We prepared custom poly(dimethylsiloxane)-based inserts (Fig. 1H) with a 5 ul volume (8 mm2 surface), seeded with ~250 HS5-CD40L-IL4 cells and ~2500 CLL cells per microwell (~12,000× lower cell numbers than in a 3.8 cm2 well of a 12-well plate). Here, CD40LG is linked to B-cell chronic lymphocytic leukemia.