What are Hydrogels - Pittsburgh Plastics …

1 Copyright Pittsburgh Plastics Manufacturing Inc. 2012. what are Hydrogels ? George A. Paleos, Pittsburgh Plastics Manufacturing, Butler, PA Introduction Pittsbu rgh Plastics Mfg. (PPM) is a contr act manufactu r er of p roducts for medical, foot care, safety and other industrial ma r kets with focus on polymeric cushioning solutions. Products for these ma r kets employ gel and/or foam to p rovide cushioning, impact/vib ration control and support. For example, PPM contr act manufactu res gel filled pouches that are designed to cushion p reemies during the critical days after birth. Needless to say, the cushioning perfor mance p roperties of the gel are essential to the comfort and well-being of the p reemie.

2 PPM has expertise with a wide range of gel polymers, including many types of polyur ethane, silicone, r ubber and ther moplastic elastomer. Several of the specialized gel for mulations were developed in-house and are exclusive to PPM. Another impo rtant type of gel is hyd rogel, and PPM is working to expand the use of lower-cost hyd rogels in cushioning applications. The purpose of this technical paper is to introduce our customers to this rema r kable material. Discussion Definition The th ree classical phases of matter on Ear th are solid, liquid or gas. Phase transitions occur with sufficient change in p ressure and/or temperatu re.

3 For example, water (liquid) transitions to ice (solid) with a d rop in temper atu re. Gelatin powder, such as Kraft Foods Jell-O, is a solid. Empty a packet of Jell-O into a mixing bowl and add boiling water. Stir until dissolved and then chill. Now the material in the bowl is neither solid nor liquid nor gas; it s a hyd rogel. Like a solid, hyd rogels do not flow. Like a liquid, small molecules diffuse th rough a hyd rogel. So what is a hydrogel ? In 1926, Dorothy Jordan Lloyd stated that the colloidal condition, the gel, is one which is easier to recognize than to define . Hydrogels are currently viewed as water insoluble, crosslinked, th ree-dimensional networks of polymer chains plus water that fills the voids between polymer chains.

4 Crosslinking facilitates insolubility in water and p rovides required mechanical strength and physical integrity. Hyd rogel is mostly water (the mass fraction of water is much greater than that of polymer). The ability of a hyd rogel to hold significant amount of water implies that the polymer chains must have at least moderate hyd rophilic char acter. Retu r ning to our discussion of Jell-O, what happened at a molecular scale that resulted in the for mation of hyd rogel? Collagen p rotein is extracted from skin, crushed bones, connective tissue and other animal par ts, and then is par tially hyd rolyzed to make gelatin, a mixtu re of peptides and p roteins.

5 When gelatin is dissolved in hot water, the long p rotein polymer chains move freely in solution. As the solution cools, the motion of the polymer chains slows down. Polymer chains encounter each other and become entangled. As the degr ee of entanglement increases, water is t rapped and immobilized. As a result, the material behaves in some ways like a liquid and other ways like a solid. Types of Hydrogels The polymer chains of a hydrogel are crosslinked (interconnected). The natu re of the connections is different for two general classes, physical and chemical. Fo r the former, the connections are weaker and more reversible; for example, heat may b reak the chain-chain link.

6 Polymers chains of physical hyd rogels are held together by electrostatic forces, hyd r ogen bonds, hyd rophobic interactions or chain entanglements. Jell-O is a physical hyd rogel. The polymer chains of chemical hyd rogels are connected by per manent covalent bonds. A covalent bond is characterized by the sharing of pairs of electrons between atoms. Copyright Pittsburgh Plastics Manufacturing Inc. 2012. A second way to categorize hyd rogels is by the star ting point for synthesis (p roduction). First, a polymer network may be p r epared from monomers (small molecules that may be linked in a repeating fashion to form high-molecular weight polymers).

7 A network is made by copolymerizing hyd rophilic monomers with crosslinkers (polyfunctonal monomers). Networks made with PEG and PPO monomers are good car riers for d r ug delivery. Second, a polymer network may be p repa red star ting with p repolymers (oligomers [low-molecular weight polymers] that are capable of further polymerization). Polyurethane networks may be p repa red using p repolymers. A non-medical application of a polyurethane hyd rogel is entr apment (immobilization) of microbial cells for wastewater treatment. Thir d, polymer networks may be prepared star ting with polymers; a network str uctu re is made by crosslinking hyd rophilic polymer chains.

8 An example is chitosan (a linear polysaccharide polymer) crosslinked with gluta r aldehyde. Two applications are scaffolding for soft tissue engineering and adhesive for peripheral nerve repair. Another way to classify hydr ogels is on the basis of polymer origin, either natu r al or synthetic. Examples of polymers from natu r al origin are p roteins like collagen and polysaccharides like chitosan, dextran and alginate. Hyd rogels from natu r al origin support cellular activities and are biocompatible and biodegradable. On the other hand, they may contain biological pathogens or evoke an immune response. Two other disadvantages ar e low mechanical strength and batch variation.

9 Synthetic polymers are made from monomers such as vinyl acetate, acrylamide, ethylene glycol and lactic acid. Not all monomer s for synthetic polymers are derived from petroleum; for example, lactic acid is made from plants such as corn and sugarcane. Synthesis of polymers can be precisely controlled and tailored to give a wide range of p roperties. Also, they have a low risk of biological pathogens and evoking an immune response. Disadvantages are low biodegradability and inherent bioactive p roperties are absent. Also, toxic substances may be p resent. Classification may be based on physical structu re of the polymer chain: amo rphous (r andom, non-crystalline), semi-crystalline (regions of par tially ordered structu re) or hyd rogen-bonded (network held together by hyd rogen bonds).

10 Another way to classify hyd rogels is by the method of p repa r ation: homopolymer (made from one type of monomer), copolymer (made from more than one type of monomer), multipolymer (more than one type of polymer) or interpenetr ating polymer (a second polymer network is polymerized around and within a first polymer network, and there are no covalent linkages between the two networks). Hyrogels may also be categorized based on ionic charges as follows: neut r al (no charge) such as dextran; anionic (negative charge) such as car r ageenan; cationic (positive charge) such as chitosan; and ampholytic (capable of behaving either positively or negatively) such as collagen Smart materials A sma rt or stimulus responsive material changes some p roperty, such as shape, in response to a change in environment.