Heparin-regulated release of growth factors in vitro and angiogenic response in vivo to implanted hyaluronan hydrogels containing VEGF and bFGF

Heparin-Functionalized Bioactive Glass to Harvest Endogenous Growth Factors for Pulp Regeneration

Pulp and periapical diseases can lead to the cessation of tooth development, resulting in compromised tooth structure and functions. Despite numerous efforts to induce pulp regeneration, effective strategies are still lacking. Growth factors (GFs) hold considerable promise in pulp regeneration due to their diverse cellular regulatory properties. However, the limited half-lives and susceptibility to degradation of exogenous GFs necessitate the administration of supra-physiological doses, leading to undesirable side effects. In this research, a heparin-functionalized bioactive glass (CaO-P2O5-SiO2-Heparin, abbreviated as PSC-Heparin) with strong bioactivity and a stable neutral pH is developed as a promising candidate to addressing challenges in pulp regeneration. Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and thermogravimetric analysis reveal the successful synthesis of PSC-Heparin. Scanning electron microscopy and X-ray diffraction show the hydroxyapatite formation can be observed on the surface of PSC-Heparin after soaking in simulated body fluid for 12 h. PSC-Heparin is capable of harvesting various endogenous GFs and sustainably releasing them over an extended duration by the enzyme-linked immunosorbent assay. Cytological experiments show that developed PSC-Heparin can facilitate the adhesion, migration, proliferation, and odontogenic differentiation of stem cells from apical papillae. Notably, the histological analysis of subcutaneous implantation in nude mice demonstrates PSC-Heparin is capable of promoting the odontoblast-like layers and pulp-dentin complex formation without the addition of exogenous GFs, which is vital for clinical applications. This work highlights an effective strategy of harvesting endogenous GFs and avoiding the involvement of exogenous GFs to achieve pulp-dentin complex regeneration, which may open a new horizon for regenerative endodontic therapy.

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.

Recent Development and Applications of Polydopamine in Tissue Repair and Regeneration Biomaterials

The various tissue damages are a severe problem to human health. The limited human tissue regenerate ability requires suitable biomaterials to help damage tissue repair and regeneration. Therefore, many researchers devoted themselves to exploring biomaterials suitable for tissue repair and regeneration. Polydopamine (PDA) as a natural and multifunctional material which is inspired by mussel has been widely applied in different biomaterials. The excellent properties of PDA, such as strong adhesion, photothermal and high drug-loaded capacity, seem to be born for tissue repair and regeneration. Furthermore, PDA combined with different materials can exert unexpected effects. Thus, to inspire researchers, this review summarizes the recent and representative development of PDA biomaterials in tissue repair and regeneration. This article focuses on why apply PDA in these biomaterials and what PDA can do in different tissue injuries.

Encapsulation of Human Spinal Cord Progenitor Cells in Hyaluronan-Gelatin Hydrogel for Spinal Cord Injury Treatment

Transplanting human induced pluripotent stem cells (iPSCs)-derived spinal cord progenitor cells (SCPCs) is a promising approach to treat spinal cord injuries. However, stem cell therapies face challenges in cell survival, cell localization to the targeted site, and the control of cell differentiation. Here, we encapsulated SCPCs in thiol-modified hyaluronan-gelatin hydrogels and optimized scaffold mechanical properties and cell encapsulation density to promote cell viability and neuronal differentiation in vitro and in vivo. Different compositions of hyaluronan-gelatin hydrogels formulated by varying concentrations of poly(ethylene glycol) diacrylate were mechanically characterized by using atomic force microscopy. In vitro SCPC encapsulation study showed higher cell viability and proliferation with lower substrate Young's modulus (200 Pa vs 580 Pa) and cell density. Moreover, the soft hydrogels facilitated a higher degree of neuronal differentiation with extended filament structures in contrast to clumped cellular morphologies obtained in stiff hydrogels (p < 0.01). When transplanted in vivo, the optimized SCPC-encapsulated hydrogels resulted in higher cell survival and localization at the transplanted region as compared to cell delivery without hydrogel encapsulation at 2 weeks postimplantation within the rat spinal cord (p < 0.01). Notably, immunostaining demonstrated that the hydrogel-encapsulated SCPCs differentiated along the neuronal and oligodendroglial lineages in vivo. The lack of pluripotency and proliferation also supported the safety of the SCPC transplantation approach. Overall, the injectable hyaluronan-gelatin hydrogel shows promise in supporting the survival and neural differentiation of human SCPCs after transplantation into the spinal cord.

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.

Construction of tissue-engineered bladders using an artificial acellular nanocomposite scaffold loaded with stromal vascular fraction secretome

Tissue engineering approaches offer promising alternative strategies for reconstructing bladder tissue; however, the low retention of transplanted cells and the possible risk of rejection limit their therapeutic efficacy. Clinical applicability is further limited by the lack of suitable scaffold materials to support the needs of various cell types. In the present study, we developed an artificial nanoscaffold system consisting of stromal vascular fraction (SVF) secretome (Sec) loaded onto zeolitic imidazolate framework-8 (ZIF-8) nanoparticles, which were then incorporated into bladder acellular matrix. This artificial acellular nanocomposite scaffold (ANS) can achieve gradient degradation and slowly release SVF-Sec to promote tissue regeneration. Furthermore, even after long-term cryopreservation, this completely acellular bladder nanoscaffold material still maintains its efficacy. In a rat bladder replacement model, ANS transplantation demonstrated potent proangiogenic ability and induced M2 macrophage polarization to promote tissue regeneration and restore bladder function. Our study demonstrates the safety and efficacy of the ANS, which can play a stem cell-like role while avoiding the disadvantages of cell therapy. Furthermore, the ANS can replace the bladder regeneration model based on cell-binding scaffold materials and has the potential for clinical application. STATEMENT OF SIGNIFICANCE: This study aimed to develop a gradient-degradable artificial acellular nanocomposite scaffold (ANS) loaded with stromal vascular fraction (SVF) secretome for rehabilitating bladders. Using various in vitro methods as well as rat- and zebrafish-based in vivo models, the developed ANS was assessed for efficacy and safety. Results indicated that the ANS achieved gradient degradation and slowly released the SVF secretome to promote tissue regeneration, even after long-term cryopreservation. Furthermore, ANS transplantation demonstrated a potent pro-angiogenic ability and induced M2 macrophage polarization to promote tissue regeneration and restore bladder function in a bladder replacement model. Our study demonstrates that ANS may replace bladder regeneration models based on cell-binding scaffold materials and have potential clinical application.

Development of a novel air-liquid interface airway tissue equivalent model for in vitro respiratory modeling studies

The human airways are complex structures with important interactions between cells, extracellular matrix (ECM) proteins and the biomechanical microenvironment. A robust, well-differentiated in vitro culture system that accurately models these interactions would provide a useful tool for studying normal and pathological airway biology. Here, we report the development and characterization of a physiologically relevant air-liquid interface (ALI) 3D airway 'organ tissue equivalent' (OTE) model with three novel features: native pulmonary fibroblasts, solubilized lung ECM, and hydrogel substrate with tunable stiffness and porosity. We demonstrate the versatility of the OTE model by evaluating the impact of these features on human bronchial epithelial (HBE) cell phenotype. Variations of this model were analyzed during 28 days of ALI culture by evaluating epithelial confluence, trans-epithelial electrical resistance, and epithelial phenotype via multispectral immuno-histochemistry and next-generation sequencing. Cultures that included both solubilized lung ECM and native pulmonary fibroblasts within the hydrogel substrate formed well-differentiated ALI cultures that maintained a barrier function and expressed mature epithelial markers relating to goblet, club, and ciliated cells. Modulation of hydrogel stiffness did not negatively impact HBE differentiation and could be a valuable variable to alter epithelial phenotype. This study highlights the feasibility and versatility of a 3D airway OTE model to model the multiple components of the human airway 3D microenvironment.

Stem cell therapy combined with controlled release of growth factors for the treatment of sphincter dysfunction

Sphincter dysfunction often occurs at the end of tubule organs such as the urethra, anus, or gastroesophageal sphincters. It is the primary consequence of neuromuscular impairment caused by trauma, inflammation, and aging. Despite intensive efforts to recover sphincter function, pharmacological treatments have not achieved significant improvement. Cell- or growth factor-based therapy is a promising approach for neuromuscular regeneration and the recovery of sphincter function. However, a decrease in cell retention and viability, or the short half-life and rapid degradation of growth factors after implantation, remain obstacles to the translation of these therapies to the clinic. Natural biomaterials provide unique tools for controlled growth factor delivery, which leads to better outcomes for sphincter function recovery in vivo when stem cells and growth factors are co-administrated, in comparison to the delivery of single therapies. In this review, we discuss the role of stem cells combined with the controlled release of growth factors, the methods used for delivery, their potential therapeutic role in neuromuscular repair, and the outcomes of preclinical studies using combination therapy, with the hope of providing new therapeutic strategies to treat incontinence or sphincter dysfunction of the urethra, anus, or gastroesophageal tissues, respectively.

Multiscale design of stiffening and ROS scavenging hydrogels for the augmentation of mandibular bone regeneration

Although biomimetic hydrogels play an essential role in guiding bone remodeling, reconstructing large bone defects is still a significant challenge since bioinspired gels often lack osteoconductive capacity, robust mechanical properties and suitable antioxidant ability for bone regeneration. To address these challenges, we first engineered molecular design of hydrogels (gelatin/polyethylene glycol diacrylate/2-(dimethylamino)ethyl methacrylate, GPEGD), where their mechanical properties were significantly enhanced via introducing trace amounts of additives (0.5 wt%). The novel hybrid hydrogels show high compressive strength (>700 kPa), stiff modulus (>170 kPa) and strong ROS-scavenging ability. Furthermore, to endow the GPEGD hydrogels excellent osteoinductions, novel biocompatible, antioxidant and BMP-2 loaded polydopamine/heparin nanoparticles (BPDAH) were developed for functionalization of the GPEGD gels (BPDAH-GPEGD). In vitro results indicate that the antioxidant BPDAH-GPEGD is able to deplete elevated ROS levels to protect cells viability against ROS damage. More importantly, the BPDAH-GPEGD hydrogels have good biocompatibility and promote the osteo differentiation of preosteoblasts and bone regenerations. At 4 and 8 weeks after implantation of the hydrogels in a mandibular bone defect, Micro-computed tomography and histology results show greater bone volume and enhancements in the quality and rate of bone regeneration in the BPDAH-GPEGD hydrogels. Thus, the multiscale design of stiffening and ROS scavenging hydrogels could serve as a promising material for bone regeneration applications.

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Trace additives of DMAEMA markedly enhanced the mechanical performances of the gelatin-based hydrogels through molecular induced multiple crosslinking structures.

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Molecular design strategy combined with bioactive nanocomposites have a synergistically effects on promoting ROS scavenging ability and osteoactivity of the biomimetic hydrogels.

Evaluation of biocompatibility and angiogenic potential of extracellular matrix hydrogel biofunctionalized with the LL-37 peptide

BACKGROUND

Biomaterials must allow revascularization for a successful tissue regeneration process. Biomaterials formulated from the extracellular matrix (ECM) have gained popularity in tissue engineering because of their superior biocompatibility, and due to their rheological properties, ECM-hydrogels can be easily applied in damaged areas, allowing cell colonization and integration into the host tissue. Porcine urinary bladder ECM (pUBM) retains functional signaling and structural proteins, being an excellent option in regenerative medicine. Even some small molecules, such as the antimicrobial cathelicidin-derived LL-37 peptide have proven angiogenic properties.

OBJECTIVE

The objective of this study was to evaluate the biocompatibility and angiogenic potential of an ECM-hydrogel derived from the porcine urinary bladder (pUBMh) biofunctionalized with the LL-37 peptide (pUBMh/LL37).

METHODS

Macrophages, fibroblasts, and adipose tissue-derived mesenchymal stem cells (AD-MSC) were exposed pUBMh/LL37, and the effect on cell proliferation was evaluated by MTT assay, cytotoxicity by quantification of lactate dehydrogenase release and the Live/Dead Cell Imaging assays. Moreover, macrophage production of IL-6, IL-10, IL-12p70, MCP-1, INF-γ, and TNF-α cytokines was quantified using a bead-based cytometric array. pUBMh/LL37 was implanted directly by dorsal subcutaneous injection in Wistar rats for 24 h to evaluate biocompatibility, and pUBMh/LL37-loaded angioreactors were implanted for 21 days for evaluation of angiogenesis.

RESULTS

We found that pUBMh/LL37 did not affect cell proliferation and is cytocompatible to all tested cell lines but induces the production of TNF-α and MCP-1 in macrophages. In vivo, this ECM-hydrogel induces fibroblast-like cell recruitment within the material, without tissue damage or inflammation at 48 h. Interestingly, tissue remodeling with vasculature inside angioreactors was seen at 21 days.

CONCLUSIONS

Our results showed that pUBMh/LL37 is cytologically compatible, and induces angiogenesis in vivo, showing potential for tissue regeneration therapies.

Bone Formation on Murine Cranial Bone by Injectable Cross-Linked Hyaluronic Acid Containing Nano-Hydroxyapatite and Bone Morphogenetic Protein

New injection-type bone-forming materials are desired in dental implantology. In this study, we added nano-hydroxyapatite (nHAp) and bone morphogenetic protein (BMP) to cross-linkable thiol-modified hyaluronic acid (tHyA) and evaluated its usefulness as an osteoinductive injectable material using an animal model. The sol (ux-tHyA) was changed to a gel (x-tHyA) by mixing with a cross-linker. We prepared two sol−gel (SG) material series, that is, x-tHyA + BMP with and without nHAp (SG I) and x-tHyA + nHAp with and without BMP (SG II). SG I materials in the sol stage were injected into the cranial subcutaneous connective tissues of mice, followed by in vivo gelation, while SG II materials gelled in Teflon rings were surgically placed directly on the cranial bones of rats. The animals were sacrificed 8 weeks after implantation, followed by X-ray analysis and histological examination. The results revealed that bone formation occurred at a high rate (>70%), mainly as ectopic bone in the SG I tests in mouse cranial connective tissues, and largely as bone augmentation in rat cranial bones in the SG II experiments when x-tHyA contained both nHAp and BMP. The prepared x-tHyA + nHAp + BMP SG material can be used as an injection-type osteoinductive bone-forming material. Sub-periosteum injection was expected.

Controlled release of vascular endothelial growth factor (VEGF) in alginate and hyaluronic acid (ALG–HA) bead system to promote wound healing in punch-induced wound rat model

For wound healing, angiogenesis is one of the main therapeutic factors for recovering the injured tissue. To address this issue, a combination of two different polymers, alginate (ALG) and hyaluronic acid (HA) in an 80:20 ratio composition is used to optimize the bead system along with the 5 IU heparin (Hep) by crosslinking into calcium chloride (CaCl₂). Encapsulation of Vascular endothelial growth factor (VEGF) in the bead system shows delayed cumulative release in phosphate buffer saline (PBS). For in vitro studies, calf pulmonary artery endothelial (CPAE) cells showed biocompatibility. ALG-HA/VEGF150 improves endothelial Vascular cell adhesion protein 1 (VCAM1) and endothelial nitric oxide synthase (eNOS) expression markers in CPAE cells. In vivo evaluation of the bead system shows around 68% of wound closure 2 weeks post-implantation in 8 mm punch wound models. The treatment group shows decreased epithelial gap between the ends of the wound and neo-epidermal regeneration. ALG-HA/VEGF150 induced significant vascularization, collagen type-1 (Col-1) and fibronectin (FN) development in the in vivo models after 2 weeks of the implantation. Hence, ALG-HA/VEGF150 beads can be used to promote wound healing.

