Networks based on biodegradable polyesters: An overview of the chemical ways of crosslinking

Elastomeric vitrimers from designer polyhydroxyalkanoates with recyclability and biodegradability

Cross-linked elastomers are stretchable materials that typically are not recyclable or biodegradable. Medium-chain-length polyhydroxyalkanoates (mcl-PHAs) are soft and ductile, making these bio-based polymers good candidates for biodegradable elastomers. Elasticity is commonly imparted by a cross-linked network structure, and covalent adaptable networks have emerged as a solution to prepare recyclable thermosets via triggered rearrangement of dynamic covalent bonds. Here, we develop biodegradable and recyclable elastomers by chemically installing the covalent adaptable network within biologically produced mcl-PHAs. Specifically, an engineered strain of Pseudomonas putida was used to produce mcl-PHAs containing pendent terminal alkenes as chemical handles for postfunctionalization. Thiol-ene chemistry was used to incorporate boronic ester (BE) cross-links, resulting in PHA-based vitrimers. mcl-PHAs cross-linked with BE at low density (<6 mole %) affords a soft, elastomeric material that demonstrates thermal reprocessability, biodegradability, and denetworking at end of life. The mechanical properties show potential for applications including adhesives and soft, biodegradable robotics and electronics.

Synthesis and Characterization of Poly(DL-lactide) Containing Fluorene Structures

9,9-bis[4-(2-hydroxy-3-acryloyloxypropoxy)phenyl]fluorene (BPF) hydroxyl groups (-OH) were used as initiators in the ring-opening polymerization reaction with DL-lactide monomers at different molar ratios to synthesize a Poly(DL-lactide) polymer containing bisphenol fluorene structure and acrylate functional groups (DL-BPF). The polymer's structure and molecular weight range were analyzed using NMR (¹H, ¹³C) and gel permeation chromatography. DL-BPF was then subjected to photocrosslinking using the photoinitiator Omnirad 1173, resulting in the formation of an optically transparent crosslinked polymer. Characterization of the crosslinked polymer involved analyzing its gel content, refractive index, thermal stability (via differential scanning thermometry (DSC) and thermogravimetric analysis (TGA)), as well as conducting cytotoxicity tests. The crosslinked copolymer exhibited a maximum refractive index of 1.5276, a maximum glass transition temperature of 61.1 °C, and cell survival rates higher than 83% in the cytotoxicity tests.

Seawater Biodegradable Poly(butylene succinate-co-adipate)-Wheat Bran Biocomposites

The present work focused on the development and characterization of biocomposites based on a fully bio-based polyester, poly(butylene succinate-co-butylene adipate) (PBSA), and wheat bran derived by flour milling. PBSA-bran composites containing 5, 10, 15, and 20 wt.% of wheat bran were produced via melt extrusion and processed by injection molding. Their thermal, rheological, morphological, and tensile properties were investigated. In addition, a biodegradation test in a natural marine environment was conducted on composite dog-bones to assess the capacity of the used filler to increase the PBSA biodegradation rate. The composites maintained similar melt processability and mechanical properties to virgin PBSA with up to 15 wt.% bran content. This result was also supported by morphological investigation, which showed good filler dispersion within the polymer matrix at low-mid bran content, whereas poor polymer-filler dispersion occurred at higher concentrations. Furthermore, the biodegradation tests showed bran's capacity to improve the PBSA biodegradation rate, probably due to the hygroscopic bran swelling, which induced the fragmentation of the dog-bone with a consequent increase in the polymeric matrix-seawater interfacial area, accelerating the degradation mechanisms. These results encourage the use of wheat bran, an abundant and low-cost agri-food by-product, as a filler in PBSA-based composites to develop products with good processability, mechanical properties, and controlled biodegradability in marine environments.

One-pot synthesis of liquid photocrosslinkable poly(l-lactide) with terminal triacrylate

We synthesized a poly(l-lactide)-pentaerythritol triacrylate (PETA) polymer modified with acrylic trifunctional groups using a one-pot method based on ring-opening polymerization of l-lactide and PETA. We calculated the molecular weight and structure of PLA-PETA using gel permeation chromatography (GPC) and nuclear magnetic resonance (NMR) (1H, 13C, heteronuclear multiple bond correlation [HMBC]) spectroscopy. Photocrosslinking PLA-PETA using the Omnirad 1173 photoinitiator yielded a transparent sample with 91% crosslinkage. The crosslinked sample was analyzed using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and thermomechanical analysis (TMA) to determine its thermal properties and thermal expansion coefficient. In vitro cell toxicity tests showed an average cell viability >90%, indicating that the PLA-PETA polymer had good biocompatibility with cells after photocrosslinking.

