The landscape of wearable electronics is on the brink of transformation, owing to groundbreaking advancements in ultra-thin, flexible energy harvesting and storage systems. Researchers have made significant strides in developing solution-processed flexible photovoltaics (OPVs), hydrogel-electrolyte-based rechargeable batteries, and printed flexible electronic circuits. These innovations promise to bring the dream of commercially viable ultrathin wearable electronics closer to reality. However, achieving efficient and self-sustaining wearable devices poses several critical challenges that need to be addressed.
Envision a simplified model of a wearable device: it consists of an energy harvesting-storage system, sensors monitoring vital human metrics (like body temperature, heart rate, blood pressure, and respiratory patterns), and a low-power electronic circuit that manages power and communicates data to external devices such as smartphones or IoT cloud systems. For a wearable electronic system to be self-sustaining, an efficient, continuous power supply is essential. This power typically comes from an energy storage unit, bolstered by an energy harvesting unit—usually a photovoltaic module—that generates electricity to recharge the battery. Power consumption by the device’s electronic circuits and sensors can be minimized by periodically switching between sleep/standby and active modes, thereby extending battery life.
A pivotal issue is the energy harvesting efficiency of flexible OPV modules in wearable devices, especially under variable light conditions. The charging rate of these systems hinges on light intensity, which directly influences the power output of OPV modules and, by extension, the battery’s charging behavior.
To better understand this, we can examine the power generation capabilities of flexible OPVs. Research data shows that under standard 1 Sun (100 mW/cm²) illumination, the power density and output of OPV cells vary significantly with active area. For example, OPV modules with an active area of around 20 cm² can produce power outputs between 250-300 mW in bright sunlight and approximately 0.82-1 mW under typical indoor lighting (1000 lx, ~ 300 µW/cm²). However, light fluctuations can significantly affect the efficiency and charging rates, potentially causing thermal issues and accelerated battery degradation.
Simulation results emphasize the necessity of high-performance photoactive materials and ultra-low-power electronic circuits and sensors to design a self-sustaining wearable system. The balance between energy harvesting and storage is crucial to counteract power fluctuations and ensure stable, efficient energy storage.
The integration of all components—flexible energy harvesting and storage systems (FEHSS), electronic control units, and sensors into an ultrathin, flexible substrate—presents considerable engineering challenges. However, recent developments by Saifi et al. have shown immense potential in overcoming these obstacles. They created a fully integrated 90 µm flexible energy harvesting and storage system that includes ultrathin OPVs and zinc-ion batteries. Notably, their freestanding OPV modules, with a thickness of just 4 µm, display an impressive power density exceeding 10 mW cm⁻². A breakthrough in reducing hydrogel electrolyte thickness from millimeters to 10 µm in zinc-ion batteries has further enhanced this system’s flexibility without compromising electrochemical performance.
This ultrathin design improves mechanical compliance, allowing the FEHSS to be comfortably worn or integrated into textiles, a vital requirement for wearable applications. Moreover, the mechanical durability of this system is noteworthy. Tests have shown that it retains over 80% efficiency even after being bent to a radius of less than 1 mm for 500 cycles and compressed to a strain of 10% for 100 cycles, underscoring its robustness under physical stress.
In essence, these advancements in flexible energy solutions are not just technical feats but pivotal steps towards revolutionizing wearable technology. As researchers continue to refine these systems, the potential for creating more efficient, durable, and truly wearable devices becomes increasingly tangible.