Tuning and Predicting Mesh Size and Protein Release from Step Growth Hydrogels.

Hydrogel-based depots are of growing interest for release of biopharmaceuticals; however, a priori selection of hydrogel compositions that will retain proteins of interest and provide desired release profiles remains elusive. Toward addressing this, in this work, we have established a new tool for the facile assessment of protein release from hydrogels and applied it to evaluate the effectiveness of mesh size estimations on predicting protein retention or release. Poly(ethylene glycol) (PEG)-based hydrogel depots were formed by photoinitiated step growth polymerization of four-arm PEG functionalized with norbornene (PEG-norbornene, 4% w/w to 20% w/w, Mn ∼ 5 to 20 kDa) and different dithiol cross-linkers (PEG Mn ∼ 1.5 kDa or enzymatically degradable peptide), creating well-defined, robust materials with a range of mesh sizes estimated with Flory-Rehner or rubber elasticity theory (∼5 to 15 nm). A cocktail of different model proteins was released from compositions of interest, and sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) was used to facilely and quantitatively analyze temporal release profiles. Mesh size was predictive of retention of relatively large proteins and release of relatively small proteins. Proteins with diameters comparable to the mesh size, which is often the case for growth factors, were released by hindered diffusion and required experimental assessment of retention and release. With this knowledge, hydrogels were designed for the controlled release of a therapeutically relevant growth factor, PDGF-BB.

Thiol-ene click hydrogels for therapeutic delivery.

Hydrogels are of growing interest for the delivery of therapeutics to specific sites in the body. For use as a delivery vehicle, hydrophilic precursors are usually laden with bioactive moieties and then directly injected to the site of interest for in situ gel formation and controlled release dictated by precursor design. Hydrogels formed by thiol-ene click reactions are attractive for local controlled release of therapeutics owing to their rapid reaction rate and efficiency under mild aqueous conditions, enabling in situ formation of gels with tunable properties often responsive to environmental cues. Herein, we will review the wide range of applications for thiol-ene hydrogels, from the prolonged release of anti-inflammatory drugs in the spine to the release of protein-based therapeutics in response to cell-secreted enzymes, with a focus on their clinical relevance. We will also provide a brief overview of thiol-ene click chemistry and discuss the available alkene chemistries pertinent to macromolecule functionalization and hydrogel formation. These chemistries include functional groups susceptible to Michael type reactions relevant for injection and radically-mediated reactions for greater temporal control of formation at sites of interest using light. Additionally, mechanisms for the encapsulation and controlled release of therapeutic cargoes are reviewed, including i) tuning the mesh size of the hydrogel initially and temporally for cargo entrapment and release and ii) covalent tethering of the cargo with degradable linkers or affinity binding sequences to mediate release. Finally, myriad thiol-ene hydrogels and their specific applications also are discussed to give a sampling of the current and future utilization of this chemistry for delivery of therapeutics, such as small molecule drugs, peptides, and biologics.