Enhanced Far-Field Thermal Radiation through a Polaritonic Waveguide.

We experimentally demonstrate the enhancement of the far-field thermal radiation between two nonabsorbent Si microplates coated with energy-absorbent silicon dioxide (SiO_{2}) nanolayers supporting the propagation of surface phonon polaritons. By measuring the radiative thermal conductance between two coated Si plates, we find that its values are twice those obtained without the SiO_{2} coating. This twofold increase results from the hybridization of polaritons with guided modes inside Si and is well predicted by fluctuational electrodynamics and an analytical model based on a two-dimensional density of polariton states. These findings could be applied to thermal management in microelectronics, silicon photonics, energy conversion, atmospheric sciences, and astrophysics.

Anisotropy Reversal of Thermal Conductivity in Silicon Nanowire Networks Driven by Quasi-Ballistic Phonon Transport.

Nanostructured semiconductors promise functional thermal management for microelectronics and thermoelectrics through a rich design capability. However, experimental studies on anisotropic in-plane thermal conduction remain limited, despite the demand for directional heat dissipation. Here, inspired by an oriental wave pattern, a periodic network of bent wires, we investigate anisotropic in-plane thermal conduction in nanoscale silicon phononic crystals with the thermally dead volume. We observed the anisotropy reversal of the material thermal conductivity from 1.2 at 300 K to 0.8 at 4 K, with the reversal temperature of 80 K mediated by the transition from a diffusive to a quasi-ballistic regime. Our Monte Carlo simulations revealed that the backflow of the directional phonons induces the anisotropy reversal, showing that the quasi-ballistic phonon transport introduces preferential thermal conduction channels with anomalous temperature dependence. Accordingly, the anisotropy of the effective thermal conductivity varied from 2.7 to 5.0 in the range of 4-300 K, indicating an anisotropic heat manipulation capability. Our findings demonstrate that the design of nanowire networks enables the directional thermal management of electronic devices.

Asymmetry of tensile versus compressive elasticity and permeability contributes to the regulation of exchanges in collagen gels.

The Starling principle describes exchanges between blood and tissues based on the balance of hydrostatic and osmotic flows. However, the permeation properties of the main constituent of tissues, namely, collagen, in response to the stress exerted by blood pressure remain poorly characterized. Here, we develop an instrument to determine the elasticity and permeability of collagen gels under tensile and compressive stress based on measuring the temporal change in pressure in an air cavity sealed at the outlet of a collagen slab. Data analysis with an analytical model reveals a drop in the permeability and enhanced strain stiffening of native collagen gels under compression versus tension, both effects being essentially lost after chemical cross-linking. Furthermore, we report the control of the permeability of native collagen gels using sinusoidal fluid injection, an effect explained by the asymmetric response in tension and compression. We lastly suggest that blood-associated pulsations could contribute to exchanges within tissues.

Electrochemical Shot Noise of a Redox Monolayer.

Redox monolayers are the base for a wide variety of devices including high-frequency molecular diodes or biomolecular sensors. We introduce a formalism to describe the electrochemical shot noise of such a monolayer, confirmed experimentally at room temperature in liquid. The proposed method, carried out at equilibrium, avoids parasitic capacitance, increases the sensitivity, and allows us to obtain quantitative information such as the electronic coupling (or standard electron transfer rates), its dispersion, and the number of molecules. Unlike in solid-state physics, the homogeneity in energy levels and transfer rates in the monolayer yields a Lorentzian spectrum. This first step for shot noise studies in molecular electrochemical systems opens perspectives for quantum transport studies in a liquid environment at room temperature as well as highly sensitive measurements for bioelectrochemical sensors.

Silicon chambers for enhanced incubation and imaging of microfluidic droplets.

Droplet microfluidics has become a powerful tool in life sciences, underlying digital assays, single-cell sequencing or directed evolution, and it is making foray in physical sciences as well. Imaging and incubation of droplets are crucial, yet they are encumbered by the poor optical, thermal and mechanical properties of PDMS, a material commonly used in microfluidics labs. Here we show that Si is an ideal material for droplet chambers. Si chambers pack droplets in a crystalline and immobile monolayer, are immune to evaporation or sagging, boost the number of collected photons, and tightly control the temperature field sensed by droplets. We use the mechanical and optical benefits of Si chambers to image ≈1 million of droplets from a multiplexed digital assay - with an acquisition rate similar to the best in-line methods. Lastly, we demonstrate their applicability with a demanding assay that maps the thermal dependence of Michaelis-Menten constants with an array of ≈150 000 droplets. The design of the Si chambers is streamlined to avoid complicated fabrication and improve reproducibility, which makes Si a complementary material to PDMS in the toolbox of droplet microfluidics.

