Creating Physicochemical Gradients in Modular Microporous Annealed Particle Hydrogels via a Microfluidic Method

Impact of Annealing Chemistry on the Properties and Performance of Microporous Annealed Particle Hydrogels

Microporous annealed particle (MAP) hydrogels are a promising class of in situ-forming scaffolds for tissue repair and regeneration. While an expansive toolkit of annealing chemistries has been described, the effects of different annealing chemistries on MAP hydrogel properties and performance have not been studied. In this study, we address this gap through a controlled head-to-head comparison of poly(ethylene glycol) (PEG)-based MAP hydrogels that were annealed using tetrazine-norbornene and thiol-norbornene click chemistry. Characterization of material properties revealed that tetrazine click annealing significantly increases MAP hydrogel shear storage modulus and results in slower in vitro degradation kinetics when microgels with a higher cross-link density are used. However, these effects are muted when the MAP hydrogels are fabricated from microgels with a lower cross-link density. In contrast, in vivo testing in murine critical-sized calvarial defects revealed that these differences in physicochemical properties do not translate to differences in bone volume or calvarial defect healing when growth-factor-loaded MAP hydrogel scaffolds are implanted into mouse calvarial defects. Nonetheless, the impact of tetrazine click annealing could be important in other applications and should be investigated further.

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 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.

Embedded Printing of Hydrogels and Watery Suspensions of Cells in Patterned Granular Baths

Bioprinting within support media has emerged as the superior alternative to conventional extrusion printing. Not only because it allows for more freedom over the shapes that can be printed but also because it allows for the printing of inks that would not retain shape fidelity in freeform deposition such as watery liquids. Apart from functioning as mechanical support during embedded printing, hydrogel microparticle support media can provide the unique advantage of offering distinct chemotactic cues to cells printed in the baths by varying the composition of the hydrogel microparticles. There is great potential in compartmentalized granular baths consisting of different hydrogel particle materials in the field of tissue engineering, as these allow for the local inclusion of properties or cues to guide tissue development. In this work, we present a method to create compartmentalized embedding baths by printing multiple granular hydrogel materials that are widely used in tissue engineering. After adapting the volume fraction (φp) of the particles in the bath, we print within them using both inks composed of hydrogel or of cells and other particles suspended in watery liquid. Our process consists of the following three steps: First, the hydrogel microparticles are packed at a φp that allows them to be extruded while being reversibly jammed, facilitating the localized deposition of the granular media to form a compartmentalized bath. Second, each granular media is deposited in succession to create a packed suspension compartment, and by adding liquid post deposition, φp is reduced to allow for embedded printing. Finally, we demonstrate the printing of multiple inks within the compartmentalized embedding bath and highlight the distinct differences between using inks composed of hydrogels or inks composed of particles suspended in watery liquid. This approach combines the advantages of embedded printing through the use of granular media with the added ability to pattern multiple bioactive granular materials to locally affect the behavior of cells printed within the bath. We expect that this workflow will allow researchers to create spatially compartmentalized, customized bioactive embedding baths that allow for the embedded printing of inks composed of hydrogels, cells, and other particles adapted to their need.

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.

Fabrication of gradient hydrogels using a thermophoretic approach in microfluidics

The extracellular matrix presents spatially varying physical cues that can influence cell behavior in many processes. Physical gradients within hydrogels that mimic the heterogenous mechanical microenvironment are useful to study the impact of these cues on cellular responses. Therefore, simple and reliable techniques to create such gradient hydrogels are highly desirable. This work demonstrates the fabrication of stiffness gradient Gellan gum (GG) hydrogels by applying a temperature gradient across a microchannel containing hydrogel precursor solution. Thermophoretic migration of components within the precursor solution generates a concentration gradient that mirrors the temperature gradient profile, which translates into mechanical gradients upon crosslinking. Using this technique, GG hydrogels with stiffness gradients ranging from 20 to 90 kPa over 600µm are created, covering the elastic moduli typical of moderately hard to hard tissues. MC3T3 osteoblast cells are then cultured on these gradient substrates, which exhibit preferential migration and enhanced osteogenic potential toward the stiffest region on the gradient. Overall, the thermophoretic approach provides a non-toxic and effective method to create hydrogels with defined mechanical gradients at the micron scale suitable forin vitrobiological studies and potentially tissue engineering applications.

