Vinh X. Truong

Simultaneous orthogonal dual-click approach to tough, in-situ-forming hydrogels for cell encapsulation.

The use of tough hydrogels as biomaterials is limited as a consequence of time-consuming fabrication techniques, toxic starting materials, and large strain hysteresis under deformation. Herein, we report the simultaneous application of nucleophilic thiol-yne and inverse electron-demand Diels-Alder additions to independently create two interpenetrating networks in a simple one-step procedure. The resultant hydrogels display compressive stresses of 14-15 MPa at 98% compression without fracture or hysteresis upon repeated load. The hydrogel networks can be spatially and temporally postfunctionalized via radical thiylation and/or inverse electron-demand Diels-Alder addition to residual functional groups within the network. Furthermore, gelation occurs rapidly under physiological conditions, enabling encapsulation of human cells.

Organocatalytic, Regioselective Nucleophilic “Click” Addition of Thiols to Propiolic Acid Esters for Polymer–Polymer Coupling

The regioselectivity of the nucleophilic addition of thiols to electron-deficient alkynes is controlled by the choice of the solvent (i.e. the polarity of the reaction mixture) and the catalyst. Both thioalkenes and dithianes can be prepared in a rapid reaction that generates no by-products (see scheme). In turn the utility of this reaction is shown for efficient end-group modification of polymers.

In situ-forming robust chitosan-poly(ethylene glycol) hydrogels prepared by copper-free azide–alkyne click reaction for tissue engineering

A water-soluble azide-functionalised chitosan was crosslinked with propiolic acid ester-functional poly(ethylene glycol) using copper-free click chemistry. The resultant hydrogel materials were formed within 5-60 min at 37 °C and resulted in mechanically robust materials with tuneable properties such as swelling, mechanical strength and degradation. Importantly, the hydrogels supported mesenchymal stem cell attachment and proliferation and were also non-toxic to encapsulated cells. As such these studies indicate that the hydrogels have potential to be used as injectable biomaterials for tissue engineering.

Photodegradable Gelatin-Based Hydrogels Prepared by Bioorthogonal Click Chemistry for Cell Encapsulation and Release

In this study, we present a method for the fabrication of in situ forming gelatin and poly(ethylene glycol)-based hydrogels utilizing bioorthogonal, strain-promoted alkyne-azide cycloaddition as the cross-linking reaction. By incorporating nitrobenzyl moieties within the network structure, these hydrogels can be designed to be degradable upon irradiation with low intensity UV light, allowing precise photopatterning. Fibroblast cells encapsulated within these hydrogels were viable at 14 days and could be readily harvested using a light trigger. Potential applications of this new class of injectable hydrogel include its use as a 3D culturing platform that allows the capture and release of cells, as well as light-triggered cell delivery in regenerative medicine.

Organocatalytic synthesis and post-polymerization functionalization of propargyl-functional poly(carbonate)s

The synthesis of well-defined propargyl-functional poly(carbonate)s was achieved via the organocatalytic ring-opening polymerization of 5-methyl-5-propargyloxycarbonyl-1,3-dioxan-2-one (MPC) using the dual catalyst system of 1-(3,5-bis(trifluoromethyl)phenyl)-3-cyclohexylthiourea (TU) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). The resulting homopolymers showed low dispersities and high end-group fidelity, with the versatility of the system being demonstrated by the synthesis of telechelic copolymers and block copolymers. The synthesized homopolymers with varying degree of polymerization were functionalized with a range of azides via copper-catalyzed Huisgen-1,3-dipolar addition or thiols via radical thiylation, to produce functional aliphatic poly(carbonate)s from a single polymeric scaffold.

Nitrile Oxide-Norbornene Cycloaddition as a Bioorthogonal Crosslinking Reaction for the Preparation of Hydrogels

This communication describes the first application of cycloaddition between an in situ generated nitrile oxide with norbornene leading to a polymer crosslinking reaction for the preparation of poly(ethylene glycol) hydrogels under physiological conditions. Hydrogels with high water content and robust physical strength are readily formed within 2-5 min by a simple two-solution mixing method which allows 3D encapsulation of neuronal cells. This bioorthogonal crosslinking reaction provides a simple yet highly effective method for preparation of hydrogels to be used in bioengineering.

Nonswelling Click-Cross-Linked Gelatin and PEG Hydrogels with Tunable Properties Using Pluronic Linkers

Swelling of hydrogels leads to a decrease in mechanical performance coupled with complications in solute diffusion. In addition, hydrogel swelling affects patient safety in biomedical applications such as compression of tissue and fluid blockage. A conventional strategy for suppressing swelling is to introduce a thermoresponsive polymer with a lower critical solution temperature (LCST) within the network structure to counter the water uptake at elevated temperature. However, altering the gels mechanical strength via modification of the network structure often affects the water uptake behavior and thus a nonswelling platform with tunable mechanical properties suitable for various biomedical applications is desirable. In this study we applied the commercially available triblock PEG-PPG-PEG (Pluronic) as a cross-linker for the preparation of nucleophilic thiol-yne click cross-linked hydrogels with suppressed swelling at physiologically relevant temperature. The mechanical properties and degradation rate of these nonswelling hydrogels can be tuned by judicious combinations of the available linkers. The Pluronic linkers can be applied to prepare biologically relevant gelatin based hydrogels with suppressed swelling under physiological conditions that support attachment of fibroblast cells in 2D culture and controlled release of albumin, paving the way for the development of reliable and better performing soft biomaterials.

Light-triggered release of ciprofloxacin from an in situ forming click hydrogel for antibacterial wound dressings

Light triggered release of an antibiotic from a click crosslinked hydrogel was developed by conjugating ciprofloxacin through a photo-cleavable linker to the hydrogel network structure. Upon irradiation of the hydrogel material with UV light (365 nm) at low intensity, native ciprofloxacin was released into the surrounding environment and could be detected by HPLC. The antimicrobial activity of the released compound on Staphylococcus aureus was demonstrated.

Polyethylene glycol–gelatin hydrogels with tuneable stiffness prepared by horseradish peroxidase-activated tetrazine–norbornene ligation

Tetrazine-norbornene ligation has previously been applied in bioorthognal polymer crosslinking to form hydrogels suitable for 3D cell culture. However, the tetrazine group is prone to reduction by the free thiol in a biological environment, reducing the crosslinking efficiency and shortening the storage of tetrazine containing linkers. Here, we introduce a method to form a tetrazine group in situ by catalytic oxidation of the dihydrogen tetrazine using horse radish peroxidase (HRP). Enzymatic oxidation is highly efficient at a low HRP concentration and does not require hydrogen peroxide, allowing for rapid gelation when HRP was added to an aqueous solution of 4-arm PEG dihydrogentetrazine and gelatin norbornene. The storage modulus of the resultant gels can be varied by changing the concentration of the crosslinker, which is in the range of 1.2-3.8 kPa. Human mesenchymal stem cells encapsulated within these gels, with varying stiffness, display varied interactions and morphologies and can be maintained with prolonged culture periods of at least 32 days of 3D culture. The enzymatic activation of tetrazine-norbornene is therefore an attractive addition to the tetrazine ligation that is highly suitable for cell related studies in tissue engineering.

Independent control of elastomer properties through stereocontrolled synthesis

Abstract In most synthetic elastomers, changing the physical properties by monomer choice also results in a change to the crystallinity of the material, which manifests through alteration of its mechanical performance. Using organocatalyzed stereospecific additions of thiols to activated alkynes, high‐molar‐mass elastomers were isolated via step‐growth polymerization. The resulting controllable double‐bond stereochemistry defines the crystallinity and the concomitant mechanical properties as well as enabling the synthesis of materials that retain their excellent mechanical properties through changing monomer composition. Using this approach to elastomer synthesis, further end group modification and toughening through vulcanization strategies are also possible. The organocatalytic control of stereochemistry opens the realm to a new and easily scalable class of elastomers that will have unique chemical handles for functionalization and post synthetic processing.