Magnetic field tuning of mechanical properties of ultrasoft PDMS-based magnetorheological elastomers for biological applications

Biomimetic Substrate to Probe Dynamic Interplay of Topography and Stiffness on Cardiac Fibroblast Activation

Materials with the ability to change properties can expand the capabilities of in vitro models of biological processes and diseases as it has become increasingly clear that static, stiff materials with smooth surfaces fall short in recapitulating the in vivo cellular microenvironment. Here, we introduce a patterned material that can be rapidly stiffened and softened in situ in response to an external magnetic field through the addition of magnetic inclusions into a soft silicone elastomer with topographic surface patterning. This substrate can be used for cell culture to investigate short-term cellular responses to dynamic stiffening or softening and the interaction with topography that encourages cells to assume a specific morphology. We investigated short-term cellular responses to dynamic stiffening or softening in human ventricular cardiac fibroblasts. Our results indicate that the combination of dynamic changes in stiffness with and without topographic cues induces different effects on the alignment and activation or deactivation of myofibroblasts. Cells cultured on patterned substrates exhibited a more aligned morphology than cells cultured on flat material; moreover, cell alignment was not dependent on substrate stiffness. On a patterned substrate, there was no significant change in the number of activated myofibroblasts when the material was temporally stiffened, but temporal softening caused a significant decrease in myofibroblast activation (50% to 38%), indicating a competing interaction of these characteristics on cell behavior. This material provides a unique in vitro platform to observe the time-dependent dynamics of cells by better mimicking more complex behaviors and realistic microenvironments for investigating biological processes, such as the development of fibrosis.

The effect of polymer stiffness on magnetization reversal of magnetorheological elastomers

Ultrasoft magnetorheological elastomers (MREs) offer convenient real-time magnetic field control of mechanical properties that provides a means to mimic mechanical cues and regulators of cells in vitro. Here, we systematically investigate the effect of polymer stiffness on magnetization reversal of MREs using a combination of magnetometry measurements and computational modeling. Poly-dimethylsiloxane-based MREs with Young's moduli that range over two orders of magnitude were synthesized using commercial polymers Sylgard™ 527, Sylgard 184, and carbonyl iron powder. The magnetic hysteresis loops of the softer MREs exhibit a characteristic pinched loop shape with almost zero remanence and loop widening at intermediate fields that monotonically decreases with increasing polymer stiffness. A simple two-dipole model that incorporates magneto-mechanical coupling not only confirms that micrometer-scale particle motion along the applied magnetic field direction plays a defining role in the magnetic hysteresis of ultrasoft MREs but also reproduces the observed loop shapes and widening trends for MREs with varying polymer stiffnesses.