F. Cicoira, S. Zhang, P. Kumar. E. Hubis
Keywords: conducting polymers, transistors, stretchable electronics, pattern transfer
Summary:Stretchable electronics, a cutting-edge technology, remains at the frontier in electronics. Such devices allow us to realize stretchable displays, and various kinds of smart electronics products. In addition, they are able to interface with irregular, soft or moving objects so that they can be plastered on skin, heart or brain tissue to monitor pressure, temperature, or even muscle movements. They can also be used for brain activity monitoring which will enable us to gain a better understanding of several diseases such as Parkinson’s, dystonia, essential tremor, obsessive-compulsive disorders, depression etc. As a result, nowadays, this innovative technology has drawn considerable attention in both academic and industrial societies due to its great commercial prospects and society’s benefit. To harvest a stretchable electronic device, the primary step is to fabricate an electrode array on elastic substrates. This can be achieved either by using a shadow mask or by photolithography. The application of a shadow mask is the most straightforward approach but the resolution is limited (ca. tens of micrometers) and is also not an ideal method for applications where additional layer alignments on electrodes are required, as is the case in multilayer devices. At present, photolithography on elastic substrates is difficult, because most of the photoresists, developers, and strippers are solutions containing organic solvents, which might lead to the swelling of elastic films (e.g., polydimethylsiloxane, PDMS). In addition, the use of toxic wet-etchants in photolithography restricts application of electronics in areas that require biocompatibility, such as smart textiles, healthcare applications any other devices interfacing directly with humans. In the promising field of organic bioelectronics, where devices need to interface with cells or the human body, complete biocompatibility is of utmost importance. Therefore, the development of a biocompatible method that can define high resolution metal arrays on elastic substrates is indispensable, and will promote the application of stretchable electronics. Here, to overcome the above technical bottlenecks, we present an easy, solvent-free, biocompatible and reliable method, named, “plastic-assisted parylene transfer-patterning”, to realize micro-electrode arrays on PDMS substrates. The following essential steps are developed and included to ensure the success of the transfer pattern: i) realization of ultrathin parylene film deposition and removal on surfactant-treated plastic; ii) microfabrication on parylene/plastic; iii) plastic-assisted parylene film transfer onto surface modified PDMS; iv) metal deposition on PDMS; v) tape-assisted parylene removal from PDMS. This method allows us to obtain electrode gap as short as 5 um. The plastic-assisted step is extremely important since it provides a mechanical flexible environment and thus avoids parylene film damage during the transfer. By taking advantage of this method, we fabricated solid electrolyte-gated, stretchable micro-conducting polymer transistors on PDMS. A fluorinated photoresist was also used to realize micro-pattern of conducting polymer on PDMS. Our micro-transistors operate at low voltages and maintain stable I-V performance under stretching.