EBME 306-Sen Gupta-Polymeric Biomaterials-Lecture 5-Fall 2023.pptx

Polymers in Medicine: Lecture 5 Hydrogels Anirban Sen Gupta, Ph.D. Email: [email protected] phone: 368-4564 Office: Room 519, Wickenden Building EBME 306 1 READ THE REFERENCE PAPERS!! EBME 306 2 Hydrogels Water swellable crosslinked polymers Crosslinks caused by: • by reaction (covalent bondin...

Polymers in Medicine: Lecture 5 Hydrogels Anirban Sen Gupta, Ph.D. Email: [email protected] phone: 368-4564 Office: Room 519, Wickenden Building EBME 306 1 READ THE REFERENCE PAPERS!! EBME 306 2 Hydrogels Water swellable crosslinked polymers Crosslinks caused by: • by reaction (covalent bonding) between “mers” • hydrogen bonds • van der Waals interactions • Ionic interactions Recall: Highly crosslinked polymer yields a thermoset Lightly crosslinked polymer yields an elastomer Lightly crosslinked hydrophilic polymer yields a hydrogel EBME 306 3 JELL-O is a HYDROGEL made from a naturally derived polymer Collagen !! EBME 306 4 Covalently Crosslinked Hydrogels Example: hydrogels for soft contact lenses pHEMA + EGDMA (Ethylene Glycol Dimethacrylate) EBME 306 5 Non-covalent Physical Hydrogels Formed by molecular entanglements and/or secondary forces such as van der Waals, H-bonding or hydrophobic forces. Example: Hydrophobic association EBME 306 6 Ionic Hydrogels Remember polyelectrolytes/ionomers EBME 306 7 Other Hydrogel Classifications Classification based on preparation method • homopolymer hydrogels (one type of hydrophilic mer) • copolymer hydrogels (two types of mers, at least one hydrophilic) • multipolymer hydrogels (more than three types of mers) • interpenetrating polymeric hydrogels (swelling a network of polmer1 in mer2 OR making intermeshing network of polymer1 and polymer2) Classification based on ionic charges • Neutral, Anionic, Cationic or Ampholytic Classification based on structure/ morphology • Amorphous (random network) • Semicrystalline (dense regions of “order”) • Non-covalently bonded hydrogels (assembly systems) Connections between chains is called a “crosslink” or “junction” EBME 306 8 Network Formation Crosslink structure: - ideal network with tetrafunctional covalent crosslinks (rare) - multifunctional junctions (asyymmetric covalent crosslinks) - molecular entanglements (permanent or semi-permanent assembly) Mc is the molecular weight between crosslinks ideal practical Defects EBME 306 entanglement ‘unreacted functionality’ and loops 9 Design of Hydrogels “crosslinked structures swellable in water”: hence need a “hydrophilic polymer” component and a “crosslinking” component”: Typical “hydrophilic component” can be monomers, oligomers or polymers bearing “polar” groups like hydroxyl (-OH), carboxyl (-COOH), amide (-CONH), sulfhydril (-SH), sulphate (-SO3H) etc. Typical “crosslinking component” can be bifunctional [di-acrylate (CH2=CHCO-O-) groups, glutaraldehyde (OHC-(CH2)3-CHO) or a diisocyanate (OCN-RNCO)] or a multifunctional “mer” Crosslinks can be induced by radiation, heat or chemical reaction • Radiation reactions include electron beams, gamma-rays, X-rays, or UV • Chemical crosslinking – small molecular weight crosslinking agents that links two chains together through its bi- or multifunctional groups – copolymerization-crosslinking reactions between the monomers and a multifunctional monomer that is present in small quantities – combination of above EBME 306 10 Swelling of Hydrogels After polymerization, the hydrophilic gel is brought in contact with water where the polymer network absorbs water and “expands” The thermodynamically driven swelling force is counterbalanced by the retractive force of the crosslinked structure Two forces become equal as equilibrium is reached EBME 306 11 Degree of Swelling Degree of swelling is important because it influences: • Solute (drug) diffusion coefficient through the hydrogel • Polymer surface properties and surface mobility • Polymer optical properties (contact lens applications) • Polymer mechanical properties Degree of swelling (also called Swelling Ratio) calculated by: • Volume ratio: ratio of sample volume in the swollen state to volume in the dry state expressed as percentage • Weight ratio: ratio of the weight of swollen sample to that of the dry sample expressed as percentage Swelling Ratio can be controlled by polymer composition and crosslink density EBME 306 12 Examples of Hydrogels Highly swollen hydrogels: • cellulose derivatives • poly(vinyl alcohol) • poly(N-vinyl 2-pyrrolidone), PNVP • poly(ethylene glycol) Moderately or poorly swollen hydrogels: • poly(hydroxyethyl methacrylate), PHEMA and derivatives One may copolymerize a highly hydrophilic monomer with other less hydrophilic monomers to achieve desired swelling properties EBME 306 13 Interpenetrating Polymer Network (IPN) A polymer comprising two or more networks which are at least partially interlaced on a molecular scale but not covalently bonded to each other and yet cannot be separated unless chemical bonds are broken. For this to happen one or both networks are formed by crosslinking in presence of each other simultaneously. As a result the components do not phase separate. Remember: A mixture of two or more preformed polymer networks is NOT an IPN. e.g.: poly(vinyl pyrrolidone)-poly(vinyl alcohol) [PVP-PVA] IPN EBME 306 14 Hydrogels for Biomedical Applications • Blood-compatible coatings: hydrophilicity prevents protein deposition and platelet adhesion • Controlled/sustained/programmable release drug delivery devices • Contact and Intra-ocular lenses • Lubricating surface coating for catheters and drainage • tubes Tissue engineering scaffolds: artificial tendon and cartilage • Wound healing dressings (Vigilon®, Hydron®, Gelperm®) • Dialysis (artificial kidney membranes) • Artificial skin, cosmetic reconstruction materials • Vocal cord replacement Reading reference: Allan S. Hoffman “Hydrogels for biomedical applications” Advanced Drug Delivery Reviews 43 (2002) 3-12 EBME 306 15