The term “biodegradable” ensures products' reliability in the environment without harmful effects. The ASTM (e.g., ASTM D5511-18 and ASTM D5526-18, American Society for Testing and Materials) defined biodegradable materials as materials that exhibited 70% degradation during 30 days under anaerobic conditions29,30. Under aerobic conditions, ASTM D6400-19 and ASTM D6868-19 require 90% mineralization degradation of the material into CO2 within 180 days31,32. Biodegradable polymers can be synthesized or extracted from different sources (Figure 1). They can be obtained from agro-resource biomass (i.e., agro-polymers). They can be extracted from microorganisms. They can be obtained from the conventional synthesis of bio-derived monomers. Petrochemicals are important sources of monomers used for the synthesis of degradable polymers. They can be classified as (Figure 1):-
(1) Natural-based materials, e.g., polysaccharides (e.g., cellulose, lignin, starch, chitin/chitosan) or proteins (e.g., collagen, Silk fibril (SF)).
(2) Synthetic polymers, fossil oil, or petroleum-derived polymers e.g., poly(ε-caprolactone) (PCL) poly(lacticacid) (PLA), poly(butylenesuccinate), poly(glycolic acid) (PGA), poly(ɛ-caprolactone) (PCL), Poly(vinyl alcohol) (PVA), Poly(vinylpyrrolidone) (PVP), polybutylene succinate (PBS), and poly(hydroxyl butyrate) (PHB)33–35.
Natural polymers can be classified as polysaccharides, polyamides, and polynucleotides (Figure 1). They can also be listed as plant-based polysaccharides (e.g., cellulose, alginate, or starch) and animal-derived polymers (e.g., collagen, silk fibroin, or chitosan). They exhibit advantages, such as high biocompatibility, low toxicity, low cost, good mechanical properties, and high biodegradability. Several linkages ensure high biodegradability (Figure 2).
Cellulose has been considered the most abundant biopolymer in nature (Figure 2)36–45. It is a linear polysaccharide biopolymer consisting of β-1,4-linked D-glucose units. It can be extracted from several sources, such as trees and cotton. The chemical structure of cellulose shows a large number of hydroxyl (‒OH) groups46. Cellulose can proceed into the nanoscale, such as nanofibers (CNFs) and nanocrystals (CNCs).
Chitosan consists of randomly distributed β-(1-4)-linked D-glucosamine and N-acetyl-d-glucosamine47–49. It is the de-acetylated form of chitin offering amino groups. It can be dissolved in an acidic solution forming a cationic polysaccharide. It exhibits good adhesive properties with high antibacterial activity.
Synthetic polymers are usually extracted or synthesized via well-known reactions such as condensation or ring-opening polymerization (ROP). Several synthesis methods were reported for the synthesis of new organic polymers. Most of these methods require the use of a catalyst. The synthesis procedures aim to polymerize monomers with suitable functional groups that tend to polymerization. There are three main steps for polymerization: initiation, propagation, and termination. MOFs are coordination polymer formed via self-asselmbly of metal ions and organic linkers 28,50–58. They can be decomposed into their constituent in harsh consition i.e. acid or base dpending on the MOFs materials.
Aliphatic polyester polylactide (PLA can be synthesized via ROP reaction of lactide using a catalyst such as tin (II) octoate. It can also be prepared via the condensation of lactic acid. It can also be extracted from corn or wheat. It displays good transparency with thermoplastic properties. It can be considered bioplastic. It is an everyday use for three-dimensional printing (3D printing)35. It can be degraded via hydrolysis, thermal decomposition, or photodegradation.
Like PLA, PCL can be obtained by ROP of 𝜀-caprolactone [26]. United States Food and Drug Administration (US-FDA) approved PCL for biomedical applications such as tissue engineering. PCL exhibits high biocompatibility, high biodegradability, good chemical resistance, and high ductility. It displays a melting point of 65 °C with tunable viscosity enabling processing using several methods. However, it has high Young’s modulus and strength.
A water-soluble polymer such as PVA can be synthesized via hydrolysis poly(vinyl acetate)59. It exhibits high transparency, high strength, good flexibility, and high biocompatibility. However, it has a high density of –OH groups; it is hard to shape via conventional processes such as melting methods. However, it can be easily blended with other materials via a mixing procedure. PVA–based materials were widely used for resistance random access memory 60–62. PVP is another water-soluble polymer. It can be synthesized via the radical polymerization of N-vinylpyrrolidone 63. PVP displays high chemical resistance, easy processability, high transparency, good biocompatibility, and low cost. PVP was used for several applications, such as wearable electronic devices and gas sensors64.
Natural polymers exhibit good intrinsic biocompatibility and enzymatic degradability, enabling intensive applications for biomedical applications (Figure 2). Biodegradable polymers are common clinical polymers, including polyglycolide, polylactides, and polycaprolactone. They can be classified as molecular, microstructural, and macroscopic. The properties of biodegradable polymers can be evaluated using UV–Vis spectroscopy, mass loss profiles, mass spectrometry, nuclear magnetic resonance (NMR), Fourier transform infrared (FT-IR) spectroscopy, Gel permeation chromatography (GPC), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM).
The properties of polymers depend on several parameters, including chemical composition, particle size, morphology, and surface structure. Small particles polymers undergo aggregation into bulk materials, especially without proper stabilization. Nanoscale polymers have high surface energy pushing the materials to aggregate to achieve high stability. Distributing the polymers uniformly in matrices can reduce or prevent the assembly.
The size of a polymer affects the properties of polymers. The morphology of the polymers can be spherical, rode, or plate, depending on the radius (R) and the thickness (t, Figure 3). Therefore, the ratio between volumes of interface material to the volume of the particle (Vinterface/Vparticle) increases with the decrease of particle size. The aspect ratio (the ratio of the diameter (2r) to the length (L)) determines the morphology of the polymers. Based on the aspect ratio, the morphology can be a plate, sphere, and rod with an aspect ratio of < 1, 1, and >1, respectively (Figure 3).