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.

Hybrid Polymer Vesicles: Controllable Preparation and Potential Applications

Hybrid polymer vesicles contain functional nanoparticles (NPs) in their walls, interfaces, coronae, or cavities. NPs render the hybrid vesicles with specific physical properties, while polymers endow them with structural stability and may significantly reduce the high toxicity of NPs. Therefore, hybrid vesicles integrate fascinating multifunctions from both NPs and polymeric vesicles, which have gained tremendous attention because of their diverse promising applications. Various types of delicate hybrid polymeric vesicles with size control and tunable localization of NPs in different parts of vesicles have been constructed via in situ and ex situ strategies, respectively. Their potential applications have been widely explored, as well. This review presents the progress of block copolymer (BCP) vesicle systems containing different types of NPs including metal NPs, magnetic NPs, and semiconducting quantum dots (QDs), etc. The strategies for controlling the location of NPs within hybrid vesicles are discussed. Typical potential applications of the elegant hybrid vesicles are also highlighted.

Self-assembled block copolymer biomaterials for oral delivery of protein therapeutics

Protein therapeutics have guided a transformation in disease treatment for various clinical conditions. They have been successful in numerous applications, but administration of protein therapeutics has been limited to parenteral routes which can decrease patient compliance as they are invasive and painful. In recent years, the synergistic relationship of novel biomaterials with modern protein therapeutics has been crucial in the treatment of diseases that were once thought of as incurable. This has guided the development of a variety of alternative administration routes, but the oral delivery of therapeutics remains one of the most desirable due to its ease of administration. This review addresses important aspects of micellar structures prepared by self-assembled processes with applications for oral delivery. These two characteristics have not been placed together in previous literature within the field. Therefore, we describe the barriers for delivery of protein therapeutics, and we concentrate in the oral/transmucosal pathway where drug carriers must overcome several chemical, physical, and biological barriers to achieve a successful therapeutic effect. We critically discuss recent research on biomaterials systems for delivering such therapeutics with an emphasis on self-assembled synthetic block copolymers. Polymerization methods and nanoparticle preparation techniques are similarly analyzed as well as relevant work in this area. Based on our own and others' research, we analyze the use of block copolymers as therapeutic carriers and their promise in treating a variety of diseases, with emphasis on self-assembled micelles for the next generation of oral protein therapeutic systems.

3D printable, thermo-responsive, self-healing, graphene oxide containing self-assembled hydrogels formed from block copolymer wormlike micelles

Graphene oxide (GO) containing block copolymer nanocomposite hydrogels formed from poly(glycerol monomethacrylate-block-hydroxypropyl methacrylate) (PGMA-PHPMA) wormlike micelles were prepared by either mixing GO and copolymer at low temperature or via in situ reversible addition-fragmentation chain-transfer (RAFT) polymerisation-induced self-assembly (PISA) of HPMA in the presence of a PGMA macromolecular chain-transfer agent and GO flakes. Hydrogels containing 15-25% w/w copolymer and 0 and 8% w/w GO, based on copolymer, were investigated and the maximum gel strength measured was ∼33 kPa for a 25% w/w copolymer gel prepared by in situ polymerisation and containing 2% w/w GO based on copolymer. This gel strength represents a fifteen-fold increase over the same copolymer gel without the addition of GO. The nanocomposite gels were found to recover efficiently after the application of high shear, with up to 98% healing efficiency within seconds. These gels are also 3D printable, self-healing, adhesive and temperature responsive on cooling and re-heating. The observed properties were both GO and copolymer concentration dependent, and tensile testing demonstrated that the nanocomposite gels had higher moduli, elongation at break and toughness than gels prepared without GO.

Synthesis of Multifunctional Polymersomes Prepared by Polymerization-Induced Self-Assembly

Polymersomes are an exciting modality for drug delivery due to their structural similarity to biological cells and their ability to encapsulate both hydrophilic and hydrophobic drugs. In this regard, the current work aimed to develop multifunctional polymersomes, integrating dye (with hydrophobic Nile red and hydrophilic sulfo-cyanine5-NHS ester as model drugs) encapsulation, stimulus responsiveness, and surface-ligand modifications. Polymersomes constituting poly(N-2-hydroxypropylmethacrylamide)-b-poly(N-(2-(methylthio)ethyl)acrylamide) (PHPMAm-b-PMTEAM) are prepared by aqueous dispersion RAFT-mediated polymerization-induced self-assembly (PISA). The hydrophilic block lengths have an effect on the obtained morphologies, with short chain P(HPMAm)₁₆ affording spheres and long chain P(HPMAm)₄₃ yielding vesicles. This further induces different responses to H₂O₂, with spheres fragmenting and vesicles aggregating. Folic acid (FA) is successfully conjugated to the P(HPMAm)₄₃, which self-assembles into FA-functionalized P(HPMAm)₄₃-b-P(MTEAM)₃₀₀ polymersomes. The FA-functionalized P(HPMAm)₄₃-b-P(MTEAM)₃₀₀ polymersomes entrap both hydrophobic Nile red (NR) and hydrophilic Cy5 dye. The NR-loaded FA-linked polymersomes exhibit a controlled release of the encapsulated NR dye when exposed to 10 mM H₂O₂. All the polymersomes formed are stable in human plasma and well-tolerated in MCF-7 breast cancer cells. These preliminary results demonstrate that, with simple and scalable chemistry, PISA offers access to different shapes and opens up the possibility of the one-pot synthesis of multicompartmental and responsive polymersomes.

Polymerization-Induced Self-Assembly: An Emerging Tool for Generating Polymer-Based Biohybrid Nanostructures

The combination of biomolecules and synthetic polymers provides an easy access to utilize advantages from both the synthetic world and nature. This is not only important for the development of novel innovative materials, but also promotes the application of biomolecules in various fields including medicine, catalysis, and water treatment, etc. Due to the rapid progress in synthesis strategies for polymer nanomaterials and deepened understanding of biomolecules' structures and functions, the construction of advanced polymer-based biohybrid nanostructures (PBBNs) becomes prospective and attainable. Polymerization-induced self-assembly (PISA), as an efficient and versatile technique in obtaining polymeric nano-objects at high concentrations, has demonstrated to be an attractive alternative to existing self-assembly procedures. Those advantages induce the focus on the fabrication of PBBNs via the PISA technique. In this review, current preparation strategies are illustrated based on the PISA technique for achieving various PBBNs, including grafting-from and grafting-through methods, as well as encapsulation of biomolecules during and subsequent to the PISA process. Finally, advantages and drawbacks are discussed in the fabrication of PBBNs via the PISA technique and obstacles are identified that need to be overcome to enable commercial application.

RAFT-mediated polymerization-induced self-assembly (RAFT-PISA): current status and future directions

Polymerization-induced self-assembly (PISA) combines polymerization and self-assembly in a single step with distinct efficiency that has set it apart from the conventional solution self-assembly processes. PISA holds great promise for large-scale production, not only because of its efficient process for producing nano/micro-particles with high solid content, but also thanks to the facile control over the particle size and morphology. Since its invention, many research groups around the world have developed new and creative approaches to broaden the scope of PISA initiations, morphologies and applications, etc. The growing interest in PISA is certainly reflected in the increasing number of publications over the past few years, and in this review, we aim to summarize these recent advances in the emerging aspects of RAFT-mediated PISA. These include (1) non-thermal initiation processes, such as photo-, enzyme-, redox- and ultrasound-initiation; the achievements of (2) high-order structures, (3) hybrid materials and (4) stimuli-responsive nano-objects by design and adopting new monomers and new processes; (5) the efforts in the realization of upscale production by utilization of high throughput technologies, and finally the (6) applications of current PISA nano-objects in different fields and (7) its future directions.

Organic–inorganic hybrid nanomaterials prepared via polymerization-induced self-assembly: recent developments and future opportunities

This review highlights recent developments in the preparation of organic–inorganic hybrid nanomaterials via polymerization-induced self-assembly.

