Which technique involves applying a charged polymer fluid onto a surface that is oppositely charged?

Electrospinning is the correct technique because it involves the use of an electric field to draw charged polymer solutions into fibers. In this process, the polymer fluid is typically applied to a collector surface that carries an opposite charge. The strong electric field produced helps to elongate the polymer solution into fine fibers as the solvent evaporates, resulting in a non-woven mat of nanofibers. This technique is critical in tissue engineering because it can create scaffolds with a high surface area-to-volume ratio and tailored porosity, which are essential for supporting cell attachment and growth.

The other techniques listed do not involve the mechanism of applying a charged fluid onto an oppositely charged surface. Freeze drying involves removing moisture from a material and is used for preserving biological samples. Porogen leaching is a method used to create pores in a scaffold by incorporating soluble materials that can be later washed away. Precision Extruding Deposition (PED) involves carefully controlling the deposition of materials but does not specifically utilize charged polymer fluids and oppositely charged surfaces in its process.