Electromagnetic Field Controlled Domain Wall Displacement for Induced Strain Tailoring in Batio3-Epoxy Nanocomposite: Development of A Contact-Less Active Toughening Mechanism

Failure in an epoxy polymer composite material is prone to initiate by the coalescence of microcracks in its polymer matrix. As such, matrix toughening via addition of a second phase as rigid or/and rubber nano/micro-particles is one of the most popular approaches to improve the fracture toughness across multiple scales in a polymer composite, which dissipates fracture energy via deformation mechanisms and microcracks arrest. Few studies have focused on tailorable and variable toughening, so-called ‘active toughening’, mainly suggesting thermally induced strains which offer slow and irreversible toughening due to polymer’s poor thermal conductivity. The research presented in the current article has developed an instantaneous, reversible active toughening composite based upon contact-less introduction of a microscopic compressive extrinsic strain field via remote electromagnetic radiation. Quantification of the extrinsic strain evolving in the composite with the microwave energy has been conducted using in-situ real-time fibre optic sensing. A theoretical constitutive equation correlating the exposure energy to micro-strains has been developed, with its solution validating the experimental data and describing their underlying physics. The research has utilised functionalised dielectric ferroelectric nanomaterials, barium titanate (BaTiO3), as a second phase dispersed in an epoxy matrix, able to introduce microscopic electro-strains to their surrounding rigid epoxy subjected to an external electric field (microwaves, herein), as result of their domain walls dipole displacements. Epoxy Araldite LY1564, a diglycidyl ether of bisphenol A (DGEBA) associated with the curing agent Aradur 3487 were embedded with the BaTiO3 nanoparticles. The silane coupling agent for the nanoparticles’ surface functionalisation was 3-glycidoxypropyl trimethoxysilane (3-GPS). Hydrogen peroxide (H2O2, 30%) and acetic acid (C2H4O2, 99.9%) used as functionalisation aids, and the ethanol (C2H6O, 99.9%) used for BaTiO3 dispersion. Firstly, the crystal microstructure of the functionalised nanoparticles and the thermal and dielectric properties of the achieved epoxy composite materials have been characterised. It has been observed that the addition of the dielectric nanoparticles has a slight impact on the curing extent of the epoxy. Secondly, the surface-bonded fibre bragg grating (FBG) sensors have been employed to investigate the real-time variation of strain and temperature in the epoxy composites exposed to microwaves at 2.45 GHz and at different exposure energy. The strains developed due to the in-situ exposure at composite, adhesive and their holding fixture material were evaluated using the FBG. The domain wall induced extrinsic strains were distinguished from the thermally induced strains, and found that the increasing exposure energy has an instantaneously increasing effect on the development of compressive strains. Post-exposure Raman spectra showed no residual field in the composite indicating no remnant strain field examined under microwave powers < 1000 W, thus suggesting a reversible strain introduction mechanism, i.e. the composite retaining its nominal properties post exposure. The dielectric composite development and quantifications presented in this article proposes a novel active toughening technology for high-performance composite applications in numerous sectors.

Metabolic responses to polychromatic LED and OLED light at night

Light exposure at night has various implications for human health, but little is known about its effects on energy metabolism during subsequent sleep. We investigated the effects of polychromatic white light using conventional light-emitting diodes (LED) and an alternative light source, organic light-emitting diodes (OLED), producing reduced spectral content in the short wavelength of blue light (455 nm). Ten male participants were exposed to either LED, OLED (1000 lx), or dim (< 10 lx) light for 4 h before sleep in a metabolic chamber. Following OLED exposure, energy expenditure and core body temperature during sleep were significantly decreased (p < 0.001). Fat oxidation during sleep was significantly reduced (p = 0.001) after the exposure to LED compared with OLED. Following exposure to OLED, fat oxidation positively correlated with the 6-sulfatoxymelatonin levels, suggesting that the role of melatonin in lipolysis differs depending on the light. These findings advance our knowledge regarding the role of light in energy metabolism during sleep and provide a potential alternative to mitigate the negative consequences of light exposure at night.

