This review discusses the topic of polymer peptide gels which can further develop into useful hydrogel properties. Hydrogels are insoluble and can be synthesised from natural or synthetic polymers or a combination of both. This review will focus on the formation of hydrogels from a combination of both natural and synthetic polymers (polymer-peptide gels). For many years scientists have been investigating the properties and capabilities of hydrogels, in spite of this these materials are still under great investigations to expand upon their employability to applications within the human body. Early studies demonstrated how either only natural or synthetic hydrogels were synthesised. However later studies established a greater desire for polymer-peptide gels, which possess both attributes of natural and synthetic polymers, allowing greater modification and adjustability to biomedical applications. The synthesis of hydrogels is introduced in the review along with its functional properties. More importantly these materials often used within biological systems are being investigated further in order to improve upon their existing uses. The review outlines the discovery of hydrogels and how they have progressed in recent years initiating a whole new generation of hydrogel motifs. To conclude, the recent and future directions of hydrogels have been evaluated.
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Certain classes of polymer materials are able to absorb excess water or biological fluids without dissolving , these are known as hydrogels (or aquagels) . It is important to point out that hydrogels can be prepared in many different ways, using a range of methods. However, this is dependent on the building blocks and preparation of such materials. Hydrogels have various applications, which can include biomaterials, introduction into drug delivery and tissue engineering. They can also maintain their structure in water due to crosslinking of their hydrophilic molecules. The cross-linked macromolecular network structure of a hydrogel can return to its natural form even after deformation during a long period of time , therefore is said to obtain elastic gel properties. The properties of hydrogels were first discovered in 1960 by Wichterle and Lim from the Czechoslovakian Academy of Sciences . They examined the material poly(2-hydroxyethyl methacrylate) which set a benchmark for other various hydrophilic polymers. The design of a hydrogel can be altered to respond in a particular environment. Certain conditions that can change these characteristics consist of temperature, solvent, pH, ionic strength, light, salt type and electrical field . It is possible for hydrogels to comprise of many different physical forms, these can include solid forms, powder matrices, microparticles, coatings, membranes/sheets, and encapsulated solids/liquids [1 & 3]. By employing such hydrogel qualities into relevant systems, the potential of the material is ultimately huge, and further investigations can be conducted to produce additional applications for future use.
2. What are hydrogels?
Hydrogels are materials which consist of a three-dimensional structure. In addition hydrogels can swell significantly in water/aqueous media  with the ability to retain a well-defined hydrophilic polymeric structure [1.7]. Hydrogels are generally synthesised by chemically crosslinking polymers, but more recently polymer-peptide gels have been introduced which can fabricate a new range of hydrogels.
2.1 Hydrogel networks
The hydrophilic polymer network of hydrogels are either chemically stable or can eventually degrade. Hydrogels exist as either chemical gels or physical gels . When gels obtain a covalently crosslinked network, they are known as chemical gels. The properties of the chemical structure are also dependant on the arrangement of the primary chains [1.7]. Physical gels are achieved by molecular entanglement or forces holding the network structures together. These forces are generally secondary forces which consist of ionic, hydrogen-bonding, or hydrophobic forces [3 & 30]. Both chemical and physical gels can form various macromolecular structures, such as networks of linear homopolymers, linear and block/graft copolymers, IPNs and polyblends.
Figure 1. Chemical and physical hydrogel structures 
Figure 1. demonstrates how once hydrogels begin absorbing water (polar), the polymer network begins to expand due to hydrophilic groups (on polymer chain) becoming more hydrated from the presence of water molecules. As a result of this water contact, additional water is absorbed by the polymer causing further swelling. In spite of this, extreme swelling is disrupted via chemical and physical crosslinking, with the polymer obtaining a flexible network with a large extent of elasticity as a result .
