Gregory P. Crawford

Strain-dependent electrical resistance of tin-doped indium oxide on polymer substrates

The increase in sheet resistance of indium–tin–oxide (ITO) films on polyethylene terephthalate with increasing tensile strain is reported. The increase in resistance is related to the number of cracks in the conducting layer which depends upon applied strain and film thickness. We propose a simple model that describes the finite but increasing resistance in the cracked ITO layer in terms of a small volume of conducting material within each crack.

Liquid-crystal diffraction gratings using polarization holography alignment techniques

A method of patterning surfaces for liquid-crystal alignment using a polarization holography exposure on a linear photopolymerizable polymer alignment layer is demonstrated. Three configurations are demonstrated which include registered planar-periodic surface boundary conditions on both surfaces (true polarization gratings), planar-periodic and uniform planary surface boundary conditions, and planar-periodic and homeotropic boundary conditions. Two-dimensional polarization gratings are also demonstrated by orientating planar-periodic alignment layers orthogonally. Passive polarization gratings are also demonstrated using reactive mesogens to capture the periodic order indefinitely. The underlying structure of the configuration is discussed, including the nature of their switching transition (threshold or thresholdless), for all three configurations. A simple phenomenological model is presented to describe the Freedericksz transition for the registered planar-periodic boundary condition case.

Drug‐screening platform based on the contractility of tissue‐engineered muscle

A tissue‐based approach to in vitro drug screening allows for determination of the cumulative positive and negative effects of a drug at the tissue rather than the cellular or subcellular level. Skeletal muscle myoblasts were tissue‐engineered into three‐dimensional muscle with parallel myofibers generating directed forces. When grown attached to two flexible microposts (μposts) acting as artificial tendons in a 96‐well plate format, the miniature bioartificial muscles (mBAMs) generated tetanic (active) forces upon electrical stimulation measured with a novel image‐based motion detection system. mBAM myofiber hypertrophy and active force increased in response to insulin‐like growth factor 1. In contrast, mBAM deterioration and weakness was observed with a cholesterol‐lowering statin. The results described in this study demonstrate the integration of tissue engineering and biomechanical testing into a single platform for the screening of compounds affecting muscle strength. Muscle Nerve, 2007

Diffusion kinetics of formation of holographic polymer-dispersed liquid crystal display materials

The underlying physical phenomena behind the increased reflection/diffraction efficiency of holographically formed polymer-dispersed liquid crystal gratings formed from blended oligomers are investigated. Using parameters determined by in situ diffraction measurements during grating formation, a phenomenological diffusion model shows that a composition modulation in the polymer network results in blended oligomer systems. Scanning electron microscope studies reveal polymer morphologies consistent with this hypothesis.

Electromechanical Properties of Transparent Conducting Substrates for Flexible Electronic Displays

As opto-electronic devices become flexible, the designing of reliable devices becomes more challenging. In this paper, we focus on the degradation of flexible transparent anodes by mechanical and thermal stresses. Indium tin oxide (ITO)-coated polyethyleneterephthalate (PET) is susceptible to cracking at low strains (<2%), but has excellent electrical and optical characteristics. Conducting polymers, such as polyethylene dioxythiophene doped with polystyrene sulfonate (PEDOT:PSS), have good mechanical properties, but are severely degraded by temperature, and have inferior optical and electronic properties. We have developed a mandrel-bending automated test system to evaluate the degradation of flexible anodes in service, and find that even when the strain is below the virgin cracking threshold, there are measurable changes in ITO resistance. Cyclic loading of ITO-coated PET shows three regimes of resistance increase: 1) an increase in resistance, due to changes in sample dimension until equilibrium width is obtained (50-100 cycles); 2) a gradual linear increase in resistance, possibly due to cracking of ITO; and 3) a catastrophic failure after 50 000 cycles due to severe cracking. For PEDOT:PSS-coated PET, the resistance does not increase significantly with increasing tensile strain, and it is also less susceptible to damage from repeated bending.

Tunable face-centered-cubic photonic crystal formed in holographic polymer dispersed liquid crystals

We report on the fabrication and electro-optic measurements of face-centered-cubic (fcc) lattices in holographic polymer dispersed liquid-crystal materials. Four linearly polarized coherent plane waves were interfered to generate a fcc optical lattice that was subsequently and indefinitely recorded as an arrayed pattern of nanometer-sized liquid-crystal droplets (approximately 50 nm) at lattice nodes within a polymer matrix. Observed transmission spectra and Kossel diffraction curves are consistent with fcc crystal structure. A completely reversible 2% wavelength shift of the (+/- 111) stop band was observed on application of an electric field.

Characterization of the cylindrical cavities of anopore and nuclepore membranes

The microscopic properties of the inner surface of the cavities of Anopore and Nuclepore membranes are investigated with scanning electron microscopy (SEM), nitrogen adsorption isotherms, and deuterium nuclear magnetic resonance (2H‐NMR). Useful information about the cavity orientation and morphology, and internal surface area is obtained. Analysis of SEM photographs yields estimates of the surface area and the porosity of these membranes which complements the adsorption results. The orientation of liquid crystals permeated in the cavities of Anopore and Nuclepore membranes is probed with the 2H‐NMR technique. It is found that the orientation of the liquid crystal molecules is governed by the confining volume and surface conditions. The 2H‐NMR spectral line shape of the confined liquid crystal also provides information on the substrate morphology and roughness that is consistent with SEM and adsorption experiments.

Influence of partial matrix fluorination on morphology and performance of HPDLC transmission gratings

The morphology and the electro-optical performance characteristics were investigated in holographic polymer-dispersed liquid crystal (HPDLC) transmission gratings with partially fluorinated polymer matrices. HPDLC transmission gratings were prepared using standard UV curable monomer mixtures along with mono-functional fluorinated acrylate monomers and a nematic liquid crystal, TL203. Partial fluorination of the host polymer matrices by incorporating hexafluoroisopropyl acrylate (HFIPA) or trifluoroethyl acrylate (TFEA) in the standard formulation has been found to influence the morphological and the electro-optical properties of the resulting HPDLC transmission gratings. Significant decrease in switching voltages and higher relaxation times were observed in fluorinated HPDLCs. Conversely, an addition of methyl acrylate (MA), a non-fluorinated monomer with a similar structure in the standard formulation, resulted in an increase in the switching voltage and produced no significant change in the relaxation time in the HPDLC gratings. Presence of fluorine atoms at the polymer-liquid crystal (LC) interface not only decreased the surface anchoring strength but also influenced the orientation of LC droplet directors.

Variable-wavelength switchable Bragg gratings formed in polymer-dispersed liquid crystals

We report a holographic polymer-dispersed liquid crystal cell whose reflection peak can be tuned as a function of applied voltage. Electro-optic results are presented which are in excellent agreement with our model based on coupled wave theory.

Characterization of holographic polymer dispersed liquid crystal transmission gratings

We investigate physical properties of polymer dispersed liquid crystal transmission gratings which influence the light diffraction efficiency behavior. Scanning electron microscopy is used to determine the orientation and shape of nanometer-sized liquid crystal droplets in a polymer host. Dependence of diffraction efficiency on light polarization suggests that the orientation of a liquid crystal within the ellipsoidal droplets is axial. The distribution of droplet orientations appears to vary for different grating spacings. The switching properties of the gratings are investigated and analyzed with a sinusoidal dielectric tensor modulation model. The analysis also takes into account orientational distribution of average directors and spatial distribution of the droplets. Temperature dependence of diffraction efficiency is tentatively explained by a structural transition within the droplets and the presence of impurities in droplets.