A deep knowledge has to be acquired on the chemical behaviour of the nanomaterials and on the interactions they establish with the bioenvironments. Finally, recent advances and emerging designs and applications will be discussed. Keywords: Biomaterials, stem cells, polymers, tissue engineering, scaffolds, nanocomposites.
Abstract: Background: Regenerative medicine deals with developing strategies to repair or substitute organ or tissue functionality; it involves any combination of specific cell types, engineered polymeric biomaterials with biochemical inducers to promote cell adhesion, migration, growth, and differentiation to restore physiological tissue function, and to stimulate regeneration of damaged and previously irreparable organs.
Journal Name: Current Organic Chemistry. Volume 22 , Issue 12 , Polylactic acid or polylactide PLA, Poly is a biodegradable thermoplastic aliphatic polyester derived from renewable resources, such as corn starch, tapioca roots, chips, starch, or sugarcane. PLA is obtainable primarily by the ionic polymerization of lactide, a ring closure of two lactic acid molecules. Lactide itself can be made through lactic acid fermentation from renewable resources such as starch by means of various bacteria.
PLA can also be produced directly from lactic acid by polycondensation. However, this process yields low molecular weight polymers, and the disposal of the solvent is a problem in the industrial production. The properties of PLA primarily depend on the molecular mass, the degree of crystallinity, and possibly the proportion of co-monomers. A higher molecular mass raises T g , as well as T m , tensile strength, elastic modulus, and lowers the strain after fracture.
Handbook of Biopolymer-Based Materials
Due to the CH 3 side group, the material has water-repellent or hydrophobic behavior. PLA is soluble in many organic solvents, such as dichloromethane or the like. PLA has higher transparency than other biodegradable polymers, and is superior in weather resistance and workability. Poly e-caprolactone PCL is a biodegradable polyester which is commonly used for preparing scaffolds for various tissue engineering applications. It is used in the manufacture of specialty polyurethanes with good water, oil, solvent, and chlorine resistance.
PCL can be used as an additive for resins to improve its impact resistance and their processing characteristics. PCL is compatible with a range of other materials which enable it to be mixed with other biodegradable polymers to lower its cost and increase biodegradability. It can also be used as a polymeric plasticizer to other polymers like PVC.
Polycaprolactone is also used for splinting, modeling, and as a feedstock for prototyping systems such as a 3D printer. PCL is biocompatible, biodegradable polymer which has shown to be capable of supporting a wide variety of cell types. PCL is also a well-known FDA-approved biomaterial which is most widely used in biomedical field [ 40 ]. The PCL is intrinsic hydrophobic in nature. Its poor surface wetting and interaction with the biological fluids avoid cells adhesion and proliferation [ 41 ].
The significance of PCL in various tissue engineering applications increased because it is degraded by hydrolysis of its ester linkages in physiological conditions.
This property makes it useful for various implantable biomaterials. Since its degradation rate is lower than that of polylactide, it is proposed to be used as long-term implantable devices.
It is commercially available as beads or as solutions in water. It is used as thickener in glues, paper-making, sizing agent in textiles, water-soluble films useful for packing, and a variety of coatings. Polyvinyl alcohol is used as an emulsion polymerization aid, as protective colloid, to make polyvinyl acetate dispersions. PVA is prepared by first polymerizing vinyl acetate, and the resulting polyvinyl acetate is converted to the PVA.
PVA has a wide range of potential applications in optical, pharmaceutical, medical, and membrane fields. It is a water-soluble polymer and allows the development of environment-friendly material processes [ 42 ]. Polyvinyl acetate PVAc belongs to polyvinyl ester family and is a thermoplastic resin produced by the polymerization of vinyl acetate monomer. Most polyvinyl acetate emulsions available in market contain various co-monomers such as n-butyl acrylate, 2-ethyl hexyl acrylate, ethylene, dibutyl maleate, and dibutyl fumarate.
Polyvinyl alcohol PVOH is produced by methanolysis or hydrolysis of polyvinyl acetates. The reaction can be controlled to produce any degree of replacement of acetate groups.
Polyvinyl acetate has immense applications in various fields like biomedical, synthesis of metal nanoparticles, sensing activity, and so on [ 44 ]. Collagen is the most abundant protein in human. It is found in the bones, muscles, skin, and tendons, where it forms a scaffold to provide strength and structure. Among this, type I collagen fibrils are stronger than steel hence is the substance that holds the whole body together. Collagen gives the skin its strength and structure, and also plays a role in the replacement of dead skin cells. Collagen in medical products can be derived from human, bovine, porcine, and ovine sources.
