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Biodegradable polymers are a special class of
polymer
A polymer (; Greek '' poly-'', "many" + ''-mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
that
breaks down after its intended purpose by bacterial decomposition process to result in natural byproducts such as gases (
CO2,
N2),
water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
,
biomass
Biomass is plant-based material used as a fuel for heat or electricity production. It can be in the form of wood, wood residues, energy crops, agricultural residues, and waste from industry, farms, and households. Some people use the terms bi ...
, and inorganic salts.
These polymers are found both naturally and
synthetically made, and largely consist of
ester
In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides ar ...
,
amide
In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a compound with the general formula , where R, R', and R″ represent organic groups or hydrogen atoms. The amide group is called a peptide bond when it is ...
, and
ether
In organic chemistry, ethers are a class of compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula , where R and R′ represent the alkyl or aryl groups. Ethers can again be c ...
functional groups. Their properties and breakdown mechanism are determined by their exact structure. These polymers are often synthesized by
condensation reaction
In organic chemistry, a condensation reaction is a type of chemical reaction in which two molecules are combined to form a single molecule, usually with the loss of a small molecule such as water. If water is lost, the reaction is also known as a ...
s,
ring opening polymerization
In polymer chemistry, ring-opening polymerization (ROP) is a form of chain-growth polymerization, in which the terminus of a polymer chain attacks cyclic monomers to form a longer polymer (see figure). The reactive center can be radical, anion ...
, and
metal catalysts. There are vast examples and applications of biodegradable polymers.
Bio-based packaging materials have been introduced as a green alternative in the past decades, among which, edible films have gained more attention due to their environmentally-friendly characteristics, vast variety and availability, non-toxicity, and low cost.
History
Biodegradable polymers have a long history, and since many are natural products, the precise timeline of their discovery and use cannot be accurately traced. One of the first medicinal uses of a biodegradable polymer was the
catgut suture
Catgut suture is a type of surgical suture that is naturally degraded by the body's own proteolytic enzymes. Absorption is complete by 90 days, and full tensile strength remains for at least 7 days. This eventual disintegration makes it good for ...
, which dates back to at least 100 AD.
The first catgut sutures were made from the intestines of sheep, but modern catgut sutures are made from purified collagen extracted from the small intestines of cattle, sheep, or goats.
The concept of synthetic
biodegradable plastics and polymers was first introduced in the 1980s.
In 1992, an international meeting was called where leaders in biodegradable polymers met to discuss a definition, standard, and testing protocol for biodegradable polymers.
Also, oversight organizations such as
American Society for Testing of Materials (ASTM) and the
International Standards Organization
The International Organization for Standardization (ISO ) is an international standard development organization composed of representatives from the national standards organizations of member countries. Membership requirements are given in Art ...
(ISO) were created. Large clothing and grocery store chains have pushed to utilize biodegradable bags in the late 2010s. Biodegradable polymers also received notice from various fields in 2012 when Professor Geoffrey Coates of Cornell University received the
Presidential Green Chemistry Challenge Award
Green chemistry, also called sustainable chemistry, is an area of chemistry and chemical engineering focused on the design of products and processes that minimize or eliminate the use and generation of hazardous substances. While environmental che ...
. As of 2013, 5-10% of the
plastic
Plastics are a wide range of synthetic or semi-synthetic materials that use polymers as a main ingredient. Their plasticity makes it possible for plastics to be moulded, extruded or pressed into solid objects of various shapes. This adaptab ...
market focused on biodegradable polymer derived plastics.
Structure and properties
The structure of biodegradable polymers is instrumental in their properties. While there are innumerable biodegradable polymers, both
synthetic and natural, there are a few commonalities among them.
Structure
Biodegradable polymers tend to consist of
ester
In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides ar ...
,
amide
In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a compound with the general formula , where R, R', and R″ represent organic groups or hydrogen atoms. The amide group is called a peptide bond when it is ...
