Controlled degradation of polyhydroxybutyrate via alcoholysis with ethylene glycol or glycerol

Enhancing the Potential of PHAs in Tissue Engineering Applications: A Review of Chemical Modification Methods

Polyhydroxyalkanoates (PHAs) are a family of polyesters produced by many microbial species. These naturally occurring polymers are widely used in tissue engineering because of their in vivo degradability and excellent biocompatibility. The best studied among them is poly(3-hydroxybutyrate) (PHB) and its copolymer with 3-hydroxyvaleric acid (PHBV). Despite their superior properties, PHB and PHBV suffer from high crystallinity, poor mechanical properties, a slow resorption rate, and inherent hydrophobicity. Not only are PHB and PHBV hydrophobic, but almost all members of the PHA family struggle because of this characteristic. One can overcome the limitations of microbial polyesters by modifying their bulk or surface chemical composition. Therefore, researchers have put much effort into developing methods for the chemical modification of PHAs. This paper explores a rarely addressed topic in review articles-chemical methods for modifying the structure of PHB and PHBV to enhance their suitability as biomaterials for tissue engineering applications. Different chemical strategies for improving the wettability and mechanical properties of PHA scaffolds are discussed in this review. The properties of PHAs that are important for their applications in tissue engineering are also discussed.

Synthesis and physical properties of polyhydroxyalkanoate (PHA)-based block copolymers: A review

Polyhydroxyalkanoates (PHAs) are a group of natural polyesters that are synthesised by microorganisms. In general, their thermoplasticity and (in some forms) their elasticity makes them attractive alternatives to petrochemical-derived polymers. However, the high crystallinity of some PHAs - such as poly(3-hydroxybutyrate) (P3HB) - results in brittleness and a narrow processing window for applications such as packaging. The production of copolymeric PHA materials is one approach to improving the mechanical and thermal properties of PHAs. Another solution is the manufacture of PHA-based block copolymers. The incorporation of different polymer and copolymer blocks coupled to PHA, and the resulting tailorable microstructure of these block copolymers, can result in a step-change improvement in PHA-based material properties. A range of production strategies for PHA-based block copolymers has been reported in the literature, including biological production and chemical synthesis. Biological production is typically less controllable, with products of a broad molecular weight and compositional distribution, unless finely controlled using genetically modified organisms. By contrast, chemical synthesis delivers relatively controllable block structures and narrowly defined compositions. This paper reviews current knowledge in the areas of the production and properties of PHA-based block copolymers, and highlights knowledge gaps and future potential areas of research.

PHB production by Bacillus megaterium strain MNSH1-9K-1 using low-cost media

Plastics are widely used for diverse applications due to their versatility. However, their negative impact on ecosystems is undeniable due to their long-term degradation. Thus, there is a rising need for developing eco-friendlier alternatives to substitute fossil-based plastics, like biopolymers. PHA are synthesized intracellularly by microorganisms under stressful conditions of growth and have similar characteristics to conventional polymers, like their melting point, transition temperatures, crystallinity, and flexibility. Although it is feasible to use biopolymers for diverse industrial applications, their elevated production cost due to the supplies needed for microbiological procedures and the low productivity yields obtained have been the main limiting factors for their commercial success. The present study assessed the ability of Bacillus megaterium strain MNSH1-9K-1 to produce biopolymers using low-cost media from different kinds of fruit-peel residues. The results show that MNSH1-9K-1 can produce up to 58 g/L of PHB when grown in a medium prepared from orange-peel residues. The data obtained provide information to enhance the scalability of these kinds of biotechnological processes.

Aminolysis of Poly-3-Hydroxybutyrate in N,N-Dimethylformamide and 1,4-Dioxane and Formation of Functionalized Oligomers

The degradation pattern of bacterial poly-3-hydroxybutyrate (PHB) in dimethylformamide (DMF) and dioxane solutions at 100 °C assisted by ethylenediamine, 1,4-diaminobutane and monoaminoethanol was studied. When diamines were introduced into the PHB solution in DMF in the amount of 1 mol of the reagent to 5 or 10 mol of PHB monomers, a rapid decrease in the molecular weight of the polymer was observed. The initial value of the weight average molecular weight (M_w) 840 kDa had decreased by 20-30 times within the first 10-20 min of the experiment, followed by its gradual decrease to several thousand Da. When a similar molar quantity of aminoethanol was added, the molecular weight decreased slower. PHB had been degrading much slower in the dioxane solution than in DMF. By varying the number of reagents, it was possible to reach stabilization of the M_w at 1000-3000 Da when using diamines and 8000-20,000 Da using aminoethanol. ¹H NMR analysis of the oligomers revealed of amino and amido groups forming in their structure. From the opposite end of the polymer chain, residues of 3-hydroxybutyric, crotonic and isocrotonic acids were formed during degradation. Differential scanning calorimetry indicated that after oligomerization there was a decrease in the melting point from 178 °C to 140-170 °C depending on the decrease in the molecular weight. The method proposed can be used for obtaining aminated PHB oligomers.

