Temporal control engineering in 4D printing materials for adaptive systems: a biomedical perspective

Four-dimensional (4D) printing introduces time as an explicit design parameter, enabling materials to undergo programmed transformations in response to environmental stimuli across timescales from milliseconds to months. Existing reviews have largely organized the field around stimulus types, material classes, or application examples; however, a clear and actionable framework for translating time-domain requirements—such as response rate, programmed delay, and multi-stage sequencing—into material and structural design strategies is still lacking. Here, we establish a temporal control engineering framework for 4D printing materials, categorizing systems into rapid-response, gradual-evolution, and sequential-response regimes based on transformation kinetics, and analyzing the physicochemical mechanisms governing their time-dependent behavior. By explicitly linking molecular architecture, transport constraints, and multiphase structural design to kinetic outcomes, this framework enables practical design rules for programming temporal behavior in adaptive systems. While the framework is broadly applicable, this review adopts a biomedical perspective, examining translational relevance in depth, with bone regeneration serving as a representative and clinically demanding model for time-critical functional adaptation. We further highlight emerging opportunities—including in situ characterization, advanced manufacturing strategies, and artificial intelligence (AI)-driven design—that may advance 4D printing from empirical material selection toward systematic temporal engineering.