Design of electrohydrodynamic sprayed polyethylene glycol hydrogel microspheres for cell encapsulation

Electrohydrodynamic spraying (EHS) has recently gained popularity for microencapsulation of cells for applications in cell delivery and tissue engineering. Some of the polymers compatible with EHS are alginate, chitosan, and other similar natural polymers, which are subject to ionotropic or physical gelation. It is desirable to further extend the use of the EHS technique beyond such polymers for wider biofabrication applications. Here, building upon our previous work of making PEG microspheres via EHS, we utilized the principles of EHS to fabricate cell-laden polyethylene glycol (PEG) hydrogel microspheres. The gelation of PEG hydrogel microspheres was achieved by forming covalent crosslinks between multiarm PEG acrylate and dithiol crosslinkers via Michael-type addition. We conducted a detailed investigation of the critical parameters of EHS, such as the applied voltage, inner needle diameter (i.d. needle), and flow rate, to obtain PEG microspheres with high cell viability and tightly-controlled diameters in the range of 70-300 μm. The polydispersity of cell-laden PEG hydrogel microspheres as measured by % coefficient of variation was between 6% and 23% for all conditions tested. We established that our method was compatible with different cell types and that all tested cell types could be encapsulated at high densities of 10⁶-10⁹ and ≥90% encapsulation efficiency. We observed cell aggregation within the hydrogel microspheres at applied voltage >5 kV. Since PEG is a synthetic polymer devoid of cell attachment sites, we could overcome this limitation by tethering Arg-Gly-Asp-Ser (RGDS) peptide to the PEG hydrogel microspheres; upon RGDS tethering, we observed uniform cell dispersion. The microencapsulated cells could be cultured in the PEG hydrogel microspheres of different sizes for up to one week without significant loss in cell viability. In conclusion, the EHS technique developed here could be used to generate cell-laden PEG hydrogel microspheres of controlled sizes for potential applications in cell delivery and organoid cultures.

The fabrication of cryogel scaffolds incorporated with poloxamer 407 for potential use in the regeneration of the nucleus pulposus

Degeneration of the nucleus pulposus (NP) is the primary cause of back pain in almost 80% of the world population. The current gold standard treatment for a degenerated NP is a spinal fusion surgery which is costly, temporary, and extremely invasive. Research has been moving towards minimally invasive methods to lessen the collateral damage created during surgery. The use of a tissue-engineered scaffold has the potential to promote a healthy and hydrated environment to regenerate the NP. Cryogels are unique polymeric scaffolds composed of a highly connected, macroporous structure, and are capable of maintaining stability under high deformations. For this study, cryogels have been developed using gelatin and poloxamer 407 (P407) at varying ratios to determine the ideal combination of stability, water retention, and pore size. For the application of NP regeneration, a gelatin-P407 cryogel should be both stable and a well hydrated carrier. The cryogels created varied from a 1:1 gelatin to P407 ratio to a 10:1 ratio. The inclusion of P407 in the cryogels resulted in a significant increase in hydrophilicity, ideal pore size for cell infiltration, mechanical stability over 28 days, and cell infiltration after just 21 days. This novel gelatin-P407 composite cryogel has the potential to be a practical alternative to the spinal fusion procedure, saving patients hundreds of thousands of dollars and, ideally, leading to improved patient outcome.

A Comparison of Tissue Engineering Scaffolds Incorporated with Manuka Honey of Varying UMF

Purpose. Manuka honey (MH) is an antibacterial agent specific to the islands of New Zealand containing both hydrogen peroxide and a Unique Manuka Factor (UMF). Although the antibacterial properties of MH have been studied, the effect of varying UMF of MH incorporated into tissue engineered scaffolds have not. Therefore, this study was designed to compare silk fibroin cryogels and electrospun scaffolds incorporated with a 5% MH concentration of various UMF. Methods. Characteristics such as porosity, bacterial clearance and adhesion, and cytotoxicity were compared. Results. Pore diameters for all cryogels were between 51 and 60 µm, while electrospun scaffolds were 10 µm. Cryogels of varying UMF displayed clearance of approximately 0.16 cm for E. coli and S. aureus. In comparison, the electrospun scaffolds clearance ranged between 0.5 and 1 cm. A glucose release of 0.5 mg/mL was observed for the first 24 hours by all scaffolds, regardless of UMF. With respect to cytotoxicity, neither scaffold caused the cell number to drop below 20,000. Conclusions. Overall, when comparing the effects of the various UMF within the two scaffolds, no significant differences were observed. This suggests that the fabricated scaffolds in this study displayed similar bacterial effects regardless of the UMF value.

