Acellular implantable and injectable hydrogels for vascular regeneration

Harnessing the potential of monocytes/macrophages to regenerate tissue-engineered vascular grafts

Cell-free tissue-engineered vascular grafts provide a promising alternative to treat cardiovascular disease, but timely endothelialization is essential for ensuring patency and proper functioning post-implantation. Recent studies from our lab showed that blood cells like monocytes (MCs) and macrophages (Mϕ) may contribute directly to cellularization and regeneration of bioengineered arteries in small and large animal models. While MCs and Mϕ are leucocytes that are part of the innate immune response, they share common developmental origins with endothelial cells (ECs) and are known to play crucial roles during vessel formation (angiogenesis) and vessel repair after inflammation/injury. They are highly plastic cells that polarize into pro-inflammatory and anti-inflammatory phenotypes upon exposure to cytokines and differentiate into other cell types, including EC-like cells, in the presence of appropriate chemical and mechanical stimuli. This review focuses on the developmental origins of MCs and ECs; the role of MCs and Mϕ in vessel repair/regeneration during inflammation/injury; and the role of chemical signalling and mechanical forces in Mϕ inflammation that mediates vascular graft regeneration. We postulate that comprehensive understanding of these mechanisms will better inform the development of strategies to coax MCs/Mϕ into endothelializing the lumen and regenerate the smooth muscle layers of cell-free bioengineered arteries and veins that are designed to treat cardiovascular diseases and perhaps the native vasculature as well.

Neurogenic Cell Behavior in 3D Culture Enhanced Within a Highly Compliant Synthetic Hydrogel Platform Formed via Competitive Crosslinking

Purpose

Scaffold materials that better support neurogenesis are still needed to improve cell therapy outcomes for neural tissue damage. We have used a modularly tunable, highly compliant, degradable hydrogel to explore the impacts of hydrogel compliance stiffness on neural differentiation. Here we implemented competitive matrix crosslinking mechanics to finely tune synthetic hydrogel moduli within soft tissue stiffnesses, a range much softer than typically achievable in synthetic crosslinked hydrogels, providing a modularly controlled and ultrasoft 3D culture model which supports and enhances neurogenic cell behavior.

Methods

Soluble competitive allyl monomers were mixed with proteolytically-degradable poly(ethylene glycol) diacrylate derivatives and crosslinked to form a matrix, and resultant hydrogel stiffness and diffusive properties were evaluated. Neural PC12 cells or primary rat fetal neural stem cells (NSCs) were encapsulated within the hydrogels, and cell morphology and phenotype were investigated to understand cell-matrix interactions and the effects of environmental stiffness on neural cell behavior within this model.

Results

Addition of allyl monomers caused a concentration-dependent decrease in hydrogel compressive modulus from 4.40 kPa to 0.26 kPa (natural neural tissue stiffness) without influencing soluble protein diffusion kinetics through the gel matrix. PC12 cells encapsulated in the softest hydrogels showed significantly enhanced neurite extension in comparison to PC12s in all other hydrogel stiffnesses tested. Encapsulated neural stem cells demonstrated significantly greater spreading and elongation in 0.26 kPa alloc hydrogels than in 4.4 kPa hydrogels. When soluble growth factor deprivation (for promotion of neural differentiation) was evaluated within the neural stiffness gels (0.26 kPa), NSCs showed increased neuronal marker expression, indicating early enhancement of neurogenic differentiation.

Conclusions

Implementing allyl-acrylate crosslinking competition reduced synthetic hydrogel stiffness to provide a supportive environment for 3D neural tissue culture, resulting in enhanced neurogenic behavior of encapsulated cells. These results indicate the potential suitability of this ultrasoft hydrogel system as a model platform for further investigating environmental factors on neural cell behavior.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12195-024-00794-2.

Hydrogel Breakthroughs in Biomedicine: Recent Advances and Implications

OBJECTIVE

The objective of this review is to present a succinct summary of the latest advancements in the utilization of hydrogels for diverse biomedical applications, with a particular focus on their revolutionary impact in augmenting the delivery of drugs, tissue engineering, along with diagnostic methodologies.

METHODS

Using a meticulous examination of current literary works, this review systematically scrutinizes the nascent patterns in applying hydrogels for biomedical progress, condensing crucial discoveries to offer a comprehensive outlook on their ever-changing importance.

RESULTS

The analysis presents compelling evidence regarding the growing importance of hydrogels in biomedicine. It highlights their potential to significantly enhance drug delivery accuracy, redefine tissue engineering strategies, and advance diagnostic techniques. This substantiates their position as a fundamental element in the progress of modern medicine.

CONCLUSION

In summary, the constantly evolving advancement of hydrogel applications in biomedicine calls for ongoing investigation and resources, given their diverse contributions that can revolutionize therapeutic approaches and diagnostic methods, thereby paving the way for improved patient well-being.

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.

The diversified hydrogels for biomedical applications and their imperative roles in tissue regeneration

Repair and regeneration of tissues after injury are complex pathophysiological processes. Microbial infection, malnutrition, and an ischemic and hypoxic microenvironment in the injured area can impede the typical healing cascade. Distinguished by biomimicry of the extracellular matrix, high aqueous content, and diverse functions, hydrogels have revolutionized clinical practices in tissue regeneration owing to their outstanding hydrophilicity, biocompatibility, and biodegradability. Various hydrogels such as smart hydrogels, nanocomposite hydrogels, and acellular matrix hydrogels are widely used for applications ranging from bench-scale to an industrial scale. In this review, some emerging hydrogels in the biomedical field are briefly discussed. The protective roles of hydrogels in wound dressings and their diverse biological effects on multiple tissues such as bone, cartilage, nerve, muscle, and adipose tissue are also discussed. The vehicle functions of hydrogels for chemicals and cell payloads are detailed. Additionally, this review emphasizes the particular characteristics of hydrogel products that promote tissue repair and reconstruction such as anti-infection, inflammation regulation, and angiogenesis.

Engineered nanovesicles from stromal vascular fraction promote angiogenesis and adipogenesis inside decellularized adipose tissue through encapsulating growth factors

Acellular matrix is a commonly used biomaterial in the field of biomedical engineering and revascularization is the key process to affect the effect of acellular matrix on tissue regeneration. The application of bioactive factors related to angiogenesis has been popular in the regulation of revascularization, but the immune system clearance, uncontrollable systemic reactions, and other factors make this method face challenges. Recent reports showed that engineered cells into nanovesicles can reorganize cell membranes and encapsulate cellular active factors, extending the in vitro preservation of cytokines. However, the problems of exogenous biological contamination and tumorigenicity restricted the clinical transformation and wide application of this method. Here, we for the first time engineer stromal vascular fraction (SVF) which is extracted from fat into nanovesicles (SVF-EVs) for angiogenesis in the acellular matrix. SVF-EVs not only promote the migration of vascular endothelial cells in vitro, but also facilitate the lipogenic differentiation of mesenchymal stem cells. In vivo, SVF-EVs enhanced the retention of decellularized adipose tissue after transplanting to the subcutaneous area of nude mice. Immunofluorescence staining further showed that SVF-EVs promoted the formation of vascular networks with large lumen diameter in the grafted acellular matrix, accompanied by adipocyte regeneration peripherally. These findings reveal that SVF-EVs can be a viable method for accelerating revascularization in acellular matrix, and this process of squeezing tissue into nanovesicles shows the potential for rapid clinical transformation.

Current Understanding of Hydrogel for Drug Release and Tissue Engineering

Due to their good absorption, satisfactory biocompatibility, and high safety, hydrogels have been widely used in the field of biomedicine, including for drug delivery and tissue regeneration. In this review, we introduce the characteristics and crosslinking methods of natural and synthetic hydrogels. Then, we highlight the design and principle of intelligent hydrogels (i.e., responsive hydrogels) used for drug release. Moreover, we introduce the application of the application of hydrogels in drug release and tissue engineering, and the limitations and research directions of hydrogel in drug release and tissue engineering are also considered. We hope that this review can provide a reference for follow-up studies in related fields.

Hydrogel-based therapeutic angiogenesis: An alternative treatment strategy for critical limb ischemia

Critical limb ischemia (CLI) is the most severe clinical manifestation of peripheral arterial disease (PAD), resulting in the total or partial loss of limb function. Although the conventional treatment strategy of CLI (e.g., medical treatment and surgery) can improve blood perfusion and restore limb function, many patients are unsuitable for these strategies and they still face the threats of amputation or death. Therapeutic angiogenesis, as a potential solution for these problems, attempts to manipulate blood vessel growth in vivo for augment perfusion without the help of extra pharmaceutics and surgery. With the rise of interdisciplinary research, regenerative medicine strategies provide new possibilities for treating many clinical diseases. Hydrogel, as an excellent biocompatibility material, is an ideal candidate for delivering bioactive molecules and cells for therapeutic angiogenesis. Besides, hydrogel could precisely deliver, control release, and keep the bioactivity of cargos, making hydrogel-based therapeutic angiogenesis a new strategy for CLI therapy. In this review, we comprehensively discuss the approaches of hydrogel-based strategy for CLI treatment as well as their challenges, and future directions.

Angiogenic Potential in Biological Hydrogels

Hydrogels are three-dimensional (3D) materials able to absorb and retain water in large amounts while maintaining their structural stability. Due to their considerable biocompatibility and similarity with the body’s tissues, hydrogels are one of the most promising groups of biomaterials. The main application of these hydrogels is in regenerative medicine, in which they allow the formation of an environment suitable for cell differentiation and growth. Deriving from these hydrogels, it is, therefore, possible to obtain bioactive materials that can regenerate tissues. Because vessels guarantee the right amount of oxygen and nutrients but also assure the elimination of waste products, angiogenesis is one of the processes at the base of the regeneration of a tissue. On the other hand, it is a very complex mechanism and the parameters to consider are several. Indeed, the factors and the cells involved in this process are numerous and, for this reason, it has been a challenge to recreate a biomaterial able to adequately sustain the angiogenic process. However, in this review the focal point is the application of natural hydrogels in angiogenesis enhancing and their potential to guide this process.

Vascularization of tissue-engineered skeletal muscle constructs

Skeletal muscle tissue can be created in vitro by tissue engineering approaches, based on differentiation of muscle stem cells. Several approaches exist and generally result in three dimensional constructs composed of multinucleated myofibers to which we refer as myooids. Engineering methods date back to 3 decades ago and meanwhile a wide range of cell types and scaffold types have been evaluated. Nevertheless, in most approaches, myooids remain very small to allow for diffusion-mediated nutrient supply and waste product removal, typically less than 1 mm thick. One of the shortcomings of current in vitro skeletal muscle organoid development is the lack of a functional vascular structure, thus limiting the size of myooids. This is a challenge which is nowadays applicable to almost all organoid systems. Several approaches to obtain a vascular structure within myooids have been proposed. The purpose of this review is to give a concise overview of these approaches.

Hydrogel vehicles for sequential delivery of protein drugs to promote vascular regeneration

In recent years, as the mechanisms of vasculogenesis and angiogenesis have been uncovered, the functions of various pro-angiogenic growth factors (GFs) and cytokines have been identified. Therefore, therapeutic angiogenesis, by delivery of GFs, has been sought as a treatment for many vascular diseases. However, direct injection of these protein drugs has proven to have limited clinical success due to their short half-lives and systemic off-target effects. To overcome this, hydrogel carriers have been developed to conjugate single or multiple GFs with controllable, sustained, and localized delivery. However, these attempts have failed to account for the temporal complexity of natural angiogenic pathways, resulting in limited therapeutic effects. Recently, the emerging ideas of optimal sequential delivery of multiple GFs have been suggested to better mimic the biological processes and to enhance therapeutic angiogenesis. Incorporating sequential release into drug delivery platforms will likely promote the formation of neovasculature and generate vast therapeutic potential.

A self-healing hydrogel as an injectable instructive carrier for cellular morphogenesis

Transplantation of progenitor cells can accelerate tissue healing and regenerative processes. Nonetheless, direct cell delivery fails to support survival of transplanted cells or long-term treatment of vascular related diseases due to compromised vasculature and tissue conditions. Using injectable hydrogels that cross-link in situ, could protect cells in vivo, but their sol-gel transition is time-dependent and difficult to precisely control. Hydrogels with self-healing properties are proposed to address these limitations, yet current self-healing hydrogels lack bio-functionality, hindering the morphogenesis of delivered cells into a tissue structure. Here we establish a gelatin (Gtn)-based self-healing hydrogel cross-linked by oxidized dextran (Odex) as an injectable carrier for delivery of endothelial progenitors. The dynamic imine cross-links between Gtn and Odex confer the self-healing ability to the Gtn-l-Odex hydrogels following syringe injection. The self-healing Gtn-l-Odex not only protects the progenitors from injected shear force but it also allows controllable spatial/temporal placement of the cells. Moreover, owing to the cell-adhesive and proteolytic sites of Gtn, the Gtn-l-Odex hydrogels support complex vascular network formation from the endothelial progenitors, both in vitro and in vivo. This is the first report of injectable, self-healing hydrogels with biological properties promoting vascular morphogenesis, which holds great promise for accelerating the success of regenerative therapies.

Controlled release of basic fibroblast growth factor for angiogenesis using acoustically-responsive scaffolds

The clinical translation of pro-angiogenic growth factors for treatment of vascular disease has remained a challenge due to safety and efficacy concerns. Various approaches have been used to design spatiotemporally-controlled delivery systems for growth factors in order to recapitulate aspects of endogenous signaling and thus assist in translation. We have developed acoustically-responsive scaffolds (ARSs), which are fibrin scaffolds doped with a payload-containing, sonosensitive emulsion. Payload release can be controlled non-invasively and in an on-demand manner using focused, megahertz-range ultrasound (US). In this study, we investigate the in vitro and in vivo release from ARSs containing basic fibroblast growth factor (bFGF) encapsulated in monodispersed emulsions. Emulsions were generated in a two-step process utilizing a microfluidic device with a flow focusing geometry. At 2.5 MHz, controlled release of bFGF was observed for US pressures above 2.2 ± 0.2 MPa peak rarefactional pressure. Superthreshold US yielded a 12.6-fold increase in bFGF release in vitro. The bioactivity of the released bFGF was also characterized. When implanted subcutaneously in mice, ARSs exposed to superthreshold US displayed up to 3.3-fold and 1.7-fold greater perfusion and blood vessel density, respectively, than ARSs without US exposure. Scaffold degradation was not impacted by US. These results highlight the utility of ARSs in both basic and applied studies of therapeutic angiogenesis.

ECM–oligourethene–silica hydrogels as a local drug release system of dexamethasone for stimulating macrophages

The incorporation of silica particles inside of extracellular matrix hydrogels supports the loading and releasing of dexamethasone, a therapeutic for modulating macrophage.

Microstructured chitosan/poly(γ-glutamic acid) polyelectrolyte complex hydrogels by computer-aided wet-spinning for biomedical three-dimensional scaffolds

The application of additive manufacturing principles to hydrogel processing represents a powerful route to develop porous three-dimensional constructs with a variety of potential biomedical applications, such as scaffolds for tissue engineering and three-dimensional in vitro tissue models. The aim of this study was to develop novel porous hydrogels based on a microstructured polyelectrolyte complex between chitosan and poly(γ-glutamic acid) by applying a computer-aided wet-spinning technique. The developed fabrication process was used to build up three-dimensional porous hydrogels by collecting microstructured layers made of chitosan/poly(γ-glutamic acid) on top of the other. Microstructured polyelectrolyte complex hydrogels were characterized and compared to chitosan/poly(γ-glutamic acid) porous hydrogels with similar composition prepared by conventional freeze-drying technique. Fourier transform infrared analysis confirmed the formation of an electrostatic interaction between the two processed polymers in all the developed chitosan/poly(γ-glutamic acid) hydrogels. The composition of the porous constructs as well as the employed processing techniques had a significant influence on the resulting swelling, thermal, and mechanical properties. In particular, the combination of the ionic interaction between chitosan/poly(γ-glutamic acid) and the defined internal microarchitecture of microstructured polyelectrolyte complex hydrogels provided a significant improvement of the compressive mechanical properties. Preliminary in vitro biological investigations revealed that microstructured polyelectrolyte complex hydrogels were suitable for the adhesion and proliferation of Balb/3T3 clone A31 mouse embryo fibroblasts. The encouraging results in terms of cytocompatibility and stability of the microstructure in aqueous solutions due to the ionic crosslinking suggest the investigation of the developed microstructured polyelectrolyte complex hydrogels as suitable scaffolds for three-dimensional cells’ culture.

Growth Factor-Mediated Migration of Bone Marrow Progenitor Cells for Accelerated Scaffold Recruitment

Tissue engineering approaches using growth factor-functionalized acellular scaffolds to support and guide repair driven by endogenous cells are thought to require a careful balance between cell recruitment and growth factor release kinetics. The objective of this study was to identify a growth factor combination that accelerates progenitor cell migration into self-assembling peptide hydrogels in the context of cartilage defect repair. A novel 3D gel-to-gel migration assay enabled quantification of the chemotactic impact of platelet-derived growth factor-BB (PDGF-BB), heparin-binding insulin-like growth factor-1 (HB-IGF-1), and transforming growth factor-β1 (TGF-β1) on progenitor cells derived from subchondral bovine trabecular bone (bone-marrow progenitor cells, BM-PCs) encapsulated in the peptide hydrogel [KLDL]3. Only the combination of PDGF-BB and TGF-β1 stimulated significant migration of BM-PCs over a 4-day period, measured by confocal microscopy. Both PDGF-BB and TGF-β1 were slowly released from the gel, as measured using their (125)I-labeled forms, and they remained significantly present in the gel at 4 days. In the context of augmenting microfracture surgery for cartilage repair, our strategy of delivering chemotactic and proanabolic growth factors in KLD may provide the necessary local stimulus to help increase defect cellularity, providing more cells to generate repair tissue.

Stem cells and injectable hydrogels: Synergistic therapeutics in myocardial repair

One of the major problems in the treatment of cardiovascular diseases is the inability of myocardium to self-regenerate. Current therapies are unable to restore the heart's function after myocardial infarction. Myocardial tissue engineering is potentially a key approach to regenerate damaged heart muscle. Myocardial patches are applied surgically, whereas injectable hydrogels provide effective minimally invasive approaches to recover functional myocardium. These hydrogels are easily administered and can be either cell free or loaded with bioactive agents and/or cardiac stem cells, which may apply paracrine effects. The aim of this review is to investigate the advantages and disadvantages of injectable stem cell-laden hydrogels and highlight their potential applications for myocardium repair.

Angiogenic gene therapy for refractory angina

INTRODUCTION

Stimulation of coronary collateral vessel growth by therapeutic angiogenesis (TA) offers an alternative treatment option for patients with refractory angina. Several TA modalities, including delivery to the heart of angiogenic growth factors (proteins or genes) and cells have been tested in clinical trials in the past two decades, but so far none of them resulted in significant therapeutic efficacy in large scale studies. This review attempts to identify the main obstacles hindering clinical success and recommends measures to overcome them in the future.

AREAS COVERED

After stating the medical need and rational for TA, and listing and briefly discussing past and current TA clinical trials, three main areas of obstacles are described: conceptual questions, technical limitations and clinical design uncertainties. Based on scientific and technical advances and lessons learned in past clinical trials, potential solutions to overcome some of these obstacles are proposed.

EXPERT OPINION

Several success criteria are identified, which apply to any TA approach of choice. It is emphasized, that each of these criteria needs to be met in future clinical trials to have a chance of therapeutic success.

Designer Hydrogels for Precision Control of Oxygen Tension and Mechanical Properties

Oxygen levels and mechanical properties provide vital cues to regulate myriad cellular functions and stem cell fate decisions. Here, we present a hybrid hydrogel system in which we can control independently oxygen levels and mechanical properties. We designed, synthesized and analyzed a hybrid hydrogel system comprised of two polymer backbones, gelatin and dextran. Both polymers were crosslinked via a laccase-mediated, oxygen consuming reaction. By specifically controlling the concentration of phenolic molecules available to react in our hydrogel, we could precisely control the time in which the hydrogel remained hypoxic (TH). We were able to achieve a range of TH from the order of minutes to greater than 10 hours. Additionally, by incorporating a secondary crosslinker, transglutaminase, mechanical properties could be adjusted in a user-defined fashion, with dynamic elastic modulus (G') values ranging from <20 Pa to >1 kPa. Importantly, oxygen levels and substrate mechanical properties could be individually tuned and decoupled in our hybrid hydrogels, while retaining the potential to study possible synergistic effects between the two parameters. By precisely controlling oxygen tension and mechanical properties, we expect that research utilizing the new hybrid hydrogels will enhance our understanding of the complex 3D cellular processes mediated by each parameter individually and may also hold clinical interest as acellular therapies.