Nonvolatile buffer coating of titanium to prevent its biological aging and for drug delivery

The osseointegration capability of titanium decreases over time. This phenomenon, defined as biological aging of titanium, is associated with the disappearance of hydrophilicity and the progressive accumulation of hydrocarbons on titanium surfaces. The objective of this study was to examine whether coating of titanium surfaces with 4-(2-Hydroxylethyl)-1-piperazineethanesulfonic acid (HEPES) buffer, a nonvolatile zwitterionic chemical buffering agent, could prevent the time-dependent degradation of the bioactivity of titanium. Commercially pure titanium samples, prepared as disks and cylinders, were acid-etched with H₂SO₄. A third of the samples were used for experiments immediately after processing (new surfaces), while another third were stored under dark ambient conditions for 3 months (3-month-old surfaces). The remaining third were coated with HEPES after acid-etching and were stored for 3 months (HEPES-coated 3-month-old surfaces). The 3-month-old surfaces were hydrophobic, while new and HEPES-coated 3-month-old surfaces were superhydrophilic. Protein adsorption and the number of osteoblasts attached during an initial culture period were substantially lower for 3-month-old surfaces than for new and HEPES-coated 3-month-old surfaces. Alkaline phosphatase activity and calcium deposition in osteoblast cultures were reduced by more than 50% on 3-month-old surfaces compared to new surfaces, whereas such degradation was not found on HEPES-coated 3-month-old surfaces. The strength of in vivo bone-implant integration for 3-month-old implants, evaluated by the push-in test, was 60% lower than that for new implants. The push-in value of HEPES-coated 3-month-old implants was equivalent to that of new implants. Coating titanium surfaces with HEPES containing an antioxidant amino acid derivative, N-acetyl cysteine (NAC), further enhanced osteoblast attachment to the surfaces, along with the increase level of intracellular glutathione reserves as a result of cellular uptake of NAC. These results suggest that HEPES coating of titanium surfaces maintained their superhydrophilicity for at least 3 months and resulted in a continuous retention of bioactivity and osteoconductivity similar to freshly prepared surfaces. This coating technology may be useful for preventing biological aging of titanium and delivering biological molecules for synergistic enhancement of bone-titanium integration.