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Fabrication and application of hydrophilic-hydrophobic micropatterned polymer surfaces

Efremov, Alexander

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Surface patterning is important in a wide spectrum of applications ranging from microelectronics, sensors design and material science to high throughput screening, tissue engineering and cell biology. A number of methods for specific patterning applications, such as photolithography, soft lithography, or electron beam and dip-pen nanolithography, have been developed. However, there is still a clear need for the development of novel methods permitting patterning of different cell types, nano- and microparticles as well as hydrogels incorporating cells. These novel patterning methods are vital for the advancement of such research fields as tissue engineering, biomaterials and for fundamental investigation of cell-cell communication, tissue and organ development. The aims of this PhD thesis were: a) develop a technique for creating droplets of liquid with defined geometries that can be used for patterning water soluble components; b) optimize the conditions for the fabrication of porous polymer surfaces for the liquid patterning; c) characterize the produced patterned polymer surfaces; d) further develop the technique for maskless generation of liquid patterns with arbitrary geometry; e) optimize the method for the patterning of different materials (chemicals, hydrogels, microparticles); f) show an application of the method for patterning of living cells and characterize their behavior on the composite surface during cultivation; g) show an application of the technology to mimic natural cell-cell communication in vitro via signaling protein propagation between patterned cell populations in co-culture. The first part of the work was devoted to the development of porous polymer layers with precise micropatterns of hydrophilic and hydrophobic areas. In order to fabricate these patterns, UV-initiated photografting of 2,2,3,3,3-pentafluoropropyl methacrylate (PFPMA) on porous poly(2-hydroxyethyl methacrylate-co-ethylene dimethacrylate) (HEMA-EDMA) was optimized. Before and after photografting, both polymer substrates were thoroughly characterized using water contact angle measurement, UV-Vis spectroscopy, scanning electron microscopy (SEM) and time of flight secondary ion mass spectrometry (ToF-SIMS). Porous properties were characterized by UV-Vis spectroscopy, SEM and dynamic light scattering techniques (DLS). Due to the high difference in wettability of the hydrophilic HEMA-EDMA polymer film and hydrophobic regions coated with PFPMA polymer brushes, aqueous solutions can be trapped in the hydrophilic areas, taking the shape of these areas. The transparency of the HEMA-EDMA monolith originated from porous properties of the polymer makes it suitable for microscopic monitoring of liquid patterns during experiments. The method was for the first time applied for the simultaneous micropatterning of multiple cell types. More than ten different cell populations separated by hydrophobic borders could be cultured in microreservoirs. After adhesion, the cells could be placed in the mutual culture medium, allowing cell-cell communication among populations. During 3 days co-culture in the mutual medium, cross-contamination was shown to be less than 1,5%, although the cells were pre-patterned in the hydrophilic areas separated by hydrophobic borders of only two to three cell diameters. The capability of cell patterning and long term cultivation opens the way for many interesting bio-applications, such as in vitro mimicking important biological processes that involve and depend on the organization of multiple cell types into complex micropatterns in vivo. As a case study, I together with Dr. Steffen Scholpp and Dipl. Eliana Stanganello (ITG, KIT) used the developed technique to visualize spreading of signaling molecules (Wnt protein) from one micropatterned population of fibroblast cells to another fibroblast population by activation of the reporter system. Thus, we were able to simulate paracrine signaling system in vitro. In addition, I further developed our technique into a new type of mask-less liquid patterning or digital liquid patterning (DLP) method. The idea of this method is similar to the working principle of a digital score board. A digital score board consists of many small bulbs, which generate light symbols on it. In the case of DLP, instead of the bulbs, small liquid droplets (digits) form a more complex liquid pattern on a substrate. The substrate for DLP is a composite surface, consisting of a grid of hydrophilic HEMA-EDMA spots divided by hydrophobic PFPMA barriers. The method allows on-demand fabrication of liquid patterns without the need to change the substrate and use an additional photomask. Patterns with customized geometries can be prepared manually by simply pipetting liquid inside the spots and successively coalescing the generated droplets to form a liquid micropattern. The DLP does not require clean room or high-precision microfabrication and allows the manual positioning of microdroplets in the range of micrometer scale. It was also shown that using superhydrophilic/superhydrophobic patterned surfaces leads to spontaneous dewetting of the coalesced microdroplets on the interface of the superhydrophobic border and the superhydrophilic spot. Hence, the usage of hydrophilic/hydrophobic patterned surface ensures the stability of liquid patterns during manipulations. Furthermore, the developed technique enables patterning of not only solutions, e.g. different chemicals, but also suspensions of living cells and microparticles, hydrogels, or formation of liquid multi-component gradients with complex geometries. Thus, this method will be especially useful for biological studies, which require the generation of complex patterns of different or the same cell types, or bioactive materials and cellular gradients without the need for sophisticated microfluidic and printing equipment, or for designing additional masks

Item Type: Dissertation
Supervisor: Grunze, Prof. Dr. Michael
Date of thesis defense: 13 June 2014
Date Deposited: 26 Jun 2014 05:02
Date: 2014
Faculties / Institutes: Fakultät für Chemie und Geowissenschaften > Institute of Physical Chemistry
Subjects: 540 Chemistry and allied sciences
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