Stimuli-responsive polymer-based systems for diagnostic applications

Stimuli-responsive polymers exhibit properties that make them ideal candidates for biosensing and molecular diagnostics. Through rational design of polymer composition combined with new polymer functionalization and synthetic strategies, polymers with myriad responsivities, e.g., responses to temperature, pH, biomolecules, CO2, light, and electricity can be achieved. When these polymers are specifically designed to respond to biomarkers, stimuli-responsive devices/probes, capable of recognizing and transducing analyte signals, can be used to diagnose and treat disease. In this review, we highlight recent state-of-the-art examples of stimuli-responsive polymer-based systems for biosensing and bioimaging.

Magnetic Nanostructure-Loaded Bicontinuous Nanospheres Support Multicargo Intracellular Delivery and Oxidation-Responsive Morphological Transitions

Magnetic nanostructures (MNS) have a wide range of biological applications due to their biocompatibility, superparamagnetic properties, and customizable composition that includes iron oxide (Fe₃O₄), Zn²⁺, and Mn²⁺. However, several challenges to the biomedical usage of MNS must still be addressed, such as formulation stability, inability to encapsulate therapeutic payloads, and variable clearance rates in vivo. Here, we enhance the utility of MNS during controlled delivery applications via encapsulation within polymeric bicontinuous nanospheres (BCNs) composed of poly(ethylene glycol)-block-poly(propylene sulfide) (PEG-b-PPS) copolymers. PEG-b-PPS BCNs have demonstrated versatile encapsulation and delivery capabilities for both hydrophilic and hydrophobic payloads due to their unique and highly organized cubic phase nanoarchitecture. MNS-embedded BCNs (MBCNs) were thus coloaded with physicochemically diverse molecular payloads using the technique of flash nanoprecipitation and characterized in terms of their structure and in vivo biodistribution following intravenous administration. Retention of the internal aqueous channels and cubic architecture of MBCNs were verified using cryogenic transmission electron microscopy and small-angle X-ray scattering, respectively. MBCNs demonstrated improvement in magnetic resonance imaging (MRI) contrast enhancement (r₂ relaxivity) as compared to free MNS, which in combination with scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy evidenced the clustering and continued access to water of MNS following encapsulation. Furthermore, MBCNs were found to be noncytotoxic and able to deliver their hydrophilic and hydrophobic small-molecule payloads both in vitro and in vivo. Finally, the oxidation sensitivity of the hydrophobic PPS block allowed MBCNs to undergo a unique, triggerable transition in morphology into MNS-bearing micellar nanocarriers. In summary, MBCNs are an attractive platform for the delivery of molecular and nanoscale payloads for diverse on-demand and sustained drug delivery applications.

Tailoring cellular microenvironments using scaffolds based on magnetically-responsive polymer brushes

A variety of polymeric scaffolds with the ability to control cell detachment has been created for cell culture using stimuli-responsive polymers. However, the widely studied and commonly used thermo-responsive polymeric substrates always affect the properties of the cultured cells due to the temperature stimulus. Here, we present a different stimuli-responsive approach based on poly(3-acrylamidopropyl)trimethylammonium chloride) (poly(APTAC)) brushes with homogeneously embedded superparamagnetic iron oxide nanoparticles (SPIONs). Neuroblastoma cell detachment was triggered by an external magnetic field, enabling a non-invasive process of controlled transfer into a new place without additional mechanical scratching and chemical/biochemical compound treatment. Hybrid scaffolds obtained in simultaneous surface-initiated atom transfer radical polymerization (SI-ATRP) were characterized by atomic force microscopy (AFM) working in the magnetic mode, secondary ion mass spectrometry (SIMS), and X-ray photoelectron spectroscopy (XPS) to confirm the magnetic properties and chemical structure. Moreover, neuroblastoma cells were cultured and characterized before and after exposure to a neodymium magnet. Controlled cell transfer triggered by a magnetic field is presented here as well.

Choline supported poly(ionic liquid) graft copolymers as novel delivery systems of anionic pharmaceuticals for anti-inflammatory and anti-coagulant therapy

New type of carriers based on grafted poly(ionic liquid)s was designed for delivery of ionically attached salicylates (Sal). Choline derived ionic liquid monomeric units were successfully introduced with various content in the side chains by the controlled radical polymerization. Properly high amounts of ionic pharmaceutics in the polymer systems were achieved by the well-fitted length and grafting degree of the side chains. In aqueous solution the graft copolymers were self-assembled into the spherical superstructures with sizes up to 73 nm. Delivery studies showed "burst" release within 4 h, after that it was slower yielding ~70% of released drug within 80 h. Proposed nanocarriers supported low toxicity against human cells (NHDF and BEAS-2B), anti-inflammation activity evaluated with the use of pro-inflammatory interleukins (IL-6 and IL-8) and antibacterial activities towards E. coli. Adjustment of ionic drug content by structural parameters of graft copolymers, including grafting degree and graft length, are advantageous to tailor nanocarriers with self-assembly properties in aqueous media. Effective release process by ionic exchange and biological activity with low toxicity are promising for further development of this type of drug delivery (DDS).

Ionic Polymethacrylate Based Delivery Systems: Effect of Carrier Topology and Drug Loading

The presented drug delivery polymeric systems (DDS), i.e., conjugates and self-assemblies, based on grafted and star-shaped polymethacrylates have been studied for the last few years in our group. This minireview is focused on the relationship of polymer structure to drug conjugation/entrapment efficiency and release capability. Both graft and linear polymers containing trimethylammonium groups showed the ability to release the pharmaceutical anions by ionic exchange, but in aqueous solution they were also self-assembled into nanoparticles with encapsulated nonionic drugs. Star-shaped polymers functionalized with ionizable amine/carboxylic groups were investigated for drug conjugation via ketimine/amide linkers. However, only the conjugates of polybases were water-soluble, giving opportunity for release studies, whereas the self-assembling polyacidic stars were encapsulated with the model drugs. Depending on the type of drug loading in the polymer matrix, their release rates were ordered as follows: Physical ≥ ionic > covalent. The studies indicated that the well-defined ionic polymethacrylates, including poly(ionic liquid)s, are advantageous for designing macromolecular carriers due to the variety of structural parameters, which are efficient for tuning of drug loading and release behavior in respect to the specific drug interactions.

Homogeneous Embedding of Magnetic Nanoparticles into Polymer Brushes during Simultaneous Surface-Initiated Polymerization

Here we present a facile and efficient method of controlled embedding of inorganic nanoparticles into an ultra-thin (<15 nm) and flat (~1.0 nm) polymeric coating that prevents unwanted aggregation. Hybrid polymer brushes-based films were obtained by simultaneous incorporation of superparamagnetic iron oxide nanoparticles (SPIONs) with diameters of 8⁻10 nm into a polycationic macromolecular matrix during the surface initiated atom transfer radical polymerization (SI-ATRP) reaction in an ultrasonic reactor. The proposed structures characterized with homogeneous distribution of separated nanoparticles that maintain nanometric thickness and strong magnetic properties are a good alternative for commonly used layers of crosslinked nanoparticles aggregates or bulk structures. Obtained coatings were characterized using atomic force microscopy (AFM) working in the magnetic mode, secondary ion mass spectrometry (SIMS), and X-ray photoelectron spectroscopy (XPS).

A Hybrid System for Magnetic Hyperthermia and Drug Delivery: SPION Functionalized by Curcumin Conjugate

Cancer is among the leading causes of death worldwide, thus there is a constant demand for new solutions, which may increase the effectiveness of anti-cancer therapies. We have designed and successfully obtained a novel, bifunctional, hybrid system composed of colloidally stabilized superparamagnetic iron oxide nanoparticles (SPION) and curcumin containing water-soluble conjugate with potential application in anticancer hyperthermia and as nanocarriers of curcumin. The obtained nanoparticulate system was thoroughly studied in respect to the size, morphology, surface charge, magnetic properties as well as some biological functions. The results revealed that the obtained nanoparticles, ca. 50 nm in diameter, were the agglomerates of primary particles with the magnetic, iron oxide cores of ca. 13 nm, separated by a thin layer of the applied cationic derivative of chitosan. These agglomerates were further coated with a thin layer of the sodium alginate conjugate of curcumin and the presence of both polymers was confirmed using thermogravimetry. The system was also proven to be applicable in magnetic hyperthermia induced by the oscillating magnetic field. A high specific absorption rate (SAR) of 280 [W/g] was registered. The nanoparticles were shown to be effectively uptaken by model cells. They were found also to be nontoxic in the therapeutically relevant concentration in in vitro studies. The obtained results indicate the high application potential of the new hybrid system in combination of magnetic hyperthermia with delivery of curcumin active agent.

Cooperative Assembly of Magneto-Nanovesicles with Tunable Wall Thickness and Permeability for MRI-Guided Drug Delivery

This article describes the fabrication of nanosized magneto-vesicles (MVs) comprising tunable layers of densely packed superparamagnetic iron oxide nanoparticles (SPIONs) in membranes via cooperative assembly of polymer-tethered SPIONs and free poly(styrene)- b-poly(acrylic acid) (PS- b-PAA). The membrane thickness of MVs could be well controlled from 9.8 to 93.2 nm by varying the weight ratio of PS- b-PAA to SPIONs. The increase in membrane thickness was accompanied by the transition from monolayer MVs, to double-layered MVs and to multilayered MVs (MuMVs). This can be attributed to the variation in the hydrophobic/hydrophilic balance of polymer-grafted SPIONs upon the insertion and binding of PS- b-PAA onto the surface of nanoparticles. Therapeutic agents can be efficiently encapsulated in the hollow cavity of MVs and the release of payload can be tuned by varying the membrane thickness of nanovesicles. Due to the high packing density of SPIONs, the MuMVs showed the highest magnetization and transverse relaxivity rate ( r₂) in magnetic resonance imaging (MRI) among these MVs and individual SPIONs. Upon intravenous injection, doxorubicin-loaded MuMVs conjugated with RGD peptides could be effectively enriched at tumor sites due to synergetic effect of magnetic and active targeting. As a result, they exhibited drastically enhanced signal in MRI, improved tumor delivery efficiency of drugs as well as enhanced antitumor efficacy, compared with groups with only magnetic or active targeting strategy. The unique nanoplatform may find applications in effective disease control by delivering imaging and therapy to organs/tissues that are not readily accessible by conventional delivery vehicles.

Biocompatible and fluorescent superparamagnetic iron oxide nanoparticles with superior magnetic properties coated with charged polysaccharide derivatives

Syntheses and characterizations of biocompatible superparamagnetic iron oxide nanoparticles with embedded curcumin and coated with ultrathin layer of hyaluronic acid-curcumin (HA-Cur) conjugate have been reported. Zeta potential measurements confirmed effective coating of native iron oxide nanoparticles stabilized by cationic derivative of chitosan (SPION-CCh) with the synthesized HA-Cur conjugate. Both SPIONs with embedded curcumin and the ones coated with HA-Cur (SPION-CCh/HA-Cur) revealed desired magnetic characteristics while fluorescent properties were much better for the coated nanoparticles. SPION-CCh/HA-Cur nanoparticles were shown to be very promising candidates for T₂ MRI contrast agents as they can easily penetrate cell membrane and their relaxivity is exceptionally high (ca. 470mM^-1s^-1). They may be also tracked using confocal fluorescence microscopy due to the presence of fluorescent curcumin in the coating. In vitro studies indicated that the obtained SPIONs-CCh/HA-Cur were non-toxic for EA.hy926 endothelial cells.

A superparamagnetic polymersome with extremely high T2 relaxivity for MRI and cancer-targeted drug delivery

Improving the relaxivity of magnetic resonance imaging (MRI) contrast agents is an important challenge for cancer theranostics. Herein we report the design, synthesis, characterization, theoretical analysis and in vivo tests of a superparamagnetic polymersome as a new MRI contrast agent with extremely high T₂ relaxivity (611.6 mM^-1s^-1). First, a noncytotoxic cancer-targeting polymersome is synthesized based on a biodegradable diblock copolymer, folic acid-poly(l-glutamic acid)-block-poly(ε-caprolactone) [FA-PGA-b-PCL]. Then, ultra-small superparamagnetic iron oxide nanoparticles (SPIONs) are in situ generated in the hydrophilic PGA coronas of polymersomes to afford magnetic polymersomes. The in vivo MRI assay revealed prominent negative contrast enhancement of magnetic polymersomes at a very low Fe dose of 0.011 mmol/kg. Moreover, this cancer-targeting magnetic polymersome can effectively encapsulate and deliver anticancer drug to inhibit the tumor growth, demonstrating promising theranostic applications in biomedicine.

A recyclable and regenerable magnetic chitosan absorbent for dye uptake

A recyclable and regenerable magnetic polysaccharide absorbent for methylene blue (MB) removal was prepared by coating magnetic polyethyleneimine nanoparticles (PEI@MNPs) with sulfonated chitosan (SCS) and further cross-linked with glutaraldehyde. The driving force for coating is the electrostactic interaction between positively charged PEI and negatively charged SCS. Infrared spectra, zeta potential, thermal gravimetric analysis and X-ray diffraction demonstrated the successful synthesis of magnetic polysaccharide absorbent. The self-assembly of polysaccharide with magnetic nanopartices did not alter the saturation magnetization value of the absorbent confirmed by vibrating sample magnetometer. The nanoparticles showed fast removal (about 30min reached equilibrium) of MB. In particular, the removal ability of MB after desorption did not reduce, demonstrating an excellent regeneration ability. Our study provides new insights into utilizing polysaccharides for environmental remediation and creating advanced magnetic materials for various promising applications.

Mini-review: fluorescence imaging in cancer cells using dye-doped nanoparticles

Fluorescence imaging has gained increased attention over the past two decades as a viable means to detect a variety of cancers.

Novel nanostructural contrast for magnetic resonance imaging of endothelial inflammation: targeting SPIONs to vascular endothelium

This study aimed to develop superparamagnetic iron oxide nanoparticles (SPIONs) targeted to the areas of vascular endothelium changed in the initial inflammation process, a first step of numerous cardiovascular diseases.