Microfabrication

The most frequently used conjugated polymer on the microscale is polypyrrole (PPy), although polyaniline (PANi) has also been used on occasion [17, 18]. Of the polypyrroles, the one that has been most often used is PPy(DBS), PPy doped with dodecylbenzenesulfo-nate, which is a bulky surfactant. This material expands in the reduced state due to cation ingress and contracts in the oxidized state. 

Using microfabrication techniques is advantageous it is possible to readily achieve micron-scale, or even smaller, lateral feature dimensions as well as excellent reproducibility of the structures and their behavior, since they are batch fabricated and not individually hand-made. Batch fabrication also makes the devices potentially low cost. The techniques described above are compatible with those used to make other MEMS devices, so conjugated polymers can be combined with them to form complex systems. As mentioned above, because of their low operating voltages, conjugated polymers can also be interfaced with standard integrated circuitry [7]. 

The cathode is composed of an HF-resistant material (e.g. platinum) which is immersed in the etch solution. If a uniform current density is required, a mesh cathode with surface area larger than the silicon wafer must be used and placed in the solution parallel to the wafer, and at a suitable distance so that the current flow at the interface between the wafer and the solution is spatially uniform. Alternatively, when a pore size gradient is pursued, a rod-like Pt cathode is immersed at a specific position within the solution [12, 19]. 

Pores (back side) Pores (cross-section) 

The size polydispersion, and the very small yield in the micron size range suitable for drug delivery applications, constitutes a significant drawback of this technique. Size uniformity is important for shared biodegradation rates, which depend on both size and porosity. Another drawback of this technique is the shape polydispersion, which affects both the cellular uptake of pSi particles and blood flow characteristics [24]. Microparticle uptake by the reticuloendothelial system (RES), as well as margination and adhesion dynamics in the bloodstream, are strongly correlated to both the shape and size of the particles [21, 25, 26). 

The electrochemical etch profile is determined by the local current intensity gradients at the silicon interface. This technique is both extremely controllable and reproducible, and allows the manufacture of particles with characteristic sizes in the micron range and monodisperse in terms of both particle and pore size. The particle shapes attainable are Umited by the 3-D patterns achievable through the combination of photolithographic techniques and the characteristic profile of the electrochemical etch. 

Once the porous siUcon microparticles have been fabricated, they must be carefully characterized to insure that the desired particle specifications have been achieved and that there is minimal batch-to-batch variation. Good Manufacturing Practice (GMP) must be maintained to ensure repHcabUity of microparticle fabrication between lots and to exclude particle structural variabUity as a source of experimental observation in biological experiments. Given the complexity and size scales involved with the internal stracture of these microparticles, a battery of techniques must be employed to quantify the important metrics, such as porosity, pore size, density, interior volume, surface area, surface charge and surface modification. 

Transport Phenomena in Microfluidic Systems, First Edition. Pradipta Kumar Panigrahi. 

This chapter introduces different fabrication methodologies of microdevices. Different functional materials used in these devices are discussed first. Subsequently, different steps adopted for photolithography-based micromanufacturing are discussed next. This is followed by the plastic-based micromanufacturing process. Subsequently, the laser-based microfabrication is introduced. Finally, different bonding processes adopted during fabrication of microdevices are discussed. 

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Figure Bl.19.17. Conunercially produced, microfabricated, V-shaped Si3N4 cantilever and tip for AFM (Taken from [215].)...

Haab B B and Mathies R A 1999 Single-molecule detection of DNA separations in microfabricated capillary electrophoresis chips employing focused molecular streams Ana/. Chem. 71 5137-45... 

Microfabrication techniques used for the production of MEMS (micro electro-mechanical systems) have been successfully used to produce highly efficient micro fluidic systems. 

Figure 3 shows an SEM micrograph of a typical SFM cantilever. The cantilevers are 100—200 pm long and 0.6 pm thick, microfabricated from low-stress Si3N4 with an integrated, pyramidal tip. Despite a minimal tip radius of about 400 A,... 

Rai-Choundhury, P. ed.. Handbook of Microlithography, Micromachining and Microfabrication. Vol. 1 microlithography, SPIE Optical Engineering Press, Washington,... 

D. W. L. Tolfree. Microfabrication using synchrotron radiation. Rep Prog Phys (57 313-351, 1998. 

J. L. Wilbur, A. Kumar, E. Kim, G. M. Whitesides. Microfabrication by microcontact printing of self-assembled monolayers. Adv Mater (5 600-604, 1994. 

Microfabrication technology has made a considerable impact on the miniaturization of electrochemical sensors and systems. Such technology allows replacement of traditional bulky electrodes and beaker-type cells with mass-producible, easy-to-use sensor strips. These strips can be considered as disposable electrochemical cells onto which the sample droplet is placed. The development of microfabricated electrochemical systems has the potential to revolutionize the field of electroanaly-tical chemistry. 

Chaudhari AM, Woudenberg TM, Albin M, Goodson KE (1998) Transient liquid crystal thermometry of microfabricated PCR vessel arrays. J Microelectromech Sys 7 345-355 Cheng P, Wu WY (2006) Mesoscale and microscale phase heat transfer. In Greene G, Cho Y, Hartnett J, Bar-Cohen A (eds) Advances in heat transfer, vol 39. Elsevier, Amsterdam Choi SB, Barron RF, Warrington RQ (1991) Fluid flow and heat transfer in micro- tubes. ASME DSC 40 89-93... 

Ross D, Gaitan M, Locascio LE (2001) Temperature measurement in microfluidic systems using a temperature-dependent fluorescent dye. Anal Chem 73 4117-4123 Sammarco TS, Bums MA (1999) ThermocapiUary pumping of discrete drops in microfabricated analysis devices. AlChE J 45 350-366... 

Li, Y. and Bhushan, B., Wear and Friction Studies of Contact Recording Interface with Microfabricated Heads," Wear, Vol. 202,1996, pp. 60-67. 

Photolithography and microfabrication can be combined to manufacture well-defined, micro-... 

Boateng S, Lateef SS, Crot C, Motlagh D, Desai T, Samarel AM, Russell B, and Hanley L. Peptides bound to silicone membranes and 3D microfabrication for cardiac cell culture. Adv Mater, 2002, 14, 461 63. 

Of course, the benefits of microfabricated and stractured reactors are also applicable to even larger scale processes. For example, Evonik s development of a production-scale microstructured reactor for vinyl acetate manufacture (150000t per annum) claims depreciation and operating cost savings of 3 million per year [25]. 

FIG. 16 SEM micrograph of a microfabricated Si3N4 tip coated with E. coli D21 cells. (Reprinted with permission from Ref. 74. Copyright 1999 American Chemical Society.)... 

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