The second layer comprises the matrix of sensing electrodes
connected to the printed circuit board underneath it, and the
third layer is made up of an ATmega328 (Atmel) microcontroller
and MTCH6102 (Microchip) capacitive touch controller.
“The microprocessor is doing all the calculations and linking
everything together,” says Kao. “Then we have the capacitive
sensing chip, which links to the electrodes and enables the
device to detect if someone is touching it or not.” The samples
are then transmitted to a laptop or phone using a Bluetooth
Low Energy chip nRF8001 (Nordic).
Beneath the circuit board is a 10 mAh lithium-polymer
battery, which powers the circuit. According to the researchers,
the battery layer was the primary challenge in terms of
miniaturization. “Its current battery life is two hours,” explains
Dementyev. “Instead of running the device all the time, we think
we could have it sleeping most of the time, and have a special
activation gesture to wake it up.”
The current battery has a mean power consumption of 4.86
mA; however, Dementyev believes that, with the right battery, it
would be possible to get the device running for up to one week
without recharging. The researchers have been in discussions
with several battery manufacturers. In fact, they think they have
found a technology that could produce a battery that fits in the
space of a thumbnail, but is only half a millimeter thick.
The nail-mounted device currently recognizes five gestures.
“Once you put NailO on your thumb, you can use your index
finger to swipe left/right, top/down, and perform a long press,”
says Dementyev. “You control the device using the small
trackpad like a cursor.”
In order to embed the software on-chip for future iterations,
the researchers chose a thresholding algorithm over machine
learning for a lightweight solution. However, after conducting
various pilot studies, they found that the baseline capacitive
sensor values differed between users, prompting the
researchers to include an initial calibration phase to find the
best parameters for each user.
The system then feeds the optimal parameters to the gesture
detection algorithm after calibration, and the software proceeds
to read in sensor data at approximately 30 frames per second,
feeding it into the algorithm.
In a user study testing the five gestures, NailO detected
gestural inputs in real time with more than 92 percent accuracy.
Ten users (five male, five female) were asked to perform the
swipe gestures with their middle finger.
“We designed NailO for the thumb, because it has more
space, and it’s easier for us to prototype,” says Kao. “But a lot
of people who tested our device wanted to put it on different
fingers, and some people wanted to put it on all their fingers
so they could use it like a scrollbar.” The researchers plan to
eventually manufacture the sensor for all ten fingers and print
electrode substrates in different sizes to fit different users.
Applications for the device can be broken down into two
categories. The first application would use NailO like a remote
control. “Imagine when both your hands are busy cooking, and
you need to look at a recipe on your mobile device – NailO
could help you navigate and scroll through it.”
In addition, she believes the users would be eager to
take advantage of the device’s small form factor. “For
example, if you’re in a meeting, but you happen to get
a text message that your child is sick, and you need to
reply, NailO is a very discrete way to send a reply.”
Despite what Kao describes as an unlimited number
of potential applications, it will take a couple of years
before the device can be brought to the market due to
limitations in current battery manufacturing technology.
The researchers realize that NailO must be able to
avoid accidental false positives. As a solution, Kao
suggests a two-second activation press before any
other gestures can be performed.
“At the MIT Media Lab, we try to imagine what
the future of technology might look like 10 to
20 years out, and we don’t just imagine it – we
build prototypes to give the world a glimpse of
the possible future,” says Kao. “We really feel
that, with technology getting closer to the human
body, it is getting more important to think about
personalization in a deeper sense.”
NailO works as an X-Y coordinate touchpad on your
fingernails and allows for swipe gestures.