Dynamic assembly of ultrasoft colloidal networks enables cell invasion within restrictive fibrillar polymers

Abstract

In regenerative medicine, natural protein-based polymers offer enhanced endogenous bioactivity and potential for seamless integration with tissue, yet form weak hydrogels that lack the physical robustness required for surgical manipulation, making them difficult to apply in practice. The use of higher concentrations of protein, exogenous cross-linkers, and blending synthetic polymers has all been applied to form more mechanically robust networks. Each relies on generating a smaller network mesh size, which increases the elastic modulus and robustness, but critically inhibits cell spreading and migration, hampering tissue regeneration. Here we report two unique observations; first, that colloidal suspensions, at sufficiently high volume fraction (ϕ), dynamically assemble into a fully percolated 3D network within high-concentration protein polymers. Second, cells appear capable of leveraging these unique domains for highly efficient cell migration throughout the composite construct. In contrast to porogens, the particles in our system remain embedded within the bulk polymer, creating a network of particle-filled tunnels. Whereas this would normally physically restrict cell motility, when the particulate network is created using ultralow cross-linked microgels, the colloidal suspension displays viscous behavior on the same timescale as cell spreading and migration and thus enables efficient cell infiltration of the construct through the colloidal-filled tunnels.fibrin | microgels | colloidal assemblies | porosity | cell migration D ecoupling stiffness, pore size, and cell infiltration is a critical hurdle in biomaterials design and has been previously addressed in synthetic hydrogels by enabling cell-mediated degradation via protease-specific peptide cross-linkers (1-4). However, even in these highly engineered systems, compared with native extracellular matrices (ECMs), the mesh size remains a critical limiting factor in host integration; this is because cells are incapable of nonproteolytic, i.e., degradation-independent, migration through such small mesh sizes (5), as illustrated in Fig. 1A.Protein-based biomaterials derived from native ECM represent an attractive alternative to synthetic hydrogels, offering significant benefits through their enhanced endogenous bioactivity. The native ECM and its derivatives act as growth factor/cytokine depots, thus providing a multivalent endogenous binding site for growth factor delivery (6). A classic example of this type of biomaterial is fibrin, the endogenous provisional matrix formed at sites of vascular injury as a result of blood coagulation (7,8). Clinically, to reach desirable mechanical properties for sealing tissues/wounds, fibrin is used at supraphysiological concentrations typically containing ∼10× more fibrinogen than physiological concentrations (9-12). When used at these artificially high concentrations, the characteristic mesh size of the network is unfortunately similar to that of synthetic PEG polymers, which is on the order of tens of nanometers, making i...

Exploring the colloid-to-polymer transition for ultra-low crosslinked microgels from three to two dimensions

“…For example, since ULC microgels are more deformable than the regular crosslinked microgels 21 , they are particularly suitable for the development of nanomaterials for selective adsorption in physiological solution 20 , or as bio-sensors 52 , as recent findings show that the softness of microgels improves their adsorption onto a solid substrate 53 . Furthermore, it has been proven that ultra-low crosslinked microgels are a perfect system to generate porous fibrin networks facilitating cell migration and growth 54 . The capability of this system to have a topography under adsorption that depends on the compression and the suppression of crystallization in two dimensions would be of interest in these fields.…”

Colloidal Gels with Tunable Mechanomorphology Regulate Endothelial Morphogenesis

“…Collectively our results support the fact that dense and mesh-like matrices prevent cell extension and migration 19,21,73 . Moreover, cells including ECs, require tracks either through aligned matrix and void space to migrate and form functional structures like tubes and lumen 74–76 . Our results show that ECs can form networks in stiffer matrices if the matrix is microstructure is loose but are likely inhibited from forming network in a soft matrix if the matrix is dense in microstructure.…”

Enhancing clot properties through fibrin-specific self-cross-linked PEG side-chain microgels

“…Noteworthy, Douglas et al analyzed the formation of fibrin-μgel constructs made of fibrinogen and ULC pNiPAm microgels at higher concentrations, 8 mg/L and 4 mg/L, respectively. [25] At these concentrations the μ gels were found to form tubular-like networks of “pores” filled with μ gel particles throughout the fibrin clot. On the other hand, no significant differences in the fibrin structure of fibrin were observed within μ gel-free regions of the fibrin clot.…”

“…The formation of these percolated colloidal networks were related to the fibrin polymerization kinetics rather to specific fiber– μ gel and μ gel– μ gel interactions. [25] The studies shown here were conducted at lower concentrations, i.e. about five times lower microgel concentration and at a fibrinogen concentration of 5 mg/mL.…”