A. Endruweit

Photo-Cross-Linked Hydrogels from Thermoresponsive PEGMEMA-PPGMA-EGDMA Copolymers Containing Multiple Methacrylate Groups: Mechanical Property, Swelling, Protein Release, and Cytotoxicity

Photo-cross-linked hydrogels from thermoresponsive polymers can be used as advanced injectable biomaterials via a combination of physical interaction (in situ thermal gelation) and covalent cross-links (in situ photopolymerization). This can lead to gels with significantly enhanced mechanical properties compared to non-photo-cross-linked thermoresponsive hydrogels. Moreover, the thermally phase-separated gels have attractive advantages over non-thermoresponsive gels because thermal gelation upon injection allows easy handling and holds the shape of the gels prior to photopolymerization. In this study, water-soluble thermoresponsive copolymers containing multiple methacrylate groups were synthesized via one-step deactivation enhanced atom transfer radical polymerization (ATRP) of poly(ethylene glycol) methyl ether methacrylate (PEGMEMA, M(n) = 475), poly(propylene glycol) methacrylate (PPGMA, M(n) = 375), and ethylene glycol dimethacrylate (EGDMA) and were used to form covalent cross-linked hydrogels by photopolymerization. The cross-linking density was found to have a significant influence on the mechanical and swelling properties of the photo-cross-linked gels. Release studies using lysozyme as a model protein demonstrated a sustained release profile that varied dependent on the copolymer composition, cross-linking density, and the temperature. Mouse C2C12 myoblast cells were cultured in the presence of the copolymers at concentrations up to 1 mg/mL. It was found that the majority of the cells remained viable, as assessed by Alamar Blue, lactate dehydrogenase (LDH), and Live/Dead cell viability/cytotoxicity assays. These studies demonstrate that thermoresponsive PEGMEMA-PPGMA-EGDMA copolymers offer potential as in situ photopolymerizable materials for tissue engineering and drug delivery applications through a combination of facile synthesis, enhanced mechanical properties, tunable cross-linking density, low cytotoxicity, and accessible functionality for further structure modifications.

Analysis of Compressibility and Permeability of Selected 3D Woven Reinforcements

For three 3D woven carbon fiber reinforcements with different architectures, the compressibility, geometrical structure, and permeability were studied. At low levels of compression, the thickness of an angle-interlock weave is reduced mainly by local reduction of the height of inter-bundle voids and permanent reordering of the fiber bundles. At higher compression levels, bundle compaction is dominant. For an orthogonal weave fabric, the main compression mechanism is compaction of the fiber bundles. The in-plane permeability is modeled by superimposing a disturbance, caused by the binder yarns, to the permeability of a regular layered fiber structure. It is characterized mainly by the dimensions of the inter-bundle voids in each layer, and the pattern and dimensions of the binder yarns. Because of the dependence on the inter-bundle void dimensions, the fabric compression behavior is reflected. The through-thickness permeability is determined by flow-enhancing channels in the structure of the reinforcement, formed around the binder yarns. It is modeled as a function of the number of binder yarns per unit surface area, and the dimensions and orientation of the binder yarns.

Mechanisms of hydrodynamically induced in plane deformation of reinforcement textiles in resin injection processes

The effects of hydrodynamically induced textile deformation during resin injection in Liquid Composites Molding processes have been experimentally investigated for various fiber volume fractions in a rectangular flow channel with linear injection gate. For layups from bi-directional ±45° engineered glass fiber fabric, three deformation effects have been identified: Wrinkling of the topmost fabric layer for low fiber volume fractions, instantaneous in-plane compression of the layup during fiber wetting for intermediate fiber volume fractions, delayed in-plane compression of the wetted part of the fabric for high fiber volume fractions. The deformation behavior is determined by the fabric stiffness and the fixation in the flow channel due to clamping. The fabric has been experimentally characterized with respect to compressibility and coefficient of static friction in order to understand the clamping effect. The injection pressure, which is critical for deformation, has been determined as function of the fiber volume fraction, i.e. as function of the clamping pressure. The characteristics of the function have been correlated with the three observed deformation effects. A simple mechanical model for description of layup displacement and compression, implying visco-elastic behavior of the fabric and clamping with friction on the flow channel walls for various impregnation states, is presented.

Textile composites with integrated optical fibres: quantification of the influence of single and multiple fibre bends on the light transmission using a Monte Carlo ray-tracing method

Monte Carlo ray-tracing simulations of the propagation of light rays along bent multimode optical fibres indicate that, for optical fibres integrated into composite components, the influence of macroscale bends induced by the component shape on the light transmission is relatively small. Mesoscale bends caused by integration of the fibres into the reinforcement fabric structure applying textile processes may cause significant transmission losses, which decrease exponentially with increasing ratio of bending radius and fibre radius and increase with increasing bending angles. Based on geometrical models of optical fibres integrated in woven fabric structures, which show multiple mesoscale bends, simulations prove that the lowest bending losses occur for fabrics with a low degree of crimp.

