{HEAD ~ the first element} and {TAIL ~ the second element} are substantially perpendicular to the substrate surface, wherein 
{ HEAD ~ the first element } and { TAIL ~ the second element } are substantially perpendicular to the substrate surface , wherein
HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH