- 1 Flow Coating
- 1.1 Background
- 1.2 Current Methods
- 1.3 Flow Coating
- 1.4 Improvements
- 1.5 Our Experiment
- 1.6 Challenges
- 1.7 Material Considerations
- 1.8 References
Technological advances in nano and micro patterning are finding their way into a number of integrated scientific fields such as materials chemistry, biology, and engineering. Developments in microfabrication, automation, microelectronics, and biology have already led to the creation of novel technologies for biological research.1 A few examples of these developments are nanoelectronics, integrated optics, biological sensors, drug delivery systems, and antifouling surfaces. Integration of research advancements between fields are likely to compromise many technological advancements in the near future. Our work is focused on implementing micropatterning of multifunctional polymers on a substrate in order to study the effects of pattern properties on bacterial attachment.
The field of patterning is large and growing. Currently, size dependence to electrical, optical, and magnetic properties of materials such metals and polymers are better understood. A few areas of research interest are in pattern generation, materials science of patterning and functional properties, and pattern transfer methods. A number of patterned features are important for the study of biological systems such as isolated features, overlapping features/stripes, dots, lines, and particles. These features can provide an array of benefits and drawbacks, so it is important to pattern features which provide optimal functionality. Depending on the design parameters of interest, the choice of material we wish to pattern onto our substrate, and the properties of the substrate itself, there are a variety of patterning approaches which must be evaluated.
Types of Materials Patterned
Nowadays there are a number of different materials that can be patterned including:
- Metals, organic polymers, organic/inorganic crystals
- Nanoparticles and nanowires/nanorods
- Metal and metal oxide nanotubes
- Deposition of clusters and nanocrystals on graphite or other semiconductive materials to obtain novel 3D nanosystems
- Single and multi-walled carbon nanotubes
- Nanobiological systems (DNA, antibodies, microorganisms, enzymes etc.)
Introduction to Techniques
Patterning can be accomplished through the direct deposition or removal of a functional material onto/from a previously conformed substrate. Deposition of a single atomic layer, or monolayer, or multiple layers of functional material is often accomplished through self-assembly or directed assembly. This means either the chemistry of the molecule can determine how the pattern assembles or another technique must be used to deposit a pattern of the material.
Numerous available techniques for patterning depending on project specifications.
Not so much a technique as the underlying concept of all microscale patterning. Without a good understanding of self-assembled monolayers (SAMS) fabrication will be much harder if not impossible. SAMS are made by simply exposing the surface to be modified to a specific compound under known conditions which cause the molecule to bind in a predictable and controllable manner. Once a surface has a monolayer present, the original surface will not be exposed to the environment and the new surface will have drastically different chemical properties. Most of the optical, mechanical and electrical properties of the surface are retained throughout the SAM fabrication process however. A common example is that of an automobile windshield which has been coated in a hydrophobic polymer. The coating reduces adhesion of the raindrops on the glass and thus they bead up and roll off much more easily, but the glass is still optically clear and will still shatter under the same conditions that it would prior to the SAM.
Electron and Atomic Beam Writing
Dip-pen-nanolithography, or DPN, is an AFM-based additive monolayer process named for its similarity to the old fashioned quill pen writing. Basically, an AFM tip is coated in a mobile chemical species (the ink) which is then transferred onto the substrate wherever the AFM tip comes in contact. The transfer of the mobile phase onto the substrate depends largely on the nanoscale meniscus formed between the substrate and the pen tip (as shown in the figure below). After lots of recent studies into this phenomenon, it is now know that feature size is critically dependent on the size of this meniscus as well as more generally on the speed at which the mobile liquid is transferred from the tip.
Subtractive lithography is the opposite of depositing a monolayer, it is exactly as it sounds where the end goal is to remove a layer. It proceeds by applying either a large enough physical force or an electric field of sufficient intensity so that the material at the surface is removed and/or deformed. In the case of a SAM, a precisely applied force will cause the bonds to break that hold the material to the substrate, but the monolayer itself will not be harmed at all, whereas applying a voltage will cause the monolayer to desorb but could also start unwanted side reactions if not properly controlled. Once a SAM is removed, the chemical properties of the original substrate are restored. In the case of nanografting, a monolayer is removed from a substrate while in the presence of another SAM-forming molecule, and the moment that the first SAM is removed the second molecule immediately begins to form a new monolayer.
The goal of the experiment that we are conducting is to produce adjacent stripes of multifunctional material that is both robust and prevents bacteria attachment. Our plan is to coat a glass surface with 5 alternating stripes of antibacterial and antifouling polymers via laminar flow coating. We designed the following photolithographic mask to be used to create our microfluidic device:
Notice how each small device on this one mask has slightly different channel dimensions and configurations. The outlet channel widths start at 80 μm on the left, and then move up to 100, 300 and finally 500 μm on the far right. The designs on the top and bottom row are those same outlet channel dimensions but with filleted corners where the inlet channels come together.