Facet of 4D printing in biomedicine

Owing to the success of three-dimensional (3D) printing in biomedical applications, the latest addition to the technology is four-dimensional (4D) printing, which has gained tremendous interest since 2012. 4D printing is being considered as an upgradation and extension of 3D that includes time as a fourth dimension with the utilization of smart biomaterials, and upon the application of any external stimulus, the shape and size of the printed structure change with time. In this review, we highlight the basic techniques involved in 4D printing, the shape memory effect, and various stimuli like light, temperature, pH, etc., that cause the shape change, leading to the transformation of the structures fabricated. 4D printing using smart materials demonstrates shape memory property and their possible applications in the field of biomedicine and regenerative medicine are discussed in detail. The authors have focused on 4D printing of various tissues, with a special highlight on bone and dental tissue. Graphical abstract