The stencil used for applying adhesive.
Peeling the backing off of the smart tattoo.
The final product. Eight conductive pads.
In September of last year, MIT Media Lab published a paper about a technology they dubbed “DuoSkin”. It was a method of rapid prototyping electronics on the human body by employing a variety of skin-friendly components.
We sought to replicate the work in the course of 36 hours while adding our own spin on how we thought this technology could be used in the real world. With the NRF52 microcontroller, we eliminated the need for an external capacitive sensing chip. Additionally, with Bluetooth Low Energy, data is transmitted with minimal power consumption and minimal work in connecting.
How it Works
Smart tattoos are constructed using normal temporary tattoo paper. First, a layer of temporary tattoo backing paper is selectively coated with an adhesive using a stencil with the desired traces. The sticky areas left on the tattoo backing paper are coated in 2-3 layers of copper leaf, with adhesive between each layer. 36GA wire wrap wire is attached to each electrical net using liquid conductive glue. Once this material cures, a layer of silicone temporary tattoo material is applied to the tattoo backing paper. A protective backing is pulled off this layer, and the tattoo is attached to the skin, using wet paper towels as per the manufacturer's’ instructions. Then, the backing was removed, leaving the smart tattoo bonded to the skin.
The wires extending from each electrical net on the smart tattoo either serve as ground planes or are connected to capacitive sensing pins on a nRF52832 microcontroller. When the capacitive sensing system detects a button press, it relays this information over a Bluetooth Low Energy (BLE) link to a computer or Android Device. While this technology has numerous applications, we chose two: a slide presenter and a game controller. We are able to advance slides and control games like Flappy Bird and Tetris using buttons assembled with the human body. On an Android device, an application can listen for changes in contact and pause or play music.
Challenges we ran into
Bluetooth Low Energy (BLE) is challenging to understand and use as both a publisher and subscriber to data. The integration of capacitive sensing and BLE was challenging. BLE is hard to integrate with Windows, causing us to switch to Ubuntu for much of the work. Integration on Android was also challenging. The large volume of BLE devices at the hackathon caused the tablet to hit the maximum number of connected devices and not pair with the microcontroller.
As this method of developing temporary tattoos is still new, we went through many iterations in order to come up with a production workflow that would maintain the physical and electrical integrity.
Accomplishments that we're proud of
This technology is new and currently being researched. Within the span of a weekend we were able to build upon the original idea with our work and we developed several demos. Our implementation was done with a minimized cost and time investment while maintaining fairly accurate results compared to the original project.
What we learned
- BLE is a difficult protocol to implement within the scope of a weekend
- When something says don’t put on the skin, don’t put it on the skin
- Hammocks have become a quintessential accessory for hackathons
- The best place to make friends is around the coffee maker
What's next for Smart Tattoos
- Miniaturize the microcontroller to a Cypress board to reduce the cost and surface area
- Improve the connection reliability on multiple platforms
- Create more documentation to allow third-party integration
- Supporting slider and wheel tattoos or more types of interactivity