Flexible and wearable thermoelectric devices represent a transformative advancement in personal energy harvesting, leveraging body heat to power small electronics. Unlike rigid thermoelectric modules based on inorganic materials like bismuth telluride, these devices prioritize conformability, stretchability, and seamless integration with textiles. Key materials include conductive polymers, elastomers, and composites, while fabrication techniques such as printing and roll-to-roll processing enable scalable production. The focus on mechanical robustness, skin compatibility, and practical power output makes them suitable for continuous wear in health monitoring, wearable electronics, and IoT applications.

Conductive polymers form the backbone of flexible thermoelectric devices due to their tunable electronic properties and inherent flexibility. Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is a standout material, offering reasonable thermoelectric performance with a power factor reaching 300 μW m⁻¹ K⁻² in optimized formulations. Blending PEDOT:PSS with elastomers like polyurethane or silicone enhances stretchability, allowing the material to withstand repeated bending and twisting without significant degradation. Other polymers, such as polyaniline and polypyrrole, are also explored, though their stability under mechanical strain remains a challenge. Composites incorporating carbon nanotubes or graphene flakes improve conductivity and mechanical resilience, enabling devices to maintain performance under dynamic stresses.

Fabrication methods for wearable thermoelectric devices prioritize low-cost, high-throughput processes compatible with flexible substrates. Screen printing and inkjet printing are widely adopted, allowing precise deposition of thermoelectric inks onto fabrics or polymer films. Roll-to-roll manufacturing further scales production, producing meters of thermoelectric material in a single run. These techniques enable the creation of patterned thermoelectric legs interconnected with conductive traces, forming flexible thermoelectric generators (TEGs). Laser cutting and transfer printing are also employed to assemble devices on stretchable substrates like polydimethylsiloxane (PDMS) or Ecoflex, ensuring adhesion and durability during wear.

Mechanical robustness is critical for wearable applications, where devices must endure bending, stretching, and compression. Elastic substrates and interconnects prevent delamination or cracking, with some designs achieving over 50% stretchability without significant performance loss. Fatigue testing reveals that devices can withstand thousands of bending cycles with minimal resistance change, provided the active materials are well-integrated with the substrate. Encapsulation layers, often made of thin polymers like parylene, protect the thermoelectric elements from moisture and mechanical abrasion while maintaining flexibility.

Skin compatibility is another essential consideration, as prolonged contact requires materials to be non-irritating and breathable. Biocompatible polymers such as PDMS and thermoplastic polyurethane (TPU) are commonly used, ensuring minimal allergic reactions. Device thickness is kept below 500 μm to avoid discomfort, and porous designs improve air permeability, reducing heat buildup on the skin. Adhesives, if used, are selected for gentle adhesion and easy removal, balancing stickiness with skin safety.

Power output from body heat harvesting is inherently limited due to the small temperature differential between skin and ambient air, typically 1-5 K. Flexible TEGs generate power densities in the range of 1-50 μW cm⁻² under realistic conditions, sufficient for low-power sensors or energy-autonomous wearables. Series-connected thermocouples enhance voltage output, while optimized thermal design minimizes heat loss. For instance, a wearable TEG with 200 thermocouples can produce 1-2 mW at a ΔT of 3 K, enough to power a heart rate monitor or environmental sensor. Energy storage integration, via micro-supercapacitors or thin-film batteries, ensures continuous operation despite fluctuations in body temperature or movement.

Integration with textiles is a key advantage of flexible thermoelectric devices. Thermoelectric fibers or patches can be woven directly into clothing, maintaining comfort and aesthetics. Embroidery techniques embed conductive threads into fabrics, creating seamless connections between thermoelectric elements. Smart garments with built-in TEGs demonstrate viability in real-world scenarios, such as powering LED indicators or transmitting sensor data wirelessly. Washability tests confirm that encapsulated devices retain functionality after multiple laundering cycles, a necessity for consumer adoption.

Applications span health monitoring, sports performance tracking, and wearable electronics. A thermoelectric wristband, for example, can harvest energy to power a temperature sensor, transmitting data to a smartphone without external batteries. Military and outdoor gear incorporates flexible TEGs to sustain GPS or communication devices in remote environments. In medical settings, wearable thermoelectric patches monitor chronic conditions while self-powered by body heat, eliminating the need for frequent battery replacements.

Challenges remain in improving efficiency, durability, and cost-effectiveness. Research focuses on polymer doping strategies to enhance the Seebeck coefficient and reduce thermal conductivity. Novel architectures, such as segmented or graded designs, aim to better exploit small temperature gradients. Advances in stretchable electronics and self-healing materials promise longer device lifetimes, even under harsh mechanical conditions.

Flexible and wearable thermoelectric devices bridge the gap between energy harvesting and human-centric technology. By prioritizing materials innovation, scalable fabrication, and user comfort, they unlock new possibilities for self-sustaining wearables. As the field progresses, these devices will play an increasingly vital role in the ecosystem of portable and autonomous electronics.