Transferring laser-induced graphene at extremely low temperatures for ultrathin bioelectronics

(Nanowerk Spotlight) Soft, thin, and biocompatible stretchable conductive nanocomposites are crucial in developing wearable skin-like devices, smart soft robots and implantable bioelectronics. Although several design strategies involving surface engineering have been reported to overcome the mechanical mismatch between the brittle electrodes and stretchable polymers, it is still challenging to realize monolithic integration of various components with diverse functionalities using the current ultrathin stretchable conductive nanocomposites. This is attributed to the lack of suitable conductive nanomaterial systems compatible with facile patterning strategies.
Laser-induced graphene (LIG), typically derived from laser irradiation of polyimide (PI), has various distinct merits, such as facile digital patterning processes, compatibility with pattern transfer approaches, as well as tunable physical and chemical features to produce diverse wearable sensors.
However, these multifunctional devices are constructed on flexible PI substrates or relatively thick elastic films owing to the mechanical limitations in transferring LIG to soft elastomers. In addition, the mechanical mismatch between the brittle LIG and elastic polymer hinders the stretchability of conductive nanocomposites.
In a recent study published in Nature Electronics ("Stretchable graphene–hydrogel interfaces for wearable and implantable bioelectronics"), researchers from Zhejiang University report an ultrathin elastic LIG-hydrogel-based nanocomposite for multifunctional on-skin and implantable bioelectronics.
First, the authors propose a new strategy to create ultrathin patterned LIG-based nanocomposite, which is formed by cryogenically (77 K) transferring LIG to a hydrogel film (minimum thickness of 1.0 µm). Second, they address the mechanical mismatch between the brittle LIG and elastic polymer, which employs the hydrogel as an energy dissipation interface and out-of-plane electrical path. Continuously deflected cracks can be induced in the LIG leading to an over fivefold enhancement in intrinsic stretchability.
Overall, this work provides a viable strategy to construct ultrathin carbon-hydrogel-based stretchable nanocomposites for integrated sensor systems, enabling diverse applications in wearable/implantable bioelectronics and human-machine interactions (Figure 1).
Design of ultrathin, antibacterial and biocompatible PPH hydrogel-enhanced stretchable nanocomposites for wearable and implantable bioelectronics
Figure 1: Design of ultrathin, antibacterial and biocompatible PPH hydrogel-enhanced stretchable nanocomposites for wearable and implantable bioelectronics. (Image courtesy of the researchers)
“Conventional LIG transfer method requires the much larger thickness (>45 µm) of elastomers or adhesive tapes to provide a strong interfacial force during the peeling-off process, hindering the conformal bioelectronics applications,” says Prof. Kaichen Xu, corresponding author of this publication. “The mechanical restrictions in transferring LIG to elastomers are overcome by a cryogenic transfer approach at -196 °C using an ultrathin and adhesive polyvinyl alcohol/phytic acid/honey (PPH) hydrogel”.
During the fast cooling process, the interfacial binding energy between defective porous graphene and the crystallized water within hydrogel is enhanced, as illustrated by molecular dynamics (MD) calculations. Such a dramatical increase in superficial binding force at 77 K was also captured in the 180° peeling test. The maximum transient peeling force of 160 N m-1 at 77 K was observed, which was much higher than that (<10 N m-1) originated from the autologous adhesion of PPH at the ambient temperature (Figure 2).
Design of ultrathin, antibacterial and biocompatible PPH hydrogel-enhanced stretchable nanocomposites for wearable and implantable bioelectronics
Figure 2: Schematic of LIG transfer on ultrathin PPH films (thickness of 1.0~1.5 μm) via a cryogenic transfer approach (left graph). The superficial binding energy was calculated based on MD simulations. Calculated binding energy of electrostatic interaction and Van der Waals interaction between the SDG, LDG and waters during the cooling process (top right). The adhesion force of PPH/PI and LIG/PI at different temperatures (293 K, 270 K, 223 K, 77 K) carried out by 180° peeling tests (bottom right). (Image courtesy of the researchers)
Furthermore, the proposed cryogenic transfer strategy allowed the transfer of LIG onto other types of adhesive or non-adhesive hydrogels, indicating the universality of this transfer technology. Nevertheless, only the adhesive hydrogel formed a mechanically stable binding interface especially under the tensile strain.
Through the facile laser direct writing and cryogenic transfer technique, multimodal sensor components are integrated as a multifunctional wearable sensor sheet for on-skin in vitro monitoring. Furthermore, the ultrathin and biocompatible characteristics of the micropatterned LIG-based nanocomposites allow to form the seamless contact with the heart of Sprague Dawley (SD) rats to in situ track cardiac signals.
Provided by Zhejiang University as a Nanowerk exclusive

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