Evaluation of innate, humoral and cell-mediated immunity in mice following in vivo implantation of electrospun polycaprolactone

Electrospun polycaprolactone (EPCL) is currently being investigated for use in tissue engineering applications such as vascular grafts. However, the effects of electrospun polymers on systemic immune responses following in vivo exposure have not previously been examined. The work presented evaluates whether EPCL in either a microfibrous or nanofibrous form affects innate, humoral and/or cell-mediated immunity using a standard immunotoxicological testing battery. Holistic in vivo endpoints examined include the antibody-forming cell assay (AFC or plaque assay) and the delayed-type hypersensitivity response to Candida albicans. In addition, natural killer cell cytotoxic activity was assessed using an ex vivo assay and splenic cell population phenotypes were analyzed by flow cytometry for material exposure-related changes. Results indicated that 28 day subcutaneous implantation of EPCL, either in microfibrous or nanofibrous form, did not affect the systemic functions of the immune system in 12-16 week old female B6C3F1 mice.

Electrospinning adipose tissue-derived extracellular matrix for adipose stem cell culture

Basement membrane-rich extracellular matrices, particularly murine sarcoma-derived Matrigel, play important roles in regenerative medicine research, exhibiting marked cellular responses in vitro and in vivo, although with limited clinical applications. We find that a human-derived matrix from lipoaspirate fat, a tissue rich in basement membrane components, can be fabricated by electrospinning and used to support cell culture. We describe practical applications and purification of extracellular matrix (ECM) from adipose tissue (At-ECM) and its use in electrospinning scaffolds and adipose stem cell (ASC) culture. The matrix composition of this purified and electrospun At-ECM was assessed histochemically for basement membrane, connective tissue, collagen, elastic fibers/elastin, glycoprotein, and proteoglycans. Each histochemical stain was positive in fat tissue, purified At-ECM, and electrospun At-ECM, and to some extent positive in a 10:90 blend with polydioxanone (PDO). We also show that electrospun At-ECM, alone and blended with PDO, supports ASC attachment and growth, suggesting that electrospun At-ECM scaffolds support ASC cultivation. These studies show that At-ECM can be isolated and electrospun as a basement membrane-rich tissue engineering matrix capable of supporting stem cells, providing the groundwork for an array of future regenerative medicine advances.

Tri-layered vascular grafts composed of polycaprolactone, elastin, collagen, and silk: Optimization of graft properties

The purpose of this study was to create seamless, acellular, small diameter bioresorbable arterial grafts that attempt to mimic the extracellular matrix and mechanical properties of native artery using synthetic and natural polymers. Silk fibroin, collagen, elastin, and polycaprolactone (PCL) were electrospun to create a tri-layered structure for evaluation. Dynamic compliance testing of the electrospun grafts ranged from 0.4-2.5%/100 mmHg, where saphenous vein (1.5%/100 mmHg) falls within this range. Increasing PCL content caused a gradual decrease in medial layer compliance, while changes in PCL, elastin, and silk content in the adventitial layer had varying affects. Mathematical modeling was used to further characterize these results. Burst strength results ranged from 1614-3500 mmHg, where some exceeded the capacity of the pressure regulator. Four week degradation studies demonstrated no significant changes in compliance or burst strength, indicating that these grafts could withstand the initial physiological conditions without risk of degradation. Overall, we were able to manufacture a multi-layered graft that architecturally mimics the native vascular wall and mechanically matches the gold standard of vessel replacement, saphenous vein.

Low-temperature electrospun silk scaffold for in vitro mucosal modeling

Electrospinning is often used to create scaffolding as a biomimetic of the extracellular matrix of tissues. A frequent limitation of this technique for three-dimensional tissue modeling is poor cell infiltration throughout the void volume of scaffolds. Here, we generated low-temperature electrospun silk scaffolds and compared these with conventional electrospun silk scaffolds in terms of mechanical properties, void volume, cell infiltration, cell viability, and potential to support mucosal models under three-dimensional culture conditions. Low-temperature electrospun silk scaffolds supported fibroblast attachment and infiltration throughout the volume of the scaffolds, while conventional electrospun scaffolds exhibited limited cell infiltration with fibroblasts attaching exclusively to the seeding surface of the scaffolds. The porosity of low-temperature electrospun scaffolds was 93% compared with 88% of conventional electrospun silk scaffolds. Uniaxial tensile testing showed a 3.5-fold reduction in strength of low-temperature electrospun silk compared with the conventional in terms of peak stress and modulus but no significant change in strain at break. Mucosal modeling with fibroblast-keratinocyte or fibroblast-carcinoma cocultures showed similar results, with cell infiltration occurring only in low-temperature electrospun scaffolds. Cell viability was confirmed using live/dead staining after 21 days in culture. Furthermore, low-temperature electrospun silk scaffolds were able to support keratinocyte differentiation, as judged by involucrin immunoreactivity. The low-temperature electrospun silk scaffold that we have developed eliminates the limitation of electrospun silk scaffolds in terms of cell infiltration and, therefore, can potentially be used for a wide range of tissue engineering purposes ranging from in vitro tissue modeling to in vivo tissue regeneration purposes.

