Single crystals are typically brittle, inelastic materials. Such mechanical responses limit their use in practical applications, particularly in flexible electronics and optical devices. Here we describe single crystals of a well-known coordination compound-copper(II) acetylacetonate-that are flexible enough to be reversibly tied into a knot. Mechanical measurements indicate that the crystals exhibit an elasticity similar to that of soft materials such as nylon, and thus display properties normally associated with both hard and soft matter. Using microfocused synchrotron radiation, we mapped the changes in crystal structure that occur on bending, and determined the mechanism that allows this flexibility with atomic precision. We show that, under strain, the molecules in the crystal reversibly rotate, and thus reorganize to allow the mechanical compression and expansion required for elasticity and still maintain the integrity of the crystal structure.

Emerging micro-scale medical devices are showing promise, whether in delivering drugs or extracting diagnostic biomarkers from skin. In progressing these devices through animal models towards clinical products, understanding the mechanical properties and skin tissue structure with which they interact will be important. Here, through measurement and analytical modelling, we advanced knowledge of these properties for commonly used laboratory animals and humans (~30 g to ~150 kg). We hypothesised that skin’s stiffness is a function of the thickness of its layers through allometric scaling, which could be estimated from knowing a species’ body mass. Results suggest that skin layer thicknesses are proportional to body mass with similar composition ratios, inter- and intra-species. Experimental trends showed elastic moduli increased with body mass, except for human skin. To interpret the relationship between species, we developed a simple analytical model for the bulk elastic moduli of skin, which correlated well with experimental data. Our model suggest that layer thicknesses may be a key driver of structural stiffness, as the skin layer constituents are physically and therefore mechanically similar between species. Our findings help advance the knowledge of mammalian skin mechanical properties, providing a route towards streamlined micro-device research and development onto clinical use.

Polyurethanes are excellent potential materials for the construction of implantable medical components due to their exceptional mechanical properties and biocompatibility. Currently, soft silicone materials are employed as insulation for implantable cochlear electrode arrays. Siloxane-based thermoplastic polyurethane (TPU) nanocomposites containing synthetic layered silicates are being investigated as new insulation materials with superior tensile and tear strength and reduced surface tack, potentially allowing for thinner insulation and more intricate electrode designs. In this work, ElastEon E5-325 (Aortech Pty Ltd.) TPU nanocomposites reinforced with 2 and 4 wt % low aspect ratio organo-hectorite and high aspect ratio organo-fluoromica (Lucentite SWN, Somasif ME100, both modified with octadecyltrimethylammonium (ODTMA)) were prepared by a solvent casting technique. The mechanical properties of the resulting nanocomposites were measured by tensile, tear, stress relaxation, and creep testing and morphologically were characterized by DSC, DMTA, XRD, TEM, and strained in situ synchrotron SAXS. We found that the hydrophobic low aspect ratio organohectorite acts as a very potent interfacial compatibilizer. At 2 wt % loading, the resulting nanocomposite displays vastly superior mechanical properties to both soft silicone and ElastEon. In addition to providing 30 nm × 1 nm synthetic nanosilicate reinforcing elements which are readily capable of orientation and reinforcement, these nanosilicates also serve to provide more cohesive hard microdomains and thus creep resistance and dimensional stability. Interestingly, at a higher (4 wt %) loading of organohectorite, gross morphological changes in the TPU microdomain texture are observed, adversely effecting the mechanical properties of the TPU.

A silk protein, fibroin, was isolated from the cocoons of the domesticated silkworm (Bombyx mori) and cast into membranes to serve as freestanding templates for tissue-engineered corneal cell constructs to be used in ocular surface reconstruction. In this study, we sought to enhance the attachment and proliferation of corneal epithelial cells by increasing the permeability of the fibroin membranes and the topographic roughness of their surface. By mixing the fibroin solution with poly(ethylene glycol) (PEG) of molecular weight 300 Da, membranes were produced with increased permeability and with topographic patterns generated on their surface. In order to enhance their mechanical stability, some PEG-treated membranes were also crosslinked with genipin. The resulting membranes were thoroughly characterized and compared to the non-treated membranes. The PEG-treated membranes were similar in tensile strength to the non-treated ones, but their elastic modulus was higher and elongation lower, indicating enhanced rigidity. The crosslinking with genipin did not induce a significant improvement in mechanical properties. In cultures of a human-derived corneal epithelial cell line (HCE-T), the PEG treatment of the substratum did not improve the attachment of cells and it enhanced only slightly the cell proliferation in the longer term. Likewise, primary cultures of human limbal epithelial cells grew equally well on both non-treated and PEG-treated membranes, and the stratification of cultures was consistently improved in the presence of an underlying culture of irradiated 3T3 feeder cells, irrespectively of PEG-treatment. Nevertheless, the cultures grown on the PEG-treated membranes in the presence of feeder cells did display a higher nuclear-to-cytoplasmic ratio suggesting a more proliferative phenotype. We concluded that while the treatment with PEG had a significant effect on some structural properties of the B. mori silk fibroin (BMSF) membranes, there were minimal gains in the performance of these materials as a substratum for corneal epithelial cell growth. The reduced mechanical stability of freestanding PEG-treated membranes makes them a less viable choice than the non-treated membranes.

The production of strong and elastic polyurethane nanocomposites toughened with nanocellulose and their widespread application in many engineering fields are currently limited by poor processability via classical industrial processing methods and/or the usage of large amount of solvents. In this report, we demonstrate a scalable, organic solvent-free incorporation of nanocellulose into thermoplastic polyurethane (TPU) and a remarkable reinforcement without compromising elastic properties. The nanocomposites were prepared via water-assisted dispersion of nanocellulose in polyether polyol by bead milling, drying and reactive extrusion of this dispersion with 2 comonomers. Upon the incorporation of nanocellulose (0.5 wt. %), as observed from infrared spectroscopic and thermal analysis, the phase mixing of hard and soft-segments in the TPU matrix and the primary relaxation temperature have slightly increased due to the hydrogen bonding, interfacial area and nucleation enhanced by long polar nanocrystals. The TPU/nanocellulose nanocomposites prepared with an appropriate stoichiometric ratio (determined through appropriate process control) showed a remarkable improvement (up to 43 %) in ultimate tensile strength without compromising the elastic properties including elongation, creep and hysteresis.