Selectins with a catch

Small minerals make strong bones Selectins with a catch ot content simply with a good fit, evolution has apparently designed adhesion molecules that stick optimally only when tugged upon. This property, may stop leukocytes and platelets from lingering in stagnant backwaters in the blood where there is little or no flow. Blood is a medium dominated by flow and shear forces. " If you think flow is disruptive, then greater flow should make cells roll faster, " says Zhu. " That's the case at the high end, but at the low end it reverses. " The Georgia Tech group found that this effect could be attributed to the behavior of individual adhesion molecules such as P-selectin and its partner P-selectin glyco-protein ligand-1 (PSGL-1). Leukocytes N But at the nanoscale mineral sizes found in virtually all biocomposites, the story is different. " At nanoscale the volume is too small, so the strain energy stored in the mineral is too small to break the bond, " says Gao. " So the crack will not grow—these flaws will not matter. " Gao says that this principle explains a case of convergent evolution: the similarly small sizes of mineral particles in a wide range of biocomposites. Nanoscale sizing can also be used in making artificial materials. Material processing, he says, " is often like cooking, " with lots of experimenting with ingredients backed up by little theory. So a principle from biology is welcome. He and his colleagues have used the nanoscale theory to study a composite coating—made of As mineral size decreases (left to right) to reach that found in tooth enamel (inset), the fracture resistance increases. grains of titanium nitrate embedded in amorphous silicon nitrate—that is almost as hard as diamond. " That is a perfect application of this principle, " he says. ᭿