Fabrication of Electrophoresis Devices

There are numerous fabrication methods available, and the complexity of possible designs is virtually limitless. A popular fabrication technique is the use of photolithographic masking in conjunction with wet or chemical etching. The simplest case would be the fabrication of a single channel in a piece of glass. First, the glass would be coated with a layer of deposited metal and subsequently photoresist, e.g. by spin coating, then the pattern mask is placed on top of the photoresist. This masked surface is subsequently exposed to ultraviolet light, which transfers the pattern on to the photoresist. The unprotected area can then be removed, along with the underlying metal surface. An etching solution, such as hydrofluoric acid/nitric acid, is used to etch away the glass, forming the channel in the chip. This surface of the chip protected by the metal and photoresist layer does not etch. This process can be seen in Figure 1. There are a few problems with this approach; the first is that only certain materials can be etched. The second is that, as the channel is etched deeper, the width also increases. This becomes more of a problem as the depth increases, resulting in channel with nonvertical sides. This problem can also create difficulties at channel intersections, which do not have true intersecting corners due to the accelerated etching of the exposed corners.

There are many alternatives to the wet etch approach. Dry etch processes include reactive ion and laser etching; these offer a way to cut precise channels of small dimensions. Silicon is gaining in popularity as an alternative substrate to glass for chip fabrication, and by employing more than one etching technique complex devices can be produced.

In order to use polymeric materials, such as silicone rubber, or fluoropolymers for chip fabrication, new approaches are required. This may take the form of stamping, imprinting or injection moulding of the polymeric material. The approach offers a significant alternative to wet etching of channels directly,

Figure 1 The fabrication process for a separation chip fabricated from silica. The first step is to place the mask on top of the silica base plate covered in deposited metal and photoresist (step 1). After this has been exposed to UV light, the chip is developed to remove the exposed photoresist and metal. It is then etched, e.g. with hydrofluoric/nitric acids (step 2); etching does not occur where the metal and photoresist remains. The final stage (step 3) is the bonding of the cover plate on to the base plate. The two etched channels can clearly be seen.

sible to mass-produce thousands of channel systems with considerable speed.

All of these methods create half the chip; the next step is to attach the cover plate, i.e. the other half of the chip. It is common to locate the holes for the necessary reservoirs in this plate; the reservoirs themselves are frequently constructed by attaching cylinders, e.g. truncated pipette tips, to the top plate. For glass and silica-based systems, it is a simple step to bond the top plate on to the channels by a heating and cooling cycle (the cooling cycle is required to avoid thermally stressing the glass). The fixing of the top plate to polymeric materials can be more complex; however, perhaps the simplest method is to use a thermally activated adhesive to laminate the top plate on to the chip. Typical channel dimensions are 200 |im wide by 60 |im deep, and vary in length from 5 mm to several centimetres. Of course, many other channel dimensions can be created. Some typical patterns can be seen in Figure 2.

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Solar Panel Basics

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