Characterization of surface microphase structures of poly(urethane urea) biomaterials by nanoscale indentation with AFM

Atomic force microscopy utilizing both tapping mode and force mode imaging is used to visualize the separated microphases in poly(urethane urea) films under ambient and aqueous conditions. The topography of the PUU surface changed upon hydration with the formation of nanometer-sized features on the surface. The surface becomes enriched in hard domains with hydration time and this enrichment is irreversible after dehydration. Force mode measurements were used to quantify mechanical properties as both indentation and modulus measurements. Analysis of the modulus during indentation reveals the three-dimensional nature of the structures, with the surface being covered by a 2-20-nm-thick soft segment overlayer under ambient conditions, while hydration leads to the loss of this overlayer. The force measurements also reveal the presence of regions having modulus values between those of the hard and soft phases and located spatially near the interface between the hard and soft domains. However, such regions with intermediate modulus were only rarely seen following hydration. Calculation of the Young's modulus from the compression data shows that hydration increases the modulus of the PUU surface by both enrichment of the amount of hard domain present and increasing the modulus of the individual hard and soft phases themselves. Direct visualization of the distribution of these different domains on the surface by nanoscale measurements provides an important path to characterizing the relationships between the surface properties of these materials and subsequent performance in biomedical applications.