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.

3D printing microporous scaffolds from modular bioinks containing sacrificial, cell-encapsulating microgels

Microgel-based biomaterials have inherent porosity and are often extrudable, making them well-suited for 3D bioprinting applications. Cells are commonly introduced into these granular inks post-printing using cell infiltration. However, due to slow cell migration speeds, this strategy struggles to achieve depth-independent cell distributions within thick 3D printed geometries. To address this, we leverage granular ink modularity by combining two microgels with distinct functions: (1) structural, UV-crosslinkable microgels made from gelatin methacryloyl (GelMA) and (2) sacrificial, cell-laden microgels made from oxidized alginate (AlgOx). We hypothesize that encapsulating cells within sacrificial AlgOx microgels would enable the simultaneous introduction of void space and release of cells at depths unachievable through cell infiltration alone. Blending the microgels in different ratios produces a family of highly printable GelMA : AlgOx microgel inks with void fractions ranging from 0.03 to 0.35. As expected, void fraction influences the morphology of human umbilical vein endothelial cells (HUVEC) within GelMA : AlgOx inks. Crucially, void fraction does not alter the ideal HUVEC distribution seen throughout the depth of 3D printed samples. This work presents a strategy for fabricating constructs with tunable porosity and depth-independent cell distribution, highlighting the promise of microgel-based inks for 3D bioprinting.

Microparticle Hydrogel Material Properties Emerge from Mixing-Induced Homogenization in a Poly(ethylene glycol) and Dextran Aqueous Two-Phase System

Polymer-polymer aqueous two-phase systems (ATPSs) are attractive for microgel synthesis, but given the complexity of phase separation, predicting microgel material properties from ATPS formulations is not trivial. The objective of this study was to determine how the phase diagram of a poly(ethylene glycol) (PEG) and dextran ATPS is related to the material properties of PEG microgel products. PEG-dextran ATPSs were prepared from four-arm 20 kDa PEG-norbornene and 40 kDa dextran in phosphate buffered saline (PBS), and the phase diagram was constructed. PEG microgels were synthesized from five ATPS formulations using an oligopeptide cross-linker and thiol-norbornene photochemistry. Thermogravimetric analysis (TGA) revealed that the polymer concentration of microgel pellets linearly correlates with the average concentration of PEG in the ATPS rather than the separated phase compositions, as determined from the phase diagram. Atomic force microscopy (AFM) and bulk rheology studies demonstrated that the mechanical properties of microgels rely on both the average concentration of PEG in the ATPS and the ATPS volume ratio as determined from the phase diagram. These findings suggest that PEG-dextran ATPSs undergo homogenization upon mixing, which principally determines the material properties of the microgels upon gelation.

Mechanical Signal-Tailored Hydrogel Microspheres Recruit and Train Stem Cells for Precise Differentiation

The aberrant mechanical microenvironment in degenerated tissues induces misdirection of cell fate, making it challenging to achieve efficient endogenous regeneration. Herein, a hydrogel microsphere-based synthetic niche with integrated cell recruitment and targeted cell differentiation properties via mechanotransduction is constructed . Through the incorporation of microfluidics and photo-polymerization strategies, fibronectin (Fn) modified methacrylated gelatin (GelMA) microspheres are prepared with the independently tunable elastic modulus (1-10Kpa) and ligand density (2 and 10 µg mL-1 ), allowing a wide range of cytoskeleton modulation to trigger the corresponding mechanobiological signaling. The combination of the soft matrix (2Kpa) and low ligand density (2 µg mL-1 ) can support the nucleus pulposus (NP)-like differentiation of intervertebral disc (IVD) progenitor/stem cells by translocating Yes-associated protein (YAP), without the addition of inducible biochemical factors. Meanwhile, platelet-derived growth factor-BB (PDGF-BB) is loaded onto Fn-GelMA microspheres (PDGF@Fn-GelMA) via the heparin-binding domain of Fn to initiate endogenous cell recruitment. In in vivo experiments, hydrogel microsphere-niche maintained the IVD structure and stimulated matrix synthesis. Overall, this synthetic niche with cell recruiting and mechanical training capabilities offered a promising strategy for endogenous tissue regeneration.

