Emerging delivery systems based on aqueous two-phase systems: A review

The aqueous two-phase system (ATPS) is an all-aqueous system fabricated from two immiscible aqueous phases. It is spontaneously assembled through physical liquid-liquid phase separation (LLPS) and can create suitable templates like the multicompartment of the intracellular environment. Delicate structures containing multiple compartments make it possible to endow materials with advanced functions. Due to the properties of ATPSs, ATPS-based drug delivery systems exhibit excellent biocompatibility, extraordinary loading efficiency, and intelligently controlled content release, which are particularly advantageous for delivering drugs in vivo . Therefore, we will systematically review and evaluate ATPSs as an ideal drug delivery system. Based on the basic mechanisms and influencing factors in forming ATPSs, the transformation of ATPSs into valuable biomaterials is described. Afterward, we concentrate on the most recent cutting-edge research on ATPS-based delivery systems. Finally, the potential for further collaborations between ATPS-based drug-carrying biomaterials and disease diagnosis and treatment is also explored.

Self-Assembly of Platelet Lysates Proteins into Microparticles by Unnatural Disulfide Bonds for Bottom-Up Tissue Engineering

There is a demand to design microparticles holding surface topography while presenting inherent bioactive cues for applications in the biomedical and biotechnological fields. Using the pool of proteins present in human-derived platelet lysates (PLs), the production of protein-based microparticles via a simple and cost-effective method is reported, exploring the prone redox behavior of cysteine (Cy-SH) amino acid residues. The forced formation of new intermolecular disulfide bonds results in the precipitation of the proteins as spherical, pompom-like microparticles with adjustable sizes (15-50 µm in diameter) and surface topography consisting of grooves and ridges. These PL microparticles exhibit extraordinary cytocompatibility, allowing cell-guided microaggregates to form, while also working as injectable systems for cell support. Early studies also suggest that the surface topography provided by these PL microparticles can support osteogenic behavior. Consequently, these PL microparticles may find use to create live tissues via bottom-up procedures or injectable tissue-defect fillers, particularly for bone regeneration, with the prospect of working under xeno-free conditions.

Recent progress in the synthesis of all-aqueous two-phase droplets using microfluidic approaches

An aqueous two-phase system (ATPS) is a system with liquid-liquid phase separation and shows great potential for the extraction, separation, purification, and enrichment of proteins, membranes, viruses, enzymes, nucleic acids, and other biomolecules because of its simplicity, biocompatibility, and wide applicability [1-4]. The clear aqueous-aqueous interface of ATPSs is highly advantageous for their implementation, therefore making ATPSs a green alternative approach to replace conventional emulsion systems, such as water-in-oil droplets. All aqueous emulsions (water-in-water, w-in-w) hold great promise in the biomedical field as glucose sensors [5] and promising carriers for the encapsulation and release of various biomolecules and nonbiomolecules [6-10]. However, the ultralow interfacial tension between the two phases is a hurdle in generating w-in-w emulsion droplets. In the past, bulk emulsification and electrospray techniques were employed for the generation of w-in-w emulsion droplets and the fabrication of microparticles and microcapsules in the later stage. Bulk emulsification is a simple and low-cost technique; however, it generates polydisperse w-in-w emulsion droplets. Another technique, electrospray, involves easy experimental setups that can generate monodisperse but nonspherical w-in-w emulsion droplets. In comparison, microfluidic platforms provide monodisperse w-in-w emulsion droplets with spherical shapes, deal with the small volumes of solutions and short reaction times and achieve portability and versatility in their design through rapid prototyping. Owing to several advantages, microfluidic approaches have recently been introduced. To date, several different strategies have been explored to generate w-in-w emulsions and multiple w-in-w emulsions and to fabricate microparticles and microcapsules using conventional microfluidic devices. Although a few review articles on ATPSs emulsions have been published in the past, to date, few reviews have exclusively focused on the evolution of microfluidic-based ATPS droplets. The present review begins with a brief discussion of the history of ATPSs and their fundamentals, which is followed by an account chronicling the integration of microfluidic devices with ATPSs to generate w-in-w emulsion droplets. Furthermore, the stabilization strategies of w-in-w emulsion droplets and microfluidic fabrication of microparticles and microcapsules for modern applications, such as biomolecule encapsulation and spheroid construction, are discussed in detail in this review. We believe that the present review will provide useful information to not only new entrants in the microfluidic community wanting to appreciate the findings of the field but also existing researchers wanting to keep themselves updated on progress in the field.

