Shape Transformation Photolithography: Self-Assembled Arrays of Suspended MEMS Structures from Patterned Polymer Membranes.

Suspended micromechanical structures are typically formed by dissolving underlying spacer material. However, capillary force-induced collapse during solvent removal can damage soft structures. If instead capillary forces are directed in the plane, they can drive liquid polymeric bridges to directly transform into suspended fibers. The various capillary force-directed methods for fabricating arrays of suspended fibers have suffered from either low manufacturing rates or an inability to produce arbitrary patterns. Shape transformation photolithography (STP) demonstrated herein is a method of producing arbitrarily patterned arrays of suspended fibers that are potentially capable of high fabrication rates. In STP, holes are prepatterned in a polymer nanofilm supported on a micropillar array, and then the film is heated above its glass transition temperature. First, the holes expand by dewetting and then capillary forces drive thinning of the polymer channels defined by the holes. Prepatterning overcomes the energy barrier for hole nucleation and ensures that all fibers form at the same time and with similar diameters. Arrays of fibers and fiber lattice networks are formed from dyed polystyrene films that are patterned with nanosecond laser pulses at 532 nm. The exposure threshold for forming holes is 10.5 mJ/cm2 for single pulses and 3.3 mJ/cm2 per pulse for repetitive pulsing, which is only about 3× larger than the dose available from current 193 nm wafer-stepping projection printers that are used in device manufacture. With the increased absorption of polystyrene at 193 nm and with additional proposed material modifications to the thin film, it may even be possible to employ STP in production wafer steppers at economically feasible manufacturing rates of over 50 wafers/h.

MoS2 actuators: reversible mechanical responses of MoS2-polymer nanocomposites to photons.

New molybdenum disulfide (MoS2)-based polymer composites and their reversible mechanical responses to light are presented, suggesting MoS2 as an excellent candidate for energy conversion. Homogeneous mixtures of MoS2/polydimethylsiloxane (PDMS) nanocomposites (0.1-5 wt.%) were prepared and their near infrared (NIR) mechanical responses studied with increasing pre-strains. NIR triggering resulted in an extraordinary change in stress levels of the actuators by ~490 times. Actuation responses of MoS2 polymer composites depended on applied pre-strains. At lower levels of pre-strains (3-9%) the actuators showed reversible expansion while at high levels (15-50%), the actuators exhibited reversible contraction. An opto-mechanical conversion (η)∼0.5-3 MPa W(-1) was calculated. The ratio of maximum stress due to photo-actuation (σmax) at 50% strain to the minimum stress due to photo-actuation (σmin) at 3% strain was found to be ∼315-322% for MoS2 actuators (for 0.1 to 5 wt.% additive), greater than single layer graphene (∼188%) and multi-wall nanotube (∼172%) photo-mechanical actuators. Unlike other photomechanical actuators, the MoS2 actuators exhibited strong light-matter interactions and an unambiguous increase in amplitude of photomechanical response with increasing strains. A power law dependence of σmax/σmin on strains with a scaling exponent of ß = 0.87-1.32 was observed, suggesting that the origin of photomechanical response is intertwined dynamically with the molecular mechanisms at play in MoS2 actuators.

Nanotube liquid crystal elastomers: photomechanical response and flexible energy conversion of layered polymer composites.

Elastomeric composites based on nanotube liquid crystals (LCs) that preserve the internal orientation of nanotubes could lead to anisotropic physical properties and flexible energy conversion. Using a simple vacuum filtration technique of fabricating nanotube LC films and utilizing a transfer process to poly (dimethyl) siloxane wherein the LC arrangement is preserved, here we demonstrate unique and reversible photomechanical response of this layered composite to excitation by near infra-red (NIR) light at ultra-low nanotube mass fractions. On excitation by NIR photons, with application of small or large pre-strains, significant expansion or contraction of the sample occurs, respectively, that is continuously reversible and three orders of magnitude larger than in pristine polymer. Schlieren textures were noted in these LC composites confirming long range macroscopic nematic order of nanotubes within the composites. Order parameters of LC films ranged from S(optical) = 0.51-0.58 from dichroic measurements. Film concentrations, elastic modulus and photomechanical stress were all seen to be related to the nematic order parameter. For the same nanotube concentration, the photomechanical stress was almost three times larger for the self-assembled LC nanotube actuator compared to actuator based on randomly oriented carbon nanotubes. Investigation into the kinetics of photomechanical actuation showed variation in stretching exponent ß with pre-strains, concentration and orientation of nanotubes. Maximum photomechanical stress of ∼ 0.5 MPa W(-1) and energy conversion of ∼ 0.0045% was achieved for these layered composites. The combination of properties, namely, optical anisotropy, reversible mechanical response to NIR excitation and flexible energy conversion all in one system accompanied with low cost makes nanotube LC elastomers important for soft photochromic actuation, energy conversion and photo-origami applications.

