INTRODUCTION:

Drug delivery systems (DDSs) are rapidly developing fields, which are underpinned by the progress in the fields of chemistry, pharmaceutics, biotechnology, nanotechnology, and etc. ControlledDDSs are designed to deliver drugs at predetermined rates for desirable times or specific sites, which have been used to achieve the improved therapeutic effects and to overcome the short coming sof conventional drug formulations. Although remarkable advances have been made in recent years, more progress is yet to be performed in the field of DDSs.

Hydrogels are a special class of polymeric networks, which can absorb and retain a large amount of water while maintaining their three-dimensional integrity. In past decades, the stimuli-sensitive hydrogels have gained increasing attention owing to their smart responsibility to the environmental stimulus, including chemical substances and changes in temperature, pH, light, pressure, electric field or etc. Among the stimuli sensitive hydrogels, the insitu gel-forming hydrogels have been widely investigated, which were sol state before administration but formed non-flowing gels after administration. The in situ gel-forming hydrogels can be formed by chemical crosslinking (covalent bonds) or physical junctions (hydrophobic interactions, electrostatic interactions, chain entanglements or etc). Compared with the permanent networks formed by chemical crosslinking, the in situ gel-forming hydrogels formed by physical junctions show a reversible phase transition behavior by varying the environmental conditions. They have several advantages, including absence of crosslinking agents, without photo irradiation, organic solvents free, and no heat released during polymerization. Especially, the thermo sensitive physical crosslinked in situ gel-forming hydrogels with sol-gel transition have been extensively studied due to their potential biomedical applications, including controlled drug delivery. In this review articles the various aspects of pharmaceutical hydrogel where compiled together and target audience are specifically the M.Pharm andB.Pharm students so that their knowledge towards the subject concern can be enhanced and also at the same time can be motivated towards the publication.

Hydrogel/ Polymers

Hydrogels are a class of biomaterials that are chemically or physically cross-linked water-soluble polymers. They can be either degradable or non-degradable as determined by their chemistry, but degradable is more desirable whenever possible. There has been great interest in hydrogels for tissue engineering purposes, because they generally possess high biocompatibility, mechanical properties similar to soft tissue, and the ability to be injected as a liquid that gels. When hydrogels are physically cross-linked they must rely on phase separation for gelation; the phase separation is temperature-dependent and reversible. Some other advantages of hydrogels are that they use only non-toxic aqueous solvents, allow infusion of nutrients and exit of waste products, and allow cells to assemble spontaneously. Hydrogels have low interfacial tension, meaning cells can easily migrate across the tissue-implant boundary. However, with hydrogels it is difficult to form a broad range of mechanical properties or structures with controlled pore size^{. [1]}

Synthetic polymer

A synthetic polymer may be non-degradable or degradable. For the purpose of neural tissue engineering.^[2] degradable materials are preferred whenever possible, because long-term effects such as inflammation and scar could severely damage nerve function. The degradation rate is dependent on the molecular weight of the polymer, its crystallinity, and the ratio of glycolic acid to lactic acid subunits. Because of a methyl group, lactic acid is more hydrophobic than glycolic acid causing its hydrolysis to be slower. Synthetic polymers have more widely mechanical properties and degradation rates that can be controlled over a wide range, and they eliminate the concern for immunogenicity. There are many different synthetic polymers currently being used in neural tissue engineering. However, the drawbacks of many of these polymers include a lack of biocompatibility and bioactivity, which prevents these polymers from promoting cell attachment, proliferation, and differentiation. Synthetic conduits have only been clinically successful for the repair of very short nerve lesion gaps less than 1–2 cm. Furthermore, nerve regeneration with these conduits has yet to reach the level of functional recovery seen with nerve auto grafts.

There are types of synthetic polymer

· Collagen-terpolymer

· Poly (lactic-co-glycolic acid) family

· Methacrylated dextran (Dex-MA) copolymerized with amino ethyl methacrylate (AEMA)

· Poly (glycerol sebacate) (PGS)

· Polyethylene glycol hydrogel

Biological polymers

There are advantages to using biological polymers over synthetic polymers. They are very likely to have good biocompatibility and be easily degraded, because they are already present in nature in some form. However, there are also several disadvantages. They have unwidely mechanical properties and degradation rates that cannot be controlled over a wide range. In addition, there is always the possibility that naturally-derived materials may cause an immune response or contain microbes. In the production of naturally-derived materials there will also be batch-to-batch variation in large-scale isolation procedures that cannot be controlled. Some other problems plaguing natural polymers are their inability to support growth across long lesion gaps due to the possibility of collapse, scar formation, and early re-absorption. Despite all these disadvantages, some of which can be overcome, biological polymers still prove to be the optimal choice in many situations^.[3] There are types of biological polymers

