Magnetic and Mechanical Properties of Ultrasoft Magnetorheological Elastomers

Abstract

Magnetorheological elastomers (MREs), composite materials consisting of magnetic particles embedded in a non-magnetic elastomeric matrix, can reversibly modulate their mechanical and magnetic properties through tuning the applied magnetic field H. Recently, ultrasoft MREs have received tremendous attention due to their great potential in biomedical applications. However, the effects of the polymer stiffness and magnetic particle concentration on the magnetic and mechanical properties of ultrasoft MREs still need to be better understood. In this dissertation, the author presents a comprehensive investigation of the magnetic and mechanical properties of ultrasoft MREs as well as their biomedical applications. The effect of polymer stiffness on magnetization reversal of MREs has been investigated using a combination of magnetometry measurements and computational modeling. 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 two-dipole model that incorporates magneto-mechanical coupling not only confirms that micron-scale particle motion along the applied magnetic field direction plays a defining role in the magnetic hysteresis but also reproduces the observed loop shapes and widening trends for MREs with varying polymer stiffnesses. Measurements of the moduli and surface roughness of ultrasoft MREs at various H’s reveal a sensitive dependence on the magnetic particle concentration and H. As increases from 0 to 23%, ultrasoft MREs at =95.5 kA/m (1200 Oe) show an increase of ≈41×,11×,and 11× in their shear storage, Young’s modulus, and surface roughness, respectively. The moduli and surface roughness can be fit to quadratic functions of and H. The presented magnetic and mechanical properties of ultrasoft MREs provides the framework for applying the MREs as dynamic platforms in biomedical engineering. Ultrasoft MREs have been applied to investigate the response of cells to 2D and 3D dynamic mechanical stimuli. Furthermore, the field-dependent particle motion observed in ultrasoft MREs has inspired an application for creating 3D heterogeneous cellular gradients. This work was performed under the guidance of the author’s thesis advisor, Professor Xuemei Cheng