Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)-based scaffolds for tissue engineering

Nucleating and reinforcing effects of nanobiochar on poly(3-hydroxybutyrate-co-3-hydroxhexanoate) bionanocomposites

This study promotes the use of nanobiochar (NBC) as an environmentally friendly substitute to conventional fillers to improve various properties of biopolymers such as their mechanical strength, thermal stability and crystallization properties. TGA analysis showed a slight increase in onset thermal degradation temperature of the composites by up to 5 °C with the addition of 4 wt% NBC. Non-isothermal DSC analysis determined that the addition of NBC into PHBHHx increases the crystallization temperature and degree of crystallinity of PHBHHx while isothermal DSC analysis demonstrated higher crystallization rate in PHBHHx/NBC composited by up to 54%. PHBHHx incorporated with NBC also exhibited superior tensile strength and modulus versus neat PHBHHx. Increase in mechanical strength was further proven via DMA where PHBHHx/NBC composites maintained higher storage modulus at higher temperatures when compared to neat PHBHHx. PHBHHx/NBC also exhibited no cytotoxicity effect against HaCat cells. This study demonstrates the ability of biochar to act as both nucleating agents and reinforcing agents in biodegradable polymers such as PHBHHx, which could be suitable for packaging application.

Natural Polyhydroxyalkanoates-An Overview of Bacterial Production Methods

Polyhydroxyalkanoates (PHAs) are intracellular biopolymers that microorganisms use for energy and carbon storage. They are mechanically similar to petrochemical plastics when chemically extracted, but are completely biodegradable. While they have potential as a replacement for petrochemical plastics, their high production cost using traditional carbon sources remains a significant challenge. One potential solution is to modify heterotrophic PHA-producing strains to utilize alternative carbon sources. An alternative approach is to utilize methylotrophic or autotrophic strains. This article provides an overview of bacterial strains employed for PHA production, with a particular focus on those exhibiting the highest PHA content in dry cell mass. The strains are organized according to their carbon source utilization, encompassing autotrophy (utilizing CO2, CO) and methylotrophy (utilizing reduced single-carbon substrates) to heterotrophy (utilizing more traditional and alternative substrates).

A Review on Electroactive Polymer-Metal Composites: Development and Applications for Tissue Regeneration

Electroactive polymer-metal composites (EAPMCs) have gained significant attention in tissue engineering owing to their exceptional mechanical and electrical properties. EAPMCs develop by combining an electroactive polymer matrix and a conductive metal. The design considerations include choosing an appropriate metal that provides mechanical strength and electrical conductivity and selecting an electroactive polymer that displays biocompatibility and electrical responsiveness. Interface engineering and surface modification techniques are also crucial for enhancing the adhesion and biocompatibility of composites. The potential of EAPMC-based tissue engineering revolves around its ability to promote cellular responses, such as cell adhesion, proliferation, and differentiation, through electrical stimulation. The electrical properties of these composites can be used to mimic natural electrical signals within tissues and organs, thereby aiding tissue regeneration. Furthermore, the mechanical characteristics of the metallic components provide structural reinforcement and can be modified to align with the distinct demands of various tissues. EAPMCs have extraordinary potential as regenerative biomaterials owing to their ability to promote beneficial effects in numerous electrically responsive cells. This study emphasizes the characteristics and applications of EAPMCs in tissue engineering.

Biomaterials-mediated CRISPR/Cas9 delivery: recent challenges and opportunities in gene therapy

The use of biomaterials in delivering CRISPR/Cas9 for gene therapy in infectious diseases holds tremendous potential. This innovative approach combines the advantages of CRISPR/Cas9 with the protective properties of biomaterials, enabling accurate and efficient gene editing while enhancing safety. Biomaterials play a vital role in shielding CRISPR/Cas9 components, such as lipid nanoparticles or viral vectors, from immunological processes and degradation, extending their effectiveness. By utilizing the flexibility of biomaterials, tailored systems can be designed to address specific genetic diseases, paving the way for personalized therapeutics. Furthermore, this delivery method offers promising avenues in combating viral illnesses by precisely modifying pathogen genomes, and reducing their pathogenicity. Biomaterials facilitate site-specific gene modifications, ensuring effective delivery to infected cells while minimizing off-target effects. However, challenges remain, including optimizing delivery efficiency, reducing off-target effects, ensuring long-term safety, and establishing scalable production techniques. Thorough research, pre-clinical investigations, and rigorous safety evaluations are imperative for successful translation from the laboratory to clinical applications. In this review, we discussed how CRISPR/Cas9 delivery using biomaterials revolutionizes gene therapy and infectious disease treatment, offering precise and safe editing capabilities with the potential to significantly improve human health and quality of life.

Three-Dimensional Scaffolds for Bone Tissue Engineering

Immobilization using external or internal splints is a standard and effective procedure to treat minor skeletal fractures. In the case of major skeletal defects caused by extreme trauma, infectious diseases or tumors, the surgical implantation of a bone graft from external sources is required for a complete cure. Practical disadvantages, such as the risk of immune rejection and infection at the implant site, are high in xenografts and allografts. Currently, an autograft from the iliac crest of a patient is considered the "gold standard" method for treating large-scale skeletal defects. However, this method is not an ideal solution due to its limited availability and significant reports of morbidity in the harvest site (30%) as well as the implanted site (5-35%). Tissue-engineered bone grafts aim to create a mechanically strong, biologically viable and degradable bone graft by combining a three-dimensional porous scaffold with osteoblast or progenitor cells. The materials used for such tissue-engineered bone grafts can be broadly divided into ceramic materials (calcium phosphates) and biocompatible/bioactive synthetic polymers. This review summarizes the types of materials used to make scaffolds for cryo-preservable tissue-engineered bone grafts as well as the distinct methods adopted to create the scaffolds, including traditional scaffold fabrication methods (solvent-casting, gas-foaming, electrospinning, thermally induced phase separation) and more recent fabrication methods (fused deposition molding, stereolithography, selective laser sintering, Inkjet 3D printing, laser-assisted bioprinting and 3D bioprinting). This is followed by a short summation of the current osteochondrogenic models along with the required scaffold mechanical properties for in vivo applications. We then present a few results of the effects of freezing and thawing on the structural and mechanical integrity of PLLA scaffolds prepared by the thermally induced phase separation method and conclude this review article by summarizing the current regulatory requirements for tissue-engineered products.

