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ELPH skin

A flexible and low-power electrophoretic (e-ink) display worn on the skin, like a temporary tattoo.

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Epidermal electronics have been in development for several years; most devices are designed to collect biometric data to be displayed on an external device or use the surface of the skin as an interface for touch input. This project aims to develop a platform in which the skin's surface becomes the display itself.

I'm using a flexible e-paper display due to its low energy consumption, lightweight and micrometer-thin components, daylight readability and ultra wide viewing angle. The grayscale qualities of e-ink make it seem like an appropriate medium to mimic tattoo designs.

This project includes research and development of a thin and flexible multilayered PCB that can be produced with commercially available equipment. The PCB would then be bonded to a layer of silicone that temporarily adheres to the skin.

I’m completing this project during my Supplyframe Design Lab residency.

Design

exploded view


The face of the tattoo is a flexible e-ink display with the circuit board and power completely concealed underneath. All of this will be adhered to a layer of silicone with a skin-friendly adhesive. Note that this sketch is not to scale, the layers are all 2mm thick or less. It will be a challenge to get all of these components sandwiched together without shorting out.

I haven't determined power consumption but I will likely need to constrain features and framerates based on the largest capacity battery that I can find that's smaller than the physical dimensions of the display. I still need to figure out the connectors I'll need to recharge the battery but I do want to avoid using rigid materials here and throughout.


Hardware Block Diagram

This is a high-level overview of the various subsystems and components for the device. Further details regarding this diagram are captured in the project log below.

p363-kawahara.pdf

Instant Inkjet Circuits: Lab-based Inkjet Printing to Support Rapid Prototyping of UbiComp Devices

Adobe Portable Document Format - 7.71 MB - 03/09/2017 at 22:58

Preview Download

  • Testing Images on an ePaper Display

    Hunter05/12/2017 at 19:24 0 comments

      While I’m waiting for my display I bought a backup version to test some graphics on. It’s the Waveshare 4.3inch e-Paper kit and is equipped with an STM32F103ZET6 microcontroller, 128MB of Nand Flash memory and a UART interface. The resolution is 800x600 and sits on top of the circuit board. It’s also ridiculously easy to use and can be purchased online for around $50US.

      Waveshare provided its own proprietary software but I opted to use this Arduino EPD library from sabas1080 on github to display bitmap images and draw some graphics onto the screen. The commands are as follows:


      Startup & communication

      epd_init() - initialize display (wakeup & reset)
      epd_reset() - reset display
      epd_wakeup() - wake up display
      epd_handhshake() - establish connection
      pd_set_baud() - set baud rate
      epd_read_baud() - read baud rate
      epd_set_memory() - Memory Mode: MEM_NAND or MEM_TF
      epd_enter_stop_mode() - stop


      Screen

      epd_screen_rotation() - set screen rotation EPD_NORMAL or EPD_INVERSION
      epd_clear() - clear screen by drawing all pixels black then white
      epd_update() - refresh screen


      Fonts

      epd_load_font() - load font from memory
      epd_set_en_font() - set English font
      epd_set_ch_font() - set Chinese font
      epd_set_color() - set font and background color (WHITE, GRAY, DARK_GRAY, BLACK)


      Drawing

      epd_draw_pixel() - draw a point at (x0, y0)
      epd_draw_line() - draw a line starting at (x0, y0) ending at (x1, y1)
      epd_fill_rect() - draw a filled rectangle starting at (x0, y0) ending at (x1, y1)
      epd_draw_circle() - draw a circle with center at (x0, y0) and radius (r)
      epd_fill_circle() - draw a filled circle with center at (x0, y0) and radius (r)
      epd_draw_triangle() - draw a triangle with point 1 at (x0, y0), point 2 at (x1, y1) and point 3 at (x2, y2)
      epd_fill_triangle() - draw a filled triangle with point 1 at (x0, y0), point 2 at (x1, y1) and point 3 at (x2, y2)
      epd_disp_char() - display a character at (x0, y0)
      epd_disp_string() - display a string starting at (x0, y0)


