Collagen () is the main structural
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...
in the
extracellular matrix of the
connective tissues of many animals. It is the most abundant protein in mammals, making up 25% to 35% of protein content.
Amino acids are bound together to form a
triple helix of elongated
fibril known as a
collagen helix. It is mostly found in
cartilage,
bone
A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, ...
s,
tendon
A tendon or sinew is a tough band of fibrous connective tissue, dense fibrous connective tissue that connects skeletal muscle, muscle to bone. It sends the mechanical forces of muscle contraction to the skeletal system, while withstanding tensi ...
s,
ligaments, and
skin
Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation.
Other animal coverings, such as the arthropod exoskeleton, have different ...
.
Vitamin C is vital for collagen synthesis.
Depending on the degree of
mineralization, collagen tissues may be rigid (bone) or compliant (tendon) or have a gradient from rigid to compliant (cartilage). Collagen is also abundant in
corneas,
blood vessel
Blood vessels are the tubular structures of a circulatory system that transport blood throughout many Animal, animals’ bodies. Blood vessels transport blood cells, nutrients, and oxygen to most of the Tissue (biology), tissues of a Body (bi ...
s, the
gut,
intervertebral discs, and the
dentin in teeth. In
muscle tissue
Muscle is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Muscle tissue gives skeletal muscles the ability to contract. ...
, it serves as a major component of the
endomysium. Collagen constitutes 1% to 2% of muscle tissue and 6% by weight of
skeletal muscle
Skeletal muscle (commonly referred to as muscle) is one of the three types of vertebrate muscle tissue, the others being cardiac muscle and smooth muscle. They are part of the somatic nervous system, voluntary muscular system and typically are a ...
. The
fibroblast is the most common cell creating collagen in animals.
Gelatin, which is used in food and industry, is collagen that was irreversibly
hydrolyzed using heat, basic solutions, or weak acids.
Etymology
The name ''collagen'' comes from the Greek
κόλλα (''kólla''), meaning "
glue", and suffix -γέν, ''-gen'', denoting "producing".
Types
As of 2011, 28 types of human collagen have been identified, described, and classified according to their structure.
This diversity shows collagen's diverse functionality.
All of the types contain at least one
triple helix.
Over 90% of the collagen in
humans is
type I & III collagen.
* Fibrillar (type I, II, III, V, XI)
* Non-fibrillar
**
FACIT (fibril-associated collagens with interrupted triple helices) (types IX, XII, XIV, XIX, XXI)
** Short-chain (types VIII, X)
**
Basement membrane (type IV)
**
Multiplexin (multiple triple helix domains with interruptions) (types XV, XVIII)
** MACIT (membrane-associated collagens with interrupted triple helices) (types XIII, XVII)
** Microfibril-forming (type VI)
**
Anchoring fibrils (type VII)
The five most common types are:
*
Type I: skin,
tendon
A tendon or sinew is a tough band of fibrous connective tissue, dense fibrous connective tissue that connects skeletal muscle, muscle to bone. It sends the mechanical forces of muscle contraction to the skeletal system, while withstanding tensi ...
, vasculature, organs,
bone
A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, ...
(main component of the organic part of bone)
*
Type II:
cartilage (main collagenous component of cartilage)
*
Type III: reticulate (main component of
reticular fibers), commonly found alongside type I
*
Type IV: forms basal lamina, the epithelium-secreted layer of the
basement membrane
*
Type V: cell surfaces, hair, and
placenta
In humans
Cardiac
The collagenous
cardiac skeleton, which includes the four
heart valve rings, is histologically, elastically and uniquely bound to cardiac muscle. The cardiac skeleton also includes the separating
septa of the heart chambers – the
interventricular septum and the
atrioventricular septum. Collagen contribution to the measure of
cardiac performance summarily represents a continuous torsional force opposed to the
fluid mechanics
Fluid mechanics is the branch of physics concerned with the mechanics of fluids (liquids, gases, and plasma (physics), plasmas) and the forces on them.
Originally applied to water (hydromechanics), it found applications in a wide range of discipl ...
of blood pressure emitted from the heart. The collagenous structure that divides the upper chambers of the heart from the lower chambers is an impermeable membrane that excludes both blood and electrical impulses through typical physiological means. With support from collagen,
atrial fibrillation never deteriorates to
ventricular fibrillation
Ventricular fibrillation (V-fib or VF) is an abnormal heart rhythm in which the Ventricle (heart), ventricles of the heart Fibrillation, quiver. It is due to disorganized electrical conduction system of the heart, electrical activity. Ventricula ...