Impact of nanopillars on phonon dispersion and thermal conductivity of silicon membranes.

The performance of silicon-based thermoelectric energy generators is limited by the high thermal conductivity of silicon. Theoretical works have long proposed reducing the thermal conductivity by resonant phonon modes in nanopillars placed on the surface of silicon films. However, these predictions have never been confirmed due to the difficulty in the nanofabrication and measurements of such nanoscale systems. In this work, we report on the fabrication and measurements of silicon films with nanopillars as small as 12 nm in diameter. Our Brillouin light scattering spectroscopy experiments revealed that nanopillars indeed host resonant phonon modes. Yet, our thermal measurements using the micro time-domain thermoreflectance technique showed only a statistically insignificant difference between the thermal properties of silicon membranes with and without nanopillars. Results of this work contrast with the predictions of a substantial reduction in the thermal conductivity due to nanopillars and suggest refining the simulations to account for realistic experimental conditions.

Noise suppression beyond the thermal limit with nanotransistor biosensors.

Transistor biosensors are mass-fabrication-compatible devices of interest for point of care diagnosis as well as molecular interaction studies. While the actual transistor gates in processors reach the sub-10 nm range for optimum integration and power consumption, studies on design rules for the signal-to-noise ratio (S/N) optimization in transistor-based biosensors have been so far restricted to 1 µm2 device gate area, a range where the discrete nature of the defects can be neglected. In this study, which combines experiments and theoretical analysis at both numerical and analytical levels, we extend such investigation to the nanometer range and highlight the effect of doping type as well as the noise suppression opportunities offered at this scale. In particular, we show that, when a single trap is active near the conductive channel, the noise can be suppressed even beyond the thermal limit by monitoring the trap occupancy probability in an approach analog to the stochastic resonance effect used in biological systems.

High Surface Phonon-Polariton in-Plane Thermal Conductance along Coupled Films.

Surface phonon-polaritons (SPhPs) are evanescent electromagnetic waves that can propagate distances orders of magnitude longer than the typical mean free paths of phonons and electrons. Therefore, they are expected to be powerful heat carriers capable of significantly enhancing the in-plane thermal conductance of polar nanostructures. In this work, we show that a SiO 2 /Si (10 µ m thick)/SiO 2 layered structure efficiently enhances the SPhP heat transport, such that its in-plane thermal conductance is ten times higher than the corresponding one of a single SiO 2 film, due to the coupling of SPhPs propagating along both of its polar SiO 2 nanolayers. The obtained results thus show that the proposed three-layer structure can outperform the in-plane thermal performance of a single suspended film while improving significantly its mechanical stability.

Direct measurement of the mechanical properties of a chromatin analog and the epigenetic effects of para-sulphonato-calix[4]arene.

By means of Silicon Nano Tweezers (SNTs) the effects on the mechanical properties of λ-phage DNA during interaction with calf thymus nucleosome to form an artificial chromatin analog were measured. At a concentration of 100 nM, a nucleosome solution induced a strong stiffening effect on DNA (1.1 N m-1). This can be compared to the effects of the histone proteins, H1, H2A, H3 where no changes in the mechanical properties of DNA were observed and the complex of the H3/H4 proteins where a smaller increase in the stiffness is observed (0.2 N m-1). Para-sulphonato-calix[4]arene, SC4, known for epigenetic activity by interacting specifically with the lysine groups of histone proteins, was studied for its effect on an artificial chromatin. Using a microfluidic SNT device, SC4 was titrated against the artificial chromatin, at a concentration of 1 mM in SC4 a considerable increase in stiffness, 15 N m-1, was observed. Simultaneously optical microscopy showed a physical change in the DNA structure between the tips of the SNT device. Electronic and Atomic Force microscopy confirmed this structural re-arrangement. Negative control experiments confirmed that these mechanical and physical effects were induced neither by the acidity of SC4 nor through nonspecific interactions of SC4 on DNA.

Current driven magnetic actuation of a MEMS silicon beam in a transmission electron microscope.

Micro-Electro-Mechanical-System (MEMS) devices associated to Transmission Electron Microscopes (TEM) have demonstrated their high potential for atomic resolution imaging of specimen while applying stress for mechanical testing. This paper introduces a novel actuation principle for the MEMS device in TEM relying on the internal magnetic field of the TEM and current flow through the device. The actuation principle is experimentally demonstrated in TEM and entirely modeled in the case of a silicon beam. The model is validated through static and dynamic experimental studies. The thermal side-effect of current flow is taken into account. The major advantages of the proposed magnetic actuation principle are the bidirectional control of the displacement of the device, the intrinsic linear displacement of the device with applied current and the potential milliNewton (mN) range force generation.

