Robust sensing is one of the main challenges for wearable physiological monitoring because of the high dependency on the placement of electrodes on the body, retaining suitable contact between electrodes and skin, and the effect of motion artifacts. In this paper, we present a wrist-worn strap that includes a 2-D array of 48 miniature electrodes covering the bottom side of the wrist with good contact with the skin. Good skin contact directly impacts the sensing robustness. The array provides local measurements between adjacent electrodes that span the whole bottom side of the wrist with an area of 6.25×4.60 cm for robust sensing. The array allows for the automatic selection of the correct electrodes at the right location regardless of changes in the device placement on the wrist. In addition, using a large number of electrodes over a large area on the wrist ensures continuous contact of some electrodes with the skin during motion since all of the electrodes will not lose contact with the skin at the same time. We measured the electrode-skin impedance of the fabricated electrodes versus frequency and compared to other types of electrodes. We demonstrated good contact between all electrodes of the array and the skin by measuring electrode-skin impedance less than 10 kΩ at 16 kHz for all locations on the wrist strap. We also conducted measurements of impedance while the wearer was bending the wrist to validate the continuous contact of at least a subset of electrodes with the skin during such movements.

Finger-based photoplethysmography (PPG) is traditionally acquired using a finger clip form factor on the fingertip. This form factor is not conducive for long-term measurements during regular everyday activities due to its inability to be easily worn while moving outside a clinical setting. However, advancements in finger-based PPG sensors over the last several decades include ring designs at the proximal phalanx of the finger, which are more conducive to being worn during everyday living conditions. The anatomy of this location is more complex relative to the tip as there is a smaller density of blood vessels. Also, the finger's proximal phalanx is comprised of more opaque tissue and bone, which affects the path of the light. In this study, a ring design that included 8 LEDs/PDs each at 45 degrees that included two wavelengths (940 nm: infrared and 655 nm: red) on the proximal phalanx of the finger was studied in terms of their relative placement to each other for detection. Specifically, the system was used to investigate how the pulsatile and DC component amplitudes of the photodiodes varied around the finger in both transmission and reflectance modes as a function of the 8 LEDs/PDs pairs at 45 degrees on the ring. All 64 combinations of LEDs/PDs located at the proximal phalanx of the finger were investigated. The results showed the locations that yielded the strongest amplitude PPG signals.

Photoplethysmography (PPG) sensors, commonly used today in wearable biomedical devices, are an easily deployable low-cost technology to estimate heart rate, heart rate variability, and, when coupled with multiple wavelengths, blood oxygenation. PPG sensors can measure these parameters by measuring volumetric changes in the blood due to light absorption. While the technology is established, its utility is hindered due to several factors, including skin contact pressure. Thus, to advance fitness trackers toward regulated medical devices, confounding factors related to contact pressure and its effect on the PPG wave morphology, intensity, and signal-to-noise ratio (SNR) need to be better understood. Toward that end, this study evaluates the effects of contact pressure between the PPG sensor and the wrist, particularly on the palmar side of the wrist along the radial artery. The amplitude of the AC and DC components of the PPG signal, the perfusion index, and the signal morphology were determined for varying contact pressures utilizing three wavelengths of light, green, red, and infrared (537, 660, and 880 nm, respectively). As pressure was applied and varied, the compression effects on the pulsatile and non-pulsatile components were observed, along with changes in the perfusion index and prominent features of the PPG signal. The results showed significant differences in morphology and intensity, which varied with each PPG wavelength. These varying effects of PPG signal morphology, intensity, and SNR as a function of contact pressure suggest that pressure must be considered with wearable sensing technology and when developing signal analysis methods in order to allow the advancement of fitness trackers toward regulated medical devices.

Photoplethysmography (PPG) is a non-invasive optical measurement that detects blood volume changes within the tissue from the skin's surface and is traditionally integrated into wearable technology to measure heart rate from the dorsal side of the wrist. The dorsal side of the wrist contains no major arteries, whereas the palmar side has two; the radial and ulnar arteries. To better understand the morphology of a PPG signal acquired along a major artery, this study investigated the change in PPG signals acquired from the distal to proximal locations of these two major arteries within the forearm. A multiwavelength (670nm, 770nm, 810nm, 850nm, 950nm) reflectance-based system was used to analyze the change in PPG morphology with the change in measurement location. Specifically, the sensor was placed into several zones down both arteries in the forearm to evaluate PPG waveform features. As the sensor traversed the arteries, changes in the morphology in each forearm zone were observed. Additionally, by using multiple wavelengths distinct waveforms were acquired from each penetration depth, as both arteries descend deeper away from the surface as you travel distal to proximal.