Micro- and nanotechnology in biomedical engineering for cartilage tissue regeneration in osteoarthritis

Osteoarthritis, which typically arises from aging, traumatic injury, or obesity, is the most common form of arthritis, which usually leads to malfunction of the joints and requires medical interventions due to the poor self-healing capacity of articular cartilage. However, currently used medical treatment modalities have reported, at least in part, disappointing and frustrating results for patients with osteoarthritis. Recent progress in the design and fabrication of tissue-engineered microscale/nanoscale platforms, which arises from the convergence of stem cell research and nanotechnology methods, has shown promising results in the administration of new and efficient options for treating osteochondral lesions. This paper presents an overview of the recent advances in osteochondral tissue engineering resulting from the application of micro- and nanotechnology approaches in the structure of biomaterials, including biological and microscale/nanoscale topographical cues, microspheres, nanoparticles, nanofibers, and nanotubes.

SAGE: Novel Therapy to Reduce Inflammation in a Naturally Occurring-Dog Model of Periodontal Disease

Objective

Methods

Results

Conclusion

Bioprinted microvasculature: progressing from structure to function

Three-dimensional (3D) bioprinting seeks to unlock the rapid generation of complex tissue constructs, but long-standing challenges with efficientin vitromicrovascularization must be solved before this can become a reality. Microvasculature is particularly challenging to biofabricate due to the presence of a hollow lumen, a hierarchically branched network topology, and a complex signaling milieu. All of these characteristics are required for proper microvascular-and, thus, tissue-function. While several techniques have been developed to address distinct portions of this microvascularization challenge, no single approach is capable of simultaneously recreating all three microvascular characteristics. In this review, we present a three-part framework that proposes integration of existing techniques to generate mature microvascular constructs. First, extrusion-based 3D bioprinting creates a mesoscale foundation of hollow, endothelialized channels. Second, biochemical and biophysical cues induce endothelial sprouting to create a capillary-mimetic network. Third, the construct is conditioned to enhance network maturity. Across all three of these stages, we highlight the potential for extrusion-based bioprinting to become a central technique for engineering hierarchical microvasculature. We envision that the successful biofabrication of functionally engineered microvasculature will address a critical need in tissue engineering, and propel further advances in regenerative medicine andex vivohuman tissue modeling.

Peptide hydrogels for affinity-controlled release of therapeutic cargo: Current and potential strategies

The development of devices for the precise and controlled delivery of therapeutics has grown rapidly over the last few decades. Drug delivery materials must provide a depot with delivery profiles that satisfy pharmacodynamic and pharmacokinetic requirements resulting in clinical benefit. Therapeutic efficacy can be limited due to short half-life and poor stability. Thus, to compensate for this, frequent administration and high doses are often required to achieve therapeutic effect, which in turn increases potential side effects and systemic toxicity. This can potentially be mitigated by using materials that can deliver drugs at controlled rates, and material design principles that allow this are continuously evolving. Affinity-based release strategies incorporate a myriad of reversible interactions into a gel network, which have affinities for the therapeutic of interest. Reversible binding to the gel network impacts the release profile of the drug. Such affinity-based interactions can be modulated to control the release profile to meet pharmacokinetic benchmarks. Much work has been done developing affinity-based control in the context of polymer-based materials. However, this strategy has not been widely implemented in peptide-based hydrogels. Herein, we present recent advances in the use of affinity-controlled peptide gel release systems and their associated mechanisms for applications in drug delivery.

Review of Recent Advances and Their Improvement in the Effectiveness of Hydrogel-Based Targeted Drug Delivery: A Hope for Treating Cancer

Using hydrogels for delivering cancer therapeutics is advantageous in pharmaceutical usage as they have an edge over traditional delivery, which is tainted due to the risk of toxicity that it imbues. Hydrogel usage leads to the development of a more controlled drug release system owing to its amenability for structural metamorphosis, its higher porosity to seat the drug molecules, and its ability to shield the drug from denaturation. The thing that makes its utility even more enhanced is that they make themselves more recognizable to the body tissues and hence can stay inside the body for a longer time, enhancing the efficiency of the delivery, which otherwise is negatively affected since the drug is identified by the human immunity as a foreign substance, and thus, an attack of the immunity begins on the drug injected. A variety of hydrogels such as thermosensitive, pH-sensitive, and magnetism-responsive hydrogels have been included and their potential usage in drug delivery has been discussed in this review that aims to present recent studies on hydrogels that respond to alterations under a variety of circumstances in "reducing" situations that mimic the microenvironment of cancerous cells.

Antioxidative and Angiogenic Hyaluronic Acid-Based Hydrogel for the Treatment of Peripheral Artery Disease

Peripheral arterial disease (PAD) is a progressive atherosclerotic disorder characterized by blockages of the arteries supplying the lower extremities. Ischemia initiates oxidative damage and mitochondrial dysfunction in the legs of PAD patients, causing injury to the tissues of the leg, significant decline in walking performance, leg pain while walking, and in the most severe cases, nonhealing ulcers and gangrene. Current clinical trials based on cells/stem cells, the trophic factor, or gene therapy systems have shown some promising results for the treatment of PAD. Biomaterial matrices have been explored in animal models of PAD to enhance these therapies. However, current biomaterial approaches have not fully met the essential requirements for minimally invasive intramuscular delivery to the leg. Ideally, a biomaterial should present properties to ameliorate oxidative stress/damage and failure of angiogenesis. Recently, we have created a thermosensitive hyaluronic acid (HA) hydrogel with antioxidant capacity and skeletal muscle-matching stiffness. Here, we further optimized HA hydrogels with the cell adhesion peptide RGD to facilitate the development of vascular-like structures in vitro. The optimized HA hydrogel reduced intracellular reactive oxygen species levels and preserved vascular-like structures against H₂O₂-induced damage in vitro. HA hydrogels also provided prolonged release of the vascular endothelial growth factor (VEGF). After injection into rat ischemic hindlimb muscles, this VEGF-releasing hydrogel reduced lipid oxidation, regulated oxidative-related genes, enhanced local blood flow in the muscle, and improved running capacity of the treated rats. Our HA hydrogel system holds great potential for the treatment of the ischemic legs of patients with PAD.

Translational Applications of Hydrogels

Advances in hydrogel technology have unlocked unique and valuable capabilities that are being applied to a diverse set of translational applications. Hydrogels perform functions relevant to a range of biomedical purposes-they can deliver drugs or cells, regenerate hard and soft tissues, adhere to wet tissues, prevent bleeding, provide contrast during imaging, protect tissues or organs during radiotherapy, and improve the biocompatibility of medical implants. These capabilities make hydrogels useful for many distinct and pressing diseases and medical conditions and even for less conventional areas such as environmental engineering. In this review, we cover the major capabilities of hydrogels, with a focus on the novel benefits of injectable hydrogels, and how they relate to translational applications in medicine and the environment. We pay close attention to how the development of contemporary hydrogels requires extensive interdisciplinary collaboration to accomplish highly specific and complex biological tasks that range from cancer immunotherapy to tissue engineering to vaccination. We complement our discussion of preclinical and clinical development of hydrogels with mechanical design considerations needed for scaling injectable hydrogel technologies for clinical application. We anticipate that readers will gain a more complete picture of the expansive possibilities for hydrogels to make practical and impactful differences across numerous fields and biomedical applications.

