Bacterial Polyester Research Articles

Biodegradable bacterial polyester, poly (hydroxybutyrate-co-hydroxyvalerate) copolymer, produced by moderately halophile bacterium Halomonas sp. PR-1 isolated from marine environment

PHA (polyhydroxyalkanoate) production by halophiles has attracted much attention in recent years. It was certified that the halophile bacteria Halomonas sp. PR-1 isolated from saltern soil synthesised poly (hydroxybutyrate-co-hydroxyvalerate) (PHBV) intracellularly from simple carbon substrate by means of FT-IR spectra analysis. The carbon and nitrogen source suitable for PHA production were selected as glucose and NH4Cl, respectively. The optimal ratio of glucose to NH4Cl was 20, when PHA content was 2.85 g/L and PHA yield was 50.85%. The optimal NaCl concentration for PHA biosynthesis was 30 g/L, when PHA yield was 63.3%. The halophile bacterium Halomonas sp. PR-1 was considered as a promising candidate for PHA production.

Natural and Synthetic Polymers for Biomedical and Environmental Applications.

Natural and synthetic polymers are a versatile platform for developing biomaterials in the biomedical and environmental fields. Natural polymers are organic compounds that are found in nature. The most common natural polymers include polysaccharides, such as alginate, hyaluronic acid, and starch, proteins, e.g., collagen, silk, and fibrin, and bacterial polyesters. Natural polymers have already been applied in numerous sectors, such as carriers for drug delivery, tissue engineering, stem cell morphogenesis, wound healing, regenerative medicine, food packaging, etc. Various synthetic polymers, including poly(lactic acid), poly(acrylic acid), poly(vinyl alcohol), polyethylene glycol, etc., are biocompatible and biodegradable; therefore, they are studied and applied in controlled drug release systems, nano-carriers, tissue engineering, dispersion of bacterial biofilms, gene delivery systems, bio-ink in 3D-printing, textiles in medicine, agriculture, heavy metals removal, and food packaging. In the following review, recent advancements in polymer chemistry, which enable the imparting of specific biomedical functions of polymers, will be discussed in detail, including antiviral, anticancer, and antimicrobial activities. This work contains the authors' experimental contributions to biomedical and environmental polymer applications. This review is a vast overview of natural and synthetic polymers used in biomedical and environmental fields, polymer synthesis, and isolation methods, critically assessessing their advantages, limitations, and prospects.

Biosynthesis, characterization, and biodegradation of elastomeric polyhydroxyalkanoates consisting of α-dimethylated monomer units

Although poly[(R)-3-hydroxybutyrate)] [P(3HB)] is a microbial polyester that can be used as a sustainable and biodegradable plastic, its use has been limited because of poor material properties due to its stiff and brittle nature. To overcome this issue, it is critical to diversify the monomeric composition of 3HB-based polymers through the introduction of new repeating units that can improve material properties. In this study, a new 3HB-based copolymer was biosynthesized with 3-hydroxypivalate (3HPi) units containing signature dimethyl groups on the α-carbon. To biosynthesize poly[(R)-3-hydroxybutyrate-co-3-hydroxypivalate] {P(3HB-co-3HPi)}, recombinant Escherichia coli LSBJ was cultured with glucose and 3-hydroxypivalic acid (3HPiA) as the carbon source and 3HPi precursor, respectively. This resulted in the successful synthesis of P(3HB-co-3HPi) copolymers with a 3HPi fraction in the range of 2–50 mol% by adjusting the precursor concentration to the microbial catalyst. Interestingly, the biosynthesized polymers exhibited an increasing molecular weight with increasing 3HPi fractions, up to 424 × 104 in weight average molecular weight. The thermal properties of P(3HB-co-3HPi) were similar to those of conventional 3HB-based copolymers, but the copolymers displayed extraordinary mechanical performance compared to other 3HB homo- and copolymers. P(3HB-co-32 mol% 3HPi) displayed an elongation at break of 1919 %, the highest value ever reported for bacterial polyesters. Given its high flexibility, P(3HB-co-3HPi) copolymers exhibited reversible deformation during tension testing, unlike other 3HB-based copolymers. Furthermore, P(3HB-co-3HPi) copolymers and monomeric 3HPiA were both biodegradable. This study has demonstrated that the incorporation of 3HPi monomers into 3HB-based copolymers results in superior mechanical properties and has introduced a promising new class of materials for use as sustainable and biodegradable plastics.

