Non-planar additive manufacturing with hydrogels: a review of flow control and toolpath strategies

Additive manufacturing (AM) has advanced rapidly, enabling the fabrication of complex, multi-material structures for engineering and biomedical applications. Among soft materials, hydrogels are promising for tissue engineering due to their high water content, biocompatibility, and tunable rheology. However, extrusion-based hydrogel printing still faces challenges including low stiffness, poor structural fidelity, and unstable flow, especially in non-planar, freeform fabrication. Here, we review the recent developments in toolpath optimization for non-planar hydrogel printing. Key progress in extrusion control, slicing algorithms, and multi-axis systems is outlined. We also highlight support-free strategies such as embedded printing, adaptive flow regulation, Eulerian path planning, and neural slicing and their applications in tissue engineering. We conclude that reliable non-planar AM of hydrogels requires coupling material behavior with real-time process control and advanced path planning. Future work should focus on adaptive algorithms for dynamic flow, collision avoidance, and in situ correction to enable precise, functional, biomimetic structures.