3D gradient printing based on digital light processing

3D gradient printing is a type of fabrication technique that builds three-dimensional objects with gradually changing properties. Gradient digital light processing based 3D printing has garnered considerable attention in recent years. This function-oriented technology precisely manipulates the performance of different positions of materials and prints them as a monolithic structure to realize specific functions. This review presents a conceptual understanding of gradient properties, covering an overview of current techniques and materials that can produce gradient structures, as well as their limitations and challenges. The principle of digital light processing (DLP) technology and feasible strategies for 3D gradient printing to overcome any barriers are also presented. Additionally, this review discusses the promising future of 4D bioprinting systems based on DLP printing.

Influence of Microgel and Interstitial Matrix Compositions on Granular Hydrogel Composite Properties

Granular hydrogels are an emerging class of biomaterials formed by jamming hydrogel microparticles (i.e., microgels). These materials have many advantageous properties that can be tailored through microgel design and extent of packing. To enhance the range of properties, granular composites can be formed with a hydrogel interstitial matrix between the packed microgels, allowing for material flow and then stabilization after crosslinking. This approach allows for distinct compartments (i.e., microgels and interstitial space) with varied properties to engineer complex material behaviors. However, a thorough investigation of how the compositions and ratios of microgels and interstitial matrices influence material properties has not been performed. Herein, granular hydrogel composites are fabricated by combining fragmented hyaluronic acid (HA) microgels with interstitial matrices consisting of photocrosslinkable HA. Microgels of varying compressive moduli (10-70 kPa) are combined with interstitial matrices (0-30 vol.%) with compressive moduli varying from 2-120 kPa. Granular composite structure (confocal imaging), mechanics (local and bulk), flow behavior (rheology), and printability are thoroughly assessed. Lastly, variations in the interstitial matrix chemistry (covalent vs guest-host) and microgel degradability are investigated. Overall, this study describes the influence of granular composite composition on structure and mechanical properties of granular hydrogels towards informed designs for future applications.

Integrated gradient tissue-engineered osteochondral scaffolds: Challenges, current efforts and future perspectives

The osteochondral defect repair has been most extensively studied due to the rising demand for new therapies to diseases such as osteoarthritis. Tissue engineering has been proposed as a promising strategy to meet the demand of simultaneous regeneration of both cartilage and subchondral bone by constructing integrated gradient tissue-engineered osteochondral scaffold (IGTEOS). This review brought forward the main challenges of establishing a satisfactory IGTEOS from the perspectives of the complexity of physiology and microenvironment of osteochondral tissue, and the limitations of obtaining the desired and required scaffold. Then, we comprehensively discussed and summarized the current tissue-engineered efforts to resolve the above challenges, including architecture strategies, fabrication techniques and in vitro/in vivo evaluation methods of the IGTEOS. Especially, we highlighted the advantages and limitations of various fabrication techniques of IGTEOS, and common cases of IGTEOS application. Finally, based on the above challenges and current research progress, we analyzed in details the future perspectives of tissue-engineered osteochondral construct, so as to achieve the perfect reconstruction of the cartilaginous and osseous layers of osteochondral tissue simultaneously. This comprehensive and instructive review could provide deep insights into our current understanding of IGTEOS.

Progress of Microfluidic Hydrogel-Based Scaffolds and Organ-on-Chips for the Cartilage Tissue Engineering

Cartilage degeneration is among the fundamental reasons behind disability and pain across the globe. Numerous approaches have been employed to treat cartilage diseases. Nevertheless, none have shown acceptable outcomes in the long run. In this regard, the convergence of tissue engineering and microfabrication principles can allow developing more advanced microfluidic technologies, thus offering attractive alternatives to current treatments and traditional constructs used in tissue engineering applications. Herein, the current developments involving microfluidic hydrogel-based scaffolds, promising structures for cartilage regeneration, ranging from hydrogels with microfluidic channels to hydrogels prepared by the microfluidic devices, that enable therapeutic delivery of cells, drugs, and growth factors, as well as cartilage-related organ-on-chips are reviewed. Thereafter, cartilage anatomy and types of damages, and present treatment options are briefly overviewed. Various hydrogels are introduced, and the advantages of microfluidic hydrogel-based scaffolds over traditional hydrogels are thoroughly discussed. Furthermore, available technologies for fabricating microfluidic hydrogel-based scaffolds and microfluidic chips are presented. The preclinical and clinical applications of microfluidic hydrogel-based scaffolds in cartilage regeneration and the development of cartilage-related microfluidic chips over time are further explained. The current developments, recent key challenges, and attractive prospects that should be considered so as to develop microfluidic systems in cartilage repair are highlighted.

