Cell-Based Chips for Biotechnology - Introduction to BioMEMS - page 262

On O

(d)

Electrode

Controlling
electrode

(a) Cell

+V -V

(e)

Induced
dipole

F dep

(b)

-V +V

Release cells
denoted by
red arrows

+V -V

100 µm

-V +V

Electric field

20 µm

(c)

Gold cylindrical
electrodes

Substrate shunt

Interconnects

Release cells
denoted by
blue arrows

100 µm

Held throughout

FIGURE 5.21 Addressable.dielectrophoretic.traps.for.single.cell.sorting..(From.Joel.Voldman,.Martha.
L..Gray,.Mehmet.Toner,.and.Martin.A..Schmidt,.“A.microfabrication-based.dynamic.array.cytom-
eter,”. Anal. Chem. . 74,. 3984-3990,. 2002.. Adapted. with. permission. of. the. American. Chemical.
Society..Contributed.by.Joel.Voldman.)

5.3.5 Micromagnetic Traps
As a general rule of thumb (so we learn in textbooks), magnetism does not miniaturize well,
because most forms of magnetism arise from the cooperation between groups of atoms—so
the smaller you try to make a device, the less magnetic poles per unit volume one has, and the
magnetization or force one will be able to generate with the device will necessarily be smaller.
Despite this seemingly losing proposition, Robert Westervelt from Harvard University fabri-
cated complementary metal-oxide semiconductor ( CMOS ) arrays that can power small coils,
each of which generates a magnetic ield powerful enough to trap a cell decorated with magnetic
beads ( Figure 5.22 ). he advantage of these traps is that the CMOS array of microelectromagnets

(a)

B [G]

30

0

20 mA 0 mA 12.5 mA -7.5 mA 10 mA -10 mA 7.5 mA -12.5 mA 0 mA -20 mA

(b)

20 µm

FIGURE 5.22 CMOS-based.micromagnetic.traps.cell.manipulation..(From.Hakho.Lee,.Yong.Liu,.Donhee.
Ham,.and.Robert.M..Westervelt,.“Integrated.cell.manipulation.system—CMOS/microluidic.hybrid,”.
Lab Chip .7,.331-337,.2007..Reproduced.with.permission.from.The.Royal.Society.of.Chemistry.)