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A hydrogel is a crosslinked hydrophilic polymer that does not dissolve in water. They are highly absorbent yet maintain well defined structures. These properties underpin several applications, especially in the biomedical area. Many hydrogels are synthetic, but some are derived from nature. The term 'hydrogel' was coined in 1894.


Chemistry


Classification

The crosslinks which bond the polymers of a hydrogel fall under two general categories: physical and chemical. Chemical hydrogels have covalent cross-linking bonds, whereas physical hydrogels have non-covalent bonds. Chemical hydrogels result in strong irreversible gels due to the covalent bonding, and they may also possess harmful properties which makes them unfavourable for medical applications. Physical hydrogels on the other hand have high biocompatibility, aren’t toxic, and are also easily reversible, by simply changing an external stimulus such as pH or temperature; thus they are favourable for use in medical applications. Physical crosslinks consist of hydrogen bonds, hydrophobic interactions, and chain entanglements (among others). A hydrogel generated through the use of physical crosslinks is sometimes called a 'reversible' hydrogel. Chemical crosslinks consist of covalent bonds between polymer strands. Hydrogels generated in this manner are sometimes called 'permanent' hydrogels. Hydrogels are prepared using a variety of
polymeric materials 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 and ...
, which can be divided broadly into two categories according to their origin: natural or synthetic polymers. Natural polymers for hydrogel preparation include
hyaluronic acid Hyaluronic acid (; abbreviated HA; conjugate base hyaluronate), also called hyaluronan, is an anionic, nonsulfated glycosaminoglycan distributed widely throughout connective, epithelial, and neural tissues. It is unique among glycosaminoglycans ...
, chitosan,
heparin Heparin, also known as unfractionated heparin (UFH), is a medication and naturally occurring glycosaminoglycan. Since heparins depend on the activity of antithrombin, they are considered anticoagulants. Specifically it is also used in the treatm ...
, alginate, and
fibrin Fibrin (also called Factor Ia) is a fibrous, non-globular protein involved in the clotting of blood. It is formed by the action of the protease thrombin on fibrinogen, which causes it to polymerize. The polymerized fibrin, together with platele ...
. Common synthetic polymers include polyvinyl alcohol, polyethylene glycol, sodium polyacrylate, acrylate polymers and copolymers thereof.


Preparation

There are two suggested mechanisms behind physical hydrogel formation, the first one being the gelation of nanofibrous peptide assemblies, usually observed for oligopeptide precursors. The precursors self-assemble into fibers, tapes, tubes, or ribbons that entangle to form non-covalent cross-links. The second mechanism involves non-covalent interactions of cross-linked domains that are separated by water-soluble linkers, and this is usually observed in longer multi-domain structures. Tuning of the supramolecular interactions to produce a self-supporting network that does not precipitate, and is also able to immobilize water which is vital for to gel formation. Most oligopeptide hydrogels have a β-sheet structure, and assemble to form fibers, although α-helical peptides have also been reported. The typical mechanism of gelation involves the oligopeptide precursors self-assemble into fibers that become elongated, and entangle to form cross-linked gels. One notable method of initiating a polymerization fuving involves the use of light as a stimulus. In this method, photoinitiators, compounds that cleave from the absorption of photons, are added to the precursor solution which will become the hydrogel. When the precursor solution is exposed to a concentrated source of light, the photoinitiators will cleave and form free radicals, which will begin a polymerization reaction that forms crosslinks between polymer strands. This reaction will cease if the light source is removed, allowing the amount of crosslinks formed in the hydrogel to be controlled. The properties of a hydrogel are highly dependent on the type and quantity of its crosslinks, making photopolymerization a popular choice for fine-tuning hydrogels. This technique has seen considerable use in cell and tissue engineering applications due to the ability to inject or mold a precursor solution loaded with cells into a wound site, then solidify it in situ.


Peptides based hydrogels

Peptides based hydrogels possess exceptional biocompatibility and biodegradability qualities, giving rise to their wide use of applications, particularly in biomedicine; as such, their physical properties can be fine-tuned in order to maximise their use. Methods to do this are: modulation of the amino acid sequence, pH,
chirality Chirality is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is distinguishable from ...
, and increasing the number of aromatic residues. The order of amino acids within the sequence is crucial for gelation, as has been shown many times. In one example, a short peptide sequence Fmoc-Phe-Gly readily formed a hydrogel, whereas Fmoc-Gly-Phe failed to do so as a result of the two adjacent aromatic moieties being moved, hindering the aromatic interactions. Altering the pH can also have similar effects, an example involved the use of the naphthalene (Nap) modified dipeptides Nap-Gly-Ala, and Nap- Ala-Gly, where a drop in pH induced gelation of the former, but led to crystallisation of the latter. 74 A controlled pH decrease method using glucono-δ-lactone (GdL), where the GdL is hydrolysed to gluconic acid in water is a recent strategy that has been developed as a way to form homogeneous and reproducible hydrogels. The hydrolysis is slow, which allows for a uniform pH change, and thus resulting in reproducible homogenous gels. In addition to this, the desired pH can be achieved by altering the amount of GdL added. The use of GdL has been used various times for the hydrogelation of Fmoc and Nap-dipeptides. In another direction, Morris et al reported the use of GdL as a ‘molecular trigger’ to predict and control the order of gelation. Chirality also plays an essential role in gel formation, and even changing the chirality of a single amino acid from its natural L-amino acid to its unnatural D-amino acid can significantly impact the gelation properties, with the natural forms not forming gels. Furthermore, aromatic interactions play a key role in hydrogel formation as a result of π- π stacking driving gelation, shown by many studies.


Other

Hydrogels also possess a degree of flexibility very similar to natural tissue due to their significant water content. As responsive " smart materials", hydrogels can encapsulate chemical systems which upon stimulation by external factors such as a change of pH may cause specific compounds such as glucose to be liberated to the environment, in most cases by a gel–sol transition to the liquid state. Chemomechanical polymers are mostly also hydrogels, which upon stimulation change their volume and can serve as actuators or
sensor A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon. In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
s. File:Hydrogel micropump.webm, A
micropump Micropumps are devices that can control and manipulate small fluid volumes. Although any kind of small pump is often referred to as micropump, a more accurate definition restricts this term to pumps with functional dimensions in the micrometer ran ...
based on a hydrogel bar (4×0.3×0.05 mm size) actuated by applied voltage. This pump can be continuously operated with a 1.5 V battery for at least 6 months. File:Short-peptide-based hydrogel, electron microscope image.jpg, A short-peptide-based hydrogel matrix, capable of holding about one hundred times its own weight in water. Developed as a medical dressing. File:Crosslinked ultrashort peptide hydrogel.jpg, Photo of the same short-peptide-based hydrogel, held in forceps to demonstrate its stiffness and transparency.


Mechanical properties

Hydrogels have been investigated for diverse applications. By modifying the polymer concentration of a hydrogel (or conversely, the water concentration), the Young's modulus, shear modulus, and
storage modulus Dynamic modulus (sometimes complex modulusThe Open University (UK), 2000. ''T838 Design and Manufacture with Polymers: Solid properties and design'', page 30. Milton Keynes: The Open University.) is the ratio of stress to strain under ''vibratory c ...
can vary from 10 Pa to 3 MPa, a range of about five orders of magnitude. A similar effect can be seen by altering the crosslinking concentration. This much variability of the mechanical stiffness is why hydrogels are so appealing for biomedical applications, where it is vital for implants to match the mechanical properties of the surrounding tissues. Characterizing the mechanical properties of hydrogels can be difficult especially due to the differences in mechanical behavior that hydrogels have in comparison to other traditional engineering materials. In addition to its rubber
elasticity Elasticity often refers to: *Elasticity (physics), continuum mechanics of bodies that deform reversibly under stress Elasticity may also refer to: Information technology * Elasticity (data store), the flexibility of the data model and the cl ...
and viscoelasticity, hydrogels have an additional time dependent deformation mechanism which is dependent on fluid flow called poroelasticity. These properties are extremely important to consider while performing mechanical experiments. Some common mechanical testing experiments for hydrogels are tension, compression (confined or unconfined), indentation,
shear Shear may refer to: Textile production *Animal shearing, the collection of wool from various species **Sheep shearing *The removal of nap during wool cloth production Science and technology Engineering *Shear strength (soil), the shear strength ...
rheometry or
dynamic mechanical analysis Dynamic mechanical analysis (abbreviated DMA) is a technique used to study and characterize materials. It is most useful for studying the viscoelastic behavior of polymers A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a subst ...
. Hydrogels have two main regimes of mechanical properties:
rubber elasticity Rubber elasticity refers to a property of crosslinked rubber: it can be stretched by up to a factor of 10 from its original length and, when released, returns very nearly to its original length. This can be repeated many times with no apparent de ...
and viscoelasticity:


