Biomedical applications of degradable polyphosphazenes

The application of hydrogels for enamel remineralization

Enamel is composed of numerous uniformly wide, well-oriented hydroxyapatite crystals. It possesses an acellular structure that cannot be repaired after undergoing damage. Therefore, remineralization after enamel defects has become a focal point of research. Hydrogels, which are materials with three-dimensional structures derived from cross-linking polymers, have garnered significant attention in recent studies. Their exceptional properties make them valuable in the application of enamel remineralization. In this review, we summarize the structure and formation of enamel, present the design considerations of hydrogels for enamel remineralization, explore diverse hydrogels types in this context, and finally, shed light on the potential future applications in this field.

Synthesis and characterization of biodegradable and antioxidant phosphazene-tannic acid nanospheres and their utilization as drug carrier material

In this study, hexachlorocyclotriphosphazene (HCCP) and tannic acid (TA) were used at different stoichiometric ratios to synthesize cyclomatrix-type polymeric materials with different surface features and dimensions. Using different reactive ratios, the structure and surface functional groups of the synthesized polymeric particles were explained using Fourier-Transform Infrared Spectroscopic (FTIR), Scanning Electron Microscope (SEM), Energy-dispersive X-ray spectroscopy (EDX), X-ray Photoelectron Spectroscopy (XPS) and Thermogravimetric (TG) analysis techniques. With morphologically fully spherical structure and mean 234.82 ± 49.37 nm dimensions, Phz-TA (4:1) nanospheres were researched for in vitro biodegradability, antioxidant features, and usability as a drug release system. In vitro biodegradability of Phz-TA (4:1) nanospheres was investigated at pH = 7.0 and pH = 1.2. Determined to degrade in 8-10 h at these pH values, nanospheres were used for releasing of Rhodamine 6G as a model drug. Due to the rich phenolic structure of the contained tannic acid units, nanospheres were determined to simultaneously have antioxidant features. Thus, this study determined that Phz-TA nanospheres with in vitro biodegradability and antioxidant features are promising polymeric materials for use as a potential drug-carrier in the future.

Generational Biodegradable and Regenerative Polyphosphazene Polymers and their Blends with Poly (lactic-co-glycolic acid)

New fields such as regenerative engineering have driven the design of advanced biomaterials with a wide range of properties. Regenerative engineering is a multidisciplinary approach that integrates the fields of advanced materials science and engineering, stem cell science, physics, developmental biology, and clinical translation for the regeneration of complex tissues. The complexity and demands of this innovative approach have motivated the synthesis of new polymeric materials that can be customized to meet application-specific needs. Polyphosphazene polymers represent this fundamental change and are gaining renewed interest as biomaterials due to their outstanding synthetic flexibility, neutral bioactivity (buffering degradation products), and tunable properties across the range. Polyphosphazenes are a unique class of polymers composed of an inorganic backbone with alternating phosphorus and nitrogen atoms. Each phosphorus atom bears two substituents, with a wide variety of side groups available for property optimization. Polyphosphazenes have been investigated as potential biomaterials for regenerative engineering. Polyphosphazenes for use in regenerative applications have evolved as a class to include different generations of degradable polymers. The first generation of polyphosphazenes for tissue regeneration entailed the use of hydrolytically active side groups such as imidazole, lactate, glycolate, glucosyl, or glyceryl groups. These side groups were selected based on their ability to sensitize the polymer backbone to hydrolysis, which allowed them to break down into non-toxic small molecules that could be metabolized or excreted. The second generation of degradable polyphosphazenes developed consisted of polymers with amino acid ester side groups. When blended with poly (lactic acid-co-glycolic acid) (PLGA), the feasibility of neutralizing acidic degradation products of PLGA was demonstrated. The blends formed were mostly partially miscible. The desire to improve miscibility led to the design of the third generation of degradable polyphosphazenes by incorporating dipeptide side groups which impart significant hydrogen bonding capability to the polymer for the formation of completely miscible polyphosphazene-PLGA blends. Blend system of the dipeptide-based polyphosphazene and PLGA exhibit a unique degradation behavior that allows the formation of interconnected porous structures upon degradation. These inherent pore-forming properties have distinguished degradable polyphosphazenes as a potentially important class of biomaterials for further study. The design considerations and strategies for the different generations of degradable polyphosphazenes and future directions are discussed.

