Carbon based functional materials enable multifunctional flexible strain sensors for wearable and implantable applications

Abstract Flexible strain sensors are essential components for wearable electronics, implantable biointerfaces, and soft human–machine systems. As application scenarios expand from epidermal monitoring toward long-term in vivo operation, increasingly stringent requirements are imposed on multifunctionality, mechanical compliance, signal stability, and biointegration. Carbon-based functional materials, owing to their tunable electrical properties, structural versatility, and favorable electromechanical compatibility, have emerged as a central materials platform for next-generation flexible strain sensors. This review presents a comprehensive, mechanism-oriented overview of carbon-enabled flexible strain sensing, encompassing piezoresistive, capacitive, and piezoelectric transduction modes. This review systematically examine how carbon material dimensionality including low-dimensional nanofillers, two-dimensional sheets, and three-dimensional porous, governing sensitivity, durability, and long-term device reliability. Particular emphasis is placed on contrasting the fundamentally different design requirements of wearable and implantable systems, including sensitivity-stability trade-offs, tissue-level mechanical matching, and operational robustness in complex biological environments. Distinct from prior material- or device-centric reviews, this review establishes a unified framework linking carbon architectures, sensing mechanisms, and application contexts, thereby clarifying critical bottlenecks and design principles for advancing multifunctional, biointegrated strain sensors toward practical and translational use.