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.

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Recent Development of Polyhydroxyalkanoates (PHA)-Based Materials for Antibacterial Applications: A Review

The diseases caused by microorganisms are innumerable existing on this planet. Nevertheless, increasing antimicrobial resistance has become an urgent global challenge. Thus, in recent decades, bactericidal materials have been considered promising candidates to combat bacterial pathogens. Recently, polyhydroxyalkanoates (PHAs) have been used as green and biodegradable materials in various promising alternative applications, especially in healthcare for antiviral or antiviral purposes. However, it lacks a systematic review of the recent application of this emerging material for antibacterial applications. Therefore, the ultimate goal of this review is to provide a critical review of the state of the art recent development of PHA biopolymers in terms of cutting-edge production technologies as well as promising application fields. In addition, special attention was given to collecting scientific information on antibacterial agents that can potentially be incorporated into PHA materials for biological and durable antimicrobial protection. Furthermore, the current research gaps are declared, and future research perspectives are proposed to better understand the properties of these biopolymers as well as their possible applications.

Fabrication of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) Fibers Using Centrifugal Fiber Spinning: Structure, Properties and Application Potential

Biobased and biodegradable polyhydroxyalkanoates (PHAs) are currently gaining momentum. Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) polymer has a useful processing window for extrusion and injection molding of packaging, agricultural and fishery applications with required flexibility. Processing PHBHHx into fibers using electrospinning or centrifugal fiber spinning (CFS) can further broaden the application area, although CFS remains rather unexplored. In this study, PHBHHx fibers are centrifugally spun from 4-12 wt.% polymer/chloroform solutions. Beads and beads-on-a-string (BOAS) fibrous structures with an average diameter (ϕ_av) between 0.5 and 1.6 µm form at 4-8 wt.% polymer concentrations, while more continuous fibers (ϕ_av = 3.6-4.6 µm) with few beads form at 10-12 wt.% polymer concentrations. This change is correlated with increased solution viscosity and enhanced mechanical properties of the fiber mats (strength, stiffness and elongation values range between 1.2-9.4 MPa, 11-93 MPa, and 102-188%, respectively), though the crystallinity degree of the fibers remains constant (33.0-34.3%). In addition, PHBHHx fibers are shown to anneal at 160 °C in a hot press into 10-20 µm compact top-layers on PHBHHx film substrates. We conclude that CFS is a promising novel processing technique for the production of PHBHHx fibers with tunable morphology and properties. Subsequent thermal post-processing as a barrier or active substrate top-layer offers new application potential.

A review on poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) [P(3HB-co-3HHx)] and genetic modifications that affect its production

Polyhydroxyalkanoates (PHAs) have garnered global attention to replace petroleum-based plastics in certain applications due to their biodegradability and sustainability. Among the different types of PHAs, poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) [P(3HB-co-3HHx)] copolymer has similar properties to commodity plastics, making them a suitable candidate to replace certain types of single-use plastics, medical devices, and packaging materials. The degradation rate of P(3HB-co-3HHx) is faster than the commercial petroleum-based plastics which take a very long time to be degraded, causing harmful pollution to both land and marine ecosystem. The biodegradability of the P(3HB-co-3HHx) is also dependent on its 3HHx molar composition which in turn influences the crystallinity of the material. Various metabolic pathways like the common PHA biosynthesis pathway, which involves phaA, phaB, and phaC, β-oxidation, and fatty acids de novo synthesis are used by bacteria to produce PHA from different carbon sources like fatty acids and sugars, respectively. There are various factors affecting the 3HHx molar composition of P(3HB-co-3HHx), like PhaCs, the engineering of PhaCs, and the metabolic engineering of strains. It is crucial to control the 3HHx molar composition in the P(3HB-co-3HHx) as it will affect its properties and applications in different fields.

Effect of Different Plasticizers on Thermal, Crystalline, and Permeability Properties of Poly(3–hydroxybutyrate–co−3–hydroxyhexanoate) Films

Poly(3−hydroxybutyrate−co−3−hydroxyhexanoate) (PHBH) films were prepared using a cast film technique. Dioxane was chosen over other polymer solvents as it resulted in homogenous films with better morphology. Several plasticizers with different molecular weights and concentrations were added to the biopolymer solution prior to casting. Thermal, crystalline, and permeability properties were analyzed by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), X−ray diffraction (XRD), and both water vapor and oxygen transmission rate analysis. In general, the addition of plasticizers decreased the glass transition temperature (Tg), cold crystallization temperatures (Tcc), melting temperatures, as well as crystallinity degrees and increased the crystallite sizes and water vapor and oxygen transmission rates. The use of isosorbide and low-molecular-weight poly(ethylene glycol) (PEG) lowered the Tg around 30 °C at the highest used concentration, also being the most effective in increasing the crystallite size. When considering isosorbide and low-molecular-weight poly(ethylene glycol) (PEG) as very good plasticizers for PHBH, the question of which plasticizer to use strongly relies on the desired PHBH application.

