Powder crystalline phase breaking: A new insight into the change in shape memory properties of poly(L-lactic acid)

Mechanism by which process parameters affect the shape memory properties of 4D-printed objects remains unclear. In this study, laser powder bed fusion (LPBF) was employed as the printing technique to systematically investigate the relationship from poly(L-lactic acid) (PLLA) raw material preparation to process-properties mapping. It was found that increasing the laser energy density led to a lower shape recovery ratio of PLLA scaffold with a lower crystallization, wherein the crystalline phase acted as netpoints remembering the permanent shape. And the phenomenon that crystalline phases were breaking down into fewer and smaller crystalline phases under a high laser energy density was intuitively observed. The mechanism was proposed that the laser broke the crystalline phase of PLLA powder during printing and enabling control over the netpoints and thereby influencing the shape memory properties of PLLA scaffold. Furthermore, based on the above mechanism, multi-phase composite scaffolds with pixel-level distribution of amorphous and semi-crystalline phases were fabricated using a single semi-crystalline PLLA as the raw material by tailoring LPBF process parameters. In conclusion, this study revealed that laser-induced breaking of PLLA crystalline phase governed the shape memory properties and provided a new paradigm for multi- phase printing via LPBF.