      Images

      epd_load_pic() - load image from memory
      epd_disp_bitmap() - display bitmap image starting at (x0, y0)


      Basic Program Structure

      In setup() call:

      1. epd_init()
      2. epd_wakeup()
      3. epd_set_memory(mode)

      In loop() call:

      1. epd_clear()
      2. code for whatever it is that you want to draw
      3. epd_update()
      4. delay(miliseconds)


      Display
      http://www.waveshare.com/4.3inch-e-paper.htm

      Wiki
      http://www.waveshare.com/wiki/4.3inch_e-Paper_Software

      Waveshare user manual
      http://www.waveshare.com/w/upload/7/71/4.3inch-e-Paper-UserManual.pdf

      Software
      http://www.waveshare.com/wiki/4.3inch_e-Paper

      Waveshare Arduino library
      https://github.com/sabas1080/LibraryEPD

  • Supplier Delays

    Hunter05/02/2017 at 17:27 0 comments

    When I started this project, I was fully aware that I had a difficult road ahead, the technical aspects of this prototype are tricky as are the material considerations for flexible circuitry.

    What I didn’t predict were the challenges with procurement. I mentioned previously that I was considering two flexible screens, one manufactured by Plastic Logic and the other by eInk. On my first day of my residency I inquired about ordering a display evaluation kit from Plastic Logic. After some hesitance from them about being a “certifiable business” Dan (Resident Engineer of the Design Lab) was finally able to submit an order with them but I still haven’t received the display.

    As a backup plan, I ordered an evaluation kit and flexible display from eInk and they shipped it right away. But out of the box it doesn’t work and since they are in Taiwan, the back and forth with their sales and technical support team has been lagging. I requested their source code and a data sheet for the display itself since I would need this anyway since I am developing my own hardware and software. They agreed on the condition that I sign an NDA, which I waiting on them to send.

    So at this point I am halfway through my residency and I don’t have a physical prototype. This is frustrating and even though I am still working full time on this project, each day I struggle with finding the motivation to continue. But I did learn a valuable (albeit sobering) lesson that manufacturers aren’t as motivated to work with small developers.

    So while I wait on the other two displays, I have ordered a third, which Todd Kurt (another Design Lab resident) recommended. It’s a rigid 4.3” ePaper display from Waveshare and is designed to be easier to work with. In an earlier post I referenced a project from davidgf.net that uses this same hardware and he added an ESP8266. Out of interest of moving forward, I am going to try it out -- even if it’s not the most direct path to my end goal. I’ll let you know how it goes.

  • Educational Resources for Designing Embedded Systems, Part I

    Hunter04/18/2017 at 19:42 0 comments

    image source: unknown

    My display and dev kit still haven’t arrived yet so I’ve had ample time to get up to speed with designing for embedded systems. My former career was in web and app development before I began working with Arduino and Raspberry Pi based systems. I consider myself an “advanced beginner” because with open source frameworks it’s easy to jump and start building without fully understanding what’s going on. I think of them as a gateway drug into engineering and computer science in that they provide the dopamine rush of building something amazing very quickly, but after a few times of doing this I developed a tolerance and like any addict, I’m continually on the hunt for my next peak experience.

    After starting this project I quickly realized that I don’t have an intuitive grasp on embedded applications and needed a thorough primer on the fundamentals. And I needed to do it fast! Here are some of the resources that have helped me get up to speed with both hardware and software design, specifically for the MSP430 microcontroller.

    Designing Embedded Hardware: Create New Computers and Devices by John Catsoulis is a very readable and high level overview of hardware architecture. In the Carrollian Wonderland of Electrical Engineering the rabbit holes (analog, DSP, FPGAs!!) but this book does an excellent job in outlining exactly what is relevant in digital electronics for computers. The chapters on peripherals and serial communication were especially helpful.

