A Drop-Printing Strategy for Low-Stress, Conformal Bioelectronics
Conformal contact between functional electronic films and biological surfaces is critical for long-term device stability, high signal sensitivity, and favorable signal-to-noise ratio. Traditional transfer methods, such as soft stamps and pad printing, often involve mechanical pressing, leading to poor conformality, localized stress concentration, or structural failure. Alternative strategies, including geometric engineering of non-stretchable materials or using stretchable organic alternatives, mitigate these issues but increase design complexity and reduce fabrication efficiency. Here, we highlight a novel 'drop-printing' strategy introduced by Li et al. that leverages capillary force to manipulate a water droplet to pick up a thin film, transfer it to a target substrate, and print it onto the surface. As the droplet evaporates, the film conformally wraps the surface. The droplet acts as a lubricating layer, while interfacial liquid penetrating microstructures generates capillary pressure, facilitating shape-adaptive deformation and significantly reducing stress concentration. The final positioning and conformality are governed by droplet behavior on the target surface. This approach achieves positional deviation of less than 20 μm via regulation of three-phase contact lines. The strategy enables damage-free conformal wrapping of non-stretchable films onto three-dimensional biological surfaces, as demonstrated by drop-printed silicon microfilm conformally wrapping on a rat brain, with successful NIR laser stimulation triggering forelimb movement and synchronized brain electrophysiological signals. This gentle, high-precision method addresses the pressing need for low-stress conformal bioelectronics, offering a general solution for diverse biological interfaces.