Assessing design-induced elasticity of 3D printed auxetic scaffolds for tissue engineering applications

Auxetic scaffolds fabricated via additive manufacturing can enable cyclic mechanical stimulation to promote the biomechanical functionalization of engineered tissues. Typical designs of additively manufactured scaffolds used in tissue engineering literature (e.g., 0/90˚ strand laydown) are not amenable to cyclic loading due to their rigidity, which is in part due to the high stiffness of biopolymers such as polycaprolactone (PCL). Auxetic scaffolds can help overcome this due to their design-induced elasticity while recapitulating negative Poisson’s ratios seen in various natural tissues. In this study, we investigated the effects of auxetic design patterns and unit cell sizes on the mechanical properties of 3D bioplotted PCL scaffolds. First, we assessed the monotonic tensile properties of two auxetic patterns – re-entrant honeycomb and missing rib (unit cell = 3 × 3 mm² for both) – in comparison to a uniaxial control (0/0˚ strand laydown) using finite element analysis (FEA) and experimental design (n = 3/group). The results showed that the scaffold design significantly impacted scaffold elasticity (p < 0.05), with the missing rib auxetic design demonstrating significantly higher yield strain (48.2 %) compared to the re-entrant honeycomb design (11.0 %) and the control (4.8 %). The missing rib design also possessed significantly lower elastic modulus and tensile strength (11.5 MPa/g and 10 MPa/g, respectively) compared to the re-entrant honeycomb (58 MPa/g and 35.7 MPa/g, respectively) (p < 0.05). For the missing rib design, we further investigated the effect of unit cell size (2 × 2, 3 × 2, 3 × 3 mm²) on the mechanical properties. Both 3 × 2 and 3 × 3 mm² unit cell scaffolds (n = 3/group) possessed similar mechanical properties whereas the 2 × 2 mm² unit cell scaffolds possessed significantly lower yield strain and higher elastic modulus and tensile strength (p < 0.05). The missing rib auxetic scaffolds were also tested under tensile cyclic loading for up to 6000 cycles at 10 % of maximum strain at 0.5 Hz. The 2 × 2 mm² unit cell scaffolds degraded significantly faster than the other two groups. Overall, the 3 × 2 mm² unit cell scaffolds performed better under cyclic loading in terms of maintaining their tensile strength. Finally, biocompatibility testing of the missing rib 3 × 2 mm² unit cell scaffolds demonstrated their ability to support the adhesion and viability of fibroblast cells. In future, this knowledge will be leveraged to engineer scaffolds for connective tissues such as tendons and cardiac muscle.