Biological Stimuli-responsive Polymer Systems: Design, Construction and Controlled Self-assembly

Gas-Responsive and Gas-Releasing Polymer Assemblies

In order to explore the unique physiological roles of gas signaling molecules and gasotransmitters in vivo, chemists have engineered a variety of gas-responsive polymers that can monitor their changes in cellular milieu, and gas-releasing polymers that can orchestrate the release of gases. These have advanced their potential applications in the field of bio-imaging, nanodelivery, and theranostics. Since these polymers are of different chain structures and properties, the morphology of their assemblies will manifest distinct transitions after responding to gas or releasing gas. In this review, we summarize the fundamental design rationale of gas-responsive and gas-releasing polymers in structure and their controlled transition in self-assembled morphology and function, as well as present some perspectives in this prosperous field. Emerging challenges faced for the future research are also discussed.

Synthetic lipo-polylysine with anti-cancer activity

Development of novel therapeutic agents that possess different anticancer mechanisms from the traditional antitumor drugs is highly attractive as no medication can cure all types of cancers. Herein, we report a rational design of antitumor lipo-polylysine polymers as synthetic mimics of biosynthetic lipopeptide surfactants featuring antimicrobial or cytotoxic activities for cancer therapy. The optimal polymer shows a wide range of anticancer activities against multiple cancer cells, including highly metastatic and drug-resistant ones, but low toxicity to normal cells. Mechanism studies show that the optimal polymer can interact with the membrane of cancer cells and induce cell necrosis by triggering cell membrane perforation, which is different from the therapeutic mechanisms of traditional anticancer drugs. In vivo studies imply that the optimal polymer efficiently inhibits tumor growth without causing obvious side effects on a C26 graft tumor model. Overall, the lipopeptide-mimicking lipo-polylysine with the advantages of easy synthesis and low cost provides a new anticancer strategy with high efficacy and biocompatibility.

Polyacrylamide-Based Block Copolymer Bearing Pyridine Groups Shows Unexpected Salt-Induced LCST Behavior

Thermal-responsive block copolymers are a special type of macromolecule that exhibit a wide range of applications in various fields. In this contribution, we report a new type of polyacrylamide-based block copolymer bearing pyridine groups of polyethylene glycol-block-poly(N-(2-methylpyridine)-acrylamide; Px) that display distinct salt-induced lower critical solution temperature (LCST) behavior. Unexpectedly, the phase-transition mechanism of the salt-induced LCST behavior of Px block copolymers is different from that of the reported LCST-featured analogues. Moreover, their thermo-responsive behavior can be significantly regulated by several parameters such as salt species and concentration, urea, polymerization degree, polymer concentration and pH values. This unique thermal behavior of pyridine-containing block copolymers provides a new avenue for the fabrication of smart polymer materials with potential applications in biomedicine.

Hydrogels-A Promising Materials for 3D Printing Technology

Hydrogels are a promising material for a variety of applications after appropriate functional and structural design, which alters the physicochemical properties and cell signaling pathways of the hydrogels. Over the past few decades, considerable scientific research has made breakthroughs in a variety of applications such as pharmaceuticals, biotechnology, agriculture, biosensors, bioseparation, defense, and cosmetics. In the present review, different classifications of hydrogels and their limitations have been discussed. In addition, techniques involved in improving the physical, mechanical, and biological properties of hydrogels by admixing various organic and inorganic materials are explored. Future 3D printing technology will substantially advance the ability to pattern molecules, cells, and organs. With significant potential for producing living tissue structures or organs, hydrogels can successfully print mammalian cells and retain their functionalities. Furthermore, recent advances in functional hydrogels such as photo- and pH-responsive hydrogels and drug-delivery hydrogels are discussed in detail for biomedical applications.

Role of chain architecture in the solution phase assembly and thermoreversibility of aqueous PNIPAM/silyl methacrylate copolymers

Stimuli-responsive polymers functionalized with reactive inorganic groups enable creation of macromolecular structures such as hydrogels, micelles, and coatings that demonstrate smart behavior. Prior studies using poly(N-isopropyl acrylamide-co-3-(trimethoxysilyl)propyl methacrylate) (P(NIPAM-co-TMA)) have stabilized micelles and produced functional nanoscale coatings; however, such systems show limited responsiveness over multiple thermal cycles. Here, polymer architecture and TMA content are connected to the aqueous self-assembly, optical response, and thermo-reversibility of two distinct types of PNIPAM/TMA copolymers: random P(NIPAM-co-TMA), and a 'blocky-functionalized' copolymer where TMA is localized to one portion of the chain, P(NIPAM-b-NIPAM-co-TMA). Aqueous solution behavior characterized via cloud point testing (CPT), dynamic light scattering (DLS), and variable-temperature nuclear magnetic resonance spectroscopy (NMR) demonstrates that thermoresponsiveness and thermoreversibility over multiple cycles is a strong function of polymer configuration and TMA content. Despite low TMA content (≤2% mol), blocky-functionalized copolymers assemble into small, well-ordered structures above the cloud point that lead to distinct transmittance behaviors and stimuli-responsiveness over multiple cycles. Conversely, random copolymers form disordered aggregates at elevated temperatures, and only exhibit thermoreversibility at negligible TMA fractions (0.5% mol); higher TMA content leads to irreversible structure formation. This understanding of the architectural and assembly effects on the thermal cyclability of aqueous PNIPAM-co-TMA can be used to improve the scalability of responsive polymer applications requiring thermoreversible behavior, including sensing, separations, and functional coatings.

