Exploring the Nanostructures Accessible to an Organic Surfactant Atmospheric Aerosol Proxy.

The composition of atmospheric aerosols varies with time, season, location, and environment. This affects key aerosol properties such as hygroscopicity and reactivity, influencing the aerosol's impact on the climate and air quality. The organic fraction of atmospheric aerosol emissions often contains surfactant material, such as fatty acids. These molecules are known to form three-dimensional nanostructures in contact with water. Different nanostructures have marked differences in viscosity and diffusivity that are properties whose understanding is essential when considering an aerosol's atmospheric impact. We have explored a range of nanostructures accessible to the organic surfactant oleic acid (commonly found in cooking emissions), simulating variation that is likely to happen in the atmosphere. This was achieved by changing the amount of water, aqueous phase salinity and by addition of other commonly coemitted compounds: sugars and stearic acid (the saturated analogue of oleic acid). The nanostructure was observed by both synchrotron and laboratory small/wide angle X-ray scattering (SAXS/WAXS) and found to be sensitive to the proxy composition. Additionally, the spacing between repeat units in these nanostructures was water content dependent (i.e., an increase from 41 to 54 Šin inverse hexagonal phase d-spacing when increasing the water content from 30 to 50 wt %), suggesting incorporation of water within the nanostructure. A significant decrease in mixture viscosity was also observed with increasing water content from ∼104 to ∼102 Pa s when increasing the water content from 30 to 60 wt %. Time-resolved SAXS experiments on levitated droplets of this proxy confirm the phase changes observed in bulk phase mixtures and demonstrate that coexistent nanostructures can form in droplets. Aerosol compositional and subsequent nanostructural changes could affect aerosol processes, leading to an impact on the climate and urban air pollution.

Extending synchrotron SAXS instrument ranges through addition of a portable, inexpensive USAXS module with vertical rotation axes.

Ultra-SAXS can enhance the capabilities of existing synchrotron SAXS/WAXS beamlines. A compact ultra-SAXS module has been developed, which extends the measurable q-range with 0.0015 ≤ q (nm-1) ≤ 0.2, allowing structural dimensions in the range 30 ≤ D (nm) ≤ 4000 to be probed in addition to the range covered by a high-end SAXS/WAXS instrument. By shifting the module components in and out on their respective motor stages, SAXS/WAXS measurements can be easily and rapidly interleaved with USAXS measurements. The use of vertical crystal rotation axes (horizontal diffraction) greatly simplifies the construction, at minimal cost to efficiency. In this paper, the design considerations, realization and synchrotron findings are presented. Measurements of silica spheres, an alumina membrane, and a porous carbon catalyst are provided as application examples.

The persistence of a proxy for cooking emissions in megacities: a kinetic study of the ozonolysis of self-assembled films by simultaneous small and wide angle X-ray scattering (SAXS/WAXS) and Raman microscopy.

Cooking emissions account for a significant proportion of the organic aerosols emitted into the urban environment and high pollution events have been linked to an increased organic content on urban particulate matter surfaces. We present a kinetic study on surface coatings of self-assembled (semi-solid) oleic acid-sodium oleate cooking aerosol proxies undergoing ozonolysis. We found clear film thickness-dependent kinetic behaviour and measured the effect of the organic phase on the kinetics for this system. In addition to the thickness-dependent kinetics, we show that significant fractions of unreacted proxy remain after extensive ozone exposure and that this effect scales approximately linearly with film thickness, suggesting that a late-stage inert reaction product may form and inhibit reaction progress - effectively building up an inert crust. We determine this by using a range of simultaneous analytical techniques; most notably Small-Angle X-ray Scattering (SAXS) has been used for the first time to measure the reaction kinetics of films of a wide range of thicknesses from ca. 0.59 to 73 µm with films <10 µm thick being of potential atmospheric relevance. These observations have implications for the evolution of particulate matter in the urban environment, potentially extending the atmospheric lifetimes of harmful aerosol components and affecting the local urban air quality and climate.

