## 5 Additively Manufactured Flexible Sensors (3D Printing)
For instance, a flexible polymer and a conductive material can be printed simultaneously to produce a multi-material object with the flexibility properties of the polymer and the conductive properties of the conductive material. Multi-material printing has enabled the growth of 4D printing technologies whereby the printed part exhibits some intelligent or smart feature such as shape memory or colour changes upon exposure to certain stimuli [56]. An FDM printer that can print two materials simultaneously was used to process a polyurethane filament in one nozzle and a carbon nanotube composite filament in another nozzle to fabricate a flexible force sensor [77]. The ability to print multiple materials saves printing time in comparison to one-material printers. It also enables innovation and more complexity in design (e.g., micro channels with conductive material within a polymer object.) Although FDM has several advantages including the low cost of printers and materials, it has two disadvantages, namely the requirement of filaments of a certain diameter and a certain range of melting points and it can only process materials with a certain range of rheological properties. On the other hand, direct ink writing (DIW) printing method overcomes these limitations. Various polymers, conductive nanofillers, elastomers, hydrogels and nanocolloids can all be printed via this method, demonstrating its superior versatility over FDM. The material can be printed at room temperature in the form of droplets, filaments or aerosols in building the part in a layer-by-layer fashion. The inks for DIW are categorised as metal, MXenes and conductive polymer composites. The conductive properties of the printed objects depend on the electron transfer ability of the printed ink. Nanoparticle inks of carbon and silver are the most prevalent in flexible sensor literature, owing to their high conductive properties, stability after printing and low cost (carbon inks). Metal nanoinks (e.g., copper, titanium, magnesium and iron) are seldom used because, although their conductive properties are superior to those of carbon/silver, they are easily oxidised in the presence of moisture which makes them lose their electrical capabilities, rendering them unusable in this case. In addition, metal leakage could be detrimental to the environment. Strain sensors based on flexible PDMS and silver nanoinks have been successfully fabricated via DIW [78]. MWCNTs have been used in flexible sensor technology. MWCNTs have the ability to entangle nicely through hydrogen bonding with polymers such as PVP and PVA, which enables the carbon nanomaterials to be well dispersed and integrated into the polymer matrix. This allows for the development of conductive polymer composites that can be used in FDM, FFF and DIW 3D printing. In another report, MWCNTs were dispersed in a chitosan matrix with citric acid, acetic acid and lactic acid and used to fabricate a strain sensor [79]. The sensor has self-healing capabilities and is water driven, enabling it to have a long operating life and potential to be self-powering. The high fabrication cost of flexible sensors had been the major concern until the introduction of additive manufacturing into the field. Nowadays, customised sensors are built rapidly with ease thanks to 3D printing given how straightforward it is to manipulate a CAD file in comparison to traditional methods such as moulding or dye casting. Force sensors are extensively used in fields such as robotics and health monitoring whereby the forces detected by the sensor are translated to information relating to the robot/human movements. Many force sensors have been fabricated via 3D printing in literature [4, 80]. Other materials that have gained incredible interest in the field of flexible sensors are conductive nanoinks (nanocolloids), owing to their superior electrical and plasmonic properties over solid materials such as wires.