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Rational design of scaffold architecture and functionalization to induce healing and tissue regeneration

Final Report Summary - DESIGN2HEAL (Rational design of scaffold architecture and functionalization to induce healing and tissue regeneration)

When materials are implanted into the body, they initiate an inflammatory response that is difficult to control. Consequently, medical implants are tolerated by the body rather than being fully integrated; the material is often sealed off from the body in a fibrotic capsule. Most recent research suggests that scaffold morphology, when controlled at cellular dimensions (i.e. porosities of 30-50 µm), is a decisive immunomodulatory trigger and may favour a healing-like reaction of the innate immune system, especially of macrophages. On the other side, it is known that surface bioactivation of materials is beneficial for specific triggering of cellular responses that are involved in the tissue regeneration after the acute innate immune response, such as mesenchymal progenitor cells.
Design2Heal proposed to combine form, as trigger for the innate immune response, with function to generate biomaterials that are designed to heal and improve implant integration. To achieve this, the project comprised a unique combination of technologies to control the morphology of fiber-based scaffolds in relevant dimensions through melt-electrowriting (MEW), as well as methods to tailor the surface chemistry, as trigger for cell adhesion in subsequent tissue regeneration. Research within Design2Heal thus aimed at unravelling the immunomodulatory potential of generic scaffold parameters (diameter, morphology) and surface functionalization (e.g. peptides) for rationally designed scaffolds in vitro with primary human innate immune cells. It further aimed at resolving the immunomodulatory effects of cellular cross talk and interaction between human immune cells, mesenchymal stem cells and eventually endothelial progenitor cells in defined geometric confinements, and finally at an in vivo proof-of-principle with selected scaffolds.
Regarding the scaffold fabrication, the young MEW technology was significantly advanced on several levels during the lifetime of Design2Heal. Altogether eight cutting-edge self-constructed high precision MEW devices have been developed, and the process was advanced regarding general phenomena during printing (etc. pulsing) and reproducible performance. Fabrication of structures with highly ordered geometry and very good long range order was pushed to physical limits regarding fiber diameters even below micrometer and the mesh-size resolution in terms of interfiber-spacing within a single layer. A library of more than 500 different scaffolds, including systematic variation of fiber diameter, spacing, angle orientation of the fibers towards each other, and overall geometry of the pores, was generated. Printing was also transferred from planar to cylindrical targets, allowing the generation of hollow tubes. Also the variation of fiber diameter during one printing process was established, that allows the generation of anisotropic and structurally hierarchical scaffolds within one printing process. MEW scaffolds were successfully explored as guidance for endothelial network formation, as meshes for cardiac tissue engineering, as structural support for tendon and ligament regeneration, and as scaffold for the controlled positioning of cell spheroids.
With respect to surface modification of fibrous meshes, the bioactivation of electrospun sheets was advanced to multimodal conjugation, and immunomodulatory effects could be achieved with that. Furthermore, anisotropic constructs with bipolar bioactivation were created that acted as an artificial basement membrane for the engineering of skin tissue. Finally, the specific bioactivation of MEW fibers was established and demonstrated in vitro. Several developments arose from the activities around the surface modification and activation of scaffolds. The hydrophilic polymers developed for this purpose were also used as a novel platform for surface modification and bioactivation of gold nanoparticles. The polymers were also evolved further to 3D printable hydrogels themselves, and several publications followed within the field of 3D bioprinting based on this line of research.
We also established single cell culture and cell co-culture models and protocols on the scaffolds and elucidated effects of scaffold geometry on cellular behaviour. We identified new modes of intercellular communication and cross talk, such as mitochondrial exchange between MSCs and macrophages, and established a direct co-culture model for primary human macrophages and hMSCs. Finally, in vivo studies were performed with the most promising scaffolds from in vitro examinations to assess the innate immune response, with the final histological analysis still ongoing.