AK, Ramakrishna S, Amirul AAA. Surface Modification of Sponge-like Porous Poly(3-hydroxybutyrate-co-4-hydroxybutyrate)/Gelatine Blend Scaffolds for Potential Biomedical Applications

In this study, we described the preparation of sponge-like porous scaffolds that are feasible for medical applications. A porous structure provides a good microenvironment for cell attachment and proliferation. In this study, a biocompatible PHA, poly(3-hydroxybutyrate-co-4-hydroxybutyrate) was blended with gelatine to improve the copolymer's hydrophilicity, while structural porosity was introduced into the scaffold via a combination of solvent casting and freeze-drying techniques. Scanning electron microscopy results revealed that the blended scaffolds exhibited higher porosity when the 4HB compositions of P(3HB-co-4HB) ranged from 27 mol% to 50 mol%, but porosity decreased with a high 4HB monomer composition of 82 mol%. The pore size, water absorption capacity, and cell proliferation assay results showed significant improvement after the final weight of blend scaffolds was reduced by half from the initial 0.79 g to 0.4 g. The pore size of 0.79g-(P27mol%G10) increased three-fold while the water absorption capacity of 0.4g-(P50mol%G10) increased to 325%. Meanwhile, the cell proliferation and attachment of 0.4g-(P50mol%G10) and 0.4g-(P82mol%G7.5) increased as compared to the initial seeding number. Based on the overall data obtained, we can conclude that the introduction of a small amount of gelatine into P(3HB-co-4HB) improved the physical and biological properties of blend scaffolds, and the 0.4g-(P50mol%G10) shows great potential for medical applications considering its unique structure and properties.

Study of the production of poly(hydroxybutyrate-co-hydroxyhexanoate) and poly(hydroxybutyrate-co-hydroxyvalerate-co-hydroxyhexanoate) in Rhodospirillum rubrum

Poly(hydroxybutyrate-co-hydroxyhexanoate) (P(HB-co-HHx)) and poly(hydroxybutyrate-co-hydroxyvalerate-co-hydroxyhexanoate) P(HB-co-HV-co-HHx) demonstrate interesting mechanical and thermal properties as well as excellent biocompatibility making them suitable for multiple applications and notably biomedical purposes. The production of such polymer was described in Rhodospirillum rubrum (Rs. rubrum), a purple non-sulfur bacteria in a nutrient-lacking environment where the HHx synthesis is triggered by the presence of hexanoate in the medium. However, the production of P(HB-co-HHx) under nutrient-balanced growth conditions has not been described so far in Rs. rubrum and the assimilation of hexanoate is poorly documented. In this study, we demonstrate using proteomic analysis and mutant fitness assay, that hexanoate assimilation involve β-oxidation and the ethylmalonyl-CoA (EMC) and methylbutanoyl-CoA (MBC) pathways, both being anaplerotic pathways already described in Rs. rubrum. Polyhydroxyalkanoate (PHA) production is likely to involve the de novo fatty acid synthesis pathway. Concerning the polymer composition, HB is the main component of the polymer, probably as acetyl-CoA and butyryl-CoA are intermediates of hexanoate assimilation pathways. When no essential nutrient is lacking in the medium, the synthesis of PHA seems to help maintain the redox balance of the cell. In this framework, we showed that the fixation of CO₂ is required to sustain the growth. An increase in the proportion of HHx in the polymer was observed when redox stress was engendered in the cell under bicarbonate limiting growth conditions. The addition of isoleucine or valerate in the medium also increased the HHx content of the polymer and allowed the production of a terpolymer of P(HB-co-HV-co-HHx). Importance The use of purple bacteria, which can assimilate volatile fatty acids for biotechnological applications has risen since they reduce the production costs of added-value compounds such as PHA. P(HB-co-HHx) and P(HB-co-HV-co-HHx) have demonstrated interesting properties notably for biomedical application. In a nutrient-lacking environment, Rs. rubrum is known to synthesize such polymer when hexanoate is used as carbon source. However, their production in non-nutrient lacking growth conditions has not been described so far in Rs. rubrum and the assimilation of hexanoate is poorly documented. As the carbon source and its assimilation directly impact the polymer composition, we studied under non-nutrient lacking growth conditions, the assimilation path of hexanoate and PHA production in Rs. rubrum. Proteomic analysis and mutant fitness assay allowed to explain PHA production and composition. Increase in HHx content of the polymer and production of P(HB-co-HV-co-HHx) was possible using the knowledge gained on metabolism under hexanoate growth conditions.

Graphene Oxide versus Carbon Nanofibers in Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) Films: Degradation in Simulated Intestinal Environments

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) is a microbial biodegradable polymer with a broad range of promising industrial applications. The effect of incorporation of low amounts (1% w/w) of carbon nanomaterials (CBNs) such as 1D carbon nanofibers (CNFs) or 2D graphene oxide (GO) nanosheets into the PHBV polymer matrix affects its degradation properties, as it is reported here for the first time. The study was performed in simulated gut conditions using two different media: an acidic aqueous medium (pH 6) and Gifu anaerobic medium. The results of this study showed that the incorporation of low amounts of filamentous 1D hydrophobic CNFs significantly increased the degradability of the hydrophobic PHBV after 3 months in simulated intestinal conditions as confirmed by weight loss (~20.5% w/w in acidic medium) and electron microscopy. We can attribute these results to the fact that the long hydrophobic carbon nanochannels created in the PHBV matrix with the incorporation of the CNFs allowed the degradation medium to penetrate at ultrafast diffusion speed increasing the area exposed to degradation. However, the hydrogen bonds formed between the 2D hydrophilic GO nanosheets and the hydrophobic PHBV polymer chains produced a homogeneous composite structure that exhibits lower degradation (weight loss of ~4.5% w/w after three months in acidic aqueous medium). Moreover, the water molecules present in both degradation media can be linked to the hydroxyl (-OH) and carboxyl (-COOH) groups present on the basal planes and at the edges of the GO nanosheets, reducing their degradation potential.