PRODUCTION AND CHARACTERIZATION OF PHB, PHBV ELECTROSPUN FIBERS AND THEIR BLENDS

Polhydroxbutyrates (PHBs) are well-known bio-based and biodegradable bacterial polyesters. In this study, the effects of polymer type, solution concentration and feeding rate on the electrospinnability of Poly(3-hydroxbutyrate) (PHB) and Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) PHBV nanofibers were investigated. First, PHB, PHBV and PHB/PHBV solutions with different polymer concentrations ranging between 5-11% wt. were prepared and characterized in terms of viscosity. Afterwards, electrospinning was performed and ultrafine fibers were produced. The surface morphology and the fiber diameters of the samples were investigated by scanning electron microscopy (SEM) analyses. Pore sizes of the samples were also calculated. In order to understand the wettability of the samples, contact angle measurements were conducted. The thermal properties and the crystallinity of the samples were investigated differential scanning calorimetry (DSC) analyses. The solution viscosities increased dramatically above %9 wt. of polymer concentration. SEM images revealed that decreasing feeding rate and increasing solution concentration resulted in fewer bead formation. On the other hand, fibers with diameters from 1.2 to 5.4 μm were produced with the increasing solution concentration and increasing voltage. All samples showed contact angle values above 90° indicating that they are hydrophobic. The PHB/PHBV blend surface showed the highest contact angle. DSC analyses showed that PHBV surface had significantly lower crystallization degree than PHB surface produced at the same concentration. It can be concluded that PHB fibers can be successfully produced by electrospinning.

A (S)-3-Hydroxybutyrate Dehydrogenase Belonging to the 3-Hydroxyacyl-CoA Dehydrogenase Family Facilitates Hydroxyacid Degradation in Anaerobic Bacteria.

Ketone bodies, including acetoacetate, 3-hydroxybutyrate, and acetone, are produced in the liver of animals during glucose starvation. Enzymes for the metabolism of (R)-3-hydroxybutyrate have been extensively studied, but little is known about the metabolism of its enantiomer (S)-3-hydroxybutyrate. Here, we report the characterization of a novel pathway for the degradation of (S)-3-hydroxybutyrate in anaerobic bacteria. We identify and characterize a stereospecific (S)-3-hydroxylbutyrate dehydrogenase (3SHBDH) from Desulfotomaculum ruminis, which catalyzes the reversible NAD(P)H-dependent reduction of acetoacetate to form (S)-3-hydroxybutyrate. 3SHBDH also catalyzes oxidation of d-threonine (2R, 3S) and l-allo-threonine (2S, 3S), consistent with its specificity for β-(3S)-hydroxy acids. Isothermal calorimetry experiments support a sequential mechanism involving binding of NADH prior to (S)-3-hydroxybutyrate. Homologs of 3SHBDH are present in anaerobic fermenting and sulfite-reducing bacteria, and experiments with Clostridium pasteurianum showed that 3SHBDH, acetate CoA-transferase (YdiF), and (S)-3-hydroxybutyryl-CoA dehydrogenase (Hbd) are involved together in the degradation of (S)-3-hydroxybutyrate as a carbon and energy source for growth. (S)-3-hydroxybutyrate is a human metabolic marker and a chiral precursor for chemical synthesis, suggesting potential applications of 3SHBDH in diagnostics or the chemicals industry. IMPORTANCE (R)-3-hydroxybutyrate is well studied as a component of ketone bodies produced by the liver and of bacterial polyesters. However, the biochemistry of its enantiomer (S)-3-hydroxybutyrate is poorly understood. This study describes the identification and characterization of a stereospecific (S)-3-hydroxylbutyrate dehydrogenase and its function in a metabolic pathway for the degradation of (S)-3-hydroxybutyrate as a carbon and energy source in anaerobic bacteria. (S)-3-hydroxybutyrate is a mammalian metabolic marker and a precursor for chemical synthesis and bioplastics, suggesting potential applications of these enzymes in diagnostics and biotechnology.

