Antonio R. Webb

Biodegradable polyester elastomers in tissue engineering.

Tissue engineering often makes use of biodegradable scaffolds to guide and promote controlled cellular growth and differentiation in order to generate new tissue. There has been significant research regarding the effects of scaffold surface chemistry and degradation rate on tissue formation and the importance of these parameters is widely recognised. Nevertheless, studies describing the role of mechanical stimuli during tissue development and function suggest that the mechanical properties of the scaffold will also be important. In particular, scaffold mechanics should be taken into account if mechanical stimulation, such as cyclic strain, will be incorporated into strategies to grow improved tissues or the target tissue to be replaced has elastomeric properties. Biodegradable polyesters, such as polyglycolide, polylactide and poly(lactide-co-glycolide), although commonly used in tissue engineering, undergo plastic deformation and failure when exposed to long-term cyclic strain, limiting their use in engineering elastomeric tissues. This review will cover the latest advances in the development of biodegradable polyester elastomers for use as scaffolds to engineer tissues, such as heart valves and blood vessels.

Biodegradable poly(diol citrate) nanocomposite elastomers for soft tissue engineering

At present, synthetic biodegradable polymers commonly used for scaffolds in tissue engineering have a limited range of mechanical properties. This limitation is a challenge to in vivo tissue engineering, as the cell-scaffold construct is expected to maintain or restore normal tissue biomechanics during new tissue formation. Herein we report the synthesis and characterization of biodegradable elastomeric nanocomposite materials whose mechanical properties can be tailored to meet the requirements of soft tissue engineering applications. The nanocomposite consists of a nanofibrous poly(L-lactic acid) (PLLA) nanophase and an elastomeric poly(diol citrate) macrophase. Incorporation of a PLLA nanophase provides reinforcement to the poly(diol citrate) as demonstrated by an increase in tensile strength, modulus, and elongation at break with minimal permanent deformation. The mechanical properties of the nanocomposite were altered with the concentration of PLLA, choice of poly(diol citrate), and polymerization conditions. More importantly, the tensile mechanical properties compare favorably to those of human cartilage, ligament, and blood vessel. Furthermore, the compressive modulus is very similar to those of human and bovine articular cartilage. These results suggest that poly(diol citrate) nanocomposite elastomers are promising candidate biomaterials for soft tissue engineering.

Citric acid‐based elastomers provide a biocompatible interface for vascular grafts

Prosthetic vascular bypass grafting is associated with poor long-term patency rates. Herein, we report on the mid-term performance of expanded polytetrafluoroethylene (ePTFE) vascular grafts modified with a citric acid-based biodegradable elastomer. Through a spin-shearing method, ePTFE grafts were modified by mechanically coating a layer of poly(1,8 octanediol citrate) (POC) onto the luminal nodes and fibrils of the ePTFE. Control and POC-ePTFE grafts were implanted into the porcine carotid artery circulation as end-to-side bypass grafts. Grafts were assessed by duplex ultrasonography, magnetic resonance angiography, and digital subtraction contrast angiography and were all found to be patent with no hemodynamically significant stenoses. At 4 weeks, POC-ePTFE grafts were found to be biocompatible and resulted in a similar extent of neointimal hyperplasia as well as leukocyte and monocyte/macrophage infiltration as control ePTFE grafts. Furthermore, POC supported endothelial cell growth. Lastly, scanning electron microscopy confirmed the presence of POC on the ePTFE grafts at 4 weeks. Thus, these data reveal that surface modification of blood-contacting surfaces with POC results in a biocompatible surface that does not induce any untoward effects or inflammation in the vasculature. These findings are important as they will serve as the foundation for the development of a drug-eluting vascular graft.

Periadventitial atRA citrate-based polyester membranes reduce neointimal hyperplasia and restenosis after carotid injury in rats

Oral all-trans retinoic acid (atRA) has been shown to reduce the formation of neointimal hyperplasia; however, the dose required was 30 times the chemotherapeutic dose, which already has reported side effects. As neointimal formation is a localized process, new approaches to localized delivery are required. This study assessed whether atRA within a citrate-based polyester, poly(1,8 octanediolcitrate) (POC), perivascular membrane would prevent neointimal hyperplasia following arterial injury. atRA-POC membranes were prepared and characterized for atRA release via high-performance liquid chromatography with mass spectrometry detection. Rat adventitial fibroblasts (AF) and vascular smooth muscle cells (VSMC) were exposed to various concentrations of atRA; proliferation, apoptosis, and necrosis were assessed in vitro. The rat carotid artery balloon injury model was used to evaluate the impact of the atRA-POC membranes on neointimal formation, cell proliferation, apoptosis, macrophage infiltration, and vascular cell adhesion molecule 1 (VCAM-1) expression in vivo. atRA-POC membranes released 12 μg of atRA over 2 wk, with 92% of the release occurring in the first week. At 24 h, atRA (200 μmol/l) inhibited [(3)H]-thymidine incorporation into AF and VSMC by 78% and 72%, respectively (*P = 0.001), with negligible apoptosis or necrosis. Histomorphometry analysis showed that atRA-POC membranes inhibited neointimal formation after balloon injury, with a 56%, 57%, and 50% decrease in the intimal area, intima-to-media area ratio, and percent stenosis, respectively (P = 0.001). atRA-POC membranes had no appreciable effect on apoptosis or proliferation at 2 wk. Regarding biocompatibility, we found a 76% decrease in macrophage infiltration in the intima layer (P < 0.003) in animals treated with atRA-POC membranes, with a coinciding 53% reduction in VCAM-1 staining (P < 0.001). In conclusion, perivascular delivery of atRA inhibited neointimal formation and restenosis. These data suggest that atRA-POC membranes may be suitable as localized therapy to inhibit neointimal hyperplasia following open cardiovascular procedures.

Effect of bilateral oophorectomy on wound healing of the rabbit vagina

We aimed to assess the effect of bilateral oophorectomy on vaginal wound healing in three groups of New Zealand White rabbits (24 rabbits each). Group 1 underwent bilateral oophorectomy, group 2 underwent a sham surgery, and group 3 served as control. Standardized vaginal tissue specimens were harvested and assessed for wound and scar surface area and tensiometric analysis before wounding, and sequentially thereafter, showing that vaginal wound closure, scar contraction, and recovery of biomechanical properties are significantly slower in oophorectomized rabbits.

Inhibiting intimal hyperplasia in prosthetic vascular grafts via immobilized all-trans retinoic acid

&NA; Peripheral arterial disease is a leading cause of morbidity and mortality. The most commonly utilized prosthetic material for peripheral bypass grafting is expanded polytetrafluoroethylene (ePTFE) yet it continues to exhibit poor performance from restenosis due to neointimal hyperplasia, especially in femoral distal bypass procedures. Recently, we demonstrated that periadventitial delivery of all‐trans retinoic acid (atRA) immobilized throughout porous poly(1,8 octamethylene citrate) (POC) membranes inhibited neointimal formation in a rat arterial injury model. Thus, the objective of this study was to investigate whether atRA immobilized throughout the lumen of ePTFE vascular grafts would inhibit intimal formation following arterial bypass grafting. Utilizing standard ePTFE, two types of atRA‐containing ePTFE vascular grafts were fabricated and evaluated: grafts whereby all‐trans retinoic acid was directly immobilized on ePTFE (atRA‐ePTFE) and grafts where all‐trans retinoic acid was immobilized onto ePTFE grafts coated with POC (atRA‐POC‐ePTFE). All grafts were characterized by SEM, HPLC, and FTIR and physical characteristics were evaluated in vitro. Modification of these grafts, did not significantly alter their physical characteristics or biocompatibility, and resulted in inhibition of intimal formation in a rat aortic bypass model, with atRA‐POC‐ePTFE inhibiting intimal formation at both the proximal and distal graft sections. In addition, treatment with atRA‐POC‐ePTFE resulted in increased graft endothelialization and decreased inflammation when compared to the other treatment groups. This work further confirms the biocompatibility and efficacy of locally delivered atRA to inhibit intimal formation in a bypass setting. Thus, atRA‐POC‐ePTFE grafts have the potential to improve patency rates in small diameter bypass grafts and warrant further investigation. Graphical abstract Figure. No caption available.

Development of poly (1,8 octanediol-co-citrate) and poly (acrylic acid) nanofibrous scaffolds for wound healing applications

Wound care is one of the leading health care problems in the United States costing billions of dollars yearly. Annually, millions of acute wounds occur due to surgical procedures or traumas such as burns and abrasions, and these wounds can become non-healing due to bacterial infection or underlying pathologies. Current wound care treatments include the use of bioinert constructs combined with topical administration of anti-bacterial agents and growth factors. However, there is a growing need for the development of bioactive wound dressing materials that are able to promote wound healing and the regeneration of healthy tissue. In this work, we evaluate and report the use of a novel electrospun polymeric scaffold consisting of poly (1,8 octanediol-co-citrate) and poly (acrylic acid) for wound healing applications. The scaffold exhibits intrinsic antibacterial activity, hydrogel-like water uptake abilities, and the ability to deliver physiologically relevant concentrations of growth factor. Additionally, the scaffold shows antibacterial function when tested with bacteria relevant to wound healing applications. Biological characterization of the electrospun scaffold shows excellent cellular adhesion, low cytotoxicity, and enhanced proliferation of skin fibroblasts. This work has potential towards the development of novel bioactive scaffolds for prevention of bacterial infiltration into the wound bed and enhanced healing.

Nanocomposites for Regenerative Medicine

This chapter describes properties and applications of nanocomposites in tissue engineering and regenerative medicine with an emphasis on the impact of the nanophase on nanocomposite function. The nanophase can be used as a means to engineer new physical properties that improve the utility of tissue engineering scaffolds. Several examples of the use of the nanophase for mechanical reinforcement or drug delivery are discussed with an emphasis on understanding how nanoparticles are used to achieve the controlled release of macromolecules. Advances in nanotechnology, knowledge of mechanical reinforcement at the nanoscale level, and new strategies for controlled drug release will contribute to the next generation of nanocomposite-based scaffolds designed for regenerative medicine.

Fabrication of a bilayer scaffold for small diameter vascular applications: Fabrication of a bilayer scaffold for small diameter vascular applications

One of the greatest challenges plaguing cardiovascular tissue engineering has been the development of a compliant vascular graft. In this work, we report the development of a synthetic vascular graft with compliance similar to native arteries at physiological pressures. A bilayer scaffold was fabricated from a solid polymeric lumen made from poly(1,8 octanediol-co-citrate) (POC) and a microfibrous medial layer composed of type I collagen, elastin, and POC. Mechanical analysis revealed dynamic compliance, ~6.9% within 1% of native vessels, 5.9%. The burst pressure was an order of magnitude lower than native vessels (~400 mmHg vs. ~3000 mmHg) but was above physiological pressure ranges. Biocompatibility studies indicated the scaffold posed no acute cytotoxic risk to relevant cell types and supported the proliferation of vascular smooth muscle cells. In addition, upon exposure of the scaffold to whole blood, there was no statistically significant hemolysis, <2%. Overall this is a promising material system and scaffold to develop a biodegradable tissue-engineered vascular graft.