What is Polyhydroxyalkanoate (PHA)?
Microorganisms produce polyhydroxyalkanoates (PHAs), which are bio-based polymers by fermenting renewable materials including sugars and waste products as well as oils. The combination of PHAs' biodegradability, biocompatibility and environmental friendliness establishes them as sustainable materials that replace traditional plastics.
Definition and General Chemical Structure of PHA
PHAs consist of linear polyesters that result from repeating (R)-3-hydroxy fatty acid monomers connected by ester bonds. The monomers of these compounds can contain carbon chains that vary between 3 and over 14 carbon atoms creating a wide structural variation. The general chemical structure can be represented as: The repeating units structure {(R)-O-CO-CH2-R'-CO-} holds R as an alkyl or alkenyl group while m and n specify the count of repeating units.
Chemical structures of PHAs
Biosynthetic Origin of PHA
Microorganisms produce PHAs primarily under conditions with limited nutrients such as nitrogen or phosphorus shortages as intracellular reserves of carbon and energy. Microorganisms which include bacteria, archaea and fungi can break down various carbon sources like sugars, fats organic residue and wastewater. SCL-PHAs primarily include 3-hydroxybutyrate (3HB) while MCL-PHAs incorporate various monomers such as 3-hydroxyvalerate (3HV) and 3-hydroxyhexanoate (3HHX).
Types of PHA
Researchers classify PHAs according to the carbon chain length of their constituent monomers.
Short-Chain-Length PHAs (SCL-PHAs)
The monomers that compose SCL-PHAs contain 3–5 carbon atoms with poly(3-hydroxybutyrate) (PHB) being an example. SCL-PHAs display thermoplastic properties that resemble conventional plastics but their brittleness and poor toughness restrict their use.
Medium/Long-Chain-Length PHAs (MCL-PHAs)
MCL-PHAs consist of monomers ranging from 6 to 14 carbon atoms which allow them to have improved mechanical properties and transparency. Medical devices, cosmetic products, and tissue engineering applications frequently use poly(3-hydroxyhexanoate) (PHBHx) because of its superior toughness and resistance to hydrolysis.
Comparison with Other Bioplastics
PHAs present multiple benefits when compared to PLA and PBAT.
| Comparison Aspect | PHAs | PLA | PBAT |
| Feedstock Flexibility | Produced from diverse renewable resources (e.g., agricultural waste, oils) | Mainly derived from plant starch (e.g., corn) | Fossil-based but biodegradable |
| Biodegradability | Fully biodegradable in soil, water, and marine environments | Requires industrial composting (>55°C) for degradation | Biodegradable in composting systems |
| Material Properties | High mechanical strength and optical clarity (suitable for premium use) | Moderate strength, lower clarity | Flexible but lower strength |
| Production Cost | Currently high, but improving with new extraction and purification methods | Relatively lower production cost | Moderate cost |
The commercial scaling of PHA production continues to face obstacles due to its high production costs and efficiency limitations.
Advances in PHA Production
Genetic Engineering of Microbial Strains
The application of genetic modification stands as an essential method for increasing PHA production. Researchers utilize CRISPR/Cas9 technology to optimize metabolic pathways which leads to increased production of targeted monomers while reducing unwanted by-products. Through synthetic biology techniques the carbon flux towards PHA synthesis increases resulting in better conversion efficiency.
Utilization of Low-Cost Feedstocks
To cut production expenses many researchers have used renewable low-cost substrates like agricultural residues and industrial by-products together with molasses and biodiesel production glycerol. The use of these resources supports environmental sustainability efforts by decreasing dependency on fossil-based feedstocks.
Continuous Fermentation and Downstream Processing
The introduction of continuous fermentation represents a major advancement that significantly improves PHA production efficiency. The continuous fermentation process provides better production yields and decreases operational expenses when compared to batch fermentation. A company based in Zhuhai, China has achieved effective scaling of continuous fermentation using 20 m3 bioreactors. Production costs have been further reduced through improved downstream processes which enhance cell recovery and purification methods.
Material Properties and Customization
Mechanical, Thermal, and Barrier Properties
PHAs show thermoplastic behavior which lets their mechanical and thermal properties adjust depending on chemical composition variations. The selection of monomers with various chain lengths allows for modifications to crystallinity as well as elasticity, toughness, and strength. The barrier ability of materials including water vapor permeability depends on both molecular weight and crystallinity but can achieve improvement through the addition of fatty acid esters.
