다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

There is a growing demand for flexible and soft electronic devices. In particular, stretchable, skin-mountable, and wearable strain sensors are needed for several potential applications including personalized health-monitoring, human motion detection, human-machine interfaces, soft robotics, and so forth. This Feature Article presents recent advancements in the development of flexible and stretchable strain sensors. The article shows that highly stretchable strain sensors are successfully being developed by new mechanisms such as disconnection between overlapped nanomaterials, crack propagation in thin films, and tunneling effect, different from traditional strain sensing mechanisms. Strain sensing performances of recently reported strain sensors are comprehensively studied and discussed, showing that appropriate choice of composite structures as well as suitable interaction between functional nanomaterials and polymers are essential for the high performance strain sensing. Next, simulation results of piezoresistivity of stretchable strain sensors by computational models are reported. Finally, potential applications of flexible strain sensors are described. This survey reveals that flexible, skin-mountable, and wearable strain sensors have potential in diverse applications while several grand challenges have to be still overcome.

Stretchable, Skin-Mountable, and Wearable Strain Sensors and Their Potential Applications: A Review

http://web.stanford.edu/group/cui_group/papers/Hui_Today_2012.pdf

Designing nanostructured Si anodes for high energy lithium ion batteries

The two-dimensional semiconducting material molybdenum disulphide shows strong piezoelectricity in its single-layered form, suggesting possible applications in nanoscale electromechanical devices for sensing and energy harvesting. Two-dimensional semiconducting materials are the focus of much research effort thanks to their unusual and potentially useful physical properties. Wenzhou Wu and colleagues now confirm theoretical expectations that one such material — molybdenum disulphide — exhibits strong piezoelectricity in its single-layered form. Such a coupling of mechanical and electrical properties suggests possible applications in nanoscale electromechanical devices for sensing and energy harvesting. The piezoelectric characteristics of nanowires, thin films and bulk crystals have been closely studied for potential applications in sensors, transducers, energy conversion and electronics1,2,3. With their high crystallinity and ability to withstand enormous strain4,5,6, two-dimensional materials are of great interest as high-performance piezoelectric materials. Monolayer MoS2 is predicted to be strongly piezoelectric, an effect that disappears in the bulk owing to the opposite orientations of adjacent atomic layers7,8. Here we report the first experimental study of the piezoelectric properties of two-dimensional MoS2 and show that cyclic stretching and releasing of thin MoS2 flakes with an odd number of atomic layers produces oscillating piezoelectric voltage and current outputs, whereas no output is observed for flakes with an even number of layers. A single monolayer flake strained by 0.53% generates a peak output of 15 mV and 20 pA, corresponding to a power density of 2 mW m−2 and a 5.08% mechanical-to-electrical energy conversion efficiency. In agreement with theoretical predictions, the output increases with decreasing thickness and reverses sign when the strain direction is rotated by 90°. Transport measurements show a strong piezotronic effect in single-layer MoS2, but not in bilayer and bulk MoS2. The coupling between piezoelectricity and semiconducting properties in two-dimensional nanomaterials may enable the development of applications in powering nanodevices, adaptive bioprobes and tunable/stretchable electronics/optoelectronics.

Piezoelectricity of single-atomic-layer MoS2 for energy conversion and piezotronics.

Flexible electronics hold great promise for wearable biomedical sensors. Here, the authors report a pressure sensor composed of gold nanowire-impregnated tissue paper, sandwiched between polydimethylsiloxane sheets, and demonstrate that the design is appropriate for large-area flexible electronics.

/pdf/a-wearable-and-highly-sensitive-pressure-sensor-with-2qjvcxrnk5.pdf

A wearable and highly sensitive pressure sensor with ultrathin gold nanowires

Strain sensors based on individual ZnO piezoelectric fine-wires (PFWs; nanowires, microwires) have been fabricated by a simple, reliable, and cost-effective technique. The electromechanical sensor device consists of a single electrically connected PFW that is placed on the outer surface of a flexible polystyrene (PS) substrate and bonded at its two ends. The entire device is fully packaged by a polydimethylsiloxane (PDMS) thin layer. The PFW has Schottky contacts at its two ends but with distinctly different barrier heights. The I-V characteristic is highly sensitive to strain mainly due to the change in Schottky barrier height (SBH), which scales linear with strain. The change in SBH is suggested owing to the strain induced band structure change and piezoelectric effect. The experimental data can be well-described by the thermionic emission-diffusion model. A gauge factor of as high as 1250 has been demonstrated, which is 25% higher than the best gauge factor demonstrated for carbon nanotubes. The strain sensor developed here has applications in strain and stress measurements in cell biology, biomedical sciences, MEMS devices, structure monitoring, and more.

