Injectable shear-thinning hydrogels used to deliver endothelial progenitor cells, enhance cell engraftment, and improve ischemic myocardium

Hyaluronic acid and fibrin from L-PRP form semi-IPNs with tunable properties suitable for use in regenerative medicine

Autologous leukocyte- and platelet-rich plasma (L-PRP) combined with hyaluronic acid (HA) has been widely used in local applications for cartilage and bone regeneration. The association between L-PRP and HA confers structural and rheological changes that differ among individual biomaterials but has not been investigated. Therefore, the standardization and characterization of L-PRP-HA are important to consider when comparing performance results to improve future clinical applications. To this end, we prepared semi-interpenetrating polymer networks (semi-IPNs) of L-PRP and HA and characterized their polymerization kinetics, morphology, swelling ratio, stability and rheological behavior, which we found to be tunable according to the HA molar mass (MM). Mesenchymal stem cells derived from human adipose tissue (h-AdMSCs) seeded in the semi-IPNs had superior viability and chondrogenesis and osteogenesis capabilities compared to the viability and capabilities of fibrin. We have demonstrated that the preparation of the semi-IPNs under controlled mixing ensured the formation of cell-friendly hydrogels rich in soluble factors and with tunable properties according to the HA MM, rendering them suitable for clinical applications in regenerative medicine.

A Hepatic Scaffold from Decellularized Liver Tissue: Food for Thought

Allogeneic liver transplantation is still deemed the gold standard solution for end-stage organ failure; however, donor organ shortages have led to extended waiting lists for organ transplants. In order to overcome the lack of donors, the development of new therapeutic options is mandatory. In the last several years, organ bioengineering has been extensively explored to provide transplantable tissues or whole organs with the final goal of creating a three-dimensional growth microenvironment mimicking the native structure. It has been frequently reported that an extracellular matrix-based scaffold offers a structural support and important biological molecules that could help cellular proliferation during the recellularization process. The aim of the present review is to underline the recent developments in cell-on-scaffold technology for liver bioengineering, taking into account: (1) biological and synthetic scaffolds; (2) animal and human tissue decellularization; (3) scaffold recellularization; (4) 3D bioprinting; and (5) organoid technology. Future possible clinical applications in regenerative medicine for liver tissue engineering and for drug testing were underlined and dissected.

Enhancing survival, engraftment, and osteogenic potential of mesenchymal stem cells

Mesenchymal stem cells (MSCs) are promising candidates for bone regeneration therapies due to their plasticity and easiness of sourcing. MSC-based treatments are generally considered a safe procedure, however, the long-term results obtained up to now are far from satisfactory. The main causes of these therapeutic limitations are inefficient homing, engraftment, and osteogenic differentiation. Many studies have proposed modifications to improve MSC engraftment and osteogenic differentiation of the transplanted cells. Several strategies are aimed to improve cell resistance to the hostile microenvironment found in the recipient tissue and increase cell survival after transplantation. These strategies could range from a simple modification of the culture conditions, known as cell-preconditioning, to the genetic modification of the cells to avoid cellular senescence. Many efforts have also been done in order to enhance the osteogenic potential of the transplanted cells and induce bone formation, mainly by the use of bioactive or biomimetic scaffolds, although alternative approaches will also be discussed. This review aims to summarize several of the most recent approaches, providing an up-to-date view of the main developments in MSC-based regenerative techniques.

Smart Hydrogels in Tissue Engineering and Regenerative Medicine

The field of regenerative medicine has tremendous potential for improved treatment outcomes and has been stimulated by advances made in bioengineering over the last few decades. The strategies of engineering tissues and assembling functional constructs that are capable of restoring, retaining, and revitalizing lost tissues and organs have impacted the whole spectrum of medicine and health care. Techniques to combine biomimetic materials, cells, and bioactive molecules play a decisive role in promoting the regeneration of damaged tissues or as therapeutic systems. Hydrogels have been used as one of the most common tissue engineering scaffolds over the past two decades due to their ability to maintain a distinct 3D structure, to provide mechanical support for the cells in the engineered tissues, and to simulate the native extracellular matrix. The high water content of hydrogels can provide an ideal environment for cell survival, and structure which mimics the native tissues. Hydrogel systems have been serving as a supportive matrix for cell immobilization and growth factor delivery. This review outlines a brief description of the properties, structure, synthesis and fabrication methods, applications, and future perspectives of smart hydrogels in tissue engineering.

Nanofibrillar Cellulose as an Enzymatically and Flow Driven Degradable Scaffold for Three-Dimensional Tissue Engineering

Nanofibrillar cellulose as a naturally biocompatible scaffold material is very promising for tissue engineering. It is shear thinning but has the downside of not being degradable in animals, it can only be degraded by cellulase enzymes. In this study, a newly developed bioreactor was used to culture fibroblast spheroids under flow conditions inside nanocellulose hydrogels with and without the presence of cellulase. The aim was to control the tissue size and ideally find a match between degradation and tissue formation within this promising material. Both the concentration of cellulase and the flow rate were varied and their influence on the activity and growth of fibroblast clusters was assessed. Cluster diameters, degradation, metabolic activity, and tissue production increase with higher cellulase concentration, although concentrations above 1 g/l does not have an additional benefit. Flow leads to more viable cells, more proliferation and migration, leading to overall larger tissue constructs compared to static conditions. This is most likely due to the shear thinning effect of flow on cellulose nanofibrils (CNFs) in addition to the increased nutrient supply through perfusion. At a constant cellulase concentration of 1 g/l, a flow of 2 ml/min proved to be optimal for tissue production. Therefore, degradation in combination with flow leads to more effective tissue production in CNF hydrogels, which is a very potent scaffold material for tissue engineering.

