Osteoblast Elastic Modulus Measured by Atomic Force Microscopy Is Substrate Dependent

Finite Element Analysis of Meniscal Anatomical 3D Scaffolds: Implications for Tissue Engineering

Solid Free-Form Fabrication (SFF) technologies allow the fabrication of anatomical 3D scaffolds from computer tomography (CT) or magnetic resonance imaging (MRI) patients’ dataset. These structures can be designed and fabricated with a variable, interconnected and accessible porous network, resulting in modulable mechanical properties, permeability, and architecture that can be tailored to mimic a specific tissue to replace or regenerate. In this study, we evaluated whether anatomical meniscal 3D scaffolds with matching mechanical properties and architecture are beneficial for meniscus replacement as compared to meniscectomy. After acquiring CT and MRI of porcine menisci, 3D fiber-deposited (3DF) scaffolds were fabricated with different architectures by varying the deposition pattern of the fibers comprising the final structure. The mechanical behaviour of 3DF scaffolds with different architectures and of porcine menisci was measured by static and dynamic mechanical analysis and the effect of these tissue engineering templates on articular cartilage was assessed by finite element analysis (FEA) and compared to healthy conditions or to meniscectomy. Results show that 3DF anatomical menisci scaffolds can be fabricated with pore different architectures and with mechanical properties matching those of natural menisci. FEA predicted a beneficial effect of meniscus replacement with 3D scaffolds in different mechanical loading conditions as compared to meniscectomy. No influence of the internal scaffold architecture was found on articular cartilage damage. Although FEA predictions should be further confirmed by in vitro and in vivo experiments, this study highlights meniscus replacement by SFF anatomical scaffolds as a potential alternative to meniscectomy.

Modulation of cellular mechanics during osteogenic differentiation of human mesenchymal stem cells

Recognition of the growing role of human mesenchymal stem cells (hMSC) in tissue engineering and regenerative medicine requires a thorough understanding of intracellular biochemical and biophysical processes that may direct the cell's commitment to a particular lineage. In this study, we characterized the distinct biomechanical properties of hMSCs, including the average Young's modulus determined by atomic force microscopy (3.2 +/- 1.4 kPa for hMSC vs. 1.7 +/- 1.0 kPa for fully differentiated osteoblasts), and the average membrane tether length measured with laser optical tweezers (10.6 +/- 1.1 microm for stem cells, and 4.0 +/- 1.1 microm for osteoblasts). These differences in cell elasticity and membrane mechanics result primarily from differential actin cytoskeleton organization in these two cell types, whereas microtubules did not appear to affect the cellular mechanics. The membrane-cytoskeleton linker proteins may contribute to a stronger interaction of the plasma membrane with F-actins and shorter membrane tether length in osteoblasts than in stem cells. Actin depolymerization or ATP depletion caused a two- to threefold increase in the membrane tether length in osteoblasts, but had essentially no effect on the stem-cell membrane tethers. Actin remodeling in the course of a 10-day osteogenic differentiation of hMSC mediates the temporally correlated dynamical changes in cell elasticity and membrane mechanics. For example, after a 10-day culture in osteogenic medium, hMSC mechanical characteristics were comparable to those of mature bone cells. Based on quantitative characterization of the actin cytoskeleton remodeling during osteodifferentiation, we postulate that the actin cytoskeleton plays a pivotal role in determining the hMSC mechanical properties and modulation of cellular mechanics at the early stage of stem-cell osteodifferentiation.

Mechanical heterogeneity of the rat hippocampus measured by atomic force microscope indentation

Knowledge of brain tissue mechanical properties may be critical for formulating hypotheses about traumatic brain injury (TBI) mechanisms and for accurate TBI simulations. To determine the local mechanical properties of anatomical subregions within the rat hippocampus, the atomic force microscope (AFM) was adapted for use on living brain tissue. The AFM provided advantages over alternative methods for measuring local mechanical properties of brain because of its high spatial resolution, high sensitivity, and ability to measure live samples under physiologic conditions. From AFM indentations, a mean pointwise or depth-dependent apparent elastic modulus, E, was determined for the following hippocampal subregions: CA1 pyramidal cell layer (CA1P) and stratum radiatum (CA1SR), CA3 pyramidal cell layer (CA3P) and stratum radiatum (CA3SR), and the dentate gyrus (DG). For all regions, E was indentation-depth-dependent, reflecting the nonlinearity of brain tissue. At an indentation depth of 3microm, E was 234 +/- 152 Pa for CA3P, 308 +/- 184 Pa for CA3SR, 137 +/- 97 Pa for CA1P, 169 +/- 52 Pa for CA1SR, and 201 +/- 133 Pa for DG (mean +/- SD). Our results demonstrate for the first time that the hippocampus is mechanically heterogeneous. Based on our findings, we discuss hypotheses accounting for experimentally observed patterns of hippocampal cell death, which can be tested with biofidelic finite element models of TBI.

