Find the culprit with MEMS

The days when fingerprints were taken with ink are long over. Modern technologies rely on fingerprint sensors based, for example, on optical sensing. The problem with this technology is its sensitivity to the moist state of the finger, and the straightforward possibility of forging fingerprints with optical images. Ultrasonic fingerprinting, however, is sensitive to the properties of skin tissue, in addition to the ridges on the finger. So far, this technology has been rather slow and the images needed to be scanned line by line. Yun-woo Nam has been granted an international patent, filed by Samsung (Korean Patent KR2003081724; 2006), that uses a microelectromechanical system (MEMS) fabrication technique for small sensors that authenticate fingerprints at high speeds. An array of piezoelectric elements generates ultrasonic waves in the sensor. At places where the finger is in contact with the sensor, these waves penetrate into the skin. If the finger does not touch the surface, the acoustic waves are instead reflected back to the device. The reflected waves are detected by the same piezoelectric elements, and the fingerprint image is generated.

Piezoelectricity in the right terms

Linear approximation, commonly used in computational materials science, is based on the idea that the variation of a physical quantity produces a proportional change in a second one. This is reasonable in most cases; but not always, at least according to what Gabriel Bester and his colleagues recently showed (Phys. Rev. Lett. 96, 187602; 2006). They estimated the effects of crystal deformation in semiconductor heterostuctures, that is, alternate layers of materials with different lattice parameters. The strain induced by the lattice mismatch produces a displacement of ions and carriers that gives rise to an electric field known as piezoelectric. In addition to the linear term connecting the strain to the field, the US-based scientists have calculated the contribution of the term proportional to the square of the strain, focusing on a thin InGaAs layer incorporated in a GaAs matrix. They found that the two terms have opposite but comparable effects, which, in some cases, almost cancel each other. Aside from its importance in the description of the specific structure considered, this result suggests that piezoelectricity is a classical example of a case in which considering only the linear term may lead to serious errors in the estimates.

Vesicle with chemical arms

Vesicles can be viewed as synthetic models for cells and for coatings of virus particles, and they are also promising as potential nanoscale reactors. Vesicles can readily be prepared from polymer analogues, but controlling the structure within their two-dimensional layers has so far proved elusive. Timothy Lodge and colleagues report the preparation of vesicles with laterally nanostructured membranes by the self-assembly of macromolecules with three chemically distinct arms (Nano Lett. doi:10.1021/nl0608700; 2006). One arm is formed from polyethylene oxide, conferring biocompatibility, colloidal stability and water dispersability, and the others comprise polyethylethylene hydrocarbons and perfluorinated polyethers. The resulting lateral structure is made of hexagonally packed fluorocarbon channels immersed in a continuous two-dimensional hydrocarbon bilayer and prevents the formation of concentric domains that are the default structure adopted by linear polymers. The assembly of these vesicles proceeds through metastable polygonal and faceted bilayer sheets, and their unprecedented and unique nanoscale periodicity makes them attractive as protocells with tunable membrane permeabilities and as drug-delivery vehicles.

One ink for all

Microcontact printing has emerged as one of the most versatile and simple methods of surface patterning. A flexible polymer stamp is 'inked' with appropriate molecules, which are then transferred to a surface simply by pressing stamp and substrate together. Bare metals, such as the 'coinage' metals gold, silver and palladium, as well as metals with oxide surfaces, such as silicon and aluminium, can be patterned in this way. However, one of the drawbacks compared with other patterning techniques is the fact that different 'inks' are needed for each type of substrate — coinage metals usually require a molecule such as an alkanethiol that contains sulphur to form a metal–sulphur bond, whereas metal oxides require acid-functionalized molecules. Burdinski et al. have dealt with this problem by simply formulating an ink that mixes the two types (Angew. Chem. Int. Edn doi:10.1002/anie.200600310; 2006). A blend of octadecanethiol and octadecylphosphonic acid in an appropriate ratio was found to produce clearly resolved patterns on both gold and aluminium, the molecules showing selectivity for their preferred substrate. The move should enable faster, simpler, and universal patterning of substrates using the technique.

Food engineering

Credit: Elsevier 2006

A new generation of engineers is turning to food to earn their living. Their task is to try and introduce additional bioactive compounds, such as vitamins, aminoacids and other dietary supplements (collectively referred to as nutriceuticals), in normal food products. To do this they need to include these compounds into microspheres that will release their load in the appropriate manner. Some compounds are best absorbed in the intestine rather than the stomach, for instance. Chen and Subirade at Université Laval, Canada, propose to use tiny whey granules (3–10 μm in diameter) included in a larger capsule (100 μm in diameter) of alginate gel through a cold-gelation method (Biomaterials 27, 4646–4654; 2006). The authors studied the release of a model bioactive compound from these systems in conditions that simulated the gastric and intestinal environment. Their results indicate that only a limited amount of the model bioactive compound will be released in the stomach, whereas its release in the intestine should be complete and rapid.