Beyond backscattering: Optical neuroimaging by BRAD

Optical coherence tomography (OCT) is a powerful technology for rapid volumetric imaging in biomedicine. The bright field imaging approach of conventional OCT systems is based on the detection of directly backscattered light, thereby waiving the wealth of information contained in the angular scattering distribution. Here we demonstrate that the unique features of few-mode fibers (FMF) enable simultaneous bright and dark field (BRAD) imaging for OCT. As backscattered light is picked up by the different modes of a FMF depending upon the angular scattering pattern, we obtain access to the directional scattering signatures of different tissues by decoupling illumination and detection paths. We exploit the distinct modal propagation properties of the FMF in concert with the long coherence lengths provided by modern wavelength-swept lasers to achieve multiplexing of the different modal responses into a combined OCT tomogram. We demonstrate BRAD sensing for distinguishing differently sized microparticles and showcase the performance of BRAD-OCT imaging with enhanced contrast for ex vivo tumorous tissue in glioblastoma and neuritic plaques in Alzheimer's disease

Open-Porous Silicon Nitride-Based Ceramics in Tubular Geometry Obtained by Slip-Casting and Gelcasting

Owing to its unique properties, silicon nitride is a frequently used materials choice in highly demanding applications in terms of thermal and mechanical load. In this work, porous silicon nitride-based support materials in hollow-tube configuration are generated through colloidal forming, and their respective properties for potential applications in the fields of membrane-based separation, filtration, or catalysis are evaluated. Shaping of the ceramics is achieved by two distinct casting techniques, slip-casting, and gelcasting, and the results of the respective methods are set in relation. Furthermore, a special focus is set on the correlation between sintering parameters and resulting porosity. Subsequently, air permeabilities of the generated structures are determined, illustrating a direct relation between processing parameters and resulting permeability. Darcian permeability values of up to 9 · 10 m are observed for samples exhibiting total porosities between 32 and 41 %. The findings allow for a predictability of suitable permeation properties for the structures’ anticipated application as complex-shaped non-oxide ceramic supports for membrane-based separation or catalysis, or as high-performance filter materials. −16 2Fonds zur Förderung der wissenschaftlichen Forschung (FWF)1151

Dense, Strong, and Precise Silicon Nitride-Based Ceramic Parts by Lithography-Based Ceramic Manufacturing

Due to the high level of light absorption and light scattering of dark colored powders connected with the high refractive indices of ceramic particles, the majority of ceramics studied via stereolithography (SLA) have been light in color, including ceramics such as alumina, zirconia and tricalcium phosphate. This article focuses on a lithography-based ceramic manufacturing (LCM) method for β-SiAlON ceramics that are derived from silicon nitride and have excellent material properties for high temperature applications. This study demonstrates the general feasibility of manufacturing of silicon nitride-based ceramic parts by LCM for the first time and combines the advantages of SLA, such as the achievable complexity and low surface roughness (Ra = 0.50 µm), with the typical properties of conventionally manufactured silicon nitride-based ceramics, such as high relative density (99.8%), biaxial strength (σf = 764 MPa), and hardness (HV10 = 1500)

Freeze-casting of highly porous cellulose-nanofiber-reinforced γ-Al2O3 monoliths

Freeze-casting is a powerful consolidation technique for the fabrication of highly porous and layered-hybrid materials, including ceramic-metal composites, and porous scaffolds for catalysis, bone substitutes and high- performance membranes. The aqueous suspensions to be freeze-casted usually contain dense particles facilitating macroporous, layered ceramics with dense (nonporous) struts. In the present study, hierarchical macro-mesoporous alumina (HMMA) monoliths were successfully prepared by freeze-casting of aqueous suspensions containing hierarchically-assembled, mesoporous γ‒Al2O3 (MA) powder and cellulose nanofibers (CNF). As- prepared monoliths were ultra-porous (93.1–99.2%), had low densities (0.01–0.25 g/cm3), and displayed relatively high surface areas (91–134 m2/g), but were still remarkably rigid with high compressive strengths (up to 52 kPa). Owing to the columnar porosity and mesoporous nature of the struts the freeze-casted HMMA monoliths exhibited high permeability and high thermal insulation, the latter ranging from 0.039 W/m∙K to 0.071 W/m∙K, depending on pore orientation

