Data analysis of the influence of microstructure, composition, and loading conditions on stress corrosion cracking behavior of Mg alloys

Stress corrosion cracking (SCC) can be a crucial problem in applying rare earth (RE) Magnesium alloys in environments where mechanical loads and electrochemical driven degradation processes interact. It has been proven already that the SCC behavior is associated with microstructural features, compositions, loading conditions, and corrosive media, especially in-vivo. However, it is still unclear when and how mechanisms acting on multiple scales and respective system descriptors predictable contribute to SCC for the wide set of existing Mg alloys. In the present work, suitable literature data along SCC of Mg alloys has been analyzed to enable the development of a reliable SCC model for MgGd binary alloys. Pearson correlation coefficient and linear fitting are utilized to describe the contribution of selected parameters to corrosion and mechanical properties. Based on our data analysis, a parameter ranking is obtained, providing information on the SCC impact with regard to ultimate tensile strength (UTS) and fracture elongation of respective materials. According to the analyzed data, SCC susceptibility can be grouped and mapped onto Ashby type diagrams for UTS and elongation of respective base materials tested in air and in corrosive media. The analysis reveals the effect of secondary phase content as a crucial materials descriptor for our analyzed materials and enables better understanding towards SCC model development for Mg-5Gd alloy based implant.

Light-Harvesting Complex II Adopts Different Quaternary Structures in Solution as Observed Using Small-Angle Scattering

The high-resolution crystal structure of the trimeric major light-harvesting complex of photosystem II (LHCII) is often perceived as the basis for understanding its light-harvesting and photoprotective functions. However, the LHCII solution structure and its oligomerization or aggregation state may generally differ from the crystal structure and, moreover, also depend on its functional state. In this regard, small-angle scattering experiments provide the missing link by offering structural information in aqueous solution at physiological temperatures. Herein, we use small-angle scattering to investigate the solution structures of two different preparations of solubilized LHCII employing the nonionic detergents n-octyl-β-d-glucoside (OG) and n-dodecyl-β-D-maltoside (β-DM). The data reveal that the LHCII-OG complex is equivalent to the trimeric crystal structure. Remarkably, however, we observe─for the first time─a stable oligomer composed of three LHCII trimers in the case of the LHCII-β-DM preparation, implying additional pigment-pigment interactions. The latter complex is assumed to mimic trimer-trimer interactions which play an important role in the context of photoprotective nonphotochemical quenching.

Influence of the Molecular Weight and the Presence of Calcium Ions on the Molecular Interaction of Hyaluronan and DPPC

Hyaluronan is an essential physiological bio macromolecule with different functions. One prominent area is the synovial fluid which exhibits remarkable lubrication properties. However, the synovial fluid is a multi-component system where different macromolecules interact in a synergetic fashion. Within this study we focus on the interaction of hyaluronan and phospholipids, which are thought to play a key role for lubrication. We investigate how the interactions and the association structures formed by hyaluronan (HA) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) are influenced by the molecular weight of the bio polymer and the ionic composition of the solution. We combine techniques allowing us to investigate the phase behavior of lipids (differential scanning calorimetry, zeta potential and electrophoretic mobility) with structural investigation (dynamic light scattering, small angle scattering) and theoretical simulations (molecular dynamics). The interaction of hyaluronan and phospholipids depends on the molecular weight, where hyaluronan with lower molecular weight has the strongest interaction. Furthermore, the interaction is increased by the presence of calcium ions. Our simulations show that calcium ions are located close to the carboxylate groups of HA and, by this, reduce the number of formed hydrogen bonds between HA and DPPC. The observed change in the DPPC phase behavior can be attributed to a local charge inversion by calcium ions binding to the carboxylate groups as the binding distribution of hyaluronan and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine is not changed.

In situ kinetics studies of Zn-Al LDH intercalation with corrosion related species

Kinetic parameters for three anion exchange reactions - Zn-LDH-NO₃→ Zn-LDH-Cl, Zn-LDH-NO₃→ Zn-LDH-SO₄ and Zn-LDH-NO₃→ Zn-LDH-VO_x- were obtained by in situ synchrotron study. The first and the second ones are two-stage reactions; the first stage is characterized by the two-dimensional diffusion-controlled reaction following deceleratory nucleation and the second stage is a one-dimensional diffusion-controlled reaction also with a decelerator nucleation effect. In the case of exchange NO₃^-→ Cl^- host anions are completely released, while in the case of NO₃^-→ SO₄^2- the reaction ends without complete release of nitrate anions. The exchange of Zn-LDH-NO₃→ Zn-LDH-VO_x is a one-stage reaction and goes much slower than the previous two cases. The latter is characterized by a one stage two-dimensional reaction with an instantaneous nucleation. As a result, at the end of this process there are two crystalline phases with different polyvanadate species, presumably V₄O₁₂^4- and V₂O₇^4-, nitrate anions were not completely released. The rate of replacing NO₃^- anions by guest ones can be represented as Cl^- > SO₄^2- > VO_x^y-.

Use of synergistic mixture of chelating agents for in situ LDH growth on the surface of PEO-treated AZ91

The principal possibility to grow layered double hydroxide (LDH) at ambient pressure on plasma electrolytic oxidation (PEO) treated magnesium alloy AZ91 in the presence of chelating agents is demonstrated for the first time. It avoids hydrothermal autoclave conditions, which strongly limit wide industrial application of such coating systems, and the presence of carbonate ions in the electrolyte, which lead to the formation of "passive" non-functionalizable LDH. A combination of chelating agents (sodium diethylenetriamine-pentaacetate (DTPA) and salicylate) were introduced to the treatment solution. The role of each additive and the influence of treatment bath composition on the LDH formation processes are discussed. A synergistic effect of DTPA and salicylate during LDH formation is discovered and its possible explanation is proposed.

Primary cell wall inspired micro containers as a step towards a synthetic plant cell

The structural integrity of living plant cells heavily relies on the plant cell wall containing a nanofibrous cellulose skeleton. Hence, if synthetic plant cells consist of such a cell wall, they would allow for manipulation into more complex synthetic plant structures. Herein, we have overcome the fundamental difficulties associated with assembling lipid vesicles with cellulosic nanofibers (CNFs). We prepare plantosomes with an outer shell of CNF and pectin, and beneath this, a thin layer of lipids (oleic acid and phospholipids) that surrounds a water core. By exploiting the phase behavior of the lipids, regulated by pH and Mg²⁺ ions, we form vesicle-crowded interiors that change the outer dimension of the plantosomes, mimicking the expansion in real plant cells during, e.g., growth. The internal pressure enables growth of lipid tubules through the plantosome cell wall, which paves the way to the development of hierarchical plant structures and advanced synthetic plant cell mimics.

Assembling synthetic plant cell is difficult due to the presence of primary cell wall. Here, the authors describe the assembly of lipid-containing bodies that can be coated with cellulose and pectin, and show how these so-called plantosomes can be manipulated by changing surrounding milieu.

Analysis of the bone ultrastructure around biodegradable Mg-xGd implants using small angle X-ray scattering and X-ray diffraction