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The syntheses of hydrogels are usually prepared from monomers, prepolymers or existing hydrophilic polymers. Methylacrylates and methylacrylamides are commonly used as monomers for such syntheses . Lim and Wichterle's study of monomer, 2-hydroxyethyl methacrylate (HEMA) and crosslinker, ethylene glycol dimtheacrylate (EGDMA) first demonstrated the usefulness of hydrogels. The production of poly(2-hydroxyethyl methacrylate) (PHEMA) came from copolymerising the monomer HEMA and crosslinker EGDMA together, in an aqueous solvent. This study has brought forward the useful application such as soft contact lenses. Table 1. illustrates the variety of chemical and physical gel formation that are possible in hydrogel configuration.
Treat collagen with glutaraldehyde or bis-epoxide
Cooling a polymer solution
Agrose/gelatine in water
Irradiate PEO in water
Warming a polymer solution
PEO-PPO-PEO block copolymers in water
Multifunctional reactive compounds
PEG + diisocyanate = PU hydrogel
Crosslinking a polymer in aqueous solutions
Freeze-thaw PVA in aqueous solution
Copolymerisation of a monomer and crosslinker
HEMA and EGDMA
Lowering the pH to form hydrogen-bonding gels
PEO and PAAc
Copolymerisation of a monomer and multifuncational macromer
Bis-methacrylate terminated PLA-PEO-PLA and photosensitiser and visible light radiation
Polyanion and polycation mixtures to form coacervate gels
Sodium alginate plus polylysine
By polymerising monomers in a different polymer (solid) to form IPN gels.
AN and starch
Combining polyelectrolyte solution with a multivalent ion of opposite charge
Na+ alginate-+ Ca2+ + 2Cl-
Converting a hydrophobic polymer into a hydrogel
Partially hydrolyse PVAc to PVA or PAN to PAN/PAAm/PAAc
Table 1. Types of chemical and physical gel formation 
If we begin with the basics, what are polymer peptides? Polymer peptides which are also known as peptide hybrids are a combination of synthetic polymers and peptide chains bonded together. Peptides employ a certain configuration based on individual R-groups present on the peptide chain and are known to commonly self assemble. They arrange in such a way that they develop well ordered structures [1.1], which is a result of the specific R groups. The peptide domain of a hydrogel hybrid may influence the formation of the hydrogel structure, whereas, the polymer (synthetic) domain can play a role in biocompatibility of the material [1.7].
Specific peptide sections can be integrated into polymers. Such peptides/proteins which are incorporated into these well arranged synthetic polymers produce hybrid macromolecules/polymer-peptide conjugates [1.4].
Figure 2. Models of peptide-polymer conjugates [1.4]
Figure 2. demonstrates how sets of macromolecules have the ability to (i) adjust the contact potential of the monodisperse fragments and furthermore develop (ii) structural and (iii) functional gaps for possible polymer assembly. In addition, (iv) peptides are capable of interacting with biosystems, resulting in further bioactive assemblies [1.4]. The peptides can further self assemble into hydrogels.
3. Synthesis methods
3.1 Synthesis of polymer peptide hydrogels
Due to the characteristics of hydrogels, they have been the centre of attention within the biomedical field. Hydrogels have been incorporated with many diverse materials to make use of their useful properties. Interactions between peptide and synthetic polymers provide the opportunity to develop a selection of new and improved hydrogel designs.
Coiled-coils and Î²-sheets found in nature are ideal models for self-assembly [1.7]. Primary structures are made up of a sequence of amino acids. These amino acids consist of hydrophilic, hydrophobic, charged or other special amino acid residues [1.1]. Hydrophobic residues are said to split into two further categories, aliphatic or aromatic residues. A hydrophobic surrounding is created from aliphatic residues, whereas aromatic residues can form Ï€-Ï€ stacking where p-orbitals overlap in the Ï€-conjugated system [1.1]. Secondary structures can be formed via self assembly of specific peptide sequences through interactions between certain amino residues . With the example of the coiled-coil, an inter-helical hydrophobic core takes place due to the hydrophobic surrounding. This results in stability of the helical structure. Hydrophillic residues can assist in hydrogen bond interactions. Charged residues on the other hand can create electrostatic interactions, supporting the stability of the coiled-coil further, plus also intervening with the organisation of the helices [1.7]. The design of the helices allows possible modifications to occur, since the stability and many other characteristics within the helices can be altered. This means that peptide sequences can be specifically designed to encourage and manipulate cell differentiation and tissue formation, ideal for tissue engineering polymer scaffolds . Figure 3. shows an example of the type of self-assembly which takes place during these interactions.
