Micro- to Nanoscale Bio-Hybrid Hydrogels Engineered by Ionizing Radiation

Bio-hybrid hydrogels consist of a water-swollen hydrophilic polymer network encapsulating or conjugating single biomolecules, or larger and more complex biological constructs like whole cells. By modulating at least one dimension of the hydrogel system at the micro- or nanoscale, the activity of the biological component can be extremely upgraded with clear advantages for the development of therapeutic or diagnostic micro- and nano-devices. Gamma or e-beam irradiation of polymers allow a good control of the chemistry at the micro-/nanoscale with minimal recourse to toxic reactants and solvents. Another potential advantage is to obtain simultaneous sterilization when the absorbed doses are within the sterilization dose range. This short review will highlight opportunities and challenges of the radiation technologies to produce bio-hybrid nanogels as delivery devices of therapeutic biomolecules to the target cells, tissues, and organs, and to create hydrogel patterns at the nano-length and micro-length scales on surfaces.

Polymerization Reactions and Modifications of Polymers by Ionizing Radiation

Ionizing radiation has become the most effective way to modify natural and synthetic polymers through crosslinking, degradation, and graft polymerization. This review will include an in-depth analysis of radiation chemistry mechanisms and the kinetics of the radiation-induced C-centered free radical, anion, and cation polymerization, and grafting. It also presents sections on radiation modifications of synthetic and natural polymers. For decades, low linear energy transfer (LLET) ionizing radiation, such as gamma rays, X-rays, and up to 10 MeV electron beams, has been the primary tool to produce many products through polymerization reactions. Photons and electrons interaction with polymers display various mechanisms. While the interactions of gamma ray and X-ray photons are mainly through the photoelectric effect, Compton scattering, and pair-production, the interactions of the high-energy electrons take place through coulombic interactions. Despite the type of radiation used on materials, photons or high energy electrons, in both cases ions and electrons are produced. The interactions between electrons and monomers takes place within less than a nanosecond. Depending on the dose rate (dose is defined as the absorbed radiation energy per unit mass), the kinetic chain length of the propagation can be controlled, hence allowing for some control over the degree of polymerization. When polymers are submitted to high-energy radiation in the bulk, contrasting behaviors are observed with a dominant effect of cross-linking or chain scission, depending on the chemical nature and physical characteristics of the material. Polymers in solution are subject to indirect effects resulting from the radiolysis of the medium. Likewise, for radiation-induced polymerization, depending on the dose rate, the free radicals generated on polymer chains can undergo various reactions, such as inter/intramolecular combination or inter/intramolecular disproportionation, b-scission. These reactions lead to structural or functional polymer modifications. In the presence of oxygen, playing on irradiation dose-rates, one can favor crosslinking reactions or promotes degradations through oxidations. The competition between the crosslinking reactions of C-centered free radicals and their reactions with oxygen is described through fundamental mechanism formalisms. The fundamentals of polymerization reactions are herein presented to meet industrial needs for various polymer materials produced or degraded by irradiation. Notably, the medical and industrial applications of polymers are endless and thus it is vital to investigate the effects of sterilization dose and dose rate on various polymers and copolymers with different molecular structures and morphologies. The presence or absence of various functional groups, degree of crystallinity, irradiation temperature, etc. all greatly affect the radiation chemistry of the irradiated polymers. Over the past decade, grafting new chemical functionalities on solid polymers by radiation-induced polymerization (also called RIG for Radiation-Induced Grafting) has been widely exploited to develop innovative materials in coherence with actual societal expectations. These novel materials respond not only to health emergencies but also to carbon-free energy needs (e.g., hydrogen fuel cells, piezoelectricity, etc.) and environmental concerns with the development of numerous specific adsorbents of chemical hazards and pollutants. The modification of polymers through RIG is durable as it covalently bonds the functional monomers. As radiation penetration depths can be varied, this technique can be used to modify polymer surface or bulk. The many parameters influencing RIG that control the yield of the grafting process are discussed in this review. These include monomer reactivity, irradiation dose, solvent, presence of inhibitor of homopolymerization, grafting temperature, etc. Today, the general knowledge of RIG can be applied to any solid polymer and may predict, to some extent, the grafting location. A special focus is on how ionizing radiation sources (ion and electron beams, UVs) may be chosen or mixed to combine both solid polymer nanostructuration and RIG. LLET ionizing radiation has also been extensively used to synthesize hydrogel and nanogel for drug delivery systems and other advanced applications. In particular, nanogels can either be produced by radiation-induced polymerization and simultaneous crosslinking of hydrophilic monomers in "nanocompartments", i.e., within the aqueous phase of inverse micelles, or by intramolecular crosslinking of suitable water-soluble polymers. The radiolytically produced oxidizing species from water, •OH radicals, can easily abstract H-atoms from the backbone of the dissolved polymers (or can add to the unsaturated bonds) leading to the formation of C-centered radicals. These C-centered free radicals can undergo two main competitive reactions; intramolecular and intermolecular crosslinking. When produced by electron beam irradiation, higher temperatures, dose rates within the pulse, and pulse repetition rates favour intramolecular crosslinking over intermolecular crosslinking, thus enabling a better control of particle size and size distribution. For other water-soluble biopolymers such as polysaccharides, proteins, DNA and RNA, the abstraction of H atoms or the addition to the unsaturation by •OH can lead to the direct scission of the backbone, double, or single strand breaks of these polymers.