Graphene/elastomer composite-based photo-thermal nanopositioners.

The addition of nanomaterials to polymers can result not only in significant material property improvements, but also assist in creating entirely new composite functionalities. By dispersing graphene nanoplatelets (GNPs) within a polydimethylsiloxane matrix, we show that efficient light absorption by GNPs and subsequent energy transduction to the polymeric chains can be used to controllably produce significant amounts of motion through entropic elasticity of the pre-strained composite. Using dual actuators, a two-axis sub-micron resolution stage was developed, and allowed for two-axis photo-thermal positioning (~100 µm per axis) with 120 nm resolution (feedback sensor limitation), and ~5 µm/s actuation speeds. A PID control loop automatically stabilizes the stage against thermal drift, as well as random thermal-induced position fluctuations (up to the bandwidth of the feedback and position sensor). Maximum actuator efficiency values of ~0.03% were measured, approximately 1000 times greater than recently reported for light-driven polymer systems.

Dielectrophoretic trapping of nanoparticles with an electrokinetic nanoprobe.

A high aspect ratio 3D electrokinetic nanoprobe is used to trap polystyrene particles (200 nm), gold nanoshells (120 nm), and gold nanoparticles (mean diameter 35 nm) at low voltages (<1 V(rms)). The nanoprobe is fabricated using room temperature self-assembly methods, without the need for nanoresolution lithography. The nanoprobe (150-500 nm in diameter, 2-150 µm in length) is mounted on the end of a glass micropipette, enabling user-specified positioning. The nanoprobe is one electrode within a point-and-plate configuration, with an indium-tin oxide cover slip serving as the planar electrode. The 3D structure of the nanoprobe enhances dielectrophoretic capture; further, electro-hydrodynamic flow enhances trapping, increasing the effective trapping region. Numerical simulations show low heating (1 K), even in biological media of moderate conductivity (1 S/m).

Scanning gate microscopy on graphene: charge inhomogeneity and extrinsic doping.

We have performed scanning gate microscopy (SGM) on graphene field effect transistors (GFET) using a biased metallic nanowire coated with a dielectric layer as a contact mode tip and local top gate. Electrical transport through graphene at various back gate voltages is monitored as a function of tip voltage and tip position. Near the Dirac point, the response of graphene resistance to the tip voltage shows significant variation with tip position, and SGM imaging displays mesoscopic domains of electron-doped and hole-doped regions. Our measurements reveal substantial spatial fluctuation in the carrier density in graphene due to extrinsic local doping from sources such as metal contacts, graphene edges, structural defects and resist residues. Our scanning gate measurements also demonstrate graphene's excellent capability to sense the local electric field and charges.

Toward wafer-scale patterning of freestanding intermetallic nanowires.

Individual metal alloy nanowires of constant diameter and high aspect ratio have previously been self-assembled at selected locations on atomic force microscope (AFM) probes by the method reported in Yazdanpanah et al (2005 J. Appl. Phys. 98 073510). This process relies on the room temperature crystallization of an ordered phase of silver-gallium. A parallel version of this method has been implemented in which a substrate, either an array of micromachined tips (similar to tips on AFM probes) or a lithographically patterned planar substrate, is brought into contact with a continuous, nearly planar film of melted gallium. In several runs, freestanding wires are fabricated with diameters of 40-400 nm, lengths of 4-80 µm, growth rates of 80-170 nm s( - 1) and, most significantly, with yields of up to 97% in an array of 422 growth sites. These results demonstrate the feasibility of developing a batch manufacturing process for the decoration of wafers of AFM tips and other structures with selectively patterned freestanding nanowires.

