Rosemarie Dittrich

Synthesis and physicochemical, in vitro and in vivo evaluation of an anisotropic, nanocrystalline hydroxyapatite bisque scaffold with parallel‐aligned pores mimicking the microstructure of cortical bone

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 m2/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. Copyright

Mineralized scaffolds for hard tissue engineering by ionotropic gelation of alginate

Alginates form gels with tube-like pores when covered with solutions of di- or trivalent cations. This phenomenon also referred to as ionotropic gelation has been known for more than 30 years. By mixing a calcium phosphate powder and an alginate as the starting material, the mineral phase of bone is incorporated. Such porous structures can be used for scaffolds in hard tissue engineering. The starting materials and stabilizing additives are dispersed in an aqueous solution. Then a solution of Ca-ions is deposited onto the surface of the slurry. The slurry can be gelled by ion exchange of Na-ions in the alginate with Ca-ions. A primary thin gel layer with the function of a membrane is immediately formed. By diffusional control of cation transport through the membrane, the slurry gradually transforms to the gel forming tube-like pores in direction of cation diffusion. Like the gelation of pure alginate the concentration of electrolyte and the kind of cations and anions influence the size (diameter and length) and size growth of the pores, but the tolerance in the preparation conditions is much smaller. The diameters of the pores can be adjusted between 50 and 500 m which fits the optimum size for cell seeding and blood capillary ingrowth very well. By selecting the proper drying method the inherent shrinkage can be controlled. Hydroxyapatite sintered at high temperatures loses the ability to be resorbed by osteoclasts in vivo. Therefore, we have developed scaffolds with channel-like pores from alginate/calcium phosphate composites without the necessity for heating them to higher temperatures.

Preparation of Al2O3, TiO2, and hydroxyapatite ceramics with pores similar to a honeycomb structure

Alginate gelation is one of few methods that can produce porous ceramics with oriented tubular pores. This process allows the production of structured alumina, titania, and hydroxyapatite ceramics with an approximately honeycomb structure as demonstrated by the authors. The sizes of the capillaries can be adjusted by changing the material, the electrolyte, and the sintering temperature. Serial sectioning and automatic image analysis are suitable methods for investigating the capillary structure of the samples.

Novel Biomaterials with Parallel Aligned Pore Channels by Directed Ionotropic Gelation of Alginate: Mimicking the Anisotropic Structure of Bone Tissue

Regenerative medicine intends to restore lost functionality by healing tissues defects. For this novel types of biodegradable implants have to be used that first foster healing and later take part in the natural remodelling cycle of the body. In this way, patient’s cells can reconstruct and adapt the tissue according to the local situation and needs. Ideally, the implant should mimic the desired tissue. That means that the biomaterial should resemble the extracellular matrix (ECM) which is expressed by specific cells and acts as the biological scaffold of living tissues. The closer an artificial scaffold material mimics the pattern the easier it can be involved in the natural healing and remodelling processes, which is why more and more researchers try to establish biomimetic approaches for the development of tissue engineering scaffolds. Biological materials are seldom isotropic and for many tissue engineering applications distinct anisotropic materials are needed. E. g. compact bone exhibits a honeycomb-like structure with overlapping, cylindrical units (osteons) with the so-called Haversian canal in the centre. Scaffolds with parallel aligned pores, mimicking the osteon structure of compact bone can be synthesised by directed ionotropic gelation of the naturally occurring polysaccharide alginate. The parallel channels are formed via a sol-gel-process when dior multivalent cations diffuse into the sol in broad front, forming an alginate hydrogel. The pore size and pore alignment of such gels is influenced by the starting materials (e.g. concentrations, additives like powders or polymers) and the preparation process (e.g. temperature, drying process). The phenomenon was discovered already in the 50th of the last century but the biomedical potential of alginate scaffolds with parallel aligned pores structured by ionotropic gelation has been explored for osteoblasts, stem cell based tissue engineering, axon guiding or co-culture of vascular and muscle cells only in the past few years.