Exploring the Potential of 3D-Printable Agar–Urea Hydrogels as an Efficient Method of Delivering Nitrogen in Agricultural Applications

Amidst population growth and challenges with existing fertilizers, the development of smart and environmentally friendly agrochemicals is imperative. While 3D printing is widespread, its potential in slow-release agrochemicals remains unexplored. This proof-of-concept study employed solvent casting and 3D printing to develop agar–urea structures. These structures, comprising 2.5% (w/w) agar, incorporated either 7% (w/w) or 13% (w/w) urea as nitrogen nutrients. Rheological, mechanical, and morphological properties and sorption capabilities were explored. Rheological analysis revealed a substantial impact of urea, enhancing material resistance to deformation. In mechanical tests, inclusion of urea showed no significant impact on compressive strength. SEM analysis confirmed the successful entrapment of urea within the agar matrix. The inclusion of urea resulted in a diminished water sorption capacity, attributed to the urea–water interactions disrupting the hydrogen bonding ability of agar. Agar–urea inks were employed in 3D printing utilizing the direct-ink writing technique, and the nitrogen release behavior was investigated. Results revealed nearly complete urea release in the positive control within 48 h. In contrast, agar–urea formulations with 7% (w/w) and 13% (w/w) achieved nitrogen release rates of 88.8% and 94.4%, respectively, suggesting potential for 3D-printed agar formulations to modify the immediate release behavior seen in conventional urea fertilizers.