Studies of the activation of FXII in both platelet poor plasma and in neat buffer solutions were undertaken for a series of mixed thiol self-assembled monolayers spanning a broad range of water wettability. A wide spectrum of carboxyl/methyl-, hydroxyl/methyl-, and amine/methyl-thiol modified surfaces were prepared, characterized, and then utilized as the procoagulant materials in a series of FXII activation studies. X-ray photoelectron spectroscopy was utilized to verify the sample surface's thiol composition and contact angles measured to determine the sample surface's wettability. These samples were then used in in vitro coagulation assays using a 50% mixture of recalcified plasma in phosphate buffered saline. Alternatively, the samples were placed into purified FXII solutions for 30 min to assess FXII activation in neat buffer solution. Plasma coagulation studies supported a strong role for anionic surfaces in contact activation, in line with the traditional models of coagulation, while the activation results in neat buffer solution demonstrated that FXIIa production is related to surface wettability with minimum levels of enzyme activation observed at midrange wettabilities, and no statistically distinguishable differences in FXII activation seen between highly wettable and highly nonwettable surfaces. Results demonstrated that the composition of the solution and the surface properties of the material all contribute to the observation of contact activation, and the activation of FXII is not specific to anionic surfaces as has been long believed.
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