The spring constant of the PEG micropillars was determined using an AFM cantilever as a reference . First , the cantilevers spring constant was determined using the integrated tool of a JPK BioAFM ( atomic force microscope , JPK Instruments AG ) . To calibrate the micropillars , the AFM cantilever was vertically fi xed in a custom - made micromanipulator such that its tip was situated approximately 1 μ m below the top of the PEG pillar . Both the cantilever and pillar were made to bend by moving highlighting the differences observed in Figure 2 ( a ) . The comparison in Figure 2 ( b ) illustrates that for forces smaller than 7 nN only a few more maximum force counts are observed on the α v β 3 -integrin antagonist than on the α 5 β 1 -integrin antagonist . In contrast , at forces above 7 nN the number of maximum force counts on the α 5 β 1 -integrin antagonist is up to four times greater than on the α v β 3 -integrin antagonist . In general , the data shows a trend towards higher maximum force counts at higher force levels for cells on the α 5 β 1 -integrin specifi c antagonist , suggesting that these cells must exert a greater pulling force on the pillars when adhering to them .