Fractography of poly(: N -isopropylacrylamide) hydrogel networks crosslinked with mechanofluorophores using confocal laser scanning microscopy

Due to their soft and brittle nature, the mechanical characterization of polymer hydrogels is a difficult task employing traditional testing equipment. Here, we endowed poly(N-isopropyl acrylamide) (PNIPAAm) hydrogel networks with Diels-Alder adducts of π-extended anthracenes as mechanofluorophore crosslinkers. After swelling the networks with varying amounts of water and subjecting them to force, we visualized the subsequent fluorescence caused by covalent bond scission with confocal laser scanning microscopy (CLSM) and related the intensities to the macroscopic fracture mechanics and the elastic moduli recorded with traditional uniaxial compression. The sensitivity of the mechanofluorophores allowed the analysis of low levels of mechanical stress produced via a hand-induced needle-puncturing process and, thus, is an alternative to conventional force application methods. The detection and precise localization of covalent bond scission via CLSM helps elucidating the interrelationship between molecular structure and the macroscopic properties of chemically crosslinked polymeric hydrogels. We believe that this micro-scale mechanophore-assisted fractography can establish a new paradigm for the mechanical analysis of soft matter in fields covering traditional polymer and life sciences. © 2019 The Royal Society of Chemistry

Investigating the Morphology and Mechanics of Biogenic Hierarchical Materials at and below Micrometer Scale

Investigating and understanding the intrinsic material properties of biogenic materials, which have evolved over millions of years into admirable structures with difficult to mimic hierarchical levels, holds the potential of replacing trial-and-error-based materials optimization in our efforts to make synthetic materials of similarly advanced complexity and properties. An excellent example is biogenic silica which is found in the exoskeleton of unicellular photosynthetic algae termed diatoms. Because of the complex micro-and nanostructures found in their exoskeleton, deter-mining the intrinsic mechanical properties of biosilica in diatoms has only partly been accomplished. Here, a general method is presented in which a combination of in situ deformation tests inside an SEM with a realistic 3D model of the frustule of diatom Craspedostauros sp. (C. sp.) obtained by electron tomography, alongside finite element method (FEM) simulations, enables quantification of the Young’s modulus (E = 2.3 ± 0.1 GPa) of this biogenic hierarchical silica. The workflow presented can be readily extended to other diatom species, biominerals, or even synthetic hierarchical materials.</p

Predicting embrittlement of polymer glasses using a hydrostatic stress criterion

In this study, the aging-induced embrittlement of three polymer glasses is investigated using a previously developed hybrid experimental–numerical method. The evolution of yield stress of unnotched tensile bars upon aging is coupled to the evolution of embrittlement of notched tensile bars using a numerical model combined with a critical hydrostatic stress criterion that determines the onset of failure. The time-to-embrittlement of notched tensile bars with a different notch geometry is predicted and in good agreement with the experimentally determined value. Next to that, the approach is extended to three polysulfone polymers, and it is shown that the value of the critical hydrostatic stress correlates well with the polymers entanglement density: : polymers with a denser entangled network display higher values, that is, a higher resistance against incipient cavitation

A novel technique for the assessment of mechanical properties of vascular tissue

Accurate estimation of mechanical properties of the different atherosclerotic plaque constituents is important in assessing plaque rupture risk. The aim of this study was to develop an experimental set-up to assess material properties of vascular tissue, while applying physiological loading and being able to capture heterogeneity. To do so, a ring-inflation experimental set-up was developed in which a transverse slice of an artery was loaded in the radial direction, while the displacement was estimated from images recorded by a high-speed video camera. The performance of the set-up was evaluated using seven rubber samples and validated with uniaxial tensile tests. For four healthy porcine carotid arteries, material properties were estimated using ultrasound strain imaging in whole-vessel-inflation experiments and compared to the properties estimated with the ring-inflation experiment. A 1D axisymmetric finite element model was used to estimate the material parameters from the measured pressures and diameters, using a neo-Hookean and Holzapfel–Gasser–Ogden material model for the rubber and porcine samples, respectively. Reproducible results were obtained with the ring-inflation experiment for both rubber and porcine samples. Similar mean stiffness values were found in the ring-inflation and tensile tests for the rubber samples as 202 kPa and 206 kPa, respectively. Comparable results were obtained in vessel-inflation experiments using ultrasound and the proposed ring-inflation experiment. This inflation set-up is suitable for the assessment of material properties of healthy vascular tissue in vitro. It could also be used as part of a method for the assessment of heterogeneous material properties, such as in atherosclerotic plaques

