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Effect of Ca²⁺ Ions on the Adhesion and Mechanical Properties of Adsorbed Layers of Human Osteopontin

Bruno Zappone ^*, Philipp J. Thurner , Jonathan Adams , Georg E. Fantner  and Paul K. Hansma 

^* Liquid Crystal Laboratory, Regional Laboratory and Center of Excellence for Functional Nanostructured Materials, Centro Nazionale delle Ricerche and Istituto Nazionale per la Fisica della Materia, Arcavacata di Rende (CS) 87036, Italy; Bioengineering Science Research Group, University of Southampton, Southampton, United Kingdom; Department of Physics, University of California, Santa Barbara, California; and Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts

Correspondence: Address reprint requests to Bruno Zappone, E-mail: zappone{at}fis.unical.it.

Using an atomic force microscope and a surface force apparatus, we measured the surface coverage, adhesion, and mechanical properties of layers of osteopontin (OPN), a phosphoprotein of the human bones, adsorbed on mica. OPN is believed to connect mineralized collagen fibrils of the bone in a matrix that dissipates energy, reducing the risk of fractures. Atomic force microscopy normal force measurements showed large adhesion and energy dissipation upon retraction of the tip, which were due to the breaking of the many OPN-OPN and OPN-mica bonds formed during tip-sample contact. The dissipated energy increased in the presence of Ca²⁺ ions due to the formation of additional OPN-OPN and OPN-mica salt bridges between negative charges. The forces measured by surface force apparatus between two macroscopic mica surfaces were mainly repulsive and became hysteretic only in the presence of Ca²⁺: adsorbed layers underwent an irreversible compaction during compression due to the formation of long-lived calcium salt bridges. This provides an energy storage mechanism, which is complementary to energy dissipation and may be equally relevant to bone recovery after yield. The prevalence of one mechanism or the other appears to depend on the confinement geometry, adsorption protocol, and loading-unloading rates.