The Science Behind The Technology
by Linh Le, Co-Founder & CEO
Since the Nobel Prize-winning discovery of graphene about 10 years ago, this exciting molecularly thin and flexible nanomaterial has received significant attention for its potential in diverse applications ranging from nanoelectronic to energy storage to biomedical devices.1-3 Our founder research team at Stevens Institute of Technology discovered that that graphene oxide (GO), upon inkjet printing and subsequent thermal reduction at 200°C, functions as mechanically flexible and molecularly thin sensors for monitoring skin temperature,4,5 heart rate,6 sweating, and muscle motion. From this invention, we have exploited the “strong” negative temperature coefficient (NTC) behavior (which is characterized by an inverse exponential relationship between temperature and electrical resistance), of GO to develop and demonstrate the most conformal (>1,000 mechanical bending cycles), precise (±0.01°C), responsive (~0.1 s), and miniaturizable (10 nm thick and 50 m wide) temperature sensor.5 Since all other known temperature sensors are ceramic-, silicon-, and metal-based, the conformal nature of GO as a temperature-sensing material is truly unique.7, 8 Because of its extraordinary flexibility and thinness, we envision that “comfortable-to-wear” GO sensor arrays can be used for constant and wireless monitoring of a variety of pathophysiological developments (e.g., fever, exhaustion, blister formation prior to diabetic foot ulcer development,9-11 etc.).
For commercialization, our inkjet printing approach offers:
- a novel means of sensor array infusion in an additive and net-shape manner
- manufacturing readiness with industrial inkjet printers developed for flexible electronics.
Because of its extraordinary flexibility and thinness, we envision that “comfortable-to-wear” graphene sensor arrays can be used for constant and wireless monitoring of a variety of pathophysiological developments
For our previous studies, we used GO which was chemically exfoliated from graphite powder and partially oxidized by the most commonly used GO production method (i.e., Hummer’s).12
GO (graphene oxide) is:
- hydrophilic because of hydroxyl functional groups
- suspension stable in water up to 5% on a weight basis
- inkjet printable
However, this material requires heat treatment at 200°C after printing to function as sensors, and critically skin patch and textile materials cannot withstand such a heat treatment. On the other hand, pre-reduced GO is not printable due to the loss of hydroxyl groups and thus suspension instability. It is important to note that, while chemical reduction of inkjet-printed GO is possible at room temperature using a toxic and explosive reagent such as hydrazine, this type of chemical reduction methods is expected to be unsuitable for wearable textile applications due to skin compatibility issues and makes roll-to-roll production of sensor arrays difficult and dangerous.
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2. Cohen-Karni, T.; Qing, Q.; Li, Q.; Fang, Y.; Lieber, C. M. Nano Letters 2010, 10, (3), 1098-1102.
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5. Kong, D.; Le, L. T.; Li, Y.; Zunino, J. L.; Lee, W. Langmuir 2012, 28, (37), 13467-13472.
6. Le, L.; Lee, W. Y.; Boon, E.; Nguyen, N. A.; Dinh, T. Wearable Graphene Sensors for Physiological Monitoring. 2015.
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8. McGee, T. D., Principles and methods of temperature measurement. John Wiley & Sons: 1988.
9. Roback, K. Expert Review of Medical Devices 2010, 7, (5), 711-718.
10. Mayfield, J. A.; Reiber, G. E.; Sanders, L. J.; Janisse, D.; Pogach, L. M. Diabetes Care 1998, 21, (12), 2161-2177.
11. Lavery, L. A.; Higgins, K. R.; Lanctot, D. R.; Constantinides, G. P.; Zamorano, R. G.; Athanasiou, K. A.; Armstrong, D. G.; Agrawal, C. M. Diabetes Care 2007, 30, (1), 14-20.
12. Hummers, W. S.; Offeman, R. E. Journal of the American Chemical Society 1958, 80, (6), 1339-1339.