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Natural stickiness inspires synthetic solutions
RESEARCH NEWS
Sugars may control bone mineralization BIOMATERIALS Bone is a remarkable and complex composite material. Its toughness comes from its organic matrix, while its stiffness arises from the hydroxyapatite-like mineral phase. The interaction between the organic and mineral phases is crucial in determining bone strength, adaptation and growth, and – importantly – diseases like osteoporosis and osteoarthritis. Researchers from the UK and Germany now show that polysaccharide chains (glycosaminoglycans or GAGs) provide the contact between the organic matrix and the mineral component [Wise et al., Chem. Mater. (2007) 19, 5055]. Previously, it has been assumed that protein stabilizes the organicmineral interface, say the team from the University of Cambridge, Addenbrookes Hospital, the Animal Health Trust in Suffolk, and BAM Federal Institute of Materials Research and Testing in Berlin. “We believe our findings will alter some fundamental preconceptions of bone biology,” says David G. Reid of the University of Cambridge. Using a solid-state nuclear magnetic resonance technique, the group looked for signals from 13C nuclei that are closest to 31P nuclei in the mineral. They find that it is sugar groups that are closest and not protein. “[This is] a very interesting approach to looking at factors in bone mineralization and biomaterials strategies,” comments Matt Dalby of the University of Glasgow. Reid and colleagues believe that GAGs play a central role in ensuring ordered biomineralization in bone and that this role for polysaccharides needs to be taken account in designing biomaterials.
Jonathan Wood
Rational route to high-performance membranes POLYMERS
The random assortment of different size and shape cavities in a polymer film, through which small gas molecules can diffuse, curtails their use as membranes. Now an international team of researchers have created a polymer in which the cavities can be tailored for the fast and selective separation of small molecules and ions [Park et al., Science (2007) 318, 254]. The researchers from Hanyang University, Korea, The University of Texas, Austin, The Commonwealth Scientific and Industrial Research Organization (CSIRO), and the Australian Synchrotron Research Program use a thermal rearrangement of soluble aromatic polyimides to create polybenzoxazole or polybenzithiazole or pyrrolone. Although these polymers are hard to process into membranes, they can easily form hollow fibers or films. “The new polymer membranes provide about 500 times better permeability and selectivity than membranes made with conventional polymers,” says Young Moo Lee of Hanyang University. Key to this improvement in performance is the formation of bottleneck-shaped cavities of an intermediate size with a narrow size distribution in these polymers. The polymers can separate mixtures of CO2/CH4, O2/N2, and H2/N2 – in all cases surpassing the separation limit of conventional polymer membranes. The researchers plan to create proof-of-principle modules for CO2 capture from flue and natural gas, as well as water purification. Doping these porosity-
Bottleneck-shaped pores give the polymer membranes superior selectivity and permeability. (Credit: CSIRO, Australia.)
tailorable polymers with acid molecules results in high proton conductivity, which the researchers believe is promising for fuel cell membranes as well. Cordelia Sealy
Natural stickiness inspires synthetic solutions SURFACE SCIENCE Stickiness seen in nature continues to influence surface science. Researchers from the Indian Institute of Technology, Kanpur, have taken a closer look at the feet of tree frogs and insects to create a new microfluidic adhesive [Majumder et al., Science (2007) 318, 258]. Meanwhile, scientists from Northwestern University have drawn inspiration from mussels to develop a new way of forming multifunctional polymer coatings [Lee et al., Science (2007) 318, 426]. The importance of surface patterns on the feet of climbing animals, such as geckos, is well known. Abhijit Majumder and colleagues have now revealed the significance of subsurface structures, such as fluid vessels and air sacs, on adhesion and reversible separation. The researchers embedded air- and oil-filled microchannels of different diameters within elastomeric adhesive layers bonded to a rigid substrate. By controlling the contents, size, and spacing of the channels, the surface adhesive strength can be increased up to 30 times. An alternative set of subsurface conditions makes the adhesive act like a quick-release coating. The next step will be to couple the effects of surface
patterning and these subsurface microstructures. Similarly, the Northwestern group has been inspired by the stickiness of proteins secreted by mussels to find a generally applicable way of depositing thin polymer films. They identified dopamine as a good mimic of a foot protein of the mussel Mytilus edulis and dissolved this in an alkaline solution of water (pH ~ 8.5) to replicate the marine environment. Objects dipped in the aqueous solution become coated with a thin layer of polydopamine. This provides a chemically reactive surface that can be functionalized easily with a secondary coating, e.g. for toxic ion removal or genomic analyses. “We have demonstrated this on 25 different materials and have yet to find one that cannot be coated by polydopamine in this manner,” says Phillip B. Messersmith. “It is simple and does not require any sophisticated polymer design and synthesis or coating formulation.” The physical and chemical durability of the polydopamine coating has yet to be determined.
