Biological structures have emerged through millennia of evolution, and nature has fine-tuned the material properties in order to optimise the structure-function relationship. Following this paradigm, polydopamine (PDA), which was found to be crucial for the adhesion of mussels to wet surfaces, was hence initially introduced as a coating substance to increase the chemical reactivity and surface adhesion properties. Structurally, polydopamine is very similar to melanin, which is a pigment of human skin responsible for the protection of underlying skin layers by efficiently absorbing light with potentially harmful wavelengths. Recent findings have shown the subsequent release of the energy (in the form of heat) upon light excitation, presenting it as an ideal candidate for photothermal applications. Thus, polydopamine can both be used to (i) coat nanoparticle surfaces and to (ii) form capsules and ultra-small (nano)particles/nanocomposites while retaining bulk characteristics (i.e., biocompatibility, stability under UV irradiation, heat conversion, and activity during photoacoustic imaging). Due to the aforementioned properties, polydopamine-based materials have since been tested in adhesive and in energy-related as well as in a range of medical applications such as for tumour ablation, imaging, and drug delivery. In this review, we focus upon how different forms of the material can be synthesised and the use of polydopamine in biological and biomedical applications. platform is highlighted ( Figure 1A). Then, theoretical and experimental proof of the chemical formation of polydopamine-based structures is summarised ( Figure 1B,C). This is then followed by descriptions of the unique (physicochemical) properties of polydopamine. Their applications as adhesive and in energy-related applications are also highlighted ( Figure 1D). Throughout, their wide range of applications in the biomedical field (thanks to their biocompatibility) is thoroughly explained. This includes examples of theranostic and drug delivery applications derived from different types of polydopamine-based configurations: adhesive, particle coating agent, capsule, and NPs/composites, respectively ( Figure 1C,E). Lastly, some challenges to make PDA into functional tools for future clinical applications are envisaged. Materials 2019, 12, x FOR PEER REVIEW 2 of 22polydopamine-based structures is summarised ( Figure 1B,C). This is then followed by descriptions of the unique (physicochemical) properties of polydopamine. Their applications as adhesive and in energy-related applications are also highlighted ( Figure 1D). Throughout, their wide range of applications in the biomedical field (thanks to their biocompatibility) is thoroughly explained. This includes examples of theranostic and drug delivery applications derived from different types of polydopamine-based configurations: adhesive, particle coating agent, capsule, and NPs/composites, respectively ( Figure 1C,E). Lastly, some challenges to make PDA into functional tools for future clinical applicatio...