, or
ether
In organic chemistry, ethers are a class of compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula , where R and R′ represent the alkyl or aryl groups. Ethers can again be c ...
bonds. In general, biodegradable polymers can be grouped into two large groups based on their structure and synthesis. One of these groups is agro-polymers, or those derived from
biomass
Biomass is plant-based material used as a fuel for heat or electricity production. It can be in the form of wood, wood residues, energy crops, agricultural residues, and waste from industry, farms, and households. Some people use the terms bi ...
.
[ The other consists of biopolyesters, which are those derived from ]microorganism
A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
s or synthetically made from either naturally or synthetic monomers.
Agro-polymers include polysaccharide
Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with wa ...
s, like starch
Starch or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants for energy storage. Worldwide, it is the most common carbohydrate in human diets ...
es found in potatoes or wood, and protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
s, such as animal based whey or plant derived gluten. Polysacharides consist of glycosidic bond
A glycosidic bond or glycosidic linkage is a type of covalent bond that joins a carbohydrate (sugar) molecule to another group, which may or may not be another carbohydrate.
A glycosidic bond is formed between the hemiacetal or hemiketal group ...
s, which take a hemiacetal
A hemiacetal or a hemiketal has the general formula R1R2C(OH)OR, where R1 or R2 is hydrogen or an organic substituent. They generally result from the addition of an alcohol to an aldehyde or a ketone, although the latter are sometimes called hemike ...
of a saccharide
In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or may ...
and binds it to an alcohol
Alcohol most commonly refers to:
* Alcohol (chemistry), an organic compound in which a hydroxyl group is bound to a carbon atom
* Alcohol (drug), an intoxicant found in alcoholic drinks
Alcohol may also refer to:
Chemicals
* Ethanol, one of sev ...
via loss of water. Proteins are made from amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
s, which contain various functional groups. These amino acids come together again through condensation reaction
In organic chemistry, a condensation reaction is a type of chemical reaction in which two molecules are combined to form a single molecule, usually with the loss of a small molecule such as water. If water is lost, the reaction is also known as a ...
s to form peptide bond
In organic chemistry, a peptide bond is an amide type of covalent chemical bond linking two consecutive alpha-amino acids from C1 (carbon number one) of one alpha-amino acid and N2 (nitrogen number two) of another, along a peptide or protein cha ...
s, which consist of amide
In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a compound with the general formula , where R, R', and R″ represent organic groups or hydrogen atoms. The amide group is called a peptide bond when it is ...
functional groups. Examples of biopolyesters include polyhydroxybutyrate
Polyhydroxybutyrate (PHB) is a polyhydroxyalkanoate (PHA), a polymer belonging to the polyesters class that are of interest as bio-derived and biodegradable plastics. The poly-3-hydroxybutyrate (P3HB) form of PHB is probably the most common type ...
and polylactic acid
Polylactic acid, also known as poly(lactic acid) or polylactide (PLA), is a thermoplastic polyester with backbone formula or , formally obtained by condensation of lactic acid with loss of water (hence its name). It can also be prepared by rin ...
.
Properties
Even though biodegradable polymers have numerous applications, there are properties that tend to be common among them. All biodegradable polymers should be stable and durable enough for use in their particular application, but upon disposal they should easily break down. Polymers, specifically biodegradable polymers, have extremely strong carbon backbones that are difficult to break, such that degradation
Degradation may refer to:
Science
* Degradation (geology), lowering of a fluvial surface by erosion
* Degradation (telecommunications), of an electronic signal
* Biodegradation of organic substances by living organisms
* Environmental degradatio ...
often starts from the end-group
End groups are an important aspect of polymer synthesis and characterization. In polymer chemistry, they are functional groups that are at the very ends of a macromolecule or oligomer (IUPAC). In polymer synthesis, like condensation polymerizati ...
s. Since the degradation begins at the end, a high surface area
The surface area of a solid object is a measure of the total area that the surface of the object occupies. The mathematical definition of surface area in the presence of curved surfaces is considerably more involved than the definition of arc ...
is common as it allows easy access for either the chemical, light, or organism. Crystallinity
Crystallinity refers to the degree of structural order in a solid. In a crystal, the atoms or molecules are arranged in a regular, periodic manner. The degree of crystallinity has a big influence on hardness, density, Transparency and translucen ...
is often low as it also inhibits access to end groups. A low degree of polymerization
The degree of polymerization, or DP, is the number of monomeric units in a macromolecule or polymer or oligomer molecule.
