Cylindrical constructs with parallel aligned pores were prepared by using ionotropic gelation of alginate/calcium phosphate hydroxyapatite (HAP) mixtures in regard to applications as scaffold for bone regeneration. The starting powder and stabilizing agents were characterized by measurement of electrosonic amplitude, particle size distribution, and specific surface. The shrinkage of the gels was investigated in dependence on the drying methods. The pore size relied on preparation conditions such as amount of HAP and concentrations of gelling agent or alginate sol. A wide field of pore sizes could be fabricated by varying the kind and concentration of additives. Micro computer tomography-investigations of freeze dried scaffolds demonstrated the pore progression over a length of 4 mm. The pore dimension and structure were adequate for cell seeding and blood capillary ingrowth. Biocompatibility was proven by in vitro experiments with human mesenchymal stem cells by fluorescence microscopy. A high stability of the wet gels was maintained under cell culture conditions for a period of 3 weeks.

Porous and mineralised scaffolds are required for various applications in hard tissue engineering. Scaffolds with oriented tube-like pores facilitate homogenous cell seeding, a sufficient nutrient supply during cell culture (even in big constructs) and a fast vascularisation after implantation. The phenomenon of ionotropic gelation has been known since more than 30 years which describes that alginate forms gels with capillary-like pores when covered with solutions of di-or trivalent cations [1]. This technique has been used here to develop scaffolds with tube-like and regular pores from alginate/calcium phosphate composites and to stabilise them by mineralisation with hydroxyapatite from solution.

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Scaffolds for bone regeneration are mostly prepared with an isotropic, sponge-like structure mimicking the architecture of trabecular bone. We have developed an anisotropic bioceramic with parallel aligned pores resembling the honeycomb arrangement of Haversian canals of cortical bone and investigated its potential as a scaffold for tissue engineering. Parallel channel-like pores were generated by ionotropic gelation of an alginate-hydroxyapatite (HA) slurry, followed by ceramic processing. Organic components were thermally removed at 650 °C, whereas the pore system was preserved in the obtained HA bioceramic in the processing stage of a bisque. Even without further sintering at higher temperatures, the anisotropic HA bisque (AHAB) became mechanically stable with a compressive strength (4.3 MPa) comparable to that of native trabecular bone. Owing to the low-temperature treatment, a nanocrystalline microstructure with high porosity (82%) and surface area (24.9 m(2)/g) was achieved that kept the material dissolvable in acidic conditions, similar to osteoclastic degradation of bone. Human mesenchymal stem cells (hMSCs) adhered, proliferated and differentiated into osteoblasts when osteogenically induced, indicating the cytocompatibility of the bisque scaffold. Furthermore, we demonstrated fusion of human monocytes to osteoclast-like cells in vitro on this substrate, similar to the natural pathway. Biocompatibility was demonstrated in vivo by implantation of the bisque ceramic into cortical rabbit femur defects, followed by histological analysis, where new bone formation inside the channel-like pores and generation of an osteon-like tissue morphology was observed.

Conventionally sintered hydroxyapatite-based materials for bone repair show poor resorbability due to the loss of nanocrystallinity. The present study describes a method to establish nanocrystalline hydroxyapatite granules. The material was prepared by ionotropic gelation of an alginate sol containing hydroxyapatite (HA) powder. Subsequent thermal elimination of alginate at 650 °C yielded non-sintered, but unexpectedly stable hydroxyapatite granules. By adding stearic acid as an organic filler to the alginate/HA suspension, the granules exhibited macropores after thermal treatment. A third type of material was achieved by additional coating of the granules with silica particles. Microstructure and specific surface area of the different materials were characterized in comparison to the already established granular calcium phosphate material Cerasorb M(®). Cytocompatibility and potential for bone regeneration of the materials was evaluated by in vitro examinations with osteosarcoma cells and osteoclasts. Osteoblast-like SaOS-2 cells proliferated on all examined materials and showed the typical increase of alkaline phosphatase (ALP) activity during cultivation. Expression of bone-related genes coding for ALP, osteonectin, osteopontin, osteocalcin and bone sialoprotein II on the materials was proven by RT-PCR. Human monocytes were seeded onto the different granules and osteoclastogenesis was examined by activity measurement of tartrate-specific acid phosphatase (TRAP). Gene expression analysis after 23 days of cultivation revealed an increased expression of osteoclast-related genes TRAP, vitronectin receptor and cathepsin K, which was on the same level for all examined materials. These results indicate, that the nanocrystalline granular materials are of clinical interest, especially for bone regeneration.

