**Preface to "3D Printing Technologies"**

The family of technologies collectively known as additive manufacturing (AM) technologies, and often called 3D-printing technologies, is rapidly revolutionizing industrial production. AM's potential to produce intricate and customized parts starting from a digital 3D model makes it one of the main pillars for the forthcoming Industry 4.0. AM also offers the opportunity to precisely engineer the properties of printed parts, with optimal material usage and an application-tailored design. Finally, the great variety of 3D-printing techniques available makes the use of many different materials possible: metals, polymers, ceramics, biomaterials and even living cells.

Thanks to these advantages over traditional manufacturing methodologies, AM finds potential applicability in virtually all production fields. As a natural consequence of this, research in this field is primarily focused on the development of novel materials and techniques for 3D printing. The importance of AM in the scientific community is attested by the thousands of papers published under the corresponding keywords in the last few years. This topic also caught the attention of major scientific publishers, resulting in the dissemination of specialized journals and dedicated books.

This Special Issue of *Technologies*, titled "3D Printing Technologies", aims at promoting the latest knowledge on materials, processes, and applications for AM. It is composed of six contributions, authored by influential scientists in the field of advanced 3D printing. The first paper, by Dr. Statnik et al., investigates the mechanical properties of metallic parts 3D printed using selective laser melting. In the second contribution, Dr. Koske et al. compare the influence of different infill designs on the mechanical properties of polymeric objects realized via fused deposition modeling. The third contribution of this Special Issue, proposed by me in collaboration with my coauthors, fits into the topic of AM for biomedical applications. In particular, it describes the 3D printing and wet metallization of untethered microdevices carrying hydrogel layers for controlled drug release. Additionally, the fourth contribution, proposed by Dr. Leonardi et al., regards the applicability of AM to the biomedical field. Indeed, Dr. Leonardi and her coauthors review the use of 3D-printed models for the preoperative planning and surgery of the pathology known as pectus excavatum. The last two contributions of this Special Issue deal with a fundamental aspect of polymeric-based AM technologies: the possibility to metallize the surface of the objects obtained. Dr. Kołczyk-Siedlecka et al. follow a wet metallization approach to realize decorated catalysts for methanol electro-oxidation, while Dr. Romani et al. use a dry approach (physical vapor deposition) for the metallization of thermoplastic polymers and composites.

The audience of this Special Issue includes professors, graduate students, researchers, engineers and specialists working in the field of AM. Finally, as the Guest Editor of this Special Issue, I would like to acknowledge the great efforts made by the authors to produce the high-quality research here presented. It takes a considerable amount of time to write, submit and review the manuscripts and recognizing this work is fundamental.

> **Roberto Bernasconi** *Editor*
