ICMCTF2015 Session TS1: Mechanical Aspects of Biointerfaces

Surface roughness is an important factor affecting cellular attachment to surfaces prior to biofilm formation. The presence of nanoscale asperities reduce the physicochemical potential barrier encountered by a bacterial cell when it approaches the surface. More recently, the effect of the surface topography on biofilm development has also been qualified . Local zones of flow recirculation introduced by the surface shapes are responsible for modifying the deposition rate defined as the ratio of the particle flux towards the wall over the wall concentration.

This work is to study the initial deposition of activated sludge bacteria from a wastewater treatment plant. The objective of the study presented here is two-fold: (1) to specify the boundary conditions of a convection diffusion equation, which are appropriate for bacterial deposition in laminar viscous flows and can be easily implemented in a Computational Fluid Dynamics (CFD) code; (2) to assess the bacterial deposition rate for different surface features, which would guide the design of appropriate material topography for wastewater treatment.

Here, bacteria are treated as inert colloidal particles that attach to surfaces according to the extended Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The deposition model accounts for bacterial cell surface structures, hydrophobicity and motility which are known to increase activated sludge bacterial adhesion. The surface asperities are modelled by a lognormal size distribution with mean μ~50 nm and standard deviation σ~3. The model is validated by comparing deposition measured in a parallel flow chamber to the CFD results for different ionic strengths and surface properties. Based on CFD simulations, the relationship between the deposition rate and the surface topography has been established.

We derive a set of phase field models for biofilms using the one-fluid two-component formulation in which the combination of extracellular polymeric substances (EPS) and the bacteria are effectively modeled as one fluid component while the collective ensemble of nutrient and the solvent are modeled as the other. The biofilm is assumed an incompressible continuum. Numerical simulations are carried out in one and two space dimension using a velocity-corrected projection method for incompressible flows. Biofilm growth, expansion, streaming, rippling, and detachment are simulated in shear cells numerically. Viscoelastic properties of the biofilm is investigated as well.

Human Mesenchymal stem cells (MSCs) have been used as primary cell lines for musculoskeletal tissue engineering. However, the translation of MSCs preclinical results has serious problems to be solved due to the significant variability of MSCs preparations and limited life-span of MSCs. To circumvent such issues, it has been suggested to immortalize these cells by genetic manipulation. Various work has demonstrated that such immortalized cells grow fast and are able to produce significant amount of extracellular matrix. However, it remains elusive how good these extracellular matrix could be.

In this study, the nanoindentation technique was adopted to evaluate the mechanical properties of the matrix formed on glass coverslips by immortalized cell line Y201 from human MSCs at different cell culture conditions. However, for the extracellular matrix formed the implant or other surfaces are often highly inhomogeneous and have a rough surface and non-uniform thickness. This would make data interpretation of nanoindentation complicated. Therefore, different nanoindentation test protocols have been designed. It has shown that multicyling test protocols are more suited to such inhomogeneous materials. Statistical analysis has revealed that the matrix produced by cells on day 14 is stronger compared to other culture periods possibly due to the denser microstructure. When the cell culture exceeds 21 days, the local cell detachment tends to affect the measured mechanical properties. In addition, surface chemistry analysis has shown that little minerals have been produced regardless fast cell growth.

Many important biological processes, such as endothelial mechanotransduction of hemodynamic forces and neutrophil extravasation, involve the transmission of stresses across a cell monolayer. During these processes, the monolayer undergoes both lateral distortion due to the in-plane traction forces generated by the cells, and bending due to the out-of-plane component of the traction forces. However, the contribution of this bending to the monolayer stresses has been neglected in the literature. Here, we present a novel technique to determine monolayer stresses that considers both lateral distortion and bending. To illustrate the method, and to quantify the relative importance of the lateral and bending stresses, we measure the monolayer stresses in micropatterned endothelial cell islands of varying sizes and shapes. The cell islands are cultured on flexible polyacrylamide gels embedded with fluorescent beads, which deform due to traction forces exerted by the cells. We measure the three-dimensional gel deformation using previously established 3D Traction Force Microscopy methods, and recover the monolayer stresses from the measured deformation using Kirchoff-Love thin plate theory. The equations are solved numerically in an efficient manner using a Fourier pseudo-spectral method. The boundary conditions corresponding to the geometry of the cell islands are enforced within the Fourier framework using a relaxation iterative method. Our results indicate that, regardless of island shape, the three-dimensional bending stresses are dominant at the center of the island while the lateral stresses are more important near the island edge. Also, comparing the results from islands of different sizes shows that the relative importance of the bending stresses increases with decreasing island size. These results suggest that it is necessary to resolve bending stresses to accurately determine the monolayer stresses, and reveal that the transmission of forces across cell junctions is three-dimensional and more complex than previously believed.