Cellular and Matrix Mechanics of Bioartificial Tissues During Continuous Cyclic Stretch

Abstract

Bioartificial tissues are useful model systems for studying cell and extra-cellular matrix mechanics. These tissues provide a 3D environment for cells and allow tissue components to be easily modified and quantified. In this study, we fabricated bioartificial tissue rings from a 1 ml solution containing one million cardiac fibroblasts and 1 mg collagen. After 8 days, rings compacted to <1% of original volume and cell number increased 2.4 fold. We initiated continuous cyclic stretching of the rings after 2, 4, or 8 days of incubation, while monitoring the tissue forces. Peak tissue force during each cycle decreased rapidly after initiating stretch, followed by further slow decline. We added 2 μM Cytochalasin-D to some rings prior to initiation of stretch to determine the force contributed by the matrix. Cell force was estimated by subtracting matrix force from tissue force. After 12 h, matrix force-strain curves were highly nonlinear. Cell force-strain curves were linear during loading and showed hysteresis indicating viscoelastic behavior. Cell stiffness increased with stretching frequency from 0.001–0.25 Hz. Cell stiffness decreased with stretch amplitude (5–25%) at 0.1 Hz. The trends in cell stiffness do not fit simple viscoelastic models previously proposed, and suggest possible strain-amplitude related changes during cyclic stretch.

A theoretical model for fibroblast-controlled growth of saccular cerebral aneurysms

“…Furthermore, the activity of adherent cells is strongly dependent upon mechanical stimuli from the surrounding ECM (Pelham and Wang, 1997;Weyts et al, 2003). More specifically, cyclic deformation (which fibroblasts embedded in an arterial wall are exposed to) is known to influence the proliferation and collagen production rate of fibroblasts (Butt et al, 1995;Eastwood et al, 1998;Lee et al, 2004;Wille et al, 2006;Wu and Chen, 2000), and both of these factors are important for the growth of aneurysmal tissue.…”

Stability enhancement of an atomic force microscope for long-term force measurement including cantilever modification for whole cell deformation

“…In comparison the last cycle (f) in the modified system starts at 0.6 nN, beginning with a rapid increase in force and then continuing along a similar force-strain curve to that observed for the first cycle. 3 For the unmodified system considered in the present study the continuous increase in force occurs due a continuously increasing level of cell compression, from 25 % at the start of the experiment to ~41 % at the end of the experiment, due to z-drift. Additionally, the increasingly negative minimum forces obtained for the unmodified system indicates the cell is being stretched at the upper end of the deformation cycle, suggesting that the passive viscoelastic behavior of the cell begins to dominate the response of the cell at such elevated applied strains (20 % higher than the desired level).…”

“…4 The data presented are similar to a broad pattern of behavior previously observed for cells cyclically stretched at 0.1 Hz within a collagen matrix. 3 In that study, isolation of the force contribution of the actin cytoskeleton from the total measured force revealed initial viscoelastic force reductions followed by slight force increases to a steady state following 30 minutes of cycling.…”

“…Force measurement has been implemented in macroscale experiments whereby cell seeded collagen scaffolds were cyclically stretched at constant strain rates. 3 The active contribution of the cells was isolated and it was demonstrated that lower strain rates resulted in higher steady state forces following several hours of cyclic deformation. This behavior cannot be explained by standard passive viscoelastic material models which predict higher stresses at higher strain rates.…”

“…4 This model provides an explanation of the strain rate effects observed by Wille et al. 3 However, in order to validate this theory, strain controlled cyclic deformation must be applied to single cell studies whereby the resultant forces are measured and the corresponding cell remodeling visualized over a physiologically relevant time period.…”