What factors determine thrombogenicity.

Q UANTITATIVE analysis forces us to note only what can be expressed in numbers, and to compare events on this very narrow basis. Arranging a pinhole, a lightmeter, and a recorder properly, we can express our entire world as a single value, plotted to give us little more than the approximate time of day. We are committing the same injustice to reality if we describe someone only in pounds, or his endothelium only in terms of surface charge, critical surface tension, or ability to disturb flow. Efforts to express thrombogenicity by net electric charge on the basis of in vivo experimental2 should take into account that even small applied voltages may damage the endothelium, causing thrombosis nonspecifically.3' Positively charged materials do appear more thrombogenic,5 but the activation of the Hageman factor (factor XII) requires a negatively charged surface, on which positively charged adsorbates are inhibitory.' The zeta potential of many solids will approach zero upon exposure to blood" because plasma will deposit protein on all surfaces, whether their charge is positive, negative, 8 or zero. The other single popular value, critical surface tension, is a very sensitive shallow probe of molecular and submolecular solid surface structures that penetrates less than 24 A,10 yet effects from below upon the behavior of this outmost surface toward proteins may be profound." On high energy surfaces, simple wettability for water serves perhaps as a measure either of cleanliness per se or of the cleaning method used; both wettability and definition of a "clean" gold surface have been disputed among experts.'2 '3 Others, less equipped, cannot hope to handle, and especially to implant, materials under better than these disputable conditions. The wettability of high energy surfaces is lowered