Macromolecular Monomers for the Synthesis of Hydrogel Niches and Their Application in Cell Encapsulation and Tissue Engineering.

Hydrogels formed from the photoinitiated, solution polymerization of macromolecular monomers present distinct advantages as cell delivery materials and are enabling researchers to three-dimensionally encapsulate cells within diverse materials that mimic the extracellular matrix and support cellular viability. Approaches to synthesize gels with biophysically and biochemically controlled microenvironments are becoming increasingly important, and require strategies to control gel properties (e.g., degradation rate and mechanism) on multiple time and size scales. Furthermore, biological responses of gel-encapsulated cells can be promoted by hydrogel degradation products, as well as by the release of tethered biologically relevant molecules.

Effects of neighboring sulfides and pH on ester hydrolysis in thiol-acrylate photopolymers.

Networks synthesized through thiol-acrylate photopolymerization or Michael-type addition step growth reactions contain esters with neighboring sulfide groups. Previous work has demonstrated that these esters are readily hydrolyzable at physiological pH. Here, the influence of the distance between the sulfide and ester, as well as the water concentration, on ester hydrolysis was characterized. These preliminary results indicate that reducing the number of carbons between the sulfide and the ester from 2 to 1 increased the rate of ester hydrolysis from 0.022+/-0.001 to 0.08+/-0.015days(-1). Increases in ester hydrolysis rates were also observed as hydrophilicity increased for oligomers prepared from a trithiol, tetrathiol and dithiol monomer (0.012+/-0.003, 0.032+/-0.004, and 0.091+/-0.003days(-1), respectively). Additionally, in bulk-eroding polymeric biomaterials, variations in pH impacted the ester hydrolysis rate. This work confirms that small variations in buffer pH predictably alter the mass loss profile of a thiol-acrylate photopolymer. More specifically, as buffer pH was changed from 7.4 to 8.0, the rate of ester hydrolysis increased from 0.074+/-0.003 to 0.28+/-0.005days(-1). The magnitude of this observed change in ester hydrolysis rate was correlated to the increase in hydroxide ion concentration that accompanied this pH change.

Development and Characterization of Degradable Thiol-Allyl Ether Photopolymers.

Degradable thiol-ene photopolymer networks were formed through radically mediated step growth reactions. Variations in the network structure were used to alter the initial and temporal moduli, mass loss profiles, and equilibrium swelling ratios. Mass loss rates varied with changes in the solvent concentration, monomer molecular weight, average monomer functionality, and concentration of degradable linkages. The time required for the networks to degrade completely ranged from 1.20 ± 0.01 to 24.5 ± 0.1 days, which corresponded to hydrolysis rates of 0.18 ± 0.01 and 0.021 ± 0.0003 days(-1). Initial moduli also varied considerably as a function of network structure, ranging from 150 ± 35 to nearly 5000 ± 100 kPa, and initial equilibrium swelling ratios ranged from 2.5 ± 0.01 to 18.7 ± 2. Collectively, these results demonstrate how the material properties and the mass loss behavior of thiol-ene networks can be independently tuned for specific applications.

Degradable thiol-acrylate photopolymers: polymerization and degradation behavior of an in situ forming biomaterial.

Degradable thiol-acrylate photopolymers are a new class of biomaterials capable of rapidly polymerizing under physiological conditions upon exposure to UV light, with or without added photoinitiators, and to depths exceeding 10 cm. These materials are formed in situ, and the versatility of their chemistry affords a high degree of control over the final material properties. For example, variations in monomer mole fractions directly affect the final network molecular structure, varying the time required to achieve complete mass loss from 25 to 100 days, the molecular weight distributions of the degradation products, and the swelling ratios and compressive moduli throughout degradation. Additionally, varying the mole fraction of multifunctional thiol monomer in the initial reaction mixture controls the concentration of reactive sites in the network available for post-polymerization modification of the polymer.

Modifying network chemistry in thiol-acrylate photopolymers through postpolymerization functionalization to control cell-material interactions.

Thiol-acrylate photopolymers often contain pendant, unreacted thiol groups even following complete reaction of the acrylate functional groups. The results presented herein demonstrate a high throughput method for quantifying pendant thiol group concentrations using FTIR spectra of thiol-acrylate microspot arrays. Using this technique, more than 25% of the original thiol groups were detected as pendant groups in microspots made from monomer solutions containing at least 40 mol % thiol functional groups. Subsequent modification reactions allowed postpolymerization tailoring of the network chemistry. The extent of modification was controlled by the concentration of the pendant thiols (ranging from 0.01 to 0.4M) and the duration of the modification reaction (0-10 min for photocoupling reactions, 0-24 h for Michael-type addition reactions). Further, when photocoupling was used to modify the networks, spatial and temporal control of the light exposure facilitated the formation of chemical patterns on the surface and throughout the material.

Controlling network structure in degradable thiol-acrylate biomaterials to tune mass loss behavior.

Degradable thiol-acrylate materials were synthesized from the mixed-mode polymerization of a diacrylate poly(ethylene glycol) (PEG) monomer with thiol monomers of varying functionalities to control the final network structure, ultimately influencing the material's degradation behavior and properties. The influence of the concentration of thiol groups and monomer functionality on the mass loss profiles were examined experimentally and theoretically. Mass loss behavior was also predicted for networks with varying extents of cyclization, PEG molecular weight, and backbone chain length distributions. Experimental results indicate that increasing the thiol concentration from 10 to 50 mol % shifted the reverse gelation time from 35 to 8 days and the extent of mass loss at reverse gelation from 75 to 40%. Similarly, decreasing the thiol functionality from 4 to 1 shifted the reverse gelation time from 18 to 8 days and the mass loss extent at reverse gelation from 70 to 45%.

Gel Permeation Chromatography Characterization of the Chain Length Distributions in Thiol-Acrylate Photopolymer Networks.

Crosslinked, degradable networks formed from the photopolymerization of thiol and acrylate monomers are explored as potential biomaterials. The degradation behavior and material properties of these networks are influenced by the molecular weight of the nondegradable thiol-polyacrylate backbone chains that form during photopolymerization. Here, gel permeation chromatography was used to characterize the thiol-polyacrylate backbone chain lengths in degraded thiol-acrylate networks. Increasing thiol functionality from 1 to 4 increased the backbone molecular weight (M̄(w) = 2.3 ± 0.07 × 10(4) Da for monothiol and 3.6 ± 0.1 × 10(4) Da for tetrathiol networks). Decreasing thiol functional group concentration from 30 to 10 mol% also increased the backbone lengths (M̄(w) = 7.3 ± 1.1 × 10(4) Da for the networks containing 10 mol% thiol groups as compared to 3.6 ± 0.1 × 10(4) Da for 30 mol% thiol). Finally, the backbone chain lengths were probed at various stages of degradation and an increase in backbone molecular weight was observed as mass loss progressed from 10 to 70%.