Progress in all-aqueous droplets generation with microfluidics: Mechanisms of formation and stability improvements

All-aqueous systems have attracted intensive attention as a promising platform for applications in cell separation, protein partitioning, and DNA extraction, due to their selective separation capability, rapid mass transfer, and good biocompatibility. Reliable generation of all-aqueous droplets with accurate control over their size and size distribution is vital to meet the increasingly growing demands in emulsion-based applications. However, the ultra-low interfacial tension and large effective interfacial thickness of the water-water interface pose challenges for the generation and stabilization of uniform all-aqueous droplets, respectively. Microfluidics technology has emerged as a versatile platform for the precision generation of all-aqueous droplets with improved stability. This review aims to systematize the controllable generation of all-aqueous droplets and summarize various strategies to improve their stability with microfluidics. We first provide a comprehensive review on the recent progress of all-aqueous droplets generation with microfluidics by detailing the properties of all-aqueous systems, mechanisms of droplet formation, active and passive methods for droplet generation, and the property of droplets. We then review the various strategies used to improve the stability of all-aqueous droplets and discuss the fabrication of biomaterials using all-aqueous droplets as liquid templates. We envision that this review will benefit the future development of all-aqueous droplet generation and its applications in developing biomaterials, which will be useful for researchers working in the field of all-aqueous systems and those who are new and interested in the field.

Cell-mediated targeting drugs delivery systems

Drug delivery systems have shown tremendous promise to improve the diagnostic and therapeutic effects of drugs due to their special property. Targeting tissue damage, tumors, or drugs with limited toxicity at the site of infection is the goal of successful pharmaceuticals. Targeted drug delivery has become significantly important in enhancing the pharmaceutical effects of drugs and reducing their side effects of therapeutics in the treatment of various disease conditions. Unfortunately, clinical translation of these targeted drug delivery system mechanisms faces many challenges. At present, only a few targeted drug delivery systems can achieve high targeting efficiency after intravenous injection, even though numerous surface markers and targeting approaches have been developed. Thus, cell-mediated drug-delivery targeting systems have received considerable attention for their enhanced therapeutic specificity and efficacy in the treatment of the disease. This review highlights the recent advances in the design of the different types of cells that have been explored for cell-mediated drug delivery and targeting mechanisms. A better understanding of cell biology orientation and a new generation of delivery strategies that utilize these endogenous approaches are expected to provide better solutions for specific site delivery and further facilitate clinical translation.

Protein-Based Hybrid Responsive Microparticles for Wound Healing

The in-depth development of biological materials, especially natural polymer materials, has injected strong vitality into clinical wound treatment. Here, a new type of controllable responsive microparticles composed of several natural polymer materials was presented for drug release and wound healing. These hybrid microparticles consisted of silk fibroin, gelatin, agarose, and black phosphorus quantum dots (BPQDs) and were loaded with growth factors and antibacterial peptides. Under near-infrared (NIR) irradiation, BPQDs could absorb the NIR light and increase the temperature of the microparticles to the melting point of gelatin. When the gelatin started to melt, the encapsulated drugs were gradually released because of the reversible phase transformation. Both in vitro and in vivo experiments have demonstrated that the BPQD-laden microparticles with a NIR-responsive feature could achieve the desired controllable release of growth factors to promote neovascularization formation. In addition, because antibacterial peptides were also mixed with the secondary hydrogel and encapsulated in the scaffolds, the microparticles are imparted with the antibacterial ability during storage and usage. These characteristics of BPQD-laden natural protein hybrid microparticles make them ideal for drug delivery and wound healing.

Self-Assembled chitosan/phospholipid nanoparticles: from fundamentals to preparation for advanced drug delivery

With the development of nanotechnology, self-assembled chitosan/phospholipid nanoparticles (SACPNs) show great promise in a broad range of applications, including therapy, diagnosis, in suit imaging and on-demand drug delivery. Here, a brief review of the SACPNs is presented, and its critical underlying formation mechanisms are interpreted with an emphasis on the intrinsic physicochemical properties. The state-of-art preparation methods of SACPNs are summarized, with particular descriptions about the classic solvent injection method. Then SACPNs microstructures are characterized, revealing the unique spherical core-shell structure and the drug release mechanisms. Afterwards, a comprehensive and in-depth depiction of their emerging applications, with special attention to drug delivery areas, are categorized and reviewed. Finally, conclusions and outlooks on further advancing the SACPNs toward a more powerful and versatile platform for investigations covering from fundamental understanding to developing multi-functional drug delivery systems are discussed.