    Despite being a voracious reader it’s hard for me to find technical books that I can consume cover to cover. Making Embedded Systems: Design Patterns for Great Software by Elecia White is among the top in this category. I can tell that Elecia is very passionate about her craft and while she provides an overview of the technical topics, she also exposes the art of designing embedded systems and I found this be the most valuable. It helps to have at least an introductory-level knowledge of computer science concepts but even if you don’t, this text should whet your appetite for more. Use it to get your bearings and you won’t feel as bewildered when referencing more in-depth materials for specific topics. The introductory chapters and the section on state machines and interrupts are excellent.


    Introduction to Embedded Systems: Using Microcontrollers and the MSP430

    I haven’t read this book in its entirety and am using it as more of a reference but this is an ideal text if you are learning the MSP430 but are still new to the world of micro controllers. It’s a textbook so there’s plenty of theory and math and problem sets but it’s also a practical and in-depth guide on architecture and processor organization, interfacing with microprocessors, peripherals and serial communication. The chapter on Assembly is solid but the subsequent chapter on C is high level so I would use another resource if you intend to learn how to tweak or write your own firmware.

  • Hardware Block Diagram

    Hunter04/03/2017 at 18:54 2 comments

    Last Monday I finished the block diagrams for the hardware and have begun hardware prototyping. These are high-level, but provide an overview of the various subsystems I will need to build.

    Architecture

    Plastic Logic is the company that manufactures the flexible e-ink display I’m using for the final prototype and I’ve based the hardware architecture on their evaluation kit. I’ve made some modifications, namely adding an ESP8266 module for wifi connectivity, using a different LDO / battery charger and simplifying all of the connections and access points so that I can fit everything within the footprint boundaries of the display itself and update the firmware wirelessly. I may end up using the ESP-12S since the footprint is only a little larger than the ESP 07 and has a 4MB flash chip. I'd like to utilize this for the MSP430 if it's possible.

    MCU

    I am using the MSP430 microcontroller to control the display itself. Since PLastic Logic also has an SDK, I am sticking with the same hardware to minimize the amount of firmware that I will have to write from scratch. If I could just run all of this off the ESP module I would, but with only a few months I want to avoid introducing any unnecessary risk and extra work and focus on just getting the thing working.

    Power Supply

    Note all of the various voltages coming out of the power supply. WTF?!?! I’ve never worked with voltages this high and the idea of placing this right next to my skin is a little unnerving (but exciting!). I did quite a bit of reading into how these e-ink displays work and in subsequent posts I will get into further detail, but the short answer is every pixel has a source voltage (+15 & -15V) and gate voltage (+25 & -32V) connected to the electrodes on the screen through thin-film transistor FETs and these need to be high enough to affect the ink particles in order to fully switch. It seems that the 9V is a reference voltage, I'm not exactly sure why this is, but I will find out.

    Data

    The display also has a temperature sensor that's needed to adjust the waveforms it uses and I2C is required for that sensor, so I will need both this and SPI in the final design.

  • One month in...

    Hunter03/29/2017 at 03:36 0 comments

    This month went by too fast, I feel like I just got here. The photo above is of a simple circuit I printed with the inkjet ( I also cheated a little by adding silver marker on top). While the traces themselves aren't so bad and the circuit did work, the conductive epoxy is awful to work with! If it was this difficult with through-hole components, I can imagine how it would be with surface-mount. I think the precision would be improved by using a syringe for the epoxy, but I will wait until later to try this out.

    The deeper I dive into this project the more I realize how ambitious it is. I knew going in that it would take a small miracle for me to pull it off, as did Dan* and Supplyframe. But when I am locked onto an idea it’s difficult for me to be realistic. It’s easier to work harder than it is to adjust my expectations. My mind has a mind of its own. Must be an ego thing.

    I'm feeling the pressure with 3 months left. I told Dan (resident engineer at the Design Lab) yesterday that I was going to start listening to him. He looked at me in disbelief and said, “Do you know how rare it is for me to hear that phrase?” I responded, “Do you know how rare it is for me to say it?”

