Introduction: The physical and mechanical properties of the extracellular matrix (ECM) are known to influence and regulate cell fate and a wide variety of biological processes such as cell migration and differentiation. However, the underlying physical mechanisms behind the interactions between cells and their microenvironment is not well understood.
Many studies have revealed profound effects of substrate elasticity on cellular responses. For instance, it has been shown that morphology, cytoskeletal structure, and cellular adhesion change in response to substrate elasticity[1]. Substrate elasticity has also been shown to guide cell migration[2] and to direct stem cell differentiation[3]. In most of these studies, purely elastic materials were used to characterize cell-biomaterial interactions, however, most biological materials are viscoelastic and exhibit time-dependent deformation to the applied force, usually referred to as creep behavior. One clear consequence of the material’s creep is energy dissipation. Unlike purely elastic materials where elastic energy is conserved, viscoelastic materials dissipate energy and constantly require input work to maintain a constant stress, commensurate with the rate of energy dissipation.
The goal of this work is to establish a new platform to study mechanobiology and cell mechanics in response to viscoelastic properties of the ECM.
Material and Method: Polydimethylsiloxane (PDMS) was purchased from Gelest. PDMS was functionalized for cell adhesion. A rheometer was used to characterize the viscoelastic properties of the substrates. Parallel plates with a diameter of 20 mm and a gap of 1000 µm were used. Frequency sweep and creep tests were performed.
NIH 3T3 fibroblasts cells were cultured in a mixture of Dulbecco’s Modified Eagle Medium, 10% fetal bovine serum, 1% Penicillin/Streptomycin at 37ºC, in 5% CO2 humidified atmosphere. Cells were disassociated using 0.25% trypsin-EDTA when the cell confluency reached 70% and were cultured on the PDMS substrates' surface.
Results: The fabricated PDMS substrates exhibit tunable viscoelastic properties. The shear and loss moduli were tunable between 4- 50 kPa and 0.2-7 kPa, respectively. Figure 1 shows the creep response of three PDMS samples. The shear stress was 2 kPa during creep test. This result indicates constant shear moduli in long time scale for these three samples but different time scale for their creep behavior. Cell culture results showed the spreading of NIH 3T3 cells on PDMS substrates.
Discussion: Tunable viscoelastic PDMS substrates can provide us with a very strong tool to understand how cells sense and respond to the mechanical properties of their surrounding matrix. This viscoelastic platform can facilitate quantitative characterization of cell-ECM interactions to understand cell behavior during development and pathological conditions.
Conclusions: PDMS-based tunable viscoelastic substrates with cell adhesion functionalities were fabricated and characterized for use in studying cell mechanics and mechanobiology.
Canada Foundation for Innovation; Canadian Institute of Health Research; Natural Sciences and Engineering Research Council of Canada
References:
[1] Yeung T et al, “Effects of substrate stiffness on cell morphology, cytoskeletal structure, and adhesion”. Cell Motil Cytoskeleton. 2005; 60, 24-34.
[2] Lo CM et al, “Cell Movement Is Guided by the Rigidity of the Substrate”, Biophysical journal, 2000, 79, 144-152.
[3] Angler AJ et al, “Matrix elasticity directs stem cell lineage specification”, Cell. 2006, 126: 677-89.