Switchable Catalysis Improves the Properties of CO2-Derived Polymers: Poly(cyclohexene carbonate-b-ε-decalactone-b-cyclohexene carbonate) Adhesives, Elastomers, and Toughened Plastics.

Carbon dioxide/epoxide copolymerization is an efficient way to add value to waste CO2 and to reduce pollution in polymer manufacturing. Using this process to make low molar mass polycarbonate polyols is a commercially relevant route to new thermosets and polyurethanes. In contrast, high molar mass polycarbonates, produced from CO2, generally under-deliver in terms of properties, and one of the most widely investigated, poly(cyclohexene carbonate), is limited by its low elongation at break and high brittleness. Here, a new catalytic polymerization process is reported that selectively and efficiently yields degradable ABA-block polymers, incorporating 6-23 wt % CO2. The polymers are synthesized using a new, highly active organometallic heterodinuclear Zn(II)/Mg(II) catalyst applied in a one-pot procedure together with biobased ε-decalactone, cyclohexene oxide, and carbon dioxide to make a series of poly(cyclohexene carbonate-b-decalactone-b-cyclohexene carbonate) [PCHC-PDL-PCHC]. The process is highly selective (CO2 selectivity >99% of theoretical value), allows for high monomer conversions (>90%), and yields polymers with predictable compositions, molar mass (from 38-71 kg mol-1), and forms dihydroxyl telechelic chains. These new materials improve upon the properties of poly(cyclohexene carbonate) and, specifically, they show good thermal stability (Td,5 ∼ 280 °C), high toughness (112 MJ m-3), and very high elongation at break (>900%). Materials properties are improved by precisely controlling both the quantity and location of carbon dioxide in the polymer chain. Preliminary studies show that polymers are stable in aqueous environments at room temperature over months, but they are rapidly degraded upon gentle heating in an acidic environment (60 °C, toluene, p-toluene sulfonic acid). The process is likely generally applicable to many other lactones, lactides, anhydrides, epoxides, and heterocumulenes and sets the scene for a host of new applications for CO2-derived polymers.

Interfibrillar stiffening of echinoderm mutable collagenous tissue demonstrated at the nanoscale.

The mutable collagenous tissue (MCT) of echinoderms (e.g., sea cucumbers and starfish) is a remarkable example of a biological material that has the unique attribute, among collagenous tissues, of being able to rapidly change its stiffness and extensibility under neural control. However, the mechanisms of MCT have not been characterized at the nanoscale. Using synchrotron small-angle X-ray diffraction to probe time-dependent changes in fibrillar structure during in situ tensile testing of sea cucumber dermis, we investigate the ultrastructural mechanics of MCT by measuring fibril strain at different chemically induced mechanical states. By measuring a variable interfibrillar stiffness (EIF), the mechanism of mutability at the nanoscale can be demonstrated directly. A model of stiffness modulation via enhanced fibrillar recruitment is developed to explain the biophysical mechanisms of MCT. Understanding the mechanisms of MCT quantitatively may have applications in development of new types of mechanically tunable biomaterials.

Hydrophobic nanoparticles promote lamellar to inverted hexagonal transition in phospholipid mesophases.

This study focuses on how the mesophase transition behaviour of the phospholipid dioleoyl phosphatidylethanolamine (DOPE) is altered by the presence of 10 nm hydrophobic and 14 nm hydrophilic silica nanoparticles (NPs) at different concentrations. The lamellar to inverted hexagonal phase transition (Lα-HII) of phospholipids is energetically analogous to the membrane fusion process, therefore understanding the Lα-HII transition with nanoparticulate additives is relevant to how membrane fusion may be affected by these additives, in this case the silica NPs. The overriding observation is that the HII/Lα boundaries in the DOPE p-T phase diagram were shifted by the presence of NPs: the hydrophobic NPs enlarged the HII phase region and thus encouraged the inverted hexagonal (HII) phase to occur at lower temperatures, whilst hydrophilic NPs appeared to stabilise the Lα phase region. This effect was also NP-concentration dependent, with a more pronounced effect for higher concentration of the hydrophobic NPs, but the trend was less clear cut for the hydrophilic NPs. There was no evidence that the NPs were intercalated into the mesophases, and as such it was likely that they might have undergone microphase separation and resided at the mesophase domain boundaries. Whilst the loci and exact roles of the NPs invite further investigation, we tentatively discuss these results in terms of both the surface chemistry of the NPs and the effect of their curvature on the elastic bending energy considerations during the mesophase transition.

