Also disclosed are methods of making the cross linkers, as well as methods of using the cross linkers, e.g., to deliver {HEAD ~ a biologically active substance} across {TAIL ~ the membranes} of selected cells in a heterogeneous cell population; once inside the cell the active substance is released, intact, by the transient, mild acidity of certain cell structures 
Also disclosed are methods of making the cross linkers , as well as methods of using the cross linkers , e.g. , to deliver { HEAD ~ a biologically active substance } across { TAIL ~ the membranes } of selected cells in a heterogeneous cell population ; once inside the cell the active substance is released , intact , by the transient , mild acidity of certain cell structures
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