Reactive oxygen-scavenging polydopamine nanoparticle coated 3D nanofibrous scaffolds for improved osteogenesis: Toward an aging in vivo bone regeneration model

Tissue engineered scaffolds aimed at the repair of critical-sized bone defects lack adequate consideration for our aging society. Establishing an effective aged in vitro model that translates to animals is a significant unmet challenge. The in vivo aged environment is complex and highly nuanced, making it difficult to model in the context of bone repair. In this work, 3D nanofibrous scaffolds generated by the thermally-induced self-agglomeration (TISA) technique were functionalized with polydopamine nanoparticles (PD NPs) as a tool to improve drug binding capacity and scavenge reactive oxygen species (ROS), an excessive build-up that dampens the healing process in aged tissues. PD NPs were reduced by ascorbic acid (rPD) to further improve hydrogen peroxide (H2O2) scavenging capabilities, where we hypothesized that these functionalized scaffolds could rescue ROS-affected osteoblastic differentiation in vitro and improve new bone formation in an aged mouse model. rPDs demonstrated improved H2O2 scavenging activity compared to neat PD NPs, although both NP groups rescued the alkaline phosphatase activity (ALP) of MC3T3-E1 cells in presence of H2O2. Additionally, BMP2-induced osteogenic differentiation, both ALP and mineralization, was significantly improved in the presence of PD or rPD NPs on TISA scaffolds. While in vitro data showed favorable results aimed at improving osteogenic differentiation by PD or rPD NPs, in vivo studies did not note similar improvements in ectopic bone formation an aged model, suggesting that further nuance in material design is required to effectively translate to improved in vivo results in aged animal models.

Nanomechanical variability in the early evolution of vertebrate dentition

Conodonts are an extinct group of primitive jawless vertebrates whose elements represent the earliest examples of a mineralized feeding apparatus in vertebrates. Their relative relationship within vertebrates remains unresolved. As teeth, conodont elements are not homologous with the dentition of vertebrates, but they exhibit similarities in mineralization, growth patterns, and function. They clearly represent an early evolutionary experiment in mineralized dentition and offer insight into analogous dentition in other groups. Unfortunately, analysis of functional performance has been limited to a handful of derived morphologies and material properties that may inform ecology and functional analysis are virtually unknown. Here we applied a nanoscale approach to evaluate material properties of conodont bioapatite by utilizing Atomic Force Microscopy (AFM) nanoindentation to determine Young's modulus (E) along multiple elements representing different ontogenetic stages of development in the coniform-bearing apparatus of Dapsilodus obliquicostatus. We observed extreme and systematic variation in E along the length (oral to aboral) of each element that largely mirrors the spatial and ontogenetic variability in the crystalline structure of these specimens. Extreme spatial variability of E likely contributed to breakage of elements that were regularly repaired/regrown in conodonts but later vertebrate dentition strategies that lacked the ability to repair/regrow likely required the development of different material properties to avoid structural failure.

Mechanical rigidity of a shape-memory metal-organic framework increases by crystal downsizing

Soft porous nanocrystals with a pronounced shape-memory effect exhibit two- to three-fold increase in elastic modulus compared to the microcrystalline counterpart as determined by atomic force microscopy nanoindentation. The increase in rigidity is consistent with the known shape-memory effect displayed by the framework solid at the nanoscale. Crystal downsizing can offer new avenues for tailoring the mechanical properties of metal-organic frameworks.

Mechanical Properties and Photomechanical Fatigue of Macro- and Nanodimensional Diarylethene Molecular Crystals

The diarylethene derivative, 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)perfluorocyclopentene, undergoes a reversible photoisomerization between its ring-open and ring-closed forms in the solid-state and has applications as a photomechanical material. Mechanical properties of macrocrystals, nanowire single crystals, and amorphous films as a function of multiple sequential UV and visible light exposures have been quantified using atomic force microscopy nanoindentation. The isomerization reaction has no effect on the elastic modulus of each solid. But going from the macro- to the nanowire crystal results in a remarkable over 3-fold decrease in the elastic modulus. The macrocrystal and amorphous solids are highly resistant to photomechanical fatigue, while nanowire crystals show clear evidence of photomechanical fatigue attributed to a transition from crystal to amorphous forms. This study provides first experimental evidence of size-dependent photomechanical fatigue in photoreactive molecular crystalline solids and suggests crystal morphology and size must be considered for future photomechanical applications.

Size-Dependent Mechanical Properties of a Metal-Organic Framework: Increase in Flexibility of ZIF-8 by Crystal Downsizing

Size engineering is an emerging strategy to modulate the mechanical properties of crystalline materials. Herein, micro- and nanodimensional single crystals of the prototypical metal-organic framework (MOF) ZIF-8 are generated using solvothermal and solution methods, respectively. Atomic force microscopy-based nanoindentation technique was used to measure the Young's modulus values of micro- and nanodimensional individual ZIF-8 crystals. We demonstrate that crystal downsizing to nanoscale dimensions results in a 40% reduction in crystal stiffness. The change is attributed to a greater contribution of surface effects to the physical properties of nanocrystalline ZIF-8. The observed change in the mechanical properties may be used to explain reported size-dependent changes in gas adsorption of ZIF-8, thought to be a result of differences in framework flexibility at the nanoscale. Our work provides an important example on how downsizing of crystalline metal-organic materials can give rise to specific and tunable physical properties.