What is a co-crystal?

This is the authors' abstract. We don't add key points for this paper.

What might very well be classified as a revolution in chemical crystallography is the emergence of crystal engineering as a discipline and the pivotal role X-ray crystallography plays in this [1]. Although dating back to the 1950's, the field has exploded in the last decade or so, and now is at the forefront of contemporary chemical and cognisant research. One of the areas in crystal engineering of greatest potential impact relates to the pharmaceutical industry – drug formulation and intellectual property – where issues relating to crystallisation, polymorphism, etc. are paramount [2]. A focus of these crystal engineering studies relates to the preparation of multi-component crystals, often referred to as co-crystals, where, for example utilizing hydrogen bonding synthons, active pharmaceutical ingredients are co-crystallised with other molecules [2]. The motivation for such studies is to improve stability ( e.g. shelf-life), bioavailability ( e.g. solubility), etc . [3].