Controlled heparin conjugation on electrospun poly(ε-caprolactone)/gelatin fibers for morphology-dependent protein delivery and enhanced cellular affinity

Improving hemocompatibility in tissue-engineered products employing heparin-loaded nanoplatforms

The enhancement of hemocompatibility through the use of nanoplatforms loaded with heparin represents a highly desirable characteristic in the context of emerging tissue engineering applications. The significance of employing heparin in biological processes is unquestionable, owing to its ability to interact with a diverse range of proteins. It plays a crucial role in numerous biological processes by engaging in interactions with diverse proteins and hydrogels. This review provides a summary of recent endeavors focused on augmenting the hemocompatibility of tissue engineering methods through the utilization of nanoplatforms loaded with heparin. This study also provides a comprehensive review of the various applications of heparin-loaded nanofibers and nanoparticles, as well as the techniques employed for encapsulating heparin within these nanoplatforms. The biological and physical effects resulting from the encapsulation of heparin in nanoplatforms are examined. The potential applications of heparin-based materials in tissue engineering are also discussed, along with future perspectives in this field.

Translational tissue-engineered vascular grafts: From bench to bedside

Cardiovascular disease is a primary cause of mortality worldwide, and patients often require bypass surgery that utilizes autologous vessels as conduits. However, the limited availability of suitable vessels and the risk of failure and complications have driven the need for alternative solutions. Tissue-engineered vascular grafts (TEVGs) offer a promising solution to these challenges. TEVGs are artificial vascular grafts made of biomaterials and/or vascular cells that can mimic the structure and function of natural blood vessels. The ideal TEVG should possess biocompatibility, biomechanical mechanical properties, and durability for long-term success in vivo. Achieving these characteristics requires a multi-disciplinary approach involving material science, engineering, biology, and clinical translation. Recent advancements in scaffold fabrication have led to the development of TEVGs with improved functional and biomechanical properties. Innovative techniques such as electrospinning, 3D bioprinting, and multi-part microfluidic channel systems have allowed the creation of intricate and customized tubular scaffolds. Nevertheless, multiple obstacles must be overcome to apply these innovations effectively in clinical practice, including the need for standardized preclinical models and cost-effective and scalable manufacturing methods. This review highlights the fundamental approaches required to successfully fabricate functional vascular grafts and the necessary translational methodologies to advance their use in clinical practice.

Bioactive Nanostructured Scaffold-Based Approach for Tendon and Ligament Tissue Engineering

An effective therapeutic strategy to treat tendon or ligament injury continues to be a clinical challenge due to the limited natural healing capacity of these tissues. Furthermore, the repaired tendons or ligaments usually possess inferior mechanical properties and impaired functions. Tissue engineering can restore the physiological functions of tissues using biomaterials, cells, and suitable biochemical signals. It has produced encouraging clinical outcomes, forming tendon or ligament-like tissues with similar compositional, structural, and functional attributes to the native tissues. This paper starts by reviewing tendon/ligament structure and healing mechanisms, followed by describing the bioactive nanostructured scaffolds used in tendon and ligament tissue engineering, with emphasis on electrospun fibrous scaffolds. The natural and synthetic polymers for scaffold preparation, as well as the biological and physical cues offered by incorporating growth factors in the scaffolds or by dynamic cyclic stretching of the scaffolds, are also covered. It is expected to present a comprehensive clinical, biological, and biomaterial insight into advanced tissue engineering-based therapeutics for tendon and ligament repair.

Development of high resilience spiral wound suture-embedded gelatin/PCL/heparin nanofiber membrane scaffolds for tendon tissue engineering

This study develops a spiral wound scaffold based on gelatin/PCL/heparin (GPH) nanofiber membranes for tendon tissue engineering. By embedding sutures in dual layers of aligned GPH nanofiber membranes, prepared from mixed electrospinning of gelatin and PCL/heparin solutions, we fabricate a high resilience scaffold intended for the high loading environment experienced by tendons. The basic fibroblast growth factor (bFGF) was anchored to GPH scaffold through bioaffinity between heparin and bFGF, aim to provide biological cues for maintenance of tenogenic phenotype. In addition, the aligned nanofiber morphology is expected to provide physical cues toward seeded tenocytes. With sustained release of bFGF, GPH-bFGF can enhance proliferation, up-regulate tenogenic gene expression, and increase synthesis of tendon-specific proteins by tenocytes in vitro. Furthermore, by properly maintaining tendon phenotypes, GPH-bFGF/tenocytes constructs showed improved mechanical properties over GPH-bFGF. From in vivo study using GPH-bFGF/tenocytes constructs to repair rabbit Achilles tendon defects, neotendon tissue formation was confirmed from histological staining and biomechanical analysis. These findings collectively demonstrate that the newly designed GPH-bFGF scaffold could provide a niche for inducing tendon tissue regeneration by effectively restoring the tendon tissue structure and function.

