Using Advanced Macromolecular Composites as Physical Models for Algal Bioplastics

The petrochemical origins and environmental persistence of commodity polymers has caused significant damage to human health and natural ecosystems. And while recycling and upcycling technologies have progressed significantly, the increasing demand for single-use plastics and the poor rate of recycling necessitates the development of plastic alternatives that are both biologically derived and biodegradable. However, many advanced bioplastics intended to replace petrochemicals require significant chemical pre-processing and do not fully decompose in natural settings. Recently, we reported the development of a fully compostable bioplastic using whole spirulina biomass. Subjecting cells to elevated heat and pressure transforms the spirulina into a robust and compostable biomatter plastic. However, because of the chemical complexity and molecular diversity of spirulina biomass, identifying the mechanism responsible for the cohesion of spirulina cells and the formation of a homogenous matrix remains a major challenge. Here, we investigate the chemical and physical interactions governing spirulina self-bonding by considering its macromolecular components. We developed analogues for spirulina biomatter by physically combining varying ratios of representative carbohydrates, proteins, and lipids using thermomechanical processing identical to that utilized for spirulina biomatter plastics. The roles of each class of macromolecules were first assessed by observing the effects of varying composition on the micromorphology and mechanical properties of biomatter analogues. The varying ratio of carbohydrates to proteins is especially isolated to compare the mechanical performance of the biomatter analogues to several species of algae, including spirulina. Next, the bonding mechanisms are assessed qualitatively by using sequential reprocessing to estimate the relative contributions of recoverable and irrecoverable bonding. Next, Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) are used to quantitatively evaluate the secondary and primary bonding interactions between different components of the biomatter analogues. Finally, molecular dynamics (MD) are implemented to simulate the combination of the representative macromolecules and to model the evolution of hydrogen bonding and protein secondary structures during the application of heat and pressure. By considering both experimental and computational results, we propose physical and chemical mechanisms governing the self-bonding of spirulina biomatter plastics.