Selectively Cross-Linked Tetra-PEG Hydrogels Provide Control over Mechanical Strength with Minimal Impact on Diffusivity

Abstract

Synthetic hydrogels formed from poly(ethylene glycol) (PEG) are widely used to study how cells interact with their extracellular matrix. These in vivo-like 3D environments provide a basis for tissue engineering and cell therapies but also for research into fundamental biological questions and disease modeling. The physical properties of PEG hydrogels can be modulated to provide mechanical cues to encapsulated cells; however, the impact of changing hydrogel stiffness on the diffusivity of solutes to and from encapsulated cells has received only limited attention. This is particularly true in selectively cross-linked "tetra-PEG" hydrogels, whose design limits network inhomogeneities. Here, we used a combination of theoretical calculations, predictive modeling, and experimental measurements of hydrogel swelling, rheological behavior, and diffusion kinetics to characterize tetra-PEG hydrogels' permissiveness to the diffusion of molecules of biologically relevant size as we changed polymer concentration, and thus hydrogel mechanical strength. Our models predict that hydrogel mesh size has little effect on the diffusivity of model molecules and instead predicts that diffusion rates are more highly dependent on solute size. Indeed, our model predicts that changes in hydrogel mesh size only begin to have a non-negligible impact on the concentration of a solute that diffuses out of hydrogels for the smallest mesh sizes and largest diffusing solutes. Experimental measurements characterizing the diffusion of fluorescein isothiocyanate (FITC)-labeled dextran molecules of known size aligned well with modeling predictions and suggest that doubling the polymer concentration from 2.5% (w/v) to 5% produces stiffer gels with faster gelling kinetics without affecting the diffusivity of solutes of biologically relevant size but that 10% hydrogels can slow their diffusion. Our findings provide confidence that the stiffness of tetra-PEG hydrogels can be modulated over a physiological range without significantly impacting the transport rates of solutes to and from encapsulated cells.

Non-Covalently Associated Streptavidin Multi-Arm Nanohubs Exhibit Mechanical and Thermal Stability in Cross-Linked Protein-Network Materials

“…Furthermore, we compared the experimental G ′ of A 2 YA 2 -A 48 -SAv to prior studies that characterized covalently cross-linked tetra-PEG hydrogels (i.e., synthetic counterpart of SAv tetramer). Despite the similar functionality between dityrosine photo-cross-linking and other covalent cross-linking mechanisms used in prior studies ( f = 2), the experimental G ′ is significantly different from covalently cross-linked tetra-PEG hydrogels where the experimental G ′ ranges from 0.2 to 200 kPa due to several factors, such as numbers of cross-linkers, polymer concentrations, and chain length/mesh sizes. − …”

Functional Hydrogels and Their Applications in Craniomaxillofacial Bone Regeneration

“…Methods to enhance stiffness include increasing the cross-linking [ 25 , 26 , 27 ], doping with nanoparticles [ 28 , 29 , 30 ], and creating hybrid hydrogels [ 15 , 31 ]. Increasing the degree of cross-linking is often accompanied by a decrease in mesh size [ 37 ], which can affect the ability of the hydrogel to load drugs and cells; therefore, there is a limit to the effectiveness of increasing the degree of cross-linking to improve stiffness. The use of nanoparticles and the preparation of hybrid hydrogels are often more popular because of the introduction of new components in the hydrogel system, such as the introduction of borax ions to prepare double-cross-linked network hydrogels [ 15 ] and the addition of nano-hydroxyapatite (nHA) [ 72 ].…”

“…For the free volume theory, the most important thing is the ratio of the solute molecule diameter to the mesh diameter. The larger the ratio, the greater the resistance to solute molecule diffusion and the slower the diffusion rate [ 36 , 37 ]. According to the hydrodynamic theory, which considers the diffusion process of drug molecules and the frictional forces between the surrounding hydrogel matrix [ 38 ], the diffusion of macromolecular drugs in the hydrogel system is slowed down by the frictional forces exerted on it by the surrounding polymer chains due to the relative displacement that occurs.…”

Editorial: Special Issue on Advanced Biomedical Hydrogels

“…The multiscale structure of hydrogels influences various properties from mechanics to biomolecule diffusivity and cell–cell communication. With regard to the molecular structure, Gentleman and colleagues report on modeling and experimental characterization of the relationship between transport properties and poly­(ethylene glycol) hydrogel stiffness . With regard to the macrostructure, Lee and co-workers developed freeze–thawing methods to introduce porosity into mussel-inspired catecholamine polymers, which then showed unique shape-recovery and sponge-like behaviors .…”

“…With regard to the molecular structure, Gentleman and colleagues report on modeling and experimental characterization of the relationship between transport properties and poly(ethylene glycol) hydrogel stiffness. 11 With regard to the macrostructure, Lee and co-workers developed freeze−thawing methods to introduce porosity into musselinspired catecholamine polymers, which then showed unique shape-recovery and sponge-like behaviors. 12 As another example of controlled porosity, Kiick and colleagues investigated the control over liquid−liquid phase separation of resilin-like polypeptides to introduce hydrogel microstructures.…”