Decellularized extracellular matrices for tissue engineering applications

Decellularization removes cellular antigens while preserving the ultrastructure and composition of extracellular matrix (ECM). Decellularized ECM (DECM) scaffolds have been widely used in various tissue engineering applications with varying levels of success. The mechanical, architectural and bioactive properties of a DECM scaffold depend largely on the method of decellularization and dictate its clinical efficacy. This article highlights the advantages and challenges associated with the clinical use of DECM scaffolds. Poor mechanical strength is a significant disadvantage of some DECM scaffolds in the repair of load-bearing tissues as well as critical-size defects, where long-term mechanical support is required for the regenerating tissue. Combining DECM scaffolds with synthetic biocompatible polymers could provide a useful strategy to circumvent the issues of poor mechanical stability. This article reviews studies that have combined DECM scaffolds from various tissues with synthetic polymers to create hybrid scaffolds using electrospinning. These hybrid scaffolds provide a mechanical backbone while retaining the bioactive properties of DECM, thus offering a significant advantage for tissue engineering and regenerative medicine applications.

Control of gelation, degradation and physical properties of polyethylene glycol hydrogels through the chemical and physical identity of the crosslinker

Tuning hydrogel properties through minor modifications of the crosslinker structure is a beneficial approach for hydrogel design that could result in hydrogels with wide range of properties to match a desired application.

Diabetic Wounds Exhibit Decreased Ym1 and Arginase Expression with Increased Expression of IL-17 and IL-20

Objective: Impaired wound healing in diabetic (DB) patients is a significant health problem; however, the roles that cytokines and innate immune cells contribute to this impaired healing are not completely understood. Approach: A mouse model was used to compare the innate immune response during DB and normal wound healing. Two 5-mm full-thickness wounds were created on the dorsal skin of BKS.Cg-m+/+Leprdb/J (DB) and C57BL/6 (wild-type) mice. Innate immune cell markers and cytokine mRNA levels were measured in wound biopsies during the first week of healing. Results: Innate immune cell influx (typified by the Gr-1 neutrophil marker and the Ym1 macrophage marker) was delayed in the DB wounds. Expression of the M2 macrophage-related genes, Ym1 and arginase 1, was significantly reduced in the DB wounds. PCR array analysis demonstrated altered cytokine expression in DB wounds. Most prominently, both interleukin (IL)-17 and IL-20 mRNA levels were significantly increased in the DB wounds. Innovation: This is the first study to identify increased levels of IL-17 and IL-20 in DB wounds. These cytokines are also elevated in the inflammatory skin disorder, psoriasis; thus, they may be potential therapeutic targets to aid in DB wound healing. Conclusion: The entire cytokine profile of DB wounds over the course of healing is not completely understood. This study suggests that the IL-17 and IL-20 families of cytokines should be further analyzed in the context of DB wound healing.

Use of electrospinning and dynamic air focusing to create three-dimensional cell culture scaffolds in microfluidic devices

Organs-on-a-chip has emerged as a powerful tool for pharmacological and physiological studies. A key part in the construction of such a model is the ability to pattern or culture cells in a biomimetic fashion. Most of the reported cells-on-a-chip models integrate cells on a flat surface, which does not accurately represent the extracellular matrix that they experience in vivo. Electrospinning, a technique used to generate sub-micron diameter polymer fibers, has been used as an in vitro cell culture substrate and for tissue engineering applications. Electrospinning of fibers directly into a fully sealed fluidic channel using a conventional setup has not been possible due to issues of confining the fibers into a discrete network. In this work, a dynamic focusing method was developed, with this approach enabling direct deposition of electrospun fibers into a fully sealed fluidic channel, to act as a matrix for cell culture and subsequent studies under continuous flowing conditions. Scanning electron microscopy of electrospun polycaprolactone fibers shows that this method enables the formation of fibrous layers on the inner wall of a 3D-printed fluidic device (mean fiber size = 1.6 ± 0.6 μm and average pore size = 113 ± 19 μm(2)). Cells were able to be cultured in this 3D scaffold without the addition of adhesion proteins. Media was pumped through the channel at high flow rates (up to 400 μL min(-1)) during a dynamic cell culture process and both the fibers and the cells were found to be strongly adherent. A PDMS fluidic device was also prepared (from a 3D printed mold) and coated with polycaprolactone fibers. The PDMS device enables optical detection and confocal imaging of cultured cells on the fibers. Finally, macrophages were cultured in the devices to study how the fibrous scaffold can affect cell behavior. It was found that under lipopolysaccharide stimulation, macrophages cultured on PCL fibers inside of a channel secreted significantly more cytokines than those cultured on a thin layer of PCL in a channel or directly on the inner channel wall. Overall, this study represents a new approach for in vitro cell studies, where electrospinning can be used to easily and quickly create 3D scaffolds that can improve the culture conditions in microfluidic devices.

