Phase separation in binary polymer solution: Gelatin/Poly(ethylene glycol) system

Exploring the potential of all-aqueous immiscible systems for preparing complex biomaterials and cellular constructs

All-aqueous immiscible systems derived from liquid-liquid phase separation of incompatible hydrophilic agents such as polymers and salts have found increasing interest in the biomedical and tissue engineering fields in the last few years. The unique characteristics of aqueous interfaces, namely their low interfacial tension and elevated permeability, as well as the non-toxic environment and high water content of the immiscible phases, confer to these systems optimal qualities for the development of biomaterials such as hydrogels and soft membranes, as well as for the preparation of in vitro tissues derived from cellular assembly. Here, we overview the main properties of these systems and present a critical review of recent strategies that have been used for the development of biomaterials with increased levels of complexity using all-aqueous immiscible phases and interfaces, and their potential as cell-confining environments for micropatterning approaches and the bioengineering of cell-rich structures. Importantly, due to the relatively recent emergence of these areas, several key design considerations are presented, in order to guide researchers in the field. Finally, the main present challenges, future directions, and adaptability to develop advanced materials with increased biomimicry and new potential applications are briefly evaluated.

Fluid dynamics alters liquid-liquid phase separation in confined aqueous two-phase systems

Liquid-liquid phase separation is key to understanding aqueous two-phase systems (ATPS) arising throughout cell biology, medical science, and the pharmaceutical industry. Controlling the detailed morphology of phase-separating compound droplets leads to new technologies for efficient single-cell analysis, targeted drug delivery, and effective cell scaffolds for wound healing. We present a computational model of liquid-liquid phase separation relevant to recent laboratory experiments with gelatin-polyethylene glycol mixtures. We include buoyancy and surface-tension-driven finite viscosity fluid dynamics with thermally induced phase separation. We show that the fluid dynamics greatly alters the evolution and equilibria of the phase separation problem. Notably, buoyancy plays a critical role in driving the ATPS to energy-minimizing crescent-shaped morphologies, and shear flows can generate a tenfold speedup in particle formation. Neglecting fluid dynamics produces incorrect minimum-energy droplet shapes. The model allows for optimization of current manufacturing procedures for structured microparticles and improves understanding of ATPS evolution in confined and flowing settings important in biology and biotechnology.

Flexible Allyl-Modified Gelatin Photoclick Resin Tailored for Volumetric Bioprinting of Matrices for Soft Tissue Engineering

Volumetric bioprinting (VBP) is a light-based 3D printing platform, which recently prompted a paradigm shift for additive manufacturing (AM) techniques considering its capability to enable the fabrication of complex cell-laden geometries in tens of seconds with high spatiotemporal control and pattern accuracy. A flexible allyl-modified gelatin (gelAGE)-based photoclick resin is developed in this study to fabricate matrices with exceptionally soft polymer networks (0.2-1.0 kPa). The gelAGE-based resin formulations are designed to exploit the fast thiol-ene crosslinking in combination with a four-arm thiolated polyethylene glycol (PEG4SH) in the presence of a photoinitiator. The flexibility of the gelAGE biomaterial platform allows one to tailor its concentration spanning from 2.75% to 6% and to vary the allyl to thiol ratio without hampering the photocrosslinking efficiency. The thiol-ene crosslinking enables the production of viable cell-material constructs with a high throughput in tens of seconds. The suitability of the gelAGE-based resins is demonstrated by adipogenic differentiation of adipose-derived stromal cells (ASC) after VBP and by the printing of more fragile adipocytes as a proof-of-concept. Taken together, this study introduces a soft photoclick resin which paves the way for volumetric printing applications toward soft tissue engineering.

Frictional properties of phase-separated agarose hydrogels in water permeation

We studied the friction coefficient between the polymer gel network and water f for thermoreversible agarose gels under various conditions of agarose concentration and gelation temperature. Since agarose gels exhibit phase separation below the gelation temperature, f strongly depends on the thermal history. We found that the friction coefficient of the phase-separated agarose gel normalized by the water viscosity, f/η, is expressed as f/η = S/ξνSD where ξSD is the frictional pore size and ν and S are constant parameters. ξSD corresponds to the correlation length of the frozen density fluctuations of the polymers via spinodal decomposition determined from small-angle light scattering. The least-squares analysis of the results shows that the exponent is ν ≃ 2 with the numerical constant of S ≃ 105/2π. The results suggest that the frictional properties of phase-separated agarose gels are dominated by the dilute regions of the bicontinuous gel structure.

Spontaneous Formation of Uniform Cell-Sized Microgels through Water/Water Phase Separation

In this study, a one-step method is discussed for producing uniform cell-sized microgels using glass capillaries filled with a binary polymer blend of polyethylene glycol (PEG) and gelatin. Upon decreasing temperature, phase separation of the PEG/gelatin blends and gelation of gelatin occur, and then the polymer blend forms linearly aligned, uniformly sized gelatin microgels in the glass capillary. When DNA is added to the polymer solution, gelatin microgels entrapping DNA are spontaneously formed, and the DNA prevents the coalescence of the microdroplets even at temperatures above the melting point. This novel method to form uniform cell-sized microgels may be applicable to other biopolymers. This method is expected to contribute to diverse materials science via biopolymer microgels and biophysics and synthetic biology through cellular models containing biopolymer gels.

Poly(ethylene oxide)/Gelatin-Based Biphasic Photocrosslinkable Hydrogels of Tunable Morphology for Hepatic Progenitor Cell Encapsulation

Macroporous hydrogels have great potential for biomedical applications. Liquid or gel-like pores were created in a photopolymerizable hydrogel by forming water-in-water emulsions upon mixing aqueous solutions of gelatin and a poly(ethylene oxide) (PEO)-based triblock copolymer. The copolymer constituted the continuous matrix, which dominated the mechanical properties of the hydrogel once photopolymerized. The gelatin constituted the dispersed phase, which created macropores in the hydrogel. The microstructures of the porous hydrogel were determined by the volume fraction of the gelatin phase. When volume fractions were close to 50 v%, free-standing hydrogels with interpenetrated morphology can be obtained thanks to the addition of a small amount of xanthan. The hydrogels displayed Young's moduli ranging from 5 to 30 kPa. They have been found to be non-swellable and non-degradable in physiological conditions. Preliminary viability tests with hepatic progenitor cells embedded in monophasic PEO-based hydrogels showed rapid mortality of the cells, whereas encouraging viability was observed in PEO-based triblock copolymer/gelatin macroporous hydrogels. The latter has the potential to be used in cell therapy.

Cell-size space effects on phase separation of binary polymer blends

Within living cells, a diverse array of biomolecules is present at high concentrations. To better understand how molecular behavior differs under such conditions (collectively described as macromolecular crowding), the crowding environment has been reproduced inside artificial cells. We have previously shown that the combination of macromolecular crowding and microscale geometries imposed by the artificial cells can alter the molecular behaviors induced by macromolecular crowding in bulk solutions. We have named the effect that makes such a difference the cell-size space effect (CSE). Here, we review the underlying biophysics of CSE for phase separation of binary polymer blends. We discuss how the cell-size space can initiate phase separation, unlike nano-sized spaces, which are known to hinder nucleation and phase separation. Additionally, we discuss how the dimensions of the artificial cell and its membrane characteristics can significantly impact phase separation dynamics and equilibrium composition. Although these findings are, of themselves, very interesting, their real significance may lie in helping to clarify the functions of the cell membrane and space size in the regulation of intracellular phase separation.

Liquid-Liquid Phase-Separated Hydrogel with Tunable Sol-Gel Transition Behavior as a Hotmelt-Adhesive Postoperative Barrier

Postoperative barriers have been widely used to prevent adhesions. However, there are currently few barriers that satisfy clinical requirements, such as tissue adhesion, operability, and biocompatibility. Inspired by the adhesion system of living organisms, we report a liquid-liquid phase-separated hydrogel as a single-syringe hotmelt-type postoperative barrier. Mixing polyethylene glycol with gelatin formed liquid-liquid phase-separated hydrogels through segregative liquid-liquid phase separation. Incorporation of a liquid-liquid phase-separated system into gelatin can enhance the sol-gel transition temperature to give a hotmelt-adhesive property to hydrogels. Hotmelt-adhesive hydrogels became a sol phase and cohered into tissue gaps when warmed and solidified at body temperature to adhere to soft tissues. The hydrogels exhibited tissue adhesion to large intestine tissues and showed improved mechanical strength, gelation time, and shear-thinning properties. In rat cecum-abdominal adhesion models, it was confirmed that the resulting hydrogels prevented abdominal adhesion and did not prevent tissue regeneration. Hotmelt-adhesive hydrogels with high tissue adhesive properties, operability, and biocompatibility have enormous potential as barriers to prevent postoperative complications.

Scalable Fabrication and Use of 3D Structured Microparticles Spatially Functionalized with Biomolecules

Microparticles with defined shapes and spatial chemical modification can interface with cells and tissues at the cellular scale. However, conventional methods to fabricate shaped microparticles have trade-offs between the throughput of manufacture and the precision of particle shape and chemical functionalization. Here, we achieved scalable production of hydrogel microparticles at rates of greater than 40 million/hour with localized surface chemistry using a parallelized step emulsification device and temperature-induced phase-separation. The approach harnesses a polymerizable polyethylene glycol (PEG) and gelatin aqueous two-phase system (ATPS) which conditionally phase separates within microfluidically generated droplets. Following droplet formation, phase separation is induced and phase separated droplets are subsequently cross-linked to form uniform crescent and hollow shell particles with gelatin functionalization on the boundary of the cavity. The gelatin localization enabled deterministic cell loading in subnanoliter-sized crescent-shaped particles, which we refer to as nanovials, with cavity dimensions tuned to the size of cells. Loading on nanovials also imparted improved cell viability during analysis and sorting using standard fluorescence activated cell sorters, presumably by protecting cells from shear stress. This localization effect was further exploited to selectively functionalize capture antibodies to nanovial cavities enabling single-cell secretion assays with reduced cross-talk in a simplified format.

Cross-linker Free Sodium Alginate and Gelatin Hydrogel: Multiscale Biomaterial Design Framework

Surface functionalization and cross-linking have been adopted extensively by researchers to customize hydrogel properties, especially in the last decade. The clinical translation of such biomaterials is in a poor state...

Perpendicular alignment of the phase-separated boundary in adhered polymer droplets

We investigated the effect of the adhered interface on the phase separation pattern using two or three adhered droplets containing a binary solution of poly(ethylene glycol) and gelatin. Under the experimental conditions, single domains of the gelatin-rich phase exhibited partial wetting to the droplet adhered interface (DAI) and nonadhered droplet surface. In the case of isolated spherical droplets, the location of the phase separation interface (PSI) of the domains was completely random owing to spatial symmetry. In the adhered droplets, the random orientation of the PSI was observed when the PSI did not contact the DAI. On the other hand, when the PSI contacted the DAI, the PSI was aligned perpendicular to the DAI. Frequency analysis showed that whether the PSI contacts the DAI is purely stochastic. However, the PSI alignment perpendicular to the DAI increases significantly with three adhered droplets, suggesting that the probability increases with increasing DAI area ratio. We explain this perpendicular pattern by the minimization of the interfacial energy and kinetics with a change in the wetting contact angle. These findings will facilitate the research on the phase separation of polymer solutions inside nonspherical micrometric spaces.

Pre-shear bioprinting of highly oriented porous hydrogel microfibers to construct anisotropic tissues

Anisotropic tissues in vivo have special structural characteristics and biological functions. Nowadays, bioprinting is widely used in tissue engineering and an effective way to process cell-laden hydrogels. However, the direct bioprinting of oriented cell-laden hydrogel structures to engineer anisotropic tissues is still difficult. Meanwhile, the inherent dense micropore network after the gelation of hydrogel-based bioinks usually limits the normal growth of encapsulated cells due to the inadequate supply of nutrient/oxygen. Herein, we proposed a pre-shear bioprinting strategy of highly oriented porous hydrogel microfibers to construct anisotropic tissues. Firstly, based on the phase separation of viscous high-molecular compound mixtures, we utilized a general viscous porous bioink paradigm, e.g., mixing a polymer thickener (PEO) with a hydrogel precursor (GelMA) with excellent biological properties. Secondly, based on the shear-oriented property of the viscous porous bioink, we designed the pre-shear in situ coaxial bioprinting of highly oriented porous hydrogel microfibers. The viscous porous bioink (GelMA/PEO) was shear-oriented through an injection tube and pumped into the inner needle of a coaxial nozzle. When GelMA/PEO passed through a transparent glass tube connected to the coaxial nozzle, GelMA can be in situ photo-crosslinked to form highly oriented porous microfibers. In addition, we showed the manufacturing of heterogeneous oriented microfibers and the manual assembly of microfibers, and within oriented microfibers, different cells or co-cultured cells exhibited highly oriented growth behaviors similar to that in vivo. As far as we know, the direct bioprinting of anisotropic tissues through high orientation induced by pre-shearing is firstly reported in our study. We believe that the pre-shear bioprinting strategy of anisotropic tissues will open more avenues for further biomedical research.

Biological phase separation: cell biology meets biophysics

Progress in development of biophysical analytic approaches has recently crossed paths with macromolecule condensates in cells. These cell condensates, typically termed liquid-like droplets, are formed by liquid-liquid phase separation (LLPS). More and more cell biologists now recognize that many of the membrane-less organelles observed in cells are formed by LLPS caused by interactions between proteins and nucleic acids. However, the detailed biophysical processes within the cell that lead to these assemblies remain largely unexplored. In this review, we evaluate recent discoveries related to biological phase separation including stress granule formation, chromatin regulation, and processes in the origin and evolution of life. We also discuss the potential issues and technical advancements required to properly study biological phase separation.

Sol-Gel Coexisting Phase of Polymer Microgels Triggers Spontaneous Buckling

Mechanical buckling is a ubiquitous phenomenon of elastic bodies like core-shell microgels. Although conventional theory predicts that sufficiently high pressure is the primary factor inducing the buckling of core-shell microgels, they often buckle spontaneously without applying pressure. We explored such spontaneous buckling of microgels by introducing interfacial tension between the gel phase of the shell and sol phase of the core. Thus, we found that the core-shell microgels in a sol-gel coexisting phase with a certain shell thickness ratio exhibit spontaneous buckling. According to our theoretical analysis, spontaneous buckling occurs due to the balance between the gel elasticity E and interfacial tension γ when the characteristic length γ/ E is comparable to the microgel size R. Moreover, we found that the ratio between γ/ E and R determines the buckling condition of the shell thickness ratio. Our findings establish an important framework for applying spontaneous buckling to the shape control of elastic bodies.

Aqueous Two-Phase Emulsion Bioink-Enabled 3D Bioprinting of Porous Hydrogels

3D bioprinting technology provides programmable and customizable platforms to engineer cell-laden constructs mimicking human tissues for a wide range of biomedical applications. However, the encapsulated cells are often restricted in spreading and proliferation by dense biomaterial networks from gelation of bioinks. Herein, a cell-benign approach is reported to directly bioprint porous-structured hydrogel constructs by using an aqueous two-phase emulsion bioink. The bioink, which contains two immiscible aqueous phases of cell/gelatin methacryloyl (GelMA) mixture and poly(ethylene oxide) (PEO), is photocrosslinked to fabricate predesigned cell-laden hydrogel constructs by extrusion bioprinting or digital micromirror device-based stereolithographic bioprinting. The porous structure of the 3D-bioprinted hydrogel construct is formed by subsequently removing the PEO phase from the photocrosslinked GelMA hydrogel. Three different cell types (human hepatocellular carcinoma cells, human umbilical vein endothelial cells, and NIH/3T3 mouse embryonic fibroblasts) within the 3D-bioprinted porous hydrogel patterns show enhanced cell viability, spreading, and proliferation compared to the standard (i.e., nonporous) hydrogel constructs. The 3D bioprinting strategy is believed to provide a robust and versatile platform to engineer porous-structured tissue constructs and their models for a variety of applications in tissue engineering, regenerative medicine, drug development, and personalized therapeutics.

Preparation of Template-Free Robust Yolk-Shell Gelled Particles from Controllably Evolved All-in-Water Emulsions

A template-free all-aqueous bulk preparation of robust hollow capsules having a gelatin shell from all-in-water double emulsions is reported. The hot (>40 °C) quaternary system water/polyethylene glycol (PEG)/gelatin/alginate is shown to spontaneously form PEG-in-gelatin-in-PEG double water emulsion droplets having a multinuclear core. These droplets are stable upon cooling below the temperature at which gelatin gelled. In contrast, above the melting temperature of gelatin, multinuclear double emulsion droplets controllably evolve into stable mononuclear yolk (aqueous PEG)-shell (gelatin) capsules dispersed in the aqueous PEG continuous phase. It is demonstrated that the gelatin shell can accommodate negatively charged latex beads and be re-enforced by glutaraldehyde or silica. These capsules are also shown to encapsulate payloads, suggesting possible applications in microencapsulation, drug delivery, and synthetic biology.

Single Micrometer-Sized Gels: Unique Mechanics and Characters for Applications

Microgels-small gels of submicron to micron size-are widely used in food, cosmetics and biomedical applications because of their biocompatibility and/or fast response to external environments. However, the properties of "single" microgels have not been characterized due to limitations in preparation technologies and measurement methods for single microgels with sizes in the multi-micrometer range. The synthesis of multiple shapes of single microgels and their characterization are important for further functionalization and application of gel-based materials. In this review, we explain the recent advancements in microgel fabrication and characterization methods for single microgels. The first topic discussed includes the self-assembly methods for single microgel fabrication using physical phenomena such as phase separation, interfacial wetting and buckling instability. The second topic deals with methods for analyzing the mechanics of single microgels and the differences between their mechanical characteristics and those of bulk gels. The recent progress in the fabrication and characterization of single microgels will bring important insights to the design and functionalization of gel-based materials.

Dynamics of Spinodal Decomposition in a Ternary Gelling System

The phase diagram and phase transitions of the ternary system of gelatin, water and poly(ethylene glycol) oligomers were studied as a function of the weight fraction of gelatin and the weight fraction and molecular weight of poly(ethylene glycol) oligomers. It was found that both phase separation and the sol-gel transition occur in this ternary system. The relative position of the phase separation line and the sol-gel transition line depends on the weight fraction and the molecular weight of the poly(ethylene glycol) oligomer that coexists in the solution. All aspects of the phase diagram are sensitive to the molecular weight of the poly(ethylene glycol) oligomer. Since the phase separation line crosses the sol-gel transition line in the phase space that is created by the temperature and the weight fraction of gelatin, the phase space is typically divided into four regions, where each region corresponds to a definite phase. The transitions between mutual phases were studied using the light-scattering technique.

Confinement Effects on Polymer Dynamics: Thermo-Responsive Behaviours of Hydroxypropyl Cellulose Polymers in Phospholipid-Coated Droplets (Water-in-Oil Emulsion)

In order to construct the artificial cells and to understand the physicochemical properties of living cells, it is important to clarify the cell-sized confinement effect on the behaviours of bio-inspired polymers. We report the dynamic behaviours of aqueous hydroxypropyl cellulose (HPC) solution coated with phospholipids in oil (water-in-oil droplets, W/O droplets), accompanied by an increase in the temperature. We directly observed the beginning of phase separation of HPC solution using a fluorescence microscope and confirmed the dependence of such phenomena on droplet size. The results indicate that the start time of phase separation is decreased with an increase in droplet size. The experimental results suggest that the confinement situation accelerates the phase separation of aqueous HPC solutions.