Micellization of Photo-Responsive Block Copolymers

Recent Progress in Implantable Drug Delivery Systems

In recent years, tremendous effort is devoted to developing platforms, such as implantable drug delivery systems (IDDSs), with temporally and spatially controlled drug release capabilities and improved adherence. IDDSs have multiple advantages: i) the timing and location of drug delivery can be controlled by patients using specific stimuli (light, sound, electricity, magnetism, etc.). Some intelligent "closed-loop" IDDS can even realize self-management without human participation. ii) IDDSs enable continuous and stable delivery of drugs over a long period (months to years) and iii) to administer drugs directly to the lesion, thereby helping reduce dosage and side effects. iv) IDDSs enable personalized drug delivery according to patient needs. The high demand for such systems has prompted scientists to make efforts to develop intelligent IDDS. In this review, several common stimulus-responsive mechanisms including endogenous (e.g., pH, reactive oxygen species, proteins, etc.) and exogenous stimuli (e.g., light, sound, electricity, magnetism, etc.), are given in detail. Besides, several types of IDDS reported in recent years are reviewed, including various stimulus-responsive systems based on the above mechanisms, radio frequency-controlled IDDS, "closed-loop" IDDS, self-powered IDDS, etc. Finally, the advantages and disadvantages of various IDDS, bottleneck problems, and possible solutions are analyzed to provide directions for subsequent research.

Controlling Growth of Poly (Triethylene Glycol Acrylate-Co-Spiropyran Acrylate) Copolymer Liquid Films on a Hydrophilic Surface by Light and Temperature

A quartz crystal microbalance with dissipation monitoring (QCM-D) was employed for in situ investigations of the effect of temperature and light on the conformational changes of a poly (triethylene glycol acrylate-co-spiropyran acrylate) (P (TEGA-co-SPA)) copolymer containing 12-14% of spiropyran at the silica-water interface. By monitoring shifts in resonance frequency and in acoustic dissipation as a function of temperature and illumination conditions, we investigated the evolution of viscoelastic properties of the P (TEGA-co-SPA)-rich wetting layer growing on the sensor, from which we deduced the characteristic coil-to-globule transition temperature, corresponding to the lower critical solution temperature (LCST) of the PTEGA part. We show that the coil-to-globule transition of the adsorbed copolymer being exposed to visible or UV light shifts to lower LCST as compared to the bulk solution: the transition temperature determined acoustically on the surface is 4 to 8 K lower than the cloud point temperature reported by UV/VIS spectroscopy in aqueous solution. We attribute our findings to non-equilibrium effects caused by confinement of the copolymer chains on the surface. Thermal stimuli and light can be used to manipulate the film formation process and the film's conformational state, which affects its subsequent response behavior.

Visible light-responsive micelles enable co-delivery of nitric oxide and antibiotics for synergistic antibiofilm applications

Tetraphenylethylene (TPE) moieties have been employed as a light-absorbing antenna for the activation of photoresponsive N-nitrosamine derivatives, enabling visible light-triggered NO release and efficient biofilm dispersal.

Visible-to-NIR-Light Activated Release: From Small Molecules to Nanomaterials

Photoactivatable (alternatively, photoremovable, photoreleasable, or photocleavable) protecting groups (PPGs), also known as caged or photocaged compounds, are used to enable non-invasive spatiotemporal photochemical control over the release of species of interest. Recent years have seen the development of PPGs activatable by biologically and chemically benign visible and near-infrared (NIR) light. These long-wavelength-absorbing moieties expand the applicability of this powerful method and its accessibility to non-specialist users. This review comprehensively covers organic and transition metal-containing photoactivatable compounds (complexes) that absorb in the visible- and NIR-range to release various leaving groups and gasotransmitters (carbon monoxide, nitric oxide, and hydrogen sulfide). The text also covers visible- and NIR-light-induced photosensitized release using molecular sensitizers, quantum dots, and upconversion and second-harmonic nanoparticles, as well as release via photodynamic (photooxygenation by singlet oxygen) and photothermal effects. Release from photoactivatable polymers, micelles, vesicles, and photoswitches, along with the related emerging field of photopharmacology, is discussed at the end of the review.

Reversible Photoswitching in Poly(2-oxazoline) Nanoreactors

This contribution reports light responsive catalytic nanoreactors based on poly(2-oxazoline) diblock copolymers. The hydrophobic block of the copolymer is a random copolymer consisting of a spiropyran functionalized 2-oxazoline (SPOx) and 2-(but-3-yn-1-yl)-4,5-dihydrooxazole (ButynOx), while the hydrophilic block is based on 2-methyl-2-oxazoline (MeOx). The block copolymer is terminated with tris(2-aminoethyl) amine (TREN) that serves as catalyst in a Knoevenagel condensation. Four block copolymers with different ButynOx/SPOx and hydrophilic/hydrophobic ratios are synthesized and self-assembled through solvent exchange. Micelles and vesicles of various sizes are observed by TEM, which undergo morphological and size changes in response to irradiation with UV light. We hypothesize that these transformations in the nanostructures are caused by increases in the hydrophilicity of the hydrophobic block when spiropyran (SP) isomerizes to merocyanine (MC) in the presence of UV light. The reversible transition from micellar to vesicular nanoreactors resulted in increased reaction kinetics through improved substrate accessibility to the catalytic site, or termination of the catalytic reaction due to polymer precipitation. These nanoreactors present a promising platform towards photoregulating reaction outcomes based on changes in nanostructure morphology.

On Demand Light-Degradable Polymers Based on 9,10-Dialkoxyanthracenes

Light induced degradation of polymers has drawn increasing interest due to the need for externally controllable modulation of materials properties. However, the portfolio of polymers, that undergo precisely controllable degradation, is limited and typically requires UV light. A novel class of backbone-degradable polymers that undergo aerobic degradation in the presence of visible light, yet remain stable against broad-spectrum light under anaerobic conditions is reported. In this design, the polymer backbone is comprised of 9,10-dialkoxyanthracene units that are selectively cleaved by singlet oxygen in the presence of green light as confirmed by NMR and UV/vis spectroscopy. The resulting polymers have been processed by electrohydrodynamic (EHD) co-jetting into bicompartmental microfibers, where one hemisphere is selectively degraded on demand.

Polymeric Photoacids Based on Naphthols-Design Criteria, Photostability, and Light-Mediated Release

The implementation of photoswitches within polymers offers an exciting toolbox in the design of light-responsive materials as irradiation can be controlled both spatially and temporally. Herein, we introduce a range of water-soluble copolymers featuring naphthol-based chromophores as photoacids in the side chain. With that, the resulting materials experience a drastic increase in acidity upon stimulation with UV light and we systematically studied how structure and distance of the photoacid from the copolymer backbone determines polymerizability, photo-response, and photostability. Briefly, we used RAFT (reversible addition-fragmentation chain transfer) polymerization to prepare copolymers consisting of nona(ethylene glycol) methyl ether methacrylate (MEO9 MA) as water-soluble comonomer in combination with six different 1-naphthol-based ("N") monomers. Thereby, we distinguish between methacrylates (NMA, NOeMA), methacrylamides (NMAm, NOeMAm), vinyl naphthol (VN), and post-polymerization modification based on [(1-hydroxynaphthalen-2-amido)ethyl]amine (NOeMAm, NAmeMAm). These P(MEO9 MAx -co-"N"y ) copolymers typically feature a 4:1 MEO9 MA to "N" ratio and molar masses in the range of 10 kg mol-1 . After synthesis and characterization by using NMR spectroscopy and size exclusion chromatography (SEC), we investigated how potential photo-cleavage or photo-degradation during irradiation depends on the type and distance of the linker to the copolymeric backbone and whether reversible excited state proton transfer (ESPT) occurs under these conditions. In our opinion, such materials will be strong assets as light-mediated proton sources in nanostructured environments, for example, for the site-specific creation of proton gradients. We therefore exemplarily incorporated NMA into an amphiphilic block copolymer and could demonstrate the light-mediated release of Nile red from micelles formed in water as selective solvent.

Block Copolymers Featuring Highly Photostable Photoacids Based on Vinylnaphthol: Synthesis and Self-Assembly

The synthesis of a photoresponsive amphiphilic diblock quarterpolymer containing 5-vinyl-1-naphthol (VN) as a photostable photoacidic comonomer is presented. The preparation is realized via a sequential reversible addition fragmentation chain transfer (RAFT) polymerization starting from a nona(ethylene glycol) methyl ether methacrylate (MEO₉ MA/"O") hydrophilic block, which is then used as a macro-RAFT agent in the terpolymerization of styrene (S), 2-vinylpyridine (2VP), and TBS-protected VN (tVN). The terpolymerization proceeds in a controlled fashion and two diblock quarterpolymers, P(O_m )-b-P(S_x -co-2VP_y -co-VN_z ), with varying functional comonomer compositions are prepared. These diblock quarterpolymers form spherical core-corona micelles in aqueous media according to dynamic light scattering (DLS) and cryogenic transmission electron microscopy (cryo-TEM). Upon irradiation, the photoacids within the micellar core experience a drastic increase in acidity causing a proton transfer from the photoacid to neighboring 2VP units. As a result, the hydrophilic/hydrophobic balance of the entire assembly is shifted, and the encapsulated cargo is released.

Polymerizations in Continuous Flow: Recent Advances in the Synthesis of Diverse Polymeric Materials

The number of reports using continuous flow technology in tubular reactors to perform precision polymerizations has grown enormously in recent years. Flow polymerizations allow highly efficient preparation of polymers exhibiting well-defined molecular characteristics, and has been applied to a slew of monomers and various polymerization mechanisms, including anionic, cationic, radical, and ring-opening. Polymerization conducted in continuous flow offers several distinct advantages, including improved efficiency, reproducibility, and enhanced safety for exothermic polymerizations using highly toxic components, high pressures, and high temperatures. The further development of this technology is thus of relevance for many industrial polymerization processes. While much progress has been demonstrated in recent years, opportunities remain for increasing the compositional and architectural complexity of polymeric materials synthesized in a continuous fashion. Extending the reactor processing principles that have heretofore been focused on optimizing homopolymerization to include multisegment block copolymers, particularly from monomers that propagate via incompatible mechanisms, represents a major challenge and coveted target for continuous flow polymerization. Likewise, the spatial and temporal control of reactivity afforded by flow chemistry has and will continue to enable the production of complex polymeric architectures. This Viewpoint offers a brief background of continuous flow polymerization focused primarily on tubular (micro)reactors and includes selected examples that are relevant to these specific developments.

Shape-Memory Polymers in Dentistry: Systematic Review and Patent Landscape Report

OBJECTIVE

To perform a systematic review (SR) of existing literature and a patent landscape report (PLR) regarding the potential applications of shape-memory polymers (SMPs) in dentistry.

SEARCH STRATEGY

Clinical and Biomedical online databases (Pubmed, Medline via Embase, Scopus, LILACS, Web of Science, Cochrane Library), Materials Science and Engineering databases (IEEE Explore, Compendex, Proquest), Material Science and Chemical database (Reaxys) so as Patents databases (Questel-Orbit, Espacenet, Patentscope) were consulted as recently as January 2019 to identify all papers and patents potentially relevant to the review. The reference lists of all eligible studies were hand searched for additional published work.

RESULTS

After duplicate selection and extraction procedures, 6 relevant full-text articles from the initial 302 and 45 relevant patents from 497 were selected. A modified Consolidated Standards of Reporting Trials (CONSORT) checklist of 14 items for reporting pre-clinical in-vitro studies was used to rate the methodological quality of the selected papers. The overall quality was judged low.

CONCLUSIONS

Despite the great potential and versatility of SMPs, it was not possible to draw evidence-based conclusions supporting their immediate employment in clinical dentistry. This was due to the weak design and a limited number of studies included within this review and reflects the fact that additional research is mandatory to determine whether or not the use of SMPs in dentistry could be effective. Nevertheless, the qualitative analysis of selected papers and patents indicate that SMPs are promising materials in dentistry because of their programmable physical properties. These findings suggest the importance of furtherly pursuing this line of research.

Responsive Polymers as Smart Nanomaterials Enable Diverse Applications

Responsive polymers undergo reversible or irreversible physical or chemical modifications in response to a change in environment or stimulus, e.g., temperature, pH, light, and magnetic or electric fields. Polymeric nanoparticles (NPs), which constitute a diverse set of morphologies, including micelles, vesicles, and core-shell geometries, have been successfully prepared from responsive polymers and have shown great promise in applications ranging from drug delivery to catalysis. In this review, we summarize pH, thermo-, photo-, and enzymatic responsiveness for a selection of polymers. We then discuss the formation of NPs made from responsive polymers. Finally, we highlight how NPs and other nanomaterials are enabling a wide range of smart applications with improved efficiency, as well as improved sustainability and recyclability of polymeric systems.

A new visible light and temperature responsive diblock copolymer

A visible light and temperature responsive diblock copolymer of poly[6-(2,6,2′,6′-tetramethoxy-4′-oxyazobenzene) hexyl methacrylate]-block-poly(N-isopropylacrylamide) (PmAzo-b-PNIPAM) was synthesized via RAFT polymerization by carefully tuning the polymerization conditions.

Synthesis and solution behaviour of dual light- and temperature-responsive poly(triethylene glycol-co-spiropyran) copolymers and block copolymers

We herein report on the synthesis and characterization of materials featuring a dual-responsive copolymer segment consisting of photo-responsive spiropyran (SPA) and temperature-responsive triethylene glycol acrylate (TEGA).

Synthesis and self-assembly of photoacid-containing block copolymers based on 1-naphthol

Photoacids experience a strong increase in acidity when absorbing light and, hence, can be considered as molecular switches. The incorporation into amphiphilic block copolymers leads to novel stimuli-responsive materials with great potential.

Dual Stimuli-Responsive P(NIPAAm-co-SPA) Copolymers: Synthesis and Response in Solution and in Films

We present the synthesis and solution properties of dual stimuli-responsive poly(N-isopropylacrylamide-co-spiropyran acrylate) (P(NIPAAm-co-SPA)) copolymers of varying composition prepared via nitroxide-mediated copolymerization. The resulting copolymers feature molar masses from 40,000 to 100,000 g/mol according to static light scattering and an SPA content of up to 5.3%. The latter was determined by ¹H NMR spectroscopy and UV⁻Vis spectroscopy. These materials exhibit reversible response upon irradiation in polymeric films for a minimum of three cycles; their response in solution to both light and temperature was also investigated in an aqueous TRIS buffer (pH 8). Irradiation was carried out using LED setups with wavelengths of 365 and 590 nm. In aqueous solution, a custom-made setup using a fiber-coupled 200 W Hg(Xe) lamp with 340 and 540 nm filters was used and additional heating of the copolymer solutions during irradiation allowed to study influence of the presence of either the spiropyran or merocyanine form on the cloud point temperature. Hereby, it was found that increasing the SPA content leads to a more pronounced difference between both states and decreasing cloud points in general.

Photoswitchable Layer-by-Layer Coatings Based on Photochromic Polynorbornenes Bearing Spiropyran Side Groups

Herein, we present the synthesis of linear photochromic norbornene polymers bearing spiropyran side groups (poly(SP-R)) and their assembly into layer-by-layer (LbL) films on glass substrates when converted to poly(MC-R) under UV irradiation. The LbL films were composed of bilayers of poly(allylamine hydrochloride) (PAH) and poly(MC-R), forming (PAH/poly(MC-R)) _n coatings. The merocyanine (MC) form presents a significant absorption band in the visible spectral region, which allowed tracking of the LbL deposition process by UV-vis spectroscopy, which showed a linear increase of the characteristic MC absorbance band with increasing number of bilayers. The thickness and morphology of the (PAH/poly(MC-R)) _n films were characterized by ellipsometry and scanning electron microscopy, respectively, with a height of ∼27.5 nm for the first bilayer and an overall height of ∼165 nm for the (PAH/poly(MC-R))₅ multilayer film. Prolonged white light irradiation (22 h) resulted in a gradual decrease of the MC band by 90.4 ± 2.9% relative to the baseline, indicating the potential application of these films as coatings for photocontrolled delivery systems.

Exploring Poly(ethylene glycol)-Polyzwitterion Diblock Copolymers as Biocompatible Smart Macrosurfactants Featuring UCST-Phase Behavior in Normal Saline Solution

Nonionic-zwitterionic diblock copolymers are designed to feature a coil-to-globule collapse transition with an upper critical solution temperature (UCST) in aqueous media, including physiological saline solution. The block copolymers that combine presumably highly biocompatible blocks are synthesized by chain extension of a poly(ethylene glycol) (PEG) macroinitiator via atom transfer radical polymerization (ATRP) of sulfobetaine and sulfabetaine methacrylates. Their thermoresponsive behavior is studied by variable temperature turbidimetry and ¹H NMR spectroscopy. While the polymers with polysulfobetaine blocks exhibit phase transitions in the physiologically interesting window of 30⁻50 °C only in pure aqueous solution, the polymers bearing polysulfabetaine blocks enabled phase transitions only in physiological saline solution. By copolymerizing a pair of structurally closely related sulfo- and sulfabetaine monomers, thermoresponsive behavior can be implemented in aqueous solutions of both low and high salinity. Surprisingly, the presence of the PEG blocks can affect the UCST-transitions of the polyzwitterions notably. In specific cases, this results in "schizophrenic" thermoresponsive behavior displaying simultaneously an UCST and an LCST (lower critical solution temperature) transition. Exploratory experiments on the UCST-transition triggered the encapsulation and release of various solvatochromic fluorescent dyes as model "cargos" failed, apparently due to the poor affinity even of charged organic compounds to the collapsed state of the polyzwitterions.

Light-fuelled reversible expansion of spiropyran-based vesicles in water

Vesicles formed by a spiropyran-based amphiphile show reversible expansion upon illumination

RAFT synthesis and micellization of a photo-, temperature- and pH-responsive diblock copolymer based on spiropyran

A photo-, temperature- and pH-responsive diblock copolymer containing a rigid spiropyran moiety was synthesized and its micellization was investigated.