The assembly of synthetic, controllable molecular mechanical systems is one of the goals of nanotechnology. Protein-based molecular machines, often driven by an energy source such as ATP, are abundant in biology. It has been shown previously that branched motifs of DNA can provide components for the assembly of nanoscale objects, links and arrays. Here we show that such structures can also provide the basis for dynamic assemblies: switchable molecular machines. We have constructed a supramolecular device consisting of two rigid DNA 'double-crossover' (DX) molecules connected by 4.5 double-helical turns. One domain of each DX molecule is attached to the connecting helix. To effect switchable motion in this assembly, we use the transition between the B and Z forms of DNA. In conditions that favour B-DNA, the two unconnected domains of the DX molecules lie on the same side of the central helix. In Z-DNA-promoting conditions, however, these domains switch to opposite sides of the helix. This relative repositioning is detected by means of fluorescence resonance energy transfer spectroscopy, which measures the relative proximity of two dye molecules attached to the free ends of the DX molecules. The switching event induces atomic displacements of 20-60 A.

Soft dielectric materials typically exhibit poor heat transfer properties due to the dynamics of phonon transport, which constrain thermal conductivity (k) to decrease monotonically with decreasing elastic modulus (E). This thermal−mechanical trade-off is limiting for wearable computing, soft robotics, and other emerging applications that require materials with both high thermal conductivity and low mechanical stiffness. Here, we overcome this constraint with an electrically insulating composite that exhibits an unprecedented combination of metal-like thermal conductivity, an elastic compliance similar to soft biological tissue (Young's modulus < 100 kPa), and the capability to undergo extreme deformations (>600% strain). By incorporating liquid metal (LM) microdroplets into a soft elastomer, we achieve a ∼25× increase in thermal conductivity (4.7 ± 0.2 W·m ) under stress-free conditions and a ∼50× increase (9.8 ± 0.8 W·m) when strained. This exceptional combination of thermal and mechanical properties is enabled by a unique thermal−mechanical coupling that exploits the deformability of the LM inclusions to create thermally conductive pathways in situ. Moreover, these materials offer possibilities for passive heat exchange in stretchable electronics and bioinspired robotics, which we demonstrate through the rapid heat dissipation of an elastomer-mounted extreme high-power LED lamp and a swimming soft robot.liquid metal | thermal conductivity | soft materials | soft robotics | stretchable electronics

A two-dimensional DNA crystal has been designed and constructed from Holliday junction analogues that contain two helical domains twisted relative to each other. The Holliday junction is not an inherently rigid system, but it can be made less flexible if it is combined into a larger construct. We have fused four junctions into a rhombus-like molecule consisting of four six-turn helices, two on an upper layer and two on a lower layer; the branch points, which define vertices, are separated by four double helical turns each. Ligation of the rhombus-like motifs produces no cyclic species, when assayed by ligation-closure experiments. Self-assembly of the rhombuses in one dimension leads to a linear pattern. The rhombuses can be directed to self-assemble by hydrogen bonding into a two-dimensional periodic array, whose spacing is six turns in each direction. The expected spacing is seen when the array is observed by atomic force microscopy (AFM). Variation of the dimensions of the repeat unit from six turns × six turns to six turns × eight turns results in the expected increase in unit cell dimensions. Hence, it is possible to assemble periodic arrays with tunable cavities using these components. This system also provides the opportunity to measure directly the angles or torsion angles between the arms of branched junctions; here we measure the torsion angle between the helical domains of the Holliday junction analogue. We find by AFM that the torsion angle between helices is 63.5°, in good agreement with previous estimates.

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Modern society is experiencing an ever-increasing demand for energy to power a vast array of electrical and mechanical devices. As hydrocarbon resources dwindle, utilization of ample nuclear energy and abundant solar energy becomes more and more attractive. For 50 years, since the invention of the transistor, semiconductor devices that convert the energy of nuclear particles [1±5] or solar photons [6,7] to electric current have been investigated. However, conventional two-dimensional (2D) planar diode structures exhibit a number of inherent deficiencies that result in relatively low energy-conversion efficiencies. A unique three-dimensional (3D) porous silicon p±n diode has been developed to form the basis of a novel betavoltaic battery. Using tritium to demonstrate the proof-ofconcept, the 3D diode geometry demonstrated a tenfold enhancement of efficiency compared to that of the usual 2D betavoltaic device geometry. Given the similarity of the energyconversion physics for betavoltaic and photovoltaic devices, significant efficiency gains due to this 3D geometry might be expected for many types of photo detectors and solar cells. The 3D diode was constructed on porous silicon (PS), which consists of a network of pores formed by electrochemical anodization of silicon substrates. According to the pore size, PS is classified as microporous (£ 2 nm), mesoporous (2±50 nm), or macroporous (> 50 nm). Such porous morphologies define a very large internal surface area, [8,9] which retains most of the characteristics associated with planar surface geometries, particularly for macropores. [10,11] Numerous investigations have been done on the physical and chemical properties of this complex material. [8,9,12] Moreover, it has been demonstrated that PS components can be integrated into microelectronic circuits in order to construct practical devices. [13] To date, however, PS has only been used as an antireflection and surface-passivation layer [14,15] in photovoltaic devices. It is believed that this work reports the first construction of conformal p±n junctions in PS. PS diodes with a 3D p±n junction structure were created as illustrated schematically in Figure 1 (see Experimental for details). The continuous p±n junction can be visualized as a 2D ªsheetº that is deformed to produce a uniform p±n junction layer on every accessible surface of the pore space. The builtin voltage [16] of the diodes was estimated to be~0.8 V, assuming an n-dopant concentration of~5 10 18 cm ±3 and an abrupt p±n junction doping profile. The metallurgical junction was about 200 nm below the surface, and the estimated depletion width on the p-side of the junction was~1.4 lm. The efficacy of the pore anodization procedure was investigated by means of scanning electron microscopy (SEM