Input Devices

This week is all about input devices and basically trying to sense all kinds of things going on in the world around us! Given my interest in flexible and soft electronics, I decided to fabricate my own flexible sensors base on functional materials that I have worked with before, such as laser-induced graphene (LIG) and Gallium Indium eutectic liquid metal (LM). I decided to make two sensors: one would utilize the resistive sensing mechanism, and the other would use the capacitive sensing mechanism, two of the most common sensing mechanisms. For the microcontroller boards, I collaborated with Ozgun to create two PCBs, one for sensing and one for capacitive, which we can use with our fabricated sensors!

For the resistive sensor, I decided to use laser-induced graphene as the sensing material that makes my sensing elemnt. I have experience working with laser-induced graphene (LIG) before for motion sensing, so I decided to re-purpose my skills to make a flex LIG sensor that can measure bend angles. I got inspired by this article , published in nature communications, where they used a resistive LIG sensor to detect small throat vibrations such as hum, cough and scream with different intensities or frequencies! Additionally LIG has a nice range of resistance values which I think can be read easily by a voltage divider circuit without the need for extra amplification or filtering.

The sensing principle is straight forward and can be seen in the below diagrams I adopted from here As When the sensor is straight, the patterned LIG particles give a resistance of about 1.3k Ohms. When the sensor is bent, the LIG particles move further apart, increasing this resistance (to about 2.4k Ohms when the sensor is bent to about 90°)

Sensor Design and Fabrication

For the design, I wanted to experiment with many resistive arrangements, including straight lines , serpentine lines, shunts , and interdigitated designs, to see the effect on the performance. However, something catastrophic happened later, which left me with one functional sensor!

The fabrication process starts by irradiating a polyamide sheet with a CO2 laser using the optimized settings. Then, I cut the fabricated sensors into small parts and taped them to a dish to prepare for casting. Afterward, I tap copper tape with conductive adhesive to make the connection later on. Finally, I cast a thin layer of PDMS on the dish and leave it to cure in the oven at 60 degrees celsius for an hour

Before moving into Fusion 360 and PCB design, I tested the output of my fabricated sensor using an Arduino board and voltage divider circuit on a breadboard. After confirming that I could get detectable reads from the ADC, we started with the PCB design. We used the SAMD11 as our microcontroller and referenced Neil's example for the Wheatstone bridge circuit for the RTD sensor as an example. Two pads were defined as the pads for our sensor shown below.

The final exported board into MODS is shown below, as well as photos of the milling and programming steps!