Liquid Metals as Initiators of Free‐Radical Polymerization of Hydrogels: A Perspective

Abstract This perspective article highlights an interesting and useful discovery: gallium‐based liquid metals can initiate free radical polymerization to form hydrogels. Normally, hydrogels are polymerized using molecular initiators that generate radicals in response to heat or light. Liquid metal particles can replace the role of molecular initiators and are interesting for several reasons: 1) Liquid metal particles are easy to make by stirring or sonicating liquid metal in a non‐solvent such as water. 2) Conventional initiators that remain in hydrogels can be problematic (e.g., toxic); in contrast, some gel formulations can slowly dissolve the liquid metal particles as salts. 3) The polymers formed using liquid metals are, in some cases, softer, more extensible, and tougher than the same polymer formed using conventional molecular initiators and crosslinkers. 4) In addition to initiating polymerization, in some cases, the liquid metal particles appear to serve as crosslinkers through interactions with the polymers or by releasing Ga 3+ ions that can ionically crosslink negatively charged polymers. The purpose of this perspective is to summarize what is and what is not understood about liquid metal initiators for forming hydrogels (and several other polymers) while highlighting opportunities for further research.