Experimental measurements and CFD modelling of hydroxyapatite scaffolds in perfusion bioreactors for bone regeneration

In the field of bone tissue engineering,particular interest is devoted to the development of 3D cultures to study bone cell proliferation under conditions similar to in vivo ones,e.g.by artificially producing mechanical stresses promoting a biological response(mechanotransduction).Of particular relevance in this context are the effects generated by the flow shear stress,which governs the nutrients delivery rate to the growing cells and which can be controlled in perfusion reactors.However,the introduction of 3D scaffolds complicates the direct measurement of the generated shear stress on the adhered cells inside the matrix,thus jeopardizing the potential of using multi-dimensional matrices.In this study,an anisotropic hydroxyapatite-based set of scaffolds is considered as a 3D biomimetic support for bone cells deposition and growth.Measurements of sample-specific flow resistance are carried out using a perfusion system,accompanied by a visual characterization of the material structure.From the obtained results,a subset of three samples is reproduced using 3D-Computational Fluid Dynamics(CFD)techniques and the models are validated by virtually replicating the flow resistance measurement.Once a good agreement is found,the analysis of flow-induced shear stress on the inner B-HA structure is carried out based on simulation results.Finally,a statistical analysis leads to a simplified expression to correlate the flow resistance with the entity and extensions of wall shear stress inside the scaffold.The study applies CFD to overcome the limitations of experiments,allowing for an advancement in multi-dimensional cell cultures by elucidating the flow conditions in 3D reactors.

Tethering of Gly-Arg-Gly-Asp-Ser-Pro-Lys Peptides on Mg-Doped Hydroxyapatite

Stem cell homing, namely the recruitment of mesenchymal stem cells （MSCs） to injured tissues, is highly effective for bone regeneration in vivo. In order to explore whether the incorporation of mimetic peptide sequences on magnesium-doped （Mg-doped） hydroxyapatite （HA） may regulate the homing of MSCs, and thus induce cell migration to a specific site, we covalently functionalized MgHA disks with two chemotactic/haptotactic factors： either the fibronectin fragment III1-C human （FF III1-C）, or the peptide sequence Gly-Arg-Gly-Asp-Ser-Pro-Lys, a fibronectin analog that is able to bind to integrin trans- membrane receptors. Preliminary biological evaluation of MSC viability, analyzed by 3-（4,5-dimethyl- thiazol-2-yl）-2,5-diphenyltetrazolium bromide （MTT） test, suggested that stem cells migrate to the MgHA disks in resoonse to the grafted haototaxis stimuli.

Flux Pinning in Y- and Ag-Doped MgB₂

High temperature superconductor research is presently concentrated upon the flux pinning properties of the Abrikosov lattice of the mixed-mode superconducting phase. The temperature thermal fluctuations, current and magnetic field unpin the flux vortices and so cause electromagnetic resistivity in high temperature superconductors. Materials with higher vortex pinning exhibit less resistivity and are more attractive for industrial uses. In the present article, we measured and correlated the pinning flux energy barrier, determined by AC magnetic measurements, and transmission electron microscopy measurements to the critical current Jc in Yttrium- and Silver-doped MgB2 superconductors. The energy of the flux vortex was evaluated as a function of the magnetic field. The energy barrier curves suggest an optimal doping level to occur in doped materials. This result only depends on the optimal size and distribution of precipitates, and not on their chemical composition. The energy barriers have been compared with that of undoped MgB2 in literature.

The effect of chemical structure of carboxylate molecules on hydroxyapatite nanoparticles.A structural and morphological study

Being the most abundant non-macromolecular organic component of bone,the role of citrate(Cit)in hydroxyapatite(HA)crystallization is of high relevance.In this work we have investigated the influence of hydroxycitrate(CitOH)and glutarate(Glr)on HA crystallization in terms of particle growth,composition,and morphology in comparison to Cit.CitOH and Glr have been selected for this work because they share the same backbone structure of Cit but bear different functional groups in the central region.Our data has revealed that CitOH strongly inhibits HA crystallization more efficiently than Cit.CitOH-HA nanoparticles are composed of platy,elongated particles similar to those of Cit-HA but they are ca.twice smaller and have a lower crystal order.On the other hand,Glr does not inhibit HA crystallization as Cit,but leads to the formation of OCP platelets that convert with maturation time to HA nanorods with larger aspect ratio than Cit-HA.In comparison to Cit-HA samples,Glr-HA nanoparticles have bigger dimensions,and higher structural order.Overall,our data reveal that the central carboxyl group of Cit is involved in the selective binding with HA crystal surface and in regulating HA crystal growth.The results of this work highlight new possibilities to control the formation of HA for designing advanced bioactive materials and give new insights on the role of the structure of Cit in regulating the HA morphology.

Liquid flow in scaffold derived from natural source:experimental observations and biological outcome

This study investigates the biological effects on a 3D scaffold based on hydroxyapatite cultured with MC3T3 osteoblasts in response to flow-induced shear stress(FSS).The scaffold adopted here(B-HA)derives from the biomorphic transformation of natural wood and its peculiar channel geometry mimics the porous structure of the bone.From the point of view of fluid dynamics,B-HA can be considered a network of micro-channels,intrinsically offering the advantages of a microfluidic system.This work,for the first time,offers a description of the fluid dynamic properties of the B-HA scaffold,which are strongly connected to its morphology.These features are necessary to determine the FSS ranges to be applied during in vitro studies to get physiologically relevant conditions.The selected ranges of FSS promoted the elongation of the attached cells along the flow direction and early osteogenic cell differentiation.These data confirmed the ability of B-HA to promote the differentiation process along osteogenic lineage.Hence,such a bioactive and naturally derived scaffold can be considered as a promising tool for bone regeneration applications.

In situ sol-gel synthesis of hyaluronan derivatives bio-nanocomposite hydrogels

The main driving idea of the present study was the comparison between two different chemical modifications of hyaluronic acid(HA)followed by the development of nanocomposite hydrogels directly in situ by biomineralization of photocrosslinkable HA polymers through sol-gel synthesis.In this way,it has been possible to overcome some limitations due to classical approaches based on the physical blending of inorganic fillers into polymer matrix.To this aim,methacrylated and maleated HA,synthesized with similar degree of substitution(DS)were compared in terms of mechanical and physico-chemical properties.The success of in situ biomineralization was highlighted by reflect Fourier transform infrared spectroscopy and thermogravimetric analysis.Furthermore,mechanical characterization demonstrated the reinforcing effect of inorganic fillers evidencing a strong correlation with DS.The swelling behavior resulted to be correlated with filler concentration.Finally,the cytotoxicity tests revealed the absence of toxic components and an increase of cell proliferation over culture time was observed,highlighting these bio-nanocomposite hyaluronan derivatives as biocompatible hydrogel with tunable properties.