In {HEAD ~ some cases}, chemical crosslinking step can include treatment of {TAIL ~ a polyelectrolyte multilayer} with a carbodiimide reagent 
In { HEAD ~ some cases } , chemical crosslinking step can include treatment of { TAIL ~ a polyelectrolyte multilayer } with a carbodiimide reagent
OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH