Flexible tactile sensors with high sensitivity and good flexibility are urgently required in modern biomimetic robots’ electronic skin, and artificial intelligence interaction. In this work, a flexible piezoelectric tactile sensor was fabricated using hydrothermally grown ZnO nanorods (ZnO NRs) on Cu-coated polyimide (PI) substrate. Structural and morphological properties of the ZnO NRs were investigated by SEM, XRD. The piezoelectric response properties of the sensors by using the pasted Cu/PI electrode and directly screen-printed Ag electrode were compared. The results show the sensor with the screen-printed Ag electrode exhibits a high sensitivity of 974 mV/N, which is higher than the pasted Cu/PI electrode, and the response of all sensors to the applied pressure is almost linear in the range of 0.1-1 N. Meanwhile, the sensor can be employed to detect human movements such as bending/stretching motion of fingers and the vibration of the vocal cords and pulse, and realize the conversion from mechanical energy to electrical energy. It is demonstrated that the sensor has great potential applications in wearable health monitoring systems and self-powered devices.

A flexible pressure sensor with co-planar electrodes was fabricated based on piezoelectric nanocomposites by the combination of polydimethylsiloxane (PDMS) base polymer with ZnO nanotetrapods (ZNTs) fillers. ZNTs were synthesized by an oxidation process of Zn powder at different temperatures over 900 °C in air, and its crystallinity and structural morphology could be controlled effectively by the growth temperature based on scanning electron microscopy, X-ray diffraction, and photoluminescence spectroscopy analyses. The effects of the growth temperature and filler concentration of ZNTs on the piezoelectric properties of the nanocomposite sensors were investigated. The sensors exhibit good linear response to external pressure less than 14 N, and the sensitivity increases with the increase of the growth temperature and ZNTs filler concentration due to the improvement in the crystallinity, the aspect ratio of T-arms and the interconnectivity between ZNTs/ZNTs interfaces in the nanocomposite film. The sensor can be employed to detect human movements such as bending/stretching motions of fingers and the vibration of vocal cords. It is demonstrated that the sensor has potential applications in hand gesture and speech recognition.

Low-voltage transparent zinc oxide (ZnO) thin-film transistor with chitosan biopolymer electrolyte as a gate dielectric was fabricated at room temperature and characterized. Due to an electric-double-layer (EDL) effect near the chitosan/ZnO interface through mobile-proton migration, the transistor exhibits a good performance with a carrier mobility of 7.88 cm ² V ⁻¹ s ⁻¹ , a threshold voltage of 1.0 V, an on-off current ratio of 5 × 10 ⁴ , a subthreshold slope of 135 mV Dec ⁻¹ , and an operating voltage of less than 3 V, respectively. Based on the EDL transistor, the excitatory post-synaptic current and paired-pulse facilitation (PPF) behavior of biological synapses are mimicked, and PPF index is related to the inter-spike interval of the paired pulse. Furthermore, the resistor-loaded inverter based on the transistor was built, showing a high voltage gain of ∼4 at a low supply voltage of 3 V, a small delay, a large noise margin and a low power consumption. The EDL transistor may find potential applications in portable electronics and synaptic electronics.

Development of high-performance nano-ZnO based piezoelectric tactile sensors with outstanding sensitivity, good linearity, flexibility, durability and biocompatibility has a great significance for smart robotics, human healthcare devices, smart sensors and electronic skins. A flexible piezoelectric pressure sensor with a sandwich structure was fabricated by using PI/Cu film as top and bottom electrodes, Mg-doped ZnO nanorods (MZO NRs) as sensing layer and polydimethylsiloxane polymer as charge coupling layer. The MZO NRs were synthesized by a one-step hydrothermal method and their structure, morphology and defect states were characterized by using scanning electron microscopy, atomic force microscope and x-ray photoelectron spectroscopy. The piezoelectric properties of the flexible tactile sensors based on MZO NRs hydrothermally synthesized from different Mg²⁺/Zn²⁺ molar ratios were investigated comparatively by evaluating their force response actions. The results indicate that the sensors exhibit a linear response in the range of 0–1 N. The device based on MZO NRs can achieve an ultra-high sensitivity of 3.1 V N⁻¹ (308.5 mV kpa⁻¹) due to higher piezoelectric coefficient of MZO NRs, which is about five times higher than that of the undoped ZnO NRs sensor. Moreover, the sensor shows excellent stability and durability by loading 2500 cycles of pressing. It is demonstrated that the sensor can used to accurately monitor human motion status and vital signs in real time, such as facial motion, joint movement, wrist pulse, cervical artery, and heartbeat.

Background: The development of high-performance piezoelectric pressure sensors with outstanding sensitivity, good linearity, flexibility, durability, and biocompatibility is of great significance for smart robotics, human healthcare devices, smart sensors, and electronic skin. Thus, considerable progress has been achieved in enhancing the piezoelectric property of PVDF-TrFEbased composite pressure sensors by adding various ZnO nanostructures in PVDF-TrFE polymer acting as a nucleating agent and dielectric material. Aims: In this work, flexible pressure sensors with a sandwich structure based on PVDFTrFE/ nano-ZnO composite sensing film were fabricated using a simple spin-coating method and post-annealing process, while electrospinning and high-voltage polarization processes were not adopted. Methods: Poly (vinylidene fluoride-trifluoroethylene) (PVDF-TrFE)/nano-ZnO composite films were prepared via spin coating to fabricate flexible piezoelectric pressure sensors. ZnO nanoparticles (ZnO NPs), tetrapod ZnO (T-ZnO) and ZnO nanorods (ZnO NRs) were used as nano-fillers for piezoelectric PVDF-TrFE, to enhance the beta-crystal ratio as well as the crystallinity of PVDF-TrFE. The structural and surface morphologies of the composite films were investigated using Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and scanning electron microscopy (SEM). Results: Among three different types of ZnO nanostructures with a concentration range (0-7.5 wt%), the sensor with 0.75 wt% ZnO NRs nanofiller exhibits a maximum output voltage of 1.73 V under an external pressure of 3 N and a maximum sensitivity of 586.3 mV/N at the range of 0-3 N. Further, the sensor can generate a clear piezoelectric voltage under bending and twisting deformation as well as compression and tensile deformation. Conclusion: To summarize, the addition of different concentrations of nano-ZnO can remarkably improve the piezoelectric performance of the composite sensor, and ZnO NRs can achieve better piezoelectric properties of the sensor as compared to ZnO NPs and T-ZnO. In addition, the sensor with 0.75 wt% ZnO NRs as nanofiller has the highest piezoelectric response, which is about 2.4 times that of the pure PVDF-TrFE sensor. It is demonstrated that the sensor has great potential applications in wearable health monitoring systems and mechanical stress measurement electronics.

Flexible piezoelectric tactile sensor with transverse planar electrodes based on hydrothermally grown ZnO nanorods (ZnO NRs) was presented by using polydimethylsiloxane (PDMS) as flexible substrate and packaging material. The effects of the content of gold nanoparticles (AuNPs) added into the precursor solution on the structural morphology of ZnO NRs and on the piezoelectric properties of the ZnO NRs tactile sensor were investigated. Tactile sensors show a linear piezoelectric response in the pressure range of 0–1 N, and the sensor for the precursor solution with AuNPs of 100 μl shows a high sensitivity of 1.42 V N⁻¹ due to the large aspect ratio of the ZnO NRs, indicating that a small amount of AuNPs addition can optimize the structural morphology of ZnO NRs and thus improve the piezoelectric response of the sensor. Meanwhile, the sensor is employed to monitor human information in real-time such as bending/stretching motion of finger and distinguish various objects.