Mechanical properties of the PHA-based composites
| Composite | Tensile Strength (MPa) | Young's Modulus (MPa) | Elongation at Break (%) | Ref. |
| PHBV | 5.82 ± 0.50 | 67.7 ± 5.2 | 50.2 ± 4.5 | [51] |
| 50PHBV/50SF | 3.87 ± 0.37 | 60.5 ± 5.0 | 29.8 ± .7 |
| PHBHHx | 11.7 ± 0.5 | | 204 ± 5 | [12] |
| PHBHHX/SF | 11.5 ± 0.5 | | 175 ± 5 |
| PHB | 6.23 ± 0.3 | | 11.74 | [50] |
| PHB/SF | 3.81 ± 0.1 | | 17.10 |
| PHB | 87 ± 3.02 | 74.45 ± 2.88 | 26 ± 1.67 | [32] |
| PHB/10 wI% CTS | 63.66 ± 6.10 | 52.79 ± 4.52 | 46 ± 4.02 |
| PHB/20 wI% CTS | 31.6 ± 3.37 | 50.74 ± 2.23 | 65.5 ± 2.25 |
| Aligned PHB | 16.2 ± 3.11 | 202.1 ± 97.6 | 7.3 ± 0.8 | [52] |
| Aligned PHB/15% wt% CTS | 8.73 ± 3.65 | 210.2 ± 90.9 | 1.45 ± 0.67 |
| Random PHB | 7.6 ± 0.8 | 164.3 ± 82.4 | 3.83 ± 0.69 |
| Random PHB/15 w% CTS | 6.41 ± 3.32 | 150.8 ± 93.6 | 1.19 ± 0.71 |
| PHBV | 4.01 ± 0.27 | 108 ± 2.61 | 56.34 ± 2.66 | [14] |
| PHBV/Col 50:50 | 2.17 ± 0.27 | 70.55 ± 1.78 | 8.17 ± 1.60 |
| PHBV | 94 | | | [11] |
| PHBV/GO | 254 | | |
| PHBV/GO/Collagen | 241 | | |
| PHB | 8.4 ± 1.9 | 554 ± 25 | 3.8 ± 1.2 | [80] |
| PHB/CTS | 8.7 ± 1.2 | 467 ± 22 | 84.1 ± 4.7 |
| PHB/CTS/BCP | 16.5 ± 0.9 | 524 ± 20 | 99.2 ± 5.1 |
| PHB | 3.8 | | 11.71 | [54] |
| PHB/CTS | 3.4 | | |
| PHBICTS/1 wt% MWCNT | 10 | | 20.99 |
| PHB | 10.67 ± 1.01 | 238 ± 52 | 7.27 ± 0.49 | [58] |
| PHB/nHA (blend) | 16.16 ± 0.86 | 397 ± 107 | 12.48 ± 1.57 |
| PHB/nHA (spray) | 5.47 ± 0.18 | 138 ± 19 | 4.90 ± 0.25 |
| PHBV | 4.41 ± 0.27 | 106.70 ± 31.33 | | [84] |
| PHBV/10 nHABR | 6.35 ± 0.38 | 158.60 ± 34.67 | |
| PHB | 1.2 ± 0.2 | | 10.6 ± 1.4 | [88] |
| PHB/25 wt% PLCL | 1.2 ± 0.2 | | 41.6 ± 0.8 |
| PHBV (100 wt%, w/w) | 0.1 | 0.34 | 108.32 | [91] |
| PHBV/PLGA(50:50 wt%, w/w) | 4.65 | 47 | 125.65 |
| PHBV/PCL(50:50 wt%, w/w) | 2.56 | 20.63 | 115 |
| PHBV/PCL (50:50 wt%, w/w)± 1 wt% CA | 1.55 | 7.47 | 210 |
| PHBV/PCL (50:50 wt%, w/w)± 10 wt% CA | 1.2 | 7.44 | 43 |
| PHB | 18.8 | | 7 | [64] |
| 40PHB/60PCL | 26.9 | | 1358 |
| PHBHHx | 10 | 220 | 102 | [92] |
| 50HBHHx/50CS-g-PCL | 19 | 390 | 148 |
| PHB | 2 | 108 | | [16] |
| PHB/0.5 wt% CNT | 5.15 | 285 | |
| 60PLA/40PHB | 11.8 | | 43.8 | [95] |
| 60PLA/40PHB/0.1 wt% HACNT | 27.87 | | 346.68 |
| PHB | 12.4 | | | [93] |
| PHB/MWCNT | 16.2 | | |
| PHB/MWCNT/hot stretching | 21.7 | | |
| PHB | 1.13 ± 0.021 | 99.41 ± 2.88 | | [102] |
| PHB/7.5 wt% nBG | 1.91 ± 1.00 | 30.59 | |
| Cancellous bone | 2-12 | 20-500 | | [28, 103] |
| Cortical bone | 100-230 | 3000-30.000 | |
| Cartilages | 3.7-10.5 | 0.7-15.3 | | [103] |
Blending and Copolymerization
The combination of PHAs with either biodegradable or non-biodegradable polymers such as PLA and PBAT improves their mechanical strength and processability. Compostable fibers with excellent mechanical performance are produced through blends of PHA and PLA. The combination of lactate or aliphatic monomers through copolymerization enables manipulation of glass transition temperature along with crystallinity and degradation rate.
