Conductive polymer PEDOT:PSS accelerates IoT society

2024/10/15

University of Yamanashi
Prof. Hidenori Okuzaki

Research summary

Organic electronics has evolved from cheap, lightweight and flexible electronics to printed electronics, stretchable electronics, and more recently wearable electronics for Internet of Things (IoT) applications. The most successful conductive polymer, poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT:PSS), has been paid considerable attention as a highly conductive, flexible, and printable electrode material. We are conducting a wide range of research from the basics to applications of organic electronics, such as soft robots and flexible sensors using PEDOT:PSS as electrodes.

Research interests

conductive polymers, organic electronics, soft robots, flexible sensors

Organic electronics opens up IoT society

Internet of Things (IoT), a system that connects people and things, and things with each other, via the Internet, is attracting attention. For example, health monitoring using heart rate sensors, remote control of home appliances using smartphones and smartwatches, and gait sensing using shoe sole sensors all aim to create a safe and secure society by connecting to the Internet. To realize the IoT society, wearable devices available to attach to the human body are essential, where the organic electronics is the key technology. In particular, soft robots and flexible sensors are essential for human-machine interfaces, and haptics technology has been paid attention as tactile feedback for smartphones, game consoles, and virtual reality [1.2].

Soft robots using conductive polymer PEDOT:PSS

Highly conductive, flexible and printable electrode materials are key in organic electronics. Poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT:PSS), the most successful conductive polymer commercially available in the form of water dispersion as colloidal particles (Fig. 1A) [3], can be utilized as electrodes in soft robots and flexible sensors. For example, a soft robot can be made by simply attaching two films of the conductive polymer PEDOT:PSS to both sides of an ion gel made of a soft and stretchable polymer containing ionic liquid (room temperature molten salt) (Fig. 1B) [4]. When DC 2 V is applied, the ionic liquid in the ion gel becomes polarized and forms an electric double layer, bending toward the anode (Fig. 1C). Furthermore, a similar response was observed when thin layers of PEDOT:PSS were coated to both surfaces of the ion gel by spin coating, spraying, or inkjet printing to create a translucent soft robot (Fig. 1D). Unlike conventional mechanical systems driven by changes in the relative positions of components, such as motors, engines, and hydraulic pumps, soft robots induce macroscopic deformations and volume changes through the accumulation of microscopic structural changes at the molecular level. Therefore, the soft robots can be used as micro/nanomachines such as micropumps, active catheters, and guidewires (medical applications), as well as flexible braille displays (welfare) and power assist suits (nursing care) [1,2].

Figure 1. Chemical structure of conductive polymer PEDOT:PSS (A), structure of soft robot (B), bending behavior at 2 V (C), and translucent soft robot with thin coatings of PEDOT:PSS (D).

Trillion sensors supporting the IoT society

Flexible sensors are generally classified as capacitive and resistive. Capacitive sensors are inexpensive, lightweight, and easy to make thin, but they are not suitable as motion sensors capable of detecting the direction of movement. On the other hand, resistive sensors have a simple structure and are easy to prepare, but there is a drawback of power consumption because a current must be flowing. In fact, as we enter a “trillion sensors” society, in which trillions of sensors are produced every year, power supply is becoming a serious issue. Therefore, we focused on “powerless sensors” available without power sources. For example, piezoelectric sensors using ferroelectric polymers and energy harvesting using frictional power generators are attracting significant attention. However, because the voltage generation is instantaneous, it is difficult to measure slow motions such as body movements.

Can soft robots act as sensors?

The soft robots made of the conductive polymer PEDOT:PSS and ion gels can generate several mV by bending, making them suitable for flexible sensors [5]. The mechanism of voltage generation differs from the conventional piezoelectric effect and is called the “piezoionic effect”. Since cations and anions have different mobilities, bending the ion gel may cause a bias in the distribution of ions (polarization), generating a voltage (Fig. 2A). Compared to metal electrodes, the conductive polymer PEDOT:PSS is more flexible, allowing the sensor to undergo large deformation and also generating a high voltage.

Figure 2. Schematic diagram of flexible sensor using the conductive polymer PEDOT:PSS as electrodes (A) and multifunctional sensor capable of simultaneously detecting bending displacement and speed (B).

Flexible sensors based on the piezoionic effect directly output a voltage without a power source due to the polarization caused by the movement of ions. Interestingly, the voltage and charge are proportional to the bending displacement and speed, respectively, indicative of a multifunctional sensor (Fig. 2B). If more information can be obtained from a single sensor, the number of sensors can be reduced. In addition, large-area flexible sensors fabricated with printing technology can be used as motion sensors in a variety of fields, including virtual reality, gaming, healthcare, automotive, aerospace, and robotics. Thus, the soft robots and flexible sensors using the conductive polymer PEDOT:PSS have the potential to become key technologies for realizing a safe and secure IoT society.

[1] K. Asaka, H. Okuzaki, and K. Suzumori, eds., Materials, Structure, and Application Technology of Soft Actuators, S&T (2016).
[2] K. Asaka and H. Okuzaki eds., Soft Actuators (2nd ed.), Springer (2019).
[3] H. Okuzaki ed., PEDOT: Material Properties and Device Applications, S&T (2012).
[4] H. Okuzaki, S. Takagi, F. Hishiki, R. Tanigawa, Sens. Actuators, B, 194, 59 (2014).
[5] Y. An, H. Yoshida, Y. Jing, T. Liang, H. Okuzaki, Soft Matter, 18, 6791 (2022).