Electrostatic forces at the cell interface affect the nature of cell adhesion and function; but there is still limited knowledge about the impact of positive or negative surface charges on cell-material interactions in regenerative medicine. Titanium surfaces with a variety of zeta potentials between −90 mV and +50 mV were generated by functionalizing them with amino polymers, extracellular matrix proteins/peptide motifs and polyelectrolyte multilayers. A significant enhancement of intracellular calcium mobilization was achieved on surfaces with a moderately positive (+1 to +10 mV) compared with a negative zeta potential (−90 to −3 mV). Dramatic losses of cell activity (membrane integrity, viability, proliferation, calcium mobilization) were observed on surfaces with a highly positive zeta potential (+50 mV). This systematic study indicates that cells do not prefer positive charges in general, merely moderately positive ones. The cell behavior of MG-63s could be correlated with the materials' zeta potential; but not with water contact angle or surface free energy. Our findings present new insights and provide an essential knowledge for future applications in dental and orthopedic surgery.

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Biomaterials, once inserted in the oral cavity, become immediately covered by a layer of adsorbed proteins that consists mostly of salivary proteins but also of plasma proteins if the biomaterial is placed close to the gingival margin or if it becomes implanted into tissue and bone. It is often this protein layer, rather than the pristine biomaterial surface, that is subsequently encountered by colonizing bacteria or attaching tissue cells. Thus, to study this important initial protein adsorption from human saliva and serum and how it might be influenced through chemical modification of the biomaterial surface, we have measured the amount of protein adsorbed and analyzed the composition of the adsorbed protein layer using gel electrophoresis and western blotting. Here, we have developed an in vitro model system based on silica surfaces, chemically modified with 7 silane-based self-assembled monolayers that span a broad range of physicochemical properties, from hydrophilic to hydrophobic surfaces (water contact angles from 15° to 115°), low to high surface free energy (12 to 57 mN/m), and negative to positive surface charge (zeta potentials from –120 to +40 mV at physiologic pH). We found that the chemical surface functionalities exerted a substantial effect on the total amounts of proteins adsorbed; however, no linear correlation of the adsorbed amounts with the physicochemical surface parameters was observed. Only the adsorption behavior of a few singular protein components, from which physicochemical data are available, seems to follow physicochemical expectations. Examples are albumin in serum and lysozyme in saliva; in both, adsorption was favored on countercharged surfaces. We conclude from these findings that in complex biofluids such as saliva and serum, adsorption behavior is dominated by the overall protein-binding capacity of the surface rather than by specific physicochemical interactions of single protein entities with the surface.

For many biomedical applications, material surfaces should not only prevent unspecific protein adsorption and bacterial attachment as in many other applications in the food, health, or marine industry, but they should also promote the adhesion of tissue cells. In order to take a first step toward the challenging development of protein and bacteria-repelling and cell-adhesion-promoting materials, polyamine and poly­(amido amine) surface coatings with terminal amine groups and varying structure (dendrimer, oligomer, polymer) were immobilized on model surfaces via silane chemistry. Physicochemical analysis showed that all modifications are hydrophilic (contact angles <60°) and possess similar surface free energies (SFEs, ∼46–54 mN/m), whereas their amine group densities and zeta potentials at physiological conditions (pH 7.4) varied greatly (−50 to +75 mV). In protein adsorption experiments with single proteins (human serum albumin (HSA) and lysozyme) as well as complex physiological fluids (fetal bovine serum (FBS) and human saliva), the amounts of adsorbed protein were found to correlate strongly with the zeta potential of the surface coatings. Both modifications based on linear polymers exhibited good protein repellency toward all proteins examined and are thus promising for testing in cell adhesion studies.