Final Activity Report Summary - MECNOR (Dynamic finite element modelling for tissue differentiation and long-term mechano-regulation processes)
The potential of 'Bone marrow stromal cells' (BMSCs) to differentiate in cell types relevant to tissue engineering has been widely proven. It has also been shown that mechanically loading BMSC cultures can lead to specific phenotypes and can improve growth factor-based inductive environments. Unfortunately, cell mechano-regulation mechanisms are still poorly understood, limiting the development of tissue engineering strategies from BMSCs. To better understand how BMSCs respond to mechanical loads, in vitro experiments could be combined with (semi-) phenomenological cell mechano-regulation theories developed in silico for bone or cartilage healing. Coupled to numerical models of mechano-stimulation systems, these theories can predict load regimes able to induce specific BMSC differentiations. Subsequently, these theoretical mechanical loadings can be tested on real cell cultures, under controlled mechanical and biochemical environments. Experimental results can then validate or invalidate the initial mechano-regulation models, allowing either a guided revision of the equations, or immediate generation of new hypotheses that in turn will be experimentally tested. This is the approach in the present project focussed on the mechano-stimulation of BMSCs in dynamically compressed fibrin scaffolds.
The work mainly focussed on the numerical methodology. The selected BMSCs mechano-regulation rules were based on octahedral shear strains and interstitial fluid flows, shown to best describe fracture healing under various loading regimes. Locally, these stimuli can initiate BMSC differentiation into chondroblast, osteoblast or fibroblast, or induce cell apoptosis. Concentration gradients may activate cell migrations and cells may also proliferate. Thus, for any cell type, local concentrations are a function of proliferation, differentiation, death, and diffusion rate constants. These rate constants, previously estimated to describe in vivo fracture healing, may substantially differ for cells cultured in fibrin. It is also unknown whether fluid flow and octahedral shear strain criteria reported for fracture healing differ from one system to another. Thus, validating the basic mechano-regulation rules and assessing the influence of the rate constants were the initial objectives of the project.
Before evaluating the selected mechano-regulation theory, the local mechanical stimuli transmitted to the cells by the fibrin scaffold needed to be accurately predicted and scaffolds had to be mechanically characterised. Fibrin carriers were shown to be possibly modelled as poroviscoelastic materials and a method was developed to analytically extract low strain constitutive parameter values, from stress relaxation experiments. Low strain approximations were sufficiently accurate to determine finite strain parameter values from non-linear numerical optimisations. Unfortunately, first biological results showed that fibrin compositions with measurable mechanical properties were not suitable for BMSC survival. Therefore, additional investigations were necessary. Maximum fibrinogen and thrombin contents, giving suitable BMSC survival/proliferation in cylindrical fibrin scaffolds, were determined and a high precision measurement setup was built to mechanically test the selected compositions. Stress relaxation experiments allowed assessing the solid- and fluid-like behaviours of the gels while mechanical properties were modified by affecting fibrin self-assembly with calcium, chloride, and factor XIII. The biologically compatible fibrin compositions initially giving fluid-like viscous gels could then be adjusted until measurable solid-like properties were obtained, giving control on further cell mechano-stimulation experiments. Finally, the poroviscoelastic fibrin model was coupled to the mechano-regulation theory, completing the theoretical tool for a further analysis of BMSC mechano-regulation together with bioreactor experiments.
The work mainly focussed on the numerical methodology. The selected BMSCs mechano-regulation rules were based on octahedral shear strains and interstitial fluid flows, shown to best describe fracture healing under various loading regimes. Locally, these stimuli can initiate BMSC differentiation into chondroblast, osteoblast or fibroblast, or induce cell apoptosis. Concentration gradients may activate cell migrations and cells may also proliferate. Thus, for any cell type, local concentrations are a function of proliferation, differentiation, death, and diffusion rate constants. These rate constants, previously estimated to describe in vivo fracture healing, may substantially differ for cells cultured in fibrin. It is also unknown whether fluid flow and octahedral shear strain criteria reported for fracture healing differ from one system to another. Thus, validating the basic mechano-regulation rules and assessing the influence of the rate constants were the initial objectives of the project.
Before evaluating the selected mechano-regulation theory, the local mechanical stimuli transmitted to the cells by the fibrin scaffold needed to be accurately predicted and scaffolds had to be mechanically characterised. Fibrin carriers were shown to be possibly modelled as poroviscoelastic materials and a method was developed to analytically extract low strain constitutive parameter values, from stress relaxation experiments. Low strain approximations were sufficiently accurate to determine finite strain parameter values from non-linear numerical optimisations. Unfortunately, first biological results showed that fibrin compositions with measurable mechanical properties were not suitable for BMSC survival. Therefore, additional investigations were necessary. Maximum fibrinogen and thrombin contents, giving suitable BMSC survival/proliferation in cylindrical fibrin scaffolds, were determined and a high precision measurement setup was built to mechanically test the selected compositions. Stress relaxation experiments allowed assessing the solid- and fluid-like behaviours of the gels while mechanical properties were modified by affecting fibrin self-assembly with calcium, chloride, and factor XIII. The biologically compatible fibrin compositions initially giving fluid-like viscous gels could then be adjusted until measurable solid-like properties were obtained, giving control on further cell mechano-stimulation experiments. Finally, the poroviscoelastic fibrin model was coupled to the mechano-regulation theory, completing the theoretical tool for a further analysis of BMSC mechano-regulation together with bioreactor experiments.