· Polysialic acid (PSA)

· Collagen Type I/III

· Spider silk fiber

· Silkworm silk fibroin

· Chitosan

· Aragonite

· Alginate

· Hyaluronic acid hydrogel

TYPES OF HYDROGELS

pH sensitive or ion sensitive hydrogels

These hydrogels respond to changes in pH of the external environment. These gels have ionic groups (which are readily ionizable side groups) attached to impart peculiar characteristics. Some of the pH sensitive polymers used in hydrogels’ preparations are polymethyl methacrylate^[4] (PMMA), polyacrylamide (PAAm), polyacrylic acid (PAA), poly dimethylaminoethyl methacrylate^[5] (PDEAEMA) and polyethylene glycol). These polymers though in nature are hydrophobic but swells in water depending upon the pH prevalent in the external environment. Any change in pH of the biological environment^[6] causes changes in the swelling behavior, for example, the hydrogel of caffeine is prepared with poly- mer PDEAEMA at pH below 6.6. As the polymer shows high swellability but when pH changes to higher side, the polymer showed shrinkage leading to drug release. The other pH sensitive hydrogels are copolymer of PMMA and polyhydroxyethyl methyl acrylate (PHEMA)^[7]which are anionic copolymers, swell high in neutral or high pH but do not swell in acidic medium. It was also observed that pH and ionic strength determines kinetics of swelling of PHEMA and guar gum. Other drugs that have been delivered through pH sensitive hydrogels. The figure 1 shown hydrogel are affected by pH, Normal state is dry, at pH 1.2 slightly in gel and at pH 10.6 it form the complete gel^.[8][9][10]

Figure 1 Effect of pH on hydrogel (Adopted from Schmaljohann, D. Thermo-and pH-responsive polymers in drug delivery. Adv. Drug Deliver. Rev., 2006)

Thermo-responsive hydrogels

Covalently linked gels. Three-dimensional covalently linked polymer networks are insoluble in all solvents, they merely swell in good solvents.^[11][12] Thermoresponsive polymer gels show a discontinuous change of the degree of swelling with temperature. At the volume phase transition temperature (VPTT) the degree of swelling changes drastically. Researchers try to exploit this behavior for temperature-induced drug delivery. In the swollen state, previously incorporated drugs are released easily by diffusion.^[13] More sophisticated "catch and release" techniques have been elaborated in combination with lithography^[14] and molecular imprinting.^[15]

Physical gels. In physical gels unlike covalently linked gels the polymers chains are not covalently linked together. That means that the gel could re-dissolve in a good solvent under some conditions. Thermoresponsive physical gels, also sometimes called Thermoresponsive Injectable gels have been used in Tissue Engineering.^[16] This involves mixing at room temperature the Thermoresponsive polymer in solution with the cells and then inject the solution to the body. Due to the temperature increase (to body temperature) the polymer creates a physical gel. Within this physical gel the cells are encapsulated.

In traumatic brain injury (TBI), a series of damaging events is initiated that lead to cell death and overall dysfunction, which cause the formation of an irregularly-shaped lesion cavity, The resulting cavity causes many problems for tissue-engineering.^[17]scaffolds because invasive implantation is required, and often the scaffold does not conform to the cavity shape. In order to get around these difficulties, thermo-responsive hydrogels have been engineered to undergo solution-gelation (sol-gel) transitions, which are caused by differences in room and physiological temperatures, to facilitate implantation through in situ gelation and conformation to cavity shape caused, allowing them to be injected in a minimally invasively manner.

Methylcellulose (MC) is a material with well-defined sol-gel transitions in the optimal range of temperatures. MC gelation occurs because of an increase in intra- and inter-molecular hydrophobic interactions as the temperature increases. The sol-gel transition is governed by the lower critical solution temperature (LCST), which is the temperature at which the elastic modulus equals the viscous modulus. The LCST must not exceed physiological temperature (37 °C) if the scaffold is to gel upon implantation, creating a minimally invasive delivery. Following implantation into a TBI lesion cavity or peripheral nerve guidance conduit, MC elicits a minimal inflammatory response.^[18] It is also very important for minimally invasive delivery that the MC solution has a viscosity at temperatures below its LCST, which allows it to be injected through a small gauge needle for implantation in in vivo applications. MC has been successfully used as a delivery agent for intra-optical and oral pharmaceutical therapies. Some disadvantages of MC include its limited propensity for protein adsorption and neuronal cellular adhesion making it a non-bioactive hydrogel. Due to these disadvantages, use of MC in neural tissue regeneration requires attaching a biologically active group onto the polymer backbone in order to enhance cell adhesion.^[19]The thermal effect on hydrogel (fig. 2), when heat is given the solution convert in to the gel and cool the gel form it convert in to solution form.

Figure 2: Conversion of sol –gel – sol by thermal effect (Adopted from R. Dinarvand, A. D'Emanuele, The use of thermoresponsive hydrogels for on-off release of molecules, Journal of Controlled Release, 1995, Volume 36)

Glucose sensitive hydrogels

These hydrogels are sugar sensitive and show variability in response depending upon the presence of glucose. One of such pharmaceutical hydrogel system is the cross linked poly (methacrylamido phenylboronic acid)- coacylamide hydrogel which liberates the drug in a controlled manner only when the concentration of glucose is high in the surrounding environment causing swelling of the hydrogel Usually glucose sensitive hydrogels are based on implantable sensor which is sensitized to glucose concentration from 0-20 mM . Insulin loaded hydrogels of cross-linked copolymers of polyethylene glycol and methyacrylic acid have been prepared by partitioning the insulin concentration. The micro particles of hydrogels showed no leakage under acidic conditions while the release was highest at pH 7.4. A similar glucose sensitive hydrogel was prepared by photo polymerization of 2- hydroxyethyl methacrylate and 3- acrylamido phenyl boronic acid. The liberation of insulin was glucose concentration dependent. The hydrogels based on sulfonamide chemistry, where the hydrogel showed maximum swelling at pH 7.4 in a local glucose environment of 0-300 mg/dl for delivery of insulin, was an enzymatic approach. The hydrogel of poly (2-hydroxythyl methacrylate^[19] –co- N, N-dimethylaminoethyl methacrylate^[19] or poly HEMA-co- DMAEMA polymer entrapped the insulin, glucose oxidaseand catalase enzymes. Under the environment of glucose, the glucose diffuses in the hydrogel from blood and gets converted to gluconic acid which raises the pH thus causes swelling of the hydrogel. Swelling of hydrogel leads to liberation of insulin which controls the glucose level in the blood. Through such controlled release devices not only the insulin release is controlled (by varying the concentration of cross linking agent) but also, the morphology of the hydrogel is regulated by oxygen uptake. Based on a similar approach of stimuli sensitive hydrogels, a conjugated polymer of monomethoxy poly (ethylene glycol) with glucose containing polymer showed reversible gel to sol phases depending on the Concentration of glucose in the external environment. The viscosity of the hydrogel decreased with the addition of glucose. Apart from gel to sol approach for glucose sensitive hydrogels, the other approach is competitively binding insulin to concanavalin A, which is a lectin protein that reacts with specific sugar residues present at terminals so that in the presence of glucose, insulin is displaced. Thus, in general, glucose sensitive hydrogels are formed by immobilizing glucose oxidase enzyme which catalyses beta D-glucoseto gluconic acid and hydrogen peroxide. The release of gluconic acid decreases pH of the external environment causing decrease in swelling behavior. This enzyme can be present in bound form or it could be attached to the polymer chain). The conducting behavior of gels which gives the idea of swelling vary with the ions liberated due to formation of gluconic acid or by ionization of amines present in the polymer (usually acrylates) used for preparation of hydrogels. Therefore, these smart biomaterials show controlled delivery of solute usually proteins like insulin, lysozyme or BSA (Bovine serum albumin) in response to external environment. Apart from temperature, pH, glucose sensitive hydrogels, other stimuli like light, electric field, chemicals and ions have been utilized in formulation of responsive hydrogels. But these have not gained considerable attention in the field of drug delivery^.[20]

The figure 3 shown glucose diffusion in enzymatic reaction

Figure 3: Glucose sensitive hydrogel

(Adopted from Chivukula P, Dusek K, Wang D, Duskova-Smrckova M, Kopeckova P, Kopecek J. Synthesis and characterization of novel aromatic azo bond-containing pH-sensitive and hydrolytically cleavable IPN hydrogels. Biomaterials 2006)

Field-responsive hydrogels

Field-responsive hydrogel respond to the application of electric, magnetic, sonic or electromagnetic fields. The additional benefit over traditional stimuli-sensitive hydrogel is their fast response time, anisotropic deformation due to directional stimuli, and also a controlled drug release rate simply by modulating the point of signal control.

Light-sensitive hydrogel

A light-sensitive hydrogel undergoes a phase transition in response to exposure to light. The major advantages of light-sensitive hydrogel are that they are water soluble, biocompatible and biodegradable. Another one is their capacity for instantaneous delivery of the sol–gel stimulus, making light-responsive hydrogel.

