Thermochromic ink (also called thermochromatic ink) is a type of dye that changes color when temperatures increase or decrease.[1][2][3] It was first used in the 1970's within novelty toys like mood rings, but has found some practical uses in things such as thermometers, product packaging, pens.[4] This ink has also found applications within the medical field for specific medical simulations in medical training. Thermochromic ink can also turn transparent when heat is applied; an example of this type of thermochromic ink is found on corners of an examination mark sheet. This proves that the sheet has not been edited or photocopied, and also on certain pizza boxes to show the temperature of the product.

Composition

There are two main variants of the ink; one composed of leuco dyes and one composed of liquid crystals.

Leuco Dyes

The leuco dye variant is typically composed of the leuco dyes with additional chemicals that adds different desired effects. It is the most commonly used type because it is easier to manufacture. They can be designed to react to changes in temperature that range from -15 °C to 60 °C. Most common applications of the ink has activation temperatures at ~ -10 °C (cold), ~ 31 °C (body-temperature), or ~ 43 °C (warm). When at lower temperatures, the ink is dyed a certain color, and once the temperature increases, the ink becomes either translucent or lightly colored, which allows hidden patterns to be seen. This gives the effect of a change in color, and the process is also reversible as you can bring the temperature down to lower levels to make the original dyed color visible again. [5][6][7]

Liquid Crystals

The properties of liquid crystals can change from liquid to solid in response to a change in its temperature. At lower temperatures, the crystals are mostly solid and hardly reflect any light, causing it to look black in this phase. As it gradually increases in temperature, the crystals become more spaced out, causing light to reflect differently and changing the color of the crystals. The temperatures at which these crystals can react range from -30 °C to 90 °C.[5]

For both types of thermochromic ink, the chemicals need to be micro-encapsulated into extremely small capsules that are around the size of 3 to 5 microns. This protects the dyes and crystals from mixing with other chemicals that might affect the functionality of the ink. This ink contains ions with m/z (grams per charge) levels of 245 m/z, 356 m/z, 467 m/z, 578 m/z, 689 m/z, and 800 m/z. These ions come from a chemical called polyvinylpyrrolidone (PVP) (Chemical formula C6H9NO). [5][8]

Applications

On June 20, 2017,[9] the United States Postal Service released the first application of thermochromic ink to postage stamps in its Total Eclipse of the Sun Forever stamp[10] to commemorate the solar eclipse of August 21, 2017. When pressed with a finger, body heat turns the black circle in the center of the stamp into an image of the full moon. The stamp image is a photo of a total solar eclipse seen in Jalu, Libya, on March 29, 2006. The photo was taken by retired NASA astrophysicist Fred Espenak, aka "Mr. Eclipse".

Medical Uses

In medical training, thermochromic ink can be used to imitate human blood because it shares its color changing property. It is currently being tested in medical simulations involving extracorporeal membrane oxygenation (ECMO). In these procedures, a change in color of blood between a dark and light red indicates blood oxygenation and blood deoxygenation, which describes the oxygen concentration levels within a person's blood sample. It's important to accurately identify this change in order to safely and correctly operate the ECMO machines. This has led to simulation-based trainings (SBT) which allows medical students to run simulations that mimic real ECMO machines before using them in serious situations. By using thermochromic ink in these simulations, the color changing effect can be realistically copied and observed without using real human blood or other costly methods. [11][12]

Artificial blood or animal blood are typically used in these simulations, however there are some advantages in using thermochromic ink as an alternative. It can be reused for multiple simulations with minimal variance in the outcomes and it is more cost effective. There are limitations to using this as the ink does not share any other properties with blood, so it's only practical use is to observe the change in color of blood. [11]

Product Packaging

Product packaging is an important aspect of maintaining the quality of consumer goods. Modern day packaging is split into 2 categories; active packaging and smart packaging. Thermochromic ink has found use in smart packaging, which is the aspect of packaging that deals with monitoring the condition of the products. Since most of the consumer goods are affected by changes in temperature, using thermochromic ink as an indicator of those temperature changes allows consumers to recognize when the quality of a product has changed. It can also be used to tell consumers the right temperatures to consume the product. [13]

Erasable Ink Pen

In 2006, Pilot Pens Inc. developed a pen with erasable ink that utilized thermochromic ink. It was composed of a solvent, a colorant, and a resin film-forming agent. At temperatures below 65 °C, the ink stayed in a colored state. Once temperatures went above 65 °C, the ink began to melt and became colorless, creating the effect of erasable ink. The ink was able to return to its colored state by cooling the temperature down to below -10 °C.[14]

