Polymer gels: properties, applications, and synthesis

Polymer Gels

History of the technology and a brief introduction of working principle

Polymer gels are made up of a cross-linked polymer complex which is filled with a solvent like water. (Schreiner, Olding and McAuley, 45) Minor changes of temperature or pH in surrounding may cause Polymer gels to reversibly swell or contract as much as 1000 times in size. Micro sized gel fibres takes milliseconds to shrink in size, however, thick polymers layers may take from few minutes to couple of hours or even days to change size. Polymer gels possess high strength which is nearly equivalent to that of a human muscle and can also exhibit considerable stress. (Maeda, Hara, Yoshida and Hashimoto, 58)

Polymeric gel

Organic polymer gel has been utilized since the start of human history and is very much related to the tissues and life activities of living beings. There are number of theories and methods that describe the working principle of polymer networks. Of these the simplest and the oldest ones are considered to be the most valuable. Several of these focus on the determination, by a number of numerical and physicochemical estimates, of the point of conversion and consequently of the progression of polymerization at the level of gel configuration and on the computation of fundamental polymer parameters result from the evaluation of the gel point.

A. Statistical Methods

Statistical methods create structures by random grouping of reacting functional groups. This process is methodical and the results are more fruitful when utilizing equilibrium-controlled reactions. Following statistical methods are considered to be more significant in polymer networking and gelation.

a. The gel theory of Carothers

b. The probabilistic gel theory of Flory -- Stockmayer

c. The cascade process theory of Gordon

d. The Miller -- Macosko recursive method.

e. The stochastic graphes theory of Bruneau

Of these the first two are the simplest ones and hence they are used most widely whereas c and d are also utilized now and again. But all of these possess some weaknesses, for instance the Carothers theory overvalues the arithmetical value of polymeric gel whereas Flory's theory undervalues it, even then both these are considered very valuable in resolving application problems. Moreover, these theories do not explain what takes place in the system in the mean time of getting the gel point and complete solidification of the network. Various other statistical theories like Percolation is also found in the field of polymerization but it is only useful in evaluation of structure growth close to the gel point.

B. Kinetic Methods

Kinetic or coagulation methods build up all chemical varieties of polymer gels through an unlimited set of kinetic differential equations. The consequential chemical variety distribution can be obtained methodically in the simplest case of random reactions only; in some other cases distributions can be attained mathematically; and finally the solution of the set of equations can also be achieved by Monte Carlo simulation techniques. The use of kinetic techniques possess severe drawbacks: the gel is regarded as a huge fragment, and hence cannot produce divisions of the structures which are trait of the gel; in addition the equations and techniques used are lengthy, difficult, and not very practical or convenient to apply. Mixture of statistical and kinetic techniques in some exceptional cases resolves the problems created due to utilization of the kinetic theory alone. (Yoshida, Uchida, Kaneko and Kiyotaka, 240) Mixture of statistical and kinetic theories too results in systems even more multifaceted and complicated that kinetic theory treatment by it self. These methods are not considered to be much successful.

2. Specific role of the polymer in the application

Application of polymer gels has been found in food manufacturing, drug delivery, bonding agents and various consumer products, they are commonly found in our everyday necessities like rechargeable batteries, make-up and clothes etc. (Hu, 416) Polyvinylalcohol (PVA), polyacrylicacid (PAA) and polyacrylonitrile (PAN) are examples of the most widely used polymer gels. Some of these are expected to be utilized in formation of synthetic muscles, robot actuators, adsorbers of lethal chamicals but at the moment, only some of these possess a commercial presence.

By working on the microstructure of polymer gels, a broad range of physical properties can be obtained ranging from tough rubbery plastics to flexible hydro gels. Silicone-based polymer gels specially are found in wide variety of consumer products like restorative implants and cooking utensils.

Cross linked structure of a polymeric hydrogen

Smart polymeric gels is a latest variety of biomaterials that are now being created at an increased rate to apply at various places like patterns for nanoscale and some biomedical tools, biosensors, actuators and scaffolds for tissue engineered prostheses. (Vepa, 412) During the past few years, tissue engineering has gone through an important modification in the structure of new biomaterials -- from static biomaterials to ECM (extracellular matrix mimetic biomaterials). A couple of major types of macromolecules constitute the extracellular matrix mimetic biomaterials:

(a) fibrous proteins such as collagen, elastin, laminin and fibronectin,

(b) GAGs (glycosaminoglycans) covalently connected with proteins -- proteoglycans (PG)

Very much hydrated, hydrogel -- like characteristic of proteoglycans molecules of the extracellular matrix mimetic biomaterials have made synthetic and semi-synthetic items of these native hydrogels useful claimants for tissue engineering. (Heitfeld, 15) Wichterle and Lim created the foremost hydrogels for biomedical purpose during late 1950s, to create the earliest variety of contact lens typically contained poly (hydroxyethyl methacrylate) crosslinked with ethylene dimethacrylate. Even though contact lens materials represent only a small part in the vast field of biomaterials, the advanced utilization of soft hydro gels by these scientists opened up a completely new chapter of explorations in the search for new biomaterials containing more useful and intelligent features. Hydro gels basically are 3D polymeric "swell gels," as they swell in liquid and do not get dissolved in it. In a dehydrated condition, the polymer chains of hydrogen are in a collapsed position, and there is little opportunity for molecular diffusion. When the hydrogel swells and reaches a stable swelling state, the pressure of swelling on the chains is neutralized by the pressure keeping the chains together, specifically, the pressure of crosslinking. At this stable state, the network mesh magnitude is the highest and molecular diffusion achieves its maximum value.

3. Unique structure-property-process relation of the polymer to make it to perform this role

Polymer gels are made up of cross linked set-up of polymers that perform like viscoelastic solids. As the polymer network is cross linked, the gel network is made up of a big branched polymer which extends to the entire gel. As gels can be flexible and soft they also maintain their texture like a solid. (Annaka, Masahiko, Tanaka and Toyoichi, 430) According to the physical arrangement of the polymer network, polymer gels can be categorized as powerful, weak or pseudo gels. Cross linked polymer gels are chemically seen as strong gels as they are stable and cannot be restructured if broken. Some biopolymer and colloidal gels are considered as weak gels for containing breakable and reformable crosslinks. Entangled polymer systems are also called pseudo gels as, with the passage of time, physical entanglements connecting polymer chains act as chemical crosslinks that gives them properties similar to gels. Nevertheless, the equilibrium reaction of pseudo gel to continuous application of pressure is to flow like a liquid.

A gel is produced by utilizing a chemical or physical crosslinker or by supramolecular connection to generate tie -- points or attachments in the polymeric matrix, that generate hydrogels or organogels. Actually, self -- assembling organogels are developing as latest functional materials that rely on supramolecular chemistry. With no such supramolecular assembling -- based entanglements or crosslinking -- based, polymers would react to stimuli by cycling between the solution and the gel conditions, rather than fluctuating between the inflated and the collapsed states. (Darmawan, Smart, Julbe, Diniz da Costa, Joao Carlos, 452) for these two types of oscillations, there are distinct and (preferably) reversible on and off conditions activated by ambient stimulus. This is sufficient to trigger a device under various system requisites and operating circumstances. Additionally, in mixing a polymeric gel with protein motifs or inert fillers, one can take assistance from the amalgamation of the most superior of different worlds -- the polymeric gel world as well as the inorganic world or the protein world. The polymeric gel world mostly exhibits the smartness (smart polymeric gels) whereas the inorganic or the protein world mostly display properties like increased mechanical force (for instance, by the amalgamation of carbon nanotubes) or self -- assembling potential (e.g., by utilizing variants of coiled -- coil motifs). In quintessence, it has been noticed that the obstructions in the existing performance potential of biomedical tools are fake; formed by the restrictions of existing technology. The pressure generated by the production of smart polymeric tools and their conjugates can clearly surpass these artificial hazards by the fast convergence of interdisciplinary technologies.

In the area of nanotechnology, progress in nanofabrication techniques necessitate a broad range of smart biomaterials. Now it has been recognized that organic tissue interfaces possess nanoscale roughness, that makes the progress in nanotechnology more significant for creating smart biomedical tools. Advancement of nanotechnology has gained significant attention in the self -- assembling characteristic of a variety of molecules, which is a vital requirement for the growing bottom -- up design of nanoscale structures. When these molecules go through molecular self -- congregation, the consequential structural elements, for instance nanotubes or vesicles, can be further transformed to give specific charactistics to the components. Like nanotubes can be covered with metals or partially -- conducting substances to fabricate nanowires.

Smart polymeric gels are classified on various structural properties. Superporous hydrogels (SPHs) are utilized to augment the responsiveness of hydrogels. In this case, the augmented responsiveness to stimuli is accomplished by manufacturing interconnected absorbent networks. Superporous hydrogels (SPHs) correspond to a rapid -- swelling group of hydrogels with pore dimensions much bigger than the usual network of a normal hydrogel. These were firstly created as modern gastric retention devices to augment the duration of drugs stay in the stomach. Normally network size of a usual hydrogel is less than 100 nm whereas the pore size of Superporous hydrogel varies from below 1 ?m to greater than 1,000 ?m. The distension kinetics of Superporous hydrogels is much quicker than that of usual hydrogels. This dissimilarity can be understood by explaining the dissimilarity in morphology of both the types of hydrogels. As the network size of usual hydrogels is diminutive, the puffiness in such fairly closed systems is restricted by dispersion of water across the glassy polymer matrix. Conversely, SPHs contain huge interconnected pores that cause the capillary ingestion of water.

Super porous Hydrogels

Shape -- memory polymers represent one more group of smart biomaterials and carry out its job by an omnipresent method present in our day -- to -- day lives. Take an example of the built-in memorizing capacity of an expandable rubber band that is stretched and then left to relax; if the entropic energy linked to the enlarged rubber band can be stored to be utilize later on, then it will be considered as shape -- memory based application. Reactions triggered by shape -- memory and stimuli -- responsiveness are interconnected. Actually, stimuli -- receptiveness can be thought as a conventional instance of the shape -- memory property in substances. Substances are said to display a shape -- memory effect if they can change their shape and get fixed into a short-term shape, and contain the capability to get the original, enduring shape only on coming in contact to an external stimulus. The shape -- memory alloys were the foremost shape -- memory objects to be discovered. This discovery was then utilized in various applications, like toys, "shrink -- to -- fit" pipe couplers for airplanes, solid state heat engines and therapeutic usage in orthopedics, orthodontics, and heart surgical procedures.

Lastly, protein hydrogels develop a complete new group of biomaterials by copying and incorporating the self -- assembling codes from surrounding in smart hybrids. Drawbacks of hydrogels manufactured by conventional methods, for instance crosslinking copolymerizaton, exhibit deficiency of accurate control of structural organization and the hysteresis connected to "on" and "off" alterations. Protein engineering put forwards potent solutions to reduce these limitations by developing distinct supramolecular structures.

protein engineering

4. Relative advantages and disadvantages of the polymer in the application

Within the field of biomaterials examination, stimuli -- sensitive hydrogels or smart 4 hydrogels are getting more stability and hence becoming more popular in its application. Reaction to a stimulus is a fundamental part of living systems. Imitating this characteristic of living systems may give a reasonable solution to a lot of the present day biomedical problems. Smart hydrogels act in response to various stimuli, that is in the form of temperature, pH, radiance, stress, electric field, chemicals, or ionic potency, or a mixture of these all. Such hydrogels possess the capability of giving response to small alterations in ambient stimuli and display remarkable property modifications. In order to react really smart, a biomaterial modification in hydrogel microstructures needs to be rapid and reversible. Nevertheless, the foremost challenge with usual stimuli -- responsive hydrogels is the slow reaction time to stimulus and the hysteres is connected to the now and then states. One technique to get rid of this disadvantage is to have slim and undersized hydrogels without considerably altering their mechanical properties. Another challenge is to create hydrogels that deteriorate as a result of proper ambient stimulus in the body. This is in comparison to the modern technology where hydrogels mortify at a fixed pace when getting implant in the body. For instance, proteolytic stimulus, that is, biochemical indicators from cells in the surrounding of an implanted biomaterial scaffold may vigorously transduce signaling cues if the scaffold can mortify and amend its infrastructure in a stimuli reactive approach. In this case, the motivation is the cascade of biochemical indicators from the cells in the surrounding area of the biomaterial.