Polysaccharide-covered nanoparticles with improved shell stability using click-chemistry strategies

Dextran-covered PLA nanoparticles have been formulated by two strategies. On one hand, dextran-g-PLA copolymers have been synthesized by click-chemistry between azide-multifunctionalized dextran (DexN3) and alkyne end-functionalized PLA chains (α-alkyne PLA); then nanoprecipitated without any additional surfactants. On the other hand, DexN3 exhibiting surfactant properties have been emulsified with unfunctionalized or α-alkyne PLA, which are dissolved in organic phase with or without CuBr. Depending on the o/w emulsion/evaporation process experimental conditions, dextran-g-PLA copolymers have been produced in situ, by click chemistry at the liquid/liquid interface during the emulsification step. Whatever the process, biodegradable core/shell polymeric nanoparticles have been obtained, then characterized. Colloidal stability of these nanoparticles in the presence of NaCl or SDS has been studied. While the physically adsorbed polysaccharide based shell has been displaced by SDS, the covalently-linked polysaccharide based shell ensures a permanent stability, even in the presence of SDS.

Enhanced bioactivity of internally functionalized cationic dendrimers with PEG cores

Hybrid dendritic-linear block copolymers based on a 4-arm poly(ethylene glycol) (PEG) core were synthesized using an accelerated AB2/CD2 dendritic growth approach through orthogonal amine/epoxy and thiol-yne chemistries. The biological activity of these 4-arm and the corresponding 2-arm hybrid dendrimers revealed an enhanced, dendritic effect with an exponential increase in cell internalization concomitant with increasing amine end groups and low cytotoxicity. Furthermore, the ability of these hybrid dendrimers to induce endosomal escape combined with their facile and efficient synthesis makes them attractive platforms for gene transfection. The 4-arm-based dendrimer showed significantly improved DNA binding and gene transfection capabilities in comparison with the 2-arm derivative. These results combined with the MD simulation indicate a significant effect of both the topology of the PEG core and the multivalency of these hybrid macromolecules on their DNA binding and delivery capablities.

Facile functionalization of polyesters through thiol-yne chemistry for the design of degradable, cell-penetrating and gene delivery dual-functional agents

Synthesis of polyesters bearing pendant amine groups with controlled molecular weights and narrow molecular weight distributions was achieved through ring-opening polymerization of 5-(4-(prop-2-yn-1-yloxy)benzyl)-1,3-dioxolane-2,4-dione, an O-carboxyanhydride derived from tyrosine, followed by thiol-yne "click" photochemistry with 2-aminoethanethiol hydrochloride. This class of biodegradable polymers displayed excellent cell penetration and gene delivery properties with low toxicities.

A biomimetic lipid library for gene delivery through thiol-yne click chemistry

The delivery of nucleic acids such as plasmid DNA and siRNA into cells is a cornerstone of biological research and is of fundamental importance for medical therapeutics. Although most gene delivery therapeutics in clinical trials are based on viral vectors, safety issues remain a major concern. Non-viral vectors, such as cationic lipids and polymers, offer safer alternatives but their gene delivery efficiencies are usually not high enough for clinical applications. Thus, there is a high demand for more efficient and safe non-viral vectors. Here, we present a facile two-step method based on thiol-yne click chemistry for parallel synthesis of libraries of new biomimetic cationic thioether lipids. A library of novel lipids was synthesized using the developed method and more than 10% of the lipids showed highly efficient transfection in different cell types, surpassing the efficiency of several popular commercial transfection reagents. One of the new lipids showed highly efficient siRNA delivery to multiple cell types and could successfully deliver DNA plasmid to difficult-to-transfect mouse embryonic stem cells (mESC). Analysis of structure-activity relationship revealed that the length of the hydrophobic alkyl groups was a key parameter for efficient cell transfection and was more important for transfection efficiency than the nature of cationic head groups. The correlation of the size and surface charge of liposomes with transfection efficiency is described.

