*3.2. Materials for Flexible PPG Devices*

Both flexible LEDs and flexible diodes have been developed to reduce the effects of motion artifacts, which have been difficult to remove with filtering alone [53]. The most common photodiodes currently in use are silicon photodiodes, as they are widely available and flexible. Kim et al. used flexible (PIN) silicon diodes in combination with near-field communication (NFC) to deliver power, eliminating the need for a battery [54]. The photodiodes were paired with red and infrared LEDs, and the signals were amplified to coils and sent to a smartphone using the NFC platform. Li et al. offered an improvement on the conventional optoelectronic architecture by designing an epidermal silicon-based device by using a specific strain-isolation design, nanodiamond thinning, and hybrid transfer printing [55]. Through the thinning process, the thickness of the LEDs and PD was reduced to 20 µm. Mechanical deformation was addressed by adding a flexible island in a sandwich structure, with PI and PDMS helping the device to show stable operation, even under a strain of 35%. This device promised the possibility for functional optoelectronic devices to be directly mounted on the skin. Gallium arsenide (GaAs) is a frequently used III-V semiconductor material that can be used as an alternative to Si-based materials based on its excellent charge-carrier mobility and high stability. Hong et al. demonstrated a GaAs-based flexible photodetector array that was hetero-epitaxially grown on a Si wafer [56]. This innovative manufacturing method showed promising results that could lower the cost of inorganic photodiodes, which are normally expensive. This platform shows promising possibilities for large-scale creation of flexible photodiodes.

Organic materials for PPG sensors have become increasingly attractive due to their low fabrication cost and environmentally friendly footprint. For example, Yokota et al. developed a flexible pulse oximeter consisting of a polymer LED (PLED) and an organic photodiode (OPD) (Figure 3a,b) [57]. The device addressed a large barrier in organic optoelectronics, which is the ability to form a high-quality passivation layer on an ultraflexible substrate by making the passivation layers very thin using a low-temperature process. The PLED was constructed using light-emitting polymers and indium tin oxide electrodes. The OPD was constructed with a poly(3-hexylthiophene) (P3HT):(6,6)-phenyl-C61-butyric acid methyl ester (PCBM) active layer, which was manufactured on a 1 µm Parylene substrate, which was used as the passive layer. The lightweight device, which is only 3 µm thick, showed robustness, even under repeated 60% compression. Khan et al. showed a flexible oximeter array in which the active materials for the organic LED (OLED) and the OPD were fabricated on polymer substrates and placed in a grid consisting of photodiodes, red LEDs, and near-infrared LEDS [14]. The device offers a solution to the fundamental problem of only being able to measure PPG signals at a single location by using a reflectance-based array and is therefore capable of measuring blood oxygenation, even in the absence of pulsatile arterial blood signal. The device was able to measure SpO2 with a mean error of 1.1%.

Exciting new optoelectronic research on quantum and nano-based materials is emerging thanks to the ability to tune the performance due to the size of the particles. As shown in Figure 3c, Polat et al. introduced a photodiode made with graphene sensitized with semiconducting quantum dots [58]. Quantum-dot-based graphene photodetectors have high responsivity due to their built-in photoconductive gain. Therefore, the readout electronics can be placed far from the sensor, preserving the form factor of the active sensing area. In addition, the detector's transparency can be changed by changing the thickness of the quantum-dot layer, which alters the responsivity. This transparent device used ambient light for low power consumption and communicated wirelessly using near-field communication circuitry, as represented by a correlation coefficient of ρ = 0.98, with a state-of-the-art clinical PPG sensor. Kim et al. demonstrated a spirally wrapped CNT-based microelectrode, which is shown in Figure 3d [59]. A CNT-based solution was printed on an

agarose hydrogel substrate, where it was then spirally wrapped around a microfiber surface, such as nylon. The CNT electrodes demonstrated a current ratio of ~10 <sup>5</sup> and a maximum field-effect mobility of 0.68 cm<sup>2</sup> V −1 s −1 , which is comparable to similar flat devices.

**Figure 3.** Photoplethysmography. (**a**) Photo of an ultra-flexible organic optical sensor. (**b**) Illustration of polymer LED (**i**) and organic photodiode (**ii**) pulse oximeters. (**c**) Photo of a graphene-based flexible sensor in a heart-rate monitoring bracelet. (**d**) Photos of CNT-based microelectrodes for a fiber optoelectronic device.

ing area. In addition, the detector's transparency can be changed by changing the thickρ = 0.98 −1 −1 For arrhythmia detection, a high signal-to-noise ratio is essential. Thus, the specific design and its optimization are crucial. Pribadi et al. optimized a flexible OLED-OPD patch using an optical simulation [60]. The group optimized the AC/DC ratio of a square-type and cross-type patch. Their results showed that the square-type OPD was the best patch due to the wide area of the OPD, with an OLED drive current between 0.1 and 0.4 mA. The flexibility of the resulting design was 130 ◦ , and the heart-rate measurement accuracy was 95%. Khan et al. optimized the geometry of OLEDs and OPDs by designing three geometries: a rectangular geometry, a bracket geometry, and a circular geometry [61]. Both the bracket geometry and the circular geometry showed clear improvement over the rectangular design, where the bracket geometry showed a 39.7% improvement in the red PPG-signal magnitude and an 18.2% improvement in the NIR-channel magnitude, while the circular geometry showed a 48.6% improvement in the red-channel magnitude and a 9.2% improvement in the NI-channel magnitude. These results show promising form-factor and geometry optimization that could increase sensor accuracy and reduce power consumption in future wearable devices.
