Recent advances in nanotechnology have enabled the development of self-powered biosensing systems that operate without external power sources. Among these, triboelectric nanogenerator (TENG)-based biosensors have emerged as a promising platform for continuous, real-time biomarker monitoring. These devices leverage the principles of contact electrification and electrostatic induction to convert mechanical energy into electrical signals while simultaneously detecting biochemical interactions. The integration of nanostructured polymers with biorecognition elements allows for highly sensitive and selective detection of biomarkers in biofluids such as sweat, offering a new paradigm in wearable health monitoring.

The fundamental working mechanism of TENG-based biosensors relies on contact electrification between dissimilar materials, typically nanostructured polymers such as polydimethylsiloxane (PDMS) and nylon. When these materials come into periodic contact and separation, electron transfer occurs at the interface due to differences in their triboelectric polarities. PDMS, with its strong electron-donating capacity, becomes positively charged upon contact with electron-accepting materials like nylon. The resulting charge separation generates a potential difference that drives current flow when connected to an external circuit. Nanostructuring these polymer surfaces through techniques like plasma etching or template replication significantly increases the contact area, enhancing both triboelectric charge generation and biosensing performance.

In biosensing applications, the presence of target biomarkers alters the charge transfer dynamics at the polymer interface. For cortisol monitoring in sweat, functionalization of the PDMS surface with cortisol-specific aptamers or antibodies creates selective binding sites. When cortisol molecules bind to these recognition elements, they modify the surface charge distribution and work function of the polymer. This biomarker-induced change directly affects the contact electrification process, leading to measurable variations in the output voltage or current of the TENG. Studies have demonstrated detection limits as low as 1 ng/mL for cortisol in artificial sweat, with response times under 5 minutes, making these sensors suitable for dynamic stress monitoring.

The self-powered nature of TENG biosensors addresses critical challenges in wearable health technology. Conventional electrochemical biosensors require bulky batteries or frequent recharging, limiting their practicality for continuous monitoring. In contrast, TENG devices harvest energy from body movements such as limb motion or respiration, generating sufficient power for both sensing and wireless data transmission. A typical TENG biosensor for cortisol monitoring can produce power densities ranging from 0.5 to 3 mW/cm2 under normal activity conditions, eliminating the need for external power sources while maintaining stable operation over extended periods.

Continuous cortisol monitoring presents unique opportunities for managing stress-related disorders, but also faces challenges from environmental interference. Humidity is a particularly significant factor, as water molecules can screen triboelectric charges and alter the dielectric properties of polymer surfaces. At relative humidity levels above 70%, the output voltage of unmodified PDMS-nylon TENGs can decrease by up to 40% due to charge dissipation through adsorbed water layers. This interference directly impacts biosensor accuracy, as humidity fluctuations in sweat may be misinterpreted as cortisol concentration changes.

Advanced encapsulation strategies have been developed to mitigate humidity effects while maintaining biomarker accessibility. Multilayer barrier coatings combining inorganic and organic materials provide effective solutions. A typical encapsulation scheme might include a 50 nm alumina layer deposited by atomic layer deposition, followed by a 1 μm parylene-C coating. This combination reduces humidity-induced signal drift by over 80% while allowing cortisol diffusion with minimal delay. Alternative approaches use hydrophobic porous membranes with pore sizes below 100 nm to block liquid water penetration while permitting vapor-phase biomarker transport.

Material engineering plays a crucial role in optimizing both triboelectric and biosensing performance. Nanocomposite polymers incorporating conductive fillers like carbon nanotubes or graphene flakes can enhance charge transfer efficiency while providing anchoring sites for biorecognition elements. For instance, PDMS doped with 0.5 wt% graphene exhibits a 150% increase in triboelectric output compared to pure PDMS, along with improved cortisol binding capacity due to increased surface roughness and chemical functionality. Similarly, nylon fibers electrospun with embedded gold nanoparticles show enhanced electron affinity and antibody immobilization efficiency.

The operational stability of TENG biosensors depends on careful consideration of mechanical and biochemical factors. Repeated contact-separation cycles can cause wear on nanostructured surfaces, gradually reducing charge generation efficiency. Cross-linked polymer networks and self-healing materials have shown promise in maintaining performance over more than 100,000 operation cycles. On the biochemical side, fouling from nonspecific protein adsorption in sweat can be minimized through surface passivation with polyethylene glycol or zwitterionic polymers, extending functional stability to over 72 hours of continuous use.

Future development directions focus on improving specificity, multiplexing capability, and integration with data processing systems. Advanced signal processing algorithms can distinguish biomarker-induced signals from motion artifacts and environmental noise, enhancing detection reliability. Multiplexed TENG arrays with different functionalized polymers enable simultaneous monitoring of cortisol alongside other biomarkers like glucose or lactate, providing comprehensive stress assessment. Integration with flexible electronics and low-power wireless modules allows for complete system autonomy in wearable formats.

The combination of energy harvesting and biosensing in a single TENG platform represents a significant advancement in wearable health technology. By eliminating power constraints while maintaining high sensitivity, these self-powered systems enable truly continuous biomarker monitoring. Cortisol detection in sweat serves as an important application case, but the underlying principles extend to numerous other biomarkers and biofluids. As material science and nanofabrication techniques continue to progress, TENG-based biosensors are poised to transform personalized health monitoring through their unique combination of autonomy, sensitivity, and wearability.

Environmental robustness remains an active area of investigation, with researchers developing new encapsulation materials and self-calibration algorithms to compensate for humidity and temperature variations. The ultimate goal is creating maintenance-free biosensing systems that operate reliably in real-world conditions while providing clinically relevant biomarker data. With continued refinement, these technologies may soon enable at-home monitoring of stress biomarkers with accuracy comparable to laboratory tests, opening new possibilities for preventive healthcare and personalized medicine.