Synthetic polymers can be structured into two different groups: biodegradables and non- biodegradables. Poly(D,L-lactic-co-glycolic acid) (PLGA) is the most commonly selected biodegradable polymer, whereas polyacrylamide and polystyrene are considered as nondegradable polymers for the preparation of micro- and nanoparticles.
Dendrimers are highly branched synthetic polymers with a diameter less than 15 nm. The polymers possess layered architectures constituting a central core, an internal region, and numerous terminal groups that determine dendrimer properties. Dendrimers can accommodate multiple functionalities at the terminal groups. A dendrimer can be prepared by multiple types of chemistry, the nature of which defines the dendrimer solubility and biological activity. Dendritic molecules, such as polyamidoamine, poly(propylene imine) polyamide, polyglycerol, and triazine, are synthesized by assembling monomeric subunits. The aggregates form uniform tree-like structures that allow for encapsulation of drug payload in the internal star structures. The branches and terminal groups can be complexed with functionalities, such as targeting ligands, imaging and diagnostic agents, as well as other drug molecules [62,63].
Nanomicelles are self-assembling nanosized (10—100 nm) colloidal dispersions with a core and shell structure . The structures are constructed with amphiphilic blocks with the core made up ofhydrophobic blocks, such as propylene oxide, L-lysine, aspartic acid, D,L-lactic acid, PLGA, and spermine, whereas the outer shell consists of hydrophilic blocks, such as PEG, polyethyleneimine, poly(N-vinyl-2-pyrrolidone), and poly(vinyl alcohol) [65,66]. Nanomicelles can encapsulate hydrophobic drugs and imaging agents in the core, thus circumventing the need for toxic organic solvents. These nanomicelles can deliver drugs by systemic, ocular, transdermal, and oral routes.
Polymeric micro/nanogels are fabricated by cross-linking polymer chains to create an inner porous space that can accommodate a large volume of payload, and therefore are conducive for simultaneous delivery of multiple treatment modalities .