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- ItemElastomeric Optical Waveguides by Extrusion Printing(Weinheim : Wiley, 2022) Feng, Jun; Zheng, Yijun; Jiang, Qiyang; Włodarczyk‐Biegun, Małgorzata K.; Pearson, Samuel; del Campo, AránzazuAdvances in optogenetics and the increasing use of implantable devices for therapies and health monitoring are driving demand for compliant, biocompatible optical waveguides and scalable methods for their manufacture. Molding, thermal drawing, and dip-coating are the most prevalent approaches in recent literature. Here the authors demonstrate that extrusion printing at room temperature can be used for continuous fabrication of compliant optical waveguides with polydimethylsiloxane (PDMS) core and crosslinked Pluronic F127-diacrylate (Pluronic-DA) cladding. The optical fibers are printed from fluid precursor inks and stabilized by physical interactions and photoinitiated crosslinking in the Pluronic-DA. The printed fibers show optical loss values of 0.13–0.34 dB cm–1 in air and tissue within the wavelength range of 405–520 nm. The fibers have a Young's Modulus (Pluronic cladding) of 150 kPa and can be stretched to more than 5 times their length. The optical loss of the fibers shows little variation with extension. This work demonstrates how printing can simplify the fabrication of compliant and stretchable devices from materials approved for clinical use. These can be of interest for optogenetic or photopharmacology applications in extensible tissues, like muscles or heart.
- ItemPrecipitate number density determination in microalloyed steels by complementary atom probe tomography and matrix dissolution(Dordrecht [u.a.] : Springer Science + Business Media B.V, 2022) Weber, Louis; Webel, Johannes; Mücklich, Frank; Kraus, TobiasParticle number densities are a crucial parameter in the microstructure engineering of microalloyed steels. We introduce a new method to determine nanoscale precipitate number densities of macroscopic samples that is based on the matrix dissolution technique (MDT) and combine it with atom probe tomography (APT). APT counts precipitates in microscopic samples of niobium and niobium-titanium microalloyed steels. The new method uses MDT combined with analytical ultracentrifugation (AUC) of extracted precipitates, inductively coupled plasma–optical emission spectrometry, and APT. We compare the precipitate number density ranges from APT of 137.81 to 193.56 × 1021 m−3 for the niobium steel and 104.90 to 129.62 × 1021 m−3 for the niobium-titanium steel to the values from MDT of 2.08 × 1021 m−3 and 2.48 × 1021 m−3. We find that systematic errors due to undesired particle loss during extraction and statistical uncertainties due to the small APT volumes explain the differences. The size ranges of precipitates that can be detected via APT and AUC are investigated by comparison of the obtained precipitate size distributions with transmission electron microscopy analyses of carbon extraction replicas. The methods provide overlapping resulting ranges. MDT probes very large numbers of small particles but is limited by errors due to particle etching, while APT can detect particles with diameters below 10 nm but is limited by small-number statistics. The combination of APT and MDT provides comprehensive data which allows for an improved understanding of the interrelation between thermo-mechanical controlled processing parameters, precipitate number densities, and resulting mechanical-technological material properties. Graphical abstract: [Figure not available: see fulltext.]
- ItemAnalysis of the compressible, isotropic, neo-Hookean hyperelastic model(Dordrecht [u.a.] : Springer Science + Business Media B.V, 2023) Kossa, Attila; Valentine, Megan T.; McMeeking, Robert M.The most widely-used representation of the compressible, isotropic, neo-Hookean hyperelastic model is considered in this paper. The version under investigation is that which is implemented in the commercial finite element software ABAQUS, ANSYS and COMSOL. Transverse stretch solutions are obtained for the following homogeneous deformations: uniaxial loading, equibiaxial loading in plane stress, and uniaxial loading in plane strain. The ground-state Poisson’s ratio is used to parameterize the constitutive model, and stress solutions are computed numerically for the physically permitted range of its values. Despite its broad application to a number of engineering problems, the physical limitations of the model, particularly in the small to moderate stretch regimes, are not explored. In this work, we describe and analyze results and make some critical observations, underlining the model’s advantages and limitations. For example, a snap-back feature of the transverse stretch is identified in uniaxial compression, a physically undesirable behavior unless validated by experimental data. The domain of this non-unique solution is determined in terms of the ground-state Poisson’s ratio and the state of stretch and stress. The analyses we perform are essential to enable the understanding of the characteristics of the standard, compressible, isotropic, neo-Hookean model used in ABAQUS, ANSYS and COMSOL. In addition, our results provide a framework for the parameter-fitting procedure needed to characterize this standard, compressible, isotropic neo-Hookean model in terms of experimental data.
