Oxidation Responsive Polymers with a Triggered Degradation via Arylboronate Self-Immolative Motifs on a Polyphosphazene Backbone

Stimuli-Responsive Polymers for Engineered Emulsions

Emulsions are complex. Dispersing two immiscible phases, thus expanding an interface, requires effort to achieve and the resultant dispersion is thermodynamically unstable, driving the system toward coalescence. Furthermore, physical instabilities, including creaming, arise due to presence of dispersed droplets of different densities to a continuous phase. Emulsions allow the formulation of oils, can act as vehicles to solubilize both hydrophilic and lipophilic molecules, and can be tailored to desirable rheological profiles, including "gel-like" behavior and shear thinning. The usefulness of emulsions can be further expanded by imparting stimuli-responsive or "smart" behaviors by inclusion of a stimuli-responsive emulsifier, polymer or surfactant. This enables manipulation like gelation, breaking, or aggregation, by external triggers such as pH, temperature, or salt concentration changes. This platform generates functional materials for pharmaceuticals, cosmetics, oil recovery, and colloid engineering, combining both smart behaviors and intrinsic benefit of emulsions. However, with increased functionality comes greater complexity. This review focuses on the use of stimuli-responsive polymers for the generation of smart emulsions, motivated by the great adaptability of polymers for this application and their efficacy as steric stabilizers. Stimuli-responsive emulsions are described according to the trigger used to provide the reader with an overview of progress in this field.

Stimuli-Responsive Polymer-Based Nanosystems for Cancer Theranostics

Stimuli-responsive polymers can respond to internal stimuli, such as reactive oxygen species (ROS), glutathione (GSH), and pH, biological stimuli, such as enzymes, and external stimuli, such as lasers and ultrasound, etc., by changing their hydrophobicity/hydrophilicity, degradability, ionizability, etc., and thus have been widely used in biomedical applications. Due to the characteristics of the tumor microenvironment (TME), stimuli-responsive polymers that cater specifically to the TME have been extensively used to prepare smart nanovehicles for the targeted delivery of therapeutic and diagnostic agents to tumor tissues. Compared to conventional drug delivery nanosystems, TME-responsive nanosystems have many advantages, such as high sensitivity, broad applicability among different tumors, functional versatility, and improved biosafety. In recent years, a great deal of research has been devoted to engineering efficient stimuli-responsive polymeric nanosystems, and significant improvement has been made to both cancer diagnosis and therapy. In this review, we summarize some recent research advances involving the use of stimuli-responsive polymer nanocarriers in drug delivery, tumor imaging, therapy, and theranostics. Various chemical stimuli will be described in the context of stimuli-responsive nanosystems. Accordingly, the functional chemical groups responsible for the responsiveness and the strategies to incorporate these groups into the polymer will be discussed in detail. With the research on this topic expending at a fast pace, some innovative concepts, such as sequential and cascade drug release, NIR-II imaging, and multifunctional formulations, have emerged as popular strategies for enhanced performance, which will also be included here with up-to-date illustrations. We hope that this review will offer valuable insights for the selection and optimization of stimuli-responsive polymers to help accelerate their future applications in cancer diagnosis and treatment.

Phosphorus and Silicon-Based Macromolecules as Degradable Biomedical Polymers

Synthetic polymers are indispensable in biomedical applications because they can be fabricated with consistent and reproducible properties, facile scalability, and customizable functionality to perform diverse tasks. However, currently available synthetic polymers have limitations, most notably when timely biodegradation is required. Despite there being, in principle, an entire periodic table to choose from, with the obvious exception of silicones, nearly all known synthetic polymers are combinations of carbon, nitrogen, and oxygen in the main chain. Expanding this to main-group heteroatoms can open the way to novel material properties. Herein the authors report on research to incorporate the chemically versatile and abundant silicon and phosphorus into polymers to induce cleavability into the polymer main chain. Less stable polymers, which degrade in a timely manner in mild biological environments, have considerable potential in biomedical applications. Herein the basic chemistry behind these materials is described and some recent studies into their medical applications are highlighted.