Stress relaxation via addition-fragmentation chain transfer in high T(g), high conversion methacrylate-based systems

To reduce shrinkage stress which arises during the polymerization of crosslinked polymers, allyl sulfide functional groups were incorporated into methacrylate polymerizations to determine their effect on stress relaxation via addition-fragmentation chain transfer (AFCT). Additionally, stoichiometrically balanced thiol and allyl sulfide-containing norbornene monomers were incorporated into the methacrylate resin to maximize the overall functional group conversion and promote AFCT while also enhancing the polymer's mechanical properties. Shrinkage stress and reaction kinetics for each of the various functional groups were measured by tensometry and Fourier-transform infrared (FTIR) spectroscopy, respectively. The glass transition temperature (T(g)) and elastic moduli (E') were measured using dynamic mechanical analysis. When the allyl sulfide functional group was incorporated into dimethacrylates, the polymerization-induced shrinkage stress was not relieved as compared with analogous propyl sulfide-containing resins. These analogous propyl sulfide containing monomers are incapable of undergoing AFCT while having similar chemical structure and crosslink density to the allyl sulfide containing methacrylates. Here, a monomethacrylate monomer that also contains a cyclic allyl sulfide (PAS) was found to increase the crosslinking density nearly 20 times as compared to an analogous monomethacrylate in which the allyl sulfide was replaced with an ethyl sulfide. Despite the much higher crosslink density, the PAS formulation exhibited no concomitant increase in stress. Thiol-norbornene resins were copolymerized in PAS to promote AFCT as well as to synergistically combine the ring opening benefits associated with the thiol-ene reaction. AFCT resulted in a 63% reduction of polymerization stress and a 45°C enhancement of the glass transition temperature in the allyl sulfide-containing thiol-norbornene-methacrylate system compared with rubbery dimethacrylates. When compared with conventional glassy dimethacrylates, this combined system has less than 10% of the typical shrinkage stress level while having similarly excellent mechanical properties.

Stress Reduction and T(g) Enhancement in Ternary Thiol-Yne-Methacrylate Systems via Addition-fragmentation Chain Transfer

Since polymerization-induced shrinkage stress is detrimental in many applications, addition-fragmentation chain transfer (AFCT) was employed to induce network relaxation and adaptation that mitigate the shrinkage stress. Here, to form high glass transition temperature, high modulus polymers while still minimizing stress, multifunctional methacrylate monomers were incorporated into allyl sulfide-containing thiol-yne resins to provide simultaneously high glass transition temperatures and a facile mechanism for AFCT throughout the network. As a negative control, in an attempt to isolate just the effects of AFCT in the polymerization, a propyl sulfide-based diyne, which has a nearly identical chemical structure though absent any AFCT-capable functional group, was synthesized and implemented in place of the allyl sulfide-based diyne. The glass transition temperature of the ternary systems increased from 39°C to 79°C as the methacrylate content increased while the shrinkage stress of the optimal ternary resin was lower than either the binary thiol-yne resin or the pure methacrylate resin. The stress relaxation benefit associated with AFCT increased with increasing allyl sulfide concentration as shown by a decrease in the relative stress from 0.98 to 0.53. The allyl sulfide-based thiol-yne-methacrylate system exhibits stress relaxation up to 55% and increased T(g) up to 40°C compared with the control, AFCT-incapable thiol-yne. This ternary system has less than 1/3 of the stress of conventional dimethacrylate monomer resins while possessing similarly outstanding mechanical behavior.

Using hyperbranched oligomer functionalized glass fillers to reduce shrinkage stress

OBJECTIVE

Fillers are widely utilized to enhance the mechanical properties of polymer resins. However, polymerization stress has the potential to increase due to the higher elastic modulus achieved upon filler addition. Here, we demonstrate a hyperbranched oligomer functionalized glass filler UV curable resin composite which is able to reduce the shrinkage stress without sacrificing mechanical properties.

METHODS

A 16-functional alkene-terminated hyperbranched oligomer is synthesized by thiol-acrylate and thiol-yne reactions and the product structure is analyzed by (1)H NMR, mass spectroscopy, and gel permeation chromatography. Surface functionalization of the glass filler is measured by thermogravimetric analysis. Reaction kinetics, mechanical properties and shrinkage stress are studied via Fourier transform infrared spectroscopy, dynamic mechanical analysis and a tensometer, respectively.

