**4.1 Material advancements**

There are several challenges associated with 3D printing, such as emission of volatile organic compounds, creation of voids, and high cost of thermoplastic polymers. To avoid all these issues, recent advancements have been done which include the use of fillers, such as carbon fibers, nanofibers, graphite, and diatomaceous earth [9, 10]. Carbon nanotube/polylactic acid composites (CNT/PLA) and multi-walled carbon nanotube/ polylactic acid composites (MWCNT/PLA) with strong mechanical properties are being explored in microelectronics [19]. The smaller particles sizes are used in composites to produce stiffness and high density in the printed products, such as hydroxyapatite-reinforced polyethylene/polyamide composites (HA-PE/PA) [20]. Carbon black/ polyamide 12 (CA/PA12) composites were fabricated which enhance the mechanical, thermal, and electrical properties of printed products [21]. Nanomaterial composites, such as nanosilica/polyamide, nanoclay/polyamide, and graphite nanoplatelets/polyamide composites, have also been fabricated leading to improved mechanical properties [9]. These composites can be used for multiple applications, such as biomedical applications, because of the high surface area of fillers present (**Figure 3**) [10].

**Metamaterials:** Metamaterials are another innovation provided by 3D printing in material science. The functional metamaterials are considered a complex machines working. For instance, there is a material developed by the Lawrence Livermore National Labs where it gets shrunk when heated up rather than expanding. USC Viterbi has created a metamaterial that can manipulate sound through the magnetic field. Metamaterial has the iron particles in its lattice structure and in the presence of a magnetic field the structure gets deformed into one which blocks the sound rather than passing them through. Similarly, researchers at Boston University have developed the metamaterial by 3d printing the plastic coil used in metamaterial which has improved the MRI scan quality and speed and also blocks 94% sound [22].

**Titanium Drones:** Titomic has recently used their Titomic Kinetic Fusion Process to 3D print the titanium drone by mechanically fusing the titanium powder. This innovative 3D-printing method allows to use of different materials or alloys in a single prototype and eliminated the problems associated with traditional manufacturing, such as welds (**Figure 4**) [22].
