Biomaterials for MEMS

Hydrogel-Based Microfluidic Cell Culture 97

5.2.6 N-isopropylacrylamide Polymers (NiPAAm)

N-isopropylacrylamide (NiPAAm) homopolymers (Fig. 5.6) and copolymers are

thermoreversible polymers which have been used for drug delivery and tissue

engineering. At low temperature, the polymer dissolves is a liquid solution since

hydrogen bonds form between polar groups in the polymer and water. The

polymer chains form extended coils surrounded by ordered water molecules. The

gel is observed at high temperature. There, the polymer precipitates out of solution

because hydrophobic interactions dominate, and the polymer chains transition to a

globular state.

36

NiPAAm homopolymer in pure water has a lower critical solution

temperature (LCST) of 32–34

◦

C, where the transition occurs between liquid and

gel. NiPAAm copolymers which contain hydrophilic groups such as polyethylene

glycol (PEG) will increase the LCST. Reversible gelation occurs due to polymer

chain entanglements and the increased hydration.

In contrast to other thermally reversible systems such as Pluronics, NiPAAm

copolymers when gelled do not revert to the liquid state upon dilution. The

NiPAAm phase transition also occurs over a narrower temperature range, permit-

ting more precise control for on-chip gelation.

Copolymers of NiPAAm have already been used for 3-D culture of chondr-

ocytes

37

and pancreatic islets.

38

Acrylic acid or polyethylene glycol (PEG) copoly-

mers are used to increase the hydration of the gel, since NiPAAm homopolymer gel

exhibits high water loss. This change in hydration with temperature has been used

to fabricate cell sheets. When cells are cultured on surfaces treated with NiPAAm

at 37

◦

C, the cells adhere to the slightly hydrophobic surface. When the temperature

is brought below the LCST, the gel swells and becomes more hydrated, and the

cells spontaneously detach. This effect has been used to create confluent sheets

of myocardial cells which can be layered to build 3-D electrically communicative

structures.

39

5.3 MICROFABRICATION

Microfabrication processes are the basis for integrated circuit (IC) manufacture.

They involve thin film deposition, patterning, and etching. In addition to fab-

rication of planar silicon-based ICs, microfabrication processes have enabled the

development of micro-electro-mechanical systems (MEMS), as well as an increas-

ing variety of glass- and polymer-based lab-on-a-chip and implantable devices for

biomedical applications.

More recently, the technique of soft lithography has been developed to cre-

ate microstructures using printing, molding, and embossing processes.

40

This

technique most commonly uses poly(dimethylsiloxane) (PDMS) as an elastomeric

mold material. Elastomeric PDMS structures are replicated from a mold master,

which is commonly either a silicon or glass substrate patterned with photolithog-

raphy (Fig. 5.7). If a high-aspect ratio photoresist such as SU-8 is used, the mold

master can be produced using only one photolithography step. Alternatively, the

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98 M. C. W. Chen and K. C. Cheung

D E

F G

H I

Figure 5.7. Microfabrication of PDMS structures. (a–b) Photoresist is spin-coated on a

silicon wafer. (c) A mask is placed in contact with the layer of photoresist. (d) The

photoresist is illuminated with ultraviolet (UV) light through the mask. An organic

solvent dissolves and removes photoresist that is not crosslinked. The master consists of

a silicon wafer with features of photoresist in bas-relief. An expanded view of one of the

microfabricated structures with its characteristic critical dimensions is shown. (e) PDMS is

poured on the master, cured thermally and peeled away. (f) The resulting layer of PDMS

has microstructures embossed in its surface.

47

For color reference, see page 261.

mold can be produced through a photolithography step followed by reactive ion

etching of the silicon substrate. PDMS is poured onto the mold master, cured,

and then removed. The PDMS structure can be used as a stamp for microcontact

printing of proteins or self-assembled monolayers. This PDMS structure can also

form closed microfluidic channels when bonded to another substrate.

PDMS has many advantages for use in biomedical applications: flexibility,

transparency, biocompatibility, and gas permeability. Prototype PDMS structures

can be made rapidly and at relatively low cost. Elastomeric structures made

in this method have been used in microstamping, microfluidic patterning, and

stencil patterning to create well-defined areas of cellular adhesives, polymers, self-

assembled monolayers (SAMs), and biomolecules to study cellular interactions on

the microscale.

41

PDMS microchambers have been used as stamps for patterning

cell attachment on surfaces,

42

chambers for DNA amplification via the polymerase

chain reaction (PCR),

43−45

and as chambers for bacterial

46

(Fig. 5.7) and mam-

malian cell culture.

47

PDMS will swell and absorb liquid media, which can be a major consideration

for cell culture applications involving small volumes. The amount of liquid

absorbed depends on the ratio of siloxane oligomer precursors used to make the

PDMS.

48

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