Filters block the big particles and allow the finer substances through, right? Not necessarily--some filters work in the opposite way. A team of chemical engineers and materials researchers has discovered a method to markedly improve these so-called reverse-selective membranes in a unexpected manner: by adding nonporous filler materials. Rather than stopping up the filter holes, though, the additives enhance the membrane's permeability to large molecules.

This result stems from how these unusual gas filters operate, according to team leader Ingo Pinnau of Membrane Technology and Research in Menlo Park, Calif. A reverse-

This article was originally published with the title Filtering in Reverse.

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