β-TCP/DCPD-PHBV (40%/60%): Biomaterial made from bioceramic and biopolymer for bone regeneration; investigation of intrinsic properties

In this study, a detailed physical, chemical, and mechanical investigation of bone substitute (β-TCP/DCPD-PHBV) was carried out. In fact, it is composed of biocompatible materials such as ceramic phosphocalcic, consisting of tricalcium phosphate (β-TCP) and dihydrated dicalcium phosphate (DCPD) and 3-hydroxybutyrate-co-3-hydroxyvalerate (PHBV) polymer having a weight fraction 40%/60%. For these analyses, diverse techniques were used, including SEM-EDS, mercury porosimeter, Fourier Transform Infrared Spectroscopy, and, finally, uniaxial compression test machine. A morphological investigation of biomaterials using MEB revealed uneven particle shape and size, as well as a rough surface with a porous and microcracked structure. In fact, this architecture promotes the development of bone within biomaterials. Compositional studies applying FTIR technology, also, revealed the existence of chemical components, comparable to those found in the mineral phase of bone (Ca²⁺, PO₄^3-, and HPO₄^2-). The following compounds prove the bioactivity of β-TCP/DCPD-PHBV. Furthermore, mechanical investigations revealed that this biomaterial has a satisfying mechanical strength (195.21 MPa), closer to bone. Nevertheless, another significant benefit of combining the two biocompatible materials used in this work is that the ductility of PHBV restricts the brittleness of β-TCP/DCPD-PHBV, compared to pure β-TCP/DCPD. The obtained results demonstrate the beneficial properties of β-TCP/DCPD-PHBV and approve the possibility of using this biomaterial as a viable material for future implantology applications.

A review on enzymes and pathways for manufacturing polyhydroxybutyrate from lignocellulosic materials

Currently, major focus in the biopolymer field is being drawn on the exploitation of plant-based resources grounded on holistic sustainability trends to produce novel, affordable, biocompatible and environmentally safe polyhydroxyalkanoate biopolymers. The global PHA market, estimated at USD 62 Million in 2020, is predicted to grow by 11.2 and 14.2% between 2020-2024 and 2020-2025 correspondingly based on market research reports. The market is primarily driven by the growing demand for PHA products by the food packaging, biomedical, pharmaceutical, biofuel and agricultural sectors. One of the key limitations in the growth of the PHA market is the significantly higher production costs associated with pure carbon raw materials as compared to traditional polymers. Nonetheless, considerations such as consumer awareness on the toxicity of petroleum-based plastics and strict government regulations towards the prohibition of the use and trade of synthetic plastics are expected to boost the market growth rate. This study throws light on the production of polyhydroxybutyrate from lignocellulosic biomass using environmentally benign techniques via enzyme and microbial activities to assess its feasibility as a green substitute to conventional plastics. The novelty of the present study is to highlight the recent advances, pretreatment techniques to reduce the recalcitrance of lignocellulosic biomass such as dilute and concentrated acidic pretreatment, alkaline pretreatment, steam explosion, ammonia fibre explosion (AFEX), ball milling, biological pretreatment as well as novel emerging pretreatment techniques notably, high-pressure homogenizer, electron beam, high hydrostatic pressure, co-solvent enhanced lignocellulosic fractionation (CELF) pulsed-electric field, low temperature steep delignification (LTSD), microwave and ultrasound technologies. Additionally, inhibitory compounds and detoxification routes, fermentation downstream processes, life cycle and environmental impacts of recovered natural biopolymers, review green procurement policies in various countries, PHA strategies in line with the United Nations Sustainable Development Goals (SDGs) along with the fate of the spent polyhydroxybutyrate are outlined.

Manipulating cell behavior on a bacterial macro-polymer poly (3-hydroxybutyrate-co-3-hydroxyhexanoate) via tuning the S-doped graphene ratio

Graphene is a material with various application potentials Graphene is a unique material with superiorities and has been applied in various fields for different purposes. Although studies on the utility of graphene oxide in the biomedical field are available, no evaluation has yet been done regarding the utility of sulfur doped (S-doped) graphene. The study focuses on the effect of blending the poly (3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) membrane with sulfur heteroatom doped graphene and the evaluation of biological responses to S-doped graphene/PHBHHx. PHBHHx membranes were blended with 1%, 0.5%, 0.1% (w/v) S-doped graphene. The morphological (SEM and Microscopy), chemical (FTIR and Raman spectroscopy), and surface area (BET) characterizations of S-doped graphene/PHBHHx membranes were performed. The presence of S groups on the surface was determined with the EDS results. Besides, the swelling profile and biodegradation tendency of the membranes were evaluated. The differentiation of protein adhesion, cell viability, cell adhesion, and cell proliferation by the increasing content of S-doped graphene was examined. The contact angle analysis revealed that modification of PHBHHx with S-doped Graphene reduced the free surface energy of PHBHHx membranes. Blending with S-doped Graphene has decreased the polarity of the PHBHHx membrane. The protein adsorption on the PHBHHx membrane was determined as 10.12 ± 0.247 mg/ml. Protein absorption on 1%, 0.5% and 0.1% S-doped graphene/PHBHHx membranes were determined as 11.34 ± 0.551 mg/ml, 9.91 ± 0.294 mg/ml and 9.48 ± 0.093 mg/ml, respectively. The cell attachment to the surface decreased with the increasing amount of S-doped graphene, however, PHBHHx membranes with graphene did not affect cytotoxicity. S-doped graphene blended PHBHHx membrane seems like a suitable patch for biomedical treatments as a hydrophobic membrane where less cell adhesion and proliferation are required like the prevention of peritoneal adhesion.

A Review of the Applications and Biodegradation of Polyhydroxyalkanoates and Poly(lactic acid) and Its Composites

Overconsumption of plastic goods and improper handling of petroleum-derived plastic waste have brought a plethora of negative impacts to the environment, ecosystem and human health due to its recalcitrance to degradation. These drawbacks become the main driving force behind finding biopolymers with the degradable properties. With the advancement in biopolymer research, polyhydroxyalkanoate (PHA) and poly(lacyic acid) (PLA) and its composites have been alluded to as a potential alternative to replace the petrochemical counterpart. This review highlights the current synthesis process and application of PHAs and PLA and its composites for food packaging materials and coatings. These biopolymers can be further ameliorated to enhance their applicability and are discussed by including the current commercially available packaging products. Factors influencing biodegradation are outlined in the latter part of this review. The main aim of this review article is to organize the scattered available information on various aspects of PHAs and PLA, and its composites for packaging application purposes. It is evident from a literature survey of about 140 recently published papers from the past 15 years that PLA and PHA show excellent physical properties as potential food packaging materials.

