Part 9-Hydrogel.pdf

1.3.2 HYDROGELS Hydrogels are Water-swollen, crosslinked polymeric structures Absorbs from 10% to thousands of times of their dry weight in water Covalently cross-linked hydrogels are usually prepared by bringing together small multifunctional molecules such as monom...

1.3.2 HYDROGELS Hydrogels are Water-swollen, crosslinked polymeric structures Absorbs from 10% to thousands of times of their dry weight in water Covalently cross-linked hydrogels are usually prepared by bringing together small multifunctional molecules such as monomers and oligomers, and reacting to form a network structure. 1. Covalent produced by the simple reaction of one or more monomers 2. Physical Molecular entanglement or secondary forces – H-bonding, ionic interactions, hydrophobic interactions, crystallites Hydrogels  Hydrogels Method of Preparation: Chemical Crosslinling 1. Copolymerization with multifunctional monomer 2. Cross-linking enzymes 3. Introduction of divalent cations (e.g. Ca ++, Mg++) 4. Temperature change (e.g. 4°C to 37°C) crosslinking can be achieved by radical methods utilizing: UV Gamma Rays  Hydrogel Key properties of gels: 1. degradability 2. responsive swelling 3. tissue-like structure/properties 4. By weight, gels are mostly liquid but behave like solids 5. Absorption of large quantities of water – 1-20% up to 1000 times their dry weight 6. Cross linkers within the fluid give a gel its structure (hardness) and contribute to stickiness (tack).  Bulk vs. Surface-Degradation Degradation refers to bond cleavage or cross-link dissolution within a network degradation occurs in one of two forms: 1.surface degradation, nonuniform surface-degrading networks maintain their cross-linking density and structural integrity throughout the degradation process, because degradation is limited to the surface of the material. advantageous for drug-delivery applications 2. uniform, bulk degradation, uniform The water that swells these gels will also homogeneously degrade the bonds present throughout the network.  Bulk vs. Surface-Degradation bulk degradation surface degradation  Hydrogels The interest in hydrogels as biomaterials from a number of advantages such as 1. The soft, rubbery nature of hydrogels minimize mechanical and frictional irritation to the surrounding tissues. 2. These polymers may have low or zero interfacial tension with surrounding biological fluids and tissues, thereby, minimizing the driving force for protein adsorption and cell adhesion 3. Hydrogels allow the permeating and diffusion of low molecular weight metabolites, waste products and salts as do living tissues.  Biomedical Applications COMPOSITION+ swelling degree + mechanical properties Blood-compatable – Non-ionic PVA, PHEMA, PNVP,PEO Contact lenses – Mechanical stability – Refractive index – Oxygen permeability Drug delivery – Controlled release – Blood circulation time Kidney membranes Artifical skin Tissue engineering Wound healing membranes  Chitosan-Based Hydrogels Polysaccharide Found in the hard outer skeleton of shellfish Chitosan can be dissolved in weak acidic solution and is positively charged in basic and neutral Chitosan can accelerate wound healing processes Used in tissue engineering, controlled delivery of proteins, and gene delivery Chitosan hydrogels can be fabricated via either physical or chemical cross-linking  Collagen and Gelatin-Based Hydrogels Collagen, a triple-helix protein, has glycine, proline and hydroxyproline amino acid is the major component of connective tissues (bone, ligament, cartilage..). Due to its physiological abundance, collagen is considered biocompatible. Collagen is limited due to its laborious, batch production, procedures as well as the inconsistency of its biological and mechanical properties Modified collagen gels are still favored for many tissue-engineering applications. Composite scaffolds such as collagen-alginate or collagen- hyaluronan [have been fabricated and used for several tissue engineering and DNA delivery applications  Collagen and Gelatin-Based Hydrogels Application: Collagen has been widely used in cosmetic surgery, as a healing aid for burn patients , used as dermal fillers for treatment of wrinkles and skin aging Collagens are widely employed in the construction of artificial skin substitutes used in the management of severe burns and wounds is used in bone grafting Wound healing Matrigel, a type IV collagen-based (as laminin, polysaccharide…) and a commercially available hydrogel, mimics the ECM environment  Collagen and Gelatin-Based Hydrogels Gelatin, a natural glycine-rich polymer derived from hydrolyzed collagen , wherein the hydrolysis reduces protein fibrils into smaller peptides gelatin is a protein product created by partially degrading collagen using heat Gelatin is widely used in food industry as well as in pharmaceutical devices for the controlled release of growth factors. Drug delivery system Biodegradable and biocompatible , the most attractive characteristic of gelatin gelatin can be adjusted to yield a positively or negatively charged polymer.  Poly (hydroxyethyl methacrylate) (PHEMA) Poly (hydroxyethyl methacrylate) (PHEMA) is a rigid acrylic polymer when dry, but it absorbs water when placed in aqueous solution and changes into an elastic gel. – Usually PHEMA Hydrogel takes up approximately 40% water, and it is transparent when wet. Non-degrading Permeable to metabolites Processable Sterilizable transparent Dry (Zerogel) form; polymer is glassy (like PMMA) Tg ~ 100°C Hydrated form (Gel); water plasticises the polymer – Tg