Introduction to Rapid Prototyping: Wearable Devices

“Wearables”, the word that defines the next revolution in consumer electronics, gadgets are moving from being portable or mobile to being simply…wearable, either embedded inside your shirt fabric or around your wrist, in your shoes or in the necklace you are wearing, and even inside your sunglasses, anything you wear or touch your body can be employed as a mount for such gadgets to either measure your biological vital signals like body temperature, pulse rate, blood pressure, motion & physical activity, muscles, heart & brain activity, and/or provide you with real-time assistance like geo-navigation, healthcare monitoring, personal assistance (appointments calendar, to-do list…etc.).

In this article we will discuss main guidelines of designing and prototyping for wearable devices, the basic rules for prototyping doesn't differ much than the ones discussed previously in the first two articles in the series “Introduction to Rapid Prototyping, adding to them the following guidelines to complete the full picture:

• It has to be light weight (both the gadget and power source).
• With very small physical profile.
• Ultra-low power consumption.
• Easy user interface.
• Flexible, so it can be shaped on human body.
• Easy to install and setup.
• Material made of has to be skin-friendly and never cause any biohazard (e.g. skin irritation, allergies…etc).
• The gadget has to be electrically safe (no electric shocks, short circuits, dissipated heat).

The three stages of a wearable device: 1. Sensing, 2. Signal processing, digitization, data processing & storage,  3. Data transmission to a remote host for further analysis and visualization.

Sensors:

Sensors are electronic devices that convert physical quantities like temperature, light, motion…etc. to electric signals so to be easily interfaced to a computer, since computers don’t understand anything except electric signals (this simple definition of sensors targets non-technical readers).

Let’s make a list of the physical quantities a wearable gadget would measure and sensors that can be used in each case:

Physical quantity	Example	Sensor	Suggested mounting
Motion (for humans or objects)	Walking, running, physical exercising.	Accelerometer can measure object acceleration. Gyroscope can measure tilting/angle of inclination.	Wristband, embedded in shoes or fabric.
Heart rate	Heart pulse rate per minute	Pulse rate sensor	Wristband, ear clip.
Fatigue and exhaustion	Heart activity	Electrocardiogram (ECG)	Embedded in fabric.
Body temperature	Skin surface temperature, ambient temperature	Thermistor & thermocouple sensor.	Wristband, embedded in fabric
Shock	Physical impact	Piezoelectric shock sensor	Embedded in shoes/footwear.
Force	Weight, muscle strength/force	Strain gauge	Embedded in shoes/footwear.
Touch	Finger tips touch	Capacitive pads	Embedded in fabric.
Light	Ambient light, reflected light from a certain source	Photo-diode.	Wristband, finger clip.
Surface bio-signals	Muscles and limbs activity	Electromyogram (EMG)	Embedded in fabric/cloth.
Mental stress/activity	Brain activity like concentration, stress, emotion	Electroencephalogram (EEG)	Embedded in eyeglasses or headwear.

Example of off-the-shelf sensors and modules:

·       MXR7900: 2-Axis accelerometer from MEMS IC can measure up to ±0.5g acceleration in X- and Y-Axis.

·       LPY503AL: 2-Axis gyroscope from ST Microelectronics can measure up to ±30° degrees in yow- and pitch-directions.

·       TMP36: Ambient temperature sensor IC from Analog Devices.

·       OPT101: Ambient light sensor IC from Texas Instruments.

Power storage

The wearable device requires a power source to keep it running for enough time to do the required measurement or assistance to the user; however the known constraints of physical profile and weight of the wearable device must be taken into consideration when choosing the power source.

So far batteries are the best option for wearables in terms of physical profile and portability, however they aren’t sustainable and require replacement or recharging on regular basis, there are three different kinds of batteries available in the market for wearables:

• Lithium Ion (Li-ion) batteries: the most commonly used type of batteries used in electronics, with the advances in technology it comes in different shapes and forms (flexible and solid).
•  Solid-state batteries: made of semiconductor materials, usually meet the size require but have a very low capacity compared to Li-ion ones.
• Super-capacitors: a high capacity electrochemical capacitors, their size and leakage problem are the main obstacles to use them in wearables. 

Energy harvesting

Another way of providing electric power to wearables is energy harvesting (AKA. energy scavenging), it is the process of converting waste energy from the surrounding environment found in the forms of heat, motion or light to usable electric power, this field is fairly new in comparison to batteries and research didn’t achieve to a revolutionary design that can replace batteries or any other form of energy storage till the date of the article. There are three types of energy harvesting based on the waste energy they harvest:

• Thermal energy harvesting: harvesting waste heat from the surrounding environment or  from human body, Thermoelectric Generator (TEG) is used for this purpose.
• Vibrations energy harvesting (VEH) is the process of harvesting waste kinetic energy resulted from motion and vibrations into electricity by the means of electromagnetic, electrostatic or piezoelectric transduction.
• Solar energy: as most of readers have read or saw it before working, Photovoltaic (PV) cells are used to convert sun light to electricity and by far it is the most successful form of energy harvesting, solid glass solar panels aren’t easy to use with wearables but flexible solar panels are more convenient and design-friendly. 