Multiscale modeling of combined deterministic and stochastic fabric non-uniformity for realistic resin injection simulation

Abstract The local fiber arrangement in a bi-directional fabric formed to a complex shape was modeled considering the stochastic arrangement of filaments within yarns, which determines axial and transverse yarn permeabilities, and the stochastic arrangement of yarns in a fabric, which determines the dimensions of interyarn gap spaces locally. To mimic the uncertainty in fabric forming, drape simulation was randomized in terms of start point and yarn start orientations. From yarn permeabilities and simulated local yarn spacing distributions, local fiber volume fractions and fabric permeabilities were approximated. This allowed resin injection into a deformed fabric to be simulated for different drape scenarios with different probabilities and different degrees of fabric randomness. The results indicate that variability in fabric properties and the forming process affects flow front shapes and times for complete impregnation of the reinforcement.

Influence of Fixation Structures on the In-Plane Permeability of Unidirectional Glass Fiber Tapes

In this study the influence of a fixation grid on the permeability of a uni-directional glass fiber tape has been experimentally investigated with respect to processing of the material applying Liquid Composites Molding technology. Two-dimensional flow experiments show that for the given material and configuration the fixation structure has a strong influence on the permeability. The ratio of the permeability principal values is found to depend on the porosity. For a critical porosity even isotropic behavior can be expected. This result is verified by additional one-dimensional flow experiments with variation of the fiber angle of the lay-ups, showing that for the critical porosity the fiber angle has no significant influence on the permeability. The ratio of the permeability principal values does not significantly differ from 1. For the glass fiber tape without fixation structure the permeability principal values have been found to differ by one order of magnitude. Comparison of the absolute permeability values for the tape with and without fixation structure suggests that (for a total of 12 material layers) each layer of the fixation grid reduces the permeability of the lay-up by 8% (parallel) and 6% (transversal) for completely parallel oriented material and by 8% for a 0°/90° alternating lay-up. For a given fiber material, the influence of the fixation structure on the permeability is predicted to increase with decreasing porosity and to decrease with increasing area related mass, i.e. overall thickness, of the tape.

Effect of specimen history on structure and in-plane permeability of woven fabrics

Before being processed into composites, reinforcement fabrics may undergo repeated involuntary deformation, the complete sequence of which is here referred to as specimen history. To mimic its effect, fabric specimens were subjected to sequences of defined shear operations. For single fabric layers with unconstrained thickness, quantitative evaluation of photographic image data indicated that repeated shear deformation results in a residual increase in inter-yarn gap width. This translates into an increase in measured fabric permeabilities in multi-layer lay-ups at given compaction levels. The extent of both interrelated effects increases with increasing yarn density in the fabric and with increasing maximum angle in the shear history. Additional numerical permeability predictions indicated that the increase in permeability may be partially reversed by through-thickness fabric compression. The observations suggest that the effect of involuntary deformation of the fabric structure can result in variations in the principal permeability values by factors of up to 2.

High-performance Composites for Applications in Medical Engineering: Susceptibility Artifacts in Magnetic Resonance Imaging:

The interaction of carbon fiber/epoxy composite parts with the magnetic fields in magnetic resonance imaging (MRI) for medical diagnostics has been quantitatively estimated for the example of the Riechert stereotactic head ring. Rings from carbon/epoxy composite have been compared with rings from other materials with respect to their influence on the applied magnetic field. For a fiber volume fraction ’ 0.5 and an angle between the fiber orientation and the field direction 90, the magnetic field disturbances caused by composite rings are similar to those caused by non-ferromagnetic metals as Cu or Al. Due to their significantly lower susceptibility values, carbon HT fibers are to be preferred to carbon HM fibers with respect to their interaction with magnetic fields. Induction of significant macroscopic eddy currents for changes of the magnetic flux through the rings is not to be expected.

Orientation of the Permeability Principal Axes in Bi-Directional Textile Fabrics

For various commercial woven glass fiber fabrics, the orientation of the permeability principal axes has been experimentally determined as function of the fiber angle, carrying out two-dimensional flow experiments with radial liquid injection. The investigated fabrics have in general been found to significantly show anisotropic properties even for the unsheared material. The orientation of the permeability principal axes of sheared fabrics is related to the degree of anisotropy, characterized by the ratio of the permeability principal values for the unsheared fabrics. The orientation of the permeability principal axes as a function of the fiber angle is described by an empirical formula, with the ratio of the permeability principal values of the unsheared fabric as parameter. Comparison of experimental and theoretical results in general indicates correspondence. Exceptions can be explained by uncertainties in the experimentally determined permeability principal values of the unsheared fabric.

Influence of the micro-structure on saturated transverse flow in fibre arrays:

This study analyses the influence of the random filament arrangement in fibre bundles on the resin flow behaviour. Transverse steady-state resin flow that occurs behind a liquid resin flow front was simulated numerically through statistically equivalent micro-structures at high-fibre volume fractions, Vf > 0.6, as observed in fibre bundles. The need of applying a minimum gap distance between neighbouring filaments was overcome by automated local mesh refinement. The derived permeability values showed significant scatter. Convergence of these values was determined at a ratio of flow length to filament radius greater than 20 for all three analysed fibre volume fractions. Mean permeabilities were between 6 and 10 times lower than those predicted for a hexagonal fibre array. A statistical model is proposed, which is able to predict the scatter of observed permeabilities based on simple micro-structural descriptors.