By Celine Teh
Do you know that bones can generate electricity? Yes, literally human bones. Besides those calcium-saturated structures in our body, crystals and ceramics such as quartz crystals (or sand) and cane sugars also do the same thing: converting mechanical energy into electricity by stress.
This type of electrical energy is known as piezoelectricity, discovered by French scientists Jacques and Pierre Curie in 1880. “Piezo” derived from Greek word “piezein” means to squeeze or press.
What’s going on here? The phenomenon is made possible by the structures of piezoelectric materials, referring to substances that are capable of converting physical tension into electricity, or vice versa. They have special arrangement of atoms that relax in an electrically neutral balance, meaning the positive charges cancel out the negative charges. Yet when tension is applied by compression, the atoms shift, forming oppositely charged terminals, creating a voltage, or difference in electrostatic charge. This phenomenon of mechanical energy being converted to electrical energy is named as direct piezoelectric effect.
On the other hand, if electrical current is applied, net charges appear, the crystal expands, therefore converts electrical energy into mechanical vibration. This is the inverse piezoelectric effect.
When the piezoelectric material is connected to two metal plates in a complete circuit, they could readily serve a plethora of purposes.
It was first considered during World War I to make the sonar system, in which a transducer produces acoustic waves into the air or water. The sound will bound off obstacles and will be received by the receptors. Then, distances were calculated by the time taken for it to return. This led to the craze of the United States, Russia and Japan to craft artificial piezoelectric materials, including ubiquitous ferroelectrics (materials that can produce far higher piezoelectric currents than natural materials) today such as PZT (lead zirconate titanate), barium titanate and lithium niobate.
Other than this, speakers in phones like Siri and Google Ask, as well as sensors like microphones, medical imaging equipment, and electrical instruments, also utilize this technology. Akin to the sonar mechanism, vibration you produce (e.g. speaking, strumming on the strings) directs current into the circuit and vice versa.
Electric-cigarette lighters and gas burners operate synonymously: a hammer hitting the piezoelectric material produces electrical current as a spark to ignite the stored fuels. Quartz in quartz clock, as mentioned before, also produces electricity that enables the clock to work.
Voracious scientists and innovators are absolutely not satisfied with the current stage of piezoelectric technology.
Research papers futuristically introduce piezoelectric roads and pavements. Piezoelectric material may be buried beneath highway pavements to be activated by the weight of passing cars, and the electricity generated could be used to power stoplights and roadside lamps. A 1 km stretch would be capable of generating 200 kW assuming 600 cars crossed in an hour, sufficient to power up to 800 houses. This design is no longer a distant dream, given that a Tokyo subway had already been using kinetic energy of crowds since 2008 to harness up to 1400 kWh per day – more than enough to run ticket gates and display signs.
Another advance proposal suggests piezoelectric biomedicine implantation harvest energy from dynamic muscle motion, for example, the lung, heart and diaphragm, during breathing. Energy yielded could recharge artificial pacemakers (a little device implanted on the heart to generate electrical signals that keep the heart pumping when the natural pacemaker is not working properly). Therefore avoiding the inconvenience in surgical replacement every 6-7 years by elongating the time it takes before the pacemaker runs out of energy.
Caption: A pacemaker is a piece of tissue that generates electrical impulses to signal the heart to beat. When the heart is beating irregularly or too slowly, an artificial pacemaker may be inserted to correct that, but generally the doctor has to open up his patient to replace the battery every 6-7 years.
Also, piezoelectricity opens up possibilities for smart wearables and garments. A few years back in 2018, Kansai University, Japan, and Teijin Limited, Japan developed the world’s first known wearable ‘e-textiles’ as braided cords that respond to complex three-dimensional motion, like twisting and folding. Later in 2020, professor Kamal Asadi and his research team at the University of Bath, United Kingdom, have discovered a way to retain the piezoelectric properties in nylon fibers, a common synthetic textile. These milestones pave the road to the days of using our T-shirts to power a device or for monitoring our health.
Despite being discovered over a century ago, piezoelectricity is yet a relatively novel field in the study of energy, as there are more exciting possibilities to explore and uncover. The sequel of the story will be told by the future generations – us.
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