Liquid Crystal Thermometers

Liquid crystal thermometers tint in response to temperature due to changes in pitch of the helix-shaped cholesteric liquid crystals of which they are formed. The pitch is the distance between each ‘rung of the ladder’, as an analogy. The change in colour is reversible which means that they respond equally well to increases and decreases in temperature. This gives a cost-effective and continuous indication of temperature which is high resolution, of acceptable accuracy and with a latency of just a few seconds. Liquid crystal thermometers are safer to use than mercury and Galinstan (gallium, indium and tin) thermometers, and easier to use than infrared thermometers which rely on accurate device pointing.

Thermotropic vs thermochromic liquid crystals

Since we are talking about temperature changes and ensuing changes in material properties, let’s clarify a couple of important terms [4]:

• Thermochromic liquid crystals exhibit a change in colour when the temperature changes. (From ‘khroma’, Ancient Greek for ‘colour’).
• Thermotropic liquid crystals exhibit a change in transparency. (From the word ‘tropic’ which comes via Latin from Ancient Greek ‘tropē’, meaning ‘to change direction’).

Both effects can exhibit themselves simultaneously within the same material and both can be reversible.

How liquid crystal thermometers work

The liquid crystals we find in thermometers are in the cholesteric phase, which means they are grouped into parallel layers, and rotated with respect to one another, similar to a spiral staircase. Just as a reminder, the image below shows the various phases of liquid crystals; namely smectic, nematic and cholesteric, which are shown below from left to right, corresponding to the phases through which a liquid crystal changes with increases in temperature. In other words, when cold, a liquid crystal will typically be in the smectic phase. As the temperature rises, it will transition into the nematic phase, then to the cholesteric phase (more correctly called the ‘chiral nematic’ phase). After that, any further temperature increases will turn the material from a liquid crystal into an isotropic liquid.

Phases of liquid crystals, appearing in the left-to-right order corresponding to temperature increase

For cholesteric liquid crystals, the helical structure expands with temperature, much like the stretching of a spring. When the distance between consecutive layers matches certain wavelengths of light, colours are produced matching those same wavelengths by diffraction of the incident light beam. The temperature changes can be the result of thermal conduction, convection or radiation. These changes in colour are also sensitive to excessive ultraviolet light and extreme temperatures, which can cause liquid crystals to degrade.

Liquid crystal thermometer strips

The most common form factor for liquid crystal thermometers are self-adhesive strips which contain a series of temperature-sensitive elements with microencapsulated thermochromic liquid crystals sitting on a black background. Each liquid crystal element has been engineered with a defined pitch (typically with additives) so that successive elements will ‘turn green’ with step changes in temperature. Thus, as the temperature rises, these elements ‘light up’ to reveal the number that is hard-wired under them. The liquid crystals are thus black below (and above) their designed temperature range, and produce the colours of the rainbow, passing through the midpoint green, as the temperature rises or falls within the temperature range. As one element of liquid crystal turns green, the one above it may turn slightly tan coloured and the one below it may become slightly blue, depending on how the molecules have been engineered.

Telatemp liquid crystal thermometer strip, image attribution

The image below shows the breakdown of a typical liquid crystal strip showing the various layers, with the liquid crystals encapsulated in the middle layer. The white graphic print has prefixed numbers and the black backing film is placed under the liquid crystals. The liquid crystals are of course the only part of the structure that actually changes colour, illuminating the white numbers located above them, one by one, as the temperature increases.

Thermochromic liquid crystal strips, by Spotsee, image attribution

Real-World Applications

Real-world applications of liquid crystal thermometers outside of medicine include continuous monitoring of refrigeration rooms, industrial processing plants, automotive, pharmaceuticals, oil & gas, freight forwarding and mining. Most manufacturers will customise products according to client needs. Various ranges of temperature can be custom engineered, for example: 0º to 30ºC, 60ºC to 90ºC, 25ºC to 100ºC, etc, each designed with a specific application in mind.

Hospital Case Study

In a study [1] conducted in a paediatric ward at Port Moresby General Hospital (Papua New Guinea) on a sample of 200 children admitted with feverish symptoms, liquid crystal thermometers were used in parallel with mercury glass thermometers to compare their accuracy and ease of use by hospital staff. Measuring body temperature is of course a crucial component of patient monitoring and the mercury glass thermometer has always been considered the reference standard. However, mercury glass thermometers are fragile and glass breakages can produce the risk of toxic chemicals being released, with subsequent injury to skin and mucous membranes. Mercury glass thermometers have been largely replaced with tympanic membrane thermometers in developed countries, but are quite expensive and some studies have reported variable accuracy [1]. Hence the interest in lower-cost temperature measurement solutions for developing countries such as Papua New Guinea. In the study, the medical staff valued ease of use, reusability, accuracy, safety and indestructibility as positive attributes of liquid crystal thermometers, with further added benefits of no toxic materials, no special storage requirements and no calibration needed. The conclusions of this study indicate that liquid crystal thermometers can be used interchangeably with glass mercury and tympanic thermometers, giving accuracies between 0.1ºC to 0.2ºC. Some disadvantages of liquid crystal thermometers include the necessity to read their temperature immediately on removal from the patient. Also, these thermometers do require good lighting conditions and good eyesight in order to read the device.

References

1. Comparison of the Use of Liquid Crystal Thermometers with Glass Mercury Thermometers in Febrile Children in a Children’s Ward at Port Moresby General Hospital, Papua New Guinea, Journal of Tropical Pediatrics, URL 2. Liquid Crystal Lab Notes, Yerkes Winter Institute, URL 3. Thermotropic and Thermochromic Polymer Based Materials for Adaptive Solar Control, National Library of Medicine, URL 4. Thermochromic and Thermotropic Materials, Arno Seeboth, Detlef Lötzsch (2013), ISBN 9789814411028, URL 5. IUPAC Compendium of Chemical Terminology, or ‘Gold Book’, International Union of Pure and Applied Chemistry, URL 6. Preparation of a Liquid Crystal Thermometer, Univ. of Wisconsin – Madison, URL 7. Thermotropic Liquid Crystals for Temperature Mapping, National Library of Medicine, URL 8. Liquid Crystal Modelling, University College London, URL