Poly(lactic acid) (PLA) and polyhydroxyalkanoates (PHAs), green alternatives to petroleum-based plastics: a review

In spite of the fact that petroleum-based plastics are convenient in terms of fulfilling the performance requirements of many applications, they contribute significantly to a number of ecological and environmental problems. Recently, the public awareness of the negative effects of petroleum-based plastics on the environment has increased. The present utilization of natural resources cannot be sustained forever. Furthermore, oil is often subjected to price fluctuations and will eventually be depleted. The increase in the level of carbon dioxide due to the combustion of fossil fuel is causing global warming. Concerns about preservation of natural resources and climate change are considered worldwide motivations for academic and industrial researchers to reduce the consumption and dependence on fossil fuel. Therefore, bio-based polymers are moving towards becoming the favorable option to be utilized in polymer manufacturing, food packaging, and medical applications. This paper represents an overview of the feasibility of both Poly Lactic Acid (PLA) and polyhydroxyalkanoates (PHAs) as alternative materials that can replace petroleum-based polymers in a wide range of industrial applications. Physical, thermal, rheological, and mechanical properties of both polymers as well as their permeability and migration properties have been reviewed. Moreover, PLA's recyclability, sustainability, and environmental assessment have been also discussed. Finally, applications in which both polymers can replace petroleum-based plastics have been explored and provided.

Oxygen Gas and UV Barrier Properties of Nano-ZnO-Coated PET and PHBHHx Materials Fabricated by Ultrasonic Spray-Coating Technique

Ultrasonic spray-coating (USSC)-a wet chemical deposition method to deposit ultrathin (down to 20 nm) coatings-is being applied as a promising alternative deposition method for functional coatings due to an economical, simple, and precise coating process with easy control over its operating parameters. In this research, zinc oxide nanoparticles (ZnO NPs) were ultrasonically spray-coated on commercial-grade polyethylene terephthalate (PET) and poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) films. The most suitable parameters for the ink composition, the ultrasonic spray-coating process, and the number of coating passes (up to 50×) were selected on the basis of a series of experiments. The oxygen gas barrier properties in terms of the oxygen transmission rate (OTR) of neat PET, and 3×, 5×, 10×, and 50× ZnO NP-coated PET and PHBHHx substrates were investigated. The OTR values for neat PET, and 3×, 5×, and 10× ZnO NP-coated PET substrates were found to be the same; however, a 5% reduction in OTR for 50× ZnO NP-coated PET substrate was observed compared to the neat PET substrate. No reduction in OTR was found for any above number of coating passes on PHBHHx substrates against the neat PHBHHx substrate. However, the ultraviolet (UV) tests of 3×, 5×, and 10× ZnO NP-coated PET and PHBHH× substrates revealed a significant decrease in percentage transmission for 10× coated PET and PHBHHx substrates as compared to their 3× and 5× ZnO NP-coated substrates, respectively. It was revealed from the study that the 50× ZnO NP coating of the PET substrate created a slight difference in OTR as compared to the reference substrate. However, the ultrasonic spray-coating method created a significant UV barrier effect for 3×, 5×, and 10× ZnO NP-coated PET and PHBHHx substrates, which demonstrates that the optimized coating method cannot be used to create a high oxygen barrier but can certainly be applied for UV barrier applications in food packaging. It is concluded that ultrasonic spray deposition of ZnO NPs on PET and PHBHHx materials has shown promising results for UV barrier properties, demonstrating the advantages of using this method compared to other coating methods with regard to cost-effectiveness, precise coating, and better process control.

In vivo and Post-synthesis Strategies to Enhance the Properties of PHB-Based Materials: A Review

The transition toward "green" alternatives to petroleum-based plastics is driven by the need for "drop-in" replacement materials able to combine characteristics of existing plastics with biodegradability and renewability features. Promising alternatives are the polyhydroxyalkanoates (PHAs), microbial biodegradable polyesters produced by a wide range of microorganisms as carbon, energy, and redox storage material, displaying properties very close to fossil-fuel-derived polyolefins. Among PHAs, polyhydroxybutyrate (PHB) is by far the most well-studied polymer. PHB is a thermoplastic polyester, with very narrow processability window, due to very low resistance to thermal degradation. Since the melting temperature of PHB is around 170-180°C, the processing temperature should be at least 180-190°C. The thermal degradation of PHB at these temperatures proceeds very quickly, causing a rapid decrease in its molecular weight. Moreover, due to its high crystallinity, PHB is stiff and brittle resulting in very poor mechanical properties with low extension at break, which limits its range of application. A further limit to the effective exploitation of these polymers is related to their production costs, which is mostly affected by the costs of the starting feedstocks. Since the first identification of PHB, researchers have faced these issues, and several strategies to improve the processability and reduce brittleness of this polymer have been developed. These approaches range from the in vivo synthesis of PHA copolymers, to the enhancement of post-synthesis PHB-based material performances, thus the addition of additives and plasticizers, acting on the crystallization process as well as on polymer glass transition temperature. In addition, reactive polymer blending with other bio-based polymers represents a versatile approach to modulate polymer properties while preserving its biodegradability. This review examines the state of the art of PHA processing, shedding light on the green and cost-effective tailored strategies aimed at modulating and optimizing polymer performances. Pioneering examples in this field will be examined, and prospects and challenges for their exploitation will be presented. Furthermore, since the establishment of a PHA-based industry passes through the designing of cost-competitive production processes, this review will inspect reported examples assessing this economic aspect, examining the most recent progresses toward process sustainability.

