Thermo- and pH-Responsive, Coacervate-Forming Hyperbranched Poly(β-amino ester)s for Selective Cell Binding

Recent progress of non-linear topological structure polymers: synthesis, and gene delivery

Currently, many types of non-linear topological structure polymers, such as brush-shaped, star, branched and dendritic structures, have captured much attention in the field of gene delivery and nanomedicine. Compared with linear polymers, non-linear topological structural polymers offer many advantages, including multiple terminal groups, broad and complicated spatial architecture and multi-functionality sites to enhance gene delivery efficiency and targeting capabilities. Nevertheless, the complexity of their synthesis process severely hampers the development and applications of nonlinear topological polymers. This review aims to highlight various synthetic approaches of non-linear topological architecture polymers, including reversible-deactivation radical polymerization (RDRP) including atom-transfer radical polymerization (ATRP), nitroxide-mediated polymerization (NMP), reversible addition-fragmentation chain transfer (RAFT) polymerization, click chemistry reactions and Michael addition, and thoroughly discuss their advantages and disadvantages, as well as analyze their further application potential. Finally, we comprehensively discuss and summarize different non-linear topological structure polymers for genetic materials delivering performance both in vitro and in vivo, which indicated that topological effects and nonlinear topologies play a crucial role in enhancing the transfection performance of polymeric vectors. This review offered a promising guideline for the design and development of novel nonlinear polymers and facilitated the development of a new generation of polymer-based gene vectors.

Multiphasic Coacervates Assembled by Hydrogen Bonding and Hydrophobic Interactions

Coacervation has emerged as a prevalent mechanism to compartmentalize biomolecules in living cells. Synthetic coacervates help in understanding the assembly process and mimic the functions of biological coacervates as simplified artificial systems. Though the molecular mechanism and mesoscopic properties of coacervates formed from charged coacervates have been well investigated, the details of the assembly and stabilization of nonionic coacervates remain largely unknown. Here, we describe a library of coacervate-forming polyesteramides and show that the water-tertiary amide bridging hydrogen bonds and hydrophobic interactions stabilize these nonionic, single-component coacervates. Analogous to intracellular biological coacervates, these coacervates exhibit "liquid-like" features with low viscosity and low interfacial energy, and form coacervates with as few as five repeating units. By controlling the temperature and engineering the molar ratio between hydrophobic interaction sites and bridging hydrogen bonding sites, we demonstrate the tuneability of the viscosity and interfacial tension of polyesteramide-based coacervates. Taking advantage of the differences in the mesoscopic properties of these nonionic coacervates, we engineered multiphasic coacervates with core-shell architectures similar to those of intracellular biological coacervates, such as nucleoli and stress granule-p-body complexes. The multiphasic structures produced from these synthetic nonionic polyesteramide coacervates may serve as a valuable tool for investigating physicochemical principles deployed by living cells to spatiotemporally control cargo partitioning, biochemical reaction rates, and interorganellar signal transport.

Emerging non-viral vectors for gene delivery

Gene therapy holds great promise for treating a multitude of inherited and acquired diseases by delivering functional genes, comprising DNA or RNA, into targeted cells or tissues to elicit manipulation of gene expression. However, the clinical implementation of gene therapy remains substantially impeded by the lack of safe and efficient gene delivery vehicles. This review comprehensively outlines the novel fastest-growing and efficient non-viral gene delivery vectors, which include liposomes and lipid nanoparticles (LNPs), highly branched poly(β-amino ester) (HPAE), single-chain cyclic polymer (SCKP), poly(amidoamine) (PAMAM) dendrimers, and polyethyleneimine (PEI). Particularly, we discuss the research progress, potential development directions, and remaining challenges. Additionally, we provide a comprehensive overview of the currently approved non-viral gene therapeutics, as well as ongoing clinical trials. With advances in biomedicine, molecular biology, materials science, non-viral gene vectors play an ever-expanding and noteworthy role in clinical gene therapy.

Biocompatible Macroion/Growth Factor Assemblies for Medical Applications

Growth factors are a class of proteins that play a role in the proliferation (the increase in the number of cells resulting from cell division) and differentiation (when a cell undergoes changes in gene expression becoming a more specific type of cell) of cells. They can have both positive (accelerating the normal healing process) and negative effects (causing cancer) on disease progression and have potential applications in gene therapy and wound healing. However, their short half-life, low stability, and susceptibility to degradation by enzymes at body temperature make them easily degradable in vivo. To improve their effectiveness and stability, growth factors require carriers for delivery that protect them from heat, pH changes, and proteolysis. These carriers should also be able to deliver the growth factors to their intended destination. This review focuses on the current scientific literature concerning the physicochemical properties (such as biocompatibility, high affinity for binding growth factors, improved bioactivity and stability of the growth factors, protection from heat, pH changes or appropriate electric charge for growth factor attachment via electrostatic interactions) of macroions, growth factors, and macroion-growth factor assemblies, as well as their potential uses in medicine (e.g., diabetic wound healing, tissue regeneration, and cancer therapy). Specific attention is given to three types of growth factors: vascular endothelial growth factors, human fibroblast growth factors, and neurotrophins, as well as selected biocompatible synthetic macroions (obtained through standard polymerization techniques) and polysaccharides (natural macroions composed of repeating monomeric units of monosaccharides). Understanding the mechanisms by which growth factors bind to potential carriers could lead to more effective delivery methods for these proteins, which are of significant interest in the diagnosis and treatment of neurodegenerative and civilization diseases, as well as in the healing of chronic wounds.

Cyclic poly(β-amino ester)s with enhanced gene transfection activity synthesized through intra-molecular cyclization

Topological structure plays a critical role in gene delivery of cationic polymers. Cyclic poly(β-amino ester)s (CPAEs) are successfully synthesized via sequential Michael addition and free radical initiating ring-closure reaction. The CPAEs exhibit superior gene transfection efficiency and safety profile compared to their linear counterparts.

Complex Coacervation of Polymerized Ionic Liquids in Non-aqueous Solvents

Oppositely charged polymerized ionic liquids (PILs) were used to form complex coacervates in two different organic solvents, 2,2,2-trifluoroethanol (TFE) and hexafluoro-2-propanol (HFIP), and the corresponding phase diagrams were constructed using UV-vis, NMR, and turbidity experiments. While previous studies on complex coacervates have focused almost exclusively on aqueous environments, the use of PILs in the current work enabled studies in solvents with substantially lower dielectric constants (27.0 for TFE, 16.7 for HFIP). The critical salt concentration required to induce complete miscibility was roughly 2-fold larger in HFIP compared with TFE, and two different PIL complexes, solidlike precipitates and liquidlike coacervates, were found in both systems. This study provides insight into the effects of low-dielectric-constant solvents on complex coacervation, which has not been widely studied because of the limited solubility of conventional polyelectrolytes in these media.

Fluorescence Switchable Block Copolymer Particles with Doubly Alternate-Layered Nanoparticle Arrays

The precise self-assembly of block copolymers (BCPs) and inorganic nanoparticles (NPs) under 3D confinement offers microparticles with programmable nanostructures and functionalities. Here, fluorescence-switchable hybrid microspheres are developed by forming doubly alternating arrays of Au NPs and CdSe/ZnS quantum dots (QDs) within polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) BCP domains. These doubly alternating arrays afford controlled nonradiative energy transfer (NRET) between the QDs and Au NPs that is dependent on the layer-to-layer distance. Solvent-selective swelling of the hybrid particles tunes the distance between layers, modulating their NRET behavior and affording switchable fluorescence. The particle fluorescence is "OFF" in water through strong NRET from the QDs to Au NPs, but is "ON" in alcohols due to the increased distance between the Au NP and QD arrays in the swollen P4VP domains. The experimentally observed NRET intensity as a function of interparticle distance shows larger quenching efficiencies than those theoretically predicted due to the enhanced quenching within a 3D-confined system. Finally, the robust and reversible fluorescence switching of the hybrid particles in different solvents is demonstrated, highlighting their potentials for bioimaging, sensing, and diagnostic applications.

Poly(β-Amino Esters): Synthesis, Formulations, and Their Biomedical Applications

Poly(β-amino ester) (abbreviated as PBAE or PAE) refers to a polymer synthesized from an acrylate and an amine by Michael addition and has properties inherent to tertiary amines and esters, such as pH responsiveness and biodegradability. The versatility of building blocks provides a library of polymers with miscellaneous physicochemical and mechanical properties. When used alone or together with other materials, PBAEs can be fabricated into different formulations in order to fulfill various requirements in drug delivery (for instance, gene, anticancer drugs, and antimicrobials delivery) and natural complex mimicry (nanochaperones). This progress report discusses the recent developments in design, synthesis, formulations, and applications of PBAEs in biomedical fields and provides a perspective view for the future of the PBAEs.

PAMAM dendrimers: blood-brain barrier transport and neuronal uptake after focal brain ischemia

Drug delivery to the central nervous system is restricted by the blood-brain barrier (BBB). However, with the onset of stroke, the BBB becomes leaky, providing a window of opportunity to passively target the brain. Here, cationic poly(amido amine) (PAMAM) dendrimers of different generations were functionalized with poly(ethylene glycol) (PEG) to reduce cytotoxicity and prolong blood circulation half-life, aiming for a safe in vivo drug delivery system in a stroke scenario. Rhodamine B isothiocyanate (RITC) was covalently tethered to the dendrimer backbone and used as a small surrogate drug as well as for tracking purposes. The biocompatibility of PAMAM was markedly increased by PEGylation as a function of dendrimer generation and degree of functionalization. The PEGylated RITC-modified dendrimers did not affect the integrity of an in vitro BBB model. Additionally, the functionalized dendrimers remained safe when in contact with the bEnd.3 cells and rat primary astrocytes composing the in vitro BBB model after hypoxia induced by oxygen-glucose deprivation. Modification with PEG also decreased the interaction and uptake by endothelial cells of PAMAM, indicating that the transport across a leaky BBB due to focal brain ischemia would be facilitated. Next, the functionalized dendrimers were tested in contact with red blood cells showing no haemolysis for the PEGylated PAMAM, in contrast to the unmodified dendrimer. Interestingly, the PEG-modified dendrimers reduced blood clotting, which may be an added beneficial function in the context of stroke. The optimized PAMAM formulation was intravenously administered in mice after inducing permanent focal brain ischemia. Twenty-four hours after administration, dendrimers could be detected in the brain, including in neurons of the ischemic cortex. Our results suggest that the proposed formulation has the potential for becoming a successful delivery vector for therapeutic application to the injured brain after stroke reaching the ischemic neurons.

Versatile Hyperbranched Poly(β-hydrazide ester) Macromers as Injectable Antioxidative Hydrogels

Synthetic reactive oxygen species (ROS)-responsive biomaterials have emerged as a useful platform for regulating critical aspects of ROS-induced pathologies and can improve such hostile microenvironments. Here, we report a series of new hyperbranched poly(β-hydrazide ester) macromers (HB-PBHEs) with disulfide moieties synthesized via an "A2 + B4" Michael addition approach. The three-dimensional structure of HB-PBHEs with multiacrylate end groups endows the macromers with rapid gelation capabilities to form (1) injectable hydrogels via cross-linking with thiolated hyaluronic acid and (2) robust UV-cross-linked hydrogels. The disulfide-containing macromers and hydrogels exhibit H₂O₂-responsive degradation compared with the counterparts synthesized by a dihydrazide monomer without disulfide moieties. The cell viability under a high ROS environment can be well-maintained under the protection of the disulfide containing hydrogels.

Soluble Hyperbranched Porous Organic Polymers

Soluble porous organic polymers (SPOPs) are currently the subject of extensive investigation due to the enhanced processability compared to insoluble counterparts. Here, a new concept for the construction of SPOPs is presented, which combines the unique topological structure of hyperbranched polymers with rigid building blocks. By using this facile, one-step strategy, a class of novel SPOPs which possess surface areas up to 646 m² g^-1 have been synthesized. The extended π-conjugated backbone affords the polymers bright fluorescence under UV irradiation. Interestingly, after dissolution in a suitable solvent that was slowly evaporated, the polymers retain a large extent of porosity. The SPOPs are potential candidates for gas storage and separation, photovoltaic, and biological applications. In particular, due to the presence of an internal porous structure and open conformations, they show high drug loading efficiency (1.91 g of ibuprofen per gram), which is considerably higher than conventional porous organic polymers.

Cooperativity Principles in Self-Assembled Nanomedicine

Nanomedicine is a discipline that applies nanoscience and nanotechnology principles to the prevention, diagnosis, and treatment of human diseases. Self-assembly of molecular components is becoming a common strategy in the design and syntheses of nanomaterials for biomedical applications. In both natural and synthetic self-assembled nanostructures, molecular cooperativity is emerging as an important hallmark. In many cases, interplay of many types of noncovalent interactions leads to dynamic nanosystems with emergent properties where the whole is bigger than the sum of the parts. In this review, we provide a comprehensive analysis of the cooperativity principles in multiple self-assembled nanostructures. We discuss the molecular origin and quantitative modeling of cooperative behaviors. In selected systems, we describe the examples on how to leverage molecular cooperativity to design nanomedicine with improved diagnostic precision and therapeutic efficacy in medicine.

Fabrication of Hybrid Capsules via CaCO3 Crystallization on Degradable Coacervate Droplets

Organic-inorganic CaCO₃ capsules were prepared by crystallization of CaCO₃ on Pickering emulsion prepared using coacervate droplets made from thermoresponsive and degradable poly(2-methylene-1,3-dioxepane- co-2-hydroxyethyl acrylate) (poly(MDO- co-HEA)) in sole aqueous medium. The diameters of CaCO₃-based Pickering emulsion could be controlled by varying several parameters: diameter of CaCO₃ powders, initial polymer concentration, and copolymer composition. The CaCO₃ Pickering emulsion was able to load low-molecular-weight hydrophobic substances at temperatures above the lower critical solution temperature (LCST) due to formation of polymer-concentrated phases, i.e., coacervate droplets. The diameter of CaCO₃ capsules prepared by crystallization also depended on the diameter of the CaCO₃ Pickering emulsion. The CaCO₃ shell was composed of calcite-type crystals, the most stable polymorph among known CaCO₃ crystals. The facially prepared CaCO₃ capsules are valuable for use in functional biomaterials, such as drug delivery carriers and cell culture scaffolds for noninvasive bone-regenerative medicine.

LCST-Type Hyperbranched Poly(oligo(ethylene glycol) with Thermo- and CO2 -Responsive Backbone

A novel hyperbranched lower critical solution temperature (LCST) polymer with sharp temperature and CO₂ -responsive behaviors is presented in this study. The target polymer of hyperbranched poly(oligo(ethylene glycol) (HBPOEG) is constructed using POEG as the backbone and tertiary amines as branch points. Phase transition of HBPOEG in aqueous solution is investigated by heating and cooling the system; the results indicate that HBPOEG in aqueous solution has a concentration-dependent phase transition behavior with excellent repeatability. Moreover, LCST of HBPOEG can be tuned by bubbling CO₂ into the solution, as the tertiary amines can be protonated and the solubility of the polymer would increase by bubbling CO₂ into the system, leading to an increase of LCST of the polymer. Further bubbling N₂ to remove CO₂ can reversibly turn back the LCST to its original value. This backbone-based hyperbranched LCST polymer with both CO₂ and temperature responsiveness can be applied in application areas like drug delivery, gene transfection, functional coatings, etc.

Dendrons and Multiarm Polymers with Thiol-Exchangeable Cores: A Reversible Conjugation Platform for Delivery

Disulfide exchange reaction has emerged as a powerful tool for reversible conjugation of proteins, peptides and thiol containing molecules to polymeric supports. In particular, the pyridyl disulfide group provides an efficient handle for the site-specific conjugation of therapeutic peptides and proteins bearing cysteine moieties. In this study, novel biodegradable dendritic platforms containing a pyridyl disulfide unit at their focal point were designed. Presence of hydroxyl groups at the periphery of these dendrons allows their elaboration to multivalent initiators that yield poly(ethylene glycol) based multiarm star polymers via controlled radical polymerization. The pyridyl disulfide unit at the core of these star polymers undergoes efficient reaction with thiol functional group containing molecules such as a hydrophobic dye, namely, Bodipy-SH, glutathione, and KLAK sequence containing peptide. While conjugation of the hydrophobic fluorescent dye to the PEG-based multiarm polymer renders it water-soluble, it can be cleaved off the construct through thiol-disulfide exchange in the presence of an external thiol such as dithiothreitol. The multiarm polymer was conjugated with a thiol group containing apoptotic peptide to increase its solubility and cellular transport. In vitro cytotoxicity and apoptosis assays demonstrated that the resultant peptide-polymer conjugate had almost five times more apoptotic potential primarily through triggering apoptosis by disrupting mitochondrial membranes of human breast cancer cell line (MDA-MB-231) compared to naked peptide. The novel dendritic platform disclosed here offers an attractive template that can be modified to multiarm polymeric constructs bearing a "tag and release" characteristic.