Microfluidic-mediated nano-drug delivery systems: from fundamentals to fabrication for advanced therapeutic applications

Nano-drug delivery systems (NDDS) are functional drug-loaded nanocarriers extensively applied in the healthcare and pharmaceutical areas. Recently, microfluidics has been demonstrated as one of the most promising techniques to fabricate high-performance NDDS with uniform morphology, size and size distribution, reduced batch-to-batch variations and controllable drug delivering capacity. Here, a brief review of the microfluidic-mediated NDDS is presented. The fundamentals of microfluidics are first interpreted with an emphasis on the fluid characteristics, design and materials for microfluidic devices. Then a comprehensive and in-depth depiction of the microfluidic-mediated fabrications of controllable NDDS with well-tailored internal structures and integrated functions for controlled encapsulation and drug release are categorized and reviewed, with particular descriptions about the underlying formation mechanisms. Afterwards, recently appreciated representative applications of the microfluidic-mediated NDDS for delivering multiple drugs are systematically summarized. Finally, conclusions and perspectives on further advancing the microfluidic-mediated NDDS toward more powerful and versatile platforms for therapeutic applications are discussed.

Flower-like droplets obtained by self-emulsification of a phase-separating (SEPS) aqueous film

Self-emulsification, referring to the spontaneous formation of droplets of one phase in another immiscible phase, is attracting growing interest because of its simplicity in creating droplets. Existing self-emulsification methods usually rely on phase inversion, temperature cycling, and solvent evaporation. However, achieving spatiotemporal control over the morphology of self-emulsified droplets remains challenging. In this work, a conceptually new approach of creating both simple and complex droplets by self-emulsification of a phase-separating (SEPS) aqueous film, is reported. The aqueous film is formed by depositing a surfactant-laden aqueous droplet onto an aqueous surface, and the fragmentation of the film into droplets is triggered by a wetting transition. Smaller and more uniform droplets can be achieved by introducing liquid-liquid phase separation (LLPS). Moreover, properly modulating quadruple LLPS and film fragmentation enables the creation of highly multicellular droplets such as flower-like droplets stabilized by the interfacial self-assembly of nanoparticles. This work provides a novel strategy to design aqueous droplets by LLPS, and it will inspire a wide range of applications such as membraneless organelle synthesis, cell mimics and delivery.

Bioinspired Helical Micromotors as Dynamic Cell Microcarriers

Micromotors have exhibited great potential in multidisciplinary nanotechnology, environmental science, and especially biomedical engineering due to their advantages of controllable motion, long lifetime, and high biocompatibility. Marvelous efforts focusing on endowing micromotors with novel characteristics and functionalities to promote their applications in biomedical engineering have been taken in recent years. Here, inspired by the flagellar motion of Escherichia coli, we present helical micromotors as dynamic cell microcarriers using simple microfluidic spinning technology. The morphologies of micromotors can be easily tailored because of the highly controllable and feasible fabrication process including microfluidic generation and manual dicing. Benefiting from the biocompatibility of the materials, the resultant helical micromotors could be ideal cell microcarriers that are suitable for cell seeding and further cultivation; the magnetic nanoparticle encapsulation imparts the helical micromotors with kinetic characteristics in response to mobile magnetic fields. Thus, the helical micromotors could be applied as dynamic cell culture blocks and further assembled to complex geometrical structures. The constructed structures out of cell-seeded micromotors could find practical potential in biomedical applications as the stack-shaped assembly embedded in the hydrogel may be used for tissue repairing and the tube-shaped assembly due to its resemblance to vascular structures in the microchannel for organ-on-a-chip study or blood vessel regeneration. These features manifest the possibility to broaden the biomedical application scope for micromotors.

Emerging aqueous two-phase systems: from fundamentals of interfaces to biomedical applications

This review summarizes recent advances of aqueous two-phase systems (ATPSs), particularly their interfaces, with a focus on biomedical applications.

Thermodynamic perspectives on liquid–liquid droplet reactors for biochemical applications

Liquid–liquid droplet reactors have garnered significant interest in biochemical applications by simulating thermodynamic systmes, ranging from closed systems, semi-closed/semi-open systems, to open systems.

A Microfluidic System for One-Chip Harvesting of Single-Cell-Laden Hydrogels in Culture Medium

Single-cell analysis has shown great potential to fully quantify the distribution of cellular behaviors among a population of individuals. Through isolation and preservation of single cells in the aqueous phase, droplet encapsulation followed by gelation enables high-throughput analysis in biocompatible microgels. However, the lack of control over the number of cells encapsulated and complicated gelation processes significantly limit its efficiency. Here, a microfluidic system for one-chip harvesting of single-cell-laden microgels is presented. Through ultraviolet irradiation, an on-chip gelation technique is seamlessly combined with droplet generation to realize high-throughput fabrication of microscale hydrogels in microfluidic channel. Moreover, a sorting module is introduced to simultaneously complete cell-laden microgel selection and transfer into culture medium. To demonstrate the efficiency of this method, two types of single cells are respectively encapsulated and collected, showing desirable single-cell encapsulation and cell viability. This technique realizes integrated droplet gelation, microgel sorting, and transfer into culture medium, allowing high-throughput analysis of single cells and comprehensive understanding of the cellular specificity.