Novel Semi-Interpenetrated Polymer Networks of Poly(3-Hydroxybutyrate-co-3-Hydroxyvalerate)/Poly (Vinyl Alcohol) with Incorporated Conductive Polypyrrole Nanoparticles

This paper reports the preparation and characterization of semi-interpenetrating polymer networks (semi-IPN) of poly(3-hydroxybutirate-co-3-hydroxyvalerate), PHBV, and poly (vinyl alcohol), PVA, with conductive polypirrole (PPy) nanoparticles. Stable hybrid semi-IPN (PHBV/PVA 30/70 ratio) hydrogels were produced by solvent casting, dissolving each polymer in chloroform and 1-methyl-2-pyrrolidone respectively, and subsequent glutaraldehyde crosslinking of the PVA chains. The microstructure and physical properties of this novel polymeric system were analysed, including thermal behaviour and degradation, water sorption, wettability and electrical conductivity. The conductivity of these advanced networks rose significantly at higher PPy nanoparticles content. Fourier transform infrared spectroscopy (FTIR) and calorimetry characterization indicated good miscibility and compatibility between all the constituents, with no phase separation and strong interactions between phases. A single glass transition was observed between those of pure PHBV and PVA, although PVA was dominant in its contribution to the glass transition process. Incorporating PPy nanoparticles significantly reduced the hydrogel swelling, even at low concentrations, indicating molecular interactions between the PPy nanoparticles and the hydrogel matrix. The PHBV/PVA semi-IPN showed higher thermal stability than the neat polymers and PHBV/PVA blend, which also remained in the tertiary systems.

Microbial Polyhydroxyalkanoates and Nonnatural Polyesters

Microorganisms produce diverse polymers for various purposes such as storing genetic information, energy, and reducing power, and serving as structural materials and scaffolds. Among these polymers, polyhydroxyalkanoates (PHAs) are microbial polyesters synthesized and accumulated intracellularly as a storage material of carbon, energy, and reducing power under unfavorable growth conditions in the presence of excess carbon source. PHAs have attracted considerable attention for their wide range of applications in industrial and medical fields. Since the first discovery of PHA accumulating bacteria about 100 years ago, remarkable advances have been made in the understanding of PHA biosynthesis and metabolic engineering of microorganisms toward developing efficient PHA producers. Recently, nonnatural polyesters have also been synthesized by metabolically engineered microorganisms, which opened a new avenue toward sustainable production of more diverse plastics. Herein, the current state of PHAs and nonnatural polyesters is reviewed, covering mechanisms of microbial polyester biosynthesis, metabolic pathways, and enzymes involved in biosynthesis of short-chain-length PHAs, medium-chain-length PHAs, and nonnatural polyesters, especially 2-hydroxyacid-containing polyesters, metabolic engineering strategies to produce novel polymers and enhance production capabilities and fermentation, and downstream processing strategies for cost-effective production of these microbial polyesters. In addition, the applications of PHAs and prospects are discussed.

Enhancing the Therapeutic Efficacy of Bortezomib in Cancer Therapy Using Polymeric Nanostructures

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Bortezomib (VELCADE®) is a boronate peptide and first-in-class proteasome inhibitor serving an important role in degenerating several intracellular proteins. It is a reversible inhibitor of the 26S proteasome, with antitumor activity and antiproliferative properties. This agent principally exerts its antineoplastic effects by inhibiting key players in the nuclear factor κB (NFκB) pathway involved in cell proliferation, apoptosis, and angiogenesis. This medication is used in the management of multiple myeloma. However, more recently, it has been used as a therapeutic option for mantle cell lymphoma. While promising, bortezomib has limited clinical applications due to its adverse effects (e.g., hematotoxicity and peripheral neuropathy) and low effectiveness in solid tumors resulting from its poor penetration into such masses and suboptimal pharmacokinetic parameters. Other limitations to bortezomib include its low chemical stability and bioavailability, which can be overcome by using nanoparticles for its delivery. Nanoparticle delivery systems can facilitate the targeted delivery of chemotherapeutic agents in high doses to the target site, while sparing healthy tissues. Therefore, this drug delivery system has provided a solution to circumvent the limitations faced with the delivery of traditional cancer chemotherapeutic agents. Our aim in this review was to describe polymer-based nanocarriers that can be used for the delivery of bortezomib in cancer chemotherapy.

Current state of fabrication technologies and materials for bone tissue engineering

A range of traditional and free-form fabrication technologies have been investigated and, in numerous occasions, commercialized for use in the field of regenerative tissue engineering (TE). The demand for technologies capable of treating bone defects inherently difficult to repair has been on the rise. This quest, accompanied by the advent of functionally tailored, biocompatible, and biodegradable materials, has garnered an enormous research interest in bone TE. As a result, different materials and fabrication methods have been investigated towards this end, leading to a deeper understanding of the geometrical, mechanical and biological requirements associated with bone scaffolds. As our understanding of the scaffold requirements expands, so do the capability requirements of the fabrication processes. The goal of this review is to provide a broad examination of existing scaffold fabrication processes and highlight future trends in their development. To appreciate the clinical requirements of bone scaffolds, a brief review of the biological process by which bone regenerates itself is presented first. This is followed by a summary and comparisons of commonly used implant techniques to highlight the advantages of TE-based approaches over traditional grafting methods. A detailed discussion on the clinical and mechanical requirements of bone scaffolds then follows. The remainder of the manuscript is dedicated to current scaffold fabrication methods, their unique capabilities and perceived shortcomings. The range of biomaterials employed in each fabrication method is summarized. Selected traditional and non-traditional fabrication methods are discussed with a highlight on their future potential from the authors' perspective. This study is motivated by the rapidly growing demand for effective scaffold fabrication processes capable of economically producing constructs with intricate and precisely controlled internal and external architectures. STATEMENT OF SIGNIFICANCE: The manuscript summarizes the current state of fabrication technologies and materials used for creating scaffolds in bone tissue engineering applications. A comprehensive analysis of different fabrication methods (traditional and free-form) were summarized in this review paper, with emphasis on recent developments in the field. The fabrication techniques suitable for creating scaffolds for tissue engineering was particularly targeted and their use in bone tissue engineering were articulated. Along with the fabrication techniques, we emphasized the choice of materials in these processes. Considering the limitations of each process, we highlighted the materials and the material properties critical in that particular process and provided a brief rational for the choice of the materials. The functional performance for bone tissue engineering are summarized for different fabrication processes and the choice of biomaterials. Finally, we provide a perspective on the future of the field, highlighting the knowledge gaps and promising avenues in pursuit of effective scaffolds for bone tissue engineering. This extensive review of the field will provide research community with a reference source for current approaches to scaffold preparation. We hope to encourage the researchers to generate next generation biomaterials to be used in these fabrication processes. By providing both advantages and disadvantage of each fabrication method in detail, new fabrication techniques might be devised that will overcome the limitations of the current approaches. These studies should facilitate the efforts of researchers interested in generating ideal scaffolds, and should have applications beyond the repair of bone tissue.

