Surface coatings Nanoscale snowmen look blank
Adv. Mater. 16, 274–277 (2004)
Moths' eyes don't show up in car headlights as they have an anti-reflective ‘stealth’ surface, which helps moths evade predators. Some artificial anti-reflective coatings mimic the microscopic structure of moth eyes, which are covered with an array of protuberances around 200 nm high. The ideal shape of these bumps is conical, as this produces a gradual change in refractive index that minimizes reflection.
Similar surface-relief patterns can be made using lithographic etching, but Hye Young Koo and colleagues have devised a cheap ‘wet’ chemical procedure that approximates the conical profiles by generating snowman-like structures in which a small spherical particle sits atop a larger one (pictured right). Both spheres are polystyrene, with diameters of 50 and 100 nm and a negative surface charge.
The nano-snowmen are made through layer-by-layer self-assembly. First, the substrate is coated with a positively charged polymer, which secures the first layer of spheres. Then the tops of the spheres are ‘inked’ with the positive polymer using a rubber stamp, enabling the second layer of smaller spheres to be attached. The resulting transparent film has about 2% reflectance: about twice that of commercial plastic coatings with conical protrusions made by a more complex moulding method.
Medical technology What a contrast!
Phys. Med. Biol. 49, 501–508 (2004)
To distinguish between structures in the body, many X-ray techniques rely on the use of a contrast agent, the injection of which is a surgical procedure carrying a small risk. As an alternative, methods are being developed that use coherent X-rays alone to create a contrast between tissues with different refractive indices.
Y. Hwu and colleagues have now shown that such a ‘phase-contrast’ technique can image blood vessels in real time and at resolutions of less than ten micrometres — a level of detail not seen before, say the authors, even with contrast agents. The team used a beam of ‘white’ X-rays (containing a range of wavelengths), generated by a synchrotron accelerator. Their images of rats show blood vessels around the heart and eyes in very fine detail.
At present the radiation dose exceeds recommended limits for humans, but Hwu et al. claim that the exposure time of about 1 millisecond could be reduced. There are, however, only a handful of specialized medical synchrotrons in the world. So, for the short term at least, the technique will be useful in fundamental studies of blood-vessel development and tumour formation, rather than in treating patients.
Atmospheric chemistry Flying detectives
J. Geophys. Res. doi:10.1029/2003JD003811 (2004)
For years scientists have simply had to assume that one of the intermediates in polar ozone destruction really exists in the atmosphere. But R. M. Stimpfle et al. at last report measurements of the chemical species concerned, the chlorine monoxide dimer ClOOCl, in the stratosphere over the Arctic.
The dimer had been identified in laboratory studies, and models of atmospheric ozone loss have relied on its presumed existence. Actually detecting ClOOCl in the atmosphere has proven elusive, however, as the molecule is thought to exist only in the particularly cold winter conditions over the poles. ClOOCl triggers ozone destruction when the absorption of sunlight breaks it into an oxygen molecule and two chlorine atoms: the latter then destroy two ozone molecules, with two chlorine monoxide radicals being produced that can then reform the chlorine monoxide dimer.
The atmospheric measurements of ClOOCl were obtained on a NASA high-altitude research aircraft deployed from Kiruna, Sweden, in the winter of 1999–2000. The results generally confirm accepted understanding of polar chlorine chemistry and will help improve modelling studies of changes in polar ozone.
Endocrinology Amylin for strong bones
J. Cell Biol. doi:10.1083/jcb.200312135 (2004)
As well as insulin, pancreatic β cells secrete a hormone called amylin, which seems to affect bone remodelling. Bones are made and broken down, or resorbed, continuously. Usually this process is in equilibrium, maintaining total bone mass.
To understand amylin function, Romain Dacquin et al. studied amylin-deficient mice. The animals' only abnormality was an increase in bone resorption; bone formation was unaffected. This imbalance resulted in low bone mass, mimicking the human condition osteoporosis, in which patients have brittle bones.
The mice lacking amylin had more osteoclasts, the cells that resorb bone. Adding amylin to bone cells in culture inhibited the production of osteoclasts, via a pathway that uses an enzyme called ERK1/2 (extracellular signal-regulated protein kinase 1/2). Amylin inhibits the process by which osteoclasts are formed, Dacquin et al. conclude, so when the hormone is in short supply, osteoclasts proliferate, engulfing more bone.
Vision Bird's-eye view
Naturwissenschaften 91, 26–29 (2004)
Oilbirds (Steatornis caripensis, right) are nocturnal, fruit-eating birds that live in the tropical rainforests of South America. They have a wing-span of around 1 m and, like some bats, they dwell in caves, forage for food at night and use echolocation to help find their way around. But they also have extraordinarily sensitive eyes.
The birds have a relatively large pupil, report Graham Martin et al., which helps the oilbird eye to collect four times more light than the human eye. The light-sensitive rod cells within the eye are stacked three deep, with a density of about 1,000,000 per mm2 — more than double the number usually found in vertebrate eyes. The tiered strategy, which has previously been found only in deep-sea fish, maximizes the chance that every photon of light entering the eye will be intercepted.
But extreme sensitivity comes at a cost. The retina's tiered structure makes it difficult for the brain to work out exactly where the light has come from, so oilbirds have a poor eye for detail. Martin and colleagues therefore suggest that these nocturnal birds rely on a combination of information from smell and echolocation, as well as from sight, to forage successfully.
Helen R. Pilcher