Precision Stealth Nanofibers via PET-RAFT Polymerisation: Synthesis, Crystallization-driven Self-assembly and Cellular Uptake Studies

Stealth precision polymer nanofibers show great promise as therapeutic delivery systems. However, existing systems are largely limited to poly(ethylene glycol) (PEG) and suffer from challenging functionalization, hampering their translation. This work develops a modular, easily functionalizable platform for biocompatible stealth nanofibers based on a combination of ring-opening polymerisation (ROP), photoinduced electron/energy transfer reversible addition-fragmentation chain transfer (PET-RAFT) polymerisation, and crystallization-driven self-assembly (CDSA). Low length-dispersity poly(fluorenetrimethylenecarbonate)-b-poly(N-(2-hydroxypropyl) methacrylamide) (PFTMC-b-PHPMA) nanofibers may be produced in a single-step via CDSA, with a length that is dependent on the PHPMA DPn. Separately, living CDSA leads to nanofibers with length control between 30 nm and ca. 700 nm. Incorporation of fluorescein into the PET-RAFT polymerization results in fluorescent PFTMC-b-PHPMA block copolymers that can undergo CDSA, forming fluorescent nanoparticles for preliminary cell studies. PFTMC-b-PHPMA nanofibers exhibited minimal toxicity to cells as well as limited cellular association, in line with previous studies on neutral polymer nanofibers. In comparison, PFTMC-b-PHPMA nanospheres exhibited no cellular association. These results indicate that the unique shape and core-crystallinity of PFTMC-b-PHPMA nanofibers ideally positions them for use as therapeutic delivery systems. Overall, the results described herein provide the basis for a modular, easily functionalizable platform for precision stealth polymer nanofibers for a variety of prospective biomedical applications.

Precise Preparation of Size-Uniform Two-Dimensional Platelet Micelles Through Crystallization-Assisted Rapid Microphase Separation Using All-Bottlebrush-Type Block Copolymers with Crystalline Side Chains

Polymer nanoparticles with low curvature, especially two-dimensional (2D) soft materials, are rich in functions and outstanding properties and have received extensive attention. Crystallization-driven self-assembly (CDSA) of linear semicrystalline block copolymers is currently a common method of constructing 2D platelets of uniform size. Although accompanied by high controllability, this CDSA method usually and inevitably requires a longer aging time and lower assembly concentration, limiting the large-scale preparation of nanoaggregates. In this study, a series of all-bottlebrush-type block copolymers, poly(octadecyl acrylate)-block-poly(oligoethylene glycol methyl ether methacrylate)s are prepared by living polymerization. Driven by the synergistic crystallization of crystalline side chains and the rapid microphase separation of bottlebrush topology, these polymers can assemble into uniform 2D circular platelet micelles in a few minutes, without being affected by a high assembly concentration. In this process, epitaxial growth of the bottlebrush molecules proceeds with rigid cylindrical molecular conformation at the micelle crystallization sites and eventually provides a sandwich-type micelle according to a head-to-head stacking mode. This is explained as a "crystallization-assisted rapid microphase separation" mechanism. The micelle structures are affected by the assembly solvent and temperature, the size of which shows a linear dependence on the assembly temperature below the melting point of the crystalline block, which can be used to precisely control the morphology of these 2D platelets. This study establishes an efficient and rapid method to prepare 2D polymer nanosoft materials, which are promising candidates for further development, preparation, and application of various nanomaterials.

Uniform block copolymer nanofibers for the delivery of paclitaxel in 2D and 3D glioblastoma tumor models

Cancer treatment has transformed in recent years, with the introduction of immunotherapy providing substantial improvements in prognoses for certain cancers. However, traditional small molecule chemotherapeutics remain the major frontline of defence, and improving their delivery to solid tumors is of utmost importance for improving potency and reducing side effects. Here, length-controlled one-dimensional seed nanofibers (ca. 25 nm, ĐL = 1.05) were generated from poly(fluorenetrimethylenecarbonate)-block-poly(dimethylaminoethylmethacrylate) via living crystallization-driven self-assembly. Paclitaxel, with an encapsulation content ranging from 1 to 100 wt%, was loaded onto the preformed nanoparticles by solvent addition and evaporation. Drug loading was quantified by dynamic light scattering and transmission electron microscopy. Drug-loaded vectors were then incubated with U87 MG glioblastoma cells in a 2D cell assay for up to 72 h, and their anticancer properties were determined. It was observed that seed nanofibers loaded with 20 wt% paclitaxel were the most advantageous combination (IC50 = 0.48 μg mL-1), while pure seed nanofibers with no loaded drug displayed much lower cytotoxicity (IC50 = 11.52 μg mL-1). The IC50 of the loaded seed nanofibers rivaled that of the commercially approved Abraxane® (IC50 = 0.46 μg mL-1). 3D tumor spheroids were then cultured and subjected to the same stresses. Live/dead cell staining revealed that once more, seed nanofibers with 20 wt% paclitaxel, Abraxane®, and paclitaxel all exhibited similar levels of potency (55% viability), whereas control samples exhibited much higher cell viability (70%) after 3 days. These results demonstrate that nanofibers contain great potential as biocompatible drug delivery vehicles for cancer treatment as they exert a similar anticancer effect to the commercially available Abraxane®.

