Contexto peruano

As mentioned, PMMA was one of the original polymer materials used by researcher developing polymer microfluidics. It is a thermoplastic, optically transparent

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lighter, less brittle alternative to glass in a wide variety of macro engineering applications

[27]. PMMA is also versatile microengineering material, and can be shaped using

thermoplastic moulding [8, 9, 28-32], traditional machining [33-39] and masked powder blasting [37-42]. However, due to its relatively brittle nature in comparison with other

polymeric microengineering materials, PMMA is used in the literature only for fluidic channels and fittings, as opposed to membranes or movable components.

PMMA is also an X-ray-sensitive material, and therefore is often used as the

working material for LIGA (lithographie, galvanoformung, abformung, or X-ray

lithography) fabricated devices [7, 43]. Further information on this application can be

found in the LIGA section later in this chapter.

After components of a microfluidic system have been fabricated, they must be

bonded together. A number of papers used conventional adhesive in this application [7-9,

32, 35], or use mechanical pressure via bolts [30, 31, 44, 45]. Yamahata et al [37-40]

produced a series of micropumps using PMMA for the body material, and used the

polymerisation of the monomer triethylene glycol dimethacrylate, which is cured in a

hotpress at 70˚C to bond the components. Shen et al [41, 42] also used this procedure.

Chloroform is a solvent for PMMA, and has been used to bond components in Irawan et

al [46]. Hsu et al [47] analysed different PMMA solvent bonding techniques, and found

that although a combination of 24.5 kPa at 100˚C for 9 minutes with ethanol as the

solvent produced the strongest bonding, a method using 24.5 kPa at 60˚C for 5 minutes

with isopropanol as the solvent produced the least shrinkage. Alternatively, Wei et al [48]

bonded 2 traditionally machined PMMA plates together by applying a pressure of 1.5

12 2.4.2 Polyimide

One of the more popular pre-PDMS materials was polyimide. There are a number

of polyimides available, which are sold under a variety of trade names. Poly(amine imide)

is known as Torlon®, and is often used in macroscale injection moulding. Poly(ether imide)

is sold as Ultem® and is used in printed circuit boards. Poly(pyromellitimide-1,4-diphenyl

ether) is thankfully also known as either Kapton® (a thin-film commercial product),

Vespel® or Apical®. It is a high-performance polymeric material used in a wide variety of

industrial applications, including wire and cable wrap and automotive polymer parts.

Some polyimides are thermoplastic materials, meaning they can be heated until melting,

before being cooled to set solid, while other formulations are photocurable [27].

Polyimide has been used in a large number of polymer-based micropumps, in

most cases as a membrane material [49-52]. The aforementioned Büstgens et al [22], who

described the first all-polymer micropump, employing a photocurable polyimide material

to form a pump and valve membrane just 2.5 µm thick. The sheet was fabricated with an

integrated titanium microhotplate using a combination of membrane transfer and

micromachining techniques, and was bonded onto the separately fabricated pump body

components using adhesive. A later paper by the same group used similar techniques to

create a thermopneumatic microvalve system [8]. Also by the same group, Goll et al [32]

in 1996 documented an interesting microvalve design using a 25 µm thick polyimide

membrane. The membrane had a central silicone rubber platelet integrated to aid valves

sealing. The membrane is made bi-stable, only requiring energy input to switch between on/off states, by becoming buckled during assembly. The adhesive process used to fix the

membrane between the moulded PMMA valve body components requires heating, which shrinks the PMMA, causing the polyimide to deform advantageously. Finally, Goll et al

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[53] in 1997 produced an interesting electrostatic microvalve concept, using a membrane

composed of a pair of 1 µm thick polyimide layers insulating a central gold layer,

fabricated using micromachining techniques.

Fluorinated polyimide can be used as a substrate for reactive ion etching (RIE), as

demonstrated by Furuya et al [54], who used the technique to create a microgrid of 100

µm tall fingers. This grid was subsequently metalized to form the electrodes of a special

effect ion drag micropump. The thermal insulation properties of the material were

employed by Kawada et al [55], who used a micromachined polyimide comb structure to

isolate a Ni/Si cantilever from the rest of the microvalve structure. Heating of the

cantilever via an integrated microhotplate caused the cantilever to deform, opening and

closing the microvalve.

2.4.3 PDMS

Polydimethylsiloxane, or PDMS, has influenced the way micropumps and

microvalves are designed more than any other polymer material. Since Unger et al [26]

and the advent of multilayer soft lithography, devices have often shifted away from

designs with discrete valve and pump chambers and membranes, to simply employing

channels with membranes incorporated into their ceilings. The flexibility of PDMS in thin

films allows pneumatic actuators to inflate the membranes like a balloon, filling even

square channels entirely. On the other hand, thicker structures are reasonably strong, allowing PDMS to be used as a bulk structural material for microfluidic devices.

PDMS is used in a range of macro-scale applications, such as in release agents, in

sealants and gaskets and in adhesives. It is favoured for its thermal stability, minimal thermal expansion and high UV radiation tolerance. However, it is known as being

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permeable to gas [27]. In the literature searches used for this review, over 70 published

papers were found using PDMS as a functional part of a micropump or valve. The majority

(50+) of these use multilayer soft lithography as the fabrication technique, and these will be discussed later in this chapter. However, other fabrication techniques have been

applied, including micromoulding [56-61], micromachining [62-64] and macroscale machining [65].

The use of PDMS with micromachining techniques allows the integration of

actuator components into thin, strong PDMS membranes. Khoo et al [62] produced a

PDMS membrane with integral permalloy blocks, in a process discussed later in this

chapter. Similarly, Yin et al [63] integrated a planar metal coil into their PDMS membrane

as part of their electromagnetic micropump. Tracey et al [66] demonstrated a PDMS-

based piezoelectric pump with a glass membrane. Under normal conditions, PDMS does

not bond to glass, but adhesion was achieved using a UV-ozone treatment of both

surfaces, followed by a 2-hour bake at 90°C.

Some of the industrial properties of PDMS described have been utilised on the

micro scale. For example, Chung et al [67] used a PDMS gasket in their otherwise

traditionally-machined piezoelectric microvalve. The gas permeability was exploited by

Eddings et al [68] in their dynamic microfluidic devices, which used a thin membrane of

PDMS to step down a macro-scale pressure to one useable at a micro level. This

permeability is however in some applications not advantageous. Hansen et al [69] found

PDMS to be inefficient as a body material for their electroosmotic special effect

micropump. Electroosmotic micropumps require ions dissolved in the working fluid to

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during operation. Wang et al [70] found their magnetohydrodynamic special effect pump

suffered from the same problem, and solved it by coating the PDMS micropump channels

with a thin layer of SU-8.

Samel et al published a pair of papers [71, 72] in which they mixed PDMS with

expandable microspheres, a mixture they called PDMS-XB. The two papers demonstrated

a single-shot drug delivery system and a thermoexpansion-actuated micropump. In both

cases, the cured PDMS-XB mixture was heated, expanding the microbeads and causing

displacement in the working fluid.