Protein adsorption

The adsorption of blood plasma proteins to biomaterial surfaces is usually a rapid process that within seconds generates a biologically active surface that may interact with other blood borne mechanisms [8, 9]. Both platelet adhesion and activation of the coagulation cascade are mediated by plasma proteins. Blood is a very complex fluid containing a large variety of molecules with different characteristics and functionalities. Proteins are one of the main constituents of blood plasma and approximately three hundred distinct proteins have so far been documented, with plasma concentrations ranging from 35-50 mg/mL for serum albumin to only 0-5 pg/mL for interleukin 6 [10]. The dynamics of protein adsorption are strictly related to the chemical and physical properties of the surface, protein and solvent, and the exothermic process is observed as a decrease in Gibbs free energy (G), i.e. ΔadsG=ΔadsH-TΔadsS <0, where H=enthalpy, S=entropy and T=temperature. This decrease generally arises from increased system entropy explained by dehydration of parts of protein and/or adsorbent surface, interactions between charged groups at the interface, and/or conformational changes in protein structure. The different modes of interaction result in selectivity between surface and proteins, i.e. proteins preferentially adsorb on surfaces exposing certain properties [11]. Proteins can be adsorbed to form a covering monolayer on the surface, resulting in a protein layer with a thickness of 2-10 nm [12]. Accumulation of proteins at the surface may yield a concentration of surface-associated proteins that is up to 1000- fold higher than the concentration of the protein in solution [13]. Adsorbed proteins are not always bound indefinitely to the surface, and the composition of proteins may be subject to change over time. This phenomenon is termed the Vroman effect and has been observed to occur preferentially on negatively charged hydrophilic surfaces [14]. Wettability (or hydrophilicity) has been characterized as a key determinant for the protein adsorption process, and it is generally accepted that hydrophobic surfaces adsorb more proteins than hydrophilic surfaces [13]. The hydrophobic surface allows interaction with hydrophobic domains and residues in the protein, the process assisted by an entropy gain during the subsequent release of unfavorably organized water at the surface [11]. Surface hydrophilicity can be determined by static and dynamic water contact angle measurements and a surface is deemed hydrophobic when generating a contact angle with water of more than 65° [15]. Protein and surface charge are also important factors in the adsorption process, both dependent on the pH of the solute. Maximal adsorption occurs when surface and proteins possess opposite net charges, i.e. at a pH between the isoelectric points (pI) of the surface and the protein [11]. Conformational changes in protein structure have also been suggested as a potential driving mechanism for protein absorption, especially under circumstances when hydrophobic interaction and electrostatic attraction is not present. However, adsorption induced conformational changes have been reported in a variety of proteins and may also affect the biological activity of the protein [13]. The effect of flow conditions on protein adsorption has been studied; however, increasing shear does not seem to affect the adsorption process, not even high shear rates of 2700 s-1 [16].