Solute Stabilization Effects of Nanoparticles Containing Boronic Acids in the Absence of Binding Pairs

Boronic acids are widely used in materials science because of their ability to reversibly bind with diol and catechol moieties through dynamic covalent interactions in a pH- and oxidative-dependent manner. Considerably fewer studies focus on property modulation of boronic acid-based materials in the absence of a biding pair. Herein, we discuss the effects of the boronic acid-containing polymer block length on solute release kinetics from nanoparticles in a stimuli-responsive manner for on-demand delivery. In this study, ABC-type linear amphiphiles of poly(d,l-lactide) and poly(2-methacryloyloxyethyl phosphorylcholine) containing a middle block functionalized with 3-aminophenylboronic acid were synthesized by a combination of ring-opening and controlled free radical polymerizations. Nile red-loaded nanoparticles were self-assembled using a multi-inlet vortex mixer in a well-controlled manner. Release was evaluated at pH above and below the pKa of the boronic acid and in the presence of hydrogen peroxide. Our results show that release kinetics from nanoparticles incorporating a boronic acid-functionalized interlayer were slower than those without it, and the rate could be modulated according to pH and oxidative conditions. These effects can be attributed to several factors, including the hydrophobicity of the boronic acid block as well as hydrogen bonding interactions existing between locally confined boronic acids. While boronic acids are generally utilized as boronic/boronate esters, their stabilizing effects in the absence of appropriate binding pairs are relevant and should be considered in the design of boronic acid-based technologies.

Characterization Techniques of Polymer Aging: From Beginning to End

Polymers have been widely applied in various fields in the daily routines and the manufacturing. Despite the awareness of the aggressive and inevitable aging for the polymers, it still remains a challenge to choose an appropriate characterization strategy for evaluating the aging behaviors. The difficulties lie in the fact that the polymer features from the different aging stages require different characterization methods. In this review, we present an overview of the characterization strategies preferable for the initial, accelerated, and late stages during polymer aging. The optimum strategies have been discussed to characterize the generation of radicals, variation of functional groups, substantial chain scission, formation of low-molecular products, and deterioration in the polymers' macro-performances. In view of the advantages and the limitations of these characterization techniques, their utilization in a strategic approach is considered. In addition, we highlight the structure-property relationship for the aged polymers and provide available guidance for lifetime prediction. This review could allow the readers to be knowledgeable of the features for the polymers in the different aging stages and provide access to choose the optimum characterization techniques. We believe that this review will attract the communities dedicated to materials science and chemistry.

Boosting Degradation of Biodegradable Polymers

Biodegradation of polymers in composting conditions is an alternative end-of-life (EoL) scenario for contaminated materials collected through the municipal solid waste management system, mainly when mechanical or chemical methods cannot be used to recycle them. Compostability certification requirements are time-consuming and expensive. Therefore, approaches to accelerate the biodegradation of these polymers in simulated composting conditions can facilitate and speed up the evaluation and selection of potential compostable polymer alternatives and inform faster methods to biodegrade these polymers in real composting. This review highlights recent trends, challenges, and future strategies to accelerate biodegradation by modifying the polymer properties/structure and the compost environment. Both abiotic and biotic methods show potential for accelerating the biodegradation of biodegradable polymers. Abiotic methods, such as the incorporation of additives, reduction of molecular weight, reduction of size and reactive blending, are potentially the most straightforward, providing a level of technology that allows for easy adoption and adaptability. Novel methods, including the concept of self-immolative and triggering the scission of polymer chains in specific conditions, are increasingly sought. In terms of biotic methods, dispersion/encapsulation of enzymes during the processing step, biostimulation of the environment, and bioaugmentation with specific microbial strains during the biodegradation process are promising to accelerate biodegradation.

Nanomaterial-based CT contrast agents and their applications in image-guided therapy

Computed tomography (CT), a diagnostic tool with clinical application, comprehensive coverage, and low cost, is used in hospitals worldwide. However, CT imaging fails to distinguish soft tissues from normal organs and tumors because their mass attenuation coefficients are similar. Various CT contrast agents have been developed in recent years to improve the sensitivity and contrast of imaging. Here, we review the progress of nanomaterial-based CT contrast agents and their applications in image-guided therapy. The CT contrast agents are classified according to their components; gold (Au)-based, bismuth (Bi)-based, lanthanide (Ln)-based, and transition metal (TM)-based nanomaterials are discussed. CT image-guided therapy of diseases, including photothermal therapy (PPT), photodynamic therapy (PDT), chemotherapy, radiotherapy (RT), gas therapy, sonodynamic therapy (SDT), immunotherapy, starvation therapy, gene therapy (GT), and microwave thermal therapy (MWTT), are reviewed. Finally, the perspectives on the CT contrast agents and their biomedical applications are discussed.