Interface & Communication:

Microcontroller Unit (MCU):

The microcontroller is the processing unit of the wearable device responsible of reading the signals from sensors, process them to become useful data, display it to the user and finally communicate it if required with any external device via wired (USB as an example) or wireless means (Bluetooth, Zigbee…etc.). The microcontroller unit has to meet the power requirement of low power consumption since the whole gadget is running on a limited power supply like a battery, the following microcontroller families are good candidates for wearable designs in the sense of low-power consumption and low cost:

• Texas Instruments MSP430, 16-bits RISC CPU.
• Microchip PIC18 and PIC12, 8-bits RISC CPU.
• ST Microelectronics STM8, 8-bits RISC CPU.
• Atmel ATmega, 8-bits RISC CPU.

Data display and communication:

Another important feature a wearable gadget must have is data display, either by sending it to an external device or on the gadget itself using a small LCD display or LED indicators, and because of the of limitations in power supply and physical profile design most wearable designers prefer to send data to an external device for display. There are two ways to communicate data to external world:

• Data is stored on an on-board memory then the user can retrieve it by connecting the gadget to a computer or a smartphone wired or wirelessly using USB, Bluetooth, Wi-Fi or Near Field Communication (NFC).
• Data is communicated in real-time to a nearby device via short range wireless technologies like Bluetooth, Wi-Fi, ZigBee or similar protocols.

And here are some wireless enabled boards and modules that can be used for wearables either stand-alone or combined with a microcontroller unit:

• Blend Micro: Bluetooth Low Energy (BLE) enabled Arduino development board from RedBear, very small form factor and low power consumption for 35$: http://redbearlab.com/blendmicro/
• Texas Instruments CC3200 SimpleLink Launchpad: Wi-Fi enabled microcontroller kit for 30$, has on-board 3-axis accelerometer and temperature sensor: http://www.ti.com/tool/cc3200-launchxl
• ESP8266: a very low cost, small form factor, low power Wi-Fi module with serial interface for wearables and IoT applications, can't operate independently, requires a microcontroller unit, starts from 5$ only: http://www.seeedstudio.com/depot/WiFi-Serial-Transceiver-Module-w-ESP8266-p-1994.html.
• Arduino Mini Pro: low cost, small form factor, Arduino compatible microcontroller kit for 10$: http://www.arduino.cc/en/Main/ArduinoBoardProMini 

Wireless communication between the wearable and display device is the preferred option for designers because it meets the portability and simplicity requirements easily, however this comes on the expense of power consumption since wireless communication modules are relatively power hungry when compared to the rest of the wearable components (MCU and sensors), however designers were able to overcome this problem by either lowering the data rate (speed), range and non-continuous transmission, low power technologies like Bluetooth Low Energy (BLE) and ZigBee use these techniques to lower power consumption and extend battery life.

Other design ideas

• As discussed previously wearables have to be small and light, also “flexible” if possible to take the shape of the human body/part it is worn on, and here comes the benefit of “flexible printed circuit board” (or Flex PCB), it is made of special polymers to be physically flexible and bend easily without any impact on the electric connections or components mounted on its sides.
• Conductive threads/plates: can be used to detect electric connectivity from an outside source or help transmitting a specific signal through clothes fabric.
• 3D printing: to accelerate prototyping of wearable gadgets 3D printing is a good option and much lower in cost in comparison of similar alternatives like mold injection and casting.

Flexible PCB
(Source: http://edablog.com/2009/03/10/utcp-wearable-electronics/)

Components manufacturers

This is a list of the most known manufacturers of sensors and semiconductors that can be used in wearable designs:

• Inertial sensors (accelerometers & gyroscopes): MEMS IC, STMicroelectronics, Analog Devices.
• Temperatures sensors: Maxim Integrates, Texas Instruments, STMicroelectronics, Analog Devices, Microchip Technologies.
• Light sensing: Texas Instruments.
• Analog Front End (AFE), Analog to Digital Converter (ADC), Sensor interface chips: Texas Instruments, Linear Technologies, Maxim Integrates, Analog Devices, Microchip Technologies.
• Energy harvesting solutions: Linear Technologies, Texas Instruments, Cymbet.
• Low-power microcontrollers: STMicroelectronics, Texas Instruments, Microchip Technologies.
• Flexible solar cells: http://www.flexsolarcells.com/PowerFilm-Solar-OEM-Components.php

For board design the following software tools are quite helpful for PCB layout, schematic capture and 3D design & modelling:

• PCB layout and schematic capture tools: DipTrace, KiCad, Altium, PCB Artist, OrCad, Mentor Graphics Pads.
• 3D design & modelling: SolidWorks, AutoCAD Inventor.

And since you have reached this point in the article then possibly you are thinking of building your own board, these companies offer PCB manufacturing services at low cost for prototyping purposes:

• Seeed Studio (also offer 3D printing services): http://www.seeedstudio.com/service/
• APCircuits: http://www.apcircuits.com/
• iMall (flexible PCB): http://imall.iteadstudio.com/open-pcb/pcb-prototyping/2layer-flexible-printed-circuits-fpc-5cm-x-5cm-max.html

Last but not least, this article is meant to be an introductory material for hobbyists and developers interested in learning rapid prototyping for wearable gadgets but it doesn’t cover everything; this article is meant to be your first step, meant to make you more hungry and curious to know more about wearables, so good luck.

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Karim El-Rayes
March 11, 2015
Vancouver, Canada