pH-Responsive Coacervate Droplets Formed from Acid-Labile Methylated Polyrotaxanes as an Injectable Protein Carrier

Coacervate Droplets as Biomimetic Models for Designing Cell-Like Microreactors

Coacervates are versatile compartments formed by liquid-liquid phase separation. Their dynamic behavior and molecularly crowded microenvironment make them ideal materials for creating cell-like systems such as synthetic cells and microreactors. Recently, combinations of synthetic and natural molecules have been exploited via simple or complex coacervation to create compartments that can be used to build hierarchical chemical systems with life-like properties. This review highlights recent advances in the design of coacervate compartments and their application as biomimetic compartments for the design of cell-like chemical reactors and cell mimicking systems. It first explores the variety of materials used for coacervation and the influence of their chemical structure on their controlled dynamic behavior. Then, the applications of coacervates as cell-like systems are reviewed, focusing on how they can be used as cell-like microreactors through their ability to sequester molecules and provide a distinct and regulatory microenvironment for chemical reactions in aqueous media.

Plasmonic Coacervate as a Droplet-Based SERS Platform for Rapid Enrichment and Microanalysis of Hydrophobic Payloads

A novel and simple coacervate microdroplet-based detection platform for the quantification of trace hydrophobic analytes is presented. Herein, taking advantage of the effective encapsulation and enrichment performance of the condensed coacervates, plasmonic metallic silver nanoparticles (AgNPs) and target hydrophobic analytes are simultaneously concentrated into a single microdroplet. The coencapsulation of AgNPs within coacervates promotes the formation of aggregates with a lot of "hot spots" for surface-enhanced Raman scattering (SERS) enhancement, facilitating the sensitive analysis of hydrophobic analytes by SERS technology. Such plasmonic coacervates are easily prepared and exhibit good reproducibility and signal uniformity. Optimized SERS performance by modulating the volume of encapsulated AgNPs enables quantitative determination of hydrophobic analytes of Nile Red, chlorpyrifos, benzo[e]pyrene, 20 and 50 nm polystyrene nanoplastics with low detection limits of 10-12 M, 10-9 M, 10-10 M, 0.05 ppb, and 0.5 ppb, and an approximately linear correlation between SERS signals and the analytical concentrations. This study opens a new convenient SERS platform for the ultrasensitive detection of hydrophobic hazardous substances, potentially becoming a rapid analysis method for extensive applications ranging from food safety to environment monitoring.

Unraveling mechanisms of protein encapsulation and release in coacervates via molecular dynamics and machine learning

Coacervates play a pivotal role in protein-based drug delivery research, yet their drug encapsulation and release mechanisms remain poorly understood. Here, we utilized the Martini model to investigate bovine serum albumin (BSA) protein encapsulation and release within polylysine/polyglutamate (PLys/PGlu) coacervates. Our findings emphasize the importance of ingredient addition sequence in coacervate formation and encapsulation rates, attributed to preference contact between oppositely charged proteins and poly(amino acid)s. Notably, coacervates composed of β-sheet poly(amino acid)s demonstrate greater BSA encapsulation efficiency due to their reduced entropy and flexibility. Furthermore, we examined the pH responsiveness of coacervates, shedding light on the dissolution process driven by Coulomb forces. By leveraging machine learning algorithms to analyze simulation results, our research advances the understanding of coacervate-based drug delivery systems, with the ultimate goal of optimizing therapeutic outcomes.

Cyclodextrins-Based Polyrotaxanes: From Functional Polymers to Applications in Electronics and Energy Storage Materials

The concept of polyrotaxane comes from the rotaxane structure in the supramolecular field. It is a mechanically interlocked supramolecular assembly composed of linear polymer chains and cyclic molecules. Over recent decades, the synthesis and application of polyrotaxanes have seen remarkable growth. Particularly, cyclodextrin-based polyrotaxanes have been extensively reported due to the low-price raw materials, good biocompatibility, and ease of modification. Hence, it is also one of the most promising mechanically interlocking supramolecules for wide industrialization in the future. Polyrotaxanes are widely introduced into materials such as elastomers, hydrogels, and engineering polymers to improve their mechanical properties or impart functionality to the materials. In these materials, polyrotaxane acts as a slidable cross-linker to dissipate energy through sliding or assist in dispersing stress concentration in the cross-linked network, thereby enhancing the toughness of the materials. Further, the unique sliding-ring effect of cyclodextrin-based polyrotaxanes has pioneered advancements in stretchable electronics and energy storage materials. This includes their innovative use in stretchable conductive composite and binders for anodes, addressing critical challenges in these fields. In this mini-review, our focus is to highlight the current progress and potential wider applications in the future, underlining their transformative impact across various domains of material science.

Regulation of enzymatic reactions by chemical composition of peptide biomolecular condensates

Biomolecular condensates are condensed intracellular phases that are formed by liquid-liquid phase separation (LLPS) of proteins, either in the absence or presence of nucleic acids. These condensed phases regulate various biochemical reactions by recruitment of enzymes and substrates. Developments in the field of LLPS facilitated new insights on the regulation of compartmentalized enzymatic reactions. Yet, the influence of condensate chemical composition on enzymatic reactions is still poorly understood. Here, by using peptides as minimalistic condensate building blocks and β-galactosidase as a simple enzymatic model we show that the reaction is restricted in homotypic peptide condensates, while product formation is enhanced in peptide-RNA condensates. Our findings also show that condensate composition affects the recruitment of substrate, the spatial distribution, and the kinetics of the reaction. Thus, these findings can be further employed for the development of microreactors for biotechnological applications.

Cyclodextrins and derivatives in drug delivery: New developments, relevant clinical trials, and advanced products

Cyclodextrins (CD) and derivatives are functional excipients that can improve the bioavailability of numerous drugs. Because of their drug solubility improving properties they are used in many pharmaceutical products. Furthermore, the stability of small molecular drugs can be improved by the incorporation in CDs and an unpleasant taste and smell can be masked. In addition to well-established CD derivatives including hydroxypropyl-β-CD, hydroxypropyl-γ-CD, methylated- β-CD and sulfobutylated- β-CD, there are promising new derivatives in development. In particular, CD-based polyrotaxanes exhibiting cellular uptake enhancing properties, CD-polymer conjugates providing sustained drug release, enhanced cellular uptake, and mucoadhesive properties, and thiolated CDs showing mucoadhesive, in situ gelling, as well as permeation and cellular uptake enhancing properties will likely result in innovative new drug delivery systems. Relevant clinical trials showed various new applications of CDs such as the formation of CD-based nanoparticles, stabilizing properties for protein drugs or the development of ready-to-use injection systems. Advanced products are making use of various benefical properties of CDs at the same time. Within this review we provide an overview on these recent developments and take an outlook on how this class of excipients will further shape the landscape of drug delivery.

Supramolecular Surface Coatings with Acetylated Polyrotaxane-Based Triblock Copolymers for Thermal Regulation of Cell Adhesion and Fabrication of Cell Sheets

Polyrotaxanes (PRXs) containing acetylated α-cyclodextrins exhibit a temperature-dependent phase transition in aqueous solutions across their lower critical solution temperature (LCST) of approximately 26.6 °C. To gain insights into the interactions of acetylated PRXs (Ac-PRXs) with biological components, thermoresponsive supramolecular surfaces were prepared by coating tissue culture polystyrene (TCPS) surfaces with Ac-PRX triblock copolymers, and their surface properties across the LCST were evaluated. The wettability and protein adsorption of Ac-PRX-coated surfaces changed significantly between 10 and 37 °C, whereas the uncoated TCPS and unmodified PRX-coated surfaces did not alter the wettability and protein adsorption at 10 and 37 °C. The adhesion, proliferation, morphology, and adhesion strength of NIH/3T3 cells on Ac-PRX-coated surfaces were found to be similar to those of the uncoated and unmodified PRX-coated surfaces. However, the adhesion strength of NIH/3T3 cells on Ac-PRX-coated surfaces decreased drastically at 10 °C. Consequently, the cells spontaneously detached from the Ac-PRX-coated surfaces without enzymatic treatment. Additionally, when incubating confluent cells at 10 °C, the cells detached from Ac-PRX-coated surfaces as cell sheets while retaining extracellular matrix proteins. The findings of this study provide new directions for the design of thermoresponsive supramolecular biointerfaces for applications in bioseparation and cell manipulation.

An Overview of Coacervates: The Special Disperse State of Amphiphilic and Polymeric Materials in Solution

Individual amphiphiles, polymers, and colloidal dispersions influenced by temperature, pH, and environmental conditions or interactions between their oppositely charged pairs in solvent medium often produce solvent-rich and solvent-poor phases in the system. The solvent-poor denser phase found either on the top or the bottom of the system is called coacervate. Coacervates have immense applications in various technological fields. This review comprises a concise introduction, focusing on the types of coacervates, and the influence of different factors in their formation, structures, and stability. In addition, their physicochemical properties, thermodynamics of formation, and uses and multifarious applications are also concisely presented and discussed.

Temporal Stimulus Patterns Drive Differentiation of a Synthetic Dipeptide-Based Coacervate

The fate of living cells often depends on their processing of temporally modulated information, such as the frequency and duration of various signals. Synthetic stimulus-responsive systems have been intensely studied for >50 years, but it is still challenging for chemists to create artificial systems that can decode dynamically oscillating stimuli and alter the systems' properties/functions because of the lack of sophisticated reaction networks that are comparable with biological signal transduction. Here, we report morphological differentiation of synthetic dipeptide-based coacervates in response to temporally distinct patterns of the light pulse. We designed a simple cationic diphenylalanine peptide derivative to enable the formation of coacervates. The coacervates concentrated an anionic methacrylate monomer and a photoinitiator, which provided a unique reaction environment and facilitated light-triggered radical polymerization─even in air. Pulsed light irradiation at 9.0 Hz (but not at 0.5 Hz) afforded anionic polymers. This dependence on the light pulse patterns is attributable to the competition of reactive radical intermediates between the methacrylate monomer and molecular oxygen. The temporal pulse pattern-dependent polymer formation enabled the coacervates to differentiate in terms of morphology and internal viscosity, with an ultrasensitive switch-like mode. Our achievements will facilitate the rational design of smart supramolecular soft materials and are insightful regarding the synthesis of sophisticated chemical cells.

Roles of interfacial water states on advanced biomedical material design

When biomedical materials come into contact with body fluids, the first reaction that occurs on the material surface is hydration; proteins are then adsorbed and denatured on the hydrated material surface. The amount and degree of denaturation of adsorbed proteins affect subsequent cell behavior, including cell adhesion, migration, proliferation, and differentiation. Biomolecules are important for understanding the interactions and biological reactions of biomedical materials to elucidate the role of hydration in biomedical materials and their interaction partners. Analysis of the water states of hydrated materials is complicated and remains controversial; however, knowledge about interfacial water is useful for the design and development of advanced biomaterials. Herein, we summarize recent findings on the hydration of synthetic polymers, supramolecular materials, inorganic materials, proteins, and lipid membranes. Furthermore, we present recent advances in our understanding of the classification of interfacial water and advanced polymer biomaterials, based on the intermediate water concept.

Stimuli-responsive cyclodextrin-based supramolecular assemblies as drug carriers

Cyclodextrins (CDs) are widely employed in biomedical applications because of their unique structures. Various biomedical applications can be achieved in a spatiotemporally controlled manner by integrating the host-guest chemistry of CDs with stimuli-responsive functions. In this review, we summarize the recent advances in stimuli-responsive supramolecular assemblies based on the host-guest chemistry of CDs. The stimuli considered in this review include endogenous (pH, redox, and enzymes) and exogenous stimuli (light, temperature, and magnetic field). We mainly discuss the mechanisms of the stimuli-responsive ability and present typical designs of the corresponding supramolecular assemblies for drug delivery and other potential biomedical applications. The limitations and perspectives of CD-based stimuli-responsive supramolecular assemblies are discussed to further promote the translation of laboratory products into clinical applications.

Biocompatible poly(N-(ω-acryloyloxy-n-alkyl)-2-pyrrolidone)s with widely-tunable lower critical solution temperatures (LCSTs): a promising alternative to poly(N-isopropylacrylamide)

The biocompatible (co)polymers undergoes a thermal stimulus-driven liquid–liquid phase separation and form coacervates above the lower critical solution temperature (LCST). The LCSTs are able to be precisely controlled between 0 °C and 100 °C.

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.