For a homopolymer, there is only one type of monomeric unit and the ''number-average'' degree of polymerization is given b ...
is normally seen, as hinted at above, as doing so allows for more accessible end groups for reaction with the degradation initiator. Another commonality of these polymers is their hydrophillicity. Hydrophobic
In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water.
Hydrophobic molecules tend to be nonpolar and, th ...
polymers and end groups will prevent an enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...
from easily interacting if the water-soluble enzyme cannot easily get in contact with the polymer.
Other properties of biodegradable polymers that are common among those used for medicinal usages include being:
* non-toxic
Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a subst ...
* capable of maintaining good mechanical integrity until degraded
* capable of controlled rates of degradation
A goal is not to elicit the immune response, and the products of degradation also need not to be toxic. These are important as biodegradable polymers are used for drug delivery where it is critical to slowly release the drug into the body over time instead of all at once and that the pill is stable in the bottle until ready to be taken. Factors controlling the rate of degradation include percent crystallinity
Crystallinity refers to the degree of structural order in a solid. In a crystal, the atoms or molecules are arranged in a regular, periodic manner. The degree of crystallinity has a big influence on hardness, density, Transparency and translucen ...
, molecular weight
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...
, and hydrophobicity
In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water.
Hydrophobic molecules tend to be nonpolar and, th ...
. The degradation rate depends on the location in the body, which influences the environment surrounding the polymer such as pH, enzymes concentration, and amount of water, among others. These are rapidly decomposed.
Synthesis
One of the most important and most studied groups of biodegradable polymers are polyesters
Polyester is a category of polymers that contain the ester functional group in every repeat unit of their main chain. As a specific material, it most commonly refers to a type called polyethylene terephthalate (PET). Polyesters include natural ...
. Polyesters can be synthesized in a number of ways including direct condensation of alcohols and acids, ring opening polymerizations (ROP), and metal-catalyzed polymerization reactions. A great disadvantage of the step-wise polymerization via condensation of an acid and an alcohol is the need to continuously remove water from this system in order to drive the equilibrium of the reaction forward. This can necessitate harsh reaction conditions and long reaction times, resulting in a wide dispersity. A wide variety of starting materials can be used to synthesize polyesters, and each monomer type endows the final polymer chain with different characteristics and properties. The ROP of cyclic dimeric glycolic or lactic acid forms α-hydroxy acids which then polymerize into poly-(α-esters). A variety of organometallic initiators can be used to start the polymerization of polyesters, including tin, zinc, and aluminum complexes. The most common is tin(II)octanoate and has been approved as a food additive by the U.S. FDA, but there are still concerns about using the tin catalysts in the synthesis of biodegradable polymers for biomedical uses. The synthesis of poly(β-esters) and poly(γ-esters) can be carried out by similar ROP or condensation methods as with poly(γ-esters). Development of metal-free process that involve the use of bacterial or enzymatic catalysis in polyester formation is also being explored. These reactions have the benefit of generally being regioselective and stereospecific but suffer from the high cost of bacteria and enzymes, long reaction times, and products of low molecular weight.
While polyesters dominate both the research and industrial focus on synthetic biodegradable polymers, other classes of polymers are also of interest. Polyanhydrides are an active area of research in drug delivery because they only degrade from the surface and so are able to release the drug they carry at a constant rate. Polyanhydrides can be made via a variety of methods also used in the synthesis of other polymers, including condensation, dehydrochlorination, dehydrative coupling, and ROP. Polyurethanes and poly(ester amide)s are used in biomaterials. Polyurethanes were initially used for their biocompatibility, durability, resilience, but are more recently being investigated for their biodegradability
Biodegradation is the breakdown of organic matter by microorganisms, such as bacteria and fungi. It is generally assumed to be a natural process, which differentiates it from composting. Composting is a human-driven process in which biodegradati ...