Dual-Crosslinked Degradable Elastomeric Networks With Self-Healing Properties: Bringing Multi(catechol) Star-Block Copolymers into Play

In the biomedical field, degradable chemically crosslinked elastomers are interesting materials for tissue engineering applications, since they present rubber-like mechanical properties matching those of soft tissues and are able to preserve their three-dimensional (3D) structure over degradation. Their use in biomedical applications requires surgical handling and implantation that can be a source of accidental damages responsible for the loss of properties. Therefore, their inability to be healed after damage or breaking can be a major drawback. In this work, biodegradable dual-crosslinked networks that exhibit fast and efficient self-healing properties at 37 °C are designed. Self-healable dual-crosslinked (chemically and physically) elastomeric networks are prepared by two methods. The first approach is based on the mix of hydrophobic poly(ethylene glycol)-poly(lactic acid) (PEG-PLA) star-shaped copolymers functionalized with either catechol or methacrylate moieties. In the second approach, hydrophobic bifunctional PEG-PLA star-shaped copolymers with both catechol and methacrylate on their structure are used. In the two systems, the supramolecular network is responsible for the self-healing properties, thanks to the dynamic dissociation/reassociation of the numerous hydrogen bonds between the catechol groups, whereas the covalent network ensures mechanical properties similar to pure methacrylate networks. The self-healable materials display mechanical properties that are compatible with soft tissues and exhibit linear degradation because of the chemical cross-links. The performances of networks from mixed copolymers versus bifunctional copolymers are compared and demonstrate the superiority of the latter. The biocompatibility of the materials is also demonstrated, confirming the potential of these degradable and self-healable elastomeric networks to be used for the design of temporary medical devices.

Highly reinforced and degradable lignocellulose biocomposites by polymerization of new polyester oligomers

Unbleached wood fibers and nanofibers are environmentally friendly bio-based candidates for material production, in particular, as reinforcements in polymer matrix biocomposites due to their low density and potential as carbon sink during the materials production phase. However, producing high reinforcement content biocomposites with degradable or chemically recyclable matrices is troublesome. Here, we address this issue with a new concept for facile and scalable in-situ polymerization of polyester matrices based on functionally balanced oligomers in pre-formed lignocellulosic networks. The idea enabled us to create high reinforcement biocomposites with well-dispersed mechanically undamaged fibers or nanocellulose. These degradable biocomposites have much higher mechanical properties than analogs in the literature. Reinforcement geometry (fibers at 30 µm or fibrils at 10–1000 nm diameter) influenced the polymerization and degradation of the polyester matrix. Overall, this work opens up new pathways toward environmentally benign materials in the context of a circular bioeconomy.

Cellulose biocomposites from nanocellulose or plant fibers with polymer matrix are often not degradable and suffer from insufficient mechanical properties to replace established materials. Here, the authors demonstrate the fabrication of hydrolytically degradable polymers through in-situ polymerization of new functionally balanced oligomers within high-content lignocellulose reinforcement networks.

Crosslinking of Polylactide by High Energy Irradiation and Photo-Curing

Polylactide (PLA) is presently the most studied bioderived polymer because, in addition to its established position as a material for biomedical applications, it can replace mass production plastics from petroleum. However, some drawbacks of polylactide such as insufficient mechanical properties at a higher temperature and poor shape stability have to be overcome. One of the methods of mechanical and thermal properties modification is crosslinking which can be achieved by different approaches, both at the stage of PLA-based materials synthesis and by physical modification of neat polylactide. This review covers PLA crosslinking by applying different types of irradiation, i.e., high energy electron beam or gamma irradiation and UV light which enables curing at mild conditions. In the last section, selected examples of biomedical applications as well as applications for packaging and daily-use items are presented in order to visualize how a variety of materials can be obtained using specific methods.

Structural and Thermo-Mechanical Properties of Poly(ε-caprolactone) Modified by Various Peroxide Initiators

The modification of poly(ε-caprolactone) (PCL) was successfully conducted during reactive processing in the presence of dicumyl peroxide (DCP) or di-(2-tert-butyl-peroxyisopropyl)-benzene (BIB). The peroxide initiators were applied in the various amounts of 0.5 or 1.0 pbw (part by weight) into the PCL matrix. The effects of the initiator type and its concentration on the structure and mechanical and thermal properties of PCL were investigated. To achieve a detailed and proper explication of this phenomenon, the decomposition and melting temperatures of DCP and BIB initiators were measured by differential scanning calorimetry. The conjecture of the branching or cross-linking of PCL structure via used peroxides was studied by gel fraction content measurement. Modification in the presence of BIB in PCL was found to effectively increase gel fraction. The result showed that the cross-linking of PCL started at a low content of BIB, while PCL modified by high DCP content was only partially cross-linked or branched. PCL branching and cross-linking were found to have a significant impact on the mechanical properties of PCL. However, the effect of used initiators on poly(ε-caprolactone) properties strongly depended on their structure and content. The obtained results indicated that, for the modification towards cross-linking/branching of PCL structure by using organic peroxides, the best mechanical properties were achieved for PCL modified by 0.5 pbw BIB or 1.0 pbw DCP, while the PCL modified by 1.0 pbw BIB possessed poor mechanical properties, as it was related to over cross-linking.