Magnesium alloys are increasingly researched as temporary biodegradable metal implants in bone applications due to their mechanical properties which are more similar to bone than conventional implant metals and the fact that Magnesium occurs naturally within the body. However, the degradation processes in vivo and in particular the interaction of the bone with the degrading material need to be further investigated. In this study we are presenting the first quantitative comparison of the bone ultrastructure formed at the interface of biodegradable Mg-5Gd and Mg-10Gd implants and titanium and PEEK implants after 4, 8 and 12 weeks healing time using two-dimensional small angle X-ray scattering and X-ray diffraction. Differences in mineralization, orientation and thickness of the hydroxyapatite are assessed. We find statistically significant (p < 0.05) differences for the lattice spacing of the (310)-reflex of hydroxyapatite between titanium and Mg-xGd materials, as well as for the (310) crystal size between titanium and Mg-5Gd, indicating a possible deposition of Mg within the bone matrix. The (310) lattice spacing and crystallite size further differ significantly between implant degradation layer and surrounding bone (p < 0.001 for Mg-10Gd), suggesting apatite formation with significant amounts of Gd and Mg within the degradation layer. STATEMENT OF SIGNIFICANCE: Biodegradable Magnesium-based alloys are emerging as a viable alternative for temporary bone implant applications. However, in order to understand if the degradation of the implant material influences the bone ultrastructure, it is necessary to study the bone structure using high-resolution techniques. We have therefore employed 2D small angle X-ray scattering and X-ray diffraction to study the bone ultrastructure surrounding Magnesium-Gadolinium alloys as well as Titanium and PEEK alloys at three different healing times. This is the first time, that the bone ultrastructure around these materials is directly compared and that a statistical evaluation is performed. We found differences indicating a possible deposition of Mg within the bone matrix as well as a local deposition of Mg and/or Gd at the implant site. DATA AVAILABILITY STATEMENT: The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

Influence of high hydrostatic pressure on solid supported DPPC bilayers with hyaluronan in the presence of Ca2+ ions

The molecular mechanisms responsible for outstanding lubrication of natural systems, like articular joints, have been the focus of scientific research for several decades. One essential aspect is the lubrication under pressure, where it is important to understand how the lubricating entities adapt under dynamic working conditions in order to fulfill their function. We made a structural investigation of a model system consisting of two of the molecules present at the cartilage interface, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine and hyaluronan, at high hydrostatic pressure. Phospholipid layers are found at the cartilage surfaces and are able to considerably reduce friction. Their behavior under load and varied solution conditions is important as pressures of 180 bar are encountered during daily life activities. We focus on how divalent ions, like Ca2+, affect the interaction between DPPC and hyaluronan, as other investigations have indicated that calcium ions influence their interaction. It could be shown that already low amounts of Ca2+ strongly influence the interaction of hyaluronan with DPPC. Our results suggest that the calcium ions increase the amount of adsorbed hyaluronan indicating an increased electrostatic interaction. Most importantly, we observe a modification of the DPPC phase diagram as hyaluronan absorbs to the bilayer which results in an Lα-like structure at low temperatures and a decoupling of the leaflets forming an asymmetric bilayer structure.

Self-assembly of block copolymers during hollow fiber spinning: an in situ small-angle X-ray scattering study

We investigated the self-assembly of block copolymers during hollow fiber membrane (HFM) fabrication by conducting in situ small angle X-ray scattering (SAXS) and ex situ scanning electron microscopy (SEM) studies. SAXS enables us to follow the structural rearrangements after extrusion at different distances from the spinning nozzle. The kinetics of the spinning process is examined as a function of the composition of block copolymer solutions and the spinning parameters. We studied the influence of the extrusion rate on the block copolymer microdomains and their self-assembly in weakly segregated and ordered solutions. The addition of magnesium acetate (MgAc2) leads to the ordering of micelles in the block copolymer solution already at lower polymer concentrations and shows an increased number of micelles with larger domain spacing as compared to the pristine solution. The SAXS data show the effect of shear within the spinneret on the self-assembly of block copolymers and the kinetics of phase separation after extrusion. It is observed that the ordering of micelles in solutions is decreased as indicated by the loss of crystallinity while high extrusion rates orient the structures perpendicular to the fiber direction. The structural features obtained from in situ SAXS experiments are correlated to the structure in the block copolymer solutions in the absence of shear and the morphologies in flat sheet and HF membranes obtained by ex situ SEM. This allows a systematic and comparative study of the effects varying the microdomain ordering within different block copolymer solutions and the formed membrane structures.