Paula Gould
DECEMBER 2007 | VOLUME 10 | NUMBER 12
15
Sugars may control bone mineralization BIOMATERIALS Bone is a remarkable and complex composite material. Its toughness comes from its organic matrix, while its stiffness arises from the hydroxyapatite-like mineral phase. The interaction between the organic and mineral phases is crucial in determining bone strength, adaptation and growth, and – importantly – diseases like osteoporosis and osteoarthritis. Researchers from the UK and Germany now show that polysaccharide chains (glycosaminoglycans or GAGs) provide the contact between the organic matrix and the mineral component [Wise et al., Chem. Mater. (2007) 19, 5055]. Previously, it has been assumed that protein stabilizes the organicmineral interface, say the team from the University of Cambridge, Addenbrookes Hospital, the Animal Health Trust in Suffolk, and BAM Federal Institute of Materials Research and Testing in Berlin. “We believe our findings will alter some fundamental preconceptions of bone biology,” says David G. Reid of the University of Cambridge. Using a solid-state nuclear magnetic resonance technique, the group looked for signals from 13C nuclei that are closest to 31P nuclei in the mineral. They find that it is sugar groups that are closest and not protein. “[This is] a very interesting approach to looking at factors in bone mineralization and biomaterials strategies,” comments Matt Dalby of the University of Glasgow. Reid and colleagues believe that GAGs play a central role in ensuring ordered biomineralization in bone and that this role for polysaccharides needs to be taken account in designing biomaterials.
Jonathan Wood
Rational route to high-performance membranes POLYMERS
The random assortment of different size and shape cavities in a polymer film, through which small gas molecules can diffuse, curtails their use as membranes. Now an international team of researchers have created a polymer in which the cavities can be tailored for the fast and selective separation of small molecules and ions [Park et al., Science (2007) 318, 254]. The researchers from Hanyang University, Korea, The University of Texas, Austin, The Commonwealth Scientific and Industrial Research Organization (CSIRO), and the Australian Synchrotron Research Program use a thermal rearrangement of soluble aromatic polyimides to create polybenzoxazole or polybenzithiazole or pyrrolone. Although these polymers are hard to process into membranes, they can easily form hollow fibers or films. “The new polymer membranes provide about 500 times better permeability and selectivity than membranes made with conventional polymers,” says Young Moo Lee of Hanyang University. Key to this improvement in performance is the formation of bottleneck-shaped cavities of an intermediate size with a narrow size distribution in these polymers. The polymers can separate mixtures of CO2/CH4, O2/N2, and H2/N2 – in all cases surpassing the separation limit of conventional polymer membranes. The researchers plan to create proof-of-principle modules for CO2 capture from flue and natural gas, as well as water purification. Doping these porosity-
Bottleneck-shaped pores give the polymer membranes superior selectivity and permeability. (Credit: CSIRO, Australia.)
tailorable polymers with acid molecules results in high proton conductivity, which the researchers believe is promising for fuel cell membranes as well. Cordelia Sealy
Natural stickiness inspires synthetic solutions SURFACE SCIENCE Stickiness seen in nature continues to influence surface science. Researchers from the Indian Institute of Technology, Kanpur, have taken a closer look at the feet of tree frogs and insects to create a new microfluidic adhesive [Majumder et al., Science (2007) 318, 258]. Meanwhile, scientists from Northwestern University have drawn inspiration from mussels to develop a new way of forming multifunctional polymer coatings [Lee et al., Science (2007) 318, 426]. The importance of surface patterns on the feet of climbing animals, such as geckos, is well known. Abhijit Majumder and colleagues have now revealed the significance of subsurface structures, such as fluid vessels and air sacs, on adhesion and reversible separation. The researchers embedded air- and oil-filled microchannels of different diameters within elastomeric adhesive layers bonded to a rigid substrate. By controlling the contents, size, and spacing of the channels, the surface adhesive strength can be increased up to 30 times. An alternative set of subsurface conditions makes the adhesive act like a quick-release coating. The next step will be to couple the effects of surface
patterning and these subsurface microstructures. Similarly, the Northwestern group has been inspired by the stickiness of proteins secreted by mussels to find a generally applicable way of depositing thin polymer films. They identified dopamine as a good mimic of a foot protein of the mussel Mytilus edulis and dissolved this in an alkaline solution of water (pH ~ 8.5) to replicate the marine environment. Objects dipped in the aqueous solution become coated with a thin layer of polydopamine. This provides a chemically reactive surface that can be functionalized easily with a secondary coating, e.g. for toxic ion removal or genomic analyses. “We have demonstrated this on 25 different materials and have yet to find one that cannot be coated by polydopamine in this manner,” says Phillip B. Messersmith. “It is simple and does not require any sophisticated polymer design and synthesis or coating formulation.” The physical and chemical durability of the polydopamine coating has yet to be determined.
Paula Gould
DECEMBER 2007 | VOLUME 10 | NUMBER 12
15