High-Tg Thiol-Click Thermoset Networks via the Thiol-Maleimide Michael Addition.

Thiol-click reactions lead to polymeric materials with a wide range of interesting mechanical, electrical, and optical properties. However, this reaction mechanism typically results in bulk materials with a low glass transition temperature (Tg ) due to rotational flexibility around the thioether linkages found in networks such as thiol-ene, thiol-epoxy, and thiol-acrylate systems. This report explores the thiol-maleimide reaction utilized for the first time as a solvent-free reaction system to synthesize high-Tg thermosetting networks. Through thermomechanical characterization via dynamic mechanical analysis, the homogeneity and Tg s of thiol-maleimide networks are compared to similarly structured thiol-ene and thiol-epoxy networks. While preliminary data show more heterogeneous networks for thiol-maleimide systems, bulk materials exhibit Tg s 80 °C higher than other thiol-click systems explored herein. Finally, hollow tubes are synthesized using each thiol-click reaction mechanism and employed in low- and high-temperature environments, demonstrating the ability to withstand a compressive radial 100 N deformation at 100 °C wherein other thiol-click systems fail mechanically.

Thiol-epoxy/maleimide ternary networks as softening substrates for flexible electronics.

Softening microelectrode arrays, or flexible bioelectronic systems which can dynamically change modulus under the application of an external stimulus such as heat or electromagnetic radiation, have been of significant interest in the literature within the previous decade. Through their ability to actively soften in vivo, these devices have shown the capacity to attenuate the neuronal damage associated with insertion of rigid microelectrode arrays into soft tissue. Thiol-click substrates specifically have shown particularly impressive results for fabricating devices requiring small-scale, high-performance electronics for neural recording. However, previous attempts to engineer increasingly lower-modulus substrates for these devices have failed due to the fundamental chemistries' (the thioether linkage) flexibility. This failure has led to substrates without sufficient mechanical rigidity for penetrating soft tissue at physiological temperatures, or sufficient softening capacity to reduce the mechanical mismatch between soft tissue and implantable device. In this work, a ternary thiol-epoxy/maleimide network is investigated as a potential substrate materials space in which the degree of softening can be modulated without sacrificing the mechanical rigidity at physiological temperatures. Using these networks as platforms for the microfabrication of electrode arrays, example implantable intracortical microelectrode arrays are fabricated on both thiol-epoxy and thiol-epoxy/maleimide networks to demonstrate the insertion capacity of microelectrode arrays on the ternary polymer networks.

Hydrolytically Stable Thiol-ene Networks for Flexible Bioelectronics.

Hydrolytically stable, tunable modulus polymer networks are demonstrated to survive harsh alkaline environments and offer promise for use in long-term implantable bioelectronic medicines known as electroceuticals. Today's polymer networks (such as polyimides or polysiloxanes) succeed in providing either stiff or soft substrates for bioelectronics devices; however, the capability to significantly tune the modulus of such materials is lacking. Within the space of materials with easily modified elastic moduli, thiol-ene copolymers are a subset of materials that offer a promising solution to build next generation flexible bioelectronics but have typically been susceptible to hydrolytic degradation chronically. In this inquiry, we demonstrate a materials space capable of tuning the substrate modulus and explore the mechanical behavior of such networks. Furthermore, we fabricate an array of microelectrodes that can withstand accelerated aging environments shown to destroy conventional flexible bioelectronics.