Thermoresponsive Pentablock Copolymer on Silica: Temperature Effects on Adsorption, Surface Forces, and Friction

The adsorption of hydrophilic or amphiphilic multiblock copolymers provides a powerful means to produce well-defined "smart" surfaces, especially if one or several blocks are sensitive to external stimuli. We focus here on an A-B-A-B-A copolymer, where A is a cationic poly((3-acrylamido-propyl)-trimethylammonium chloride) (PAMPTMA) block containing 15 (end blocks) or 30 (middle block) repeat units and B is a neutral thermosensitive water-soluble poly(2-isopropyl-2-oxazoline) (PIPOZ) block with 50 repeat units. X-ray reflectivity and quartz crystal microbalance with dissipation monitoring were employed to study the adsorption of PAMPTMA₁₅-PIPOZ₅₀-PAMPTMA₃₀-PIPOZ₅₀-PAMPTMA₁₅ on silica surfaces. The latter technique was employed at different temperatures up to 50 °C. Surface forces and friction between the two silica surfaces across aqueous pentablock copolymer solutions at different temperatures were determined with the atomic force microscopy colloidal probe force and friction measurements. The cationic pentablock copolymer was found to have a high affinity to the negatively charged silica surface, leading to a thin (2 nm) and rigid adsorbed layer. A steric force was encountered at a separation of around 3 nm from hard wall contact. A capillary condensation of a polymer-rich phase was observed at the cloud point of the solution. The friction forces were evaluated using Amontons' rule modified with an adhesion term.

Chelating agent-assisted in situ LDH growth on the surface of magnesium alloy

In situ formation of layered double hydroxides (LDH) on metallic surfaces has recently been considered a promising approach for protective conversion surface treatments for Al and Mg alloys. In the case of Mg-based substrates, the formation of LDH on the metal surface is normally performed in autoclave at high temperature (between 130 and 170 °C) and elevated pressure conditions. This hampers the industrial application of MgAl LDH to magnesium substrates. In this paper, the growth of MgAl LDH conversion coating directly on magnesium alloy AZ91 at ambient conditions (25 °C) or elevated temperatures is reported in carbonate free electrolyte for the first time. The direct LDH synthesis on Mg alloys is enabled by the presence of organic chelating agents (NTA and EDTA), which control the amount of free and/or hydroxyl bound Mg2+ and Al3+ in the solution. The application of the chelating agents help overcoming the typical technological limitations of direct LDH synthesis on Mg alloys. The selection of chelators and the optimization of the LDH treatment process are supported by the analysis of the thermodynamic chemical equilibria.

Macromolecular pHPMA-Based Nanoparticles with Cholesterol for Solid Tumor Targeting: Behavior in HSA Protein Environment

Nanoparticles (NPs) that form by self-assembly of amphiphilic poly(N-(2-hydroxypropyl)-methacrylamide) (pHPMA) copolymers bearing cholesterol side groups are potential drug carriers for solid tumor treatment. Here, we investigate their behavior in solutions of human serum albumin (HSA) in phosphate buffered saline. Mixed solutions of NPs, from polymer conjugates with or without the anticancer drug doxorubicin (Dox) bound to them, and HSA at concentrations up to the physiological value are characterized by synchrotron small-angle X-ray scattering and isothermal titration calorimetry. When Dox is absent, a small amount of HSA molecules bind to the cholesterol groups that form the core of the NPs by diffusing through the loose pHPMA shell or get caught in meshes formed by the pHPMA chains. These interactions are strongly hindered by the presence of Dox, which is distributed in the pHPMA shell, meaning that the delivery of Dox by the NPs in the human body is not affected by the presence of HSA.

