Fracture and toughening of high volume fraction ceramic particle reinforced metals

This work contributes to the fundamental understanding of fracture properties of Particle Reinforced Metal Matrix Composites (PRMMCs), by identifying the key microstructural parameters that control fracture. To this end, PRMMCs with a high volume fraction of ceramic reinforcement (40-60 vol.%) are produced by gas-pressure infiltration. These composites are considered as model ductile/brittle twophase materials in that (i): the particles are homogeneously distributed in the matrix, (ii): the matrix microstructure is kept simple, and (iii) the composites are free of processing defects. The reinforcements used are alumina (Al2O3) particles of various shape (angular, polygonal) and size (5 to 60 µm), and boron carbide (B4C) particles (5 to 60 µm). The matrix materials are (i): pure Al, (ii): Al-Cu2% alloy, and (iii): Al-Cu4.5% alloy, all being chosen in order to obtain a single-phase matrix (Cu in solid-solution for the Al-Cu alloys), and to minimize chemical reactivity with the reinforcement. Pure Al matrix composites exhibit marked R-curve behaviour; they are characterized by J-integral fracture testing. The fracture toughness increases with the interparticle distance. At a given particle size, polygonal Al2O3 particle composites are the toughest, followed by B4C particle composites, and by angular Al2O3 particle composites. Al-Cu matrix composites feature a flatter R-curve, and are tested by a Linear Elastic Fracture Mechanics (LEFM) method: the chevron-notch test. Again, polygonal particles yield tougher composites than angular ones. In the as-cast condition, coarse intermetallics formed at the interface matrix/reinforcement during solidification are strongly detrimental to the toughness. After heat-treatment, on the other hand, toughness of the alloyed matrix composites is improved and increases as the matrix is strengthened by raising the Cu content in the matrix. Using an arrested-crack technique, it is found that the dominant micromechanisms of fracture of pure Al matrix composites are strongly dependent on the particle type, shape and size: the stronger the reinforcement, the more the crack tends to propagate by a ductile mechanism of nucleation, growth, and coalescence of micro-cavities. With weaker particles, cracking of the composite is promoted by premature particle cracking. A stereoscopic method coupled with Scanning Electron Microscopy (SEM) imaging is used to reconstruct the fracture surfaces in 3D. The final dimple size (diameter, depth) is found to depend on the microstructural length scale of the composites, i.e. the interparticle distance. Data obtained from two types of measurement (quantitative metallography, dimple depth) are used to estimate the local energy necessary to create the fracture profile, by using simple micromechanical models. At the global scale, surface strain fields are revealed by photoelasticity. The observed crack-tip strain fields are fully confirmed by 3D Finite Element (FE) computations. Although most of the fracture energy is spent in the plastic zone, it is shown that toughness is controlled by the local fracture energy that is dissipated in the crack-tip process zone: the macroscopic fracture toughness is an "amplification" of the local fracture energy. This simple and linear correlation breaks down when, for a given ceramic particle type and size, a transition in the dominant micromechanism of fracture occurs as the matrix is strengthened. The local/global correlation is discussed in more detail, using a simplified approach based on the Cohesive Zone Model (CZM) for ductile fracture: the fundamental parameters allowing to achieve attractive toughness are identified as: (i) the intrinsic particle strength, and (ii) the high local stress triaxiality between the closely spaced particles, made possible by the strong interfacial bonds between matrix and reinforcement. Overall, the composites feature very high toughness for materials made of up to 60 vol.% of brittle phase. The toughest pure Al matrix composites feature a KJeq as high as 40 MPa·m1/2. For Al-Cu matrix composites, KIv (the plane-strain chevron notch fracture toughness) exceeds 30 MPa·m1/2 (a value, to our knowledge, never reported for this class of materials) together with a Young's modulus of 180 GPa, a yield strength of 400 MPa and an ultimate tensile strength approaching 500 MPa. This combination of values gives an interesting potential for these composites as engineering materials

Structural, Nanomechanical, and Computational Characterization of d , l -Cyclic Peptide Assemblies

The rigid geometry and tunable chemistry of d,l-cyclic peptides makes them an intriguing building-block for the rational design of nano- and microscale hierarchically structured materials. Herein, we utilize a combination of electron microscopy, nanomechanical characterization including depth sensing-based bending experiments, and molecular modeling methods to obtain the structural and mechanical characteristics of cyclo-[(Gln-d-Leu)4] (QL4) assemblies. QL4 monomers assemble to form large, rod-like structures with diameters up to 2 μm and lengths of tens to hundreds of micrometers. Image analysis suggests that large assemblies are hierarchically organized from individual tubes that undergo bundling to form larger structures. With an elastic modulus of 11.3 ± 3.3 GPa, hardness of 387 ± 136 MPa and strength (bending) of 98 ± 19 MPa the peptide crystals are among the most robust known proteinaceous micro- and nanofibers. The measured bending modulus of micron-scale fibrils (10.5 ± 0.9 GPa) is in the same range as the Young’s modulus measured by nanoindentation indicating that the robust nanoscale network from which the assembly derives its properties is preserved at larger length-scales. Materials selection charts are used to demonstrate the particularly robust properties of QL4 including its specific flexural modulus in which it outperforms a number of biological proteinaceous and nonproteinaceous materials including collagen and enamel. The facile synthesis, high modulus, and low density of QL4 fibers indicate that they may find utility as a filler material in a variety of high efficiency, biocompatible composite materials.Engineering and Applied Science

pH-dependent interactions of coacervate-forming histidine-rich peptide with model lipid membranes

Peptide-based liquid droplets (coacervates) produced by spontaneous liquid-liquid phase separation (LLPS), have emerged as a promising class of drug delivery systems due to their high entrapping efficiency and the simplicity of their formulation. However, the detailed mechanisms governing their interaction with cell membranes and cellular uptake remain poorly understood. In this study, we investigated the interactions of peptide coacervates composed of HBpep—peptide derived from the histidine-rich beak proteins (HBPs) of the Humboldt squid—with model cellular membranes in the form of supported lipid bilayers (SLBs). We employed quartz crystal microbalance with dissipation monitoring (QCM-D), neutron reflectometry (NR) and atomistic molecular dynamics (MD) simulations to reveal the nature of these interactions in the absence of fluorescent labels or tags. HBpep forms small oligomers at pH 6 whereas it forms µm-sized coacervates at physiological pH. Our findings reveal that both HBpep oligomers and HBpep-coacervates adsorb onto SLBs at pH 6 and 7.4, respectively. At pH 6, when the peptide carries a net positive charge, HBpep oligomers insert into the SLB, facilitated by the peptide’s interactions with the charged lipids and cholesterol. Importantly, however, HBpep coacervate adsorption at physiological pH, when it is largely uncharged, is fully reversible, suggesting no significant lipid bilayer rearrangement. HBpep coacervates, previously identified as efficient drug delivery vehicles, do not interact with the lipid membrane in the same manner as traditional cationic drug delivery systems or cell-penetrating peptides. Based on our findings, HBpep coacervates at physiological pH cannot cross the cell membrane by a simple passive mechanism and are thus likely to adopt a non-canonical cell entry pathway

Fast and Green Synthesis of an Oligo-Hydrocaffeic Acid-Based Adhesive

A green, mussel-inspired bioadhesive based on oligomerization of hydrocaffeic acid was synthesized in water by an ultrafast one-step reaction in the presence of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide as an activating agent. The resulting oligomers exhibited strong wet adhesion when applied to different substrates including glass, stainless steel, and aluminum. Compared to most commercial adhesives, this bioinspired adhesive is produced via a sustainable and green process, i.e., aqueous-based synthesis, one-step reaction, and in the absence of any purification step to obtain the final functional adhesive.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Published versio

Integrating materials and life sciences toward the engineering of biomimetic materials

Research in the field of biological and biomimetic materials constitutes a case study of how traditional research boundaries are becoming increasingly obsolete. Positioned at the intersection of life and physical sciences, it is becoming more and more evident that future development in this area will require extensive interaction between materials and life scientists. To highlight this cross-talking, we provide a brief overview of the field, intended to illustrate how these disciplines can be integrated. We start with a short historical perspective, emphasizing the role of biologists in initiating early studies in the field. In the second part of the paper, a summary of important biochemical concepts and techniques relevant to biological materials is presented, with the goal of guiding nonspecialists towards the relevant techniques and knowledge required to investigate potential model systems. In the third part, we describe two case studies that emphasize the critical role of biosynthesis in understanding structure–function–property relationships in biological materials. We conclude with some remarks related to our own perception of how integration of materials and life sciences will lead to future developments in the field

Complex coacervates of oppositely charged co-polypeptides inspired by the sandcastle worm glue

