A device is arranged to couple electromagnetic radiation into or out of a biological material. The device comprises a first metamaterial comprising: a substrate component 101 having a thickness no greater than a first wavelength of the electromagnetic radiation； and a plurality of elements 103 supported by the substrate Component. Each element has a first dimension no greater than a first wavelength of the electromagnetic radiation and at least two of the elements are non-identical. The metamaterial may be used to resonate at a specific required wavelength. The device may be used as a sensor with a transmitter and receiver to couple wireless radiation via antennae. The senor may be worn on the body as a non-invasive glucose monitoring system for diabetes or for attachment to foodstuff containers. Low frequency (10-300 GHz), long wavelength (around 5mm in air) radiation may be used to allow penetration of human tissue.
