Leveraging Isothermal Crystallization to Actuate the Stress-Free Two-Way Shape Memory Effect within a Narrow and Biocompatible Temperature Window

Two-way shape memory polymers capable of reversible shape changes under a thermal stimulus without applied load remain largely constrained by broad, non-biocompatible actuation windows. In this work, an unconventional strategy is introduced to achieve isothermal, stress-free two-way shape memory actuation in a biocompatible polycaprolactone network without altering its chemistry, after proper programming. This approach exploits isothermal crystallization, the key mechanism underlying the effect, to program reversible deformation. Remarkably, significant elongation is induced by isothermally holding the partially molten network at temperatures well above its peak crystallization temperature. Crystallization kinetics was characterized directly via thermal analysis and indirectly through stress-free two-way shape memory tests. By tuning thermo-mechanical programming conditions while keeping chemistry constant, the actuation window can be confined to a narrow, controlled temperature interval between 37 °C and crystallization temperatures of 10–30 °C. These findings establish a pathway for designing intrinsically reversible polymer networks capable of actuation within tightly defined thermal windows.