Textile Displays

Recently, the role of clothing has evolved from merely body protection, maintaining the body temperature, and fashion, to advanced functions such as various types of information delivery, communication, and even augmented reality. Using Bluetooth or wireless connection or displaying the information on the fabric our clothes are starting to become intelligent. Textile displays are poised to revolutionize current electronic devices and reshape the future of electronics and related fields such as biomedicine and soft robotics. If the idea of fabric displays has been around since 2005 when Philips Research has demonstrated photonic textiles that integrate multi-coloured LEDs into the fabric, today a few technologies exist to create fabric displays. Among them, we can enumerate Electroluminescent Fibers¹, OLED² and PLED³ but also thermochromic paint⁴. They can even be self-powered and washable⁵. However, they remain relatively unavailable due to the difficulty of directly constructing electroluminescent or simple colour-changing devices onto the textile-like substrate to really display desired programmable patterns.

The one that caught my interest this time was the thermochromic paint.

Thermochromic paint

Thermochromic ink alongside liquid crystal, photochromic, hydrochromic, holographic, glow in the dark, invisible, magnetic, and conductive inks are considered as smart material technologies. Because of their unique appeal, they are usually used for advertising or packaging, but they also have their place in the industry as markers or for electronics manufacturing and announce an interesting era for cosmetics as shown by MIT Medialab trough their ChromoSkin⁶ project.

Thermochromic material is a combination of Leuco Dyes (also known as Colour Formers), Colour Developers and Melt Materials that are all blended and then microencapsulated with an impervious polymeric wall. This produces an aqueous slurry or concentrate which changes from colour to colourless as the temperature rises. The change can be reversible, reversible with memory effect or irreversible.

Goal of the tutorial :

This tutorial will allow creating a simple thermochromic display using reversible screen-printing thermochromic paint. We are going to create heat using conductive thread and conductive paint.

Working principle:

The conductive ink has a resistance of 55 Ω/Sq @ 50 microns. So depending on the number of layers we add, the paint is going to be more or less resistive. But remember that the more a material is resistive the more it heats when an electrical current is applied to it. So we’re going to use this property to create a heating system.

[layers and WP collage]

Material used :

SFXC Thermochromic Color Changing Screen Ink for Textiles
Barepaint Electric Paint
Vinyl polyurethane PromaFlex Pu
Hot press
Sewing machine
Vinyl plotter

Steps :

Create the stencil to apply the paint using the plotter. As the paints are both liquid, you want to be sure they wont mix trough the textile. So we’re going to separate them using a vinyl layer. We’re going to cut the stencil with a higher force applied to cut almost trough the whole material (90g).
Remove the excess of vinyl and thermically apply the “pixels” on the inside of the fabric.
From the plastic support resulted after the 2nd step remove the pixels using a scalpel. This is going to be your screen-printing stencil.
Using the sewing machine (00 or 01 sewing) and conductive thread as the bottom bobbin, make 2 parallel lines on the pixels as in the picture below. Go forth, stop when reaching the end, but don't cut the thread. Stop the machine with the needle down, release the footer and turn around the fabric. Lower the footer and go back. Do 2 back and forth on each line.
In a transparent glass mix the screen-printing ink in a 50:50 ratio (base / thermochromic pigment) and apply it on the outside of the fabric using the stencil. Match the pattern with the one on the inside! Let it dry.
Using the same stencil, after cleanup, apply the conductive paint on the inside. Be careful to cover the conductive wires. Don’t apply too much but let the layer have a medium thickness (0.5 - 1 mm). Let it dry.
READ THE NOTE Connect it to the power source. Use the stabilized DC generator not a battery the first time. Once you have the green light on the generator choose a battery accordingly to the values.

NOTE: depending on the humidity left in the fabric your conductivity won’t be the same during time. Also, from one pixel to another your impedance is different so first, you'll need to make sure about the values of voltage and amperage that you’ll need.

Results and upcoming improvements :

At this point, you have a semi-flexible / semi-stretchable thermochromic display.

Be careful: exposed to the air, after a certain amount of time the conductive paint will crack and fall. Keep it in a hermetical box.

I am currently working on the design of a flexible control card and looking for a way to protect the paint layers both on the inside and the outside.

REFERENCES

Zhang, Z., Cui, L., Shi, X., Tian, X., Wang, D., Gu, C., Chen, E., Cheng, X., Xu, Y., Hu, Y., Zhang, J., Zhou, L., Fong, H. H., Ma, P., Jiang, G., Sun, X., Zhang, B., Peng, H., Adv. Mater. 2018, 30, 1800323. https://doi.org/10.1002/adma.201800323
Choi, S., Kwon, S., Kim, H. et al. Highly Flexible and Efficient Fabric-Based Organic Light-Emitting Devices for Clothing-Shaped Wearable Displays. Sci Rep7, 6424 (2017). https://doi.org/10.1038/s41598-017-06733-8
S. Kwon, W. Kim, H. Kim, S. Choi, B. C. Park, S. H. Kang, and K. C. Choi*, Adv. Electron. Mater., 2015, 1, https://doi.org/10.1002/aelm.201500103
Xiaoqian Ji ab, Wenwen Liu a, Yunjie Yin a, Chaoxia Wang * and Felice Torrisi, Journal of Materials Chemistry C , 2020, 44, https://doi.org/10.1039/D0TC03144E
Eun Gyo Jeong, Yongmin Jeon, Seok Ho Cho and Kyung Cheol Choi*, Energy and Environmental Science 2019, 6, https://doi.org/10.1039/C8EE03271H
Hsin-Liu (Cindy) Kao, Manisha Mohan, Chris Schmandt, Joseph A. Paradiso, and Katia Vega. 2016. ChromoSkin: Towards Interactive Cosmetics Using Thermochromic Pigments. In <i>Proceedings of the 2016 CHI Conference Extended Abstracts on Human Factors in Computing Systems</i> (<i>CHI EA '16</i>). Association for Computing Machinery, New York, NY, USA, 3703–3706. https://doi.org/10.1145/2851581.2890270

How to create a simple thermochromic display