Developing roll-to-roll manufacturing system for flexible and stretchable electronics by direct stamping of silver nano-ink

Direct stamping of silver nanoparticle based ink has been developed for cost-effective and process-effective manufacturing of flexible or stretchable electronic devices. Facile removal of residual layer from deposited silver nanoparticles (AgNPs) layer results in high fidelity of final silver electrode without further post-processes. Scanning electron microscopy (SEM) images and energy dispersive x-ray spectroscopic analysis have revealed residue-free transfer of microscale inter-digitated capacitors onto flexible or stretchable substrates. Enhanced adhesion with viscous UV-curable adhesive enables perfect contact between the substrate and AgNPs in the patterned trenches. As a result, adhesion between them is maximized enough to transfer AgNPs from the trenches to the target substrates. The direct stamping also leads to densification of AgNPs in the patterned trenches by pressing force for contacting the substrate and the stamp. The densification results in lower annealing temperature (~130°C) of AgNPs than normally required annealing temperature (160°C) and better mechanical stability of resulting patterns than one from other ink-based printing techniques. SEM and atomic force microscopy show highly reduced number of pores and less roughness of the layer of AgNPs as it confirms densification of AgNPs. For further understanding of the densification in the microscopic view, simulation of the direct stamping has been performed as normal pressure is applied vertically to the stamp. The simulation reveals that by external pressure during direct stamping, the layer of AgNPs is compressed in the direction of the external pressure and is extended in the normal direction to the pressure. Extension provides room for AgNPs to move helping compression of the AgNPs’ layer by the external pressure. In addition, restoring force of the PDMS stamp enables further densification of AgNPs in the x-direction after removal of the pressure to the stamp. Finally, a table-top roll-to-roll (R2R) manufacturing system has been also successfully built to demonstrate high throughput manufacturing of flexible and stretchable electronic devices by the developed direct stamping method. Strain sensors, touch pads and radio-frequency identification (RFID) tags have been fabricated for highly sensitive flexible and stretchable sensor applications by the R2R direct stamping with the maximum speed of 1 m/minute. The fabricated sensors are still operational even with 8 mm bending radius and after 10,000 cycles of bending. And the wireless and stretchable RFID tags have shown strain sensing behavior up to 7% of stretching strain.