The stability of silver nanoparticles in a model of pulmonary surfactant.

The growing use of silver nanoparticles (AgNPs) in consumer products has raised concerns about their potential impact on the environment and human health. Whether AgNPs dissolve and release Ag(+) ions, or coarsen to form large aggregates, is critical in determining their potential toxicity. In this work, the stability of AgNPs in dipalmitoylphosphatidylcholine (DPPC), the major component of pulmonary surfactant, was investigated as a function of pH. Spherical, citrate-capped AgNPs with average diameters of 14 ± 1.6 nm (n = 200) were prepared by a chemical bath reduction. The kinetics of Ag(+) ion release was strongly pH-dependent. After 14 days of incubation in sodium perchlorate (NaClO4) or perchloric acid (HClO4) solutions, the total fraction of AgNPs dissolved varied from ∼10% at pH 3, to ∼2% at pH 5, with negligible dissolution at pH 7. A decrease in pH from 7 to 3 also promoted particle aggregation and coarsening. DPPC (100 mg·L(-1)) delayed the release of Ag(+) ions, but did not significantly alter the total amount of Ag(+) released after two weeks. In addition, DPPC improved the dispersion of the AgNPs and inhibited aggregation and coarsening. TEM images revealed that the AgNPs were coated with a DPPC layer serving as a semipermeable layer. Hence, lung lining fluid, particularly DPPC, can modify the aggregation state and kinetics of Ag(+) ion release of inhaled AgNPs in the lung. These observations have important implications for predicting the potential reactivity of AgNPs in the lung and the environment.

Continuous-Flow Laboratory SAXS for In Situ Determination of the Impact of Hydrophilic Block Length on Spherical Nano-Object Formation during Polymerization-Induced Self-Assembly.

In situ small-angle X-ray scattering (SAXS) is a powerful technique for characterizing block-copolymer nano-object formation during polymerization-induced self-assembly. To work effectively in situ, it requires high intensity X-rays which enable the short acquisition times required for real-time measurements. However, routine access to synchrotron X-ray sources is expensive and highly competitive. Flow reactors provide an opportunity to obtain temporal resolution by operating at a consistent flow rate. Here, we equip a flow-reactor with an X-ray transparent flow-cell at the outlet which facilitates the use of a low-flux laboratory SAXS instrument for in situ monitoring. The formation and morphological evolution of spherical block copolymer nano-objects was characterized during reversible addition fragmentation chain transfer polymerization of diacetone acrylamide in the presence of a series of poly(dimethylacrylamide) (PDMAm) macromolecular chain transfer agents with varying degrees of polymerization. SAXS analysis indicated that during the polymerization, highly solvated, loosely defined aggregates form after approximately 100 s, followed by expulsion of solvent to form well-defined spherical particles with PDAAm cores and PDMAm stabilizer chains, which then grow as the polymerization proceeds. Analysis also indicates that the aggregation number (Nagg) increases during the reaction, likely due to collisions between swollen, growing nanoparticles. In situ SAXS conducted on PISA syntheses using different PDMAm DPs indicated a varying conformation of the chains in the particle cores, from collapsed chains for PDMAm47 to extended chains for PDMAm143. At high conversion, the final Nagg decreased as a function of increasing PDMAm DP, indicating increased steric stabilization afforded by the longer chains which is reflected by a decrease in both core diameter (from SAXS) and hydrodynamic diameter (from DLS) for a constant core DP of 400.

Correction to "Continuous-Flow Laboratory SAXS for In Situ Determination of the Impact of Hydrophilic Block Length on Spherical Nano-Object Formation during Polymerization-Induced Self-Assembly".

