Which method involves the injection of a charged polymer fluid onto an oppositely charged surface?

The method that involves the injection of a charged polymer fluid onto an oppositely charged surface is electrospinning. This technique utilizes an electrically charged solution or melt of polymer to form fibers. When the charged fluid is ejected from a nozzle, the electrostatic forces accelerate the fluid towards a collector surface that is oppositely charged. As the fluid travels through the air, it undergoes stretching and solidifies to form fine fibers upon reaching the collector. This process allows for the production of nanofibers with varied diameters and structural properties, suitable for various applications in tissue engineering, such as scaffolding to support cellular growth and proliferation.

In comparison, direct cell writing is a technique that uses a printer-like device to deposit living cells in specific patterns but does not involve the dynamics of charged fluids and surfaces. Stereolithography employs a light source to cure photo-sensitive resins layer by layer, which does not relate to the charged polymer fluid interaction. Freeze drying is a dehydration process used to preserve biological materials, which also does not involve electrostatic forces or charged surfaces.