## 3 Nanomaterials
### Carbon Nanomaterials and Others
(**a**) PDMS/Ag nanoparticles on PDMS layer, reprinted with permission from [48]. (**b**) Carbon nanomaterials on a shape memory polymer capable of detecting temperature changes by opening/closing an LED circuit. The sensor changes shape when the temperatures rises/falls, thereby switching a circuit on/off, reprinted with permission from [59]. Figure 6: A shape transforming sensor triggered by light, reprinted with permission from [58]. (**a**) Drop-casting fabrication method, nanocolloid is deposited onto a flexible substrate. (**b**) Different shape transformations of the sensor depending on the position on which the sensor is exposed to light. sensor to have versatile uses including stimuli-responsive soft robotics, agricultural and biomedical actuators and tissue engineering. The research team used the CELLINK BIO X 3D printer (CELLINK, Sweden) to print 10 \(\times\) 10 mm cubes with various infill density (10-75%). The nanoparticle concentration was varied from 7.5-15% \(w/w\). It was found that an increase in nanoparticle concentration increased the responsiveness (measured as bending angle towards the magnet) of the fabricated sensor when subjected to a rectangular-shaped neodymium magnet (20 \(\times\) 50 \(\times\) 5 mm), as shown in Figure 7. The bend angle of the sensor towards the magnet increased with decreasing infill (lower amount of hydrogel). The sensor has potential for remote actuation and can be made into various shapes by means of 3D printing. Various analytical techniques, including TEM, SEM, XRD, attenuated total reflectance-Fourier transform infrared (ATR-FTIR), were used to analyse the nanoparticle size/morphology/chemical composition. Magnetic measurements were performed using a MPMS-XL (Quantum Design) SQUID magnetometer. Other characterisation techniques included compression tests, viscosity measurements and a thermal stability test. All the aforementioned characterisation techniques gave insights to the stability, usability and robustness of the sensor in a real-world application. Gold nanomaterials have good mechanical properties and have better electrical conductivity than carbon nanomaterials. However, gold is much more expensive than carbon and its superiority in electrical properties usually does not always justify its massive cost in comparison to carbon. In a published paper, Gold nanowires (Au NWs) impregnated in a tissue paper were sandwiched with PDMS films to fabricate a flexible pressure sensor with a fast response time of less than 17 ms [3]. The sensor can detect pressures down to 13 Pa, has a high sensitivity of over 1.14 KPa\({}^{-1}\) and has a high stability of over 50,000 loading-unloading cycle. In another report, a silver nanoparticle-based temperature sensor was fabricated via inkjet printing on paper. The sensor can sense temperatures in the range of \(-20\) to 60\({}^{\circ}\) with excellent linearity [14]. Similarly, spherical silver nanoparticles impregnated in a poly(N-isopropylacrylamide) (PNIPAM) matrix were used to fabricate a light  
Figure 7: A magnetically actuated sensor based on flexible hydrogel (alginate and methylcellulose hydrogel) and magnetic nanoparticles (MNPs). The responsiveness of the sensor (measured as bending angle towards the magnet) is proportional to the concentration of the MNPs, reprinted with permission from [60]. sensor that can transform to multiple shapes depending on the light pattern [61]. In another report, Au nanorods incorporated in PNIPAM matrix were used to develop a light/thermal responsive sensor with competitive response speed and potential for remote actuation [62]. Similarly, gold nanoparticles were used to fabricate electrodes that were incorporated into a PNIPAM bi-layer to develop a flexible sensor with temperature and pH responsivities [63].