Polymeric Nanoparticles for Drug Delivery

The recent emergence of nanomedicine has revolutionized the therapeutic landscape and necessitated the creation of more sophisticated drug delivery systems. Polymeric nanoparticles sit at the forefront of numerous promising drug delivery designs, due to their unmatched control over physiochemical properties such as size, shape, architecture, charge, and surface functionality. Furthermore, polymeric nanoparticles have the ability to navigate various biological barriers to precisely target specific sites within the body, encapsulate a diverse range of therapeutic cargo and efficiently release this cargo in response to internal and external stimuli. However, despite these remarkable advantages, the presence of polymeric nanoparticles in wider clinical application is minimal. This review will provide a comprehensive understanding of polymeric nanoparticles as drug delivery vehicles. The biological barriers affecting drug delivery will be outlined first, followed by a comprehensive description of the various nanoparticle designs and preparation methods, beginning with the polymers on which they are based. The review will meticulously explore the current performance of polymeric nanoparticles against a myriad of diseases including cancer, viral and bacterial infections, before finally evaluating the advantages and crucial challenges that will determine their wider clinical potential in the decades to come.

Self-Immolative Hydrogels with Stimulus-Mediated On-Off Degradation

Hydrogels are of interest for a wide range of applications from sensors to drug delivery and tissue engineering. Self-immolative polymers, which depolymerize from end-to-end following a single backbone or end-cap cleavage, offer advantages such as amplification of the stimulus-mediated cleavage event through a cascade degradation process. It is also possible to change the active stimulus by changing only a single end-cap or linker unit. However, there are very few examples of self-immolative polymer hydrogels, and the reported examples exhibited relatively poor stability in their nontriggered state or slow degradation after triggering. Described here is the preparation of hydrogels composed of self-immolative poly(ethyl glyoxylate) (PEtG) and poly(ethylene glycol) (PEG). Hydrogels formed from 2 kg/mol 4-arm PEG and 1.2 kg/mol PEtG with a light-responsive linker end-cap had high gel content (90%), an equilibrium water content of 89%, and a compressive modulus of 26 kPa. The hydrogel degradation could be turned on and off repeatedly through alternating cycles of irradiation and dark storage. Similar cycles could also be used to control the release of the anti-inflammatory drug celecoxib. These results demonstrate the potential for self-immolative hydrogels to afford a high degree of control over responses to stimuli in the context of smart materials for a variety of applications.

Self-immolative Amphiphilic Diblock Copolymers with Individually Triggerable Blocks

Self-immolative polymers are a growing class of degradable polymers that undergo end-to-end depolymerization after the stimuli-responsive cleavage of an end-cap or backbone unit. Their incorporation into amphiphilic block copolymers can lead to functions such as the disintegration of copolymer nanoassemblies when depolymerization is triggered. However, diblock copolymers have not yet been developed where both blocks are self-immolative. Described here is the synthesis, self-assembly, and triggered depolymerization of self-immolative block copolymers with individually triggerable hydrophilic and hydrophobic blocks. Neutral and cationic hydrophilic polyglyxoylamides (PGAm) with acid-responsive end caps were synthesized and coupled to an ultraviolet (UV) light-triggerable poly(ethyl glyoxylate) (PEtG) hydrophobic block. The resulting block copolymers self-assembled to form nanoparticles in aqueous solution, and their depolymerization in response to acid and UV light was studied by techniques including light scattering, NMR spectroscopy, and electron microscopy. Acid led to selective depolymerization of the PGAm blocks, leading to aggregation, while UV light led to selective depolymerization of the PEtG block, leading to disassembly. This self-immolative block copolymer system provides an enhanced level of control over smart copolymer assemblies and their degradation.

Degradable polyprodrugs: design and therapeutic efficiency

Prodrugs are developed to increase the therapeutic properties of drugs and reduce their side effects. Polyprodrugs emerged as highly efficient prodrugs produced by the polymerization of one or several drug monomers. Polyprodrugs can be gradually degraded to release therapeutic agents. The complete degradation of polyprodrugs is an important factor to guarantee the successful disposal of the drug delivery system from the body. The degradation of polyprodrugs and release rate of the drugs can be controlled by the type of covalent bonds linking the monomer drug units in the polymer structure. Therefore, various types of polyprodrugs have been developed based on polyesters, polyanhydrides, polycarbonates, polyurethanes, polyamides, polyketals, polymetallodrugs, polyphosphazenes, and polyimines. Furthermore, the presence of stimuli-responsive groups, such as redox-responsive linkages (disulfide, boronate ester, metal-complex, and oxalate), pH-responsive linkages (ester, imine, hydrazone, acetal, orthoester, P-O and P-N), light-responsive (metal-complex, o-nitrophenyl groups) and enzyme-responsive linkages (ester, peptides) allow for a selective degradation of the polymer backbone in targeted tumors. We envision that new strategies providing a more efficient synergistic therapy will be developed by combining polyprodrugs with gene delivery segments and targeting moieties.

Synthesis and depolymerization of self-immolative poly(disulfide)s with saturated aliphatic backbones

Self-immolative polymers (SIPs) are a class of degradable stimuli-responsive polymers, which, upon removal of labile end-caps, depolymerize selectively and stepwise to small molecules.

Stimuli-responsive temporary adhesives: enabling debonding on demand through strategic molecular design

Stimuli-responsive temporary adhesives constitute a rapidly developing class of materials defined by the modulation of adhesion upon exposure to an external stimulus or stimuli. Engineering these materials to shift between two characteristic properties, strong adhesion and facile debonding, can be achieved through design strategies that target molecular functionalities. This perspective reviews the recent design and development of these materials, with a focus on the different stimuli that may initiate debonding. These stimuli include UV light, thermal energy, chemical triggers, and other potential triggers, such as mechanical force, sublimation, electromagnetism. The conclusion discusses the fundamental value of systematic investigations of the structure-property relationships within these materials and opportunities for unlocking novel functionalities in future versions of adhesives.

Polymerization-Induced Hierarchical Self-Assembly: From Monomer to Complex Colloidal Molecules and Beyond

The nanoscale hierarchical design that draws inspiration from nature's biomaterials allows the enhancement of material performance and enables multifarious applications. Self-assembly of block copolymers represents one of these artificial techniques that provide an elegant bottom-up strategy for the synthesis of soft colloidal hierarchies. Fast-growing polymerization-induced self-assembly (PISA) renders a one-step process for the polymer synthesis and in situ self-assembly at high concentrations. Nevertheless, it is exceedingly challenging for the fabrication of hierarchical colloids via aqueous PISA, simply because most monomers produce kinetically trapped spheres except for a few PISA-suitable monomers. We demonstrate here a sequential one-pot synthesis of hierarchically self-assembled polymer colloids with diverse morphologies via aqueous PISA that overcomes the limitation. Complex formation of water-immiscible monomers with cyclodextrin via "host-guest" inclusion, followed by sequential aqueous polymerization, provides a linear triblock terpolymer that can in situ self-assemble into hierarchical nanostructures. To access polymer colloids with different morphologies, three types of linear triblock terpolymers were synthesized through this methodology, which allows the preparation of AX_n-type colloidal molecules (CMs), core-shell-corona micelles, and raspberry-like nanoparticles. Furthermore, the phase separations between polymer blocks in nanostructures were revealed by transmission electron microscopy and atomic force microscopy-infrared spectroscopy. The proposed mechanism explained how the interfacial tensions and glass transition temperatures of the core-forming blocks affect the morphologies. Overall, this study provides a scalable method of the production of CMs and other hierarchical structures. It can be applied to different block copolymer formulations to enrich the complexity of morphology and enable diverse functions of nano-objects.

Sonochemical preparation of polymer-metal nanocomposites with catalytic and plasmonic properties

Polymer-metal nanocomposites are of increasing interest for a wide range of applications; however, the preparation of these nanocomposites often requires the addition of external initiation and reducing agents for the synthesis of polymer and metal nanoparticles, respectively. Herein, we demonstrate the preparation of polymer-metal nanocomposites for improved catalytic performance by utilizing ultrasound as both the initiation and reducing source. Specifically, synthesis of the macro-RAFT agent containing poly[2-(dimethylamino)ethyl methacrylate], followed by ultrasound-initiated polymerization-induced self-assembly (sono-PISA), provides triblock copolymer nanoparticles containing tertiary amine groups. These polymer nanoparticles were further used as the scaffold for the in situ reduction of metal ions (Au and Pd ions) by radicals generated via sonolysis of water without additional reducing agents. The immobilization of metal nanoparticles has been confirmed by TEM and electron diffraction patterns. Polymer-Au nanocomposites with stepwise-grown AuNPs can be applied as surface-enhanced Raman scattering (SERS) substrates for 4-aminothiophenol (4-ATP) detection. Furthermore, the catalytic performances of these prepared polymer-Au and polymer-Pd nanocomposites were examined for aerobic alcohol oxidation and the Suzuki-Miyaura cross-coupling reaction, respectively. Overall, this strategy is expected to greatly expand the utility of ultrasound in the preparation of polymer-metal nanocomposites and promote the catalytic applications of these nanocomposites.

Triggered Degradable Colloidal Particles with Ordered Inverse Bicontinuous Cubic and Hexagonal Mesophases

We herein report a facile strategy to prepare triggered degradable block copolymer nano/macro-objects, ranging from typical micelles, worms, jellyfish, and vesicles to rarely achieved spongosomes, cubosomes, and hexosomes via RAFT-mediated polymerization-induced self-assembly (PISA). The morphological transitions from a simple spherical micelle to a spongosome, ordered Im3¯m cubosome, and p6mm hexosome were captured and demonstrated by TEM, SEM, and synchrotron SAXS. In addition, morphological phase diagrams including important factors, such as solid contents, degree of polymerization (DP), and stabilizer block chain length, were constructed to unveil the formation mechanism and guide the scalable preparation of complex morphologies with packing parameter (P) > 1. This study not only represents an example that achieved inverse mesophases via acrylate-based monomers with high conversion but also reports a triggered degradable system in the most extended morphological range via PISA. The facile synthesis and stimuli-responsiveness of our system should greatly expand the utility of polymer inverse mesophases for triggered releasing, templating, and many other applications.

Poly(carboxypyrrole)s That Depolymerize from Head to Tail in the Solid State in Response to Specific Applied Signals

This Article describes the design, synthesis, and analysis of a new class of polymer that is capable of depolymerizing continuously, completely, and cleanly from head to tail when a detection unit on the head of the polymer is exposed to a specific applied signal. The backbone of this polymer consists of 1,3-disubstituted pyrroles and carboxy linkages similar to polyurethanes. Diverse side chains or reactive end-groups can be introduced readily, which provides modular design of polymer structure. The designed depolymerization mechanism proceeds through spontaneous release of carbon dioxide and azafulvene in response to a single triggering reaction with the detection unit. These poly(carboxypyrrole)s depolymerize readily in nonpolar environments, and even in the bulk as solid-state plastics.

Room temperature synthesis of block copolymer nano-objects with different morphologies via ultrasound initiated RAFT polymerization-induced self-assembly (sono-RAFT-PISA)

The first room temperature synthesis of diblock copolymer nano-objects with different morphologies using ultrasound (990 kHz) initiated reversible addition-fragmentation chain transfer PISA (sono-RAFT-PISA) in aqueous system.

Self-immolative polymers in biomedicine

Biomedical use cases for self-immolative polymers.

Synthesis of Unzipping Polyester and a Study of its Photochemistry

Preparation of an unzipping polyester is reported. The monomer was prepared from benzoic acid in a four-step sequence. Step growth polymerization of the monomer provides the target polymer. Efficient depolymerization upon irradiation at 254 nm was confirmed with a quantum yield of >0.8. The photolysis mechanism was investigated, and the results of radical trapping experiments are consistent with an initial Norrish type I like homolysis followed by a radical mediated depropagation reaction driven by aromatization.

Neuro- and hepatic toxicological profile of (S)-2,4-diaminobutanoic acid in embryonic, adolescent and adult zebrafish

(S)-2,4-Diaminobutanoic acid (DABA) is a noncanonical amino acid often co-produced by cyanobacteria along with β-N-methylamino-l-alanine (BMAA) in algal blooms. Although BMAA is a well-established neurotoxin, the toxicity of DABA remains unclear. As part of our development of biocompatible materials, we wish to make use of DABA as both a building block and as the end-product of enzymatically induced depolymerization; however, if it is toxic at very low concentrations, this would not be possible. We examined the toxicity of DABA using both in vivo embryonic and adult zebrafish models. At higher sublethal concentrations (700 μm), the fish demonstrated early signs of cardiotoxicity. Adolescent zebrafish were able to tolerate a higher concentration. Post-mortem histological analysis of juvenile zebrafish showed no liver or brain abnormalities associated with hepato- or neurotoxicity. Combined, these results show that DABA exhibits no overt toxicity at concentrations (100-300 μm) within an order of magnitude of those envisioned for its application. This study further highlights the low cost and ease of using zebrafish as an early-stage toxicological screening tool.