Fully bio-based composite and modular metastructures

Metamaterials and metastructures are traditionally made using fossil-based components. We describe here an eco-friendly and sustainable alternative to build metastructures with load-bearing characteristics by using a novel and modular trussed cell concept from natural composite rods derived from the giant bamboo ( Dendrocalamus asper ) and bi-phasic plant-based polymeric joints derived from soybean ( Glycine max ) and castor oil ( Ricinus communis ). The metastructure cells are engineered to exhibit structural chiral behaviour by rotating upon compression to absorb energy. The trussed unit cells (50×50×50 mm³) can then be integrated into large-scale trussed beams (400×50×50 mm³). The raw materials undergo comprehensive testing to assess their physical, chemical, and mechanical properties. The structural performance of the trussed cells is evaluated under compression, and the trussed beam is tested using four-point bending. The trussed metastructure is also used as the core of a sandwich configuration made with balsa wood skins, and prototypes of these sandwich beams are also subjected to four-point bending tests. The metastructure trussed cell, with a mass of ~ 30 g, supports up to 700 kg under compression, with a displacement of ~ 2 mm, a rotation of 4°, and an energy absorption of ~ 750 μJ/mm³; exhibiting an equivalent zero Poisson ratio and a force-displacement slope of up to ~ 7,200 N/mm within the elastic regime, which persists up to a displacement of 1 mm. The trussed and sandwich beams demonstrate equivalent densities of ~ 0.19 g/cm³ and ~ 0.21 g/cm³, respectively. The trussed beam supports nearly 2,000 N under bending, corresponding to a maximum bending moment of ~ 103 kN∙mm, absorbing ~ 158 μJ/mm³ prior to failure. The sandwich beam achieves a loading capacity of nearly 3,600 N, with a maximum bending moment of ~ 188 kN∙mm and energy absorption up to failure of ~ 193 μJ/mm³. The experimental data obtained are used to validate finite element models that simulate the elastic behaviour of these structures, which are assessed for additional loading configurations; showcasing that the trussed cell depicts a response torque of ~ 7300 N∙mm for 1° of angle twist, while the trussed and sandwich beams have a homogenised flexural modulus under cantilever of ~ 623 MPa and ~ 751 MPa, respectively. These findings suggest a promising direction for the development of renewable, high-strength, and lightweight materials suitable for a range of applications from civil construction to aerospace engineering.