- Light weight
- Low- or zero-power
Liquid crystals (LCs) are an intermediate state of matter between liquids and crystals. Platypus LC sensors use nematic LCs, which have no positional order, but self-align to generate long-range directional order (see image nearby). They are “birefringent”, a property that can be exploited to control how light passes through them. Light passes largely unaffected through a thin film of LCs when travelling parallel to the long axis of rod-shaped nematic LC molecules. Consequently, if such a film is located between a pair of crossed polarizers, no light will pass through. However, if the LC molecules are perpendicular to the light path, the LCs rotate the polarization of light, and the light now passes through crossed polarizers.
In TV and computer displays, the orientation of the LCs is controlled by changes in electric fields, thereby controlling whether light is transmitted. Platypus LC sensors instead use chemistry to control LC molecular orientation. As a result, sensors can be fabricated with extraordinary properties:
- No electrical power is needed to observe a response. When the sensor is placed between crossed polarizers, ambient light is all that is needed to read the response.
- Sensors can be robust, small and very light weight, making them suitable for diverse applications such as deployment on small drones and robots, and as discreet wearable items on people.
- Sensors can be inexpensive, facilitating broader use. For example, it will be possible to deploy LC environmental sensors at many more geographic locations than was previously affordable with existing sensor designs.
Platypus Technologies is developing LC sensors for toxic gases for use in environmental, industrial hygiene and military applications.
Simple LC Sensors
The simplest LC sensors consist of uniform thin LC films spread on small disks of glass. In the cartoon nearby, a reactive binding chemistry orients the LC molecules perpendicular to the glass substrate. When viewed in ambient light through crossed polarizers, the sensors are dark. However, when the target gas reacts with the binding chemistry, the binding chemistry no longer holds the LC orientation, and the molecules rearrange planar to the substrate surface. The planar LC molecules now rotate the polarization of light, and the sensor becomes bright when viewed through crossed polarizers.
Simle sensors can be integrated into electronic monitors to provide audible and visual alerts when toxic gas concentrations approach levels of concern.
LC dosimeters work on the same principle as simple LC sensors, but here the sensor is fully enclosed except for a ~20 µm gap at one end. When target gas diffuses in through the narrow gap, it reacts irreversibly with the binding chemistry, as shown in the nearby cartoon. As a result, the LC film reorients in the area of the sensor that has reacted, and the sensor becomes bright. The length of the bright area of the sensor is indicative of the total chemical exposure dose – i.e. a simple measure of the length of the bright front provides the exposure dose in ppm x hrs. This is how the ClearSense™ H₂S dosimeter works.
Current capabilities include LC sensing of H₂S, NH₃, NO₂, nerve gases, volatile organics and pesticides. Platypus technologies also offers custom solutions for diverse sensor needs. Please for more information.
Platypus Technologies owns or exclusively licenses over 40 patents and over 20 pending applications related to liquid crystal sensors. If you are interested in licensing opportunities, please for more information.