A thermal responsive, water soluble copolymer is disclosed, which comprises the co-polymerization product of: (a) 0.5.about.20 wt % of N-isopropyl acrylamide (NIPAAm); (b) 0.5.about.20 wt % of 1-vinyl-2-pyrrolidinone (VPD); (c) 0.01.about.10 wt % of acrylic acid (AA); and (d) 80.about.90 wt % of acrylamide. The viscosity of.the co-polymerization product is adjustable by temperature. Moreover, phase-separating does not occur in the copolymer while temperature is changing and the structure as well as the fluidity are maintained, therefore, the copolymer of the present invention is suitable for the application of a micro-fluidic chip.