Biomedical applications of carbohydrate-based polyurethane: From biosynthesis to degradation

The foremost common natural polymers are carbohydrate-based polymers or polysaccharides, having a long chain of monosaccharide or disaccharide units linked together via a glycosidic linkage to form a complex structure. There are several uses of carbohydrate-based polymers in biomedical sector due to its attractive features including less toxicity, biocompatibility, biodegradability, high reactivity, availability, and relatively inexpensive. The aim of our study was to explore the synthetic approaches for the preparation of numerous carbohydrate-based polyurethanes (PUs) and their wide range of pharmaceutical and biomedical applications. The data summarized in this study shows that the addition of carbohydrates in the structural skeleton of PUs not only improve their suitability but also effect the applicability for employing them in biological applications. Carbohydrate-based units are incorporated into the PUs, which is the most convenient method for the synthesis of novel biocompatible and biodegradable carbohydrate-based PUs to use in various biomedical applications.

Collagen fibrous scaffolds for sustained delivery of growth factors for meniscal tissue engineering

Aim: To mimic the ultrastructural morphology of the meniscus with nanofiber scaffolds coupled with controlled growth factor delivery to modulate cellular performance for tissue engineering of menisci. Methods: The authors functionalized collagen nanofibers by conjugating heparin to the following growth factors for sustained release: PDGF-BB, TGF-β1 and CTGF. Results: Incorporating growth factors increased human meniscal and synovial cell viability, proliferation and infiltration in vitro, ex vivo and in vivo; upregulated key genes involved in meniscal extracellular matrix synthesis and enhanced generation of meniscus-like tissue. Conclusion: The authors' results indicate that functionalizing collagen nanofibers can create a cell-favorable micro- and nanoenvironment and can serve as a system for sustained release of bioactive factors.

Syndecan-4 functionalization of tissue regeneration scaffolds improves interaction with endothelial progenitor cells

Key to most implanted cell free scaffolds for tissue regeneration is the ability to sequester and retain undifferentiated mesenchymal stem cells at the repair site. In this report, syndecan-4, a heparan sulfate containing proteoglycan, was investigated as a unique molecule for use in scaffold functionalization. An electrospun hybrid scaffold comprised of poly (glycerol) sebacate (PGS), silk fibroin and type I collagen (PFC) was used as a model scaffold to develop a procedure and test the hypothesis that functionalization would result in increased scaffold binding of endothelial progenitor cells (EPCs). For these studies both Syndecan-4 and stromal derived factor-1α (SDF-1α) were used in functionalization PFC. Syndecan-4 functionalized PFC bound 4.8 fold more SDF-1α compared to nonfunctionalized PFC. Binding was specific as determined by heparin displacement studies. After culture for 7 days, significantly, more EPCs were detected on PFC scaffolds having both syndecan-4 and SDF-1α compared to scaffolds of PFC with only syndecan-4, or PFC adsorbed with SDF-1α, or PFC alone. Taken together, this study demonstrates that EPCs can be bound to and significantly expanded on PFC material through syndecan-4 mediated growth factor binding. Syndecan-4 with a multiplicity of binding sites has the potential to functionalize and expand stem cells on a variety of scaffold materials for use in tissue regeneration.

Blood biocompatibility enhancement of biomaterials by heparin immobilization: a review

Blood contacting materials are concerned with biocompatibility including thrombus formation, decrease blood coagulation time, hematology, activation of complement system, platelet aggression. Interestingly, recent research suggests that biocompatibility is increasing by incorporating various materials including heparin using different methods. Basic of heparin including uses and complications was mentioned, in which burst release of heparin is major issue. To minimize the problem of biocompatibility and unpredictable heparin release, present review article potentially reviews the reported work and investigates the various immobilization methods of heparin onto biomaterials, such as polymers, metals, and alloys. Detailed explanation of different immobilization methods through different intermediates, activation, incubation method, plasma treatment, irradiations and other methods are also discussed, in which immobilization through intermediates is the most exploitable method. In addition to biocompatibility, other required properties of biomaterials like mechanical and corrosion resistance properties that increase by attachment of heparin are reviewed and discussed in this article.

Sulfated carboxymethylcellulose conjugated electrospun fibers as a growth factor presenting system for tissue engineering

Inspired by the natural electrostatic interaction of cationic growth factors with anionic sulfated glycosaminoglycans in the extracellular matrix, we developed electrospun poly(hydroxybutyrate)/gelatin (PG) fibers conjugated with anionic sulfated carboxymethylcellulose (sCMC) to enable growth factor immobilization via electrostatic interaction for tissue engineering. The fibrous scaffold bound cationic molecules, was cytocompatible and exhibited a remarkable morphological and functional stability. Transforming growth factor-β1 immobilized on the sCMC conjugated fibers was retained for at least 4 weeks with negligible release (3%). Immobilized fibroblast growth factor-2 and connective tissue growth factor were bioactive and induced proliferation and fibrogenic differentiation of infrapatellar fat pad derived mesenchymal stem cells respectively with efficiency similar to or better than free growth factors. Taken together, our studies demonstrate that sCMC conjugated PG fibers can immobilize and retain function of cationic growth factors and hence show potential for use in various tissue engineering applications.

Development of Novel Amphotericin B-Immobilized Nitric Oxide-Releasing Platform for the Prevention of Broad-Spectrum Infections and Thrombosis

Indwelling medical devices currently used to diagnose, monitor, and treat patients invariably suffer from two common clinical complications: broad-spectrum infections and device-induced thrombosis. Currently, infections are managed through antibiotic or antifungal treatment, but the emergence of antibiotic resistance, the formation of recalcitrant biofilms, and difficulty identifying culprit pathogens have made treatment increasingly challenging. Additionally, systemic anticoagulation has been used to manage device-induced thrombosis, but subsequent life-threatening bleeding events associated with all available therapies necessitates alternative solutions. In this study, a broad-spectrum antimicrobial, antithrombotic surface combining the incorporation of the nitric oxide (NO) donor S-nitroso-N-acetylpenicillamine (SNAP) with the immobilization of the antifungal Amphotericin B (AmB) on polydimethylsiloxane (PDMS) was developed in a two-step process. This novel strategy combines the key advantages of NO, a bactericidal agent and platelet inhibitor, with AmB, a potent antifungal agent. We demonstrated that SNAP-AmB surfaces significantly reduced the viability of adhered Staphylococcus aureus (99.0 ± 0.2%), Escherichia coli (89.7 ± 1.0%), and Candida albicans (93.5 ± 4.2%) compared to controls after 24 h of in vitro exposure. Moreover, SNAP-AmB surfaces reduced the number of platelets adhered by 74.6 ± 3.9% compared to controls after 2 h of in vitro porcine plasma exposure. Finally, a cytotoxicity assay validated that the materials did not present any cytotoxic side effects toward human fibroblast cells. This novel approach is the first to combine antifungal surface functionalization with NO-releasing technology, providing a promising step toward reducing the rate of broad-spectrum infection and thrombosis associated with indwelling medical devices.

Accelerating neovascularization and kidney tissue formation with a 3D vascular scaffold capturing native vascular structure

Establishing an adequate vascularization of three-dimensional (3D) bioengineered tissues remains a critical challenge. We previously fabricated a vascular scaffold using the vascular corrosion casting technique, which provides a similar 3D geometry of native kidney vasculature. In this study, we functionalized the collagen vascular scaffold with a controlled release of vascular endothelial growth factor (VEGF vascular scaffold) to further promote vascularization. The VEGF vascular scaffold showed improved angiogenic capability in 2-dimensional (2D) and 3D in vitro settings. Implantation of the VEGF vascular scaffold seeded with human renal cells into a rat kidney demonstrated enhanced implant vascularization and reduced apoptosis of implanted human renal cells. Hybrid renal tubule-like structures composed of implanted human and migrated host renal cells were formed. This work highlights the critical role of early vascularization of the geometrically mimetic vascular scaffold using the VEGF incorporated vascular scaffold in reducing apoptosis of implanted cells as well as the formation of renal tissue structures.

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.

Meniscal tissue repair with nanofibers: future perspectives

The knee menisci are critical to the long-term health of the knee joint. Because of the high incidence of injury and degeneration, replacing damaged or lost meniscal tissue is extremely clinically relevant. The multiscale architecture of the meniscus results in unique biomechanical properties. Nanofibrous scaffolds are extremely attractive to replicate the biochemical composition and ultrastructural features in engineered meniscus tissue. We review recent advances in electrospinning to generate nanofibrous scaffolds and the current state-of-the-art of electrospun materials for meniscal regeneration. We discuss the importance of cellular function for meniscal tissue engineering and the application of cells derived from multiple sources. We compare experimental models necessary for proof of concept and to support translation. Finally, we discuss future directions and potential for technological innovations.