Weakly acidic carboxy group-grafted β-cyclodextrin-threaded acid-degradable polyrotaxanes for modulating protein interaction and cellular internalization

To improve the therapeutic potential of β-cyclodextrin (β-CD)-threaded acid-degradable polyrotaxanes (β-CD PRXs) in cholesterol-related metabolic disorders, we investigated the effect of carboxylation of β-CD PRXs on intracellular uptake. In this study, we established a synthetic method for the modification of carboxylalkyl carbamates on β-CD PRXs without degradation and synthesized three series of carboxyalkyl carbamate group-modified β-CD PRXs with different alkyl spacer lengths. The modification of carboxymethyl carbamate (CMC), carboxyethyl carbamate (CEC), and carboxypropyl carbamate (CPC) on the β-CD PRXs slightly reduced the interaction of the PRXs with the lipid layer model compared with the modification of 2-(2-hydroxyethoxy)ethyl carbamate (HEE-PRX), which was used in our previous studies. However, all the carboxylated β-CD PRXs showed a significantly stronger interaction with a protein model compared with HEE-PRX. The carboxylated β-CD PRXs showed significantly high intracellular uptake, through macrophage scavenger receptor A (MSR-A)-mediated endocytosis, in MSR-A-positive RAW 264.7 cells compared with HEE-PRX. Interestingly, the carboxylated β-CD PRXs also showed significantly higher intracellular uptake even in MSR-A-negative cells compared with HEE-PRX. Carboxylated β-CD PRXs are considered to strongly interact with other membrane proteins, resulting in high intracellular uptake. The length of the alkyl spacer affected the intracellular uptake levels of carboxylated PRXs, however, this relationship was varied for different cell types. Furthermore, none of the carboxylated β-CD PRXs exhibited cytotoxicity in the RAW 264.7 and NIH/3T3 cells. Altogether, carboxylation of β-CD PRXs is a promising chemical modification approach for their therapeutic application because carboxylated β-CD PRXs exhibit high cellular internalization efficiency in MSR-A-negative cells and negligible toxicity.

Incorporation of proteins into complex coacervates

Complex coacervates have found a renewed interest in the past few decades in various fields such as food and personal care products, membraneless cellular compartments, the origin of life, and, most notably, as a mode of transport and stabilization of drugs. Here, we describe general methods for characterizing the phase behavior of complex coacervates and quantifying the incorporation of proteins into these phase separated materials.

Protein Encapsulation Using Complex Coacervates: What Nature Has to Teach Us

Protein encapsulation is a growing area of interest, particularly in the fields of food science and medicine. The sequestration of protein cargoes is achieved using a variety of methods, each with benefits and drawbacks. One of the most significant challenges associated with protein encapsulation is achieving high loading while maintaining protein viability. This difficulty is exacerbated because many encapsulant systems require the use of organic solvents. By contrast, nature has optimized strategies to compartmentalize and protect proteins inside the cell-a purely aqueous environment. Although the mechanisms whereby aspects of the cytosol is able to stabilize proteins are unknown, the crowded nature of many newly discovered, liquid phase separated "membraneless organelles" that achieve protein compartmentalization suggests that the material environment surrounding the protein may be critical in determining stability. Here, encapsulation strategies based on liquid-liquid phase separation, and complex coacervation in particular, which has many of the key features of the cytoplasm as a material, are reviewed. The literature on protein encapsulation via coacervation is also reviewed and the parameters relevant to creating protein-containing coacervate formulations are discussed. Additionally, potential opportunities associated with the creation of tailored materials to better facilitate protein encapsulation and stabilization are highlighted.

Emerging Nanoassembly of Polyrotaxanes Comprising Acetylated α-Cyclodextrins and High-Molecular-Weight Axle Polymer

Acetylated α-cyclodextrin (α-CD)/poly(ethylene glycol) (PEG)-based polyrotaxanes (Ac-PRXs) with varying degrees of acetylation (DA) and molecular weight of axle PEG were synthesized and their solubility in aqueous solutions was investigated. Ac-PRXs with low DA (less than 35%) were dissolved in aqueous solutions without considering the molecular weight of axle PEG, whereas Ac-PRXs with high DA (more than 40%) and low molecular weight of axle PEG (less than 35000) were precipitated into the solutions. Interestingly, Ac-PRXs with high DA and high molecular weight of axle PEG (100000) exhibited a colloidal dispersion in aqueous solutions. It is considered that the threaded acetylated α-CDs formed hydrophobic microenvironments via hydrophobic interactions and the noncovered segments of axle PEGs provided colloidal stability. Furthermore, the potential application of Ac-PRX_100k as a drug carrier was examined and it was established that Ac-PRX_100k can encapsulate a hydrophobic drug. Accordingly, acetylation of PRXs is a viable approach to promote solubility in aqueous solutions and prepare self-assembled nanoparticles.