance of optical sensor are preliminarily investigated.

Blood pressure is a primary candidate to be remotely characterized by continuous monitoring devices, as high blood pressure is frequently a symptom of chronic conditions such as diabetes, obesity, and cardiovascular disease while also leading to morbidity itself. Much work has been done to characterize an electrocardiogram (EKG)-photoplethysmogram (PPG) continuous monitor, however a dual PPG continuous monitor may yield more accurate results due to the lack of the inherent pre-ejection period. To predict the anticipated signal of a remote dual-photoplethysmogram (PPG) blood pressure monitor, we utilized parallelized multi-layer Monte Carlo modelling to estimate light transport through the index finger at the proximal phalange. Monte Carlo models of the digital artery and surrounding tissue were created that represent various skin tones as well as the presence of arterioles. Additionally, the signal contribution by arteriole and artery was also determined. By varying incident wavelength, source-detector separation, skin tone, and arterial diameter as a function of pulse propagation; the feasibility of a proximal phalange PPG can be estimated and further optimized to aid development of a dual-PPG blood pressure monitor. The modelling results indicate that at a separation of 3.0mm, 700nm wavelength provides signal resolution that is indicative of contribution from the artery and not the arterioles, which could be beneficial for estimating pulse transit time toward calculating blood pressure. Further work should be completed to optimize sourcedetector separation for arterial contribution to signal and explore various locations on the body, as much optical property variation exists within literature.

Systolic and diastolic blood pressure values can be used as an indicator of an individual's risk for cardiovascular disease. The common practice of blood pressure (BP) measurement using a cuff-based system provides a snapshot of blood pressure at a single instance in time and can be inconvenient and intrusive. The development of optical methods to determine blood pressure could provide continuous monitoring of blood pressure through techniques such as pulse transit time (PTT) or pulse arrival time (PAT) when used with echocardiogram. Cuff based BP devices are known to have variation and inaccuracies when applied to larger arm sizes as seen in individuals with obesity but little is known of the influence of obesity in the PPG/PTT and PAT signals. We propose that accurate waveform replication is required for the derivation of blood pressure applied to individuals with obesity. Here we use the Monte Carlo framework to develop the PPG waveform as a means to derive blood pressure through cuff less techniques. The development of a simulated waveform incorporates realistic changes in the artery related to its biomechanical properties as a pressure wave is propagated through the vessel. It is shown that a change in vessel pressure and geometry directly affects the captured optical signal. The system can account for variations in body-mass index to compensate for geometrical changes in adipose tissue layer and changes in optical properties.

Continuous monitoring of Blood Pressure (BP) is desirable in the management of cardiovascular disease (CVD). Novel tools are exploring Photoplethysmography (PPG) to determine a patient BP. We utilized a Monte Carlo (MC) framework to connect the cardiovascular biomechanics and light transport associated with a PPG signal. Optical properties specific of obese and non-obese groups were used to determine their influence on the PPG waveform. Our simulation shows that the PPG signal is altered in individuals suffering from obesity, which could limit their use in BP monitoring.

Patients with spinal cord injuries (SCI) are often subject to continues stationary pressure on prominent bony areas which, over and extended period of time, become high risk regions for pressure ulcers. Pressure ulcers are as prevalent as 30% in those with SCI and are one of the leading causes of re-hospitalization. In order to develop a better understanding of the methods of treating and preventing these pressure ulcers, we have developed a voxilization based model of the ischial tuberosity based on a Monte Carlo framework. The 3D model of the underside of the pelvis, the ischial tuberosity, allows for variation of the layers of the skin, fat, and muscle created in Solidworks. From the 3D model, the simulation was done with Monte Carlo eXtreme, a GPU-based Monte Carlo model. Tissue layer changes due to movement of the muscle and compression when sitting which results in a significant reduction of the muscle thickness have been accounted for in our model. Skin and muscle profusion of SCI patients is not well known and limited experimental work has been conducted in this environment. This model can aid in the study of methods of improving and maintaining skin health and help with rehabilitation and prevention of pressure ulcers.