RESEARCH NEWS

September 200516

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.

(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.Jonathan Wood

Biomimeticadhesion usingnanotubesNANOTECHNOLOGY

Despite the inherent brittleness of glass, the deep-sea Euplectella sponge has evolved in such a waythat its glassy silica skeleton is durable.Researchers from Bell Laboratories, the Universityof California, Santa Barbara and the Max PlanckInstitute of Colloids and Interfaces in Potsdam,Germany report that the sponge achievesmechanical stability through a highly complexstructure from the nano-to-macro scales [Aizenberget al., Science (2005) 309, 275].“The resultant structure might be regarded as atextbook example in mechanical engineering,” saythe authors. “The seven hierarchical levels in thesponge skeleton represent major fundamentalconstruction strategies, such as laminatedstructures, fiber-reinforced composites, bundledbeams, and diagonally reinforced square-grid cells,to name a few.”This natural phenomenon is not uncommon. Thesehighly complex building principles are found inalmost all biomineralized materials, including human

bone. With Euplectella, however, the developmentprocess is more complex than those higher on thefood chain. At the first structural level, silica nanospheresconsolidate around protein filaments. Next, thinorganic protective films form to enhance mechanicalrigidity. Spicules of alternating organic and silicalayers are formed and bundled into parallel griddedgroups in levels three and four. The gridded area iscemented with a layered silica matrix and resemblesfiber-reinforced polymers. In the final levels ofstructural hierarchy, the grid wraps into a curvedcylinder and ridges form. This provides an increasedstiffness and torsion resistance. “Glass is widely used as a building material in thebiological world, despite its fragility. Organisms haveevolved means to effectively reinforce this inherentlybrittle material,” say the researchers. The teamhopes that understanding the synthesis of thesponge skeleton will lead to new material concepts. Patrick Cain

Sponge makes strong case for brittle glassMECHANICAL BEHAVIOR

Nanofluidic detection of DNA moleculesNANOTECHNOLOGY

By integrating inorganic nanotubes withinmicrofluidic systems, University of California,Berkeley researchers have developed devicescapable of sensing single DNA molecules (Fan et al., Nano Lett. (2005), doi:10.1021/nl0509677).The new systems offer three distinctadvantages over traditional devices thatmeasure translocation through a nanopore,note lead researchers Arun Majumdar andPeidong Yang. The nanotubes can confine theentire DNA molecule, resulting in newtranslocation characteristics. Second, thenanotube devices have a planar geometry,

which could allow simultaneous optical andelectrical probing. Third, the geometry iscompatible with lab-on-a-chip systems. A Si nanotube with an inner diameter of 50 nm is used to bridge two microfluidicchannels, each filled with buffer solution. A bias is applied and the ionic currentrecorded. Transient ionic current changesindicate DNA translocation events throughthe nanotube. The nanotube nanofluidic device extends thetime scale of single molecule transportevents greatly compared to nanoporedevices. As a result, the current change,duration, and decay characteristics atdifferent ionic concentrations and bias couldall provide useful information on the behaviorof biomolecules within a confined geometry. The nanotube devices represent a new meansto study single biomolecule translocation andhave the potential to be integrated intonanofluidic circuits. The researchers are nowworking on a new apparatus to enablesimultaneous optical and electrical probingthat could provide a better understanding ofDNA translocation events.John K. Borchardt

Nanofluidic device based on an inorganic nanotube can sense

individual DNA molecules. (Courtesy of Peidong Yang, University

of California, Berkeley.)