. Polyurethanes are typically synthesized using a diisocyanate, a diol, and a polymer chain extender. The initial reaction is carried out between the diisocyanate and the diol, with the diisocyanate in excess to ensure that the ends of the new polymer chain are isocyanate groups. This polymer can then be reacted with either a diol or a diamine to form urethane or urethane-urea end groups, respectively. The choice of terminal groups affects the properties of the resulting polymer. Additionally, the use of vegetable oil and biomass in the formation of polyurethanes is an active area of research.
The mechanical properties of biodegradable polymers can be enhanced with the addition of fillers or other polymers to make a composite, blend, or copolymer. Some fillers are natural fiber reinforcements such as silk nanofibers, bamboo, jute, in addition to nano-clay, and carbon nanotubes as alternatives to name a few. Each of these enhancements have a unique property that not only improve strength, but also processability, through humidity resistance, reduced gas permeability, and have shape memory/recovery. Some examples, such as the polyhydroxyalkanoates
Polyhydroxyalkanoates or PHAs are polyesters produced in nature by numerous microorganisms, including through bacterial fermentation of sugars or lipids. When produced by bacteria they serve as both a source of energy and as a carbon store. M ...
/polylactic acid
Polylactic acid, also known as poly(lactic acid) or polylactide (PLA), is a thermoplastic polyester with backbone formula or , formally obtained by condensation of lactic acid with loss of water (hence its name). It can also be prepared by rin ...
blend, shows an exceptional increase in the toughness without sacrificing optical clarity, and the copolymer poly(L-lactide-co-ε-caprolactone) has shown shape memory behavior depending on the concentration of poly-ε-caprolactone added.
Mechanism of breakdown
In general, biodegradable polymers break down to form gases, salts, and biomass
Biomass is plant-based material used as a fuel for heat or electricity production. It can be in the form of wood, wood residues, energy crops, agricultural residues, and waste from industry, farms, and households. Some people use the terms bi ...
. Complete biodegradation
Biodegradation is the breakdown of organic matter by microorganisms, such as bacteria and fungi. It is generally assumed to be a natural process, which differentiates it from composting. Composting is a human-driven process in which biodegrada ...
is said to occur when there are no oligomer
In chemistry and biochemistry, an oligomer () is a molecule that consists of a few repeating units which could be derived, actually or conceptually, from smaller molecules, monomers.Quote: ''Oligomer molecule: A molecule of intermediate relativ ...
s or monomer
In chemistry, a monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process called polymerization.
Classification
Mo ...
s left. The breakdown of these polymers depend on a variety of factors including the polymer and also, the environment the polymer is in. Polymer properties that influence degradation are bond
Bond or bonds may refer to:
Common meanings
* Bond (finance), a type of debt security
* Bail bond, a commercial third-party guarantor of surety bonds in the United States
* Chemical bond, the attraction of atoms, ions or molecules to form chemica ...
type, solubility
In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution.
The extent of the solubil ...
, and copolymer
In polymer chemistry, a copolymer is a polymer derived from more than one species of monomer. The polymerization of monomers into copolymers is called copolymerization. Copolymers obtained from the copolymerization of two monomer species are some ...
s among others. The surrounding environment of the polymer is just as important as the polymer structure itself. These factors included items such as the pH, temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied o ...
, microorganism
A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
s present, and water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
are just a few examples.
There are two primary mechanisms through which biodegradation
Biodegradation is the breakdown of organic matter by microorganisms, such as bacteria and fungi. It is generally assumed to be a natural process, which differentiates it from composting. Composting is a human-driven process in which biodegrada ...
can occur. One is through physical decomposition through reactions such as hydrolysis
Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...
and photodegradation Photodegradation is the alteration of materials by light. Commonly, the term is used loosely to refer to the combined action of sunlight and air, which cause oxidation and hydrolysis. Often photodegradation is intentionally avoided, since it destroy ...
