Biodegradable Materials and Application of “Smart Polymers” as Biomaterials Flashcards
Polymer architecture
Polymerization route
Molecular weight
Morphology
Hydrophobicity/hydrophilicity
Hydrolytic rates of degradable functional groups (most to least)
polyanhydrides, polycarbonates, polyesters, polyurethanes, polyorthoesters, polyamide
Polymer architecture
Varying the architecture of a polymer, rather than its backbone chemistry and pendant functional groups, is equally important for controlling the final degradation profiles.
Polymerization Routes
composition (homopolymer), topology, structure
Different polymerization routes enable controlled synthesis of copolymers with different compositions (e.g., gradient, block copolymers) and topologies (e.g., graft, dendritic, and star polymers)
higher ratio of hydrophilic monomers will accelerate the degradation rate
Polymerization Routes: composition Copolymers
periodic copolymer, gradient copolymer, block copolymer
can self-assemble into different structures, e.g., micelles, vesicles, nanoparticles, or hydrogel, facilitating sustained release of a drug or layer-by-layer degradation of a nanoparticle
Polymerization Routes: topology
linear, graft, star, branched
Polymerization Routes: structure
vesicle, micelle, nanoparticle, hydrogel
Polymer architecture: Molecule weight
The degradation rate decreases as molecular weight increases due to reduced accessibility of functional groups and greater chain entanglement.
in general increasing MW, decreases the degradation rate
PLA methyl group make it harder to degrade
For example, in polymers like PLGA, the degradation occurs mainly through hydrolysis of ester bonds. As molecular weight increases, these ester bonds may be less accessible because they are buried in the dense polymer matrix, not more accessible.
Polymer architecture: Morphology
Tg: polymer transitions from a “glassy” to a “rubbery” state
above Tg, faster degradation rate
Polymer architecture: Morphology Crystallinity
crystalline domains resist the infiltration of the grading species
Stereochemistry: changes crystallinity (e.g., water, enzyme) because the polymer chains are densely packed.
ex. Both PLA and PGA have higher degrees of crystallinity, but the copolymers lead to decreased organization and lower degrees of crystallinity. (PLGA)
Polymer architecture: Hydrophobicity/hydrophilicity
increase hydrophilic monomer increases the degradation rate
Degradation Routes
Three mechanisms of chemical degradation by which cleavage of bond leads to the formation of water-soluble polymers or oligomers:
Cleavage of crosslinks between polymer chains
Cleavage of side chains
Cleavage of the polymer backbone
In reality, a combination of these routes is used for the degradation of polymeric biomaterials
Application of “Smart Polymers” as Biomaterials: environmental stimuli
Physical, temp, ionic strength, solvents, radiation, electric fields, mech stress, high pressure, sonic radiation, magnetic fields, chemical, pH, specific ions, chem agents, biochem, enzyme substrates, affinity ligands, biological agents
All systems are reversible when the stimulus is reversed.
Thermo-responsive poly(N- isopropyl acrylamide) (PNIPAM)
Soluble in water below 32°C, and precipitates sharply as the temperature is raised above 32°C.
The precipitation temperature is called the lower critical solution temperature (LCST).
pH-responsive polymers
One way to have pH-responsive polymers is to incorporate carboxylic acid-derived monomers, such as acrylic acid or methacrylic acid, to impart pH sensitivity in a variety of copolymers
By copolymerizing these monomers with more hydrophobic monomers, the pH transition can be turned to higher pH values.
