Dual-Stimuli Responsive and Sustainable PLA/APHA/TPU Blend for 4D Printing.

4D printing of shape memory polymers (SMPs) offers transformative potential for patient-specific medical devices, yet current SMPs often face a trade-off between mechanical toughness and low-temperature activation. This study presents a novel PLA/APHA/TPU blend filament for 3D printing that overcomes this limitation by combining high strength and flexibility with low-temperature shape memory activation-features not previously achieved in PLA-based SMPs. The uniform dispersion of TPU and APHA in the PLA matrix creates a composite with enhanced tensile strength, modulus, and elongation, addressing the brittleness typical of neat PLA. The optimized 60/20/20 wt.% formulation enables rapid shape recovery at ∼39.5°C, significantly below PLA's glass transition, with near-complete shape fixity (∼100%) and high recovery ratios (>92%) under both thermal and mechanical stimuli. This dual-responsive behavior is driven by the synergistic roles of TPU (providing ductility) and APHA (enhancing flexibility and thermal sensitivity). The composite also retains excellent printability and biocompatibility, making it ideal for next-generation biomedical SMP applications such as 4D-printed orthopedic braces, soft robotic actuators, and adaptive implants. Using bio-based, biodegradable polymers, this work advances eco-friendly, high-performance SMPs for additive manufacturing, setting a new benchmark for PLA-based 4D-printable materials.