. Collagen is layered in variable densities with smooth muscle mass. The mass, distribution, age, and density of collagen all contribute to the
compliance required to move blood back and forth. Individual cardiac valvular leaflets are folded into shape by specialized collagen under variable
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...
. Gradual
calcium deposition within collagen occurs as a natural function of aging. Calcified points within collagen matrices show contrast in a moving display of blood and muscle, enabling methods of
cardiac imaging technology to arrive at ratios essentially stating blood in (
cardiac input) and blood out (
cardiac output). Pathology of the collagen underpinning of the heart is understood within the category of
connective tissue disease.
Bone grafts
As the skeleton forms the structure of the body, it is vital that it maintains its strength, even after breaks and injuries. Collagen is used in bone grafting because its triple-helix structure makes it a very strong molecule. It is ideal for use in bones, as it does not compromise the structural integrity of the skeleton. The triple helical structure prevents collagen from being broken down by enzymes, it enables adhesiveness of cells and it is important for the proper assembly of the extracellular matrix.
Tissue regeneration
Collagen scaffolds are used in tissue regeneration, whether in sponges, thin sheets, gels, or fibers. Collagen has favorable properties for tissue regeneration, such as pore structure, permeability,
hydrophilicity, and stability in vivo. Collagen scaffolds also support deposition of cells, such as
osteoblasts and
fibroblasts, and once inserted, facilitate growth to proceed normally.
Reconstructive surgery
Collagens are widely used in the construction of
artificial skin substitutes used for managing severe
burns and wounds.
These collagens may be derived from cow, horse, pig, or even human sources; and are sometimes used in combination with
silicone
In Organosilicon chemistry, organosilicon and polymer chemistry, a silicone or polysiloxane is a polymer composed of repeating units of siloxane (, where R = Organyl group, organic group). They are typically colorless oils or elastomer, rubber ...
s,
glycosaminoglycans, fibroblasts,
growth factors and other substances.
Wound healing
Collagen is one of the body's key natural resources and a component of skin tissue that can benefit all stages of
wound healing.
When collagen is made available to the wound bed, closure can occur. This avoids wound deterioration and procedures such as amputation.
Collagen is used as a natural wound dressing because it has properties that artificial wound dressings do not have. It resists bacteria, which is vitally important in wound dressing. As a burn dressing, collagen helps it heal fast by helping
granulation tissue to grow over the burn.
Throughout the four phases of wound healing, collagen performs the following functions:
* Guiding:
collagen fibers guide fibroblasts because they migrate along a connective tissue matrix.
*
Chemotaxis
Chemotaxis (from ''chemical substance, chemo-'' + ''taxis'') is the movement of an organism or entity in response to a chemical stimulus. Somatic cells, bacteria, and other single-cell organism, single-cell or multicellular organisms direct thei ...
: collagen fibers have a large surface area which attracts fibrogenic cells which help healing.
*
Nucleation
In thermodynamics, nucleation is the first step in the formation of either a new Phase (matter), thermodynamic phase or Crystal structure, structure via self-assembly or self-organization within a substance or mixture. Nucleation is typically def ...
: in the presence of certain neutral salt molecules, collagen can act as a nucleating agent causing formation of fibrillar structures.
*
Hemostasis: Blood
platelet
Platelets or thrombocytes () are a part of blood whose function (along with the coagulation#Coagulation factors, coagulation factors) is to react to bleeding from blood vessel injury by clumping to form a thrombus, blood clot. Platelets have no ...
s interact with the collagen to make a hemostatic plug.
Use in basic research
Collagen is used in
laboratory studies for
cell culture, studying cell behavior and cellular interactions with the
extracellular environment. Collagen is also widely used as a
bioink for
3D bioprinting and
biofabrication of 3D tissue models.
Biology
The collagen protein is composed of a triple helix, which generally consists of two identical chains (α1) and an additional chain that differs slightly in its chemical composition (α2). The amino acid composition of collagen is atypical for proteins, particularly with respect to its high
hydroxyproline content. The most common motifs in collagen's amino acid sequence are
glycine
Glycine (symbol Gly or G; ) is an amino acid that has a single hydrogen atom as its side chain. It is the simplest stable amino acid. Glycine is one of the proteinogenic amino acids. It is encoded by all the codons starting with GG (G ...
-
proline-X and glycine-X-hydroxyproline, where X is any amino acid other than glycine, proline or hydroxyproline.
The table below lists average amino acid composition for fish and mammal skin.
Synthesis
First, a three-dimensional stranded structure is assembled, mostly composed of the amino acids glycine and proline. This is the collagen precursor procollagen. Then, procollagen is modified by the addition of
hydroxyl groups to the amino acids proline and
lysine. This step is important for later
glycosylation and the formation of collagen's triple helix structure. Because the
hydroxylase enzymes performing these reactions require
vitamin C as a
cofactor, a long-term deficiency in this vitamin results in impaired collagen synthesis and
scurvy. These hydroxylation reactions are catalyzed by the enzymes
prolyl 4-hydroxylase and
lysyl hydroxylase. The reaction consumes one ascorbate molecule per hydroxylation. Collagen synthesis occurs inside and outside cells.
The most common form of collagen is fibrillary collagen. Another common form is meshwork collagen, which is often involved in the formation of filtration systems. All types of collagen are triple helices, but differ in the make-up of their alpha peptides created in step 2. Below we discuss the formation of fibrillary collagen.
# Transcription of mRNA: Synthesis begins with turning on genes associated with the formation of a particular alpha peptide (typically alpha 1, 2 or 3). About 44 genes are associated with collagen formation, each coding for a specific mRNA sequence, and are typically named with the "''COL''" prefix.
# Pre-pro-peptide formation: The created mRNA exits the cell nucleus into the cytoplasm. There, it links with the ribosomal subunits and is translated into a peptide. The peptide goes into the endoplasmic reticulum for post-translational processing. It is directed there by a
signal recognition particle
The signal recognition particle (SRP) is an abundant, cytosolic, universally conserved ribonucleoprotein (protein-RNA complex) that recognizes and targets specific proteins to the endoplasmic reticulum in eukaryotes and the plasma membrane ...
on the
endoplasmic reticulum, which recognizes the peptide's signal sequence (the early part of the sequence). The processed product is a
pre-pro-peptide called preprocollagen.
# Pro-collagen formation: Three modifications of the pre-pro-peptide form the alpha peptide:
## The signal peptide on the N-terminal is removed. This molecule is called ''propeptide.''
## Lysines and prolines are hydroxylated by the enzymes 'prolyl hydroxylase' and 'lysyl hydroxylase', producing hydroxyproline and hydroxylysine. This helps in cross-linking the alpha peptides. This enzymatic step requires vitamin C as a cofactor. In scurvy, the lack of hydroxylation of prolines and lysines causes a looser triple helix (which is formed by three alpha peptides).
## Glycosylation occurs by adding either glucose or galactose monomers onto the hydroxyl groups that were placed onto lysines, but not on prolines.
## Three of the hydroxylated and glycosylated propeptides twist into a triple helix (except for its ends), forming procollagen. It is packaged into a transfer vesicle destined for the Golgi apparatus.
# Modification and secretion: In the
Golgi apparatus, the procollagen goes through one last post-translational modification, adding oligosaccharides (not monosaccharides as in step 3). Then it is packaged into a secretory vesicle to be secreted from the cell.
# Tropocollagen formation: Outside the cell, membrane-bound enzymes called collagen peptidases remove the unwound ends of the molecule, producing tropocollagen. Defects in this step produce various collagenopathies called
Ehlers–Danlos syndrome. This step is absent when synthesizing type III, a type of fibrillar collagen.
# Collagen fibril formation:
Lysyl oxidase, a
copper-dependent enzyme, acts on lysines and hydroxylysines, producing aldehyde groups, which eventually form covalent bonds between tropocollagen molecules. This polymer of tropocollagen is called a collagen fibril.
Amino acids
Collagen has an unusual amino acid composition and sequence:
* Glycine is found at almost every third
residue.
* Proline makes up about 17% of collagen.
* Collagen contains two unusual derivative amino acids not directly inserted during
translation. These amino acids are found at specific locations relative to glycine and are modified post-translationally by different enzymes, both of which require vitamin C as a cofactor.
**
Hydroxyproline derived from proline
**
Hydroxylysine derived from lysine – depending on the type of collagen, varying numbers of hydroxylysines are
glycosylated (mostly having
disaccharides attached).
Cortisol stimulates
degradation of (skin) collagen into amino acids.
Collagen I formation
Most collagen forms in a similar manner, but the following process is typical for type I:
# Inside the cell
## Two types of alpha chains – alpha-1 and alpha 2, are formed during
translation on ribosomes along the
rough endoplasmic reticulum (RER). These peptide chains known as preprocollagen, have registration peptides on each end and a
signal peptide
A signal peptide (sometimes referred to as signal sequence, targeting signal, localization signal, localization sequence, transit peptide, leader sequence or leader peptide) is a short peptide (usually 16–30 amino acids long) present at the ...
.
## Polypeptide chains are released into the lumen of the RER.
## Signal peptides are cleaved inside the RER and these are known as pro-alpha chains.
##
Hydroxylation of lysine and proline amino acids occurs inside the lumen. This process is dependent on and consumes
ascorbic acid (vitamin C) as a
cofactor.
##
Glycosylation of specific hydroxylysine residues occurs.
## Triple alpha helical structure is formed inside the endoplasmic reticulum from two alpha-1 chains and one alpha-2 chain.
##
Procollagen is shipped to the
Golgi apparatus, where it is packaged and secreted into the extracellular space by
exocytosis.
# Outside the cell
## Registration peptides are cleaved, and tropocollagen is formed by
procollagen peptidase.
## Multiple tropocollagen molecules form collagen fibrils, via covalent cross-linking (
aldol reaction) by
lysyl oxidase which links hydroxylysine and lysine residues. Multiple collagen fibrils form into collagen fibers.
## Collagen may be attached to cell membranes via several types of protein, including
fibronectin,
laminin,
fibulin, and
integrin.
Molecular structure
A single collagen molecule, tropocollagen, is used to make up larger collagen aggregates, such as fibrils. It is approximately 300
nm long and 1.5 nm in diameter, and it is made up of three
polypeptide strands (called alpha peptides, see step 2), each of which has the conformation of a left-handed
helix – this should not be confused with the right-handed
alpha helix
An alpha helix (or α-helix) is a sequence of amino acids in a protein that are twisted into a coil (a helix).
The alpha helix is the most common structural arrangement in the Protein secondary structure, secondary structure of proteins. It is al ...
. These three left-handed helices are twisted together into a right-handed triple helix or "super helix", a cooperative
quaternary structure stabilized by many
hydrogen bonds. With type I collagen and possibly all fibrillar collagens, if not all collagens, each triple-helix associates into a right-handed super-super-coil referred to as the collagen microfibril. Each microfibril is
interdigitated with its neighboring microfibrils to a degree that might suggest they are individually unstable, although within collagen fibrils, they are so well ordered as to be crystalline.
A distinctive feature of collagen is the regular arrangement of amino acids in each of the three chains of these collagen subunits. The sequence often follows the pattern
Gly-
Pro-X or Gly-X-
Hyp, where X may be any of various other amino acid residues.
Proline or hydroxyproline constitute about 1/6 of the total sequence. With glycine accounting for the 1/3 of the sequence, this means approximately half of the collagen sequence is not glycine, proline or hydroxyproline, a fact often missed due to the distraction of the unusual GX
1X
2 character of collagen alpha-peptides. The high glycine content of collagen is important with respect to stabilization of the collagen helix, as this allows the very close association of the collagen fibers within the molecule, facilitating hydrogen bonding and the formation of intermolecular cross-links.
This kind of regular repetition and high glycine content is found in only a few other fibrous proteins, such as silk
fibroin.
Collagen is not only a structural protein. Due to its key role in the determination of cell phenotype, cell adhesion, tissue regulation, and infrastructure, many sections of its non-proline-rich regions have cell or matrix association/regulation roles. The relatively high content of proline and hydroxyproline rings, with their geometrically constrained
carboxyl and (secondary)
amino groups, along with the rich abundance of glycine, accounts for the tendency of the individual polypeptide strands to form left-handed helices spontaneously, without any intrachain hydrogen bonding.
