Template-free 3D programmable magnetization of soft millirobots induced by interlayer stress.

Soft magnetic miniature devices are crucial for applications in minimally invasive medicine, soft electronics, and robotics. While substantial progress has been made, current magnetic programming techniques are inherently tied to template-based and sequential fabrication processes. These processes limit scalability, precision, and programmability. Here, we present a template-free, integrative strategy that leverages interlayer stress-induced 3D shape morphing in xerogel-PDMS bilayer materials triggered by temperature variations. This process induces preprogrammed deformation and fixes the 3D structure via interlayer stress and solid-liquid phase transition. It is akin to an insect encased in amber, resulting in a soft machine with precisely tailored magnetic domains upon saturated magnetization. The approach eliminates the need for predesigned molds, which offers scalable, template-free programmable magnetization, reducing time and labor costs. The versatility of this method is demonstrated through reconfigurable mechanical behavior in kirigami metamaterial structures, information encryption, and multilegged millirobots. Moreover, by incorporating a nonmagnetic PDMS layer, laser-based engraving and ablation allow simultaneous control of interlayer stress and material properties. This facilitates precise regulation of stress-induced deformation and magnetically responsive regions with 20 μm resolution and over 1.8 T magnetization strength. This template-free 3D magnetization strategy significantly enhances design flexibility, machining precision, and mass production. It paves the way for advanced multiscale and programmable soft magnetic devices.