Engineering small-caliber vascular grafts from collagen filaments and nanofibers with comparable mechanical properties to native vessels

Engineering collagen-based biomaterials for cardiovascular medicine

Cardiovascular diseases have been the leading cause of global mortality and disability. In addition to traditional drug and surgical treatment, more and more studies investigate tissue engineering therapeutic strategies in cardiovascular medicine. Collagen interweaves in the form of trimeric chains to form the physiological network framework of the extracellular matrix of cardiac and vascular cells, possessing excellent biological properties (such as low immunogenicity and good biocompatibility) and adjustable mechanical properties, which renders it a vital tissue engineering biomaterial for the treatment of cardiovascular diseases. In recent years, promising advances have been made in the application of collagen materials in blood vessel prostheses, injectable cardiac hydrogels, cardiac patches, and hemostatic materials, although their clinical translation still faces some obstacles. Thus, we reviewed these findings and systematically summarizes the application progress as well as problems of clinical translation of collagen biomaterials in the cardiovascular field. The present review contributes to a comprehensive understanding of the application of collagen biomaterials in cardiovascular medicine.

Graphical abstract

Propelling Minimally Invasive Tissue Regeneration With Next-Era Injectable Pre-Formed Scaffolds

The growing aging population, with its associated chronic diseases, underscores the urgency for effective tissue regeneration strategies. Biomaterials play a pivotal role in the realm of tissue reconstruction and regeneration, with a distinct shift toward minimally invasive (MI) treatments. This transition, fueled by engineered biomaterials, steers away from invasive surgical procedures to embrace approaches offering reduced trauma, accelerated recovery, and cost-effectiveness. In the realm of MI tissue repair and cargo delivery, various techniques are explored. While in situ polymerization is prominent, it is not without its challenges. This narrative review explores diverse biomaterials, fabrication methods, and biofunctionalization for injectable pre-formed scaffolds, focusing on their unique advantages. The injectable pre-formed scaffolds, exhibiting compressibility, controlled injection, and maintained mechanical integrity, emerge as promising alternative solutions to in situ polymerization challenges. The conclusion of this review emphasizes the importance of interdisciplinary design facilitated by synergizing fields of materials science, advanced 3D biomanufacturing, mechanobiological studies, and innovative approaches for effective MI tissue regeneration.

Computational and Experimental Characterization of Aligned Collagen across Varied Crosslinking Degrees

Collagen-based scaffolds have been widely used in tissue engineering. The alignment of collagen fibers and the degree of crosslinking in engineering tissue scaffolds significantly affect cell activity and scaffold stability. Changes in microarchitecture and crosslinking degree also impact the mechanical properties of collagen scaffolds. A clear understanding of the effects of collagen alignment and crosslinking degrees can help properly control these critical parameters for fabricating collagen scaffolds with desired mechanical properties. In this study, combined uniaxial mechanical testing and finite element method (FEM) were used to quantify the effects of fiber alignment and crosslinking degree on the mechanical properties of collagen threads. We have fabricated electrochemically aligned collagen (ELAC) and compared it with randomly distributed collagen at varying crosslinking degrees, which depend on genipin concentrations of 0.1% or 2% for crosslinking durations of 1, 4, and 24 h. Our results indicate that aligned collagen fibers and higher crosslinking degree contribute to a larger Young's modulus. Specifically, aligned fiber structure, compared to random collagen, significantly increases Young's modulus by 112.7% at a 25% crosslinking degree (0.1% (4 h), i.e., 0.1% genipin concentration with a crosslinking duration of 4 h). Moreover, the ELAC Young's modulus increased by 90.3% as the crosslinking degree doubled by changing the genipin concentration from 0.1% to 2% with the same 4 h crosslinking duration. Furthermore, verified computational models can predict mechanical properties based on specific crosslinking degrees and fiber alignments, which facilitate the controlled fabrication of collagen threads. This combined experimental and computational approach provides a systematic understanding of the interplay among fiber alignment, crosslinking parameters, and mechanical performance of collagen scaffolds. This work will enable the precise fabrication of collagen threads for desired tissue engineering performance, potentially advancing tissue engineering applications.

Electrospun silk fibroin/fibrin vascular scaffold with superior mechanical properties and biocompatibility for applications in tissue engineering

Electrospun scaffolds play important roles in the fields of regenerative medicine and vascular tissue engineering. The aim of the research described here was to develop a vascular scaffold that mimics the structural and functional properties of natural vascular scaffolding. The mechanical properties of artificial vascular tissue represent a key issue for successful transplantation in small diameter engineering blood vessels. We blended silk fibroin (SF) and fibrin to fabricate a composite scaffold using electrospinning to overcome the shortcomings of fibrin with respect to its mechanical properties. Subsequently, we then carefully investigated the morphological, mechanical properties, hydrophilicity, hemocompatibility, degradation, cytocompatibility and biocompatibility of the SF/fibrin (0:100), SF/fibrin (15:85), SF/fibrin (25:75), and SF/fibrin (35:65) scaffolds. Based on these in vitro results, we implanted SF/fibrin (25:75) vascular scaffold subcutaneously and analyzed its in vivo degradation and histocompatibility. The fiber structure of the SF/fibrin hybrid scaffold was smooth and uniform, and its fiber diameters were relatively small. Compared with the fibrin scaffold, the SF/fibrin scaffold clearly displayed increased mechanical strength, but the hydrophilicity weakened correspondingly. All of the SF/fibrin scaffolds showed excellent blood compatibility and appropriate biodegradation rates. The SF/fibrin (25:75) scaffold increased the proliferation and adhesion of MSCs. The results of animal experiments confirmed that the degradation of the SF/fibrin (25:75) scaffold was faster than that of the SF scaffold and effectively promoted tissue regeneration and cell infiltration. All in all, the SF/fibrin (25:75) electrospun scaffold displayed balanced and controllable biomechanical properties, degradability, and good cell compatibility. Thus, this scaffold proved to be an ideal candidate material for artificial blood vessels.

