Inductive tuning of Fano-resonant metasurfaces using plasmonic response of graphene in the mid-infrared.

Graphene is widely known for its anomalously strong broadband optical absorptivity of 2.3% that enables seeing its single-atom layer with the naked eye. However, in the mid-infrared part of the spectrum graphene represents a quintessential lossless zero-volume plasmonic material. We experimentally demonstrate that, when integrated with Fano-resonant plasmonic metasurfaces, single-layer graphene (SLG) can be used to tune their mid-infrared optical response. SLG's plasmonic response is shown to induce large blue shifts of the metasurface's resonance without reducing its spectral sharpness. This effect is explained by a generalized perturbation theory of SLG-metamaterial interaction that accounts for two unique properties of the SLG that set it apart from all other plasmonic materials: its anisotropic response and zero volume. These results pave the way to using gated SLG as a platform for dynamical spectral tuning of infrared metamaterials and metasurfaces.

A Three-dimensional Polymer Scaffolding Material Exhibiting a Zero Poisson's Ratio.

Poisson's ratio describes the degree to which a material contracts (expands) transversally when axially strained. A material with a zero Poisson's ratio does not transversally deform in response to an axial strain (stretching). In tissue engineering applications, scaffolding having a zero Poisson's ratio (ZPR) may be more suitable for emulating the behavior of native tissues and accommodating and transmitting forces to the host tissue site during wound healing (or tissue regrowth). For example, scaffolding with a zero Poisson's ratio may be beneficial in the engineering of cartilage, ligament, corneal, and brain tissues, which are known to possess Poisson's ratios of nearly zero. Here, we report a 3D biomaterial constructed from polyethylene glycol (PEG) exhibiting in-plane Poisson's ratios of zero for large values of axial strain. We use digital micro-mirror device projection printing (DMD-PP) to create single- and double-layer scaffolds composed of semi re-entrant pores whose arrangement and deformation mechanisms contribute the zero Poisson's ratio. Strain experiments prove the zero Poisson's behavior of the scaffolds and that the addition of layers does not change the Poisson's ratio. Human mesenchymal stem cells (hMSCs) cultured on biomaterials with zero Poisson's ratio demonstrate the feasibility of utilizing these novel materials for biological applications which require little to no transverse deformations resulting from axial strains. Techniques used in this work allow Poisson's ratio to be both scale-independent and independent of the choice of strut material for strains in the elastic regime, and therefore ZPR behavior can be imparted to a variety of photocurable biomaterial.

Three-Dimensional Polymer Constructs Exhibiting a Tunable Negative Poisson's Ratio.

Young's modulus and Poisson's ratio of a porous polymeric construct (scaffold) quantitatively describe how it supports and transmits external stresses to its surroundings. While Young's modulus is always non-negative and highly tunable in magnitude, Poisson's ratio can, indeed, take on negative values despite the fact that it is non-negative for virtually every naturally occurring and artificial material. In some applications, a construct having a tunable negative Poisson's ratio (an auxetic construct) may be more suitable for supporting the external forces imposed upon it by its environment. Here, three-dimensional polyethylene glycol scaffolds with tunable negative Poisson's ratios are fabricated. Digital micromirror device projection printing (DMD-PP) is used to print single-layer constructs composed of cellular structures (pores) with special geometries, arrangements, and deformation mechanisms. The presence of the unit-cellular structures tunes the magnitude and polarity (positive or negative) of Poisson's ratio. Multilayer constructs are fabricated with DMD-PP by stacking the single-layer constructs with alternating layers of vertical connecting posts. The Poisson's ratios of the single- and multilayer constructs are determined from strain experiments, which show (1) that the Poisson's ratios of the constructs are accurately predicted by analytical deformation models and (2) that no slipping occurrs between layers in the multilayer constructs and the addition of new layers does not affect Poisson's ratio.

Suppression of long-range collective effects in meta-surfaces formed by plasmonic antenna pairs.

The collective effects in a periodic array of plasmonic double-antenna meta-molecules are studied. We experimentally observe that the collective behavior in this structure substantially differs from the one observed in their single-antenna counterparts. This behavior is explained using an analytical dipole model. We find that in the double-antenna case the effective dipole-dipole interaction is significantly modified and the transverse long-range interaction is suppressed, giving rise to the disappearance of Wood's anomalies. Numerical calculations also show that such suppression of long-range interaction results in an anomalous spatial dispersion of the electric-dipolar mode, making it insensitive to the angle of incidence. In contrast, the quadrupolar mode of the antenna pair experiences strong spatial dispersion. These results show that collective effects in plasmonic metamaterials are very sensitive to the design and topology of meta-molecules. Our findings envision the possibility of suppressing the spatial dispersion effects to weaken the dependence of the metamaterials' response on the incidence angle.

A Continuous-Flow Polymerase Chain Reaction Microchip With Regional Velocity Control.

This paper presents a continuous-flow polymerase chain reaction (PCR) microchip with a serpentine microchannel of varying width for "regional velocity control." Varying the channel width by incorporating expanding and contracting conduits made it possible to control DNA sample velocities for the optimization of the exposure times of the sample to each temperature phase while minimizing the transitional periods during temperature transitions. A finite element analysis (FEA) and semi-analytical heat transfer model was used to determine the distances between the three heating assemblies that are responsible for creating the denaturation (96 degrees C), hybridization (60 degrees C), and extension (72 degrees C) temperature zones within the microchip. Predictions from the thermal FEA and semi-analytical model were compared with temperature measurements obtained from an infrared (IR) camera. Flow-field FEAs were also performed to predict the velocity distributions in the regions of the expanding and contracting conduits to study the effects of the microchannel geometry on flow recirculation and bubble nucleation. The flow fields were empirically studied using micro particle image velocimetry (mu-PIV) to validate the flow-field FEA's and to determine experimental velocities in each of the regions of different width. Successful amplification of a 90 base pair (bp) bacillus anthracis DNA fragment was achieved.

