The term eutectic, or eutaxia, is not just a recent occurrence within the scientific community. Reports from Frederick Guthrie, a British physicist in 1884 coined the term to describe binary (or multiple component) systems which had “a lower temperature of liquefaction than that given by any other proportion” i.e., melting at a lower temperature than either one of its parent components [1]. He combined two Greek terms “ευ” meaning easy and “τηκτος” which means melting and formed the term eutectic to refer to the common feature of melting point (MP) depression of multicomponent systems that are produced upon combination in specific ratios. Such MP depression is commonly thought to be due to certain types of non-covalent interactions between the components. Eutectic changes in MP are classified into simple/ideal depressions that usually produce solid ideal eutectic mixtures and non-ideal depressions where the eutectic point is far below MP of original components [2].

Eutectics gained much interest recently as a useful mean to improve performance in different fields like pharmacy, polymer chemistry, analytical separation, biology and chemical reactions. Pharmaceutically, eutectics are known to enhance the biopharmaceutical behaviour of drugs, improve drug dissolution and permeability and to act as drug carriers. Moreover, eutectics have clear thermodynamic advantage over the commonly applied drug enabling techniques. They are recognizable as stable system that neither have tendency to revert to the original state, nor need extra stabilization by incorporating polymers. Adding this to their simple preparation approaches, they represent excellent candidate for drug characteristics manipulation [3], [4], [5], [6], [7], [8]. Since the growing number of reports employing eutectic property in the pharmaceutical field, a clear distinction between different types of eutectics should be made. Similar distinction should be made between eutectics and other binary pharmaceutical systems like non-covalently bonded co-crystals or ionically bonded salts and ionic liquids (ILs) [9], [10].

In the report by Abbott and co-workers, the term deep was used to describe the profound non-ideal depression in MP when choline chloride (MP 302 oC) and urea (MP 133 oC) were combined at 1:2 molar ratio. The depression was deep enough to transform these solids into clear liquid at ambient temperatures which solidified only when cooled to 12 oC or below, hence they first introduced the term deep eutectic solvents (DES) to stand out the novel characteristics of this liquid and its solvent properties [11]. Liquid systems are usually neglected during pharmaceutical production, and they were discarded or perceived as products to be avoided during traditional manufacturing procedures due to challenges associated with liquid handling and characterization and the lack of knowledge about their benefits. Moreover, comprehensive investigation of microstructure and thermodynamic functions of therapeutic eutectics especially their liquid products is usually overlooked in the pharmaceutical reports and reviews [12], [13]. Accordingly, this review will focus on the liquid deep eutectics that possess melting transition temperature (Tm) below the ambient temperature i.e., deep eutectic solvents, and the abbreviation DES will be used to refer to this class, while giving an in-depth look at their thermodynamics, microstructure and therapeutic applications.