Mechanical properties of the epidermis and stratum corneum determined by submicron indentation in vitro

The outer skin layers are important drug and vaccine delivery targets in the treatment of diseases. These skin layers possess some important characteristics making them favorable sites for pain-free delivery with minimal damage: a rich population of immunologically sensitive cells as well as the lack of blood vessels and sensory nerve endings [1]. Until today, however, the development of effective cell targeting methods is acquainted with many challenges. A collective shortcoming is a poor understanding of the key mechanical properties of the outer skin layers, e.g. the stratum corneum and epidermis. The anisotropic, dynamic and very complex nature of skin makes it difficult to perform proper mechanical testing as well as to obtain reliable, reproducible data. The stratum corneum is an effective physical barrier of dead cells with a “brick-and-mortar” structure, while the viable epidermis mainly consists of actively migrating keratinocytes constantly undergoing massive morphological and compositional changes. As a consequence, the structure differences among the skin layers lead to significant variations in mechanical properties. Since there is no method available to determine the mechanical behavior of isolated viable epidermis in vivo or in vitro, the mechanical behavior of epidermis and stratum corneum only are investigated here. A commercially available indentation system has been adapted to enable the measurement of these thin soft tissues in an in vitro set up. Combining the outcomes of the two skin layer types leads to an assessment of the contribution of the viable epidermis to the mechanical behavior of skin. To our knowledge, no data have been published yet regarding mechanical bulk properties of (viable) epidermis, while no consistency exists with respect to those of the stratum corneum

Investigating the Morphology and Mechanics of Biogenic Hierarchical Materials at and below Micrometer Scale

Investigating and understanding the intrinsic material properties of biogenic materials, which have evolved over millions of years into admirable structures with difficult to mimic hierarchical levels, holds the potential of replacing trial-and-error-based materials optimization in our efforts to make synthetic materials of similarly advanced complexity and properties. An excellent example is biogenic silica which is found in the exoskeleton of unicellular photosynthetic algae termed diatoms. Because of the complex micro-and nanostructures found in their exoskeleton, deter-mining the intrinsic mechanical properties of biosilica in diatoms has only partly been accomplished. Here, a general method is presented in which a combination of in situ deformation tests inside an SEM with a realistic 3D model of the frustule of diatom Craspedostauros sp. (C. sp.) obtained by electron tomography, alongside finite element method (FEM) simulations, enables quantification of the Young’s modulus (E = 2.3 ± 0.1 GPa) of this biogenic hierarchical silica. The workflow presented can be readily extended to other diatom species, biominerals, or even synthetic hierarchical materials

Shaping and properties of thermoplastic scaffolds in tissue regeneration: The effect of thermal history on polymer crystallization, surface characteristics and cell fate

Abstract: Thermoplastic semi-crystalline polymers are excellent candidates for tissue engineering scaffolds thanks to facile processing and tunable properties, employed in melt-based additive manufacturing. Control of crystallization and ultimate crystallinity during processing affect properties like surface stiffness and roughness. These in turn influence cell attachment, proliferation and differentiation. Surface stiffness and roughness are intertwined via crystallinity, but never studied independently. The targeted stiffness range is besides difficult to realize for a single thermoplastic. Via correlation of thermal history, crystallization and ultimate crystallinity of vitamin E plasticized poly(lactide), surface stiffness and roughness are decoupled, disclosing a range of surface mechanics of biological interest. In osteogenic environment, human mesenchymal stromal cells were more responsive to surface roughness than to surface stiffness. Cells were particularly influenced by overall crystal size distribution, not by average roughness. Absence of mold-imposed boundary constrains makes additive manufacturing ideal to spatially control crystallization and henceforward surface roughness of semi-crystalline thermoplastics. Graphic abstract: [Figure not available: see fulltext.