RESULTS

Silica nanoparticles are functionalized with a flexible 16-functional alkene-terminated hyperbranched oligomer which is synthesized by multistage thiol-ene/yne reactions. 93% of the particle surface was covered by this oligomer and an interfacial layer ranging from 0.7 nm to 4.5 nm thickness is generated. A composite system with these functionalized silica nanoparticles incorporated into the thiol-yne-methacrylate resin demonstrates 30% reduction of shrinkage stress (from 0.9 MPa to 0.6 MPa) without sacrificing the modulus (3100 ± 300 MPa) or glass transition temperature (62 ± 3°C). Moreover, the shrinkage stress of the composite system builds up at much later stages of the polymerization as compared to the control system.

SIGNIFICANCE

Due to the capability of reducing shrinkage stress without sacrificing mechanical properties, this composite system will be a great candidate for dental composite applications.

Enhanced Two-Stage Reactive Polymer Network Forming Systems

In this study, we develop thiol/acrylate two-stage reactive network forming polymer systems that exhibit two distinct and orthogonal stages of curing. Using a thiol-acrylate system with excess acrylate functional groups, a first stage polymer network is formed via a 1 to 1 stoichiometric thiol-acrylate Michael addition reaction (stage 1). At a later point in time, the excess acrylate functional groups are homopolymerized via a photoinitiated free radical polymerization to form a second stage polymer network (stage 2). By varying the monomers within the system as well as the stoichiometery of the thiol to acrylate functional groups, we demonstrate the ability of the two-stage polymer network forming systems to encompass a wide range of properties at the end of both the stage 1 and stage 2 polymerizations. Using urethane di- and hexa-acrylates within the formulations led to two-stage reactive polymeric systems with stage 1 T(g)s that ranged from -12 to 30 °C. The systems were then photocured, upon which the T(g) of the systems increases by up to 90 °C while also achieving a nearly 20 fold modulus increase.

Thiol-yne coupling: revisiting old concepts as a breakthrough for up-to-date applications

Radical thiol-yne coupling (TYC) has emerged as one of the most appealing click chemistry procedures, appearing as a sound candidate for replacing/complementing other popular click reactions such as the thiol-ene coupling (TEC) and the Cu-catalysed azide-alkyne cycloaddition (CuAAC). Radical TYC is indeed a metal-free reaction suitable for biomedical applications, and its mechanistic features often make it more efficient than its TEC sister reaction and more suitable for multifaceted derivatisations in the materials chemistry and bioconjugation realms. This article reviews the fascinating results obtained in those fields in very recent years.

Thiol-yne reaction on boron-doped diamond electrodes: application for the electrochemical detection of DNA-DNA hybridization events

Boron-doped diamond (BDD) interfaces were chemically functionalized through the catalyst free thiol-yne reaction. Different thiolated precursors (e.g., perfluorodecanethiol, 6-(ferrocenyl)-hexanethiol, DNA) were successfully "clicked" to alkynyl-terminated BDD by irradiating the interface at 365 nm for 30 min. Thiolated oligonucleotide strands were immobilized using the optimized reaction conditions, and the surface concentration was tuned to obtain a surface coverage of 3.1 × 10(12) molecules cm(-2). Electrochemical impedance spectroscopy (EIS) was employed to follow the kinetics of hybridization and dehybridization events. The sensitivity of the oligonucleotide modified BDD interface was assayed, and a detection limit of 1 nM was obtained.

In situ photopolymerization of biomaterials by thiol-yne click chemistry

The thiol-yne click chemistry reaction has been used for the in situ photocrosslinking of an aliphatic hyperbranched polyester. The biocompatibility of the resulting networks has been studied and marked cytotoxicity was not found for HeLa (human cervical carcinoma) tumoral cells and COS7 fibroblasts. The photoinduced thiol-yne process allows the generation of patterned structures with different geometries in films by DLW and these materials can be used as substrates for cell adhesion. The influence of the substrate geometry on cell adhesion has been studied by culturing cells onto these substrates and a preference for the photopatterned polymeric material can be seen in some of the structures by contrast phase microscopy. Actin and vinculin fluorescent staining revealed different adhesion behavior for HeLa cells and COS7 fibroblasts and this could be assigned to the different motility of cells. The thiol-yne photoreaction has proven to be an attractive approach for the preparation of micropatterned biomaterials.