Emerging trends in additive manufacturing of shape memory and stimuli-responsive materials: 3D and 4D printing technologies explored – a critical review

Four-dimensional (4D) printing represents a transformative paradigm within additive manufacturing, wherein time-dependent programmable materials are integrated into three-dimensional constructs to create structures capable of dynamic morphological evolution. This advanced approach combines state-of-the-art 3D printing techniques with intelligent materials exhibiting stimuli-responsive behaviours such as shape memory and autonomous self-repair. These capabilities enable fabrication of adaptive, multifunctional architectures that reconfigure or recover form in response to external cues, eliminating manual intervention in inaccessible or extreme environments, including extraterrestrial and harsh climatic conditions. This review outlines the current trajectory of innovation in 4D printable smart materials, critically assessing their physicochemical responsiveness to stimuli including thermal gradients, humidity, pH variations, light activation, magnetic fields, and electrical input. Key material classes – shape memory polymers and alloys, architected metamaterials, and intrinsically self-healing systems – are analysed regarding activation mechanisms and functional performance. Furthermore, potential applications are examined, ranging from autonomous health-monitoring systems and reconfigurable electronics to deployable aerospace structures, biomimetic soft robotics, and tunable mechanical metamaterials. Collectively, these advances highlight the pivotal role of 4D printing in advancing next-generation adaptive systems across diverse engineering disciplines.