Elastic materials for tissue engineering applications: Natural, synthetic, and hybrid polymers

Elastin and collagen are the two main components of elastic tissues and provide the tissue with elasticity and mechanical strength, respectively. Whereas collagen is adequately produced in vitro, production of elastin in tissue-engineered constructs is often inadequate when engineering elastic tissues. Therefore, elasticity has to be artificially introduced into tissue-engineered scaffolds. The elasticity of scaffold materials can be attributed to either natural sources, when native elastin or recombinant techniques are used to provide natural polymers, or synthetic sources, when polymers are synthesized. While synthetic elastomers often lack the biocompatibility needed for tissue engineering applications, the production of natural materials in adequate amounts or with proper mechanical strength remains a challenge. However, combining natural and synthetic materials to create hybrid components could overcome these issues. This review explains the synthesis, mechanical properties, and structure of native elastin as well as the theories on how this extracellular matrix component provides elasticity in vivo. Furthermore, current methods, ranging from proteins and synthetic polymers to hybrid structures that are being investigated for providing elasticity to tissue engineering constructs, are comprehensively discussed.

STATEMENT OF SIGNIFICANCE

Tissue engineered scaffolds are being developed as treatment options for malfunctioning tissues throughout the body. It is essential that the scaffold is a close mimic of the native tissue with regards to both mechanical and biological functionalities. Therefore, the production of elastic scaffolds is of key importance to fabricate tissue engineered scaffolds of the elastic tissues such as heart valves and blood vessels. Combining naturally derived and synthetic materials to reach this goal proves to be an interesting area where a highly tunable material that unites mechanical and biological functionalities can be obtained.

Poly(3-Hydroxybutyrate-co-3-Hydroxyvalerate): Enhancement Strategies for Advanced Applications

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate), PHBV, is a microbial biopolymer with excellent biocompatible and biodegradable properties that make it a potential candidate for substituting petroleum-derived polymers. However, it lacks mechanical strength, water sorption and diffusion, electrical and/or thermal properties, antimicrobial activity, wettability, biological properties, and porosity, among others, limiting its application. For this reason, many researchers around the world are currently working on how to overcome the drawbacks of this promising material. This review summarises the main advances achieved in this field so far, addressing most of the chemical and physical strategies to modify PHBV and placing particular emphasis on the combination of PHBV with other materials from a variety of different structures and properties, such as other polymers, natural fibres, carbon nanomaterials, nanocellulose, nanoclays, and nanometals, producing a wide range of composite biomaterials with increased potential applications. Finally, the most important methods to fabricate porous PHBV scaffolds for tissue engineering applications are presented. Even though great advances have been achieved so far, much research needs to be conducted still, in order to find new alternative enhancement strategies able to produce advanced PHBV-based materials able to overcome many of these challenges.

Polyester elastomers for soft tissue engineering

Polyester elastomers are soft, biodegradable and biocompatible and are commonly used in various biomedical applications, especially in tissue engineering. These synthetic polyesters can be easily fabricated using various techniques such as solvent casting, particle leaching, molding, electrospinning, 3-dimensional printing, photolithography, microablation etc. A large proportion of tissue engineering research efforts have focused on the use of allografts, decellularized animal scaffolds or other biological materials as scaffolds, but they face the major concern of triggering immunological responses from the host, on top of other issues. This review paper will introduce the recent developments in elastomeric polyesters, their synthesis and fabrication techniques, as well as their application in the biomedical field, focusing primarily on tissue engineering in ophthalmology, cardiac and vascular systems. Some of the commercial and near-commercial polyesters used in these tissue engineering fields will also be described.

Electroactive polymers for tissue regeneration: Developments and perspectives

Human body motion can generate a biological electric field and a current, creating a voltage gradient of -10 to -90 mV across cell membranes. In turn, this gradient triggers cells to transmit signals that alter cell proliferation and differentiation. Several cell types, counting osteoblasts, neurons and cardiomyocytes, are relatively sensitive to electrical signal stimulation. Employment of electrical signals in modulating cell proliferation and differentiation inspires us to use the electroactive polymers to achieve electrical stimulation for repairing impaired tissues. Electroactive polymers have found numerous applications in biomedicine due to their capability in effectively delivering electrical signals to the seeded cells, such as biosensing, tissue regeneration, drug delivery, and biomedical implants. Here we will summarize the electrical characteristics of electroactive polymers, which enables them to electrically influence cellular function and behavior, including conducting polymers, piezoelectric polymers, and polyelectrolyte gels. We will also discuss the biological response to these electroactive polymers under electrical stimulation. In particular, we focus this review on their applications in regenerating different tissues, including bone, nerve, heart muscle, cartilage and skin. Additionally, we discuss the challenges in tissue regeneration applications of electroactive polymers. We conclude that electroactive polymers have a great potential as regenerative biomaterials, due to their ability to stimulate desirable outcomes in various electrically responsive cells.