See also

References

  1. ↑ Thamrin, T S (2022). "Thermochromic ink as a smart indicator on cold product packaging - review". IOP Conf. Series: Earth and Environmental Science. 1063 (1): 012021. Bibcode:2022E&ES.1063a2021T. doi:10.1088/1755-1315/1063/1/012021. S2CID 251368125.
  2. ↑ Yan, Xiaoxingng (2020). "Effect of Concentration of Thermochromic Ink on Performance of Waterborne Finish Films for the Surface of Cunninghamia Lanceolata". Polymers. 12 (552): 552. doi:10.3390/polym12030552. PMC 7182879. PMID 32138270.
  3. ↑ Kulˇcar, Rahela (2011). "Dynamic colorimetric properties of mixed thermochromic printing inks". Color Technology. 127 (6): 411–417. doi:10.1111/j.1478-4408.2011.00338.x. Retrieved 24 December 2022.
  4. ↑ "How Thermochromic Ink Works". HowStuffWorks. 2012-05-08. Retrieved 2023-10-24.
  5. 1 2 3 "How Thermochromic Ink Works". HowStuffWorks. 2012-05-08. Retrieved 2023-10-24.
  6. ↑ Kooroshnia, Marjan (2013). "Leuco Dye-based thermochromic inks : recipes as a guide for designing textile surfaces". AUTEX World Textile Conference. {{cite journal}}: Cite journal requires |journal= (help)
  7. ↑ Kulčar, Rahela; Friškovec, Mojca; Gunde, Marta Klanjšek; Knešaurek, Nina (2011-10-07). "Dynamic colorimetric properties of mixed thermochromic printing inks". Coloration Technology. 127 (6): 411–417. doi:10.1111/j.1478-4408.2011.00338.x. ISSN 1472-3581.
  8. ↑ Khatami, Amin; Prova, Shamina S.; Bagga, Aafreen K.; Yan Chi Ting, Michelle; Brar, Gurnoor; Ifa, Demian R. (2017-06-30). "Detection and imaging of thermochromic ink compounds in erasable pens using desorption electrospray ionization mass spectrometry". Rapid Communications in Mass Spectrometry. 31 (12): 983–990. Bibcode:2017RCMS...31..983K. doi:10.1002/rcm.7867. ISSN 0951-4198. PMID 28370721.
  9. ↑ "Total Eclipse of the Sun to be commemorated on a Forever Stamp". United States Postal Service. 2017-04-27. Retrieved 2017-07-10.
  10. ↑ "Total Eclipse of the Sun". United States Postal Service (store). Archived from the original on 2017-09-24. Retrieved 2017-06-27.
  11. 1 2 Alsalemi, Abdullah; Aldisi, Mohammed; Alhomsi, Yahya; Ahmed, Ibrahim; Bensaali, Faycal; Alinier, Guillaume; Amira, Abbes (2017-02-14). "Using thermochromic ink for medical simulations". Qatar Medical Journal. 2017 (1 - Extracorporeal Life Support Organisation of the South and West Asia Chapter 2017 Conference Proceedings): 63. doi:10.5339/qmj.2017.swacelso.63. ISSN 0253-8253. PMC 5474631.
  12. ↑ Noorizadeh, Mohammad; Alsalemi, Abdullah; Alhomsi, Yahya; Sayed, Aya Nabil Khalaf Mohamed; Bensaali, Faycal; Meskin, Nader; Hssain, Ali Ait (2021-07-11). "Advanced Thermochromic Ink System for Medical Blood Simulation". Membranes. 11 (7): 520. doi:10.3390/membranes11070520. ISSN 2077-0375. PMC 8306066. PMID 34357170.
  13. ↑ Thamrin, E. S.; Warsiki, E.; Bindar, Y.; Kartika, I. A. (2022). "Thermochromic ink as a smart indicator on cold product packaging - review". IOP Conference Series: Earth and Environmental Science. 1063: 012021. Bibcode:2022E&ES.1063a2021T. doi:10.1088/1755-1315/1063/1/012021. S2CID 251368125.
  14. ↑ Khatami, Amin; Prova, Shamina S.; Bagga, Aafreen K.; Yan Chi Ting, Michelle; Brar, Gurnoor; Ifa, Demian R. (2017-06-30). "Detection and imaging of thermochromic ink compounds in erasable pens using desorption electrospray ionization mass spectrometry". Rapid Communications in Mass Spectrometry. 31 (12): 983–990. Bibcode:2017RCMS...31..983K. doi:10.1002/rcm.7867. ISSN 0951-4198. PMID 28370721.
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