Nanoindentation Response of 3D Printed PEGDA Hydrogels in a Hydrated Environment

Abstract

Hydrogels are commonly used materials in tissue engineering and organ-on-chip devices. This study investigated the nanomechanical properties of monolithic and multilayered poly(ethylene glycol) diacrylate (PEGDA) hydrogels manufactured using bulk polymerization and layer-by-layer projection lithography processes, respectively. An increase in the number of layers (or reduction in layer thickness) from 1 to 8 and further to 60 results in a reduction in the elastic modulus from 5.53 to 1.69 and further to 0.67 MPa, respectively. It was found that a decrease in the number of layers induces a lower creep index (CIT) in three-dimensional (3D) printed PEGDA hydrogels. This reduction is attributed to mesoscale imperfections that appear as pockets of voids at the interfaces of the multilayered hydrogels attributed to localized regions of unreacted prepolymers, resulting in variations in defect density in the samples examined. An increase in the degree of cross-linking introduced by a higher dosage of ultraviolet (UV) exposure leads to a higher elastic modulus. This implies that the elastic modulus and creep behavior of hydrogels are governed and influenced by the degree of cross-linking and defect density of the layers and interfaces. These findings can guide an optimal manufacturing pathway to obtain the desirable nanomechanical properties in 3D printed PEGDA hydrogels, critical for the performance of living cells and tissues, which can be engineered through control of the fabrication parameters.

Mechanical Behavior of 3D Printed Poly(ethylene glycol) Diacrylate Hydrogels in Hydrated Conditions Investigated Using Atomic Force Microscopy

“…The observed tearing suggests that the interface between the layers is indeed the weakest point across the entire structure due to its highest heterogeneity in cross-linking density. This agrees with our previous study that suggests, due to the nature of the LbL cross-linking process, there are inherent structural imperfections in projection based printing processes . For each layer upon cross-linking, localized areas of unreacted monomer are formed which affect the stiffness of the material at the interface.…”

“…On the other hand, the multilayer sample, which had the highest concentration of QY photoabsorber, exhibited a lower degree of cross-linking as well as E . This observation suggests that a higher concentration of QY photoabsorber in the prepolymer solution leads to a reduction in the degree of cross-linking in the cross-linked hydrogel, as shown in Figures id,iid, and a,b . We have also showed the effect of cross-linking on water and sol–gel content in the monolithic and multilayer samples with equilibrium water content of 72.6% and 75%, respectively .…”

“…For each layer upon cross-linking, localized areas of unreacted monomer are formed which affect the stiffness of the material at the interface. These pockets of voids or defects could result in variations in the defect density and lower E at the interface of the multilayer sample . NMR characterization carried out in our studies suggests that the presence of localized unreacted prepolymer regions is mainly caused by the insufficiency of the UV light dose of 120 mJ/cm 2 to fully cross-link the exposed prepolymer solution in the presence of the QY photoabsorber with a concentration of 9 mg/mL .…”

“…On the other hand, the multilayer sample, which had the highest concentration of QY photoabsorber, exhibited a lower degree of cross-linking as well as E. This observation suggests that a higher concentration of QY photoabsorber in the prepolymer solution leads to a reduction in the degree of cross-linking in the cross-linked hydrogel, as shown in Figures 2id,iid, and 3a,b. 48 We have also showed the effect of cross-linking on water and sol−gel content in the monolithic and multilayer samples with equilibrium water content of 72.6% and 75%, respectively. 38 Gel fractions were measured to be 27.4% and 25%, while sol percentages were 2.5% and 5.9% for the monolithic and multilayer samples, respectively.…”

“…Thus, we employed AFM to verify the existence of cross-linking and measure the spatial variation across the layers. Note that the observed reduction in modulus could also be attributed to the presence of voids or defects, suggesting that the layers may not be fully covalently cross-linked at the interfaces . It is crucial to investigate and find the optimum amount of LAP photoinitiator, QY photoabsorber, and UV dosage required to minimize the chance of the appearance of these pockets of voids and increase the adhesion at the interfaces of the cross-linked layers.…”