Cyclo- and Polyphosphazenes for Biomedical Applications

Cyclic and polyphosphazenes are extremely interesting and versatile substrates characterized by the presence of -P=N- repeating units. The chlorine atoms on the P atoms in the starting materials can be easily substituted with a variety of organic substituents, thus giving rise to a huge number of new materials for industrial applications. Their properties can be designed considering the number of repetitive units and the nature of the substituent groups, opening up to a number of peculiar properties, including the ability to give rise to supramolecular arrangements. We focused our attention on the extensive scientific literature concerning their biomedical applications: as antimicrobial agents in drug delivery, as immunoadjuvants in tissue engineering, in innovative anticancer therapies, and treatments for cardiovascular diseases. The promising perspectives for their biomedical use rise from the opportunity to combine the benefits of the inorganic backbone and the wide variety of organic side groups that can lead to the formation of nanoparticles, polymersomes, or scaffolds for cell proliferation. In this review, some aspects of the preparation of phosphazene-based systems and their characterization, together with some of the most relevant chemical strategies to obtain biomaterials, have been described.

Factors Controlling Degradation of Biologically Relevant Synthetic Polymers in Solution and Solid State

The field of biodegradable synthetic polymers, which is central for regenerative engineering and drug delivery applications, encompasses a multitude of hydrolytically sensitive macromolecular structures and diverse processing approaches. The ideal degradation behavior for a specific life science application must comply with a set of requirements, which include a clinically relevant kinetic profile, adequate biocompatibility, benign degradation products, and controlled structural evolution. Although significant advances have been made in tailoring materials characteristics to satisfy these requirements, the impacts of autocatalytic reactions and microenvironments are often overlooked resulting in uncontrollable and unpredictable outcomes. Therefore, roles of surface versus bulk erosion, in situ microenvironment, and autocatalytic mechanisms should be understood to enable rational design of degradable systems. In an attempt to individually evaluate the physical state and form factors influencing autocatalytic hydrolysis of degradable polymers, this Review follows a hierarchical analysis that starts with hydrolytic degradation of water-soluble polymers before building up to 2D-like materials, such as ultrathin coatings and capsules, and then to solid-state degradation. We argue that chemical reactivity largely governs solution degradation while diffusivity and geometry control the degradation of bulk materials, with thin "2D" materials remaining largely unexplored. Following this classification, this Review explores techniques to analyze degradation in vitro and in vivo and summarizes recent advances toward understanding degradation behavior for traditional and innovative polymer systems. Finally, we highlight challenges encountered in analytical methodology and standardization of results and provide perspective on the future trends in the development of biodegradable polymers.

Biodegradable AuNP-Based Plasmonic Nanogels as Contrast Agents for Computed Tomography and Photoacoustics

Gold nanoparticles (AuNP) are well-established contrast agents in computed tomography (CT) and photoacoustic imaging (PAI). A wide variety of AuNP sizes, shapes, and coatings have been reported for these applications. However, for clinical translation, AuNP should be excretable to avoid long-term accumulation and possible side effects. Sub-5 nm AuNP have the benefit to be excretable through kidney filtration, therefore their loading in biodegradable nanogels holds promise to result in contrast agents that have long circulation times in the vasculature and subsequent biodegradation for excretion. Polyphosphazenes are intrinsically biodegradable polymers capable of forming nanogels with high payloads, and to release their payloads upon degradation. The significant development in polyphosphazenes that have tailored degradation kinetics, and their formulation with drugs or contrast agents, has shown potential as a biodegradable platform for imaging vasculature and endogenous molecules, by combination of CT and PA modalities. Therefore, we herein present methods for the formulation of AuNP assemblies loaded in nanogels composed of biodegradable polyphosphazenes, with a size range from 50 to 200 nm. We describe protocols for their characterization by UV-vis spectroscopy, Fourier-transform infrared spectroscopy, various microscopy techniques, elemental quantification by induced coupling plasma optical emission spectroscopy and contrast production in both CT and PAI. Finally, we detail the methods to investigate their effect on cells, distribution in cells and imaging properties for detection of endogenous molecules.

Recent research progress on polyphosphazene-based drug delivery systems

In recent years, synthetic polymer materials have become a research hotspot in the field of drug delivery. Compared with natural polymer materials, synthetic polymer materials have more flexible structural adjustability, and can be designed to obtain clinically required delivery vehicles. Polyphosphazenes are one of the most promising biomedical materials in the future due to their controllable degradation properties and structural flexibility. These materials can be designed by controlling the hydrophilic and hydrophobic balance, introducing functional groups or drugs to form different forms of administration, such as nanoparticles, polyphosphazene-drug conjugates, injectable hydrogels, coatings, etc. In addition, the flexible backbone of polyphosphazenes and the flexibility of substitution enable them to meet researchers' design requirements in terms of stereochemistry, nanostructures, and topologies. At present, researchers have achieved a lot of successful practices in the field of targeted delivery of anticancer drugs/proteins/genes, bone tissue engineering repair, cell imaging tracking, photothermal therapy, and immunologic preparations. This review provides a summary of the progress of the recent 10 years of polyphosphazene-based drug delivery systems in terms of of chemical structure and functions.

Smart Responsive Quercetin-Conjugated Glycol Chitosan Prodrug Micelles for Treatment of Inflammatory Bowel Diseases

The incidence and progression of inflammatory bowel disease are closely related to oxidative stress caused by excessive production of reactive oxygen species (ROS). To develop an efficacious and safe nanotherapy against inflammatory bowel diseases (IBD), we designed a novel pH/ROS dual-responsive prodrug micelle GC-B-Que as an inflammatory-targeted drug, which was comprised by active quercetin (Que) covalently linked to biocompatible glycol chitosan (GC) by aryl boronic ester as a responsive linker. The optimized micelles exhibited well-controlled physiochemical properties and stability in a physiological environment. Time-dependent NMR spectra traced the changes in the polymer structure in the presence of H₂O₂, confirming the release of the drug. The in vitro drug release studies indicated a low release rate (<20 wt %) in physiological conditions, but nearly complete release (>95 wt % after 72 h incubation) in a pH 5.8 medium containing 10 μM H₂O₂, exhibiting a pH/ROS dual-responsive property and sustained release behavior. Importantly, the negligible drug release in a simulated gastric environment in 1 h allowed us to perform intragastric administration, which has potential to achieve the oral delivery by mature enteric-coating modification in future. Further in vivo activities and biodistribution experiments found that the GC-B-Que micelles tended to accumulate in intestinal inflammation sites and showed better therapeutic efficacy than the free drugs (quercetin and mesalazine) in a colitis mice model. Typical inflammatory cytokines including TNF-α, IL-6, and iNOS were significantly suppressed by GC-B-Que micelle treatment. Our work promoted inflammatory-targeted delivery and intestinal drug accumulation for active single drug quercetin and improved the therapeutic effect of IBD. The current study also provided an alternative strategy for designing a smart responsive nanocarrier for a catechol-based drug to better achieve the target drug delivery.

Polyphosphazenes: Phosphorus in Inorganic-Organic Polymers

Although the best-known examples of synthetic polymers are derived from carbon-based monomers, there exists another large and growing family of macromolecules based on the chemistry of phosphorus. These are the poly(organophosphazenes): polymers with a backbone of alternating phosphorus and nitrogen atoms and with two organic side groups attached to each phosphorus. The methods of synthesis of these polymers allow access to property combinations not found in all-organic counterparts, and this provides pathways to new materials that are important in biomedical research, energy generation and storage, aerospace materials, and numerous other specialized applications.

Oxidative instability of boronic acid-installed polycarbonate nanoparticles

Oxidative stress, caused by the overproduction of reactive oxygen species (ROS), is often observed in degenerative and/or metabolic diseases, tumors, and inflamed tissues. Boronic acids are emerging as a unique class of responsive biomaterials targeting ROS because of their reactivity toward H2O2. Herein, we examine the oxidative reactivity of nanoparticles from a boronic acid-installed polycarbonate. The extent of oxidation under different concentrations of H2O2 was tracked by the change in fluorescence intensity of an encapsulated solvatochromic reporter dye, demonstrating their sensitivity to biologically-relevant concentrations of hydrogen peroxide. Oxidation-triggered particle destabilization, however, was shown to be highly dependent on the concentration of the final oxidized polymer product, and was only achieved if it fell below polymer critical micelle concentration. Our results indicate that these nanocarriers serve as an excellent dual pH/H2O2 responsive vehicle for drug delivery.

Activatable Hybrid Polyphosphazene-AuNP Nanoprobe for ROS Detection by Bimodal PA/CT Imaging

Overproduction of reactive oxygen species (ROS) is often related to inflammation or cancer and can cause tissue damage. Probes that have been previously reported to image ROS typically rely on imaging techniques that have low depth penetration in tissue, thus limiting their use to superficial disease sites. We report herein a novel formulation of hybrid nanogels loaded with gold nanoparticles (AuNP) to produce contrast for computed tomography (CT) and photoacoustics (PA), both being deep-tissue imaging techniques. The polyphosphazene polymer has been designed to selectively degrade upon ROS exposure, which triggers a switch-off of the PA signal by AuNP disassembly. This ROS-triggered degradation of the nanoprobes leads to a significant decrease in the PA contrast, thus allowing ratiometric ROS imaging by comparing the PA to CT signal. Furthermore, ROS imaging using these nanoprobes was applied to an in vitro model of inflammation, that is, LPS-stimulated macrophages, where ROS-triggered disassembly of the nanoprobe was confirmed via reduction of the PA signal. In summary, these hybrid nanoprobes are a novel responsive imaging agent that have the potential to image ROS overproduction by comparing PA to CT contrast.

Stimuli-Responsive Phosphorus-Based Polymers

This microreview details recent developments in stimuli-responsive polymers with phosphorus in the main-chain, in particular polyphosphazenes and polyphosphoesters. The presence of phosphorus in the polymers endows unique properties onto the macromolecules, which can be utilized for the preparation of materials capable of physically responding to specific stimuli. Achieving the desired responsiveness has been much facilitated by recent developments in synthetic polymer chemistry, in particular controlled synthesis and backbone functionalization phosphorus-based polymers, in order to achieve the required properties and hence responsiveness of the materials. The development of phosphorus-based polymers which respond to the most important stimuli are discussed, namely, pH, oxidation, reduction, temperature and biological triggers. The polymers are placed in the context not just of each other but also with reference to state-of-the-art organic polymers.

Polyphosphazene-based nanocarriers for the release of agrochemicals and potential anticancer drugs

The synthesised polyphosphazene-based nanocarriers allowed sustained diosgenin and brassinosteroid release over 4 days, with strong to moderate MCF-7 cytotoxicity and good agrochemical activity at medium and low concentrations.

Coumarin-Caged Polyphosphazenes with a Visible-Light Driven On-Demand Degradation

Polymers that, upon photochemical activation with visible light, undergo rapid degradation to small molecules are described. Through functionalization of a polyphosphazene backbone with pendant coumarin groups sensitive to light, polymers which are stable in the dark could be prepared. Upon irradiation, cleavage of the coumarin moieties exposes carboxylic acid moieties along the polymer backbone. The subsequent macromolecular photoacid is found to catalyze the rapid hydrolytic degradation of the polyphosphazene backbone. Water-soluble and non-water-soluble polymers are reported, which due to their sensitivity toward light in the visible region could be significant as photocleavable materials in biological applications.

Degradable, Dendritic Polyols on a Branched Polyphosphazene Backbone

Herein, we present the design, synthesis, and characterization of fully degradable, hybrid, star-branched dendritic polyols. First multiarmed polyphosphazenes were prepared as a star-branched scaffold which upon functionalization produced globular branched hydroxyl-functionalized polymers with over 1700 peripheral functional end groups. These polyols with unique branched architectures could be prepared with controlled molecular weights and relatively narrow dispersities. Furthermore, the polymers are shown to undergo hydrolytic degradation to low molecular weight degradation products, the rate of which could be controlled through postpolymerization functionalization of the phosphazene backbone.

Synthesis of a phenylboronic ester-linked PEG-lipid conjugate for ROS-responsive drug delivery

A kind of phenylboronic ester-linked PEG-lipid conjugate was designed and synthesized for ROS-responsive drug delivery.