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.

Biomaterials with Stiffness Gradient for Interface Tissue Engineering

Interface tissue engineering is a rapidly growing field that aims to develop engineered tissue alternates with the goal of promoting integration between multiple tissue types. Engineering interface tissues is a complex process, which requires a specialized biomaterials with organized material composition, stiffness, cell types, and signaling molecules. Among these, stiffness-controllable substrates have been developed to investigate the effect of stiffness on cell behavior. Especially these substrates with graded stiffness are advantageous since they allow the differentiation of multiple cell phenotypes and subsequent tissue development. In this review, we highlight the various types of manufacturing techniques that can be leveraged to fabricate scaffolds with stiffness gradient, discuss methods to characterize them, and gradient biomaterials for controlling cellular behavior including attachment, migration, proliferation, and differentiation. We also address fundamentals of interface tissue organization, and stiffness gradient biomaterials for interface tissue regeneration. Potential challenges and future directions in this emerging field are also discussed.

Pushing the Natural Frontier: Progress on the Integration of Biomaterial Cues toward Combinatorial Biofabrication and Tissue Engineering

The engineering of fully functional, biological-like tissues requires biomaterials to direct cellular events to a near-native, 3D niche extent. Natural biomaterials are generally seen as a safe option for cell support, but their biocompatibility and biodegradability can be just as limited as their bioactive/biomimetic performance. Furthermore, integrating different biomaterial cues and their final impact on cellular behavior is a complex equation where the outcome might be very different from the sum of individual parts. This review critically analyses recent progress on biomaterial-induced cellular responses, from simple adhesion to more complex stem cell differentiation, looking at the ever-growing possibilities of natural materials modification. Starting with a discussion on native material formulation and the inclusion of cell-instructive cues, the roles of shape and mechanical stimuli, the susceptibility to cellular remodeling, and the often-overlooked impact of cellular density and cell-cell interactions within constructs, are delved into. Along the way, synergistic and antagonistic combinations reported in vitro and in vivo are singled out, identifying needs and current lessons on the development of natural biomaterial libraries to solve the cell-material puzzle efficiently. This review brings together knowledge from different fields envisioning next-generation, combinatorial biomaterial development toward complex tissue engineering.

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.

Programming hydrogels to probe spatiotemporal cell biology

The recapitulation of complex microenvironments that regulate cell behavior during development, disease, and wound healing is key to understanding fundamental biological processes. In vitro, multicellular morphogenesis, organoid maturation, and disease modeling have traditionally been studied using either non-physiological 2D substrates or 3D biological matrices, neither of which replicate the spatiotemporal biochemical and biophysical complexity of biology. Here, we provide a guided overview of the recent advances in the programming of synthetic hydrogels that offer precise control over the spatiotemporal properties within cellular microenvironments, such as advances in the control of cell-driven remodeling, bioprinting, or user-defined manipulation of properties (e.g., via light irradiation).

Methods to Characterize Granular Hydrogel Rheological Properties, Porosity, and Cell Invasion

Granular hydrogels are formed through the packing of hydrogel microparticles and are emerging for various biomedical applications, including as inks for 3D printing, substrates to study cell-matrix interactions, and injectable scaffolds for tissue repair. Granular hydrogels are suited for these applications because of their unique properties including inherent porosity, shear-thinning and self-healing behavior, and tunable design. The characterization of their material properties and biological response involves technical considerations that are unique to modular systems like granular hydrogels. Here, we describe detailed methods that can be used to quantitatively characterize the rheological behavior and porosity of granular hydrogels using reagents, tools, and equipment that are typically available in biomedical engineering laboratories. In addition, we detail methods for 3D cell invasion assays using multicellular spheroids embedded within granular hydrogels and describe steps to quantify features of cell outgrowth (e.g., endothelial cell sprouting) using standard image processing software. To illustrate these methods, we provide examples where features of granular hydrogels such as the size of hydrogel microparticles and their extent of packing during granular hydrogel formation are modulated. Our intent with this resource is to increase accessibility to granular hydrogel technology and to facilitate the investigation of granular hydrogels for biomedical applications.

