3. Challenges and Perspectives of the Polymer-Induced Liquid-Precursor Process: The Pathway from Liquid-Condensed Mineral Precursors to Mesocrystalline Products

The polymer-induced liquid-precursor (PILP) process falls within the category of nonclassical pathways of crystallization and proceeds in a colloid-mediated fashion via a liquid amorphous intermediate. By addition of tiny amounts of polyionic polymers like polyaspartate, polyamines or selected biomineralization proteins, classical nucleation of a solid crystalline phase is suppressed which, in turn, promotes the formation of a liquid-condensed phase of mineral precursor. This unusual ion-enriched liquid-amorphous phase becomes the crucial agent of the precipitation reaction; the process of mineralization is converted from a solution crystallization process to a pseudomorphic solidification process. This change of pathway provides efficient means to synthesize an impressive multitude of mineral morphologies, many of which mimic the features long considered enigmatic in biominerals. Mosaic and mesocrystalline thin films, replicas, hierarchical microspheres or fibrous mineral structures – all these non-equilibrium and non-facetted morphologies can be readily generated by means of the PILP process. In this chapter, we will review our current state of knowledge of this extraordinary crystallization pathway with a special regard to the as of yet unanswered questions. We will discuss the mechanistic foundations of the PILP process and highlight its ultimate provenance in the unexpected liquid/liquid phase separation of mineral solutions, which even occur under additive-free conditions, but do not seem suitable for morphosynthetic exploitation if generated in the absence of polymeric additives.