Poly(ethylene oxide) (PEO) chains have been grafted onto a thermally sensitive poly(N-isopropylacrylamide) (PNIPAM) spherical microgel to form a novel quasi-planar polymer brush. The shrinking of the microgel at temperatures higher than 32 degreesC dramatically reduces its surface area, providing a convenient way to vary the grafting density. At the initial stage of the shrinking, we observed an expected steric repulsion-driven chain stretching, in which the hydrodynamic volume of the grafted PEO chains is incompressible, contrary to previously reported results. A further increase of the grafting density to similar to 150 nm(2) per PEO chain led to an unexpected collapse of the stretched chains, which could be attributed to the long-predicted n-clustering attraction among densely packed brush chains. The stretching and collapse of the grafted chain in the heating-and-cooling cycle was fully reversible, and there was no hysteresis.