Scaffold design for artificial tissue with bone marrow stem cells

OBJECTIVE

The aim of this study was to test polymeric materials (collagen, fibrin, polyimide film, and polylactic acid) for single- and multi-layer scaffold formation.

MATERIALS AND METHODS

In our study, we used rabbit bone marrow stem cells (rBMSCs) and human mesenchymal stem cells (hMSCs) with materials of a different origin for the formation of an artificial scaffold, such as a collagen scaffold, fibrin scaffold produced from clotted rabbit plasma, electrospun poly(lactic acid) (PLA) mats, polyimide film (PI), and the combination of the latter two. Cell imaging was performed 3-14 days after cell cultivation in the scaffolds. Time-lapse imaging was used to determine hMSC mobility on the PI film.

RESULTS

Cell incorporation in collagen and clotted fibrin scaffolds was evaluated after 2-week cultivation in vitro. Histological analysis showed that cells penetrated only external layers of the collagen scaffold, while the fibrin clot was populated with rBMSCs through the entire scaffold thickness. As well, cell behavior on the laser micro-structured PI film was analyzed. The mobility of hMSCs on the smooth PI film and the micro-machined surface was 20±2μm/h and 18±4μm/h, respectively. After 3-day cultivation, hMSCs were capable of spreading through the whole 100±10μm-thick layer of the electrospun PLA scaffold and demonstrated that the multilayer scaffold composed of PI and PLA materials ensured a suitable environment for cell growth.

CONCLUSIONS

The obtained results suggest that electrospinning technology and femtosecond laser micro-structuring could be employed for the development of multi-layer scaffolds. Different biopolymers, such as PLA, fibrin, and collagen, could be used as appropriate environments for cell inhabitation and as an inner layer of the multi-layer scaffold. PI could be suitable as a barrier blocking cell migration from the scaffold. However, additional studies are needed to determine optimal parameters of inner and outer scaffold layers.

Emerging bone tissue engineering via Polyhydroxyalkanoate (PHA)-based scaffolds

Polyhydroxyalkanoates (PHAs) are a class of biodegradable polymers derived from microorganisms. On top of their biodegradability and biocompatibility, different PHA types can contribute to varying mechanical and chemical properties. This has led to increasing attention to the use of PHAs in numerous biomedical applications over the past few decades. Bone tissue engineering refers to the regeneration of new bone through providing mechanical support while inducing cell growth on the PHA scaffolds having a porous structure for tissue regeneration. This review first introduces the various properties PHA scaffold that make them suitable for bone tissue engineering such as biocompatibility, biodegradability, mechanical properties as well as vascularization. The typical fabrication techniques of PHA scaffolds including electrospinning, salt-leaching and solution casting are further discussed, followed by the relatively new technology of using 3D printing in PHA scaffold fabrication. Finally, the recent progress of using different types of PHAs scaffold in bone tissue engineering applications are summarized in intrinsic PHA/blends forms or as composites with other polymeric or inorganic hybrid materials.

Electrospun fibers of chitosan-grafted polycaprolactone/poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) blends

Chitosan-grafted polycaprolactone (CS-g-PCL)/poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHH_x) fiber blends were prepared via wet electrospinning, and their morphology, hydrophilicity, water absorption, biodegradation, cytotoxicity, and thermal, mechanical and antibacterial properties were analyzed. IR spectra revealed strong H-bonding interactions between CS-g-PCL and PHBHH_x. SEM and DSC analysis confirmed the immiscibility of the blends at all compositions studied. As the proportion of CS-g-PCL increased, the overall crystallinity of the blends increased, the melting temperature of PCL decreased, and each component promoted the crystallization of the others. The hydrophilicity, water absorption and weight loss in buffered solution decreased as the PHBHH_x content increased. DMA and tensile tests indicated a synergistic effect on the mechanical properties at a blend composition of 70/30, leading to an optimal combination of stiffness, strength, ductility and toughness. The fibers retained adequate rigidity and strength under physiological conditions. The 70/30 blend exhibited the highest biocide action against Gram-positive and Gram-negative bacteria. The fibers did not induce toxicity over human dermal fibroblasts. These biodegradable, biocompatible electrospun fibers could be used as scaffolds for tissue engineering.

Differential osteogenic potential of human adipose-derived stem cells co-cultured with human osteoblasts on polymeric microfiber scaffolds

The osteogenic potential of human adipose-derived stem cells (HADSCs) co-cultured with human osteoblasts (HOBs) using selected HADSCs/HOBs ratios of 1:1, 2:1, and 1:2, respectively, is evaluated. The HADSCs/HOBs were seeded on electrospun three-dimensional poly[(R)-3-hydroxybutyric acid] (PHB) blended with bovine-derived hydroxyapatite (BHA). Monocultures of HADSCs and HOBs were used as control groups. The effects of PHB-BHA scaffold on cell proliferation and cell morphology were assessed by AlamarBlue assay and field emission scanning electron microscopy. Cell differentiation, cell mineralization, and osteogenic-related gene expression of co-culture HADSCs/HOBs were examined by alkaline phosphatase (ALP) assay, alizarin Red S assay, and quantitative real time PCR, respectively. The results showed that co-culture of HADSCs/HOBs, 1:1 grown into PHB-BHA promoted better cell adhesion, displayed a significant higher cell proliferation, higher production of ALP, extracellular mineralization and osteogenic-related gene expression of run-related transcription factor, bone sialoprotein, osteopontin, and osteocalcin compared to other co-culture groups. This result also suggests that the use of electrospun PHB-BHA in a co-culture HADSCs/HOBs system may serve as promising approach to facilitate osteogenic differentiation activity of HADSCs through direct cell-to-cell contact with HOBs.