Antibody-Conjugated Electrospun Vascular Scaffolds to Enhance In Situ Endothelialization

Tissue-engineered vascular grafts (TEVGs) are promising alternatives to small-diameter prosthetic grafts. Previous methods of seeding tubular scaffolds with autologous vascular cells have been successful; however, these methods require significant preparation time. Endothelial cell (EC) growth on the luminal surface of vascular scaffolds may be critical for the integration of a TEVG to the host environment. An alternative approach for TEVGs includes the in situ endothelialization of acellular scaffolds by capturing circulating endothelial progenitor cells (EPCs) and ECs from the bloodstream through the biofunctionalization of the vascular scaffolds. In this study, fibrous scaffolds were electrospun with a 1:1 poly(ε-caprolactone) (PCL)/collagen blend solution. The electrospun fibrous scaffolds were surface-modified by immobilizing EC-specific antibodies: CD31, vascular endothelial cadherin (VE-CAD), vascular endothelial growth factor receptor 2 (VEGFR2), and von Willebrand factor (vWF). Antibodies most efficacious at capturing ECs were then paired to examine their potential synergistic cell-capturing capabilities. The study demonstrated that vascular scaffolds bioconjugated with dual antibodies demonstrated synergistic capture efficacy compared to bioconjugation with a single antibody. The capture of circulating EPCs and ECs can be optimized with bioconjugation of one or more antibodies on the luminal surface of TEVGs.

Mimicking the Endothelium: Dual Action Heparinized Nitric Oxide Releasing Surface

The management of thrombosis and bacterial infection is critical to ensure the functionality of medical devices. While administration of anticoagulants is the current antithrombotic clinical practice, a variety of complications, such as uncontrolled hemorrhages or heparin-induced thrombocytopenia, can occur. Additionally, infection rates remain a costly and deadly complication associated with use of these medical devices. It has been hypothesized that if a synthetic surface could mimic the biochemical mechanisms of the endothelium of blood vessels, thrombosis could be reduced, anticoagulant use could be avoided, and infection could be prevented. Herein, the interfacial biochemical effects of the endothelium were mimicked by altering the surface of medical grade silicone rubber (SR). Surface modification was accomplished via heparin surface immobilization (Hep) and the inclusion of a nitric oxide (NO) donor into the SR polymeric matrix to achieve synergistic effects (Hep-NO-SR). An in vitro bacteria adhesion study revealed that Hep-NO-SR exhibited a 99.46 ± 0.17% reduction in viable bacteria adhesion compared to SR. An in vitro platelet study revealed Hep-NO-SR reduced platelet adhesion by 84.12 ± 6.19% compared to SR, while not generating a cytotoxic response against fibroblast cells. In a 4 h extracorporeal circuit model without systemic anticoagulation, all Hep-NO-SR samples were able to maintain baseline platelet count and device patency; whereas 66% of SR samples clotted within the first 2 h of study. Results indicate that Hep-NO-SR creates a more hemocompatible and antibacterial surface by mimicking two key biochemical functions of the native endothelium.

Preparation and characterization of functionalized heparin-loaded poly-Ɛ-caprolactone fibrous mats to prevent infection with human papillomaviruses

In this study, heparin-loaded poly-ɛ-caprolactone (PCL) fibrous mats were prepared and characterized based on their physical, cytotoxic, thermal, and biological properties. The main objective of the work described here was to test the hypothesis that incorporation of heparin into a PCL carrier could serve as bio-compatible material capable of inhibiting Human Papillomavirus (HPV) infection. The idea of firmly anchoring heparin to capture soluble virus, vs. a slow heparin release to inhibit a virus in solution was tested. Thus, one material was produced via conventional heparin matrix encapsulation and electrohydrodynamic fiber processing in one step. A second type of material was obtained via heparin crosslinking. This was achieved by running a carbodiimide/N-hydroxysuccinimide (EDC/NHS) coupling reaction on preformed PCL fibers. In vitro HPV16 L1 protein binding capacity studies were performed. Infectivity assays were done using HPV16 pseudoviruses (PsVs) carrying a GFP plasmid to directly test the ability of the fibrous mats to prevent internalization of HPV PsVs. The crosslinked heparin material presented a dissociation constant (K_d) value comparable to those found in the literature for different heparin-protein L1 peptide interactions. Both materials significantly reduced internalization of HPV PsVs, with a reduction of 94% of PsVs internalization when matrix encapsulated heparin-loaded material was present. Differences in performance between the two proposed structures are discussed.

Hemocompatible and Bioactive Heparin-Loaded PCL-α-TCP Fibrous Membranes for Bone Tissue Engineering

The combination of bioactive components such as calcium phosphates and fibrous structures are encouraging niche-mimetic keys for restoring bone defects. However, the importance of hemocompatibility of the membranes is widely ignored. Heparin-loaded nanocomposite poly(ε-caprolactone) (PCL)-α-tricalcium phosphate (α-TCP) fibrous membranes are developed to provide bioactive and hemocompatible constructs for bone tissue engineering. Nanocomposite membranes are optimized based on bioactivity, mechanical properties, and cell interaction. Consequently, various concentrations of heparin molecules are loaded within nanocomposite fibrous membranes. In vitro heparin release profiles reveal a sustained release of heparin over the period of 14 days without an initial burst. Moreover, heparin encapsulation enhances mesenchymal stem cell (MSC) attachment and proliferation, depending on the heparin content. It is concluded that the incorporation of heparin within TCP-PCL fibrous membranes provides the most effective cellular interactions through synergistic physical and chemical cues.

Heparin-Eluting Electrospun Nanofiber Yarns for Antithrombotic Vascular Sutures

The surgical connection of blood vessels, anastomosis, is a critical procedure in many reparative, transplantation, and reconstructive surgical procedures. However, effective restoration of circulation is complicated by pathological clotting (thrombosis) or progressive occlusion due to excess cell proliferation that often leads to additional surgeries and increases morbidity and mortality risk for patients. Pharmaceutical agents have been tested to prevent these complications, but many have unacceptable systemic side effects. Therefore, an alternative approach to deliver these drugs at the site of injury in a controlled manner is necessary. The objective of this study was to develop electrospun nanofibers composed of polyester poly(lactide- co-glycolide) (PLGA), poly(ethylene oxide) (PEO), and positively charged copolymer, poly(lactide- co-glycolide)- graft-polyethylenimine (PgP) for electrostatic binding and release of heparin for application as an antithrombotic microvascular suture. PgP was synthesized with different coupling ratios between PLGA and branched polyethylenimine (bPEI) to obtain PgP₁ (∼1 PLGA grafted to 1 bPEI) and PgP_3.7 (∼3.7 PLGA grafted to 1 bPEI). Nanofiber yarns (PLGA/PEO/PgP₁ and PLGA/PEO/PgP_3.7) were fabricated by electrospinning. Heparin immobilization on the positively charged nanofiber yarns was visualized using fluorescein-conjugated heparin (F-Hep), and the amount of immobilized F-Hep was higher on both PLGA/PEO/PgP_3.7 and PLGA/PEO/PgP₁ than yarns without PgP (PLGA/PEO). We also found that F-Hep was released from both PgP-containing yarns in a sustained manner over 20 days, while over 60% of F-Hep was released within 4 h from PLGA/PEO. Finally, we observed that heparin-eluting nanofiber yarns with both PgP₁ and PgP_3.7 showed significantly longer clotting times than nanofiber yarns without PgP. The clotting time of PLGA/PEO/PgP_3.7 was not significantly different than that of free heparin (0.5 μg/mL). These results show that heparin-eluting electrospun nanofiber yarns may offer a basis for the development of microvascular sutures with anticoagulant activity.

A compliant and biomimetic three-layered vascular graft for small blood vessels

Engineering a small diameter vascular graft with mechanical and biological properties comparable to living tissues remains challenging. Often, current devices lead to thrombosis and unsatisfactory long-term patency as a result of poor blood compatibility and a mismatch between the mechanical properties of the living tissue and the implanted biomaterial. Addressing all these requirements is essential to produce scaffolds able to survive throughout the life of the patient. For this purpose, we fabricated a novel three-layered vascular graft by combining electrospinning and braiding. Mirroring the structure of human blood vessels, the proposed device is composed of three layers: the intima, the media, and the adventitia. The intima and media layers were obtained by sequentially electrospinning silk fibroin (SF) and poly(L-lactide-co-ε-caprolactone), with ratios selected to match the mechanical properties of the native tissue. For the outer layer, the adventitia, SF yarns were braided on top of the electrospun tubes to create a structure able to withstand high pressures. Compliance, Young's modulus and deformability of the obtained scaffold were similar to that of human blood vessels. Additionally, cytocompatibility of the two layers, media and intima, was assessed in vitro by analysing cell metabolic activity and proliferation of endothelial cells and smooth muscle cells, respectively. Furthermore, heparin functionalization of the scaffolds led to improved anticoagulant properties upon incubation in whole blood. The obtained results indicate a potential application of the herewith designed three-layered construct as a vascular graft for small diameter blood vessel engineering.