Rubber elasticity

In the unswollen state, hydrogels can be modelled as highly crosslinked chemical gels, in which the system can be described as one continuous polymer network. In this case: G=N_kT= where ''G'' is the shear modulus, ''k'' is the Boltzmann constant, ''T'' is temperature, ''Np'' is the number of polymer chains per unit volume, ''ρ'' is the density, ''R'' is the ideal gas constant, and \overline_ is the (number) average molecular weight between two adjacent cross-linking points. \overline_ can be calculated from the swell ratio, ''Q'', which is relatively easy to test and measure. For the swollen state, a perfect gel network can be modeled as: G_=GQ^ In a simple uniaxial extension or compression test, the true stress, \sigma _, and engineering stress, \sigma _, can be calculated as: \sigma _=G_\left ( \lambda ^-\lambda ^ \right ) \sigma _=G_\left ( \lambda -\lambda ^ \right ) where \lambda =l_/l_ is the stretch.


Viscoelasticity

For hydrogels, their elasticity comes from the solid polymer matrix while the viscosity originates from the polymer network mobility and the water and other components that make up the aqueous phase. Viscoelastic properties of a hydrogel is highly dependent on the nature of the applied mechanical motion. Thus, the time dependence of these applied forces is extremely important for evaluating the viscoelasticity of the material. Physical models for viscoelasticity attempt to capture the elastic and viscous material properties of a material. In an elastic material, the stress is proportional to the strain while in a viscous material, the stress is proportional to the strain rate. The Maxwell model is one developed mathematical model for linear viscoelastic response. In this model, viscoelasticity is modeled analogous to an electrical circuit with a Hookean spring, that represents the Young's modulus, and a Newtonian dashpot that represents the viscosity. A material that exhibit properties described in this model is a Maxwell material. Another physical model used is called the Kelvin-Voigt Model and a material that follow this model is called a Kelvin–Voigt material. In order to describe the time-dependent creep and stress-relaxation behavior of hydrogel, a variety of physical lumped parameter models can be used. These modeling methods vary greatly and are extremely complex, so the empirical Prony Series description is commonly used to describe the viscoelastic behavior in hydrogels. In order to measure the time-dependent viscoelastic behavior of polymers
dynamic mechanical analysis Dynamic mechanical analysis (abbreviated DMA) is a technique used to study and characterize materials. It is most useful for studying the viscoelastic behavior of polymers A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a subst ...
is often performed. Typically, in these measurements the one side of the hydrogel is subjected to a sinusoidal load in shear mode while the applied stress is measured with a stress transducer and the change in sample length is measured with a strain transducer. One notation used to model the sinusoidal response to the periodic stress or strain is: :G = G' + iG'' in which G' is the real (elastic or storage) modulus, G" is the imaginary (viscous or loss) modulus.


Poroelasticity

Poroelasticity is a characteristic of materials related to the migration of solvent through a porous material and the concurrent deformation that occurs. Poroelasticity in hydrated materials such as hydrogels occurs due to friction between the polymer and water as the water moves through the porous matrix upon compression. This causes a decrease in water pressure, which adds additional stress upon compression. Similar to viscoelasticity, this behavior is time dependent, thus poroelasticity is dependent on compression rate: a hydrogel shows softness upon slow compression, but fast compression makes the hydrogel stiffer. This phenomenon is due to the friction between the water and the porous matrix is proportional to the flow of water, which in turn is dependent on compression rate. Thus, a common way to measure poroelasticity is to do compression tests at varying compression rates. Pore size is an important factor in influencing poroelasticity. The
Kozeny–Carman equation The Kozeny–Carman equation (or Carman–Kozeny equation or Kozeny equation) is a relation used in the field of fluid dynamics to calculate the pressure drop of a fluid flowing through a packed bed of solids. It is named after Josef Kozeny and ...
has been used to predict pore size by relating the pressure drop to the difference in stress between two compression rates. Poroelasticity is described by several coupled equations, thus there are few mechanical tests that relate directly to the poroelastic behavior of the material, thus more complicated tests such as indentation testing, numerical or computational models are utilized. Numerical or computational methods attempt to simulate the three dimensional permeability of the hydrogel network.


Environmental response

The most commonly seen environmental sensitivity in hydrogels is a response to temperature. Many polymers/hydrogels exhibit a temperature dependent phase transition, which can be classified as either an upper critical solution temperature (UCST) or
lower critical solution temperature The lower critical solution temperature (LCST) or lower consolute temperature is the critical temperature below which the components of a mixture are miscible in all proportions. The word ''lower'' indicates that the LCST is a lower bound to a t ...
(LCST). UCST polymers increase in their water-solubility at higher temperatures, which lead to UCST hydrogels transitioning from a gel (solid) to a solution (liquid) as the temperature is increased (similar to the melting point behavior of pure materials). This phenomenon also causes UCST hydrogels to expand (increase their swell ratio) as temperature increases while they are below their UCST. However, polymers with LCSTs display an inverse (or negative) temperature-dependence, where their water-solubility decreases at higher temperatures. LCST hydrogels transition from a liquid solution to a solid gel as the temperature is increased, and they also shrink (decrease their swell ratio) as the temperature increases while they are above their LCST. Applications can dictate for diverse thermal responses. For example, in the biomedical field, LCST hydrogels are being investigated as drug delivery systems due to being injectable (liquid) at room temp and then solidifying into a rigid gel upon exposure to the higher temperatures of the human body. There are many other stimuli that hydrogels can be responsive to, including: pH, glucose, electrical signals, light, pressure, ions,
antigens In immunology, an antigen (Ag) is a molecule or molecular structure or any foreign particulate matter or a pollen grain that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune response. ...
, and more.


Additives

The mechanical properties of hydrogels can be fine-tuned in many ways beginning with attention to their hydrophobic properties. Another method of modifying the strength or elasticity of hydrogels is to graft or surface coat them onto a stronger/stiffer support, or by making superporous hydrogel (SPH) composites, in which a cross-linkable matrix swelling additive is added. Other additives, such as
nanoparticles A nanoparticle or ultrafine particle is usually defined as a particle of matter that is between 1 and 100 nanometres (nm) in diameter. The term is sometimes used for larger particles, up to 500 nm, or fibers and tubes that are less than 1 ...
and
microparticles Microparticles are particles between 0.1 and 100 μm in size. Commercially available microparticles are available in a wide variety of materials, including ceramics, glass, polymers, and metals. Microparticles encountered in daily life includ ...
, have been shown to significantly modify the stiffness and gelation temperature of certain hydrogels used in biomedical applications.


Processing techniques

While a hydrogel's mechanical properties can be tuned and modified through crosslink concentration and additives, these properties can also be enhanced or optimized for various applications through specific processing techniques. These techniques include electro-spinning, 3D/
4D printing 4-dimensional printing (4D printing; also known as 4D bioprinting, active origami, or shape-morphing systems) uses the same techniques of 3D printing through computer-programmed deposition of material in successive layers to create a three-dimensio ...
, self-assembly, and
freeze-casting Freeze-casting, also frequently referred to as ''ice-templating'', or ''freeze alignment'', is a technique that exploits the highly anisotropic solidification behavior of a solvent (generally water) in a well-dispersed slurry to controllably temp ...
. One unique processing technique is through the formation of multi-layered hydrogels to create a spatially-varying matrix composition and by extension, mechanical properties. This can be done by polymerizing the hydrogel matrixes in a layer by layer fashion via UV polymerization. This technique can be useful in creating hydrogels that mimic articular cartilage, enabling a material with three separate zones of distinct mechanical properties. Another emerging technique to optimize hydrogel mechanical properties is by taking advantage of the
Hofmeister series The Hofmeister series or lyotropic series is a classification of ions in order of their lyotrophic properties, which is the ability to salt out or salt in proteins. The effects of these changes were first worked out by Franz Hofmeister, who stud ...
. Due to this phenomenon, through the addition of salt solution, the polymer chains of a hydrogel aggregate and crystallize, which increases the toughness of the hydrogel. This method, called " salting out", has been applied to poly(vinyl alcohol) hydrogels by adding a
sodium sulfate Sodium sulfate (also known as sodium sulphate or sulfate of soda) is the inorganic compound with formula Na2SO4 as well as several related hydrates. All forms are white solids that are highly soluble in water. With an annual production of 6 milli ...
salt solution. Some of these processing techniques can be used synergistically with each other to yield optimal mechanical properties.
Directional freezing Directional freezing freezes from only one direction. Directional freezing can freeze water, from only one direction or side of a container, into clear ice. Directional freezing in a domestic freezer can be done by putting water in a insulated ...
or
freeze-casting Freeze-casting, also frequently referred to as ''ice-templating'', or ''freeze alignment'', is a technique that exploits the highly anisotropic solidification behavior of a solvent (generally water) in a well-dispersed slurry to controllably temp ...
is another method in which a directional temperature gradient is applied to the hydrogel is another way to form materials with anisotropic mechanical properties. Utilizing both the freeze-casting and salting-out processing techniques on poly(vinyl alcohol) hydrogels to induce hierarchical morphologies and anisotropic mechanical properties. Directional freezing of the hydrogels helps to align and coalesce the polymer chains, creating anisotropic array honeycomb tube-like structures while salting out the hydrogel yielded out a nano-fibril network on the surface of these honeycomb tube-like structures. While maintaining a water content of over 70%, these hydrogels' toughness values are well above those of water-free polymers such as polydimethylsiloxane (PDMS), Kevlar, and
synthetic rubber A synthetic rubber is an artificial elastomer. They are polymers synthesized from petroleum byproducts. About 32-million metric tons of rubbers are produced annually in the United States, and of that amount two thirds are synthetic. Synthetic rubbe ...
. The values also surpass the toughness of natural tendon and spider silk.


Applications


Soft contact lenses

The dominant material for contact lenses are acrylate- siloxane hydrogels. They have replaced hard contact lenses. One of their most attractive properties is oxygen permeability, which is required since the cornea lacks vasculature.


