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Biomimetic adhesion using nanotubes
RESEARCH NEWS
Nanofluidic detection of DNA molecules NANOTECHNOLOGY
Nanofluidic device based on an inorganic nanotube can sense individual DNA molecules. (Courtesy of Peidong Yang, University of California, Berkeley.)
By integrating inorganic nanotubes within microfluidic systems, University of California, Berkeley researchers have developed devices capable of sensing single DNA molecules (Fan et al., Nano Lett. (2005), doi: 10.1021/nl0509677). The new systems offer three distinct advantages over traditional devices that measure translocation through a nanopore, note lead researchers Arun Majumdar and Peidong Yang. The nanotubes can confine the entire DNA molecule, resulting in new translocation characteristics. Second, the nanotube devices have a planar geometry,
which could allow simultaneous optical and electrical probing. Third, the geometry is compatible with lab-on-a-chip systems. A Si nanotube with an inner diameter of 50 nm is used to bridge two microfluidic channels, each filled with buffer solution. A bias is applied and the ionic current recorded. Transient ionic current changes indicate DNA translocation events through the nanotube. The nanotube nanofluidic device extends the time scale of single molecule transport events greatly compared to nanopore devices. As a result, the current change, duration, and decay characteristics at different ionic concentrations and bias could all provide useful information on the behavior of biomolecules within a confined geometry. The nanotube devices represent a new means to study single biomolecule translocation and have the potential to be integrated into nanofluidic circuits. The researchers are now working on a new apparatus to enable simultaneous optical and electrical probing that could provide a better understanding of DNA translocation events.
September 2005
Geckos are well known for their ability to climb any vertical surface or hang from a ceiling by one toe. Mimicking these capabilities could lead to new dry adhesives for applications in space, microelectronics, and information technology. Researchers at The University of Akron in Ohio and Rensselaer Polytechnic University have now made synthetic nanotube structures with strong nanometer-level adhesion based on gecko foot hairs [Yurdumakan et al., Chem. Commun.
bone. With Euplectella, however, the development process is more complex than those higher on the food chain. At the first structural level, silica nanospheres consolidate around protein filaments. Next, thin organic protective films form to enhance mechanical rigidity. Spicules of alternating organic and silica layers are formed and bundled into parallel gridded groups in levels three and four. The gridded area is cemented with a layered silica matrix and resembles fiber-reinforced polymers. In the final levels of structural hierarchy, the grid wraps into a curved cylinder and ridges form. This provides an increased stiffness and torsion resistance. “Glass is widely used as a building material in the biological world, despite its fragility. Organisms have evolved means to effectively reinforce this inherently brittle material,” say the researchers. The team hopes that understanding the synthesis of the sponge skeleton will lead to new material concepts. Patrick Cain
Jonathan Wood
John K. Borchardt
MECHANICAL BEHAVIOR
16
NANOTECHNOLOGY
(2005) (30), 3799]. Gecko feet have five toes that are covered with microscopic, elastic hairs called setae. The end of each hair splits further into spatulae. It is the aspect ratio, nanoscale dimensions, stiffness, and density of the spatulae that provide a van der Waals force sufficient to hold the gecko in contact with a surface. Carbon nanotubes have very similar dimensions to the spatulae, and this prompted Ali Dhinojwala and colleagues to try and mimic gecko foot hairs. Multiwalled nanotubes (MWNTs) 50-100 µm long are grown on quartz or Si substrates by chemical vapor deposition. The vertically aligned nanotubes are then embedded in a polymer matrix. The composite sheets are peeled from the substrate and the MWNT brushes are exposed by etching the polymer matrix. A scanning probe microscope tip was used to measure adhesive forces on retraction from the MWNT brushes. The typical forces are 200 times those of a single gecko foot hair. The scientists attribute this to the combination of van der Waals forces and energy dissipation during elongation of the MWNTs.
Sponge makes strong case for brittle glass Despite the inherent brittleness of glass, the deepsea Euplectella sponge has evolved in such a way that its glassy silica skeleton is durable. Researchers from Bell Laboratories, the University of California, Santa Barbara and the Max Planck Institute of Colloids and Interfaces in Potsdam, Germany report that the sponge achieves mechanical stability through a highly complex structure from the nano-to-macro scales [Aizenberg et al., Science (2005) 309, 275]. “The resultant structure might be regarded as a textbook example in mechanical engineering,” say the authors. “The seven hierarchical levels in the sponge skeleton represent major fundamental construction strategies, such as laminated structures, fiber-reinforced composites, bundled beams, and diagonally reinforced square-grid cells, to name a few.” This natural phenomenon is not uncommon. These highly complex building principles are found in almost all biomineralized materials, including human
Biomimetic adhesion using nanotubes
Nanofluidic detection of DNA molecules NANOTECHNOLOGY
Nanofluidic device based on an inorganic nanotube can sense individual DNA molecules. (Courtesy of Peidong Yang, University of California, Berkeley.)
