The barrier may be {HEAD ~ either a fluid permeable matrix} or {TAIL ~ a fluid gap} with a thin, fluid permeable membrane for blocking convection currents but otherwise admitting fluid and analyte 
The barrier may be { HEAD ~ either a fluid permeable matrix } or { TAIL ~ a fluid gap } with a thin , fluid permeable membrane for blocking convection currents but otherwise admitting fluid and analyte
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH