Spatial Micropatterning of Growth Factors in 3D Hydrogels for Location-Specific Regulation of Cellular Behaviors

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.

Engineering Innervated Musculoskeletal Tissues for Regenerative Orthopedics and Disease Modeling

Musculoskeletal (MSK) disorders significantly burden patients and society, resulting in high healthcare costs and productivity loss. These disorders are the leading cause of physical disability, and their prevalence is expected to increase as sedentary lifestyles become common and the global population of the elderly increases. Proper innervation is critical to maintaining MSK function, and nerve damage or dysfunction underlies various MSK disorders, underscoring the potential of restoring nerve function in MSK disorder treatment. However, most MSK tissue engineering strategies have overlooked the significance of innervation. This review first expounds upon innervation in the MSK system and its importance in maintaining MSK homeostasis and functions. This will be followed by strategies for engineering MSK tissues that induce post-implantation in situ innervation or are pre-innervated. Subsequently, research progress in modeling MSK disorders using innervated MSK organoids and organs-on-chips (OoCs) is analyzed. Finally, the future development of engineering innervated MSK tissues to treat MSK disorders and recapitulate disease mechanisms is discussed. This review provides valuable insights into the underlying principles, engineering methods, and applications of innervated MSK tissues, paving the way for the development of targeted, efficacious therapies for various MSK conditions.

Hydrogel-Based Growth Factor Delivery Platforms: Strategies and Recent Advances

Growth factors play a crucial role in regulating a broad variety of biological processes and are regarded as powerful therapeutic agents in tissue engineering and regenerative medicine in the past decades. However, their application is limited by their short half-lives and potential side effects in physiological environments. Hydrogels are identified as having the promising potential to prolong the half-lives of growth factors and mitigate their adverse effects by restricting them within the matrix to reduce their rapid proteolysis, burst release, and unwanted diffusion. This review discusses recent progress in the development of growth factor-containing hydrogels for various biomedical applications, including wound healing, brain tissue repair, cartilage and bone regeneration, and spinal cord injury repair. In addition, the review introduces strategies for optimizing growth factor release including affinity-based delivery, carrier-assisted delivery, stimuli-responsive delivery, spatial structure-based delivery, and cellular system-based delivery. Finally, the review presents current limitations and future research directions for growth factor-delivering hydrogels.

Advances in Regulating Cellular Behavior Using Micropatterns

Micropatterns, characterized as distinct physical microstructures or chemical adhesion matrices on substance surfaces, have emerged as a powerful tool for manipulating cellular activity. By creating specific extracellular matrix microenvironments, micropatterns can influence various cell behaviors, including orientation, proliferation, migration, and differentiation. This review provides a comprehensive overview of the latest advancements in the use of micropatterns for cell behavior regulation. It discusses the influence of micropattern morphology and coating on cell behavior and the underlying mechanisms. It also highlights future research directions in this field, aiming to inspire new investigations in materials medicine, regenerative medicine, and tissue engineering. The review underscores the potential of micropatterns as a novel approach for controlling cell behavior, which could pave the way for breakthroughs in various biomedical applications.

Natural Polymer-Based Hydrogels: From Polymer to Biomedical Applications

Hydrogels prepared from natural polymer have attracted extensive attention in biomedical fields such as drug delivery, wound healing, and regenerative medicine due to their good biocompatibility, degradability, and flexibility. This review outlines the commonly used natural polymer in hydrogel preparation, including cellulose, chitosan, collagen/gelatin, alginate, hyaluronic acid, starch, guar gum, agarose, and dextran. The polymeric structure and process/synthesis of natural polymers are illustrated, and natural polymer-based hydrogels including the hydrogel formation and properties are elaborated. Subsequently, the biomedical applications of hydrogels based on natural polymer in drug delivery, tissue regeneration, wound healing, and other biomedical fields are summarized. Finally, the future perspectives of natural polymers and hydrogels based on them are discussed. For natural polymers, novel technologies such as enzymatic and biological methods have been developed to improve their structural properties, and the development of new natural-based polymers or natural polymer derivatives with high performance is still very important and challenging. For natural polymer-based hydrogels, novel hydrogel materials, like double-network hydrogel, multifunctional composite hydrogels, and hydrogel microrobots have been designed to meet the advanced requirements in biomedical applications, and new strategies such as dual-cross-linking, microfluidic chip, micropatterning, and 3D/4D bioprinting have been explored to fabricate advanced hydrogel materials with designed properties for biomedical applications. Overall, natural polymeric hydrogels have attracted increasing interest in biomedical applications, and the development of novel natural polymer-based materials and new strategies/methods for hydrogel fabrication are highly desirable and still challenging.

Mimicking Molecular Pathways in the Design of Smart Hydrogels for the Design of Vascularized Engineered Tissues

Biomaterials are pivotal in supporting and guiding vascularization for therapeutic applications. To design effective, bioactive biomaterials, understanding the cellular and molecular processes involved in angiogenesis and vasculogenesis is crucial. Biomaterial platforms can replicate the interactions between cells, the ECM, and the signaling molecules that trigger blood vessel formation. Hydrogels, with their soft and hydrated properties resembling natural tissues, are widely utilized; particularly synthetic hydrogels, known for their bio-inertness and precise control over cell-material interactions, are utilized. Naturally derived and synthetic hydrogel bases are tailored with specific mechanical properties, controlled for biodegradation, and enhanced for cell adhesion, appropriate biochemical signaling, and architectural features that facilitate the assembly and tubulogenesis of vascular cells. This comprehensive review showcases the latest advancements in hydrogel materials and innovative design modifications aimed at effectively guiding and supporting vascularization processes. Furthermore, by leveraging this knowledge, researchers can advance biomaterial design, which will enable precise support and guidance of vascularization processes and ultimately enhance tissue functionality and therapeutic outcomes.

Hydrogels with stiffness-degradation spatial patterns control anisotropic 3D cell response

In nature, tissues are patterned, but most biomaterials used in human applications are not. Patterned biomaterials offer the opportunity to mimic spatially segregating biophysical and biochemical properties found in nature. Engineering such properties allows to study cell-matrix interactions in anisotropic matrices in great detail. Here, we developed alginate-based hydrogels with patterns in stiffness and degradation, composed of distinct areas of soft non-degradable (Soft-NoDeg) and stiff degradable (Stiff-Deg) material properties. The hydrogels exhibit emerging patterns in stiffness and degradability over time, taking advantage of dual crosslinking: Diels-Alder covalent crosslinking (norbornene-tetrazine, non degradable) and UV-mediated peptide crosslinking (matrix metalloprotease sensitive peptide, enzymatically degradable). The materials were mechanically characterized using rheology for single-phase and surface micro-indentation for patterned materials. 3D encapsulated mouse embryonic fibroblasts (MEFs) allowed to characterize the anisotropic cell-matrix interaction in terms of cell morphology by employing a novel image-based quantification tool. Live/dead staining showed no differences in cell viability but distinct patterns in proliferation, with higher cell number in Stiff-Deg materials at day 14. Patterns of projected cell area became visible already at day 1, with larger values in Soft-NoDeg materials. This was inverted at day 14, when larger projected cell areas were identified in Stiff-Deg. This shift was accompanied by a significant decrease in cell circularity in Stiff-Deg. The control of anisotropic cell morphology by the material patterns was also confirmed by a significant increase in filopodia number and length in Stiff-Deg materials. The novel image-based quantification tool was useful to spatially visualize and quantify the anisotropic cell response in 3D hydrogels with stiffness-degradation spatial patterns. Our results show that patterning of stiffness and degradability allows to control cell anisotropic response in 3D and can be quantified by image-based strategies. This allows a deeper understanding of cell-matrix interactions in a multicomponent material.

Small Molecule-Templated DNA Hydrogel with Record Stiffness Integrates and Releases DNA Nanostructures and Gene Silencing Nucleic Acids

Deoxyribonucleic acid (DNA) hydrogels are a unique class of programmable, biocompatible materials able to respond to complex stimuli, making them valuable in drug delivery, analyte detection, cell growth, and shape-memory materials. However, unmodified DNA hydrogels in the literature are very soft, rarely reaching a storage modulus of 10³  Pa, and they lack functionality, limiting their applications. Here, a DNA/small-molecule motif to create stiff hydrogels from unmodified DNA, reaching 10⁵  Pa in storage modulus is used. The motif consists of an interaction between polyadenine and cyanuric acid-which has 3-thymine like faces-into multimicrometer supramolecular fibers. The mechanical properties of these hydrogels are readily tuned, they are self-healing and thixotropic. They integrate a high density of small, nontoxic molecules, and are functionalized simply by varying the molecule sidechain. They respond to three independent stimuli, including a small molecule stimulus. These stimuli are used to integrate and release DNA wireframe and DNA origami nanostructures within the hydrogel. The hydrogel is applied as an injectable delivery vector, releasing an antisense oligonucleotide in cells, and increasing its gene silencing efficacy. This work provides tunable, stimuli-responsive, exceptionally stiff all-DNA hydrogels from simple sequences, extending these materials' capabilities.

Electrohydrodynamic printing of submicron-microscale hybrid scaffolds with improved cellular adhesion and proliferation behaviors

Electrohydrodynamic (EHD) printing has been considered as a mature strategy to mimic the hierarchical microarchitectures in native extracellular matrix (ECM). Most of the EHD-printed scaffolds possess single-dimensional fibrous structures, which cannot mimic the multi-dimensional architectures for enhanced cellular behaviors. Here we developed a two-nozzle EHD printing system to fabricate hybrid scaffolds involving submicron and microscale features. The polyethylene oxide- polycaprolactone (PEO-PCL) submicron fibers were fabricated via solution-based EHD printing with a width of 527 ± 56 nm. The PCL microscale fibers were fabricated via melt-based EHD printing with a width of 11.2 ± 2.3μm. The hybrid scaffolds were fabricated by printing the submicron and microscale fibers in a layer-by-layer manner. The microscale scaffolds were utilized as a control group. Rat myocardial cells (H9C2 cells) were cultured on the two kinds of scaffolds for the culturing period of 1, 3 and 5 d. Biological results indicated that H9C2 cells showed enhanced adhesion and proliferation behaviors on the hybrid scaffold than those on the pure microscale scaffold. This work offers a facile and scalable strategy to fabricate multiscale synthetic scaffolds, which might be further explored to regulate cellular behaviors in the fields of tissue regeneration and biomedical engineering.

The Auxiliary Role of Heparin in Bone Regeneration and its Application in Bone Substitute Materials

Bone regeneration in large segmental defects depends on the action of osteoblasts and the ingrowth of new blood vessels. Therefore, it is important to promote the release of osteogenic/angiogenic growth factors. Since the discovery of heparin, its anticoagulant, anti-inflammatory, and anticancer functions have been extensively studied for over a century. Although the application of heparin is widely used in the orthopedic field, its auxiliary effect on bone regeneration is yet to be unveiled. Specifically, approximately one-third of the transforming growth factor (TGF) superfamily is bound to heparin and heparan sulfate, among which TGF-β1, TGF-β2, and bone morphogenetic protein (BMP) are the most common growth factors used. In addition, heparin can also improve the delivery and retention of BMP-2 in vivo promoting the healing of large bone defects at hyper physiological doses. In blood vessel formation, heparin still plays an integral part of fracture healing by cooperating with the platelet-derived growth factor (PDGF). Importantly, since heparin binds to growth factors and release components in nanomaterials, it can significantly facilitate the controlled release and retention of growth factors [such as fibroblast growth factor (FGF), BMP, and PDGF] in vivo. Consequently, the knowledge of scaffolds or delivery systems composed of heparin and different biomaterials (including organic, inorganic, metal, and natural polymers) is vital for material-guided bone regeneration research. This study systematically reviews the structural properties and auxiliary functions of heparin, with an emphasis on bone regeneration and its application in biomaterials under physiological conditions.

A review on 3D printing in tissue engineering applications

In tissue engineering, 3D printing is an important tool that uses biocompatible materials, cells, and supporting components to fabricate complex 3D printed constructs. This review focuses on the cytocompatibility characteristics of 3D printed constructs, made from different synthetic and natural materials. From the overview of this article, inkjet and extrusion-based 3D printing are widely used methods for fabricating 3D printed scaffolds for tissue engineering. This review highlights that scaffold prepared by both inkjet and extrusion-based 3D printing techniques showed significant impact on cell adherence, proliferation, and differentiation as evidenced by in vitro and in vivo studies. 3D printed constructs with growth factors (FGF-2, TGF-β1, or FGF-2/TGF-β1) enhance extracellular matrix (ECM), collagen I content, and high glycosaminoglycan (GAG) content for cell growth and bone formation. Similarly, the utilization of 3D printing in other tissue engineering applications cannot be belittled. In conclusion, it would be interesting to combine different 3D printing techniques to fabricate future 3D printed constructs for several tissue engineering applications.

Advances in Hydrogels for Stem Cell Therapy: Regulation Mechanisms and Tissue Engineering Applications

Stem cell therapy has shown unparalleled potential in tissue engineering, but it still faces challenges in the regulation of stem cell fate. Inspired by the native stem cell niche, a...

4D biofabrication via instantly generated graded hydrogel scaffolds

Formation of graded biomaterials to render shape-morphing scaffolds for 4D biofabrication holds great promise in fabrication of complex structures and the recapitulation of critical dynamics for tissue/organ regeneration. Here we describe a facile generation of an adjustable and robust gradient using a single- or multi-material one-step fabrication strategy for 4D biofabrication. By simply photocrosslinking a mixed solution of a photocrosslinkable polymer macromer, photoinitiator (PI), UV absorber and live cells, a cell-laden gradient hydrogel with pre-programmable deformation can be generated. Gradient formation was demonstrated in various polymers including poly(ethylene glycol) (PEG), alginate, and gelatin derivatives using various UV absorbers that present overlap in UV spectrum with that of the PI UV absorbance spectrum. Moreover, this simple and effective method was used as a universal platform to integrate with other hydrogel-engineering techniques such as photomask-aided microfabrication, photo-patterning, ion-transfer printing, and 3D bioprinting to fabricate more advanced cell-laden scaffold structures. Lastly, proof-of-concept 4D tissue engineering was demonstrated in a study of 4D bone-like tissue formation. The strategy's simplicity along with its versatility paves a new way in solving the hurdle of achieving temporal shape changes in cell-laden single-component hydrogel scaffolds and may expedite the development of 4D biofabricated constructs for biological applications.

Reversible dynamic mechanics of hydrogels for regulation of cellular behavior

The mechanical properties of the native extracellular matrix play a key role in regulating cell behavior during developmental, healing and homeostatic processes. Since these properties change over time, it may be valuable to have the capacity to dynamically vary the mechanical properties of engineered hydrogels used in tissue engineering strategies to better mimic the dynamic mechanical behavior of native extracellular matrix. However, in situ repeatedly reversible dynamic tuning of hydrogel mechanics is still limited. In this study, we have engineered a hydrogel system with reversible dynamic mechanics using a dual-crosslinkable alginate hydrogel. The effect of reversible mechanical signals on encapsulated stem cells in dynamically tunable hydrogels has been demonstrated. In situ stiffening of hydrogels decreases cell spreading and proliferation, and subsequent softening of hydrogels gives way to an increase in cell spreading and proliferation. The hydrogel stiffening and softening, and resulting cellular responses are repeatedly reversible. This hydrogel system provides a promising platform for investigating the effect of repeatedly reversible changes in extracellular matrix mechanics on cell behaviors. STATEMENT OF SIGNIFICANCE: Since the mechanical properties of native extracellular matrix (ECM) change over time during development, healing and homeostatic processes, it may be valuable to have the capacity to dynamically control the mechanics of biomaterials used in tissue engineering and regenerative medicine applications to better mimic this behavior. Unlike previously reported biomaterials whose mechanical properties can be changed by the user only a limited number of times, this system provides the capacity to induce unlimited alterations to the mechanical properties of an engineered ECM for 3D cell culture. This study presents a strategy for on-demand dynamic and reversible control of materials' mechanics by single and dual-crosslinking mechanisms using oxidized and methacrylated alginates. By demonstrating direct changes in encapsulated human mesenchymal stem cell morphology, proliferation and chondrogenic differentiation in response to multiple different dynamic changes in hydrogel mechanics, we have established a repeatedly reversible 3D cellular mechanosensing system. This system provides a powerful platform tool with a wide range of stiffness tunability to investigate the role of dynamic mechanics on cellular mechanosensing and behavioral responses.

Micro-and-nanometer topological gradient of block copolymer fibrous scaffolds towards region-specific cell regulation

Regulating cell behavior and function by surface topography has drawn significant attention in tissue engineering. Herein, a gradient fibrous scaffold comprising anisotropic aligned fibers and isotropic annealed fibers was developed to provide a controllable direction of cell migration, adhesion, and spreading. The electrospun aligned fibers were engraved to create surface gradients with micro-and-nanometer roughness through block copolymer (BCP) self-assembly induced by selective solvent vapor annealing (SVA). The distinct manipulation of cell behavior by annealed fibrous scaffolds with tailored self-assembled nanostructure and welded fibrous microstructure has been illustrated by in situ/ex situ small angle X-ray scattering (SAXS), scanning electron microscopy (SEM), atomic force microscopy (AFM) and in vitro cell culture. Further insights into the effect of integrated gradient fibrous scaffold were gained at the level of protein expression. From the perspective of gradient topology, this region-specific scaffold based on BCP fibers shows the prospect of guiding cell migration, adhesion and spreading and provides a generic method for designing biomaterials for tissue-engineering.

Self-Organogenesis from 2D Micropatterns to 3D Biomimetic Biliary Trees

BACKGROUND AND AIMS

Globally, liver diseases account for 2 million deaths per year. For those with advanced liver disease the only curative approach is liver transplantation. However, less than 10% of those in need get a liver transplant due to limited organ availability. To circumvent this challenge, there has been a great focus in generating a bioengineered liver. Despite its essential role in liver functions, a functional biliary system has not yet been developed. In this framework, exploration of epithelial cell self-organogenesis and microengineering-driven geometrical cell confinement allow to envision the bioengineering of a functional biomimetic intrahepatic biliary tract.

APPROACH

three-dimensional (3D) bile ducts were built in vitro by restricting cell adhesion to two-dimensional (2D) patterns to guide cell self-organization. Tree shapes mimicking the configuration of the human biliary system were micropatterned on glass slides, restricting cell attachment to these areas. Different tree geometries and culture conditions were explored to stimulate self-organogenesis of normal rat cholangiocytes (NRCs) used as a biliary cell model, either alone or in co-culture with human umbilical endothelial cells (HUVECs).

RESULTS

Pre-seeding the micropatterns with HUVECs promoted luminogenesis with higher efficiency to yield functional branched biliary tubes. Lumen formation, apico-basal polarity, and preservation of the cholangiocyte phenotype were confirmed. Moreover, intact and functional biliary structures were detached from the micropatterns for further manipulation.

CONCLUSION

This study presents physiologically relevant 3D biliary duct networks built in vitro from 2D micropatterns. This opens opportunities for investigating bile duct organogenesis, physiopathology, and drug testing.

Engineering stromal heterogeneity in cancer

Based on our exponentially increasing knowledge of stromal heterogeneity from advances in single-cell technologies, the notion that stromal cell types exist as a spectrum of unique subpopulations that have specific functions and spatial distributions in the tumor microenvironment has significant impact on tumor modeling for drug development and personalized drug testing. In this Review, we discuss the importance of incorporating stromal heterogeneity and tumor architecture, and propose an overall approach to guide the reconstruction of stromal heterogeneity in vitro for tumor modeling. These next-generation tumor models may support the development of more precise drugs targeting specific stromal cell subpopulations, as well as enable improved recapitulation of patient tumors in vitro for personalized drug testing.

Bioengineering in vitro models of embryonic development

Stem cell-based in vitro models of embryonic development have been established over the last decade. Such model systems recapitulate aspects of gametogenesis, early embryonic development, or organogenesis. They enable experimental approaches that have not been possible previously and have the potential to greatly reduce the number of animals required for research. However, each model system has its own limitations, with certain aspects, such as morphogenesis and spatiotemporal control of cell fate decisions, diverging from the in vivo counterpart. Targeted bioengineering approaches to provide defined instructive external signals or to modulate internal cellular signals could overcome some of these limitations. Here, we present the latest technical developments and discuss how bioengineering can further advance the optimization and external control of stem cell-based embryo-like structures (ELSs). In vitro models combined with sophisticated bioengineering tools will enable an even more in-depth analysis of embryonic development in the future.

Advances in Engineering Human Tissue Models

Research in cell biology greatly relies on cell-based in vitro assays and models that facilitate the investigation and understanding of specific biological events and processes under different conditions. The quality of such experimental models and particularly the level at which they represent cell behavior in the native tissue, is of critical importance for our understanding of cell interactions within tissues and organs. Conventionally, in vitro models are based on experimental manipulation of mammalian cells, grown as monolayers on flat, two-dimensional (2D) substrates. Despite the amazing progress and discoveries achieved with flat biology models, our ability to translate biological insights has been limited, since the 2D environment does not reflect the physiological behavior of cells in real tissues. Advances in 3D cell biology and engineering have led to the development of a new generation of cell culture formats that can better recapitulate the in vivo microenvironment, allowing us to examine cells and their interactions in a more biomimetic context. Modern biomedical research has at its disposal novel technological approaches that promote development of more sophisticated and robust tissue engineering in vitro models, including scaffold- or hydrogel-based formats, organotypic cultures, and organs-on-chips. Even though such systems are necessarily simplified to capture a particular range of physiology, their ability to model specific processes of human biology is greatly valued for their potential to close the gap between conventional animal studies and human (patho-) physiology. Here, we review recent advances in 3D biomimetic cultures, focusing on the technological bricks available to develop more physiologically relevant in vitro models of human tissues. By highlighting applications and examples of several physiological and disease models, we identify the limitations and challenges which the field needs to address in order to more effectively incorporate synthetic biomimetic culture platforms into biomedical research.

Composition and Mechanism of Three-Dimensional Hydrogel System in Regulating Stem Cell Fate

Three-dimensional (3D) hydrogel systems integrating different types of stem cells and scaffolding biomaterials have an important application in tissue engineering. The biomimetic hydrogels that pattern cell suspensions within 3D configurations of biomaterial networks allow for the transport of bioactive factors and mimic the stem cell niche in vivo, thereby supporting the proliferation and differentiation of stem cells. The composition of a 3D hydrogel system determines the physical and chemical characteristics that regulate stem cell function through a biological mechanism. Here, we discuss the natural and synthetic hydrogel compositions that have been employed in 3D scaffolding, focusing on their characteristics, fabrication, biocompatibility, and regulatory effects on stem cell proliferation and differentiation. We also discuss the regulatory mechanisms of cell-matrix interaction and cell-cell interaction in stem cell activities in various types of 3D hydrogel systems. Understanding hydrogel compositions and their cellular mechanisms can yield insights into how scaffolding biomaterials and stem cells interact and can lead to the development of novel hydrogel systems of stem cells in tissue engineering and stem cell-based regenerative medicine. Impact statement Three-dimensional hydrogel system of stem cell mimicking the stemcell niche holds significant promise in tissue engineering and regenerative medicine. Exactly how hydrogel composition regulates stem cell fate is not well understood. This review focuses on the composition of hydrogel, and how the hydrogel composition and its properties regulate the stem cell adhesion, growth, and differentiation. We propose that cell-matrix interaction and cell-cell interaction are important regulatory mechanisms in stem cell activities. Our review provides key insights into how the hydrogel composition regulates the stem cell fate, untangling the engineering of three-dimensional hydrogel systems for stem cells.

Dual alginate crosslinking for local patterning of biophysical and biochemical properties

Hydrogels with patterned biophysical and biochemical properties have found increasing attention in the biomaterials community. In this work, we explore alginate-based materials with two orthogonal crosslinking mechanisms: the spontaneous Diels-Alder reaction and the ultraviolet light-initiated thiol-ene reaction. Combining these mechanisms in one material and spatially restricting the location of the latter using photomasks, enables the formation of dual-crosslinked hydrogels with patterns in stiffness, biomolecule presentation and degradation, granting local control over cell behavior. Patterns in stiffness are characterized morphologically by confocal microscopy and mechanically by uniaxial compression and microindentation measurement. Mouse embryonic fibroblasts seeded on stiffness-patterned substrates attach preferably and attain a spread morphology on stiff compared to soft regions. Human mesenchymal stem cells demonstrate preferential adipogenic differentiation on soft surfaces and osteogenic differentiation on stiff surfaces. Patterns in biomolecule presentation reveal favored attachment of mouse pre-osteoblasts on stripe regions, where thiolated cell-adhesive biomolecules have been coupled. Patterns in degradation are visualized by microindentation measurement following collagenase exposure. Patterned tissue infiltration into degradable regions on the surface is discernible in n=5/12 samples, when these materials are implanted subcutaneously into the backs of mice. Taken together, these results demonstrate that our hydrogel system with patterns in biophysical and biochemical properties enables the study of how environmental cues affect multiple cell behaviors in vitro and could be applied to guide endogenous tissue growth in diverse healing scenarios in vivo. STATEMENT OF SIGNIFICANCE: Hydrogels with patterns in biophysical and biochemical properties have been explored in the biomaterials community in order to spatially control or guide cell behavior. In our alginate-based system, we demonstrate the effect of local substrate stiffness and biomolecule presentation on the in vitro cell attachment, morphology, migration and differentiation behavior of two different mouse cell lines and human primary cells. Additionally, the effect of degradation patterns on the in vivo tissue infiltration is analyzed following subcutaneous implantation into a mouse model. The achievement of patterned tissue infiltration following the hydrogel template represents an important step towards guiding endogenous healing responses, thus inviting application in various tissue engineering contexts.

Spatially-directed cell migration in acoustically-responsive scaffolds through the controlled delivery of basic fibroblast growth factor

Hydrogels are commonly used in regenerative medicine for the delivery of growth factors (GFs). The spatial and temporal presentations of GFs are critical for directing regenerative processes, yet conventional hydrogels do not enable such control. We have developed a composite hydrogel, termed an acoustically-responsive scaffold (ARS), where release of a GF is non-invasively and spatiotemporally-controlled using focused ultrasound. The ARS consists of a fibrin matrix doped with a GF-loaded, phase-shift emulsion. The GF is released when the ARS is exposed to suprathreshold ultrasound via a mechanism termed acoustic droplet vaporization. In this study, we investigate how different spatial patterns of suprathreshold ultrasound can impact the biological response upon in vivo implantation of an ARS containing basic fibroblast growth factor (bFGF). ARSs were fabricated with either perfluorohexane (bFGF-C6-ARS) or perflurooctane (bFGF-C8-ARS) within the phase-shift emulsion. Ultrasound generated stable bubbles in bFGF-C6-ARS, which inhibited matrix compaction, whereas transiently stable bubbles were generated in bFGF-C8-ARS, which decreased in height by 44% within one day of implantation. The rate of bFGF release and distance of host cell migration were up to 6.8-fold and 8.1-fold greater, respectively, in bFGF-C8-ARS versus bFGF-C6-ARS. Ultrasound increased the formation of macropores within the fibrin matrix of bFGF-C8-ARS by 2.7-fold. These results demonstrate that spatially patterning suprathreshold ultrasound within bFGF-C8-ARS can be used to elicit a spatially-directed response from the host. Overall, these findings can be used in developing strategies to spatially pattern regenerative processes. STATEMENT OF SIGNIFICANCE: Hydrogels are commonly used in regenerative medicine for the delivery of growth factors (GFs). The spatial and temporal presentations of GFs are critical for directing regenerative processes, yet conventional hydrogels do not enable such control. We have developed a composite hydrogel, termed an acoustically-responsive scaffold (ARS), where GF release is non-invasively and spatiotemporally-controlled using focused ultrasound. The ARS consists of a fibrin matrix doped with a phase-shift emulsion loaded with GF, which is released when the ARS is exposed to ultrasound. In this in vivo study, we demonstrate that spatially patterning ultrasound within an ARS containing basic fibroblast growth factor (bFGF) can elicit a spatially-directed response from the host. Overall, these findings can be used in developing strategies to spatially pattern regenerative processes.

