4D printing of soft actuators using functionally graded hydrogels with shape-morphing behavior

Purpose The purpose of this study is to understand the influence of hydrogel concentration and printer parameters on the swelling and compression behaviors of 3D-printed PEGDA hydrogels. These functionally graded hydrogels lead to a smart response, however, there have been few studies regarding the characteristics and applications of 4D-printed functionally graded hydrogels. The influence of printer parameters on their properties also needs further exploration. Design/methodology/approach In this work, the swelling and compression properties of functionally graded Polyethylene glycol diacrylate (PEGDA) hydrogels were studied and compared with the properties of single-composition hydrogels. The shape morphing behavior of 4D-printed thin functionally graded PEGDA hydrogels was investigated with respect to the gradation in polymer concentration, the thickness of the sample and the environmental temperature. Findings The swelling behavior was found to vary greatly depending on the PEGDA concentration of the hydrogel. The printer parameters did not have such a significant effect on swelling behavior. On the other hand, compression properties were found to depend on both PEGDA concentration and printer parameters. 4D printing of passive soft actuators were achieved using functionally graded hydrogels which could repeatably perform gripping and release actions. Originality/value This study provides a detailed evaluation of the variation in swelling and compression behaviors of 3D-printed PEGDA hydrogels with hydrogel concentration and printer parameters. The study demonstrates the effective use of this property variation in 4D printing functionally graded hydrogel soft actuators.