Harnessing Natural Polymers for Nano-Scaffolds in Bone Tissue Engineering: A Comprehensive Overview of Bone Disease Treatment

Numerous surgeries are carried out to replace tissues that have been harmed by an illness or an accident. Due to various surgical interventions and the requirement of bone substitutes, the emerging field of bone tissue engineering attempts to repair damaged tissues with the help of scaffolds. These scaffolds act as template for bone regeneration by controlling the development of new cells. For the creation of functional tissues and organs, there are three elements of bone tissue engineering that play very crucial role: cells, signals and scaffolds. For the achievement of these aims, various types of natural polymers, like chitosan, chitin, cellulose, albumin and silk fibroin, have been used for the preparation of scaffolds. Scaffolds produced from natural polymers have many advantages: they are less immunogenic as well as being biodegradable, biocompatible, non-toxic and cost effective. The hierarchal structure of bone, from microscale to nanoscale, is mostly made up of organic and inorganic components like nanohydroxyapatite and collagen components. This review paper summarizes the knowledge and updates the information about the use of natural polymers for the preparation of scaffolds, with their application in recent research trends and development in the area of bone tissue engineering (BTE). The article extensively explores the related research to analyze the advancement of nanotechnology for the treatment of bone-related diseases and bone repair.

Integrated biophysical matching of bacterial nanocellulose coronary artery bypass grafts towards bioinspired artery typical functions

Revascularization via coronary artery bypass grafting (CABG) to treat cardiovascular disease is established as one of the most important lifesaving surgical techniques worldwide. But the shortage in functionally self-adaptive autologous arteries leads to circumstances where the clinical reality must deal with fighting pathologies coming from the mismatching biophysical functionality of more available venous grafts. Synthetic biomaterial-based CABG grafts did not make it to the market yet, what is mostly due to technical hurdles in matching biophysical properties to the complex demands of the CABG niche. But bacterial Nanocellulose (BNC) Hydrogels derived by growing biofilms hold a naturally integrative character in function-giving properties by its freedom in designing form and intrinsic fiber architecture. In this study we use this integral to combine impacts on the luminal fiber matrix, biomechanical properties and the reciprocal stimulation of microtopography and induced flow patterns, to investigate biomimetic and artificial designs on their bio-functional effects. Therefore, we produced tubular BNC-hydrogels at distinctive designs, characterized the structural and biomechanical properties and subjected them to in vitro endothelial colonization in bioreactor assisted perfusion cultivation. Results showed clearly improved functional properties and gave an indication of successfully realized stimulation by artery-typical helical flow patterns.

Natural and Synthetic Polymeric Biomaterials for Application in Wound Management

Biomaterials are at the forefront of the future, finding a variety of applications in the biomedical field, especially in wound healing, thanks to their biocompatible and biodegradable properties. Wounds spontaneously try to heal through a series of interconnected processes involving several initiators and mediators such as cytokines, macrophages, and fibroblasts. The combination of biopolymers with wound healing properties may provide opportunities to synthesize matrices that stimulate and trigger target cell responses crucial to the healing process. This review outlines the optimal management and care required for wound treatment with a special focus on biopolymers, drug-delivery systems, and nanotechnologies used for enhanced wound healing applications. Researchers have utilized a range of techniques to produce wound dressings, leading to products with different characteristics. Each method comes with its unique strengths and limitations, which are important to consider. The future trajectory in wound dressing advancement should prioritize economical and eco-friendly methodologies, along with improving the efficacy of constituent materials. The aim of this work is to give researchers the possibility to evaluate the proper materials for wound dressing preparation and to better understand the optimal synthesis conditions as well as the most effective bioactive molecules to load.

Development and characterization of biological sutures made of cell-assembled extracellular matrix

Most vascular surgical repair procedures, such as vessel anastomoses, requires using suture materials that are mechanically efficient and accepted by the patient's body. These materials are essentially composed of synthetic polymers, such as polypropylene (ProleneTM) or polyglactin (VicrylTM). However, once implanted in patients, they are recognized as foreign bodies, and the patient's immune system will degrade, encapsulate, or even expel them. In this study, we developed innovative biological sutures for cardiovascular surgical repairs using Cell-Assembled extracellular Matrix (CAM)-based ribbons. After a mechanical characterization of the CAM-based ribbons, sutures were made with hydrated or twisted/dried ribbons with an initial width of 2 or 3 mm. These biological sutures were mechanically characterized and used to anastomoseex vivoanimal aortas. Data showed that our biological sutures display lower permeability and higher burst resistance than standard ProleneTMsuture material.In vivocarotid anastomoses realized in sheep demonstrated that our biological sutures are compatible with standard vascular surgery techniques. Echography confirmed the absence of thrombus and perfect homeostasis with no blood leakage was obtained within the first 10 min after closing the anastomosis. Finally, our findings confirmed the effectiveness and clinical relevance of these innovative biological sutures.

Three-dimensional bioprinting of artificial blood vessel: Process, bioinks, and challenges

The coronary artery bypass grafting is a main treatment for restoring the blood supply to the ischemic site by bypassing the narrow part, thereby improving the heart function of the patients. Autologous blood vessels are preferred in coronary artery bypass grafting, but their availability is often limited by due to the underlying disease. Thus, tissue-engineered vascular grafts that are devoid of thrombosis and have mechanical properties comparable to those of natural vessels are urgently required for clinical applications. Most of the commercially available artificial implants are made from polymers, which are prone to thrombosis and restenosis. The biomimetic artificial blood vessel containing vascular tissue cells is the most ideal implant material. Due to its precision control ability, three-dimensional (3D) bioprinting is a promising method to prepare biomimetic system. In the 3D bioprinting process, the bioink is at the core state for building the topological structure and keeping the cell viable. Therefore, in this review, the basic properties and viable materials of the bioink are discussed, and the research of natural polymers in bioink, including decellularized extracellular matrix, hyaluronic acid, and collagen, is emphasized. Besides, the advantages of alginate and Pluronic F127, which are the mainstream sacrificial material during the preparation of artificial vascular graft, are also reviewed. Finally, an overview of the applications in the field of artificial blood vessel is also presented.

