Sensing and Applications

Generators—Triboelectric Based Flexible Electronics

In the past three years, the research team under Dr. Z.L. Wang launched the triboelectric effect into the field of flexible electronics. With unique applicability resulting from its distinctive working mechanism, triboelectric effect-based devices open up a new technique in the field of wearable/ flexible electronics.

A demonstrator relying on the coupling effect of triboelectrification and electrostatic induction, produced a 125 μm thickness, rollable, paper-based microphone. Owing to the superior advantages of a broad working bandwidth, thin structure, and flexibility, the rolled self-powered microphone is capable of recording the sound without an angular dependence.

The team also demonstrated an intelligent and self-powered keyboard as an advanced security safeguard against unauthorized access to computers. Based on the triboelectric effect between human fingers and keys, the intelligent keyboard (IKB) could convert typing motions into localized electric signals that could be identified as personalized physiological information. Given its exceptional authentication capability, the IKB was able to identify the individual typing characteristics, making it practical as a highly secured authentication system based on behavioral biometrics.

Harvesters—Flexible and Stretchable Biomechanical Energy Harvester

In the past two years, Prof. Z.L. Wang group demonstrated several flexible and stretchable biomechanical energy harvesters, with rational structure designs and material selections. Based on the coupling of triboelectrification and electrostatic induction, these energy harvesters can be used to utilize mechanical energy from the human body.

A shape-adaptive triboelectric nanogenerator (TENG) unit composed of a conductive liquid electrode and an elastic polymer cover. Taking advantage of the unique conformability of the liquid electrode, as well as the high flexibility of the rubber cover, the saTENG is capable of withstanding a strain of as large as 300% without degradation of the electrical properties. In addition, water can act as the liquid electrode for the TENG, which considerably broadens the range of its applications. The TENG was applied as a wearable energy scavenger and a self-powered biomechanical motion sensor by incorporating it with the human body. Furthermore, the TENG was used for large-area energy harvesting and energy conversion out of mechanical motion using flowing water as the electrode. This work opens up new design opportunities for energy harvesters, stretchable electronics, wearable devices, and self-powered sensors. (Science Advances, 2:e1501624, 2016)

Highly stretchable TENGs by using the traditional kirigami patterns were also developed, whose stretchability originates from the designed structures instead of constituent materials. This method enables stretchable TENGs to be made from materials without intrinsic stretchability, such as paper, fluorinated ethylene propylene (FEP), polytetrafluoroethylene (PTFE), poly (ethylene terephthalate) (PET), and Kapton and, thus, is very versatile. The fabricated devices sustained an ultrahigh tensile strain up to 100% and were capable of harvesting energy from various types of motions such as stretching, pressing, and twisting. Simple hand clapping on the device could generate a maximum open-circuit voltage of 115.49 V and a maximum transferred charge quantity of 39.87 nC. Furthermore, the KTENG has been demonstrated for a broad range of applications, such as powering an LCD screen, lighting LED arrays, self-powered acceleration sensing, and self-powered sensing of book opening and closing. This work presents the progress of stretchable TENGs for application in stretchable and flexible electronics, and it will shed light on future directions of kirigami-based devices. (ACS Nano, 10(4):4652, 2016)

These flexible and stretchable TENGs can be mounted in various positions on the human body for biomechanical energy harvesting, or directly integrated with wearable electronics as self-powered systems. To increase the total current output, rectified current outputs from multiple TENGs can be connected in parallel. Due to the high voltage (over hundreds of volts) output characteristics, TENGs can simultaneously drive numerous electronics connected in series, such as hundreds of LED bulbs as shown in the figure. These characteristics and performances pave the road for utilizing flexible TENGs for harvesting biomechanical energy towards self-powered wearable systems and the internet of things (IoT).

Sensors

• Chemical – Thin-Film Transistors (TFT) for Chemical Sensing

The research group of Prof. Oliver Brand is exploring thin-film transistors based on InGaZnO semiconducting films for chemical sensing applications. Low process and annealing temperatures enable the use of (flexible) glass and polymeric substrates. Special emphasis is placed on passivation schemes for the bottom-gate TFTs that minimize performance degradation in gas- and liquid-phase chemical sensing applications. InGaZnO TFT passivated with TiOx films deposited by Atomic Layer Deposition show improved stability even in liquids and a pH sensor based on a dual-gate InGaZnO TFT with Super Nernstian behavior has been demonstrated.

• Chemical and Temperature – All-Soft Liquid-Metal-Based Chemical Sensors

The research group of Prof. Oliver Brand is developing all-soft electronics based on PDMS molds and liquid metal conductors. An improved liquid-metal patterning and vertical integration strategy enable simultaneous fabrication of eutectic gallium indium alloy (EGaIn) structures ranging from 2µm to 2mm in width. Besides demonstrating integrated passives (resistors, capacitors, and inductors) and their co-integration with surface-mount devices, such as LEDs, gas- and liquid-phase chemical sensors, and temperature sensors have been demonstrated using this technology.

• Gas Rapid Low Power Thermal Conductivity Based Gas Sensors

Micro-thermal conductivity detector (mTCD) gas sensors work based on detecting modulation of the thermal conductivity of the surrounding medium and are used as detectors in many applications such as gas chromatography systems but at a larger scale. In the lab of Professor Peter Hesketh, the research team has developed a new measurement method based on processing the transient response of a low thermal mass mTCD.

• Detection and quantification of volatile compounds in an air sample using MEMS-GC and ultra-low power gas sensor.
• Distributed wireless gas sensors for ambient air quality monitoring.
• Detection of volatile compounds in an air sample to identify the presence of infection and fungus on plants.
  Dr. Hesketh’s research is in collaboration with KWJ Engineering.

• Flexible Carbon Nanotube (CNT)-Based Gas Sensor

The Tentzeris group has created a proof-of-concept prototype chemical/gas sensor which features the use of multiwalled CNT’s (MWCNT) as the basis for a thin-film sensing material, which was deposited using a water-based MWCNT ink that was inkjet-printed on a flexible polyimide Kapton substrate. NO2 and NH3, diluted in N2, were chosen as the test gases due to their extensive industrial use, toxicity, and relative ease of acquisition. The effect of these gases on the MWCNT material properties was tested using a KIN-TEK FlexStream gas standard generator, capable of providing very stable and accurate analyte concentrations into a carrier gas down to very low concentrations, using gas permeation tubes of various gas delivery rates. The changes in the resistance of the tested sensor were monitored as a sensing indicator. Before any test, almost complete desorption of chemical species potentially previously absorbed by the sensing material was achieved by placing the sensors under a flow of pure nitrogen (N2) for 5 min. Sensitivities of 21.7% and 9.4% were achieved for 10 ppm NO2 and 4 ppm NH3, respectively, at 864 MHz; these values are higher than those achieved in recent efforts for a higher concentration (100 ppm) of NH3.

• Bio—Sample Preconcentration for Detection of Bacteria in Food Samples

Detection of low concentrations of bacteria in food samples is a challenging process. The key to this process is the separation of the target from the food matrix. We have been investigating the use of functionalized magnetic beads and vibrating cilia which are manipulated by an external magnetic field. The magnetic features and cilia are magnetized by an external field trapping the functionalized magnetic beads. The extent of mixing induced in micro-channels by the oscillating magnetic beads and cilia has been investigated and was found to be significant. We are studying methods to improve capture efficiency for these systems using fluorescently labeled microparticles to simulate the presence of Salmonella. This work by the Hesketh group at Georgia Tech is in collaboration with Dr. Marilyn Erickson in the Department of Food Science and Technology at the University of Georgia.