Progress in the development of flexible and stretchable materials has led to the development of next-generation electronic devices. The unique ability of these materials to stretch, bend, and twist without hampering their functionality has led to the development of innovative devices. This article discusses flexible and stretchable materials that have revolutionized the world of electronics.
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Flexible and Stretchable Electronics - Overview
Flexible and stretchable electronics have gained significance owing to their ability to be compressed, twisted, and molded into non-planar surfaces. Soft, flexible, and stretchable materials facilitate the development of next-generation wearable devices for energy, healthcare, and military applications.
The emergence of flat panel displays and solar cells, created using standard microfabrication and thin films, is a proven example of the success of stretchable electronics. The fabrication of thin-film circuits on plastic substrates enables them to fit non-planar surfaces with permanent deformation, and those fabricated on rubber-like substrates facilitate reversible deformation.
These technologies with permanent and reversible deformations have burgeoned to include conformable displays, electronic muscles, sensors, and many more that can be stretched, bent, and shaped. Recent advancements in this category include the creation of circuits on biodegradable substrates, whose dissolution leaves a conformable circuit on a living organism.
Historical Background of Flexible and Stretchable Electronics
Flexible and stretchable electronics have originated in the 1960s with the fabrication of flexible solar cell arrays of 100 μm thickness, wherein flexibility was offered by arranging single-crystal silicon wafer cells on plastic substrates, opening a wide scope for flexible electronics.
Later, the invention of thin-film transistors (TFT) in 1968, active-matrix liquid-crystal displays (AMLCD) in the mid-1980s, and flexible polycrystalline silicon (poly-Si) TFTs assembled on a plastic substrate by the late 1990s led to significant progress in flexible electronic devices.
The demand for intelligent and wearable electronic devices has led to the investigation of stretchable electronics that can withstand strain deformations without affecting their inherent performance.
Recently, flexible and stretchable electronics have found extensive application in the fabrication of electronic textiles and skins, revealing the potential of conformally shaped displays and sensors.
Characteristics of Flexible and Stretchable Materials
Young’s modulus (E) of stretchable materials used in electronics is in the range of 12 orders of magnitude. Under mechanical deformation, stretchable materials undergo changes in entropy and internal energy. Materials used in stretchable electronics include soft and elastic materials, liquids, and brittle materials.
Elastomers are highly stretchable materials commonly used in wearable electronics with E between 1-100 MPa and are similar to polymers. Although materials with a high E ( gold, silicon, and diamond-like) are not stretchable, they are combined with elastic, plastic, and brittle materials to impart stretchability. Stretchable electronics can withstand stretching in multiple directions.
Flexible materials used in electronics include polymer substrates ( such as polyethylene naphthalate (PEN), polyethylene terephthalate (PET), and polyimide (PI)) that can withstand bending, twisting, or rolling without hampering their inherent properties.
Additionally, conductive polymers, such as poly(3,4-ethylene-dioxythiophene) polystyrene sulfonate (PEDOT: PSS) and polyaniline (PANI), can be used to construct solar cells, flexible displays, and batteries.
Applications of Flexible and Stretchable Materials
These materials have been extensively used in the manufacturing of various next-generation electronic devices. A few are listed below:
- Wearable Electronic Devices: From health monitors to fitness trackers, wearable electronics have gained popularity in recent years. The use of these devices includes step counting, sleep monitoring, blood pressure monitoring, and pulse recording. These wearable electronics have flexible and stretchable materials that provide a convenient human-machine interface to monitor physiological properties.
- Soft Robotics: Soft robots made of elastomers have flexible or stretchable sensors that undergo deformations, such as bending, torsion, or elongation, to generate motion and are used as actuators. Thus, they act as microorganisms, making them safer inside the human body.
- Medical Implants: Medical implants and electronic skins that use flexible or stretchable electronic materials are widely used in personalized medicine applications and health-monitoring platforms. Conductive polymers, carbon-based polymeric materials, and nanomaterials have made skin bioelectronic robust systems.
- Flexible Displays: The use of flexible or stretchable materials has enabled the manufacture of flexible and rollable displays. Many manufacturers of consumer electronics have integrated these materials into mobile phones, e-readers, and other consumer electronics to allow for the rolling of screens without impeding functionality.
Limitations of Flexible or Stretchable Materials
Although flexible and stretchable materials have gained significant application in electronics, a few medical devices have shown reduced device performance compared to rigid electronics. Hence, there is a need to develop novel, robust human-friendly medical systems based on stretchable or flexible electronics.
Another limitation in this field is the three-dimensional (3D) printing of these materials, which is limited to small-sized production and to organic materials, polymers, and a few metals. Additionally, there is a need to develop flexible devices based on inorganic materials to enhance heat tolerance.
Conclusion
In conclusion, although flexible or stretchable materials face a few limitations compared to the traditional rigid materials used in electronics, they have huge potential to revolutionize the field of next-generation electronics.
The healthcare field is rapidly adapting these materials to manufacture human-friendly medical devices that inherently interface with the soft and curvilinear structures of the human body.
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References and Further Reading:
Zhou, Z., Zhang, H., Liu, J., & Huang, W. (2021). Flexible electronics from intrinsically soft materials. Giant, 6, 100051. https://doi.org/10.1016/j.giant.2021.100051
Corzo, D., Tostado-Blázquez, G., & Baran, D. (2020). Flexible electronics: status, challenges and opportunities. Frontiers in Electronics, 1, 594003. https://doi.org/10.3389/felec.2020.594003
What Are Flexible Electronics? [Online] Available at https://www.semi.org/en/communities/flextech/what-are-flexible-electronics (Accessed on 4 August 2023).
Flexible Display. [Online] Available at https://academic-accelerator.com/encyclopedia/flexible-display. (Accessed on 5 August 2023).
Wu, Z., Huang, Y., & Chen, R. (2017). Opportunities and challenges in flexible and stretchable electronics: A panel discussion at ISFSE2016. Micromachines, 8(4), 129. https://doi.org/10.3390/mi8040129.
Cheng, IC., Wagner, S. (2009). Overview of Flexible Electronics Technology. In: Wong, W.S., Salleo, A. (eds) Flexible Electronics. Electronic Materials: Science & Technology, vol 11. Springer, Boston, MA. https://doi.org/10.1007/978-0-387-74363-9_1
Wei Wu (2019) Stretchable electronics: functional materials, fabrication strategies and applications, Science and Technology of Advanced Materials, 20:1, 187-224. https://doi.org/10.1080/14686996.2018.1549460
Hao, Y., Zhang, S., Fang, B. et al. A Review of Smart Materials for the Boost of Soft Actuators, Soft Sensors, and Robotics Applications. Chin. J. Mech. Eng. 35, 37 (2022). https://doi.org/10.1186/s10033-022-00707-2
Gillan, L., Hiltunen, J., Behfar, M. H., & Rönkä, K. (2022). Advances in design and manufacture of stretchable electronics. Japanese Journal of Applied Physics, 61. https://iopscience.iop.org/article/10.35848/1347-4065/ac586f#references
Baeg, K. J., & Lee, J. (2020). Flexible electronic systems on plastic substrates and textiles for smart wearable technologies. Advanced Materials Technologies, 5(7), 2000071. https://doi.org/10.1002/admt.202000071
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