http://www.nanoscience.gatech.edu/zlwang/paper/2008new/08_2367t.pdf

Flexible piezotronic strain sensor

We have applied the perturbation theory for calculating the piezoelectric potential distribution in a nanowire (NW) as pushed by a lateral force at the tip. The analytical solution given under the first-order approximation produces a result that is within 6% from the full numerically calculated result using the finite element method. The calculation shows that the piezoelectric potential in the NW almost does not depend on the z-coordinate along the NW unless very close to the two ends, meaning that the NW can be approximately taken as a “parallel plated capacitor”. This is entirely consistent to the model established for nanopiezotronics, in which the potential drop across the nanowire serves as the gate voltage for the piezoelectric field effect transistor. The maximum potential at the surface of the NW is directly proportional to the lateral displacement of the NW and inversely proportional to the cube of its length-to-diameter aspect ratio. The magnitude of piezoelectric potential for a NW of diameter 50 nm and length 600 nm is 0.3 V. This voltage is much larger than the thermal voltage (25 mV) and is high enough to drive the metal-semiconductor Schottky diode at the interface between atomic force microscope tip and the ZnO NW, as assumed in our original mechanism for the nanogenerators. Developing novel technologies for wireless nanodevices and nanosystems is of critical importance for applications in biomedical sensing, environmental monitoring, and even personal electronics. Miniaturization of a power package and self-powering of these tiny devices are some key challenges for their applications. Various approaches have been developed for harvesting energy from the environment based on approaches such as thermoelectricity and piezoelectricity. Innovative nanotechnologies are being developed for converting mechanical energy (such as body movement, muscle stretching), vibration energy (such as acoustic/ultrasonic wave), and hydraulic energy (such as body fluid and blood flow) into electric energy that will be used to power nanodevices that operate at low power. Recently, using piezoelectric ZnO nanowire (NW) arrays, a novel approach has been demonstrated for converting nanoscale mechanical energy into electric energy. 1-3 The single nanowire nanogenerator (NG) relies on the bending of a NW by a conductive atomic force microscope (AFM) tip, which transfers the displacement energy from the tip to the elastic bending energy of the NW. The coupled piezoelectric and semiconducting properties of the NW perform a charge creation, accumulation, and discharge process. Most recently, this approach has been extensively developed to produce continuous direct-current output with the use of aligned NWs that were covered by a zigzag top electrode, and the nanogenerator was driven by ultrasonic wave, establishing the platform of producing usable power output for nanodevices by harvesting energy from the environment. 4 Furthermore, based on the coupled piezoelectric and semiconducting properties of the NW, a new field of nanopiezotronics has been created, 5,6 which is the basis for fabricating piezoelectric field effect transistors, 7 piezoelectric diode, 8 piezoelectric force/humidity/chemical sensors, 9 and more. The theoretical background for the nanogenerator and nanopiezotronics is based on a voltage drop created across the cross section of the NW when it is laterally deflected, with the tensile side surface in positive voltage and compressive side in negative voltage. 1,5 It is essential to quantitatively calculate and even develop analytical equations that can give a direct calculation of the voltage at the two side surfaces of the NW, which is important to calculating the efficiency of the nanogenerator and the operation voltage of the nanopiezotronics. In the literature, numerous theories for onedimensional (1D) nanostructure piezoelectricity have been proposed, including first-principles calculations, 10,11 molecular dynamics (MD) simulations, 12 and continuum models. 13

Electrostatic potential in a bent piezoelectric nanowire. The fundamental theory of nanogenerator and nanopiezotronics

Using phosphorus-doped ZnO nanowire (NW) arrays grown on silicon substrate, energy conversion using the p-type ZnO NWs has been demonstrated for the first time. The p-type ZnO NWs produce positive output voltage pulses when scanned by a conductive atomic force microscope (AFM) in contact mode. The output voltage pulse is generated when the tip contacts the stretched side (positive piezoelectric potential side) of the NW. In contrast, the n-type ZnO NW produces negative output voltage when scanned by the AFM tip, and the output voltage pulse is generated when the tip contacts the compressed side (negative potential side) of the NW. In reference to theoretical simulation, these experimentally observed phenomena have been systematically explained based on the mechanism proposed for a nanogenerator.

Piezoelectric nanogenerator using p-type ZnO nanowire arrays.

We have investigated the behavior of free charge carriers in a bent piezoelectric semiconductive nanowire under thermodynamic equilibrium conditions. For a laterally bent n-type ZnO nanowire, with the stretched side exhibiting positive piezoelectric potential and the compressed side negative piezoelectric potential, the conduction band electrons tend to accumulate at the positive side. The positive side is thus partially screened by free charge carriers while the negative side of the piezoelectric potential preserves as long as the donor concentration is not too high. For a typical ZnO nanowire with diameter 50 nm, length 600 nm, donor concentration N(D) = 1 x 10(17) cm(-3) under a bending force of 80 nN, the potential in the positive side is <0.05 V and is approximately -0.3 V at the negative side. The theoretical results support the mechanism proposed for a piezoelectric nanogenerator. Degeneracy in the positive side of the nanowire is significant, but the temperature dependence of the potential profile is weak for the temperature range of 100-400 K.

http://www.nanoscience.gatech.edu/zlwang/paper/2009/nano_lett_1103.pdf

Equilibrium Potential of Free Charge Carriers in a Bent Piezoelectric Semiconductive Nanowire

Using a two-end bonded ZnO piezoelectric-fine-wire (PFW) (nanowire, microwire) on a flexible polymer substrate, the strain-induced change in I−V transport characteristic from symmetric to diode-type has been observed. This phenomenon is attributed to the asymmetric change in Schottky-barrier heights at both source and drain electrodes as caused by the strain-induced piezoelectric potential-drop along the PFW, which have been quantified using the thermionic emission−diffusion theory. A new piezotronic switch device with an “on” and “off” ratio of ∼120 has been demonstrated. This work demonstrates a novel approach for fabricating diodes and switches that rely on a strain governed piezoelectric-semiconductor coupling process.

http://www.nanoscience.gatech.edu/zlwang/paper/2008new/2008_ppc.pdf

Yifan Gao

Papers

Flexible piezotronic strain sensor

Electrostatic potential in a bent piezoelectric nanowire. The fundamental theory of nanogenerator and nanopiezotronics

Piezoelectric nanogenerator using p-type ZnO nanowire arrays.

Equilibrium Potential of Free Charge Carriers in a Bent Piezoelectric Semiconductive Nanowire

Piezoelectric-potential-controlled polarity-reversible Schottky diodes and switches of ZnO wires