Energy harvesting from human motion presents a promising avenue for powering wearable electronics and medical devices without relying on traditional batteries. Recent advances in nanomaterial-based systems have enabled the efficient conversion of mechanical energy from activities like walking and breathing into usable electrical energy. These systems leverage flexible and stretchable nanomaterials, biocompatible designs, and seamless integration with textiles to create durable and efficient energy-harvesting solutions.

Flexible and stretchable nanomaterials are critical for capturing energy from human motion due to their ability to conform to dynamic biomechanical movements. Silver nanowire networks have emerged as a leading material due to their high conductivity, transparency, and mechanical resilience. When embedded in elastomeric substrates, these nanowires form percolation networks that maintain electrical connectivity even under repeated stretching. Recent studies demonstrate that Ag nanowire-polymer composites can sustain strains exceeding 50% while retaining over 90% of their initial conductivity after thousands of stretching cycles. Such durability is essential for wearable applications where mechanical fatigue is a major concern.

Piezoelectric polymers, such as polyvinylidene fluoride (PVDF) and its copolymers, are another key component in motion energy harvesting. These materials generate electrical charges in response to mechanical deformation, making them ideal for capturing energy from low-frequency human motions. Nanostructuring PVDF into electrospun fibers or porous films enhances its piezoelectric response by promoting the formation of the polar beta-phase. Recent developments include the incorporation of barium titanate nanoparticles or graphene oxide into PVDF matrices, which further improve charge generation and mechanical flexibility. These composites exhibit power densities in the range of 10 to 100 µW/cm² under biomechanical loading, sufficient for low-power wearable sensors.

Biocompatibility is a critical requirement for energy-harvesting systems intended for medical applications or long-term wear. Materials like polydimethylsiloxane (PDMS) and polyurethane are widely used as substrates due to their skin-friendly properties and flexibility. Advances in nanostructured elastomers have enabled the development of hybrid systems where piezoelectric or triboelectric nanomaterials are embedded in biocompatible matrices without compromising performance. For instance, zinc oxide nanowires grown on PDMS substrates have demonstrated efficient energy harvesting from subtle motions like breathing, with no adverse effects in cytotoxicity tests. Similarly, carbon nanotube-elastomer composites exhibit both high stretchability and biocompatibility, making them suitable for implantable or skin-adherent devices.

Integration with textiles is a major focus for wearable energy harvesters, as clothing provides a natural platform for unobtrusive energy capture. Conductive nanofibers and nanowires can be woven or printed directly onto fabrics, creating energy-harvesting textiles that remain flexible and breathable. Recent work has shown that triboelectric nanogenerators (TENGs) made from nylon and polytetrafluoroethylene (PTFE) nanofibers can be embedded into clothing to harvest energy from walking or arm movements. These textile-based systems generate power outputs in the range of 1 to 10 µW/cm², enough to power small electronic devices like fitness trackers or health monitors. Additionally, the use of nanostructured coatings ensures washability and long-term wear resistance.

Optimizing performance at low frequencies remains a significant challenge, as human motion typically occurs below 10 Hz. Nanomaterial-based systems address this through resonant design and nonlinear mechanical structures. For example, buckled or serpentine nanowire architectures enhance strain tolerance while maintaining sensitivity to low-frequency inputs. Recent advances include the use of kirigami-inspired designs, where precisely patterned cuts in nanomaterial films allow for large deformations without material failure. These designs enable energy harvesting from motions as subtle as pulse waves or diaphragmatic movement during breathing.

Fatigue resistance is another critical factor, as wearable and implantable devices must endure millions of motion cycles over their lifetimes. Nanostructured elastomers with self-healing properties have shown promise in addressing this challenge. Materials incorporating dynamic covalent bonds or hydrogen-bonding networks can autonomously repair microcracks that develop during cyclic loading. For instance, self-healing polyurethane composites with embedded silver nanowires recover over 80% of their conductivity after damage, ensuring long-term functionality in energy-harvesting applications.

Applications of these systems span wearable electronics, medical devices, and smart textiles. In wearables, energy-harvesting nanomaterials can power sensors for heart rate monitoring, activity tracking, or environmental sensing without requiring battery replacements. Medical applications include self-powered pacemakers or neural stimulators that draw energy from bodily movements. Recent prototypes have demonstrated the feasibility of nanogenerators that harvest sufficient energy from walking to power wireless transmission of physiological data.

The field continues to evolve with innovations in material synthesis, device architecture, and system integration. Future directions include the development of hybrid systems that combine multiple energy-harvesting mechanisms, such as piezoelectric, triboelectric, and thermoelectric effects, to maximize efficiency. Advances in scalable fabrication techniques, such as roll-to-roll printing or 3D printing of nanomaterials, will further accelerate the adoption of these technologies in consumer and medical markets. By addressing the dual challenges of mechanical durability and low-frequency performance, nanomaterial-based energy harvesters are poised to enable a new generation of self-sustaining wearable and implantable devices.