Anisotropic Rod-Shaped Particles Influence Injectable Granular Hydrogel Properties and Cell Invasion

Granular hydrogels have emerged as a new class of injectable and porous biomaterials that improve integration with host tissue when compared to solid hydrogels. Granular hydrogels are typically prepared using spherical particles and this study considers whether particle shape (i.e., isotropic spheres vs anisotropic rods) influences granular hydrogel properties and cellular invasion. Simulations predict that anisotropic rods influence pore shape and interconnectivity, as well as bead transport through granular assemblies. Photo-cross-linkable norbornene-modified hyaluronic acid is used to produce spherical and rod-shaped particles using microfluidic droplet generators and formed into shear-thinning and self-healing granular hydrogels, with particle shape influencing mechanics and injectability. Rod-shaped particles form granular hydrogels that have anisotropic and interconnected pores, with pore size and number influenced by particle shape and degree of packing. Robust in vitro sprouting of endothelial cells from embedded cellular spheroids is observed with rod-shaped particles, including higher sprouting densities and sprout lengths when compared to hydrogels with spherical particles. Cell and vessel invasion into granular hydrogels when injected subcutaneously in vivo are significantly greater with rod-shaped particles, whereas a gradient of cellularity is observed with spherical particles. Overall, this work demonstrates potentially superior functional properties of granular hydrogels with rod-shaped particles for tissue repair.

Micropatterns and peptide gradient on the inner surface of a guidance conduit synergistically promotes nerve regeneration in vivo

Both of the surface topographical features and distribution of biochemical cues can influence the cell-substrate interactions and thereby tissue regeneration in vivo. However, they have not been combined simultaneously onto a biodegradable scaffold to demonstrate the synergistic role so far. In this study, a proof-of-concept study is performed to prepare micropatterns and peptide gradient on the inner wall of a poly (D,L-lactide-co-caprolactone) (PLCL) guidance conduit and its advantages in regeneration of peripheral nerve in vivo. After linear ridges/grooves of 20/40 μm in width are created on the PLCL film, its surface is aminolyzed in a kinetically controlled manner to obtain the continuous gradient of amino groups, which are then transferred to CQAASIKVAV peptide density gradient via covalent coupling of glutaraldehyde. The Schwann cells are better aligned along with the stripes, and show a faster migration rate toward the region of higher peptide density. Implantation of the nerve guidance conduit made of the PLCL film having both the micropatterns and peptide gradient can significantly accelerate the regeneration of sciatic nerve in terms of rate, function recovery and microstructures, and reduction of fibrosis in muscle tissues. Moreover, this nerve conduit can also benefit the M2 polarization of macrophages and promote vascularization in vivo.

Articular cartilage and osteochondral tissue engineering techniques: Recent advances and challenges

In spite of the considerable achievements in the field of regenerative medicine in the past several decades, osteochondral defect regeneration remains a challenging issue among diseases in the musculoskeletal system because of the spatial complexity of osteochondral units in composition, structure and functions. In order to repair the hierarchical tissue involving different layers of articular cartilage, cartilage-bone interface and subchondral bone, traditional clinical treatments including palliative and reparative methods have showed certain improvement in pain relief and defect filling. It is the development of tissue engineering that has provided more promising results in regenerating neo-tissues with comparable compositional, structural and functional characteristics to the native osteochondral tissues. Here in this review, some basic knowledge of the osteochondral units including the anatomical structure and composition, the defect classification and clinical treatments will be first introduced. Then we will highlight the recent progress in osteochondral tissue engineering from perspectives of scaffold design, cell encapsulation and signaling factor incorporation including bioreactor application. Clinical products for osteochondral defect repair will be analyzed and summarized later. Moreover, we will discuss the current obstacles and future directions to regenerate the damaged osteochondral tissues.

