Micro- and nanofabricated devices with feature sizes comparable to biological cells are ideal platform for manipulating biological cells for advanced applications. In this talk, we present several microfluidic devices to achieve: 1) deterministic filtering and fractionation of cells according to their sizes for cancer diagnostics; 2) trapping of individual cells of different types for heterotypic cell fusion for biological research; and 3) surface micro and nanostructures for controlling cell growth. In this talk, we focus on the device design, fabrication and testing. Current devices are implemented with traditional channel-based continuous fluid flow. The possibility of achieving similar functionalities using droplet-based active-matrix-driven digital microfluidics will also be discussed.