By integrating inorganic nanotubes within microfluidic systems, University of California, Berkeley researchers have developed devices capable of sensing single DNA molecules (Fan et al., Nano Lett. (2005), doi: 10.1021/nl0509677). The new systems offer three distinct advantages over traditional devices that measure translocation through a nanopore, note lead researchers Arun Majumdar and Peidong Yang. The nanotubes can confine the entire DNA molecule, resulting in new translocation characteristics. Second, the nanotube devices have a planar geometry,
which could allow simultaneous optical and electrical probing. Third, the geometry is compatible with lab-on-a-chip systems. A Si nanotube with an inner diameter of 50 nm is used to bridge two microfluidic channels, each filled with buffer solution. A bias is applied and the ionic current recorded. Transient ionic current changes indicate DNA translocation events through the nanotube. The nanotube nanofluidic device extends the time scale of single molecule transport events greatly compared to nanopore devices. As a result, the current change, duration, and decay characteristics at different ionic concentrations and bias could all provide useful information on the behavior of biomolecules within a confined geometry. The nanotube devices represent a new means to study single biomolecule translocation and have the potential to be integrated into nanofluidic circuits. The researchers are now working on a new apparatus to enable simultaneous optical and electrical probing that could provide a better understanding of DNA translocation events.
September 2005
Geckos are well known for their ability to climb any vertical surface or hang from a ceiling by one toe. Mimicking these capabilities could lead to new dry adhesives for applications in space, microelectronics, and information technology. Researchers at The University of Akron in Ohio and Rensselaer Polytechnic University have now made synthetic nanotube structures with strong nanometer-level adhesion based on gecko foot hairs [Yurdumakan et al., Chem. Commun.
bone. With Euplectella, however, the development process is more complex than those higher on the food chain. At the first structural level, silica nanospheres consolidate around protein filaments. Next, thin organic protective films form to enhance mechanical rigidity. Spicules of alternating organic and silica layers are formed and bundled into parallel gridded groups in levels three and four. The gridded area is cemented with a layered silica matrix and resembles fiber-reinforced polymers. In the final levels of structural hierarchy, the grid wraps into a curved cylinder and ridges form. This provides an increased stiffness and torsion resistance. “Glass is widely used as a building material in the biological world, despite its fragility. Organisms have evolved means to effectively reinforce this inherently brittle material,” say the researchers. The team hopes that understanding the synthesis of the sponge skeleton will lead to new material concepts. Patrick Cain
Jonathan Wood
John K. Borchardt
MECHANICAL BEHAVIOR
16
NANOTECHNOLOGY
(2005) (30), 3799]. Gecko feet have five toes that are covered with microscopic, elastic hairs called setae. The end of each hair splits further into spatulae. It is the aspect ratio, nanoscale dimensions, stiffness, and density of the spatulae that provide a van der Waals force sufficient to hold the gecko in contact with a surface. Carbon nanotubes have very similar dimensions to the spatulae, and this prompted Ali Dhinojwala and colleagues to try and mimic gecko foot hairs. Multiwalled nanotubes (MWNTs) 50-100 µm long are grown on quartz or Si substrates by chemical vapor deposition. The vertically aligned nanotubes are then embedded in a polymer matrix. The composite sheets are peeled from the substrate and the MWNT brushes are exposed by etching the polymer matrix. A scanning probe microscope tip was used to measure adhesive forces on retraction from the MWNT brushes. The typical forces are 200 times those of a single gecko foot hair. The scientists attribute this to the combination of van der Waals forces and energy dissipation during elongation of the MWNTs.
Sponge makes strong case for brittle glass Despite the inherent brittleness of glass, the deepsea Euplectella sponge has evolved in such a way that its glassy silica skeleton is durable. Researchers from Bell Laboratories, the University of California, Santa Barbara and the Max Planck Institute of Colloids and Interfaces in Potsdam, Germany report that the sponge achieves mechanical stability through a highly complex structure from the nano-to-macro scales [Aizenberg et al., Science (2005) 309, 275]. “The resultant structure might be regarded as a textbook example in mechanical engineering,” say the authors. “The seven hierarchical levels in the sponge skeleton represent major fundamental construction strategies, such as laminated structures, fiber-reinforced composites, bundled beams, and diagonally reinforced square-grid cells, to name a few.” This natural phenomenon is not uncommon. These highly complex building principles are found in almost all biomineralized materials, including human
Biomimetic adhesion using nanotubes