Movement implies energy. When you move, it requires energy in the muscles to make the motion, but the motion itself contains energy (mechanical) and that can be transferred to something else. That something else could be a silicon rubber patch with an embedded piezoelectric material, recently developed by research at Princeton University (New Jersey, USA) that uses the motion of the body to produce electrical energy, enough energy to power small electrical devices.
The idea of converting body movement into electricity is an old idea that over the decades has found many approaches. Most of them went nowhere, mainly because they were not efficient – either they produced too little useful electricity, or in order to produce enough electricity they were too cumbersome. Riding a bike with a generator attached to it, for example, produces lots of electricity – but not enough to justify spending hours doing it. Efficiency is the name of the micro-electricity game; and in this game piezoelectricity is the top player.
Piezoelectricity is the capacity of some materials to generate an electric field under stress. ‘Piezo’ means to squeeze or press in Greek and that about covers it. Of all the materials that exhibit piezoelectric capability (quartz crystals, certain ceramics, even bone) the most efficient known is lead zirconate titanate (PZT), a ceramic. PZT can convert up to 80% of the mechanical energy applied to it into electrical energy. The Princeton team is the first to combine a silicon rubber patch with nanoribbons of PZT. The nanoribbons, a hundred of which side by side don’t equal a millimeter, pack the silicon patch so that even the smallest movement (such as breathing) will generate electricity.
The silicon rubber patch (or sheet) is biologically neutral (think silicon breast implants), and can be used inside the body without rejection. This gives the PZT patch an arm and a leg up in medical applications such as powering pacemakers and defibrillators.
“PZT is 100 times more efficient than quartz, another piezoelectric material,” said Michael McAlpine, a professor of mechanical and aerospace engineering, at Princeton, who led the project. “You don’t generate that much power from walking or breathing, so you want to harness it as efficiently as possible.”
In addition to generating electricity when it is flexed, the opposite is true: the material flexes when electrical current is applied to it. This opens the door to other kinds of applications, such as use for microsurgical devices, McAlpine said.
“The beauty of this is that it’s scalable,” said Yi Qi, a postdoctoral researcher who works with McAlpine. “As we get better at making these chips, we’ll be able to make larger and larger sheets of them that will harvest more energy.”
The Princeton team has taken the most efficient piezoelectric material, embedded it in biologically neutral silicon, and found a way to tap its electrical output. How much output at what size of patch? Comments suggest that size matters, at least at this level of experimentation. When the efficiency demonstrates practicality – the electrical output and its storage can drive real electrical devices – then it becomes much bigger news.