857 research outputs found

    Injectable biodegradable poly(ester-co-ether) methacrylate monomers for bone tissue engineering and drug delivery applications.

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    The aim of this project was to produce strong, fast photocuring polymer adhesives and composites for biomedical applications that degrade and can release drugs at a controllable rate after set. Five ABA triblock poly(lactide-co-propylene glycol-co- lactide)s with 7, 17 or 34 propylene glycol and 2, 4 or 8 lactic acid units in each B and A block respectively, end capped with methacrylate groups, were prepared. Using FTIR, Raman and NMR, the relative lactide, polypropylene glycol and methacrylate levels in the monomer were proved controllable and as expected from reactant ratios. Polymerisation rates upon blue light exposure for 60, 120 or 240 s using 0.5, 1 or 2 wt% initiators were determined. The shortest monomer with 0.5 wt% initiator achieved 96 % conversion by 120 s after start of 60 s illumination, forming a semi-rigid polymer that in water degraded almost linearly with time with 19 wt% material loss over 14 weeks. Raising initiator concentration reduced polymerisation rate on the lower surface of samples. Increasing the number of lactic acid units in each A block from 2 to 8 enhanced water sorption and increased average total mass loss in 14 weeks to 60 wt% but degradation rate decreased with time. Monomers produced with longer polypropylene glycol B blocks required longer periods of light exposure for full cure and the final more flexible polymers exhibited slower non-linear degradation. Drug release was controlled by varying monomer composition and drug loading level. With hydrophobic ketoprofen, release was more enhanced from rapid-eroding, high water-absorbing polymers. Release of the more water-soluble chlorhexidine diacetate and prednisolone were affected more by polymer swelling and drug diffusion rates through the polymer. Two highly soluble phosphate glasses (67 wt%) were added to one monomer producing composites without losing the rapid set capacity of the polymer. Within a few days in water the glass was leached out providing a means to generate a porous structure. Replacement of phosphate glass with p-tricalcium phosphate and monocalcium phosphate monohydrate filler increased the composite modulus by an order of magnitude upon water sorption, buffered the acidic polymer degradation products and raised the polymer erosion rate significantly. Through further investigations, these polymers and composites should potentially provide a new range of injectable biodegradable slow drug-releasing adhesive materials for various applications in bone tissue engineering and drug delivery

    XRD, AFM and UV-Vis optical studies of PbSe thin films produced by chemical bath deposition method.

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    PbSe thin �films have been deposited on microscope glass substrates by chemical bath deposition technique. The chemical bath consisted of lead nitrate, sodium selenate and triethanolamine solutions. The influence of bath temperature on the properties of PbSe �lms was investigated. The X-ray diff�raction, atomic force microscope and UV/Vis Spectrophotometer were used to obtain the structural characterization, surface morphological and absorbance data, respectively. Based on the X-ray diff�raction results, the thin �films obtained were found to be polycrystalline in nature with cubic structure. The intensity of the (111) peak showed a signifi�cant increased as the bath temperature was increased from 40 to 80C. The �films deposited at 80C indicated that the crystallinity was improved and more PbSe peaks were observed. On the other hand, the grain size, fi�lm thickness and surface roughness were increased while band gap energy decreased as could be observed in atomic force microscope and UV-Vis optical studies, respectively

    Approximate k-state solutions to the Dirac-Yukawa problem based on the spin and pseudospin symmetry

