## 4 Polymers
### Poly(N-Isopropylacrylamide) (PNIPAM)  
Poly(N-isopropylacrylamide) (PNIPAM) is a temperature responsive polymer, making it a good candidate for temperature sensors. Its low density of 1.1 g/cm\({}^{3}\) and flexibility have led to its use in flexible sensors [61; 70; 71]. The main challenge when using PNIPAM in sensors is to achieve fast responses and integrity during shape changes. To counteract this, researchers have been incorporating nanomaterials of graphene oxide [70], silver [61], gold [62; 72] and others [71] to develop new advanced composites. These composites have better physicochemical properties than the isolated components. The added nanomaterials increase conductivity thereby increasing the sensitivity of the sensor and, in some cases, the added particles introduce a new functionality such as light or pH responsiveness. Such is the case in a study by [PERSON] et al. [62], when gold nanorods were incorporated into the polymer matrix via electrospinningning to develop a composite that is not only temperature responsive but also light responsive due to the surface plasmon resonance of gold nanorods. The added nanomaterials also increased the sensitivity of the sensor. The sensor could then increase its temperature from room temperature to 34.5 \({}^{\circ}\)C in 1 s and further increase this temperature to 60 \({}^{\circ}\)C in 5 s during laser irradiation. This sensitivity upgrade is due to the remarkable property of gold nanomaterials being able to absorb near-infrared (NIR) irradiation and turn it into heat energy in a short period of time. This heat energy is then used to heat up the thermoresponsive polymer to achieve a quick response. The research team further concluded that these Au/PNIPAM composite-based sensors can be easily mass produced due to the high feed rate of 0.6 mL/h and high productivity rate of over 20 cm\({}^{2}\) /h via electrospinning.  
In another report, PNIPAM impregnated with silver nanoparticles was used to fabricate a light responsive sensor capable of transforming into many shapes, including boat-like, hoof-like, and helical and saddle-like structures [61]. The research team claim that the sensor can transform to an infinite number of shapes depending on the light pattern (see Figure 6b). The sensor can revert to its original flat shape upon removal of the light source. The sensor responds to near-infrared (NIR) light. NIR light is preferable to UV or visible light as an actuation method because it is less harmful. A laser of wavelength 808 nm was used to irradiate the sensor for actuation and depending on the light irradiation pattern. A unique feature of this sensor was the introduction of pores (200 nm-1 \(\upmu\)m) which led to the reduction in response time of the sensor. The pores increased the rate of water molecules transport when the polymer undergoes swelling or shrinkage thereby increasing the speed of shape transformation. The speed of shape transformation was also increased by the introduction of the silver nanoparticles. Silver nanoparticles can absorb NIR light and can turn it into heat energy quickly. The heat is then used to locally heat the polymer thereby inducing shape changes when the temperature goes above 32 \({}^{\circ}\)C, the lower critical solution temperature of PNIPAM. The pores were fabricated by the addition of poly-(ethylene glycol) (pore-creating reagent) to the hydrogel and followed by UV photopolymerisation and washing of the composite thereafter to remove the PEG, leaving sub-micron pores. The advantages of this sensor are its competitive response time, potential for remote actuation and the low actuation temperature (32 \({}^{\circ}\)C). Its disadvantage is the need for a constant stimulus for a transformed shape to remain. However, the sensor can be used in normally open switching mechanisms.