An electrochemical apparatus 1 permits electric-field-assisted fluidic assembly of objects 2 on a patterned silicon substrate 11 by means of electrical addressing. Charged objects 2 such as beads and live cells are moved electrokinetically, like as in electrophoresis, through a solution, typically water 3, towards a micro-patterned charged semiconductor electrode, such as a silicon electrode 11 patterned with silicon dioxide, silicon nitride or agarose gel. The charged objects 2 are thus localized and assembled, most typically into arrays of multiple or single particles, in accordance with the patterning of the electrode 11. Correlating with theoretical predictions, negatively charged polystyrene beads of 20 .mu.m diameter, or live mammalian cells of 20-30 .mu.m diameter, can be assembled and disassembled on 100 .mu.m feature size micro-patterned substrates by means of electrical addressing. The apparatus 1 has applications in creation of active cellular arrays for cell biology research, drug discovery and tissue engineering.