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    Using an approximation scheme to deal with the centrifugal (pseudo-centrifugal) term, we solve the Dirac equation with the screened Coulomb (Yukawa) potential for any arbitrary spin-orbit quantum number {\kappa}. Based on the spin and pseudospin symmetry, analytic bound state energy spectrum formulas and their corresponding upper- and lower-spinor components of two Dirac particles are obtained using a shortcut of the Nikiforov-Uvarov method. We find a wide range of permissible values for the spin symmetry constant C_{s} from the valence energy spectrum of particle and also for pseudospin symmetry constant C_{ps} from the hole energy spectrum of antiparticle. Further, we show that the present potential interaction becomes less (more) attractive for a long (short) range screening parameter {\alpha}. To remove the degeneracies in energy levels we consider the spin and pseudospin solution of Dirac equation for Yukawa potential plus a centrifugal-like term. A few special cases such as the exact spin (pseudospin) symmetry Dirac-Yukawa, the Yukawa plus centrifugal-like potentials, the limit when {\alpha} becomes zero (Coulomb potential field) and the non-relativistic limit of our solution are studied. The nonrelativistic solutions are compared with those obtained by other methods.Comment: 21 pages, 6 figure

    Estimating nonresponse bias and mode effects in a mixed mode survey

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    In mixed-mode surveys, it is difficult to separate sample selection differences from mode-effects that can occur when respondents respond in different interview settings. This paper provides a framework for separating mode-effects from selection effects by matching very similar respondents from different survey modes using propensity score matching. The answer patterns of the matched respondents are subsequently compared. We show that matching can explain differences in nonresponse and coverage in two Internet-samples. When we repeat this procedure for a telephone and Internet-sample however, differences persist between the samples after matching. This indicates the occurrence of mode-effects in telephone and Internet surveys. Mode-effects can be problematic; hence we conclude with a discussion of designs that can be used to explicitly study mode-effects

    Cooperative Ring Exchange and Quantum Melting of Vortex Lattices in Atomic Bose-Einstein Condensates

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    Cooperative ring-exchange is suggested as a mechanism of quantum melting of vortex lattices in a rapidly-rotating quasi two dimensional atomic Bose-Einstein condensate (BEC). Using an approach pioneered by Kivelson et al. [Phys. Rev. Lett. {\bf 56}, 873 (1986)] for the fractional quantized Hall effect, we calculate the condition for quantum melting instability by considering large-correlated ring exchanges in a two-dimensional Wigner crystal of vortices in a strong `pseudomagnetic field' generated by the background superfluid Bose particles. BEC may be profitably used to address issues of quantum melting of a pristine Wigner solid devoid of complications of real solids.Comment: 7 pages, 1 figure, to appear in Physical Review

    Free expansion of lowest Landau level states of trapped atoms: a wavefunction microscope

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    We show that for any lowest-Landau-level state of a trapped, rotating, interacting Bose gas, the particle distribution in coordinate space in a free expansion (time of flight) experiment is related to that in the trap at the time it is turned off by a simple rescaling and rotation. When the lowest-Landau-level approximation is valid, interactions can be neglected during the expansion, even when they play an essential role in the ground state when the trap is present. The correlations in the density in a single snapshot can be used to obtain information about the fluid, such as whether a transition to a quantum Hall state has occurred.Comment: 5 pages, no figures. v2: discussion of neglect of interactions during expansion improved, refs adde

    Spontaneous decay in the presence of dispersing and absorbing bodies: general theory and application to a spherical cavity

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    A formalism for studying spontaneous decay of an excited two-level atom in the presence of dispersing and absorbing dielectric bodies is developed. An integral equation, which is suitable for numerical solution, is derived for the atomic upper-state-probability amplitude. The emission pattern and the power spectrum of the emitted light are expressed in terms of the Green tensor of the dielectric-matter formation including absorption and dispersion. The theory is applied to the spontaneous decay of an excited atom at the center of a three-layered spherical cavity, with the cavity wall being modeled by a band-gap dielectric of Lorentz type. Both weak coupling and strong coupling are studied, the latter with special emphasis on the cases where the atomic transition is (i) in the normal-dispersion zone near the medium resonance and (ii) in the anomalous-dispersion zone associated with the band gap. In a single-resonance approximation, conditions of the appearance of Rabi oscillations and closed solutions to the evolution of the atomic state population are derived, which are in good agreement with the exact numerical results.Comment: 12 pages, 6 figures, typos fixed, 1 figure adde
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