The adhesion energy per unit area (E Adh , mJ/m 2 ) measured between receptor and ligand monolayers is quantified from the pull-off force required to separate the adherent sample surfaces (F po ) 28 and use of the DerjaguinÀM€ ullerÀToporov model for adhesion between undeformed surfaces: E a = F po /2πR. Here, R is the geometric average radius of the two discs: R =(R 1 R 2 ) 1/2 . Further normalizing the adhesion energy per area by the protein surface coverage (number of proteins/area) accounts for differences in protein densities across experiments.