Disposable polydimethylsiloxane/silicon hybrid chips for protein detection.

This paper presents disposable protein analysis chips with single- or four-chamber-constructed from poly(dimethylsiloxane) (PDMS) and silicon. The chips are composed of a multilayer stack of PDMS layers that sandwich a silicon microchip. This inner silicon chip features an etched array of micro-cavities hosting polymeric beads. The sample is introduced into the fluid network through the top PDMS layer, where it is directed to the bead chamber. After reaction of the analyte with the probe beads, the signal generated on the beads is captured with a CCD camera, digitally processed, and analyzed. An established bead-based fluorescent assay for C-reactive protein (CRP) was used here to characterize these hybrid chips. The detection limit of the single-chamber protein chip was found to be 1 ng/ml. Additionally, using a back pressure compensation method, the signals from each chamber of the four-chamber chip were found to fall within 10% of each other.

Selective axonal growth of embryonic hippocampal neurons according to topographic features of various sizes and shapes.

PURPOSE: Understanding how surface features influence the establishment and outgrowth of the axon of developing neurons at the single cell level may aid in designing implantable scaffolds for the regeneration of damaged nerves. Past studies have shown that micropatterned ridge-groove structures not only instigate axon polarization, alignment, and extension, but are also preferred over smooth surfaces and even neurotrophic ligands. METHODS: Here, we performed axonal-outgrowth competition assays using a proprietary four-quadrant topography grid to determine the capacity of various micropatterned topographies to act as stimuli sequestering axon extension. Each topography in the grid consisted of an array of microscale (approximately 2 µm) or submicroscale (approximately 300 nm) holes or lines with variable dimensions. Individual rat embryonic hippocampal cells were positioned either between two juxtaposing topographies or at the borders of individual topographies juxtaposing unpatterned smooth surface, cultured for 24 hours, and analyzed with respect to axonal selection using conventional imaging techniques. RESULTS: Topography was found to influence axon formation and extension relative to smooth surface, and the distance of neurons relative to topography was found to impact whether the topography could serve as an effective cue. Neurons were also found to prefer submicroscale over microscale features and holes over lines for a given feature size. CONCLUSION: The results suggest that implementing physical cues of various shapes and sizes on nerve guidance conduits and other advanced biomaterial scaffolds could help stimulate axon regeneration.

Hippocampal neurons respond uniquely to topographies of various sizes and shapes.

A number of studies have investigated the behavior of neurons on microfabricated topography for the purpose of developing interfaces for use in neural engineering applications. However, there have been few studies simultaneously exploring the effects of topographies having various feature sizes and shapes on axon growth and polarization in the first 24 h. Accordingly, here we investigated the effects of arrays of lines (ridge grooves) and holes of microscale (approximately 2 microm) and nanoscale (approximately 300 nm) dimensions, patterned in quartz (SiO2), on the (1) adhesion, (2) axon establishment (polarization), (3) axon length, (4) axon alignment and (5) cell morphology of rat embryonic hippocampal neurons, to study the response of the neurons to feature dimension and geometry. Neurons were analyzed using optical and scanning electron microscopy. The topographies were found to have a negligible effect on cell attachment but to cause a marked increase in axon polarization, occurring more frequently on sub-microscale features than on microscale features. Neurons were observed to form longer axons on lines than on holes and smooth surfaces; axons were either aligned parallel or perpendicular to the line features. An analysis of cell morphology indicated that the surface features impacted the morphologies of the soma, axon and growth cone. The results suggest that incorporating microscale and sub-microscale topographies on biomaterial surfaces may enhance the biomaterials' ability to modulate nerve development and regeneration.

Micro-well texture printed into PEG hydrogels using the FILM nanomanufacturing process affects the behavior of preadipocytes.

To date, biomaterial scaffolds for adipose tissue engineering have focused on macro- and upper micro-scale fabrication, biocompatibility, and biodegradation, but have failed to recapitulate the sub-micron dimensions of native extracellular matrix (ECM) and, therefore, have not optimized material-cell interactions. Here, we report the findings from a study investigating the effects of a quasi-mimetic sub-micron (< 1 micrometer) surface texture on the qualitative behavior of preadipocytes (PAs). We found that PAs in contact with tread-like micro-well structures exhibit a different morphology relative to PAs seeded onto control smooth glass surfaces. Additionally, the micro-well topography induced isolated PAs to undergo adipogenesis, which usually occurs in the presence of aggregates of contact-inhibited PAs. The micro-well structures were printed into polyethylene glycol dimethacrylate (PEGDMA) using the recently reported nanomanufacturing process called Flash Imprint Lithography Using a Mask Aligner (FILM). FILM is a simple process that can be utilized to fabricate micro- and nanostructures in UV-curable materials (D.Y. Fozdar, W. Zhang, M. Palard, C.W. Patrick Jr., S.C. Chen, Flash Imprint Lithography Using A Mask Aligner (FILM): A Method for Printing Nanostructures in Photosensitive Hydrogels for Tissue Engineering. Nanotechnology 19, 2008). We demonstrate the utilization of the FILM process for a tissue engineering application for the first time. The micro-well topographical theme is characterized by contact angle and surface energy analysis and the results were compared with those for smooth glass and unpatterned PEGDMA surfaces. Based on our observations, we believe that the micro-well texture may ultimately be beneficial on implantable tissue scaffolds.