[This corrects the article DOI: 10.1021/acs.macromol.3c00585.].

A Cross-Sectional Study of Bone Nanomechanics in Hip Fracture and Aging.

Bone mechanics is well understood at every length scale except the nano-level. We aimed to investigate the relationship between bone nanoscale and tissue-level mechanics experimentally. We tested two hypotheses: (1) nanoscale strains were lower in hip fracture patients versus controls, and (2) nanoscale mineral and fibril strains were inversely correlated with aging and fracture. A cross-sectional sample of trabecular bone sections was prepared from the proximal femora of two human donor groups (aged 44-94 years): an aging non-fracture control group (n = 17) and a hip-fracture group (n = 20). Tissue, fibril, and mineral strain were measured simultaneously using synchrotron X-ray diffraction during tensile load to failure, then compared between groups using unpaired t-tests and correlated with age using Pearson's correlation. Controls exhibited significantly greater peak tissue, mineral, and fibril strains than the hip fracture (all p < 0.05). Age was associated with a decrease in peak tissue (p = 0.099) and mineral (p = 0.004) strain, but not fibril strain (p = 0.260). Overall, hip fracture and aging were associated with changes in the nanoscale strain that are reflected at the tissue level. Data must be interpreted within the limitations of the observational cross-sectional study design, so we propose two new hypotheses on the importance of nanomechanics. (1) Hip fracture risk is increased by low tissue strain, which can be caused by low collagen or mineral strain. (2) Age-related loss of tissue strain is dependent on the loss of mineral but not fibril strain. Novel insights into bone nano- and tissue-level mechanics could provide a platform for the development of bone health diagnostics and interventions based on failure mechanisms from the nanoscale up.

Collagen pre-strain discontinuity at the bone-Cartilage interface.

The bone-cartilage unit (BCU) is a universal feature in diarthrodial joints, which is mechanically-graded and subjected to shear and compressive strains. Changes in the BCU have been linked to osteoarthritis (OA) progression. Here we report existence of a physiological internal strain gradient (pre-strain) across the BCU at the ultrastructural scale of the extracellular matrix (ECM) constituents, specifically the collagen fibril. We use X-ray scattering that probes changes in the axial periodicity of fibril-level D-stagger of tropocollagen molecules in the matrix fibrils, as a measure of microscopic pre-strain. We find that mineralized collagen nanofibrils in the calcified plate are in tensile pre-strain relative to the underlying trabecular bone. This behaviour contrasts with the previously accepted notion that fibrillar pre-strain (or D-stagger) in collagenous tissues always reduces with mineralization, via reduced hydration and associated swelling pressure. Within the calcified part of the BCU, a finer-scale gradient in pre-strain (0.6% increase over ~50µm) is observed. The increased fibrillar pre-strain is linked to prior research reporting large tissue-level residual strains under compression. The findings may have biomechanical adaptative significance: higher in-built molecular level resilience/damage resistance to physiological compression, and disruption of the molecular-level pre-strains during remodelling of the bone-cartilage interface may be potential factors in osteoarthritis-based degeneration.

Quantification of collagen organization in the peripheral human cornea at micron-scale resolution.

The collagen microstructure of the peripheral cornea is important in stabilizing corneal curvature and refractive status. However, the manner in which the predominantly orthogonal collagen fibrils of the central cornea integrate with the circumferential limbal collagen is unknown. We used microfocus wide-angle x-ray scattering to quantify the relative proportion and orientation of collagen fibrils over the human corneolimbal interface at intervals of 50 µm. Orthogonal fibrils changed direction 1-1.5 mm before the limbus to integrate with the circumferential limbal fibrils. Outside the central 6 mm, additional preferentially aligned collagen was found to reinforce the cornea and limbus. The manner of integration and degree of reinforcement varied significantly depending on the direction along which the limbus was approached. We also employed small-angle x-ray scattering to measure the average collagen fibril diameter from central cornea to limbus at 0.5 mm intervals. Fibril diameter was constant across the central 6 mm. More peripherally, fibril diameter increased, indicative of a merging of corneal and scleral collagen. The point of increase varied with direction, consistent with a scheme in which the oblique corneal periphery is reinforced by chords of scleral collagen. The results have implications for the cornea's biomechanical response to ocular surgeries involving peripheral incision.