Biodegradable Poly[(Amino Acid Ester)phosphazenes] for Biomedical Applications

Polyphosphazene derivatives with amino acid ester side groups were prepared by reacting poly(dichlorophosphazene) with ethyl esters of amino acids and were characterized by 1H, 31P-NMR and DSC analyses. The in vitro rate of degradation of these polymers depended on the nature of the amino acids while introducing small amounts of depsipeptide ester co-substituents increased degradation rates. The rate of hydrolytic degradation of the poly[(organo)phosphazene] materials could be controlled by the amino acid ester by the depsipeptide ester side group content and by blending poly[(amino acid ester)phosphazenes] with poly[(amino acid ester)-co-(depsipeptide ester)phosphazenes]. Poly[(glycine ethyl ester)phosphazenes] prepared from poly[(dichloro)phosphazenes] degradation rates were independent of molecular weight. Degradations of polymer blends indicate that an intramolecular catalysis ofthe polymer by the pendent carboxylic acid.

Biodegradable polyphosphazene biomaterials for tissue engineering and delivery of therapeutics

Degradable biomaterials continue to play a major role in tissue engineering and regenerative medicine as well as for delivering therapeutic agents. Although the chemistry of polyphosphazenes has been studied extensively, a systematic review of their applications for a wide range of biomedical applications is lacking. Polyphosphazenes are synthesized through a relatively well-known two-step reaction scheme which involves the substitution of the initial linear precursor with a wide range of nucleophiles. The ease of substitution has led to the development of a broad class of materials that have been studied for numerous biomedical applications including as scaffold materials for tissue engineering and regenerative medicine. The objective of this review is to discuss the suitability of poly(amino acid ester)phosphazene biomaterials in regard to their unique stimuli responsive properties, tunable degradation rates and mechanical properties, as well as in vitro and in vivo biocompatibility. The application of these materials in areas such as tissue engineering and drug delivery is discussed systematically. Lastly, the utility of polyphosphazenes is further extended as they are being employed in blend materials for new applications and as another method of tailoring material properties.

Water-Soluble, Biocompatible Polyphosphazenes with Controllable and pH-Promoted Degradation Behavior

The synthesis of a series of novel, water-soluble poly(organophosphazenes) prepared via living cationic polymerization is presented. The degradation profiles of the polyphosphazenes prepared are analyzed by GPC, 31P NMR spectroscopy, and UV-Vis spectroscopy in aqueous media and show tunable degradation rates ranging from days to months, adjusted by subtle changes to the chemical structure of the polyphosphazene. Furthermore, it is observed that these polymers demonstrate a pH-promoted hydrolytic degradation behavior, with a remarkably faster rate of degradation at lower pH values. These degradable, water soluble polymers with controlled molecular weights and structures could be of significant interest for use in aqueous biomedical applications, such as polymer therapeutics, in which biological clearance is a requirement and in this context cell viability tests are described which show the non-toxic nature of the polymers as well as their degradation intermediates and products.

Temperature-sensitive polymers for drug delivery

The ability to undergo rapid changes in response to subtle environmental cues make stimuli- responsive materials attractive candidates for minimally invasive, targeted and personalized drug delivery applications. This special report aims to highlight and provide a brief description of several of the significant natural and synthetic temperature-responsive materials that have clinical relevance for drug delivery applications. This report examines the advantages and disadvantages of natural versus synthetic materials and outlines various scaffold architectures that can be utilized with temperature-sensitive drug delivery materials. The authors provide a commentary on the current state of the field and provide their insight into future expectations for temperature-sensitive drug delivery, emphasizing the importance of the emergence of dual and multiresponsive systems capable of responding precisely to an expanding set of stimuli, thereby allowing the development of disease-specific drug delivery vehicles.

