3D, 4D Printing, and Bioprinting of Hydrogels

3D printing of hydrogels has become an attractive option for biomedical researchers. Numerous additive manufacturing techniques have been utilized to produce hydrogels appropriate for tissue engineering such as digital light processing, and 3D printing systems based on laser, inkjet, and nozzle or extrusion. The notion behind the development of 4D printing began in 2014 and the fabrication process can be described as the amalgamation of additive manufacturing and stimuli-responsive material. Presently, smart materials available are limited because 4D printing is still at its infancy and majority of them are either hydrogels or shape memory polymers. Currently, there are several production techniques being developed and examined for 4D printing, and in essence, they are all based on the same additive manufacturing techniques used in 3D printing. Stimulus-responsive hydrogels can be further categorized according to the type of stimulus used such as temperature, light, moisture, or an external magnetic field. Distinct from conventional 3D printing approaches, bioprinting requires a different technical methodology that is compatible with depositing living cells. Deposition approaches such as laser-induced forward transfer (LIFT) and inkjet-based printing have been investigated for the biofabrication of hydrogels as a carrier for cells and/or bioactive compounds.