Introduction: Limitations in current procedures such as secondary trauma for autografts and disease transmission in allografts have resulted in an increased push to develop synthetic tissue scaffolds[1]. Additive manufacturing (AM) provides precise control over scaffold morphology. Complex internal architectures can be predesigned with specific properties. However challenges include the limited number of AM compatible biomaterials[2]. This work aims to investigate AM techniques to understand process-structure-performance relationships for clinically relevant bioresorbable polymers in order to tailor degradation rates and optimise release kinetics of incorporated bioactive agents. Particular attention was focused on identifying degradation effects throughout the manufacturing process, determining implant performance.
Materials and Methods: Poly(L-lactic acid) (PLLA) (4043D Natureworks) was cryogenically ground before being processed into filament using a Haake twin-screw extruder (Thermo Fisher Scientific). An extruder temperature of 170oC-175oC, draw off speed of 0.11 m/s, feed rate of 12% and screw speed of 300 rpm were applied. An air ring and chilled air ring were positioned alongside rollers to control draw down and stabilise extrudate. Composites of PLLA incorporating 0.2µm beta-tricalcium phosphate (β-TCP) (Plasma Biotal Ltd) were mixed in weight percentages of 5 and 20% and extruded into filament.
A RepRap Tricolour Mendel (RepRapPro) fused deposition modelling technique was used to produce scaffolds. Various 3-D structures with porous geometries were created using computer aided design (Solidworks). To evaluate print performance scaffolds were manufactured from the tailored filament using optimised RepRap parameters. A 0.3mm nozzle extruded filament at 190oC onto a 60oC heated bed at 30mm/s. Scanning electron microscopy (Oxford Instruments) and Micro CT analysis (Scanco) were used to examine the structure of printed scaffolds.
Results and Discussion: Optimisation of the extrusion process produced FDM compatible filament at 1.75 (+0.15/-0.08) mm. Processing of composite β-TCP filaments required minor adjustments of the draw off speed to maintain consistency of diameter.
FDM demonstrated suitable processing capability, parameter adaptability, and material compatibility. The ability to print a range of different morphologies compatible with the deposition technique has been demonstrated, maintaining pore geometry within the recommended range for hard tissue[3]. Initial results in processing 0/90o laydown patterns provided clear pore channels. Controlled adaptation of RepRap parameters allowed the equipment to be functionalised towards the manufacture of scaffolds with improved control over pore geometry, creating pores >200µm. Scaffolds manufactured from all three tailored filaments produced accurate and repeatable results (Fig.1) with minimal morphology variation. Micro CT revealed micropore inclusions of 130µm typical diameter throughout the structure (Fig.2) suggesting void formation through extrusion or FDM processes.
Conclusion: Work completed highlights the potential of this innovative approach for manufacturing bioresorbable hard tissue scaffolds. With the aim of developing scaffolds with clinically relevant resorption rates, this study has provided the foundation for processing other resorbable polymers.
References:
[1] Lichte P et al. Injury. 42(6):569–73, 2011.
[2] Lam CX et al. Mater Sci Eng C. 20(1-2):49–56, 2002.
[3] Shor L et al. Biofabrication. 1(1), 2009.