This study focuses on the development of flexible and biocompatible polyurethane/Co@C composite films with weakly negative permittivity. Co nanoparticles (NPs) encapsulated in carbon particles (Co@C) were synthesized by carbonizing metal-organic frameworks (MOFs). The negative permittivity with a small absolute value was achieved by dispersing Co@C into a polyurethane (PU) matrix. At a Co@C content of 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 a weakly negative permittivity (< -3300). Additionally, an alternating current (AC) conductivity percolation phenomenon was observed as the Co@C content increased from 15 wt.% to 20 wt.%. The PU/Co@C metamaterials exhibit excellent biocompatibility and flexibility, making them suitable for applications such as underwater detection and wearable devices.This study focuses on the development of flexible and biocompatible polyurethane/Co@C composite films with weakly negative permittivity. Co nanoparticles (NPs) encapsulated in carbon particles (Co@C) were synthesized by carbonizing metal-organic frameworks (MOFs). The negative permittivity with a small absolute value was achieved by dispersing Co@C into a polyurethane (PU) matrix. At a Co@C content of 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 a weakly negative permittivity (< -3300). Additionally, an alternating current (AC) conductivity percolation phenomenon was observed as the Co@C content increased from 15 wt.% to 20 wt.%. The PU/Co@C metamaterials exhibit excellent biocompatibility and flexibility, making them suitable for applications such as underwater detection and wearable devices.