    So, when you suspect that the ship is bearing too much weight, putting you and your precious cargo at risk, what’s the first thing you do? Unload some of your cargo in order to preserve the remainder. I’m putting the research & dev on a flexible circuit board on hold so that I can focus fully on hardware and software development. While the inkjet tests were fun and somewhat promising and I have so many more paths to explore with other materials (Kapton, Pyralux), and methods (silkscreen, lasercutting, chemical etching) I also know that it could take months or longer to come up with a robust and working solution to this problem, if it's even possible. There are manufacturers I can reach out to that have already developed a reliable and flexible (and multilayer!) circuit board and seeing how they do it may give me more ideas. But first, I need to design and build the thing.

  • Hardware Research

    Hunter03/21/2017 at 00:47 0 comments

    Plastic Logic's e-paper display


    There are several others before me that have attempted to drive TFT active matrix electrophoretic displays and have graciously documented their experience:

    Petteri Aimonen http://essentialscrap.com/eink/

    ED060SC4 display with STM32 microcontroller programmed with C

    Spritetm http://spritesmods.com/?art=einkdisplay&page=1

    ED060SC4 display with ESP8266 and PhantomJS pixel server

    David Guillen Fandos https://davidgf.net/page/41/e-ink-wifi-display

    GDE043A2 display with an STM32, ESP8266 and PHP

    Jara https://hackaday.io/project/1181-pip-watch

    GDE021A1 display with STM32 and Bluetooth


    j0z0r pwn4tr0n https://hackaday.io/project/10161-6-inch-pi-e-ink-display

    ED060SC4 display with Raspberry Pi

    Sapir https://hackaday.io/project/7443-e-ink-display-adapter

    ED060SC4 display STM32

    This week I am drafting some block diagrams and starting the schematics for the power management circuit while I wait for my test displays to arrive. Reading about how these displays work is fascinating. There's something about their appearance that feels not quite digital and that's due to the millions of microcapsules of ink that are charged with 15V and -15V source voltages. The displays I'm considering have a TFT (thin film transistor) array just like LCDs. Each pixel is attached to a transistor and capacitor that store the current state while the other pixels are being addressed. The image is retained even when all power sources are removed.

    As of now my plan is to use a flexible display (Plastic Logic's or E-ink) with an ESP8266. At a minimum, I'd like to be able to update a bitmap image wirelessly, ideally I would like animation or write a program that draws to the screen directly. All of this will be highly dependent on power consumption and the amount of time I have. We'll see how far I get during my residency, but a functioning prototype with all electronics integrated into a single on-skin device is my primary goal, no matter what path I take to get there.

  • Materials Research

    Hunter03/20/2017 at 08:25 0 comments

    a few of the papers I reviewed and test plan notes

    I've been switching back and forth this week between development of the flexible circuit board** and designing the driver board hardware so I am wading in white papers and data sheets. I'm just a one-person team but parallel pathing this project is necessary since these two streams depend on each other. Research is by far my favorite part of the process, there's a lot of uncertainty about whether or not my design will work. I also learn a lot of interesting information that is completely irrelevant to my project. I'm a diligent procrastinator.

    I've converted an inkjet printer to become a circuit printer and after reading some research from others who have attempted the same or something similar, I have some ideas on what to test and measure and techniques to get better quality prints. I'll be sharing some photos of the conversion and results from the testing in my next post.

    I also learned about how rigid circuit boards are made and it's a pretty wasteful process. One of the reasons why I chose inkjet over other flexible circuit options is that it's an additive method and generates less waste. It's also very fast. Theoretically. Since we have an Othermill CNC here in the lab perhaps I can pit the two processes against each other. If I do I'll broadcast it live.

    ** using the word "board" when referring to a flexible substrate is a contradiction; I need to come up with a new descriptor

  • First shipment!

    Hunter03/09/2017 at 22:56 0 comments

    Got my initial shipment of supplies that will enable me to rapidly prototype with printed circuits. I will be using an inkjet printer to extrude silver nanoparticle ink so that I can print traces onto thin transparent film. Several layers of this film sandwiched together will form the circuit for my display driver.

    I'm using the research paper "Inkjet Printed Circuits" as reference for best practices for inkjet printed circuits.

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