- ItemMicroscopic Softening Mechanisms of an Ionic Liquid Additive in an Electrically Conductive Carbon-Silicone Composite(Weinheim : Wiley, 2022) Zhang, Long; Schmidt, Dominik S.; González‐García, Lola; Kraus, TobiasThe microstructural changes caused by the addition of the ionic liquid (IL) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide to polydimethylsiloxane (PDMS) elastomer composites filled with carbon black (CB) are analyzed to explain the electrical, mechanical, rheological, and optical properties of IL-containing precursors and composites. Swelling experiments and optical analysis indicate a limited solubility of the IL in the PDMS matrix that reduces the cross-linking density of PDMS both globally and locally, which reduces the Young's moduli of the composites. A rheological analysis of the precursor mixture shows that the IL reduces the strength of carbon–carbon and carbon–PDMS interactions, thus lowering the filler–matrix coupling and increasing the elongation at break. Electromechanical testing reveals a combination of reversible and irreversible piezoresistive responses that is consistent with the presence of IL at microscopic carbon–carbon interfaces, where it enables re-established electrical connections after stress release but reduces the absolute conductivity.
- ItemTuning the Release Force of Microfibrillar Adhesives by Geometric Design(Weinheim : Wiley-VCH, 2022) Barnefske, Lena; Rundel, Fabian; Moh, Karsten; Hensel, René; Zhang, Xuan; Arzt, EduardSwitchable micropatterned adhesives exhibit high potential as novel resource-efficient grippers in future pick-and-place systems. In contrast with the adhesion acting during the “pick” phase, the release during the “place” phase has received little research attention so far. For objects smaller than typically 1 mm, release may become difficult as gravitational and inertial forces are no longer sufficient to allow shedding of the object. A compressive overload can initiate release by elastic buckling of the fibrils, but the switching ratio (ratio between high and low adhesion force) is typically only 2–3. In this work, new microfibrillar designs are reported exhibiting directional buckling with high switching ratios in the order of 20. Their functionality is illustrated by in situ optical observation of the contact signatures. Such micropatterns can enable the successful release of small objects with high placement accuracy.
- ItemNanoscale disintegration kinetics of mesoglobules in aqueous poly(N-isopropylacrylamide) solutions revealed by small-angle neutron scattering and pressure jumps(Cambridge : RSC Publ., 2021) Niebuur, Bart-Jan; Chiappisi, Leonardo; Jung, Florian A.; Zhang, Xiaohan; Schulte, Alfons; Papadakis, Christine M.Identification and control of the disintegration mechanism of polymer nanoparticles are essential for applications in transport and release including polymer delivery systems. Structural changes during the disintegration of poly(N-isopropylacrylamide) (PNIPAM) mesoglobules in aqueous solution are studied in situ and in real time using kinetic small-angle neutron scattering with a time resolution of 50 ms. Simultaneously length scales between 1 and 100 nm are resolved. By initiating phase separation through fast pressure jumps across the coexistence line, 3 wt% PNIPAM solutions are rapidly brought into the one-phase state. Starting at the same temperature (35.1 °C) and pressure (17 MPa) the target pressure is varied over the range 25–48 MPa, allowing to systematically alter the osmotic pressure of the solvent within the mesoglobules. Initially, the mesoglobules have a radius of gyration of about 80 nm and contain a small amount of water. Two disintegration mechanisms are identified: (i) for target pressures close to the coexistence line, single polymers are released from the surface of the mesoglobules, and the mesoglobules decrease in size, which takes ∼30 s. (ii) For target pressures more distant from the coexistence line, the mesoglobules are swollen by water, and subsequently the chains become more and more loosely associated. In this case, disintegration proceeds within less than 10 s, controlled by the osmotic pressure of the solvent.