Probing the structure of the crystalline core of field-aligned, monodisperse, cylindrical polyisoprene-block-polyferrocenylsilane micelles in solution using synchrotron small- and wide-angle X-ray scattering.

The self-assembly of block copolymers in selective solvents represents a powerful approach to functional core-shell nanoparticles. Crystallization of the core can play a critical role in directing self-assembly toward desirable, nonspherical morphologies with low mean interfacial curvature. Moreover, epitaxial growth processes have been implicated in recent advances that permit access to monodisperse cylinders, cylindrical block comicelles with segmented cores and/or coronas, and complex hierarchical architectures. However, how the core-forming block crystallizes in an inherently curved nanoscopic environment has not been resolved. Herein we report the results of synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) studies of well-defined, monodisperse crystalline-coil polyisoprene-block-polyferrocenylsilane cylindrical micelles aligned in an electric field. WAXS studies of the aligned cylinders have provided key structural information on the nature of the PFS micelle core together with insight into the role of polymer crystallinity in the self-assembly of these and potentially related crystalline-coil block copolymers.

Reversible changes in the 3D collagen fibril architecture during cyclic loading of healthy and degraded cartilage.

Biomechanical changes to the collagen fibrillar architecture in articular cartilage are believed to play a crucial role in enabling normal joint function. However, experimentally there is little quantitative knowledge about the structural response of the Type II collagen fibrils in cartilage to cyclic loading in situ, and the mechanisms that drive the ability of cartilage to withstand long-term repetitive loading. Here we utilize synchrotron small-angle X-ray scattering (SAXS) combined with in-situ cyclic loading of bovine articular cartilage explants to measure the fibrillar response in deep zone articular cartilage, in terms of orientation, fibrillar strain and inter-fibrillar variability in healthy cartilage and cartilage degraded by exposure to the pro-inflammatory cytokine IL-1ß. We demonstrate that under repeated cyclic loading the fibrils reversibly change the width of the fibrillar orientation distribution whilst maintaining a largely consistent average direction of orientation. Specifically, the effect on the fibrillar network is a 3-dimensional conical orientation broadening around the normal to the joint surface, inferred by 3D reconstruction of X-ray scattering peak intensity distributions from the 2D pattern. Further, at the intrafibrillar level, this effect is coupled with reversible reduction in fibrillar pre-strain under compression, alongside increase in the variability of fibrillar pre-strain. In IL-1ß degraded cartilage, the collagen rearrangement under cyclic loading is disrupted and associated with reduced tissue stiffness. These finding have implications as to how changes in local collagen nanomechanics might drive disease progression or vice versa in conditions such as osteoarthritis and provides a pathway to a mechanistic understanding of such diseases. STATEMENT OF SIGNIFICANCE: Structural deterioration in biomechanically loaded musculoskeletal organs, e.g., joint osteoarthritis and back pain, are linked to breakdown and changes in their collagen-rich cartilaginous tissue matrix. A critical component enabling cartilage biomechanics is the ultrastructural collagen fibrillar network in cartilage. However, experimental probes of the dynamic structural response of cartilage collagen to biomechanical loads are limited. Here, we use X-ray scattering during cyclic loading (as during walking) on joint tissue to show that cartilage fibrils resist loading by a reversible, three-dimensional orientation broadening and disordering mechanism at the molecular level, and that inflammation reduces this functionality. Our results will help understand how changes to small-scale tissue mechanisms are linked to ageing and osteoarthritic progression, and development of biomaterials for joint replacements.

Past, present and future-sample environments for materials research studies in scattering and spectroscopy; a UK perspective.

