Controlled Self-Assembly of Biobased Materials at Aqueous Interfaces

Abstract

The self-assembly of biobased materials in water is a fundamental pillar of life, with natural molecules and biocolloids assembling into well-organized structures displaying a myriad of functions. Wetting forces and geometry of the associated interfaces often guide the interfacial interactions and consequent structure formation of these natural materials. Therefore, this thesis aims at using non-wetting and highly wetting surfaces to form new hierarchical assemblies from some of the most important biocolloids available. First, nanocellulose-producing bacteria were used to form robust 3D biofilms, where hydrophobic particles and superhydrophobic moulds guided the aerobic biofabrication at the air-water interface.The resulting 3D objects were hollow, seamless constructs of varied morphologies. Gradients of thickness and topographical features were tethered to biofabrication time and hydrostatic pressure, respectively, while nanocellulose fibres were observed to be aligned at the lower sections of objects and at a parallel orientation to the gradient of hydrostatic pressure and oxygen availability. These well-controlled morphological features may find important applications in tissue engineering and other biomedical applications. We demonstrated that this approach can also be used for the encapsulation of functional particles, for the formation of multicompartmentalized structures, and for a self-healing functionality. Similarly, highly wetting interfaces also generated hierarchical self-assembly of biocolloids. By drying cellulose nanocrystals (CNC) and chitin nanocrystals (ChNC) suspensions between glass substrates, lamellar structures were formed at the vicinity of the edges of the bond. The anisotropic particles were well-aligned within the lamellae in a parallel orientation. This arrangement, mostly driven by capillary flow, resulted in high lap-shear strengths (ca. 300 N with only 0.8 mg of the adhesive). Suspensions of hen egg white lysozyme (HEWL) amyloids and short amyloids, however, were more affected by Marangoni flow and did not generate well-ordered structures and high adhesive strengths. When mixed with ChNC, however, the lamellar structure was maintained up to ca. 1:10 HEWL amyloids to ChNC ratio, and a synergistic interaction generated ultimate lap-shear loads ca. 25% higher than that of the strongest individual building block. These adhesives also displayed high anisotropy, with an out of plane adhesion ca.10 to 100 times lower than the in plane. This anisotropy is reminiscent of gecko feet, and may find applications e.g. in green adhesives that provide robust adhesion, easy disassembling, and full biodegradability. Overall, via biofabrication and biopolymeric assembly, non- and highly-wetting interfaces can serve as important guides for the fabrication of well-ordered hierarchical assemblies displaying a unique set of properties and functionalities. Micro-structured substrates, with controlled wetting states and topographical features, are expected to have an important role in the fabrication of the sustainable materials of the future.