Bioinspired polypropylene-based functionally graded materials and metamaterials modeling the mistletoe-host interface

Biological systems and their structural and functional adaptations provide valuable insights into increasing the longevity of engineered materials. A striking example is the hemiparasitic European mistletoe (Viscum album), which forms a lifelong (over 20 years) connection with its host tree, providing physiological supply and mechanical anchorage. The V-shaped interface between mistletoe and host is characterized by a lignification and cell wall gradient that bridges the mechanical differences between the adjacent tissues. These characteristics of the mistletoe-host interface can be transferred to functionally graded polymeric materials. Using extrusion molding and hot pressing, we developed a material system that combines pure and glass-fiber-reinforced polypropylene and exhibits a continuously graded mistletoe-inspired V-shaped interface. Microtomographic analyses quantified the gradual transition of the glass fiber content along one specimen from 0 to 30%, further revealing the random fiber orientation in the polymer matrix. Tensile tests showed that both Young's modulus (by 38%) and ultimate tensile strength (by 62%) could be increased by introducing V-shaped interfaces. Digital image correlation analysis and the fracture images showed that the positioning of the area with the highest glass fiber content can lead to spatial control over local strain behavior and the failure point. Moreover, this phenomenon was transferred to metamaterial structures where the material gradient counteracts the geometric gradient (beam thickness). The results highlight the effective anchoring method of mistletoe through graded structuring of the interface with the host branch and provide a framework for creating bioinspired functionally graded material systems with programmable local strain and failure behavior.