Optimized Synthesis of Biodegradable Elastomer PEGylated Poly(glycerol sebacate) and Their Biomedical Application

Poly(glycerol sebacate) (PGS), a biodegradable elastomer, has been extensively explored in biomedical applications for its favorable mechanical properties and biocompatibility. Efforts have been made to fabricate multifunctional PGS copolymer in recent years, in particular PGS-co-PEG (poly(glycerol sebacate)-co-polyethylene glycol) polymers. However, rare research has been systematically conducted on the effect of reactant ratios on physicochemical properties and biocompatibility of PGS copolymer till now. In this study, a serial of PEGylated PGS (PEGS) with PEG content from 20% to 40% and carboxyl to hydroxyl from 0.67 to 2 were synthesized by thermal curing process. The effects of various PEGS on the mechanical strength and biological activity were further compared and optimized. The results showed that the PEGS elastomers around 20PEGS-1.0C/H and 40PEGS-1.5C/H exhibited the desirable hydrophilicity, degradation behaviors, mechanical properties and cell viability. Subsequently, the potential applications of the 20PEGS-1.0C/H and 40PEGS-1.5C/H in bone repair scaffold and vascular reconstruction were investigated and the results showed that 20PEGS-1.0C/H and 40PEGS-1.5C/H could significantly improve the mechanical strength for the calcium phosphate scaffolds and exhibited preferable molding capability for fabrication of the vascular substitute. These results confirmed that the optimized PEGS elastomers should be promising multifunctional substrates in biomedical applications.

Effect of Ethylenediaminetetraacetic Acid on Unsaturated Poly(Butylene Adipate-Co-Butylene Itaconate) Copolyester with Low-Melting Point and Controllable Hardness

A series of copolyesters, poly(butylene adipate-co-butylene itaconate) (PBABI), was synthesized using melt polycondensation from adipic acid (AA), itaconic acid (IA), 1,4-butanediol (1,4-BDO), and ethylenediaminetetraacetic acid (EDTA). ¹H-NMR, FT-IR, GPC, DSC, TGA, DMA, XRD, Shore D, and tensile test were used to systematically characterize the structural and composition/physical properties of the copolyesters. It was found that the melting point (T_m) and crystallization temperature (T_c) of the copolyesters were, respectively, between 21.1 to 57.5 °C and −6.7 to 29.5 °C. The glass transition (T_g) and the initial thermal decomposition (T_d-5%) temperatures of the PBABI copolyesters were observed to be between −53.6 to −55.8 °C and 313.6 and 342.1 °C at varying ratios of butylene adipate (BA) and butylene itaconate (IA), respectively. The XRD feature peak was identified at the 2θ values of 21.61°, 22.31°, and 23.96° for the crystal lattice of (110), (020), and (021), respectively. Interestingly, Shore D at various IA ratios had high values (between 51.3 to 62), which indicated that the PBABI had soft plastic properties. The Young’s modulus and elongation at break, at different IA concentrations, were measured to be at 0.77–128.65 MPa and 71.04–531.76%, respectively, which could be attributed to a close and compact three-dimensional network structure formed by EDTA as a crosslinking agent. There was a significant bell-shaped trend in a BA/BI ratio of 8/2, at different EDTA concentrations—the ∆H_m increased while the EDTA concentration increased from 0.001 to 0.05 mole% and then decreased at an EDTA ratio of 0.2 mole%. Since the PBABI copolymers have applications in the textile industry, these polymers have been adopted to reinforce 3D air-permeable polyester-based smart textile. This kind of composite not only possesses the advantage of lower weight and breathable properties for textiles, but also offers customizable, strong levels of hardness, after UV curing of the PBABI copolyesters, making its potential in vitro orthopedic support as the “plaster of the future”.

Reversible networks of degradable polyesters containing weak covalent bonds

The synthesis of reversible polyester networks based mainly on the Diels–Alder chemistry, alkene [2 + 2] cycloaddition or transesterification reactions and studies of their reversibility and its consequences are reviewed.

Current development in the formulations of non-injection administration of paclitaxel

Paclitaxel (PTX) belongs to a class of taxane anti-tumor drug used for the clinic treatment of breast cancer, ovarian cancer, non-small-cell lung cancer, and so on. PTX has poor water solubility and oral bioavailability. It is generally administered via intravenous (i.v.) infusion. Traditional PTX injectable preparations contain Cremophor-EL and ethanol to improve its solubility, which would result in adverse reactions like severe hypersensitivity, neutropenia, etc. Adverse reactions can be reduced only by complicated pretreatment with glucocorticoid and antihistamines drugs and followed by PTX slow infusion for three hours, which has brought significant inconvenience to the patients. Though, a new-generation PTX formulation, Abraxane, free of Cremophor-EL and ethanol, is still being administrated by frequent i.v. infusions and extremely expensive. Therefore, non-injection administration of PTX is urgently needed to avoid the side effects as well as reduce inconvenience to the patients. Recently, a variety of non-injection drug delivery systems (DDSs) of PTX have been developed. This review aims to discuss the progress of non-injectable administration systems of PTX, including oral administration systems, vaginal administration systems, implantable DDSs, transdermal DDSs and intranasal administration for the future study and clinical applications.