Modelling the central nervous system: tissue engineering of the cellular microenvironment

With the increasing prevalence of neurodegenerative diseases, improved models of the central nervous system (CNS) will improve our understanding of neurophysiology and pathogenesis, whilst enabling exploration of novel therapeutics. Studies of brain physiology have largely been carried out using in vivo models, ex vivo brain slices or primary cell culture from rodents. Whilst these models have provided great insight into complex interactions between brain cell types, key differences remain between human and rodent brains, such as degree of cortical complexity. Unfortunately, comparative models of human brain tissue are lacking. The development of induced Pluripotent Stem Cells (iPSCs) has accelerated advancement within the field of in vitro tissue modelling. However, despite generating accurate cellular representations of cortical development and disease, two-dimensional (2D) iPSC-derived cultures lack an entire dimension of environmental information on structure, migration, polarity, neuronal circuitry and spatiotemporal organisation of cells. As such, researchers look to tissue engineering in order to develop advanced biomaterials and culture systems capable of providing necessary cues for guiding cell fates, to construct in vitro model systems with increased biological relevance. This review highlights experimental methods for engineering of in vitro culture systems to recapitulate the complexity of the CNS with consideration given to previously unexploited biophysical cues within the cellular microenvironment.

Generalizing hydrogel microparticles into a new class of bioinks for extrusion bioprinting

Hydrogel microparticles (HMPs) are an emerging bioink that can allow three-dimensional (3D) printing of most soft biomaterials by improving physical support and maintaining biological functions. However, the mechanisms of HMP jamming within printing nozzles and yielding to flow remain underexplored. Here, we present an in-depth investigation via both experimental and computational methods on the HMP dissipation process during printing as a result of (i) external resistance from the printing apparatus and (ii) internal physicochemical properties of HMPs. In general, a small syringe opening, large or polydisperse size of HMPs, and less deformable HMPs induce high resistance and closer HMP packing, which improves printing fidelity and stability due to increased interparticle adhesion. However, smooth extrusion and preserving viability of encapsulated cells require low resistance during printing, which is associated with less shear stress. These findings can be used to improve printability of HMPs and facilitate their broader use in 3D bioprinting.

Influence of Microgel Fabrication Technique on Granular Hydrogel Properties

Bulk hydrogels traditionally used for tissue engineering and drug delivery have numerous limitations, such as restricted injectability and a nanoscale porosity that reduces cell invasion and mass transport. An evolving approach to address these limitations is the fabrication of hydrogel microparticles (i.e., "microgels") that can be assembled into granular hydrogels. There are numerous methods to fabricate microgels; however, the influence of the fabrication technique on granular hydrogel properties is unexplored. Herein, we investigated the influence of three microgel fabrication techniques (microfluidic devices (MD), batch emulsions (BE), and mechanical fragmentation by extrusion (EF)) on the resulting granular hydrogel properties (e.g., mechanics, porosity, and injectability). Hyaluronic acid (HA) modified with various reactive groups (i.e., norbornenes (NorHA), pentenoates (HA-PA), and methacrylates (MeHA)) were used to form microgels with an average diameter of ∼100 μm. The MD method resulted in homogeneous spherical microgels, the BE method resulted in heterogeneous spherical microgels, and the EF method resulted in heterogeneous polygonal microgels. Across the various reactive groups, microgels fabricated with the MD and BE methods had lower functional group consumption when compared to microgels fabricated with the EF method. When microgels were jammed into granular hydrogels, the storage modulus (G') of EF granular hydrogels (∼1000-3000 Pa) was consistently an order of magnitude higher than G' for MD and BE granular hydrogels (∼50-200 Pa). Void space was comparable across all groups, although EF granular hydrogels exhibited an increased number of pores and decreased average pore size when compared to MD and BE granular hydrogels. Furthermore, granular hydrogel properties were tuned by varying the amount of cross-linker used during microgel fabrication. Lastly, granular hydrogels were injectable across formulations due to their general shear-thinning and self-healing properties. Taken together, this work thoroughly characterizes the influence of the microgel fabrication technique on granular hydrogel properties to inform the design of future systems for biomedical applications.

Chips for Biomaterials and Biomaterials for Chips: Recent Advances at the Interface between Microfabrication and Biomaterials Research

In recent years, the use of microfabrication techniques has allowed biomaterials studies which were originally carried out at larger length scales to be miniaturized as so-called "on-chip" experiments. These miniaturized experiments have a range of advantages which have led to an increase in their popularity. A range of biomaterial shapes and compositions are synthesized or manufactured on chip. Moreover, chips are developed to investigate specific aspects of interactions between biomaterials and biological systems. Finally, biomaterials are used in microfabricated devices to replicate the physiological microenvironment in studies using so-called "organ-on-chip," "tissue-on-chip" or "disease-on-chip" models, which can reduce the use of animal models with their inherent high cost and ethical issues, and due to the possible use of human cells can increase the translation of research from lab to clinic. This review gives an overview of recent developments at the interface between microfabrication and biomaterials science, and indicates potential future directions that the field may take. In particular, a trend toward increased scale and automation is apparent, allowing both industrial production of micron-scale biomaterials and high-throughput screening of the interaction of diverse materials libraries with cells and bioengineered tissues and organs.

Photoclick Chemistry: A Bright Idea

At its basic conceptualization, photoclick chemistry embodies a collection of click reactions that are performed via the application of light. The emergence of this concept has had diverse impact over a broad range of chemical and biological research due to the spatiotemporal control, high selectivity, and excellent product yields afforded by the combination of light and click chemistry. While the reactions designated as "photoclick" have many important features in common, each has its own particular combination of advantages and shortcomings. A more extensive realization of the potential of this chemistry requires a broader understanding of the physical and chemical characteristics of the specific reactions. This review discusses the features of the most frequently employed photoclick reactions reported in the literature: photomediated azide-alkyne cycloadditions, other 1,3-dipolarcycloadditions, Diels-Alder and inverse electron demand Diels-Alder additions, radical alternating addition chain transfer additions, and nucleophilic additions. Applications of these reactions in a variety of chemical syntheses, materials chemistry, and biological contexts are surveyed, with particular attention paid to the respective strengths and limitations of each reaction and how that reaction benefits from its combination with light. Finally, challenges to broader employment of these reactions are discussed, along with strategies and opportunities to mitigate such obstacles.

Augmenting Tendon-to-Bone Repair with Functionally Graded Scaffolds

Tendon-to-bone repair often fails because the functionally graded attachment is not regenerated during the healing process. Biomimetic scaffolds that recapitulate the unique features of the native tendon-to-bone attachment hold great promise for enhancing the healing process. Among various types of scaffolds that are developed and evaluated for tendon-to-bone repair, those with gradations (in either a stratified or a continuous fashion) in composition, structure, mechanical properties, and cell phenotype have gained the most attention. In this progress report, the recent efforts in the rational design and fabrication of functionally graded scaffolds based upon electrospun nanofiber mats and inverse opal structures, as well as the evaluation of their applications in augmenting tendon-to-bone repair, are reviewed. This report concludes with perspectives on the necessary future steps for clinical translation of the scaffolds.

Enzyme functionalized microgels enable precise regulation of dissolved oxygen and anaerobe culture

Anaerobes are a major constituent of the gut microbiome and profoundly influence the overall health of humans. However, the lack of a simple, cost-effective, and scalable system that mimics the anaerobic conditions of the human gut is hindering research on the gut microbiome and the development of therapeutics. Here, we address this gap by using glucose oxidase and catalase containing gelatin microparticles (GOx-CAT-GMPs) to precisely regulate dissolved oxygen concentration [O2] via GOx-mediated consumption of oxygen. Fluorescence images generated using conjugated polymer afterglow nanoparticles showed that [O2] can be tuned from 257.9 ​± ​6.2 to 0.0 ​± ​4.0 ​μM using GOx-CAT-GMPs. Moreover, when the obligate anaerobe Bacteroides thetaiotaomicron was inoculated in media containing GOx-CAT-GMPs, bacterial growth under ambient oxygen was comparable to control conditions in an anaerobic chamber (5.4 ​× ​105 and 8.8 ​× ​105 colony forming units mL-1, respectively). Finally, incorporating GOx-CAT-GMPs into a bioreactor that permitted continuous radial diffusion of oxygen and glucose generated a gut-mimetic [O2] gradient of 132.4 ​± ​2.6 ​μM in the outer ring of the reactor to 7.9 ​± ​1.7 ​μM at the core. Collectively, these results indicate that GOx-CAT-GMPs are highly effective oxygen-regulating materials. These materials can potentially be leveraged to advance gut microbiome research and fecal microbiota transplantation, particularly in low-resource settings.

Hierarchical and heterogeneous hydrogel system as a promising strategy for diversified interfacial tissue regeneration

A state-of-the-art review on the design and preparation of hierarchical and heterogeneous hydrogel systems for interfacial tissue regeneration.

