Biomechanical properties are an excellent health marker of biological tissues, however they are challenging to be measured in-vivo. Non-invasive approaches to assess tissue biomechanics have been suggested, but there is a clear need for more accurate techniques for diagnosis, surgical guidance and treatment evaluation. Recently air-puff systems have been developed to study the dynamic tissue response, nevertheless the experimental geometrical observations lack from an analysis that addresses specifically the inherent dynamic properties. In this study a viscoelastic finite element model was built that predicts the experimental corneal deformation response to an air-puff for different conditions. A sensitivity analysis reveals significant contributions to corneal deformation of intraocular pressure and corneal thickness, besides corneal biomechanical properties. The results show the capability of dynamic imaging to reveal inherent biomechanical properties in vivo. Estimates of corneal biomechanical parameters will contribute to the basic understanding of corneal structure, shape and integrity and increase the predictability of corneal surgery.

/pdf/corneal-viscoelastic-properties-from-finite-element-analysis-4q1csflqe2.pdf

Corneal viscoelastic properties from finite-element analysis of in vivo air-puff deformation.

During development, epithelial tissues undergo extensive morphogenesis based on coordinated changes of cell shape and position over time. Continuum mechanics describes tissue mechanical state and shape changes in terms of strain and stress. It accounts for individual cell properties using only a few spatially averaged material parameters. To determine the mechanical state and parameters in the Drosophila pupa dorsal thorax epithelium, we severed in vivo the adherens junctions around a disc-shaped domain comprising typically a hundred cells. This enabled a direct measurement of the strain along different orientations at once. The amplitude and the anisotropy of the strain increased during development. We also measured the stress-to-viscosity ratio and similarly found an increase in amplitude and anisotropy. The relaxation time was of the order of 10 s. We propose a space–time, continuous model of the relaxation. Good agreement with experimental data validates the description of the epithelial domain as a continuous, linear, visco-elastic material. We discuss the relevant time and length scales. Another material parameter, the ratio of external friction to internal viscosity, is estimated by fitting the initial velocity profile. Together, our results contribute to quantify forces and displacements, and their time evolution, during morphogenesis.

https://royalsocietypublishing.org/doi/pdf/10.1098/rsif.2012.0263

Mechanical state, material properties and continuous description of an epithelial tissue

Improving the fracture resistance of oxide glasses through adjustment of the chemical composition remains a challenging task, although composition-mechanical property relations have been established for simple model systems. The glass mechanical properties are, among other methods, conventionally tested using instrumented indentation, which is a fast and convenient technique that mimics the real-life damage for certain applications, although interpretation can be challenging due to the complex stress fields that develop under the indenter. Early indentation experiments have shown that oxide glasses exhibit pronounced tendency to densify under compressive load compared to metals and ceramics. After decades of investigations, it is now known that the extent of densification is strongly dependent on the glass' chemical composition and in turn its atomic packing density and Poisson's ratio. Spectroscopic techniques have shed light on the mechanism of densification, which include changes in the bond angle distributions as well as an increase in the coordination number of the network-forming cations. Knowledge of such details is crucial for understanding the link between chemical composition and resistance to cracking in oxide glasses, since densification is an efficient way to dissipate the elastic energy applied to the material during indentation. Here, we review the experimental work on identification and quantification of indentation deformation in glasses, as well as on probing the accompanying structural changes in the glassy network. We also include the conclusions drawn from computer simulation studies, which can provide atomistic details of the indentation deformation mechanisms. Finally, we discuss the link between the mechanism of deformation and the crack resistance.

https://vbn.aau.dk/files/294535277/Indentation_deformation_in_oxide_glasses_Quantification_structural_changes_and_relation_to_cracking.pdf

Indentation deformation in oxide glasses: Quantification, structural changes, and relation to cracking

https://ro.ecu.edu.au/cgi/viewcontent.cgi?article=6986&context=ecuworkspost2013

Deformation and strength characteristics of Laves phases in titanium alloys

In the course of animal development, the shape of tissue emerges in part from mechanical and biochemical interactions between cells. Measuring stress in tissue is essential for studying morphogenesis and its physical constraints. For that purpose, a possible new approach is force inference (up to a single prefactor) from cell shapes and connectivity. It is non-invasive and can provide space-time maps of stress in a whole tissue, unlike existing methods. To validate this approach, three force-inference methods, which differ in their approach of treating indefiniteness in an inverse problem between cell shapes and forces, were compared. Tests using two artificial and two experimental data sets consistently indicate that our Bayesian force inference, by which cell-junction tensions and cell pressures are simultaneously estimated, performs best in terms of accuracy and robustness. Moreover, by measuring the stress anisotropy and relaxation, we cross-validated the force inference and the global annular ablation of tissue, each of which relies on different prefactors. A practical choice of force-inference methods in different systems of interest is discussed.

/pdf/comparative-study-of-non-invasive-force-and-stress-inference-5446ym34yd.pdf

Comparative study of non-invasive force and stress inference methods in tissue.

Fracture toughness, fracture energy and slow crack growth of glass as investigated by the Single-Edge Precracked Beam (SEPB) and Chevron-Notched Beam (CNB) methods

It is getting increasingly evident that physical properties such as elastoviscoplastic properties of living materials are quite important for the process of tissue development, including regulation of genetic pathways. Measuring such properties in vivo is a complicated and challenging task. In this paper, we present an instrument, a scanning air puff tonometer, which is able to map point by point the viscoelastic properties of flat or gently curved soft materials. This instrument is an improved version of the air puff tonometer used by optometrists, with important modifications. The instrument allows one to obtain a direct insight into gradients of material properties in vivo. The instrument capabilities are demonstrated on substances with known elastoviscoplastic properties and several biological objects. On the basis of the results obtained, the role of the gradients of elastoviscoplastic properties is outlined for the process of angiogenesis, limb development, bacterial colonies expansion, etc. which is important for bridging the gaps in the theory of the tissue development and highlighting new possibilities for tissue engineering, based on a clarification of the role of physical features in developing biological material.

/pdf/introducing-the-scanning-air-puff-tonometer-for-biological-xsjmijdrxb.pdf

Introducing the scanning air puff tonometer for biological studies.

The role of temperature (T) and moisture on fracture toughness (KIc) of glasses, as measured by the single-edge precracked beam method (SEPB), is investigated. The brittle to ductile temperature, shown by an increase of KIc, is ~Tg. At T below 0.9Tg, the decrease of the bond strength with T results in the decrease of both the fracture surface energy and Young's modulus, so that KIc decreases with T. From above Tg to below 1.1Tg, the work of fracture increases because of dissipative processes, including confined viscoplasticity and blunting at the crack front. From 1.1Tg, viscous flow extends to the whole specimen and becomes more uniform, resulting in the macroscopic deformation of the specimen, eventually preventing against fracture. The results on humidity sensitive glasses show that both precracking and final fracture steps need to be performed in inert environment to meet the intrinsic KIc value, unless the test is fast enough.

Environment dependence of KIc of glass

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Fracture toughness, fracture energy and slow crack growth of glass as investigated by the Single-Edge Precracked Beam (SEPB) and Chevron-Notched Beam (CNB) methods

Introducing the scanning air puff tonometer for biological studies.

Environment dependence of KIc of glass