Patterned and switchable surfaces for biomaterial applications

Andrew Hook – Patterned and switchable surfaces for biomaterial applications5.4. Results and discussion5.4.1. SPR imaging of a PNIPAAm microarrayInitially, SPRi was utilised for studying the switchable nature of PNIPAAm.Surface plasmons were excited using the Kretschmann configuration (Figure 5.1A).The reflectivity of light at a wavelength of 800 nm was selected for probing theexcitation of surface plasmons. A longer wavelength was utilised in this case, ascompared to commonly used light of wavelength 635 nm [248, 269], due to thehigher sensitivity attained due to the smaller angle shifts and sharper minima, whichalso allows for measurements with thicker films [248]. However, the excited surfaceplasmons have a longer propagation length, thus, decreasing the lateral resolution toapproximately 25 m [248]. However, for the typical dimensions of surface featuresbeing investigated (spot diameter of 400 m) this resolution was adequate to resolvesurface patterns.An array of PNIPAAm from solutions of varied concentration was formed forSPRi analysis. A typical SPR image of the resultant array is shown as Figure 5.2.Spots of equivalent thickness were selected (Figure 5.2, 4 th column), thus, fixedangle analysis with the SPR instrument was possible. PNIPAAm is an interestingmaterial for biomaterial applications due to its high water content and switchabilitybetween a collapsed state, which sustains biomolecular adsorption, and a swollenstate, which resists biomolecular adsorption, about its lower critical solutiontemperature (LCST) [270]. For PNIPAAm this is approximately 32 °C [70].Production of a patterned PNIPAAm surface would allow for both spatial andtemporal control of biomolecular surface adsorption. Hydrogel surfaces havefrequently been patterned using photolithographic techniques [271-273]. However,5-169