Evaluation of triacetin on mechanical strength and free surface energy of PHBHHx: The prevention of intra-abdominal adhesion

Several polymers are used for the preparation of biomaterials as membranes and films for tissue engineering applications. The most common plasticizer is PEG to obtain polymer-based biomaterials. On the other hand, triacetin is a non-toxic, FDA-approved plasticizer mostly used in the food industry. In this study, we used triacetin as a plasticizer to obtain hydrophobic membranes for the prevention of intra-abdominal adhesion. We selected a well-known polymer named PHBHHx which is a bacterial polyester generally used as supporting material for cell attachments in regenerative tissue applications. We evaluated the triacetin as a plasticizer and its effect on mechanical, thermal, surface area, pore size, and surface energy. The hydrophobic/hydrophilic contrast of a biomaterial surface determines the biological response. Surface hydrophobicity is critical for the cellular response. The contact angle tests of PHBHHx revealed that the hydrophilicity of the membrane was decreased following triacetin blending. Modification of the PHBHHx membrane by blending with triacetin caused a significant decrease in cell adhesion. The cell attachment rates of PHBHHx membranes were as 95 ± 5% on the first day, 34.5 ± 0.9% on third day, and 23 ± 1.5% on the fifth day, respectively. The rates of cell attachments on PHBHHx/triacetin membranes were determined as 79 ± 2.5% for the first day, 33 ± 2.7% for the third day, and 13 ± 2.1% for the fifth day, respectively. Besides, triacetin blending decreased the surface area from 38.790 to 32.379 m2/g. The elongation at breaks was observed as 128% for PHBHHx and 171% for PHBHHx/triacetin. Graphical abstract [Formula: see text]

Synthesis and properties of biobased polyurethane tissue adhesives from bacterial polyester

Tissue adhesives provide an alternative method for painless wound closure compared to sutures and clips. However, the ideal tissue adhesive does not exist yet. Biocompatibility, short curing time, limited exothermic reaction and tailored properties to the final tissue are for instance key criteria to answer. In this study, biobased two-component polyurethane (PU) tissue adhesives were developed. They were composed of NCO-terminated prepolymers and a chain extender. The renewable prepolymers were synthesized from biopolyesters (oligomers obtained from poly(3-hydroxybutyrate)) and biobased or potentially biobased diisocyanates. Prepolymers chemical and physico-chemical properties were assessed, as well as their in vitro cytocompatibility. Then, PU bi-component adhesives bonding strength on muscle tissues and curing times were evaluated. Adhesion strength of 2–11 kPa were obtained, which are in the range of conventional fibrin adhesives. Curing times were in the range but slightly lower than other NCO-terminated PU adhesives found in the literature. Finally, cured prepolymer-chain extender systems were analyzed and exhibited a good in vitro cytocompatibility and low in vitro hydrolytic degradation, associated with a large range of physico-chemical and mechanical properties from hard and brittle to elastomeric behavior.

Surface Modification of PHBV Fibrous Scaffold via Lithium Borohydride Reduction.

In this study, lithium borohydride (LiBH4) reduction was used to modify the surface chemistry of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) fibers. Although the most common reaction employed in the surface treatment of polyester materials is hydrolysis, it is not suitable for fiber modification of bacterial polyesters, which are highly resistant to this type of reaction. The use of LiBH4 allowed the formation of surface hydroxyl groups under very mild conditions, which was crucial for maintaining the fibers' integrity. The presence of these groups resulted in a noticeable improvement in the surface hydrophilicity of PHBV, as revealed by contact angle measurements. After the treatment with a LiBH4 solution, the electrospun PHBV fibrous mat had a significantly greater number of viable osteoblast-like cells (SaOS-2 cell line) than the untreated mat. Moreover, the results of the cell proliferation measurements correlated well with the observed cell morphology. The most flattened SaOS-2 cells were found on the surface that supported the best cell attachment. Most importantly, the results of our study indicated that the degree of surface modification could be controlled by changing the degradation time and concentration of the borohydride solution. This was of great importance since it allowed optimization of the surface properties to achieve the highest cell-proliferation capacity.

Mechanical, thermal and hydrodegradation behavior of poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) composites with agricultural fibers as reinforcing fillers

Fully biodegradable polymer composites based on the bacterial polyester, poly (3-hydroxybutyrate-co-3-hydroxyvalerate), (PHBV), were produced using injection molding. Agricultural reinforcements in the form of nettle fibers (stalks and leaves), pine cones flour, and walnut shells flour were used in the amount of 15 wt%. The impact of natural additives on mechanical properties (tensile, flexural, and impact) at three different temperatures (−24, 23, and 80 °C) has been studied. For all the tested composites, Young's modulus was improved by ~20% with a slight decrease in the tensile strength. The mechanical properties correspond well with the DSC results. The values of the degree of crystallinity for the composites are higher or equal to those for neat PHBV, only the composites with stalk nettle fibers were characterized by a reduced degree of crystallinity. Research has shown that composites have higher stiffness stability at elevated temperatures.Furthermore, the materials were subjected to hydrolytic degradation at 38 °C to determine the synergistic effect between water uptake and water temperature on tensile and flexural properties. The work reveals that the addition of natural fillers increases the possibility of water absorption and decreases the value of the tested properties by ~40% after two weeks of water incubation.Recent work has shown that it is possible to produce fully biodegradable polymer composites using agricultural wastes. The addition of fibers not only reduces the price of materials but increases the stiffness of the polymer whilst controlling the biodegradation process. Moreover, in this approach, waste materials will be able to act as an efficient filler, what will lead to their effective management.

Synthesis of Biobased and Hybrid Polyurethane Xerogels from Bacterial Polyester for Potential Biomedical Applications.

Organic–inorganic xerogel networks were synthesized from bacterial poly (3-hydroxybutyrate) (PHB) for potential biomedical applications. Since silane-based networks usually demonstrate increased biocompatibility and mechanical properties, siloxane groups have been added onto polyurethane (PU) architectures. In this work, a diol oligomer (oligoPHB-diol) was first prepared from bacterial poly(3-hydroxybutyrate) (PHB) with an environmentally friendly method. Then, hexamethylene diisocyanate or biobased dimeryl diisocyanate was used as diisocyanate to react with the short oligoPHB-diol for the synthesis of different NCO-terminated PU systems in a bulk process and without catalyst. Various PU systems containing increasing NCO/OH molar ratios were prepared. Siloxane precursors were then obtained after reaction of the NCO-terminated PUs with (3-aminopropyl)triethoxysilane, resulting in silane-terminated polymers. These structures were confirmed by different analytical techniques. Finally, four series of xerogels were prepared via a sol–gel process from the siloxane precursors, and their properties were evaluated depending on varying parameters such as the inorganic network crosslinking density. The final xerogels exhibited adequate properties in connection with biomedical applications such as a high in vitro degradation up to 15 wt% after 12 weeks.

Green and controlled synthesis of short diol oligomers from polyhydroxyalkanoate to develop fully biobased thermoplastics

Polyhydroxyalkanoates (PHAs) are a biodegradable and biobased family of bacterial polyesters with different architectures. Among them, the poly-3-hydroxybutyrate (PHB) is one of the most conventional and easily obtained by biotechnologies from different bioresources. However, this thermoplastic polyester known since more than 50 years presents until now only very limited applications due to a certain number of drawbacks such as poor mechanical properties, high degree of crystallinity, and low thermal stability. One major way to valorize this bacterial polyester could be to develop controlled building blocks for the synthesis of new generation of biobased polymers. In this way, short linear PHB diols oligomers (oligoPHB-diol) with controlled low molar masses have been successfully obtained from a new synthetic pathway using reactive solvents in a green way. Different transesterification reaction parameters were investigated to control the PHB molar mass decrease, such as the catalyst and reactive solvent contents, the temperature and the reactive solvent type. Obtained oligomers were then analyzed in detail and further incorporated into fully biobased thermoplastics. In this work, thermoplastic polyurethanes (TPU) were elaborated as an example of second-generation biobased thermoplastics. To obtain TPUs with high biobased content, a dimer fatty acid-based diisocyanate, the dimeryl diisocyanate, and another biobased diol, the isosorbide, were used. Two series of TPUs were prepared with (i) varying the isosorbide/oligoPHB-diol ratio, and (ii) different oligoPHB-diol molar mass. Materials were fully evaluated for their structural, thermal and mechanical properties, as well as their crystalline behavior. A large range of properties were obtained for these innovative thermoplastics which could open a large range of applications with a green and sustainable approach.

