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    4D Biofabrication of fibrous artificial nerve graft for neuron regeneration
    (Bristol : IOP Publ., 2020) Apsite, Indra; Constante, Gissela; Dulle, Martin; Vogt, Lena; Caspari, Anja; Boccaccini, Aldo R.; Synytska, Alla; Salehi, Sahar; Ionov, Leonid
    In this paper, we describe the application of the 4D biofabrication approach for the fabrication of artificial nerve graft. Bilayer scaffolds consisting of uniaxially aligned polycaprolactone-poly(glycerol sebacate) (PCL-PGS) and randomly aligned methacrylated hyaluronic acid (HA-MA) fibers were fabricated using electrospinning and further used for the culture of PC-12 neuron cells. Tubular structures form instantly after immersion of fibrous bilayer in an aqueous buffer and the diameter of obtained tubes can be controlled by changing bilayer parameters such as the thickness of each layer, overall bilayer thickness, and medium counterion concentration. Designed scaffolds showed a self-folded scroll-like structure with high stability after four weeks of real-time degradation. The significance of this research is in the fabrication of tuneable tubular nerve guide conduits that can simplify the current existing clinical treatment of neural injuries. © 2020 The Author(s). Published by IOP Publishing Ltd.
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    MWCNT induced negative real permittivity in a copolyester of Bisphenol-A with terephthalic and isophthalic acids
    (Bristol : IOP Publ., 2020) Özdemir, Zeynep Güven; Daşdan, Dolunay Şakar; Kavak, Pelin; Pionteck, Jürgen; Pötschke, Petra; Voit, Brigitte; SüngüMısırlıoğlu, Banu
    In the present study, the negative real permittivity behavior of a copolyester of bisphenol-A with terephthalic acid and isophthalic acid (PAr) containing 1.5 to 7.5 wt% multi-walled carbon nanotubes (MWCNTs) have been investigated in detail. The structural and morphological analysis of the melt-mixed composites was performed by Fourier transform infrared spectroscopy using attenuated total reflection (FTIR-ATR), atomic force microscopy (AFM), X-ray diffraction (XRD), and light microscopy. The influences of the MWCNT filler on the AC impedance, complex permittivity, and AC conductivity of the PAr polymer matrix were investigated at different operating temperatures varied between 296 K and 373 K. The transition from a negative to positive real permittivity was observed at different crossover frequencies depending on the MWCNT content of the composites whereas pure PAr showed positive values at all frequencies. The negative real permittivity characteristic of the composites was discussed in the context of Drude model. © 2020 The Author(s). Published by IOP Publishing Ltd.
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    Elastic properties of single crystal Bi12SiO20 as a function of pressure and temperature and acoustic attenuation effects in Bi12 MO20 (M = Si, Ge and Ti)
    (Bristol : IOP Publ., 2020) Haussühl, Eiken; Reichmann, Hans Josef; Schreuer, Jürgen; Friedrich, Alexandra; Hirschle, Christian; Bayarjargal, Lkhamsuren; Winkler, Björn; Alencar, Igor; Wiehl, Leonore; Ganschow, Steffen
    A comprehensive study of sillenite Bi12SiO20 single-crystal properties, including elastic stiffness and piezoelectric coefficients, dielectric permittivity, thermal expansion and molar heat capacity, is presented. Brillouin-interferometry measurements (up to 27 GPa), which were performed at high pressures for the first time, and ab initio calculations based on density functional theory (up to 50 GPa) show the stability of the sillenite structure in the investigated pressure range, in agreement with previous studies. Elastic stiffness coefficients c 11 and c 12 are found to increase continuously with pressure while c 44 increases slightly for lower pressures and remains nearly constant above 15 GPa. Heat-capacity measurements were performed with a quasi-adiabatic calorimeter employing the relaxation method between 2 K and 395 K. No phase transition could be observed in this temperature interval. Standard molar entropy, enthalpy change and Debye temperature are extracted from the data. The results are found to be roughly half of the previous values reported in the literature. The discrepancy is attributed to the overestimation of the Debye temperature which was extracted from high-temperature data. Additionally, Debye temperatures obtained from mean sound velocities derived by Voigt-Reuss averaging are in agreement with our heat-capacity results. Finally, a complete set of electromechanical coefficients was deduced from the application of resonant ultrasound spectroscopy between 103 K and 733 K. No discontinuities in the temperature dependence of the coefficients are observed. High-temperature (up to 1100 K) resonant ultrasound spectra recorded for Bi12 MO20 crystals revealed strong and reversible acoustic dissipation effects at 870 K, 960 K and 550 K for M = Si, Ge and Ti, respectively. Resonances with small contributions from the elastic shear stiffness c 44 and the piezoelectric stress coefficient e 123 are almost unaffected by this dissipation. © 2020 The Author(s). Published by IOP Publishing Ltd.