Polyphosphazenes: Multifunctional, Biodegradable Vehicles for Drug and Gene Delivery

Poly[(organo)phosphazenes] are a unique class of extremely versatile polymers with a range of applications including tissue engineering and drug delivery, as hydrogels, shape memory polymers and as stimuli responsive materials. This review aims to divulge the basic principles of designing polyphosphazenes for drug and gene delivery and portray the huge potential of these extremely versatile materials for such applications. Polyphosphazenes offer a number of distinct advantages as carriers for bioconjugates; alongside their completely degradable backbone, to non-toxic degradation products, they possess an inherently and uniquely high functionality and, thanks to recent advances in their polymer chemistry, can be prepared with controlled molecular weights and narrow polydispersities, as well as self-assembled supra-molecular structures. Importantly, the rate of degradation/hydrolysis of the polymers can be carefully tuned to suit the desired application. In this review we detail the recent developments in the chemistry of polyphosphazenes, relevant to drug and gene delivery and describe recent investigations into their application in this field.

Polímeros usados como sistemas de transporte de princípios ativos

Os diferentes sistemas de transporte têm evidenciado potencial terapêutico para uma grande variedade de princípios ativos, satisfazendo vários requisitos, como a prevenção da sua eliminação rápida do organismo, a redução da sua toxicidade sistêmica, a estabilização e a otimização do seu metabolismo, e o direcionamento específico ao local alvo e os mecanismos de defesa. No entanto, têm sido reconhecidos vários outros desafios associados à liberação específica do princípio ativo ao local alvo, pelo que, para ultrapassar os obstáculos químicos e biológicos, a seleção do polímero utilizado para a preparação do sistema de transporte é de importância crucial. O presente trabalho apresenta um relato sobre os principais polímeros naturais e sintéticos utilizados para a preparação de sistemas de transporte de princípios ativos in vivo.

Recent advances in synthetic bioelastomers

This article reviews the degradability of chemically synthesized bioelastomers, mainly designed for soft tissue repair. These bioelastomers involve biodegradable polyurethanes, polyphosphazenes, linear and crosslinked poly(ether/ester)s, poly(ε-caprolactone) copolymers, poly(1,3-trimethylene carbonate) and their copolymers, poly(polyol sebacate)s, poly(diol-citrates) and poly(ester amide)s. The in vitro and in vivo degradation mechanisms and impact factors influencing degradation behaviors are discussed. In addition, the molecular designs, synthesis methods, structure properties, mechanical properties, biocompatibility and potential applications of these bioelastomers were also presented.

Electrospun polyphosphazene nanofibers forin vitro rat endothelial cells proliferation

A large variety of natural and synthetic polymers have been explored as scaffolds for the seeding and growth of different types of cells. To fabricate a scaffold that can be used as a synthetic extracellular matrix (ECM), it is important to replicate the nanoscale dimensions of natural ECM. The electrospinning process allows to produce ultrathin fibers so that this method represents a suitable approach to scaffold fabrication for tissue engineering applications. In this work, the feasibility of obtaining flat or tubular matrices from biocompatible poly[(ethyl phenylalanato)(1.4) (ethyl glycinato)(0.6) phosphazene] by electrospinning was evaluated and the effect of process parameters on the diameter of nanofibers was examined. The adhesion and growth of rat neuromicrovascular endothelial cells cultured on sheets and tubes composed by the polymer with an average fiber diameter of 850 +/- 150 nm were also reported. Microscopic examination of the seeded tubes demonstrated that, after 16 days of incubation, endothelial cells formed a monolayer on the whole surface. These results are the first step to demonstrate that tubes of biodegradable polyphosphazenes might be a feasible model to construct human tissues such as vessels or cardiac valves.

In vitro evaluation of poly[bis(ethyl alanato)phosphazene] as a scaffold for bone tissue engineering

Polyphosphazenes with amino acid ester as side groups are biocompatible polymers that could provide valid scaffolds for cell growth. In the present study we investigate the adhesion and growth of osteoblasts obtained from rat bone marrow on matrices composed of thin fibers of poly[bis(ethyl alanato)phosphazene] (PAlaP), poly(d,l-lactic acid) (PDLLA), or PAlaP/PDLLA blend. Our data show that scaffolds of PAlaP or PAlaP/PDLLA blend enhanced the cell adhesion and growth in comparison with that observed in cultures seeded on polystyrene tissue culture plates. Although collagenase-digestible protein synthesis remained unchanged, all scaffolds induced a decrease in alkaline phosphatase activity, suggesting that osteoblasts are in the proliferation phase. Both PAlaP and PAlaP blended with PDLLA may represent a new and interesting substrate for bone tissue engineering.

