A novel bioink and a dispensing technique for 3D tissue‐engineering applications are presented. The technique incorporates a coaxial extrusion needle using a low‐viscosity cell‐laden bioink to produce highly defined 3D biostructures. The extrusion system is then coupled to a microfluidic device to control the bioink arrangement deposition, demonstrating the versatility of the bioprinting technique. This low‐viscosity cell‐responsive bioink promotes cell migration and alignment within each fiber organizing the encapsulated cells.

The availability of mannuronan and mannuronan C-5 epimerases allows the production of a strictly alternating mannuronate-guluronate (MG) polymer and the MG-enrichment of natural alginates, providing a powerful tool for the analysis of the role of such sequences in the calcium-alginate gel network. In view of the calcium binding properties of long alternating sequences revealed by circular dichroism studies which leads eventually to the formation of stable hydrogels, their direct involvement in the gel network is here suggested. In particular, 1H NMR results obtained from a mixed alginate sample containing three polymeric species, G blocks, M blocks, and MG blocks, without chemical linkages between the block structures, indicate for the first time the formation of mixed junctions between G and MG blocks. This is supported by the analysis of the Young's modulus of hydrogels from natural and epimerized samples obtained at low calcium concentrations. Furthermore, the "zipping" of long alternating sequences in secondary MG/MG junctions is suggested to account for the shrinking (syneresis) of alginate gels in view of its dependence on the length of the MG blocks. As a consequence, a partial network collapse, macroscopically revealed by a decrease in the Young's modulus, occurred as the calcium concentration in the gel was increased. The effect of such "secondary" junctions on the viscoelastic properties of alginate gels was evaluated measuring their creep compliance under uniaxial compression. The experimental curves, fitted by a model composed of a Maxwell and a Voigt element in series, revealed an increase in the frictional forces between network chains with increasing length of the alternating sequences. This suggests the presence of an ion mediated mechanism preventing the shear of the gel.

In this work we demonstrate how to print 3D biomimetic hydrogel scaffolds for cartilage tissue engineering with high cell density (>10(7) cells ml(-1)), high cell viability (85 ÷ 90%) and high printing resolution (≈100 μm) through a two coaxial-needles system. The scaffolds were composed of modified biopolymers present in the extracellular matrix (ECM) of cartilage, namely gelatin methacrylamide (GelMA), chondroitin sulfate amino ethyl methacrylate (CS-AEMA) and hyaluronic acid methacrylate (HAMA). The polymers were used to prepare three photocurable bioinks with increasing degree of biomimicry: (i) GelMA, (ii) GelMA + CS-AEMA and (iii) GelMA + CS-AEMA + HAMA. Alginate was added to the bioinks as templating agent to form stable fibers during 3D printing. In all cases, bioink solutions were loaded with bone marrow-derived human mesenchymal stem cells (BM-MSCs). After printing, the samples were cultured in expansion (negative control) and chondrogenic media to evaluate the possible differentiating effect exerted by the biomimetic matrix or the synergistic effect of the matrix and chondrogenic supplements. After 7, 14, and 21 days, gene expression of the chondrogenic markers (COL2A1 and aggrecan), marker of osteogenesis (COL1A1) and marker of hypertrophy (COL10A1) were evaluated qualitatively by means of fluorescence immunocytochemistry and quantitatively by means of RT-qPCR. The observed enhanced viability and chondrogenic differentiation of BM-MSCs, as well as high robustness and accuracy of the employed deposition method, make the presented approach a valid candidate for advanced engineering of cartilage tissue.

Gelatin is a natural protein with many desirable properties for application as a biomaterial, including scaffolding for tissue engineering. In this work gelatin A with a molecular weight in the range 50-100 kg mol-1 was modified with methacrylic anhydride and processed into a concentrated oil-in-water emulsion. Polymerization of the continuous phase gave rise to a polyHIPE, a porous material possessing a highly interconnected, trabecular morphology. In the paper, we focused on the goal of obtaining matrixes characterized by suitable sizes of both voids and interconnects, to allow an in depth colonization from transplanted cells. In this respect, we investigated the role of the volume percentage of the dispersed phase and the effect of additives. It was established that high pore volumes (>or=90%) are to be preferred, because they allow the production of solid foams characterized by average void and interconnect diameters of approximately 20 and 10 microm, respectively. These values are still inadequate for the intended application of these scaffolds but represent a good starting point for further improvements. These were achieved through the use of additives, namely sodium chloride and dimethyl sulfoxide, which partially destabilized the precursor emulsion and allowed a solid foam to be obtained with void and interconnect diameters in the range of 30-150 microm and 10-50 microm, respectively.