Current Strategies for Engineered Vascular Grafts and Vascularized Tissue Engineering

Blood vessels not only transport oxygen and nutrients to each organ, but also play an important role in the regulation of tissue regeneration. Impaired or occluded vessels can result in ischemia, tissue necrosis, or even life-threatening events. Bioengineered vascular grafts have become a promising alternative treatment for damaged or occlusive vessels. Large-scale tubular grafts, which can match arteries, arterioles, and venules, as well as meso- and microscale vasculature to alleviate ischemia or prevascularized engineered tissues, have been developed. In this review, materials and techniques for engineering tubular scaffolds and vasculature at all levels are discussed. Examples of vascularized tissue engineering in bone, peripheral nerves, and the heart are also provided. Finally, the current challenges are discussed and the perspectives on future developments in biofunctional engineered vessels are delineated.

Adipose-Derived Stem Cells in Reinforced Collagen Gel: A Comparison between Two Approaches to Differentiation towards Smooth Muscle Cells

Scaffolds made of degradable polymers, such as collagen, polyesters or polysaccharides, are promising matrices for fabrication of bioartificial vascular grafts or patches. In this study, collagen isolated from porcine skin was processed into a gel, reinforced with collagen particles and with incorporated adipose tissue-derived stem cells (ASCs). The cell-material constructs were then incubated in a DMEM medium with 2% of FS (DMEM_part), with added polyvinylalcohol nanofibers (PVA_part sample), and for ASCs differentiation towards smooth muscle cells (SMCs), the medium was supplemented either with human platelet lysate released from PVA nanofibers (PVA_PL_part) or with TGF-β1 + BMP-4 (TGF + BMP_part). The constructs were further endothelialised with human umbilical vein endothelial cells (ECs). The immunofluorescence staining of alpha-actin and calponin, and von Willebrand factor, was performed. The proteins involved in cell differentiation, the extracellular matrix (ECM) proteins, and ECM remodelling proteins were evaluated by mass spectrometry on day 12 of culture. Mechanical properties of the gels with ASCs were measured via an unconfined compression test on day 5. Gels evinced limited planar shrinkage, but it was higher in endothelialised TGF + BMP_part gel. Both PVA_PL_part samples and TGF + BMP_part samples supported ASC growth and differentiation towards SMCs, but only PVA_PL_part supported homogeneous endothelialisation. Young modulus of elasticity increased in all samples compared to day 0, and PVA_PL_part gel evinced a slightly higher ratio of elastic energy. The results suggest that PVA_PL_part collagen construct has the highest potential to remodel into a functional vascular wall.

Drug loaded implantable devices to treat cardiovascular disease

INTRODUCTION

It is widely acknowledged that cardiovascular diseases (CVDs) continue to be the leading cause of death globally. Furthermore, CVDs are the leading cause of diminished quality of life for patients, frequently as a result of their progressive deterioration. Medical implants that release drugs into the body are active implants that do more than just provide mechanical support; they also have a therapeutic role. Primarily, this is achieved through the controlled release of active pharmaceutical ingredients (API) at the implementation site.

AREAS COVERED

In this review, the authors discuss drug-eluting stents, drug-eluting vascular grafts, and drug-eluting cardiac patches with the aim of providing a broad overview of the three most common types of cardiac implant.

EXPERT OPINION

Drug eluting implants are an ideal alternative to traditional drug delivery because they allow for accurate drug release, local drug delivery to the target tissue, and minimise the adverse side effects associated with systemic administration. Despite the fact that there are still challenges that need to be addressed, the ever-evolving new technologies are making the fabrication of drug eluting implants a rewarding therapeutic endeavour with the possibility for even greater advances.

Current biofabrication methods for vascular tissue engineering and an introduction to biological textiles

Cardiovascular diseases are the leading cause of mortality in the world and encompass several important pathologies, including atherosclerosis. In the cases of severe vessel occlusion, surgical intervention using bypass grafts may be required. Synthetic vascular grafts provide poor patency for small-diameter applications (< 6 mm) but are widely used for hemodialysis access and, with success, larger vessel repairs. In very small vessels, such as coronary arteries, synthetics outcomes are unacceptable, leading to the exclusive use of autologous (native) vessels despite their limited availability and, sometimes, quality. Consequently, there is a clear clinical need for a small-diameter vascular graft that can provide outcomes similar to native vessels. Many tissue-engineering approaches have been developed to offer native-like tissues with the appropriate mechanical and biological properties in order to overcome the limitations of synthetic and autologous grafts. This review overviews current scaffold-based and scaffold-free approaches developed to biofabricate tissue-engineered vascular grafts (TEVGs) with an introduction to the biological textile approaches. Indeed, these assembly methods show a reduced production time compared to processes that require long bioreactor-based maturation steps. Another advantage of the textile-inspired approaches is that they can provide better directional and regional control of the TEVG mechanical properties.

Poly(ferulic acid)-hybrid nanofibers for reducing thrombosis and restraining intimal hyperplasia in vascular tissue engineering

Small-diameter blood vascular transplantation failure is mainly caused by the vascular materials' unreliable hemocompatibility and histocompatibility and the unmatched mechanical properties, which will cause unstable blood flow. How to solve the problems of coagulation and intimal hyperplasia caused by the above factors is formidable in vascular replacement. In this work, we have synthesized poly(ferulic acid) (PFA) and prepared poly(ester-urethane)urea (PEUU)/silk fibroin (SF)/poly(ferulic acid) (PFA) hybrid nanofibers vascular graft (PSPG) by random electrospinning and post-double network bond crosslinking for process optimization. The results in vitro demonstrated that the graft is of significant anti-oxidation, matched mechanical properties, reliable cytocompatibility, and blood compatibility. Replacing resected rat abdominal aorta and rabbit carotid artery models with PSPG vascular grafts indicated that the grafts are capable of homogeneous hybrid PFA significantly promoted the stabilization of endothelial cells and the ingrowth of smooth muscle cells, meanwhile stabilizing the immune microenvironment. This research demonstrates the PSPG vascular graft with substantial patency, indicating their potential for injured vascular healing.