Endothelial cell scaffolds generated by 3D direct writing of biodegradable polymer microfibers.

The engineering of large (thickness > 100 µm) tissues requires a microvascular network to supply nutrients and remove waste. To produce microvasculature in vitro, a scaffold is required to mechanically support and stimulate endothelial cell (EC) adhesion and growth. Scaffolds for ECs are currently produced by patterning polymers or other biomaterials into configurations which often possess isotropic morphologies such as porous films and fibrous mats. We propose a new "direct-write" process for fabricating scaffolds composed of suspended polymer microfibers that are precisely oriented in 3D, providing directional architecture for selectively guiding cell growth along a desired pathway. The diameters of the fibers produced with this process were predictably and repeatably controlled through modulation of the system parameters, enabling production of fibers with microvascular-scale diameters (5-20 µm) from a variety of biodegradable polymers. These scaffolds were successfully seeded with ECs, which conformed to the geometry of the fibers and proliferated over the course of one week.

Characterization of silver-gallium nanowires for force and mass sensing applications.

We investigate the mechanical properties of cantilevered silver-gallium (Ag(2)Ga) nanowires using laser Doppler vibrometry. From measurements of the resonant frequencies and associated operating deflection shapes, we demonstrate that these Ag(2)Ga nanowires behave as ideal Euler-Bernoulli beams. Furthermore, radial asymmetries in these nanowires are detected through high resolution measurements of the vibration spectra. These crystalline nanowires possess many ideal characteristics for nanoscale force and mass sensing, including small spring constants (as low as 10(-4) N m(-1)), high frequency bandwidth with resonance frequencies in the 0.02-10 MHz range, small suspended mass (picograms), and relatively high Q-factors (approximately 2-50) under ambient conditions. We evaluate the utility of Ag(2)Ga nanowires for nanocantilever applications, including ultrasmall mass and high frequency bandwidth piconewton force detection.

The association of continuity of care on the diagnosis of hypertension in children and adolescents.

Hypertension among pediatric patients is an underdiagnosed condition. As continuity of care has been found to increase quality of pediatric care, we undertook this study to assess effect of continuity on diagnosis of pediatric hypertension. This is a retrospective analysis of 774 hypertensive patients, ages 3-18 years between June 1999 and October 2007 within the MetroHealth System in northeastern Ohio. The proportion of hypertensive patients diagnosed was assessed using coding within the electronic medical record. Continuity was assessed using the usual provider of care, defined as the number of visits to the most frequent provider divided by the total number of visits in the study period. Overall continuity did not have a statistically significant association with diagnosis (OR 0.7, CI 0.4-1.4). Our research indicates that continuity does not significantly affect diagnosis of hypertension in pediatric patients. Other approaches should be investigated to improve the significant underdiagnosis of pediatric hypertension.

The effects of racemic albuterol versus levalbuterol in very low birth weight infants.

Bronchodilators have been used in premature infants. Levalbuterol (LEV) an R-isomer of Albuterol has fewer hemodynamic side effects than Racemic Albuterol (RAC) in adults and children. In a retrospective study we sought to investigate the effects of LEV (0.31 mg) versus RAC (1.25 mg) in very low-birth weight infants (VLBW) who were treated with a beta-2 agonist for > or =2 weeks. Medical records (between January 2001 and December 2006) were reviewed for patients' demographics, medications use, hemodynamic and respiratory parameters, hypokalemia and hyperglycemia. Among 811 VLBW infants who were admitted to our NICU, 16 infants received RAC and 31 infants received LEV for > or =2 weeks. Infants who received RAC were younger, smaller, and received less Ipratropium Bromide (IB) than infants who received LEV [26.1 +/- 1.2 weeks vs. 28.1 +/- 3.7 weeks (P = 0.01), 817 +/- 211 g vs. 1,127 +/- 589 g (P = 0.01) and 2/16 (12%) vs. 15/31 (48%; P = 0.01); respectively]. In infants treated exclusively with RAC or LEV without IB, mean arterial blood pressures were lower in the RAC (n = 14) than the LEV group (n = 16, P = 0.05 by general linear model with repeated measures); however there were no differences in daily heart rates, oxygen supplementations, oxygen saturations, or respiratory rates. Also there were no differences between the two groups in hypokalemia or hyperglycemia. We conclude that LEV at a dose of 0.31 mg might have an indication in VLBW infants who are at risk for hemodynamic instability.

