Patterned and switchable surfaces for biomaterial applications

Chapter 5 – Surface plasmon resonance imaging of polymer microarraysof interest is the facile integration of PNIPAAm patterns with protein and cellmicroarrays, which can be achieved by forming a PNIPAAm array using a contactprinter [215]. Thus, the formation and behaviour of a PNIPAAm microarray printedusing a robotic spotter was assessed, with a particular focus on its ability to switchbetween a swollen and collapsed state about its LCST whilst in an arrayed format.SPR signal intensity was set up to look spatially at a PNIPAAm spot andsimultaneously at uncoated gold as a negative control. The resulting reported changein SPR signal intensity is the difference between the signal measured at thePNIPAAm spot and the uncoated gold.Initially, the array was allowed to reach a stable background with constant bufferflow at 1 l/s at an initial temperature of 20 °C. The temperature was then increasedto 40 °C. The resulting sensorgram is reported as Figure 5.3. The initial spike is dueto inhomogeneous sample heating resulting in localised differences in temperature.Upon the temperature reaching the polymers LCST (ca. 32 °C), the intensity of thereflected light was enhanced due to a displacement of the resonance angle to higherangles as a result of a local increase in the refractive index (Figure 5.3) [263]. Thiscan be explained by a net increase in polymer at close proximity to the surface and anet rise in the polymer’s refractive index as a result of the polymer transitioning froma well-solvated random coil state to a densely packed globular state, with theexpulsion of water allowing for intermolecular hydrophobic interactions [274]. Thisswitchable behaviour, above the polymers LCST, is an effect of the entropic penaltydominating over the exothermic enthalpy for the hydrogen bonding between watermolecules and the polymer’s polar groups, resulting in the solvation of the polymerbecoming thermodynamically unfavourable [275].5-170