Dual-Sensing Piezoresponsive Foam for Dynamic and Static Loading.

Polymeric foams, embedded with nano-scale conductive particles, have previously been shown to display quasi-piezoelectric (QPE) properties; i.e., they produce a voltage in response to rapid deformation. This behavior has been utilized to sense impact and vibration in foam components, such as in sports padding and vibration-isolating pads. However, a detailed characterization of the sensing behavior has not been undertaken. Furthermore, the potential for sensing quasi-static deformation in the same material has not been explored. This paper provides new insights into these self-sensing foams by characterizing voltage response vs frequency of deformation. The correlation between temperature and voltage response is also quantified. Furthermore, a new sensing functionality is observed, in the form of a piezoresistive response to quasi-static deformation. The piezoresistive characteristics are quantified for both in-plane and through-thickness resistance configurations. The new functionality greatly enhances the potential applications for the foam, for example, as insoles that can characterize ground reaction force and pressure during dynamic and/or quasi-static circumstances, or as seat cushioning that can sense pressure and impact.

Correlation of chromatographic performance with morphological features of organic polymer monoliths.

Monoliths are considered to be a low pressure alternative to particle packed columns for liquid chromatography (LC). However, the chromatographic performance of organic monoliths, in particular, has still not reached the level of particle packed columns. Since chromatographic performance can be attributed to morphological features of the monoliths, in-situ characterization of the monolith structure in three dimensions would provide valuable information that could be used to help improve performance. In this work, serial sectioning and imaging were performed with a dual-beam scanning electron microscope for reconstruction and quantitative characterization of poly(ethylene glycol) diacrylate (PEGDA) monoliths inside a capillary column. Chord lengths, homogeneity factors, porosities and tortuosities were calculated from three-dimensional (3D) reconstructions of two PEGDA monoliths. Chromatographic efficiency was better for the monolith with smaller mean chord length (i.e., 5.23µm), porosity (i.e., 0.49) and tortuosity (i.e., 1.50) compared to values of 5.90µm, 0.59 and 2.34, respectively, for the other monolithic column. Computational prediction of tortuosity (2.32) was found to be in agreement with the experimentally measured value (2.34) for the same column. The monoliths were found to have significant radial heterogeneity since the homogeneity factor decreased from 5.39 to 4.89 (from center to edge) along the column radius.

Vertical impedance measurements on concrete bridge decks for assessing susceptibility of reinforcing steel to corrosion.

Corrosion is a pressing problem for aging concrete infrastructure, especially bridge decks. Because of its sensitivity to factors that affect corrosion of reinforcing steel in concrete, resistivity is an important structural health indicator for reinforced concrete structures. In this research, an instrument was developed to measure vertical impedance on concrete bridge decks. Measurements of vertical impedance on slabs prepared in the laboratory, on slabs removed from decommissioned bridge decks, and on an in-service bridge deck in the field demonstrate the utility of the new apparatus.

Quantitation of pH-induced aggregation in binary protein mixtures by dielectric spectroscopy.

This paper presents a quantitative approach for measuring pH-controlled protein aggregation using dielectric spectroscopy. The technique is demonstrated through two aggregation experiments, the first between ß-lactoglobulin (ß-Lg) and hen lysozyme (HENL) and the second between bovine serum albumin (BSA) and HENL. In both experiments, the formation of aggregates is strongly dependent on the solution pH and is clearly indicated by a decrease in the measured permittivity when the second protein is added. A quantifiable lower-bound on the ratio of proteins involved in the aggregation process is obtained from the permittivity spectra. Lower-bound aggregation ratios of 83 % for ß-Lg/HENL at pH 6.0 and 48 % for BSA/HENL at pH 9.2 were consistent with turbidity measurements made on the same solutions.

Paper-based, capacitive touch pads.

Metallized paper is patterned to create touch pads of arrayed buttons that are sensitive to contact with both bare and gloved fingers. The paper-based keypad detects the change in capacitance associated with the touch of a finger to one of its buttons. Mounted on an alarmed cardboard box, the keypad requires the appropriate sequence of touches to disarm the system.

Increased bandwidth for dielectric spectroscopy of proteins through electrode surface preparation.

Dielectric spectroscopy measurements of liquids are often limited by electrode polarization. The influence of surface polishing and deposition of the conducting polymer polypyrrole/polystyrenesulfonate (PPy/PSS) on the polarization impedance is investigated. A quantitative description of the electrode polarization contribution to the real-valued permittivity spectrum is derived. This description explains the origin of the ω(-const). (const.>1) dependency commonly observed in permittivity measurements. Electrode surface roughness is correlated with both the magnitude and phase of the constant phase element. Generally, rougher electrodes have better performance, and an order of magnitude bandwidth improvement is achieved using PPy/PSS electrodes.

Influence of pK(a) shifts on the calculated dipole moments of proteins.

The protein dipole moment is a low-resolution parameter that characterizes the second-order charge organization of a biomolecule. Theoretical approaches to calculate protein dipole moments rely on pK(a) values, which are either computed individually for each ionizable residue or obtained from model compounds. The influence of pK(a) shifts are evaluated first by comparing calculated and measured dipole moments of ß-lactoglobulin. Second, calculations are made on a dataset of 66 proteins from the Protein Data Bank, and average differences are determined between dipole moments calculated with model pK(a)s, pK(a)s derived using a Poisson-Boltzmann approach, and empirically-calculated pK(a)s. Dipole moment predictions that neglect pK(a) shifts are consistently larger than predictions in which they are included. The importance of pK(a) shifts are observed to vary with protein size, internal permittivity, and solution pH.

Note: electrode polarization of Galinstan electrodes for liquid impedance spectroscopy.

