The microfluidic chip structure and cell classification steps. (a) Structure hierarchical diagram of the polydimethylsiloxane (PDMS) microfluidic chips. The PDMS scaffold (top layer) is bonded to a glass substrate (bottom layer), forming three channels (middle layer) separated by two arrays of micro-sized pillars (upper sides 200- μ m long, bottom side 400- μ m long, and high 200 μ m) that center spacing 400 μ m. Tumor cells are injected into the middle channel (7200- μ m long and 3000- μ m wide) with a collagen solution, which gelatinizes after incubation for 30 min at 37 °C and 5.0% CO₂. The two shoulder channels are supplied with culture medium pools in experiments. This is also illustrated in the middle-right insert. Bottom-right insert displays a representative photo of a microfluidic chip. (b) and (c) Images of cell movement in collagen (scale bar, 50 μ m ) and trajectory diagram, respectively. (d) and (e) Cell classification process diagram, based on VACV with different persistent times ( τ p ) of cells and power spectra, respectively.