The AFM colloidal probe technique (29) was used for adhesion force and adhesion energy measurements. This technique is based on quantification of the adhesion interactions between adhesive material (preabsorbed on a planar mica surface in the experiments) and a colloidal sphere (with well-defined size). It essentially measures the asymmetric adhesion of materials, which prebind firmly to mica and bind temporarily to the AFM tip surface during measurement. We used the continuous measurement mode in our experiments. The continuous measurement mode enabled us to assess the cyclic behaviors of contacts between materials and AFM tip surfaces.

AFM force measurements were carried out at room temperature in buffered solutions [20 mM tris-HCl and 50 mM NaCl (pH 5 for tyrosinase-modified protein or pH 7 for unmodified protein)] using an Asylum MFP-3D AFM (Asylum Research). Force measurements (force-displacement curves) were made at a rate of 1 Hz, using Si3N4 or Si cantilevers modified with a silica sphere coated with gold layers with a radius of 10 μm and calibrated spring constants of 8.9 N/m (NovaScan). Image processing and analysis of force curves were performed using IGOR Pro (WaveMetrics) data analysis software and Origin 8.0 software (OriginLab Corporation). We used continuous measurement mode for each type of adhesive material tested. Data from adhesion tests at five spots (each spot with a total of five continuous measurements) were used for statistical assessments in this study.

For each measurement, we also applied control tests by directly measuring adhesion on a clean mica surface under the same conditions. These procedures thus ensured that all the presented force curves were measured on real samples rather than contaminated surfaces. Typical force curves measured on a clean mica surface (as a negative control) are shown in fig. S7C. Cooler heater module and BioHeater module were applied to control solution temperature for adhesion performance assessment at varied temperature.

Last, measured adhesion forces, Fad, taken at the point of maximum force on the retraction curves, were related to the adhesion energy per area (Ead) according to the Johnson-Kendall-Roberts (29) theory for deformable surfaces.

For a similar sphere/sphere systemFad=1.5π REad

For a dissimilar sphere/flat system, which is the case in our studyFad=3π REadwhere R is the radius of contact.

The reported forces and adhesion energies in this paper are given as absolute values. Data are presented in Fig. 3 (B, C, F, and G) and fig. S7 (A and B). Note that, except those specifically noted adhesion measurements that were carried out at different temperatures, all the adhesion measurements by an AFM colloidal probe were carried out in aqueous solution at 25°C.

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