Mechanical characterization of human umbilical arteries by thick-walled models: Enhanced vascular compliance by removing an abluminal lining

The decellularized human umbilical artery (HUA) is considered as a promising option for small-diameter, tissue-engineered vascular grafts (TEVGs). Our previous study showed that the HUA bears a thin, watertight lining on its outermost abluminal surface. Removal of this abluminal lining layer improves efficacy of the perfusion-assisted decellularization of the HUA and increases its compliance. As stress across the wall is believed to affect growth and remodeling of the TEVG, it is imperative to mechanically characterize the HUA using thick-walled models. Combining inflation experiments and computational methods, we investigate the mechanical properties of the HUA before and after the abluminal lining removal to examine the HUA's wall mechanics. The inflation tests of five HUAs were performed to obtain the mechanical and geometrical response of the vessel wall before and after the lining layer removal. Using nonlinear hyperelastic models, the same responses are obtained computationally using the thick-walled models. The experimental data are incorporated into the computational models to estimate the mechanical and orientation parameters of the fibers and isotropic matrix of different layers in the HUAs. The parameter fitting of both thick-walled models (before and after the abluminal lining removal) results in most of the R-squared values for measuring the goodness of fitting to be over 0.90 for all samples. The compliance of the HUA increases from a mean value of 2.60% per 100 mmHg before the removal of the lining to a mean value of 4.21% per 100 mmHg after the removal. The results reveal that, although the abluminal lining is thin, it is stiff and capable of supporting majority of the high luminal pressure, and that the inner layer is far less stressed than the abluminal lining. Computational simulations also show that removal of the abluminal lining increases the circumferential wall stress by up to 280 kPa under the in vivo luminal pressure. The integrated computational and experimental approaches provide more accurate estimates of the material behaviors of HUAs employed in grafts and, in turn, the study enhances our understanding of interactions between the graft and the native vessel on vascular growth and remodeling.

A textile-reinforced composite vascular graft that modulates macrophage polarization and enhances endothelial cell migration, adhesion and proliferation in vitro

At the present time, there is no successful off-the-shelf small-caliber vascular graft (<6 mm) for the repair or bypass of the coronary or carotid arteries. In this study, we engineer a textile-reinforced hydrogel vascular graft. The textile fibers are circularly knitted into a flexible yet robust conduit to serve as the backbone of the composite vascular graft and provide the primary mechanical support. It is embedded in the hydrogel matrix which seals the open structure of the knitted reinforcement and mediates cellular response toward a faster reendothelialization. The mechanical properties of the composite vascular graft, including bursting strength, suture retention strength and radial compliance, significantly surpass the requirement for the vascular graft application and can be adjusted by altering the structure of the textile reinforcement. The addition of hydrogel matrix, on the other hand, improves the survival, adhesion and proliferation of endothelial cells in vitro. The composite vascular graft also enhances macrophage activation and upregulates M1 and M2 related gene expression, which further improves the endothelial cell migration that might favor the reendothelialization of the vascular graft. Taken together, the textile-reinforced hydrogel shows it potential to be a promising scaffold material to fabricate a tissue engineered vascular graft.

Vascular elasticity measurement of the great saphenous vein based on optical coherence elastography

Vascular elasticity is important in physiological and clinical problems. The mechanical properties of the great saphenous vein (GSV) deserve attention. This research aims to measure the radial elasticity of ex vivo GSV using the optical coherence elasticity (OCE). The finite element model of the phantom is established, the displacement field is calculated, the radial mechanical characteristics of the simulation body are obtained. Furthermore, we performed OCE on seven isolated GSVs. The strain field is obtained by combining the relationship between strain and displacement to obtain the radial elastic modulus of GSVs. In the phantom experiment, the strain of the experimental region of interest is mainly between 0.1 and 0.4, while the simulation result is between 0.06 and 0.40. The radial elastic modulus of GSVs ranged from 3.83 kPa to 7.74 kPa. This study verifies the feasibility of the OCE method for measuring the radial elastic modulus of blood vessels.

Republication: In Vitro and Ex Vivo Analysis of Collagen Foams for Soft and Hard Tissue Regeneration

BACKGROUND/AIM

The aim of this study was the conception, production, material analysis and cytocompatibility analysis of a new collagen foam for medical applications.

MATERIALS AND METHODS

After the innovative production of various collagen sponges from bovine sources, the foams were analyzed ex vivo in terms of their structure (including pore size) and in vitro in terms of cytocompatibility according to EN ISO 10993-5/-12. In vitro, the collagen foams were compared with the established biomaterials cerabone and Jason membrane. Materials cerabone and Jason membrane.

RESULTS

Collagen foams with different compositions were successfully produced from bovine sources. Ex vivo, the foams showed a stable and long-lasting primary structure quality with a bubble area of 1,000 to 2,000 μm2 In vitro, all foams showed sufficient cytocompatibility.

CONCLUSION

Collagen sponges represent a promising material for hard and soft tissue regeneration. Future studies could focus on integrating and investigating different additives in the foams.

Tailoring micro/nano-fibers for biomedical applications

Nano/micro fibers have evoked much attention of scientists and have been researched as cutting edge and hotspot in the area of fiber science in recent years due to the rapid development of various advanced manufacturing technologies, and the appearance of fascinating and special functions and properties, such as the enhanced mechanical strength, high surface area to volume ratio and special functionalities shown in the surface, triggered by the nano or micro-scale dimensions. In addition, these outstanding and special characteristics of the nano/micro fibers impart fiber-based materials with wide applications, such as environmental engineering, electronic and biomedical fields. This review mainly focuses on the recent development in the various nano/micro fibers fabrication strategies and corresponding applications in the biomedical fields, including tissue engineering scaffolds, drug delivery, wound healing, and biosensors. Moreover, the challenges for the fabrications and applications and future perspectives are presented.

