Jammed Microgel Inks for 3D Printing Applications

Micropore structure engineering of injectable granular hydrogels via controlled liquid-liquid phase separation facilitates regenerative wound healing in mice and pigs

Biomaterials can play a crucial role in facilitating tissue regeneration, but their application is often limited by that they induce scarring rather than complete tissue restoration. Hydrogels with microporous architectures, engineered via 3D printing techniques or particle packing (granular hydrogels), have shown promise in providing a conducive microenvironment for cellular infiltration and favorable immune response. Nonetheless, there is a notably lacking in studies that demonstrate scarless regeneration solely through pore structure engineering. In this study, we demonstrate that optimizing micropore structure of injectable granular hydrogels via controlled liquid-liquid phase separation facilitates scarless wound healing. The building block particles are fabricated by precisely controlling the separation kinetics of two immiscible aqueous phases (gelling and porogenic) and timely arresting phase separation, to generate bicontinuous, hollow or closed porous structure. Employing a murine model, we reveal that the optimized pore structure significantly facilitates mature vascular network boosts pro-regenerative macrophage polarization (M2/M1) and CD4+/Foxp3+ regulatory T cells, culminating in scarless skin regeneration enriched with hair follicles. Moreover, our hydrogels outperform the clinical gold-standard collagen/proteoglycan scaffolds in a porcine model, showcasing superior cell infiltration, epidermal integration, and dermal regeneration. Micropore structure engineering of biomaterials presents a promising and biologics free pathway for tissue regeneration.

Peanut oil body as a food-grade ink for 3D printing: Preparation, characterization and performance

Peanut oil body (POB) refers to intact micron-sized organelles in plants that store lipids, functioning as natural oil-in-water emulsion systems. This characteristic has garnered significant interest in the food industry due to the excellent emulsifying properties and safety of POB. In this study, POB was utilized as the ink for direct 3D printing of soft food products without the addition of any cross-linking agent, and the underlying mechanisms were investigated. First, POB-based inks with favorable rheological and self-supporting properties were prepared from two types of peanuts with varying oleic acid contents, exhibiting behavior akin to high internal phase emulsions. Second, direct 3D printing of POB was achieved for the first time by modulating the interfacial properties of POB through pH adjustment, which influenced the composition and molecular interactions of the ink phase components. Notably, the ink derived from high oleic acid peanuts demonstrated superior printability under elevated pH conditions. Third, a novel mechanism for the direct 3D printability of POB is proposed. This mechanism involves the elution of interfacial proteins, decreased hydration and alterations in the phase composition of the ink system, thereby enhancing printing performance. Additionally, changes in interfacial properties improved hydrophobic interactions and electrostatic forces, leading to the formation of network structures with elastoplastic rheological properties that enhanced self-supporting capabilities. In summary, this study successfully achieved direct 3D printing of POB ink for the first time, elucidated the mechanisms underlying the printability of POB, and proposed a novel strategy for the 3D printing of high-oil food-grade inks. These findings offer valuable insights for the future development of 3D printing applications of POB in the food industry.

Soft and responsive: rheological insights into PNIPAM based microgels and applications

In this Review, we synthesize and critically evaluate past and recent findings on the rheology of microgels based on poly(N-isopropylacrylamide) (PNIPAM), versatile soft materials with tunable properties, providing a comprehensive analysis of their flow behaviour. Differences and similarities are pointed out depending on their structure: homopolymeric, core-shell, copolymeric and interpenetrate polymer networks microgels. We discuss rotational and oscillatory shear rheology measurements and examine the influence of crosslinker density, temperature, pH, and polymer concentration. Additionally, we highlight the practical implications in the knowledge of the rheological properties for applications. Through this review, therefore, we aim to create a solid starting point for all scientists involved in this research area with a powerful source identifying current challenges and potential future research directions.

An overview of recent advancements in 4D printing of alginate hydrogels for tissue regeneration

4D printing of alginate hydrogels has emerged as a transformative strategy in tissue engineering, enabling the fabrication of stimuli-responsive scaffolds that recapitulate the temporal and spatial complexities of native tissues. Leveraging alginate's tunable crosslinking, biocompatibility, and easy modification, recent research has demonstrated the successful design of constructs capable of programmable shape morphing in response to physiological stimuli. This review highlights recent advances in polymer design, including methacrylated, oxidized, and ligand-functionalized alginate derivatives, and cutting-edge 4D printing technologies such as extrusion-based and photopolymerization-based printing technologies. Notably, these systems have shown promising outcomes in regenerating cartilage, bone, vascular, and neural tissues. However, key challenges remain, including the standardization of shape-morphing quantification, enhancement of mechanical robustness, improvement of host tissue integration, and the replication of native tissue complexity. This review concludes with a critical evaluation of current limitations and future directions, highlighting the potential of integrating 4D alginate hydrogel systems with emerging technologies such as artificial intelligence, machine learning, organoid models, and bioelectronic interfaces to accelerate innovation and broaden their application in tissue engineering. By synthesizing recent advancements and offering insights into the implementation of 4D alginate hydrogels, this review aims to stimulate continued progress in this rapidly evolving field.

3D-Printable Granular Hydrogel Composed of Hyaluronic Acid-Chitosan Hybrid Polyelectrolyte Complex Microgels

When it comes to 3D printing of hydrogels, optimizing rheological properties is crucial to ensure (i) a smooth flow of the ink through the nozzle during printing and (ii) structural integrity postprinting. Granular hydrogels offer excellent printability due to their intrinsic yield-stress properties; however, they typically lack postprinting integrity in aqueous environments. To address this limitation, a novel approach is proposed to integrate the yield-stress behavior of granular hydrogels with polyelectrolyte complexes, which exhibit tunable mechanical properties and structural integrity in aqueous media. In this approach, hybrid microgels composed of chemically co-cross-linked hyaluronic acid-chitosan polyelectrolytes are prepared in a high-salt medium to shield electrostatic interactions and then jammed to form a printable granular hydrogel. By reducing the salt concentration below the critical threshold for electrostatic association, intra- and interparticle electrostatic interactions are activated, causing both the microgels and the granular hydrogel to shrink. This process yields a hydrogel with tunable stiffness, packing density, and dimensions. Overall, this strategy paves the way for the design of 3D hydrogel constructs with dynamic properties.

High-Resolution 3D Printing of Stretchable Granular Hydrogel Filaments for Fabricating Robust and Durable Tissue Phantoms with Tunable Mechanical Strength

Granular hydrogels are a promising class of 3D-printable inks but often suffer from low printing resolution due to large microgel sizes (>100 µm) and weak mechanical performance from lower packing density. To overcome these limitations, a novel whey protein microgels-based granular hydrogel (WMGH) is developed, consisting of uniform, size-controllable microgels (1, 6, and 20 µm) via protein-polysaccharide segregative phase separation. The smaller microgels enable WMGH to stretch like continuous liquid inks by adjusting printing speed and pressure, achieving high-resolution 3D-printing (200 µm) with minimal ink spreading (≈5%) using a 25G nozzle (260 µm). This allows the fabrication of intricate structures like human ear and aortic valve models. Incorporating a polyacrylamide (PAM) second percolating network transforms WMGH inks into double-network hydrogels (DN-WMGH), showing up to 36 fold increase in toughness (1.45 MJ m- 3) compared to PAM hydrogels. Controlling microgel size provides a new approach for tailoring mechanical strength (6-300 kPa) while maintaining durability, exhibiting full recovery after 100 tensile cycles at 100% strain. DN-WMGH from biopolymers demonstrated good compatibility. This high-resolution 3D-printing of robust DN-WMGH replicates the mechanical properties of various tissues, from brain (<10 kPa) to intestine (≈300 kPa), demonstrating new possibilities for tissue-mimicking applications in surgical training, implantable devices, and drug-delivery systems.

Compositional Control of Stereocomplexed Hydrogel Microparticle Network Formation and Physical Properties

Granular hydrogel scaffolds composed of many discrete hydrogel microparticles (HMPs) have demonstrated significant advantages over bulk hydrogels, including injectability and the flexibility to incorporate diverse chemistries, physical properties, and bioactive payloads. Herein, we demonstrate the ability to tune HMP properties through varying the length of poly(ethylene glycol) (PEG) arms and stereocomplexed poly(lactic acid) (SC PLA) cross-links within PEG-based HMPs to further understand the networks' structure-property relationships and utility in a model prodrug delivery system. DSC and WAXS revealed that hydrogels with shorter PEG arms were able to form stereocomplex domains to a greater extent than longer PEG arms. Additionally, as the SC PLA length increased, the HMPs were more thermally and mechanically stable. HMPs were also loaded with model prodrug, doxorubicin, to characterize compositional variations' effects on release profiles. These studies suggest that variations in the cross-linker concentration influence the crystallinity of each HMP formulation, allowing for tunable drug loading and release.

Electrostatically Reinforced Double Network Granular Hydrogels

Rapid advances in biomedical applications and soft robotics demand load-bearing soft materials that can be processed into complex 3D shapes. Direct ink writing (DIW) enables the fabrication of customizable shapes with locally varying compositions. Hydrogels that are formulated as microgels meet the rheological requirements that DIW imparts on the inks if they are jammed. However, most granular hydrogels are soft because inter-particle interactions are weak. These hydrogels can be reinforced with a second hydrogel, yielding double network granular hydrogels (DNGHs). Yet, DNGHs suffer from low fracture energy. This limitation is addressed by electrostatically reinforcing them. The resulting materials exhibit Young's moduli and fracture energies similar to values of cartilage and muscles. An empirical model is proposed to predict the fracture energy of these reinforced DNGHs, based on the dissipation zone size, contact area, and adhesion energy. These DNGHs can be 3D-N, N-methylene bisacrylamideprinted into free-standing structures exhibiting tuneable mechanical properties at the centimeter scale without the need for supporting structures.

In Situ-Formed Tissue-Adhesive Macroporous Scaffolds Enhance Cell Infiltration and Tissue Regeneration

Macroporous hydrogels have shown significant promise in tissue engineering and regenerative medicine. However, conventional macroporous scaffold fabrications are complex and incompatible with in situ customization and fabrication. Here, we propose a highly translational approach for the in situ formation of adhesive macroporous scaffolds through microfluidic homogenization of gas into a self-crosslinkable gelatin and transglutaminase (TG) mixture using a double syringe system. Using this strategy, the tissue defect can be evaluated, and the precursor, with the desired composition and volume, foamed and administered in situ. The TG-induced crosslinking stabilizes the pores, leading to strong tissue adhesion and accurate defect geometry approximation. We demonstrate precise control over the porosity, by changing the foaming parameters, and crosslinking kinetics, by adjusting the concentration of gelatin and TG. The resulting foam scaffolds offer controlled pore distribution, flexibility, tissue adhesion, stability, sustained protein release profile, and cell permissibility, with a faster biodegradation profile compared to bulk hydrogel compartments. Consequently, enhanced cell infiltration and reduced fibrous capsule formation are observed upon subcutaneous injection of foams compared to bulk hydrogels. Finally, the scaffolds demonstrate significant improvements in the rate and quality of the healing compared to the bulk hydrogels for the treatment of full-thickness cutaneous wounds in mice. STATEMENT OF SIGNIFICANCE: A highly translational method is presented for the in situ formation of tissue-adhesive macroporous scaffolds through microfluidic homogenization of gas into a self-crosslinkable hydrogel precursor using a double syringe system. This approach allows precise control over porosity and pore size, facilitating cell infiltration, tissue integration, and improved wound healing compared to bulk hydrogels, highlighting their potential in regenerative medicine.

Injectable microgel and micro-granular hydrogels for bone tissue engineering

Injectable microgels, made from both natural and synthetic materials, are promising platforms for the encapsulation of cells or bioactive agents, such as drugs and growth factors, for delivery to injury sites. They can also serve as effective micro-scaffolds in bone tissue engineering (BTE), offering a supportive environment for cell proliferation or differentiation into osteoblasts. Microgels can be injected in the injury sites individually or in the form of aggregated/jammed ones named micro-granular hydrogels. This review focuses on common materials and fabrication techniques for preparing injectable microgels, as well as their characteristics and applications in BTE. These applications include their use as cell carriers, delivery systems for bioactive molecules, micro-granular hydrogels, bio-inks for bioprinting, three-dimensional microarrays, and the formation of microtissues. Furthermore, we discuss the current and potential future applications of microgels in bone tissue regeneration.

I-PRF functionalized gelatin methacrylate microspheres for improving the proliferation and osteogenic differentiation of human periodontal ligament stem cells

Although human periodontal ligament stem cell (hPDLSC)-based tissue engineering have been promising for regenerating periodontal bone tissue, their effectiveness is limited by the lack of an optimal delivery vehicle for these cells. Therefore, this study reports a gelatin methacryloyl microsphere system, incorporating injectable platelet-rich fibrin and nano-hydroxyapatite (nHA) as carriers for hPDLSCs. These hybrid microspheres were effectively produced using droplet microfluidics technology, achieving a size range of 100-300 μm. Importantly, the release profile of multiple growth factors (GFs) from the microspheres was significantly extended, lasting up to 28 d. Moreover, the released GFs and nHA considerably enhanced the proliferation of encapsulated hPDLSCs along with their spreading and osteogenic differentiation. The microspheres facilitated the development of Spheroid-like cell aggregates within two weeks of culture, demonstrating a promising approach for advanced periodontal bone tissue regeneration.

4D Printing: A Comprehensive Review of Technologies, Materials, Stimuli, Design, and Emerging Applications

4D printing is a groundbreaking technology that seamlessly integrates additive manufacturing with smart materials, enabling the creation of multiscale objects capable of changing shapes and/or functions in a controlled and programmed manner in response to applied energy inputs. Printing technologies, mathematical modeling, responsive materials, stimuli, and structural design constitute the blueprint of 4D printing, all of which have seen rapid advancement in the past decade. These advancements have opened up numerous possibilities for dynamic and adaptive structures, finding potential use in healthcare, textiles, construction, aerospace, robotics, photonics, and electronics. However, current 4D printing primarily focuses on proof-of-concept demonstrations. Further development is necessary to expand the range of accessible materials and address fabrication complexities for widespread adoption. In this paper, we aim to deliver a comprehensive review of the state-of-the-art in 4D printing, probing into shape programming, exploring key aspects of resulting constructs including printing technologies, materials, structural design, morphing mechanisms, and stimuli-responsiveness, and elaborating on prominent applications across various fields. Finally, we discuss perspectives on limitations, challenges, and future developments in the realm of 4D printing. While the potential of this technology is undoubtedly vast, continued research and innovation are essential to unlocking its full capabilities and maximizing its real-world impact.

Highly Extensible Physically Crosslinked Hydrogels for High-Speed 3D Bioprinting

Hydrogels have emerged as promising materials for bioprinting and many other biomedical applications due to their high degree of biocompatibility and ability to support and/or modulate cell viability and function. Yet, many hydrogel bioinks have suffered from low efficiency due to limitations on accessible printing speeds, often limiting cell viability and/or the constructs which can be generated. In this study, a highly extensible bioink system created by modulating the rheology of physically crosslinked hydrogels comprising hydrophobically-modified cellulosics and either surfactants or cyclodextrins is reported. It is demonstrated that these hydrogels are highly shear-thinning with broadly tunable viscoelasticity and stress-relaxation through simple modulation of the composition. Rheological experiments demonstrate that increasing concentration of rheology-modifying additives yields hydrogel materials exhibiting extensional strain-to-break values up to 2000%, which is amongst the most extensible examples of physically crosslinked hydrogels of this type. The potential of these hydrogels for use as bioinks is demonstrated by evaluating the relationship between extensibility and printability, demonstrating that greater hydrogel extensibility enables faster print speeds and smaller print features. The findings suggest that optimizing hydrogel extensibility can enhance high-speed 3D bioprinting capabilities, reporting over 5000 fold enhancement in speed index compared to existing works reported for hydrogel-based bioinks in extrusion-based printing.

Highly elastic bioactive bR-GelMA micro-particles: synthesis and precise micro-fabrication via stop-flow lithography

Osteoporosis poses a significant public health challenge, necessitating advanced bone regeneration solutions. While gelatin methacrylate (GelMA) hydrogels show promise, conventional fabrication methods using aqueous two-phase systems (ATPS) often result in inconsistent mechanical properties and structural irregularities. This study presents an approach synthesizing new methods and parameters for bR-GelMA, utilizing stop-flow lithography (SFL) to fabricate highly elastic micro-particles incorporating bioactive glass particles. SFL, in contrast to ATPS, offers precise control over micro-particle formation, enabling the production of uniform and stable structures ideal for biomedical applications. The resulting elastic micro-particles demonstrate rapid degradation, enhanced cell proliferation, and improved mechanical strength without compromising flexibility. This innovative approach using SFL to fabricate GelMA-based micro-particles holds significant promise for bone regeneration and other critical therapeutic applications.

3D Printing of Hydrogel Polysaccharides for Biomedical Applications: A Review

Additive manufacturing, also known as 3D printing, is becoming more and more popular because of its wide range of materials and flexibility in design. Layer by layer, 3D complex structures can be generated by the revolutionary computer-aided process known as 3D bioprinting. It is particularly crucial for youngsters and elderly patients and is a useful tool for tailored pharmaceutical therapy. A lot of research has been carried out recently on the use of polysaccharides as matrices for tissue engineering and medication delivery. Still, there is a great need to create affordable, sustainable bioink materials with high-quality mechanical, viscoelastic, and thermal properties as well as biocompatibility and biodegradability. The primary biological substances (biopolymers) chosen for the bioink formulation are proteins and polysaccharides, among the several resources utilized for the creation of such structures. These naturally occurring biomaterials give macromolecular structure and mechanical qualities (biomimicry), are generally compatible with tissues and cells (biocompatibility), and are harmonious with biological digesting processes (biodegradability). However, the primary difficulty with the cell-laden printing technique (bioprinting) is the rheological characteristics of these natural-based bioinks. Polysaccharides are widely used because they are abundant and reasonably priced natural polymers. Additionally, they serve as excipients in formulations for pharmaceuticals, nutraceuticals, and cosmetics. The remarkable benefits of biological polysaccharides-biocompatibility, biodegradability, safety, non-immunogenicity, and absence of secondary pollution-make them ideal 3D printing substrates. The purpose of this publication is to examine recent developments and challenges related to the 3D printing of stimuli-responsive polysaccharides for site-specific medication administration and tissue engineering.

Minimally Invasive Syringe-Injectable Hydrogel with Angiogenic Factors for Ischemic Stroke Treatment

Ischemic stroke (IS) accounts for most stroke incidents and causes intractable damage to brain tissue. This condition manifests as diverse aftereffects, such as motor impairment, emotional disturbances, and dementia. However, a fundamental approach to curing IS remains unclear. This study proposes a novel approach for treating IS by employing minimally invasive and injectable jammed gelatin-norbornene nanofibrous hydrogels (GNF) infused with growth factors (GFs). The developed GNF/GF hydrogels are administered to the motor cortex of a rat IS model to evaluate their therapeutic effects on IS-induced motor dysfunction. GNFs mimic a natural fibrous extracellular matrix architecture and can be precisely injected into a targeted brain area. The syringe-injectable jammed nanofibrous hydrogel system increased angiogenesis, inflammation, and sensorimotor function in the IS-affected brain. For clinical applications, the biocompatible GNF hydrogel has the potential to efficiently load disease-specific drugs, enabling targeted therapy for treating a wide range of neurological diseases.

Controlling Microparticle Aspect Ratio via Photolithography for Injectable Granular Hydrogel Formation and Cell Delivery

Granular hydrogels are injectable and inherently porous biomaterials assembled through the packing of microparticles. These particles typically have a symmetric and spherical shape. However, recent studies have shown that asymmetric particles with high aspect ratios, such as fibers and rods, can significantly improve the mechanics, structure, and cell-guidance ability of granular hydrogels. Despite this, it remains unknown how controlled changes in the particle aspect ratio influence the injectability, porosity, and cell-instructive capabilities of granular hydrogels. Part of the challenge lies in obtaining microparticles with precisely tailored dimensions using fabrication methods such as flow-focusing microfluidics or extrusion fragmentation. In this work, we leveraged facile photolithography and photocurable hyaluronic acid to fabricate rod-shaped microparticles with widths and heights of 130 μm and lengths that varied from 260 to 1300 μm to obtain aspect ratios (ARs) of 2, 4, 6, 8, and 10. All AR microparticles formed porous and injectable granular hydrogels after centrifugation jamming. Interestingly, the longest microparticles neither clogged the needle nor fractured after extrusion from a syringe. This was attributed to a relatively low elastic modulus that permitted microparticle pliability and reversible deformation under shear. Cells (NIH/3T3 fibroblasts) mixed with the jammed microparticles and injected into molds remained viable, adhered to the particles' surface, and showed a significant and rapid rate of proliferation over a period of 7 days compared to bulk hydrogels. The proliferation rate and morphology of the cells were significantly influenced by the particle AR, with higher cell numbers observed with intermediate ARs, likely attributable to the surface area available for cell adhesion. These findings showcase the utility of injectable granular hydrogels made with high-aspect-ratio microparticles for biomedical applications.

Interparticle Crosslinked Ion-responsive Microgels for 3D and 4D (Bio)printing Applications

Microgels offer unique advantages over bulk hydrogels due to their improved diffusion limits for oxygen and nutrients. Particularly, stimuli-responsive microgels with inherently bioactive and self-supporting properties emerge as highly promising biomaterials. This study unveils the development of interparticle-crosslinked, self-supporting, ion-responsive microgels tailored for 3D and 4D (bio)printing applications. A novel strategy was proposed to develop microgels that enabled interparticle crosslinking, eliminating the need for filler hydrogels and preserving essential microscale void spaces to support cell migration and vascularization. Additionally, these microgels possessed unique, ion-responsive shrinking behavior primarily by the Hofmeister effect, reversible upon the removal of the stimulus. Two types of microgels, spherical (µS) and random-shaped (µR), were fabricated, with µR exhibiting superior mechanical properties and higher packing density. Fabricated microgel-based constructs supported angiogenesis with tunable vessel size based on interstitial void spaces while demonstrating excellent shear-thinning and self-healing properties and high print fidelity. Various bioprinting techniques were employed and validated using these microgels, including extrusion-based, embedded, intraembedded, and aspiration-assisted bioprinting, facilitating the biofabrication of scalable constructs. Multi-material 4D printing was achieved by combining ion-responsive microgels with non-responsive microgels, enabling programmable shape transformations upon exposure to ionic solutions. Utilizing 4D printing, complex, dynamic structures were generated such as coiling filaments, grippers, and folding sheets, providing a foundation for the development of advanced tissue models and devices for regenerative medicine and soft robotics, respectively.

Direct Ink Writing of Rigid Microparticles

Direct ink writing (DIW) enables 3D printing of macroscopic objects with well-defined structures and compositions that controllably change over length scales of order 100 µm. Unfortunately, only a limited number of materials can be processed through DIW because it imparts stringent rheological requirements on inks. This limitation can be overcome for soft materials, if they are formulated as microparticles that, if jammed, fulfill the rheological requirements to be printed. By contrast, densely packed rigid microparticles with stiffnesses exceeding 2 MPa do not exhibit appropriate rheological properties that enable DIW. Here, an ink composed of up to 60 vol% rigid microparticles with core stiffnesses up to 50 MPa is introduced. To achieve this goal, rigid microparticles possessing soft hydrogel shells are produced. The 3D printed fragile granular structure is transformed into a load-bearing granular material through the formation of a 2nd network within the soft shells and in the interstitial spaces. The potential of these particles is demonstrated to be printed into intricate 3D structures, such as a trophy cup, or cast into flexible macroscopic photonic films.

Bioinstructive Liquefied Pockets in Hierarchical Hydrogels and Bioinks

This study proposes a novel, versatile, and modular platform for constructing porous and heterogeneous microenvironments based on the embedding of liquefied-based compartments in hydrogel systems. Using a bottom-up approach, microgels carrying the necessary cargo components, including cells and microparticles, are combined with a hydrogel precursor to fabricate a hierarchical structured (HS) system. The HS system possesses three key features that can be fully independently controlled: I) liquefied pockets enabling free cellular mobility; II) surface modified microparticles facilitating 3D microtissue organization inside the liquefied pockets; III) at a larger scale, the pockets are jammed in the hydrogel, forming a macro-sized construct. After crosslinking, the embedded microgels undergo a liquefaction process, forming a porous structure that ensures high diffusion of small biomolecules and enables cells to move freely within their miniaturized compartmentalized volume. More importantly, this platform allows the creation of multimodular cellular microenvironments within a hydrogel with controlled macrostructures, while decoupling micro- and macroenvironments. As a proof of concept, the enhancement of cellular functions using the HS system by encapsulating human adipose-derived mesenchymal stem cells (hASCs) is successfully demonstrated. Finally, the potential application of this system as a hybrid bioink for bioprinting complex 3D structures is showcased.

Engineering Granular Hydrogels without Interparticle Cross-Linking to Support Multicellular Organization

Advancing three-dimensional (3D) tissue constructs is central to creating in vitro models and engineered tissues that recapitulate biology. Materials that are permissive to cellular behaviors, including proliferation, morphogenesis of multicellular structures, and motility, will support the emergence of tissue structures. Granular hydrogels in which there is no interparticle cross-linking exhibit dynamic properties that may be permissive to such cellular behaviors. However, designing granular hydrogels that lack interparticle cross-linking but support cellular self-organization remains underexplored relative to granular systems stabilized by interparticle cross-linking. In this study, we developed a polyethylene glycol-based granular hydrogel system, with average particle diameters under 40 μm. This granular hydrogel exhibited bulk stress-relaxing behaviors and compatibility with custom microdevices to sustain cell cultures without degradation. The system was studied in conjunction with cocultures of endothelial cells and fibroblasts, known for their spontaneous network formation. Cross-linking, porosity, and cell-adhesive ligands (such as RGD) were manipulated to control system properties. Toward supporting cellular activity, increased porosity was found to enhance the formation of cellular networks, whereas RGD reduced network formation in the system studied. This research highlights the potential of un-cross-linked granular systems to support morphogenetic processes, like vasculogenesis and tissue maturation, offering insights into material design for 3D cell culture systems.

A Universal Strategy to Construct High-Performance Homo- and Heterogeneous Microgel Assembly Bioinks

Three dimensional (3D) extrusion bioprinting aims to replicate the complex architectures and functions of natural tissues and organs. However, the conventional hydrogel and new-emerging microgel bioinks are both difficult in achieving simultaneously high shape-fidelity and good maintenance of cell viability/function, leading to limited amount of qualified hydrogel/microgel bioinks. Herein, a universal strategy is reported to construct high-performance microgel assembly (MA) bioinks by using epigallocatechin gallate-modified hyaluronic acid (HA-EGCG) as coating agent and phenylboronic acid grafted hyaluronic acid (HA-PBA) as assembling agent. HA-EGCG can spontaneously form uniform coating on the microgel surface via mussel-inspired chemistry, while HA-PBA quickly forms dynamic phenylborate bonds with HA-EGCG, conferring the as-prepared MA bioinks with excellent rheological properties, self-healing, and tissue-adhesion. More importantly, this strategy is applicable to various microgel materials, enabling the preparation of homo- and heterogeneous MA (homo-MA and hetero-MA) bioinks and the hierarchical printing of complicated structures with high fidelity by integration of different microgels containing multiple materials/cells in spatial and compositional levels. It further demonstrates the printing of breast cancer organoid in vitro using homo-MA and hetero-MA bioinks and its preliminary application for drug testing. This universal strategy offers a new solution to construct high-performance bioinks for extrusion bioprinting.

Selective Segregation of Thermo-Responsive Microgels via Microfluidic Technology

Separation of equally sized particles distinguished solely by material properties remains still a very challenging task. Here a simple separation of differently charged, thermo-responsive polymeric particles (for example microgels) but equal in size, via the combination of pressure-driven microfluidic flow and precise temperature control is proposed. The separation principle relies on forcing thermo-responsive microgels to undergo the volume phase transition during heating and therefore changing its size and correspondingly the change in drift along a pressure driven shear flow. Different thermo-responsive particle types such as different grades of ionizable groups inside the polymer matrix have different temperature regions of volume phase transition temperature (VPTT). This enables selective control of collapsed versus swollen microgels, and accordingly, this physical principle provides a simple method for fractioning a binary mixture with at least one thermo-responsive particle, which is achieved by elution times in the sense of particle chromatography. The concepts are visualized in experimental studies, with an intend to improve the purification strategy of the broad distribution of charged microgels into fractioning to more narrow distribution microgels distinguished solely by slight differences in net charge.

All-Aqueous Embedded 3D Printing for Freeform Fabrication of Biomimetic 3D Constructs

All-aqueous embedded 3D printing, which involves extruding inks in an aqueous bath, has emerged as a transformative platform for the freeform fabrication of 3D constructs with precise control. The use of a supporting bath not only enables the printing of arbitrarily designed 3D constructs but also broadens ink selection for various soft matters, advancing the wide application of this technology. This review focuses on recent progress in the freeform preparation of 3D constructs using all-aqueous embedded 3D printing. It begins by discussing the significance of ultralow interfacial tension in all-liquid embedded printing and highlights the fundamental concepts and properties of all-aqueous system. The review then introduces recent advances in all-aqueous embedded 3D printing and clarifies the key factors affecting printing stability and shape fidelity, aiming to guide expansion and assessment of emerging printing systems used for various representative applications. Furthermore, it proposes the potential scope and applications of this technology, including in vitro models, cytomimetic microreactors, and soft ionic electronics. Finally, the review discusses the challenges facing current all-aqueous embedded 3D printing and offers future perspectives on possible improvements and developments.

Using a Supramolecular Monomer Formulation Approach to Engineer Modular, Dynamic Microgels, and Composite Macrogels

Microgels show advantages over bulk hydrogels due to convenient control over microgel size and composition, and the ability to use microgels to modularly construct larger hierarchical scaffold hydrogel materials. Here, supramolecular chemistry is used to formulate supramolecular polymer, dynamic microgels solely held together by non-covalent interactions. Four-fold hydrogen bonding ureido-pyrimidinone (UPy) monomers with different functionalities are applied to precisely tune microgel properties in a modular way, via variations in monomer concentration, bifunctional crosslinker ratio, and the incorporation of supramolecular dyes and peptides. Functionalization with a bioactive supramolecular cell-adhesive peptide induced selectivity of cells toward the bioactive microgels over non-active, non-functionalized versions. Importantly, the supramolecular microgels can also be applied as microscale building blocks into supramolecular bulk macrogels with tunable dynamic behavior: a robust and weak macrogel, where the micro- and macrogels are composed of similar molecular building blocks. In a robust macrogel, microgels act as modular micro-building blocks, introducing multi-compartmentalization, while in a weak macrogel, microgels reinforce and enhance mechanical properties. This work demonstrates the potential to modularly engineer higher-length-scale structures using small molecule supramolecular monomers, wherein microgels serve as versatile and modular micro-building units.

Clickable PEG-norbornene microgels support suspension bioprinting and microvascular assembly

The development of perfusable and multiscale vascular networks remains one of the largest challenges in tissue engineering. As such, there is a need for the creation of customizable and facile methods to produce robustly vascularized constructs. In this study, secondarily crosslinkable (clickable) poly(ethylene glycol)-norbornene (PEGNB) microbeads were produced and evaluated for their ability to sequentially support suspension bioprinting and microvascular self-assembly towards the aim of engineering hierarchical vasculature. The clickable PEGNB microbead slurry exhibited mechanical behavior suitable for suspension bioprinting of sacrificial bioinks, could be UV crosslinked into a granular construct post-print, and withstood evacuation of the bioink and subsequent perfusion of the patterned void space. Endothelial and stromal cells co-embedded within jammed RGD-modified PEGNB microbead slurries assembled into capillary-scale vasculature after secondary crosslinking of the beads into granular constructs, with endothelial tubules forming within the interstitial space between microbeads and supported by the perivascular association of the stromal cells. Microvascular self-assembly was not impacted by printing sacrificial bioinks into the cell-laden microbead support bath before UV crosslinking. Collectively, these results demonstrate that clickable PEGNB microbeads are a versatile substrate for both suspension printing and microvascular culture and may be the foundation for a promising methodology to engineer hierarchical vasculature.

The Assembly of Miniaturized Droplets toward Functional Architectures

Recent explorations of bioengineering have generated new concepts and strategies for the processing of soft and functional materials. Droplet assembly techniques can address problems in the construction of extremely soft architectures by expanding the manufacturing capabilities using droplets containing liquid or hydrogels including weak hydrogels. This Perspective sets out to provide a brief overview of this growing field, and discusses the challenges and opportunities ahead. The study highlights the recent key advances of materials and architectures from hitherto effective droplet-assembly technologies, as well as the applications in biomedical and bioengineering fields from artificial tissues to bioreactors. It is envisaged that these assembled architectures, as nature-inspired models, will stimulate the discovery of biomaterials and miniaturized platforms for interdisciplinary research in health, biotechnology, and sustainability.

Engineered Shape-Morphing Transitions in Hydrogels Through Suspension Bath Printing of Temperature-Responsive Granular Hydrogel Inks

4D printing of hydrogels is an emerging technology used to fabricate shape-morphing soft materials that are responsive to external stimuli for use in soft robotics and biomedical applications. Soft materials are technically challenging to process with current 4D printing methods, which limits the design and actuation potential of printed structures. Here, a simple multi-material 4D printing technique is developed that combines dynamic temperature-responsive granular hydrogel inks based on hyaluronic acid, whose actuation is modulated via poly(N-isopropylacrylamide) crosslinker design, with granular suspension bath printing that provides structural support during and after the printing process. Granular hydrogels are easily extruded upon jamming due to their shear-thinning properties and their porous structure enables rapid actuation kinetics (i.e., seconds). Granular suspension baths support responsive ink deposition into complex patterns due to shear-yielding to fabricate multi-material objects that can be post-crosslinked to obtain anisotropic shape transformations. Dynamic actuation is explored by varying printing patterns and bath shapes, achieving complex shape transformations such as 'S'-shaped and hemisphere structures. Furthermore, stepwise actuation is programmed into multi-material structures by using microgels with varied transition temperatures. Overall, this approach offers a simple method to fabricate programmable soft actuators with rapid kinetics and precise control over shape morphing.

Porosity dominates over microgel stiffness for promoting chondrogenesis in zwitterionic granular hydrogels

Granular hydrogels comprised of jammed, crosslinked microgels offer great potential as biomaterial scaffolds for cell-based therapies, including for cartilage tissue regeneration. As stiffness and porosity of hydrogels affect the phenotype of encapsulated cells and the extent of tissue regeneration, the design of tunable granular hydrogels to control and optimize these parameters is highly desirable. We hypothesized that chondrogenesis could be modulated using a granular hydrogel platform based on biocompatible, zwitterionic materials with independent intra- and inter-microgel crosslinking mechanisms. Microgels are made with mechanical fragmentation of photocrosslinked zwitterionic carboxybetaine acrylamide (CBAA) and sulfobetaine methacrylate (SBMA) hydrogels, and secondarily crosslinked in the presence of cells using horseradish peroxide (HRP) to produce cell-laden granular hydrogels. We varied the intra-microgel crosslinking density to produce microgels with varied stiffnesses (1-3 kPa) and swelling properties. These microgels, when resuspended at the same weight fraction and secondarily crosslinked, resulted in granular hydrogels with distinct porosities (5-40%) due to differing swelling properties. The greatest extent of chondrogenesis was achieved in scaffolds with the highest microgel stiffness and highest porosity. However, when scaffold porosity was kept constant and just microgel stiffness varied, cell phenotype and chondrogenesis were similar across scaffolds. These results indicate the dominant role of granular scaffold porosity on chondrogenesis, whereas microgel stiffness appears to play a relatively minor role. These observations are in contrast to cells encapsulated within conventional bulk hydrogels, where stiffness has been shown to significantly affect chondrocyte response. In summary, we introduce chemically-defined, zwitterionic biomaterials to fabricate versatile granular hydrogels allowing for tunable scaffold porosity and microgel stiffness to study and influence chondrogenesis.

On the Relation between the Viscoelastic Properties of Granular Hydrogels and Their Performance as Support Materials in Embedded Bioprinting

Granular hydrogels, formed by jamming microgels suspension, are promising materials for three-dimensional bioprinting applications. Despite their extensive use as support materials for embedded bioprinting, the influence of the particle's physical properties on the macroscale viscoelasticity on one hand and on the printing performance on the other hand remains unclear. Herein, we investigate the linear and nonlinear rheology of κ-carrageenan granular hydrogel through small- and large-amplitude oscillatory shear measurements. We tuned the granular hydrogel's properties by changing the stiffness (soft, medium, stiff) and the packing density of the individual microgels. Characterizations in the linear viscoelasticity regime revealed that the storage modulus of granular hydrogels is not a simple function of microgel stiffness and depends on the microgel packing density. At larger strains, increasing the microgel stiffness reduced the energy dissipation of the granular beds and increased the solid-fluid transition point. To understand how the different rheological properties of granular support materials influence embedded bioprinting, we examined the printing fidelity and cellular filament shrinkage within the granular beds. We found that microgels with low packing density diminished the printing quality, while stiff microgels promoted filament roughness. In addition, we found that highly packed stiff microgels significantly reduced the postprinting contraction of cellular filaments. Overall, this work provides a comprehensive knowledge of the rheology of granular hydrogels that can be used to rationally design support beds for bioprinting applications with specific characteristics.

Granular Hydrogels Improve Myogenic Invasion and Repair after Volumetric Muscle Loss

Skeletal muscle injuries including volumetric muscle loss (VML) lead to excessive tissue scarring and permanent functional disability. Despite its high prevalence, there is currently no effective treatment for VML. Bioengineering interventions such as biomaterials that fill the VML defect to support cell and tissue growth are a promising therapeutic strategy. However, traditional biomaterials developed for this purpose lack the pore features needed to support cell infiltration. The present study investigates for the first time, the impact of granular hydrogels on muscle repair - hypothesizing that their flowability will permit conformable filling of the defect site and their inherent porosity will support the invasion of native myogenic cells, leading to effective muscle repair. Small and large microparticle fragments are prepared from photocurable hyaluronic acid polymer via extrusion fragmentation and facile size sorting. In assembled granular hydrogels, particle size and degree of packing significantly influence pore features, rheological behavior, and injectability. Using a mouse model of VML, it is demonstrated that, in contrast to bulk hydrogels, granular hydrogels support early-stage (satellite cell invasion) and late-stage (myofiber regeneration) muscle repair processes. Together, these results highlight the promising potential of injectable and porous granular hydrogels in supporting endogenous repair after severe muscle injury.

Embedded 3D Bioprinting of Collagen Inks into Microgel Baths to Control Hydrogel Microstructure and Cell Spreading

Microextrusion-based 3D bioprinting into support baths has emerged as a promising technique to pattern soft biomaterials into complex, macroscopic structures. It is hypothesized that interactions between inks and support baths, which are often composed of granular microgels, can be modulated to control the microscopic structure within these macroscopic-printed constructs. Using printed collagen bioinks crosslinked either through physical self-assembly or bioorthogonal covalent chemistry, it is demonstrated that microscopic porosity is introduced into collagen inks printed into microgel support baths but not bulk gel support baths. The overall porosity is governed by the ratio between the ink's shear viscosity and the microgel support bath's zero-shear viscosity. By adjusting the flow rate during extrusion, the ink's shear viscosity is modulated, thus controlling the extent of microscopic porosity independent of the ink composition. For covalently crosslinked collagen, printing into support baths comprised of gelatin microgels (15-50 µm) results in large pores (≈40 µm) that allow human corneal mesenchymal stromal cells (MSCs) to readily spread, while control samples of cast collagen or collagen printed in non-granular support baths do not allow cell spreading. Taken together, these data demonstrate a new method to impart controlled microscale porosity into 3D printed hydrogels using granular microgel support baths.

Engineering Microgel Packing to Tailor the Physical and Biological Properties of Gelatin Methacryloyl Granular Hydrogel Scaffolds

Granular hydrogel scaffolds (GHS) are fabricated via placing hydrogel microparticles (HMP) in close contact (packing), followed by physical and/or chemical interparticle bond formation. Gelatin methacryloyl (GelMA) GHS have recently emerged as a promising platform for biomedical applications; however, little is known about how the packing of building blocks, physically crosslinked soft GelMA HMP, affects the physical (pore microarchitecture and mechanical/rheological properties) and biological (in vitro and in vivo) attributes of GHS. Here, the GHS pore microarchitecture is engineered via the external (centrifugal) force-induced packing and deformation of GelMA HMP to regulate GHS mechanical and rheological properties, as well as biological responses in vitro and in vivo. Increasing the magnitude and duration of centrifugal force increases the HMP deformation/packing, decreases GHS void fraction and median pore diameter, and increases GHS compressive and storage moduli. MDA-MB-231 human triple negative breast adenocarcinoma cells spread and flatten on the GelMA HMP surface in loosely packed GHS, whereas they adopt an elongated morphology in highly packed GHS as a result of spatial confinement. Via culturing untreated or blebbistatin-treated cells in GHS, the effect of non-muscle myosin II-driven contractility on cell morphology is shown. In vivo subcutaneous implantation in mice confirms a significantly higher endothelial, fibroblast, and macrophage cell infiltration within the GHS with a lower packing density, which is in accordance with the in vitro cell migration outcome. These results indicate that the packing state of GelMA GHS may enable the engineering of cell response in vitro and tissue response in vivo. This research is a fundamental step forward in standardizing and engineering GelMA GHS microarchitecture for tissue engineering and regeneration.

Granular Hydrogels in Biofabrication: Recent Advances and Future Perspectives

Granular hydrogels, which are formed by densely packing microgels, are promising materials for bioprinting due to their extrudability, porosity, and modularity. However, the multidimensional parameter space involved in granular hydrogel design makes material optimization challenging. For example, design inputs such as microgel morphology, packing density, or stiffness can influence multiple rheological properties that govern printability and the behavior of encapsulated cells. This review provides an overview of fabrication methods for granular hydrogels, and then examines how important design inputs can influence material properties associated with printability and cellular responses across multiple scales. Recent applications of granular design principles in bioink engineering are described, including the development of granular support hydrogels for embedded printing. Further, the paper provides an overview of how key physical properties of granular hydrogels can influence cellular responses, highlighting the advantages of granular materials for promoting cell and tissue maturation after the printing process. Finally, potential future directions for advancing the design of granular hydrogels for bioprinting are discussed.

Granular Biphasic Colloidal Hydrogels for 3D Bioprinting

Granular hydrogels composed of hydrogel microparticles are promising candidates for 3D bioprinting due to their ability to protect encapsulated cells. However, to achieve high print fidelity, hydrogel microparticles need to jam to exhibit shear-thinning characteristics, which is crucial for 3D printing. Unfortunately, this overpacking can significantly impact cell viability, thereby negating the primary advantage of using hydrogel microparticles to shield cells from shear forces. To overcome this challenge, a novel solution: a biphasic, granular colloidal bioink designed to optimize cell viability and printing fidelity is introduced. The biphasic ink consists of cell-laden polyethylene glycol (PEG) hydrogel microparticles embedded in a continuous gelatin methacryloyl (GelMA)-nanosilicate colloidal network. Here, it is demonstrated that this biphasic bioink offers outstanding rheological properties, print fidelity, and structural stability. Furthermore, its utility for engineering complex tissues with multiple cell types and heterogeneous microenvironments is demonstrated, by incorporating β-islet cells into the PEG microparticles and endothelial cells in the GelMA-nanosilicate colloidal network. Using this approach, it is possible to induce cell patterning, enhance vascularization, and direct cellular function. The proposed biphasic bioink holds significant potential for numerous emerging biomedical applications, including tissue engineering and disease modeling.

Advancing Synthetic Hydrogels through Nature-Inspired Materials Chemistry

Synthetic extracellular matrix (ECM) mimics that can recapitulate the complex biochemical and mechanical nature of native tissues are needed for advanced models of development and disease. Biomedical research has heavily relied on the use of animal-derived biomaterials, which is now impeding their translational potential and convoluting the biological insights gleaned from in vitro tissue models. Natural hydrogels have long served as a convenient and effective cell culture tool, but advances in materials chemistry and fabrication techniques now present promising new avenues for creating xenogenic-free ECM substitutes appropriate for organotypic models and microphysiological systems. However, significant challenges remain in creating synthetic matrices that can approximate the structural sophistication, biochemical complexity, and dynamic functionality of native tissues. This review summarizes key properties of the native ECM, and discusses recent approaches used to systematically decouple and tune these properties in synthetic matrices. The importance of dynamic ECM mechanics, such as viscoelasticity and matrix plasticity, is also discussed, particularly within the context of organoid and engineered tissue matrices. Emerging design strategies to mimic these dynamic mechanical properties are reviewed, such as multi-network hydrogels, supramolecular chemistry, and hydrogels assembled from biological monomers.

Photo-annealable agarose microgels for jammed microgel printing: Transforming thermogelling hydrogel to a functional bioink

Three-dimensional (3D) printing of hydrogel structures using jammed microgel inks offer distinct advantages of improved printing functionalities, as these inks are strain-yielding and self-recovering types. However, interparticle binding in granular hydrogel inks is a challenge to overcome the limited integrity and reduced macroscale modulus prevalent in the 3D printed microgel scaffolds. In this study, we prepared chemically annealable agarose microgels through a process of xerogel rehydration, applying a low-cost and high throughput method of spray drying. The crosslinked jammed microgel matrix is found to have superior mechanical properties with a Young's modulus of 2.23 MPa and extensibility up to 7.2%, surpassing those of traditional biopolymer-based and microgel-based inks. Furthermore, this study addresses the complexities encountered in the existing system of printing thermoresponsive agarose bioink using this jammed microgel printing approach. The jammed agarose microgel ink exhibited to be self-recovering, yield stress fluid and validated the temperature-independent printing. Furthermore, the 3D printed jammed microgel scaffold demonstrated good cell responsiveness as evaluated through the viability and morphological study in-vitro with mesenchymal stem cells cultured in it. This unique fabrication approach offers exciting possibilities to expand on microgel printing for varied requirements in tissue engineering.

Laponite nanoclay loaded microgel suspensions as supportive matrices for osteogenesis

Microscale carriers have emerged as promising materials for nurturing cell growth and as delivery vehicles for regenerative therapies. Carriers based on hydrogels have proved advantageous, where "microgels" can be formulated to have a broad range of properties to guide the behavior of adherent cells. Here we demonstrate the fabrication of osteogenic microgels through incorporation of laponite nanoclays. Forming a jammed suspension provides a scaffolding where cells can adhere to the surface of the microgels, with pathways for migration and proliferation fostered by the interstitial volume. By varying the content and type of laponite-RD and XLG-the degree of osteogenesis can be tuned in embedded populations of adipose derived stem cells (ADSCs). The nano- micro-structured composite materials enhance osteogenesis at the transcript and protein level, leading to increased deposition of bone minerals and an increase in the compressive modulus of the assembled scaffold. Together, these microgel suspensions are promising materials for encouraging osteogenesis with scope for delivery via syringe injection and stabilization to bone-mimetic mechanical properties after matrix deposition.

Injectable Radiopaque Hyaluronic Acid Granular Hydrogels for Intervertebral Disc Repair

Injectable hydrogels offer minimally-invasive treatment options for degenerative disc disease, a prevalent condition affecting millions annually. Many hydrogels explored for intervertebral disc (IVD) repair suffer from weak mechanical integrity, migration issues, and expulsion. To overcome these limitations, an injectable and radiopaque hyaluronic acid granular hydrogel is developed. The granular structure provides easy injectability and low extrusion forces, while the radiopacity enables direct visualization during injection into the disc and non-invasive monitoring after injection. The radiopaque granular hydrogel is injected into rabbit disc explants to investigate restoration of healthy disc mechanics following needle puncture injury ex vivo and then delivered in a minimally-invasive manner into the intradiscal space in a clinically-relevant in vivo large animal goat model of IVD degeneration initiated through degradation by chondroitinase. The radiopaque granular hydrogel successfully halted loss of disc height due to degeneration. Further, the hydrogel not only enhanced proteoglycan content and reduced collagen content in the nucleus pulposus (NP) region compared to degenerative discs, but also helped to maintain the structural integrity of the disc and promote healthy segregation of the NP and annulus fibrosus regions. Overall, this study demonstrates the great potential of an injectable radiopaque granular hydrogel for treatment of degenerative disc disease.

Improving tumor microenvironment assessment in chip systems through next-generation technology integration

The tumor microenvironment (TME) comprises a diverse array of cells, both cancerous and non-cancerous, including stromal cells and immune cells. Complex interactions among these cells play a central role in driving cancer progression, impacting critical aspects such as tumor initiation, growth, invasion, response to therapy, and the development of drug resistance. While targeting the TME has emerged as a promising therapeutic strategy, there is a critical need for innovative approaches that accurately replicate its complex cellular and non-cellular interactions; the goal being to develop targeted, personalized therapies that can effectively elicit anti-cancer responses in patients. Microfluidic systems present notable advantages over conventional in vitro 2D co-culture models and in vivo animal models, as they more accurately mimic crucial features of the TME and enable precise, controlled examination of the dynamic interactions among multiple human cell types at any time point. Combining these models with next-generation technologies, such as bioprinting, single cell sequencing and real-time biosensing, is a crucial next step in the advancement of microfluidic models. This review aims to emphasize the importance of this integrated approach to further our understanding of the TME by showcasing current microfluidic model systems that integrate next-generation technologies to dissect cellular intra-tumoral interactions across different tumor types. Carefully unraveling the complexity of the TME by leveraging next generation technologies will be pivotal for developing targeted therapies that can effectively enhance robust anti-tumoral responses in patients and address the limitations of current treatment modalities.

Granular polyrotaxane microgels as injectable hydrogels for corneal tissue regeneration

Corneal diseases, a leading cause of global vision impairment, present challenges in treatment due to corneal tissue donor scarcity and transplant rejection. Hydrogel biomaterials in the form of corneal implants for tissue regeneration, while promising, have faced obstacles related to cellular and tissue integration. This study develops and investigates the potential of granular polyrotaxane (GPR) hydrogels as a scaffold for corneal keratocyte growth and transparent tissue generation. Employing host-guest driven supramolecular interactions, we developed injectable, cytocompatible hydrogels. By optimizing cyclodextrin (CD) concentrations in thiol-ene crosslinked PEG microgels, we observed improved mechanical properties and thermoresponsiveness while preserving injectability. These microgels, adaptable for precise defect filling, 3D printing or tissue culture facilitate enhanced cellular integration with corneal keratocytes and exhibit tissue-like structures in culture. Our findings demonstrate the promise of GPR hydrogels as a minimally invasive avenue for corneal tissue regeneration. These results have the potential to address transplantation challenges, enhance clinical outcomes, and restore vision.

Size-Controllable and pH-Sensitive Whey Protein Microgels as High-Performance Aqueous Biolubricants

Developing efficient aqueous biolubricants has become a significant focus of research due to their prevalence in biotribological contacts and enormous potential in soft matter applications. In this study, size-controllable, pH-sensitive whey protein microgels were prepared using a water-in-water emulsion template method from protein-polysaccharide phase separation. The granular hydrogel from the protein microgels exhibited superior lubricity, obtaining 2.7-fold and 1.7-fold reductions in coefficient of friction (μ) compared to native protein and human saliva (μ = 0.30 compared to 0.81 and 0.52, respectively). The microgels also exhibited outstanding load-bearing capabilities, sustaining lubrication under normal forces up to 5 N. Microgels with a smaller size (1 μm) demonstrated better lubricating performance than 6 and 20 μm microgels. The exceptional lubricity was from a synergistic effect of the ball-bearing mechanism and the hydration state of the microgels. Particularly at pH 7.4, the hydration layer surrounding highly negative charges contributed to the electrostatic repulsion among the swollen microgels, leading to an improved buffer ability to separate contact surfaces and effective rolling behavior. Such pH-dependent repulsion was evidenced using a surface forces apparatus that the adhesion between the whey protein-coated surfaces and protein-mica surfaces decreased from 4.49 to 0.97 mN/m and from 7.89 to 0.36 mN/m, respectively, with pH increasing from the isoelectronic point to 7.4. Our findings fundamentally elucidated the tribo-rheological properties and lubrication mechanisms of the whey protein microgels with excellent biocompatibility and environmental responsiveness, offering novel insights for their food and biomedical applications requiring aqueous biolubrication.

Advances in 3D bioprinting for environmental remediation and hazardous materials treatment

The high-throughput method based on the micron-level structure that 3D bioprinting technology offers for various environmental microbiological engineering applications is made possible by its several printing paths and precision programming control. This versatility makes it an on-demand manufacturing technology. A novel 3D manufacturing technique called 3D bioprinting may be used to precisely uptake and disperse bacteria to create microbial active substances with a variety of intricate functionalities for environmental applications. The technological challenges that the current 3D bioprinting technology must face include the mechanical properties of materials, the creation of specific bioinks to adapt to different strains, and the exploration of 4D bioprinting for intelligent applications. Therefore, this analysis delves deeply into the core technological ideas of 3D bioprinting, bioink materials, and their environmental applications. It also offers recommendations about the challenges and opportunities associated with 3D bioprinting. Combined with the present advancements in microbe enhancement technology, 3D bioprinting will provide an enabling platform for multifunctional microorganisms and facilitate the management of in situ directional responses in the environmental domain. This review highlights the applications of 3D bioprinting in the environmental monitoring and bioremediation. 3D printing in solid waste management is also discussed in detail.

Localized Ionic Reinforcement of Double Network Granular Hydrogels

Nature produces soft materials with fascinating combinations of mechanical properties. For example, the mussel byssus embodies a combination of stiffness and toughness, a feature that is unmatched by synthetic hydrogels. Key to enabling these excellent mechanical properties are the well-defined structures of natural materials and their compositions controlled on lengths scales down to tens of nanometers. The composition of synthetic materials can be controlled on a micrometer length scale if processed into densely packed microgels. However, these microgels are typically soft. Microgels can be stiffened by enhancing interactions between particles, for example through the formation of covalent bonds between their surfaces or a second interpenetrating hydrogel network. Nonetheless, changes in the composition of these synthetic materials occur on a micrometer length scale. Here, 3D printable load-bearing granular hydrogels are introduced whose composition changes on the tens of nanometer length scale. The hydrogels are composed of jammed microgels encompassing tens of nm-sized ionically reinforced domains that increase the stiffness of double network granular hydrogels up to 18-fold. The printability of the ink and the local reinforcement of the resulting granular hydrogels are leveraged to 3D print a butterfly with composition and structural changes on a tens of nanometer length scale.

Formation of Self-Healing Granular Eutectogels through Jammed Carbopol Microgels in Supercooled Deep Eutectic Solvent

Typically, gel-like materials consist of a polymer network structure in a solvent. In this work, a gel-like material is developed in a deep eutectic solvent (DES) without the presence of a polymer network, achieved simply by adding microgels. The DES is composed of choline chloride and citric acid and remains stably in a supercooled state at room temperature, exhibiting Newtonian fluid behavior with high viscosity. When the microgel (Carbopol) concentration exceeds 2 wt %, the DES undergoes a transition from a liquid to a soft gel state, characterized as a granular eutectogel. The soft gel characteristics of eutectogels exhibit a yield stress, and their storage moduli exceed the loss moduli. The yield stress and storage moduli are observed to increase with increasing microgel concentration. In contrast, the ion conductivity decreases with increasing microgel concentration but eventually levels off. Because the eutectogel can dissolve completely in excess water, it is a physical gel-like material, attributed to the densely packed structure of microgels in the supercooled DES. Due to the absence of networks, the granular eutectogel has the capability to self-heal simply by being pushed together after being cut into two pieces.

Surface-Functionalized Microgels as Artificial Antigen-Presenting Cells to Regulate Expansion of T Cells

Artificial antigen-presenting cells (aAPCs) are currently used to manufacture T cells for adoptive therapy in cancer treatment, but a readily tunable and modular system can enable both rapid T cell expansion and control over T cell phenotype. Here, it is shown that microgels with tailored surface biochemical properties can serve as aAPCs to mediate T cell activation and expansion. Surface functionalization of microgels is achieved via layer-by-layer coating using oppositely charged polymers, forming a thin but dense polymer layer on the surface. This facile and versatile approach is compatible with a variety of coating polymers and allows efficient and flexible surface-specific conjugation of defined peptides or proteins. The authors demonstrate that tethering appropriate stimulatory ligands on the microgel surface efficiently activates T cells for polyclonal and antigen-specific expansion. The expansion, phenotype, and functional outcome of primary mouse and human T cells can be regulated by modulating the concentration, ratio, and distribution of stimulatory ligands presented on microgel surfaces as well as the stiffness and viscoelasticity of the microgels.

3D-printed polyether-ether ketone/carboxymethyl cellulose scaffolds coated with Zn-Mn doped mesoporous bioactive glass nanoparticles

Patient-specific fabrication of scaffold/implant requires an engineering approach to manufacture the ideal scaffold. Herein, we design and 3D print scaffolds comprised of polyether-ether-ketone (PEEK) and sodium-carboxymethyl cellulose (Na-CMC). The fabricated scaffold was dip coated with Zn and Mn doped bioactive glass nanoparticles (Zn-Mn MBGNs). The synthesized ink exhibit suitable shear-thinning behavior for direct ink write (DIW) 3D printing. The scaffolds were crafted with precision, featuring 85% porosity, 0.3 mm layer height, and 1.5 mm/s printing speed at room temperature. Scanning electron microscopy images reveal a well-defined scaffold with an average pore size of 600 ± 30 μm. The energy dispersive X-ray spectroscopy analysis confirmed a well dispersed/uniform coating of Zn-Mn MBGNs on the PEEK/Na-CMC scaffold. Fourier transform infrared spectroscopy approved the presence of PEEK, CMC, and Zn-Mn MBGNs. The tensile test revealed a Young's modulus of 2.05 GPa. Antibacterial assays demonstrate inhibition zone against Staphylococcus aureus and Escherichia Coli strains. Chick Chorioallantoic Membrane assays also present significant angiogenesis potential, owing to the antigenic nature of Zn-Mn MBGNs. WST-8 cell viability assays depicted cell proliferation, with a 103% viability after 7 days of culture. This study suggests that the PEEK/Na-CMC scaffolds coated with Zn-Mn MBGNs are an excellent candidate for osteoporotic fracture treatment. Thus, the fabricated scaffold can offer multifaceted properties for enhanced patient outcomes in the bone tissue regeneration.

Enhanced osteogenesis of mesenchymal stem cells encapsulated in injectable microporous hydrogel

Delivery of therapeutic stem cells to treat bone tissue damage is a promising strategy that faces many hurdles to clinical translation. Among them is the design of a delivery vehicle which promotes desired cell behavior for new bone formation. In this work, we describe the use of an injectable microporous hydrogel, made of crosslinked gelatin microgels, for the encapsulation and delivery of human mesenchymal stem cells (MSCs) and compared it to a traditional nonporous injectable hydrogel. MSCs encapsulated in the microporous hydrogel showed rapid cell spreading with direct cell-cell connections whereas the MSCs in the nonporous hydrogel were entrapped by the surrounding polymer mesh and isolated from each other. On a per-cell basis, encapsulation in microporous hydrogel induced a 4 × increase in alkaline phosphatase (ALP) activity and calcium mineral deposition in comparison to nonporous hydrogel, as measured by ALP and calcium assays, which indicates more robust osteogenic differentiation. RNA-seq confirmed the upregulation of the genes and pathways that are associated with cell spreading and cell-cell connections, as well as the osteogenesis in the microporous hydrogel. These results demonstrate that microgel-based injectable hydrogels can be useful tools for therapeutic cell delivery for bone tissue repair.

Microgels for Cell Delivery in Tissue Engineering and Regenerative Medicine

Microgels prepared from natural or synthetic hydrogel materials have aroused extensive attention as multifunctional cells or drug carriers, that are promising for tissue engineering and regenerative medicine. Microgels can also be aggregated into microporous scaffolds, promoting cell infiltration and proliferation for tissue repair. This review gives an overview of recent developments in the fabrication techniques and applications of microgels. A series of conventional and novel strategies including emulsification, microfluidic, lithography, electrospray, centrifugation, gas-shearing, three-dimensional bioprinting, etc. are discussed in depth. The characteristics and applications of microgels and microgel-based scaffolds for cell culture and delivery are elaborated with an emphasis on the advantages of these carriers in cell therapy. Additionally, we expound on the ongoing and foreseeable applications and current limitations of microgels and their aggregate in the field of biomedical engineering. Through stimulating innovative ideas, the present review paves new avenues for expanding the application of microgels in cell delivery techniques.

3D bioprinting of dense cellular structures within hydrogels with spatially controlled heterogeneity

Embedded bioprinting is an emerging technology for precise deposition of cell-laden or cell-only bioinks to construct tissue like structures. Bioink is extruded or transferred into a yield stress hydrogel or a microgel support bath allowing print needle motion during printing and providing temporal support for the printed construct. Although this technology has enabled creation of complex tissue structures, it remains a challenge to develop a support bath with user-defined extracellular mimetic cues and their spatial and temporal control. This is crucial to mimic the dynamic nature of the native tissue to better regenerate tissues and organs. To address this, we present a bioprinting approach involving printing of a photocurable viscous support layer and bioprinting of a cell-only or cell-laden bioink within this viscous layer followed by brief exposure to light to partially crosslink the support layer. This approach does not require shear thinning behavior and is suitable for a wide range of photocurable hydrogels to be used as a support. It enables multi-material printing to spatially control support hydrogel heterogeneity including temporal delivery of bioactive cues (e.g. growth factors), and precise patterning of dense multi-cellular structures within these hydrogel supports. Here, dense stem cell aggregates are printed within methacrylated hyaluronic acid-based hydrogels with patterned heterogeneity to spatially modulate human mesenchymal stem cell osteogenesis. This study has significant impactions on creating tissue interfaces (e.g. osteochondral tissue) in which spatial control of extracellular matrix properties for patterned stem cell differentiation is crucial.