Important for various engineering and biomedical applications.^[21]Light-responsive hydrogel are very attractive for triggering drug release because of the ability to control the spatial and temporal triggering of the release. This means that the encapsulated drug can be released or active after irradiation with a light source from outside the body. Limitations of light-sensitive hydrogel include inconsistent response due to the leaching of no covalently-bound chromospheres’ during swelling or contraction of the system, and a slow response of hydrogel towards the stimulus. Dark toxicity is also one of the drawbacks of light-responsive polymeric systems.

PROPERTIES OF HYDROGEL

Hydrophilic gels called hydrogels receive considerable attention for their use in the field of pharmaceutical and biomedical engineering.^[21] This material can be used as a carrier for drug and other therapeutic bio-molecule only if it is biodegradable, biocompatible and non-toxic in situ. Thus once the biomaterials are prepared one must evaluate the characteristic properties like swelling behavior, mechanical properties and toxicity studies etc so that the hydrogel could be used successfully in the concerned biomedical field.

Swelling properties

All polymer chains in hydrogels are cross linked to each other either physically or chemically and thus, considered as one molecule regardless of its size. For this reason, there is no concept of molecular weight of hydrogels and therefore, sometimes called infinitely large molecules or super macromolecules. A small change in environmental condition may trigger fast and reversible changes in hydrogel. The alteration in environmental parameters like pH, temperature, electric signal, presence of enzyme or other ionic species may lead to a change in physical texture of the hydrogel. These changes may occur at macroscopic level as precipitate formation, changes in size and water content of hydrogels. The difference in concentration of mobile ions in the hydrogel interior relative to external solution (osmotic pressure), changes in solvent pH, drives the volume change. Hydrogels with acidic or basic functional groups respond to the fluctuations in the external environmental pH. Degree of ionization of the functional groups dictates its swelling profile and hence the volume change. Polyacrylic acid is such type of pH sensitive hydrogel where swelling ratio changes due to the ionization of carboxyl groups on the polymer chain.

In other experiment, temperature-induced phase transitions and microenvironment of PNIPAM based hydrogels were studied in water using 9-(4-N,N-dimethylaminophenyl) phenanthrene (DP) as an intramolecular fluorescence probe. Fluorescence behavior of the DP-labeled PNIPAM gels depended on the concentrations of monomer and cross-linker. Thermo-responsive behavior of the PNIPAM hydrogel was affected by copolymerization of NIPAM with ahydrophilic monomer N,N-dimethylacrylamide (DMAM) and a hydrophobic monomer methyl methacrylate (MMA). Incorporation of DMAM raised the lower critical solution temperature (LCST) of the PNIPAM hydrogel and MMA lowered it. The results indicate that the NIPAM-DMAM copolymer hydrogel with higher LCST are more open with water-swollen nature above their LCST and the NIPAM-MMA co-polymer hydrogels with lower LCST are less open along with water shrunken nature below their LCST when compared with PNIPAM homo-polymer hydrogel.^[21] The fig. 4 is shown swellen network properties of hydrogel.^[22]

Figure4: Effect of temperature on the swelling of covalently linked networks.

(Alexandro Castellanos; Samuel J. DuPont; August J. Heim II; Garrett Matthews; Peter G. Stroot; Wilfrido Moreno; Ryan G. Toomey (2007). "Size-Exclusion "captures and release")

Mechanical properties

Mechanical properties of hydrogels are very important from the pharmaceutical and biomedical point of view. The evaluation of mechanical property is essential in various biomedical applications viz. ligament and tendon repair, wound dressing material, matrix for drug delivery, tissue engineering and as cartilage replacement material. The mechanical properties of hydrogels should be such that it can maintain its physical texture during the delivery of therapeutic moieties for the predetermined period of time. Changing the degree of crosslinking the desired mechanical property of the hydrogel could be achieved. Increasing the degree of crosslinking a stronger hydrogel could be achieved though the higher degree of crosslinking decreases the % elongation of the hydrogels creates a more brittle structure. Hence there is an optimum degree of crosslinking to achieve a relatively strong and yet elastic hydrogel. Copolymerization with co-monomer, may result into hydrogen bonding within the hydrogel which has also been utilized by many researchers to achieve desired mechanical properties. Recently, determined the mechanical properties of calcium alginate hydrogel.^[23] The mechanical characterization consisted of the relaxation experiments (normal stress relaxation at constant deformation) to determine the hydrogel linear viscoelastic range and to define the relaxation spectra and Young modulus by using the generalized Maxwell model. On the basis of Young modulus and Flory’s theory, it was possible to determine the hydrogels cross-linking.^[24] density. This value was then used to estimate the average polymeric mesh size according to the equivalent network theory. The fig. 5 is shown the mechanical properties of calcium alginate hydrogel.

Biocompatible properties

It is important for the hydrogels to be biocompatible and nontoxic in order to make it applicable in biomedical field. Most polymers used for this purpose must pass cytotoxicity and in-vivo toxicity tests. Biocompatibility is the ability of a material to perform with an appropriate host response in a specific application. Biocompatibility consists basically of two elements: (a) bio-safety i.e. appropriate host response not only systemic but also local (the surrounding tissue), the absence of cytotoxicity, mutagenesis, and/or carcinogenesis and (b) bio-functionality i.e. the ability of material to perform the specific task for which it is intended.

Figure 5. Mechanical properties of PNIPAAM based hydrogel

(Adopted from Zhang XZ, Chu CC. Fabrication and characterization of microgel-impregnated, thermosensitive PNIPAAm hydrogels. Polymer 2005)

This definition is particularly relevant in tissue engineering since the nature of tissue construct is to continuously interact with the body through the healing and cellular regeneration process as well as during scaffold degradation.^[25] If this requirement is not met, the hydrogel can be fouled or there may be damage and scarring to connected tissues, whether those tissues are immediately adjacent or linked by vasculature. Toxic chemicals that may be used in the polymerization of synthetic hydrogels present a challenge for in vivo biocompatibility if conversion is not 100%. Furthermore, initiators, organic solvents, stabilizers, emulsifiers, unreacted monomers and cross-linkers used in polymerization and hydrogel synthesis may be toxic to host cells if they seep out to tissues or encapsulated cells. For example, Irgacure 2959, a typical photo-initiator used in many free radical photo-polymerizations, has been shown to decrease cell viability when used in concentrations over 0.1%. To remove hazardous chemicals from preformed gels, various purification processes should be followed such as solvent washing or dialysis. In situ gelation of scaffolds, usually with oligomers and pre-polymers, presents a special challenge since reactants used to synthesize the gel are injected into the body while still in a pre-polymer solution. Utilization of this technique is ideal for its minimal invasiveness but requires special attention to ensure all components used are safe regarded to have superior biocompatibility over synthetic one, yet the presence of synthetic cross-linkers and initiators used in the polymerizations of naturally derived monomers and pre-polymers are subject to the same toxicity concerns as purely synthetic hydrogels.

Evaluation of biocompatibility

In vitro cell culture tests are often used to screen the tissue compatibility of implantable devices. The cell culture methods are also known as cytotoxicity tests. Three primary cell culture assays are used to evaluate biocompatibility of the hydrogels include:

a) Elution (extract dilution) b) direct contact c) agar diffusion. These assays are described in the US Pharmacopeia and in standards published by the International Standards Organization. These are morphological assays and the outcome is measured by observation of changes in cell morphology.

The in vivo assessment of tissue compatibility of a hydrogel is the knowledge of chemical composition of the biomaterial and the conditions of tissue exposure (including nature, degree, frequency and duration of exposure). Principles generally applied to the biological evaluation of hydrogels are described as follows: The material(s) of manufacture; Intended additives, process contaminants, and residues; Leachable substances; Degradation products; Other components and their interactions in the final product; The properties and characteristics of the final product.

Most of the problems associated with hydrogel regarding toxicity, are the unreacted monomers, oligomers and initiators that leach out during application. So it is important to evaluate the toxicity of the hydrogel components like monomers, initiators and other building blocks used for its synthesis. Modifying the kinetics of polymerization and extensive washing of the resulting hydrogel can reduce the toxicity. The formation of hydrogels without any initiators and using alternate path like radiation may eliminate the problem of the residual initiator. PVA hydrogels synthesized by freeze-thawing method to induce crystallization also reduces cytotoxicity. The crystals formed act as physical crosslink’s and are capable to withstand the load applied to the hydrogels. In a study, a biodegradable poly(ethylene glycol)-poly(epsilon-caprolactone)-poly (ethylene glycol) (PECE) triblock copolymer was successfully synthesized, which was flowing at low temperature (sol state) and turned to non-flowing state at body temperature (gel state). The toxicity of in situ forming PECE hydrogel as a potential opthalmic sustained drug delivery system was evaluated. The biodegradation of the hydrogel was studied in the eye compartment, its effect on cultured human lens epithelia, intraocular pressure and ocular tissues are also been included in the study. The results were in good agreement from biocompatibility and toxicity point of view.^[26]

PREPARATION METHODS OF HYDROGEL

Based on the methods of preparation, hydrogels may be classified as (1) homo-polymer (2) copolymer (3) Semi-interpenetrating network (4) interpenetrating network. Homo-polymer hydrogels are cross-linked networks of one type of hydrophilic monomer unit, whereas copolymer hydrogels are produced by cross-linking of two co-monomer units, at least one of which must be hydrophilic to render them swellable. Finally, interpenetrating polymeric hydrogels are produced by preparing a first network that is then swollen in a monomer. The latter reacts to form a second intermeshing network structure.

Homo-polymeric Hydrogel

Homopolymers are referred to polymer networks derived from single species of monomer. It is the basic structural unit and comprising of any polymer network . Homopolymers may have cross-linked skeletal structure depending on the nature of the monomer and polymerization technique. Cross-linked homopolymers are used in drug delivery system and in contact lenses. One possible way of preparing homo-polymeric hydrogel film is the use of poly (2-hydroxyethyl methacrylate) (polyHEMA) as a monomer, polyethylene glycol dimethacrylate as cross-linking agent and benzoin isobutyl ether as the UV-sensitive initiator. The film was prepared in de-ionised water and treated with UV radiation (λ = 253.7 nm, 11 mm distance from the source for 20 minutes). The film was then immersed for 24 h in water until it is fully saturated in order to remove toxic or unreacted substances that could damage a living tissue. Besides contact lenses, pHEMA can also be applied in artificial skin manufacturing and burn dressings, as it ensures good wound-healing conditions. It is also used for bone marrow and spinal cord cell regeneration, scaffolds for promoting cell adhesion and in artificial cartilage production [8-10]. Another low molecular weight cross-linking agent used in the synthesis of polyHEMA hydrogel is 1,1,1-trimethylol propane trimethacrylate. The hydrogel obtained with this agent is soft and contains 30-40 % of water and distinguished by its high oxygen permeability. These properties have translated its use in contact lenses, as matrices for drug delivery system and soft tissue implants. If the mechanical properties of the hydrogel are improved its application could further be extended.

Polyvinyl alcohol (PVA) hydrogels can be obtained by alternating cycles of freezing and thawing. The PVA material prepared by this method has a greater mechanical strength than that obtained by UV radiation as the cross-linking agent. Functional groups of PVA are more accessible which broadens its range of application.

Polyvinyl pyrrolidone (PVP) hydrogels obtained by radiation technique could be applied for wound healing applications. Prepared PVP solutions and carried out the irradiation using a 60Co source at a dose rate of 3.2 Gy/minute.

Polyacrylic acid (PAA) is another homopolymeric hydrogel. Its commercial version contains 2.5 % of PAA and 97.5 % of water. It is stable and has optimal elasticity property. When used as an endoprosthesis, it was designed to be non-toxic, non-inflammatory and to imitate surrounding soft tissue

Co-polymeric hydrogel

Co-polymeric hydrogels are composed of two types of monomer in which atleast one is hydrophilic in nature. Synthesized the biodegradable triblock poly(ethylene glycol)-poly(ε-caprolactone)- poly(ethylene glycol) (PECE) co-polymeric hydrogel for the development of drug delivery system. The mechanism involve here is the ring-opening copolymerization of ε-caprolactone. In the triblock synthesis mPEG was used as initiator, stannous octoate as catalyst and hexamethylene diisocyanate as coupling agent. This co-polymeric block is capable to form hydrogel when it is applied in-situ. The study reveals that the hydrogel was bio-degradable and bio-compatible. It was capable of releasing both the hydrophobic and hydrophilic drugs including proteins over a sustained period of time. This thermosensitive hydrogel has also been evaluated for cell encapsulation and tissue repair applications

In another study, Kim and his co-workers attempted PEG based hydrogels to evaluate its feasibility to be used as a drug delivery system. They prepared copolymers of methacrylic acid (MAA) with PEG-PEGMA by free-radical photopolymerization using tetra (ethylene glycol) dimethacrylate as cross-linker. The cross-linking occurred in the presence of an initiator, 1-hydroxycyclohexyl phenyl ketone in nitrogen atmosphere for 30-minutes under UV light. The hydrogel was successfully loaded with insulin. The authors claim that the swelling behavior and consequently the rate of release strongly depend on the molecular weight of PEG. A thermoplastic co-polymeric hydrogel based on γ-benzyl L-glutamate (BLG) and poloxamer was synthesized by polymerization of BLG N-carboxyanhydride, which was initiated by diamine groups located at the ends of poly(ethylene oxide) chains of the poloxamer. The resulting hydrogel was pH and temperature sensitive and characterized for drug delivery application ^[26]. The melting temperature (Tm) of the poloxamer in the copolymer was reduced with an increase of the PBLG block which is indicative of a thermoplastic property. The water contents of the hydrogel were dependent on the poloxamer content in the copolymers. Hydrogels water content was 31 and 41 wt % when the poloxamer quantum was 48.7 and 57.5 mol % respectively.