Construction of spider silk protein small-caliber tissue engineering vascular grafts based on dynamic culture and its performance evaluation

Tissue engineering is an alternative method for preparing small-caliber (<6 mm) vascular grafts. Dynamic mechanical conditioning is being researched as a method to improve mechanical properties of tissue engineered blood vessels. This method attempts to induce unique reaction in implanted cells that regenerate the matrix around them, thereby improving the overall mechanical stability of the grafts. In this study, we used a bioreactor to seed endothelial cells and smooth muscle cells into the inner and outer layers of the electrospun spider silk protein scaffold respectively to construct vascular grafts. The cell proliferation, mechanical properties, blood compatibility and other indicators of the vascular grafts were characterized in vitro. Furthermore, the vascular grafts were implanted in Sprague Dawley rats, and the vascular grafts' patency, extracellular matrix formation, and inflammatory response were evaluated in vivo. We aimed to construct spider silk protein vascular grafts with the potential for in vivo implantation by using a pulsating flow bioreactor. The results showed that, when compared with the static culture condition, the dynamic culture condition improved cell proliferation on vascular scaffolds and enhanced mechanical function of vascular scaffolds. In vivo experiments also showed that the dynamic culture of vascular grafts was more beneficial for the extracellular matrix deposition and anti-thrombogenesis, as well as reducing the inflammatory response of vascular grafts. In conclusion, dynamic mechanical conditioning aid in the resolution of challenges impeding the application of electrospun scaffolds and have the potential to construct small-caliber blood vessels with regenerative function for cardiovascular tissue repair.

Preparation of Collagen/Hydroxyapatite Composites Using the Alternate Immersion Method and Evaluation of the Cranial Bone-Forming Capability of Composites Complexed with Acidic Gelatin and b-FGF

Bone-substitute materials are essential in dental implantology. We prepared collagen (Col)/hydroxyapatite (Hap)/acidic gelatin (AG)/basic fibroblast growth factor (b-FGF) constructs with enhanced bone-forming capability. The Col/Hap apatite composites were prepared by immersing Col sponges alternately in calcium and phosphate ion solutions five times, for 20 and 60 min, respectively. Then, the sponges were heated to 56 °C for 48 h. Scanning electron microscopy/energy-dispersive X-ray spectroscopy, Fourier-transform infrared spectroscopy, and X-ray diffraction analyses showed that the Col/Hap composites contained poorly crystalline Hap precipitates on the Col matrix. Col/Hap composite granules were infiltrated by AG, freeze-dried, and immersed in b-FGF solution. The wet quaternary constructs were implanted in rat cranial bone defects for 8 weeks, followed by soft X-ray measurements and histological analysis. Animal studies have shown that the constructs moderately increase bone formation in cranial bone defects. We found that an alternate immersion time of 20 min led to the greatest bone formation (p < 0.05). Constructs placed inside defects slightly extend the preexisting bone from the defect edges and lead to the formation of small island-like bones inside the defect, followed by disappearance of the constructs. The combined use of Col, Hap, AG, and b-FGF might bring about novel bone-forming biomaterials.

Matrix Regeneration Ability In Situ Induced by a Silk Fibroin Small-Caliber Artificial Blood Vessel In Vivo

The success of a small-caliber artificial vascular graft in the host in order to obtain functional tissue regeneration and remodeling remains a great challenge in clinical application. In our previous work, a silk-based, small-caliber tubular scaffold (SFTS) showed excellent mechanical properties, long-term patency and rapid endothelialization capabilities. On this basis, the aim of the present study was to evaluate the vascular reconstruction process after implantation to replace the common carotid artery in rabbits. The new tissue on both sides of the SFTSs at 1 month was clearly observed. Inside the SFTSs, the extracellular matrix (ECM) was deposited on the pore wall at 1 month and continued to increase during the follow-up period. The self-assembled collagen fibers and elastic fibers were clearly visible in a circumferential arrangement at 6 months and were similar to autologous blood vessels. The positive expression rate of Lysyl oxidase-1 (LOXL-1) was positively correlated with the formation and maturity of collagen fibers and elastic fibers. In summary, the findings of the tissue regeneration processes indicated that the bionic SFTSs induced in situ angiogenesis in defects.

Electrospun PCL/collagen hybrid nanofibrous tubular graft based on post-network bond processing for vascular substitute

Inhibiting thrombus formation and intimal hyperplasia is essential for orthotopic tissue-engineered vascular grafts. The matching mechanical properties of autologous blood vessels and inhibition of platelet aggregation are considered as two points to improve the success rate of transplantation. The poly(ε-caprolactone)/collagen/heparin composite vascular graft (PCLHC) with three-dimensional network structure were constructed by electrospinning, which can mimic natural vascular biomechanics and enhance the viability of cells viability in vitro. The hybrid collagen matrix network nanofibers formed by electrospinning exhibited uniform and smooth morphology. The results of mechanical experiments showed that PCLHC had similar mechanical properties to natural blood vessels. And the addition of heparin enhanced the anticoagulation of PCLHC. Simultaneous three-component hybrid nanofibers showed a potentially reliable ability to promote the proliferation of human umbilical vein endothelial cells (HUVECs). In summary, all the results showed that the three-dimensional network structure of PCLHC presented the potential to heal injured vessels.

Review of Polymeric Biomimetic Small-Diameter Vascular Grafts to Tackle Intimal Hyperplasia

Small-diameter artificial vascular grafts (SDAVG) are used to bypass blood flow in arterial occlusive diseases such as coronary heart or peripheral arterial disease. However, SDAVGs are plagued by restenosis after a short while due to thrombosis and the thickening of the neointimal wall known as intimal hyperplasia (IH). The specific causes of IH have not yet been deduced; however, thrombosis formation due to bioincompatibility as well as a mismatch between the biomechanical properties of the SDAVG and the native artery has been attributed to its initiation. The main challenges that have been faced in fabricating SDAVGs are facilitating rapid re-endothelialization of the luminal surface of the SDAVG and replicating the complex viscoelastic behavior of the arteries. Recent strategies to combat IH formation have been mostly based on imitating the natural structure and function of the native artery (biomimicry). Thus, most recently, developed grafts contain a multilayered structure with a designated function for each layer. This paper reviews the current polymeric, biomimetic SDAVGs in preventing the formation of IH. The materials used in fabrication, challenges, and strategies employed to tackle IH are summarized and discussed, and we focus on the multilayered structure of current SDAVGs. Additionally, the future aspects in this area are pointed out for researchers to consider in their endeavor.