Therapeutic potential of exosomes in rotator cuff tendon healing

Rotator cuff tears are common musculoskeletal injuries that can cause significant pain and disability. While the clinical results of rotator cuff repair can be good, failure of tendon healing remains a significant problem. Molecular mechanisms underlying structural failure following surgical repair remain unclear. Histologically, enhanced inflammation, disorganization of the collagen fibers, calcification, apoptosis and tissue necrosis affect the normal healing process. Mesenchymal stem cells (MSCs) have the ability to provide improved healing following rotator cuff repair via the release of mediators from secreted 30-100 nm extracellular vesicles called exosomes. They carry regulatory proteins, mRNA and miRNA and have the ability to increase collagen synthesis and angiogenesis through increased expression of mRNA and release of proangiogenic factors and regulatory proteins that play a major role in proper tissue remodeling and preventing extracellular matrix degradation. Various studies have shown the effect of exosomes on improving outcome of cutaneous wound healing, scar tissue formation, degenerative bone disease and Duchenne Muscular Dystrophy. In this article, we critically reviewed the potential role of exosomes in tendon regeneration and propose the novel use of exosomes alone or seeded onto biomaterial matrices to stimulate secretion of favorable cellular factors in accelerating the healing response following rotator cuff repair.

Self-Healing Hydrogels: The Next Paradigm Shift in Tissue Engineering?

Given their durability and long-term stability, self-healable hydrogels have, in the past few years, emerged as promising replacements for the many brittle hydrogels currently being used in preclinical or clinical trials. To this end, the incompatibility between hydrogel toughness and rapid self-healing remains unaddressed, and therefore most of the self-healable hydrogels still face serious challenges within the dynamic and mechanically demanding environment of human organs/tissues. Furthermore, depending on the target tissue, the self-healing hydrogels must comply with a wide range of properties including electrical, biological, and mechanical. Notably, the incorporation of nanomaterials into double-network hydrogels is showing great promise as a feasible way to generate self-healable hydrogels with the above-mentioned attributes. Here, the recent progress in the development of multifunctional and self-healable hydrogels for various tissue engineering applications is discussed in detail. Their potential applications within the rapidly expanding areas of bioelectronic hydrogels, cyborganics, and soft robotics are further highlighted.

Sustained release of endothelial progenitor cell-derived extracellular vesicles from shear-thinning hydrogels improves angiogenesis and promotes function after myocardial infarction

Aims

Previous studies have demonstrated improved cardiac function following myocardial infarction (MI) after administration of endothelial progenitor cells (EPCs) into ischaemic myocardium. A growing body of literature supports paracrine effectors, including extracellular vesicles (EVs), as the main mediators of the therapeutic benefits of EPCs. The direct use of paracrine factors is an attractive strategy that harnesses the effects of cell therapy without concerns of cell engraftment or viability. We aim to reproduce the beneficial effects of EPC treatment through delivery of EPC-derived EVs within a shear-thinning gel (STG) for precise localization and sustained delivery.

Methods and results

EVs were harvested from EPCs isolated from adult male Rattus norvegicus (Wistar) rats and characterized by electron microscopy, nanoparticle tracking analysis (NTA), and mass spectrometry. EVs were incorporated into the STG and injected at the border zone in rat models of MI. Haemodynamic function, angiogenesis, and myocardial remodelling were analyzed in five groups: phosphate buffered saline (PBS) control, STG control, EVs in PBS, EVs in STG, and EPCs in STG. Electron microscopy and NTA of EVs showed uniform particles of 50-200 nm. EV content analysis revealed several key angiogenic mediators. EV uptake by endothelial cells was confirmed and followed by robust therapeutic angiogenesis. In vivo animal experiments demonstrated that delivery of EVs within the STG resulted in increased peri-infarct vascular proliferation, preservation of ventricular geometry, and improved haemodynamic function post-MI.

Conclusions

EPC-derived EVs delivered into ischaemic myocardium via an injectable hydrogel enhanced peri-infarct angiogenesis and myocardial haemodynamics in a rat model of MI. The STG greatly increased therapeutic efficiency and efficacy of EV-mediated myocardial preservation.

Injectable Supramolecular Hydrogel/Microgel Composites for Therapeutic Delivery

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

Therapeutic neovascularization promoted by injectable hydrogels

The aim of therapeutic neovascularization is to repair ischemic tissues via formation of new blood vessels by delivery of angiogenic growth factors, stem cells or expansion of pre-existing cells. For efficient neovascularization, controlled release of growth factors is particularly necessary since bolus injection of molecules generally lead to a poor outcome due to inadequate retention within the injured site. In this regard, injectable hydrogels, made of natural, synthetic or hybrid biomaterials, have become a promising solution for efficient delivery of angiogenic factors or stem and progenitor cells for in situ tissue repair, regeneration and neovascularization. This review article will broadly discuss the state-of-the-art in the development of injectable hydrogels from natural and synthetic precursors, and their applications in ischemic tissue repair and wound healing. We will cover a wide range of in vitro and in vivo studies in testing the functionalities of the engineered injectable hydrogels in promoting tissue repair and neovascularization. We will also discuss some of the injectable hydrogels that exhibit self-healing properties by promoting neovascularization without the presence of angiogenic factors.

Delivery of progenitor cells with injectable shear-thinning hydrogel maintains geometry and normalizes strain to stabilize cardiac function after ischemia

OBJECTIVES

The ventricle undergoes adverse remodeling after myocardial infarction, resulting in abnormal biomechanics and decreased function. We hypothesize that tissue-engineered therapy could minimize postischemic remodeling through mechanical stress reduction and retention of tensile myocardial properties due to improved endothelial progenitor cell retention and intrinsic biomechanical properties of the hyaluronic acid shear-thinning gel.

METHODS

Endothelial progenitor cells were harvested from adult Wistar rats and resuspended in shear-thinning gel. The constructs were injected at the border zone of ischemic rat myocardium in an acute model of myocardial infarction. Myocardial remodeling, tensile properties, and hemodynamic function were analyzed: control (phosphate-buffered saline), endothelial progenitor cells, shear-thinning gel, and shear-thinning gel + endothelial progenitor cells. Novel high-resolution, high-sensitivity ultrasound with speckle tracking allowed for global strain analysis. Uniaxial testing assessed tensile biomechanical properties.

RESULTS

Shear-thinning gel + endothelial progenitor cell injection significantly increased engraftment and retention of the endothelial progenitor cells within the myocardium compared with endothelial progenitor cells alone. With the use of strain echocardiography, a significant improvement in left ventricular ejection fraction was noted in the shear-thinning gel + endothelial progenitor cell cohort compared with control (69.5% ± 10.8% vs 40.1% ± 4.6%, P = .04). A significant normalization of myocardial longitudinal displacement with subsequent stabilization of myocardial velocity with shear-thinning gel + endothelial progenitor cell therapy compared with control was also evident (0.84 + 0.3 cm/s vs 0.11 ± 0.01 cm/s, P = .03). A significantly positive and higher myocardial strain was observed in shear-thinning gel + endothelial progenitor cell (4.5% ± 0.45%) compared with shear-thinning gel (3.7% ± 0.24%), endothelial progenitor cell (3.5% ± 0.97%), and control (8.6% ± 0.3%, P = .05). A resultant reduction in dynamic stiffness was noted in the shear-thinning gel + endothelial progenitor cell cohort.

CONCLUSIONS

This novel injectable shear-thinning hyaluronic acid hydrogel demonstrates stabilization of border zone myocardium with reduction in adverse myocardial remodeling and preservation of myocardial biomechanics. The cellular construct provides a normalization of strain measurements and reduces left ventricular dilatation, thus resulting in improvement of left ventricular function.

Advanced Material Catheter (AMCath), a minimally invasive endocardial catheter for the delivery of fast-gelling covalently cross-linked hyaluronic acid hydrogels

Injectable hydrogels that aim to mechanically stabilise the weakened left ventricle wall to restore cardiac function or to deliver stem cells in cardiac regenerative therapy have shown promising data. However, the clinical translation of hydrogel-based therapies has been limited due to difficulties injecting them through catheters. We have engineered a novel catheter, Advanced Materials Catheter (AMCath), that overcomes translational hurdles associated with delivering fast-gelling covalently cross-linked hyaluronic acid hydrogels to the myocardium. We developed an experimental technique to measure the force required to inject such hydrogels and determined the mechanical/viscoelastic properties of the resulting hydrogels. The preliminary in vivo feasibility of delivering fast-gelling hydrogels through AMCath was demonstrated by accessing the porcine left ventricle and showing that the hydrogel was retained in the myocardium post-injection (three 200 μL injections delivered, 192, 204 and 183 μL measured). However, the mechanical properties of the hydrogels were reduced by passage through AMCath (≤20.62% reduction). We have also shown AMCath can be used to deliver cardiopoietic adipose-derived stem cell-loaded hydrogels without compromising the viability (80% viability) of the cells in vitro. Therefore, we show that hydrogel/catheter compatibility issues can be overcome as we have demonstrated the minimally invasive delivery of a fast-gelling covalently cross-linked hydrogel to the beating myocardium.

Dynamic Hydrogels from Host-Guest Supramolecular Interactions

Hydrogel biomaterials are pervasive in biomedical use. Applications of these soft materials range from contact lenses to drug depots to scaffolds for transplanted cells. A subset of hydrogels is prepared from physical cross-linking mediated by host-guest interactions. Host macrocycles, the most recognizable supramolecular motif, facilitate complex formation with an array of guests by inclusion in their portal. Commonly, an appended macrocycle forms a complex with appended guests on another polymer chain. The formation of poly(pseudo)rotaxanes is also demonstrated, wherein macrocycles are threaded by a polymer chain to give rise to physical cross-linking by secondary non-covalent interactions or polymer jamming. Host-guest supramolecular hydrogels lend themselves to a variety of applications resulting from their dynamic properties that arise from non-covalent supramolecular interactions, as well as engineered responsiveness to external stimuli. These are thus an exciting new class of materials.

Synthesis and Biomedical Applications of Self-healing Hydrogels

Hydrogels, which are crosslinked polymer networks with high water contents and rheological solid-like properties, are attractive materials for biomedical applications. Self-healing hydrogels are particularly interesting because of their abilities to repair the structural damages and recover the original functions, similar to the healing of organism tissues. In addition, self-healing hydrogels with shear-thinning properties can be potentially used as the vehicles for drug/cell delivery or the bioinks for 3D printing by reversible sol-gel transitions. Therefore, self-healing hydrogels as biomedical materials have received a rapidly growing attention in recent years. In this paper, synthesis methods and repair mechanisms of self-healing hydrogels are reviewed. The biomedical applications of self-healing hydrogels are also described, with a focus on the potential therapeutic applications verified through in vivo experiments. The trends indicate that self-healing hydrogels with automatically reversible crosslinks may be further designed and developed for more advanced biomedical applications in the future.

The use of hydrogels for cell-based treatment of chronic kidney disease

Chronic kidney disease (CKD) is a major and growing public health concern with increasing incidence and prevalence worldwide. The therapeutic potential of stem cell therapy, including mesenchymal stem cells (MSCs) and endothelial progenitor cells (EPCs) holds great promise for treatment of CKD. However, there are significant bottlenecks in the clinical translation due to the reduced number of transplanted cells and the duration of their presence at the site of tissue damage. Bioengineered hydrogels may provide a route of cell delivery to enhance treatment efficacy and optimise the targeting effectiveness while minimising any loss of cell function. In this review, we highlight the advances in stem cell therapy targeting kidney disease and discuss the emerging role of hydrogel delivery systems to fully realise the potential of adult stem cells as a regenerative therapy for CKD in humans. MSCs and EPCs mediate kidney repair through distinct paracrine effects. As a delivery system, hydrogels can prolong these paracrine effects by improving retention at the site of injury and protecting the transplanted cells from the harsh inflammatory microenvironment. We also discuss the features of a hydrogel, which may be tuned to optimise the therapeutic potential of encapsulated stem cells, including cell-adhesive epitopes, material stiffness, nanotopography, modes of gelation and degradation and the inclusion of bioactive molecules. This review concludes with a discussion of the challenges to be met for the widespread clinical use of hydrogel delivery system of stem cell therapy for CKD.

It's All in the Delivery: Designing Hydrogels for Cell and Non-viral Gene Therapies

Hydrogels provide a regenerative medicine platform with their ability to create an environment that supports transplanted or endogenous infiltrating cells and enables these cells to restore or replace the function of tissues lost to disease or trauma. Furthermore, these systems have been employed as delivery vehicles for therapeutic genes, which can direct and/or enhance the function of the transplanted or endogenous cells. Herein, we review recent advances in the development of hydrogels for cell and non-viral gene delivery through understanding the design parameters, including both physical and biological components, on promoting transgene expression, cell engraftment, and ultimately cell function. Furthermore, this review identifies emerging opportunities for combining cell and gene delivery approaches to overcome challenges to the field.

Self-Healing Zwitterionic Microgels as a Versatile Platform for Malleable Cell Constructs and Injectable Therapies

Injectable and malleable hydrogels that combine excellent biocompatibility, physiological stability, and ease of use are highly desirable for biomedical applications. Here, a simple and scalable strategy is reported to make injectable and malleable zwitterionic polycarboxybetaine hydrogels, which are superhydrophilic, nonimmunogenic, and completely devoid of nonspecific interactions. When zwitterionic microgels are reconstructed, the combination of covalent crosslinking inside each microgel and supramolecular interactions between them gives the resulting zwitterionic injectable pellet (ZIP) constructs supportive moduli and tunable viscoelasticity. ZIP constructs can be lyophilized to a sterile powder that fully recovers its strength and elasticity upon rehydration, simplifying storage and formulation. The lyophilized powder can be reconstituted with any aqueous suspension of cells or therapeutics, and rapidly and spontaneously self-heals into a homogeneous composite construct. This versatile and highly biocompatible platform material shows great promise for many applications, including as an injectable cell culture scaffold that promotes multipotent stem cell expansion and provides oxidative stress protection.

Sequentially Moldable and Bondable Four-Dimensional Hydrogels Compatible with Cell Encapsulation

Hydrogels have captivated the attention of several research and industry segments, including bioengineering, tissue engineering, implantable/wearable sensors and actuators, bioactive agent delivery, food processing, and industrial processes optimization. A common limitation of these systems is their fixed shape. The concept of hydrogel moldability is often assigned to the injectability potential of liquid precursors, and this feature is often lost right after hydrogel formation. Hydrogel modulation is a recent trend that advocates the importance of designing materials with shape fitting ability targeting on-demand responses or defect filling purposes. Here, we present a compliant and cell encapsulation-compatible hydrogel prepared from unmodified natural origin polymers with the ability to undergo extreme sequential shape alterations with high recovery of its mechanical properties. Different fragments of these hydrogels could be bonded together in spatiotemporally controlled shape- and formulation-morphing structures. This material is prepared with affordable off-the-shelf polysaccharides of natural origin using a mild and safe processing strategy based solely on polyelectrolyte complexation followed by an innovative partial coacervate compaction and dehydration step. These unique hydrogels hold potential for multifield industrial and healthcare applications. In particular, they may find application as defect filling agents or highly compliant wound healing patches for cargo release and/or cell delivery for tissue regeneration and cell-based therapies.

Bioengineering strategies to accelerate stem cell therapeutics

Although only a few stem cell-based therapies are currently available to patients, stem cells hold tremendous regenerative potential, and several exciting clinical applications are on the horizon. Biomaterials with tuneable mechanical and biochemical properties can preserve stem cell function in culture, enhance survival of transplanted cells and guide tissue regeneration. Rapid progress with three-dimensional hydrogel culture platforms provides the opportunity to grow patient-specific organoids, and has led to the discovery of drugs that stimulate endogenous tissue-specific stem cells and enabled screens for drugs to treat disease. Therefore, bioengineering technologies are poised to overcome current bottlenecks and revolutionize the field of regenerative medicine.

3D and 4D Bioprinting of the Myocardium: Current Approaches, Challenges, and Future Prospects

3D and 4D bioprinting of the heart are exciting notions in the modern era. However, myocardial bioprinting has proven to be challenging. This review outlines the methods, materials, cell types, issues, challenges, and future prospects in myocardial bioprinting. Advances in 3D bioprinting technology have significantly improved the manufacturing process. While scaffolds have traditionally been utilized, 3D bioprinters, which do not require scaffolds, are increasingly being employed. Improved understanding of the cardiac cellular composition and multiple strategies to tackle the issues of vascularization and viability had led to progress in this field. In vivo studies utilizing small animal models have been promising. 4D bioprinting is a new concept that has potential to advance the field of 3D bioprinting further by incorporating the fourth dimension of time. Clinical translation will require multidisciplinary collaboration to tackle the pertinent issues facing this field.

TGF-β1/CD105 signaling controls vascular network formation within growth factor sequestering hyaluronic acid hydrogels

Cell-based strategies for the treatment of ischemic diseases are at the forefront of tissue engineering and regenerative medicine. Cell therapies purportedly can play a key role in the neovascularization of ischemic tissue; however, low survival and poor cell engraftment with the host vasculature following implantation limits their potential to treat ischemic diseases. To overcome these limitations, we previously developed a growth factor sequestering hyaluronic acid (HyA)-based hydrogel that enhanced transplanted mouse cardiosphere-derived cell survival and formation of vasculature that anastomosed with host vessels. In this work, we examined the mechanism by which HyA hydrogels presenting transforming growth factor beta-1 (TGF-β1) promoted proliferation of more clinically relevant human cardiosphere-derived cells (hCDC), and their formation of vascular-like networks in vitro. We observed hCDC proliferation and enhanced formation of vascular-like networks occurred in the presence of TGF-β1. Furthermore, production of nitric oxide (NO), VEGF, and a host of angiogenic factors were increased in the presence of TGF-β1. This response was dependent on the co-activity of CD105 (Endoglin) with the TGF-βR2 receptor, demonstrating its role in the process of angiogenic differentiation and vascular organization of hCDC. These results demonstrated that hCDC form vascular-like networks in vitro, and that the induction of vascular networks by hCDC within growth factor sequestering HyA hydrogels was mediated by TGF-β1/CD105 signaling.

Bioorthogonal Strategies for Engineering Extracellular Matrices

Hydrogels are commonly used as engineered extracellular matrix (ECM) mimics in applications ranging from tissue engineering to in vitro disease models. Ideal mechanisms used to crosslink ECM-mimicking hydrogels do not interfere with the biology of the system. However, most common hydrogel crosslinking chemistries exhibit some form of cross-reactivity. The field of bio-orthogonal chemistry has arisen to address the need for highly specific and robust reactions in biological contexts. Accordingly, bio-orthogonal crosslinking strategies have been incorporated into hydrogel design, allowing for gentle and efficient encapsulation of cells in various hydrogel materials. Furthermore, the selective nature of bio-orthogonal chemistries can permit dynamic modification of hydrogel materials in the presence of live cells and other biomolecules to alter matrix mechanical properties and biochemistry on demand. In this review, we provide an overview of bio-orthogonal strategies used to prepare cell-encapsulating hydrogels and highlight the potential applications of bio-orthogonal chemistries in the design of dynamic engineered ECMs.

Injectable network biomaterials via molecular or colloidal self-assembly

Self-assembly is a powerful tool to create functional materials. A specific application for which self-assembled materials are ideally suited is in creating injectable biomaterials. Contrasting with traditional biomaterials that are implanted through surgical means, injecting biomaterials through the skin offers numerous advantages, expanding the scope and impact for biomaterials in medicine. In particular, self-assembled biomaterials prepared from molecular or colloidal interactions have been frequently explored. The strategies to create these materials are varied, taking advantage of engineered oligopeptides, proteins, and nanoparticles as well as affinity-mediated crosslinking of synthetic precursors. Self-assembled materials typically facilitate injectability through two different mechanisms: i) in situ self-assembly, whereby materials would be administered in a monomeric or oligomeric form and self-assemble in response to some physiologic stimulus, or ii) self-assembled materials that, by virtue of their dynamic, non-covalent interactions, shear-thin to facilitate flow within a syringe and subsequently self-heal into its reassembled material form at the injection site. Indeed, many classes of materials are capable of being injected using a combination of these two mechanisms. Particular utility has been noted for self-assembled biomaterials in the context of tissue engineering, regenerative medicine, drug delivery, and immunoengineering. Given the controlled and multifunctional nature of many self-assembled materials demonstrated to date, we project a future where injectable self-assembled biomaterials afford improved practice in advancing healthcare.

Protein-engineered hydrogels enhance the survival of induced pluripotent stem cell-derived endothelial cells for treatment of peripheral arterial disease

A key feature of peripheral arterial disease (PAD) is damage to endothelial cells (ECs), resulting in lower limb pain and restricted blood flow. Recent preclinical studies demonstrate that the transplantation of ECs via direct injection into the affected limb can result in significantly improved blood circulation. Unfortunately, the clinical application of this therapy has been limited by low cell viability and poor cell function. To address these limitations we have developed an injectable, recombinant hydrogel, termed SHIELD (Shear-thinning Hydrogel for Injectable Encapsulation and Long-term Delivery) for cell transplantation. SHIELD provides mechanical protection from cell membrane damage during syringe flow. Additionally, secondary in situ crosslinking provides a reinforcing network to improve cell retention, thereby augmenting the therapeutic benefit of cell therapy. In this study, we demonstrate the improved acute viability of human induced pluripotent stem cell-derived endothelial cells (iPSC-ECs) following syringe injection delivery in SHIELD, compared to saline. Using a murine hind limb ischemia model of PAD, we demonstrate enhanced iPSC-EC retention in vivo and improved neovascularization of the ischemic limb based on arteriogenesis following transplantation of iPSC-ECs delivered in SHIELD.

An injectable conductive hydrogel encapsulating plasmid DNA-eNOs and ADSCs for treating myocardial infarction

Myocardial infarction (MI) leads to the mass death of cardiomyocytes accompanying with the unfavorable alternation of microenvironment, a fibrosis scar deprived of electrical communications, and the lack of blood supply in the infarcted myocardium. The three factors are inextricably intertwined and thus result in a conservative MI therapy efficacy in clinic. A holistic approach pertinently targeted to these three key points would be favorable to rebuild the heart functions. Here, an injectable conductive hydrogel was constructed via in situ Michael addition reaction between multi-armed conductive crosslinker tetraaniline-polyethylene glycol diacrylate (TA-PEG) and thiolated hyaluronic acid (HA-SH). The resultant soft conductive hydrogel with equivalent myocardial conductivity and anti-fatigue performance was loaded with plasmid DNA encoding eNOs (endothelial nitric oxide synthase) nanocomplexes and adipose derived stem cells (ADSCs) for treating MI. The TA-PEG/HA-SH/ADSCs/Gene hydrogel-based holistic system was injected into the infarcted myocardium of SD rats. We demonstrated an increased expression of eNOs in myocardial tissue the heightening of nitrite concentration, accompanied with upregulation of proangiogenic growth factors and myocardium related mRNA. The results of electrocardiography, cardiogram, and histological analysis convincingly revealed a distinct increase of ejection fraction (EF), shortened QRS interval, smaller infarction size, less fibrosis area, and higher vessel density, indicating a significant improvement of heart functions. This conception of combination approach by a conductive injectable hydrogel loaded with stem cells and gene-encoding eNOs nanoparticles will become a robust therapeutic strategy for the treatment of MI.

Methods To Assess Shear-Thinning Hydrogels for Application As Injectable Biomaterials

Injectable hydrogels have gained popularity as a vehicle for the delivery of cells, growth factors, and other molecules to localize and improve their retention at the injection site, as well as for the mechanical bulking of tissues. However, there are many factors, such as viscosity, storage and loss moduli, and injection force, to consider when evaluating hydrogels for such applications. There are now numerous tools that can be used to quantitatively assess these factors, including for shear-thinning hydrogels because their properties change under mechanical load. Here, we describe relevant rheological tests and ways to measure injection force using a force sensor or a mechanical testing machine toward the evaluation of injectable hydrogels. Injectable, shear-thinning hydrogels can be used in a variety of clinical applications, and as an example we focus on methods for injection into the heart, where an understanding of injection properties and mechanical forces is imperative for consistent hydrogel delivery and retention. We discuss methods for delivery of hydrogels to mouse, rat, and pig hearts in models of myocardial infarction, and compare methods of tissue postprocessing for hydrogel preservation. Our intent is that the methods described herein can be helpful in the design and assessment of shear-thinning hydrogels for widespread biomedical applications.

Endothelial progenitor cells for cellular angiogenesis and repair: lessons learned from experimental animal models

Stem/progenitor cell-based therapy has been extensively studied for angiomyogenic repair of the ischemic heart by regeneration of the damaged myocytes and neovascularization of the ischemic tissue through biological bypassing. Given their inherent ability to assume functionally competent endothelial phenotype and release of broad array of proangiogenic cytokines, endothelial progenitor cells (EPCs)-based therapy is deemed as most appropriate for vaculogenesis in the ischemic heart. Emulating the natural repair process that encompasses mobilization and homing-in of the bone marrow and peripheral blood EPCs, their reparability has been extensively studied in the animal models of myocardial ischemia with encouraging results. Our literature review is a compilation of the lessons learned from the use of EPCs in experimental animal models with emphasis on the in vitro manipulation and delivery strategies to enhance their retention, survival and functioning post-engraftment in the heart.

Angiogenesis precedes cardiomyocyte migration in regenerating mammalian hearts

OBJECTIVE

Although the mammalian heart's ability to fully regenerate is debated, its potential to extensively repair itself is gaining support. We hypothesized that heart regeneration relies on rapid angiogenesis to support myocardial regrowth and sought to characterize the timeline for angiogenesis and cell proliferation in regeneration.

METHODS

One-day-old CD-1 mice (P1, N = 60) underwent apical resection or sham surgery. Hearts were explanted at serial time points from 0 to 30 days postresection and analyzed with immunohistochemistry to visualize vessel ingrowth and cardiomyocyte migration into the resected region. Proliferating cells were labeled with 5-ethynyl-2'-deoxyuridine injections 12 hours before explant. 5-Ethynyl-2'-deoxyuridine-positive cells were counted in both the apex and remote areas of the heart. Masson's trichrome was used to assess fibrosis.

RESULTS

By 30 days postresection, hearts regenerated with minimal fibrosis. Compared with sham surgery, apical resection stimulated a significant increase in proliferation of preexisting cardiomyocytes between 3 and 11 days after injury. Capillary migration into the apical thrombus was detected as early as 2 days postresection, with development of mature arteries by 5 days postresection. New vessels became perfused by 5 days postresection as evidenced by lectin injection. Vessel density and diameter significantly increased within the resected area over 21 days, and vessel ingrowth always preceded cardiomyocyte migration, with coalignment of most migrating cardiomyocytes with ingrowing vessels.

CONCLUSIONS

Endothelial cells migrate into the apical thrombus early after resection, develop into functional arteries, and precede cardiomyocyte ingrowth during mammalian heart regeneration. This endogenous neonatal response emphasizes the importance of expeditious angiogenesis required for neomyogenesis.

Local and sustained miRNA delivery from an injectable hydrogel promotes cardiomyocyte proliferation and functional regeneration after ischemic injury

MicroRNA-based therapies that target cardiomyocyte proliferation have great potential for the treatment of myocardial infarction (MI). In previous work, we showed that the miR-302/367 cluster regulates cardiomyocyte proliferation in the prenatal and postnatal heart. Here, we describe the development and application of an injectable hyaluronic acid (HA) hydrogel for the local and sustained delivery of miR-302 mimics to the heart. We show that the miR-302 mimics released in vitro promoted cardiomyocyte proliferation over one week, and that a single injection of the hydrogel in the mouse heart led to local and sustained cardiomyocyte proliferation for two weeks. After MI, gel/miR-302 injection caused local clonal proliferation and increased cardiomyocyte numbers in the border zone of a Confetti mouse model. Gel/miR-302 further decreased cardiac end-diastolic (39%) and end-systolic (50%) volumes, and improved ejection fraction (32%) and fractional shortening (64%) four weeks after MI and injection, compared to controls. Our findings suggest that biomaterial-based miRNA delivery systems can lead to improved outcomes in cardiac regeneration.

Cyclodextrins as versatile building blocks for regenerative medicine

Cyclodextrins (CDs) are one of the most versatile substances produced by nature, and it is in the aqueous biological environment where the multifaceted potential of CDs can be completely unveiled. CDs form inclusion complexes with a variety of guest molecules, including polymers, producing very diverse biocompatible supramolecular structures. Additionally, CDs themselves can trigger cell differentiation to distinct lineages depending on the substituent groups and also promote salt nucleation. These features together with the affinity-driven regulated release of therapeutic molecules, growth factors and gene vectors explain the rising interest for CDs as building blocks in regenerative medicine. Supramolecular poly(pseudo)rotaxane structures and zipper-like assemblies exhibit outstanding viscoelastic properties, performing as syringeable implants. The sharp shear-responsiveness of the supramolecular assemblies is opening new avenues for the design of bioinks for 3D printing and also of electrospun fibers. CDs can also be transformed into polymerizable monomers to prepare alternative nanostructured materials. The aim of this review is to analyze the role that CDs may play in regenerative medicine through the analysis of the last decade research. Most applications of CD-based scaffolds are focussed on non-healing bone fractures, cartilage reparation and skin recovery, but also on even more challenging demands such as neural grafts. For the sake of clarity, main sections of this review are organized according to the architecture of the CD-based scaffolds, mainly syringeable supramolecular hydrogels, 3D printed scaffolds, electrospun fibers, and composites, since the same scaffold type may find application in different tissues.

A methylcellulose and collagen based temperature responsive hydrogel promotes encapsulated stem cell viability and proliferation in vitro

With the number of stem cell-based therapies emerging on the increase, the need for novel and efficient delivery technologies to enable therapies to remain in damaged tissue and exert their therapeutic benefit for extended periods, has become a key requirement for their translation. Hydrogels, and in particular, thermoresponsive hydrogels, have the potential to act as such delivery systems. Thermoresponsive hydrogels, which are polymer solutions that transform into a gel upon a temperature increase, have a number of applications in the biomedical field due to their tendency to maintain a liquid state at room temperature, thereby enabling minimally invasive administration and a subsequent ability to form a robust gel upon heating to physiological temperature. However, various hurdles must be overcome to increase the clinical translation of hydrogels as a stem cell delivery system, with barriers including their low tensile strength and their inadequate support of cell viability and attachment. In order to address these issues, a methylcellulose based hydrogel was formulated in combination with collagen and beta glycerophosphate, and key development issues such as injectability and sterilisation processes were examined. The polymer solution underwent thermogelation at ~36 °C as determined by rheological analysis, and when gelled, was sufficiently robust to resist significant disintegration in the presence of phosphate buffered saline (PBS) while concomitantly allowing for diffusion of methylene blue dye solution into the gel. We demonstrate that human mesenchymal stem cells (hMSCs) encapsulated within the gel remained viable and showed raised levels of dsDNA at increasing time points, an indication of cell proliferation. Mechanical testing showed the "injectability", i.e. force required for delivery of the polymer solution through devices such as a syringe, needle or catheter. Sterilisation of the freeze-dried polymer wafer via gamma irradiation showed no adverse effects on the formed hydrogel characteristics. Taken together, these results indicate the potential of this gel as a clinically translatable delivery system for stem cells and therapeutic molecules in vivo.

Shear-thinning and self-healing hydrogels as injectable therapeutics and for 3D-printing

The design of injectable hydrogel systems addresses the growing demand for minimally invasive approaches for local and sustained delivery of therapeutics. We developed a class of hyaluronic acid (HA) hydrogels that form through noncovalent guest-host interactions, undergo disassembly (shear-thinning) when injected through a syringe and then reassemble within seconds (self-healing) when shear forces are removed. Its unique properties enable the use of this hydrogel system for numerous applications, such as injection in vivo (including with cells and therapeutic molecules) or as a 'bioink' in 3D-printing applications. Here, we describe the functionalization of HA either with adamantanes (guest moieties) via controlled esterification or with β-cyclodextrins (host moieties) through amidation. We also describe how to modify the HA derivatives with methacrylates for secondary covalent cross-linking and for reaction with fluorophores for in vitro and in vivo imaging. HA polymers are rationally designed from relatively low-molecular-weight starting materials, with the degree of modification controlled, and have matched guest-to-host stoichiometry, allowing the preparation of hydrogels with tailored properties. This procedure takes 3-4 weeks to complete. We detail the preparation and characterization of the guest-host hydrogels, including assessment of their rheological properties, erosion and biomolecule release in vitro. We furthermore demonstrate how to encapsulate cells in vitro and provide procedures for quantitative assessment of in vivo hydrogel degradation by imaging of fluorescently derivatized materials.

Ventricular wall biomaterial injection therapy after myocardial infarction: Advances in material design, mechanistic insight and early clinical experiences

Intramyocardial biomaterial injection therapy for myocardial infarction has made significant progress since concept initiation more than 10 years ago. The interim successes and progress in the first 5 years have been extensively reviewed. During the last 5 years, two phase II clinical trials have reported their long term follow up results and many additional biomaterial candidates have reached preclinical and clinical testing. Also in recent years deeper investigations into the mechanisms behind the beneficial effects associated with biomaterial injection therapy have been pursued, and a variety of process and material parameters have been evaluated for their impact on therapeutic outcomes. This review explores the advances made in this biomaterial-centered approach to ischemic cardiomyopathy and discusses potential future research directions as this therapy seeks to positively impact patients suffering from one of the world's most common sources of mortality.

The Diverse Roles of Hydrogel Mechanics in Injectable Stem Cell Transplantation

Stem cell delivery by local injection has tremendous potential as a regenerative therapy but has seen limited clinical success. Several mechanical challenges hinder therapeutic efficacy throughout all stages of cell transplantation, including mechanical forces during injection and loss of mechanical support post-injection. Recent studies have begun exploring the use of biomaterials, in particular hydrogels, to enhance stem cell transplantation by addressing the often-conflicting mechanical requirements associated with each stage of the transplantation process. This review explores recent biomaterial approaches to improve the therapeutic efficacy of stem cells delivered through local injection, with a focus on strategies that specifically address the mechanical challenges that result in cell death and/or limit therapeutic function throughout the stages of transplantation.

Injectable, Guest-Host Assembled Polyethylenimine Hydrogel for siRNA Delivery

While siRNA has tremendous potential for therapeutic applications, advancement is limited by poor delivery systems. Systemically, siRNAs are rapidly degraded, may have off-target silencing, and necessitate high working concentrations. To overcome this, we developed an injectable, guest-host assembled hydrogel between polyethylenimine (PEI) and polyethylene glycol (PEG) for local siRNA delivery. Guest-host modified polymers assembled with siRNAs to form polyplexes that had improved transfection and viability compared to PEI. At higher concentrations, these polymers assembled into shear-thinning hydrogels that rapidly self-healed. With siRNA encapsulation, the assemblies eroded as polyplexes which were active and transfected cells, observed by Cy3-siRNA uptake or GFP silencing in vitro. When injected into rat myocardium, the hydrogels localized polyplex release, observed by uptake of Cy5.5-siRNA and silencing of GFP for 1 week in a GFP-expressing rat. These results illustrate the potential for this system to be applied for therapeutic siRNA delivery, such as in cardiac pathologies.

Evolution of hierarchical porous structures in supramolecular guest-host hydrogels

Macromolecular interactions are used to form supramolecular assemblies, including through the interaction of guest-host chemical pairs. Microstructural heterogeneity has been observed within such physical hydrogels; yet, systematic investigation of the microstructure and its determining inputs are lacking. Herein, we investigated the hierarchical self-assembly of hyaluronic acid (HA) modified by the guest-host pair adamantane (Ad-HA, guest) and β-cyclodextrin (CD-HA, host), as well as with methacrylate groups to both tether fluorescent agents and to covalently stabilize the material structure. We observed microporous materials in the hydrated state, which temporally arose from initially homogenous hydrogels composed of the two polymers. Independent fluorescent labeling of Ad-HA and CD-HA demonstrated spatiotemporal co-localization, indicative of guest-host polymer condensation on the microscale. The hydrogel void fractions and pore diameters were independently tuned through incubation time (0-7 days), polymer concentration (1.25-10 wt%), and polymer modification (25-50% Ad-HA modification). Void fractions as great as 93.3 ± 2.4% were achieved and pore diameters ranged from 2.1 ± 0.5 to 1025.4 ± 209.4 μm. The segregation of discrete solid and solute phases was measured with both atomic force microscopy and diffusive microparticle tracking analysis, where the solute phase contained only dilute polymer. The study represents a systematic investigation of hierarchical self-assembly in binary associating hydrogels, and provides insights on mechanisms that control microstructure within supramolecular hydrogels.

Design of Injectable Materials to Improve Stem Cell Transplantation

Stem cell-based therapies are steadily gaining traction for regenerative medicine approaches to treating disease and injury throughout the body. While a significant body of work has shown success in preclinical studies, results often fail to translate in clinical settings. One potential cause is the massive transplanted cell death that occurs post injection, preventing functional integration with host tissue. Therefore, current research is focusing on developing injectable hydrogel materials to protect cells during delivery and to stimulate endogenous regeneration through interactions of transplanted cells and host tissue. This review explores the design of targeted injectable hydrogel systems for improving the therapeutic potential of stem cells across a variety of tissue engineering applications with a focus on hydrogel materials that have progressed to the stage of preclinical testing.

Self-Healing Supramolecular Hydrogels Based on Reversible Physical Interactions

Dynamic and reversible polymer networks capable of self-healing, i.e., restoring their mechanical properties after deformation and failure, are gaining increasing research interest, as there is a continuous need towards extending the lifetime and improving the safety and performance of materials particularly in biomedical applications. Hydrogels are versatile materials that may allow self-healing through a variety of covalent and non-covalent bonding strategies. The structural recovery of physical gels has long been a topic of interest in soft materials physics and various supramolecular interactions can induce this kind of recovery. This review highlights the non-covalent strategies of building self-repairing hydrogels and the characterization of their mechanical properties. Potential applications and future prospects of these materials are also discussed.

Convergence of Highly Resolved and Rapid Screening Platforms with Dynamically Engineered, Cell Phenotype-Prescriptive Biomaterials

Biophysical and biochemical cues from the cellular microenvironment initiate intracellular signaling through cellular membrane receptors and trigger specific cell developmental programs. Extracellular substrates and matrix scaffolds engineered to mimic cell's native physiological environment must incorporate the multifactorial parameters (composition, micro and nanoscale organization and topography) of the extracellular matrix as well as the dynamic nature of the matrix. The design of such engineered biomaterials is challenged by the inherent complexity and dynamic nature of the cell-extracellular matrix reciprocity, while the validation of robust microenvironments requires a deeper, higher content phenotypic resolution of cell-matrix interactions alongside a rapid screening capability. To this end, high-throughput platforms are integral to facilitating the screening and optimization of complex engineered microenvironments for directing desired cell developmental pathway. This review highlights the recent advances in biomaterial platforms that present dynamic cues and enable high throughput screening of cell's response to a combination of micro-environmental factors. We also address some newer techniques involving high content image informatics to elucidate emergent cellular behaviors with a focus on stem cell regenerative endpoints.

Delivery of interleukin-10 via injectable hydrogels improves renal outcomes and reduces systemic inflammation following ischemic acute kidney injury in mice

Injectable hydrogels can be used to deliver drugs in situ over a sustained period of time. We hypothesized that sustained delivery of interleukin-10 (IL-10) following acute kidney injury (AKI) would mitigate the local and systemic proinflammatory cascade induced by AKI and reduce subsequent fibrosis. Wild-type C57BL/6 mice underwent ischemia-reperfusion AKI with avertin anesthesia. Three days later, mice were treated with either hyaluronic acid injectable hydrogel with or without IL-10, or IL-10 suspended in saline, injected under the capsule of the left kidney, or hydrogel with IL-10 injected subcutaneously. Untreated AKI served as controls. Serial in vivo optical imaging tracked the location and degradation of the hydrogel over time. Kidney function was assessed serially. Animals were killed 28 days following AKI and the following were evaluated: serum IL-6, lung inflammation, urine neutrophil gelatinase-associated lipocalin, and renal histology for fibroblast activity, collagen type III deposition and fibrosis via Picrosirius Red staining and second harmonic imaging. Our model shows persistent systemic inflammation, and renal inflammation and fibrosis 28 days following AKI. The hydrogels are biocompatible and reduced serum IL-6 and renal collagen type III 28 days following AKI even when delivered without IL-10. Treatment with IL-10 reduced renal and systemic inflammation, regardless of whether the IL-10 was delivered in a sustained manner via the injectable hydrogel under the left kidney capsule, as a bolus injection via saline under the left kidney capsule, or via the injectable hydrogel subcutaneously. Injectable hydrogels are suitable for local drug delivery following renal injury, are biocompatible, and help mitigate local and systemic inflammation.

ECM and ECM-like materials - Biomaterials for applications in regenerative medicine and cancer therapy

Regenerative strategies such as stem cell-based therapies and tissue engineering applications are being developed with the aim to replace, remodel, regenerate or support damaged tissues and organs. In addition to careful cell type selection, the design of appropriate three-dimensional (3D) scaffolds is essential for the generation of bio-inspired replacement tissues. Such scaffolds are usually made of degradable or non-degradable biomaterials and can serve as cell or drug carriers. The development of more effective and efficient drug carrier systems is also highly relevant for novel cancer treatment strategies. In this review, we provide a summary of current approaches that employ ECM and ECM-like materials, or ECM-synthetic polymer hybrids, as biomaterials in the field of regenerative medicine. We further discuss the utilization of such materials for cell and drug delivery, and highlight strategies for their use as vehicles for cancer therapy.

Supramolecular Guest-Host Interactions for the Preparation of Biomedical Materials

Supramolecular chemistry has emerged as an important technique for the formation of biomaterials, including nano- and microparticles and hydrogels. One specific class of supramolecular chemistry is the direct association of guest-host pairs, which involves host macrocycles such as cyclodextrins and cucurbit[n]urils and a wide range of guest molecules, where association is typically driven by molecule size and hydrophobicity. These systems are of particular interest in the biomedical field due to their dynamic nature, chemical diversity, relative ease of synthesis, and ability to interact with biological or synthetic molecules. In this review, we discuss aspects of polymeric material assembly mediated by guest-host interactions, including the fundamentals of assembly into functional biomedical materials. Additionally, applications of biomaterials that utilize guest-host interactions are discussed with a focus on injectable material formulations, the sequestration and delivery of encapsulated cargo (i.e., drugs, biomolecules), and the investigation of cell-material interactions (i.e., adhesion, differentiation, and delivery). While methodologies for guest-host mediated assembly and biological interaction have rapidly evolved in recent years, they remain far from realizing their full potential in the biomaterials field.