Design, fabrication and application of responsive hydrogel micropatterns
Wünnemann, Patrick; Pich, Andrij (Thesis advisor); Böker, Alexander (Thesis advisor)
Dissertation / PhD Thesis
Dissertation, RWTH Aachen University, 2018
The large-scale fabrication of highly responsive micropattern via a hydrogel imprint process is demonstrated in the present work; firstly, in its structural variability and secondly in its chemical and therefore responsive flexibility in the swelling behavior. Beside the quantitative and qualitative analysis of the transfer of the pattern, namely the stability and swelling/deswelling in an aqueous environment via SFM this work discusses the application of the geometrically defined gels for the manufacture of continuous and conducting metallic gold nanostructures. Chapter 2 presents the idea of the process and sets the fabrication procedure in context to the existing literature. The generation and variation of the wrinkled templates is realized by the interactions between the oxidized thin hard silica-like film and the elastomeric polydimethylsiloxane substrate in chapter 3. Both differ in their mechanical properties and deformation (shrinkage) during relaxation of the stretched substrate. Herein pattern of different geometry and dimension, i.e. pocket like and sun-like pattern, were fabricated by the assignment of distinct flat and (sub-) microstructured polydimethylsiloxane substrates and by the utilization of different stretching devices. In case of sun-like pattern, the data shows that while the number of cavity adjoining wrinkles could be mainly increased by shorter plasma exposure times and increasing applied strain, the corona-width is decreased at the same time. In Chapter 4 the print process, the formation of responsive hydrogel pattern based on poly(N-isopropylacrylamide) and poly(N-vinylimidazole), is discussed. The focus lies in the visualization of the transfer to responsive gel-pattern as well as their stability and swelling behavior in several aqueous environments, with differing ionic strength, pH value and temperature. The comparison between the microgel print and the imprint of a monomer-crosslinker-initiator-mixture to form patterned hydrogel points out(highlights), that the usage of the aqueous mixture evolves the print and leads to a higher physical and chemical variability. Considering the transfer, a minimal loss in height between the stamp and its negative was observed. The transfer from one-dimensional, two-dimensional and radial pattern, visualized via SFM, revealed that complex structures down to 6-10 nm were achieved in this process. Liquid-cell SFM analysis demonstrated the stability of one-dimensional pattern from air into aqueous surrounding. Further measurements demonstrate the responsiveness of one dimensional poly(N-isopropylacrylamide) and poly(N-vinylimidazole) pattern in dependence of their crosslinker concentration and their surroundings. In case of poly(N-isopropylacrylamide) the content of N,N’-methylenebisacrylamide was varied from 5 mol%, 7.5 mol% to 10 mol%, which results in a shift to a higher volume phase transition temperature (VPTT) and a lower swelling degree. While poly(N-isopropylacrylamide) is inert to the change of the pH, the poly(N-vinylimidazole) swells at low and shrinks at high pH values. A copolymerization of both in equalmolarities reveals the stronger influence of the stronger Coulomb interactions between the coordinated charges than the influence of the increased hydrophobic interactions induced by an increase of the temperature. In chapter 5 a two-chamber SFM measurement cell was developed, where the redox-responsivity of P(NIPAAm-co-MAPTAC) microgels in presence of ferri- and ferrocyanates could be mimicked in concentrated solutions comparable to experiments and results from 3D-DLS. Revealing the swelling behavior of the gels on silicon and functionalized gold surfaces shows the strong influence of the surface on the size and swelling behavior of the gel-particles. The hydrogel guided formation to gold nanowires was achieved for the first time due to the successful coordination of the metal-precursor tetrachloroauric acid into the microstructured PVIM-network as described in chapter 6. Here, the incubation of the ions leads to a significant decrease in responsivity of the hydrogel-ion-complex due to the repulsive Coulomb interactions between the induced charges. In a second step the gold was reduced to its elementary form by means of reducing agent or plasma. Hereby the reduction by a reducing agent sustains the polymer-network, whereas the reduction by cold plasma leads to polycrystalline gold pre-structures including a minority of organic residues. The plasma exposed and developed gold structures were analyzed by situ conductivity experiments and revealed resistivities of individual gold nanowires within the range of ρ = 2.2 10-6 Ω·m to 1.4 10-7 Ω·m.