Evaluation of biological activity of bone morphogenetic proteins on exposure to commonly used electrospinning solvents

Bone tissue engineering is one of the emerging strategies for developing functionally viable bone substitutes. The recent trend in bone tissue engineering is to combine the benefits of a three-dimensional nanofibrous scaffold with biologically active molecules and responsive stem cells. Electrospinning is the most versatile of the scaffold fabrication strategies and may involve the use of an organic solvent at one stage or another. In spite of all distinct advantages of electrospinning, valid concerns about potentially denaturing interactions between the organic solvent and the biomolecules exist. Efforts are ongoing to incorporate osteoinductive molecules, such as bone morphogenetic proteins (BMPs), during the electrospinning process. The challenge lies in ensuring that the biological activity of these incorporated molecules survives the process. This study was specifically designed to investigate the effects of exposure to commonly used organic solvents on heterodimeric BMP-2/7 using slot-blot assay quantified by infrared imaging and on embryonic myoblasts stably transfected with BMP-specific response element linked to a luciferase reporter – C2C12BRA. Overall, the biological activity of these molecules significantly decreased when exposed to organic solvents but can be restored to their original values by increasing the polarity of the solvent. It was found that an aqueous buffer can effectively overcome the deleterious effects of organic solvents on BMPs, thus generating osteoinductive bone scaffolds.

Incorporating platelet-rich plasma into electrospun scaffolds for tissue engineering applications

Platelet-rich plasma (PRP) therapy has seen a recent spike in clinical interest due to the potential that the highly concentrated platelet solutions hold for stimulating tissue repair and regeneration. The aim of this study was to incorporate PRP into a number of electrospun materials to determine how growth factors are eluted from the structures, and what effect the presence of these factors has on enhancing electrospun scaffold bioactivity. PRP underwent a freeze-thaw-freeze process to lyse platelets, followed by lyophilization to create a powdered preparation rich in growth factors (PRGF), which was subsequently added to the electrospinning process. Release of protein from scaffolds over time was quantified, along with the quantification of human macrophage and adipose-derived stem cell (ADSC) chemotaxis and proliferation. Protein assays demonstrated a sustained release of protein from PRGF-containing scaffolds at up to 35 days in culture. Scaffold bioactivity was enhanced as ADSCs demonstrated increased proliferation in the presence of PRGF, whereas macrophages demonstrated increased chemotaxis to PRGF. In conclusion, the work performed in this study demonstrated that the incorporation of PRGF into electrospun structures has a significant positive influence on the bioactivity of the scaffolds, and may prove beneficial in a number of tissue engineering applications.

Modulation of mast cell adhesion, proliferation, and cytokine secretion on electrospun bioresorbable vascular grafts

Mast cells synthesize several potent angiogenic factors and can also stimulate fibroblasts, endothelial cells, and macrophages. An understanding of how they participate in wound healing and angiogenesis is important to further our knowledge about in situ vascular prosthetic regeneration. The adhesion, proliferation, and cytokine secretion of bone marrow-derived murine mast cells (BMMC) on electrospun polydioxanone, polycaprolactone, and silk scaffolds, as well as tissue culture plastic, has been investigated in the presence or absence of IL-3, stem cell factor, IgE and IgE with a crosslinking antigen, dinitrophenol-conjugated albumin (DNP). It was previously believed that only activated BMMCs exhibit adhesion and cytokine secretion. However, this study shows nonactivated BMMC adhesion to electrospun scaffolds. Silk scaffold was not found to be conducive for mast cell adhesion and cytokine secretion. Activation by IgE and DNP significantly enhanced mast cell adhesion, proliferation, migration, and secretion of tumor necrosis factor alpha, macrophage inflammatory protein-1α, and IL-13. This indicates that mast cells might play a role in the process of biomaterial integration into the host tissue, regeneration, and possibly angiogenesis.

