Flexible and wearable energy storage devices: Nanomaterials, device architectures, and bio-integrated applications

The rapid evolution of wearable and bio-integrated electronics has intensified the demand for high-performance, deformable energy storage systems that can seamlessly conform to the human body while maintaining electrochemical efficiency and mechanical durability. This review critically synthesizes recent advancements in flexible energy storage devices (FESDs), emphasizing cutting-edge developments from 2022 to 2025. It begins by exploring material innovations, including carbon-based nanomaterials like graphene, carbon nanotubes, and MXenes; metal nanowires and oxides; and hybrid composites, detailing their contributions to conductivity, flexibility, and energy storage performance. The discussion progresses to novel device architectures, such as planar, fiber-shaped, and origami-inspired geometries for both supercapacitors and flexible batteries, with special attention to electrode design, substrate selection, and encapsulation techniques that ensure resilience under bending, twisting, and stretching. Integration into real-world applications is analyzed across textile-based platforms, skin-mounted and implantable systems, and self-powered hybrid configurations that combine triboelectric, piezoelectric, or photovoltaic modules for autonomous operation. Experimental validations through real-time use cases in health monitoring, athletic performance, and military wearables underscore the feasibility of these technologies. This review also rigorously evaluates the core challenges impeding widespread adoption, including the trade-off between energy density and flexibility, cycling stability under mechanical stress, safety concerns, toxicity of active materials, and barriers in large-scale manufacturing and cost. Looking ahead, it identifies key research trajectories such as biodegradable electronics, AI-enabled energy systems, and edge-computing integration, and calls for intensified interdisciplinary collaborations spanning materials science, bioengineering, and human–machine interfacing. By articulating both the technological progress and strategic research pathways, this article presents a forward-thinking vision to guide academia and industry toward a new era of smart, energy-autonomous wearable systems.