Analysis of the Formation Mechanism for Thermoresponsive-Type Coacervate with Functional Copolymers Consisting of N-Isopropylacrylamide and 2-Hydroxyisopropylacrylamide

Thermoresponsive Brush Coatings for Cell Sheet Engineering with Low Protein Adsorption above the Polymers' Phase Transition Temperature

Thermoresponsive polymer coatings on cell culture substrates enable noninvasive cell detachment and cell sheet fabrication for biomedical applications. Optimized coatings should support controlled culture and detachment of various cell types and allow chemical modifications, e.g., to introduce specific growth factors for enhanced gene expression. Furthermore, the sterilization and storage stability of the coatings must be assessed for translational attempts. Poly(glycidyl ether) (PGE) brush coatings with short alkoxy side chains provide a versatile platform for cell culture and detachment, but their polyether backbones are susceptible to oxidation and degradation. Thus, we rationally designed potential alternatives with thermoresponsive glycerol-based block copolymers comprising a stable polyacrylate or polymethacrylate backbone and an oligomeric benzophenone (BP)-based anchor. The resulting poly(ethoxy hydroxypropyl acrylate-b-benzophenone acrylate) (pEHPA-b-BP) and poly(ethoxy hydroxypropyl methacrylate-b-benzophenone methacrylate) (pEHPMA-b-BP) block copolymers preserve the short alkoxy-terminated side chains of the PGE derived structure on a stable, but hydrophobic, aliphatic backbone. The amphiphilicity balance is maintained through incorporated hydroxyl groups, which simultaneously can be used for chemical modification. The polymers were tailored into brush coatings on polystyrene surfaces via directed adsorption using the BP oligomer anchor. The resulting coatings with thickness values up to ∼3 nm supported efficient adhesion and proliferation of human fibroblasts despite minimal protein adsorption. The conditions for cell sheet fabrication on pEHPA-b-BP were gentler and more reliable than on pEHPMA-b-BP, which required additional cooling. Hence, the stability of pEHPA-b-BP and PGE coatings was evaluated post gamma and formaldehyde (FO) gas sterilization. Gamma sterilization partially degraded PGE coatings and hindered cell detachment on pEHPA-b-BP. In contrast, FO sterilization only slowed detachment on PGE coatings and had no adverse effects on pEHPA-b-BP, maintaining their efficient performance in cell sheet fabrication.

Terminal Structure of Triethylene Glycol-Tethered Chains on β-Cyclodextrin-Threaded Polyrotaxanes Dominates Temperature Responsivity and Biointeractions

Pharmacological and biomedical applications of cyclodextrin (CD)-threaded polyrotaxanes (PRXs) have gained increasing attention. We had previously investigated the therapeutic effects of oligo(ethylene glycol) (OEG)-modified β-CD PRXs in congenital metabolic disorders. Although the chemical modification of PRXs is crucial for these applications, the influences of the chemical structure of OEG modified on PRXs were not completely understood. The current study focuses on the terminal group structures of triethylene glycol (TEG)-tethered chains, wherein three series of TEG-tethered PRXs (TEG-PRXs) with various TEG terminal group structures (hydroxy, methoxy, and ethoxy) were synthesized to investigate their physicochemical properties and biointeractions. The methoxy and ethoxy-terminated TEG-PRXs exhibited temperature-dependent phase transitions in phosphate buffer saline and formed coacervate droplets above their cloud points. A comprehensive analysis revealed that the hydrophobicity of the terminal group structures of the TEG-tethered chains played a dominant role in exhibiting temperature-dependent phase transition. Furthermore, the hydrophobicity of the terminal group structures of TEG-tethered chains on PRXs also affected the interactions with lipids and proteins, with the hydrophobic ethoxy-terminated TEG-tethered chains showing the highest interactions. However, in normal human skin fibroblasts, the moderately hydrophobic methoxy-terminated TEG-modified PRXs showed the highest intracellular uptake levels. As a result, we concluded that methoxy-terminated TEG is a suitable chemical modification for the biomedical applications of PRXs due to the negligible temperature responsivity around physiological temperature and significant intracellular uptake levels. The findings of this study shall contribute significantly to the rational design of PRXs and CD-based materials for future pharmacological and biomedical applications.

Facile preparation of multi-stimuli-responsive degradable hydrogels for protein loading and release

Functional materials that can recognize the tumor microenvironment, characterized by acidic or reducing conditions, are needed for the designing of drug delivery carriers for cancer treatment. Hydrogels are potential protein drug carriers because they contain a large amount of water and stimuli-responsive functions can easily be introduced in them. However, it is difficult to introduce multi-stimuli-responsive functions and degradability at the same time. Here, we synthesized thermo- and pH-responsive hydrogels via a coupling reaction between poly(ethylene glycol) diglycidyl ether (PEGDE) and cystamine (CA). The prepared hydrogels showed lower critical solution temperature-type thermoresponsive behavior and pH-responsive swelling changes due to the protonation of secondary and/or tertiary amino groups arising from the crosslinking agent CA. Under reducing conditions, the hydrogels were degraded via the thiol exchange reaction in the presence of dithiothreitol or glutathione. The loading and release properties of FITC-labeled model proteins from the hydrogels were investigated. The loaded amount of the protein increased with decreasing molecular weight or hydrodynamic radius, which is based on the size of the network structure of the hydrogels. Notably, loaded proteins in the hydrogels were released only under reducing conditions, which mimic the tumor microenvironment. Thus, the prepared multi-responsive degradable hydrogels are expected to be used as functional drug delivery carriers for cancer treatment.

Formation of Single Double-Layered Coacervate of Poly(N,N-diethylacrylamide) in Water by a Laser Tweezer

We demonstrate liquid-liquid phase separation involving both coacervation and coil-to-globule phase transition of a thermoresponsive polymer. By focusing a near-infrared laser beam into an aqueous solution of poly(N-isopropylacrylamide) (PNIPAM), a single phase-separated polymer microdroplet can be formed and stably trapped at the focal point. Such droplet formation is induced by a local elevation in temperature (induced by a photothermal effect) and an optical force. The technique allows us to selectively analyze a single polymer droplet trapped at the focal point. In this study, we applied this technique to poly(N,N-diethylacrylamide) (PDEA) in water and generated a double-layered PDEA droplet. Such an inhomogeneous and complex microstructure has not been previously observed both in steady-state heating of a PDEA solution and in the PNIPAM system. Moreover, we used micro-Raman spectroscopy to clarify that PDEA underwent dehydration due to a coil-to-globule phase transition. Despite this, the polymer concentration (C_poly) of the trapped PDEA droplet was very low and was around 30 wt %. C_poly depended on the molecular weight of PDEA and the laser power that regulates the temperature elevation. These results strongly indicate that PDEA undergoes coacervation in addition to a coil-to-globule phase transition. This study will help provide us with a fundamental understanding of the phase separation mechanisms of thermoresponsive polymers.

Efficient Protein Encapsulation within Thermoresponsive Coacervate-Forming Biodegradable Polyesters

Presented here is a novel method for encapsulating proteins into biodegradable, thermoresponsive coacervate-type polyesters. Bovine serum albumin (BSA) was efficiently incorporated into coacervate droplets via a simple thermoresponsive encapsulation mechanism. Tunable modular systems for encapsulation such as the one presented here may be useful in a range of protein delivery applications.

Recent Advances in Dual Temperature Responsive Block Copolymers and Their Potential as Biomedical Applications

The development of stimuli responsive polymers has progressed significantly with novel preparation techniques, which has allowed access to new materials with unique properties. Dual thermoresponsive (double temperature responsive) block copolymers are particularly of interest as their properties can change depending on the lower critical solution temperature (LCST) or upper critical solution temperature (UCST) of each segment. For instance, these block copolymers can change from being hydrophilic, to amphiphilic or to hydrophobic simply by changing the solution temperature without any additional chemicals and the block copolymers can change from being fully solubilized to self-assembled structures to macroscopic aggregation/precipitation. Based on the unique solution properties, these dual thermo-responsive block copolymers are expected to be suitable for biomedical applications. This review is divided into three parts; LCST-LCST types of block copolymers, UCST-LCST types of block copolymers, and their potential as biomedical applications.