Surface Modification and Functionalization
Techniques like chemical grafting and physical coating enable the enhancement of PHA surface properties through surface modification. Graphene oxide (GO) nanofillers improve both mechanical strength and thermal stability when introduced into materials. Surface engineering enables the addition of functionalities including antimicrobial properties and accelerated degradation under certain conditions.
Biodegradability in Various Environments
PHAs demonstrate effective degradation properties in soil environments as well as marine and compost conditions. Scientists can control degradation rates of PHAs by modifying their molecular weight and monomer composition. The degradation rate of SCL-PHAs is faster in marine environments compared to MCL-PHAs which have a slower degradation rate for certain applications.
Diverse Application Potential
The mechanical strength along with biocompatibility and biodegradability of PHAs make them suitable for packaging, agricultural, and healthcare applications. PHAs function as fully compostable films and components for medical scaffolds as well as drug delivery systems.
Applications of PHA-Based Materials
Packaging and Disposable Plastics
PHAs serve as eco-conscious materials in packaging applications including biodegradable bags, films, foams and food wrapping solutions. The natural decomposition process of these materials results in CO2 and water production which helps minimize environmental pollution over time. PHAs prove effective as materials for disposable tableware along with containers and shopping bags.
Applications of polyhydroxyalkanoates (PHAs) from biomass
Biomedical Applications
PHAs serve as materials for surgical sutures and drug delivery systems thanks to their outstanding biocompatibility and biodegradability which also makes them suitable for tissue engineering scaffolds bone implants cardiovascular stents and wound dressings. PHA sutures naturally break down in the body which means they do not require surgical removal. When used as drug carriers these materials provide both controlled drug release and enhanced treatment results.
Agricultural Films and Controlled-Release Fertilizers
PHAs serve the agricultural sector by producing both mulch films and encapsulation materials for both fertilizers and pesticides. These materials lower environmental pollution while extending product lifespan and enhancing nutrient absorption. PHAs-based controlled-release fertilizers decrease chemical usage and lessen environmental damage.
3D Printing and Sustainable Textiles
PHAs have become popular as printing materials for 3D printing because they offer good printability and natural biodegradability. These materials provide excellent performance for both prototyping processes and precision manufacturing tasks. PHAs serve as raw materials for producing sustainable fibers and nonwoven fabrics used in both clothing and medical textile manufacturing.
Other Applications
PHAs find applications in cosmetic packaging materials and personal care products as well as tableware and electronic components. PHAs function as product integrity preservers in cosmetic packaging but degrade completely when used for household utensils. Through fermentation processes PHAs function as precursors for biofuels.
PHA stands as a renewable bio-based material with extensive application possibilities due to its low carbon footprint. PHA supports environmental protection and sustainable development across multiple industries including packaging and agriculture but faces persistent challenges related to production costs and scalability which demand continued innovation.
References
- Taguchi, Seiichi, and Ken'ichiro Matsumoto. "Evolution of polyhydroxyalkanoate synthesizing systems toward a sustainable plastic industry." Polymer Journal 53.1 (2021): 67-79.
- Pryadko, A., et al. "Review of hybrid materials based on polyhydroxyalkanoates for tissue engineering applications." Polymers 13.11 (2021): 1738.
- Adeleye, A. T., et al. "Sustainable synthesis and applications of polyhydroxyalkanoates (PHAs) from biomass." Process biochemistry 96 (2020): 174-193.
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