Injectable hyaluronic acid and platelet lysate-derived granular hydrogels for biomedical applications

Towards the repair of damaged tissues, numerous scaffolds have been fabricated to recreate the complex extracellular matrix (ECM) environment to support desired cell behaviors; however, it is often challenging to design scaffolds with the requisite cell-anchorage sites, mechanical stability, and tailorable physicochemical properties necessary for many applications. To address this and to improve on the properties of hyaluronic acid (HA) hydrogels, we combined photocrosslinkable norbornene-modified HA (NorHA) with human platelet lysate (PL). These PL-NorHA hybrid hydrogels supported the adhesion of cells when compared to NorHA hydrogels without PL, exhibited tailorable physicochemical properties based on the concentration of individual components, and released proteins over time. Using microfluidic techniques with on-chip mixing of NorHA and PL and subsequent photocrosslinking, spherical PL-NorHA microgels with a hierarchical fibrillar network were fabricated that exhibited the sustained delivery of PL proteins. Microgels could be jammed into granular hydrogels that exhibited shear-thinning and self-healing properties, enabling ejection from syringes and the fabrication of stable 3D constructs with 3D printing. Again, the inclusion of PL enhanced cellular interactions with the microgel structures. Overall, the combination of biomolecules and fibrin self-assembly arising from the enriched milieu of PL-derived proteins improved the bioactivity of HA-based hydrogels, enabling the formation of dynamic systems with modular design. The granular systems can be engineered to meet the complex demands of functional tissue repair using versatile processing techniques, such as with 3D printing.

Functionally graded biomaterials for use as model systems and replacement tissues

The heterogeneity of native tissues requires complex materials to provide suitable substitutes for model systems and replacement tissues. Functionally graded materials have the potential to address this challenge by mimicking the gradients in heterogeneous tissues such as porosity, mineralization, and fiber alignment to influence strength, ductility, and cell signaling. Advancements in microfluidics, electrospinning, and 3D printing enable the creation of increasingly complex gradient materials that further our understanding of physiological gradients. The combination of these methods enables rapid prototyping of constructs with high spatial resolution. However, successful translation of these gradients requires both spatial and temporal presentation of cues to model the complexity of native tissues that few materials have demonstrated. This review highlights recent strategies to engineer functionally graded materials for the modeling and repair of heterogeneous tissues, together with a description of how cells interact with various gradients.

Recent advances in bio-orthogonal and dynamic crosslinking of biomimetic hydrogels

In recent years, dynamic, 'click' hydrogels have been applied in numerous biomedical applications. Owing to the mild, cytocompatible, and highly specific reaction kinetics, a multitude of orthogonal handles have been developed for fabricating dynamic hydrogels to facilitate '4D' cell culture. The high degree of tunability in crosslinking reactions of orthogonal 'click' chemistry has enabled a bottom-up approach to install specific biomimicry in an artificial extracellular matrix. In addition to click chemistry, highly specific enzymatic reactions are also increasingly used for network crosslinking and for spatiotemporal control of hydrogel properties. On the other hand, covalent adaptable chemistry has been used to recapitulate the viscoelastic component of biological tissues and for formulating self-healing and shear-thinning hydrogels. The common feature of these three classes of chemistry (i.e., orthogonal click chemistry, enzymatic reactions, and covalent adaptable chemistry) is that they can be carried out under ambient and aqueous conditions, a prerequisite for maintaining cell viability for in situ cell encapsulation and post-gelation modification of network properties. Due to their orthogonality, different chemistries can also be applied sequentially to provide additional biochemical and mechanical control to guide cell behavior. Herein, we review recent advances in the use of orthogonal click chemistry, enzymatic reactions, and covalent adaptable chemistry for the development of dynamically tunable and biomimetic hydrogels.

Gradient Hydrogels-The State of the Art in Preparation Methods

Gradient hydrogels refer to hydrogel materials with a gradual or abrupt change in one or some of their properties. They represent examples of more sophisticated gel materials in comparison to simple, native gel networks. Here, we review techniques used to prepare gradient hydrogels which have been reported in literature over the last few years. A variety of simple preparation methods are available, most of which can be relatively easily utilized in standard laboratories.