, which can lead to partial or complete degradation. The second mechanistic route is through biological
Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary in ...
processes which can be further broken down into aerobic
Aerobic means "requiring air," in which "air" usually means oxygen.
Aerobic may also refer to
* Aerobic exercise, prolonged exercise of moderate intensity
* Aerobics, a form of aerobic exercise
* Aerobic respiration, the aerobic process of cel ...
and anaerobic
Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to:
* Anaerobic adhesive, a bonding a ...
processes. The first involves aerobic biodegradation, where oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
is present and important. In this case, the general equation seen below where Cresidue represents smaller fragments of the initial polymer such as oligomers.
The second mechanism of biodegradation is by anaerobic processes, where oxygen is not present.
There are numerous organisms that have the ability to break down natural polymers. There are also synthetic polymers
Some familiar household synthetic polymers include: Nylons in textiles and fabrics, Teflon in non-stick pans, Bakelite for electrical switches, polyvinyl chloride (PVC) in pipes, etc. The common PET bottles are made of a synthetic polymer, polye ...
that have only been around for a hundred years with new features that microorganisms do not have the capability to break down. It will take millions of years before organisms can adapt
ADAPT (formerly American Disabled for Attendant Programs Today) is a United States grassroots disability rights organization with chapters in 30 states and Washington, D.C. They use nonviolent direct action in order to bring about disability just ...
to degrade all of these new synthetic polymers. Typically, after physical processes carry out the initial breakdown of the polymer, microorganisms will then take what is left and break down the components into even simpler units. These microorganisms normally take polymer fragments, such as oligomers or monomers, into the cell where enzymes work to make adenosine triphosphate
Adenosine triphosphate (ATP) is an organic compound that provides energy to drive many processes in living cells, such as muscle contraction, nerve impulse propagation, condensate dissolution, and chemical synthesis. Found in all known forms of ...
(ATP) and polymer end products carbon dioxide, nitrogen gas, methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...
, water, mineral
In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. ( ...
s, and biomass. These enzymes act in a variety of ways to break down polymers including through oxidation
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
or hydrolysis. Examples of key enzymes include protease
A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalyzes (increases reaction rate or "speeds up") proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the ...
s, esterase
An esterase is a hydrolase enzyme that splits esters into an acid and an alcohol in a chemical reaction with water called hydrolysis.
A wide range of different esterases exist that differ in their substrate specificity, their protein structure, ...
s, glycosidase
Glycoside hydrolases (also called glycosidases or glycosyl hydrolases) catalyze the hydrolysis of glycosidic bonds in complex sugars. They are extremely common enzymes with roles in nature including degradation of biomass such as cellulose (cel ...
s, and manganese peroxidase
Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...
s.
Applications and uses
Biodegradable polymers are of significant interest to a variety of fields including medicine, agriculture, and packaging. One of the most active areas of research in biodegradable polymer is in controlled drug delivery and release.
Medical
Biodegradable polymers have an innumerable uses in the biomedical
Biomedicine (also referred to as Western medicine, mainstream medicine or conventional medicine) field, particularly in the fields of tissue engineering
Tissue engineering is a biomedical engineering discipline that uses a combination of Cell (biology), cells, engineering, Materials science, materials methods, and suitable biochemistry, biochemical and physicochemical factors to restore, maintai ...
and drug delivery
Drug delivery refers to approaches, formulations, manufacturing techniques, storage systems, and technologies involved in transporting a pharmaceutical compound to its target site to achieve a desired therapeutic effect. Principles related to d ...
. In order for a biodegradable polymer to be used as a therapeutic, it must meet several criteria: 1) be non-toxic in order to eliminate foreign body response; 2) the time it takes for the polymer to degrade is proportional to the time required for therapy; 3) the products resulting from biodegredation are not cytotoxic
Cytotoxicity is the quality of being toxic to cells. Examples of toxic agents are an immune cell or some types of venom, e.g. from the puff adder (''Bitis arietans'') or brown recluse spider (''Loxosceles reclusa'').