Superior thermal stability and fast crystallization behavior of a novel, biodegradable \u03b1-methylated bacterial polyester

Given their ubiquity in modern society, the development of biodegradable and renewably sourced plastics is essential for the creation of an environmentally sustainable society. One of the drawbacks for currently available biodegradable plastics such as poly(l-lactic acid) (PLLA) and polyhydroxyalkanoates (PHAs) is that it is difficult to simultaneously achieve mechanical flexibility and certain crystallization behavior in these materials, which limits their use as replacements for established petroleum-based plastics such as isotactic polypropylene (iPP). Here, we report the synthesis and characterization of a new biodegradable plastic, poly(3-hydroxy-2-methylbutyrate) [P(3H2MB)], which is a member of the bacterial PHA family whose members include an α-methylated monomer unit. Biosynthesis of P(3H2MB) was achieved using recombinant Escherichiacoli expressing an engineered pathway. Biosynthesized P(3H2MB) exhibited the highest melting temperature (197 °C) among the biosynthesized PHAs and improved thermal resistance. It also exhibited improved crystallization behavior and mechanical flexibility nearly equal to those of iPP. The primary nucleation rate of P(3H2MB) was faster than that of P(3HB), and the spherulite morphology of P(3H2MB) was much finer than that of P(3HB). This crystal morphology may result in more rapid crystallization behavior, increased transparency, and enhanced mechanical properties. The superior physical properties of P(3H2MB) have the potential to open new avenues for the production of high-performance biodegradable plastics for replacing petroleum-based bulk commodity plastics.

Valorization of agro-wastes for the biosynthesis and characterization of polyhydroxybutyrate by Bacillus sp. isolated from rice bran dumping yard.

Investigations have been made to determine the usage of inexpensive agro-waste products as an alternative carbon source for the production of degradable bacterial polyester. Among 33 bacterial isolates, a gram-positive bacterium PPECLRB-16 isolated from rice bran dumping yard was found to accumulate a relatively higher quantity of PHB and identified as Bacillus sp. through 16S rRNA gene sequence analysis. The higher PHB producing bacterial isolate was grown with different inexpensive agro-wastes to determine the suitable carbon source for its growth and PHB production. The one-factor-at-a-time approach comparatively enhanced PHB yield (5.64g/L) when grown for 48h with 1.5% (w/v) of defatted oil cake at a pH of 7.0. The bacterially accumulated PHB was isolated from the cells, purified, and characterized using solid-state 13C NMR, FT-IR, Powder XRD, TGA, GPC, Tensile and HR-SEM analyses. The hydrophobicity and printing accessibility of recovered PHB were demonstrated using contact angle measurement by coating on different surfaces. The results obtained in the present investigation have thrown light on the potential usage of agro-waste by-products, mainly oil cake, as an appropriate carbon source for the commercial production of PHB by Bacillus sp. in a cost-effective way.

Biodegradation of Hemicellulose-Cellulose-Starch-Based Bioplastics and Microbial Polyesters

The volume of discarded solid wastes, especially plastic, which accumulates in large quantities in different environments, has substantially increased. Population growth and the consumption pattern of societies associated with unsustainable production routes have caused the pollution level to increase. Therefore, the development of materials that help mitigate the impacts of plastics is fundamental. However, bioplastics can result in a misunderstanding about their properties and environmental impacts, as well as incorrect management of their final disposition, from misidentifications and classifications. This chapter addresses the aspects and factors surrounding the biodegradation of bioplastics from natural (plant biomass (starch, lignin, cellulose, hemicellulose, and starch) and bacterial polyester polymers. Therefore, the biodegradation of bioplastics is a factor that must be studied, because due to the increase in the production of different bioplastics, they may present differences in the decomposition rates.

