Macroporous ceramics with pore sizes from 400 nm to 4 mm and porosity within the range 20%–97% have been produced for a number of well-established and emerging applications, such as molten metal filtration, catalysis, refractory insulation, and hot gas filtration. These applications take advantage of the unique properties achieved through the incorporation of macropores into solid ceramics. In this article, we review the main processing routes that can be used for the fabrication of macroporous ceramics with tailored microstructure and chemical composition. Emphasis is given to versatile and simple approaches that allow one to control the microstructural features that ultimately determine the properties of the macroporous material. Replica, sacrificial template, and direct foaming techniques are described and compared in terms of microstructures and mechanical properties that can be achieved. Finally, directions to future investigations on the processing of macroporous ceramics are proposed.

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Processing Routes to Macroporous Ceramics: A Review

Interaction of nanoparticles with proteins is the basis of nanoparticle bio-reactivity. This interaction gives rise to the formation of a dynamic nanoparticle-protein corona. The protein corona may influence cellular uptake, inflammation, accumulation, degradation and clearance of the nanoparticles. Furthermore, the nanoparticle surface can induce conformational changes in adsorbed protein molecules which may affect the overall bio-reactivity of the nanoparticle. In depth understanding of such interactions can be directed towards generating bio-compatible nanomaterials with controlled surface characteristics in a biological environment. The main aim of this review is to summarise current knowledge on factors that influence nanoparticle-protein interactions and their implications on cellular uptake.

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Interaction of nanoparticles with proteins: relation to bio-reactivity of the nanoparticle

Keywords: Droplets Bubbles Microfluidics Encapsulation Emulsion a b s t r a c t In this paper, we emphasize our long series of experiments proving that the physical processes along fluid interfaces can be exploited for creating unusual fluidic objects. We report for the first time a couple of new fluidic objects so-called “liquid onions” and “mayonnaise” droplets. The study starts from the observation of antibubbles, exhibiting unstable liquid‐air‐liquid interfaces. We show that the lifetime of such a system has the same origin as floating/coalescing droplets on liquid surfaces. By analyzing such behaviours, we created droplets bouncing on a liquid bath. The methods and physical phenomena collected in this paper provide a basis for the development of a discrete microfluidics. Open questions are underlined, experimental challenges and future applications are proposed.

Colloids and Surfaces A: Physicochemical and Engineering Aspects

The conformational changes of bovine serum albumin (BSA) in the albumin:gold nanoparticle bioconjugates were investigated in detail by various spectroscopic techniques including UV-vis absorption, fluorescence, circular dichroism, and Fourier transform infrared spectroscopies. Our studies suggested that albumin in the bioconjugates that was prepared by the common adsorption method underwent substantial conformational changes at both secondary and tertiary structure levels. BSA was found to adopt a more flexible conformational state on the boundary surface of gold nanoparticles as a result of the conformational changes in the bioconjugates. The conformational changes at pH 3.8, 7.0, and 9.0, which corresponded to different isomeric forms of albumin, were investigated, respectively, to probe the pH effect on the conformational changes of BSA in the bioconjugates. The results showed that the pH of the medium influenced the changes greatly and that fluorescence and circular dichroism studies further indicated that the changes were larger at higher pH.

pH-dependent protein conformational changes in albumin:gold nanoparticle bioconjugates : A spectroscopic study

Cellular ceramics are a class of highly porous materials that covers a wide range of structures, such as foams, honeycombs, interconnected rods, interconnected fibres, interconnected hollow spheres. Recently, there has been a surge of activity in this field, because these innovative materials have started to be used as components in special and advanced engineering applications. These include filtering liquids and particles in gas streams, porous burners, biomedical devices, lightweight load-bearing structures, etc. Improvements in conventional processing methods and the development of innovative fabrication approaches are required because of the increasing specific demands on properties and morphology (cell size, size distribution and interconnection) for these materials, which strictly depend on the application considered. This paper will cover the main fabrication methods for cellular ceramics, focusing primarily on foams, offering some insight into novel fabrication processes and recent developments.

Conventional and novel processing methods for cellular ceramics

Adsorption-induced conformational changes of proteins onto ceramic particles: Differential scanning calorimetry and FTIR analysis

This project uses the change of functional properties of proteins for ceramic and powder metallurgical shaping. Albumin (Bovine Serum Albumin, BSA) as a major constituent of blood was added as a model binder to an aqueous powder suspension, which then showed a sufficiently low viscosity for mould filling. The flow behaviour showed a structural viscosity being best described by a Herschel–Bulkley model. Temperatures higher than 66 °C lead to a significant increase of viscosity caused by irreversible changes in the spatial-structure of the protein molecule. Albumin, however, had a second effect in this process. Since albumin has amphiphilic properties its solutions are prone to foaming. A fine cellular foam structure of approximately 50–300 μm cell diameter was formed, probably due to a stable arrangement at the liquid gas interface. The combination of foaming and increase of stiffness lead to a stable protein-ceramic foam structure. After burn out and sintering final densities in the range from 8 to 20% t.d. were achieved. Fine cellular structures have more isolated pores while larger cells are typically interconnected. Typical applications would be high temperature insulation, catalyst carriers or scaffolds for cell technology.

