Coagulative Granular Hydrogels with an Enzyme Catalyzed Fibrin Network for Endogenous Tissue Regeneration

Background

Tissue engineering has largely relied on engineered hydrogels that have cellular units to fabricate tissue like constructs. These hydrogel matrices are certainly made up of polymer biomaterials (natural or synthetic) with tunable properties. Often these hydrogel matrices are bulk and macro in size which makes them harder to inject, especially for in situ applications. Hence a relatively new class of material design known as granular hydrogels has come to the forefront to overcome the limitation of bulk hydrogel matrices. However, these granular systems often possess weak mechanical stability, due to interstitial void formation and poor injectability.

In this preprint the authors report a new approach to stabilize the interstitial porosity-driven mechanical instability different from previously reported methods like photo annealing, and dynamic chemistries, including host-guest interactions. The resulting coagulant granular hydrogels use surface-functionalized thrombin to catalyze fibrinogen-to-fibrin conversion that glues granular units together to form a cell-laden scaffold

Key findings

Thrombin as an effective catalyzer of fibrinogen to glue granular microgel units

First the authors functionalized thrombin onto the microgel surface using surface modification of GelMA granular hydrogels. The controlled thrombin functionalization reported in this study is highly tunable and the enzymatic activity can be upscaled depending on the requirements. The thrombin-functionalized granular microgel units were used as active catalyzers to engulf the surrounding fibrinogen into fibrin gel to promote cellular infiltration.

Interconnecting stability-induced enhancement in scaffold mechanics

The inter-microgel gluing created by fibrinogen-converted fibrin gel enhanced the total mechanics of the scaffold. The total shear modulus of this modular scaffold was (~1.5 fold) increased compared to the bulk hydrogels’ mechanical property. Moreover, the modular coagulative scaffold that was formed had enhanced injectability suitable for in-situ injectable granular systems.

Fibrin glue promoted cellular infiltration and vascular sprouting

The authors validated the cellular adaptability of interstitial fibrin glue between the granular microgel units by assessing the activity of human umbilical vein endothelial cells (HUVECs). The modular scaffold with fibrin-glued granular units revealed enhanced cell proliferation with higher metabolic activity and more cell spreading compared to control conditions. To check the robustness of this modular design, the authors further explored controlled vascular sprouting through the fibrin-glued interstitial space. Multicellular spheroids with HUVECs and MSCs (mesenchymal stem cells) exhibited enhanced vascular sprouting compared to conditions without fibrinogen with an outgrowth of (5.6 fold) respectively.

Why do I like this preprint ?

This work denotes a scalable and injectable granular system that can provide mechanical stability to the scaffold. Moreover, such a modular design with enhanced vascular endothelial sprouting can aid vascularization when injected in vivo. Even though, thrombin cross-linkable hydrogels have been established previously but the concept of gluing granular systems using thrombin as a catalyzer is novel and can open up new ways in jam-packed granular microgel applications.

Tags: coagulant, granular

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