Elucidating the mechanism of the considerable mechanical stiffening of DNA induced by the couple Zn2+/Calix[4]arene-1,3-O-diphosphorous acid.

The couple Calix[4]arene-1,3-O-diphosphorous acid (C4diP) and zinc ions (Zn2+) acts as a synergistic DNA binder. Silicon NanoTweezer (SNT) measurements show an increase in the mechanical stiffness of DNA bundles by a factor of >150, at Zn2+ to C4diP ratios above 8, as compared to Zinc alone whereas C4diP alone decreases the stiffness of DNA. Electroanalytical measurements using 3D printed devices demonstrate a progression of events in the assembly of C4diP on DNA promoted by zinc ions. A mechanism at the molecular level can be deduced in which C4diP initially coordinates to DNA by phosphate-phosphate hydrogen bonds or in the presence of Zn2+ by Zn2+ bridging coordination of the phosphate groups. Then, at high ratios of Zn2+ to C4diP, interdigitated dimerization of C4diP is followed by cross coordination of DNA strands through Zn2+/C4diP inter-strand interaction. The sum of these interactions leads to strong stiffening of the DNA bundles and increased inter-strand binding.

Pick-and-Place Assembly of Single Microtubules.

Intracellular transport is affected by the filament network in the densely packed cytoplasm. Biophysical studies focusing on intracellular transport based on microtubule-kinesin system frequently use in vitro motility assays, which are performed either on individual microtubules or on random (or simple) microtubule networks. Assembling intricate networks with high flexibility requires the manipulation of 25 nm diameter microtubules individually, which can be achieved through the use of pick-and-place assembly. Although widely used to assemble tiny objects, pick-and-place is not a common practice for the manipulation of biological materials. Using the high-level handling capabilities of microelectromechanical systems (MEMS) technology, tweezers are designed and fabricated to pick and place single microtubule filaments. Repeated picking and placing cycles provide a multilayered and multidirectional microtubule network even for different surface topographies. On-demand assembly of microtubules forms crossings at desired angles for biophysical studies as well as complex networks that can be used as nanotransport systems.

A rapid and practical technique for real-time monitoring of biomolecular interactions using mechanical responses of macromolecules.

Monitoring biological reactions using the mechanical response of macromolecules is an alternative approach to immunoassays for providing real-time information about the underlying molecular mechanisms. Although force spectroscopy techniques, e.g. AFM and optical tweezers, perform precise molecular measurements at the single molecule level, sophisticated operation prevent their intensive use for systematic biosensing. Exploiting the biomechanical assay concept, we used micro-electro mechanical systems (MEMS) to develop a rapid platform for monitoring bio/chemical interactions of bio macromolecules, e.g. DNA, using their mechanical properties. The MEMS device provided real-time monitoring of reaction dynamics without any surface or molecular modifications. A microfluidic device with a side opening was fabricated for the optimal performance of the MEMS device to operate at the air-liquid interface for performing bioassays in liquid while actuating/sensing in air. The minimal immersion of the MEMS device in the channel provided long-term measurement stability (>10 h). Importantly, the method allowed monitoring effects of multiple solutions on the same macromolecule bundle (demonstrated with DNA bundles) without compromising the reproducibility. We monitored two different types of effects on the mechanical responses of DNA bundles (stiffness and viscous losses) exposed to pH changes (2.1 to 4.8) and different Ag(+) concentrations (1 µM to 0.1 M).

Direct electrical and mechanical characterization of in situ generated DNA between the tips of silicon nanotweezers (SNT).

Previously, we reported the application of micromachined silicon nanotweezers (SNT) integrated with a comb-drive actuator and capacitive sensors for capturing and mechanical characterization of DNA bundles. Here, we demonstrate direct DNA amplification on such a MEMS structure with subsequent electrical and mechanical characterization of a single stranded DNA (ssDNA) bundle generated between the tips of SNT via isothermal rolling circle amplification (RCA) and dielectrophoresis (DEP). An in situ generated ssDNA bundle was visualized and evaluated via electrical conductivity (I-V) and mechanical frequency response measurements. Colloidal gold nanoparticles significantly enhanced (P < 0.01) the electrical properties of thin ssDNA bundles. The proposed technology allows direct in situ synthesis of DNA with a predefined sequence on the tips of a MEMS sensor device, such as SNT, followed by direct DNA electrical and mechanical characterization. In addition, our data provides a "proof-of-principle" for the feasibility of the on-chip label free DNA detection device that can be used for a variety of biomedical applications focused on sequence specific DNA detection.

Real-time mechanical characterization of DNA degradation under therapeutic X-rays and its theoretical modeling.