Rational synthesis of novel biocompatible thermoresponsive block copolymer worm gels

It is well known that reversible addition-fragmentation chain transfer (RAFT) aqueous dispersion polymerization of 2-hydroxypropyl methacrylate (HPMA) enables the rational design of diblock copolymer worm gels. Moreover, such hydrogels can undergo degelation on cooling below ambient temperature as a result of a worm-to-sphere transition. However, only a subset of such block copolymer worms exhibit thermoresponsive behavior. For example, PMPC26-PHPMA280 worm gels prepared using a poly(2-(methacryloyloxy)ethyl phosphorylcholine) (PMPC26) precursor do not undergo degelation on cooling to 6 °C (see S. Sugihara et al., J. Am. Chem. Soc., 2011, 133, 15707-15713). Informed by our recent studies (N. J. Warren et al., Macromolecules, 2018, 51, 8357-8371), we decided to reduce the mean degrees of polymerization of both the PMPC steric stabilizer block and the structure-directing PHPMA block when targeting a pure worm morphology. This rational approach reduces the hydrophobic character of the PHPMA block and hence introduces the desired thermoresponsive character, as evidenced by the worm-to-sphere transition (and concomitant degelation) that occurs on cooling a PMPC15-PHPMA150 worm gel from 40 °C to 6 °C. Moreover, worms are reconstituted on returning to 40 °C and the original gel modulus is restored. This augurs well for potential biomedical applications, which will be examined in due course. Finally, small-angle X-ray scattering studies indicated a scaling law exponent of 0.67 (≈2/3) for the relationship between the worm core cross-sectional diameter and the PHPMA DP for a series of PHPMA-based worms prepared using a range of steric stabilizer blocks, which is consistent with the strong segregation regime for such systems.

Polymerisation-induced self-assembly (PISA) as a straightforward formulation strategy for stimuli-responsive drug delivery systems and biomaterials: recent advances

Stimuli-responsive amphiphilic block copolymers have emerged as promising nanocarriers for enhancing site-specific and on-demand drug release in response to a range of stimuli such as pH, the presence of redox agents, and temperature. The formulation of amphiphilic block copolymers into polymeric drug-loaded nanoparticles is typically achieved by various methods (e.g. oil-in-water emulsion solvent evaporation, solid dispersion, microphase separation, dialysis or microfluidic separation). Despite much progress that has been made, there remain many challenges to overcome to produce reliable polymeric systems. The main drawbacks of the above methods are that they produce very low solid contents (<1 wt%) and involve multiple-step procedures, thus limiting their scope. Recently, a new self-assembly methodology, polymerisation-induced self-assembly (PISA), has shown great promise in the production of polymer-derived particles using a straightforward one-pot approach, whilst facilitating high yield, scalability, and cost-effectiveness for pharmaceutical industry protocols. We therefore focus this review primarily on the most recent studies involved in the design and preparation of PISA-generated nano-objects which are responsive to specific stimuli, thus providing insight into how PISA may become an effective formulation strategy for the preparation of precisely tailored drug delivery systems and biomaterials, while some of the current challenges and limitations are also critically discussed.

Enthalpic incompatibility between two steric stabilizer blocks provides control over the vesicle size distribution during polymerization-induced self-assembly in aqueous media

Over the past two decades, block copolymer vesicles have been widely used by many research groups to encapsulate small molecule drugs, genetic material, nanoparticles or enzymes. They have also been used to design examples of autonomous self-propelled nanoparticles. Traditionally, such vesicles are prepared via post-polymerization processing using a water-miscible co-solvent such as DMF or THF. However, such protocols are invariably conducted in dilute solution, which is a significant disadvantage. In addition, the vesicle size distribution is often quite broad, whereas aqueous dispersions of relatively small vesicles with narrow size distributions are highly desirable for potential biomedical applications. Alternatively, concentrated dispersions of block copolymer vesicles can be directly prepared via polymerization-induced self-assembly (PISA). Moreover, using a binary mixture of a relatively long and a relatively short steric stabilizer block enables the convenient PISA synthesis of relatively small vesicles with reasonably narrow size distributions in alcoholic media (C. Gonzato et al., JACS, 2014, 136, 11100-11106). Unfortunately, this approach has not yet been demonstrated for aqueous media, which would be much more attractive for commercial applications. Herein we show that this important technical objective can be achieved by judicious use of two chemically distinct, enthalpically incompatible steric stabilizer blocks, which ensures the desired microphase separation across the vesicle membrane. This leads to the formation of well-defined vesicles of around 200 nm diameter (size polydispersity = 13-16%) in aqueous media at 10% w/w solids as judged by transmission electron microscopy, dynamic light scattering and small-angle X-ray scattering.

The rise of bio-inspired polymer compartments responding to pathology-related signals

Self-organized nano- and microscale polymer compartments such as polymersomes, giant unilamellar vesicles (GUVs), polyion complex vesicles (PICsomes) and layer-by-layer (LbL) capsules have increasing potential in many sensing applications. Besides modifying the physicochemical properties of the corresponding polymer building blocks, the versatility of these compartments can be markedly expanded by biomolecules that endow the nanomaterials with specific molecular and cellular functions. In this review, we focus on polymer-based compartments that preserve their structure, and highlight the key role they play in the field of medical diagnostics: first, the self-assembling abilities that result in preferred architectures are presented for a broad range of polymers. In the following, we describe different strategies for sensing disease-related signals (pH-change, reductive conditions, and presence of ions or biomolecules) by polymer compartments that exhibit stimuli-responsiveness. In particular, we distinguish between the stimulus-sensitivity contributed by the polymer itself or by additional compounds embedded in the compartments in different sensing systems. We then address necessary properties of sensing polymeric compartments, such as the enhancement of their stability and biocompatibility, or the targeting ability, that open up new perspectives for diagnostic applications.

Thermoresponsive Block Copolymer Vesicles by Visible Light-Initiated Seeded Polymerization-Induced Self-Assembly for Temperature-Regulated Enzymatic Nanoreactors

Block copolymer vesicles loaded with active compounds have been employed as decent candidates to mimic complex biological systems that attract considerable interest in different research communities. We herein report a visible light-initiated seeded reversible addition-fragmentation chain transfer (RAFT)-mediated polymerization-induced self-assembly (PISA) for in situ preparation of enzyme-loaded cross-linked block copolymer vesicles without compromising the bioactivity. Permeability of the vesicular membrane can be regulated through changing the solution temperature, allowing further control over the enzymatic reaction rate of enzyme-loaded vesicles. Finally, non-cross-linked thermoresponsive block copolymer vesicles that can transform into worm-like micelles at low temperature are also prepared by this method, allowing the release of bimacromolecules from the vesicles under mild conditions.

Cross-linking approaches for block copolymer nano-assemblies via RAFT-mediated polymerization-induced self-assembly

This minireview summarizes the current cross-linking approaches to stabilize block copolymer nano-assemblies obtained via RAFT-mediated PISA process.

Tuning the membrane permeability of polymersome nanoreactors developed by aqueous emulsion polymerization-induced self-assembly

Polymeric vesicles (or polymersomes) are hollow bilayer structures consisting of an inner aqueous compartment enclosed by a hydrophobic membrane. Vesicular constructs are ubiquitous in nature and perform a variety of functions by compartmentalizing molecules into disparate environments. For polymer chemists, the synthesis of vesicles can be readily accomplished using polymerization-induced self-assembly (PISA), whereby pure vesicle morphologies can be easily accessed by tuning initial reaction parameters. Research into polymersomes is motivated primarily by the fact that hydrophilic cargo such as drug molecules, DNA, or enzymes can be encapsulated and protected from the often harsh conditions of the surrounding environment. A key factor governing the capability of vesicles to retain and protect their cargo is the permeability of their hydrophobic membrane. Herein, we demonstrate that membrane permeability of enzyme-loaded epoxy-functionalized polymersomes synthesized by aqueous emulsion PISA can be modulated via epoxide ring-opening with various diamine cross-linkers and hydrophobic primary amines. In general, membrane cross-linking or amine conjugation resulted in increased polymersome membrane thickness. Membrane modification was also found to decrease permeability in all cases, as measured by enzymatically-catalysed oxidation of an externally administered substrate. Functionalization with hydrophobic amines resulted in the largest reduction in enzyme activity, suggesting significant blocking of substrate diffusion into the central aqueous compartment. This procedurally facile strategy yields meaningful insight into how the chemical structure of the membrane influences permeability and thus could be generally applied to the formulation of polymeric vesicles for therapeutic applications.

