Solution Polymeric Optical Temperature Sensors with Long-Term Memory Function Powered by Supramolecular Chemistry

Smart and Biomimetic 3D and 4D Printed Composite Hydrogels: Opportunities for Different Biomedical Applications

In recent years, smart/stimuli-responsive hydrogels have drawn tremendous attention for their varied applications, mainly in the biomedical field. These hydrogels are derived from different natural and synthetic polymers but are also composite with various organic and nano-organic fillers. The basic functions of smart hydrogels rely on their ability to change behavior; functions include mechanical, swelling, shaping, hydrophilicity, and bioactivity in response to external stimuli such as temperature, pH, magnetic field, electromagnetic radiation, and biological molecules. Depending on the final applications, smart hydrogels can be processed in different geometries and modalities to meet the complicated situations in biological media, namely, injectable hydrogels (following the sol-gel transition), colloidal nano and microgels, and three dimensional (3D) printed gel constructs. In recent decades smart hydrogels have opened a new horizon for scientists to fabricate biomimetic customized biomaterials for tissue engineering, cancer therapy, wound dressing, soft robotic actuators, and controlled release of bioactive substances/drugs. Remarkably, 4D bioprinting, a newly emerged technology/concept, aims to rationally design 3D patterned biological matrices from synthesized hydrogel-based inks with the ability to change structure under stimuli. This technology has enlarged the applicability of engineered smart hydrogels and hydrogel composites in biomedical fields. This paper aims to review stimuli-responsive hydrogels according to the kinds of external changes and t recent applications in biomedical and 4D bioprinting.

Hydrogels as Emerging Materials for Cornea Wound Healing

Hydrogel biomaterials have many favorable characteristics including tuneable mechanical behavior, cytocompatibility, optical properties suitable for regeneration and restoration of the damaged cornea tissue. The cornea is a tissue susceptible to various injuries and traumas with a complicated healing cascade, in which conserving its transparency and integrity is critical. Accordingly, the hydrogels' known properties along with the stimulation of nerve and cell regeneration make them ideal scaffold for corneal tissue engineering. Hydrogels have been used extensively in clinical applications for the repair and replacement of diseased organs. The development and optimizing of novel hydrogels to repair/replace corneal injuries have been the main focus of researches within the last decade. This research aims to critically review in vitro, preclinical, as well as clinical trial studies related to corneal wound healing using hydrogels in the past 10 years, as this is considered as an emerging technology for corneal treatment. Several unique modifications of hydrogels with smart behaviors have undergone early phase clinical trials and showed promising outcomes. Financially, this considers a multibillion dollars industry and with huge interest from medical devices as well as pharmaceutical industries with several products may emerge within the next five years.

Effect of Host-Guest Complexation on the Thermoresponsive Behavior of Poly(oligo ethylene glycol acrylate)s Functionalized with Dialkoxynapththalene Guest Side Chains

The combination of thermoresponsive polymers with supramolecular host-guest interactions enables accurate tuning of the phase transition temperature, while also providing additional response mechanisms based on host-guest complexation. Most studies focused on a single thermoresponsive polymer to demonstrate the effect of host-guest complexation on the responsive behavior. In this work, the effect of the polymer structure on the host-guest complexation and thermoresponsive behavior is reported. Therefore, different poly(oligoethylene glycol acrylate)s, namely, poly(2-hydroxyethylacrylate) (PHEA), poly(methoxy diethylene glycol acrylate), poly(methoxy triethylene glycol acrylate), and poly(methoxy tetraethylene glycol acrylate), are synthesized functionalized with 1,5-dialkoxynaphthalene guest molecules in the side chain. Their complexation with the cyclobis(paraquat-p-phenylene) tetrachloride host is studied to understand the effect of polymer structure on the supramolecular association and the polymer phase transition, revealing that the oligoethylene glycol side chains lead to weaker host-guest complexation and also have a smaller increase in the cloud point temperature compared to PHEA.

Adamantane Functionalized Poly(2-oxazoline)s with Broadly Tunable LCST-Behavior by Molecular Recognition

Smart or adaptive materials often utilize stimuli-responsive polymers, which undergo a phase transition in response to a given stimulus. So far, various stimuli have been used to enable the modulation of drug release profiles, cell-interactive behavior, and optical and mechanical properties. In this respect, molecular recognition is a powerful tool to fine-tune the stimuli-responsive behavior due to its high specificity. Within this contribution, a poly(2-oxazoline) copolymer bearing adamantane side chains was synthesized via triazabicyclodecene-catalyzed amidation of the ester side chains of a poly(2-ethyl-2-oxazoline-stat-2-methoxycarbonylpropyl-2-oxazoline) statistical copolymer. Subsequent complexation of the pendant adamantane groups with sub-stoichiometric amounts (0-1 equivalents) of hydroxypropyl β-cyclodextrin or β-cyclodextrin enabled accurate tuning of its lower critical solution temperature (LCST) over an exceptionally wide temperature range, spanning from 30 °C to 56 °C. Furthermore, the sharp thermal transitions display minimal hysteresis, suggesting a reversible phase transition of the complexed polymer chains (i.e., the β-cyclodextrin host collapses together with the polymers) and a minimal influence by the temperature on the supramolecular association. Analysis of the association constant of the polymer with hydroxypropyl β-cyclodextrin via 1H NMR spectroscopy suggests that the selection of the macrocyclic host and rational polymer design can have a profound influence on the observed thermal transitions.

Smart Hydrogels - Synthetic Stimuli-Responsive Antitumor Drug Release Systems

Among modern drug formulations, stimuli-responsive hydrogels also called "smart hydrogels" deserve a special attention. The basic feature of this system is the ability to change their mechanical properties, swelling ability, hydrophilicity, bioactive molecules permeability, etc., influenced by various stimuli, such as temperature, pH, electromagnetic radiation, magnetic field and biological factors. Therefore, stimuli-responsive matrices can be potentially used in tissue engineering, cell cultures and technology of innovative drug delivery systems (DDSs), releasing the active substances under the control of internal or external stimuli. Moreover, smart hydrogels can be used as injectable DDSs, due to gel-sol transition connected with in situ cross-linking process. Innovative smart hydrogel DDSs can be utilized as matrices for targeted therapy, which enhances the effectiveness of tumor chemotherapy and subsequently limits systemic toxicity. External stimulus sensitivity allows remote control over the drug release profile and gel formation. On the other hand, internal factors provide drg accumulation in tumor tissue and reduce the concentration of active drug form in healthy tissue. In this report, we summarise the basic knowledge and chemical strategies for the synthetic smart hydrogel DDSs applied in antitumor therapy.

Biobased thermosensitive polyrotaxanes constructed by polymerization of cyclodextrin-triterpenoid inclusion complexes

Three biobased thermosensitive polyrotaxanes with alternating multiblock structures have been constructed through polymerization of inclusion complexes in a convenient tandem way.

The Pathway to Intelligence: Using Stimuli-Responsive Materials as Building Blocks for Constructing Smart and Functional Systems

Systems that are intelligent have the ability to sense their surroundings, analyze, and respond accordingly. In nature, many biological systems are considered intelligent (e.g., humans, animals, and cells). For man-made systems, artificial intelligence is achieved by massively sophisticated electronic machines (e.g., computers and robots operated by advanced algorithms). On the other hand, freestanding materials (i.e., not tethered to a power supply) are usually passive and static. Hence, herein, the question is asked: can materials be fabricated so that they are intelligent? One promising approach is to use stimuli-responsive materials; these "smart" materials use the energy supplied by a stimulus available from the surrounding for performing a corresponding action. After decades of research, many interesting stimuli-responsive materials that can sense and perform smart functions have been developed. Classes of functions discussed include practical functions (e.g., targeting and motion), regulatory functions (e.g., self-regulation and amplification), and analytical processing functions (e.g., memory and computing). The pathway toward creating truly intelligent materials can involve incorporating a combination of these different types of functions into a single integrated system by using stimuli-responsive materials as the basic building blocks.

Thermoadaptive Supramolecular α-Cyclodextrin Crystallization-Based Hydrogels via Double Hydrophilic Block Copolymer Templating

Supramolecular hydrogels play a prominent role in contemporary research of hydrophilic polymers. Especially, hydrogels based on α-cyclodextrin/poly(ethylene glycol) (α-CD/PEG) complexation and crystal formation are studied frequently. Here, the effect of double hydrophilic block copolymers (DHBCs) on α-CD/PEG hydrogel properties is investigated. Therefore, a novel DHBC, namely poly(N-vinylpyrrolidone)-b-poly(oligo ethylene glycol methacrylate) (PVP-b-POEGMA), was synthesized via a combination of reversible deactivation radical polymerization and modular conjugation methods. In the next step, hydrogel formation was studied after α-CD addition. Interestingly, DHBC-based hydrogels showed a significant response to thermal history. Heating of the gels to different temperatures led to different mechanical properties after cooling to ambient temperature, i.e., gels with mechanical properties similar to the initial gels or weak flowing gels were obtained. Thus, the hydrogels showed thermoadaptive behavior, which might be an interesting property for future applications in sensing.

Self-Templated Generation of Triggerable and Restorable Nonequilibrium Micelles

Conditional variations can lead to micellar transformations resulting in various (equilibrium) morphologies. However, creating differently shaped assemblies under the same final conditions (same ingredients, composition, temperature, etc.) is challenging. We present a thermoresponsive polyelectrolyte system allowing a pathway-dependent preparation of kinetically stable spherical star-like or cylindrical micelles. In more detail, a temperature-induced structure switch is used to generate equilibrated interpolyelectrolyte complex (IPEC) micelles of different morphologies (templates) below and above the lower critical solution temperature in the presence of plasticizer (salt). Then, lowering the salt concentration at a specific temperature kinetically freezes the formed IPECs, keeping the respective microstructural information encoded in the frozen IPEC also at other temperatures. Hence, different nonequilibrium morphologies at the same final conditions are provided. The salt-triggered transition from nonequilibrium to equilibrium micelles can be repeated for the same sample, highlighting a system with an on-demand changeable and restorable structure.

Triple Noncovalent-Interaction-Containing Supramolecular Polymer Vesicle Chemosensors with Dynamically Tunable Detection Ranges

Chemosensors (CSs) with dynamically tunable detection ranges have important significance for their expansion in practical applications; however, most CSs possess an unchangeable detection limit. In this work, we report the first example of a supramolecular polymer vesicle (SPV) chemosensor with a dynamically tunable detection range. SPVs containing porphyrin (PP) moieties and β-cyclodextrin (β-CD)/azobenzene (Azo) host-guest interactions were first constructed. The obtained SPVs were used to detect Zn²⁺ with a high selectivity and sensitivity over a wide detection limit range of 8.67×10^-9 to 1.99×10^-11 under UV light irradiation. The corresponding sensing mechanism was attributed to the synergistic effects of the triple noncovalent interactions, including the metal-ligand coordination of PP/Zn²⁺ and the double host-guest interactions of β-CD/Azo and β-CD/PP.

Thermoresponsive poly(2-oxazoline)s, polypeptoids, and polypeptides

Recent advances in thermoresponsive poly(2-oxazoline)s, polypeptoids, and polypeptides, with a specific focus on structure–property relationships, self-assembly, and applications, are reviewed.

Multiresponsive Behavior of Functional Poly(p-phenylene vinylene)s in Water

The multiresponsive behavior of functionalized water-soluble conjugated polymers (CPs) is presented with potential applications for sensors. In this study, we investigated the aqueous solubility behavior of water-soluble CPs with high photoluminescence and with a particular focus on their pH and temperature responsiveness. For this purpose, two poly(phenylene vinylene)s (PPVs)-namely 2,5-substituted PPVs bearing both carboxylic acid and methoxyoligoethylene glycol units-were investigated, with different amount of carboxylic acid units. Changes in the pH and temperature of polymer solutions led to a response in the fluorescence intensity in a pH range from 3 to 10 and for temperatures ranging from 10 to 85 °C. Additionally, it is demonstrated that the polymer with the largest number of carboxylic acid groups displays upper critical solution temperature (UCST)-like thermoresponsive behavior in the presence of a divalent ion like Ca2+. The sensing capability of these water-soluble PPVs could be utilized to design smart materials with multiresponsive behavior in biomedicine and soft materials.

Synthesis of novel boronic acid-decorated poly(2-oxazoline)s showing triple-stimuli responsive behavior

The synthesis of novel boronic-acid decorated poly(2-oxazoline)s showing a glucose- and pH dependent thermal transition is reported.

Enzyme and Thermal Dual Responsive Amphiphilic Polymer Core-Shell Nanoparticle for Doxorubicin Delivery to Cancer Cells

Dual responsive polymer nanoscaffolds for administering anticancer drugs both at the tumor site and intracellular compartments are made for improving treatment in cancers. The present work reports the design and development of new thermo- and enzyme-responsive amphiphilic copolymer core-shell nanoparticles for doxorubicin delivery at extracellular and intracellular compartments, respectively. A hydrophobic acrylate monomer was tailor-made from 3-pentadecylphenol (PDP, a natural resource) and copolymerized with oligoethylene glycol acrylate (as a hydrophilic monomer) to make new classes of thermo and enzyme dual responsive polymeric amphiphiles. Both radical and reversible addition-fragmentation chain transfer (RAFT) methodologies were adapted for making the amphiphilic copolymers. These amphiphilic copolymers were self-assembled to produce spherical core-shell nanoparticles in water. Upon heating, the core-shell nanoparticles underwent segregation to produce larger sized aggregates above the lower critical solution temperature (LCST). The dual responsive polymer scaffold was found to be capable of loading water insoluble drug, such as doxorubicin (DOX), and fluorescent probe-like Nile Red. The drug release kinetics revealed that DOX was preserved in the core-shell assemblies at normal body temperature (below LCST, ≤ 37 °C). At closer to cancer tissue temperature (above LCST, ∼43 °C), the polymeric scaffold underwent burst release to deliver 90% of loaded drugs within 2 h. At the intracellular environment (pH 7.4, 37 °C) in the presence of esterase enzyme, the amphiphilic copolymer ruptured in a slow and controlled manner to release >95% of the drugs in 12 h. Thus, both burst release of cargo at the tumor microenvironment and control delivery at intracellular compartments were accomplished in a single polymer scaffold. Cytotoxicity assays of the nascent and DOX-loaded polymer were carried out in breast cancer (MCF-7) and cervical cancer (HeLa) cells. Among the two cell lines, the DOX-loaded polymers showed enhanced killing in breast cancer cells. Furthermore, the cellular uptake of the DOX was studied by confocal and fluorescence microscopes. The present investigation opens a new enzyme and thermal-responsive polymer scaffold approach for DOX delivery in cancer cells.

Tuning temperature responsive poly(2-alkyl-2-oxazoline)s by supramolecular host-guest interactions

A poly[(2-ethyl-2-oxazoline)-ran-(2-nonyl-2-oxazoline)] random copolymer was synthesized and its thermoresponsive behavior in aqueous solution modulated by the addition of different supramolecular host molecules. The macrocycles formed inclusion complexes with the nonyl aliphatic side-chains present in the copolymer, increasing its cloud point temperature. The extent of this temperature shift was found to depend on the cavitand concentration and on the strength of the host-guest complexation. The cloud point temperature could be tuned in an unprecedented wide range of 30 K by supramolecular interactions. Since the temperature-induced breakage of the inclusion complexes constitutes the driving force for the copolymer phase transition, the shift in cloud point temperature could be utilized to estimate the association constant of the nonyl side chains with the cavitands.

Thermoresponsive interplay of water insoluble poly(2-alkyl-2-oxazoline)s composition and supramolecular host-guest interactions

A series of water insoluble poly[(2-ethyl-2-oxazoline)-ran-(2-nonyl-2-oxazoline)] amphiphilic copolymers was synthesized and their solubility properties in the presence of different supramolecular host molecules were investigated. The resulting polymer-cavitand assemblies exhibited a thermoresponsive behavior that could be modulated by variation of the copolymer composition and length. Interestingly, the large number of hydrophobic nonyl units across the polymer chain induced the formation of kinetically-trapped nanoparticles in solution. These nanoparticles further agglomerate into larger aggregates at a temperature that is dependent on the polymer composition and the cavitand type and concentration. The present research expands the understanding on the supramolecular interactions between water insoluble copolymers and supramolecular host molecules.