Strategies for inclusion of growth factors into 3D printed bone grafts

There remains a critical need to develop new technologies and materials that can meet the demands of treating large bone defects. The advancement of 3-dimensional (3D) printing technologies has allowed the creation of personalized and customized bone grafts, with specific control in both macro- and micro-architecture, and desired mechanical properties. Nevertheless, the biomaterials used for the production of these bone grafts often possess poor biological properties. The incorporation of growth factors (GFs), which are the natural orchestrators of the physiological healing process, into 3D printed bone grafts, represents a promising strategy to achieve the bioactivity required to enhance bone regeneration. In this review, the possible strategies used to incorporate GFs to 3D printed constructs are presented with a specific focus on bone regeneration. In particular, the strengths and limitations of different methods, such as physical and chemical cross-linking, which are currently used to incorporate GFs to the engineered constructs are critically reviewed. Different strategies used to present one or more GFs to achieve simultaneous angiogenesis and vasculogenesis for enhanced bone regeneration are also covered in this review. In addition, the possibility of combining several manufacturing approaches to fabricate hybrid constructs, which better mimic the complexity of biological niches, is presented. Finally, the clinical relevance of these approaches and the future steps that should be taken are discussed.

Neovascularization of engineered tissues for clinical translation: Where we are, where we should be?

One of the key challenges in engineering three-dimensional tissue constructs is the development of a mature microvascular network capable of supplying sufficient oxygen and nutrients to the tissue. Recent angiogenic therapeutic strategies have focused on vascularization of the constructed tissue, and its integration in vitro; these strategies typically combine regenerative cells, growth factors (GFs) with custom-designed biomaterials. However, the field needs to progress in the clinical translation of tissue engineering strategies. The article first presents a detailed description of the steps in neovascularization and the roles of extracellular matrix elements such as GFs in angiogenesis. It then delves into decellularization, cell, and GF-based strategies employed thus far for therapeutic angiogenesis, with a particularly detailed examination of different methods by which GFs are delivered in biomaterial scaffolds. Finally, interdisciplinary approaches involving advancement in biomaterials science and current state of technological development in fabrication techniques are critically evaluated, and a list of remaining challenges is presented that need to be solved for successful translation to the clinics.

Rebuilding the Vascular Network: In vivo and in vitro Approaches

As the material transportation system of the human body, the vascular network carries the transportation of materials and nutrients. Currently, the construction of functional microvascular networks is an urgent requirement for the development of regenerative medicine and in vitro drug screening systems. How to construct organs with functional blood vessels is the focus and challenge of tissue engineering research. Here in this review article, we first introduced the basic characteristics of blood vessels in the body and the mechanism of angiogenesis in vivo, summarized the current methods of constructing tissue blood vessels in vitro and in vivo, and focused on comparing the functions, applications and advantages of constructing different types of vascular chips to generate blood vessels. Finally, the challenges and opportunities faced by the development of this field were discussed.

Cell-Laden Gradient Hydrogel Scaffolds for Neovascularization of Engineered Tissues

Gradients in mechanical properties, physical architecture and biochemical composition exist in a variety of complex tissues, yet 3D in vitro models that enable investigation of these cues on cellular processes, especially those contributing to vascularization of engineered tissues are limited. Here, a photopolymerization approach to create cell-laden hydrogel biomaterials with decoupled and combined gradients in modulus, immobilized cell adhesive peptide (RGD) concentration, and proteolytic degradation enabling spatial encapsulation of vascular spheroids is reported to elucidate their impact on vascular sprouting in 3D culture. Vascular spheroids encapsulated in these gradient scaffolds exhibit spatial variations in total sprout length. Scaffolds presenting an immobilized RGD gradient promote biased vascular sprouting toward increasing RGD concentration. Importantly, biased sprouting is found to be dependent on immobilized RGD gradient characteristics, including magnitude and slope, with increases in these factors contributing to significant enhancements in biased sprouting responses. Conversely, reduction in biased sprouting responses is observed in combined gradient scaffolds possessing opposing gradients in RGD and modulus. The presented work is the first to demonstrate the use of a cell-laden biomaterial platform to systematically investigate the role of multiple scaffold gradients as well as gradient slope, magnitude and orientation on vascular sprouting responses in 3D culture.

bFGF and SDF-1α Improve In Vivo Performance of VEGF-Incorporating Small-Diameter Vascular Grafts

Tissue-engineered vascular grafts are widely tested as a promising substitute for both arterial bypass and replacement surgery. We previously demonstrated that incorporation of VEGF into electrospun tubular scaffolds from poly(3-hydroxybutyrate-co-3-hydroxyvalerate)/poly(ε-caprolactone) enhances formation of an endothelial cell monolayer. However, an overdose of VEGF can induce tumor-like vasculature; thereby, other bioactive factors are needed to support VEGF-driven endothelialization and successful recruitment of smooth muscle cells. Utilizing emulsion electrospinning, we fabricated one-layer vascular grafts with either VEGF, bFGF, or SDF-1α, and two-layer vascular grafts with VEGF incorporated into the inner layer and bFGF and SDF-1α incorporated into the outer layer with the following structural evaluation, tensile testing, and in vivo testing using a rat abdominal aorta replacement model. The latter graft prototype showed higher primary patency rate. We found that the two-layer structure improved surface topography and mechanical properties of the grafts. Further, the combination of bFGF, SDF-1α, and VEGF improved endothelialization compared with VEGF alone, while bFGF induced a rapid formation of a smooth muscle cell layer. Taken together, these findings show that the two-layer structure and incorporation of bFGF and SDF-1α into the vascular grafts in combination with VEGF provide a higher primary patency and therefore improved in vivo performance.

Construction of nanofibrous scaffolds with interconnected perfusable microchannel networks for engineering of vascularized bone tissue

Vascularization and bone regeneration are two closely related processes during bone reconstruction. A three-dimensional (3D) scaffold with porous architecture provides a suitable microenvironment for vascular growth and bone formation. Here, we present a simple and general strategy to construct a nanofibrous poly(l-lactide)/poly(ε-caprolactone) (PLLA/PCL) scaffold with interconnected perfusable microchannel networks (IPMs) based on 3D printing technology by combining the phase separation and sacrificial template methods. The regular and customizable microchannel patterns within the scaffolds (spacings: 0.4 mm, 0.5 mm, and 0.6 mm; diameters: 0.8 mm, 1 mm, and 1.2 mm) were made to investigate the effect of microchannel structure on angiogenesis and osteogenesis. The results of subcutaneous embedding experiment showed that 0.5/0.8-IPMs (spacing/diameter = 0.5/0.8) and 0.5/1-IPMs (spacing/diameter = 0.5/1) scaffolds exhibited more vascular network formation as compared with other counterparts. After loading with vascular endothelial growth factor (VEGF), VEGF@IPMs-0.5/0.8 scaffold prompted better human umbilical vein endothelial cells (HUVECs) migration and neo-blood vessel formation, as determined by Transwell migration, scratch wound healing, and chorioallantoic membrane (CAM) assays. Furthermore, the microangiography and rat cranial bone defects experiments demonstrated that VEGF@IPMs-0.5/0.8 scaffold exhibited better performance in vascular network formation and new bone formation compared to VEGF@IPMs-0.5/1 scaffold. In summary, our results suggested that the microchannel structure within the scaffolds could be tailored by an adjustable caramel-based template strategy, and the combination of interconnected perfusion microchannel networks and angiogenic factors could significantly enhance vascularization and bone regeneration.

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3D-printed sacrificial templates are used to construct the scaffold with interconnected perfusable microchannel networks.

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The microchannel structure within scaffolds can be tailored by changing the template specifications.