Semi- Inter Penetrating Network (Semi-IPN)

If one polymer is linear and penetrates another cross-linked network without any other chemical bonds between them, it is called a semi-inter penetrating network. Semi-IPNs can more effectively preserve rapid kinetic response rates to pH or temperature due to the absence of restricting interpenetrating elastic network, while still providing the benefits like modified pore size and slow drug release etc. One example to justify the situation is the entrapment of linear cationic polyallylammonium chloride in acrylamide/ acrylic acid copolymer hydrogel which imparted both higher mechanical strength and fully reversible pH switching of theophylline release. This pH sensitive semi-IPN was synthesized by template copolymerization in the presence of N, N′-methylene.^[27]

CHARACTERIZATION OF THERMORESPONSIVE HYDROGELS

The PNIPAAm-PEG-DA hydrogel is an ideal polymer because it has unique biocompatibility and polymerization characteristics. It is soluble in water and is readily cleared by the body. It can be immobilized either chemically or physically, it is highly resistant to protein adsorption and cell adhesion, and it is not readily recognized by the immune system. Acrylates are used as end groups because they undergo very rapid photopolymerization. By incorporating PEG-DA with PNIPAAm in the polymerization process, a nondegradable formulation was achieved. Sample images of the hydrogel at room temperature and at body temperature (37ºC) are seen in Figure 6. At room temperature, the hydrogel exists in a liquid gel−like phase; however, once the temperature was raised to 37ºC, a solid gel rapidly formed (within 1 minute). Bright-field images of the gel surface and edge show a relatively uniform pore surface created by the cross-linking process.^[28] (Fig. 6).

BIOMEDICAL APPLICATION OF HYDROGEL

Hydrogels have been successfully used in biomedical fields due to their high water content and the consequent biocompatibility. Potential applications of hydrogels in tissue engineering, synthetic extracellular matrix (ECM) and three dimensional scaffolds are well highlighted in a recent work. The proliferation and differentiation of mesenchymal stem cells (MSC) in a three dimensional (3-D) network of nanofibres formed by self-assembly of peptide-amphiphile (PA) molecules. A 3-D network of nanofibreswas formed by mixing cell suspensions in media with dilute aqueous solution of PA. In another work, a hybrid scaffold consists of two biopolymers, a hydrogel formed through self-assembly of peptide-amphiphile with cell suspensions in media and a collagen sponge reinforced with poly(glycolic acid) fibre incorporation, were used successfully to enhance bone formation. A novel injectable 3-D scaffolds with encapsulated growth factor was formed by mixing of PA aqueous solution with basic fibroblast growth factor (bFGF) suspension. when aqueous solution of PA was subcutaneously injected together with bFGF suspension into the back of mice, a transparent 3-D hydrogel was formed at the injected site and induced significant angiogenesis around the injected site, in marked contrast to bFGF injection alone or PA injection alone. Materials controlling the activity of enzymes, phospholipids bilayer destabilizing agents, materials controlling reversible cell attachment, nanoreactors with precisely placed reactive groups in three-dimensional space, smart micro fluidics with responsive hydrogels, and energy-conversion systems are the promising applications of hydrogels in biomedical and pharmaceutical areas. The soft and hydrophilic nature of hydrogels makes them particularly suitable as novel drug delivery systems.^[29]

EVALUATION

Viscosity study

The measurement of viscosity of the prepared gel was done with a Brookfield Viscometer. The gels were rotated at 0.3, 0.6 and 1.5 rotations per minute. At each speed, the corresponding dial reading was noted. The viscosity of the gel was obtained by multiplication of the dial reading with factor given in the Brookfield Viscometer catalogues.

Spreadability

It indicates the extent of area to which gel readily spreads on application to skin or affected part. The therapeutic potency of a formulation also depends upon its spreading value. Spreadability is expressed in terms of time in seconds taken by two slides to slip off from gel which is placed in between the slides under the direction of certain load. Lesser the time taken for the separation of two slides, better the spreadability. It is calculated by using following formula,

S = M. L / T

Where, M = wt. tied to upper slide L = length of glass slides T = time taken to separate the slides.^[30]

Figure 6: Images of cross-linked thermoresponsive gels: vial containing the hydrogel at room temperature (top left) and at 37ºC(top right), and bright-field images of gel surface (bottom left) and gel edge (bottom right).( Kretlow, James D.; Klouda, Leda; Mikos, Antonios G. (2007-05-30). "Injectable matrices and scaffolds for drug delivery in tissue engineering" Advanced Drug Delivery Reviews. Matrices and Scaffolds for Drug Delivery in Tissue Engineering.)

