Toward 4D printed functional soft tissues

The repair of soft tissue defects remains a leading clinical challenge for patients with active lifestyles, unintentional falls and injuries, cancer, and age-related diseases. Tissue engineering and 3D printing have been developed over the last decades as strategies to create personalized tissue mimics by precisely depositing biomaterials and cells to fabricate static constructs. However, long-term clinical solutions call for increasing the complexity of engineered models to incorporate bioactive processes that mimic the dynamic nature of human tissues. 4D printing has therefore become a growing strategy for building soft tissue constructs that exert function with time. The critical challenge lies in balancing biologically relevant tissue-specific function with programmable material capabilities in response to environmental stimuli. This review highlights the technological advancements that have improved progress in soft tissue engineering to build complex skin, cardiovascular, nerve, skeletal muscle, and connective tissue constructs. We first discuss mechanisms for 4D material actuation through external stimuli, which, when combined with advanced additive manufacturing tools, can assemble and program responsive tissue mimics. We next address progress in engineering functional soft tissues, which are characterized by tissue type, and discuss their limitations. Finally, the challenges associated with the fabrication of next generation 4D printed soft tissues are defined, and emerging frontiers are highlighted. STATEMENT OF SIGNIFICANCE: Soft tissue regeneration remains a clinical reconstructive challenge due to the hierarchical nature and intricate mechanics of native tissue. While 3D printing is an effective strategy for short-term healing outcomes, most tissues in the human body rely on dynamic properties to support normal physiological function. 4D printing strategies offer improvements in complexity to embed tissue-specific function into bioprinted constructs. Many existing reviews thoroughly cover 4D printing technologies and stimuli; however, their applications in soft tissue engineering toward prototyping functional tissue mimics remain underexplored. This review explores programmable stimuli for 4D printed soft tissues, advancements and limitations in function classified by soft tissue type, and insights and strategies for future challenges to work toward 4D printed functional, engineered soft tissues.