Burst inflation test for measuring biomechanical properties of rat abdominal walls.

PURPOSE: Evaluation of potential grafts to improve upon current strategies for abdominal wall (AW) repair in small animal models typically involves mechanical testing using methods that currently are inadequate to assess physiologically relevant parameters. This study introduces burst inflation testing as a more relevant assessment of the mechanical integrity of the AW compared to traditional tensile testing. METHODS: AWs were excised from 14 healthy adult Fischer 344 rats and tested using either a custom burst inflation device or an Instron tensile testing system. Modulus outcomes from both testing methods were compared. RESULTS: Mechanical analyses of native AW using burst and tensile testing methods resulted in similar average tissue moduli, but with the burst test, there was significantly less variability among specimens. CONCLUSIONS: The burst test had greater repeatability compared to tensile testing and has the ability to test repaired AWs without compromising the integrity of the repair site, making it a useful tool for assessing graft repairs.

Use of adipose-derived stem cells to fabricate scaffoldless tissue-engineered neural conduits in vitro.

Peripheral nerve injuries resulting from trauma or disease often necessitate surgical intervention. Although the gold standard for such repairs uses nerve autografts, alternatives that do not require invasive harvesting of autologous nerve tissues are currently being designed and evaluated. We previously established the use of scaffoldless engineered neural conduits (ENCs) fabricated from primary cells as one such alternative in sciatic nerve repair in rats [Baltich et al. (2010) In Vitro Cell Dev Biol Anim 46(5):438-444]. The present study establishes protocols for fabricating neural conduits from adipose-derived stem cells (ASCs) differentiated to either a fibroblast or neural lineage and co-cultured into a three-dimensional (3-D) scaffoldless tissue-ENC. Addition of ascorbic acid-2-phosphate and fibroblast growth factor (FGF)-2 to the medium induced and differentiated ASCs to a fibroblast lineage in more than 90% of the cell population, as confirmed by collagen I expression. ASC-differentiated fibroblasts formed monolayers, delaminated, and formed 3-D conduits. Neurospheres were formed by culturing ASCs on non-adherent surfaces in serum-free neurobasal medium with the addition of epidermal growth factor (EGF) and FGF-2. The addition of 10 ng EGF and 10 ng FGF-2 produced larger and more numerous neurospheres than treatments of lower EGF and FGF-2 concentrations. Subsequent differentiation to glial-like cells was confirmed by the expression of S100. ASC-derived fibroblast monolayers and neurospheres were co-cultured to fabricate a 3-D scaffoldless tissue-ENC. Their nerve-like structure and incorporation of glial-like cells, which would associate with regenerating axons, may make these novel, stem cell-derived neural conduits an efficacious technology for repairing critical gaps following peripheral nerve injury.

Perspectives on biological growth and remodeling.

The continuum mechanical treatment of biological growth and remodeling has attracted considerable attention over the past fifteen years. Many aspects of these problems are now well-understood, yet there remain areas in need of significant development from the standpoint of experiments, theory, and computation. In this perspective paper we review the state of the field and highlight open questions, challenges, and avenues for further development.

The micromechanics of fluid-solid interactions during growth in porous soft biological tissue.

In this paper, we address some modelling issues related to biological growth. Our treatment is based on a formulation for growth that was proposed within the context of mixture theory (J Mech Phys Solids 52:1595-1625, 2004). We aim to make this treatment more appropriate for the physics of porous soft tissues, paying particular attention to the nature of fluid transport, and mechanics of fluid and solid phases. The interactions between transport and mechanics have significant implications for growth and swelling. We also reformulate the governing differential equations for reaction-transport of solutes to represent the incompressibility constraint on the fluid phase of the tissue. This revision enables a straightforward implementation of numerical stabilisation for the advection-dominated limit of these equations. A finite element implementation with operator splitting is used to solve the coupled, non-linear partial differential equations that arise from the theory. We carry out a numerical and analytic study of the convergence of the operator splitting scheme subject to strain- and stress-homogenisation of the mechanics of fluid-solid interactions. A few computations are presented to demonstrate aspects of the physical mechanisms, and the numerical performance of the formulation.

Finite element modeling of human skin using an isotropic, nonlinear elastic constitutive model.

The collagen network in skin is largely responsible for the nonlinear mechanical stress-strain response of skin. We hypothesize that the force-stretch response of collagen is governed by the entropics of long-chain molecules. We show that a constitutive model derived from the statistical mechanics of long-chain molecules, corresponding to the fibrous collagen network in skin, captures the mechanical response of skin. A connection between the physiologically meaningful parameters of network molecular chain density and free length of collagen fibers and the constitutively significant parameters of initial modulus and limiting stretch is thus established. The relevant constitutive law is shown to have predictive capabilities related to skin histology by replicating in vivo and in vitro experimental results. From finite element simulations, this modeling approach predicts that the collagen network in hypertrophic scars is more dense and the constituent collagen fibers have shorter free lengths than in healthy skin. Additionally, the model is shown to predict that as rat skin ages, collagen network density increases and fiber free length decreases. The importance of knowledge of the in situ stress state for analyzing skin response and validating constitutive laws is also demonstrated.