Nanopores make sterile filtration more reliable

Jul 01, 2010
This mechanically stabilized nanoporous filter membrane exhibits a regular pore structure. At the same time, the pore size distribution is very tight and even. (© Fraunhofer IWM)

Irregular pores, low flow rates: The plastic membrane filters used in sterile filtration do not always ensure that conditions are really sterile. Filter membranes of aluminum oxide are more reliable - the size of the nanopores can be determined with precision. Even the smallest viruses cannot pass through the membrane.

The good ones are kept, the bad ones done away with - that, in a nutshell, is the principle behind sterile filtration: A filtration frees liquids of unwanted particles and germs. Nothing larger than the filter’s , only a few ten-thousandths of a millimeter in diameter, can pass through. Conventional membranes, usually made of plastic, come with limitations: Their pores are not evenly distributed and are occasionally too wide - and particles slip through after all. Conventional filtration membranes also have virtually no way of stopping viruses: Because most viruses are smaller than the pores, this technology offers no way to filter them out.

Now, researchers at the Fraunhofer Institute for Mechanics of Materials IWM in Halle, Germany, have created a new generation of filtration membranes: They developed ceramic membranes with a uniform pore structure and a very tight and even pore size distribution. "Compared to the ceramic membranes we have seen previously, they offer better mechanical stability and considerably higher flow rates. As a result, for the first time they are also able to replace polymer membranes", notes Annika Thormann, project manager at IWM. These membranes guarantee much more reliable filtration results than polymer membranes do. images of the membranes prove: The pores are regularly aligned alongside one another like the honeycombs in a beehive, one identical to the next.

To produce such filtration membranes, what is required first is the right raw material: "We use highly pure aluminum that we mold to the desired shape using extrusion equipment and thermomechanical structuring", Thormann explains. But just how can you create tiny pores on an aluminum plate with such precision? "A chemical reaction does the job", Thormann says. The molded aluminum part is placed in an acid bath where anodic oxidation takes place. An oxide layer just a few microns thick forms on the surface during electrolysis. "Tiny pores form in the aluminum during oxidation," Thormann explains. These nanopores are honeycomb-shaped, vertical to the surface, and are arrayed parallel to one another. "To set the pore size, we have to keep the voltage and the concentration of the acid stable", Thormann notes. The thickness of the nanoporous layer - and hence the flow rate of the membrane itself - can be fine-tuned as well via the duration of the oxidation process. In the end, the only step remaining is to open up the pores. This step is accomplished with chemical etching to remove unneeded residual aluminum.

The result: High-precision filtration membranes with a high porosity level. "We can vary pore diameters between 15 and 450 nanometers", says Thormann. At 15 nanometers, even the smallest viruses don’t stand a chance of slipping through. The new filtration membranes are particularly beneficial to biotechnology. Aside from use of the filtration properties to produce sterile media the membranes can also facilitate tissue engineering - the cultivation of artificial tissue - thanks to their high porosity.

Explore further: The latest fashion: Graphene edges can be tailor-made

add to favorites email to friend print save as pdf

Related Stories

Artificial Nanopores Take Analyte Pulse

Jul 31, 2007

Resistive pulse sensing represents a very attractive method for identifying and quantifying biomedical species such as drugs, DNA, proteins, and viruses in solution.

Nanomaterials to Mimic Cells

Aug 23, 2005

Mimicking a real living cell by combining artificial membranes and nanomaterials in one construction is the aim of a new research grant at UC Davis. The Nanoscale Integrated Research Team grant, funded by the National Science ...

Recommended for you

The latest fashion: Graphene edges can be tailor-made

Jan 23, 2015

Theoretical physicists at Rice University are living on the edge as they study the astounding properties of graphene. In a new study, they figure out how researchers can fracture graphene nanoribbons to get ...

Nanotechnology changes behavior of materials

Jan 23, 2015

One of the reasons solar cells are not used more widely is cost—the materials used to make them most efficient are expensive. Engineers are exploring ways to print solar cells from inks, but the devices ...

Gold 'nano-drills'

Jan 22, 2015

Spherical gold particles are able to 'drill' a nano-diameter tunnel in ceramic material when heated. This is an easy and attractive way to equip chips with nanopores for DNA analysis, for example. Nanotechnologists ...

The importance of building small things

Jan 22, 2015

Strong materials, such as concrete, are usually heavy, and lightweight materials, such as rubber (for latex gloves) and paper, are usually weak and susceptible to tearing and damage. Julia R. Greer, professor ...

Graphene brings quantum effects to electronic circuits

Jan 22, 2015

Research by scientists attached to the EC's Graphene Flagship has revealed a superfluid phase in ultra-low temperature 2D materials, creating the potential for electronic devices which dissipate very little ...

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.