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The introduction of VEGF in the microchannel of scaffold promotes the vascular network formation.

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Microchannel structure and angiogenic factor within scaffold significantly enhance vascularization and bone regeneration.

Semisynthetic Hyaluronic Acid-Based Hydrogel Promotes Recovery of the Injured Tibialis Anterior Skeletal Muscle Form and Function

Volumetric muscle loss (VML) injuries are characterized by a degree of tissue loss that exceeds the endogenous regenerative capacity of muscle, resulting in permanent structural and functional deficits. Such injuries are a consequence of trauma, as well as a host of congenital and acquired diseases and disorders. Despite significant preclinical research with diverse biomaterials, as well as early clinical studies with implantation of decellularized extracellular matrices, there are still significant barriers to more complete restoration of muscle form and function following repair of VML injuries. In fact, identification of novel biomaterials with more advantageous regenerative profiles is a critical limitation to the development of improved therapeutics. As a first step in this direction, we evaluated a novel semisynthetic hyaluronic acid-based (HyA) hydrogel that embodies material features more favorable for robust muscle regeneration. This HyA-based hydrogel is composed of an acrylate-modified HyA (AcHyA) macromer, an AcHyA macromer conjugated with the bsp-RGD(15) peptide sequence to enhance cell adhesion, a high-molecular-weight heparin to sequester growth factors, and a matrix metalloproteinase-cleavable cross-linker to allow for cell-dependent remodeling. In a well-established, clinically relevant rat tibialis anterior VML injury model, we report observations of robust functional recovery, accompanied by volume reconstitution, muscle regeneration, and native-like vascularization following implantation of the HyA-based hydrogel at the site of injury. These findings have important implications for the development and clinical application of the improved biomaterials that will be required for stable and complete functional recovery from diverse VML injuries.

Silk Hydrogels with Controllable Formation of Dityrosine, 3,4-Dihydroxyphenylalanine, and 3,4-Dihydroxyphenylalanine-Fe3+ Complexes through Chitosan Particle-Assisted Fenton Reactions

The oxidation of tyrosine residues of silk fibroin involves the generation of dityrosine and 3,4-dihydroxyphenylalanine (DOPA). However, it remains a challenge to selectively control the reaction pathway to produce dityrosine or DOPA in a selective fashion. Here, silk hydrogels with controllable formation of not only dityrosine and DOPA but also DOPA-Fe³⁺ complexes within the cross-linked networks were developed. The use of chitosan particles in the Fenton reaction allowed the interaction of Fe³⁺ ions with silk fibroin to be limited through the adsorption of Fe³⁺ ions onto chitosan particles by manipulating contact time between the reaction medium and chitosan particles. This led to significant suppression of the premature formation of β-sheet structures that cause steric hindrance to the collisions between tyrosyl radicals and thus enabled higher selectivity toward the formation of dityrosine than DOPA. Remarkably, the addition of ethylenediaminetetraacetic acid (EDTA) to the chitosan particle-assisted Fenton reactions resulted in hydrogels that significantly favored the formation of DOPA over dityrosine due to the increase in the hydroxylation of phenol in the presence of EDTA. Despite the existence of Fe³⁺-EDTA complexes, Raman spectra indicated the DOPA-Fe³⁺ complexation in the hydrogels. Mechanistically, the hydrogel networks with small-sized and uniformly distributed β-sheet structures as well as the abundance of DOPA appear to make non-EDTA-chelated Fe³⁺ ions more accessible to complexation with DOPA. These findings have important implications for understanding the oxidation of tyrosine residues of silk fibroin by metal-catalyzed oxidation systems with potential benefits for future studies on silk protein-based hydrogels capable of generating intrinsic adhesive features as well as for exploring dual-cross-linked silk hydrogels constructed by chemical cross-linking and metal-coordinate complexation.

Effects of Biotin-Avidin Interactions on Hydrogel Swelling

Affinity interactions between the small molecule biotin and the protein avidin have been used extensively to functionalize biomaterials. More recently, researchers have leveraged the changes in biotin-avidin affinity that occur upon biotin conjugation to larger molecules to control the release of biotinylated drugs and proteins. However, the effects of biotin-avidin interactions on hydrogel properties have not been thoroughly investigated. The objective of this study was to evaluate the effect of increasing biotin and avidin concentrations on hydrogel swelling properties, as an indicator of crosslinking. Gelatin, selected as a model hydrogel material, was biotinylated at increasing fold molar excesses of biotin with a PEG linker using N-hydroxysuccinimide chemistry. Afterwards, biotinylated gelatin was formed into hydrogels and stabilized with glutaraldehyde. Swelling properties of the biotinylated hydrogels were investigated by conducting swelling studies in different avidin solutions. Increasing the degree of biotinylation caused significant decreases in swelling ratios of the hydrogels in a dose-dependent manner, suggesting increases in crosslinking of the hydrogels. However, increasing avidin concentrations in excess of biotin content did not significantly affect swelling ratios. Moving hydrogels to phosphate-buffered saline following avidin incorporation resulted in increased swelling ratios for hydrogels prepared with a lower concentration of biotin. However, hydrogels prepared with the highest concentration of biotin did not experience increased swelling ratios, implying that the stability of biotin-avidin-mediated crosslinking depends on the number of biotin molecules available for binding. Collectively, these results demonstrate that biotin-avidin interactions control hydrogel swelling properties, and that the magnitude and stability of the effects depend on the biotin concentration. These results have important implications for affinity-based controlled release of biotinylated drugs or proteins from biotin-avidin-crosslinked hydrogels.

Heparin-conjugated collagen as a potent growth factor-localizing and stabilizing scaffold for regenerative medicine

Introduction

Growth factors are crucial bioactive molecules in vitro and in vivo. Among them, basic fibroblast growth factor (bFGF) has been used widely for various applications such as cell culture and regenerative medicine. However, bFGF has extremely poor stability in aqueous solution; thus, it is difficult to maintain its high local concentration. Heparin-conjugated materials have been studied recently as promising scaffold-immobilizing growth factors for biological and medical applications. The previous studies have focused on the local concentration maintenance and sustained release of the growth factors from the scaffold.

Methods

In this paper, we focused on the biological stability of bFGF immobilized on the heparin-conjugated collagen (hep-col) scaffold. The stability of the immobilized bFGF was quantitatively evaluated at physiological temperature (37 °C) using cell culture and ELISA.

Results

The immobilized bFGF had twice higher stability than the bFGF solution. Furthermore, the hep-col scaffold was able to immobilize not only bFGF but also other growth factors (i.e., vascular endothelial growth factor and hepatocyte growth factor) at high efficiency.

Conclusions

The hep-col scaffold can localize several kinds of growth factors as well as stabilize bFGF under physiological temperature and is a promising potent scaffold for regenerative medicine.

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Heparin-conjugated collagen scaffold immobilized bFGF, VEGF, and HGF with a high efficiency of 80–90% even at 100 ng/mL.

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Cell proliferation of HUVECs was promoted depending on the bFGF amount on the scaffold, and slowed by pre-incubation at 37 °C.

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Growth factor-immobilization on the scaffold stabilized bFGF and maintained its bioactivity longer than bFGF solution.

