H14 - The scaling-up effect on the mechanical performance of the auxetic implant for breast reconstruction

Introduction: : We recently developed a novel 3D auxetic architecture for large volume soft tissue engineering applications. This auxetic architecture was fabricated using selective laser sintering (SLS)-based 3D printing of PCL and demonstrated its great potential as a clinically viable implant, exhibiting significant flexibility, nonlinear stiffening behavior, and high permeability. We scaled up the auxetic design by increasing strut width (SW) and the corresponding strut interval (SI) to further enhance its structural, mechanical, and biological performance, in terms of flexibility, durability, permeability, and tissue regeneration capability, leading to successful large volume breast reconstruction.

Materials and

Methods: : Two scaled-up auxetic cubes, one with a 0.7 mm SW and 1.8 mm SI, and the other with a 1.0 mm SW and 3.9 mm SI, were designed and used for nonlinear FE simulations to evaluate their compressive modulus and nonlinear behavior under uniaxial compression, compared to the original auxetic design (0.5 mm SW and 1.2 mm SI). The scaled-up auxetic cubes were fabricated using SLS based 3D printing of PCL. Mechanical testing of the printed scaled-up auxetic cubes, including uniaxial compression and cyclic loading, was performed to evaluate mechanical performance in comparison with the original design. In addition, 3D models of the printed single struts for each scaled-up auxetic design were constructed through µCT scanned data and used for Stokes flow homogenization FE simulation to assess micro-scale permeability of the single strut, compared to that of the original auxetic design.

Results, Conclusions, and Discussions:: Despite the increased SW, homogenization FE simulation showed that the scaled-up auxetic unit cells had significantly higher flexibility and permeability than the original auxetic design. FEBio simulation results also showed the scaled-up auxetic cubes had significantly increased flexibility while maintaining nonlinear stiffening behavior. Mechanical testing results confirmed that the printed scaled-up auxetic cubes exhibited significantly greater flexibility under uniaxial compression, directly comparable to the native adipose tissue. Interestingly, the printed scaled-up auxetic cubes of 1 mm SW withstood a significantly larger number of the repeated compressive loads than the original design. Increasing the SW increased the bending durability of the struts, ultimately improving the fatigue resistance of the scaled-up auxetic cubes against the cyclic compressive loading. However, despite the inherent characteristics of SLS, stokes flow homogenization FE simulation revealed that the printed single struts of 1 mm SW had a lower internal void fraction, which restricted the flow infiltration through the struts and suppressed the bimodal permeability effect of the printed scaled-up auxetic cube. This limitation resulted from the increased number of laser scanning paths required to print a 1 mm strut. The final strut width will be determined by considering the number of laser scanning paths and the resulting changes in the flexibility of the printed auxetic cube.

Acknowledgements and/or References (Optional):: This research was supported by a US National Institutes of Health (NIH) (R21CA290907).