Mechanical Characterization of Photo-crosslinkable Hydrogels with AFM
Photo-crosslinkable hydrogel films are versatile materials for controlled drug delivery devices (Duncan, 2003), three-dimensional micro-assemblies, and components in microfluidic systems (Beebe et al., 2000). A hydrogel is a flexible network of polymer molecules which swells when placed in water but will not dissolve because of the chemical or physical interconnections, called crosslinks, between the long chains of polymer molecules. Because of the high water content, hydrogels are pliable and respond to environmental conditions since the degree of swelling in these networks depends on environmental conditions such as pH and temperature (Hilt et al., 2003). This flexibility and environmental sensitivity renders hydrogels versatile materials for applications. For instance, the probability that they will damage delicate system components during micro-assembly is lower, and they can be tailored to respond to specific environments. Consequently, it is important to understand both the mechanical properties and the dynamics responses of these materials when designing such applications. Therefore, my thesis uses an atomic force microscope (AFM) to measure indentation with each applied force and then determines the best model with which to determine the mechanical properties from these data. Hydrogels also have poroelastic and viscoelastic properties which affect stress relaxation and thus impact their behavior in the different applications above. On short time scales, viscoelastic materials respond to stresses as elastic solids. On longer time scales, however, viscoelastic materials behave under stress as viscous fluids (Lin, 51). In poroelastic materials, the fluid in the material migrates simultaneously with the deformation of the porous network (Cai et al., 2010). Relatively recently, poroelasticity has been identified as an important factor in the mechanical behavior of polymer gels (Galli et al., 2008). Photo-crosslinkable hydrogels are unique because they crosslink – the polymer chains bond to form the network – when exposed to ultraviolet (UV) light. Different dosages of UV light produce different mechanical properties, a feature which can be utilized when designing systems (Toomey et al., 2004). Although the mechanical properties of hydrogels have been studied extensively, the mechanical properties of photo-crosslinkable hydrogels have not been investigated thoroughly. To design accurate controlled drug delivery devices and other applications, we must both understand the stress relaxation of photo-patterned hydrogels and correlate the UV dosage and mechanical properties. We use AFM to examine the mechanical properties of poly(N-isopropylacrylamide) (PNIPAm), which is attached to a silanized silicon substrate with a HEMA adhesive layer. We primarily use the AFM to collect force versus indentation data. In order to extract mechanical properties, particularly the Young’s modulus (the ratio of stress to strain) of the materials, from these data, we will evaluate existing mechanical models. Originally, the Hertz model (Hertz, 1881) was used to correlate the indentation and force data to the mechanical properties of the materials. This model, however, does not consider the surface forces between the two contacting surfaces, rendering it inaccurate for hydrogel thin films, where surface forces are significant. The Johnson, Kendall, and Roberts model (Johnson et al., 1971), which includes surface forces, is more appropriate but does not account for viscoelastic and poroelastic effects. Therefore, we will evaluate these models and modify them as needed to account for the poroelasticity of hydrogels. Simultaneously, we study the stress relaxation in these materials in the context of the viscoelastic and poroelastic relaxation models.