Abstract
Living animal cells are strongly influenced by the mechanical properties of their environment. To model physiological conditions ultrasoft cell culture substrates, in some instances with elasticity (Young's modulus) of only 1 kPa, are mandatory. Due to their long shelf life PDMS-based elastomers are a popular choice. However, uncertainty about additives in commercial formulations and difficulties to reach very soft materials limit their use. Here, we produced silicone elastomers from few, chemically defined and commercially available substances. Elastomers exhibited elasticities in the range from 1 kPa to 55 kPa. In detail, a high molecular weight (155 kg/mol), vinyl-terminated linear silicone was crosslinked with a multifunctional (f = 51) crosslinker (a copolymer of dimethyl siloxane and hydrosilane) by a platinum catalyst. The following different strategies towards ultrasoft materials were explored: sparse crosslinking, swelling with inert silicone polymers, and, finally, deliberate introduction of dangling ends into the network (inhibition). Rheological experiments with very low frequencies led to precise viscoelastic characterizations. All strategies enabled tuning of stiffness with the lowest stiffness of ~1 kPa reached by inhibition. This system was also most practical to use. Biocompatibility of materials was tested using primary cortical neurons from rats. Even after several days of cultivation no adverse effects were found.
Highlights
Reaction to external signals is one of the hallmarks of living systems
G0 is again the equilibrium shear modulus, i.e., the low frequency limit of G’ originating from permanent crosslinks, Gr is the rubber plateau modulus resulting at high frequencies from transient crosslinks formed by entanglements, τ is the characteristic Maxwell time that we found to be different for both modules and α, β are the two exponents describing width and asymmetry of the distribution function
In the remaining sections we focus on how the equilibrium shear modulus G0 is tunable by the control parameters catalyst concentration, stoichiometric ratio, filler molecular weight and amount of inhibitor
Summary
Reaction to external signals is one of the hallmarks of living systems. Research on living cells has focused on chemical and genetic factors. Recently mechanical factors have emerged as additional potent control parameters [1, 2]. Many animal cells strongly change their phenotypes in response to the elasticity of their substrates. Cells have been shown to generate mechanical forces and to react to them [3, 4]. Such mechanobiological effects are well documented, amongst others, for endothelial cells, smooth muscle cells, stem cells, and fibroblasts [5,6,7,8,9]
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