Satoru Kidoaki

To determine the adhesion force of representative blood plasma proteins (albumin (Alb), immunoglobulin G (IgG), and fibrinogen (Fib)) to foreign material surfaces, force-versus-distance curves were measured using an atomic force microscope (AFM) between the protein covalently immobilized AFM tips and the well-defined model surfaces of self-assembled monolayers (SAMs) of alkanethiolates terminated with different functional groups (CH3, NH2, OH, and COOH). From the f−d curves measured between the protein-immobilized tip and the SAM surface, the mechanical adhesion forces of the protein to the SAM surface (detachment force of protein from the surface) were determined. From the adhesion forces determined between the protein tip and the SAM surface and between like SAM-coated tips and surfaces (i.e., tip/surface combinations: CH3/CH3, NH2/NH2, OH/OH, and COOH/COOH), the thermodynamic adhesion strength (work of adhesion and surface pressure of SAM on a protein surface) was also determined according to the Dupré equation and Johnson, Kendall, and Roberts adhesion theory. The relative strength of thermodynamic adhesion of the proteins to the SAM surfaces was found with statistical significance to be in the following orders: (a) For Alb and IgG, CH3- ≫ (OH-, NH2-) > COOH-SAM surface; for Fib, CH3 ≫ OH > NH2 > COOH. (b) On CH3-, NH2-, and OH-SAM surfaces, Fib exhibits higher adhesion than Alb and IgG.

To directly characterize the thermoresponsive structural changes of a poly(N-isopropylacrylamide) (PNIPAAm) graft layer at the microscopic level, the force−distance curve (f−d curve) was measured on a well-tailored end-grafted PNIPAAm surface in aqueous solution at 25 and 40 °C, using an atomic force microscope (AFM). The PNIPAAm surface was prepared by an iniferter-based photograft polymerization technique. The approach trace of the f−d curve exhibited a steric repulsion profile at 25 °C, while the range of repulsion decreased 1/10 to 1/20 at 40 °C, confirming the ascending-heat-induced collapse of the PNIPAAm graft layer. The change in thickness of the graft layer was complementarily measured from the scanning images of the boundary between the grafted and nongrafted regions under well-defined scanning forces. The thermoresponsive characteristics of the PNIPAAm graft layer including its interaction with proteins and the applied-load dependence of the measured graft thickness are discussed.