Figure 3. An example of self-assembly peptide PEO conjugates [1.4]
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Artificial protein hydrogels have been synthesised using recombinant DNA procedures. One of the many studies used to develop greater understanding of these particular preparations of peptide hydrogels was conducted by Nowak et al . Diblock copolypeptide amphilphiles containing hydrophobic and charged segments were synthesised. The hydrophilic block being poly(L-lysine)/poly(L-glutamic acid), and the hydrophobic block as poly(L-valine)/poly(D/L-leucine) [1.7]. The interesting advantage of this was that not only did hydrogels form, but the peptide gels also preserved their mechanical strength during high temperatures of 90 Â°C . This acknowledgment can further expand the uses of hydrogels, as the work can be used as a basis for adjustment. Additionally, further investigation into such properties demonstrated how the self-assembly of triblock amphiphilic copolymers were consistent with that of diblock copolymers. However, in comparison, hydrogels produced by triblock copolymers possess greater mechanical strength and improvement in salt tolerance whilst in the same concentration [1.7].
3.2 Different methods of hydrogel preparation
Hydrogels prepared by irradiation or freeze-thawing methods have also been investigated. Chitosan is a material used for biomedical applications, since it has terrific biocompatibility, hemostatic and wound healing properties, and antibacterial activity . Chitosan is a deacetylated deritive of chitin, a highly insoluble material consisting of 2-acetamido-2-deoxy-Î²-D-glucose . Poly(vinylalcohol) (PVA) is a synthetic polymer admired for its biocompatibility, chemical resistance and easy preparation. If chitosan and PVA are combined they can provide useful biomaterials with a good selection of properties. It was reported that hydrogels prepared by freeze thawing and then followed by irradiation produce better results in terms of mechanical stability. Yang et al designed a series of novel bilayer PVA/chitosan/glycerol hydrogels which consisted of an irradiation layer and freeze thawing, and then followed by another irradiation layer. The study showed that these bilayer hydrogels improve in stability even when dispersed in water for greater than 8 months .
Table 1. (section 2.1) & Schemes 1, 2, 3 demonstrate the various routes in which can be applied to produce these hydrogel networks.
Scheme 1 . Free radical methods for synthesis of crosslinked network gels
Scheme 2. Condensation methods for synthesis of Scheme 3. Free radical methods of IPNs
crosslinked network gels
4. Investigations into hydrogel properties
As explained previously, hydrogels have a crosslinked network which when placed into water and other similar fluids will swell without dissolving. However, hydrogels possess an unwanted characteristic of low mechanical properties, and therefore are limited in their uses within the body. Many studies have been conducted to try and overcome this problem of weak mechanical strength. One solution to avoiding low mechanical stability is to 'blend' conducting polymers together with predictable hydrogels. In spite of this, at times this process can modify the physical characteristics of the polymer, and still obtain a weak mechanical strength . Later discoveries of double-network (DN) and triple-network (TN) methods were established. These methods are key since they have been created so that the mechanical stability of hydrogels remains high.
Some polymer materials that contain hydrophobic side groups can be converted into hydrogels, when hydrophilic side groups are introduced into the polymer in order to conquer the hydrophobicity of the material. Poly(dimethylsiloxane) (PDMS) is an example of this type of hydrogel formation technique. It is of interest due to its widespread use in bodily devices , such as microfluid and drug delivery systems. This material is known to have good optical transparency, high gas permeability, biocompatibility, stability, and high flexibility. However, since it consists of many hydrophobic groups, its uses in the body are limited. In order to overcome this problem, scientists such as Zhang et al [1.5], have end linked hydrophilic components. In this case, Zhang et al end linked PDMS with TEOS and a hydrophilic crosslinker. End results determined that the end product was within the same range as contact lens material.
