Spheroidal beads present an exterior surface of a hydrophilic hydrogel,
which is an isocyanate-functional polymer that is polymerized by urethane
bonds and cross-linked by urethane and urea bonds. Sequestering agents
present at the surface are covalently bound to isocyanate groups or to
intermediate linkers that are so bound. These beads allow sequestering
agents to retain their native three-dimensional configuration, and as a
result of such surface characteristics and hydrophilicity, they achieve
highly effective capture of very small subpopulations of rare cells from
bodily fluids or the like and very effectively deter nonspecific binding
of other biomaterials present in such bodily fluid. They may be
all-hydrogel spheroids or hydrogel-coated substrates.