Purpose This paper aims to provide a review of four-dimensional (4D) printing of shape memory polymers using inkjet printing technology. 4D printing refers to the three-dimensional (3D) printing of smart materials capable of shape change or function modification with respect to time when activated by external stimuli. Inkjet printing has gained popularity because of the technical advantages such as non-contact deposition, multi-material printing, high resolution, high speed of printing and minimal post processing. This review will serve as a platform for understanding the inkjet 4D printing process and the shape memory capability of the polymer structures printed using inkjet printing. Design/methodology/approach The approach used in this review was to search for and review research works related to inkjet 4D printing of shape memory polymers. The search period was limited for the duration 2013 to 2021 as the 4D printing technology came into light later in 2013. With the review of inkjet 4D printing of shape memory polymers, the shape memory capability of the inkjet-printed structures were also studied. Findings With the available research documents, it was found that the inkjet 4D printing technology gained momentum from 2016, three years after the introduction of the 4D printing technology. The key findings of this review show that inkjet 4D printing of shape memory polymers were primarily performed using commercial inkjet printers and polymer inks linked to the printers. Even though the inkjet printing technology is matured enough to print multiple materials, development of shape memory polymer inks for inkjet printability remains complex. To realize the full potential of inkjet 4D printing, novel polymer inks specific for inkjet printing needs development. Research limitations/implications The major limitation to this review was the availability of research papers for review. Even though inkjet printing technology has grown to popularity in the graphics printing and publishing industry since its inception in the 19th century, the technology still needs to evolve in the printing of 3D structures due to the limitations in synthesizing inks that are inkjet printable. However, this research will serve as a platform for understating the current status of inkjet 4D printing and the limitations of the technology. Originality/value This review focuses only on the inkjet 4D printing of shape memory polymers among the generally summarized 4D printing review papers available. Currently, 4D printing of shape memory polymers is carried out using only the commercially available polymer printers. Also, researchers do not have the flexibility of modifying the polymer inks linked to the printers. This review can spur more research into the development of novel polymer inks specific for inkjet printing.

The current study investigated the inkjet printability and shape memory performance of shape memory polymers prepared from commercially available ultraviolet light curable polymers named Vero PureWhite (RGD837) and Agilus30 (FLX985) for four-dimensional (4-D) printing. The rheological properties of the samples, such as density, dynamic viscosity, and surface tension, were measured to calculate the dimensionless number “Z.” The value of “Z” calculated for all the samples was within the typical inkjet printability value of 1 < Z < 10. From the calculated dimensionless numbers, the Weber number (We) should be greater than 4 for droplets to be generated. Satellite formation with the main droplet was observed for Samples S3 (0.5 RGD837 + 0.5 FLX985), S4 (0.9 RGD837 + 0.1 FLX985), and S5 (0.3 RGD837 + 0.7 FLX985) in the numerical simulation. However, as the main drop progressed further, the satellite drops coalesced with the main droplet, resulting in a single stable droplet. The developed numerical model using the level set method supports the validation of drop characteristics prior to experimentation with various inkjet nozzles. Further, the numerical model was validated with an experimental study by ejecting the inks through a 0.1 mm diameter nozzle using a 27 mm piezo disk. Sample strips printed using the prepared inks demonstrated complete shape recovery at ∼45°C in bridge and cantilever mode at 9 and 7 s, respectively, with a shape recovery ratio of 99.4 %. The investigation validated that it is possible to inkjet 4-D print with the prepared shape memory polymers because 4-D printing that uses the inkjet method demands inks with highly controlled rheological properties. The properties in achieving the drop-on-demand performance of an ink such as diameter, velocity, and volume exhibited good correlation, indicating that the drop-on-demand performance of a polymer ink can be validated numerically, along with the printability diagrams.