A Critical Review of Thermo-Mechanical Behaviour of Reinforced 4D-Printed Shape-Memory Polymers

Additive manufacturing (AM) combined with smart materials has enabled 4D printing of structures capable of programmable, stimulus-driven shape transformation. Among these, particle-reinforced shape-memory polymers (SMPs) have emerged as a key platform for achieving enhanced thermo-mechanical performance and multi-stimuli actuation. While existing reviews have broadly surveyed 4D printing technologies or shape-memory materials in isolation, no prior work has critically examined how the characterisation toolkit, dynamic mechanical thermal analysis (DMTA), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and thermomechanical analysis (TMA), collectively governs process-structure–property-performance relationships in particle-reinforced 4D-printed SMP composites. This review addresses that gap by providing a critical, cross-platform assessment of thermo-mechanical behaviour across fused deposition modelling (FDM), direct ink writing (DIW), stereolithography (SLA), and digital light processing (DLP). Particular attention is given to how filler type, dispersion quality, and interfacial bonding govern glass transition behaviour, storage modulus, viscoelastic response, shape fixity, recovery kinetics, and thermal stability. The review critically evaluates the negative consequences of excessive filler concentration, highlights inconsistencies in experimental protocols that hinder cross-study comparison, and discusses constitutive modelling, industrial implementation, and AI-driven optimisation. A process-structure–property-performance framework is proposed as a structured basis for designing next-generation 4D-printed smart material systems.