On a phase-field approach to model fracture of small intestine walls

A comparative study of constitutive relations and variational formulations for modeling gastrointestinal peristalsis

Gastrointestinal (GI) peristalsis is a vital process for food transport and digestion. Many methods have been formulated to model this process computationally in recent years. One such approach is the finite element (FE) method, which is efficient and robust to model peristalsis in a single framework. However, to construct a FE model, a suitable constitutive relation is required to represent the intrinsic stress-strain behavior of the tissue. Furthermore, as the GI tissues experience large deformation, an efficient variational formulation is needed to model finite deformation without numerical instabilities and volume locking. Therefore, the objective of this work was to examine the nearly incompressible and purely incompressible versions of different constitutive models and determine the most suited constitutive model for GI tissue characterization. Furthermore, we investigated various variational principles to decide on an appropriate FE approach for modeling GI peristalsis. In our study, the incompressible Humphrey's material model was efficient in recreating experimental observations, whereas the two-field formulation for an incompressible material was an adequate variational formulation for modeling large deformation. Our model was able to recreate the experimental stress-strain data accurately with R2>0.99.

Material properties of the embryonic small intestine during buckling morphogenesis

During embryonic development, tissues undergo dramatic deformations as functional morphologies are stereotypically sculpted from simple rudiments. Formation of healthy, functional organs therefore requires tight control over the material properties of embryonic tissues during development, yet the biological basis of embryonic tissue mechanics is poorly understood. The present study investigates the mechanics of the embryonic small intestine, a tissue that is compactly organized in the body cavity by a mechanical instability during development, wherein differential elongation rates between the intestinal tube and its attached mesentery create compressive forces that buckle the tube into loops. The wavelength and curvature of these loops are tightly conserved for a given species. Focusing on the intestinal tube, we combined micromechanical testing with histologic analyses and enzymatic degradation experiments to conclude that elastic fibers closely associated with intestinal smooth muscle layers are responsible for the bending stiffness of the tube, and for establishing its pronounced mechanical anisotropy. These findings provide insights into the developmental role of elastic fibers in controlling tissue stiffness, and raise new questions on the physiologic function of elastic fibers in the intestine during adulthood. STATEMENT OF SIGNIFICANCE: The functional form of adult organs is established during embryogenesis through the action of physical forces on tissues with precise material properties. Despite this, however, biological control of material properties during embryogenesis is poorly understood. Focusing on the small intestine, we identified elastic fibers - rather than oriented smooth muscle - as defining bending stiffness, prescribing the lengthy intestine to be buckled precisely into compact loops for proper placement within the body cavity. We revealed a role for elastin in storing elastic energy during cell contraction, highlighting a potential role for elastin in gut motility through the ability to resist cyclic deformations associated with peristalsis. These results provide insights into intestinal development and adult function, and highlight elastin's diverse roles during organogenesis.

Elephants develop wrinkles through both form and function

The trunks of elephants have prominent wrinkles from their base to the very tip. But neither the obvious differences in wrinkles between elephant species nor their development have been studied before. In this work, we characterize the lifelong development of trunk wrinkles in Asian and African elephants. Asian elephants have more dorsal major, meaning deep and wide, trunk wrinkles (approx. 126 ± 25 s.d.) than African elephants (approx. 83 ± 13 s.d.). Both species have more dorsal than ventral major trunk wrinkles and a closer wrinkle spacing distally than proximally. In Asian elephants, wrinkle density is high in the 'trunk wrapping zone'. Wrinkle numbers on the left and right sides of the distal trunk differed as a function of trunk lateralization, with frequent bending in one direction causing wrinkle formation. Micro-computed tomography (microCT) imaging and microscopy of newborn elephants' trunks revealed a constant thickness of the putative epidermis, whereas the putative dermis shrinks in the wrinkle troughs. During fetal development, wrinkle numbers double every 20 days in an early exponential phase. Later wrinkles are added slowly, but at a faster rate in Asian than African elephants. We discuss the relationship of species differences in trunk wrinkle distribution and number with behavioural, environmental and biomechanical factors.

Mechanical experimentation of the gastrointestinal tract: a systematic review

The gastrointestinal (GI) organs of the human body are responsible for transporting and extracting nutrients from food and drink, as well as excreting solid waste. Biomechanical experimentation of the GI organs provides insight into the mechanisms involved in their normal physiological functions, as well as understanding of how diseases can cause disruption to these. Additionally, experimental findings form the basis of all finite element (FE) modelling of these organs, which have a wide array of applications within medicine and engineering. This systematic review summarises the experimental studies that are currently in the literature (n = 247) and outlines the areas in which experimentation is lacking, highlighting what is still required in order to more fully understand the mechanical behaviour of the GI organs. These include (i) more human data, allowing for more accurate modelling for applications within medicine, (ii) an increase in time-dependent studies, and (iii) more sophisticated in vivo testing methods which allow for both the layer- and direction-dependent characterisation of the GI organs. The findings of this review can also be used to identify experimental data for the readers' own constitutive or FE modelling as the experimental studies have been grouped in terms of organ (oesophagus, stomach, small intestine, large intestine or rectum), test condition (ex vivo or in vivo), number of directions studied (isotropic or anisotropic), species family (human, porcine, feline etc.), tissue condition (intact wall or layer-dependent) and the type of test performed (biaxial tension, inflation-extension, distension (pressure-diameter), etc.). Furthermore, the studies that investigated the time-dependent (viscoelastic) behaviour of the tissues have been presented.

Investigation of microstructural failure in the human cornea through fracture tests

Fracture toughness of the human cornea is one of the critical parameters in suture-involved corneal surgeries and the development of bioengineered mimetics of the human cornea. The present article systematically studied the fracture characteristics of the human cornea to evaluate its resistance to tear in the opening (Mode-I) and trouser tear mode (Mode-III). Tear experiments reveal the dependency of the fracture behavior on the notch size and its location created in the corneal specimens. The findings indicate lamellar tear and collagen fiber pull-out as a failure mechanism in trouser tear and opening mode tests, respectively. Experimental results have shown a localized variation of tear behavior in trouser tear mode and indicated an increasing resistance to tear from the corneal center to the periphery. This article demonstrated the complications of evaluating fracture toughness in opening mode and showed that the limbus was weaker than the cornea and sclera against tearing. The overall outcomes of the present study help in designing experiments to understand the toughness of the diseased tissues, understanding the effect of the suturing location and donor placement, and creating numerical models to study parameters affecting corneal replacement surgery.