From Smart Phones to Smart Clothes: Mobile Gas Monitoring Using Color Changing Fabric

Title: Colorimetric Gas Sensing Washable Threads for Smart Textiles

Authors: Rachel E. Owyeung, Matthew J. Panzer and Sameer R. Sonkusale

Year: 2019

Journal: Scientific Reports

DOI: https://doi.org/10.1038/s41598-019-42054-8

Featured image and figures reprinted under a Creative Commons License (https://creativecommons.org/licenses/by/4.0/) from Oweyeung, R.E. et. Al., Scientific Reports, 2019, 9, 5607.

As technology has advanced greatly over the last few decades, smart devices are now a significant part of our lives, with smart phones being the most prevalent and common item used. In addition to smart phones, it is now common to see other similar devices such as smart TVs, smart watches and home assistants such as Siri (Google Home) or Alexa (Amazon).

Looking at these, it is no surprise to see that advances in sensing technologies are also following this route, making use of the functionalities provided to us with these smart devices and using that in tandem with an electrical or optical sensitizer to determine levels of specific chemicals.

In this paper, Owyeung and co-workers have taken an optical sensitizer and imbued it into threads for weaving clothing. The color changes that these dyes undergo can be detected by the naked eye and can also be analyzed using the optical capability of a smart phone camera. This results in a novel sensing approach that involves fabrics that can be used in chemical and electrical sensing, which benefits from the high surface are to volume ratio of the threads.

Figure 1a. Diagram of the coating process, b. Photographs of thread after each dip-dry, c. SEM image of final thread (After PDMS coat)

To coat the threads with the desired sensitizing dye, the group employed a 3 step dip and dry method (Figure 1.). Regular cotton thread was first soaked in the dye, followed by an acetic acid rinse to increase the roughness of the surface, which allows for better adhesion of the final layer, as well as potentially removing any pre-existing thread coating. Finally, the thread was coated with a polymer, polydimethylsiloxane (PDMS), which allows the thread to maintain the flexibility while protecting the attached dyes from being removed in the regular clothes washing process. PDMS was chosen for its elastomeric properties, its hydrophilicity and its gas permeability.

They then demonstrated the effectiveness of this sensor through a series of tests. In this test, the threads were treated with 3 different dyes; bromothymol blue (BTB), methyl red (MR) and 5,10,15,20-tetraphenyl-21H,23H-porphine manganese (III) chloride (MnTPP). These threads were then exposed to varying concentration of gaseous ammonia and hydrochloric acid, as shown in Figure 2. Simply observing it with the naked eye, we can notice a distinct change in the colors of BTB and MR coated threads. This was confirmed using RGB intensities that were taken from the images of the threads, which showed a change that corresponded to the visual data. Additionally, there was no noticeable leaching of the dyes from the material when they were subjected to washing, which included simulated agitation to mimic the motions of a washing machine.

Figure 2. Images of coated threads (Left to right: BTB, MR, MnTPP) a. after treatment with various levels of ammonia gas, b. after treatment with various levels of gaseous hydrochloric acid, c&d. corresponding RGB data extracted from images in a&b.

While this is a very promising technology, one of the main things to consider when developing such materials is the sensitivity or reaction time. It would be a meaningless application if the threads took a long time to change color and was only noticeable after significant exposure to toxic gases. The group noted that the reaction time was directly related to the thickness of the polymer coating as that affects the kinetics of the gases passing through to reach the sensitizer.  

With this initial discovery, there is significant potential for the development of fabrics as sensors, which could prove very useful in fields that have significant potential exposure to toxic gases such as CO or benzene. While the current study only involves ammonia and hydrochloric gas, further development of these materials to cover a wider range of gases could mean a newer, safer way of simple detection of harmful, volatile gases.

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