Thin films based on dodecylamine stabilized gold nanoparticles interlinked with different organic molecules are prepared by automatic layer-by-layer self-assembly in a microfluidic quartz crystal microbalance (QCM) cell, to obtain an in situ insight on the film formation by ligand/linker exchange reactions. The influence of interlinking functional groups and the length of the organic linker molecule on the assembly behavior is investigated. Alkyldithiols with different lengths are compared to alkyldiamines and alkylbisdithiocarbamates with a C8 alkylic molecular backbone. The stepwise layer-by-layer assembly occurs independently of the linker molecule, while the largest frequency changes always correspond to the gold nanoparticle step. During the solvent rinsing and ligand/linker exchange reaction step, the frequency is almost constant with slight increases or decreases dependent on the molar mass of the linker compared to the exchanged ligand. The assembly efficiency is higher for shorter molecules and for molecules with stronger interacting functional groups. The densities of the composite films are calculated from QCM data and independent thickness measurements. They reflect the higher fraction of organic material in the films comprising longer organic linkers. The plasmon resonance band of the gold nanoparticles in the final assemblies is measured with UV/vis spectroscopy. Band positions in films prepared from dithiols and diamines of comparable lengths are very similar, while the spectrum of the bisdithiocarbamate film exhibits a distinct blue-shift. This observation is explained by the longer molecular structure of the linker due to a larger binding group, in conjunction with a delocalization of particle charge on the organic molecule. Obtained results play an essential role in the understanding of thin film layer-by-layer self-assembly processes, and enable the formation of new gold nanoparticle networks with organic diamine and bisdithiocarbamate molecules.

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Alginate gelation is one of few methods that can produce porous ceramics with oriented tubular pores. Alginates are well known as inorganic polymers that can be gelled by cross-linking with multivalent metal ions. This process allows the production of structured alumina, titania, and hydroxyapatite ceramics with an approximately honeycomb structure.The primary thin layer has the function of a selective membrane through which the slurry gradually transforms into the gel.The gel-like substance was dried by various methods such as air drying, supercritical drying, and freeze drying. The main problem is very high shrinkage occurring during the transition from the wet gel to the sintered body. The samples were fired at various temperatures and had a high porosity of up to 80 %. The bulk density, tensile splitting strength, and a narrow pore and capillary size distribution were measured. The ceramics have a bimodal pore size distribution when measured by mercury porosimetry. The first peak represents the pores between the capillaries, the second is caused by the capillaries themselves. The pores between the capillaries almost disappear with increasing sintering temperature.The spatial pore structure was investigated by serial sectioning and image analysis. An increase in pore size and homogenization of pore arrangement led to the formation of capillaries.This process offers new possibilities for preparing porous ceramics with uniform parallel capillaries, which might be useful as membranes, catalyst supports, gas or chemical sensors, or for implants.The starting powder was dispersed in aqueous solution at pH 6 with Na alginate to give a slurry. It is important to control the pH to produce stable slurries. Na alginate slurry is stable at pH 5±10. The average particle sizes of the powders are shown in Table 1 (Microtrac Ultrafine Particle Analyzer; Leeds & Northrup). The isoelectric points of the materials are also given in Table 1.Then a solution of divalent metal ions (Me 2+ ) are deposited onto the surface of the slurry.The slurry can be gelled by ion exchange of Na + in the alginate with divalent metal ions such as Cu 2+ , Cd 2+ , Pb 2+ , Ca 2+ , Zn 2+ , and Sr 2+ . The gelation reaction is 2 Na alginate (aq.) + Me 2+ ® Me alginate 2 + 2 Na + (1) Immediately a primary thin gel layer is formed. The primary gel layer has the function of a selective membrane, allowing the passage of the metal ions (Me 2+ ) but not other ions (such as ions of Na alginate and Al 2 O 3 ). Owing to diffusional control of Me 2+ transport through the membrane, the slurry gradually transforms to the gel resulting in the formation of capillaries in the direction of Me 2+ diffusion. The metal ions and also the anions have an effect on the size of the capillaries. [1] The concentration of electrolyte also influenced the capillaries: the higher the concentration the smaller and fewer the capillaries.After gelation, the primary membrane and the gel bottom are cut away. To remove Me 2+ impurities, ion exchange is carried out. [2±4] Figure 1 shows ...