Synthesis of poly-3-hydroxybutyrate (PHB) by Bacillus cereus using grape residues as sole carbon source

ABSTRACT Polyhydroxyalkanoates (PHA) are bacterial polyesters that have been described as one of the most promising sustainable alternatives to petroleum-based plastics. A major challenge for PHA to become market-competitive, is to decrease the associated production costs. According to several studies, about 20–50% of the total production costs is attributed to the raw materials and therefore, the use of different types of organic residues have been evaluated as substrates for the microorganisms. In this study, the strain Bacillus cereus ATCC 14579 was cultivated in shake flasks using a defined medium and pretreated grape peel as sole carbon source. The pretreatment of the grape peels was performed by a hydrolysis step with diluted sulfuric acid yielding up to 52.9% (w/w) of total sugars from the dry residue. PHB extraction using dry and wet biomass was performed and evaluated showing up to 3.7-higher yields when using the latter. PHB accumulation of 18.79% (w/w), corresponding to 0.53 gPHB/L, was achieved using the grape peel hydrolyzate at 72 h of cultivation. Eventually, the possibility of producing PHB by the use of non-treated lignocellulosic biomass (LCB) with high contents of fermentable sugars is presented as an interesting alternative to be optimized and projected for sustainable bioplastic production processes.

The effect of wet spinning conditions on the structure and properties of poly-4-hydroxybutyrate fibers.

Polyhydroxyalkanoates (PHAs), also known as bacterial polyesters, are considered novel polymers for fabricating biomedical products, such as sutures and hernia meshes, because of their biocompatibility and slow biodegradability. Poly-4-hydroxybutyrate (P4HB) is a commonly used PHA that was explored in this study as an absorbable biomaterial for several medical applications, including controlled drug delivery. Currently, P4HB is melt spun and drawn into filaments at high processing temperatures (~200°C), precluding the incorporation of thermally sensitive drugs within the polymer during melt spinning. Post-spinning drug incorporation can potentially cause nonuniform drug absorption that leads to an uneven release profile. This raises the need for a low temperature spinning process for these polymers. Until now, there has been no defined procedure to produce P4HB fibers through a low temperature solution spinning process. This study focuses on determining suitable wet spinning conditions to form continuous P4HB fibers. After several preliminary tests, it was found that a chloroform-based spin dope with 10-15% polymer concentration facilitated the extrusion of continuous stretchable fibers into a coagulation bath containing reagent alcohol. Subsequently, several P4HB fibers were spun with various spin dope concentrations, coagulation bath temperatures, and spin draw ratios to assess their effect on fiber structure and properties.

Bacterial Polyesters: Microbial Polyhydroxyalkanoates and Nonnatural Polyesters (Adv. Mater. 35/2020)

Polyhydroxyalkanoates (PHAs) are a family of polyesters synthesized and intracellularly accumulated as distinct granules in many microorganisms. PHAs are promising materials to substitute the petroleum-based plastics currently in use. In article 1907138, Sang Yup Lee and co-workers provide a comprehensive overview of PHAs, including their biosynthesis, overall production process, and applications.

Nanocomposites from functionalized bacterial cellulose and poly(3-hydroxybutyrate-co-3-hydroxyvalerate)

Bacterial cellulose (BC) sponges are valuable materials for tissue engineering and regenerative medicine due to their biocompatibility and nano-sized fibrous network with interconnected open porosity. However, their instability in physiological environment and poor mechanical properties are the main issues that need to be solved in order to obtain appropriate three-dimensional scaffolds for tissue formation. In this work, a bacterial polyester, poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and a simple impregnation method were used to improve the properties of BC sponges for biomedical application. Highly hydrophilic BC was surface functionalized by amination (BCA) to improve its affinity to PHBV. PHBV uptake in BC and BCA sponges depended on the PHBV concentration and was confirmed by Fourier transform infrared spectroscopy and by the increase in density after impregnation. SEM investigation showed that PHBV was deposited on the BC nanofibrous network, in some conditions forming a tridimensional honeycomb ordered structure with uniform micrometer pores. Thermogravimetric and kinetic analyses showed a delay in the thermal degradation for the BCA nanocomposites sponges compared to the BC ones and an increase in the activation energy of degradation compared to neat PHBV. Better compression strength was obtained for BCA/PHBV nanocomposite sponges due to the increased interactions between the polymer and the aminated cellulose substrate. Swelling tests showed that BC and BCA sponges did not resist and completely disintegrated in 90 min of incubation in phosphate-buffered saline, but a good stability in this simulated physiological environment was obtained after impregnation with PHBV. The swelling degree varied between 1200% and 2400% for BC/PHBV and between 700% and 1200% for BCA/PHBV sponges, which are high enough to allow the diffusion of water and the transport of nutrients. Therefore, these easily obtained BC/PHBV and BCA/PHBV nanocomposite sponges, with improved properties, could be a promising option for tissue engineering scaffolds.