Multifunctional Islet Transplantation Hydrogel Encapsulating A20 High-Expressing Islets

Islet transplantation is regarded as the most promising treatment for type 1 diabetes (T1D). However, the function of grafted islet could be damaged on account of transplant rejection and/or hypoxia several years later after transplantation. We proposed a hypothetical functionalized hydrogel model, which encapsulates sufficient A20 high-expressing islets and supporting cells, and performs as a drug release system releasing immunosuppressants and growth factors, to improve the outcome of pancreatic islet transplantation. Once injected in vivo, the hydrogel can gel and offer a robust mechanical structure for the A20 high-expressing islets and supporting cells. The natural biomaterials (eg, heparin) added into the hydrogel provide adhesive sites for islets to promote islets’ survival. Furthermore, the hydrogel encapsulates various supporting cells, which can facilitate the vascularization and/or prevent the immune system attacking the islet graft. Based on the previous studies that generally applied one or two combined strategies to protect the function of islet graft, we designed this hypothetical multifunctional encapsulation hydrogel model with various functions. We hypothesized that the islet graft could survive and maintain its function for a longer time in vivo compared with naked islets. This hypothetical model has a limitation in terms of clinical application. Future development work will focus on verifying the function and safety of this hypothetical islet transplantation hydrogel model in vitro and in vivo.

Release mechanisms and applications of drug delivery systems for extended-release

INTRODUCTION

Drug delivery systems with extended-release profiles are ideal in improving patient compliance with enhanced efficacy. To develop devices capable of a prolonged delivery kinetics, it is crucial to understand the various underlying mechanisms contributing to extended drug release and the impact thereof on modulating the long-term performance of such systems in a practical application environment.

AREAS COVERED

This review article intends to provide a comprehensive summary of release mechanisms in extended-release drug delivery systems, particularly polymer-based systems; however, other material types will also be mentioned. Selected current research in the delivery of small molecule drugs and macromolecules is highlighted. Emphasis is placed on the combined impact of different release mechanisms and drug properties on the long-term release kinetics in vitro and in vivo.

EXPERT OPINION

The development of drug delivery systems over an extended duration is promising but also challenging when considering the numerous interrelated delivery-related parameters. Achieving a well-controlled extended drug release requires advanced techniques to minimize burst release and lag phase, a better understanding of the dynamic interrelationship between drug properties and release profiles over time, and a thorough elucidation of the impact of multiple in vivo conditions to methodically evaluate the eventual clinical efficacy.

Controlled Growth Factor Release in 3D-Printed Hydrogels

Growth factors (GFs) are critical components in governing cell fate during tissue regeneration. Their controlled delivery is challenging due to rapid turnover rates in vivo. Functionalized hydrogels, such as heparin-based hydrogels, have demonstrated great potential in regulating GF release. While the retention effects of various concentrations and molecular weights of heparin have been investigated, the role of geometry is unknown. In this work, 3D printing is used to fabricate GF-embedded heparin-based hydrogels with arbitrarily complex geometry (i.e., teabag, flower shapes). Simplified cylindrical core-shell structures with varied shell thickness are printed, and the rates of GF release are measured over the course of 28 days. Increasing the shell layers' thickness decreases the rate of GF release. Additionally, a mathematical model is developed, which is found capable of accurately predicting GF release kinetics in hydrogels with shell layers greater than 0.5 mm thick (R2 > 0.96). Finally, the sequential release is demonstrated by printing two GFs in alternating radial layers. By switching the spatial order, the delivery sequence of the GFs can be modulated. This study demonstrates how 3D printing can be utilized to fabricate user-defined structures with unique geometry in order to control the rate of GF release in hydrogels.

Induced cell migration based on a bioactive hydrogel sheet combined with a perfused microfluidic system

Endothelial cell migration is a crucial step in the process of new blood vessel formation-a necessary process to maintain cell viability inside thick tissue constructs. Here, we report a new method for maintaining cell viability and inducing cell migration using a perfused microfluidic platform based on collagen gel and a gradient hydrogel sheet. Due to the helpful role of the extracellular matrix components in cell viability, we developed a hydrogel sheet from decellularized tissue (DT) of the bovine heart and chitosan (CS). The results showed that hydrogel sheets with an optimum weight ratio of CS/DT = 2 possess a porosity of around 75%, a mechanical strength of 23 kPa, and display cell viability up to 78%. Then, we immobilized a radial gradient of vascular endothelial growth factor (VEGF) on the hydrogel sheet to promote human umbilical vein endothelial cell migration. Finally, we incorporated the whole system as an entirety on the top of the microfluidic platform and studied cell migration through the hydrogel sheet in the presence of soluble and immobilized VEGF. The results demonstrated that immobilized VEGF stimulated cell migration in the hydrogel sheet at all depths compared with soluble VEGF. The results also showed that applying a VEGF gradient in both soluble and immobilized states had a significant effect on cell migration at limited depths (<100 μm). The main finding of this study is a significant improvement in cell migration using an in vivo imitating, cost-efficient and highly reproducible platform, which may open up a new perspective for tissue engineering applications.

Heparin-based hydrogel scaffolding alters the transcriptomic profile and increases the chemoresistance of MDA-MB-231 triple-negative breast cancer cells

The tumor microenvironment plays a critical role in the proliferation and chemoresistance of cancer cells. Growth factors (GFs) are known to interact with the extracellular matrix (ECM) via heparin binding sites, and these associations influence cell behavior. In the present study, we demonstrate the ability to define signals presented by the scaffold by pre-mixing growth factors, such as epidermal growth factor, into the heparin-based (HP-B) hydrogel prior to gelation. In the 3D biomimetic microenvironment, breast cancer cells formed spheroids within 24 hours of initial seeding. Despite higher number of proliferating cells in 2D cultures, 3D spheroids exhibited a higher degree of chemoresistance after 72 hours. Further, our RNA sequencing results highlighted the phenotypic changes influenced by solid-phase GF presentation. Wnt/β-catenin and TGF-β signaling were upregulated in the cells grown in the hydrogel, while apoptosis, IL2-STAT5 and PI3K-AKT-mTOR signaling were downregulated. With emerging technologies for precision medicine in cancer, this nature of fine-tuning the microenvironment is paramount for cultivation and downstream characterization of primary cancer cells and rare circulating tumor cells (CTCs), and effective screening of chemotherapeutic agents.

Polyhedral Oligomeric Silsesquioxane-Incorporated Gelatin Hydrogel Promotes Angiogenesis during Vascularized Bone Regeneration

Many approaches have been made toward the development of scaffolds with good biocompatibility and appreciable physicochemical properties to facilitate stem cell adhesion, osteogenic differentiation, and vascularization in tissue engineering. Nowadays, vascularization is a main bottleneck in tissue engineering strategies that is needed to be overcome and developed. Herein, we construct a series of polyhedral oligomeric silsesquioxane (POSS)-modified porous gelatin hydrogels with different POSS concentrations from 0 to 5 wt %, defined as X% POSS hydrogels (X = 0, 1, 2, 3, 4, 5) to support vascularized bone repair. The introduction of POSS into gelatin effectively promoted adhesive protein adsorption and integrin α5β1 expression, subsequently leading to enhanced adhesion of both rat bone marrow mesenchymal stem cells and human umbilical vein endothelial cells (HUVECs). In vitro experiments further demonstrated that POSS-containing hybrid hydrogels more effectively support the angiogenic tube and network formation in HUVECs than the 0% POSS hydrogel. Besides, POSS-containing hybrid hydrogels showed desirable performance as a sustained release system of vascular endothelial growth factor (VEGF) and bone morphogenetic protein-2 (BMP-2), and they further accelerated vascular network establishment and the formation of a new bone in defect regions. When the hydrogels were implanted into critical-sized rat calvarial defects in vivo, the VEGF/BMP-2-coupled 3% POSS group gained a higher blood vessel volume in the bone defect regions (5.49 ± 0.35 mm³) than the 3% POSS group (3.12 ± 0.20 mm³) and the 0% POSS group (1.57 ± 0.25 mm³), suggesting that the 3% POSS hydrogel with VEGF/BMP-2 would expedite vascularization. Based on these evaluations, our results indicated that the POSS-incorporated gelatin hydrogel would provide a promising bone graft scheme in potential clinical application of large bone defect repair.