PLA/PHB-Based Materials Fully Biodegradable under Both Industrial and Home-Composting Conditions

In order to make bioplastics accessible for a wider spectrum of applications, ready-to-use plastic material formulations should be available with tailored properties. Ideally, these kinds of materials should also be “home-compostable” to simplify their organic recycling. Therefore, materials based on PLA (polylactid acid) and PHB (polyhydroxybutyrate) blends are presented which contain suitable additives, and some of them contain also thermoplastic starch as a filler, which decreases the price of the final compound. They are intended for various applications, as documented by products made out of them. The produced materials are fully biodegradable under industrial composting conditions. Surprisingly, some of the materials, even those which contain more PLA than PHB, are also fully biodegradable under home-composting conditions within a period of about six months. Experiments made under laboratory conditions were supported with data obtained from a kitchen waste pilot composter and from municipal composting plant experiments. Material properties, environmental conditions, and microbiology data were recorded during some of these experiments to document the biodegradation process and changes on the surface and inside the materials on a molecular level.

Detachment of bovine corneal endothelial cell sheets by cooling-induced surface hydration of poly[(R)-3-hydroxybutyrate]-based thermoresponsive copolymer coating

A smart surface was prepared by non-covalently coating of a thermoresponsive copolymer via a simple drop-casting method. The smart surface was conducive to cell culture, from which intact cell sheets could be effectively detached by cooling.

Effect of calcium stearate as a lubricant and catalyst on the thermal degradation of poly(3-hydroxybutyrate)

Poly(3-hydroxybutyrate) (PHB) is a promising substitute to petroleum-based polymers in packaging and biomedical applications provided that its melt processability and degradability are improved. A new method to control the properties of PHB by using cheap calcium stearate (CS) as a lubricant and decomposition catalyst in melt-mixed PHB-CS compounds was first used. CS is composed of a metallic cation, which promotes PHB degradation, and a hydrophobic anion that improves the compatibility with PHB and processability. An environmentally friendly melt mixing technique was employed to obtain the PHB-CS compounds. Incorporation of 0.5 or 5 wt% CS reduced the melt viscosity and molecular weight of PHB, decreased the melting temperature with up to 5 °C, the crystallization temperature with more than 25 °C, and the degradation temperature with 15 and 40 °C, respectively. In small amounts (0.05 wt%), CS improved the processability and mechanical properties of PHB. In higher amount (0.5 wt%), CS slightly improved the Young's modulus, reduced the tensile strength and enhanced degradation. A better control of thermal and mechanical properties of PHB is, thus, possible by using different CS amount and processing conditions. These results are relevant for PHB application in the context of the global transition to biodegradable packaging.

Properties and Degradation Performances of Biodegradable Poly(lactic acid)/Poly(3-hydroxybutyrate) Blends and Keratin Composites

From environmental aspects, the recovery of keratin waste is one of the important needs and therefore also one of the current topics of many research groups. Here, the keratin hydrolysate after basic hydrolysis was used as a filler in plasticized polylactic acid/poly(3-hydroxybutyrate) blend under loading in the range of 1–20 wt%. The composites were characterized by infrared spectroscopy, and the effect of keratin on changes in molar masses of matrices during processing was investigated using gel permeation chromatography (GPC). Thermal properties of the composites were investigated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The effect of keratin loading on the mechanical properties of composite was investigated by tensile test and dynamic mechanical thermal analysis. Hydrolytic degradation of matrices and composites was investigated by the determination of extractable product amounts, GPC, DSC and NMR. Finally, microbial growth and degradation were investigated. It was found that incorporation of keratin in plasticized PLA/PHB blend provides material with good thermal and mechanical properties and improved degradation under common environmental conditions, indicating its possible application in agriculture and/or packaging.