Development and characterization of heparin-immobilized polycaprolactone nanofibrous scaffolds for tissue engineering using gamma-irradiation

Polycaprolactone (PCL) has been considered a useful material for orthopedic devices and osseous implants because of its biocompatibility and bone-forming activity.

Progress and perspectives in bioactive agent delivery via electrospun vascular grafts

The review discusses the progress in the design and synthesis of bioactive agents incorporated into vascular grafts obtained by the electrospinning process.

Electrospun polycaprolactone/gelatin composites with enhanced cell-matrix interactions as blood vessel endothelial layer scaffolds

During the fabrication of tissue engineering scaffolds and subsequent tissue regeneration, surface bioactivity is vital for cell adhesion, spreading, and proliferation, especially for endothelium dysfunction repair. In this paper, synthetic polymer polycaprolactone (PCL) was blended with natural polymer gelatin at four different weight ratios followed by crosslinking (i.e., 100:0, 70:30, 50:50, 30:70, labeled as PCL-C, P7G3-C, P5G5-C, and P3G7-C) to impart enhanced bioactivity and tunable mechanical properties. The PCL/gelatin blends were first dissolved in 2,2,2-trifluroethanol (TFE) and supplementary acetic acid (1% relative to TFE) solvent, electrospun, and then cross-linked to produce PBS-proof fibrous scaffolds. Scanning electron micrographs (SEM) indicated that fibers of each sample were smooth and homogeneous, with the fiber diameters increasing from 1.01±0.51μm to 1.61±0.46μm as the content of gelatin increased. While thermal resistance and crystallization of the blends were affected by the presence of gelatin, as reflected by differential scanning calorimetry (DSC) results, water contact angle (WCA) tests confirmed that the scaffold surfaces became more hydrophilic. Tensile tests showed that PCL-C and P7G3-C scaffolds had mechanical properties comparable to those of human coronary arteries. As for cytocompatibility, skeleton staining images showed that human mesenchymal stem cells (hMSCs) had more favorable binding sites on PCL/gelatin scaffolds than those on PCL scaffolds. Cell proliferation assays revealed that P7G3-C scaffolds could support the most number of hMSCs. The results of this study demonstrated the enhanced cell-matrix interactions and potential use of electrospun PCL/gelatin scaffolds in the tissue engineering field, especially in wound dressings and endothelium regeneration.

A novel tissue-engineered trachea with a mechanical behavior similar to native trachea

A novel tissue-engineered trachea was developed with appropriate mechanical behavior and substantial regeneration of tracheal cartilage. We designed hollow bellows scaffold as a framework of a tissue-engineered trachea and demonstrated a reliable method for three-dimensional (3D) printing of monolithic bellows scaffold. We also functionalized gelatin sponge to allow sustained release of TGF-β1 for stimulating tracheal cartilage regeneration and confirmed that functionalized gelatin sponge induces cartilaginous tissue formation in vitro. A tissue-engineered trachea was then created by assembling chondrocytes-seeded functionalized gelatin sponges into the grooves of bellows scaffold and it showed very similar mechanical behavior to that of native trachea along with substantial regeneration of tracheal cartilage in vivo. The tissue-engineered trachea developed here represents a novel concept of tracheal substitute with appropriate mechanical behavior similar to native trachea for use in reconstruction of tracheal stenosis.

Naturally derived and synthetic scaffolds for skeletal muscle reconstruction

Skeletal muscle tissue has an inherent capacity for regeneration following injury. However, severe trauma, such as volumetric muscle loss, overwhelms these natural muscle repair mechanisms prompting the search for a tissue engineering/regenerative medicine approach to promote functional skeletal muscle restoration. A desirable approach involves a bioscaffold that simultaneously acts as an inductive microenvironment and as a cell/drug delivery vehicle to encourage muscle ingrowth. Both biologically active, naturally derived materials (such as extracellular matrix) and carefully engineered synthetic polymers have been developed to provide such a muscle regenerative environment. Next generation naturally derived/synthetic "hybrid materials" would combine the advantageous properties of these materials to create an optimal platform for cell/drug delivery and possess inherent bioactive properties. Advances in scaffolds using muscle tissue engineering are reviewed herein.

Enhanced vascularization in hybrid PCL/gelatin fibrous scaffolds with sustained release of VEGF

Creating a long-lasting and functional vasculature represents one of the most fundamental challenges in tissue engineering. VEGF has been widely accepted as a potent angiogenic factor involved in the early stages of blood vessel formation. In this study, fibrous scaffolds that consist of PCL and gelatin fibers were fabricated. The gelatin fibers were further functionalized by heparin immobilization, which provides binding sites for VEGF and thus enables the sustained release of VEGF. In vitro release test confirms the sustained releasing profile of VEGF, and stable release was observed over a time period of 25 days. In vitro cell assay indicates that VEGF release significantly promoted the proliferation of endothelial cells. More importantly, in vivo subcutaneous implantation reflects that vascularization has been effectively enhanced in the PCL/gelatin scaffolds compared with the PCL counterpart due to the sustained release of VEGF. Therefore, the heparinized PCL/gelatin scaffolds developed in this study may be a promising candidate for regeneration of complex tissues with sufficient vascularization.

Quantitative imaging of electrospun fibers by PeakForce Quantitative NanoMechanics atomic force microscopy using etched scanning probes

Electrospun polymeric submicron and nanofibers can be used as tissue engineering scaffolds in regenerative medicine. In physiological conditions fibers are subjected to stresses and strains from the surrounding biological environment. Such stresses can cause permanent deformation or even failure to their structure. Therefore, there is a growing necessity to characterize their mechanical properties, especially at the nanoscale. Atomic force microscopy is a powerful tool for the visualization and probing of selected mechanical properties of materials in biomedical sciences. Image resolution of atomic force microscopy techniques depends on the equipment quality and shape of the scanning probe. The probe radius and aspect ratio has huge impact on the quality of measurement. In the presented work the nanomechanical properties of four different polymer based electrospun fibers were tested using PeakForce Quantitative NanoMechanics atomic force microscopy, with standard and modified scanning probes. Standard, commercially available probes have been modified by etching using focused ion beam (FIB). Results have shown that modified probes can be used for mechanical properties mapping of biomaterial in the nanoscale, and generate nanomechanical information where conventional tips fail.

Immobilization of heparin/poly-(L)-lysine nanoparticles on dopamine-coated surface to create a heparin density gradient for selective direction of platelet and vascular cells behavior

Restenosis, thrombosis formation and delayed endothelium regeneration continue to be problematic for coronary artery stent therapy. To improve the hemocompatibility of the cardiovascular implants and selectively direct vascular cell behavior, a novel kind of heparin/poly-l-lysine (Hep/PLL) nanoparticle was developed and immobilized on a dopamine-coated surface. The stability and structural characteristics of the nanoparticles changed with the Hep:PLL concentration ratio. A Hep density gradient was created on a surface by immobilizing nanoparticles with various Hep:PLL ratios on a dopamine-coated surface. Antithrombin III binding quantity was significantly enhanced, and in plasma the APTT and TT times as coagulation tests were prolonged, depending on the Hep density. A low Hep density is sufficient to prevent platelet adhesion and activation. The sensitivity of vascular cells to the Hep density is very different: high Hep density inhibits the growth of all vascular cells, while low Hep density could selectively inhibit smooth muscle cell hyperplasia but promote endothelial progenitor cells and endothelial cell proliferation. These observations provide important guidance for modification of surface heparinization. We suggest that this method will provide a potential means to construct a suitable platform on a stent surface for selective direction of vascular cell behavior with low side effects.

Nano/microfibrous polymeric constructs loaded with bioactive agents and designed for tissue engineering applications: a review

Nano/microfibrous polymeric constructs present various inherent advantages, such as highly porous architecture and high surface to volume ratio, making them attractive for tissue engineering purposes. Electrospinning is the most preferred technique for the fabrication of polymeric nanofibrous assemblies that can mimic the physical functions of native extracellular matrix greatly favoring cells attachment and thus influencing their morphology and activities. Different approaches have been developed to apply polymeric microfiber fabrication techniques (e.g. wet-spinning) for the obtainment of scaffolds with a three-dimensional network of micropores suitable for effective cells migration. Progress in additive manufacturing technology has led to the development of complex scaffold's shapes and microfibrous structures with a high degree of automation, good accuracy and reproducibility. Various loading methods, such as direct blending, coaxial electrospinning and microparticles incorporation, are enabling to develop customized strategies for the biofunctionalization of nano/microfibrous scaffolds with a tailored kinetics of release of different bioactive agents, ranging from small molecules, such as antibiotics, to protein drugs, such as growth factors, and even cells. Recent activities on the combination of different processing techniques and loading methods for the obtainment of biofunctionalized polymeric constructs with a complex multiscale structure open new possibilities for the development of biomimetic scaffolds endowed with a hierarchical architecture and a sophisticated release kinetics of different bioactive agents. This review is aimed at summarizing current advances in technologies and methods for manufacturing nano/microfibrous polymeric constructs suitable as tissue engineering scaffolds, and for their combination with different bioactive agents to promote tissue regeneration and therapeutic effects.