Effect of surface nanoscale topography on elastic modulus of individual osteoblastic cells as determined by atomic force microscopy

Mechanical stimulation of osteoblasts by fluid flow promotes a variety of pro-differentiation effects and improving the efficiency of these mechanical signals could encourage specific differentiation pathways. One way this could be accomplished is by altering mechanical properties of osteoblasts. In this study, murine osteoblastic MC3T3-E1 cells were cultured on surfaces covered with nanometer-sized islands to examine the hypothesis that the elastic modulus of osteoblastic cells is affected by nanoscale topography. Nanoislands were produced by polymer demixing of polystyrene and poly(bromostyrene), which leads to a segregated polymer system and formation of nanometer-sized topographical features. The elastic modulus of MC3T3-E1 cells was determined using atomic force microscopy in conjunction with the Hertz mathematical model. Osteoblastic cells cultured on nanotopographic surfaces (11-38 nm high islands) had a different distribution of cellular modulus values, e.g., the distribution shifted toward higher modulus values, relative to cells on flat control surfaces. There were also differences in cell modulus distribution between two flat controls as surface chemistry was changed between polystyrene and glass. Taken together, our results demonstrate that both surface nanotopography and chemistry affect the mechanical properties of cells and may provide new methods for altering the response of cells to external mechanical signals.

A thin-layer model for viscoelastic, stress-relaxation testing of cells using atomic force microscopy: do cell properties reflect metastatic potential?

Atomic force microscopy has rapidly become a valuable tool for quantifying the biophysical properties of single cells. The interpretation of atomic force microscopy-based indentation tests, however, is highly dependent on the use of an appropriate theoretical model of the testing configuration. In this study, a novel, thin-layer viscoelastic model for stress relaxation was developed to quantify the mechanical properties of chondrosarcoma cells in different configurations to examine the hypothesis that viscoelastic properties reflect the metastatic potential and invasiveness of the cell using three well-characterized human chondrosarcoma cell lines (JJ012, FS090, 105KC) that show increasing chondrocytic differentiation and decreasing malignancy, respectively. Single-cell stress relaxation tests were conducted at 2 h and 2 days after plating to determine cell mechanical properties in either spherical or spread morphologies and analyzed using the new theoretical model. At both time points, JJ012 cells had the lowest moduli of the cell lines examined, whereas FS090 typically had the highest. At 2 days, all cells showed an increase in stiffness and a decrease in apparent viscosity compared to the 2-h time point. Fluorescent labeling showed that the F-actin structure in spread cells was significantly different between FS090 cells and JJ012/105KC cells. Taken together with results of previous studies, these findings indicate that cell transformation and tumorigenicity are associated with a decrease in cell modulus and apparent viscosity, suggesting that cell mechanical properties may provide insight into the metastatic potential and invasiveness of a cell.

Advanced Tools for Tissue Engineering: Scaffolds, Bioreactors, and Signaling

This article contains the collective views expressed at the second session of the workshop "Tissue Engineering--The Next Generation,'' which was devoted to the tools of tissue engineering: scaffolds, bioreactors, and molecular and physical signaling. Lisa E. Freed and Farshid Guilak discussed the integrated use of scaffolds and bioreactors as tools to accelerate and control tissue regeneration, in the context of engineering mechanically functional cartilage and cardiac muscle. Edward Guo focused on the opportunities that tissue engineering generates for studies of mechanobiology and on the need for tissue engineers to learn about mechanical forces during tissue and organ genesis. Martha L. Gray focused on the potential of biomedical imaging for noninvasive monitoring of engineered tissues and on the opportunities biomedical imaging can generate for the development of new markers. Robert Tranquillo reviewed the approach to tissue engineering of a spectrum of avascular habitually loaded tissues- blood vessels, heart valves, ligaments, tendons, cartilage, and skin. Jeffrey W. Holmes offered the perspective of a "reverse paradigm''--the use of tissue constructs in quantitative studies of cell-matrix interactions, cell mechanics, matrix mechanics, and mechanobiology. Milica Radisic discussed biomimetic design of tissue-engineering systems, on the example of synchronously contractile cardiac muscle. Michael V. Sefton proposed a new, simple approach to the vascularization of engineered tissues. This session stressed the need for advanced scaffolds, bioreactors, and imaging technologies and offered many enlightening examples on how these advanced tools can be utilized for functional tissue engineering and basic research in medicine and biology.