Biodegradable Polyurethanes Based on Castor Oil and Poly (3-hydroxybutyrate)

Biodegradable polyurethanes (PUs) were produced from castor oil (CO) and poly (3-hydroxybutyrate) diol (PHBD) using hexamethylene diisocyanate as a crosslinking agent. PHBDs of different molecular weights were synthesized through transesterification of bacterial PHB and ethylene glycol by changing the reaction time. The synthesized PHBDs were characterized in terms of Fourier transform infrared and proton nuclear magnetic resonance spectroscopy. A series of PUs at different NCO/OH and CO/PHBD ratios were prepared. The resulting CO/PHBD-based PUs were then characterized in terms of mechanical and thermal properties. Increasing PHBD content significantly increased the tensile strength of CO/PHBD-based PUs by 300% compared to neat CO-based PU. CO/PHBD-based PUs synthetized from short chain PHBD exhibited higher tensile strength compared to those produced from long chain PHBD. As revealed by scanning electron microscopy analysis, such improvement in stiffness of the resulting PUs is due to the good compatibility between CO and PHBD. Increasing PHBD content also increased the crystallinity of the resulting PUs. In addition, higher degradation rates were obtained for CO/PHBD-based PUs synthetized from long chain PHBD compared to neat CO PU and PUs produced from short chain PHBD.

Cascade strategies for the full valorisation of Garganega white grape pomace towards bioactive extracts and bio-based materials

Agro-waste reduction and reuse are among the current main social challenges. In this perspective, the present research was aimed at the complete valorisation of Garganega grape pomace by recovering bioactive phenol extracts and by testing the solid fibre extract residues in composite formulation for packaging applications. The pomace was derived from white wine production, therefore, respect to red pomace, it was promptly removed from must after pressing, and its exploitation can be particularly interesting and valuable as still rich in active compounds. Phenol extracts were obtained both via solvent-based and pressurised liquid extractions and their phytochemical compositions were compared in terms of total amount of phenols, flavonoids, flavanols, anthocyanins, hydroxycinnamic acids, and reducing sugars. Antioxidant activity and detailed phenol profiles were also achieved. The highest phenol yield was obtained via solvent-based extraction with 75% acetone (v/v), solid/liquid ratio 1:5, 2h incubation at 50°C (77.9 gGAeq/kgDW). The fibrous solid residue of the extraction was characterized via thermogravimetric analysis and used for composite preparation by melt mixing with the renewable and biodegradable PHBV polymer through a green approach (solvent-less process). The composites resulted thermally stable at high temperatures, showing initial degradation processes only at temperatures higher than 250°C. Differential scanning calorimetry analyses were carried out to study melting and crystallization phenomena, while mechanical properties were investigated by tensile tests. The materials finally showed properties similar to those of the matrix. The bio-composites can be considered as an alternative to plain PHBV, since they are less expensive and eco-friendlier thanks to a reduced polymeric content, and they could represent a suitable way for full agro-waste exploitation.

Mitigating the Impact of Cellulose Particles on the Performance of Biopolyester-Based Composites by Gas-Phase Esterification

Materials that are both biodegradable and bio-sourced are becoming serious candidates for substituting traditional petro-sourced plastics that accumulate in natural systems. New biocomposites have been produced by melt extrusion, using bacterial polyester (poly(3-hydroxybutyrate-co-3-hydroxyvalerate)) as a matrix and cellulose particles as fillers. In this study, gas-phase esterified cellulose particles, with palmitoyl chloride, were used to improve filler-matrix compatibility and reduce moisture sensitivity. Structural analysis demonstrated that intrinsic properties of the polymer matrix (crystallinity, and molecular weight) were not more significantly affected by the incorporation of cellulose, either virgin or grafted. Only a little decrease in matrix thermal stability was noticed, this being limited by cellulose grafting. Gas-phase esterification of cellulose improved the filler's dispersion state and filler/matrix interfacial adhesion, as shown by SEM cross-section observations, and limiting the degradation of tensile properties (stress and strain at break). Water vapor permeability, moisture, and liquid water uptake of biocomposites were increased compared to the neat matrix. The increase in thermodynamic parameters was limited in the case of grafted cellulose, principally ascribed to their increased hydrophobicity. However, no significant effect of grafting was noticed regarding diffusion parameters.

Cellulose defibrillation and functionalization by plasma in liquid treatment

Submerged liquid plasma (SLP) is a new and promising method to modify powder materials. Up to now, this technique has been mostly applied to carbonaceous materials, however, SLP shows great potential as a low-cost and environmental-friendly method to modify cellulose. In this work we demonstrate the modification of microcrystalline cellulose (MCC) by applying the SLP combined with ultrasonication treatments. The plasma generated either in an inert (argon) or reactive (argon: oxygen or argon:nitrogen) gas was used in MCC dispersions in water or acetonitrile:water mixtures. An enhanced defibrillation of MCC has been observed following the application of SLP. Furthermore, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy have been applied to investigate the surface functionalization of MCC with oxygen or nitrogen moieties. Depending on the plasma treatment applied, poly (3-hydroxybutyrate) composites fabricated with the plasma modified cellulose fibers showed better thermal stability and mechanical properties than pristine PHB. This submerged liquid plasma processing method offers a unique approach for the activation of cellulose for defibrillation and functionalization, aiming towards an improved reinforcing ability of biopolymers.

