Transparent and Transient Flexible Electronics

Transparent electronics has gained tremendous attention in recent years because of the growing demand for see-through devices in applications such as displays, windscreens, and wearables. These applications require transparent electronics in a large area and flexible form factors along with performances at par with conventional electronics. Additionally, the controlled transience and degradability of electronics are desired to reduce the end-of-life challenges. Attaining these attributes simultaneously is challenging as inherent material properties do not always align well, and there are technological limitations such as thermal budget issues in the case of flexible substrates. As a result, several materials and structures, including 1D nanowires, 2D nanosheets, metal oxides, and polymers etc., are explored. This comprehensive review discusses these developments related to transparent electronics as well as the challenges associated with the development of flexible and transient transparent electronics over large areas. Potential solutions to overcome these challenges and various resource-efficient deposition and printing technologies are also presented along with examples of reported transparent circuits, sensors, actuators, and energy devices. Finally, potential future directions are discussed for flexible transient transparent electronics as their ever-growing demand could lead to the emergence of new materials, fabrication techniques, and applications

Transparent piezoelectric nanogenerator for self-powered force sensing applications

Piezoelectric materials have been widely used as dynamic force sensors, accelerometers, and energy harvesters. Herein, we report the poly(vinylidene fluoride-co-tetrafluoroethylene) (P(VDF-TrFE)) based transparent piezoelectric nanogenerator (PENG) as a self-powered force sensor. The 2 x 2 array of PENGs was fabricated by sandwiching the P(VDF-TrFE) film between two indium tin oxide-based electrodes. The device demonstrated good stable and repeatable piezoelectric response (sensitivity of 0.3 V N -1 ), which increased linearly with externally applied compressive force. The output voltage of the devices varied with the frequency of the external force application, and they showed a sensitivity of 0.25 V Hz -1 between the 2 to 10 Hz frequency. The excellent dynamic force sensing response, transparency, and self-power feature make presented devices ideally suited for applications such as see-through smart plaster that allow wound healing monitoring without removing the plaster

Capacitive-triboelectric based hybrid sensor system for human-like tactile perception

Human skin contains slowly-adapting (SA) and rapidly-adapting mechanoreceptors (RA) through which it can discriminate between static and dynamic tactile stimuli. Bio-mimicking of such human tactile sensing systems using flexible and reliable sensors has recently gained importance for developing future robots and prosthetics with better sensory capabilities. In this work, a hybrid flexible sensor system consisting of a capacitive pressure sensor (CPS) (mimicking SA mechanoreceptors) firmly stacked over a triboelectric nanogenerator (TENG) (mimicking RA mechanoreceptors) was developed. CPS consisted of porous Ecoflex as the dielectric material, while the two layers used in TENG were made using copper-nickel conducting fabric and ITO-coated PET sheet. This hybrid sensor system was characterized and showed good sensitivity for CPS and voltage response for the TENG device. Later, three distinct scenarios for the hybrid system have been demonstrated, in which it was used for the qualitative hardness assessment, slip detection and impact/vibration detection. In summary, the signals from both CPS and TENG complement each other, making this hybrid sensor system capable of simultaneously detecting both static and dynamic pressure signals

GC-MS analysis of yellow pigmented Macrococcus equipercicus isolated from alfalfa rhizosphere soil fields of Coimbatore

The rhizosphere of plant possesses important microflora, which secretes wide chemical compounds including secondary metabolites necessary for plant growth and development. The microbial flora of alfalfa plant rhizosphere soil region was explored for functional activity and we found upto ten different pigmented colonies. Due to good functional diversity, this yellow pigmented colony was taken for further studies. Thus, the culture was molecularly characterized and identified for potent bioactive components responsible for antimicrobial activity. The selected culture mass was cultured and secondary metabolites were produced and extracted using ethyl acetate and subjected to GC-MS analysis. The antimicrobial study revealed selective activity against Streptococcus pneumonia, and Proteus sp with zone of inhibition to be 18 and 20 mm respectively.  Molecular identification of the isolate by 16S rRNA sequencing showed the isolate as Macrococcus equipercicus with 100 % similarity. Based on GC-MS analysis report 25bioactive compounds were identified and 13-docosenamide, hexadecanoic acid esters and quercetin were found in ethyl acetate extract. Conclusion: Thus the yellow pigmented gram positive cocci M.equipercicus isolated from Medicago sativa possessed wide antibacterial activity due to presence of quercetin. Through the studies, we were able to identify potent antibacterial compound producing bacteria from M. sativa plant rhizosphere soil

Self‐powered e‐Skin based on integrated flexible organic photovoltaics and transparent touch sensors

There is a growing interest in the large area, lightweight, low-power electronic skin (e-Skin), consisting of a multitude of sensors over conformable surfaces. The use of multifunctional sensors is always challenging, especially when their energy requirements are considered. Herein, the heterogeneous integration of custom-made flexible organic photovoltaic (OPV) cells is demonstrated with a large area touch sensor array. The OPV can offer power density of more than 0.32 μW cm−2 at 1500 lux, which is sufficient to meet the instantaneous demand of the array of touch sensors. In addition to energy harvesting, it is shown that the OPVs can perform shadow sensing for proximity and gesture recognition, which are crucial features needed in the e-Skin, particularly for safe interaction in the industrial domain. Along with pressure sensing (sensitivity of up to 0.26 kPa−1 in the range of 1–10 kPa) and spatial information, the touch sensors made of indium tin oxide and monolayer graphene have shown >70% transparency, which allow light to pass through them to reach the bottom OPV layer. With better resource management and space utilization, the presented stacked integration of transparent touch-sensing layer and OPVs can evolve into a futuristic energy-autonomous e-Skin that can “see” and “feel.

Fully degradable, transparent, and flexible photodetectors using ZnO nanowires and PEDOT:PSS based nanofibres

Transparent light detection devices are attractive for emerging see-through applications such as augmented reality, smart windows and optical communications using light fidelity (Li-Fi). Herein, we present flexible and transparent photodetectors (PDs) using conductive poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS): Ag nanowires (NWs) based nanofibres and zinc oxide (ZnO) NWs on a transparent and degradable cellulose acetate (CA) substrate. The electrospun (PEDOT:PSS): Ag NW-based nanofibres exhibit a sheet resistance of 11 Ω/sq and optical transmittance of 79% (at 550 nm of wavelength). The PDs comprise of ZnO NWs, as photosensitive materials, bridging the electrode based on conductive nanofibres on CA substrate. The developed PDs exhibit high responsivity (1.10 ×10⁶ A/W) and show excellent stability under dynamic exposure to ultraviolet (UV) light, and on both flat and curved surfaces. The eco-friendly PDs present here can degrade naturally at the end of life – thus offering an electronic waste-free solution for transparent electrodes and flexible optoelectronics applications