Solution structure of monomeric and trimeric photosystem I of Thermosynechococcus elongatus investigated by small-angle X-ray scattering

The structure of monomeric and trimeric photosystem I (PS I) of Thermosynechococcus elongatus BP1 (T. elongatus) was investigated by small-angle X-ray scattering (SAXS). The scattering data reveal that the protein-detergent complexes possess radii of gyration of 58 and 78 Å in the cases of monomeric and trimeric PS I, respectively. The results also show that the samples are monodisperse, virtually free of aggregation, and contain empty detergent micelles. The shape of the protein-detergent complexes can be well approximated by elliptical cylinders with a height of 78 Å. Monomeric PS I in buffer solution exhibits minor and major radii of the elliptical cylinder of about 50 and 85 Å, respectively. In the case of trimeric PS I, both radii are equal to about 110 Å. The latter model can be shown to accommodate three elliptical cylinders equal to those describing monomeric PS I. A structure reconstitution also reveals that the protein-detergent complexes are larger than their respective crystal structures. The reconstituted structures are larger by about 20 Å mainly in the region of the hydrophobic surfaces of the monomeric and trimeric PS I complexes. This seeming contradiction can be resolved by the addition of a detergent belt constituted by a monolayer of dodecyl-β-D-maltoside molecules. Assuming a closest possible packing, a number of roughly 1024 and 1472 detergent molecules can be determined for monomeric and trimeric PS I, respectively. Taking the monolayer of detergent molecules into account, the solution structure can be almost perfectly modeled by the crystal structures of monomeric and trimeric PS I.

Complex solutions under shear and pressure: a rheometer setup for X-ray scattering experiments

A newly developed high-pressure rheometer for in situ X-ray scattering experiments is described. A commercial rheometer was modified in such a way that X-ray scattering experiments can be performed under different pressures and shear. First experiments were carried out on hyaluronan, a ubiquitous biopolymer that is important for different functions in the body such as articular joint lubrication. The data hint at a decreased electrostatic interaction at higher pressure, presumably due to the increase of the dielectric constant of water by 3% and the decrease of the free volume at 300 bar.

Studying solutions at high shear rates: a dedicated microfluidics setup

The development of a dedicated small-angle X-ray scattering setup for the investigation of complex fluids at different controlled shear conditions is reported. The setup utilizes a microfluidics chip with a narrowing channel. As a consequence, a shear gradient is generated within the channel and the effect of shear rate on structure and interactions is mapped spatially. In a first experiment small-angle X-ray scattering is utilized to investigate highly concentrated protein solutions up to a shear rate of 300000 s(-1). These data demonstrate that equilibrium clusters of lysozyme are destabilized at high shear rates.

Structure of DPPC-hyaluronan interfacial layers - effects of molecular weight and ion composition

Hyaluronan and phospholipids play an important role in lubrication in articular joints and provide in combination with glycoproteins exceptionally low friction coefficients. We have investigated the structural organization of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) Langmuir layers at the solution-air interface at different length scales with respect to the adsorption of hyaluronan (HA). This allows us to assemble a comprehensive picture of the adsorption and the resulting structures, and how they are affected by the molecular weight of HA and the presence of calcium ions. Brewster angle microscopy and grazing incident diffraction were used to determine the lateral structure at the micro- and macro scale. The data reveals an influence of HA on both the macro and micro structure of the DPPC Langmuir layer, and that the strength of this effect increases with decreasing molecular weight of HA and in presence of calcium ions. Furthermore, from X-ray reflectivity measurements we conclude that HA adsorbs to the hydrophilic part of DPPC, but data also suggest that two types of interfacial structures are formed at the interface. We argue that hydrophobic forces and electrostatic interactions play important rules for the association between DPPC and HA. Surface pressure area isotherms were used to determine the influence of HA on the phase behavior of DPPC while electrophoretic mobility measurements were used to gain insight into the binding of calcium ions to DPPC vesicles and hyaluronan.

