Stretchable graphene inks for skin-compatible wearables

Printing stretchable and skin-friendly conductors with low thickness coefficient and high durability is challenging. An article published in the journal Materials Chemistry introduces a skin-compatible graphene nanoplatelet (GNP)-based colloidalinkutilizing a thermoplastic polyurethane (TPU) binder with tunable rheology to Preparation of graphene conductors with excellent properties.

  In addition to being highly conductive even at 100% strain, graphene conductors also exhibit high fatigue resistance to cyclic strains of 20-50%. These graphene conductors showed a low sheet resistance of 34 ohms per square when heated at 120 degrees Celsius.

  Furthermore, photonic annealing at several energy levels reduces the sheet resistance to below 10 ohms per square. However, the fatigue resistance and stretchability of graphene conductors are preserved and can be tuned. Thus, the combination of stretchability, high conductivity, and cycle stability with scalable ink production with tunable rheology demonstrates the scope for high-volume production of stretchable wearable devices.

  Graphene conductors in wearable devices

  Conductive components in printed electronics are based on metals. However, suitable metals are scarce or prone to electromigration. While metals such as silver (Ag), gold (Au) and copper (Cu) are ideal for printed electronics, they are either expensive (Ag/Au) or toxic and extremely prone to oxidation (Cu).

  Flexible conductive polymers are an alternative to metals that could be integrated into wearable conductors. However, their stability issues hinder their practical applications. The carbon allotrope graphene is another mechanically strong, environmentally inert, abundant and electrically conductive alternative.

  Graphene, a one-atom-thick layer of carbon atoms, is transparent, conductive, bendable, and yet one of the strongest materials known. However, using graphene as a conductor remains a great challenge because the lowest value of its sheet resistance demonstrated so far is higher than that of commercially available indium tin oxide (ITO) (i.e., at 85% optical transmittance per squared 10 ohms).

  GNPs are produced in a large-scale and cost-effective manner, which facilitates the production of GNP-based inks, which are alternatives to metallic inks for wearable technology. Various industrial printing methods require inks with different physical properties, including surface tension, rheology and drying time.

  Although inkjet printing is efficient in achieving high-resolution deposition, it requires low-viscosity (i.e., low-concentration) inks, which limits the conductivity of the printed tracks. In contrast, screen printing and flexographic printing technologies are flexible, simple, fast and scalable methods for producing wearable electronics. Screen printing can print thick layers that can accommodate a wide variety of inks and substrates.

 Printed stretchable graphene conductors based on GNP ink

  Previously, graphene-based strain sensors were fabricated by screen printing, while stretchable supercapacitors were printed using conductive polymer adhesive-based inks containing small amounts of graphene.