Improving Biocompatibility of Polyester Fabrics through Polyurethane/Gelatin Complex Coating for Potential Vascular Application

Medical apparatus and instruments, such as vascular grafts, are first exposed to blood when they are implanted. Therefore, blood compatibility is considered to be the critical issue when constructing a vascular graft. In this regard, the coating method is verified to be an effective and simple approach to improve the blood compatibility as well as prevent the grafts from blood leakage. In this study, polyester fabric is chosen as the substrate to provide excellent mechanical properties while a coating layer of polyurethane is introduced to prevent the blood leakage. Furthermore, gelatin is coated on the substrate to mimic the native extracellular matrix together with the improvement of biocompatibility. XPS and FTIR analysis are performed for elemental and group analysis to determine the successful coating of polyurethane and gelatin on the polyester fabrics. In terms of blood compatibility, hemolysis and platelet adhesion are measured to investigate the anticoagulation performance. In vitro cell experiments also indicate that endothelial cells show good proliferation and morphology on the polyester fabric modified with such coating layers. Taken together, such polyester fabric coated with polyurethane and gelatin layers would have a promising potential in constructing vascular grafts with expected blood compatibility and biocompatibility without destroying the basic mechanical requirements for vascular applications.

The effect of surface morphology on endothelial and smooth muscle cells growth on blow-spun fibrous scaffolds

This study aimed to analyze the growth of two types of blood vessel building cells: endothelial cells (ECs) and smooth muscle cells (SMCs) on surfaces with different morphology. Two types of materials, differing in morphology, were produced by the solution blow spinning technique. One-layer materials consisted of one fibrous layer with two fibrous surfaces. Bi-layer materials consisted of one fibrous-solid layer and one fibrous layer, resulting in two different surfaces. Additionally, materials with different average fiber diameters (about 200, 500, and 900 nm) were produced for each group. It has been shown that it is possible to obtain structures with a given morphology by changing the selected process parameters (working distance and polymer solution concentration). Both morphology (solid versus fibrous) and average fiber diameter (submicron fibers versus microfibers) of scaffolds influenced the growth of ECs. However, this effect was only visible after an extended period of culture (6 days). In the case of SMCs, it was proved that the best growth of SMCs is obtained for micron fibers (with an average diameter close to 900 nm) compared to the submicron fibers (with an average diameter below 900 nm).

In Vitro and Ex Vivo Analysis of Collagen Foams for Soft and Hard Tissue Regeneration

BACKGROUND/AIM

The aim of this study was the conception, production, material analysis and cytocompatibility analysis of a new collagen foam for medical applications.

MATERIALS AND METHODS

After the innovative production of various collagen sponges from bovine sources, the foams were analyzed ex vivo in terms of their structure (including pore size) and in vitro in terms of cytocompatibility according to EN ISO 10993-5/-12. In vitro, the collagen foams were compared with the established soft and hard tissue materials cerabone and Jason membrane (both botiss biomaterials GmbH, Zossen, Germany).

RESULTS

Collagen foams with different compositions were successfully produced from bovine sources. Ex vivo, the foams showed a stable and long-lasting primary structure quality with a bubble area of 1,000 to 2,000 μm² In vitro, all foams showed sufficient cytocompatibility.

CONCLUSION

Collagen sponges represent a promising material for hard and soft tissue regeneration. Future studies could focus on integrating and investigating different additives in the foams.

Fabrication of Highly Oriented Cylindrical Polyacrylonitrile, Poly(lactide-co-glycolide), Polycaprolactone and Poly(vinyl acetate) Nanofibers for Vascular Graft Applications

Small-diameter vascular grafts fabricated from synthetic polymers have found limited applications so far in vascular surgeries, owing to their poor mechanical properties. In this study, cylindrical nanofibrous structures of highly oriented nanofibers made from polyacrylonitrile, poly (lactide-co-glycolide) (PLGA), polycaprolactone (PCL) and poly(vinyl acetate) (PVAc) were investigated. Cylindrical collectors with alternate conductive and non-conductive segments were used to obtain highly oriented nanofibrous structures at the same time with better mechanical properties. The surface morphology (orientation), mechanical properties and suture retention of the nanofibrous structures were characterized using SEM, mechanical tester and universal testing machine, respectively. The PLGA nanofibrous cylindrical structure exhibited excellent properties (tensile strength of 9.1 ± 0.6 MPa, suture retention strength of 27N and burst pressure of 350 ± 50 mmHg) when compared to other polymers. Moreover, the PLGA grafts showed good porosity and elongation values, that could be potentially used for vascular graft applications. The combination of PLGA nanofibers with extracellular vesicles (EVs) will be explored as a potential vascular graft in future.

Challenges and strategies for in situ endothelialization and long-term lumen patency of vascular grafts

Vascular diseases are the most prevalent cause of ischemic necrosis of tissue and organ, which even result in dysfunction and death. Vascular regeneration or artificial vascular graft, as the conventional treatment modality, has received keen attentions. However, small-diameter (diameter < 4 mm) vascular grafts have a high risk of thrombosis and intimal hyperplasia (IH), which makes long-term lumen patency challengeable. Endothelial cells (ECs) form the inner endothelium layer, and are crucial for anti-coagulation and thrombogenesis. Thus, promoting in situ endothelialization in vascular graft remodeling takes top priority, which requires recruitment of endothelia progenitor cells (EPCs), migration, adhesion, proliferation and activation of EPCs and ECs. Chemotaxis aimed at ligands on EPC surface can be utilized for EPC homing, while nanofibrous structure, biocompatible surface and cell-capturing molecules on graft surface can be applied for cell adhesion. Moreover, cell orientation can be regulated by topography of scaffold, and cell bioactivity can be modulated by growth factors and therapeutic genes. Additionally, surface modification can also reduce thrombogenesis, and some drug release can inhibit IH. Considering the influence of macrophages on ECs and smooth muscle cells (SMCs), scaffolds loaded with drugs that can promote M2 polarization are alternative strategies. In conclusion, the advanced strategies for enhanced long-term lumen patency of vascular grafts are summarized in this review. Strategies for recruitment of EPCs, adhesion, proliferation and activation of EPCs and ECs, anti-thrombogenesis, anti-IH, and immunomodulation are discussed. Ideal vascular grafts with appropriate surface modification, loading and fabrication strategies are required in further studies.

Challenges and Possibilities of Cell-Based Tissue-Engineered Vascular Grafts

There is urgent demand for biologically compatible vascular grafts for both adult and pediatric patients. The utility of conventional nonbiodegradable materials is limited because of their thrombogenicity and inability to grow, while autologous vascular grafts involve considerable disadvantages, including the invasive procedures required to obtain these healthy vessels from patients and insufficient availability in patients with systemic atherosclerosis. All of these issues could be overcome by tissue-engineered vascular grafts (TEVGs). A large body of evidence has recently emerged in support of TEVG technologies, introducing diverse cell sources (e.g., somatic cells and stem cells) and novel fabrication methods (e.g., scaffold-guided and self-assembled approaches). Before TEVG can be applied in a clinical setting, however, several aspects of the technology must be improved, such as the feasibility of obtaining cells, their biocompatibility and mechanical properties, and the time needed for fabrication, while the safety of supplemented materials, the patency and nonthrombogenicity of TEVGs, their growth potential, and the long-term influence of implanted TEVGs in the body must be assessed. Although recent advances in TEVG fabrication have yielded promising results, more research is needed to achieve the most feasible methods for generating optimal TEVGs. This article reviews multiple aspects of TEVG fabrication, including mechanical requirements, extracellular matrix components, cell sources, and tissue engineering approaches. The potential of periodic hydrostatic pressurization in the production of scaffold-free TEVGs with optimal elasticity and stiffness is also discussed. In the future, the integration of multiple technologies is expected to enable improved TEVG performance.

Evaluation of an electrochemically aligned collagen yarn for textile scaffold fabrication

Collagen is the major component of the extracellular matrix in human tissues and widely used in the fabrication of tissue engineered scaffolds for medical applications. However, these forms of collagen gels and films have limitations due to their inferior strength and mechanical performance and their relatively fast rate of degradation. A new form of continuous collagen yarn has recently been developed for potential usage in fabricating textile tissue engineering scaffolds. In this study, we prepared the continuous electrochemical aligned collagen yarns from acid-soluble collagen that was extracted from rat tail tendons (RTTs) using 0.25 M acetic acid. Sodium dodecyl sulfate polyacrylamide gel electrophoresis and Fourier transform infrared spectroscopy confirmed that the major component of the extracted collagen contained alpha 1 and alpha 2 chains and the triple helix structure of Type 1 collagen. The collagen solution was processed to monofilament yarns in continuous lengths by using a rotating electrode electrochemical compaction device. Exposing the non-crosslinked collagen yarns and the collagen yarns crosslinked with 1-ethyl-3-(-3-dimethyl-aminopropyl) carbodiimide hydrochloride to normal physiological hydrolytic degradation conditions showed that both yarns were able to maintain their tensile strength during the first 6 weeks of the study. Cardiosphere-derived cells showed significantly enhanced attachment and proliferation on the collagen yarns compared to synthetic polylactic acid filaments. Moreover, the cells were fully spread and covered the surface of the collagen yarns, which confirmed the superiority of collagen in terms of promoting cellular adhesion. The results of this work indicated that the aligned RTT collagen yarns are favorable for fabricating biotextile scaffolds and are encouraging for further studies of various textile structure for different tissue engineering applications.

Electrospun tubular vascular grafts to replace damaged peripheral arteries: A preliminary formulation study

Polymeric tubular vascular grafts represent a likely alternative to autologous vascular grafts for treating peripheral artery occlusive disease. This preliminary research study applied cutting-edge electrospinning technique for manufacturing prototypes with diameter ≤ 6 mm and based on biocompatible and biodegradable polymers such as polylactide-polycaprolactone, polylactide-co-glycolide and polyhydroxyethylmethacrylate combined in different design approaches (layering and blending). Samples were characterized about fiber morphology, diameter, size distribution, porosity, fluid uptake capability, and mechanical properties. Biocompatibility and cell interaction were evaluated by in vitro test. Goal of this preliminary study was to discriminate among the prototypes and select which composition and design approach could better suit tissue regeneration purposes. Results showed that electrospinning technique is suitable to obtain grafts with a diameter < 6 mm and thickness between 140 ± 7-175 ± 4 μm. Scanning electron microscopy analysis showed fibers with suitable micrometric diameters and pore size between 5 and 35 μm. polyhydroxyethylmethacrylate provided high hydrophilicity (≃ 100°) and optimal cell short term proliferation (cell viability ≃ 160%) in accordance with maximum fluid uptake ability (300-350%). Moreover, addition of polyhydroxyethylmethacrylate lowered suture retention strength at value < 1 N. Prototypes obtaining combining polylactide-co-glycolide and polylactide-coglycolide/ polyhydroxyethylmethacrylate with polylactide-polycaprolactone in a bilayered structure showed optimal mechanical behavior resembling native bovine vessel.

Vascular Tissue Engineering: Polymers and Methodologies for Small Caliber Vascular Grafts

Cardiovascular disease is the most common cause of death in the world. In severe cases, replacement or revascularization using vascular grafts are the treatment options. While several synthetic vascular grafts are clinically used with common approval for medium to large-caliber vessels, autologous vascular grafts are the only options clinically approved for small-caliber revascularizations. Autologous grafts have, however, some limitations in quantity and quality, and cause an invasiveness to patients when harvested. Therefore, the development of small-caliber synthetic vascular grafts (<5 mm) has been urged. Since small-caliber synthetic grafts made from the same materials as middle and large-caliber grafts have poor patency rates due to thrombus formation and intimal hyperplasia within the graft, newly innovative methodologies with vascular tissue engineering such as electrospinning, decellularization, lyophilization, and 3D printing, and novel polymers have been developed. This review article represents topics on the methodologies used in the development of scaffold-based vascular grafts and the polymers used in vitro and in vivo.