Highly Flexible and Photo-Activating Acryl-Polyurethane for 3D Steric Architectures

An acryl-functionalized polyurethane (PU) series was successfully synthesized using poly(tetramethylene ether) glycol-methylene diphenyl diisocyanate (PTMG-MDI) oligomer based on urethane methacrylates to control the flexibility of photo-cured 3D printing architectures. The mass ratio of acryl-urethane prepolymer: 1,4-butanediol (BD) chain-extender: diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide (TPO) photoinitiator was 10:0.25:1. To produce suitably hard and precisely curved 3D architectures, the optimal UV absorbance and exposure energy of the acryl-PTMG-MDI resin were controlled precisely. Owing to the optimized viscosity of the acryl-PTMG-MDI resins, they could be printed readily by digital light processing (DLP) to form precisely curved 3D architectures after mixing with 1,6-hexanediol diacrylate (HDDA). The acryl-PTMG-MDI formulations showed much better flexural resolution than the neat resins. The printed 3D structure exhibited high surface hardness, good mechanical strength, and high elasticity for flexible applications in consumer/industrial and biomedical fields.

A Facile Fabrication Route of Poly(Ethylene Glycol Phenyl Ether Acrylate) Photopolymers with Efficient Optical Response for Holographic Storage

A series of photopolymers based on ethylene glycol phenyl ethyl arylate (EGPEA) monomers and poly(methyl methacrylate) (PMMA) matrix with varying initiator concentrations and sample thicknesses have been synthesized and their optical performance characterized in this study. The advantages of lowering the initiator concentration, including a rather short initiation time within a few seconds; a sharp rising optical response; and a stable saturated diffraction efficiency are demonstrated. The variation in the diffraction efficiency and response time with the exposure energy under various sample thickness and initiator concentrations is examined; a diffraction efficiency as high as 80% and a relatively short response time of 12–39 s are attainable. The dependence of the normalized optical parameter “sensitivity” on the exposure time is depicted, and the peak value of S ranges vastly from about 0.2 to 1.2 × 104 cm/J within a period of 15 s or so, with a maximum value of nearly 1.2 × 104. Favorable evidence of low initiator concentration can further be found when the dependence of the saturated diffraction efficiency with the exposure energy is examined. The data from this study using a low initiator concentration cover a range of exposure energy from 100 to 800 mJ/cm2 and a saturated diffraction efficiency from about 15% to 80%. The successful image reconstruction of 6-membered-ring metal nuts on a hologram based on this EGPEA/PMMA photopolymer system using a reflective holographic recording setup is demonstrated to verify its applicability to holographic storage.

Effect of design geometry, exposure energy, cytophilic molecules, cell type and load in fabrication of single-cell arrays using micro-contact printing

In this study a range of factors influencing the fabrication of single-cell arrays (SCAs) are identified and investigated. Micro-contact printing was used to introduce spots coated with polyethyleneimine or Matrigel on glass surfaces pre-coated with polyethylene glycol. Unmodified E. coli, Synechococcus sp., Chlamydomonas reinhardtii as well as diverse mammalian cells including HUVEC, AAV293, U87, OHS, PC3, SW480, HepG2 and AY-27 were successfully immobilised onto the chemically coated spots. The developed SCAs show high cell viability and probability for capturing single-cells. A discrepancy between the size and shape of the squares described in the design file and the actual structures obtained in the final PDMS structure is characterised and quantified. The discrepancy is found to be depending on the exposure energy used in the photolithography process as well as the size of the squares and their separation distance as detailed in the design file. In addition to these factors, the effect of the cell density loaded onto the patterned surfaces is also characterised. The systematic characterisation of key parameters that need to be optimised prior to the fabrication of SCAs is essential in order to increase the efficiency and reproducibility of future fabrication of SCAs for single-cell studies.

