Introduction: The use of commercially available and low cost 3D printing machines, such as fused filament fabrication (FFF), offers the ability to fabricate 3D porous scaffolds. However, one of the drawbacks of the FFF process is that fabricated 3D structures are often characterised by closed edges instead of open porosity, which is crucial for promoting tissue growth. Laser cutting can offer an effective, precise and fast solution for obtaining scaffolds with desired-shape and open porosity from a pre-fabricated 3D printed porous bar. Therefore, this study aims at manufacturing a polylactic acid (PLA) scaffold by a two-step route: 1) 3D printing of a PLA porous bar and 2) laser cut PLA scaffolds from the 3D printed bar.To determine the applicability of the fabricated scaffolds for bone tissue engineering, morphology, mechanical behaviour and in vitro degradation behaviour were assessed.
Materials and Methods: PLA filament (PLA, 4032D, Nature Works®) was the material used for the experimental work. Fig.1 summarizes the scaffold fabrication route used during this study. The molecular weight (Mw) distribution and thermal properties were determined at each point in the processing route. Morphological analysis of laser cut PLA scaffolds was carried out (Hitachi TM3030 SEM) by measuring the transversal and axial pore size (PS) and filament width (FW). Compression samples were prepared by 2 routes: 1) porous samples were 3D printed and 2) samples were laser cut from printed porous bars. An in vitro degradation study of the porous laser cut PLA scaffolds was carried out in PBS solution under pH 7.0 at 37ºC over 10 weeks. At two week intervals the compressive properties of 3 samples still in a wet state were assessed and another group of 3 samples was vacuum dried for 48h and Mw and dry weight were determined.
Results: During FFF processing the Mw and glass transition temperature of PLA decreased significantly. The laser cut PLA scaffold presented a well-defined structure with a uniform open pore distribution with a PS in XYZ-axes of 550-620 µm and 60% porosity (Fig.2). After laser cutting, a decrease of ~27% was observed in the apparent compressive modulus and a ~35% decrease in the apparent yield stress. After 10 weeks immersion in PBS, the PLA scaffolds scaffold dry weight, Mw and compressive properties values did not show any significant decrease.
Discussion: A decrease in the glass transition temperature and Mw values during FFF fabrication can be attributed to degradation of the polymer during extrusion[1].The lower compressive properties observed after laser cutting can be explained by differences in morphology. The more open porosity means that there is some “redundant” material at the edges of the scaffold, which would not be load bearing. However, a relatively small sacrifice in mechanical properties for the higher open porosity could enhance vascularization and cell survival into the inner part of the scaffold[2].
Conclusion: Overall the two step manufacturing route used in this work led to the fabrication of mechanically stable PLA scaffolds with well-defined and open architectures, with potential for trabecular bone replacement applications.
MeDe Innovation (the EPSRC Centre for Innovative Manufacture in Medical Devices)
References:
[1] Dietmar, D., C.C. Sandra, and R. Dominik, Suitability of PLA/TCP for fused deposition modeling. Rapid Prototyping Journal, 2012. 18(6): p. 500-507.
[2] Understanding the effect of mean pore size on cell activity in collagen-glycosaminoglycan scaffolds. Cell Adhesion & Migration, 2010. 4(3): p. 377-381.