Another study into the mechanical strength was conducted by Abdurrahmanoglu et al , whereby polyacrylamide (PAAm) hydrogels were designed using hydrophobic modifications to increase the toughness of such polymer hydrogels. PAAm was prepared by crosslinking monomer, acrylamide, and crosslinker, N,N'-methylenebis(acrylamide), with certain hydrophobic comonomers . Extremely swollen polymer hydrogels have been identified to be exceptionally brittle, restricting their uses for further applications.
Hydrogels can be designed so that they depend greatly on its surroundings. For example, conditions such as the pH, temperature, ionic strength, solvents, and electrical field  can reversibly change hydrogel properties. Such hydrogels are known as stimuli-sensitive or intelligent hydrogels . Yan et al blended carboxymethylchitosan (CMCS) and poly(vinylmethyl ether) (PVME) together with (i) electron beam irradiation or (ii) in the presence of crosslinker glutaraldehyde (GA) at room temperature. The study determined that the blends for both methods showed an instant decrease in swelling once temperatures increased from 25 °C to 37°C. During low and high pH the hydrogels possess greater swelling, however at approximately pH 3 the hydrogels begin to shrink .
5. Advantages and disadvantages of hydrogels
5.1 Mechanical stability
Hydrogels play a large role in tissue engineering, drug delivery and wound dressing. The materials can be designed in order that they degrade or are capable of allowing entry of living cells into their structure . Hydrogels have such unique properties, for example, in the presence of fluid they have low interfacial tension and reduce mechanical and frictional contact between certain tissues due to their rubbery characteristics.
One of the problems encountered with incorporating such hydrogel materials into such systems, is that they possess low mechanical strength. However, depending on the application in which the hydrogel is used for, scientists may be able to overcome this problem. Research has been conducted, whereby polymers have been 'blended' with insulating hydrogels in order to obtain high mechanical characteristics . However, it is possible for hydrogels to lose their physical properties from this process, and so other options are to graft a hydrogel with greater mechanical stability. The biomaterial changes only its surface properties whilst its bulk properties remain untouched. This can be achieved through coupling, polymerisation, physical adsorption and physical entrapment .
Hydrogels produced by irradiation can yield hydrogel formation and sterilisation in just one single step, but possess poor mechanical strength. On the other hand the free thawing technique used to prepare hydrogels have a good mechanical strength, therefore the two techniques have been combined to improve the mechanical properties of hydrogels . These processes are also easy to control and do not require any initiators or crosslinkers. The only problem with the hydrogels produced is that they usually have a limited swelling capacity and thermal stability, along with being opaque in appearance.
5.2 Other hydrogel characteristics and behaviour
Generally polymeric hydrogels which are based on polysaccharides and such derivatives consist of many exceptional properties. Scientists are looking more into such hydrogels with the intention of applying them into devices for the body. Blended hydrogels consist of a rubbery nature and can consequently act as material in replace of body tissue .
Depending on how the hydrogel is synthesised, then it is a possibility that their conformation may alter due to alteration in pH, temperature, and concentrations . Temperature and pH are the most investigated parameters for hydrogels as these environmental features are important in the body .
Thermosensitive polymers such as N-isopropylacrylamide and PEO-PPO-PEO are often used in drug delivery. However this creates drawbacks since they are not biodegradable and consist of toxic properties. Often injectable and implant systems are introduced into the body, but the use of organic solvents can denature protein encapsulated drugs .
6. Potential applications
Protein based hydrogels often imitate biological structures within the body. Their most admired function being their hydrophilic networks, allowing a large water capacity, and allowing the material to remain intact, and the way they assist in aiding the use of drug delivery, tissue scaffolding, wound dressings and other similar treatments.
6.1.1 Tissue engineering
Looking further into the employability of hydrogels, much research focuses on how to apply such potential biomaterials for the use of tissue engineering. Even though both natural and synthetic materials can be used to assist in tissue engineering, natural materials are generally more favoured due to their availability and cost effectiveness. In previous studies, hen egg white lysozyme (HEWL) proteins were discovered to produce hydrogels at physiological pH when a reductant (dithioreitol, DTT) was added. However, in order for such substances to even be considered for tissue engineering purposes, cells must be absorbed by the polymer hydrogel. Hui Yan et al  investigated the arrangement of self-supporting, fibrillar hydrogels from HEWL with human dermal fibroblast (hDF) media. DTT was later added to the gel matrix with the intention that gelatin formation would activate. The results proved that preparation of such hydrogels were cytocompatible, therefore encouraging the dispersing and attachment of cells, with no cell adhesive peptide sequences necessary. These hydrogel modifications have resulted in potential substitutes for three dimensional tissue engineering, due to their ability to construct protein building blocks in place of original materials.
Kisiday et al conducted studies on self assembling peptide hydrogels promoting chondrocyte ECM production and cell division, which is regarded as implications for cartilage repair . Figure 4. below shows how three possible routes for tissue repair. Certain tissues with natural healing abilities can be restored by incorporating biodegradable polymer-peptide hybrid materials in order to avoid unwanted cells from reproducing; this is known as 'conduction of repair.' If a filler is required for tissue repair, release systems can be inserted into scaffolds with bioactive features placed in to persuade cell growth from nearby tissues (induction of repair). Cell transplantation is the most complex approach out of all three and requires pre-chosen cells, which are then embedded into an appropriate scaffold/release system.
Figure 4. Tissue engineering strategies [2.2]
6.1.2 Drug delivery
The use of crosslinked hydrogels are under investigation for drug delivery system purposes. These hydrogels have the ability to manipulate release and transportation of molecules . Markland et al prepared a polypeptide hydrogel which was pH responsive. The hydrogel synthesised from PLG and PEG showed a fluctuation in its swelling behaviour due to the alteration of amount of hydrophobic groups along the polypeptide chain, and external pH . Within the experiment, the variables PLA concentration, PEG loading and PEG molecular weight affect the degree of hydrogel swelling. The hydrogel swells and then shrinks in the presence of external pH. In this particular experiment, using a swelling diffusion method, lysozyme was inserted into the hydrogel. The approach becomes useful in biodegradable drug delivery systems as it assists in model protein drugs release. Once the hydrogel begins to contract due to change in external conditions, the principle is that the protein drug will have chance to escape/diffuse out into the body and attach onto appropriate active sites.
As previously explained, changes in the environment can modify the shape and properties of a hydrogel . Conditions such as temperature and pH responsive hydrogels are explored more greatly because these are the most important factors which occur within the body.
After the work conducted by Zhang et al [1.7] was able to develop fibrillar scaffolds from self complimentary oliopeptides in the presence of aqueous buffers, in a similar way hybrid copolymers were prepared from water-soluble synthetic polymers which consisted of coiled-coil protein domains [1.5]. Recent studies have established a real desire for the fabrication of injectable hydrogels to auto-crosslink once inserted into the body. Due to such methods, surgical implants are no longer a necessity. Physical hydrogels has gained more recent attention because of their injectable nature and mild preparation procedures .
Figure 5. Single and dual component systems 
Chung et al  demonstrated how single and dual component systems both enable crosslinking to occur once hydrogels are injected into the body. Figure 5. illustrates how (A) a single component system comprised of drugs and cells within a sol state during room temperature/acidic pH, can change from a sol state and into a gel state immediately after injection depending on temperature or pH. (B) illustrates how two sol state water soluble polymers are injected into the body and form crosslinks via mixing together.
6.2 Recent developments
Hydrogels are useful in orthopaedics and materials such as UHMWPE are commonly used in artificial joint applications for bearing surfaces. However, studies have uncovered that bone which surrounds the implant begins to deteriorate (osteolysis) caused by the wearing of UHMWPE . Researchers are currently investigating routes to reduce wear factor in such applications.
Valvular interstitial cells (VICs) possess temporary fibroblast and myofibroblast properties and preserve heart valve structures. Benton et al  developed an enzymatically degradable hydrogel system for VIC function to produce possible replacement valves.
Scheme 1. shows the structure and synthesis of the hydrogel system used in this particular experiment. There are four-arm synthetic PEG chains bonded with crosslinker MMP-degradable peptide sequences.
PNIPAAm is a useful polymer material which responds to temperature, and is useful in drug delivery, especially in hyperthermia for cancer treatment. Meenach et al  combined magnetic nanoparticles (iron oxide) into hydrogels. The materials synthesised were adjustable nanocomposites and could be manipulated by direction of magnetic field.
The diagram illustrates how a NIPAAm/iron oxide-based hydrogel nanocomposite appears when it is influenced via magnetic field. Once the gel is heated by this magnetic field, the hydrogel system breaks down above its LCST (lower critical solution temperature). Due to little literature regarding the biocompatibility of hydrogel nanocomposites, this area of research is currently being further investigated.
Î±vÎ²3 integrin participates majorly in tumour-induced angiogenesis and tumour metastasis [1.6]. Studies conducted by Borgman et al [1.3] determine that an increase in accumulation in solid tumours is a result of HPMA copolymer-RGDfK conjugates. However, the drawback of this study was that non-target organs were also affected by this accumulation. HPMA is regularly used in drug delivery systems due to their ability to alter the distribution of active ingredients within the body [1.6]. The study continued to try and develop an improved delivery system by attempting to construct HMPA copolymer-cyclo-RGD conjugates with lower molecular weights and charges. Further investigation into the modification of molecular design is required [1.3]. The synthetic polymer is able to increase selective delivery to targeting and recent studies have shown how cyclised Arg-Gly-Asp (RGD) peptides have been used to target the Î±vÎ²3 integrin in tumours. The cyclisation of these structures offers greater binding to specific groups and enhances stability. Mitra et al [1.6] studied the effects of mono- and bi-cyclic Î±vÎ²3 binding peptides. Figure... shows bi-cyclic (B) peptide RGD4C and mono-cyclic (C) peptide RGDfK.
Both polymeric conjugates were suitable for targeting tumour sites and happen to produce similar results, therefore offering an appropriate therapeutic guide for applications such as radiotherapy or chemotherapy.
6.3 Other uses of hydrogels
A large extent of dye is used within chemical industries; therefore it is crucial to remove these harmful dyes from waste water before they disturb the development of sea-life. These dyes are frequently removed using polymer based hydrogels and organic-inorganic hydrid gels. Adhikari et al  conducted an investigation into short synthetic based hydrogels.
Adhikari synthesised five water soluble peptides (gelator peptides) *diagram* in which three of them can cleverly self-assemble at basic pH (11.5-13.5) constructing sensitive hydrogels which are thermoreversible to pH. The gels are capable of adsorbing the toxic dyes.
In the above example, the diagram shows how after two days the dyes are completely absorbed by the gelator peptides. Gelator molecules consisting of hydrophobic and hydrophilic are known to be very effective in adsorbing dye molecules, and therefore are used to adsorb organic dyes. Recycling of such gelator peptides is possible by changing the pH.
7. Future directions
Hydrogels are well known for their development and uses for medical and surgical devices. Biodegradable hydrogels are desired due to having the ability to decompose and eliminate itself from the body, without the need of performing surgery . Because of this, it is believed that scientists are developing more advanced biodegradable systems to allow the use for controlled and better drug delivery systems.
Figure . demonstrates how injectable hydrogels can serve great purpose for release of drugs and tissue repairs. Hydrogels are potential candidates for soft tissue replacements; however certain hydrogels can only operate in certain conditions and may not always achieve its target. Therefore it is very important that scientists continue to develop new approaches to overcome these difficulties.
It is certain that with the current understanding and amount of literature available hydrogels extensive use will broaden even further. Even though much research on hydrogels has demonstrated the successful use of hydrogels much work is still required to improve upon existing research, such as cancer treatments.
The future of hydrogels looks promising with researchers continuously trying to develop new and improved methods to integrate hydrogels into systems within the body. It is believed that there is still more to discover as medical technology and the demand for new surgical devices increases.
Straightforward biological concepts have encouraged scientists to investigate in new methods to connect the space between block copolymers and proteins. As a result, mixtures of peptides and synthetic polymers have been fabricated to enhance polymer properties to conquer any possible drawbacks. This review has discussed the many uses of polymer-peptide gels and has explained how they have been applied to overcome the complexity of biological systems. Such hydrogels are not only limited to uses within the body but they can be employed into other areas, such as cosmetics and waste industry.