Lactic Acid Suppresses IL-33-Mediated Mast Cell Inflammatory Responses via Hypoxia-Inducible Factor-1α-Dependent miR-155 Suppression

Lactic acid (LA) is present in tumors, asthma, and wound healing, environments with elevated IL-33 and mast cell infiltration. Although IL-33 is a potent mast cell activator, how LA affects IL-33-mediated mast cell function is unknown. To investigate this, mouse bone marrow-derived mast cells were cultured with or without LA and activated with IL-33. LA reduced IL-33-mediated cytokine and chemokine production. Using inhibitors for monocarboxylate transporters (MCT) or replacing LA with sodium lactate revealed that LA effects are MCT-1- and pH-dependent. LA selectively altered IL-33 signaling, suppressing TGF-β-activated kinase-1, JNK, ERK, and NF-κB phosphorylation, but not p38 phosphorylation. LA effects in other contexts have been linked to hypoxia-inducible factor (HIF)-1α, which was enhanced in bone marrow-derived mast cells treated with LA. Because HIF-1α has been shown to regulate the microRNA miR-155 in other systems, LA effects on miR-155-5p and miR-155-3p species were measured. In fact, LA selectively suppressed miR-155-5p in an HIF-1α-dependent manner. Moreover, overexpressing miR-155-5p, but not miR-155-3p, abolished LA effects on IL-33-induced cytokine production. These in vitro effects of reducing cytokines were consistent in vivo, because LA injected i.p. into C57BL/6 mice suppressed IL-33-induced plasma cytokine levels. Lastly, IL-33 effects on primary human mast cells were suppressed by LA in an MCT-dependent manner. Our data demonstrate that LA, present in inflammatory and malignant microenvironments, can alter mast cell behavior to suppress inflammation.

Characterization of slow-gelling alginate hydrogels for intervertebral disc tissue-engineering applications

Reversal of intervertebral disc degeneration can have a potentially monumental effect on spinal health. As such, the goal of this research is to create an injectable, cellularized alginate-based nucleus pulposus that will restore disc function; with the primary goal of creating an alginate gel with tailorable rates of gelation to improve functionality over standard CaCl2 crosslinking techniques. Gelation characteristics of 1% sodium alginate were analyzed over various molar concentrations of a 1:2 ratio of CaCO3:glucono-δ-lactone (GDL), with 10% CaCl2 as the control crosslinker. Alginate construct characterization for all concentrations was performed via ultimate and cyclic compressive testing over a 28day degradation period in PBS. Dehydration, swell testing, and albumin release kinetics were determined, and cytotoxicity and cell homogeneity tests showed promise for cellularization strategies. Overall, the 30 and 60mM GDL alginate concentrations presented the most viable option for use in further studies, with a gelation time between 10 and 30min, low hysteresis over control, low percent change in thickness and weight under both PBS degradation and swelling conditions, and stable mechanical properties over 28days in vitro.

The Lipid Portion of Activated Platelet-Rich Plasma Significantly Contributes to Its Wound Healing Properties

Objective: Platelet-rich plasma (PRP) is a popular choice for the treatment of chronic wounds. Current dogma attributes these healing properties to the peptide growth factors of PRP. However, PRP is also rich in bioactive lipids whose contribution to healing has not been characterized and warrants investigation due to the protease-rich environment of chronic wounds. Approach: The lipid fraction of PRP was tested with respect to proliferation and migration of primary adult human dermal fibroblasts (HDFa)±exposure to chronic wound fluid (CWF). This fraction was also characterized via LC-MS/MS for bioactive lipids. A synthetic formulation of the bioactive lipid composition was developed and tested for the ability to overcome proliferative growth arrest induced by CWF. Results: The data demonstrate the ability of the lipid fraction of PRP to significantly enhance the migration and proliferation of HDFa, and to overcome the proliferative growth arrest induced by CWF. Furthermore, the synthetic lipid formulation generated following characterization of the PRP lipidome demonstrated a similar ability to overcome proliferative arrest of HDFa in the presence of CWF. Innovation: For the first time, we demonstrate the relevance of the lipid fraction of PRP toward the biology of wound healing. These studies open the possibility of altering the lipid profile of PRP via diet or exogenous pathway manipulation to obtain a better healing outcome. Conclusion: The lipid fraction of PRP is under investigated and yet relevant component in wound healing. The current study demonstrates the relevance of this fraction in wound healing by PRP.

