Figure 1. Principle of Single-Cell Printing.

(A) shows a scheme of the bright-field optical detection system as implemented in this work. Light of a high-power blue LED passes two apertures and illuminates the dispenser chip through a 10x objective. Light rays reflected from the chip pass a half mirror and are detected by a high-resolution camera to image the dispenser chip nozzle. The single-cell printhead (B) comprises a piezo-stack actuator, the optical detection system, and the disposable cartridge including a sample reservoir and the microfluidic dispenser chip. A small mirror in front of the dispenser chip allows for tilting the optical path by 90°. On actuating the piezo-stack actuator, the piston displaces a constant volume within the chip generating a single droplet of 35 pl ejected from the nozzle. An algorithm coupled to the optical feedback of the camera decides whether the volume is expected to contain a single cell to print the droplet to the target or to remove it otherwise by a vacuum suction. A consecutive image series (C) from each single printed cell is stored on the PC. In this study the instrument was used for single-cell patterning on agar plates and deposition of single cells into a microwell plate like illustrated (D).