Because glycine is the smallest amino acid with no side chain, it plays a unique role in fibrous structural proteins. In collagen, Gly is required at every third position because the assembly of the triple helix puts this residue at the interior (axis) of the helix, where there is no space for a larger side group than glycine's single
hydrogen
Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
atom. For the same reason, the rings of the Pro and Hyp must point outward. These two amino acids help stabilize the triple helix – Hyp even more so than Pro because of a stereoelectronic effect; a lower concentration of them is required in animals such as fish, whose
body temperatures are lower than most warm-blooded animals. Lower proline and hydroxyproline contents are characteristic of cold-water, but not warm-water fish; the latter tend to have similar proline and hydroxyproline contents to mammals.
The lower proline and hydroxyproline contents of cold-water fish and other
poikilotherm animals lead to their collagen having a lower thermal stability than mammalian collagen.
This lower thermal stability means that
gelatin derived from fish collagen is not suitable for many food and industrial applications.
The tropocollagen
subunits spontaneously
self-assemble, with regularly staggered ends, into even larger arrays in the
extracellular
This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms. It is intended as introductory material for novices; for more specific and technical definitions ...
spaces of tissues.
Additional assembly of fibrils is guided by fibroblasts, which deposit fully formed fibrils from fibripositors. In the fibrillar collagens, molecules are staggered to adjacent molecules by about 67
nm (a unit that is referred to as 'D' and changes depending upon the hydration state of the aggregate). In each D-period repeat of the microfibril, there is a part containing five molecules in cross-section, called the "overlap", and a part containing only four molecules, called the "gap".
These overlap and gap regions are retained as microfibrils assemble into fibrils, and are thus viewable using electron microscopy. The triple helical tropocollagens in the microfibrils are arranged in a quasihexagonal packing pattern.

There is some
covalent crosslinking within the triple helices and a variable amount of covalent crosslinking between tropocollagen helices forming well-organized aggregates (such as fibrils). Larger fibrillar bundles are formed with the aid of several different classes of proteins (including different collagen types), glycoproteins, and proteoglycans to form the different types of mature tissues from alternate combinations of the same key players.
Collagen's
insolubility was a barrier to the study of monomeric collagen until it was found that tropocollagen from young animals can be extracted because it is not yet fully
crosslinked. However, advances in microscopy techniques (i.e. electron microscopy (EM) and atomic force microscopy (AFM)) and X-ray diffraction have enabled researchers to obtain increasingly detailed images of collagen structure ''in situ''. These later advances are particularly important to better understanding the way in which collagen structure affects cell–cell and cell–matrix communication and how tissues are constructed in growth and repair and changed in development and disease. For example, using AFM–based nanoindentation it has been shown that a single collagen fibril is a heterogeneous material along its axial direction with significantly different mechanical properties in its gap and overlap regions, correlating with its different molecular organizations in these two regions.
Collagen fibrils/aggregates are arranged in different combinations and concentrations in various tissues to provide varying tissue properties. In bone, entire collagen triple helices lie in a parallel, staggered array. 40 nm gaps between the ends of the tropocollagen subunits (approximately equal to the gap region) probably serve as nucleation sites for the deposition of long, hard, fine crystals of the mineral component, which is hydroxylapatite (approximately) Ca
10(OH)
2(PO
4)
6. Type I collagen gives bone its
tensile strength
Ultimate tensile strength (also called UTS, tensile strength, TS, ultimate strength or F_\text in notation) is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials, the ultimate ...
.
Associated disorders
Collagen-related diseases most commonly arise from genetic defects or nutritional deficiencies that affect the biosynthesis, assembly, posttranslational modification, secretion, or other processes involved in normal collagen production.
In addition to the above-mentioned disorders, excessive deposition of collagen occurs in
scleroderma.
Diseases
One thousand mutations have been identified in 12 out of more than 20 types of collagen. These mutations can lead to various diseases at the tissue level.
Osteogenesis imperfecta – Caused by a mutation in ''type 1 collagen'', dominant autosomal disorder, results in weak bones and irregular connective tissue, some cases can be mild while others can be lethal. Mild cases have lowered levels of collagen type 1 while severe cases have structural defects in collagen.
Chondrodysplasias – Skeletal disorder believed to be caused by a mutation in ''type 2 collagen'', further research is being conducted to confirm this.
Ehlers–Danlos syndrome – Thirteen different types of this disorder, which lead to deformities in connective tissue, are known. Some of the rarer types can be lethal, leading to the rupture of arteries. Each syndrome is caused by a different mutation. For example, the vascular type (vEDS) of this disorder is caused by a mutation in ''collagen type 3''.