Microfabrication of Ni-Fe Mold Insert via Hard X-ray Lithography and Electroforming Process

In this research, a Ni-Fe mold insert for the efficient replication of high aspect-ratio microstructure arrays was fabricated via hard X-ray lithography and an electroforming process. For the X-ray exposure on a photoresist, a gold-based X-ray mask was prepared with conventional UV photolithography. The gold thickness was designed to be over 15 μm to prevent development underneath the absorber and to enhance the adhesion strength between the photoresist and substrate. By using the X-ray mask, a positive-type photoresist was selectively exposed to X-ray under an exposure energy of 4 kJ/cm3. Thereafter, the exposed region was developed in a downward direction to effectively remove the residual photoresist from the substrate. During the evaporation process, deionized water mixed with a surface additive prevented the bending and clustering of the photoresist microstructure arrays by lowering the capillary force, resulting in a defect-free mother structure for electroforming. Lastly, the mother structure was uniformly Ni-Fe electroformed on a conductive substrate without the formation of any pores or detachment from the substrate. Based on the proposed microfabrication process, a Ni-Fe mold insert with a 183 μm pattern size, 68 μm gap size, 550 μm height, 2116 microcavities and a hardness of 585 Hv was precisely manufactured. It can be utilized for the mass production of high aspect ratio metal and ceramic microstructure arrays in micro molding technologies.

Effects of the Layer Height and Exposure Energy on the Lateral Resolution of Zirconia Parts Printed by Lithography-Based Additive Manufacturing

Lithography-based ceramics manufacturing (LCM) processes enable the sophisticated 3 dimensional (3D) shaping of ceramics by additive manufacturing (AM). The build-up occurs, like many other AM processes, layer by layer, and is initiated by light. The built-in digital mirror device (DMD) enables the specific exposure of desired pixels for every layer, giving as a consequence a first estimation of the printing resolution in the x and y axes. In this work, a commercial zirconia slurry and the CeraFab 7500, both from Lithoz GmbH (Vienna, Austria), were used to investigate the potential of reaching this resolution. The results showed that the precision of a part is strongly dependent on the applied exposure energy. Higher exposure energies resulted in oversized dimensions of a part, whereas too low energy was not able to guarantee the formation of a stable part. Furthermore, the investigation of the layer thickness showed that the applied exposure energy (mJ/cm2) was acting in a volume, and the impact is visible in x, y, and z dimensions. The lowest applied exposure energy was 83 mJ/cm2 and showed the most accurate results for a layer thickness of 25 μm. With this energy, holes and gaps smaller than 500 μm could be printed; however, the measurements differed significantly from the dimensions defined in the design. Holes and gaps larger than 500 μm showed deviations smaller than 50 μm from the design and could be printed reliably. The thinnest printable gaps were between 100 and 200 μm. Concerning the wall thickness, the experiments were conducted to a height of 1 cm. Taking into account the stability and deformation of the walls as well, the best results after sintering were achieved with thicknesses of 200–300 μm.

Dopant occupancy and exposure energy of Zn(1 mol.%, 3 mol.%, 5 mol.%, 7 mol.%):Yb:Nd:LiNbO3 crystals

A series of Zn:Yb:Nd:LiNbO3 crystals with various concentrations of Zn[Formula: see text](1 mol.%, 3 mol.%, 5 mol.% and 7 mol.%) were grown by Czochralski method. The dopant occupancy and light-induced scattering ability of Zn:Yb:Nd:LiNbO3 crystals were measured and discussed by infrared spectra and exposure energy flux method. The results show that the infrared absorption peak of the Zn(7 mol.%):Yb:Nd:LiNbO3 crystal is blueshift and [Formula: see text] ion reaches threshold concentration. The light-induced scattering of Zn(7 mol.%):Yb:Nd:LiNbO3 crystals is increased by two-orders of magnitude compared to Zn(1 mol.%):Yb:Nd:LiNbO3 crystals.