Release of bioactive compounds from delivery systems by stimuli-responsive approaches; triggering factors, mechanisms, and applications

Recent advances in emerging nanocarriers and stimuli-responsive (SR) delivery systems have brought about a revolution in the food and pharmaceutical industries. SR carriers are able to release the encapsulated bioactive compounds (bioactives) upon an external trigger. The potential of releasing the loaded bioactives in site-specific is of great importance for the pharmaceutical industry and medicine that can deliver the cargo in an appropriate condition. For the food industry, release of encapsulated bioactives is considerably important in processing or storage of food products and can be used in their formulation or packaging. There are various stimuli to control the favorite release of bioactives. In this review, we will shed light on the effect of different stimuli such as temperature, humidity, pH, light, enzymatic hydrolysis, redox, and also multiple stimuli on the release of encapsulated cargo and their potential applications in the food and pharmaceutical industries. An overview of cargo release mechanisms is also discussed. Furthermore, various alternatives to manipulate the controlled release of bioactives from carriers and the perspective of more progress in these SR carriers are highlighted.

Polymer-based thermoresponsive hydrogels for controlled drug delivery

INTRODUCTION

controlled drug delivery through hydrogels is generally limited by the poor barrier that polymeric network can create to diffusion mechanism. Stimuli responsive polymers can help in this way guaranteeing that delivery can be sustained and finely controlled using an external stimulus.

AREA COVERED

this review provides an overview of recent studies about the use of temperature as an external stimulus able to work as an efficient new route of drug's administration. Thermoresponsive hydrogels are discussed and compared in terms of physical properties and mechanism of drug release considering their classification in intrinsically (formed by thermosensitive polymers) and non-intrinsically (polymers with thermosensitive moieties) hydrogels.

EXPERT OPINION

thermoresponsive hydrogels can be developed by using different polymers added or not with micro/nanoparticles of organic or inorganic origin. In both cases the final system represents an innovative way for the local and sustained drug delivery in a specific site of the body. In particular, it is possible to obtain an on-demand release of drug by applying a local increase of temperature to the system.

Precise delivery of multi-stimulus-responsive nanocarriers based on interchangeable visual guidance

Cancer treatment is imminent, and controlled drug carriers are an important development direction for future clinical chemotherapy. Visual guidance is a feasible means to achieve precise treatment, reduce toxicity and increase drug efficacy. However, the existing visual control methods are limited by imaging time-consuming, sensitivity and side effects. In addition, the ability of the carrier to respond to environmental stimuli in vivo is another difficulty that limits its application. Here, we propose a highly stimulus-responsive GC liposome with precise tracing and sensitive feedback capabilities. It combines magnetic resonance imaging and fluorescence imaging, and addresses the need for precise visualization by alternating imaging modalities. More importantly, GC liposomes are a carrier that can accumulate stimuli. In this paper, by tracking the fragmentation process of empty GC and drug-loaded D-GC liposomes, we confirm the synergistic effect between multiple stimuli, which can result in a more efficient drug release performance. Finally, in mice models we examined the GC liposome imaging approach and the D-GC + UV group guided by this visualization exhibited the highest tumor inhibition efficiency (6.85-fold). This study highlights the advantages of alternate visualization-guided and co-stimulation treatment strategies, and provides design ideas and potential materials for efficient and less toxic cancer treatments.

Thermoresponsive Polymers Based on Tertiary Amine Moieties

Thermoresponsive polymers exhibiting unique reversible phase transition properties in aqueous solution in response to temperature stimuli have been extensively investigated. In the past two decades, thermoresponsive polymers based on tertiary amine moieties have achieved considerable progress and become an important family of thermoresponsive polymers, including tertiary amine functionalized poly((meth)acrylamide)s, poly((meth)acrylate)s, poly(styrene)s, poly(vinyl alcohol)s, and poly(ethylene oxide)s, which exhibit lower critical solution temperature and/or upper critical solution temperature in water or aliphatic alcohols. Their phase transition behavior can be modulated by the solution pH and CO₂ due to the protonation of tertiary amine moieties in acidic condition and deprotonation in alkaline condition and the charged ammonium bicarbonate formed by the tertiary amine moieties and CO₂ . The aim of this review is to summarize the recent progress in the thermoresponsive polymers based on tertiary amine moieties.

Composition of α-tocopheryl acetate with micellar nanocarriers and the possibility of its use as a biologically active additive

Based on the asymmetric diblock copolymer (DBC) poly(ethylene oxide)/polyacrylic acid, effective, biocompatible and biodegradable micellar carriers were obtained for the delivery of vitamin E analogue, α-tocopheryl acetate (α-TOCA), in living organisms. The monitoring of the stability of micellar structures of the block copolymer and its composition with α-TOCA over time, in a saline solution and when the pH of the solution changes, was carried out. The stability of DBC micelles over time at pH = 3.5, partial disaggregation of micelles at pH = 9 and an increase in their aggregation in physiological solution were shown. The high stability of the α-TOCA/DBC composition formed in situ in time in the range of pH=3.5-9 and a significant decrease in its solutions of salting out effects in the presence of NaCl were established. The thermodynamic parameters of the process of the micelle formation of the pure α-TOCA in water/ethanol solution (95/5 v/v) as well as the size and morphology of its micellar structures were determined by light scattering and TEM methods. The initial α-TOCA micelles in water/ethanol solution were stable over a wide pH range, but their stability was much lower and the sensitivity to the presence of NaCl was much higher than that of DBC micelles. The dialysis method revealed the gradual release of the drug from the micellar carrier through a semipermeable membrane into the surrounding aqueous and aqueous-saline media. However, the rate and efficiency of α-TOCA release from the DBC micelles in an aqueous medium were significantly lower compared to a similar process of drug release from the pure α-TOCA dispersion. Thus, a possibility of providing of long-term controlled release of α-TOCA in the living organism due to the use of DBC micelles has been proven. Based on in vivo tests of the biological action of the composition on pregnant sows, its high bioavailability, rapid absorption, active participation in metabolic processes and positive effect on the reproductive qualities of sows compared to pure α-TOCA, were displayed, which improves the safety and productivity of newborn piglets. Key words: diblock copolymer, α-tocopheryl acetate, micellar carrier, encapsulation/release, biological action.

Sulfur dioxide signaling molecule-responsive polymeric nanoparticles

Stimuli-responsive polymer nano-capsules toward a specific signaling molecule show great potential in the fabrication of smart and efficient controlled/targeted drug vehicles. Herein, we design and synthesize a PEG45-b-PVPOP14 diblock copolymer (PEG = poly(ethylene glycol) and PVPOP = poly(4-vinylphenyl 4-oxopentanoate), the subscripts representing the number of repeat units of each block) with levulinate-protected phenol side groups. The PEG45-b-PVPOP14 diblock copolymer could self-assemble to form large compound micelles in aqueous media. Since the core of the large compound micelles formed contains both hydrophilic PEG and hydrophobic PVPOP domains, this kind of micelle is able to load both hydrophobic and hydrophilic species within the core. The ester moiety of levulinate-protected phenol can be selectively cleaved upon incubation with a sulfite, a derivative of SO2 in aqueous media, to give phenol groups. Thus, the sulfite exhibits the ability to alter the amphiphilicity and further the self-assembled behavior of PEG45-b-PVPOP14. The release of payloads in the core of micelles can be accelerated by triggering of the sulfite. Significantly, the nano-capsule of PEG45-b-PVPOP14 shows specific response to the sulfite (SO2) with slight interference of other bio-species, such as Cys, GSH and Hcy. As far as we are aware, this is the first example of a nano-capsule with sulfite (SO2) specific responsiveness. We envisage that this polymer model could broaden the scope of biological signaling molecule responsive macromolecular systems and provide a new platform to fabricate SO2-responsive biomedicine materials.

Polycations for Gene Delivery: Dilemmas and Solutions

Gene therapy has been a promising strategy for treating numerous gene-associated human diseases by altering specific gene expressions in pathological cells. Application of nonviral gene delivery is hindered by various dilemmas encountered in systemic gene therapy. Therefore, solutions must be established to address the unique requirements of gene-based treatment of diseases. This review will particularly highlight the dilemmas in polycation-based gene therapy by systemic treatment. Several promising strategies, which are expected to overcome these challenges, will be briefly reviewed. This review will also explore the development of polycation-based gene delivery systems for clinical applications.

Synthesis, self-assembly and drug release behaviors of reduction-labile multi-responsive block miktobrush quaterpolymers with linear and V-shaped grafts

Stimuli-tunable topological/morphological transitions and drug release properties based on novel disulfide-functionalized coil–comb–coil quaterpolymers were revealed.

Short-term urea cycle inhibition in rat liver cells induced by polyethylene glycol

We illuminate the biological effect of PEG on a specific cellular pathway: the urea cycle at a molecular level.