Cell physiology
Treating cells ...
and are readily eliminated from the body; 4) the material must be easily processed in order to tailor the mechanical properties for the required task; 5) be easily sterilized; and 6) have acceptable shelf life
Shelf life is the length of time that a commodity may be stored without becoming unfit for use, consumption, or sale. In other words, it might refer to whether a commodity should no longer be on a pantry shelf (unfit for use), or no longer on a ...
.
Biodegradable polymers are of great interest in the field of drug delivery
Drug delivery refers to approaches, formulations, manufacturing techniques, storage systems, and technologies involved in transporting a pharmaceutical compound to its target site to achieve a desired therapeutic effect. Principles related to d ...
and nanomedicine
Nanomedicine is the medical application of nanotechnology. Nanomedicine ranges from the medical applications of nanomaterials and BioBrick, biological devices, to Nanoelectronics, nanoelectronic biosensors, and even possible future applicatio ...
. The great benefit of a biodegradable drug delivery system is the ability of the drug carrier to target the release of its payload to a specific site in the body and then degrade into nontoxic materials that are then eliminated from the body via natural metabolic pathways
In biochemistry, a metabolic pathway is a linked series of chemical reactions occurring within a cell. The reactants, products, and intermediates of an enzymatic reaction are known as metabolites, which are modified by a sequence of chemical re ...
. The polymer slowly degrades into smaller fragments, releasing a natural product, and there is controlled ability to release a drug. The drug slowly releases as polymer degrades. For example, polylactic acid
Polylactic acid, also known as poly(lactic acid) or polylactide (PLA), is a thermoplastic polyester with backbone formula or , formally obtained by condensation of lactic acid with loss of water (hence its name). It can also be prepared by rin ...
, poly(lactic-co-glycolic) acid, and poly(caprolactone), all of which are biodegradable, have been used to carry anti-cancer drugs. Encapsulating the therapeutic in a polymer and adding targeting agents decreases the toxicity of the drug to healthy cells.
Biodegradable polymers and biomaterials
A biomaterial is a substance that has been engineered to interact with biological systems for a medical purpose, either a therapeutic (treat, augment, repair, or replace a tissue function of the body) or a diagnostic one. As a science, biomateria ...
are also of significant interest for tissue engineering
Tissue engineering is a biomedical engineering discipline that uses a combination of Cell (biology), cells, engineering, Materials science, materials methods, and suitable biochemistry, biochemical and physicochemical factors to restore, maintai ...
and regeneration. Tissue engineering is the ability to regenerate tissue with the help of artificial materials. The perfection of such systems can be used to grow tissues and cells ''in vitro'' or use a biodegradable scaffold to construct new structures and organs ''in vitro''. For these uses, a biodegradable scaffold is obviously preferred as it reduces the risk of immunological reaction and rejection of the foreign object. While many of the more advanced systems are not ready for human therapeutics, there is significant positive research in animal studies. For example, it was possible to successfully grow rat smooth muscle tissue on a polycaprolactone/polylactide scaffold. Further research and development may allow for this technology to be used for tissue replacement, support, or enhancement in humans. One of the ultimate goals of tissue engineering is the creation of organs, such as the kidney, from basic constituents. A scaffolding is necessary to grow the entity into a functioning organ, after which the polymer scaffold would degrade and be safely eliminated from the body. There are reports of using polyglycolic acid
Polyglycolide or poly(glycolic acid) (PGA), also spelled as polyglycolic acid, is a biodegradable, thermoplastic polymer and the simplest linear, aliphatic polyester. It can be prepared starting from glycolic acid by means of polycondensation or r ...
and polylactic acid
Polylactic acid, also known as poly(lactic acid) or polylactide (PLA), is a thermoplastic polyester with backbone formula or , formally obtained by condensation of lactic acid with loss of water (hence its name). It can also be prepared by rin ...
to engineer vascular tissue for heart repair. The scaffold can be used to help create undamaged arteries and vessels.
