Modest size, monumental impact

by Surabhi Joshi and Sherman Hung

Richard Feynman, considered the father of nanotechnology, once said that “there’s plenty of room at the bottom.” One can understand his fascination with nanoscale and micrometers: The change in properties of ordinary materials at these scales is truly fascinating, be it the self-lubrication of silicon carbide (which gets smoother the more you run it) or the appearance of gold at nanoscale (it turns into the color red!).

In terms of practical applications, Microelectromechanical systems (MEMS) and nanoscale devices will have unprecedented value. For instance, nanoscale particles can be used in solar cells instead of the traditional material pure silicon, which is very expensive to produce. Products such as smart clothes, cosmetics and appliances can be produced. Nanomembranes are handy objects that can be used to obtain clean water in poorer countries. Moreover, pressure sensors, air bag deployment, micromirrors for optical communication already utilize MEMS technology.

This branch of engineering is attractive because it has the perfect mix: physics, engineering, solid state chemistry, material science, electronics, in addition to the marvelous benefits and outstanding applications. One such application includes the “smart dust” concept, which, if successful, could be as important and powerful a phenomena as the Internet is.

Smart Dust
Fourteen years ago, the researcher Kris Pister dreamed of an idea where a network of wireless sensors as tiny as dust particles could retrieve data autonomously to a computer used to monitor everything. You take a bunch of these “smart dust” or “motes”, scatter them in any location, and they collect the desired data (temperature, humidity, light intensity, etc.) from humans and their environment. There are countless applications involving these tiny sensors. Imagine depositing these devices within the concrete of a bridge; they could then monitor vibration, stress and cracks, therefore alerting engineers of the current condition of the infrastructure and problems that might arise from it.

Another practical use of “smart dust” is to detect landslides. Upon detection of small movements in the soil, the information will be instantly sent back to a monitored computer. Providing people with notifications ahead of time can prevent the occurrence of calamities. Other applications include observing animals that face extinction and tracking, for example, their movements and body temperature, thereby allowing biologists to better understand them in their natural habitat. The sky is the limit for applications involving the “smart dust” concept but their implementation currently faces a practical challenge, namely, providing these sensors with a constant power source. Batteries generally last one to two years so imagine replacing millions of those mini sensors distributed all around a field with a new set of batteries; that process is not feasible.

This summer, we analyzed what might be the first step towards achieving a perpetually powered sensor. Texas instruments have come up with a recent product, namely the solar energy harvester, that makes use of the new thin film batteries called EnerChips (manufactured by CYMBET).

These batteries can be recharged a number of times without any detrimental effects on the battery, thus eliminating the need to replace them periodically. The kit has a solar cell that doesn’t require very bright light; it can be powered by indoor lighting. The batteries come into play when there is too little or no light available to send data to the computer. The data transmissions are carried out using radio waves. This kit, gives information such as the temperature reading and the strength of the signal. However other sensors such as vibration sensor, pressure sensor can also be connected externally and the corresponding measurements recorded.

We looked into the range of the device, that is, how far one can go before the strength of the signal reduces significantly. We learnt to calibrate the device, so as to give an accurate temperature reading. We explored the fluctuations in voltage under different conditions (this is important if one needs to power a device using the output voltage). We calculated how much energy is obtained by the solar panel and how much is needed per transmission. Furthermore, we computed the number of transmissions undertaken by the battery under no light condition. It turns out that this number (400 transmissions) is directly connected to the battery capacity. Only 17% of the battery capacity gets used up during these 400 transmissions. This means that a low period of exposure to light is needed to recharge the battery fully.

In conclusion, if this thin film battery does indeed live up to its expectations along with the solar panel, it will overcome one of the biggest challenges we face today. The next step will be to decrease the package size so that they are inconspicuous yet powerful (currently, the limiting factor is the size of the batteries). In the kit we examined, the solar panel was the biggest component. However, the energy obtained by the solar panel was found to be 3.5 times the energy needed for one transmission. Hence, the solar panel can possibly be made smaller without any drastic effects. It must be noted that the complexity of these devices is bound to increase, especially when dealing with millions of such similar devices but on smaller scales. Therefore, to obtain this global objective of interactive sensors, one has to ensure that these nanoscale ‘motes’ can communicate with each other in a network and function satisfactorily using long lasting batteries and/or vibration or solar energies. The approach to energy storage at these miniscule scales is still in its infancy and thus an attractive area for research. This might lead to new materials, new manufacturing processes and indispensable results