Electrode polarization is a significant obstacle in the impedance measurements of ionic liquids. An atomically smooth electrode surface could potentially reduce unwanted impedance contributions from electrode polarization. Liquid metal electrodes were formed by adhering Galinstan to acrylic plates in a parallel-plate capacitor arrangement. Electrode polarization was compared to a similar cell with stainless steel electrodes. The impedance of salt and protein solutions (ß-lactoglobulin) was measured from 40 Hz to 110 MHz. Because of oxide layer formation, the performance of the Galinstan electrode is significantly different than the theoretical ideal.

Transient deflection response in microcantilever array integrated with polydimethylsiloxane (PDMS) microfluidics.

We report the integration of a nanomechanical sensor consisting of 16 silicon microcantilevers with polydimethylsiloxane (PDMS) microfluidics. For microcantilevers positioned near the bottom of a microfluidic flow channel, a transient differential analyte concentration for the top versus bottom surface of each microcantilever is created when an analyte-bearing fluid is introduced into the flow channel (which is initially filled with a non-analyte containing solution). We use this effect to characterize a bare (nonfunctionalized) microcantilever array in which the microcantilevers are simultaneously read out with our recently developed high sensitivity in-plane photonic transduction method. We first examine the case of non-specific binding of bovine serum albumin (BSA) to silicon. The average maximum transient microcantilever deflection in the array is -1.6 nm, which corresponds to a differential surface stress of only -0.23 mN m(-1). This is in excellent agreement with the maximum differential surface stress calculated based on a modified rate equation in conjunction with finite element simulation. Following BSA adsorption, buffer solutions with different pH are introduced to further study microcantilever array transient response. Deflections of 20-100 nm are observed (2-14 mN m(-1) differential surface stress). At a flow rate of 5 µL min(-1), the average measured temporal width (FWHM) of the transient response is 5.3 s for BSA non-specific binding and 0.74 s for pH changes.

Method for estimating the internal permittivity of proteins using dielectric spectroscopy.

Protein charge organization is dependent on the low-permittivity region in the hydrophobic core of the molecule. We suggest a novel approach to estimate the dielectric constant of this region by comparing measured and simulated first- and second-order charge moments. Here, the dipole moment is measured as a function of pH using dielectric spectroscopy. The results are compared to dipole moments based on Poisson-Boltzmann estimates of pK(a) shifts calculated from structures in the Protein Data Bank. Structures are additionally refined using CHARMM molecular dynamics simulations. The best estimate for the internal permittivity is found by minimizing the root-mean-square residual between measured and predicted charge moments. Using the protein ß-lactoglobulin, a core dielectric constant in the range of 6-7 is estimated.

Dielectric spectroscopy of molecular interactions based on the avidin-biotin complex.

Dielectric spectroscopy is used to probe the electrical properties of biomolecules dissolved in liquids. A 40 µl cell is constructed out of acrylic with polished, stainless steel electrodes. Experiments are performed on avidin and biotin-labeled BSA, showing characteristics of aggregation. Experiments with avidin and biotin demonstrate shifts in dielectric relaxation of the avidin associated with changes in the dipole moment and size of the molecule due to biotin binding. These shifts are analyzed in the context of biomolecular changes. These experiments demonstrate the utility of impedance spectroscopy to detect changes due to small molecules binding to proteins.

Broadband RF impedance spectroscopy in micromachined microfluidic channels.

Impedance spectroscopy in the radio frequency range from 100 MHz to 20 GHz can reveal the dielectric relaxations of biological and chemical solutions. S-parameters for a coplanar waveguide are derived. To perform these measurements, a coplanar waveguide device was fabricated on a conventional FR-4 substrate for fluid interrogation. The microfluidic channel was formed by milling conventional waveguides and laser-cutting channels in the dielectric substrate. Measurements using this device were performed on standards: deionized water, isopropyl alcohol, and air. These measurements were compared to those taken with a conventional dielectric probe. The results demonstrate the ability of the fabricated device to extract varying transmission parameters due to changing sample properties.

Contactless conductivity detection of small ions in a surface micro-machined CE chip.

A microchip is presented which is capable of CE separations and is built using exclusively thin film deposition techniques, fully compatible with microelectronics batch processing. Standard photolithography provides control of the spacing between electrodes used in conductivity measurement and overall channel geometry. Fluid channels are arranged as a double-T injector with a 50 microm offset at the arm intersection. The chip's performance was tested using concentrations of sodium chloride and calcium chloride ranging from 1 microM to 1 mM in a 5-mM MES/histidine buffer. Separations were performed by applying different voltages to reservoirs positioned at the four fluid channel openings. Conductivity detection was performed by applying a small AC voltage (1 Vrms) to the insulated electrodes positioned inside the fluid channels. A computer running LabVIEW controlled the AC signal generation, data acquisition and storage. Measurements indicated that the chip's detection limit was below 1 microM for both sodium and calcium cations.

Temperature-stable parallel-plate dielectric cell for broadband liquid impedance measurements.

A liquid impedance cell for broadband impedance measurements up to 110 MHz is presented. The design incorporates temperature control and minimizes parasitic capacitance and inductance. The cell is simple to fabricate and uses chemically resistant materials, stainless steel, and Teflon. This dielectric cell can be used in a variety of liquid measurements, particularly those related to impedance measurements of biological objects in solution. Temperature control is illustrated in measurements of the permittivity of deionized water from 5 to 55 °C. Numerical fitting procedures employed on the relaxation curves indicate good agreement with previous studies on beta-lactoglobulin and hen lysozyme. Titration capability is demonstrated through dielectric titration of hen lysozyme and beta-lactoglobulin.