## 5 Additively Manufactured Flexible Sensors (3D Printing)  
Additive manufacturing (AM), also called 3D printing, has many advantages over other manufacturing methods, such as subtractive manufacturing, dye casting and moulding. These advantages include its flexibility in design and material savings. The field of AM has grown so much that there now exists a copious amount of AM methods including fused deposition modelling (FDM), selective laser melting (SLM), electron beam melting (EBM), aerosol jetting, inkjet printing and many others. The development of new AM methods has allowed various materials to be processed, including polymers, metallic nanoinks, ceramics, nanocomposites and alloys. Various AM methods are being used to produce flexible sensors. Extrusion methods such as FDM/FFF are the most used AM methods for processing polymers. FDM involves the melting and extrusion of a polymer filament onto the print bed in a layer-by-layer fashion to build the part according to the CAD file. FFF has the same format except that the filament in FFF is usually custom made to produce certain mechanical/electrical/chemical properties in the filament before the printing process. During FFF of flexible sensors, the molten polymer filament is usually mixed with 56 conductive nanomaterials, such as graphene or silver nanoparticles, before printing the sensor according to the CAD file. By controlling the polymer-nanomaterial ratio, the CAD file (printing pattern) and the printing parameters (print speed, bed temperature, etc. ), one can directly control the properties of the printed sensor such as resistivity, sensitivity, size and rigidity. This enables the development of complex shaped and tailored sensors, which is otherwise difficult to achieve with conventional methods. Many examples of 3D printed sensors exist in the literature; the field has grown very much in the last two decades due to the increased availability and declining costs of 3D printers. One can now purchase a standard FDM desktop 3D printer online at under EUR 500, and the cost is predicted to continue falling due to the increased number of 3D printer manufacturers. An interesting example of the use of AM is an inductive sensor that was 3D printed via coaxial extrusion method [73]. The sensor was fabricated by extruding silicon rubber and gallium-indium alloy liquid at the same time. The sensor was installed on a human finger and could capture different degrees of bending. A similar example involved an FDM 3D printer that was used to print structures which were then combined with liquid metal to pattern conductive patterns with microstructures [74]. Flexible sensors can be easily fabricated by combining 3D printed flexible structures with basic micro resistors, capacitors and inductors. Commercial polymer filaments for FDM include acrylonitrile butadiene styrene (ABS), polylactic acid (and versions of these with carbon nanoparticles incorporated), polycarbonate (PC), PU, polyphenylsulfone, poly (ether ether ketone) and poly(ether imide)s. In one report, FDM was used in fabricating capacitive and piezoresistive sensors composed of polycaprolactone infused with carbon black to induce electrical properties [75]. First, the polymer and the carbon black were mixed and moulded into a filament of 1.5 mm diameter to match the requirements of a desktop 3D printer. The resulting filament was then extruded and printed to form a sensor that exhibited piezoresistive properties. Another research team developed a polymer nanocomposite composed of polyurethane infused with MWCNTs. FDM was then used to print the nanocomposite into piezoresistive strain sensors [76]. The printing of multiple materials simultaneously to fabricate a multi-material object has gained much attention in the literature. This is because multi-material objects enable the fabrication of objects that have versatile properties.