Combining the power of heat and light: temperature-programmed photoinitiated RAFT dispersion polymerization to tune polymerization-induced self-assembly

A novel temperature-programmed photo-PISA method which combines the power of heat and light is developed for the preparation of a diverse set of morphologies.

Recent advances in colloidal nanocomposite designviaheterogeneous polymerization techniques

Recent advances in colloidal nanocomposite design by heterogeneous polymerization are reviewed, with a specific focus on encapsulation and particle-based stabilization for specific materials applications.

Anionic block copolymer vesicles act as Trojan horses to enable efficient occlusion of guest species into host calcite crystals

We report a versatile 'Trojan Horse' strategy using highly anionic poly(methacrylic acid)-poly(benzyl methacrylate) vesicles to incorporate two types of model payloads, i.e. either silica nanoparticles or an organic dye (fluorescein), within CaCO3 (calcite). Uniform occlusion of silica-loaded vesicles was confirmed by scanning electron microscopy, while thermogravimetry studies indicated extents of vesicle occlusion of up to 9.4% by mass (∼33% by volume). Efficient dye-loaded vesicle occlusion produces highly fluorescent calcite crystals as judged by fluorescence microscopy. In control experiments, silica nanoparticles alone are barely occluded, while only very weakly fluorescent calcite crystals are obtained when using just the fluorescein dye. This new 'Trojan Horse' strategy opens up a generic route for the efficient occlusion of various nanoparticles and organic molecules within inorganic host crystals.

CO2 -Breathing Polymer Assemblies via One-Pot Sequential RAFT Dispersion Polymerization

ABC triblock copolymer assemblies with reversible "breathing" behaviors based on poly[oligo(ethylene glycol) methyl ether methacrylate]-b-poly(benzyl methacrylate)-b-poly[2-(diethylamino)ethyl methacrylate] (POEGMA-b-PBnMA-b-PDEA) are fabricated via one-pot sequential reverisble addition-fragmentation chain transfer dispersion polymerization. Using a POEGMA as the macromolecular chain transfer agent, chain extension with BnMA and DEA is conducted in ethanol, where PBnMA acts as the core-forming block, and the PDEA block endows the solvophilicity and CO₂ -responsiveness. With the increment of the DP of PBnMA, the morphology of the assemblies evolves from spheres to worms, and to vesicles, while it degenerates from conglutinated vesicles to spheres as the DP of PDEA increases. After replacing ethanol with water, the morphologies of these assemblies remain unchanged, while their size decreases due to the collapse of the hydrophobic PDEA chains. Interestingly, due to the protonation and deprotonation of PDEA blocks, both the spheres and vesicles manifest a reversible expansion/shrinkage upon alternative CO₂ /Ar stimulation, exhibiting distinctive breathing feature.

Photochemistry for Well-Defined Polymers in Aqueous Media: From Fundamentals to Polymer Nanoparticles to Bioconjugates

This review article highlights recent developments in the field of photochemistry and photochemical reversible deactivation radical polymerization applied to aqueous polymerizations. Photochemistry is a topic of significant interest in the fields of organic, polymer, and materials chemistry because it allows challenging reactions to be performed under mild conditions. Aqueous polymerization is of significant interest because water is an environmentally benign solvent, and the use of water enables complex polymer self-assembly and bioconjugation processes to occur. This review focuses on powerful new developments in photochemical aqueous polymerization reactions and their applications to the synthesis of well-defined polymer nano-objects and bioconjugates. It is anticipated that these aqueous photopolymerizations will enable the next generation of self-assembled structures and biohybrid materials to be developed under mild and environmentally friendly conditions.

Expanding the Scope of Polymerization-Induced Self-Assembly: Z-RAFT-Mediated Photoinitiated Dispersion Polymerization

In this communication, we developed the first well-controlled Z-RAFT (RAFT = reversible addition-fragmentation chain transfer) mediated polymerization-induced self-assembly (PISA) formulation based on photoinitiated RAFT dispersion polymerization of tert-butyl acrylate (tBA) in ethanol/water (60/40, w/w) at room temperature using a Z-type macromolecular chain transfer agent (macro-CTA). Polymerizations proceeded rapidly via the exposure of visible-light irradiation (405 nm, 0.45 mW/cm²) with high monomer conversion (>95%) being achieved within 1 h. A variety of polymer nano-objects (spheres, worms, and vesicles) with narrow molar mass distributions were prepared by this Z-RAFT mediated PISA formulation. Silver nanoparticles were loaded with the vesicles via in situ reduction, which can be used as a catalyst for the reduction of methylene blue (MB) in the presence of NaBH₄. Finally, gel permeation chromatography (GPC) analysis demonstrated that the corona block and the core-forming block could be cleaved by treating with excess initiator. This novel PISA formulation will greatly expand the scope of PISA and provide more mechanistic insights into the PISA research.

Enzyme catalysis-induced RAFT polymerization in water for the preparation of epoxy-functionalized triblock copolymer vesicles

Enzyme catalysis-induced aqueous reversible addition–fragmentation chain transfer (RAFT) polymerization was conducted at room temperature for the preparation of epoxy-functionalized triblock copolymer vesicles.

Carboxyl-Functionalized Polymeric Microspheres Prepared by One-Stage Photoinitiated RAFT Dispersion Polymerization

Herein, we report a photoinitiated reversible addition-fragmentation chain transfer (RAFT) dispersion copolymerization of methyl methacrylate (MMA) and methyl methacrylic (MAA) for the preparation of highly monodisperse carboxyl-functionalized polymeric microspheres. High rates of polymerization were observed, with more than 90% particle yields being achieved within 3 h of UV irradiation. Effects of reaction parameters (e.g., MAA concentration, RAFT agent concentration, photoinitiator concentration, and solvent composition) were studied in detail, and highly monodisperse polymeric microspheres were obtained in most cases. Finally, silver (Ag) composite microspheres were prepared by in situ reduction of AgNO₃ using the carboxyl-functionalized polymeric microspheres as the template. The obtained Ag composite microspheres were able to catalyze the reduction of methylene blue (MB) with NaBH₄ as a reductant.

Rapid synthesis of well-defined all-acrylic diblock copolymer nano-objects via alcoholic photoinitiated polymerization-induced self-assembly (photo-PISA)

A series of well-defined all-acrylic poly(hydroxyethyl acrylate)-poly(isobornyl acrylate) (PHEA-PIBOA) diblock copolymer nano-objects were prepared by photoinitiated polymerization-induced self-assembly (photo-PISA).

Adding a solvophilic comonomer to the polymerization-induced self-assembly of block copolymer and homopolymer: a cooperative strategy for preparing large compound vesicles

We report a RAFT dispersion polymerization of styrene and 4-vinylpyridine in methanol/water at 70 °C. The polymerization was mediated by a binary mixture of DDMAT and mPEG₄₅-DDMAT.

Synthesis of fluorinated nanoparticles via RAFT dispersion polymerization-induced self-assembly using fluorinated macro-RAFT agents in supercritical carbon dioxide

Nano-sized fluorinated block copolymer particles are prepared by RAFT dispersion polymerization with polymerization-induced self-assembly proceeding in supercritical carbon dioxide.