Voxel-programmed single-layer hydrogels for multifunctional self-folding microrobots

<h2>Summary</h2> The development of self-folding microrobots is currently constrained by a reliance on multi-material architectures, which introduce fabrication complexity and interfacial failure modes. Here, we overcome this paradigm by creating multifunctional microrobots from a single-layer hydrogel, where sophisticated shape-morphing is programmed directly into the microstructure. Our 4D printing strategy employs voxel-level control to engineer intrinsic stress gradients that, when combined with macroscopic hinge designs, enable rapid (∼3 s) and predictable transformation of 2D sheets into complex 3D configurations, such as ellipsoids and flowers. This single-material platform exhibits multi-stimulus responsiveness, dynamically reacting to pH, magnetic fields, and solvents, and features up to 500% reversible volumetric expansion under pH cycling. We demonstrate its multifunctional capabilities through targeted drug release (up to 55%), cargo transport, ultrasound-based environmental sensing, and programmable modular assembly. This work establishes voxel-programmed hydrogels as versatile and robust material platforms for creating functional microrobots, bypassing the need for complex composite architectures.