Alport syndrome – Can be passed on genetically, usually as X-linked dominant, but also as both an autosomal dominant and autosomal recessive disorder, those with the condition have problems with their kidneys and eyes, loss of hearing can also develop during the childhood or adolescent years.
Knobloch syndrome – Caused by a mutation in the
COL18A1 gene that codes for the production of collagen XVIII. Patients present with protrusion of the brain tissue and degeneration of the retina; an individual who has family members with the disorder is at an increased risk of developing it themselves since there is a hereditary link.
Animal harvesting
When not synthesized, collagen can be harvested from animal skin. This has led to deforestation as has occurred in Paraguay where large collagen producers buy large amounts of cattle hides from regions that have been
clear-cut for cattle grazing.
Characteristics
Collagen is one of the long,
fibrous structural proteins whose functions are quite different from those of
globular proteins, such as
enzyme
An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...
s. Tough bundles of collagen called ''collagen fibers'' are a major component of the
extracellular matrix that supports most tissues and gives cells structure from the outside, but collagen is also found inside certain cells. Collagen has great
tensile strength
Ultimate tensile strength (also called UTS, tensile strength, TS, ultimate strength or F_\text in notation) is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials, the ultimate ...
, and is the main component of
fascia,
cartilage,
ligaments,
tendon
A tendon or sinew is a tough band of fibrous connective tissue, dense fibrous connective tissue that connects skeletal muscle, muscle to bone. It sends the mechanical forces of muscle contraction to the skeletal system, while withstanding tensi ...
s,
bone
A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, ...
and skin. Along with
elastin
Elastin is a protein encoded by the ''ELN'' gene in humans and several other animals. Elastin is a key component in the extracellular matrix of gnathostomes (jawed vertebrates). It is highly Elasticity (physics), elastic and present in connective ...
and soft
keratin, it is responsible for skin strength and elasticity, and its degradation leads to
wrinkles that accompany
aging.
[Dermal Fillers , The Ageing Skin](_blank)
. Pharmaxchange.info. Retrieved on 21 April 2013. It strengthens
blood vessel
Blood vessels are the tubular structures of a circulatory system that transport blood throughout many Animal, animals’ bodies. Blood vessels transport blood cells, nutrients, and oxygen to most of the Tissue (biology), tissues of a Body (bi ...
s and plays a role in
tissue development. It is present in the
cornea and lens of the eye in
crystalline form. It may be one of the most abundant proteins in the fossil record, given that it appears to fossilize frequently, even in bones from the
Mesozoic
The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...
and
Paleozoic.
Mechanical properties
Collagen is a complex hierarchical material with
mechanical properties that vary significantly across different scales.
On the molecular scale,
atomistic and
course-grained modeling simulations, as well as numerous experimental methods, have led to several estimates of the
Young's modulus of collagen at the molecular level. Only above a certain strain rate is there a strong relationship between elastic modulus and strain rate, possibly due to the large number of atoms in a collagen molecule.
The length of the molecule is also important, where longer molecules have lower tensile strengths than shorter ones due to short molecules having a large proportion of hydrogen bonds being broken and reformed.
On the
fibrillar scale, collagen has a lower modulus compared to the molecular scale, and varies depending on geometry, scale of observation, deformation state, and hydration level.
By increasing the
crosslink density from zero to 3 per molecule, the maximum stress the fibril can support increases from 0.5 GPa to 6 GPa.
Limited tests have been done on the tensile strength of the collagen fiber, but generally it has been shown to have a lower Young's modulus compared to fibrils.
When studying the mechanical properties of collagen,
tendon
A tendon or sinew is a tough band of fibrous connective tissue, dense fibrous connective tissue that connects skeletal muscle, muscle to bone. It sends the mechanical forces of muscle contraction to the skeletal system, while withstanding tensi ...
is often chosen as the ideal material because it is close to a pure and aligned collagen structure. However, at the macro, tissue scale, the vast number of structures that collagen fibers and fibrils can be arranged into results in highly variable properties. For example, tendon has primarily parallel fibers, whereas skin consists of a net of wavy fibers, resulting in a much higher strength and lower ductility in tendon compared to skin. The mechanical properties of collagen at multiple hierarchical levels is given.
Collagen is known to be a viscoelastic solid. When the collagen fiber is modeled as two Kelvin-Voigt models in series, each consisting of a spring and a dashpot in parallel, the strain in the fiber can be modeled according to the following equation: