University of Texas at Austin

A chemical engineer notes that not all membrane pores are made equal; some are more equal than others.

Few cheap, man-made membranes have holes of uniform size. This makes them either inefficient or unreliable sieves of particles such as viruses. But membranes are also one of the least energy-intensive separation devices. As fuel costs rise, many of the billion or so people without access to safe drinking water find it harder to sanitize what water they have. This is why I view the low-cost manufacture of isoporous membranes as a holy grail in the field.

Recently, some scientists in Germany unearthed a path to this chalice by tinkering with a technique known as 'phase inversion'. This is often used to make synthetic membranes: a polymer solution is immersed in a liquid, often water, which diffuses into the solution and causes a thin, porous membrane of hydrophobic polymer to form. The solid polymer is a twisted, irregular matrix, full of odd-shaped pores.

Klaus-Viktor Peinemann and his co-workers started with a polymer in which the chain-like molecules have a hydrophobic and a hydrophilic end, and allowed the solvent solution to evaporate. As this happened, they think that the polymer assembled into connected cylinders, with the hydrophobic and hydrophilic parts of different molecules coming together. The researchers then plunged this nascent membrane into water, which moved through the hydrophilic cylinders, opening them up and thus creating identical and aligned pores (K.-V. Peinemann et al. Nature Mater. 6, 992–996; 2007).

The pores were all about 10 nanometres wide — roughly the right size to separate hepatitis B virus from water. Picking other polymers with hydrophobic and hydrophilic parts should allow the development of membranes with uniform-diameter pores of various sizes. That could be a boon for industry as well as public health.