In addition to tissue engineering
Tissue engineering is a biomedical engineering discipline that uses a combination of Cell (biology), cells, engineering, Materials science, materials methods, and suitable biochemistry, biochemical and physicochemical factors to restore, maintai ...
, biodegradable polymers are being used in orthopedic applications, such as bone and joint replacement. A wide variety of non-biodegradable polymers have been used for orthopedic applications including silicone rubber Silicone rubber is an elastomer (rubber-like material) composed of silicone—itself a polymer—containing silicon together with carbon, hydrogen, and oxygen. Silicone rubbers are widely used in industry, and there are multiple formulations ...
, polyethylene
Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic. It is a polymer, primarily used for packaging ( plastic bags, plastic films, geomembranes and containers including bo ...
, acrylic resins, polyurethane
Polyurethane (; often abbreviated PUR and PU) refers to a class of polymers composed of organic chemistry, organic units joined by carbamate (urethane) links. In contrast to other common polymers such as polyethylene and polystyrene, polyurethan ...
, polypropylene
Polypropylene (PP), also known as polypropene, is a thermoplastic polymer used in a wide variety of applications. It is produced via chain-growth polymerization from the monomer propylene.
Polypropylene
belongs to the group of polyolefins and ...
, and polymethylmethacrylate
Poly(methyl methacrylate) (PMMA) belongs to a group of materials called engineering plastics. It is a transparent thermoplastic. PMMA is also known as acrylic, acrylic glass, as well as by the trade names and brands Crylux, Plexiglas, Acrylite ...
. The primary role of many of these polymers was to act as a biocompatible cement in the fixation of prostheses and in the replacement of joints. Newer biologically compatible synthetic and natural biodegradable polymers have been developed; these include polyglycolide, polylactide, polyhydroxobutyrate, chitosan
Chitosan is a linear polysaccharide composed of randomly distributed β-(1→4)-linked Glucosamine, D-glucosamine (deacetylated unit) and N-Acetylglucosamine, ''N''-acetyl-D-glucosamine (acetylated unit). It is made by treating the chitin shell ...
, hyaluronic acid, and hydrogels
A gel is a semi-solid that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady-state, although the liquid phase may still dif ...
. In particular, poly(2-hydroxyethyl-methacrylate), poly(ethylene glycol), chitosan
Chitosan is a linear polysaccharide composed of randomly distributed β-(1→4)-linked Glucosamine, D-glucosamine (deacetylated unit) and N-Acetylglucosamine, ''N''-acetyl-D-glucosamine (acetylated unit). It is made by treating the chitin shell ...
, and hyaluronic acid have been used extensively in the repair of cartilage, ligaments, and tendons. For example, poly(L-lactide) (PLA), is used to make screws and darts for meniscal repair and is marketed under the trade name Clearfix Mensical Dart/Screw. PLA is a slow degrading polymer and requires times greater than two years to degrade and be absorbed by the body.
Packaging and materials
In addition to medicine, biodegradable polymers are often used to reduce the volume of waste in packaging materials. There is also significant effort to replace materials derived from petrochemicals with those that can be made from biodegradable components. One of the most commonly used polymers for packaging purposes is polylactic acid
Polylactic acid, also known as poly(lactic acid) or polylactide (PLA), is a thermoplastic polyester with backbone formula or , formally obtained by condensation of lactic acid with loss of water (hence its name). It can also be prepared by rin ...
, PLA. The production of PLA has several advantages, the most important of which is the ability to tailor the physical properties of the polymer through processing methods. PLA is used for a variety of films, wrappings, and containers (including bottles and cups). In 2002, FDA ruled that PLA was safe to use in all food packaging. BASF markets a product called ecovio® which is a biobased blend of the company's certified compostable and biodegradable co-polyester ecoflex® and PLA. An application for this certified compostable and bio-based material is for any kind of plastic films such as shopping bags or organic waste bags. ecovio® can also be used in other applications, like thermoformed and injection moulded articles. Even paper-coating or particle foamed products can be produced by this very versatile biopolymer.
