A flexible and stretchable polypyrrole/knitted cotton for electrothermal heater

Highlights

• A flexible and conductive textile-electronic PKCF is prepared.

• Parallel current collectors are used to enhance electrothermal temperature.

• PKCF shows good stretchability and maintains heating function well after stretched.

• PKCF exhibits great wearable electrothermal application in warming-garment.

• PKCF has potential in new display technique when matched with IR thermal imager.

Abstract

Electrothermal heater (ETH) is very promising in wearable electronic area for its potential thermotherapy and electrical-warming garment applications. To fabricate a suitable conductive material for wearable ETH, knitted cotton is used as substrate for in-situ polymerization of pyrrole monomers, producing conductive polypyrrole/knitted cotton fabric (PKCF) with good flexibility and stretchability. A parallel strategy for current collectors is taken to enhance the temperatures of prepared PKCF from 37.0 to 51.0 °C at 8 V. This PKCF provides a service time about 13.5 h in a continuous electrothermal process, keeping a temperature above 35 °C. Besides, the PKCF still shows good electrothermal performances after bent or stretched several thousand times, and can be sewn on fabric to realize its electrothermal function in garments, which indicates a promising wearable market.

Introduction

Flexible electronics, ranging from on-skin sensors, bendable energy-storage devices, roll-up smart cards to soft heaters, have evoked great interest of researchers [[1], [2], [3], [4], [5], [6], [7]]. As energy conversion electronics, electrothermal heaters (ETH) are desirable heat sources in numerous aspects such as soft actuator [[8], [9], [10]], abrupt insulator-metal transition [11] and body health (thermotherapy and electrical-warming garments) because of their controllable convertibility from electric power to heat energy [6,12]. Possessing satisfactory conductivity, metals or their oxides species (like copper, silver or ITO) [8,[13], [14], [15], [16]], carbon materials (like carbon nanotubes and graphene) [12,[17], [18], [19], [20]] and transition-metal carbide (MXene) [6] have been widely used for ETH. However, heavy-weight (of metals), fragile nature and high production cost (of ITO), complicated preparation and poor processability (of MXene and carbon materials) restrict their anticipated applications [6,14,18,21]. Polydimethylsiloxane (PDMS) has gained high opinion as flexible and transparent substrate to support conductive materials [[22], [23], [24]], but high cost and weak adhesive forces with conductive materials become obstacle to its scale application in ETH [15]. Besides, the integration between PDMS and garments should be taken into account when involved in wearable smart clothing [4,25].

Conductive textiles, usually based on the combination of non-conductive fabric substrate and conductive materials, overcome the problems of flexibility and integration, presenting great potential in wearable sensors, energy storage devices and heaters [5,[26], [27], [28]]. Conducting polymers due to their light weight, facile preparation and good conductivity are considered promising conductive materials [3,28,29]. For instance, Yun [28] prepared a conducting polymer-fabric ETH based on (poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) with temperatures about 50–100 °C at applied voltage of 6–12 V. However, the high cost of PEDOT often restricts its wide application, especially for a large-scale production. Polypyrrole (PPy) with mild synthesis conditions, relatively high conductivity and moderate raw material cost has great potential in ETH application. For example, Wang [29] prepared a PPy/PVA-co-PE/PET fabric via facile in-situ polymerization which exhibited wide electrothermal temperature range about 30–200 °C at applied voltage of 1–6 V. Cotton not only possesses advantages of light-weight, flexibility, high wettability and permeability [30,31], but also contains multi-hydroxy structures that results in a good adsorption for pyrrole monomers owing to the interaction between cellulose –OH and pyrrole NH; therefore, cotton is a favorable substrate for the in-situ polymerization of pyrrole monomers. Knitted cotton due to its specific fabric structures exhibits good stretchability, which is regarded as a promising stretchable substrate.

Serving as gate (inlet or outlet) for electrons flowing from conductive materials to power source or conversely, current collector plays a significant role in maximizing device's electrothermal performances. Usually, metals are employed as current collectors for their very low resistances, which reduces energy losses and scatters current (decreasing current density) [[32], [33], [34], [35]]. Copper tape is favorable ETH current collectors [[36], [37], [38], [39]], but the glue on the tape results in the ascent of contact resistance between copper and conductive materials. To minimize the electrical contact loss, “direct connecting” is more effective. In addition, the fact that sample's resistance “in the vertical direction of the line of two relative current collectors” becomes higher and higher along the edges should be taken into consideration. For instance, Luo [7] tested the electrothermal performances of their prepared rectangle carbon nanotube fiber fabrics (length × width is 28 cm × 1.5 cm) using two clips to clamp two opposite edges (28 cm), with phenomenon that only the nearby area of the clips presented electrothermal response. The longer distance along this edge (28 cm) leads to a higher resistance, thus the farther place has a lower current and only a limited region near the clips is effective.

Herein, we report a flexible and stretchable polypyrrole/knitted cotton (PKCF) for ETH. To make the best use of PKCF, a parallel connection strategy has been taken to drive more electrons directionally migrating at the same voltage, enhancing the electrothermal region and temperature of PKCF. It should be emphasized that the parallel connection strategy not only maintains flexibility but also improves electrothermal performances of the ETH especially for device with large area. The assembled ETH is also easily sewn onto textiles or clothing and function as electrical-warming garments at a low&safe voltage (8 V).