Research On Design And Fabrication Of Extrusion-based 3D Printed Biomimetic Gradient Tissue Engineering Scaffolds

An ideal artificial tissue engineering scaffold should have bioactivity,biocompatibility,mechanical properties,and pore structure comparable to natural tissues.Extrusion-based 3D printing is widely used in various fields,especially in tissue engineering,due to its low cost,good scalability,and the variety of materials available.Extrusion-based 3D printed tissue engineering scaffolds provide a solution to the challenge of severe shortage of clinically available tissue for transplantation.Natural tissues have a complex gradient pore structure,such as cancellous bone with loose and large pores,and cortical bone with dense and small pores.For the design and fabrication of biomimetic gradient scaffolds,although extrusion-based 3D printing technology can build axial gradient pore scaffolds,there are still great challenges in its research and exploration of radial biomimetic gradient tissue engineering scaffolds.Therefore,this paper proposes two new strategies: fractal design strategy and variable fiber diameter design strategy based on extrusion-based 3D printing technology for the tissue engineering needs of bionic gradient scaffolds.In addition,theoretical analysis and experimental data are combined to build a parametric "design-fabrication" workflow to obtain principle prototypes of bone,meniscus and vascular scaffolds with controlled gradient pores,aiming to provide technical support for the design and fabrication of extrusion-based 3D printed bionic gradient tissue engineering scaffolds.A new strategy for fractal design of biomimetic gradient tissue engineering scaffolds is proposed based on the fractal iteration rule of Koch curve according to the characteristics of extrusion-based 3D printing fiber layer-by-layer stacking molding.The strategy adjusts the pore characteristics of the fractal scaffold model by design parameters including the number of fractal iterations,the number of circumferential arrays,and the fiber diameter,and the results show that the fractal scaffold with three iterations has pore characteristics that gradually decrease along the radial direction.A parametric "fractal design-fabrication" workflow for fractal bionic gradient tissue engineering scaffolds is built with a commercial extrusion 3D printer and CAD software Grasshopper for efficient one-click adjustment of model gradient porosity and acquisition of matching fabrication code.To address the need for a radial gradient porous scaffold for critical bone defects in the femur,a fractal bionic gradient bone tissue engineering scaffold mimicking the "cancellous-cortical bone" structure was constructed based on the new fractal design strategy mentioned above.The feasibility of extrusion 3D printing preparation of this fractal bionic gradient bone tissue engineering scaffold was demonstrated using solvent dissolved material(Poly(lactic-coglycolic acid),PLGA),heated molten material(Beta-tricalcium phosphate/polycaprolactone,β-TCP/PCL),conventional ink material(Alginate,Alg)and bio-ink material(Gelatin methacryloyl/Alginate/human mesenchymal stem cells,Gel MA/Alg/h MSCs),respectively.Further,the fractal bone tissue engineering scaffold was evaluated for its gradient characteristics in terms of pore structure,permeability properties,and mechanical properties by experimental tests,and its advantages in terms of permeability and mechanical properties were verified using simulations.In addition,a new strategy of variable fiber diameter design is proposed to break through the constraint that the fiber diameter is the same everywhere during the processing of traditional extrusion 3D printing technology.Specifically,the fiber state and fiber diameter are precisely controlled by adjusting the printing speed(approximately 2-20 times the ink extrusion speed)and the nozzle height(approximately 0.4-1.0 times the inside diameter of nozzle),which in turn enables the construction of gradient pore support samples by controlling the diameter size of the fiber deposited at each point of the printing path.Meanwhile,a parametric "variable diameter design-fabrication" workflow is built to match the variable diameter strategy for its smooth implementation.Based on the above new strategy of variable fiber diameter design,with the help of parametric "variable diameter design-fabrication" workflow,this paper explores the new strategy of variable fiber diameter design in design and fabrication of controlled gradient porous scaffolds for traditional 0°/90° fiber cross-fill patterns and complex fill patterns respectively,and obtains point,line and surface interference structures,spiral structure,embedded "HIT" structure and metastructure with gradient porosity characteristics.Further,based on the structural characteristics of meniscus physiology and anatomy,we designed and prepared a principle prototype of a biomimetic meniscus PCL scaffold with radial gradient pore size;based on coaxial printing technology,we obtained a principle prototype of a human lung right lobe main vessel scaffold.The potential applications of the new strategy of variable fiber diameter design in tissue engineering of meniscus and blood vessels were explored.