Manufacturing and Properties of Binary Blend from Bacterial Polyester Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) and Poly(caprolactone) with Improved Toughness.

Polyhydroxyalkanoates (PHAs) represent a promising group of bacterial polyesters for new applications. Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) is a very promising bacterial polyester with potential uses in the packaging industry; nevertheless, as with many (almost all) bacterial polyesters, PHBH undergoes secondary crystallization (aging) which leads to an embrittlement. To overcome or minimize this, in the present work a flexible petroleum-derived polyester, namely poly(ε-caprolactone), was used to obtain PHBH/PCL blends with different compositions (from 0 to 40 PCL wt %) using extrusion followed by injection moulding. The thermal analysis of the binary blends was studied by means of differential scanning calorimetry (DSC) and thermogravimetry (TGA). Both TGA and DSC revealed immiscibility between PHBH and PCL. Mechanical dynamic thermal analysis (DMTA) allowed a precise determination of the glass transition temperatures (Tg) as a function of the blend composition. By means of field emission scanning electron microscopy (FESEM), an internal structure formed by two phases was observed, with a PHBH-rich matrix phase and a finely dispersed PCL-rich phase. These results confirmed the immiscibility between these two biopolymers. However, the mechanical properties obtained through tensile and Charpy tests, indicated that the addition of PCL to PHBH considerably improved toughness. PHBH/PCL blends containing 40 PCL wt % offered an impact resistance double that of neat PHBH. PCL addition also contributed to a decrease in brittleness and an improvement in toughness and some other ductile properties. As expected, an increase in ductile properties resulted in a decrease in some mechanical resistant properties, e.g., the modulus and the strength (in tensile and flexural conditions) decreased with increasing wt % PCL in PHBH/PCL blends.

Development and Characterization of Sustainable Composites from Bacterial Polyester Poly(3-Hydroxybutyrate-co-3-hydroxyhexanoate) and Almond Shell Flour by Reactive Extrusion with Oligomers of Lactic Acid.

Eco-efficient Wood Plastic Composites (WPCs) have been obtained using poly(hydroxybutyrate-co-hexanoate) (PHBH) as the polymer matrix, and almond shell flour (ASF), a by-product from the agro-food industry, as filler/reinforcement. These WPCs were prepared with different amounts of lignocellulosic fillers (wt %), namely 10, 20 and 30. The mechanical characterization of these WPCs showed an important increase in their stiffness with increasing the wt % ASF content. In addition, lower tensile strength and impact strength were obtained. The field emission scanning electron microscopy (FESEM) study revealed the lack of continuity and poor adhesion among the PHBH-ASF interface. Even with the only addition of 10 wt % ASF, these green composites become highly brittle. Nevertheless, for real applications, the WPC with 30 wt % ASF is the most attracting material since it contributes to lowering the overall cost of the WPC and can be manufactured by injection moulding, but its properties are really compromised due to the lack of compatibility between the hydrophobic PHBH matrix and the hydrophilic lignocellulosic filler. To minimize this phenomenon, 10 and 20 phr (weight parts of OLA-Oligomeric Lactic Acid per one hundred weight parts of PHBH) were added to PHBH/ASF (30 wt % ASF) composites. Differential scanning calorimetry (DSC) suggested poor plasticization effect of OLA on PHBH-ASF composites. Nevertheless, the most important property OLA can provide to PHBH/ASF composites is somewhat compatibilization since some mechanical ductile properties are improved with OLA addition. The study by thermomechanical analysis (TMA), confirmed the increase of the coefficient of linear thermal expansion (CLTE) with increasing OLA content. The dynamic mechanical characterization (DTMA), revealed higher storage modulus, E’, with increasing ASF. Moreover, DTMA results confirmed poor plasticization of OLA on PHBH-ASF (30 wt % ASF) composites, but interesting compatibilization effects.