Modulus Mapping of 3D Printed PEGDA Hydrogels in Hydrated Condition

Three-dimensional (3D) printed hydrogels, particularly those based on poly(ethylene glycol) diacrylate (PEGDA), have emerged as promising materials in tissue engineering and organ-on-chip devices due to their biocompatibility, ease of fabrication, and ability to form complex structures. However, a major challenge in using these materials is the inherent structural heterogeneity that arises during the layer-by-layer (LbL) photo-cross-linking process, which can lead to variations in mechanical properties. These heterogeneities, specifically in the elastic modulus (E), can affect how biological cells interact with the hydrogel surfaces, influencing cell adhesion, proliferation, and tissue formation. This study investigates the variation in the modulus of LbL PEGDA hydrogels under hydrated conditions.

Hydrogel samples were fabricated using projection lithography, a precise LbL technique that relies on UV light to cross-link the prepolymer solution one layer at a time [1]. Nanoindentation and AFM were used to measure the nanomechanical properties of the printed hydrogels in their fully hydrated state, mimicking physiological conditions. We observed a significant increase in the modulus as the indentation moved from the bottom to the top of a single printed layer. This variation is attributed to the spatial decay of light intensity as it penetrates the hydrogel layers during the photo-cross-linking process, resulting in a cross-link density gradient within each layer. We also observed that the modulus continues to increase throughout the entire multilayer structure, indicating that mechanical heterogeneity is not limited to individual layers but extends across the entire printed structure [2]. Such heterogeneity can be attributed to interfacial defects, voids, and uneven cross-linking, which are direct consequences of imperfect UV light absorption during LbL printing process [3].

This study demonstrates the importance of understanding and controlling the photo-cross-linking process in 3D printed hydrogels. By optimizing factors such as UV light intensity, exposure time, and photoabsorber concentration, it may be possible to minimize the degree of mechanical variation within each layer and across the entire structure. Overall, this investigation provides valuable insights into the spatial mechanical properties of 3D printed PEGDA hydrogels in hydrated conditions, highlighting the importance of modulus mapping for improving the performance and reproducibility of hydrogel-based systems.

[1] Khalili et al., Polym. Degrad. Stab. 2022, 195, 109805
[2] Khalili et al., ACS Appl. Polym. Mater. 2023, 5, 4, 3034–3042
[3] Khalili et al., ACS Appl. Polym. Mater. 2023, 5, 2, 1180–1190