Biomaterials for MEMS

120 V. Bazargan and B. Stoeber

consisting of PEO-PPO block copolymers belong to this group and are commer-

cially available and commonly used to form TRFs.

6.4.2 Properties of Pluronic Solutions

6.4.2.1 Phase Transition of Pluronic Solutions

Pluronics are biocompatible symmetric triblock copolymers of poly(ethylene

oxide)

x

-poly(propylene oxide)

y

-poly(ethylene oxide)

x

;P(EO)

x

–P(PO)

y

–P(EO)

x

.

These amphiphilic copolymers have hydrophilic EO

(x)

and hydrophobic PO

(y)

units where x and y can take on specific values depending on the particular kind

of Pluronic. Aqueous Pluronic solutions undergo reversible gel formation at a

specific critical solution temperature that depends on the polymer concentration,

its molecular weight and the polymer architecture.

29,32,90,91,93−95

The sol-gel tran-

sition of Pluronic solutions occurs through three dimensional packing of polymer

micelles into a lattice, to achieve a hydrophilic/hydrophobic balance.

34

Above the critical micelle concentration (CMC) the polymers aggregate into

multi molecular structures with a hydrophobic central core (PO) and a hydrophilic

shell(EO)facingtheexternalmedium.

96,97

At higher concentrations, above the

critical gel concentration (CGC) the micelles can take on a cubic lattice structure.

34

Elevated temperatures lead to dehydration of the hydrophobic core and also

increase the hydrophobicity of the hydrophilic blocks, which eventually leads to

gel formation.

92,98

The aggregation and phase behavior of Pluronics have been

extensively studied using several techniques including light scattering, NMR,

SANS, SAXS, DSC, microscopy, and rheology.

34,99−104

Pluronic F127, or Poloxamer 407, has previously been used for several appli-

cations in drug delivery. Its chemical formula is (PEO)

106

(PPO)

70

(PEO)

106

,itisa

nonionic surfactant, and in aqueous solution it can thermo-reversibly change from

its liquid phase into a gel structure at a gelation temperature T

G

, which depends

on its concentration. Aqueous solutions of Pluronic F127 are non-Newtonian

fluids with thermo-thickening and shear thinning properties.

26,99

The rheological

properties of Pluronic F127 solutions including the effect of salts on gelation as

well as its biocompatibility will be further discussed.

6.4.2.2 Rheological Characterization of Pluronic Solutions

Pluronic F127 solutions are thermo thickening materials. This means that upon

heating their viscosity increases gradually over a wide range of temperatures

until to their gel point, where the viscosity appears to increase dramatically.

At higher concentration of Pluronic, gel forms at lower temperature. Stoeber

et al. have shown this behavior using rheometry for different concentrations

of Pluronic F127.

26

Continuous cone and plate vioscometry characterizes this

phase transition under conditions that are relevant for the application to fluids

streaming in microfluidic channels. This change in viscosity or phase change is

completely reversible and after one thermal cycle, the shear viscosity returns to its

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Flow Control in Biomedical Microdevices using Thermally Responsive Fluids 121

dĞŵƉĞ ƌĂƚƵƌĞ΀Σ΁

sŝƐĐŽƐŝƚLJ΀WĂ Ɛ΁

Figure 6.1. Viscosity as a function of temperature for different concentrations of Pluronic

F127 in water from cone and plate viscometry at controlled shear stress (0.6 Pa s).

26

For color

reference, see page 265.

original value. The phase transition between liquid and gel occurs over a narrow

temperature range (0.5

◦

C) as shown in Fig. 6.1.

Pluronic F127 solutions also show pseudo plastic flow behavior (i.e. shear

thinning). Their viscosity occurs to decrease for increasing shear stress at a

constant temperature as previously shown using rheometry.

26

6.4.2.3 Effect of Ions on the Gelation Temperature of Pluronic Solutions

The effect of salt on micellization and also on the gelation temperature of Pluronic

F127 has previously been shown.

105−108

In general, a lowering of the micellization

and gelation temperature is observed upon addition of salt except for salts with

large polarizable anions, such as SCN

−

,andI

−

, where an initial increase in the

gelation temperature is often encountered. The effect is often discussed in terms

of “salting in” and “salting out”, of “structure-making” and ”structure-breaking”,

additives. The addition of salt is expected to further dehydrate PPO blocks and

induce some dehydration of the PEO blocks.

108

Almgren et al. have used a simple

but effective approach to qualitatively predict the effects of additives: type one,

those that increase the polarity difference between the solvent and the polymer

will favor phase separation and decrease the micellization temperature. Salts such

as NaCl, MgSO

4

and KF are depleted in the immediate vicinity of the polymer

with its low polarizability, and increase the polarity of water. Phase separation is

facilitated and so micellization occurs at a lower temperature. For salts of type

2, such as NaI or NaSCN on the other hand, the large, polarizable anion seems

to partition slightly in favor of the polymer, increasing its polarity and thereby

reducing the polarity difference. The polymer becomes more compatible with

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