This study presents flexible and biocompatible polyurethane/Co@C composite films with weakly negative permittivity. Co nanoparticles (NPs) were encapsulated into carbon particles (Co@C) by carbonizing metal–organic frameworks (MOFs). The Co@C was then dispersed into a polyurethane (PU) matrix to create PU/Co@C composites. When the Co@C content reached 20 wt.%, a continuous network of Co@C was formed, leading to induced electric dipole resonance in carbon and plasma oscillation in Co NPs, resulting in weakly negative permittivity (<-3300). Alternating current conductivity percolation was observed when the Co@C content increased from 15 wt.% to 20 wt.%. The PU/Co@C metamaterials exhibited remarkable biocompatibility and flexibility, suggesting potential applications in underwater detection and wearable devices. Electromagnetic metamaterials have attracted attention due to their unique properties and applications in perfect absorption, antennas, and wireless power transmission. Traditional metamaterials adjust their electromagnetic parameters by tuning structural units. Epsilon-negative materials (ENMs) have developed rapidly due to their adjustable negative parameters. Metals are often used in ENM design to generate negative permittivity through plasma oscillation. However, high carrier concentrations in metals lead to high absolute values of negative permittivity, limiting practical applications. Carbon materials derived from MOF derivatives can form conductive networks and act as metal carriers, enabling low plasma frequencies with low filler content. In this work, Co NPs were incorporated into carbon particles derived from MOFs (Co@C) to serve as ENM constituents. PU was chosen as the matrix due to its flexibility and biocompatibility. At 20 wt.% Co@C content, the material exhibited weakly negative permittivity and AC conductivity percolation. The PU/Co@C metamaterials showed excellent biocompatibility and flexibility, indicating potential for diverse applications.This study presents flexible and biocompatible polyurethane/Co@C composite films with weakly negative permittivity. Co nanoparticles (NPs) were encapsulated into carbon particles (Co@C) by carbonizing metal–organic frameworks (MOFs). The Co@C was then dispersed into a polyurethane (PU) matrix to create PU/Co@C composites. When the Co@C content reached 20 wt.%, a continuous network of Co@C was formed, leading to induced electric dipole resonance in carbon and plasma oscillation in Co NPs, resulting in weakly negative permittivity (<-3300). Alternating current conductivity percolation was observed when the Co@C content increased from 15 wt.% to 20 wt.%. The PU/Co@C metamaterials exhibited remarkable biocompatibility and flexibility, suggesting potential applications in underwater detection and wearable devices. Electromagnetic metamaterials have attracted attention due to their unique properties and applications in perfect absorption, antennas, and wireless power transmission. Traditional metamaterials adjust their electromagnetic parameters by tuning structural units. Epsilon-negative materials (ENMs) have developed rapidly due to their adjustable negative parameters. Metals are often used in ENM design to generate negative permittivity through plasma oscillation. However, high carrier concentrations in metals lead to high absolute values of negative permittivity, limiting practical applications. Carbon materials derived from MOF derivatives can form conductive networks and act as metal carriers, enabling low plasma frequencies with low filler content. In this work, Co NPs were incorporated into carbon particles derived from MOFs (Co@C) to serve as ENM constituents. PU was chosen as the matrix due to its flexibility and biocompatibility. At 20 wt.% Co@C content, the material exhibited weakly negative permittivity and AC conductivity percolation. The PU/Co@C metamaterials showed excellent biocompatibility and flexibility, indicating potential for diverse applications.