Engineering thymus viscoelasticity and microstructure integrating chaotropic salts in gels

The thymus has a pivotal function in the generation of immunocompetent T cells. Despite this function strictly relies on its complex extracellular matrix, to date, the organization of its 3D mesh structure, as well as its mechanical properties have been poorly investigated. This missing gap hampers any attempt to model and engineer this organ.

For this reason, in a previous work, a thorough mechanical characterization of the bovine thymus (from a local slaughterhouse) was performed to derive its fundamental viscoelastic descriptors.

We found that thymic tissue is featured with a high viscoelastic nature, with local instantaneous and equilibrium elastic modulus (12 and 2.9 kPa, respectively, as median) significantly higher than those of the bulk tissue (3.2 and 0.6 kPa, respectively, as median), probably due to surface roughness and sample anisotropy.

Thymic tissue was also characterized in terms of its microstructure, through scanning electron microscopy (SEM) and histological analyses, so to quantify essential parameters (e.g., fibers and pores diameter distributions) in view of a more efficient in vitro replication of the organ.

Here, hydrogels were fabricated and optimized in order to replicate thymic mechanical and structural features. More specifically, urea (2-8% w/v) was added to a 40 mg/mL gelatin solution, as chaotropic salt to modulate peptide solubility and gelation network, whereas 0.1% w/v Pluronic F-127 was deployed to tune hydrogel viscous component.

Mechanical tests and SEM analysis showed that gelatin hydrogels closely mimic thymic mechanical and structural features, with next steps focused on the biological testing of thymic relevant cell lines seeded on the hydrogel itself.

On the one hand, thymus characterization provides researchers with a portfolio of mechanical and structural data for engineering this organ in view of future in vitro studies or clinical applications; on the other, the herein fabricated gelatin hydrogels represent the first successful attempt to reproduce thymic mechanostructure in vitro.