Fabrication of Hexagonal-Prismatic Granular Hydrogel Sheets

Natural soft materials are often composed of proteins that self-assemble into well-defined structures and display mechanical properties that cannot be matched by manmade materials. These materials are frequently mimicked with hydrogels whose mechanical properties depend on their composition and the type and density of cross-links. Protocols to tune these parameters are well established and routinely used. The mechanical properties of hydrogels also depend on their structure; this parameter is more difficult to control. In this paper, we present a method to produce hexagonal-prismatic granular hydrogel sheets with well-defined structures and heterogeneous cross-link densities. The hydrogel sheets are made of self-assembled covalently cross-linked 40–120 μm diameter hexagonal-prismatic hydrogel particles that display a narrow size distribution. The structure and microscale surface roughness of the hydrogels sheets can be tuned with the polymerization conditions, their chemical composition with that of the individual hydrogel particles, and their mechanical properties with the cross-link density. Remarkably, the hydrogels composed of hexagonal-prismatic microparticles are significantly stiffer than unstructured counterparts. These results demonstrate that the stiffness of hydrogels can be tuned by controlling their micrometer-scale structure without altering their composition. Thereby, they open up new possibilities to design soft materials whose performance more closely resembles that of natural ones.

Water: its influence on the formation and crystallization of amorphous calcium carbonate

Huachuan Du

Nature produces CaCO3-based materials that display fascinating mechanical properties. These properties are a result of the exquisite control over the formation of CaCO3 crystals that nature possesses. Such a high level of control is likely achieved, at least in parts, by controlling the crystallization of CaCO3 via a transient phase, amorphous CaCO3 (ACC), with certain soluble additives and insoluble organic scaffolds present in living organisms. Inspired by the excellent properties of natural CaCO3-based materials, a lot of research has been devoted to the fabrication of synthetic counterparts with similar properties. Yet, we are far from obtaining the same degree of control over the properties of these synthetic materials. A contributing reason for this inferior control is our incomplete understanding of the influence of the processing conditions, soluble additives, and insoluble scaffolds on the formation of CaCO3 and thereby the composition, structure and properties of resulting materials. This thesis is devoted to gaining a better understanding of the early stages of the CaCO3 formation. To achieve this goal, we introduce a new, organic solvent-free method to quench this process with a high temporal resolution. We produce ACC particles using a microfluidic spray-dryer and characterize them with different techniques. This method allows us to demonstrate that the amount of mobile water contained in ACC particles increases during their growth. As a result of the higher amount of mobile water, larger particles display a lower kinetic stability against the temperature-induced crystallization than smaller counterparts. We also reveal that certain additives reduce the amount of mobile water contained in ACC particles, thereby increasing their kinetic stability if exposed to elevated temperatures. To study the influence of additives on the kinetic stability of ACC against crystallization in more detail, we expose ACC particles functionalized with different additives to a humid environment or elevated pressures. We monitor the evolution of the structure and degree of hydration of these ACC particles during their crystallization. We show that the humidity-induced crystallization of ACC follows a distinctly different pathway than the pressure-induced one. In both pathways, the amount of water and its mobility that depends on its interaction strength with additives can influence the crystallization kinetics of ACC. The resulting CaCO3 crystals display a difference in their size, morphology, orientation, and structure. To process ACC particles into bulk CaCO3-based materials that possess similar mechanical properties to natural ones, insoluble organic scaffolds that offer a control over the local composition and structures of these materials across multiple length scales are typically required. To open up new possibilities of achieving this goal, we develop a new method to fabricate two dimensional (2D) structured hydrogel sheets. These 2D structured hydrogel sheets are composed of self-assembled, crosslinked hexagonal prismatic hydrogel particles. Using this method, we can vary the microscale structure and composition of these hydrogel sheets with the size, composition and arrangement of individual particles, their surface morphology with the polymerization conditions, and their mechanical properties with the crosslink density.