The killing of tumor cells by ionizing radiation beams in cancer radiotherapy is currently based on a rather empirical understanding of the basic mechanisms and effectiveness of DNA damage by radiation. By contrast, the mechanical behaviour of DNA encompassing sequence sensitivity and elastic transitions to plastic responses is much better understood. A novel approach is proposed here based on a micromechanical Silicon Nanotweezers device. This instrument allows the detailed biomechanical characterization of a DNA bundle exposed to an ionizing radiation beam delivered here by a therapeutic linear particle accelerator (LINAC). The micromechanical device endures the harsh environment of radiation beams and still retains molecular-level detection accuracy. In this study, the first real-time observation of DNA damage by ionizing radiation is demonstrated. The DNA bundle degradation is detected by the micromechanical device as a reduction of the bundle stiffness, and a theoretical model provides an interpretation of the results. These first real-time observations pave the way for both fundamental and clinical studies of DNA degradation mechanisms under ionizing radiation for improved tumor treatment.

Ratchet effect study in Si/SiGe heterostructures in the presence of asymmetrical antidots for different polarizations of microwaves.

In this work, we studied the photovoltage response of an antidot lattice to microwave radiation for different antidot parameters. The study was carried out in a Si/SiGe heterostructure by illuminating the antidot lattice with linearly polarized microwaves and recording the polarity of induced photovoltage for different angles of incidence. Our study revealed that with increased antidot density and etching depth, the polarity of induced photovoltage changed when the angle of incidence was rotated 90 degrees. In samples with large antidot density and/or a deeply etched antidot lattice, scattering was dominated by electron interaction with the asymmetrical potential created by semicircular antidots. The strong electron-electron interaction prevailed in other cases. Our study provides insight into the mechanism of interaction between microwaves and electrons in an antidot lattice, which is the key for developing an innovative ratchet-based device. Moreover, we present an original and fundamental example of antidot lattice etching through the use of a two-dimensional electron gas. This system deals with a hole lattice instead of an electron depletion in the antidot lattice region.

A programmable and reconfigurable microfluidic chip.

This article reports an original concept enabling the rapid fabrication of continuous-flow microfluidic chips with a programmable and reconfigurable geometry. The concept is based on a digital microfluidic platform featuring an array of individually addressable electrodes. A selection of electrodes is switched on sequentially to create a de-ionized (DI) water finger specific pattern, while the surrounding medium consists of liquid-phase paraffin. The water displacement is induced by both electrowetting on dielectric and liquid dielectrophoresis phenomena. Once the targeted DI water pattern is obtained, the chip temperature is lowered by turning on an integrated thermoelectric cooler, forming channel structures made of solidified paraffin with edges delimitated by the DI water pattern. As a result, the chip can be used afterwards to conduct in-flow continuous microfluidic experiments. This process is resettable and reversible by heating up the chip to melt the paraffin and reconfigure the microchannel design on demand, offering the advantages of cost, adaptability, and robustness. This paper reports experimental results describing the overall concept, which is illustrated with typical and basic fluidic geometries.

Integrated MEMS platform with silicon nanotweezers and open microfluidic device for real-time and routine biomechanical probing on molecules and cells.

This paper describes an integrated biomechanical platform for real-time molecular or cellular assays. This platform is composed of silicon nanotweezers to manipulate the biological samples and an open microfluidic to handle solution and reactive agents. The tweezers are fabricated by standard Silicon-On-Insulator based micromachining processes (2 masks +1 additional mask for special tips) and integrate actuator, trapping tips and sensor. The microfluidic device is produced from common polydimethylsiloxane (PDMS) micromolding and integrates active valves for controlling the biological medium. Combining both technologies, a versatile experimental setup, built up in an enclosed space (< 10 cm(3)), enables direct interrogation of molecules or cells in solution. The silicon nanotweezers sense slight biological modifications of the trapped molecules or cell by monitoring the mechanical resonance response, which keeps a high Q factor (over 20) in liquid. Biomolecular assays (molecule trapping and enzymatic reaction kinetics) as well as characterizations of cells are reported here. The system provide molecular level resolution and is sensitive enough to capture cell biomechano-transduction activities. Moreover as the system is handy, it may be an easy, fast and quantitative alternative to existing methods.

Silicon Nanotweezers with a microfluidic cavity for the real time characterization of DNA damage under therapeutic radiation beams.

We report the biomechanical characterization of λ-DNA bundle exposed to a therapeutic radiation beam by silicon Nanotweezers. The micromechanical device endures the harsh environment of radiation beams, and still retains molecular-level detection accuracy. The real-time DNA bundle degradation is observed in terms of biomechanical stiffness and viscosity reduction, both in air and in solution. These results pave the way for both fundamental and clinical studies of DNA degradation mechanisms under ionizing radiation for improved tumor treatment.