Direct laser-writing of graphene on Kevlar makes protective clothing 'smart'
(Nanowerk Spotlight) Kevlar is a well-known high-strength polymer that, thanks to its excellent mechanical performance, has found a variety of important security applications, from textiles (e.g. bullet-proof vests or firefighters' protective clothing) to tough composites (such as car armor plating).
Get ready for the next generation of protective clothing to become multi-functional, i.e. 'smart'. These materials will not only be able to protect the human body from injury but also possess intelligent functions such as monitoring physiological signals and detecting potential hazard such as gases, pathogens, or radiation.
In order to fabricate smart clothing, it is necessary to combine functional materials with fibers or textiles, usually in the form of nanofinishing where a range of nanomaterials such as carbon nanotubes, graphene, or silver nanowires can be deposited onto textiles.
"Generally, these methods require multi-step routes and time-consuming precursor preparation processing," Yingying Zhang, an Associate Professor in the Department of Chemistry & Center for Nano and Micro Mechanics (CNMM) at Tsinghua University in Beijing, explains to Nanowerk. "Therefore, it is still challenging to fabricate intelligent protective clothing through a straightforward approach, especially those with arbitrary patterns or custom-designed functions."
Back in 2014, researchers at Rice University created flexible, patterned sheets of multilayer graphene from a cheap polymer by burning it with a computer-controlled laser, a technique they called laser-induced graphene (LIG). This high-yield and low-cost graphene synthesis process works in air at room temperature and eliminates the need for hot furnaces and controlled environments, and it makes graphene that is suitable for electronics or energy storage.
"This laser writing method for preparing graphene on polyimide, wood, or paper is simple, efficient, and design-flexible," notes Zhang. "On the basis of the fact that there are similar polymer structures in textiles, we hypothesized that the laser writing technique may be applied on polymer textiles, which enables the facile fabrication of graphene-based textile electronics."
Formation of graphene on a Kevlar textile induced by laser writing. (a) Schematic of the fabrication of the graphene/Kevlar textile from Kevlar. (b) SEM image of LIG patterned into the shape of a bird. (c) Digital image of LIG patterned into a human side face shape. (Reprinted with permission by American Chemical Society) (click on image to enlarge)
According to the team, the transformation of Kevlar into graphene can be attributed to the photothermal effect induced by CO2 laser irradiation. Specifically, this resulted in high localized temperature, leading to the ablation and depolymerization of the Kevlar fiber. The remaining carbon atoms are recombined and 'recrystallized' into graphene.
What they termed the Janus graphene/Kevlar textile –which has porous graphene on the front side and Kevlar fibers on the back side – can be prepared in air. To prevent graphene from exfoliating from the textile, the researchers dropped pre-cured Ecoflex onto the surface of the processed textile to form a very thin coating to fix and encapsulate the graphene.
The direct writing of graphene from commercial textiles in air conditions is simple, efficient, and design-flexible and paves a new way for the development of cost-effective and custom-designed textile electronics. For example, it provides a versatile and rapid route for the fabrication of textile electronics such as flexible supercapacitors and sensors for the in situ monitoring of human physiological conditions.
"Before this work, we didn't know that textiles can be transformed to graphene in air conditions," Zhang points out. "Our research group focuses on the design and fabrication of novel flexible and wearable material and electronics. In recent years we have been very interested in developing fiber and textile electronic materials made from nanocarbons and silk materials. It is our goal to develop flexible electronics that are ergonomically correct for the human body and have excellent performance – which encouraged us to experiment with the laser writing process on Kevlar."
Based on this technique, it becomes feasible to prepare various types of flexible electronics on different commercial textiles such as silk and cotton. This will enable the efficient and customized preparation of multi-functional textile electronics.
The laser direct writing of graphene from commercial textiles in air conditions also provides a versatile and rapid route for the fabrication of textile electronics such as flexible supercapacitors and sensors.
With the popularization of rapidly emerging technologies such as biotechnologies, virtual reality, and artificial intelligence, the demand for wearable devices is surging. They will change the way we communicate, monitoring physiological functions and environmental data, and administer medical treatment.
"Future wearable electronics need to be ergonomically correct and bio-safe for the human body, and even implantable," Zhang concludes. "Researchers active in this field need to further explore how to integrate individual electronics such as sensors, displays, batteries, wireless transmitters and so on to a complete flexible system and finally integrate it with clothing or even medical implants. This ambitious goal requires researchers and engineers from different backgrounds to work together to solve problems with all aspects of material preparation, energy devices, electronics design and fabrication, data processing, packaging technology, medical technology etc."