Small angle x-ray scattering and x-ray absorption fine structure are two techniques that have been employed at synchrotron sources ever since their inception. Over the course of the development of the techniques, the introduction of sample environments for added value experiments has grown dramatically. This article reviews past successes, current developments and an exploration of future possibilities for these two x-ray techniques with an emphasis on the developments in the United Kingdom between 1980-2020.

Tomographic X-ray scattering based on invariant reconstruction: analysis of the 3D nanostructure of bovine bone.

Small-angle X-ray scattering (SAXS) is an effective characterization technique for multi-phase nanocomposites. The structural complexity and heterogeneity of biological materials require the development of new techniques for the 3D characterization of their hierarchical structures. Emerging SAXS tomographic methods allow reconstruction of the 3D scattering pattern in each voxel but are costly in terms of synchrotron measurement time and computer time. To address this problem, an approach has been developed based on the reconstruction of SAXS invariants to allow for fast 3D characterization of nanostructured inhomogeneous materials. SAXS invariants are scalars replacing the 3D scattering patterns in each voxel, thus simplifying the 6D reconstruction problem to several 3D ones. Standard procedures for tomographic reconstruction can be directly adapted for this problem. The procedure is demonstrated by determining the distribution of the nanometric bone mineral particle thickness (T parameter) throughout a macroscopic 3D volume of bovine cortical bone. The T parameter maps display spatial patterns of particle thickness in fibrolamellar bone units. Spatial correlation between the mineral nano-structure and microscopic features reveals that the mineral particles are particularly thin in the vicinity of vascular channels.

Molecular to Macroscale Energy Absorption Mechanisms in Biological Body Armour Illuminated by Scanning X-ray Diffraction with In Situ Compression.

Determining multiscale, concurrent strain, and deformation mechanisms in hierarchical biological materials is a crucial engineering goal, to understand structural optimization strategies in Nature. However, experimentally characterizing complex strain and displacement fields within a 3D hierarchical composite, in a multiscale full-field manner, is challenging. Here, we determined the in situ strains at the macro-, meso-, and molecular-levels in stomatopod cuticle simultaneously, by exploiting the anisotropy of the 3D fiber diffraction coupled with sample rotation. The results demonstrate the method, using the mineralized 3D α-chitin fiber networks as strain sensors, can capture submicrometer deformation of a single lamella (mesoscale), can extract strain information on multiple constituents concurrently, and shows that α-chitin fiber networks deform elastically while the surrounding matrix deforms plastically before systematic failure under compression. Further, the results demonstrate a molecular-level prestrain gradient in chitin fibers, resulting from different mineralization degrees in the exo- and endo cuticle.

Triblock polyester thermoplastic elastomers with semi-aromatic polymer end blocks by ring-opening copolymerization.

Thermoplastic elastomers benefit from high elasticity and straightforward (re)processability; they are widely used across a multitude of sectors. Currently, the majority derive from oil, do not degrade or undergo chemical recycling. Here a new series of ABA triblock polyesters are synthesized and show high-performances as degradable thermoplastic elastomers; their composition is poly(cyclohexene-alt-phthalate)-b-poly(ε-decalactone)-b-poly(cyclohexene-alt-phthalate) {PE-PDL-PE}. The synthesis is accomplished using a zinc(ii)/magnesium(ii) catalyst, in a one-pot procedure where ε-decalactone ring-opening polymerization yielding dihydroxyl telechelic poly(ε-decalatone) (PDL, soft-block) occurs first and, then, addition of phthalic anhydride/cyclohexene oxide ring-opening copolymerization delivers semi-aromatic polyester (PE, hard-block) end-blocks. The block compositions are straightforward to control, from the initial monomer stoichiometry, and conversions are high (85-98%). Two series of polyesters are prepared: (1) TBPE-1 to TBPE-5 feature an equivalent hard-block volume fraction (f hard = 0.4) and variable molar masses 40-100 kg mol-1; (2) TBPE-5 to TBPE-9 feature equivalent molar masses (∼100 kg mol-1) and variable hard-block volume fractions (0.12 < f hard < 0.4). Polymers are characterized using spectroscopies, size-exclusion chromatography (SEC), thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA). They are amorphous, with two glass transition temperatures (∼-51 °C for PDL; +138 °C for PE), and block phase separation is confirmed using small angle X-ray scattering (SAXS). Tensile mechanical performances reveal thermoplastic elastomers (f hard < 0.4 and N > 1300) with linear stress-strain relationships, high ultimate tensile strengths (σ b = 1-5 MPa), very high elongations at break (ε b = 1000-1900%) and excellent elastic recoveries (98%). There is a wide operating temperature range (-51 to +138 °C), an operable processing temperature range (+100 to +200 °C) and excellent thermal stability (T d,5% ∼ 300 °C). The polymers are stable in aqueous environments, at room temperature, but are hydrolyzed upon gentle heating (60 °C) and treatment with an organic acid (para-toluene sulfonic acid) or a common lipase (Novozyme® 51032). The new block polyesters show significant potential as sustainable thermoplastic elastomers with better properties than well-known styrenic block copolymers or polylactide-derived elastomers. The straightforward synthesis allows for other commercially available and/or bio-derived lactones, epoxides and anhydrides to be developed in the future.

