Alcohol-based highly conductive polymer for conformal nanocoatings on hydrophobic surfaces toward a highly sensitive and stable pressure sensor

• Published in: NPG Asia materials Vol. 12; no. 1
• Main Authors: Lee, Jung Joon, Gandla, Srinivas, Lim, Byeongjae, Kang, Sunju, Kim, Sunyoung, Lee, Sunjong, Kim, Sunkook
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 2020; Nature Publishing Group
• Subjects: 639/166/898; 639/301/1005/1009; 639/925/357/995; Biomaterials; Chemistry and Materials Science; Coating; Conducting polymers; Contact angle; Contact pressure; Cracks; Electric contacts; Electrical resistivity; Energy Systems; Ethanol; Exchanging; Hydrophobicity; Industrial applications; Low pressure; Materials Science; Optical and Electronic Materials; Polydimethylsiloxane; Polymer films; Polymers; Polystyrene resins; Pressure sensors; Pulse rate; Response time; Sensor arrays; Sensors; Solvents; Structural Materials; Substrates; Surface and Interface Science; Thin Films; Ultrafiltration; Wettability
• ISSN: 1884-4049; 1884-4057
• DOI: 10.1038/s41427-020-00238-z

Conformal and ultrathin coating of highly conductive PEDOT:PSS on hydrophobic uneven surfaces is essential for resistive-based pressure sensor applications. For this purpose, a water-based poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) solution was successfully exchanged to an organic solvent-based PEDOT:PSS solution without any aggregation or reduction in conductivity using the ultrafiltration method. Among various solvents, the ethanol (EtOH) solvent-exchanged PEDOT:PSS solution exhibited a contact angle of 34.67°, which is much lower than the value of 96.94° for the water-based PEDOT:PSS solution. The optimized EtOH-based PEDOT:PSS solution exhibited conformal and uniform coating, with ultrathin nanocoated films obtained on a hydrophobic pyramid polydimethylsiloxane (PDMS) surface. The fabricated pressure sensor showed high performances, such as high sensitivity (−21 kPa −1 in the low pressure regime up to 100 Pa), mechanical stability (over 10,000 cycles without any failure or cracks) and a fast response time (90 ms). Finally, the proposed pressure sensor was successfully demonstrated as a human blood pulse rate sensor and a spatial pressure sensor array for practical applications. The solvent exchange process using ultrafiltration for these applications can be utilized as a universal technique for improving the coating property (wettability) of conducting polymers as well as various other materials. Sensors: a flexible way to connect A method for attaching flexible conducting contacts to polymer pressure sensors has been developed by researchers in South Korea. Flexible pressure sensors can be integrated into wearable electronics and tactile robotic systems. One possible material for these devices is a conducting polymer, which is stable, highly sensitive and quickly responds to mechanical changes. However, it is difficult to connect the necessary flexible electrical contacts to them because they are hydrophobic, i.e., they repel water. A team led by Sunjong Lee, Korea Institute of Industrial Technology, Cheonan, and Sunkook Kim, Sungkyunkwan University, Suwon, have developed an ultrafiltration method that enabled them to coat a hydrophobic substrate with a flexible conducting polymer film. The team used the technique to create a sensitive pressure sensor which was tested over 10,000 cycles without failure or cracking. Water of aqueous PEDOT:PSS solution was successfully exchanged with ethylene glycol or ethanol solvents using ultrafiltration method. This novel solvent exchange process enables uniform and conformal nano-coating of PEDOT:PSS on hydrophobic substrates without the addition of surfactants. This process can be widely used in fabrication of various sensors or electronic devices using conducting polymers.