SPHERESCAFF · The Manufacturing of Scaffolds from Novel Coated Microspheres via Additive Manufacturing Techniques for Temporomandibular Joint Tissue Engineering

The Manufacturing of Scaffolds from Novel Coated Microspheres via Additive Manufacturing Techniques for Temporomandibular Joint Tissue Engineering

The SphereScaff project aimed at producing tissue engineering scaffolds via 3D printing of microspheres. Microspheres are being used to create scaffolds that allow controlled release of encapsulated matter (e.g. cells, growth factors, other phases) and with predetermined porosity depending on the size of the spheres. Within the scope of this project, the target area for such scaffolds is the temporomandibular (TMJ) joint, while the application of the technique is not limited to this area. 3D Printing with Microspheres To allow 3D printing of microspheres, these must be mixed with a carrier phase which enables extrusion through a printing head and keeps the printed strands in shape until the spheres have been sintered. Microspheres were produced from poly(lactic-co-glycolic acid) (PLGA) in various sizes between 50 and 110 µm. The investigated carrier phases were alginate, agarose, a hyaluronic acid hydrogel, and carboxymethyl cellulose (CMC). Syringe extruders were made and attached to the “RepRapPro Mendel 3 Tricolour” 3D printer to extrude the microsphere pastes. Extrusion from a syringe is influenced by the paste’s viscosity, drying time, microsphere diameter and gel strength of the carrier phase. Generally, a solids:carrier ratio of 2:1 or higher is necessary to preserve the shape of the extruded material. Extrusions of microspheres with alginate or agarose dried quickly and allowed easy transfer to the sintering chamber. The microsphere/hydrogel paste was cured by UV light before transfer. CMC does not dry as quickly and the stay extrusions stayed soft for a prolonged time. Sintering of the pastes A system to sinter biomaterials with subcritical CO2 was designed and installed at NUIG. To allow sintering, the amount of the carrier phase has to be kept low. Ideally, the spheres show only a very thin layer of the carrier phase, which sticks them together to keep the shape of the extruded material until dry and/or sintered. To increase flow, however, higher amounts of the carrier phase, or a lower viscosity is necessary. While the first option impedes the sintering (less contact between spheres), the latter causes significant flow of the paste also on the printing bed, so the printed structures lose shape. When extruding the paste from a syringe is was observed that a larger amount of paste is necessary to print. It appeared that the solids in the paste are compacted, while the carrier phase is pressed out. While material flows from the syringe tip, not all material could be extruded. With a larger amount of paste, more structures could be printed, however, the microsphere production that was used only allowed for minor amounts of spheres (up to 2 g) per batch, and batches could not be mixed due to different sphere sizes. Another issue that was observed is the amount of force that was needed to extrude the paste from the syringe. High shear forces within the paste make it difficult to extrude from small volume syringes. Again, high amounts of microspheres are needed to prepare a bigger amount of paste. To extrude the paste from the syringe in the printer, the syringe plunger was pushed by using the filament and filament drive of the printer. A syringe extruder was also build to directly push the plunger. Both systems did not provide enough force to continuously extrude the material. Sintering of microspheres with subcritical CO2 was also possible in presence of a carrier phase. Problems that occurred, such as bubble formation, were due to suboptimal sintering parameters. After reducing the sintering time and decreasing de-pressurization, the shape/size of the extruded material did not change significantly. Naturedly, the amount of carrier phase has to be low enough to allow necking between the microspheres for a stable construct. Good results were achieved with a ratio of 2:1 (solids:carrier). Conclusions 3D printing can be used to print with microsphere pastes. The printing capability depends on the viscosity and amount of the carrier, and the extrusion force that can be applied. Due to high shear forces within the material, larger syringe diameters are recommended. A mixture of sphere sizes may increase the flow of the material during extrusion as well as the sintering capability. This technique allows to produce scaffolds with enhanced new tissue formation through a designed internal architecture to address mechanical requirements and the use of microspheres, which allow, e.g., controlled release of growth factors. It also allows spatial control of the placement of phases in multiphasic scaffolds. This provides researchers with a new tool to produce constructs for tissue engineering that Impact The researcher expanded his knowledge and skills in microsphere fabrication, subcritical CO2 sintering of biomaterials, cell harvesting and culturing, and tissue engineering techniques and procedures. He received first-hand information from leading TMJ surgeons on current procedures and the need for tissue engineering products for the temporomandibular joint. The research was presented at two major conferences and will be further disseminated by full research paper publications in the near future. Outreach activities addressed at students and pupils attracted significant numbers from those audiences to inquire about 3D printing and the research performed. Links that have been established by the researcher throughout the project include three international universities and several TMJ surgeons who are interested in the research.

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