Research

*
Breast implant A breast implant is a prosthesis used to change the size, shape, and contour of a person's breast. In reconstructive plastic surgery, breast implants can be placed to restore a natural looking breast following a mastectomy, to correct congenital ...
s *
Contact lens Contact lenses, or simply contacts, are thin lenses placed directly on the surface of the eyes. Contact lenses are ocular prosthetic devices used by over 150 million people worldwide, and they can be worn to correct vision or for cosmetic ...
es (
silicone A silicone or polysiloxane is a polymer made up of siloxane (−R2Si−O−SiR2−, where R = organic group). They are typically colorless oils or rubber-like substances. Silicones are used in sealants, adhesives, lubricants, medicine, cooking ...
hydrogels, polyacrylamides,
polymacon Polymacon is a non-proprietary (i.e., generic) term for a hydrophilic polymer A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many ...
) * Disposable diapers where they absorb urine, or in sanitary napkins * Dressings for healing of burn or other hard-to-heal
wound A wound is a rapid onset of injury that involves laceration, lacerated or puncture wound, punctured skin (an ''open'' wound), or a bruise, contusion (a ''closed'' wound) from blunt force physical trauma, trauma or compression. In pathology, a '' ...
s.
Wound gel A hydrocolloid dressing is an opaque or transparent dressing for wounds. A hydrocolloid dressing is biodegradable, breathable, and adheres to the skin, so no separate taping is needed. The active surface of the dressing is coated with a cross-l ...
s are excellent for helping to create or maintain a moist environment. * EEG and
ECG Electrocardiography is the process of producing an electrocardiogram (ECG or EKG), a recording of the heart's electrical activity. It is an electrogram of the heart which is a graph of voltage versus time of the electrical activity of the hear ...
medical electrodes using hydrogels composed of cross-linked polymers ( polyethylene oxide, polyAMPS and polyvinylpyrrolidone) * Encapsulation of quantum dots * Environmentally sensitive hydrogels (also known as 'smart gels' or 'intelligent gels'). These hydrogels have the ability to sense changes of pH, temperature, or the concentration of metabolite and release their load as result of such a change. *
Fibers Fiber or fibre (from la, fibra, links=no) is a natural or artificial substance that is significantly longer than it is wide. Fibers are often used in the manufacture of other materials. The strongest engineering materials often incorporate ...
*
Glue Adhesive, also known as glue, cement, mucilage, or paste, is any non-metallic substance applied to one or both surfaces of two separate items that binds them together and resists their separation. The use of adhesives offers certain advant ...
* Granules for holding soil moisture in arid areas * Air bubble-repellent (superaerophobicity). Can improve the performance and stability of electrodes for water
electrolysis In chemistry and manufacturing, electrolysis is a technique that uses direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of elements from n ...
. * Culturing cells: Hydrogel-coated wells have been used for cell culture. * Biosensors: Hydrogels that are responsive to specific molecules, such as glucose or antigens, can be used as biosensors, as well as in DDS. *Cell carrier: Injectable hydrogels can be used to carry drugs or cells (tissue regeneration/engineering). *Investigate cell biomechanical functions combined with
holotomography Holotomography (HT) is a laser technique to measure three-dimensional refractive index (RI) tomogram of a microscopic sample such as biological cells and tissues. Because the RI can serve as an intrinsic imaging contrast for transparent or phase ob ...
microscopy * Provide absorption, desloughing and debriding of necrotic and fibrotic tissue * Tissue engineering scaffolds. When used as scaffolds, hydrogels may contain human cells to repair tissue. They mimic 3D microenvironment of cells. Materials include agarose, methylcellulose,
hyaluronan Hyaluronic acid (; abbreviated HA; conjugate base hyaluronate), also called hyaluronan, is an anionic, nonsulfated glycosaminoglycan distributed widely throughout connective, epithelial, and neural tissues. It is unique among glycosaminoglycans ...
,
elastin-like polypeptides Elastin-like polypeptides (ELPs) are synthetic biopolymers with potential applications in the fields of cancer therapy, tissue scaffolding, metal recovery, and protein purification. For cancer therapy, the addition of functional groups to ELPs can ...
, and other naturally derived polymers. * Sustained-release drug delivery systems. Ionic strength, pH and temperature can be used as a triggering factor to control the release of the drug. * Window coating/replacement: Hydrogels are under consideration for reducing infrared light absorption by 75%. Another approach reduced interior temperature using a temperature-responsive hydrogel. * Thermodynamic electricity generation: When combined with ions allows for heat dissipation for electronic devices and batteries and converting the heat exchange to an electrical charge. *
Water gel explosives A water-gel explosive is a fuel sensitized explosive mixture consisting of an aqueous ammonium nitrate solution that acts as the oxidizer. Water gels that are cap-insensitive are referred to under United States safety regulations as blasting age ...
* Controlled release of agrochemicals (pesticides and fertilizer) *
Talin Talin may refer to: Places *Talin, Armenia, a city *Tálín, a municipality and village in the Czech Republic *Tallinn, capital of Estonia *Talin, Iran, a village in West Azerbaijan Province *Talin, Syria, a village in Tartus Governorate Other *Ta ...
Shock Absorbing Materials - protein-based hydrogels that can absorb supersonic impacts


Pharmaceuticals

Hydrogels have been investigated for drug delivery. Polymeric drug delivery systems have overcome challenge due to their biodegradability, biocompatibility, and anti-toxicity. Materials such as
collagen Collagen () is the main structural protein in the extracellular matrix found in the body's various connective tissues. As the main component of connective tissue, it is the most abundant protein in mammals, making up from 25% to 35% of the whole ...
, chitosan, cellulose, and poly (lactic-co-glycolic acid) have been implemented extensively for drug delivery to organs such as eye, nose, kidneys, lungs, intestines, skin and brain. Future work is focused on reducing toxicity, improving biocompatibility, expanding assembly techniques Hydrogels have been considered as vehicles for drug delivery. They can also be made to mimic animal mucosal tissues to be used for testing mucoadhesive properties. They have been examined for use as reservoirs in topical drug delivery; particularly ionic drugs, delivered by
iontophoresis Iontophoresis is a process of transdermal drug delivery by use of a voltage gradient on the skin. Molecules are transported across the stratum corneum by electrophoresis and electroosmosis and the electric field can also increase the permeability ...
.


References


Further reading

* {{refend Colloidal chemistry Gels Water chemistry