Mechanism of Action and Design of Potent Antibacterial Block Copolymer Nanoparticles

Self-assembled polymer nanoparticles are promising antibacterials, with nonspherical morphologies of particular interest as recent work has demonstrated enhanced antibacterial activity relative to their spherical counterparts. However, the reasons for this enhancement are currently unclear. We have performed a multifaceted analysis of the antibacterial mechanism of action of 1D nanofibers relative to nanospheres by the use of flow cytometry, high-resolution microscopy, and evaluations of the antibacterial activity of pristine and tetracycline-loaded nanoparticles. Low-length dispersity, fluorescent diblock copolymer nanofibers with a crystalline poly(fluorenetrimethylenecarbonate) (PFTMC) core (length = 104 and 472 nm, height = 7 nm, width = 10-13 nm) and a partially protonated poly(dimethylaminoethyl methacrylate) (PDMAEMA) corona (length = 12 nm) were prepared via seeded growth living crystallization-driven self-assembly. Their behavior was compared to that of analogous nanospheres containing an amorphous PFTMC core (diameter of 12 nm). While all nanoparticles were uptaken into Escherichia coli W3110, crystalline-core nanofibers were observed to cause significant bacterial damage. Drug loading studies indicated that while all nanoparticle antibacterial activity was enhanced in combination with tetracycline, the enhancement was especially prominent when small nanoparticles (ca. 15-25 nm) were employed. Therefore, the identified differences in the mechanism of action and the demonstrated consequences for nanoparticle size and morphology control may be exploited for the future design of potent antibacterial agents for overcoming antibacterial resistance. This study also reinforces the requirement of morphological control over polymer nanoparticles for biomedical applications, as differences in activity are observed depending on their size, shape, and core-crystallinity.

Rationally designed block copolymer-based nanoarchitectures: An emerging paradigm for effective drug delivery

Various polymeric materials have been investigated to produce unique modes of delivery for drug modules to achieve either temporal or spatial control of bioactives delivery. However, after intravenous administration, phagocytic cells quickly remove these nanostructures from the systemic circulation via the reticuloendothelial system (RES). To overcome these concerns, ecofriendly block copolymers are increasingly being investigated as innovative carriers for the delivery of bioactives. In this review, we discuss the design, fabrication techniques, and recent advances in the development of block copolymers and their applications as drug carrier systems to improve the physicochemical and pharmacological attributes of bioactives.

Uniform Two-Dimensional Crystalline Platelets with Tailored Compositions for pH Stimulus-Responsive Drug Release

Two-dimensional, size-tunable, water-dispersible particle micelles with spatially defined chemistries can be obtained by using "living" crystallization-driven self-assembly (CDSA) approach. Nevertheless, a major obstacle of crystalline particles in drug delivery application is the difficulty in accessing to cargo within crystalline cores. In the present work, we design four different types of biocompatible two-dimensional platelets with a crystalline poly(ε-caprolactone) (PCL) core, a hydrophobic poly(4-vinylprydine) (P4VP) segment, and a water dispersible poly(N,N-dimethyl acrylamide) (PDMA) block in ethanol by seeded growth method. Transferring those uniform platelets with tailored compositions to an aqueous solution in the presence of a hydrophobic drug leads to efficient encapsulation of the cargo in the P4VP segments via hydrophobic interactions. These drug-loaded platelets exhibit pH-responsive release behavior in aqueous media due to the protonated-deprotonated process of P4VP blocks in acidic and neutral solutions. This work provides initial insight into biocompatible PCL platelets with low dispersity and precise chemistry control in stimulus-responsive drug delivery fields.

Length-Controlled Nanofiber Micelleplexes as Efficient Nucleic Acid Delivery Vehicles

Micelleplexes show great promise as effective polymeric delivery systems for nucleic acids. Although studies have shown that spherical micelleplexes can exhibit superior cellular transfection to polyplexes, to date there has been no report on the effects of micelleplex morphology on cellular transfection. In this work, we prepared precision, length-tunable poly(fluorenetrimethylenecarbonate)-b-poly(2-(dimethylamino)ethyl methacrylate) (PFTMC₁₆-b-PDMAEMA₁₃₁) nanofiber micelleplexes and compared their properties and transfection activity to those of the equivalent nanosphere micelleplexes and polyplexes. We studied the DNA complexation process in detail via a range of techniques including cryo-transmission electron microscopy, atomic force microscopy, dynamic light scattering, and ζ-potential measurements, thereby examining how nanofiber micelleplexes form, as well the key differences that exist compared to nanosphere micelleplexes and polyplexes in terms of DNA loading and colloidal stability. The effects of particle morphology and nanofiber length on the transfection and cell viability of U-87 MG glioblastoma cells with a luciferase plasmid were explored, revealing that short nanofiber micelleplexes (length < ca. 100 nm) were the most effective delivery vehicle examined, outperforming nanosphere micelleplexes, polyplexes, and longer nanofiber micelleplexes as well as the Lipofectamine 2000 control. This study highlights the potential importance of 1D micelleplex morphologies for achieving optimal transfection activity and provides a fundamental platform for the future development of more effective polymeric nucleic acid delivery vehicles.