Flexible thermoelectric generators and coolers represent a significant advancement in wearable energy harvesting and personal thermal management. Unlike rigid thermoelectric systems, which rely on brittle inorganic materials like bismuth telluride, flexible versions leverage polymer-based and hybrid materials to maintain functionality under mechanical deformation. These devices convert temperature gradients into electrical energy or vice versa, enabling applications in self-powered wearables, localized cooling, and medical monitoring.

Polymer-based thermoelectric materials are central to flexibility. Conducting polymers such as poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and polyaniline (PANI) exhibit reasonable thermoelectric properties while remaining mechanically compliant. PEDOT:PSS, for example, achieves power factors up to 300 μW m⁻¹ K⁻² when optimized with dopants or secondary fillers. Its flexibility stems from the entangled polymer chains that accommodate strain without fracturing. However, polymers generally suffer from low Seebeck coefficients and high thermal conductivity, limiting their thermoelectric efficiency. To address this, researchers incorporate nanostructured additives like carbon nanotubes or graphene, which enhance charge transport while preserving flexibility.

Hybrid materials combine polymers with inorganic thermoelectric nanoparticles, offering a balance between performance and mechanical resilience. For instance, composites of PEDOT:PSS with tellurium nanowires or bismuth antimony telluride particles demonstrate improved power factors exceeding 500 μW m⁻¹ K⁻². The inorganic phase boosts the Seebeck coefficient, while the polymer matrix ensures bendability. These hybrids can withstand bending radii as small as 5 mm without significant performance degradation, making them suitable for integration into textiles or skin-mounted devices.

Performance metrics under bending are critical for wearable applications. Key parameters include the power factor retention, electrical conductivity stability, and fatigue resistance after repeated flexing. Studies show that well-designed polymer composites retain over 80% of their initial power factor after 1,000 bending cycles at a 10 mm radius. The degradation mechanisms often involve microcrack formation in the conductive filler network or delamination at the polymer-particle interface. Strategies like elastomer blending or fiber reinforcement mitigate these issues by distributing strain more evenly.

Thermal conductivity is another consideration. Flexible thermoelectric materials must maintain low thermal conductivity to preserve the temperature gradient driving energy conversion. Polymers inherently exhibit low thermal conductivity, often below 0.5 W m⁻¹ K⁻¹, but adding high-thermal-conductivity fillers can inadvertently increase it. Engineering porous structures or using hollow nanoparticles helps counteract this effect, keeping the thermal conductivity below 1 W m⁻¹ K⁻¹ while maintaining mechanical flexibility.

Wearable energy harvesting relies on exploiting small temperature differences between the body and ambient environment. Typical human skin temperatures range from 30°C to 35°C, while ambient temperatures vary between 20°C and 25°C, creating gradients of 5–10 K. Flexible thermoelectric generators can convert these gradients into usable power, with reported outputs of 1–10 μW cm⁻² under realistic conditions. While this is insufficient for high-power devices, it is adequate for low-power sensors or intermittent data transmission in health monitors.

Device architecture plays a crucial role in wearables. Thin-film designs dominate due to their lightweight and conformability. Common configurations include segmented or sandwich structures, where alternating p-type and n-type legs are connected electrically in series and thermally in parallel. Printing techniques like screen printing or inkjet deposition enable scalable fabrication of these patterns on flexible substrates such as polyimide or polyethylene terephthalate. Recent advances include stretchable interconnects and serpentine layouts that enhance durability under strain.

Flexible thermoelectric coolers operate on the Peltier effect, offering localized cooling for wearable applications. Unlike generators, coolers require high electrical conductivity and low thermal resistance to maximize heat pumping efficiency. Polymer-inorganic hybrids with optimized filler alignment can achieve cooling efficiencies of 20–30% of conventional rigid devices while maintaining flexibility. Cooling capacities range from 0.5 to 5 W cm⁻², suitable for targeted thermal relief in medical patches or smart clothing.

Challenges remain in scaling up production and improving long-term stability. Environmental factors like humidity and repeated washing can degrade polymer-based devices, necessitating encapsulation strategies. Additionally, achieving uniform material properties over large areas is difficult with current fabrication methods. Advances in roll-to-roll processing and self-healing materials may address these limitations in the future.

The integration of flexible thermoelectrics with other wearable technologies is an emerging trend. Combining them with energy storage devices like supercapacitors or batteries enables continuous operation, while pairing them with sensors creates self-powered monitoring systems. For example, a thermoelectric-powered pulse oximeter could operate indefinitely without external charging, leveraging body heat as an energy source.

In summary, flexible thermoelectric generators and coolers based on polymer and hybrid materials offer unique advantages for wearable applications. Their ability to maintain performance under mechanical deformation, coupled with advances in material design and fabrication, positions them as key components in the future of personal electronics and health monitoring. Continued research into material optimization and device integration will further enhance their practicality and adoption.