Recent research in the field of nanometre-scale electronics has focused on two fundamental issues: the operating principles of small-scale devices, and schemes that lead to their realization and eventual integration into useful circuits. Experimental studies on molecular to submicrometre quantum dots and on the electrical transport in carbon nanotubes have confirmed theoretical predictions of an increasing role for charging effects as the device size diminishes. Nevertheless, the construction of nanometre-scale circuits from such devices remains problematic, largely owing to the difficulties of achieving inter-element wiring and electrical interfacing to macroscopic electrodes. The use of molecular recognition processes and the self-assembly of molecules into supramolecular structures might help overcome these difficulties. In this context, DNA has the appropriate molecular-recognition and mechanical properties, but poor electrical characteristics prevent its direct use in electrical circuits. Here we describe a two-step procedure that may allow the application of DNA to the construction of functional circuits. In our scheme, hybridization of the DNA molecule with surface-bound oligonucleotides is first used to stretch it between two gold electrodes; the DNA molecule is then used as a template for the vectorial growth of a 12 microm long, 100 nm wide conductive silver wire. The experiment confirms that the recognition capabilities of DNA can be exploited for the targeted attachment of functional wires.

The combination of their electronic properties and dimensions makes carbon nanotubes ideal building blocks for molecular electronics. However, the advancement of carbon nanotube-based electronics requires assembly strategies that allow their precise localization and interconnection. Using a scheme based on recognition between molecular building blocks, we report the realization of a self-assembled carbon nanotube field-effect transistor operating at room temperature. A DNA scaffold molecule provides the address for precise localization of a semiconducting single-wall carbon nanotube as well as the template for the extended metallic wires contacting it.Individual single-wall carbon nanotubes (SWNT) have been used to realize molecular-scale electronic devices such as singleelectron (1) and field-effect transistors (FET) (2). Several SWNT-based devices have been successfully integrated into logic circuits (3) and transistor arrays (4 ). However, the difficulty in precise localization and interconnection of nanotubes impedes further progress toward larger-scale integrated circuits.Self-assembly based on molecular recognition provides a promising approach for constructing complex architectures from molecular building blocks, such as SWNTs, bypassing the need for precise nanofabrication and mechanical manipulations (5). Biology, with its inherent self-assembly capabilities (6-13), is particularly attractive for this task. Biological recognition has been imparted to carbon nanotubes (14-18), but their self-assembly into functional devices and circuits has not yet been demonstrated. We present a framework for self-assembly of carbon nanotube-based electronics using DNA and homologous genetic recombination. A semiconducting SWNT was localized at a desired address on a DNA scaffold molecule using homologous recombination by the RecA protein from Escherichia coli bacteria (19). DNA metallization, with the RecA doubling as a sequence-specific resist (13), led to the formation of extended conductive wires that electrically contact the SWNT. The conduction through the SWNT was controlled by a voltage applied to the substrate supporting the structure.The SWNT-FET ( Fig. 1) was assembled via a three-strand homologous recombination reaction between a long double-stranded DNA (dsDNA) molecule serving as a scaffold and a short, auxiliary single-stranded DNA (ssDNA) (20). The assembly process was guided by the information encoded in these DNA molecules. The short ssDNA molecule was synthesized so that its sequence is identical to the dsDNA at the designated location of the FET. RecA proteins were first polymerized on the auxiliary ssDNA molecules to form nucleoprotein filaments (Fig. 1, step i), which were then mixed with the scaffold dsDNA molecules. A nucleoprotein filament bound a dsDNA molecule according to the sequence homology between the ssDNA and the designated address on the dsDNA (Fig. 1, step ii). The RecA later helped localize a SWNT at that address and protect the covered DNA segment against metallization (13). Figure ...

Recent advances in the realization of individual molecular-scale electronic devices emphasize the need for novel tools and concepts capable of assembling such devices into large-scale functional circuits. We demonstrated sequence-specific molecular lithography on substrate DNA molecules by harnessing homologous recombination by RecA protein. In a sequence-specific manner, we patterned the coating of DNA with metal, localized labeled molecular objects and grew metal islands on specific sites along the DNA substrate, and generated molecularly accurate stable DNA junctions for patterning the DNA substrate connectivity. In our molecular lithography, the information encoded in the DNA molecules replaces the masks used in conventional microelectronics, and the RecA protein serves as the resist. The molecular lithography works with high resolution over a broad range of length scales from nanometers to many micrometers.

Animal morphogenesis arises from the complex interplay between multiple mechanical and biochemical processes with mutual feedback. Developing an effective, coarse-grained description of morphogenesis is essential for understanding how these processes are coordinated across scales to form robust, functional outcomes. Here we show that the nematic order of the supra-cellular actin fibers in regenerating Hydra defines a slowlyvarying field, whose dynamics provide an effective description of the morphogenesis process. We show that topological defects in this field, which are long-lived yet display rich dynamics, act as organization centers with morphological features developing at defect sites. These observations suggest that the nematic orientation field can be considered a "mechanical morphogen" whose dynamics, in conjugation with various biochemical and mechanical signaling processes, result in the robust emergence of functional patterns during morphogenesis.Animal morphogenesis involves multiple mechanical and biochemical processes, spanning several orders of magnitude in space and time, from local dynamics at the molecular level to global, organism-scale morphology. How these numerous processes are coordinated and integrated across scales to form robust, functional outcomes remains an outstanding question [1][2][3][4] .Developing an effective, coarse-grained description of morphogenesis can provide essential insights towards addressing this important challenge. Here, we focus on whole-body regeneration in Hydra, a small fresh-water predatory animal, and provide an effective description of the morphogenesis process that is based on the dynamic organization of the supra-cellular actin fibers in regenerating tissues 5,6 .

Understanding how mechanics complement bio-signaling in defining patterns during morphogenesis is an outstanding challenge. Here, we utilize the multicellular polyp Hydra to investigate the role of the actomyosin cytoskeleton in morphogenesis. We find that the supra-cellular actin fiber organization is inherited from the parent Hydra and determines the body axis in regenerating tissue segments. This form of structural inheritance is non-trivial because of the tissue folding and dynamic actin reorganization involved. We further show that the emergence of multiple body axes can be traced to discrepancies in actin fiber alignment at early stages of the regeneration process. Mechanical constraints induced by anchoring regenerating Hydra on stiff wires suppressed the emergence of multiple body axes, highlighting the importance of mechanical feedbacks in defining and stabilizing the body axis. Together, these results constitute an important step toward the development of an integrated view of morphogenesis that incorporates mechanics.