Droplet-Mediated Deterministic Microtransfer Printing: Water as a Temporary Adhesive.

Deterministic transfer printing of solid objects has been introduced and demonstrated, where capillary force based on water droplet has been utilized. Droplets on pickup head form capillary bridges with chips to be picked, where the capillarity provides enough force to grip or lift the chip. Release or printing of chips can be achieved by simply drying droplet while the chip is in contact with receiver substrate. That is, water droplet acts as a temporary adhesive, adheres onto solid chip by capillary force, and releases it upon drying. The technique has unique features such as self-alignment during pickup, self-correction on pickup head by short exposure to water mist, and pickup capability of solids having a nonflat, corrugated surface, all of which originated from the fluidic nature of water droplet. The technique has been successfully applied for the fabrication of stretchable micro-light-emitting diode chip arrays. The proposed technique can find wide applications in many fields such as displays, sensors, printed electronics, photovoltaics, etc.