Flexible and biocompatible poly (vinyl alcohol)/multi-walled carbon nanotubes hydrogels with epsilon-near-zero properties

Abstract

• Flexible PVA/MWCNTs hydrogels were fabricated by solution method, and cytotoxicity experiments proved that the process was environmentally friendly and hydrogels were biocompatible. • Epsilon-near-zero properties were realized at radio frequency from PVA/MWCNTs hydrogels, originated from the resonance of the induced electric dipole. • After the frequency achieving epsilon-near-zero, negative permittivity which was different from Drude model but related to interband transition, was observed. Epsilon-near-zero (ENZ) material has been a research hotspot in recent years due to unique physical properties such as inverse Doppler effect and negative refractive index, showing great potentials in the fields of flexible electronics, wearable devices, sensors, etc. The ENZ materials are mostly reported at visible, infrared and terahertz wavelengths, while the report about ENZ materials at radio frequency is rare. In this work, flexible and biocompatible poly (vinyl alcohol)/multi-walled carbon nanotubes (PVA/MWCNTs) hydrogels, which were successfully fabricated by an environmentally friendly method, were used as ENZ materials at radio frequency for the first time. Cytotoxicity experiments proved that the experimental process and products are green, environmentally friendly and biocompatible. The microstructure, crystalline structure, chemical composition and dielectric properties were investigated. Two different water states, which were free water molecules and bound water molecules, coexisted in these PVA/MWCNTs hydrogels and accounted for about 37.5 wt% and 15.9 wt% respectively by analyzing the thermogravimetric analysis curves. When the MWCNTs content reached 12 wt% and 15 wt%, the continuous conductive MWCNTs network was formed in the hydrogel, and ENZ phenomenon was observed at about 760 kHz and 580 kHz respectively, which was attributed to the interband transition. Considering the flexibility and non-toxicity of PVA/MWCNTs hydrogels, the ENZ properties of this structure at the radio frequency can be well used in wearable invisibility cloak, flexible electronics, skin sensors and other wearable devices.