Microstructure and roughness of photopolymerized poly(ethylene glycol) diacrylate hydrogel as measured by atomic force microscopy in amplitude and frequency modulation mode

In this study, atomic force microscopy (AFM) has been employed to image a photopolymerized poly(ethylene glycol) diacrylate (PEG-DA) hydrogel. The same area was imaged both in amplitude modulation (AM) and in frequency modulation (FM) mode and the latter allowed for excellent resolution of the hydrogel microstructure. It shows globular domains with typical diameters in the range of ~10-100 nm. The hydrogel morphology has been analysed using grain size analysis as well as roughness analysis. Based on AFM topography images of hydrogel nano-domains, a set of roughness parameters has been identified which can be readily used as descriptors for spatial resolution. It includes the density of summits, Sds, the mean summit curvature, Ssc, the surface area ratio, Sdr, and the correlation length parameter, Scl37. The latter describes the length over which the autocorrelation function decays to 37% of its peak value. These parameters allow for better discrimination than the widely used root-mean-square (RMS) roughness, Sq, and are available with common image processing software packages. Systematic variation of the virtual tilt angle has indicated that these parameters are robust to small variations in plane levelling. Such image processing is frequently needed to separate the inherent surface microstructure from the global topography related to sample tilt or surface waviness. Hydrogels are an important group of biomaterials as they find numerous applications in biomedical engineering, ranging from adhesives, to controlled release of water-soluble drugs, to encapsulation of cells, to tissue engineering. Optimisation of their interactions with bioentities, such as bacteria, cells or proteins, requires accurate surface characterisation.