2.5D direct laser engraving of silicone microfluidic channels for stretchable electronics

Abstract

Stretchable and bendable sensors have become increasingly relevant as the technology behind them matures rapidly from lab based to industrially applicable production principles. In a broader sense, stretchable electronics promises to increase the way we are surrounded by and interact with our devices. Electronic circuits will be deployed in environments where we require them to dynamically flex, bend, stretch, compress, twist and - quite possibly - even fold; where they have to demonstrate a certain conformability towards the substrates they are attached to. Microchannels primarily originate from their usage in biosensing applications where they transport analyte. Low temperature solders have furthermore been applied in combination with these microchannels to create conductive, low temperature healing electronic structures. By instead using room temperature liquid conductors we can take this a step further and create self-healing conductive geometries. However most of these RT liquid conductors which have good conductivity exhibit either toxic, unstable or even explosive behavior in ambient atmosphere. Only the Gallium alloys, eutectic Gallium-Indium and Galinstan, are well conducting, non-toxic and stable at normal operating conditions and were used to create the self-healing conductive microchannel structures presented here. Conventional patterning of microchannels consists of creating a master mould or stamp out of photoresist using photolithography. This is a very costly and time consuming approach but delivers channels of highest accuracy and detail. Faster, less expensive but also less precise alternatives range from hand-cutting scotch tape moulds, over lasercutting them, to direct knife or laser ablation of cured PDMS. For the results presented herein, Sylgard 184 PDMS was engraved using a typical fablab lasercutter, the Trotec Speedy 100R, to obtain line-space features of ~100µm. Thereafter these channels were sealed with a second layer of PDMS, punctured with in- and outlets and filled with RT liquid Gallium alloy. Stretch tests were succeedingly performed to characterize resistance-strain performance and the self-healing property. The main contribution of this work lies in the characterization of laser engraving depths for different amounts of passes and power while keeping an eye on channel uniformity and repeatability.