Abstract
Poly-N-isopropylacrylamide (PNIPAAm) hydrogels, known for their sensor and actuator capabilities, can be photolithographically structured for microsystem applications. For usage in microsystems, the preparation, and hence the characteristics, of these hydrogels (e.g., degree of swelling, size, cooperative diffusion coefficient) are key features, and have to be as reproducible as possible. A common method of hydrogel fabrication is free radical polymerisation using a thermally-initiated system or a photoinitiator system. Due to the reaction quenching by oxygen, the polymer solution has to be rinsed with protective inert gases like nitrogen or argon before the polymerisation process. In this paper, we focus on the preparation reproducibility of PNIPAAm hydrogels under different conditions, and investigate the influence of oxygen and the UV light source during the photopolymerisation process. The flushing of the polymer solution with inert gas is not sufficient for photostructuring approaches, so a glove box preparation resulting in better quality. Moreover, the usage of a wide-band UV light source yields higher reproducibility to the photostructuring process compared to a narrow-band UV source.
Highlights
Hydrogels are 3D polymer networks with the capability to swell in a swelling agent
By measuring the UV light spots regarding their local intensities and comparing these results with the NMR tubes were filled under different conditions and exposed to UV light for 30 s
We investigated the influence of different oxygen content in the polymerisation setup and the influence of different UV radiation sources, where we introduce a pulsed laser as UV source (51 kHz, λ = 355 nm)
Summary
Hydrogels are 3D polymer networks with the capability to swell in a swelling agent (e.g., water [1]). The gel can take up multiples of its dry mass of water [2] This type of polymer can be chemically tuned regarding its chemical, physical, or mechanical properties [3]. Popular hydrogels for microsystem engineering approaches are PNIPAAm (Poly-N-isopropylacrylamide)-based hydrogels [16,17] Such smart hydrogels react to environmental condition changes with an altered degree of swelling [18,19,20]. A common method to produce polymers in general, including hydrogels is free radical polymerisation, because of its convenient and time-effective synthesis procedure. The cooperative diffusion coefficients and the degree of swelling were measured as characteristics to get insights into the reproducibility of the polymerisation process and the hydrogel preparation
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