## 1 Introduction
### Polyhydroxyalkanoates  
Polyhydroxyalkanoates (PHAs) are biodegradable biopolymers metabolically produced in nature in small quantities by various prokaryote organisms and some eukaryotes [12] as a form of storage component. They are stored in bacteria in the form of intracellular granules (Figure 1) [13]. Their diverse monomeric structure guarantees these polymers' variable physical characteristics so that they serve different applications [14]. PHA is characterized into three groups according to the number of carbon atoms that make up their monomers: short-chain PHA, with 3-5 carbons in their structure; medium-chain PHA, with 6-14 carbons in their structure; and long-chain PHA, possessing a number of carbons greater than 14 [15]. PHAs are a class of polyesters characterized by ester bonds linking the carboxyl group of one monomer with the hydroxyl group of another. The majority of PHAs are produced in the R form [16]. Polyhydroxyalkanoate (PHA) molecules typically consist of a range of 600 to 35,000 monomer units. Approximately 150 distinct monomers have been recognized thus far, and this count is expected to increase as new types of PHAs are developed through chemical or physical modifications of natural PHA or via the utilization of genetically modified organisms [17]. The critical attributes exhibited by PHAs are the piezoelectric effect, nontoxicity, thermoplasticity, gas impermeability, hydrophobicity, enantiomeric purity, and a high degree of polymerization [18]. The multiple mechanical characteristics of polyhydroxyalkanoate (PHAs) arise from the range of monomers that may be included in their polymer structure, resulting in variations in flexibility, strength, and crystallinity. These properties exhibit variation based on factors such as the organism responsible for production, the number of carbons in a monomeric unit, and the specific growth circumstances [19]. Within microbial cells, PHA is present in the form of granules, also referred to as carbonaceous (Figure 1). These granules are complex molecules organized into layers, which are composed of different polypeptides, as well as proteins. Among the proteins present in the layers, phasins stand out, which can fill up to \(\frac{3}{4}\) of the granule space, exercising a structural function by maintaining the spherical shape of the granules, in addition to participating in cell metabolism. The size of these granules varies between 100-500 nm, depending on the organism and the composition of the phasins that constitute the granule [20]. This versatility is essential in synthesizing and depolymerizing PHA molecules, promoting polymer accumulation, and determining granules' size, quantity, and distribution. Other examples of phasins include PhaP1 from _Cupriavidus necator_, which can increase the activity of PhaC1 class II and PhaC2 from _Pseudomonas aeruginosa_ in vitro, HAF from _Pseudomonas putida_, which regulates PHA synthesis, and ApdA from _Rhodospirillum rubrum_ which plays a role in activating the depolymerization of Poly-(3-hydroxybutyrate) [20; 21]. Short-chain PHAs (SCL-PHAs) exhibit high crystallinity, hardness, and fragility, but with slow crystallization rates and low impact resistance, along with rapid aging, making their use difficult. Poly-(3-hydroxybutyrate) (PHB), a homopolymer with four carbon subunits of 3-hydroxybutyrate (3 HB), is the most frequent SCL-PHA, but other representatives of this group are poly(3-hydroxyactanoate) (PHO), poly(4-hydroxybutyrate) (P4 HB), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (PHB-4 HB), and poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) copolymers (PHBHHx), as shown in Figure 2[22]. Figure 1: PHA production and cellular functions. Medium-chain-length PHA (MCL-PHA) polyhydroxyalkanoatees exhibit distinct characteristics in comparison to SCL-PHA. This group is inherently elastomeric, more flexible than SCL-PHA, and has a lower melting point and is less crystalline.