Measurement of pH

The pH of various gel formulations was determined by using digital pH meter. One gram of gel was dissolved in 100 ml distilled water and stored for two hours. The measurement of pH of each formulation was done in triplicate and average values are calculated

Drug content

1 g of the prepared gel was mixed with 100ml of suitable solvent. Aliquots of different concentration were prepared by suitable dilutions after filtering the stock solution and absorbance was measured. Drug content was calculated using the equation, which was obtained by linear regression analysis of calibration curve.

Stability

The stability studies were carried out for all the gel formulation by freeze - thaw cycling. here, by subjecting the product to a temperature of 4° C for 1 month, then at 25°C for 1 month and then at 40°C for 1month, syneresis was observed. After this, the gel is exposed to ambient room temperature and liquid exudate separating is noted.

Skin irritation study

Guinea pigs (400-500 g) of either sex were used for testing of skin irritation. The animals were maintained on standard animal feed and had free access to water. The animals were kept under standard conditions. Hair was shaved from back of guinea pigs and area of 4 cm2 was marked on both the sides, one side served as control while the other side was test gel was applied (500 mg / guinea pig) twice a day for 7 days and the site was observed for any sensitivity and the reaction if any, was graded as 0, 1, 2, 3 for no reaction, slight patchy erythema, slight but confluent or moderate but patchy erythema and severe erythema with or without edema, respectively.^[34]

In vivo studies

Inhibition of carrageen an - induced rat paw odema– three groups of 6 male wistaralbino rats were used one for marketed sample (reference), other for test formulation and one group for control. The volume of unilateral hind paw test animal were measured. On each paw, 100 mg of preparation was carefully rubbed twice at 1and 2 hrs. Before carrageen an administration. They were placed in cages with copography meshes. 0.1 Ml of 1 % w/v carrageen an was injected subcutaneously into the paw and volume of hind paw measured at hourly interval for 5 hrs using a mercury plethysmometer. Percentage of inhibition was calculated.

FUTURE PROSPECTS

The specific requirements of advanced drug delivery could easily be met by hydrogels. Wide array of methods for the synthesis of these novel biomaterials has extended its application from drug delivery system to tissue engineering scaffolds, wound dressing material, bioseparators, gene delivery device and biosensors etc. Further delve into the fundamentals of multi-polymer based hydrogel and their properties, may give raise a novel approach for implementing the biomaterials in the biomedical field in a better way.

CONCLUSION

The polymer based hydrogel very useful in novel drug delivery system. The stimuli-sensitive hydrogels have gained increasing attention owing to their smart responsibility to the environmental stimulus, including chemical substances and changes in temperature, pH, light, pressure, electric field or etc. Among the stimuli sensitive hydrogels, the insitu gel-forming hydrogels have been widely investigated, which were sol state before administration but formed non-flowing gels after administration. This material can be used as a carrier for drug and other therapeutic bio-molecule only if it is biodegradable, biocompatible and non-toxic in situ. The alteration in environmental parameters like pH, temperature, electric signal, presence of enzyme or other ionic species may lead to a change in physical texture of the hydrogel. These changes may occur at macroscopic level as precipitate formation, changes in size and water content of hydrogels. Most polymers used for this purpose must pass cytotoxicity and in-vivo toxicity tests. Biocompatibility is the ability of a material to perform with an appropriate host response in a specific application. This application is biocompatibility, mutagenesis, and/or carcinogenesis. With this compilation we assure that the content of the article would be useful tool to understand in depth knowledge of the subject.

ACKNOWLEDGEMENT

Authors want to acknowledge the facilities provided by the Rungta College of Pharmaceutical Sciences and Research, Kohka, Kurud Road, Bhilai, Chhattisgarh, India. The authors are also grateful to the e-library of Pt. Ravishankar Shukla University, Raipur, Chhattisgarh, India, 490001 for providing UGC-INFLIBNET facility. The authors acknowledge Chhattisgarh Council of Science and Technology (CGCOST) for providing financial assistance under mini research project (MRP) vide letter no. 1124/CCOST/MRP/2015; Dated: September 4, 2015 and 1115/CCOST/MRP/2015; Dated: September 4, 2015.

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Received on 02.01.2017 Modified on 18.01.2017

Research J. Pharm. and Tech. 2017; 10(3): 944-953.

DOI: 10.5958/0974-360X.2017.00173.1