Mixed Layers of Nonionic Dendritic Amphiphiles and DPPC at the Water Surface

Nonionic dendritic amphiphiles that self-assemble into defined supramolecular aggregates are useful for the efficient solubilization of active agents, for example, in drug delivery. We investigated a new class of dendritic amphiphiles based on a hydrophilic polyol dendron head connected to a two-chain hydrophobic block. In analogy to phospholipids, these molecules form well-organized layers in bulk (vesicles) or at the water surface (Langmuir monolayer). The actual study focuses on the phase behavior and microscopic structure of mixed Langmuir layers of theses dendritic amphiphiles with the well-known phospholipid DPPC. The combination of surface pressure area isotherms with X-ray grazing incident diffraction and Brewster angle microscopy gives us information on the phase behavior of the mixed monolayers and the orientation of the amphiphiles inside the condensed domains with molecular resolution. We could prove that the dendritic generation and, by this, the headgroup size of the amphiphilic molecules have a significant influence on their interaction with DPPC at the air-water interface. Thus, our findings are important for the understanding of mixed lipid membranes in general as well as for the preparation of artificial membranes and vesicles with adjustable properties, e.g., their drug delivery potential.

Formation of iron containing aggregates at the liquid-air interface

The early stages of the formation of inorganic aggregates, composed of iron compounds at the solution-air interface, were investigated in situ. The properties of the solution-air interface were changed by using different Langmuir layers. In order to get insight into the evolution of the sample system in situ, the processes were studied by X-ray scattering and spectroscopy techniques. The formation of aggregates was detected under cationic as well as under anionic Langmuir layers. The observed compounds lack long range order which indicates the formation of amorphous structures. This is supported by extended X-ray absorption fine structure measurements showing only minor order in the formed aggregates.

Dissolution of iron oxide nanoparticles inside polymer nanocapsules

The structure of poly(organosiloxane) nanocapsules partially filled with iron oxide cores of different sizes was revealed by small angle X-ray scattering and X-ray diffraction. The nanocapsules are synthesized by the formation of a poly(organosiloxane) shell around iron oxide nanoparticles and the simultaneous partial dissolution of these cores. Due to the high scattering contrast of the iron oxide cores compared to the polymer shell, the particle size distribution of the cores inside the capsules can be measured by small angle X-ray scattering. Additional information can be revealed by X-ray diffraction, which gives insights into the formation of the polymer network and the structure of the iron oxide cores. The study shows how the crystallinity and size of the nanoparticles as well as the shape and width of the size distribution can be altered by the synthesis parameters.

Nonlinear pressure dependence of the interaction potential of dense protein solutions

The influence of pressure on the structure and protein-protein interaction potential of dense protein solutions was studied and analyzed using small-angle x-ray scattering in combination with a liquid state theoretical approach. The structural as well as the interaction parameters of dense lysozyme solutions are affected by pressure in a nonlinear way. The structural properties of water lead to a modification of the protein-protein interactions below 4 kbar, which might have significant consequences for the stability of proteins in extreme natural environments.

High-pressure SAXS study of folded and unfolded ensembles of proteins

A structural interpretation of the thermodynamic stability of proteins requires an understanding of the structural properties of the unfolded state. High-pressure small-angle x-ray scattering was used to measure the effects of temperature, pressure, denaturants, and stabilizing osmolytes on the radii of gyration of folded and unfolded state ensembles of staphylococcal nuclease. A set of variants with the internal Val-66 replaced with Ala, Tyr, or Arg was used to examine how changes in the volume and polarity of an internal microcavity affect the dimensions of the native state and the pressure sensitivity of the ensemble. The unfolded state ensembles achieved for these proteins with high pressure were more compact than those achieved at high temperature, and were all very sensitive to the presence of urea and glycerol. Substitutions at the hydrophobic core detectably altered the conformation of the protein, even in the folded state. The introduction of a charged residue, such as Arg, inside the hydrophobic interior of a protein could dramatically alter the structural properties, even those of the unfolded state. The data suggest that a charge at an internal position can interfere with the formation of transient hydrophobic clusters in the unfolded state, and ensure that the pressure-unfolded form of a protein occupies the maximum volume possible. Only at high temperatures does the radius of gyration of the unfolded state ensemble approach the value for a statistical random coil.