Micro-wilhelmy and related liquid property measurements using constant-diameter nanoneedle-tipped atomic force microscope probes.

The micro-Wilhelmy method is a well-established method of determining surface tension by measuring the force of withdrawing a tens of microns to millimeters in diameter cylindrical wire or fiber from a liquid. A comparison of insertion force to retraction force can also be used to determine the contact angle with the fiber. Given the limited availability of atomic force microscope (AFM) probes that have long constant diameter tips, force-distance (F-D) curves using probes with standard tapered tips have been difficult to relate to surface tension. In this report, constant diameter metal alloy nanowires (referred to as "nanoneedles") between 7.2 and 67 microm in length and 108 and 1006 nm in diameter were grown on AFM probes. F-D and Q damping AFM measurements of wetting and drag forces made with the probes were compared against standard macroscopic models of these forces on slender cylinders to estimate surface tension, contact angle, meniscus height, evaporation rate, and viscosity. The surface tensions for several low molecular weight liquids that were measured with these probes were between -4.2% and +8.3% of standard reported values. Also, the F-D curves show well-defined stair-step events on insertion and retraction from partial wetting liquids, compared to the continuously growing attractive force of standard tapered AFM probe tips. In the AFM used, the stair-step feature in F-D curves was repeatably monitored for at least 0.5 h (depending on the volatility of the liquid), and this feature was then used to evaluate evaporation rates (as low as 0.30 nm/s) through changes in the surface height of the liquid. A nanoneedle with a step change in diameter at a known distance from its end produced two steps in the F-D curve from which the meniscus height was determined. The step features enable meniscus height to be determined from distance between the steps, as an alternative to calculating the height corresponding to the AFM measured values of surface tension and contact angle. All but one of the eight measurements agreed to within 13%. The constant diameter of the nanoneedle also is used to relate viscous damping of the vibrating cantilever to a macroscopic model of Stokes drag on a long cylinder. Expected increases in drag force with insertion depth and viscosity are observed for several glycerol-water solutions. However, an additional damping term (associated with drag of the meniscus on the sidewalls of the nanoneedle) limits the sensitivity of the measurement of drag force for low-viscosity solutions, while low values of Q limit the sensitivity for high-viscosity solutions. Overall, reasonable correspondence is found between the macroscopic models and the measurements with the nanoneedle-tipped probes. Tighter environmental control of the AFM and treatments of needles to give them more ideal surfaces are expected to improve repeatability and make more evident subtle features that currently appear to be present on the F-D and Q damping curves.

Visual force sensing with flexible nanowire buckling springs.

A calibrated method of force sensing is demonstrated in which the buckled shape of a long flexible metallic nanowire, referred to as a 'nanoneedle', is interpreted to determine the applied force. An individual needle of 157 nm diameter by 15.6 µm length is grown on an atomic force microscope (AFM) cantilever with a desired orientation (by the method of Yazdanpanah et al 2005 J. Appl. Phys. 98 073510). Using a nanomanipulator the needle is buckled in the chamber of a scanning electron microscope (SEM) and the buckled shapes are recorded in SEM images. Force is determined as a function of deflection for an assumed elastic modulus by fitting the shapes using the generalized elastica model (De Bona and Zelenika 1997 Proc. Inst. Mech. Eng. C 211 509-17). In this calibration the elastic modulus (68.3 GPa) was determined using an auxiliary AFM measurement, with the needle in the same orientation as in the SEM. Following this calibration the needle was used as a sensor in a different orientation than the AFM coordinates to deflect a suspended PLLA polymer fiber from which the elastic modulus (2.96 GPa) was determined. The practical value of the sensing method does depend on the reliability and ruggedness of the needle. In this study the same needle remained rigidly secured to the AFM cantilever throughout the entire SEM/AFM calibration procedure and the characterization of the nanofiber.