A cocktail of growth factors released from a heparin hyaluronic-acid hydrogel promotes the myogenic potential of human urine-derived stem cells in vivo

Traditional cell therapy technology relies on the maximum expansion of primary stem cells in vitro, through multiple passages and potential differentiation protocols, in order to generate the abundance of cells needed prior to transplantation in vivo. Implantation of in vitro over-expanded and pre-differentiated cells typically results in poor cell survival and reduced regeneration capacity for tissue repair in vivo. We hypothesized that implantation of primary stem cells, after a short time culture in vitro (passage number ≤p3), in combination with controlled release of relevant growth factors would improve in vivo cell viability, engraftment and tissue regeneration. The goal of this study was to determine whether the release of myogenic growth factors from a heparin-hyaluronic acid gel (hp-HA gel) could enhance in vivo cell survival, in-growth and myogenic differentiation of human urine-derived stem cells (USC) with a corresponding enhancement in graft vascularization, innervation and regenerative properties. Human USC were obtained from healthy adult donors (n = 6), expanded and then mixed with a hp-HA gel containing sets of growth factors known to enhance myogenesis (IGF1, HGF, PDGF-BB), neurogenesis (NGF, FGF) and angiogenesis (VEGF), or a cocktail with a combination of growth factors. Primary cultured USC (p3) mixed with the hp-HA gel and the various combinations of growth factors, were subcutaneously injected into athymic mice. In vivo cell survival, engraftment and functional differentiation within the host tissue were assessed. The implanted grafts containing USC and the growth factor cocktail showed the greatest number of surviving cells as well as increased numbers of cells that expressed myogenic and endothelial cell markers as compared to other groups 4 weeks after implantation. Moreover, the graft with USC and the growth factor cocktail showed increased numbers of blood vessels and infiltrating neurons. Thus, growth factors released in a controlled manner from an hp-HA gel containing USC efficiently improved in vivo cell survival and supported vascularization and myogenic differentiation within the grafts. This study provides evidence for the use of primary USC and growth factors in a hydrogel as a novel mode of cell therapy for the promotion of myogenic differentiation for the treatment of injured muscle tissue. STATEMENT OF SIGNIFICANCE: Cell therapies are a promising treatment option for neuromuscular dysfunction disorders. However, major limitations in cell retention and engraftment after implantation remain a hindrance to the use of stem cell therapy for the treatment of muscle injuries or diseased tissues. Implanted long-term in vitro cultured cells tend to demonstrate low rates of survival and tissue engraftment, lessened paracrine effects, and poor homing and differentiation. Human USC are an easily obtainable stem cell source that possess stem cell characteristics such as a robust proliferative potential, paracrine effects on neighboring cells, and multi-potential differentiation. In this study, we demonstrated that a combination of primary human USC with a cocktail of growth factors combined in a hyaluronic gel was optimal for cell survival and engraftment, including myogenic differentiation potential of USC, angiogenesis and host nerve fiber recruitment in vivo. The present study also demonstrated that the use of primary urine derived stem cells at early passages, without in vitro pre-differentiation, implanted in a hyaluronic-heparin hydrogel containing a cocktail of growth factors, provided an alternative safe site-specific delivery method for cell therapy.

Temperature-Responsive Multifunctional Protein Hydrogels with Elastin-like Polypeptides for 3-D Angiogenesis

Supramolecular protein hydrogels with tunable properties represent promising candidates for advanced designer extracellular matrices (ECMs). To control cellular functions, ECMs should be able to spatiotemporally regulate synergistic signaling between transmembrane receptors and growth factor (GF) receptors. In this study, we developed genetically engineered temperature-responsive multifunctional protein hydrogels. The designed hydrogel was fabricated by combining the following four peptide blocks: thermosensitive elastin-like polypeptides (ELPs), a polyaspartic acid (polyD) chain to control aggregation and delivery of GFs, a de novo-designed helix peptide that forms antiparallel homotetrameric coiled-coils, and a biofunctional peptide. The resultant coiled-coil unit bound ELPs (CUBEs) exhibit a controllable sol-gel transition with tunable mechanical properties. CUBEs were functionalized with bone sialoprotein-derived RGD (bRGD), and human umbilical vein endothelial cells (HUVECs) were three-dimensionally cultured in bRGD-modified CUBE (bRGD-CUBE) hydrogels. Proangiogenic activity of HUVECs was promoted by bRGD. Moreover, heparin-binding angiogenic GFs were immobilized to bRGD-CUBEs via electrostatic interactions. HUVECs cultured in GF-tethered bRGD-CUBE hydrogels formed three-dimensional (3-D) tubulelike structures. The designed CUBE hydrogels may demonstrate utility as advanced smart biomaterials for biomedical applications. Further, the protein hydrogel design strategy may provide a novel platform for constructing designer 3-D microenvironments for specific cell types.

A hydrophobic gelatin fiber sheet promotes secretion of endogenous vascular endothelial growth factor and stimulates angiogenesis

In tissue engineering and regenerative medicine, the formation of vascular beds is an effective method to supply oxygen and nutrients to implanted cells or tissues to improve their survival and promote normal cellular functions. Various types of angiogenic materials have been developed by incorporating growth factors, such as vascular endothelial growth factor, in biocompatible materials. However, these exogenous growth factors suffer from instability and inactivation under physiological conditions. In this study, we designed a novel angiogenic electrospun fiber sheet (C16-FS) composed of Alaska pollock-derived gelatin (ApGltn) modified with hexadecyl (C16) groups to induce localized and sustained angiogenesis without growth factors. C16-FS was thermally crosslinked to enhance its stability. We demonstrated that C16-FS swells in phosphate-buffered saline for over 24 h and resists degradation. Laser doppler perfusion imaging showed that C16-FS induced increased blood perfusion when implanted subcutaneously in rats compared with unmodified ApGltn-fiber sheets (Org-FS) and the sham control. Furthermore, angiogenesis was sustained for up to 7 days following implantation. Immunohistochemical studies revealed elevated nuclear factor-κB and CD31 levels around the C16-FS implantation site compared with the Org-FS implantation site and the control incision site. These results demonstrate that C16-FS is a promising angiogenic material to promote the formation of vascular beds for cell and tissue transplantation without the need for growth factors.

In vivo long-term growth factor-free angiogenesis by LPS-mimicking C16-modified gelatin based electrospun fiber sheet.

Dual Action of Sulfated Hyaluronan on Angiogenic Processes in Relation to Vascular Endothelial Growth Factor-A

Pathological healing characterized by abnormal angiogenesis presents a serious burden to patients’ quality of life requiring innovative treatment strategies. Glycosaminoglycans (GAG) are important regulators of angiogenic processes. This experimental and computational study revealed how sulfated GAG derivatives (sGAG) influence the interplay of vascular endothelial growth factor (VEGF)₁₆₅ and its heparin-binding domain (HBD) with the signaling receptor VEGFR-2 up to atomic detail. There was profound evidence for a HBD-GAG-HBD stacking configuration. Here, the sGAG act as a “molecular glue” leading to recognition modes in which sGAG interact with two VEGF₁₆₅-HBDs. A 3D angiogenesis model demonstrated the dual regulatory role of high-sulfated derivatives on the biological activity of endothelial cells. While GAG alone promote sprouting, they downregulate VEGF₁₆₅-mediated signaling and, thereby, elicit VEGF₁₆₅-independent and -dependent effects. These findings provide novel insights into the modulatory potential of sGAG derivatives on angiogenic processes and point towards their prospective application in treating abnormal angiogenesis.

