Patterned and switchable surfaces for biomaterial applications

Andrew Hook – Patterned and switchable surfaces for biomaterial applicationsThe absence of a Si signal on the Si-ALAPP surfaces (Table 2.1) stronglysuggests that pinhole-free films with a thickness of more than 10 nm (theapproximate penetration depth of XPS for polymers) were formed. The oxygencontent of this film is due to the uptake of oxygen once it is re-exposed to air byreaction with long-lived reactive species in the polymer film such as free radicals[183, 184]. Upon subsequent grafting of PEG to the amino groups of the ALAPPcoating, a significant decrease in the nitrogen content (11.0% to 5.7%) and anaccompanying rise in the oxygen content (9.0% to 21.0%) was observed.Furthermore, the C 1s spectrum in Figure 2.4B shows a large increase in the C-O/C-N component and a relative decrease in the aliphatic carbon peak and higher oxidisedcarbon components as compared with the ALAPP film (Figure 2.4A; Table 2.2).These results suggest not only that PEG was successfully grafted to the ALAPP, butalso that amino groups were indeed present on the ALAPP surface. The presence of anitrogen signal for the PEG grafted surface suggests that this layer is less than 10 nmthick. The theoretical length of a fully extended PEG chain with a molecular weightof 5000 is approximately 42 nm [65]. The large difference between this theoreticalvalue and the observed value is due both to the low persistence length of PEGrandom coils and to the fact that the XPS measurements were taken in the dry statewhere the grafted polymer is collapsed. A recent study by Zdryko et al., 2003 [65]reports a greater than 3 fold swelling of the PEG layer in an aqueous environment ascompared to a dry sample.2-77