Nature offers a variety of innovative solutions to engineering challenges, and this study draws inspiration from the structural gradients observed in plant systems, particularly at the interface between the parasitic mistletoe and its host tree. The gradual transition of material properties in this natural system, including gradients in lignin concentration and cell wall thickness along with the V-shaped interface, increases their damage resistance and longevity. Our research explores the application of this concept to bioinspired functionally graded metamaterials, focusing on the use of gradients to program mechanical properties and failure behavior. In this context, the adaptation of the mistletoe-host interface concept has been applied to a polypropylene-based material system. Polypropylene and glass-fiber reinforced polypropylene, which have different mechanical properties, are gradually connected via straight or V-shaped bio-inspired interfaces. The material system is produced by extrusion of a polymer blend and hot pressing with compartments of polypropylene with varying glass fiber content. Analysis of the fracture behavior under tensile loading shows that the samples fail at the locations with the highest glass fiber content, which can be used to program the failure characteristics. Micro-computed tomography scans quantified the glass fiber content along the specimens, revealing the gradual progression of glass fiber content. Digital image correlation technique was used to assess the local strain behavior during mechanical loading with respect to glass fiber content.
The proposed method for fabrication of sheets of graded polymers by using polymer extrusion and hot compression, can then be laser-cut into the desired (2D) structure. Accordingly, we fabricated metamaterials with the described material gradient to program their behavior. Gradients in metamaterials are usually limited to structural gradients due to the challenges additively manufactured metamaterial structures. This novel approach expands the scope of gradient metamaterials and provides a scalable and efficient method for incorporating gradients of material properties into their structures. In summary, the bioinspired functionally graded metamaterials developed in this study demonstrate the potential of mimicking natural systems for improved engineering material interfaces, helping to expand and diversify the possibilities of multi-material metamaterials.