(This is an edited version of a story that first appeared in the October 2000 issue of the Materials Processing Center Industry Collegium Report.)
Membranes that filter larger materials from others are key to wastewater treatment and a variety of other processes. Yet the membranes currently on the market are often easily clogged.
Associate Professor Anne Mayes decided that the field was ripe for fresh ideas. She's developed a way not only to make better filtration membranes, but also to give those membranes additional applications. For example, her team is modifying the membranes to encourage the attachment of living cells -- a key goal of tissue engineering. Dr. Mayes is associate professor of polymer physics in the Department of Materials Science and Engineering.
Black art
To date, the manufacture of filtration membranes has been something of a black art, principally using a process known as immersion precipitation. A concentrated polymer solution is spread on a moving belt, skimmed to a thin and level coating, then immersed in a water-containing bath. There the polymer precipitates out of solution, forming a membrane with a surprisingly ideal structure for filtration: small surface pores, with larger channels in the bulk membrane structure.
Unfortunately, this process leaves a very small number of pores on the surface, and the materials that are sufficiently strong for filtering applications are hydrophobic, or water-repelling. Oil- or protein-containing solutions that are passed through such filters tend to clog or foul them.
In the wastewater treatment industry, cleaning and replacement costs associated with fouling amount to some 47 percent of the total process costs. Granted, further treatments can lead to a fouling-resistant surface, but these extra fabrication steps drive up costs and don't always work well, since they treat only the outer surface of the membrane. The inner surfaces of the membrane channels remain foulant attractors.
Polymer 'combs'
Professor Mayes's novel idea was to mix into the bulk membrane polymer solution a "comb" polymer composed of a hydrophobic backbone lined with short hydrophilic (water-loving) "bristles."
When the membrane is cast from the mixed polymer solution, the different affinities of the comb and bulk polymers cause the combs to segregate to the membrane surface. Thus the membrane ends up with a hydrophilic outer surface and even a hydrophilic lining on the internal membrane channels.
"It turns out that this works extraordinarily well," said Professor Mayes. The presence of hydrophilic groups on the filtrant-exposed surfaces helps the membrane resist fouling.
What's more, the comb-containing membranes have a larger number of pores on the surface than membranes that don't include the additive. This increases the amount of solution that can be passed through the membranes. And the membranes heal themselves: if damaged by a caustic solution, a comb-containing membrane can be heated at approximately 90���C in water to re-segregate excess combs from the bulk and reline the damaged surfaces with hydrophilic material.
New chemistries
Professor Mayes and her students are now working on comb backbones made of different materials in the hope of further adapting their approach to other membrane chemistries.
"We've been branching out in terms of our chemistry," she said. "We've developed a new route for the synthesis of these comb additives, and recently applied for a patent on the materials.
"We have other applications, too," Professor Mayes continued. These include tethering chelating agents to the comb "bristles" to trap heavy metals from a filtrant, and tethering cell-signaling ligands to the bristles to encourage cell attachment to, for instance, a membrane being used as an artificial tissue material.
To explore bioengineering possibilities, she has teamed up with Professor Linda Griffith of the Department of Chemical Engineering and the Division of Bioengineering and Environmental Health to show that comb polymer additives modified with the cell-adhesion peptide RGD can be used to modulate cell attachment.
"We wanted to make a system where we could control the spatial distribution of ligands on the surface more specifically. Often cell signaling events require clustering of ligand receptors on the surface of cells, and that's what we hope to induce by mixing comb polymers modified with multiple ligands with those having no ligands," Professor Mayes said. The researchers have demonstrated comb surface segregation in the manufacture of tissue-engineering scaffolds.
Professor Mayes's work is sponsored by the Office of Naval Research, the National Institutes of Health and the Whitaker Foundation.
A version of this article appeared in MIT Tech Talk on December 6, 2000.