The heterogeneous selective reduction of PHB as a useful method for preparation of oligodiols and surface modification

A selective heterogeneous reduction of natural PHB with lithium borohydride as a reducing agent has been described.

Complex study of the physical properties of a poly(lactic acid)/poly(3-hydroxybutyrate) blend and its carbon black composite during various outdoor and laboratory ageing conditions

Carbon black improved retention of mechanical properties of compostable PLA/PHB – based foil during ageing making it suitable as mulching material.

Biocompatible terpolyesters containing polyhydroxyalkanoate and sebacic acid structural segments – synthesis and characterization

A convenient synthetic route of poly(3HB-co-3HH-co-SEB) terpolyesters has been reported.

Selective reduction of PHA biopolyesters and their synthetic analogues to corresponding PHA oligodiols proved by structural studies

A highly selective method is described for controlling the degradation of polyhydroxyalkanoates, PHA, via a reduction reaction that uses lithium borohydride. Using this method, oligo(hydroxyalkanoate)diols derived from a poly(3-hydroxybutyrate-co-4-hydroxybutyrate) biopolyester [poly(3HB-co-4HB)] and from synthetic atactic poly[(R,S)-3-hydroxybutyrate] (a-PHB) were obtained. The structural characterization of the oligo(hydroxyalkanoate)diols was conducted using NMR and ESI-mass spectrometry analyses, which confirmed that oligomers that were terminated by two hydroxyl end groups were formed. The reduction of the ester groups occurred in a statistical way regardless of the chemical structure of the comonomer units or of the microstructure of the polyester chain. The presented method can be used to synthesize various PHA oligodiols that are potentially useful in the further synthesis of tailor-made biodegradable materials.

Redução da massa molecular e funcionalização do poli(3-hidroxibutirato-co-3-hidroxivalerato) (PHBHV) via hidrólise ácida e transesterificação com glicóis

Neste trabalho foi realizado um estudo da redução da massa molecular do poli (3-hidroxibutirato-co-3-hidroxivalerato) (PHBHV) usando duas metodologias: hidrólise ácida com ácido clorídrico e transesterificação com etilenoglicol e hexilenoglicol. Foram investigados os parâmetros do processo: tempo, temperatura e concentração de catalisadores. Todas as metodologias estudadas geraram biopolímeros com massa molecular reduzida e funcionalizados com grupos hidroxila e carboxila terminais. Foram comparadas as metodologias estudadas, onde foi possível determinar a metodologia mais eficaz na funcionalização do PHBHV. A redução da massa molecular, associada a estratégias de funcionalização, é extremamente útil para promover alterações na taxa de degradação do PHBHV, ampliando assim as suas aplicações como biomateriais.

Síntese e caracterização do copolímero poli(3-hidroxibutirato-co-ε-caprolactona) a partir de poli (3-hidroxibutirato) e poli(ε-caprolactona)

O copolímero poli(3-hidroxibutirato-co-ε-caprolactona) foi sintetizado por transesterificação, a partir dos homopolímeros PHB e PCL, usando acetilacetonato de zircônio(IV), como catalisador, nas concentrações de 20, 50 e 80% de PHB em massa. Os copolímeros foram caracterizados por GPC, métodos espectroscópicos (RMN-¹H, RMN-13C e IV-FT) e métodos térmicos (TG e DSC). A rota de síntese utilizada mostrou-se eficaz na síntese dos copolímeros P(HB-co-CL), os quais mostraram diminuição das cadeias poliméricas, apresentando Mw inferior a 24.000Daltons. Todos os copolímeros obtidos são termicamente mais estáveis que o PHB e com menor cristalinidade que os homopolímeros de partida. Esses materiais são bons candidatos para utilização como biomateriais em matrizes para liberação controlada de fármacos ou mesmo como compatibilizante em blendas PHB/PCL.

Estudo cinético da redução da massa molar do poli(3-hidroxibutirato-co-3-hidroxivalerato) (PHBHV)

Neste trabalho foi realizado um estudo cinético da redução da massa molar do poli(3-hidroxibutirato-co-3-hidroxivalerato) (PHBHV) em três diferentes metodologias: hidrólise ácida com ácido clorídrico, transesterificação com hexilenoglicol ou etilenoglicol e redução na presença do borohidreto de sódio (NaBH4). Foram investigados os parâmetros de processo: tempo, temperatura, tipo e concentração de catalisadores. Os resultados de redução de massa molar foram analisados pelo modelo cinético de ordem n. A redução com NaBH4 foi mais eficiente do ponto de vista cinético, tendo em vista os parâmetros: constante de reação k e energia de ativação (Ea), uma vez que os valores observados foram significativamente maiores em comparação com os calculados para os demais métodos.