Microdevice arrays formed by magnetic assembly

A magnetic arraying device, comprising:

• an arraying chip comprising a substrate having embedded magnetizable magnetic elements forming an array of discrete regions, wherein each element exerts magnetic forces as induced by an external magnetic field, and wherein the magnetic elements comprise alternating longer and shorter magnetic bars;
• wherein the shorter bars are each positioned in a gap between the ends of adjacent longer bars along the long axis of the longer bars, at least some of the longer bars having an average length of less than 500 μM, and at least some of the shorter bars having an average length of less than 50% of the longest bars;
• and
• an arrayed plurality of microdevices placed upon the arraying chip, wherein at least some of the microdevices comprise a magnetic bar having a length greater than the gap between ends of adjacent long bars in the arraying chip, wherein the microdevices are configured to be moved on the substrate by a field created by the longer bars and shorter bars, and wherein at least some of the microdevices further comprise at least one of a chemically active site, a magnetic coding space, mutually distinct polymers, or mutually distinct codes.