Advanced Materials

Conjugated polymer actuators developed over the last decade have now reached the early stages of commercialization, particularly for use in biomedical devices, such as the blood vessel connector shown in the Figure. This article reviews the motivation for using this class of actuator, the types of devices that have been fabricated, and some of the biomedical applications that are being developed. Recommendations are also presented for future work.

Substituted hexabenzocoronenes (HBCs) form films with supramolecularly ordered columnar stacks that are uniaxially oriented onto poly(tetrafluoroethylene) alignment layers (see Figure). In field-effect transistor (FET) tests, mobilities of up to 10^–3 cm² V^–1 s^–1 and high on–off ratios of more than 10⁴ were derived for these aligned HBC films, characteristics superior to FETs prepared from isotropic HBC layers.

A novel inorganic wrapping technique has been used to create stable, three-dimensional spherical assemblies of gold nanoparticles. Uniform, crack-free TiO₂ layers of 5 nm thickness could be wrapped around the nanoparticle assemblies using a sol–gel technique. Upon addition of a suitable surfactant these particles are stable in organic media (see Figure: with (right) and without (left) suitable surfactant).

A broadly wavelength tunable polyferrocenylsilane–silica composite photonic crystal with a fast response is presented. Polychromic variation in the reflection wavelength is effected via the swelling by solvent of a crosslinked redox-active metallopolymer gel network incorporated in the voids of a planarized silica colloidal crystal film. The Figure demonstrates the fast mechanical response of the composite photonic crystals.

For biomedical applications, zirconia-toughened alumina ceramics (see Figure) would be very appropriate materials, provided that the zirconia content is kept below the percolation threshold (upper limit of 16 vol.-% 3Y-TZP inside an alumina matrix). That the concept of a percolation threshold is relevant when talking about aging degradation, is demonstrated by comparing specular IR reflectance measurements with aging experiments.

Modification of the mesoporous surface of SBA-15 with platinum has been successfully carried out by means of γ-radiation treatment. This novel treatment allows the selective growth of platinum in the SBA-15 micropores (see Figure). This is in contrast to traditional temperature treatment, in which platinum particles or rods also grow in the mesopores.

A capillary surface instability caused by a high temperature gradient is used to replicate sub-micrometer patterns (see Figure for an example). As opposed to convection effects, the film instability is driven by the diffusion of heat across the polymer film. By lateral modulation of the temperature gradient, this instability can be harnessed as a lithographic technique.

A simple one-step process to fabricate three- dimensional (3D) microstructures is reported. Based on two-photon excited-state chemistry, a chemically amplified positive tone photoresist has been developed and the lithographic process investigated. The results, as indicated by the fabricated material (see Figure) suggest that positive tone two-photon 3D microfabrication may be a valuable route to precision microfluidic and microoptical devices.

A group of iron-doped novel oxides of cerium, terbium, and praseodymium are reported to produce hydrogen at relatively low temperatures through a cycling two-step reaction process without catalyst. Iron doping in the oxides not only stimulates surface activity, but also lowers the reduction temperature of water vapor (to ∼ 375 °C) while preserving efficient hydrogen production. The two-step reactions eliminate the problem of catalyst deactivation due to coke. Thus the cycle can be carried out continuously without replacing oxide.

Multiple quantum well nanorods have been fabricated via heteroepitaxial growth of ZnO and ZnMgO (see Figure and also cover). Simple yet accurate thickness control allows the realization of nanosized well structures in individual nanorods that are tunable through the effects of quantum confinement. This approach should be readily extendible to other heteroepitaxial semiconductor nanorods.

Nanoporous metallic nickel colloids (∼8.3 nm, see Figure) are produced by the thermal decomposition of nickel-doped poly(vinyl alcohol) films. A small amount of carbon remaining on the surface protects the particles against further degradation. Adsorption isotherms indicate the colloids have both mesopores and micropores.

Uniform thin films of gold nanoparticles have been fabricated on gold substrates using a layer-by-layer self-assembly approach (see Figure). The optical properties of these nanoparticle films were characterized using variable-angle spectroscopic ellipsometry, and the data modeled using a Lorentz oscillator dispersion model. Both the thickness and refractive indices were evaluated simultaneously over a wide wavelength range.

The third-order nonlinear optical property χ⁽³⁾_I of a film of single-walled carbon nanotubes was studied in the infrared region. The film combines a high χ⁽³⁾_I (–0.85 × 10^–7 esu at resonant wavelength) with a rapid transmittance recovery (from differential transmission spectra, see Figure), making it suitable for highly efficient and ultrafast devices for optical telecommunications.

Sol–gel precursors of poly[bis(methoxyethoxyethoxy)phosphazene] have been synthesized by incorporating triethoxysilane in the polymer side groups. Studies of the water-swelling (see Figure, dry left, water-swollen, right) behavior and dye-release of the hybrid network indicate that the polyphosphazene–silicate network has potential as a biomaterial and for use in both passive and responsive membranes.

Low-pressure cold welding has been used to fabricate 200 μm diameter organic light-emitting devices (OLEDs) (see Figure). The contacts are made by transferring a metal film from a patterned, soft elastomeric stamp onto unpatterned organic and metal layers on a substrate. The process does not adversely affect the OLED performance when compared with those fabricated by shadow mask patterning.