Composition and layered co-continuous structure co-regulate shape memory properties

Abstract

4D printed shape memory implant is highly promising for the realization of minimally invasive, while challenged by the poor shape memory effect (SME) of commonly used biodegradable shape memory polymer (SMP) such as poly(L-lactic acid) (PLLA) and thermoplastic polyurethane (TPU). Herein, the shape memory bone scaffold was fabricated by laser powder bed fusion (LPBF) with layered co-continuous structures containing PLLA and TPU. And SME of the scaffold was ameliorated by regulating the material composition and constructing a special layered co-continuous structure for the first time. The layered co-continuous structure could avoid the impact of morphology on SME due to the immiscibility between PLLA and TPU, thus broadening the window of tuning the SME. As the ratio of PLLA and TPU decreased gradually, the shape fixity ratio (R_f) decreased and the shape recovery ratio (R_r) increased. This was attributed to the combination of changes in the ratio of "switching segment and netpoint" and the reverse stiffness effect between PLLA and TPU. Besides, due to the efficient stress transfer in the layered co-continuous structure, the R_f and R_r would be considerably changed only when the content threshold of the switching segment or netpoint was reached. A good SME was obtained when the ratio of TPU to PLLA was 2:1, with R_f of 96.5 % and R_r of 96.71 % compared to the pure PLLA. Additionally, the scaffold exhibited sufficient compressive strength and benign cytocompatibility. This study proposed a new and simple but effective strategy to prepare bone scaffold with excellent shape memory properties.