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    The effect of oxygen admixture on the properties of microwave generated plasma in Ar-O2: A modelling study
    (Bristol : IOP Publ., 2021) Baeva, M; Stankov, M; Trautvetter, T; Methling, R; Hempel, F; Loffhagen, D; Foest, R
    This work presents results of a self-consistent modelling analysis on microwave plasma generated in Ar-O2 mixtures at a frequency of 2.45 GHz at atmospheric pressure. The study focuses on how the plasma properties are influenced by the increase of the oxygen fraction in the gas mixture. The oxygen admixture is increased from 1% up to 95% in mass for values of the input microwave power of 1 and 1.5 kW. The results show that for a power of 1 kW and gradually increasing the oxygen admixture from 1% to 25% the electron density drops by a factor of more than four due to the energy lost by the electrons due to dissociation of oxygen molecules and the gas heating. An analysis of the number densities of species produced in the Ar-O2 plasma is presented. Oxygen admixtures of above 50% are considered along with an increase of the input microwave power in order to supply the discharge with electron number density values of the order of 1019 m-3. Gas temperatures above 3700 K are obtained in the plasma core along with a strong production of oxygen atoms with a number density of the order of 1023 m-3.
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    X-ray emission from stainless steel foils irradiated by femtosecond petawatt laser pulses
    (Bristol : IOP Publ., 2018) Alkhimova, M.A.; Faenov, A.Ya.; Pikuz, T.A.; Skobelev, I.Yu.; Pikuz, S.A.; Nishiuchi, M.; Sakaki, H.; Pirozhkov, A.S.; Sagisaka, S.; Dover, N.P.; Kondo, Ko.; Ogura, K.; Fukuda, Y.; Kiriyama, H.; Esirkepov, T.; Bulanov, S V.; Andreev, A.; Kando, M.; Zhidkov, A.; Nishitani, K.; Miyahara, T.; Watanabe, Y.; Kodama, R.; Kondo, K.
    We report about nonlinear growth of x-ray emission intensity emitted from plasma generated by femtosecond petawatt laser pulses irradiating stainless steel foils. X-ray emission intensity increases as ∼ I 4.5 with laser intensity I on a target. High spectrally resolved x-ray emission from front and rear surfaces of 5 μm thickness stainless steel targets were obtained at the wavelength range 1.7-2.1 Å, for the first time in experiments at femtosecond petawatt laser facility J-KAREN-P. Total intensity of front x-ray spectra three times dominates to rear side spectra for maximum laser intensity I ≈ 3.21021 W/cm2. Growth of x-ray emission is mostly determined by contribution of bremsstrahlung radiation that allowed estimating bulk electron plasma temperature for various magnitude of laser intensity on target.
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    Looking inside the tunnelling barrier: II. Co- and counter-rotating electrons at the ‘tunnelling exit’
    (Bristol : IOP Publ., 2018-08-03) Kaushal, Jivesh; Smirnova, Olga
    The initial conditions for electron trajectories at the exit from the tunnelling barrier are often used in strong field models, for example to bridge the first and the second steps of the three-step model celebrated in this issue. Since the analytical R-matrix theory does not rely on the three-step model or the concept of the tunnelling barrier in coordinate space, obtaining the initial conditions for electron trajectories at the barrier exit is, strictly speaking, not necessary to calculate standard observables. Not necessary, but possible—especially when motivated by the occasion of this issue. The opportunity to evaluate such initial conditions emerges as a corollary of analysing sub-barrier kinematics, which includes the interplay of laser and Coulomb fields on the sub-cycle scale (see the companion paper I). We apply our results to discuss the difference in such initial conditions for co- and counter-rotating electrons liberated during strong field ionisation. We derive quantum orbits and classical trajectories describing ionization dynamics of co- and counter-rotating electrons in long-range potentials.
