## 8 Conclusions  
This review paper highlights the advantages of flexible sensors. These advantages include the ease of fabrication of flexible sensors owing to the advancements in 3D printing methods and the ability to self-heal. Several examples of 3D printing methods that are currently used in manufacturing flexible sensors were discussed, including FDM, FFF, inkjet printing, aerosol jetting, SLM, DIW, SLS and others. Current materials being used in the manufacturing of flexible sensors were discussed, including polymers such as PDMS, PNIPAM, PVP, PET, PEN, PVA and Kapton. Nanomaterials are being incorporated into polymer matrixes to induce electrical properties including nanomaterials of carbon, silver and gold. Hydrogels have also gained much attention in flexible sensor literature, mainly due to their self-healing properties. Self-healing enables the sensor to recover from damages such as cuts and bends incurred during use, and this enables the sensor to have a longer operating life and higher sensitivity. Wearables have become part of our day-to-day life; these include smart watches and biomedical devices that provide real-time health and fitness data. Flexible sensors have become a topic of interest due to the rising demand in various fields, including stretchable wearables, health monitoring, packaging, soft robotics, \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Type of Sensor**} & **Sensitivity (Smallest** & **Stability/Minimum** & **Mean Response Time** \\  & **Detectable Quantity)** & **Bending Cycles** & **17 ms \\ \hline Pressure sensor. [12] & \(>\)1.14/ kPa & 50,000 bending cycles & 17 ms \\ \hline Respiration monitoring. [15] & 44\% relative humidity & 20 bending cycles & 8.9 s \\ \hline Capacitance pressure sensor. [27] & 0.86 pF/bar & n/a & n/a \\ \hline Piezoresistive pressure sensor. [28] & 8.5 mV/bar & 22 bending cycles & 15 s \\ \hline Pressure sensors. [29] & 0.02-0.55 /kPa & n/a & 1 ms–10 s \\ \hline Temperature sensors. [36] & 0.4 mV\({}^{\circ}\)/C & 600 h & n/a \\ \hline Temperature sensor. [41] & \(2.23\times 10^{-3}/^{\circ}\)C & \(-\)269 and 400 \({}^{\circ}\)C & n/a \\ \hline Pressure sensors. [47] & 1.80/kPa & 3000 bending cycles & 200 ms \\ \hline Temperature sensor. [48] & 0.214 \(\Omega^{\circ}\)/C & 35 to 45 \({}^{\circ}\)C & 1.6 s \\ \hline Pressure sensor. [50] & 0.7 kPa\({}^{-1}\) (up to 25 kPa) & 100,000 bending cycles & 50 ms \\ \hline Temperature sensor. [50] & 0.83\%/K & 22–70 \({}^{\circ}\)C & 100 ms \\ \hline Piezoelectric-heartrate monitor with & 3312 pC/N & 2500 bending cycles & n/a \\ \hline \multirow{2}{*}{Strain sensing with self-healing. [98]} & n/a & 1503\% stretchability, 184.1 kPa strength & 60 min (healing time) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Examples of wearables and some key characteristics studied in literature. electronic skins and weather monitoring. The most utilised polymer in the fabrication of flexible sensors is PDMS due to its biocompatibility, transparency, non-flammable properties, non-toxicity, self-healing, hydrophobic nature and stretchability. The most used nanomaterials are carbon-based due to the high conductivity and stability of carbon nanomaterials yet being low cost in comparison to silver or gold nanomaterials, which are also being utilised to induce electrical properties in the flexible sensors. FDM, FFF, inkjet printing and direct ink writing are the most used additive manufacturing methods for flexible sensors owing to their low cost, cheapness of materials and ease of fabrication. The main challenge in these printing methods is getting the materials to be in the correct range of rheological properties acceptable by the printer. Conceptualisation, A.N., M.V. and D.B. ; methodology, A.N., M.V. and D.B. ; validation, M.V., S.C., B.F. and D.B. ; formal analysis, M.V. and D.B; investigation, A.N., M.V., S.C., B.F. and D.B. ; resources, M.V., S.C., B.F. and D.B. ; data curation, A.N., M.V. and D.B. ; writing--original draft preparation, A.N.