Sandcastle worms secrete a water-resistant proteinaceous glue that is used to bind mineral particulates into their protective tubing. Previous proteomics studies have shown that the constitutive proteins of the glue are oppositely charged co-polypeptides that form a complex coacervate precursor phase, which is critical for stable underwater delivery of the adhesive. Using ring-opening polymerization (ROP) from N-carboxyanhydride (NCA) monomers, we synthesized oppositely charged co-polypeptides that mimic the amino acid composition and molecular weight of the native glue-forming proteins. The synthesis strategy enabled the incorporation of non-standard phosphoserine (pSer) and 3,4-Dihydroxyphenylalanine (Dopa) amino acids in the co-polypeptides, thereby duplicating chemical functionalities of the native glue that are key for electrostatic complexation and adhesion. Complex coacervates were obtained from these oppositely charged co-polypeptides, thus mimicking the self-assembly process of the native adhesive secreted by the sandcastle worm. Varying the relative ratio of the co-polypeptides enabled the fine-tuning of coacervation conditions such as pH and ionic strength. Wetting and rheological characterization demonstrated that our oppositely charged co-polypeptide complexes exhibited the key features associated with coacervates, namely, low surface tension, shear thinning behaviour, and viscoelastic response, making these sandcastle worm glue-inspired polypeptide coacervates a suitable modality for water-resistant bioadhesives.NRF (Natl Research Foundation, S’pore

Phase-separating peptides recruiting aggregation-induced emission fluorogen for rapid E. coli detection

Rationally designed biomolecular condensates have found applications primarily as drug-delivery systems, thanks to their ability to self-assemble under physico-chemical triggers (such as temperature, pH, or ionic strength) and to concomitantly trap client molecules with exceptionally high efficiency (>99%). However, their potential in (bio)sensing applications remains unexplored. Here, we describe a simple and rapid assay to detect E. coli by combining phase-separating peptide condensates containing a protease recognition site, within which an aggregation-induced emission (AIE)-fluorogen is recruited. The recruited AIE-fluorogen’s fluorescence is easily detected with the naked eye when the samples are viewed under UV-A light. In the presence of E. coli, the bacteria’s outer membrane protease (OmpT) cleaves the phase-separating peptides at the encoded protease recognition site, resulting in two shorter peptide fragments incapable of liquid-liquid phase separation. As a result, no condensates are formed and the fluorogen remains non-fluorescent. The assay feasibility was first tested with recombinant OmpT reconstituted in detergent micelles and subsequently confirmed with E. coli K-12. In its current format, the assay can detect E. coli K-12 (10^8 CFU) within 2 h in spiked water samples and 1-10 CFU/mL with the addition of a 6-7 h pre-culture step. In comparison, most commercially available E. coli detection kits can take anywhere from 8 to 24 h to report their results. Optimizing the peptides for OmpT’s catalytic activity can significantly improve the detection limit and assay time. Besides detecting E. coli, the assay can be adapted to detect other Gram-negative bacteria as well as proteases having diagnostic relevance.Ministry of Education (MOE)Submitted/Accepted versionThis research was funded by the Ministry of Education (MOE), Singapore, through an Academic Research Fund (AcRF) Tier 3 grant (GrantNo. MOE2019-T3-1-012)

Synthesis of biomimetic co-polypeptides with tunable degrees of phosphorylation

Phosphorylated polypeptides represent promising biomimetic macromolecules for various regenerative applications. However to date, large-scale synthesis of phosphorylated polypeptides with controlled degrees of phosphorylation has not been achieved, restricting research in phosphorylated proteins to their central roles in biomineralization pathways. Here, we present a co-polypeptide synthesis strategy based on the Ring-Opening Polymerization (ROP) of N-carboxyanhydrides (NCAs), followed by controlled phosphorylation of serine (Ser) residues. The molecular design, including amino acid composition and molecular weight of polypeptides, mimicked the intriguing phosphorylated protein Pc-3 secreted by the sandcastle tube worm Phragmatopoma californica, which is a major constituent of the glue produced by the animal to bind hard particles together for their protective tubes. Pc-3 is comprised of mostly Ser and tyrosine (Tyr), with up to 70% of Ser residues phosphorylated into phospho-serine (pSer) giving rise to the high net negative charge of Pc-3. Three NCA monomers were synthesized, namely Ser with free –OH groups, and Ser and Tyr with protected –OH groups, and subsequently polymerized with various feeding ratios in order to obtain a broad range of final amino acid compositions. In the final step, phosphorylation targeting free –OH groups of Ser was conducted. With this strategy, the degree of phosphorylation is governed by the initial amount of unprotected –OH groups of the precursor Ser–NCA, and the final co-polypeptides contain relative amounts of Tyr and pSer that can be tailored, yielding a composition and molecular weight (MW) that closely match those of Pc-3. This control of phosphorylation leads to polypeptides exhibiting a wide range of zeta potential values between −20 and −50 mV. Using analytical assays, including Dynamic Light Scattering (DLS), Surface Plasmon Resonance (SPR), and Quartz Crystal Microbalance with Dissipation (QCM-D), we demonstrate that these phosphorylated polypeptides exhibit affinity towards divalent ions such as Ca2+, thus opening the door for their usage as scaffolds for mineralized tissue repair or as a major component of biocompatible adhesives