Side-by-side comparison of Raman spectra of anchored and suspended carbon nanomaterials.

Raman spectra of ordered carbon nanomaterials are quite sensitive to surface perturbations, including trace residues, structural defects and residual stress. This is demonstrated by a series of experiments with carbon nanotubes and graphene. Their spectra change due to subtle changes in preparation and attachment to the substrate and to each other. Differences are most clearly seen by forming a material into an air bridge and probing it in the air gap and at the anchor points. A monolayer graphene sheet, shows a larger disorder band at the anchor points than in the air gap. However, a bundle or rope of parallel-aligned single-wall nanotubes shows a larger disorder band in the gap than at the anchor points. For the graphene sheet the substrate surface deforms the graphene, leading to increases in the disorder band. For the rope, the close proximity of the nanotubes to each other appears to produce a larger stress than the rope resting on the substrate.

Oriented nanomaterial air bridges formed from suspended polymer-composite nanofibers.

In a two-step method, carbon nanotubes, inorganic nanowires, or graphene sheets are connected between two anchor points to form nanomaterial air bridges. First, a recently developed method of forming directionally oriented polymer nanofibers by hand-application is used to form suspended composite polymer-nanomaterial fibers. Then, the polymer is sacrificed by thermally induced depolymerization and vaporization, leaving air bridges of the various materials. Composite fibers and bundles of nanotubes as thin as 10 nm that span 1 microm gaps have been formed by this method. Comparable bridges are observed by electrospinning solutions of the same nanomaterial-polymer composites onto micrometer-scale corrugated surfaces. This method for assembling nanomaterial air-bridges provides a convenient way to suspend nanomaterials for mechanical and other property determinations, and for subsequent device fabrication built up from the suspended nanosubstrates.

Effects of (r,r)- and (r,r/s,s)-formoterol on airway relaxation and contraction in an experimental rat model.

BACKGROUND: Racemic (R,R/S,S)-formoterol is a long-acting ß-agonist composed of a 50:50 mixture of (R,R)- and (S,S)-enantiomers. OBJECTIVE: The aim of this study was to determine whether (R,R)-formoterol and (R,R/S,S)-formoterol have differing effects on airway contraction and relaxation in vitro. METHODS: Cylindrical airway segments 3-mm long were isolated from the mid-trachea of healthy Sprague-Dawley rats and placed in a modified Krebs-Henseleit solution. Dose-response curves of bethanechol-induced contraction (measured as milligrams of tension) and the concentration of bethanechol that elicited 50% to 75% of maximal contraction (EC50-75) were determined. The air-way cylinders were then precontracted with bethanechol at the EC50-75 and exposed to different concentrations of (R,R)-formoterol (0.0001-1.0 µM) or (R,R/S,S)-formoterol (0.0002-2.0 µM). Each concentration of the 2 formoterol formulations contained the same amount of (R,R)-enantiomer (eg, [R,R]-formoterol 0.0001 µM and [R,R/S,S]-formoterol 0.0002 1JM contained the same amount of [R,R]-enantiomer). The relaxation percentage in response to formoterol was calculated as a reduction in tension (in milligrams) in relation to baseline tension in the precontracted state, with each tracheal cylinder serving as its own control. To determine the effect of (R,R)-formoterol on airway contraction, tracheal cylinders were incubated with (R,R)- or (R,R/S,S)-formoterol before electrical field stimulation (EFS). RESULTS: Tracheae from 56 three-week-old Sprague-Dawley rats were used in the study. The relaxation percentage of precontracted trachea was significantly greater after exposure to (R,R)-formoterol than to (R,R/S,S)-formoterol at a 2-fold higher concentration (P = 0.03; general linear model with repeated measures analysis comparing the 2 groups of animals). However, in a post hoc analysis, the mean (SE) relaxation percentage of precontracted trachea was significantly greater only after exposure to (R,R)-formoterol 0.01 µM than to (R,R/S,S)-formoterol 0.02 µM (15.6% [5.8%] vs 39.0% [5.6%]; P < 0.05, unpaired t test). EFS-induced airway contraction was significantly less in tracheal cylinders incubated in (R,R)-formoterol compared with those incubated in (R,R/S,S)-formoterol at a 2-fold higher concentration (P = 0.05; general linear model with repeated measures analysis comparing the 2 groups of animals). However, in the post hoc analysis, mean (SE) EFS-induced tracheal contraction was significantly less only in (R,R)-formoterol 0.01 µM compared with (R,R/S,S)-formoterol 0.02 µM at 10 V (1070 [55] mgvs 1225 [28] mg; P < 0.05, unpaired t test). CONCLUSION: We found that (R,R)-formoterol may induce greater relaxation of precontracted airway smooth muscle cells than (R,R/S,S)-formoterol and that (R,R)-formoterol may have a greater inhibitory effect on the endogenous cholinergic and excitatory nonadrenergic, noncholinergic contractile airway responses than (R,R/S,S)-formoterol. We speculate that the presence of the (S,S)-enantiomer in (R,R/S,S)-formoterol may impair airway relaxation of pre-contracted trachea in rats.