Clot-forming: the use of proteins as binders for producing ceramic foams

Behaviour of BSA and of BSA-derivatives at the air/water interface

Porous ceramics made of alumina and hydroxyapatite were created using a protein foaming method. Porosity and pore size distribution were successfully varied by means of chemical modification of the foaming protein Bovine serum albumin (BSA). The effectiveness of the BSA and of its chemical modifications as well as the influence of the dispersing agent were investigated using synchrotron tomography. Resulting porous ceramic materials were used as three-dimensional substrates for the cultivation of human peripheral stem cells. The cells proliferated and differentiated in culture. Five cell lines consistent with human blood cell lines were observed.

Biocompatible porous ceramics for the cultivation of hematopoietic cells.

The interaction of a cell with a surface of an artificial material is always mediated by a protein adsorption layer. The properties of this adsorption layer are responsible for the following biological response to the cultured surface. The adsorption of the three very different physiological model proteins, bovine serum albumin (BSA), fibrinogen (FBG) and lysozyme (LSZ), onto ceramic particles (Al2O3; hydroxyapatite) was studied as a function of pH, electrolyte concentration and time. Their adsorption behaviour on various ceramic surfaces is related to the different molecular and physicochemical properties as well as the influence of surface chemistry and charge. The more flexible and bigger proteins BSA and fibrinogen show their maximal adsorption at their own isoelectric point (IEP) irrespective of the ceramic surface. The smaller rigid protein lysozyme is driven primary by surface-protein interaction and has its maximum of adsorption at the IEP of the ceramic material. The addition of non-buffering electrolyte results in a reduction of the adsorbed amount of lysozyme caused by the reduction of lateral protein-protein interactions irrespective of the available surface. BSA shows almost no dependence on electrolyte concentration whereas fibrinogen increases its adsorbed amount presumably caused by additional salt bridges or charge compensation in adsorption layer by increasing salt concentration. The strength of adsorptive bonding is, as expected from structural rigidity and molecular size, the smallest for lysozyme irrespective of the ceramic surface. Whereas the smaller but more flexible BSA adsorbs more firmly, than the bigger fibrinogen. Future Prospects Further investigations will complete these adsorption studies by the determination of structural changes by spectroscopic methods and calorimetric data of the adsorbed proteins onto the ceramic surfaces. Proteins will be modified to estimate the influence of ionic, polar or hydrophobic interactions by the change of the adsorption profile. Acknowledgement The authors thank the Deutsche Forschungsgesellschaft (DFG) for the financial support of the project KR 1452/7-1 and KR 1452/7-2. References [1] Barroug A., Fastrez J., Lemaitre J., Rouxhet P.: Adsorption of succinylated lysozyme on hydroxyapatite. J. Colloid Interface Sci. 189: 37-42, 1997 [2] Bergström L., Ernstsson M., Gruvin B., Brage R., Nyberg B., Carlström E.: The Effect of Wet and Dry Milling on the Surface Properties of Silicon Nitride Powders. in Proceedings of 7th CIMTEC, Ceramics Today Tomorrow's Ceramics, Elsevier: 1005-1014, 1991 [3] Brynda E., Cepalova N. A., Stol M.: Equilibrium adsorption of human serum albumin and human fibrinogen an hydrophobic and hydrophilic surfaces. J. Biomed. Mat. Res. 18: 685-693, 1984 [4] Buijs J., Hlady V.: Adsorption kinetics, conformation, and mobility of the growth hormone and lysozyme on solid surfaces, studied with TIRF. J. Colloid Interface Sci. 190: 171-181, 1997 [5] Garrn I., Reetz C., Brandes N., Kroh L. W., Schubert H.: Figure 4: Strength of adsorptive bonding expressed as desorbed amount of protein in three consecutive washing steps from the ceramic surfaces HAP (▲), HP (❍ ), AKP50 ( ), AKP50ip (–), and AKP50eg (X)

Natascha Brandes

Papers

Adsorption-induced conformational changes of proteins onto ceramic particles: Differential scanning calorimetry and FTIR analysis

Clot-forming: the use of proteins as binders for producing ceramic foams

Behaviour of BSA and of BSA-derivatives at the air/water interface

Biocompatible porous ceramics for the cultivation of hematopoietic cells.

Adsorption physiologischer Proteine auf verschiedenen Keramikpartikeln