This Power-Generating Shoe Isn’t Ready for Prime Time Yet, but This Kid’s Project is Still Pretty Cool

This is a video by a Angelo Casimiro, a 15-year-old Filipino participating in this year’s Google science fair. And he has seriously tweaked his shoes to do something cool: they spark. And I don’t mean spark like  those kids’ shoes that have stripes that dimly light as you walk that you really wanted to try but evidently you couldn’t wear because none of them could support your ankles (okay, that last part may not have applied to everyone else…). Angelo’s new shoes actually generate a little bit of electricity each time he takes a step. This is incredibly cool.

But just because I can, I’m going to bury the lede for a bit, because I want to contextualize this. Angelo did this as a test to see if it could work AT ALL, and he says he’s nowhere near a final product that you might buy. So before dreams of daily jogging to power your iPhone and laptop dance in your head, we need to look at the electricity we can create and how much we actually use.

Duracell’s basic alkaline non-renewable AA battery has a charge of about 2500-3000 miliampere-hours (mAh), which I estimated based on multiplying the number of hours it was used by the constant currents applied in the graphs on the first page here. The two basic rechargeable NiMH AA batteries have charges of 1700 and 2450 mAh. The battery in my Android smartphone has a charge of 1750 mAh, based on dividing the energy (6.48 watt-hours) by its operating voltage (3.7 V). Based on Angelo’s best reported current of 11 mA on his Google science fair page, it would take 159 hours to fully charge my phone. That’s nearly a week of non-stop running! (Literally! There’s only 168 hours in a week. You could only spend 9 hours doing anything besides running that week if you wanted to charge the phone, or replace one of the two AA batteries it takes to power my digital camera) However, I might be overestimating based on his averages. At around the 3:50 mark in the video, an annotation says that Angelo was able to charge a 400 mAh battery after 8 hours of jogging. That would translate to about 33 hours of jogging to charge my cell phone. No one I know would want to do that, but that is significantly less than jogging non-stop for almost 7 days.

But as Angelo points out, while you not be able to power your phone with his shoe, lots of sensors and gadgets that could go into smart clothes could be powered by this. In the video, he says he was able to power an Arduino board. An Arduino is a common mini-CPU board with extras people often use to make nifty devices, from how Peter Parker locks his room door in The Amazing Spider-Man movie to laser harps you can play by touching beams of light (note that the Arduino isn’t necessarily powering all the other components it is controlling in these cases), so you could potentially control smart clothes that respond to your moving.  A study by MIT’s Media Lab also looked at putting piezoelectric material in shoes and found they could power an RFID transmitter, which can be used to broadcast information to either devices. So perhaps your gym shoes could also act as your gym ID. The 400 mAh battery Angelo mentions is pretty close to the charge of batteries in small blood sugar monitors and over double the charge of some smaller hearing aid batteries.

But in relation to another recent science fair controversy, let’s put Angelo in context. No, he did not “invent” a new way to “charge your phone with your shoes“. Angelo himself points out that his work is more like a proof of concept than anything close to a product, and his numbers show you really won’t want to charge energy heavy devices with it. And MIT and DARPA, that branch of the US Department of Defense that funds crazy research schemes, have both looked at similar systems. (DARPA has looked at piezo-boots that could help power soldiers’ electronics.) Angelo and DARPA also both realize the limits of this: with our current materials, there’s only so much you can stuff into footwear before you run out of room or make it harder to walk. So instead, people have shifted to different goals for piezoelectricity: instead of having the material move with a single person who has to provide all the energy, we can place it where we know lots of people will walk and split the work. In Europe, high foot traffic areas have been covered with piezoelectric sidewalks to power lights, and in Japan, commuters walking through turnstiles in Tokyo and Shibuya stations help power ticket readers and the signboards that guide them to their trains.

Two distinct images. The left image shows a turnstile for ticketing. There is a black strip of material running through it. The right image shows a figure with an explanation in Japanese describing the power-generating nature of the strip.

Piezoelectric strip in ticket turnstile in Japanese subway station, from 2008

But none of this means that Angelo hasn’t done good technical work. It’s just that his effort falls more on the engineering side than the science side. Which is perfectly fine, because Google has categories for electronics and inventions and that other big science fair everyone talks about is technically a science AND engineering fair. Angelo’s shoe modification is posted on instructables and is something you could do in your home with consumer materials. The MIT Media Lab study still worked with custom-made piezoelectrics from colleagues in another lab. So the fact that Angelo could still manage to charge a battery in a reasonable (if you don’t need power right away) amount of time is incredibly impressive. And he also seems quite skilled at designing the circuits he used. As a 15 year old, he easily seems to know more about the various aspects of his circuit he needs to consider than I did through most of my time in college (granted, you didn’t need to know any particularly complicated circuity to be a physic majors). He’s definitely on to a great start if he wants to study engineering or science in college.

Walling in Wi-Fi

A roll of wallpaper is shown. A pattern of triangles arranged in hexagons is seen on the paper.A world of ubiquitous wireless networking poses numerous opportunities for theft.  Hence the fancy new trend of RFID wallets to secure credit cards.  Of course, new credit cards aren’t the only thing people want to protect.  Whether you’re incredibly concerned about someone hacking into your home wireless network or maybe you own a Panera and wish everyone at the neighboring burger bar would quit stealing your free Wi-Fi (I might have done this when working out of town one summer…), the stumbling block to applying this to your home was our society’s inability to make wallets that can hold people.  Or maybe you didn’t feel like making your home  follow neo-survivalist decor by covering your walls in chicken wire and/or aluminum foil (I’m scared to link to sites on that one, so just Google that on your own).

A more detailed view of the pattern is shown. It is a set of triangles, with a more ornate pattern in each.

Detailed Faraday cage pattern of ink

So leave it to the French to create a fashionable Faraday cage.  Researchers at the Grenoble Institute of Technology have made a wallpaper containing electrically conductive ink.  The ink, which contains silver particles, forms a snowflake pattern on the wallpaper.  This pattern is similar to something like chicken wire and so it forms a selective Faraday cage that prevents the electromagnetic waves of a Wi-Fi signal from transmitting through the paper.  It doesn’t affect the network in the room (or perhaps your house if you only paper exterior walls).  Even better, the team expects this to be no more expensive than original wallpaper.  And if metal snowflakes aren’t your thing, you can cover this with an additional layer of wallpaper without ruining the Faraday effect.

The researchers also make another claim about cell phone signals.  The original article is in French, which I don’t understand, but looking at it through Google Translate says they claim it doesn’t interfere with “emergency service” calls, but does block three kinds of signals.  Another English article I found says the wallpaper shouldn’t interfere with cell phone signals at all.  The claim about emergency services strikes me as weird because a call to 911 isn’t physically different from a call to your friend.  But I also don’t buy that it shouldn’t affect any cell phone signals.  If the photo is a somewhat accurate representation of the wallpaper, the pattern looks like there’s only about an inch of space between lines.  Since the “cage” isn’t solid, it does allow electromagnetic waves smaller than this space to pass through.  Wi-Fi uses radio frequencies in the 2400-2485 MHz range and cells phones work somewhere between 1700 and 2155 MHz (the exact range depending on your carrier).  Dividing the speed of light (300,000,000 meters a second) by the frequency, we see that the wavelength of Wi-Fi signals is about 12.5 cm while your phone’s signal has something between a 17.6 and 14 cm wavelength.  So based on my quick and dirty math, it doesn’t seem like either signal should get through ever.

P.S. The French researchers do also point out the survivalist/paranoid applications of this, saying you could also wallpaper your house in this to prevent some electromagnetic radiation from getting in.