Polysaccharide-modified scaffolds for controlled lentivirus delivery in vitro and after spinal cord injury

Gene delivering biomaterials have increasingly been employed to modulate the cellular microenvironment to promote tissue regeneration, yet low transduction efficiency has been a persistent challenge for in vivo applications. In this report, we investigated the surface modification of poly(lactide-co-glycolide) (PLG) scaffolds with polysaccharides, which have been implicated in binding lentivirus but have not been used for delivery. Chitosan was directly conjugated onto PLG scaffolds, whereas heparin and hyaluronan were indirectly conjugated onto PLG scaffolds with multi-amine crosslinkers. The addition of chitosan and heparin onto PLG promoted the association of lentivirus to these scaffolds and enhanced their transduction efficiency in vitro relative to hyaluronan-conjugated and control scaffolds that had limited lentivirus association and transduction. Transduction efficiency in vitro was increased partly due to an enhanced retention of virus on the scaffold as well as an extended half-life of viral activity. Transduction efficiency was also evaluated in vivo using porous, multiple channel PLG bridges that delivered lentivirus to the injured mouse spinal cord. Transgene expression persisted for weeks after implantation, and was able to enhance axon growth and myelination. These studies support gene-delivering PLG scaffolds for in vivo regenerative medicine applications.

Protein-hydrogel interactions in tissue engineering: mechanisms and applications

Recent advances in our understanding of the sophistication of the cellular microenvironment and the dynamics of tissue remodeling during development, disease, and regeneration have increased our appreciation of the current challenges facing tissue engineering. As this appreciation advances, we are better equipped to approach problems in the biology and therapeutics of even more complex fields, such as stem cells and cancer. To aid in these studies, as well as the established areas of tissue engineering, including cardiovascular, musculoskeletal, and neural applications, biomaterials scientists have developed an extensive array of materials with specifically designed chemical, mechanical, and biological properties. Herein, we highlight an important topic within this area of biomaterials research, protein-hydrogel interactions. Due to inherent advantages of hydrated scaffolds for soft tissue engineering as well as specialized bioactivity of proteins and peptides, this field is well-posed to tackle major needs within emerging areas of tissue engineering. We provide an overview of the major modes of interactions between hydrogels and proteins (e.g., weak forces, covalent binding, affinity binding), examples of applications within growth factor delivery and three-dimensional scaffolds, and finally future directions within the area of hydrogel-protein interactions that will advance our ability to control the cell-biomaterial interface.

The effect of controlled release of PDGF-BB from heparin-conjugated electrospun PCL/gelatin scaffolds on cellular bioactivity and infiltration

Heparin-conjugated electrospun poly(ε-caprolactone) (PCL)/gelatin scaffolds were developed to provide controlled release of platelet-derived growth factor-BB (PDGF-BB) and allow prolonged bioactivity of this molecule. A mixture of PCL and gelatin was electrospun into three different morphologies. Next, heparin molecules were conjugated to the reactive surface of the scaffolds. This heparin-conjugated scaffold allowed the immobilization of PDGF-BB via electrostatic interaction. In vitro PDGF-BB release profiles indicated that passive physical adsorption of PDGF-BB to non-heparinized scaffolds resulted in an initial burst release of PDGF-BB within 5 days, which then leveled off. However, electrostatic interaction between PDGF-BB and the heparin-conjugated scaffolds gave rise to a sustained release of PDGF-BB over the course of 20 days without an initial burst. Moreover, PDGF-BB that was strongly bound to the heparin-conjugated scaffolds enhanced smooth muscle cell (SMC) proliferation. In addition, scaffolds composed of 3.0 μm diameter fibers that were immobilized with PDGF-BB accelerated SMC infiltration into the scaffold when compared to scaffolds composed of smaller diameter fibers or scaffolds that did not release PDGF-BB. We concluded that the combination of the large pore structure in the scaffolds and the heparin-mediated delivery of PDGF-BB provided the most effective cellular interactions through synergistic physical and chemical cues.