<p>Health monitoring is becoming increasingly critical for disease prevention, early diagnosis, and high-quality living. Polymeric materials, with their mechanical flexibility, biocompatibility, and tunable biochemical properties, offer unique advantages for creating next-generation personalized devices. In recent years, flexible polymer-based platforms have shown remarkable potential to capture diverse physiological signals in both daily and clinical contexts, including electrophysiological, biochemical, mechanical, and thermal indicators. In this review, we introduce a safety-level-oriented framework to evaluate material and device strategies for health monitoring, spanning the continuum from noninvasive wearables to deeply embedded implants. Physiological signals are systematically classified by use case, and application-specific requirements such as stability, comfort, and long-term compatibility are highlighted as critical factors guiding the selection of polymers, interfacial designs, and device architectures. Special emphasis is placed on mapping material types—including hydrogels, elastomers, and conductive composites—to their most suitable applications. Finally, we propose design principles for developing safe, functional, and adaptive polymer-based systems, aiming at reliable integration with the human body and enabling personalized, preventive healthcare.</p><p></p>