Potassium iodide reduces the stability of triple-cation perovskite solar cells.

The addition of alkali metal halides to hybrid perovskite materials can significantly impact their crystallisation and hence their performance when used in solar cell devices. Previous work on the use of potassium iodide (KI) in active layers to passivate defects in triple-cation mixed-halide perovskites has been shown to enhance their luminescence efficiency and reduce current-voltage hysteresis. However, the operational stability of KI passivated perovskite solar cells under ambient conditions remains largely unexplored. By investigating perovskite solar cell performance with SnO2 or TiO2 electron transport layers (ETL), we propose that defect passivation using KI is highly sensitive to the composition of the perovskite-ETL interface. We reconfirm findings from previous reports that KI preferentially interacts with bromide ions in mixed-halide perovskites, and - at concentrations >5 mol% in the precursor solution - modifies the primary absorber composition as well as leading to the phase segregation of an undesirable secondary non-perovskite phase (KBr) at high KI concentration. Importantly, by studying both material and device stability under continuous illumination and bias under ambient/high-humidity conditions, we show that this secondary phase becomes a favourable degradation product, and that devices incorporating KI have reduced stability.

Proteoglycan degradation mimics static compression by altering the natural gradients in fibrillar organisation in cartilage.

Structural and associated biomechanical gradients within biological tissues are important for tissue functionality and preventing damaging interfacial stress concentrations. Articular cartilage possesses an inhomogeneous structure throughout its thickness, driving the associated variation in the biomechanical strain profile within the tissue under physiological compressive loading. However, little is known experimentally about the nanostructural mechanical role of the collagen fibrils and how this varies with depth. Utilising a high-brilliance synchrotron X-ray source, we have measured the depth-wise nanostructural parameters of the collagen network in terms of the periodic fibrillar banding (D-period) and associated parameters. We show that there is a depth dependent variation in D-period reflecting the pre-strain and concurrent with changes in the level of intrafibrillar order. Further, prolonged static compression leads to fibrillar changes mirroring those caused by removal of extrafibrillar proteoglycans (as may occur in aging or disease). We suggest that fibrillar D-period is a sensitive indicator of localised changes to the mechanical environment at the nanoscale in soft connective tissues. STATEMENT OF SIGNIFICANCE: Collagen plays a significant role in both the structural and mechanical integrity of articular cartilage, allowing the tissue to withstand highly repetitive loading. However, the fibrillar mechanics of the collagen network in cartilage are not clear. Here we find that cartilage has a spatial gradient in the nanostructural collagen fibril pre-strain, with an increase in the fibrillar pre-strain with depth. Further, the fibrillar gradient changes similarly under compression when compared to an enzymatically degraded tissue which mimics age-related changes. Given that the fibrils potentially have a finite capacity to mechanically respond and alter their configuration, these findings are significant in understanding how collagen may alter in structure and gradient in diseased cartilage, and in informing the design of cartilage replacements.