Multi-material additive manufacturing of polymeric composite structures with seamless soft-hard interface integration from molecular bonding

Multi-material additive manufacturing enables three-dimensional (3D) structures with spatially programmed properties, yet achieving robust bonding between soft and stiff materials remains a major challenge. Here, we introduce a molecularly bonded vat photopolymerization (VP) strategy that seamlessly integrates polymer networks spanning nearly three orders of magnitude in stiffness (~60 kPa – ~50 MPa). Foldable bottlebrush networks sharing identical photochemistry form covalently continuous interfaces, preventing delamination at soft–hard interfaces. Using a custom VP platform, we print multi-material structures with exceptional stretchability (breaking strain >800%) and graded mechanical transitions that are accurately captured by continuous mechanics models accounting for stiffness contrast between regions. This molecularly bonded VP approach establishes a versatile framework for 3D printing polymer architectures with large stiffness contrasts, opening new avenues for advanced structural, soft robotic, and bio‐integrated devices