Diagnostico Físico Urbano

There are a number of situations where it is beneficial to be able to cure a

polymer within an otherwise complete device. This is especially true of polymers that are

damaged or destroyed by the high temperatures or the use of solvents required by the

later fabrication steps used during the construction of the device, such as the baking of

SU-8. The main field in which in-situ photopolymerisation has been developed is in the

fabrication of devices actuated by hydrogels.

The precise nature of hydrogels and their mechanisms of actuation are discussed

in the next chapter. There are however a number of papers that focus directly on the

processes of in-situ photopolymerisation. The first example in the literature employing

the technique in the fabrication of a polymer-based micropump was Liu et al [150] in

2002. This paper describes 4 separate devices, all hydrogel actuated, in this case by an

acrylic acid material. All the hydrogel actuators are fabricated by in-situ

photopolymerisation within pre-fabricated PDMS devices. All the devices are forms of valves, apart from one that is closer in mechanism to a throttle. The throttle device uses a

novel laminar flow method to produce two layers of solidified hydrogel on either side of a

microfluidic channel. A flowing layer of glycerine was used to separate two flows of hydrogel inserted from either side of the throttle channel. The hydrogel flows were cured

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using a UV light source, creating the actuation layers. Further devices were created using

simple masking techniques, creating precision-fabricated hydrogel features. Eddington et

al [6] also describe a device actuated using in-situ photopolymerised hydrogel features, including a microvalve and a pump chamber.

Kim et al [155] describes a complete system for producing features within PDMS

microfluidics devices. This consists of an optical microscope with a CCD camera,

connected to a computer to visualise the mask and prefabricated device. The microscope

and camera are mounted on a rotating frame along with a UV source, which can be

rotated into position above the device once the mask has been aligned using an X/Y

mobile stage. The mask is stationary while the device is manoeuvred into position with

the stage. Both the device and the mask are held in place using negative pressure from a

vacuum pump. The liquid pre-polymer, in this case 4-hydroxylbutyl acrylate (4-HBA), is

inserted into the microfluidic channels using a syringe pump. The pre-polymer is cured

using the UV source, before the unused material is flushed from the finished device by

deionised water. Using these techniques, Kim et al fabricated passive check valves to

rectify flow in a micropump device, and fabricated the actuation pillar for a bi-fluid active

plug microvalve.

A pair of later papers by the same group [159, 160] demonstrated a method of

creating hydrogel spheres for use in microfluidic devices. Liquid prepolymer 4-HBA was

slowly injected into a sheath flow of mineral oil using a sharp pipette. This formed

droplets of the prepolymer within the sheath flow, which were carried with the flow

downstream. The droplets then passed through a UV source, which cured the pre-

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varying the sheath fluid flow rate. The beads were then used to produce a pH-sensitive

microfluidic valve. Much like Liu et al [150], a bead was placed behind a thin PDMS layer,

held in position by a series of posts. A secondary fluid, separate from the working fluid, was passed across the microsphere to initiate expansion and contraction against the

membrane, causing the valve to close and open respectively.

2.5.7 Other

A series of papers by Yamahata and Gijs et al [37-42] describe a number of microfluidic

devices, in part made of PMMA shaped using masked powder blasting. This technique

uses gas-flow accelerated micro-scale particles such as alumina to erode bulk material

through a laser-cut metal mask. This process can be used to rapid prototype multiple

sheets of PMMA, which can then be adhered together to make a multi-level device.

However, the blasting powder also erodes the metal mask at a rate of around 0.2 μm/s,

decreasing accuracy on masks used multiple times. The surface roughness of the final part

is around 1.25 µm [40].

Ultrasonic welding is a technique used commonly in the macro-scale world for the

bonding of polymer components made out of materials such as PEEK that cannot be

solvent bonded. It is however not commonly used in the fabrication of micropumps and

valves. Truckenmüller et al [82] used a standard ultrasonic welding machine in the

assembly of their piezoelectric micropump, using both PMMA and PEEK. Metal components such as the inlet and outlet tubing were also successfully integrated using

35 2.5.8 Stereolithography

Stereolithography is a form of rapid manufacturing technology that is additive in

nature – layers of material are deposited in series to build a true 3D model. The materials

used include metal powder, which is laser sintered into a solid form, or the repeated

layering of cut paper sheets. However, the major form of the technology involves the

automated curing of multiple layers of photosensitive resins. A 3D model is created using

a computer-aided design (CAD) program. This model is then sliced into multiple layers

using specialised software, which then creates a mask for each layer. These masks are

then used to pattern sequential layers of the resin. The layer to be patterned is normally

defined by the surface of the resin, as in most laser-based SLA (stereolithography

apparatus), or as a layer between two glass plates, as seen in the projector-based

systems. As such, polymer-based stereolithography it can be seen as a form of automated

surface micromachining [11], and also has similarities with multi-layer soft lithography.

The first use of this technology in the fabrication of a micropump was reported by

Carozza et al in 1995 [161], who produced a piezoelectrically-driven ball-valve micropump

with stereolithography fabricated pump body and fluidics, although the exact form of SLA

technology employed was not reported. Since then, the field has gone quiet, with the

next paper in the literature being Han et al in 2005 [115], followed by Hasegawa et al in

2008 [162], both of whom created microvalves with an SLA fabricated body. Both papers

create only fluidic components using the SLA systems; devices with functional

components built using stereolithography are an exciting potential area of future research.

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