In vitro comparison of two different mechanical circulatory support devices installed in series and in parallel

This study investigates the novel approach of placing a ventricular assist pump in the descending aorta in series configuration with the heart and compares it with the two traditional approaches of left-ventricle-to-ascending-aorta (LV-AA) and left-ventricle-to-descending-aorta (LV-DA) placement in parallel with the heart. Experiments were conducted by using the in-house simulator of the cardiovascular blood-flow loop (SCVL). The results indicate that the use of the LV-AA in-parallel configuration leads to a significant improvement in the systemic and pulmonic flow as the level of continuous flow is increased; however, this approach is considered highly invasive. The use of the LV-DA in-parallel configuration leads to an improvement in the systemic and pulmonic flow at lower levels of continuous flow but at higher levels of pump support leads to retrograde flow. In both in-parallel configurations, increasing the level of pump continuous flow leads to a decrease in pulsatility to a certain extent. The results of placing the pump in the descending aorta in series configuration show that the pressure drop upstream of the pump facilitates cardiac output as a result of afterload reduction. In addition, the pressure rise downstream of the pump may assist with renal perfusion. However, at the same time, the pressure drop generated at the proximal part of the descending aorta induces a slight drop in carotid perfusion, which would be autoregulated by the brain in a native cardiovascular system. The pulse wave analysis shows that placing the pump in the descending aorta leads to improved pulsatility in comparison with the traditional in-parallel configurations.

A preliminary study on amelogenin-loaded electrospun scaffolds

Amelogenin is a major enamel matrix protein onto which developing enamel forms. In the realm of tissue engineering, amelogenin has been studied and applied to periodontal and wound healing applications. This study introduces the first attempts of incorporating amelogenin within an electrospun scaffold. Amelogenin was extracted from porcine unerupted tooth buds and electrospun with poly(glycolic acid) and poly(ϵ-caprolactone). Protein release kinetics, mechanical properties, fiber diameter, mineralization potential, and cell adhesion properties of the amelogenin-blended scaffolds were studied and compared to the electrospun poly(glycolic acid) and poly(ϵ-caprolactone) controls. Electrospun scaffolds loaded with amelogenin were incubated in phosphate buffer saline. Protein quantification and morphological and mechanical analyses were conducted on the degraded scaffolds, and the incubated phosphate buffer saline was also tested for protein content. Fresh scaffolds were incubated overnight in conventional simulated body fluid to evaluate mineralization potential of the incorporated electrospun amelogenin. Human dermal fibroblasts were seeded onto scaffolds, incubated overnight, cryosectioned, and stained with 4′,6-diamidino-2-phenylindole to determine cellular adhesive properties. The incorporation of 5 mg/mL amelogenin into electrospun scaffolds improved mechanical properties (in poly(ϵ-caprolactone) scaffolds), increased fiber mineralization (in poly(glycolic acid) scaffolds), and improved human dermal fibroblast adhesion (in poly(ϵ-caprolactone) scaffolds). The presented results suggest that amelogenin can be used for multiple tissue engineering applications in the form of an additive to an electrospun scaffold.

The influence of platelet-rich plasma on myogenic differentiation

The ability to expand and direct both precursor and stem cells towards a differential fate is considered extremely advantageous in tissue engineering. Platelet-rich plasma (PRP) possesses a milieu of growth factors and cytokines, which have the potential to have either a differentiative or proliferative influence on the cell type tested. Here, we investigated the effect of PRP on C2C12 myoblasts. A range of PRP concentrations in differentiation media was used to determine whether a concentration dependence existed, while PRP embedded in fibres of aligned electrospun polydioxanone and polycaprolactone was used to determine whether this presence of fibres would cause any differences in response. In both cases, it was found that late myogenic markers were suppressed after 7 days in culture. However, an early differentiation marker, MyoD, was upregulated during this same time period. The results from this study represent the ability of PRP to have an influence over both myogenic proliferation and differentiation, a factor which could prove useful in future studies involved with skeletal muscle tissue engineering.

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.

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.

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.

A case report on the use of sustained release platelet-rich plasma for the treatment of chronic pressure ulcers

BACKGROUND/OBJECTIVES

Chronic pressure ulcers affect patient health, emotional state, and quality of life, causing considerable morbidity and mortality in addition to contributing to significant health care costs from lengthy hospitalizations to advanced home care and surgical care costs. The conventional treatment of these wounds can be slow due to their chronic inflammatory state and the senescence of local reparative cells. Platelet-rich plasma (PRP) therapy has been growing as a viable treatment alternative for a number of clinical applications and has potential benefit for use in chronic wounds. The sustained release of large quantities of autologous growth factors, cytokines, and other mediators found in PRP plus the favorable mononuclear cell profile of PRP may help us to stimulate wound healing and resolve chronic inflammation.

METHODS

Three veterans with spinal cord injury (SCI), presenting with chronic stage IV pressure ulcers, were treated with a sustained release PRP therapy to stimulate wound healing.

RESULTS

PRP treatment consistently resulted in the formation of granulation tissue and improved vascularity for each of the three patients treated, while reducing the overall ulcer area and volume.

CONCLUSION

The controlled release of growth factors from PRP demonstrated a positive stimulatory effect on the healing rate of chronic pressure ulcers in individuals with SCI.

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.

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.