Green one-pot synthesis and processing of polyimide–silica hybrid materials

Inorganic–organic hybrid materials allow for combining features typical of the inorganic component with those of the organic component in one material. Generally, the preparation of organic and inorganic compounds requires considerably different synthesis conditions. Hence, the development of one-pot routes to inorganic–organic hybrid materials is challenging. We herein report a fully green one-pot synthesis of polyimide/silica (PI/SiO2) hybrids. Specifically, we co-condense both components hydrothermally, using nothing but the respective precursors and water. Furthermore, we show that the PI and the SiO2 component can be covalently connected under hydrothermal conditions, using the compatibilizer (3-aminopropyl)-triethoxysilane. We thoroughly investigate the effect of different reaction conditions, including temperature, pH, precursor concentration and reaction time on the morphology and crystallinity of the final materials. The polyimide component, poly(hexamethylene pyromellitimide) was chosen for its thermoplasticity, which allows for processing both the PI and the PI/SiO2via sintering. For being a solvent-free method, sintering qualifies as a green processing technique. This work is the first report of the simultaneous hydrothermal condensation of an inorganic and an organic material.publishe

High modulus polyimide particle-reinforcement of epoxy composites

In this work, a novel class of fully organic, lightweight composite materials was prepared by incorporating highly crystalline, hydrothermally synthesized poly(p-phenylene pyromellitimide) (PPPI) microparticles into a commercial epoxy matrix. Particle loadings of up to 15 vol% could be achieved. Microstructural, mechanical, and thermal properties of these composites were investigated by scanning electron microscopy, three-point bending and Vickers hardness testing, dynamic mechanical analysis, nanoindentation, and thermogravimetric analysis. The incorporation of the PPPI filler particles into the epoxy matrix was found to be homogeneous. Powder X-ray diffraction shows that PPPI's crystallinity is retained in the composites, and infrared spectroscopy indicates a covalent bonding of PPPI to the epoxy matrix. Flexural modulus and storage modulus were increased by the PPPI addition, while the flexural strain at break was reduced. In contrast to that, the flexural strength remained unaffected by the incorporation of PPPI filler particles. Raising the filler content also resulted in an improvement of hardness. Furthermore, a decrease in glass-transition temperature with increasing PPPI content was observed, as well as a pronounced increase in thermal stability of the composites in comparison to the unfilled cured epoxy resin. These results indicate the high potential of this new class of composites with prospective applicability e.g. in the fields of sports equipment, aerospace, and automotive technology.publishe

Influence of DVB as linker molecule on the micropore formation in polymer-derived SiCN ceramics

In this work, the formation of transient microporosity during the polymer-to-ceramic conversion in polymer-derived ceramics was studied using a commercially available poly(vinyl)silazane precursor which was modified with divinylbenzene (DVB) as linker molecule during crosslinking. After pyrolysis treatments between 400 and 700 °C, the resulting materials not only showed distinct changes in elemental composition and structural features upon introduction of the linker molecule, but also a shift in micropore size, e.g. shifting from 0.84 nm to 0.70 nm after pyrolysis at 600 °C. Due to hindrance of transamination reactions in the low-temperature region, the nitrogen content in linker-containing samples was significantly higher, leading to a different composition of micropore-forming entities and, in case of the system studied, to smaller pores. These findings are a first step towards the clarification of the mechanisms leading to the pore formation in PDCs during pyrolytic conversion, which is essential for their use in prospective applications.Fonds zur Förderung der wissenschaftlichen Forschung (FWF)329233021

Polymer-derived Ni/SiOC materials structured by vat-based photopolymerization with catalytic activity in CO2 methanation

A new concept for the additive manufacturing of nickel-modified polymer-derived ceramics via vat-based photopolymerization is presented. A photoactive polysiloxane resin system modified by nickel nitrate via methacrylic acid complexation was developed and modified to facilitate vat-based photopolymerization. Through pyrolysis of the Ni-modified preceramic polymer at temperatures between 600 and 800 °C, amorphous SiOC components with well-dispersed Ni nanoparticles can be obtained. The modified polymer and the fabricated structures were characterized by photorheology, thermal analysis, scanning and transmission electron microscopy, optical coherence tomography, and powder X-ray diffraction. In addition, the effect of pyrolysis temperature on specific surface area, crystallinity, and shrinkage was investigated. The developed material systems enable additive manufacturing of porous SiOC structures containing crystalline, uniformly distributed, and bimodally sized Ni nanoparticles, exhibiting catalytic activity suitable for CO2 methanation. The developed printable SiOC/Ni materials represent a promising approach for combining metal-modified polymer-derived ceramic systems and additive manufacturing for prospective catalysis applications

Polarization sensitive optical coherence tomography of melanin provides tissue inherent contrast based on depolarization

Polarization sensitive optical coherence tomography (PS-OCT) was used to investigate the polarization properties of melanin. Measurements in samples with varying melanin concentrations revealed polarization scrambling, i.e. depolarization. The results indicate that the depolarizing appearance of pigmented structures like, for instance, the retinal pigment epithelium (RPE) is likely to be caused by the melanin granules contained in these cells.Austrian Science Fund (FWF grants P19624-B02 and P19751-N20