Single-Step Transfer-Printing Microfabrication of Soft Bioelectronics Using PDMS with Tuned Adhesion Forces

Recent soft and stretchable bioelectronics for various wearable applications generally require special equipment and facilities for microelectromechanical systems (MEMS). As an alternative, simple photolithography-free microfabrication methods have been proposed based on transfer printing onto soft substrates; however, limitations remain in that precise control is involved for modulating adhesion forces. In this study, a simple, rapid, cost-effective transfer-printing-based microfabrication process is demonstrated without requiring any MEMS process or sophisticated transfer control. We utilized ethoxylated polyethylenimine (PEIE) to tune the adhesion properties of polydimethylsiloxane (PDMS) such that thin sensor patterns fabricated by laser-machined gold leaves are easily transferred from the low-adhesion donor PDMS onto the high-adhesion receiver PDMS layer. The microfabrication steps were optimized based on the electrical and mechanical analysis of the transferred patterns, thereby enabling stable fabrication of thin gold lines of 100 μm width. An additional advantage of the sticky PDMS (sPDMS) is presented for stronger bonding of the cover layer and gold layer around the opening windows, which may allow for greater long-term stability in aqueous conditions. The sPDMS also enables adhesive-free attachment to the skin without losing its adhesion forces over repeated applications. The feasibility of the simple microfabrication process is verified by successfully demonstrating a multifunctional wearable patch for electrical (electromyography) and mechanical (strain) monitoring. This process provides a complete set of efficient microfabrication procedures for soft bioelectronics, from metal deposition to patterning and selective encapsulation, which can be utilized in a wide range of multifunctional and wearable physiological monitoring.