Date post: | 11-Nov-2014 |
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Health & Medicine |
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Three-dimensional networks of hydrophilic polymer chains that do not dissolve but can swell in water
both solid like and liquid like properties
high biocompatibility environmental stimuli respondent
(temperature, pH, light, specific molecules)
Ideal for controlled drug delivery
- various criteria for the classification of hydrogels
Origin Natural
Synthetic
Water content or degree of swelling
Low swelling
Medium swelling
High swelling
Superabsorbent
Porosity Nonporous
Microporous
Macroporous
Superporous
Cross-linking Chemical (covalent bonding)
Physical (noncovalent bonding)
Biodegradability Biodegradable
Nondegradable
Rs = (Ws-Wd) / Wd
Rs = swelling ratio
Ws = weight of swollen hydrogels
Wd = weight of dried hydrogels
Swelling property is influenced by:•type and composition of monomers•other environmental factors such as :
temperature, pH, ionic strength•cross-linking Mechanical Mechanical
strength and strength and permeabilitypermeability
Cross-linking and/or copolymerization with hydrophobic comonomers density↑, mechanical strength↑, swelling property↓
Chemical hydrogels
Physical hydrogels
▪ Hydrogen bonding
▪ hydrophobic interaction
▪ crystallinity
▪ stereocomplex formation
▪ ionic complexation
Covalently crosslinked
Noncovalently crosslinked
Thermoset hydrogels
Thermoplastic hydrogels
Volume phase transition
Sol-gel phase transition
Reliable shape stability and memory
Limited shape stability and memory
+
Monomer
Crosslinker
Vinyl group-containing water-soluble polymers
Copolymerization
Polymerization
Hydrogel network
Chemical crosslinking
Polymerization of water soluble monomers in the presence of bi- or multifunctional cross-linking agent
or
Physical crosslinking
• Ionic hydrogel
Chemical and Physical crosslinking
• Cross-linking without chemical reaction
• ionic interaction, hydrogen bonding, antigen-antibody interaction, supramolecular association
Monomer abbreviation Monomer
HEMA Hydroxyethyl methacrylateHEEMA Hydroxyethoxyethyl methacrylateHDEEMA Hydroxydiethoxyethyl methacrylateMEMA Methoxyethyl methacrylateMEEMA Methoxyethoxyethyl methacrylateMDEEMA Methoxydiethoxyethyl methacrylateEGDMA Ethylene glycol dimethacrylateNVP N-vinyl-2-pyrrolidoneNIPAAm N-isopropyl AamAA Acrylic acidHPMA N-(2-hydroxypropyl) methacrylamideEG Ethylene glycolPEGMA PEG methacrylate
- Benefits of controlled drug delivery more effective therapies with reduced side effects the maintenance of effective drug concentration
levels in the blood patient’s convenience as medicines hence increased
patient compliance
- Release mechanisms of drug molecules
: diffusion, dissolution, osmosis, ion exchange
- Diffusion controlled Drug Delivery(1) Polymer matrix
Water-insolublePolymer matrices
(2) Reservoir system
time
Water-insolublePolymer membrane
time
Environment-Sensitive Hydrogels
• respond to environmental change : temperature, pH, specific molecule• reversible volume phase transition or sol-gel phase transition• “intelligent” or “smart” hydrogel
Drug-loaded gelChange in pH for gel swelling
Drug release through the swollen network
Drug release by the squeezing action
Change in temperature for gel collapse
Environmental Factor
Typical polymers Main Mechanism Applications
Temperature
PNIPAAm, PDEAAm, PEO-PPO block copolymers
Competition between hydrophobic interaction and hydrogen bonding
On/off drug release, squeezing device
pH Polyelectrolytes, PAA, PDEAEM
Ionization of polymer chains upon pH change
pH-dependent oral drug delivery
Glucose pH-sensitive hydrogels; Concanavalin A-grafted polymers; polymers containing phenylborate groups
pH change caused by glucose oxidase; reversible interaction between glucose-containing polymers and Concanavalin A; reversible solgel transformation
Self-regulated insulin delivery
Electric signal
Polyelectrolytes (pH-sensitive)
Reversible swelling or deswelling in the presence of electric field
Actuator, artificial muscle, on off drug release
Light Copolymer of PNIPAAm and light sensitive chromophore, such as triphenylmethane and leuco derivatives
Temperature change via the incorporated photosensitive molecules; dissociation into ion pairs by UV irradiation
Optical switches, ophthalmic drug delivery
Antigen Semi-IPN with grafted antibodies or antigens
Competition between polymer-grafted antigen and free antigen
Modulated drug release in the presence of a specific antigen; sensor for immunoassay and antigen
Environmental-Sensitive Hydrogels used for Drug Delivery
Specific applications of Hydrogels in Oral Drug Delivery
Fast-disintegrating tablet formulations fast swelling and superabsorbent properties microparticles of superporous hydrogels
mucoadhesive hydrogels
• buccal drug deliveryGastric emptying of oral dosage forms
• The physical properties (size and density) of the oral dosage form
• The presence of food in the stomach Intragastric floating system
• low density
• float on top of the gastric juice
Specific applications of Hydrogels in Oral Drug Delivery
Mucoadhesive system • cross-linked PAA • highly mucoadhesive at pH (1-3) of the
stomach (∵ hydrogen bonding) Superporous hydrogels
• Swell to a very large size• High swelling ratio ( > a few hundreds)• Fast swelling property - avoid premature
emptying by the housekeeper waves• Gastric retention device (ex. Dogs-from
several hours to a day)• Ac-Di-Sol (cross-linked carboxymethylcellulose
sodium)
Specific applications of Hydrogels in Oral Drug Delivery
- Hydrotropic Hydrogels for delivery of poorly soluble drug
-Hydrotropic agent : Diverse class of water soluble compounds at high
concentration, enhance water solubilities of poorly soluble solutes
ex) N,N-dimethylnicotinamide (3.5M), N,N-diethylnicotinamide
- Many drugs : poorly soluble in water
: low absorption and low bioavailability
- Low-molecular-weight hydrotropes : high concentration
- Polymeric forms of hydrotropes (e.g., hydrotropic hydrogels)
Specific applications of Hydrogels in Oral Drug Delivery
• Hydrogels have played role in the development of various controlled-release formulation
• biocompatible and increasing the solubility of poorly soluble drug
• Hydrogels with novel properties will continue to play important role in drug delivery
• smart hydrogels and new controlled-release formulation