A hybrid vascular graft harnessing the superior mechanical properties of synthetic fibers and the biological performance of collagen filaments

Tissue-engineered small caliber vascular grafts have attracted much research attention as a viable alternative to traditional vascular grafts with their biocompatibility and potential to achieve complete healing. However, the major challenge is to fabricate a scaffold with both satisfactory mechanical properties and fast endothelialization. In this study, a hybrid tubular vascular tissue engineered scaffold has been circular-knitted using novel electrochemically aligned collagen (ELAC) filaments plied together with traditional poly(lactic acid) (PLA) yarn. The collagen component was able to promote the recruitment and proliferation of endothelial cells by increasing the initial cell adhesion 10-fold and the eventual cell population 3.2 times higher than the PLA scaffold alone. At the same time, the PLA yarn was able to provide sufficient mechanical strength and structural stability, as well as facilitate scaffold fabrication on high speed textile production equipment. The tubular hybrid scaffold exhibited excellent bursting strength (1.89 ± 0.43 MPa) and suture retention strength (10.86 ± 0.49 N), and had comparable compliance (3.98 ± 1.94%/100 mmHg) to that of the coronary artery (3.8 ± 0.3%/100 mmHg) under normotensive pressure. With its excellent mechanical and biological performance, this prototype hybrid scaffold is a promising candidate for the construction of a clinically successful and easily translatable tissue-engineered small caliber vascular graft.

Comparison of small-diameter decellularized scaffolds from the aorta and carotid artery of pigs

AIM

Tissue-specific extracellular matrix promotes tissue regeneration and repair. We aimed to identify the optimal decellularized matrices for tissue-engineered vascular graft (TEVG).

METHODS

Decellularized aorta of fetal pigs (DAFP, n = 6, group A), decellularized aorta of adult pigs (DAAP, n = 6, group B), and decellularized carotid artery of adult pigs (DCAP, n = 6, group C) were prepared. Scaffolds were compared using histology and ultrastructure. Endothelial cell (EC) and myofibroblast (MFB) infiltration assessments were performed in vitro. Cell infiltration was measured in vivo. Biomechanical properties were also determined.

RESULTS

Almost original cells were removed by the acellularization procedure, while the construction of the matrix basically remained. In vitro, monolayer ECs and multi-layer MFBs were formed onto the internal surface of the specimens after 3 weeks. In vivo, cell infiltration in group A significantly increased at the 6th and 8th week when compared with groups B and C (p < 0.01). The infiltrated cells were mainly MFBs and a few CD4+ T-lymphocytes/macrophages in the specimens. Groups A and B showed greater axial compliance than group C (p < 0.01).

CONCLUSION

DAFP was the most suitable for use as a small-caliber vascular graft.

Biodegradable Polymers as the Pivotal Player in the Design of Tissue Engineering Scaffolds

Biodegradable polymers play a pivotal role in in situ tissue engineering. Utilizing various technologies, researchers have been able to fabricate 3D tissue engineering scaffolds using biodegradable polymers. They serve as temporary templates, providing physical and biochemical signals to the cells and determining the successful outcome of tissue remodeling. Furthermore, a biodegradable scaffold also presents the fourth dimension for tissue engineering, namely time. The properties of the biodegradable polymer change over time, presenting continuously changing features during the degradation process. These changes become more complicated when different materials are combined together to fabricate a composite or heterogeneous scaffold. This review undertakes a systematic analysis of the basic characteristics of biodegradable polymers and describe recent advances in making composite biodegradable scaffolds for in situ tissue engineering and regenerative medicine. The interaction between implanted biodegradable biomaterials and the in vivo environment are also discussed, including the properties and functional changes of the degradable scaffold, the local effect of degradation on the contiguous tissue and their evaluation using both in vitro and in vivo models.

Construction and application of textile-based tissue engineering scaffolds: a review

Tissue engineering (TE) provides a practicable method for tissue and organ repair or substitution. As the most important component of TE, a scaffold plays a critical role in providing a growing environment for cell proliferation and functional differentiation as well as good mechanical support. And the restorative effects are greatly dependent upon the nature of the scaffold including the composition, morphology, structure, and mechanical performance. Medical textiles have been widely employed in the clinic for a long time and are being extensively investigated as TE scaffolds. However, unfortunately, the advantages of textile technology cannot be fully exploited in tissue regeneration due to the ignoring of the diversity of fabric structures. Therefore, this review focuses on textile-based scaffolds, emphasizing the significance of the fabric design and the resultant characteristics of cell behavior and extracellular matrix reconstruction. The structure and mechanical behavior of the fabrics constructed by various textile techniques for different tissue repairs are summarized. Furthermore, the prospect of structural design in the TE scaffold preparation was anticipated, including profiled fibers and some unique and complex textile structures. Hopefully, the readers of this review would appreciate the importance of structural design of the scaffold and the usefulness of textile-based TE scaffolds in tissue regeneration.

Electrospun PET/PCL small diameter nanofibrous conduit for biomedical application

In recent years, the mortality rate caused by cardiovascular diseases has increased dramatically around the world. Tissue engineering is considered as a novel and efficient approach to offer a substituent of engineered tissues for defective body tissues. For this purpose, fabrication of the scaffold that resembles the physical and mechanical properties of natural body vessels, and culturing appropriate cells seems to be a promising approach. Due to the fibrous structure of the vascular wall, the nanofibrous scaffold produced by electrospinning could be a proper choice for vascular tissue engineering. One of the main properties of artificial vessels is its mechanical properties consistency with the native one in order to mimic its natural characteristics. To do so, in present study two biocompatible polymers, polyethylene terephthalate (PET) and polycaprolactone (PCL) with different blend ratio were electrospun into a tubular nanofibrous structure with 6 mm internal diameter and the mechanical properties such as tensile strength, modulus, compliance, bursting pressure, elastic recovery, and suture retention were investigated. The results revealed that PET/PCL (1:3) had better similar properties with the reported natural one as its longitudinal and transverse tensile strength was about 9.47 and 6.38 MPa, respectively. The longitudinal strain at break, compliance, bursting pressure, and suture retention were 205.88 ± 51.12%, 4.19 ± 0.78%/100 mmHg, 6378.76 ± 2159.20 mmHg, and 287.73 ± 13.10 gmf, respectively. The elasticity of this studied sample was 60.21 ± 12.49% as it was relieved, and this may be a good candidate for the artificial vessel in this size, as the MTT test confirmed its appropriate substrate for cell culture.