Collagen dressings attract new skin cells to wound sites [ 46 ]. Controllable factors that damage the production of collagen include sunlight, smoking, and high sugar consumption. Poly 3-hydroxybutyrate-cohydroxyvalerate PHBV, is a polyhydroxyl- alkanoate- type thermoplastic linear aliphatic polyester.
PHBV Fig. It is biodegradable, nontoxic, and biocompatible plastic which can be produced from glucose and propionate by the recombinant Escherichia coli strains [ 47 ]. PHBV is a copolymer of 3-hydroxybutanoic acid and 3-hydroxypentanoic acid and can also be synthesized from butyrolacone and valerolactone in presence of oligomeric aluminoxane as catalyst. PHBV has many applications especially for the development of implanted medical devices for dental, orthopedic, hernioplastic, and skin surgery. Various potential medical devices like bioresorbable surgical sutures, biodegradable screws and plates for cartilage and bone fixation, biodegradable membranes for periodontal treatment, surgical meshes with PHBV coating for hernioplastic surgery, wound coverings, and the like have been developed using this biopolymer [ 48 ].
Biopolymers, due to its biocompatible and biodegradable nature, can be used to improve the performances of other biologically active molecules in a product. They can also be modified to suite various potential applications which include the following. Nanotechnology is the science of nanomaterials which deals with its synthesis, characterization, and applications. Researchers are currently focusing on developing more eco-friendly processes for the synthesis of nanoparticles. Metal nanoparticles, due to their quantum size effects, possess various novel properties.
However, most of their synthesis protocol imposes a major threat to the environment [ 49 ]. In common synthetic methods, the reducing agents used which include organic solvents and toxic-reducing agents like hydrazine, N-dimethylformamide, and sodium borohydride are considered to be highly toxic for the environment [ 50 ]. All these chemicals are highly reactive and pose potential environmental and biological risks. Biopolymers have been extensively used as capping and reducing agent for the synthesis of various nanoparticles. Biopolymers like chitosan, heparin, soluble starch, cellulose, gelatin, PVA, PVP, and so on can be used to replace various toxic regents in synthesizing different nanoparticles [ 51 — 53 ].
In recent years, biopolymer materials have aroused great interest because of their biomedical applications, such as those in tissue engineering, pharmaceutical carriers, and medical devices [ 54 , 55 ]. A common biopolymer, gelatin, was widely applied in medicine for dressing wounds, as an adhesive, and so on. Porous gelatin scaffolds and films were produced with the help of solvents or gases as simple porogens, which enable the scaffolds to hold drug or nutrients to be supplied to the wound for healing [ 56 ].
Electrospun PLGA-based scaffolds have been applied extensively in biomedical engineering, such as tissue engineering and drug-delivery system [ 57 ]. MWCNT-incorporated electrospun nanofibers with high surface area-to-volume ratio and porous characteristics have also shown potential applications in many aspects of tissue engineering [ 58 , 59 ]. Biomaterials made from proteins, polysaccharides, and synthetic biopolymers are preferred but lack the mechanical properties and stability in aqueous environments necessary for medical applications.
Cross-linking improves the properties of the biomaterials, but most cross-linkers either cause undesirable changes to the functionality of the biopolymers or result in cytotoxicity.
ACS Symposium Series (ACS Publications)
Glutaraldehyde, the most widely used cross-linking agent, is difficult to handle and contradictory views have been presented on the cytotoxicity of glutaraldehyde-cross-linked materials [ 60 ]. Replacing the oil-based packaging materials with biobased films and containers might give not only a competitive advantage due to more sustainable and greener image but also some improved technical properties.
Biopolymers are currently used in food coatings, food packaging materials, and encapsulation matrices for functional foods. They provide unique solutions to enhance product shelf life while also reducing the overall carbon footprint related to food packaging [ 61 ]. Within food-related applications, these biobased materials are particularly useful in three main areas: food packaging, food coating, and edible films for food and encapsulation.
The most commercially viable materials in food packaging are certain biodegradable polyesters and thermoplastics like starch, PLA, PHA, and so on, which can be processed by conventional equipment. These materials are already used in a number of monolayer and multilayer applications in the food-packaging field. Starch and PLA biopolymers are potentially the most attractive types of biodegradable material. This is due to the balance of their properties and the fact that they have become commercially available. PLA is of particular interest in food packaging, due to its excellent transparency and relatively good water resistance.
The challenge for these specific biomaterials is to improve their barrier and thermal properties so that they perform like polyethylene terephthalate PET. Other materials extracted from biomass resources, such as proteins e. The inherent high rigidity and the difficulty of processing them in conventional equipment are the main drawbacks of these types of materials.