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