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    Terahertz magnetic field enhancement in an asymmetric spiral metamaterial
    (Bristol : IOP Publ., 2018-10-25) Polley, Debanjan; Hagström, Nanna Zhou; Schmising, Clemens von Korff; Eisebitt, Stefan; Bonetti, Stefano
    We use finite element simulations in both the frequency and the time-domain to study the terahertz resonance characteristics of a metamaterial (MM) comprising a spiral connected to a straight arm. The MM acts as a RLC circuit whose resonance frequency can be precisely tuned by varying the characteristic geometrical parameters of the spiral: inner and outer radius, width and number of turns. We provide a simple analytical model that uses these geometrical parameters as input to give accurate estimates of the resonance frequency. Finite element simulations show that linearly polarized terahertz radiation efficiently couples to the MM thanks to the straight arm, inducing a current in the spiral, which in turn induces a resonant magnetic field enhancement at the center of the spiral. We observe a large (approximately 40 times) and uniform (over an area of ∼10 μm2) enhancement of the magnetic field for narrowband terahertz radiation with frequency matching the resonance frequency of the MM. When a broadband, single-cycle terahertz pulse propagates towards the MM, the peak magnetic field of the resulting band-passed waveform still maintains a six-fold enhancement compared to the peak impinging field. Using existing laser-based terahertz sources, our MM design allows to generate magnetic fields of the order of 2 T over a time scale of several picoseconds, enabling the investigation of nonlinear ultrafast spin dynamics in table-top experiments. Furthermore, our MM can be implemented to generate intense near-field narrowband, multi-cycle electromagnetic fields to study generic ultrafast resonant terahertz dynamics in condensed matter.
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    Phase- and intensity-resolved measurements of above threshold ionization by few-cycle pulses
    (Bristol : IOP Publ., 2018-06-11) Kübel, M.; Arbeiter, M.; Burger, C.; Kling, Nora G.; Pischke, T.; Moshammer, R.; Fennel, T.; Kling, M.F.; Bergues, B.
    We investigate the carrier-envelope phase (CEP) and intensity dependence of the longitudinal momentum distribution of photoelectrons resulting from above threshold ionization of argon by few-cycle laser pulses. The intensity of the pulses with a center wavelength of 750 nm is varied in a range between 0.7 × 1014 and . Our measurements reveal a prominent maximum in the CEP-dependent asymmetry at photoelectron energies of 2 U P (U P being the ponderomotive potential), that is persistent over the entire intensity range. Further local maxima are observed around 0.3 and 0.8 U P. The experimental results are in good agreement with theoretical results obtained by solving the three-dimensional time-dependent Schrödinger equation. We show that for few-cycle pulses, the amplitude of the CEP-dependent asymmetry provides a reliable measure for the peak intensity on target. Moreover, the measured asymmetry amplitude exhibits an intensity-dependent interference structure at low photoelectron energy, which could be used to benchmark model potentials for complex atoms.
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    Delayed relaxation of highly excited naphthalene cations
    (Bristol : IOP Publ., 2020) Reitsma, G.; Hummert, J.; Dura, J.; Loriot, V.; Vrakking, M.J.J.; Lépine, F.; Kornilov, O.
    The efficiency of energy transfer in ultrafast electronic relaxation of molecules depends strongly on the complex interplay between electronic and nuclear motion. In this study we use wavelength-selected XUV pulses to induce relaxation dynamics of highly excited cationic states of naphthalene. Surprisingly, the observed relaxation lifetimes increase with the cationic excitation energy. We propose that this is a manifestation of a quantum mechanical population trapping that leads to delayed relaxation of molecules in the regions with a high density of excited states. © 2019 Published under licence by IOP Publishing Ltd.
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    Laser-induced surface modification of biopolymers - Micro/nanostructuring and functionalization
    (Bristol : IOP Publ., 2018) Stankova, N.E.; Atanasov, P.A.; Nedyalkov, N.N.; Tatchev, Dr.; Kolev, K.N.; Valova, E.I.; Armyanov, St.A.; Grochowska, K.; Śliwiński, G.; Fukata, N.; Hirsch, D.; Rauschenbach, B.
    The medical-grade polydimethylsiloxane (PDMS) elastomer is a widely used biomaterial in medicine for preparation of high-tech devices because of its remarkable properties. In this paper, we present experimental results on surface modification of PDMS elastomer by using ultraviolet, visible, and near-infrared ns-laser system and investigation of the chemical composition and the morphological structure inside the treated area in dependence on the processing parameters - wavelength, laser fluence and number of pulses. Remarkable chemical transformations and changes of the morphological structure were observed, resulting in the formation of a highly catalytically active surface, which was successfully functionalized via electroless Ni and Pt deposition by a sensitizing-activation free process. The results obtained are very promising in view of applying the methods of laser-induced micro- and nano-structuring and activation of biopolymers' surface and further electroless metal plating to the preparation of, e.g., multielectrode arrays (MEAs) devices in neural and muscular surface interfacing implantable systems.