Application of instant assembly of collagen to bioprint cardiac tissues.

Advancing cardiac tissue engineering requires innovative fabrication techniques, including 3D bioprinting and tissue maturation, to enable the generation of new muscle for repairing or replacing damaged heart tissue. Recent advances in tissue engineering have highlighted the need for rapid, high-resolution bioprinting methods that preserve cell viability and maintain structural fidelity. Traditional collagen-based bioinks gel slowly, limiting their use in bioprinting. Here, we implement TRACE (tunable rapid assembly of collagenous elements), a macromolecular crowding-driven bioprinting technique that enables the immediate gelation of collagen bioinks infused with cells. This overcomes the need for extended incubation, allowing for direct bioprinting of engineered cardiac tissues with high fidelity. Unlike methods that rely on high-concentration acidic collagen or fibrin for gelation, TRACE achieves rapid bioink stabilization without altering the biochemical composition. This ensures greater versatility in bioink selection while maintaining functional tissue outcomes. Additionally, agarose slurry provides stable structural support, preventing tissue collapse while allowing nutrient diffusion. This approach better preserves complex tissue geometries during culture than gelatin-based support baths or polydimethylsiloxane (PDMS) molds. Our results demonstrate that TRACE enables the bioprinting of structurally stable cardiac tissues with high resolution. By supporting the fabrication of biomimetic tissues, TRACE represents a promising advancement in bioprinting cardiac models and other engineered tissues.