Extrusion of Heterogeneous Filament-like Structures: A New Paradigm in Fabricating Soft Mechanical Gradient with Long Span.

Soft-to-hard material interfaces found in multimaterial systems, such as microelectronics, prosthetics, body armor, and soft robotics, often suffer from mechanical mismatches that compromise their structural integrity overtime. These mismatches occur due to significant differences in mechanical properties, such as stiffness, between soft materials (e.g., polymers and biological tissues) and hard materials (e.g., metals and ceramics). In this study, an extrusion-based approach is presented to fabricate continuous stiffness gradient materials using polydimethylsiloxane and thermoplastic expandable microspheres (EM). Morphological characterization shows the intended distribution of EM content along the length of the filament and the corresponding variation in tensile and bending stiffness. The gradient mechanical properties can be tuned by varying the EM expansion temperature. Compared to traditional fabrication techniques, this method allows for precise control over gradient magnitude and span, even post-fabrication, offering greater flexibility for various applications. This work demonstrates a scalable and efficient solution for mitigating the mechanical mismatch at soft-hard material junctions, offering the potential for advanced material design in both industrial and biomedical applications.