Which material is considered non linear communication channel?
Linearity in the medium is usually limited by the variation of the dielectric constant Dk.  For cables with Dk reducing somewhat above 1Ghz and for optical fibre the same is true for single and multi mode glass fibres which causes diffraction due to the Dk value. So variations in Dk cause group delay skew variable and optical diffraction  variances.

I have tested my body up to 6GHz with an Anritsu VNA and and HP spectrum analyzers and  found no such cutaneous or subcutaneous external harmonic distortion or peculiar resonances other than natural inductive leads with variable capacitance.  However there must be some dielectric variation effects found in Nuclear Imaging but this is beyond my bandwidth to explain or professional experience although I have collected and created many such datasets in both CAT and MRI for my amusement in 3D analysis.  (most diagnostics are done in 2D)