Elastomeric composites and nanostructured materials have emerged as critical enablers for stretchable energy harvesters in wearable applications. These materials must simultaneously achieve mechanical compliance, electrical conductivity, and durability under repeated deformation, presenting unique challenges in material design and fabrication. The integration of conductive fillers into elastomeric matrices or the use of intrinsically stretchable nanostructures allows energy harvesters to maintain functionality while conforming to dynamic human motion.

A key consideration in stretchable energy harvesters is the selection of elastomeric matrices. Polydimethylsiloxane (PDMS) is widely used due to its high elasticity, biocompatibility, and chemical stability, with typical elongation at break exceeding 150%. Alternative elastomers like styrene-ethylene-butylene-styrene (SEBS) and polyurethane (PU) offer tunable stiffness and better adhesion to certain conductive materials. The choice of matrix affects not only stretchability but also interfacial bonding with conductive components, which is crucial for long-term performance.

Conductive fillers, such as carbon nanotubes (CNTs), graphene, silver nanowires, and conductive polymers, are incorporated to impart electrical conductivity. CNT-elastomer composites achieve conductivities up to 1000 S/m at loading fractions around 5-10 wt%, but higher filler concentrations can compromise stretchability. Percolation thresholds—the minimum filler concentration needed for continuous conductive pathways—vary with filler morphology. For instance, 1D nanowires form networks at lower loadings compared to 0D nanoparticles. Hybrid filler systems, combining CNTs with silver flakes, have demonstrated improved conductivity-stretchability trade-offs, reaching conductivities of 2000 S/m while maintaining 50% stretchability.

Nanostructured materials offer alternative approaches to stretchable conductors. Wavy or serpentine metal traces on pre-strained elastomers retain conductivity under stretching by unfolding rather than fracturing. Liquid metal embeddings, such as eutectic gallium-indium (EGaIn), provide self-healing capabilities and stable conductivity up to 500% strain. However, liquid metals face challenges in encapsulation and oxidation prevention. Conductive polymers like poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) modified with plasticizers achieve stretchability over 100% while retaining conductivities around 100 S/m.

Durability is a critical metric for wearable energy harvesters, which undergo cyclic mechanical stress. Fatigue resistance depends on filler dispersion, interfacial adhesion, and matrix resilience. CNT-elastomer composites subjected to 10,000 stretch cycles at 20% strain show less than 20% increase in resistance if well-dispersed. In contrast, poorly bonded fillers exhibit rapid resistance degradation due to microcrack formation. Self-healing elastomers, incorporating dynamic bonds or reversible polymers, recover conductivity after damage, extending operational lifetimes. For example, hydrogen-bonded polyurethane composites restore 90% of initial conductivity post-fracture after heating at 60°C for 10 minutes.

Energy harvesting mechanisms compatible with stretchable materials include piezoelectric, triboelectric, and thermoelectric systems. Piezoelectric elastomers, such as polyvinylidene fluoride (PVDF) blended with BaTiO3 nanoparticles, generate voltages up to 10 V under bending but require poling and suffer from low current outputs. Triboelectric nanogenerators (TENGs) leverage contact electrification between elastomers and electrodes, producing power densities of 0.5-3 W/m². Stretchable TENGs often use microstructured PDMS surfaces to enhance charge transfer, though long-term wear alters surface properties. Thermoelectric materials face intrinsic trade-offs between stretchability and the Seebeck coefficient; nanocomposites like Bi2Te3-PU achieve ZT values near 0.2 at 50% strain.

Integration strategies ensure that stretchable harvesters function reliably in wearable systems. Island-bridge designs localize rigid energy-harvesting components on stiff islands interconnected by stretchable conductors, minimizing strain on active materials. Textile integration embeds harvesters into fabrics via knitting or printing, but washability and abrasion resistance remain challenges. Encapsulation layers, often thin PDMS or polyimide films, protect against moisture and mechanical wear while maintaining flexibility.

Future advancements may focus on multi-functional composites that combine energy harvesting with sensing or energy storage. Machine learning-assisted material optimization could accelerate the discovery of compositions balancing conductivity, stretchability, and durability. Environmental considerations will drive the adoption of biodegradable elastomers and non-toxic fillers without compromising performance.

In summary, elastomeric composites and nanostructured materials enable stretchable energy harvesters by carefully balancing conductivity, mechanical compliance, and durability. Material selection, hybridization strategies, and robust integration methods are pivotal for realizing wearable systems that endure real-world use while efficiently converting ambient energy into usable power.