A breakthrough in biomimetic membrane technology using aquaporins (AQPs) can potentially reduce industrial water treatment costs by 30%, say researchers at the National University of Singapore (NUS) Environmental Research Institute.
AQPs are membrane proteins that selectively conduct water molecules in and out of cells, preventing the passage of ions and other solutes. These channels are present in all living things from bacteria to mangrove plants and human kidneys and are examples of membrane structures that allow high volume of water molecules to pass through a small surface area at very low pressures, leaving behind impurities such as salt.
The NUS team designed and fabricated a new AQP-incorporated biomimetic membrane water purification and treatment system. Its studies have shown that the osmotic permeability of a single AQP is in the range of 6×10-14 to 24×10-14 m3 s-1, and that the permeability of an AQP-reconstituted biomimetic membrane is 167 µm s-1 bar-1 — which the researchers say is up to two orders of magnitude greater than those of commercial polymeric membranes.
It has long been thought that biomimetic membranes composed of AQPs and amphiphilic phospholipids or copolymers may have great use in high-flux and low-energy-consumption water purification processes. However, ultrathin biomimetic layers often are too fragile to withstand the high hydraulic or osmotic pressures in industrial water purification processes.
The NUS team says it’s among the first in the world to have succeeded in placing AQP proteins onto polymer membranes to act as channels that allow only water to go through very quickly, at low pressures and with low energy use. In addition, the team observed that the membrane exhibits high mechanical strength and stability during the water filtration. This makes it suitable for industrial applications in water treatment and desalination.
“The biomimetic membrane is constructed to mimic the layers of cells on the roots of mangrove trees by embedding nano-sized aquaporin-vesicles onto a stable and functional ultrafiltration substrate membrane using an innovative, yet simple and easy-to-implement surface imprinting technology. We found that the resultant aquaporin-incorporated biomimetic membrane allows water to pass through it faster and also display lower salt leakage than a membrane without aquaporin,” explains lead researcher and associate professor Tong Yen Wah of the Department of Chemical and Biomolecular Engineering at NUS.
The researchers currently are in discussions with a U.S.-based company to develop a pilot-scale module to test the feasibility of the membranes in the next two years.
According to Yen Wah, the team’s technique of producing the biomimetic membranes also can be applied in biological and biomedical research where the study of any other biological membrane protein requires its unique characteristics and functions to be expressed and properly placed onto synthetic membranes.
In fact, AWAK Technologies, Burbank, Calif., currently is working with the team to engineer similar biomimetic membranes for incorporation into wearable kidney dialysis devices. AWAK has developed a patented sorbent technology that reportedly revolutionizes dialysis by reducing the amount of dialysate needed for treatment by continuously recycling and regenerating the dialysis fluid. The technology enables the development of lightweight, compact, automated, wearable and portable medical devices.
Furthermore, last year, an international team of researchers at Penn State, University Park, Pa., announced development of a more-stable and easier-to-manufacture second-generation water channel that improves on earlier attempts to mimic AQPs. The team noted that its peptide-appended pillararenes (PAPs) are more easily produced and aligned than carbon nanotubules, which also are under investigation for membrane separation.
“Nature does things very efficiently and transport proteins are amazing machines present in biological membranes,” says Manish Kumar, assistant professor of chemical engineering at Penn State. “They have functions that are hard to replicate in synthetic systems.”
“We were surprised to see transport rates approaching the ‘holy grail’ number of a billion water molecules per channel per second. We also found that these artificial channels like to associate with each other in a membrane to make two-dimensional arrays with a very high pore density,” he adds.
The Penn State researchers believe the PAP membranes are an order of magnitude better than the first-generation artificial water channels reported to date. The propensity for these channels to automatically form densely packed arrays leads to a variety of engineering applications.
“The most obvious use of these channels is perhaps to make highly efficient water purification membranes,” believes Kumar.
Seán Ottewell is Chemical Processing's Editor at Large. You can email him at firstname.lastname@example.org.