Introduction: Wearable Skin Temperature Logger
As part of CU Boulder's CSCI 7000 Physical Computing for Good course I created a wearable device measure/record skin temperature and the wearer's thermal comfort (hot, warm, neutral, cool, cold). The goal is to eventually develop a system that can recognize the wearer's thermal state, based on skin temperature, and turn on heating or cooling to maintain thermal comfort.
The prototype described in this tutorial is broken up into two separate parts: the skin temperature sensing platform and the data acquisition module. For this project I designed a wearable arm sleeve to measure the wrist temperature and have the data acquisition module located on the upper arm. The data acquisition module receives user thermal input through the use of swipe gestures and records the skin temperature and the thermal state onto a micro SD Card.
The swipe gestures are labeled as the following :
2 Down Swipes = Cold,
1 Down Swipe = Cool,
No Swipe = Neutral,
1 Up Swipe = Warm,
2 Up Swipe = Hot.
This project can easily be modified to accommodate other sensors or better electronic parts.
Step 1: List of Materials
The following are some materials needed to put together the two pieces of the prototype.
Skin Temperature Platform Materials:
(1) Surface Thermistor http://www.omega.com/pptst/ON-409_ON-909.html
(1) Vibration Motor (Lilypad Vibe Board)
(.5" x 2') Heat Transfer Vinyl
(~2') Ribbon Cable
Female & Male Headers
Data Acquisition Module Materials
(1) SparkFun Power Cell - LiPo Charger/Booster
(1) SparkFun RGB and Gesture Sensor - APDS-9960
(1) Arduino Pro Mini
(1) microSD breakout board
(1) Heat Press Machine
Step 2: Data Acquisition Module: Description
The following is a PCB diagram of the data acquisition board I created in Fritizing. The PCB board includes the Arduino Pro Mini, mini SD Card, LiPo Charger, and header connections for the vibe motor, temperature sensor, and (optional heating pad). The footprint of the PCB is small enough to fit inside of an enclosure, but also includes additional space to incorporate other features in the future. I decided to use header connections for the vibe board and the temperature sensor so that the temperature sensing sleeve can be detached and washed if necessary.
The Arduino Pro Mini can be powered either by an FTDI cable from the top or by a 3.3V battery connected to LiPo breakout board for portability. The breakout board also allows for USB charging, which is located at the bottom of the board.
The PCB board can be created by following this tutorial https://www.instructables.com/id/PCB-making-guide/ or sent to a PCB manufacturer http://www.ladyada.net/library/pcb/manufacturers.h...
Step 3: Temperature Sensing Platform: Description
The next step is create the temperature sensor platform. To get a good skin temperature measurement, the sensor should constantly be in direct contact with the skin at all times. I decided to use a thermistor, which is a ceramic material that changes resistance depending on the measured temperature. The data acquisition module determines the skin temperature by indirectly measuring the resistance of the thermistor using a voltage divider and calculates the temperature using the Steinhart Equation. For a more in depth understanding of measuring temperature with thermistors check out: https://learn.adafruit.com/thermistor/using-a-ther...
To ensure the thermistor is constantly in contact with the skin, I chose to embed the thermistor into an athletic arm sleeve. These arm sleeves are made of elastane, polyester, rayon materials that are tight fitting on the body, but also stretchy to conform to various body types. The stretchiness of the materials also makes it difficult to sew using a regular sewing machine. Instead of I using conductive wires or sewing the ribbon cables into the fabric, I used heat transfer vinyl to seal/encase the wires into the fabric. I wouldn't have to worry about short circuits or loose connections by sticking with the ribbon cables.
Some disadvantages of the embedding wires with heat transfer vinyl:
- Rayon, elastane, polyester fabric are temperature sensitive and can melt easily. I tried to use a commercial iron to heat the heat transfer vinyl, but ended up burning my fabric. (Avoid burning fabric like I did!) I recommend sticking with a conventional heat press, which can ensure temperature control and the needed applied pressure to embed the wires.
- The heat transfer vinyl can adhere too well onto the fabric and reduce the stretchiness of the fabric. So there is a balance between using enough heat transfer vinyl to keep the wires embedded and without sacrificing stretchiness of the fabric.
- Once the heat transfer vinyl is melted onto the fabric it is extremely difficult to remove the vinyl or the wires out of the fabric without compromising the visual look or possibly structural integrity of the fabric. This makes it difficult to fix short circuits or change locations of the wire once embedded.
Step 4: Temperature Sensing Platform: Ironing/pressing
First layout the where you want the ribbon cables and sensors embedded into the arm sleeve. I've placed the temperature sensor and vibration motor closest to the wrist area, and the wires in the middle of the fabric.
Cut out the heat transfer vinyl that will be melted on top of the ribbon cables, leaving the vibe motor board and temperature sensor exposed. I suggest cutting the heat transfer vinyl with 1" - .25" extended outside of the ribbon cable to ensure a good bond with the fabric without being too stiff.
The heat transfer vinyl has two surfaces. One side is the actual vinyl, which is a matte surface, and will be placed onto the fabric. The other side is a plastic shiny/smooth surface to protect the vinyl from attaching to the heat press machine. The plastic side should face up and be in contact with the heat press machine surface. Use a sharp item and pick at both sides of the vinyl to determine which is vinyl and which is plastic. The vinyl side will peel off while the plastic will just be indented.
The vinyl will be set in using a heat press machine, which can be found in a t-shirt shop or purchased online http://www.uscutter.com/Heat-Presses?view_all for about $200-$300. The heat press should be set to approximately 310 - 320F. Placing the fabric, wires, and heat transfer vinyl in the heat press machine, the heat press heated platen is lowered to compress and heat up the materials. The materials should be compressed for approximately 10-20 seconds to ensure the vinyl has completely melted.
After 10-20 seconds, raise the heat press machine platen and remove the fabric from the machine. Let vinyl cool off and set into the fabric. After the fabric has cooled off (5-10 minutes), the plastic layer of the heat transfer vinyl can be peeled off. Pulling off the plastic before the vinyl has cooled may result in peeling off the vinyl as well.
Step 5: Data Acquisition Module Enclosure
Attached are the two .STL files to 3D print the cover panel and main enclosure. The cover slides on top of the enclosure to protect the electronics inside. Large cutouts on the exterior of the enclosure allows connections and access to the micro SD card, sensor header connections, FTDI connection and micro USB port for charging.
Since this was my first prototype and first 3D model created in SolidWorks, the location of the screw holes on the interior of the enclosure was not placed in the right position. They were suppose to provide a location to fix the PCB board onto the enclosure, however they were too close to the corners of the PCB to be drilled. The PCB board is also slightly smaller than the interior of the enclosure, so it slightly rattles inside the enclosure since it is not screwed down. Further adjustments to the enclosure design can be modified in the future.
Print out model using a 3D printer.
Clean off 3D printed support material using water jet.
Place PCB Board Inside enclosure
Slide in Elastic band sewed with velcro into the slot on the back of the enclosure.