Development of Stretchable Antenna using Laser-induced Graphene (LIG) for Wearable Devices

Abstract

Wearable and flexible wireless devices have become very important in modern technology over the past few years particularly for health monitoring sports and other IoT fields of applications. Therefore, there arises a special requirement for antennas that the antennas used in such systems can comply with curved or moving surfaces like human skin or apparel. Most conventional antennas however are fabricated on rigid substrates thus limiting their eventual use in stretchable as well as body-worn applications. Researchers have recently been inspired to investigate novel materials together with techniques of fabrication that would allow both flexibility and robust performance of an antenna. Laser-induced graphene (LIG) is one such material, a conductive carbon form directly written onto polymers by means of a laser beam possessing, which provides good conductivity, mechanical flexibility as well as ease in fabrication for the next-generation wearable communication systems.

The main goal of this work is aimed at designing, developing and performing the analysis over a stretchable microstrip patch antenna which will be based on laser-induced graphene. The antenna is fabricated by laser scribing Kapton polyimide film, where high energy CO2 laser irradiation turns its surface into a porous conductive graphene layer. No extra metals or chemical inks are involved in this process thus making it simple low cost as well environmentally friendly. The formed LIG is analysed through multiple testing like sheet resistance, Raman spectroscopy and field-emission scanning electron microscopy. Once, the structural quality and degree of graphitization are achieved for the LIG formed, it is then transferred onto flexible as well as stretchable polydimethylsiloxane (PDMS) substrate using soft transfer printing method. The strong adhesion force between porous LIG and PDMS surface enables bending or stretching of structure without losing any electrical property. A thin copper and PEDOT:PSS layer, in different iterations, is also deposited on back side of PDMS to act as ground plane for patch antenna.

This work also focuses on the fabrication process of developing patch antenna, which comprises of CO2 laser engraving, soft transfer printing and blade coatings etc. Therefore, a simple patch antenna is designed, simulated and optimized by using an electromagnetic solver (HFSS) for fine impedance matching and good stable operation around the 2.4 GHz centre frequency, widely used in Bluetooth and Wi-Fi communication. The footprint of the proposed antenna is approximately 52 mm by 54 mm, with a PDMS substrate having about a dielectric constant of 3.0 and thickness of 2 mm. The LIG layer serving as the radiating patch normally has a thickness within 5 to 20 micrometres. The formation of graphene is confirmed by Raman spectroscopy through the appearance of characteristic D, G, and 2D peaks that indicate partially ordered carbon network conductivity good for being conductive. FESEM displays more details as being three-dimensional porous interconnected structure of LIG.

Experiment results of return loss and radiation pattern for the bending and stretching of antenna show that it still keeps stable performance. S11 is less than -19 dB at its working frequency with a steadily maintained radiation pattern within permissible limits hence proving the fact that this antenna works well under mechanical deformation to suit perfectly wearable as well as stretchable communication systems. LIG possesses good mechanical strength besides durability coming from its porous structure.

In conclusion, the experimented results clearly prove a possibility to create a stretchable antenna using laser-induced graphene (LIG) on stretchable substrate. Furthermore, this approach is favoured due to its key advantages like low cost and environmentally friendly besides not involving any complicated processing steps or toxic materials. The strong electrical conductivity combined with mechanical flexibility which makes LIG perfectly suitable for future applications in wearable health monitoring as well as soft robotics together with IoT devices. Summing up these findings, it can be stated that LIG based antennas may become an essential part of developing the next generation flexible, conformal and sustainable wireless systems.