EPFL2019

In situ Studies of Peptide and Protein Assemblies at the Solid-Liquid Interface

Bart Willem Stel

At the interface between science and engineering, there is the field of biomimicry: Innovations inspired by the observation of natural, evolutionary optimized biological structures and processes. To understand the challenges of biomimicry, the concept of âMolecular Tectonicsâ is particularly useful. It describes how different self-assembling processes are spatially and dynamically integrated to form successively more complex structures. This integration makes it difficult to isolate the underlying mechanisms, which are often on the scale of individual molecules. Molecular biomimicry therefore critically depends on the observation of natural systems at the molecular scale. Advances in scanning probe microscopy have enabled the study of molecular self-assembly in unprecedented detail. However, many of these advances critically depend on the use of Ultra High Vacuum (UHV) conditions. To study natural systems at the molecular scale, a transition from UHV towards solvated systems is required. In the first part of this thesis, small synthetic peptides are used as model system for molecular self-assembly. The self-assembly of peptides at interfaces has been studied by systematically varying the chemical structure and deposition method. By using ElectroSpray Ion Beam Deposition (ES-IBD) as well as in situ drop casting, the self-assembly at the solid-vacuum interface (UHV conditions) and at the solid-liquid interface was directly compared. This comprehensive study on the effect of solvent, structure and deposition method on the assembly of peptides at gas-solid and solid-liquid biointerfaces has shown that the same AcFA5K peptide can organize into various structures, from Î²-sheets to tubular and globular assemblies, depending on their local environment. The study of molecular self-assembly was extended to 2D protein crystals. By using high-speed and high-resolution and Atomic Force Microscopy (AFM), it was possible to resolve both spatially and dynamically the mechanisms involved in S-layer self-assembly at the solid-liquid interface, such as monomer adsorption, condensation into high-density clusters, crystallization and finally conformational changes of existing S-layer crystals. In the second part of this thesis, the integration of different self-assembling systems is explored and their functional properties are investigated. It combines the concepts of self-assembly and molecular tectonics. Self-assembled striped PS-b-PEO block copolymer (BCP) thin film substrates were used to confine and align S-layer self-assembly, without affecting the internal structure. Furthermore, the nucleation rate and S-layer growth was greatly enhanced on PS-b-PEO substrates, compared to the pure PS or PEO constituents. The hierarchical use of different self-assembling systems can be seen as a functional approach to the concept of molecular tectonics. The catalytic properties of S-layers regarding the formation of CaCO3 were studied. By studying the catalytic properties of S-layers ex vivo, their role can be investigated independently from the internal metabolism of their parent bacteria. Synchrotron based in situ X-ray Absorption Spectroscopy (XAS) and AFM are combined with continuous flow conditions, demonstrating that S-layers are able to stabilize amorphous forms of CaCO3 and catalyze the nucleation of calcite at extremely low supersaturations.

EPFL2018

3D to 2D reorganization of silver-thiol nanostructures, triggered by solvent vapor annealing

Sergio Allegri, Francesco Stellacci

Metal-organic composites are of great interest for a wide range of applications. The control of their structure remains a challenge, one of the problems being a complex interplay of covalent and supramolecular interactions. This paper describes the self-assembly, thermal stability and phase transitions of ordered structures of silver atoms and thiol molecules spanning from the molecular to the mesoscopic scale. Building blocks of molecularly defined clusters formed from 44 silver atoms, each particle coated by a monolayer of 30 thiol ligands, are used as ideal building blocks. By changing solvent and temperature it is possible to tune the self-assembled 3D crystals of pristine nanoparticles or, conversely, 2D layered structures, with alternated stacks of Ag atoms and thiol monolayers. The study investigates morphological, chemical and structural stability of these materials between 25 and 300 degrees C in situ and ex situ at the nanoscale by combining optical and electronic spectroscopic and scattering techniques, scanning probe microscopies and density-functional theory (DFT) calculations. The proposed wet-chemistry approach is relatively cheap, easy to implement, and scalable, allowing the fabricated materials with tuned properties using the same building blocks.