Thermal Conductivity of Covalent-Organic Frameworks.

Covalent-organic frameworks (COFs) are a highly promising class of materials that can provide an excellent platform for thermal management applications. In this Perspective, we first review previous works on the thermal conductivities of COFs. Then we share our insights on achieving high, low, and switchable thermal conductivities of future COFs. To obtain the desired thermal conductivity, a comprehensive understanding of their thermal transport mechanisms is necessary but lacking. We discuss current limitations in atomistic simulations, synthesis, and thermal conductivity measurements of COFs and share potential pathways to overcoming these challenges. We hope to stimulate collective, interdisciplinary efforts to study the thermal conductivity of COFs and enable their wide range of thermal applications.

Collagen piezoelectricity in osteogenesis imperfecta and its role in intrafibrillar mineralization.

Intrafibrillar mineralization plays a critical role in attaining desired mechanical properties of bone. It is well known that amorphous calcium phosphate (ACP) infiltrates into the collagen through the gap regions, but its underlying driving force is not understood. Based on the authors' previous observations that a collagen fibril has higher piezoelectricity at gap regions, it was hypothesized that the piezoelectric heterogeneity of collagen helps ACP infiltration through the gap. To further examine this hypothesis, the collagen piezoelectricity of osteogenesis imperfecta (OI), known as brittle bone disease, is characterized by employing Piezoresponse Force Microscopy (PFM). The OI collagen reveals similar piezoelectricity between gap and overlap regions, implying that losing piezoelectric heterogeneity in OI collagen results in abnormal intrafibrillar mineralization and, accordingly, losing the benefit of mechanical heterogeneity from the fibrillar level. This finding suggests a perspective to explain the ACP infiltration, highlighting the physiological role of collagen piezoelectricity in intrafibrillar mineralization.

Finite element analysis of the impact of bone nanostructure on its piezoelectric response.

The piezoelectric response of bone at the submicron scale is analyzed under mechanical loadings using the finite element (FE) method. A new algorithm is presented to virtually reconstruct realistic bone nanostructures, consisting of collagen fibrils embedded in a hydroxyapatite mineral network. This algorithm takes into account potential misalignments between fibrils, as well the porous structure of the mineral phase. A parallel non-iterative mesh generation algorithm is utilized to create high-fidelity FE models for several representative volume elements (RVEs) of the bone with various fibrils volume fractions and misalignments. The piezoelectric response of each RVE is simulated under three types of loading: the longitudinal compression, lateral compression, and shear. The resulting homogenized stress and electric field in RVEs with aligned fibrils showed a linear variation with the fibrils volume fraction under all loading conditions. For RVEs with misaligned fibrils, although more oscillations were observed in homogenized results, their difference with the results of RVEs with aligned fibrils subject to lateral compression and shear loadings were negligible. However, under longitudinal compression, the electric field associated with RVEs with misaligned fibrils was notably higher than that of RVEs with aligned fibrils for the same volume fraction.

Piezoelectric Heterogeneity in Collagen Type I Fibrils Quantitatively Characterized by Piezoresponse Force Microscopy.

Piezoelectricity of Type I collagen can provide the stress-generated potential that is considered to be one of the candidate mechanisms to explain bone's adaptation to loading. However, it is still challenging to quantify piezoelectricity because of its heterogeneity and small magnitude. In this study, resonance-enhanced piezoresponse force microscopy (PFM) was utilized to amplify a weak piezoresponse of a single collagen fibril with a carefully calibrated cantilever. The quantitative PFM, combined with a dual-frequency resonance-tracking method, successfully identified the anisotropic and heterogenous nature of the piezoelectric properties in the collagen fibril. The profile of shear piezoelectric coefficient (d15) was obtained to be periodic along the collagen fibril, with a larger value in the gap zone (0.51 pm/V) compared to the value in the overlap zone (0.29 pm/V). Interestingly, this piezoelectric profile corresponds to the periodic profile of mechanical stiffness in a mineralized collagen fibril having a higher stiffness in the gap zone. Considering that apatite crystals are nucleated at the gap zone and subsequently grown along the collagen fibril, the heterogeneous and anisotropic nature of piezoelectric properties highlights the physiological importance of the collagen piezoelectricity in bone mineralization.

Characterization of Cyanophages in Lake Erie: Interaction Mechanisms and Structural Damage of Toxic Cyanobacteria.

Cyanophages are abundant in aquatic environments and play a critical role in bloom dynamics, including regulation of cyanobacteria growth and photosynthesis. In this study, cyanophages from western Lake Erie water samples were screened for lytic activities against the host cell (Microcystis aeruginosa), which was also originated from Lake Erie and identified with real-time sequencing (Nanopore sequencing). M. aeruginosa was mixed with the cyanophages and their dynamic interactions were examined over two weeks using atomic force microscopy (AFM) as well as transmission electron microscopy (TEM), qPCR, phycocyanin and chlorophyll-a production, and optical absorbance measurements. The TEM images revealed a short-tailed virus (Podoviridae) in 300 nm size with unique capsid, knob-like proteins. The psbA gene and one knob-like protein gene, gp58, were identified by PCR. The AFM showed a reduction of mechanical stiffness in the host cell membranes over time after infection, before structural damage became visible. Significant inhibition of the host growth and photosynthesis was observed from the measurements of phycocyanin and chlorophyll-a concentrations. The results provide an insight into cyanobacteria-cyanophage interactions in bloom dynamics and a potential application of cyanophages for bloom control in specific situations.