Rational design of a comprehensive cancer therapy platform using temperature-sensitive polymer grafted hollow gold nanospheres: simultaneous chemo/photothermal/photodynamic therapy triggered by a 650 nm laser with enhanced anti-tumor efficacy

Combining multi-model treatments within one single system has attracted great interest for the purpose of synergistic therapy. In this paper, hollow gold nanospheres (HAuNs) coated with a temperature-sensitive polymer, poly(oligo(ethylene oxide) methacrylate-co-2-(2-methoxyethoxy)ethyl methacrylate) (p(OEGMA-co-MEMA)), co-loaded with DOX and a photosensitizer Chlorin e6 (Ce6) were successfully synthesized. As high as 58% DOX and 6% Ce6 by weight could be loaded onto the HAuNs-p(OEGMA-co-MEMA) nanocomposites. The grafting polymer brushes outside the HAuNs play the role of "gate molecules" for controlled drug release by 650 nm laser radiation owing to the temperature-sensitive property of the polymer and the photothermal effect of HAuNs. The HAuNs-p(OEGMA-co-MEMA)-Ce6-DOX nanocomposites with 650 nm laser radiation show effective inhibition of cancer cells in vitro and enhanced anti-tumor efficacy in vivo. In contrast, control groups without laser radiation show little cytotoxicity. The nanocomposite demonstrates a way of "killing three birds with one stone", that is, chemotherapy, photothermal and photodynamic therapy are triggered simultaneously by the 650 nm laser stimulation. Therefore, the nanocomposites show the great advantages of multi-modal synergistic effects for cancer therapy by a remote-controlled laser stimulus.

A coacervate-forming biodegradable polyester with elevated LCST based on bis-(2-methoxyethyl)amine

This work details a thermoresponsive, biodegradable coacervate-forming polyester, which exhibits tunable T_cp under various conditions.

Temperature-responsive electrospun nanofibers for 'on-off' switchable release of dextran

We propose a new type of 'smart' nanofiber (NF) with dynamically and reversibly tunable properties for the 'on-off' controlled release of the polysaccharide dextran. The fibers are produced by electrospinning copolymers of N-isopropylacrylamide (NIPAAm) and N-hydroxymethylacrylamide (HMAAm). The OH groups of HMAAm are subsequently crosslinked by thermal curing. The copolymers were successfully fabricated into a well-defined nanofibrous structure with a diameter of about 600-700 nm, and the fibers preserved their morphology even after thermal curing. The resulting crosslinked NFs showed rapid and reversible volume changes in aqueous media in response to cycles of temperature alternation. The fibrous morphology was maintained for the crosslinked NFs even after the cycles of temperature alternation, while non-crosslinked NFs collapsed and dispersed quickly in the aqueous solution. Dextran-containing NFs were prepared by electrospinning the copolymers blended with fluorescein isothiocyanate (FITC)-dextran, and the 'on-off' switchable release of FITC-dextran from the crosslinked NFs was observed. Almost all the FITC-dextran was released from the NFs after six heating cycles, whereas only a negligible amount of FITC-dextran was evolved during the cooling process. The reported incorporation of smart properties into NFs takes advantage of their extremely large surface area and porosity and is expected to provide a simple platform for on-off drug delivery.

Surface design with self-heating smart polymers for on-off switchable traps

We have developed a novel self-heating, temperature-responsive chromatography system for the effective separation of biomolecules. Temperature-responsive poly(N-isopropylacrylamide-co-N-hydroxymethylacrylamide), poly(NIPAAm-co-HMAAm), was covalently grafted onto the surface of magnetite/silica composites as 'on-off' switchable surface traps. The lower critical solution temperature (LCST) of the poly(NIPAAm-co-HMAAm)s was controlled from 35 to 55 °C by varying the HMAAm content. Using the heat generated by magnetic particles in an alternating magnetic field (AMF) we were able to induce the hydrophilic to hydrophobic phase separation of the grafted temperature-responsive polymers. To assess the feasibility of the poly(NIPAAm-co-HMAAm)-grafted magnetite/silica particles as the stationary phase for chromatography, we packed the particles into the glass column of a liquid chromatography system and analyzed the elusion profiles for steroids. The retention time for hydrophobic steroids markedly increased in the AMF, because the hydrophobic interaction was enhanced via self-heating of the grafted magnetite/silica particles, and this effect could be controlled by changing the AMF irradiation time. Turning off the AMF shortened the total analysis time for steroids. The proposed system is useful for separating bioactive compounds because their elution profiles can be easily controlled by an AMF.

Designing temperature-responsive biocompatible copolymers and hydrogels based on 2-hydroxyethyl(meth)acrylates

Free-radical copolymerization of 2-hydroxyethyl methacrylate with 2-hydroxyethyl acrylate can be successively utilized for the synthesis of water-soluble polymers and hydrogels with excellent physicochemical properties, thus showing promise for pharmaceutical and biomedical applications. In the work presented it has been demonstrated that water-soluble copolymers based on 2-hydroxyethyl methacrylate and 2-hydroxyethyl acrylate exhibit lower critical solution temperature in aqueous solutions, whereas the corresponding high molecular weight homopolymers do not have this unique property. The temperature-induced transitions observed upon heating the aqueous solutions of these copolymers proceed via liquid-liquid phase separation. The hydrogels were also synthesized by copolymerizing 2-hydroxyethyl methacrylate and 2-hydroxyethyl acrylate in the absence of a bifunctional cross-linker. The cross-linking of these copolymers during copolymerization is believed to be due to the presence of bifunctional admixtures or transesterification reactions. Transparency, swelling behavior, mechanical properties, and porosity of the hydrogels are dependent upon the monomer ratio in the copolymers. Hydrogel samples containing more 2-hydroxyethyl methacrylate are less transparent, have lower swelling capacity, higher elastic moduli, and pores of smaller size. The assessment of the biocompatibility of the copolymers using the slug mucosal irritation test revealed that they are also less irritant than poly(acrylic acid).

Stimuli-responsive coacervate induced in binary functionalized poly(N-isopropylacrylamide) aqueous system and novel method for preparing semi-ipn microgel using the coacervate

We describe a novel method for preparing a stimuli-responsive semi-interpenetrating polymer network (semi-IPN) hydrogel microsphere using a thermoresponsive-type coacervation. The coacervate droplets were formed in the two-component nonionic poly(N-isopropylacrylamide-co-2-hydroxyisopropylacrylamide) (poly(NIPAAm-co-HIPAAm)) and ionic poly(NIPAAm-co-2-carboxyisopropylacrylamide) (poly(NIPAAm-co-CIPAAm)) aqueous system by heating the solution above the lower critical solution temperature. The resulting coacervate droplets included both kinds of polymer chains. Divinyl sulfone, which cross-links the hydroxyl groups of the poly(NIPAAm-co-HIPAAm), was added to the coacervate droplets. In this way, the stimuli-responsive semi-IPN hydrogel microsphere consisting of the poly(NIPAAm-co-HIPAAm) gel matrix and the linear poly(NIPAAm-co-CIPAAm) chains could be prepared, and their sizes were relatively homogeneous. That is, by utilizing the thermoresponsive coacervate droplets induced in the binary system, we could successfully prepare the fine stimuli-responsive semi-IPN hydrogel microsphere and it was prepared in a simple and easy method without any additives.

Novel Thermoresponsive and pH-Responsive Aggregates from Self-Assembly of Triblock Copolymer PSMA-b-PNIPAAm-b-PSMA

A series of novel triblock copolymers of poly(stearyl methacrylate)-b-poly(N-isopropylacrylamide)-b-poly(stearyl methacrylate) (PSMA-b-PNIPAAm-b-PSMA) with different molecular weights was synthesized through carboxyl-terminated trithiocarbonates as a highly efficient RAFT agent via reversible addition-fragmentation chain transfer (RAFT) polymerization. The resultant polymers were characterized by 1H NMR, FT-IR spectroscopy, and GPC. By varying the organic solvent used in the self-assembly procedure and adjusting the copolymer composition, multiple morphologies ranging from vesicles and core-shell spherical aggregates with different dimensions to pearl-necklace-like aggregates were obtained. The aggregates showed thermoresponsive and pH-responsive properties through the lower critical solution temperature (LCST) of PNIPAAm and the two carboxyl end groups of the copolymer.