Aliphatic T1-Positive Contrast Agents’ Behaviors and Magnetic Resonance Imaging Performances in Triblock Copolymer Polymeric Implant Microphase-Separated Structures

Abstract

Polymeric materials have been widely used to fabricate various implant materials, but such implants may undergo purposeful or accidental degradation, displacement, or performance changes in vivo. Therefore, it is essential to monitor conditions and performances of polymer implants. For medical imaging, magnetic resonance imaging (MRI) is highly valued for its advantages of being radiation-free and offering high resolution. However, MRI cannot directly detect the positive contrast signal from polymeric implants. Thus, additional contrast agents in or on the polymeric implants are required to provide clear location information on the body. The traditional contrast agents are mainly metal-based, such as gadolinium (Gd), manganese (Mn), and iron oxide, facing stability and safety issues and thus causing allergic reactions. Therefore, in this study, stearate methacrylate (SMA), an aliphatic molecule with higher biocompatibility than metal-based contrast agents, is chosen as the T₁ contrast agent. For better contrast agent encapsulation, phase-separated triblock copolymer polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene (SEBS) is used as a polymeric matrix. Herein, this study investigates the mechanical properties and MRI performance of the designed polymeric implant model, SEBS/SMA electrospun fibers and thin films. To investigate the effects of SMA on the microphase-separated structures of the SEBS matrix, small-angle neutron scattering (SANS) technique was applied to determine the self-assembled structures of SEBS. The SEBS/SMA fibers prepared by electrospinning sequentially formed lamella, hexagonal close-packed cylinder and face-centered cubic (FCC) phase-separated structures by increasing SMA loading, while thin films prepared by blade-coating also formed lamella and then FCC as the SMA content increased. And it was confirmed that the different microphase-separated structures truly affect the mechanical properties and the MRI performance. Further analysis demonstrated that SMA tumbling was impeded by the SEBS matrix, which was corresponding to the confined crystallization finding confirmed by differential scanning calorimetry.