Stretchable substrates and Elastomeric materials
By Dr. S. S. VERMA, Department of Physics, S.L.I.E.T., Longowal, Distt.-Sangrur (Punjab)-148 106 E-mail: ssverma@fastmail.fm
Electronic components historically have been flat and unbendable because silicon, the principal component of all electronics, is brittle and inflexible. Any significant bending or stretching renders an electronic device useless. Flexible and stretchable electronics are the future of mobile electronics and beyond traditional electronics, potential stretchable applications include biomedical, wearable, portable and sensory devices, such as cyber skin for robotic devices and implantable electronics. Advances in soft and stretchable substrates and elastomeric materials have given rise to an entirely new field. Stretchable circuitry would be able to do many things that its rigid counterpart cannot. For example, an electronic “skin” could help robots pick up delicate objects without breaking them,and stretchable displays and antennas could make cell phones and other electronic devices stretch and compress without affecting their performance. However, the first step toward making such applications possible is to produce conductors that are elastic and able to effectively and reliably transmit electric signals regardless of whether they are deformed. The piezoelectrically modulated resistive memory (PRM) devices take advantage of the fact that the resistance of piezoelectric semiconducting materials such as zinc oxide (ZnO) can be controlled through the application of strain from a mechanical action. The change in resistance can be detected electronically, providing a simple way to obtain an electronic signal from a mechanical action.
Mimicking the sense of touch electronically has been challenging, and is now done by measuring changes in resistance prompted by mechanical touch. Piezoelectric based devices developed rely on a different physical phenomenon — tiny polarization charges formed when piezoelectric materials such as zinc oxide are moved or placed under strain. In the piezotronic transistors, the piezoelectric charges control the flow of current through the wires just as gate voltages do in conventional three-terminal transistors. The technique only works in materials that have both piezoelectric and semiconducting properties. These properties are seen in nanowires and thin films created from the wurtzite and zinc blend families of materials, which includes zinc oxide, gallium nitride and cadmium sulfide. As computers and other electronic devices become more personalized and human-like, there is a need to develop new types of signals, interfacing mechanical actions to electronics and piezoelectric materials provide the most sensitive way to translate these gentle mechanical actions into electronic signals that can be used by electronic devices. Flexible electronics can provide the interface between biology and electronics. This technology, which is based on zinc oxide nanowires, allows communication between a mechanical action in the biological world and conventional devices in the electronic world.
Working principle
In conventional transistors, the flow of current between a source and a drain is controlled by a gate voltage applied to the device. That gate voltage determines whether the device is on or off.The piezotronic memory devices take advantage of the fact that piezoelectric materials like zinc oxide produce a charge potential when they are mechanically deformed or otherwise put under strain. These PRM devices use the piezoelectric charge created by the deformation to control the current flowing through the zinc oxide nanowires that are at the heart of the devices — the basic principle of piezotronics. The charge creates polarity in the nanowires — and increases the electrical resistance much like gate voltage in a conventional transistor. The application of an external voltage is replaced with the production of an internal voltage. As zinc oxide is both piezoelectric and semiconducting, and straining the material with a mechanical action, create a piezopotential. This piezopotential tunes the charge transport across the interface — instead of controlling channel width as in conventional field effect transistors. The mechanical strain could come from mechanical activities as diverse as signing a name with a pen, the motion of an actuator on a nanorobot, or biological activities of the human body such as a heart beating. The piezotronic switching affects current flowing in just one direction, depending whether the strain is tensile or compressive. That means the memory stored in the piezotronic devices has both a sign and a magnitude. The information in this memory can be read, processed and stored through conventional electronic means.
Development status
Flexible and stretchable electronics has increased the stretching range (as much as 140 percent) and allows the user to subject circuits to extreme twisting. This emerging technology promises new flexible sensors, transmitters, new photovoltaic and microfluidic devices, and other applications for medical and athletic use. The researchers used a chemical growth technique at approximately 85 to 90 degrees Celsius, which allowed them to fabricate arrays of strain-gated vertical piezotronic transistors on substrates that are suitable for microelectronics applications. The transistors are made up of bundles of approximately 1,500 individual nanowires, each nanowire between 500 and 600 nanometers in diameter. Using bundles of vertical zinc oxide nanowires, researchers have fabricated arrays of piezotronic transistors capable of converting mechanical motion directly into electronic controlling signals. The arrays include more than 8,000 functioning piezotronic transistors, each of which can independently produce an electronic controlling signal when placed under mechanical strain. The vertically-aligned taxels operate with two-terminal transistors. Instead of a third gate terminal used by conventional transistors to control the flow of current passing through them, taxels control the current with a technique called “strain-gating.” Strain-gating based on the piezotronic effect uses the electrical charges generated at the Schottky contact interface by the piezoelectric effect when the nanowires are placed under strain by the application of mechanical force.
Applications
These touch-sensitive transistors — dubbed “taxels” — could provide significant improvements in resolution, sensitivity and active/adaptive operations compared to existing techniques for tactile sensing. Any mechanical motion, such as the movement of arms or the fingers of a robot, could be translated to control signals. This could make artificial skin smarter and more like the human skin. It would allow the skin to feel activity on the surface. Their sensitivity is comparable to that of a human fingertip. The arrays could help give robots a more adaptive sense of touch, provide better security in handwritten signatures and offer new ways for humans to interact with electronic devices. The arrays are transparent, which could allow them to be used on touch-pads or other devices for fingerprinting. They are also flexible and foldable, expanding the range of potential uses.
• Multidimensional signature recording, in which not only the graphics of the signature would be included, but also the pressure exerted at each location during the creation of the signature, and the speed at which the signature is created.
• Shape-adaptive sensing in which a change in the shape of the device is measured. This would be useful in applications such as artificial/prosthetic skin, smart biomedical treatments and intelligent robotics in which the arrays would sense what was in contact with them.
• Active tactile sensing in which the physiological operations of mechanoreceptors of biological entities such as hair follicles or the hairs in the cochlea are emulated. Because the arrays would be used in real-world applications, the researchers evaluated their durability. The devices still operated after 24 hours immersed in both saline and distilled water.
Future work
This is a fundamentally new technology that allows us to control electronic devices directly using mechanical agitation.This could be used in a broad range of areas, including robotics, MEMS, human-computer interfaces and other areas that involve mechanical deformation. Future work will include producing the taxel arrays from single nanowires instead of bundles, and integrating the arrays onto CMOS silicon devices. Using single wires could improve the sensitivity of the arrays by at least three orders of magnitude. These piezotronic memory elements provide another component needed for fabricating complete self-powered nanoelectromechanical systems (NEMS) on a single chip. Researchers have already demonstrated other key elements such as nanogenerators, sensors and wireless transmitters. They are taking another step toward the goal of self-powered complete systems and the challenges now are to make them small enough to be integrated onto a single chip.
Acknowledgement: The use of information retrieved through various references/sources of internet in this article is highly acknowledged.