Thermosensitive poly(organophosphazene) hydrogels for a controlled drug delivery

Thermosensitive poly(organophosphazenes) were synthesized for a controlled release of hydrophilic polymeric model drugs such as dextran and albumin in this study. The solutions of the present polymers bearing both hydrophobic side groups of L-isoleucine ethyl ester (IleOEt) and hydrophilic groups of alpha-amino-omega-methoxy-PEG (Mw 550) (AMPEG550) exhibited reversible sol-gel transition behaviors with changes of temperature. Viscometric measurement indicated that the thermosensitive hydrogels with good strength could be formed from the solutions in the range of the concentrations of 7-15 wt% around body temperature. For increasing their biodegradabililites, depsipeptides of ethyl-2-(O-glycyl)lactate (GlyLacOEt) were also introduced to the polymer, showing enhanced degradation of hydrogels. In vitro release behaviors of hydrophilic FITC-dextran (Mw 71,600) and human serum albumin from these polymer hydrogels were sustained for about 2 weeks while those from poloxamer (Pluronic F-127) hydrogel showed a distinct initial burst. The release of FITC-dextran exhibited concentration-dependent behavior ranging from 7 to 15 wt% of the polymer solution while it was almost independent of the concentration of FITC-dextran.

Novel micro-crosslinked poly(organophosphazenes) with improved mechanical properties and controllable degradation rate as potential biodegradable matrix

As biodegradable materials, linear polyphosphazenes undergo rapid hydrolysis degradation but exhibit poor mechanical properties. Blending with biodegradable polyesters or inorganic particles strengthen their mechanical properties but give rise to slower degradation rate. To balance the mechanical properties and the degradation rate, micro-crosslinked polyphosphazenes were synthesized in this study. Their glass transition temperatures, mechanical properties, and in vitro degradation behavior were investigated. 2-hydroxyethyl methacrylate (HEMA) was firstly attached to the side chain along with glycine ethyl ester to prepare co-substituted poly(organophosphazene) with pendant ethenyl substituents. The co-substituted poly(organophosphazene) was blended with HEMA or acrylic acid (AA) followed by a free radical polymerization to prepare micro-crosslinked poly(organophosphazenes). The resulting crosslinked polymers showed two separate glass transition temperatures depending on the HEMA or AA feed. Incorporation of crosslinking affected the mechanical properties positively. Crosslinked poly(organophosphazenes) showed an approximately 11-17 fold increase in terms of modulus of elasticity when compared to the linear counterpart. In vitro degradation tests indicated that HEMA-crosslinked polymers hydrolyzed at a retarded rate while AA-crosslinked polymers hydrolyzed at a moderate rate compared to linear polymers.

Tyrosine-Bearing Polyphosphazenes

Tyrosine-functionalized polyphosphazenes were synthesized, and their hydrolytic stability, pH-sensitive behavior, and hydrogel-forming capabilities were investigated. The physical and chemical properties of the polymers varied with the type of linkage between the tyrosine unit and phosphazene backbone. Poly[(ethyl glycinat-N-yl)(ethyl tyrosinat-N-yl)phophazenes] (linkage via the amino group of tyrosine) were found to be hydrolytically erodible. The rate of hydrolysis was dependent on the ratio of the two side groups, the slowest rate being associated with the highest concentration of tyrosine. The hydrolysis products were identified as phosphates, tyrosine, glycine, ammonia, and ethanol derived from the ester group. The hydrolytically stable phenolic-linked tyrosine derivatives were prepared from N-t-BOC-L-tyrosine methyl ester and alkoxy-based cosubstituents. Polyphosphazenes with both propoxy and phenolic-linked tyrosine side groups showed a pH-sensitive solubility behavior, which was dependent on the ratio and nature of the two side groups. The polymer was soluble in aqueous media below pH 3 and above pH 4. From pH 3-4, the polymer was insoluble. Replacement of propoxy by trifluoroethoxy units yielded a polymer that was insoluble in aqueous media at all pH values. Replacement of propoxy by methoxyethoxyethoxy groups gave a polymer that was soluble at all pH values. Exposure of both the propoxy and methoxyethoxyethoxy polymers to calcium ions in aqueous media caused gel formation due to ionic cross-linking through the carboxylate groups.