Biomaterial Based Strategies for Engineering New Lymphatic Vasculature

The lymphatic system is essential for tissue regeneration and repair due to its pivotal role in resolving inflammation, immune cell surveillance, lipid transport, and maintaining tissue homeostasis. Loss of functional lymphatic vasculature is directly implicated in a variety of diseases, including lymphedema, obesity, and the progression of cardiovascular diseases. Strategies that stimulate the formation of new lymphatic vessels (lymphangiogenesis) could provide an appealing new approach to reverse the progression of these diseases. However, lymphangiogenesis is relatively understudied and stimulating therapeutic lymphangiogenesis faces challenges in precise control of lymphatic vessel formation. Biomaterial delivery systems could be used to unleash the therapeutic potential of lymphangiogenesis for a variety of tissue regenerative applications due to their ability to achieve precise spatial and temporal control of multiple therapeutics, direct tissue regeneration, and improve the survival of delivered cells. In this review, the authors begin by introducing therapeutic lymphangiogenesis as a target for tissue regeneration, then an overview of lymphatic vasculature will be presented followed by a description of the mechanisms responsible for promoting new lymphatic vessels. Importantly, this work will review and discuss current biomaterial applications for stimulating lymphangiogenesis. Finally, challenges and future directions for utilizing biomaterials for lymphangiogenic based treatments are considered.

Biomimetic hydrogels with spatial- and temporal-controlled chemical cues for tissue engineering

Biomimetic hydrogels have emerged as the most useful tissue engineering scaffold materials. Their versatile chemistry can recapitulate multiple physical and chemical features to integrate cells, scaffolds, and signaling molecules for tissue regeneration. Due to their highly hydrophilic nature hydrogels can recreate nutrient-rich aqueous environments for cells. Soluble regulatory molecules can be incorporated to guide cell proliferation and differentiation. Importantly, the controlled dynamic parameters and spatial distribution of chemical cues in hydrogel scaffolds are critical for cell-cell communication, cell-scaffold interaction, and morphogenesis. Herein, we review biomimetic hydrogels that provide cells with spatiotemporally controlled chemical cues as tissue engineering scaffolds. Specifically, hydrogels with temporally controlled growth factor-release abilities, spatially controlled conjugated bioactive molecules/motifs, and targeting delivery and reload properties for tissue engineering applications are discussed in detail. Examples of hydrogels that possess clinically favorable properties, such as injectability, self-healing ability, stimulus-responsiveness, and pro-remodeling features, are also covered.

The Delivery of RNA-Interference Therapies Based on Engineered Hydrogels for Bone Tissue Regeneration

RNA interference (RNAi) is an efficient post-transcriptional gene modulation strategy mediated by small interfering RNAs (siRNAs) and microRNAs (miRNAs). Since its discovery, RNAi has been utilized extensively to diagnose and treat diseases at both the cellular and molecular levels. However, the application of RNAi therapies in bone regeneration has not progressed to clinical trials. One of the major challenges for RNAi therapies is the lack of efficient and safe delivery vehicles that can actualize sustained release of RNA molecules at the target bone defect site and in surrounding cells. One promising approach to achieve these requirements is encapsulating RNAi molecules into hydrogels for delivery, which enables the nucleic acids to be delivered as RNA conjugates or within nanoparticles. Herein, we reviewed recent investigations into RNAi therapies for bone regeneration where RNA delivery was performed by hydrogels.

The use of bioactive matrices in regenerative therapies for traumatic brain injury

Functional deficits due to neuronal loss are a common theme across multiple neuropathologies, including traumatic brain injury (TBI). Apart from mitigating cell death, another approach to treating brain injuries involves re-establishing the neural circuitry at the lesion site by utilizing exogeneous and/or endogenous stem cells to achieve functional recovery. While there has been limited success, the emergence of new bioactive matrices that promote neural repair introduces new perspectives on the development of regenerative therapies for TBI. This review briefly discusses current development on cell-based therapies and the use of bioactive matrices, hydrogels in particular, when incorporated in regenerative therapies. Desirable characteristics of bioactive matrices that have been shown to augment neural repair in TBI models were identified and further discussed. Understanding the relative outcomes of newly developed biomaterials implanted in vivo can better guide the development of biomaterials as a therapeutic strategy, for biomaterial-based cellular therapies are still in their nascent stages. Nonetheless, the value of bioactive matrices as a treatment for acute brain injuries should be appreciated and further developed. STATEMENT OF SIGNIFICANCE: Cell-based therapies have received attention as an alternative therapeutic strategy to improve clinical outcome post-traumatic brain injury but have achieved limited success. Whilst the incorporation of newly developed biomaterials in regenerative therapies has shown promise in augmenting neural repair, studies have revealed new hurdles which must be overcome to improve their therapeutic efficacy. This review discusses the recent development of cell-based therapies with a specific focus on the use of bioactive matrices in the form of hydrogels, to complement cell transplantation within the injured brain. Moreover, this review consolidates in vivo animal studies that demonstrate relative functional outcome upon the implantation of different biomaterials to highlight their desirable traits to guide their development for regenerative therapies in traumatic brain injury.

Mechanics-Controlled Dynamic Cell Niches Guided Osteogenic Differentiation of Stem Cells via Preserved Cellular Mechanical Memory

Stem cells sense and respond to their local dynamic mechanical niches, which further regulate the cellular behaviors. While in naturally, instead of instantly responding to real-time mechanical changes of their surrounding niches, stem cells often present a delayed cellular response over a time scale, namely cellular mechanical memory, which may finally influence their lineage choice. Here, we aim to build a dynamic mechanical niche model with alginate-based hydrogel, therein the dynamic mechanical switching can be easily realized via the introduce or removal of Ca²⁺. The results show that stiffening hydrogel (from soft to stiff) suppresses osteogenic differentiation of human mesenchymal stem cells (hMSCs) early on, though it finally promoted osteogenic differentiation over a long time period. Instead, softening hydrogel (from stiff to soft) still retains the strong osteogenic differentiation in the early days, though it finally showed a lower level of osteogenic differentiation compared with stiff hydrogel. Further, microRNA miR-21 has been found as a long-term mechanical memory sensor of the osteogenic program in hMSCs, as its level remains to match early mechanics of substrate over a period of time. Regulation of miR-21 level is efficient to erase the past mechanical memory and resensitize hMSCs to subsequent substrate mechanics. Our findings highlight cellular mechanical memory effect as a key factor of cell and cellular microenvironment interactions, which has been largely neglected before, and as a crucial design element of biomaterials for cell culture.

Electrical stimulation as a novel tool for regulating cell behavior in tissue engineering

Recently, electrical stimulation as a physical stimulus draws lots of attention. It shows great potential in disease treatment, wound healing, and mechanism study because of significant experimental performance. Electrical stimulation can activate many intracellular signaling pathways, and influence intracellular microenvironment, as a result, affect cell migration, cell proliferation, and cell differentiation. Electrical stimulation is using in tissue engineering as a novel type of tool in regeneration medicine. Besides, with the advantages of biocompatible conductive materials coming into view, the combination of electrical stimulation with suitable tissue engineered scaffolds can well combine the benefits of both and is ideal for the field of regenerative medicine. In this review, we summarize the various materials and latest technologies to deliver electrical stimulation. The influences of electrical stimulation on cell alignment, migration and its underlying mechanisms are discussed. Then the effect of electrical stimulation on cell proliferation and differentiation are also discussed.

Bioprinters for organs-on-chips

Recent advances in bioprinting technologies have enabled rapid manufacturing of organ-on-chip models along with biomimetic tissue microarchitectures. Bioprinting techniques can be used to integrate microfluidic channels and flow connections in organ-on-chip models. We review bioprinters in two categories of nozzle-based and optical-based methods, and then discuss their fabrication parameters such as resolution, replication fidelity, fabrication time, and cost for micro-tissue models and microfluidic applications. The use of bioprinters has shown successful replicates of functional engineered tissue models integrated within a desired microfluidic system, which facilitates the observation of metabolism or secretion of models and sophisticated control of a dynamic environment. This may provide a wider order of tissue engineering fabrication in mimicking physiological conditions for enhancing further applications such as drug development and pathological studies.

Engineered materials for organoid systems

Organoids are 3D cell culture systems that mimic some of the structural and functional characteristics of an organ. Organoid cultures provide the opportunity to study organ-level biology in models that mimic human physiology more closely than 2D cell culture systems or non-primate animal models. Many organoid cultures rely on decellularized extracellular matrices as scaffolds, which are often poorly chemically defined and allow only limited tunability and reproducibility. By contrast, the biochemical and biophysical properties of engineered matrices can be tuned and optimized to support the development and maturation of organoid cultures. In this Review, we highlight how key cell-matrix interactions guiding stem-cell decisions can inform the design of biomaterials for the reproducible generation and control of organoid cultures. We survey natural, synthetic and protein-engineered hydrogels for their applicability to different organoid systems and discuss biochemical and mechanical material properties relevant for organoid formation. Finally, dynamic and cell-responsive material systems are investigated for their future use in organoid research.