The Israeli anesthesiology physician workforce.

BACKGROUND: Anesthesiology is a vital specialty that permits the safe and humane performance of painful procedures. Most Israeli anesthesiologists are immigrants, while only a minimal number of Israeli medical school graduates enter the specialty. Unfortunately, the supply of immigrant physicians is declining due to falling immigration rates. OBJECTIVES: To examine the current Israeli anesthesiology workforce and project future needs. METHODS: Demographic and professional information about Israeli hospital anesthesiologists was solicited from anesthesiology department heads. Data were also gathered about the past, present and projected future growth, age distribution and birth rate of the Israeli population. Needs and demand-based analyses were used to project future anesthesiology workforce requirements. RESULTS: Data on 711 anesthesiologists were obtained from 30 hospital anesthesiology department heads. Eighty-seven anesthesiologists (12.2%) graduated from Israeli medical schools and 459 (64.6%) graduated from medical schools in the former Soviet Union. Among the 154 anesthesiology residents < or = 40 years old, only 13 (8.4%) graduated from Israeli medical schools. There are approximately 10.8 anesthesiologists per 100,000 population. Projections for 2005-2015 revealed a need for 250-300 new anesthesiologists. CONCLUSIONS: The anesthesiology workforce is predominantly composed of immigrants. This has vast implications for the future viability of the specialty because of the continuing reduction in immigration, the lack of interest in the specialty by Israeli medical school graduates, and the projected need for many new anesthesiologists to replace retirees and to provide care to a growing and aging population.

Spiking voltages for faster switching of nematic liquid-crystal light modulators.

Electrical address circuits developed for driving fast-switching ferroelectric liquid-crystal spatial light modulators (SLM) can be programmed to increase the speed of much slower responding nematic liquid-crystal SLMs. Using an addressing circuit that can switch as fast as 0.164 ms, voltages are programmed for values of phase that exceed the desired phase, and when the phase reaches the desired value, the voltage is switched to the required steady-state voltage. For a SLM that has a phase range of 3.5pi and that is programmed over a 2pi range, switching speed is reduced from 400 ms to between 71 and 77 ms. The speedup algorithm is applied to each pixel of the SLM together with a digital correction for a spatially nonuniform phase.

Phase calibration of spatially nonuniform spatial light modulators.

A new 512 x 512 pixel phase-only spatial light modulator (SLM) has been found to deviate from being flat by several wavelengths. Also, the retardation of the SLM relative to voltage varies across the device by as much as 0.25 wavelength. The birefringence of each pixel as a function of address voltage is measured from the intensity of the SLM between crossed polarizers. To these responses are added a reference spatial phase measured by phase shifting interferometry for a single address voltage. Fits to the measured data facilitate the compensation of the SLM to a root-mean-square wave-front error of 0.06 wavelength. The application of these corrections to flatten the full aperture of the SLM sharpens the focal plane spot and reduces the distortion of computer-designed diffraction patterns.