Inkjet printing is familiar as a method of printing text and images onto porous surfaces. In the last few years it has been used as a free-form fabrication method for building threedimensional parts and is being explored as a way of printing electrical and optical devices, especially where these involve organic components. Inkjet printers are also being used to produce arrays of proteins and nucleic acids. The need for a versatile inkjet technology for free-forming materials and for multilayer devices raises a number of materials problems that do not apply to conventional printing of images. Higher resolutions will be needed if organic transistors are to be printed. Also, it must be possible to print pinhole-free layers to avoid shorting of devices. Multiple layers must be printed such that they mix and react to form a single material or such that they form discrete unmixed layers. Printing on dense rather than porous substrates will be the norm. This article reviews the range of materials that has been printed and the issues that arise as the ink interacts with the substrate.

Hydrogels have applications in surgery and drug delivery, but are never considered alongside polymers and composites as materials for mechanical design. This is because synthetic hydrogels are in general very weak. In contrast, many biological gel composites, such as cartilage, are quite strong, and function as tough, shock‐absorbing structural solids. The recent development of strong hydrogels suggests that it may be possible to design new families of strong gels that would allow the design of soft biomimetic machines, which have not previously been possible.

Many organisms construct structural ceramic (biomineral) composites from seemingly mundane materials; cell-mediated processes control both the nucleation and growth of mineral and the development of composite microarchitecture. Living systems fabricate biocomposites by: (i) confining biomineralization within specific subunit compartments; (ii) producing a specific mineral with defined crystal size and orientation; and (iii) packaging many incremental units together in a moving front process to form fully densified, macroscopic structures. By adapting biological principles, materials scientists are attempting to produce novel materials. To date, neither the elegance of the biomineral assembly mechanisms nor the intricate composite microarchitectures have been duplicated by nonbiological processing. However, substantial progress has been made in the understanding of how biomineralization occurs, and the first steps are now being taken to exploit the basic principles involved.

news and views 210 NATURE | VOL 399 | 20 MAY 1999 | www.nature.com than clarified by these important structures. First, how does binding of Ran•GTP to exportins increase -rather than disruptsubstrate binding ( Fig. 1)? Second, how can an importin Ȋ molecule lacking the conserved IBB domain still be active in Ran•GTPdependent nuclear import 12,13 ? Finally, given the limited conservation of the outer surfaces of Kap ȋ2 and, in particular, importin ȋ, what mediates the specific interactions required for translocation through the nuclear pore complex? Time will tell. Luckily, the field of nucleocytoplasmic transport moves considerably faster than a snail.

riers in the 6T wire. [3] Holes are trapped by defects in the crystal, which reduces the number of mobile carriers and hinders the movement of untrapped positive-charge carriers. Charge trapping can be reversed, however, by applying a positive-gate voltage (backbiasing) to force trapped holes out of trap states via electrostatic repulsion. As seen from the trace on the right-hand side of Figure 3B, hole current was temporarily restored to nearly 100 % of its original value by applying a sufficient backbias. However, trapping immediately resumed when the normal operating voltages were re-established.In conclusion, we have shown that we can use AFM nanoshaving to fabricate functional semiconducting wires from the molecular semiconductor sexithiophene. Because there is nothing particularly special about sexithiophene per se, there is no reason why the method cannot be applied to other organic semiconductors, provided they are soft enough for an AFM tip to carve. We have shown that wires carved to 300 and 70 nm widths ( Figures 1B and 1D, respectively) are obtainable, but based on our work and that of others, [5a] we believe that it would be possible to carve wires as thin as 20 nm. Through photoconductivity and temperature-dependent transport measurements, we have demonstrated that the electrical properties of the wires are consistent with what we have observed for single grains of 6T. Finally, we have investigated time-dependent transport in the wires, and our data were consistent with our previous assertion [3] that 6T undergoes reversible trapping of positive-charge carriers.The ability to create long, narrow wires of organic semiconductors can be further exploited in transport studies. For example, we estimate that using the gate electrode we can controllably induce 100±500 positive charges in a 20 nm wide, 500 nm long 6T wire. Fabrication of thin wires thus provides opportunities to observe discrete events such as charge trapping or Coulomb effects in organic semiconductors, as has been reported for carbon nanotubes. [13] Such studies will be the subject of future work.