Stimulus-responsive shape-shifting polymers^1,2,3 have shown unique promise in emerging applications, including soft robotics^4,5,6,7, medical devices⁸, aerospace structures⁹ and flexible electronics¹⁰. Their externally triggered shape-shifting behaviour offers on-demand controllability essential for many device applications. Ironically, accessing external triggers (for example, heating or light) under realistic scenarios has become the greatest bottleneck in demanding applications such as implantable medical devices⁸. Certain shape-shifting polymers rely on naturally present stimuli (for example, human body temperature for implantable devices)⁸ as triggers. Although they forgo the need for external stimulation, the ability to control recovery onset is also lost. Naturally triggered, yet actively controllable, shape-shifting behaviour is highly desirable but these two attributes are conflicting. Here we achieved this goal with a four-dimensional printable shape memory hydrogel that operates via phase separation, with its shape-shifting kinetics dominated by internal mass diffusion rather than by heat transport used for common shape memory polymers^8,9,10,11. This hydrogel can undergo shape transformation at natural ambient temperature, critically with a recovery onset delay. This delay is programmable by altering the degree of phase separation during device programming, which offers a unique mechanism for shape-shifting control. Our naturally triggered shape memory polymer with a tunable recovery onset markedly lowers the barrier for device implementation.

刺激响应变形聚合物^1,2,3在新兴应用中显示出独特的前景，包括软机器人^4,5,6,7、医疗设备⁸、航空航天结构⁹和柔性电子产品¹⁰。它们的外部触发变形行为提供了许多设备应用所必需的按需可控性。讽刺的是，在现实场景下访问外部触发（例如，加热或光）已成为植入式医疗设备等要求苛刻的应用中的最大瓶颈⁸。某些变形聚合物依赖于自然存在的刺激（例如，可植入设备的人体温度）⁸作为触发器。尽管他们不再需要外部刺激，但也失去了控制恢复开始的能力。自然触发但可主动控制的变形行为是非常理想的，但这两个属性是相互冲突的。在这里，我们通过四维可打印形状记忆水凝胶实现了这一目标，该水凝胶通过相分离进行操作，其变形动力学由内部质量扩散主导，而不是由常见形状记忆聚合物使用的热传输主导^8,9,10,11。这种水凝胶可以在自然环境温度下发生形状转变，关键是恢复开始延迟。该延迟可通过在器件编程期间改变相分离程度来进行编程，这为变形控制提供了独特的机制。我们的自然触发形状记忆聚合物具有可调节的恢复起始点，显着降低了设备实施的障碍。