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Stretchable LIG-Based Composites for Biointegrated Applications
Alexandra Serebrennikova1,Alexander Dallinger1,David Grafinger1,Qamar Abbas1,Francesco Greco1
Graz University of Technology1
The recently developed laser induced graphene (LIG) process allows scribing electrically conductive circuits directly on a polyimide film. Its versatility is enabling a variety of designs, suitable for a broad range of electro-mechanical applications . Silicone rubber layers incorporating LIG patterns were proposed to produce strain responsive devices such as strain gauges . Nevertheless, the sensors had an overall thickness in the mm range, which makes challenging their use for thin, wearable, biointegrated devices. Here conformability plays a main role in determining a seamless interface with skin . In this work we report the results on new LIG-based composites with low thickness and enhanced conformability to skin. The fabricated materials were used to develop various stretchable electrical devices.
Various silicone as well as polyurethane (PU) materials were investigated as the elastic carriers for LIG patterns. The latter were produced either in the form of a porous flat structure or as bundles of graphene fibers, by fine tuning of laser rastering parameters. The research included thin films with/without acrylic adhesives of various compositions and types of finish (including spin coating, blown film extrusion, etc.) The influence of laser rastering direction on the final functional properties of the composites was also investigated. One of the main challenges addressed in this work was an optimization of the transfer process of an electrically integer LIG circuit onto the investigated film. That was done in order to preserve low sheet resistance (100-1000 Ω/sq) and to achieve replicability among specimens. Developing a proper electrical connection suitable for flexible applications was another part of the work. A comprehensive electro-mechanical testing of composites has been carried out.
PDMS-based LIG devices have shown fully recoverable linear elastic behaviour and linear electrical resistance response up to 25% strain, as well as maximum stretchability up to 70%. These results allowed us to develop highly responsive thin strain sensors. A thin and soft (~30 µm, E ~ 15 MPa) wearable respiratory sensor based on biofriendly and medically approved flexible PU film served as another example for the proposed future applications.
The spectrum of patterned LIG applications can be further enhanced by designing compact energy efficient power storage devices such as microsupercapacitors (MSCs)  which could be suitable for sports and medical applications, among others. To this aim we worked on creating thin flat electrodes with high surface area for electrochemical applications. These could be integrated into the last generation of bio- and environmentally friendly electrochemical micro supercapacitors (MSCs), based on a low-cost and skin-friendly aqueous electrolyte. Since the surface area of the electrode material plays a crucial role in the charge storage processes, LIG material in form of a scribed powder was investigated by means of BET technique in order to confirm its remarkably high surface area of ~400 m2/g. Thin and soft LIG electrodes integrating electrical contacts were tested for MSC application. The obtained results of electrochemical measurements (cyclic voltammetry, galvanostatic charge/discharge curves) have shown high potential for further applications of the developed devices in the form of miniaturized power suppliers.
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