Construction of a vascularized bladder with autologous adipose-derived stromal vascular fraction cells combined with bladder acellular matrix via tissue engineering

The formation of an effective vascular network can promote peripheral angiogenesis, ensuring an effective supply of blood, oxygen, and nutrients to an engineered bladder, which is important for bladder tissue engineering. Stromal vascular fraction cells (SVFs) promote vascularization and improve the function of injured tissues. In this study, adipose tissue-derived SVFs were introduced as an angiogenic cell source and seeded into the bladder acellular matrix (BAM) to generate a SVF-BAM complex for bladder reconstruction. The morphological regeneration and functional restoration of the engineered bladder were evaluated. In addition, we also explored the role of the Wnt5a/sFlt-1 noncanonical Wnt signaling pathway in regulating the angiogenesis of SVFs, and in maintaining the rational capability of SVFs to differentiate into vasculature in regenerated tissues. Histological assessment indicated that the SVF-BAM complex was more effective in promoting smooth muscle, vascular, and nerve regeneration than BAM alone and subsequently led to the restoration of bladder volume and bladder compliance. Moreover, exogenous Wnt5a was able to enhance angiogenesis by increasing the activity of MMP2, MMP9, and VEGFR2. Simultaneously, the expression of sFlt-1 was also increased, which enhanced the stability of the SVFs angiogenic capability. SVFs may be a potential cell source for tissue-engineered bladders. The Wnt5a/sFlt-1 pathway is involved in the regulation of autologous vascular formation by SVFs. The rational regulation of this pathway can promote neo-microvascularization in tissue-engineered bladders.

Co-immobilization of CD133 antibodies, vascular endothelial growth factors, and REDV peptide promotes capture, proliferation, and differentiation of endothelial progenitor cells

Capture of endothelial progenitor cells (EPCs) in situ has been considered as a promising strategy for the rapid endothelialization and long-term patency of artificial blood vessels and implant devices. In this study, a CD133⁺ EPC capture surface was fabricated by grafting CD133 antibody (a more specific EPC surface marker than CD34) and Arg-Glu-Asp-Val (REDV) peptideon the methacrylate-grafted hyaluronic acid (MA-HA) and heparin-hybridized (MA-HA&Heparin) resisting layer. Vascular endothelial growth factor (VEGF) was further conjugated to the immobilized heparin. This engineered surface showed good hemocompatibility and significantly higher ability of capturing CD133⁺ EPCs from human peripheral blood mononuclear cells (PBMCs) and obviously upregulated the expression of endothelial cell (EC) marker genes of EPCs such as VEGF receptor 2 (VEGFR2), CD31, VE-cadherin, and von Willebrand factor (vWF), facilitating the differentiation of EPCs into ECs. The dramatically enhanced EPC proliferation on this surface was dependent on the integrin-VEGFR synergistic signaling, as ERK1/2 phosphorylation was only significantly enhanced on the REDV and VEGF co-immobilized surface. This study highlights a new surface coating strategy for blood-contact materials based on the specific EPC capturing and rapid endothelialization. STATEMENT OF SIGNIFICANCE: Capture of endothelial progenitor cells (EPCs) in situ is a promising strategy for the rapid endothelialization and long-term patency of artificial blood vessels and scaffolds. More specific capture of EPCs by targeting CD133 rather than CD34 can better reduce the risk of inflammation and restenosis. On the other hand, an appropriate microenvironment for EPC proliferation is equally important for endothelialization, which is rarely considered by the existing EPC capture strategies. In this study, the capture ratio of EPCs was significantly increased by simultaneously grafting CD133 antibody and VEGF on a MA-HA and heparin-hybridized antifouling layer. Further, proliferation of EPCs after capture was significantly promoted by grafting VEGF and REDV peptide through the integrin-VEGFR synergistic signaling. This study highlights a new strategy for the surface coating of blood-contact materials based on specific EPC capture and rapid endothelialization.

Kidney regeneration with biomimetic vascular scaffolds based on vascular corrosion casts

We have developed a biomimetic renal vascular scaffold based on a vascular corrosion casting technique. This study evaluated the feasibility of using this novel biomimetic scaffold for kidney regeneration in a rat kidney cortical defect model. Vascular corrosion casts were prepared from normal rat kidneys by perfusion with 10% polycaprolactone (PCL) solution, followed by tissue digestion. The corrosion PCL cast was coated with collagen, and PCL was removed from within the collagen coating, leaving only a hollow collagen-based biomimetic vascular scaffold. The fabricated scaffolds were pre-vascularized with MS1 endothelial cell coating, incorporated into 3D renal constructs, and subsequently implanted either with or without human renal cells in the renal cortex of nude rats. The implanted collagen-based vascular scaffold was easily identified and integrated into native kidney tissue. The biomimetic vascular scaffold coated with endothelial cells (MS1) showed significantly enhanced vascularization, as compared to the uncoated scaffold and hydrogel only groups (P < 0.001). Along with the improved vascularization effects, the MS1-coated scaffolds showed a significant renal cell infiltration from the neighboring host tissue, as compared to the other groups (P < 0.05). Moreover, addition of human renal cells to the MS1-coated scaffold resulted in further enhancement of vascularization and tubular structure regeneration within the implanted constructs. The biomimetic collagen vascular scaffolds coated with endothelial cells are able to enhance vascularization and facilitate the formation of renal tubules after 14 days when combined with human renal cells. This study shows the feasibility of bioengineering vascularized functional renal tissues for kidney regeneration. STATEMENT OF SIGNIFICANCE: Vascularization is one of the major hurdles affecting the survival and integration of implanted three-dimensional tissue constructs in vivo. A novel, biomimetic, collagen-based vascular scaffold that is structurally identical to native kidney tissue was developed and tested. This biomimetic vascularized scaffold system facilitates the development of new vessels and renal cell viability in vivo when implanted in a partial renal defect. The use of this scaffold system could address the challenges associated with vascularization, and may be an ideal treatment strategy for partial augmentation of renal function in patients with chronic kidney disease.

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.

Supramolecular Nanofibrous Peptide/Polymer Hydrogels for the Multiplexing of Bioactive Signals

The ability to provide multiple functions within a single scaffold biomaterial is a major goal in tissue engineering. Self-assembling peptide-based hydrogels are gaining significant attention as three-dimensional biomaterials because they provide a network of nanofibers similar to the native extracellular matrix while allowing the presentation of multiple biochemical cues for cell signaling. Herein, we combine a positively charged peptide amphiphile (PA) and the negatively charged synthetic polymer poly(sodium 4-styrenesulfonate) (PSS) to fabricate hybrid hydrogels through supramolecular self-assembly. PSS/PA hydrogels show rather high mechanical stiffness while being stable in buffered environment. The sulfonate functionality in PSS promotes hydrogel mineralization which can be controlled if undertaken in standard osteogenic medium. Loading proteins with different charges in the hydrogels reveals their ability to retain and sustain their release and indicates their potential for the controlled delivery of growth factors. Human mesenchymal stem cells encapsulated in PSS/PA hydrogels remain viable. The biomimetic nanofibrous structure of the hydrogels, together with multiplexing of bioactive signals, can provide a suitable environment for stem cell differentiation.

State-of-the-Art Strategies for the Vascularization of Three-Dimensional Engineered Organs

Engineering three-dimensional (3D) implantable tissue constructs is a promising strategy for replacing damaged or diseased tissues and organs with functional replacements. However, the efficient vascularization of new 3D organs is a major scientific and technical challenge since large tissue constructs or organs require a constant blood supply to survive in vivo. Current approaches to solving this problem generally fall into the following three major categories: (a) cell-based, (b) angiogenic factor-based, and (c) scaffold-based. In this review, we summarize state-of-the-art technologies that are used to develop complex, stable, and functional vasculature for engineered 3D tissue constructs and organs; additionally, we have suggested directions for future research.