Notable examples
2012 Presidential Green Chemistry Challenge
Each year hundreds of millions of tons of plastic
Plastics are a wide range of synthetic or semi-synthetic materials that use polymers as a main ingredient. Their plasticity makes it possible for plastics to be moulded, extruded or pressed into solid objects of various shapes. This adaptab ...
s are produced from petroleum
Petroleum, also known as crude oil, or simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological formations. The name ''petroleum'' covers both naturally occurring unprocessed crud ...
. Most of these plastics will remain in landfill
A landfill site, also known as a tip, dump, rubbish dump, garbage dump, or dumping ground, is a site for the disposal of waste materials. Landfill is the oldest and most common form of waste disposal, although the systematic burial of the waste ...
s for years to come or litter
Litter consists of waste products that have been discarded incorrectly, without consent, at an unsuitable location. Litter can also be used as a verb; to litter means to drop and leave objects, often man-made, such as aluminum cans, paper cups, ...
the environment posing significant health risks to animals; however, the average person's lifestyle would be impractical without them (see Applications
Application may refer to:
Mathematics and computing
* Application software, computer software designed to help the user to perform specific tasks
** Application layer, an abstraction layer that specifies protocols and interface methods used in a c ...
). One solution to this conundrum lies in biodegradable polymers. These polymers have the distinct advantage that over time they will break down. Dr. Geoffrey Coates headed research to create catalysts that can not only efficiently create these biodegradable polymers, but the polymers also incorporate the greenhouse gas
A greenhouse gas (GHG or GhG) is a gas that Absorption (electromagnetic radiation), absorbs and Emission (electromagnetic radiation), emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse ...
and global warming
In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
contributor, CO2, and, environmentally present ground-ozone
Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the lo ...
producer, CO. These two gases can be found or produced in high concentrations from agricultural waste, coal
Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen.
Coal is formed when dea ...
, and industrial applications as byproducts. Not only do the catalysts utilize these normally wasted and environmentally unfriendly gases, but they also do it extremely efficiently with high turnover number Turnover number has two different meanings:
In enzymology, turnover number (also termed ''k''cat) is defined as the maximum number of chemical conversions of substrate molecules per second that a single active site will execute for a given enzym ...
s and frequencies in addition to good selectivity. These catalysts have been actively used by Novomer Inc to make polycarbonates that can replace the current coating bisphenol A
Bisphenol A (BPA) is a chemical compound primarily used in the manufacturing of various plastics. It is a colourless solid which is soluble in most common organic solvents, but has very poor solubility in water. BPA is produced on an industrial s ...
(BPA) found in many food and drink packaging. Novomer's analysis shows that if used in all cases, these biodegradable polymer coatings could not only sequester, but also avoid further production of CO2 in hundreds of millions of metric tons in just a single year.
Future concerns and potential problems
First, the properties such as weight capacity of biodegradable polymer are different from the traditional polymer, which may be unfavorable in many daily applications. Second, engineering issues. Biodegradable polymers are mostly plant-base materials, which means they originally come from organic source like soybean or corn. These organic plants have the chance to be sprayed with pesticides which contain chemicals which can contaminate the crops and be transferred into the final finished product. Third, low biodegradation rate. Compared to the traditional deposition way, the biodegradation for polymer has a longer degradation period. Polyhydroxyalkanoatesas an example, have a degradation period for up to three to six months. Last, the cost issue. The production technology of biodegradable polymer is still immature, the cost of resources such as labor and raw materials in large production quantity scale will be comparable high.
References
{{Reflist
External links
Polyketals
- esciencenews.com
"New emerging trends in synthetic biodegradable polymers – Polylactide: A critique."
''European Polymer Journal'', 2007, 43 4053-4074
Polymers