Fluorescent Dye Labeling Changes the Biodistribution of Tumor-Targeted Nanoparticles

Fluorescent dye labeling is a common strategy to analyze the fate of administered nanoparticles in living organisms. However, to which extent the labeling processes can alter the original nanoparticle biodistribution has been so far neglected. In this work, two widely used fluorescent dye molecules, namely, ATTO488 (ATTO) and Sulfo-Cy5 (S-Cy5), have been covalently attached to a well-characterized CXCR4-targeted self-assembling protein nanoparticle (known as T22-GFP-H6). The biodistribution of labeled T22-GFP-H6-ATTO and T22-GFP-H6-S-Cy5 nanoparticles has been then compared to that of the non-labeled nanoparticle in different CXCR4+ tumor mouse models. We observed that while parental T22-GFP-H6 nanoparticles accumulated mostly and specifically in CXCR4+ tumor cells, labeled T22-GFP-H6-ATTO and T22-GFP-H6-S-Cy5 nanoparticles showed a dramatic change in the biodistribution pattern, accumulating in non-target organs such as liver or kidney while reducing tumor targeting capacity. Therefore, the use of such labeling molecules should be avoided in target and non-target tissue uptake studies during the design and development of targeted nanoscale drug delivery systems, since their effect over the fate of the nanomaterial can lead to considerable miss-interpretations of the actual nanoparticle biodistribution.

Engineering Nanogels for Drug Delivery to Pathogenic Fungi Aspergillus fumigatus by Tuning Polymer Amphiphilicity

Invasive aspergillosis is a serious threat to immunodeficient and critically ill patients caused mainly by the fungus Aspergillus fumigatus. Here, poly(glycidol)-based nanogels (NGs) are proposed as delivery vehicles for antifungal agents for sustained drug release. NGs are formed by simple self-assembly of random copolymers, followed by oxidative cross-linking of thiol functionalities. We investigate the impact of copolymer amphiphilicity on NG interaction with mature fungal hyphae in order to select the optimal drug delivery system for model antifungal drug amphotericin B. The results show that drug-loaded NGs decrease minimal inhibitory concentration (MIC) for around four times and slow down the fungal biofilm synthesis at concentrations lower than MIC. Our results suggest that amphiphilicity of nanoparticle's polymer matrix is an important factor in understanding the action of nanocarriers toward fungal cells and should be considered in the development of nanoparticle-based antifungal therapy.

Small, Traceable, Endosome-Disrupting, and Bioresponsive Click Nanogels Fabricated via Microfluidics for CD44-Targeted Cytoplasmic Delivery of Therapeutic Proteins

Nanogels (NG) are among the most ideal cytoplasmic protein delivery vehicles; however, their performance is suboptimal, partly owing to relatively big size, poor cell uptake, and endosomal entrapment. Here, we developed small, traceable, endosome-disrupting, and bioresponsive hyaluronic acid NG (HA-NG) for CD44-targeted intracellular delivery of therapeutic proteins. With microfluidics and catalyst-free photo-click cross-linking, HA-NG with hydrodynamic diameters of ca. 80 and 150 nm, strong green fluorescence and efficient loading of various proteins including saporin (Sap), cytochrome C, herceptin, immunoglobulin G (IgG), and bovine serum albumin could be fabricated. Interestingly, 80 nm-sized HA-NG revealed clearly better cellular uptake than its 150 nm counterparts in both CD44-negative U87 cancer cells and CD44-positive 4T1 and MDA-MB-231 cells. Moreover, small NG exhibited accelerated endosomal escape, which was further boosted by introducing GALA, a pH-sensitive fusogenic peptide. Accordingly, Sap-loaded small and GALA-functionalized HA-NG showed the highest cytotoxicity in CD44-positive MDA-MB-231, 4T1, A549, and SMMC-7721 cancer cells. The biodistribution studies demonstrated that 80 nm-sized HA-NG displayed significantly greater tumor uptake as well as penetration in MDA-MB-231 human breast tumor xenografts than its 150 nm counterparts, whereas the introduction of GALA had no detrimental effect on tumor accumulation. Small, endosome-disrupting, and bioresponsive HA-NG with easy and controlled fabrication hold a great potential for targeted protein therapy.