Synthesis of monodisperse, covalently cross-linked, degradable "smart" microgels using microfluidics

Hydrogels Based on Proteins Cross-Linked with Carbonyl Derivatives of Polysaccharides, with Biomedical Applications

Adding carbonyl groups into the hydrogel matrix improves the stability and biocompatibility of the hydrogels, making them suitable for different biomedical applications. In this review article, we will discuss the use of hydrogels based on polysaccharides modified by oxidation, with particular attention paid to the introduction of carbonyl groups. These hydrogels have been developed for several applications in tissue engineering, drug delivery, and wound healing. The review article discusses the mechanism by which oxidized polysaccharides can introduce carbonyl groups, leading to the development of hydrogels through cross-linking with proteins. These hydrogels have tunable mechanical properties and improved biocompatibility. Hydrogels have dynamic properties that make them promising biomaterials for various biomedical applications. This paper comprehensively analyzes hydrogels based on cross-linked proteins with carbonyl groups derived from oxidized polysaccharides, including microparticles, nanoparticles, and films. The applications of these hydrogels in tissue engineering, drug delivery, and wound healing are also discussed.

Microfluidic Fabrication and Applications of Microgels and Hybrid Microgels

Smart microgels have gained much attention because of their wide range of applications in the field of biomedical, environmental, nanotechnological and catalysis sciences. Most of the applications of microgels are strongly affected by their morphology, size and size distribution. Various methodologies have been adopted to obtain polymer microgel particles. Droplet microfluidic techniques have been widely reported for the fabrication of highly monodisperse microgel particles to be used for various applications. Monodisperse microgel particles of required size and morphology can be achieved via droplet microfluidic techniques by simple polymerization of monomers in the presence of suitable crosslinker or by gelation of high molecular weight polymers. This report gives recent research progress in fabrication, characterization, properties and applications of microgel particles synthesized by microfluidic methods.

Preparation and Characteristics of Alginate Microparticles for Food, Pharmaceutical and Cosmetic Applications

Alginates are the most widely used natural polymers in the pharmaceutical, food and cosmetic industries. Usually, they are applied as a thickening, gel-forming and stabilizing agent. Moreover, the alginate-based formulations such as matrices, membranes, nanospheres or microcapsules are often used as delivery systems. Alginate microparticles (AMP) are biocompatible, biodegradable and nontoxic carriers, applied to encapsulate hydrophilic active substances, including probiotics. Here, we report the methods most frequently used for AMP production and encapsulation of different actives. The technological parameters important in the process of AMP preparation, such as alginate concentration, the type and concentration of other reagents (cross-linking agents, oils, emulsifiers and pH regulators), agitation speed or cross-linking time, are reviewed. Furthermore, the advantages and disadvantages of alginate microparticles as delivery systems are discussed, and an overview of the active ingredients enclosed in the alginate carriers are presented.

4D Printing of Extrudable and Degradable Poly(Ethylene Glycol) Microgel Scaffolds for Multidimensional Cell Culture

Granular synthetic hydrogels are useful bioinks for their compatibility with a variety of chemistries, affording printable, stimuli-responsive scaffolds with programmable structure and function. Additive manufacturing of microscale hydrogels, or microgels, allows for the fabrication of large cellularized constructs with percolating interstitial space, providing a platform for tissue engineering at length scales that are inaccessible by bulk encapsulation where transport of media and other biological factors are limited by scaffold density. Herein, synthetic microgels with varying degrees of degradability are prepared with diameters on the order of hundreds of microns by submerged electrospray and UV photopolymerization. Porous microgel scaffolds are assembled by particle jamming and extrusion printing, and semi-orthogonal chemical cues are utilized to tune the void fraction in printed scaffolds in a logic-gated manner. Scaffolds with different void fractions are easily cellularized post printing and microgels can be directly annealed into cell-laden structures. Finally, high-throughput direct encapsulation of cells within printable microgels is demonstrated, enabling large-scale 3D culture in a macroporous biomaterial. This approach provides unprecedented spatiotemporal control over the properties of printed microporous annealed particle scaffolds for 2.5D and 3D tissue culture.

Microfluidics Fabrication of Micrometer-Sized Hydrogels with Precisely Controlled Geometries for Biomedical Applications

Micrometer-sized hydrogels are cross-linked three-dimensional network matrices with high-water contents and dimensions ranging from several to hundreds of micrometers. Due to their excellent biocompatibility and capability to mimic physiological microenvironments in vivo, micrometer-sized hydrogels have attracted much attention in the biomedical engineering field. Their biological properties and applications are primarily influenced by their chemical compositions and geometries. However, inhomogeneous morphologies and uncontrollable geometries limit traditional micrometer-sized hydrogels obtained by bulk mixing. In contrast, microfluidic technology holds great potential for the fabrication of micrometer-sized hydrogels since their geometries, sizes, structures, compositions, and physicochemical properties can be precisely manipulated on demand based on the excellent control over fluids. Therefore, micrometer-sized hydrogels fabricated by microfluidic technology have been applied in the biomedical field, including drug encapsulation, cell encapsulation, and tissue engineering. This review introduces micrometer-sized hydrogels with various geometries synthesized by different microfluidic devices, highlighting their advantages in various biomedical applications over those from traditional approaches. Overall, emerging microfluidic technologies enrich the geometries and morphologies of hydrogels and accelerate translation for industrial production and clinical applications.

Simultaneous One-Pot Interpenetrating Network Formation to Expand 3D Processing Capabilities

The incorporation of a secondary network into traditional single-network hydrogels can enhance mechanical properties, such as toughness and loading to failure. These features are important for many applications, including as biomedical materials; however, the processing of interpenetrating polymer network (IPN) hydrogels is often limited by their multistep fabrication procedures. Here, a one-pot scheme for the synthesis of biopolymer IPN hydrogels mediated by the simultaneous crosslinking of two independent networks with light, namely: i) free-radical crosslinking of methacrylate-modified hyaluronic acid (HA) to form the primary network and ii) thiol-ene crosslinking of norbornene-modified HA with thiolated guest-host assemblies of adamantane and β-cyclodextrin to form the secondary network, is reported. The mechanical properties of the IPN hydrogels are tuned by changing the network composition, with high water content (≈94%) hydrogels exhibiting excellent work of fracture, tensile strength, and low hysteresis. As proof-of-concept, the IPN hydrogels are implemented as low-viscosity Digital Light Processing resins to fabricate complex structures that recover shape upon loading, as well as in microfluidic devices to form deformable microparticles. Further, the IPNs are cytocompatible with cell adhesion dependent on the inclusion of adhesive peptides. Overall, the enhanced processing of these IPN hydrogels will expand their utility across applications.

Compression and swelling of hydrogels in polymer solutions: A dominant-mode model

The swelling and compression of hydrogels in polymer solutions can be understood by considering hydrogel-osmolyte-solvent interactions which determine the osmotic pressure difference between the inside and the outside of a hydrogel particle and the changes in effective solvent quality for the hydrogel network. Using the theory of poroelasticity, we find the exact solution to hydrogel dynamics in a dilute polymer solution, which quantifies the effect of diffusion and partitioning of osmolyte and the related solvent quality change to the volumetric changes of the hydrogel network. By making a dominant-mode assumption, we propose a model for the swelling and compression dynamics of (spherical) hydrogels in concentrated polymer solutions. Osmolyte diffusion induces a biexponential response in the size of the hydrogel radius, whereas osmolyte partitioning and solvent quality effects induce monoexponential responses. Comparison of the dominant-mode model to experiments provides reasonable values for the compressive bulk modulus of a hydrogel particle, the permeability of the hydrogel network, and the diffusion constant of osmolyte molecules inside the hydrogel network. Our model shows that hydrogel-osmolyte interactions can be described in a conceptually simple manner, while still capturing the rich (de)swelling behaviors observed in experiments. We expect our approach to provide a roadmap for further research into and applications of hydrogel dynamics induced by, for example, changes in the temperature and the pH.

Injectable Supramolecular Hydrogel/Microgel Composites for Therapeutic Delivery

Shear-thinning hydrogels are useful for biomedical applications, from 3D bioprinting to injectable biomaterials. Although they have the appropriate properties for injection, it may be advantageous to decouple injectability from the controlled release of encapsulated therapeutics. Toward this, composites of hydrogels and encapsulated microgels are introduced with microgels that are fabricated via microfluidics. The microgel cross-linker controls degradation and entrapped molecule release, and the concentration of microgels alters composite hydrogel rheological properties. For the treatment of myocardial infarction (MI), interleukin-10 (IL-10) is encapsulated in microgels and released from composites. In a rat model of MI, composites with IL-10 reduce macrophage density after 1 week and improve scar thickness, ejection fraction, cardiac output, and the size of vascular structures after 4 weeks when compared to saline injection. Improvements are also observed with the composite without IL-10 over saline, emphasizing the role of injectable hydrogels alone on tissue repair.

Comparative cytocompatibility of multiple candidate cell types to photoencapsulation in PEGNB/PEGDA macroscale or microscale hydrogels

The encapsulation of live cells into photopolymerized hydrogel scaffolds has the potential to augment or repair tissue defects, establish versatile regenerative medicine strategies, and be developed as well-defined, yet tunable microenvironments to study fundamental cellular behavior. However, hydrogel fabrication limitations constrain most studies to macroscale hydrogel scaffolds encapsulating millions of cells. These macroscale materials possess regions of heterogeneous photopolymerization conditions and are therefore poor platforms to identify the response of individual cells to encapsulation. Recently, microfluidic droplet-based hydrogel miniaturization and cell encapsulation offers high-throughput, reproducible, and continuous fabrication. Reports of post-encapsulation cell viability, however, vary widely among specific techniques. Furthermore, different cell types often exhibit different level of tolerance to photoencapsulation-induced toxicity. Accordingly, we evaluate the cellular tolerance of various encapsulation techniques and photopolymerization parameters for four mammalian cell types, with potential applications in tissue regeneration, using polyethylene glycol diacrylate or polyethylene glycol norbornene (PEGNB) hydrogels on micro- and macro-length scales. We found PEGNB provides excellent cellular tolerance and supports long-term cell survival by mitigating the deleterious effects of acrylate photopolymerization, which are exacerbated at diminishing volumes. PEGNB, therefore, is an excellent candidate for hydrogel miniaturization. PEGNB hydrogel properties, however, were found to have variable effects on encapsulating different cell candidates. This study could provide guidance for cell encapsulation practices in tissue engineering and regenerative medicine research.

Fabricating Degradable Thermoresponsive Hydrogels on Multiple Length Scales via Reactive Extrusion, Microfluidics, Self-assembly, and Electrospinning

While various smart materials have been explored for a variety of biomedical applications (e.g., drug delivery, tissue engineering, bioimaging, etc.), their ultimate clinical use has been hampered by the lack of biologically-relevant degradation observed for most smart materials. This is particularly true for temperature-responsive hydrogels, which are almost uniformly based on polymers that are functionally non-degradable (e.g., poly(N-isopropylacrylamide) (PNIPAM) or poly(oligoethylene glycol methacrylate) (POEGMA)). As such, to effectively translate the potential of thermoresponsive hydrogels to the challenges of remote-controlled or metabolism-regulated drug delivery, cell scaffolds with tunable cell-material interactions, theranostic materials with the potential for both imaging and drug delivery, and other such applications, a method is required to render the hydrogels (if not fully degradable) at least capable of renal clearance following the required lifetime of the material. To that end, this protocol describes the preparation of hydrolytically-degradable hydrazone-crosslinked hydrogels on multiple length scales based on the reaction between hydrazide and aldehyde-functionalized PNIPAM or POEGMA oligomers with molecular weights below the renal filtration limit. Specifically, methods to fabricate degradable thermoresponsive bulk hydrogels (using a double barrel syringe technique), hydrogel particles (on both the microscale through the use of a microfluidics platform facilitating simultaneous mixing and emulsification of the precursor polymers and the nanoscale through the use of a thermally-driven self-assembly and cross-linking method), and hydrogel nanofibers (using a reactive electrospinning strategy) are described. In each case, hydrogels with temperature-responsive properties similar to those achieved via conventional free radical cross-linking processes can be achieved, but the hydrazone cross-linked network can be degraded over time to re-form the oligomeric precursor polymers and enable clearance. As such, we anticipate these methods (which may be generically applied to any synthetic water-soluble polymer, not just smart materials) will enable easier translation of synthetic smart materials to clinical applications.

Microfluidic production of degradable thermoresponsive poly(N-isopropylacrylamide)-based microgels

Highly monodisperse and hydrolytically degradable thermoresponsive microgels on the tens-to-hundreds of micron size scale have been fabricated based on simultaneous on-chip mixing and emulsification of aldehyde and hydrazide-functionalized poly(N-isopropylacrylamide) precursor polymers. The microfluidic chip can run for extended periods without upstream gelation and can produce monodisperse (<10% particle size variability) microgels on the size range of ∼30-90 μm, with size tunable according to the flow rate of the oil continuous phase. Fluorescence analysis indicates a uniform distribution of each reactive pre-polymer inside the microgels while micromechanical testing suggests that smaller microfluidic-produced microgels exhibit significantly higher compressive moduli compared to bulk hydrogels of the same composition, an effect we attribute to improved mixing (and thus crosslinking) of the precursor polymer solutions within the microfluidic device. The microgels retain the reversible volume phase transition behavior of conventional microgels but can be hydrolytically degraded back into their oligomeric precursor polymer fragments, offering potential for microgel clearance following use in vivo.

Compression and Reswelling of Microgel Particles after an Osmotic Shock

We use dedicated microfluidic devices to expose soft hydrogel particles to a rapid change in the externally applied osmotic pressure and observe a surprising, nonmonotonic response: After an initial rapid compression, the particle slowly reswells to approximately its original size. We theoretically account for this behavior, enabling us to extract important material properties from a single microfluidic experiment, including the compressive modulus, the gel permeability, and the diffusivity of the osmolyte inside the gel. We expect our approach to be relevant to applications such as controlled release, chromatography, and responsive materials.

Enhanced Pulsatile Drug Release from Injectable Magnetic Hydrogels with Embedded Thermosensitive Microgels

Nanocomposite in situ-gelling hydrogels containing both superparamagnetic iron oxide nanoparticles (SPIONs) and thermoresponsive microgels are demonstrated to facilitate pulsatile, high-low release of a model drug (4 kDa fluorescein-labeled dextran). The materials can be injected through a minimally invasive route, facilitate a ∼4-fold enhancement of release when pulsed on relative to the off state, and, in contrast to previous gel-based systems, can maintain pulsatile release properties over multiple cycles and multiple days instead of only hours. Optimal pulsatile release is achieved when the microgel transition temperature is engineered to lie just above the (physiological) incubation temperature. Coupled with the demonstrated degradability of the nanocomposites and the cytocompatibility of all nanocomposite components, we anticipate these nanocomposites have potential to facilitate physiologically relevant, controlled pulsatile drug delivery.

In situ cross-linking of stimuli-responsive hemicellulose microgels during spray drying

Chemical cross-linking during spray drying offers the potential for green fabrication of microgels with a rapid stimuli response and good blood compatibility and provides a platform for stimuli-responsive hemicellulose microgels (SRHMGs). The cross-linking reaction occurs rapidly in situ at elevated temperature during spray drying, enabling the production of microgels in a large scale within a few minutes. The SRHMGs with an average size range of ∼ 1-4 μm contain O-acetyl-galactoglucomannan as a matrix and poly(acrylic acid), aniline pentamer (AP), and iron as functional additives, which are responsive to external changes in pH, electrochemical stimuli, magnetic field, or dual-stimuli. The surface morphologies, chemical compositions, charge, pH, and mechanical properties of these smart microgels were evaluated using scanning electron microscopy, IR, zeta potential measurements, pH evaluation, and quantitative nanomechanical mapping, respectively. Different oxidation states were observed when AP was introduced, as confirmed by UV spectroscopy and cyclic voltammetry. Systematic blood compatibility evaluations revealed that the SRHMGs have good blood compatibility. This bottom-up strategy to synthesize SRHMGs enables a new route to the production of smart microgels for biomedical applications.

Microscale Bioadhesive Hydrogel Arrays for Cell Engineering Applications

Bioengineered hydrogels have been explored in cell and tissue engineering applications to support cell growth and modulate its behavior. A rationally designed scaffold should allow for encapsulated cells to survive, adhere, proliferate, remodel the niche, and can be used for controlled delivery of biomolecules. Here we report a microarray of composite bioadhesive microgels with modular dimensions, tunable mechanical properties and bulk modified adhesive biomolecule composition. Composite bioadhesive microgels of maleimide functionalized polyethylene glycol (PEG-MAL) with interpenetrating network (IPN) of gelatin ionically cross-linked with silicate nanoparticles were engineered by integrating microfabrication with Michael-type addition chemistry and ionic gelation. By encapsulating clinically relevant anchorage-dependent cervical cancer cells and suspension leukemia cells as cell culture models in these composite microgels, we demonstrate enhanced cell spreading, survival, and metabolic activity compared to control gels. The composite bioadhesive hydrogels represent a platform that could be used to study independent effect of stiffness and adhesive ligand density on cell survival and function. We envision that such microarrays of cell adhesive microenvironments, which do not require harsh chemical and UV crosslinking conditions, will provide a more efficacious cell culture platform that can be used to study cell behavior and survival, function as building blocks to fabricate 3D tissue structures, cell delivery systems, and high throughput drug screening devices.

Microfluidic-based generation of size-controlled, biofunctionalized synthetic polymer microgels for cell encapsulation

Cell and islet microencapsulation in synthetic hydrogels provides an immunoprotective and cell-supportive microenvironment. A microfluidic strategy for the genaration of biofunctionalized, synthetic microgel particles with precise control over particle size and molecular permeability for cell and protein delivery is presented. These engineered capsules support high cell viability and function of encapsulated human stem cells and islets.

Multiphase flow microfluidics for the production of single or multiple emulsions for drug delivery

Considerable effort has been directed towards developing novel drug delivery systems. Microfluidics, capable of generating monodisperse single and multiple emulsion droplets, executing precise control and operations on these droplets, is a powerful tool for fabricating complex systems (microparticles, microcapsules, microgels) with uniform size, narrow size distribution and desired properties, which have great potential in drug delivery applications. This review presents an overview of the state-of-the-art multiphase flow microfluidics for the production of single emulsions or multiple emulsions for drug delivery. The review starts with a brief introduction of the approaches for making single and multiple emulsions, followed by presentation of some potential drug delivery systems (microparticles, microcapsules and microgels) fabricated in microfluidic devices using single or multiple emulsions as templates. The design principles, manufacturing processes and properties of these drug delivery systems are also discussed and compared. Furthermore, drug encapsulation and drug release (including passive and active controlled release) are provided and compared highlighting some key findings and insights. Finally, site-targeting delivery using multiphase flow microfluidics is also briefly introduced.

Microfluidics-assisted in vitro drug screening and carrier production

Microfluidic platforms provide several unique advantages for drug development. In the production of drug carriers, physical properties such as size and shape, and chemical properties such as drug composition and pharmacokinetic parameters, can be modified simply and effectively by tuning the flow rate and geometries. Large numbers of carriers can then be fabricated with minimal effort and with little to no batch-to-batch variation. Additionally, cell or tissue culture models in microfluidic systems can be used as in vitro drug screening tools. Compared to in vivo animal models, microfluidic drug screening platforms allow for high-throughput and reproducible screening at a significantly lower cost, and when combined with current advances in tissue engineering, are also capable of mimicking native tissues. In this review, various microfluidic platforms for drug and gene carrier fabrication are reviewed to provide guidelines for designing appropriate carriers. In vitro microfluidic drug screening platforms designed for high-throughput analysis and replication of in vivo conditions are also reviewed to highlight future directions for drug research and development.

Dripping and jetting in microfluidic multiphase flows applied to particle and fiber synthesis

Dripping and jetting regimes in microfluidic multiphase flows have been investigated extensively, and this review summarizes the main observations and physical understandings in this field to date for three common device geometries: coaxial, flow-focusing and T-junction. The format of the presentation allows for simple and direct comparison of the different conditions for drop and jet formation, as well as the relative ease and utility of forming either drops or jets among the three geometries. The emphasis is on the use of drops and jets as templates for microparticle and microfiber syntheses, and a description is given of the more common methods of solidification and strategies for achieving complex multicomponent microparticles and microfibers.

Advanced materials and processing for drug delivery: the past and the future

Design and synthesis of efficient drug delivery systems are of vital importance for medicine and healthcare. Materials innovation and nanotechnology have synergistically fueled the advancement of drug delivery. Innovation in material chemistry allows the generation of biodegradable, biocompatible, environment-responsive, and targeted delivery systems. Nanotechnology enables control over size, shape and multi-functionality of particulate drug delivery systems. In this review, we focus on the materials innovation and processing of drug delivery systems and how these advances have shaped the past and may influence the future of drug delivery.