Electrospinning jets and nanofibrous structures

Electrospinning is a process that creates nanofibers through an electrically charged jet of polymer solution or melt. This technique is applicable to virtually every soluble or fusible polymer and is capable of spinning fibers in a variety of shapes and sizes with a wide range of properties to be used in a broad range of biomedical and industrial applications. Electrospinning requires a very simple and economical setup but is an intricate process that depends on several molecular, processing, and technical parameters. This article reviews information on the three stages of the electrospinning process (i.e., jet initiation, elongation, and solidification). Some of the unique properties of the electrospun structures have also been highlighted. This article also illustrates some recent innovations to modify the electrospinning process. The use of electrospun scaffolds in the field of tissue engineering and regenerative medicine has also been described.

Biomedical Nanoscience: Electrospinning Basic Concepts, Applications, and Classroom Demonstration

Electrospinning is an old polymer processing technique that has recently been rediscovered. It allows for the easy creation of nano- to micro-fibers that can be collected to form a non-woven structure, which can then be used to fabricate novel structures for various applications including tissue engineering scaffolds, clothing, drug delivery vehicles, and filtration media. Current research in our laboratories is focused on the processing of synthetic and biological polymers to create materials with tailored properties and functions for tissue engineering scaffolds and various other medical applications. This technology is revolutionizing the biomaterials and nanotechnology fields and has prompted us to incorporate its history, basic concepts, and applications into diverse courses such as Biomaterials, Tissue Engineering, Polymers in Medicine, and Senior Design in Chemical and Biomedical Engineering. This Innovation of the Curriculum is timely and crucial for multiple reasons. There is a need for a systematic approach to course structure that ties historical concepts to new materials and processes and, ultimately, to practical applications. Combining this lecture organization with active learning in the forms of open discussions and hands-on experiments/demonstrations will enhance learning outcomes (including retention and critical thinking) at all levels of education. At the undergraduate and graduate levels in the courses mentioned, discussions of electrospinning can create a classroom atmosphere of creative thinking, and an actual demonstration of nanomaterial fabrication can serve as a visual aid to the students. More importantly, this curriculum innovation can be used at the high school level to demonstrate nanotechnology and its applications to medicine, which will aid in sparking the interest of future generations of tissue engineers, biomaterial scientists, nanotechnologists, and scientists and engineers in general.

A Novel Electrospun Dendrimer-Gelatin Hybrid Nanofiber Scaffold for Tissue Regeneration and Drug Delivery

Gelatin has been widely used to develop tissue engineering scaffolds because it has many attractive properties. Dendrimer provides a versatile, compositionally and structurally controlled architecture to construct nanomedicine. This study was aimed at developing a novel electrospun dendrimer-gelatin nanofiber scaffold to best mimic natural extracellular matrix (ECM) to promote tissue formation and serve as a reservoir for controlled drug delivery. Starburst™ polyamidoamine (PAMAM) dendrimer G3.5 was covalently bonded to the gelatin backbone and electrospun into nanofibers. Doxycycline (DC), which is an effective antibiotic that has the ability to inhibit matrix metalloproteinase, was encapsulated into the nanofiber scaffold. The electrospun DC-gelatin scaffold provides a bacterial free environment for cell growth and tissue regeneration. The resulting dendrimer-gelatin nanofiber scaffold achieved a unique structural configuration where covalently bound three-dimensional dendritic nanospheres were evenly distributed along the elongated dimension of the nanofiber, and both dendrimer and gelatin had numerous functional groups suitable for accommodating multiple functional entities and high payload of drugs. The development of this new scaffold with the capability of delivering multiple functional entities was an important step towards the use of bioactive nanofibers to facilitate tissue regeneration and controlled drug release.

Tri-layered electrospinning to mimic native arterial architecture using polycaprolactone, elastin, and collagen: a preliminary study

Throughout native artery, collagen and elastin play an important role, providing a mechanical backbone, preventing vessel rupture, and promoting recovery under pulsatile deformations. The goal of this study was to mimic the structure of native artery by fabricating a multi-layered electrospun conduit composed of poly(caprolactone) (PCL) with the addition of elastin and collagen with blends of 45-45-10, 55-35-10, and 65-25-10 PCL-ELAS-COL to demonstrate mechanical properties indicative of native arterial tissue, while remaining conducive to tissue regeneration. Whole grafts and individual layers were analyzed using uniaxial tensile testing, dynamic compliance, suture retention, and burst strength. Compliance results revealed that changes to the middle/medial layer changed overall graft behavior with whole graft compliance values ranging from 0.8 - 2.8 % / 100 mmHg, while uniaxial results demonstrated an average modulus range of 2.0 - 11.8 MPa. Both modulus and compliance data displayed values within the range of native artery. Mathematical modeling was implemented to show how changes in layer stiffness affect the overall circumferential wall stress, and as a design aid to achieve the best mechanical combination of materials. Overall, the results indicated that a graft can be designed to mimic a tri-layered structure by altering layer properties.

Two pole air gap electrospinning: Fabrication of highly aligned, three-dimensional scaffolds for nerve reconstruction

We describe the structural and functional properties of three-dimensional (3D) nerve guides fabricated from poly-ε-caprolactone (PCL) using the air gap electrospinning process. This process makes it possible to deposit nano-to-micron diameter fibers into linear bundles that are aligned in parallel with the long axis of a cylindrical construct. By varying starting electrospinning conditions it is possible to modulate scaffold material properties and void space volume. The architecture of these constructs provides thousands of potential channels to direct axon growth. In cell culture functional assays, scaffolds composed of individual PCL fibers ranging from 400 to 1500 nm supported the penetration and growth of axons from rat dorsal root ganglion. To test the efficacy of our guide design we reconstructed 10mm lesions in the rodent sciatic nerve with scaffolds that had fibers 1 μm in average diameter and void volumes >90%. Seven weeks post implantation, microscopic examination of the regenerating tissue revealed dense, parallel arrays of myelinated and non-myelinated axons. Functional blood vessels were scattered throughout the implant. We speculate that end organ targeting might be improved in nerve injuries if axons can be directed to regenerate along specific tissue planes by a guide composed of 3D fiber arrays.

Modulation of murine innate and acquired immune responses following in vitro exposure to electrospun blends of collagen and polydioxanone

In light of cell sourcing issues and the lack of a bioreactor comparable to the body, many in the field of tissue engineering have focused their efforts on designing biomaterials capable of in situ regeneration. The theory is that, by using the body as both the bioreactor and the source for cell infiltration, scaffolds composed of bioresorbable materials can be remodeled into native tissue. Thus, research into the effects of such materials on the host immune response is increasingly important. This study applies an immunotoxicological approach to evaluate the effects of electrospun blends of polydioxanone (PDO) and collagen type I on murine innate and acquired immune responses. Results indicated that these materials had few effects on innate immune responses, yet they produced significant immunomodulatory effects in multiple endpoints evaluating both branches of acquired immunity (i.e., cell-mediated and humoral immunity). Specifically, collagen content appeared to be responsible for suppression of cell-mediated immunity, while blends of PDO and collagen appeared to be more suppressive of antibody-forming cell responses than either PDO or collagen alone. These results demonstrate the importance of completing evaluations into the immunotoxicological effects of biomaterials, and they suggest that such testing should become a primary focus when evaluating a material's potential foruse in tissue engineering applications.

Evaluation of thrombogenic potential of electrospun bioresorbable vascular graft materials: acute monocyte tissue factor expression

The purpose of this study was to quantify the acute expression of tissue factor (TF) by monocytes on interaction with electrospun bioresorbable constructs. A minimal expression of TF will demonstrate the potential for scaffolds to be used as a vascular graft without enhanced risk of failure from acute thrombotic occlusion. Polydioxanone (PDO) (60, 80, 120, and 160 mg/mL) and polycaprolactone (PCL) (80, 10, and 160 mg/mL) dissolved in 1,1,1,3,3,3 hexafluoro-2-propanol (HFP) were electrospun to form fibrous scaffolds. Circular discs (10 mm diameter) of each scaffold were disinfected and seeded with human monocytes (50,000 cells/well). The discs were statically cultured under standard conditions (37 degrees C and 5% CO2), and removed after 24 h for TF analysis with an In-Cell Western assay. Fiber diameter was calculated through ImageTool analysis of scanning electron micrographs. Acute monocyte interaction with scaffolds of PCL (120 mg/mL) resulted in the lowest amount of TF expressed (4 ng/disc), whereas scaffolds of 160 mg/mL PDO elicited the highest amount of TF expressed (51 ng/disc). TF levels expressed on all scaffolds were comparable with the amount expressed on e-PTFE (20 ng/disc). Preliminary data for TF expression on scaffolds of silk (70 mg/mL and 150 mg/mL) and silk:PCL (100 mg/mL, v/v) blends (50:50 and 70:30) resulted in values of TF expression ranging from 0 to 24 ng. Results from this study reveal electrospun grafts composed of PDO and PCL provide no greater risk of failure from an acute thrombotic occlusion due to TF expression when compared with that of the standard e-PTFE graft.

Nanotechnology in the design of soft tissue scaffolds: innovations in structure and function

Engineered scaffolds function to supplement or replace injured, missing, or compromised tissue or organs. The current direction in this research area is to create scaffolds that mimic the structure and function of the native extracellular matrix (ECM). It is believed that the fabrication of a scaffold that has both structural integrity and allows for normal cellular function and interaction will bring scaffolds closer to clinical relevance. Nanotechnology innovations have aided in the development of techniques for the production of nanofiber scaffolds. The three major processing techniques, self-assembly, phase separation, and electrospinning, produce fibers that rival the size of those found in the native ECM. However, the simplicity, versatility, and scalability of electrospinning make it an attractive processing method that can be used to reproduce aspects of the complexity that characterizes the native ECM. Novel electrospinning strategies include alterations of scaffold composition and architecture, along with the addition and encapsulation of cells, pharmaceuticals and growth factors within the scaffold. This article reviews the major nanofiber fabrication technologies as well as delves into recent significant contributions to the conception of a meaningful and practical electrospun scaffold.

Electrospinning of collagen/biopolymers for regenerative medicine and cardiovascular tissue engineering

The process of electrospinning has seen a resurgence of interest in the last few decades which has led to a rapid increase in the amount of research devoted to its use in tissue engineering applications. Of this research, the area of cardiovascular tissue engineering makes up a large percentage, with substantial resources going towards the creation of bioresorbable vascular grafts composed of electrospun nanofibers of collagen and other biopolymers. These bioresorbable grafts have compositions that allow for the in situ remodeling of the structure, with the eventual replacement of the graft with completely autologous tissue. This review will highlight some of the work done in the field of electrospinning for cardiovascular applications, with an emphasis on the use of biopolymers such as collagens, elastin, gelatin, fibrinogen, and silk fibroin, as well as biopolymers used in combination with resorbable synthetic polymers.

Science of nanofibrous scaffold fabrication: strategies for next generation tissue-engineering scaffolds

Native extracellular matrix (ECM) provides structural support to the multicellular organism on a macroscopic scale and establishes a unique microenvironment (niche) to tissue- and organ-specific cell types. Both these functions are critical for optimal function of the organism. These natural ECMs comprise predominantly fibrillar proteins, collagen and elastin and are synthesized as monomers but undergo hierarchical organization into well-defined nanoscaled structural units. The interaction between the cells and ECM is dynamic, reciprocal and essential for tissue development, maintenance of function, repair and regeneration processes. Tissue-engineering scaffolds are synthetic, biomimetic ECM analogues that have great promise in regenerative medicine. Ongoing efforts in mimicking the native ECM in terms of composition and dimension have resulted in three strategies that permit the generation of scaffolds in nanometer dimensions. Although excellent reviews regarding the applications of these strategies in tissue engineering are available, a comprehensive review of the science behind these fabrication techniques does not exist. This review intends to fill this critical gap in the existing knowledge in the fast-expanding field of nanofibrous scaffolds. A thorough understanding of the fabrication processes would enable us to better exploit available technologies to produce superior tissue-engineering scaffolds.

Regulation of material properties in electrospun scaffolds: Role of cross-linking and fiber tertiary structure

We cross-linked scaffolds of electrospun collagen to varying degrees with glutaraldehyde using an ethanol-based solvent system and subsequently defined how the percentage of cross-linking impacts bulk and microscale material properties and fiber structure. At hydration, electrospun fibers underwent coiling; the extent of coiling was proportional to the percentage of cross-linking introduced into the samples and was largely suppressed as cross-linking approached saturation. These data suggest that electrospun collagen fibers are not deposited in a minimal energy state; fiber coiling may reflect a molecular reorganization. This result has functional/structural implications for protein-based electrospun scaffolds. Changes in fiber topology that develop during post-electrospinning processing may alter monomer organization, mask or unmask receptor binding sites, and/or change the biological properties of these nanomaterials. Hydrated scaffolds were mounted into a custom stretching device installed on a microscope stage and photographed after incremental changes in strain. Changes in fiber alignment were measured using the two-dimensional fast Fourier transform method. Fibers in all scaffolds underwent alignment in response to strain; however, the rate and extent of alignment that could be achieved varied as a function of cross-linking. We propose four distinct modes of scaffold response to strain: fiber uncoiling, fiber reorientation, fiber elongation and interfiber sliding. We conclude that bulk material properties and local microscale architecture must be simultaneously considered to optimize the performance of electrospun scaffolds.