## 5 Additively Manufactured Flexible Sensors (3D Printing)
The inks have an added advantage because they can be processed via various AM methods, including cheap and easy to implement methods such as inkjet printing. Standard desktop inkjet printers can be used to print conductive patterns, even on simple substrates such as paper. The main challenge in inkjet printing conductive nanocolloids is getting the ink to a particular range of viscosity values acceptable by the printer. Highly viscous organic liquids such as glycerol have been used in pursuit to control the viscosity of the conductive ink, however care has to be taken such that the glycerol does not inhibit the conductive properties of the ink. In another report, six functional inks, including a piezoelectric and a conductive ink, were used in fabricating soft strain gauge sensors within micro-channels [74]. These sensors have potential use in the fields of toxicology and drug screening. In another study, nanoinks of carbon, silver and manganese dioxide were inkjet printed on cellulose-based paper to fabricate supercapacitors and other sensors [81]. Kapton was also used as a substrate in the inkjet printing of silver nanoinks to develop a sensor [41]. Another innovative flexible sensor fabricating technique involves using an AM method such as SLA to 3D print flexible devices from polymers which are then coated with a conductive ink of silver/carbon nanoparticles to introduce electrical properties. The polymers can have shape memory effects which makes the device intelligent and self-powering. For example, flexible temperature sensors were fabricated by coating a 3D printed shape memory polymer with a silver nanoink via a sintering technique at room temperature [59]. The shape of the sensor changes upon increasing/decreasing the temperature. The sensor is shown in Figure 5b. It is shaped in such a way that when the temperature is increased its shape changes from the top shape in Figure 5b to the bottom shape, enabling for a conductive path to be created. The sensor can be used in detecting temperature changes by opening/closing a circuit. It can be used as a thermometer or as a safety feature at a production plant or in a device. Polymers with shape memory behaviour enable the fabrication of intelligent devices that can self-power and be controlled remotely. The ability to self-power further reduces the volume and bulkiness of the device which opens new application opportunities for sensors whereby weight reduction is imperative (e.g., the aerospace industry). A shape memory sensor based on polyurethane and carbon black was printed via FDM [82]. The device's shape memory effect is triggered by sunlight which enables it to be self-powering. The device has potential use in temperature sensing and weather monitoring. A review of current state of the art shape memory nanocomposite devices that are currently used in various fields, including heath monitoring, energy storage/harvesting and sensing technologies, was published in early 2021 [56]. This review touches on the area of 4D printing, an interesting topic at the moment. In another report, a 3D printed flexible glove was embedded with a temperature and other sensors to create a smart wearable medical device which is tailored to the condition of the patient [83]. The glove has conductive channels that accommodate a heater, sensors, actuators, resistors, antennas and capacitors, generating a smart, flexible and tailored medical device. The glove is used for comfort thermotherapy of the patient's hand. The heat produced by the electronics could help treat injured areas by increasing blood-flow and relieving pain. Similarly, a flexible prosthetic hand made from Tango plus (a flexible material) was 3D printed using a commercial 3D Polylet printer (Eden260V) [84]. A flexible temperature sensor was embedded onto the surface of the prosthetic via hydrogen bonding. The prosthetic hand is capable of retaining its original electrical properties at various hand signals. The 3D printed hand could sense the temperature of a human hand accurately.