Biodegradable Elastomers with Antioxidant and Retinoid-like Properties

Urotensin II receptor deficiency ameliorates ligation-induced carotid intimal hyperplasia partially through the RhoA-YAP1 pathway

Intimal hyperplasia (IH) is a common pathological feature of vascular proliferative diseases, such as atherosclerosis and restenosis after angioplasty. Urotensin II (UII) and its receptor (UTR) are widely expressed in cardiovascular tissues. However, it remains unclear whether the UII/UTR system is involved in IH. Right unilateral common carotid artery ligation was performed and maintained for 21 days to induce IH in UTR knockout (UTR-/-) and wild-type (WT) mice. Histological analysis revealed that compared with WT mice, UTR-deficient mice exhibited a decreased neointimal area, angiostenosis and intima-media ratio. Immunostaining revealed fewer smooth muscle cells (SMCs), endothelial cells and macrophages in the lesions of UTR-/- mice than in those of WT mice. Protein interaction analysis suggested that the UTR may affect cell proliferation by regulating YAP and its downstream target genes. In vitro experiments revealed that UII can promote the proliferation and migration of SMCs, and western blotting also revealed that UII increased the protein expression of RhoA, CTGF, Cyclin D1 and PCNA and downregulated p-YAP protein expression, while these effects could be partly reversed by urantide. To evaluate the translational value of UTRs in IH management, WT mice were also treated with two doses of urantide, a UTR antagonist, to confirm the benefit of UTR blockade in IH progression. A high dose of urantide (600 μg/kg/day), rather than a low dose (60 μg/kg/day), successfully improved ligation-induced IH compared with that in mice receiving vehicle. The results of the present study suggested that the UII/UTR system may regulate IH partly through the RhoA-YAP signaling pathway.

Electrospun Scaffolds are Not Necessarily Always Made of Nanofibers as Demonstrated by Polymeric Heart Valves for Tissue Engineering

In the last 30 years, there are ≈60 000 publications about electrospun nanofibers, but it is still unclear whether nanoscale fibers are really necessary for electrospun tissue engineering scaffolds. The present report puts forward this argument and reveals that compared with electrospun nanofibers, microfibers with diameter of ≈3 µm (named as "oligo-micro fiber") are more appropriate for tissue engineering scaffolds owing to their better cell infiltration ability caused by larger pores with available nuclear deformation. To further increase pore sizes, electrospun poly(ε-caprolactone) (PCL) scaffolds are fabricated using latticed collectors with meshes. Fiber orientation leads to sufficient mechanical strength albeit increases porosity. The latticed scaffolds exhibit good biocompatibility and improve cell infiltration. Under aortic conditions in vitro, the performances of latticed scaffolds are satisfactory in terms of the acute systolic hemodynamic functionality, except for the higher regurgitation fraction caused by the enlarged pores. This hierarchical electrospun scaffold with sparse fibers in macropores and oligo-micro fibers in filaments provides new insights into the design of tissue engineering scaffolds, and tissue engineering may provide living heart valves with regenerative capabilities for patients with severe valve disease in the future.

Cell-free biodegradable electroactive scaffold for urinary bladder regeneration

Tissue engineering heavily relies on cell-seeded scaffolds to support the complex biological and mechanical requirements of a target organ. However, in addition to safety and efficacy, translation of tissue engineering technology will depend on manufacturability, affordability, and ease of adoption. Therefore, there is a need to develop scalable biomaterial scaffolds with sufficient bioactivity to eliminate the need for exogenous cell seeding. Herein, we describe synthesis, characterization, and implementation of an electroactive biodegradable elastomer for urinary bladder tissue engineering. To create an electrically conductive and mechanically robust scaffold to support bladder tissue regeneration, we developed a phase-compatible functionalization method wherein the hydrophobic conductive polymer poly(3,4-ethylenedioxythiophene) (PEDOT) was polymerized in situ within a similarly hydrophobic citrate-based elastomer poly(octamethylene-citrate-co-octanol) (POCO) film. We demonstrate the efficacy of this film as a scaffold for bladder augmentation in athymic rats, comparing PEDOT-POCO scaffolds to mesenchymal stromal cell-seeded POCO scaffolds. PEDOT-POCO recovered bladder function and anatomical structure comparably to the cell-seeded POCO scaffolds and significantly better than non-cell seeded POCO scaffolds. This manuscript reports: (1) a new phase-compatible functionalization method that confers electroactivity to a biodegradable elastic scaffold, and (2) the successful restoration of the anatomy and function of an organ using a cell-free electroactive scaffold.

Chromatin reprogramming and bone regeneration in vitro and in vivo via the microtopography-induced constriction of cell nuclei

Topographical cues on cells can, through contact guidance, alter cellular plasticity and accelerate the regeneration of cultured tissue. Here we show how changes in the nuclear and cellular morphologies of human mesenchymal stromal cells induced by micropillar patterns via contact guidance influence the conformation of the cells' chromatin and their osteogenic differentiation in vitro and in vivo. The micropillars impacted nuclear architecture, lamin A/C multimerization and 3D chromatin conformation, and the ensuing transcriptional reprogramming enhanced the cells' responsiveness to osteogenic differentiation factors and decreased their plasticity and off-target differentiation. In mice with critical-size cranial defects, implants with micropillar patterns inducing nuclear constriction altered the cells' chromatin conformation and enhanced bone regeneration without the need for exogenous signalling molecules. Our findings suggest that medical device topographies could be designed to facilitate bone regeneration via chromatin reprogramming.

Materials and Design Approaches for a Fully Bioresorbable, Electrically Conductive and Mechanically Compliant Cardiac Patch Technology

Myocardial infarction (MI) is one of the leading causes of death and disability. Recently developed cardiac patches provide mechanical support and additional conductive paths to promote electrical signal propagation in the MI area to synchronize cardiac excitation and contraction. Cardiac patches based on conductive polymers offer attractive features; however, the modest levels of elasticity and high impedance interfaces limit their mechanical and electrical performance. These structures also operate as permanent implants, even in cases where their utility is limited to the healing period of tissue damaged by the MI. The work presented here introduces a highly conductive cardiac patch that combines bioresorbable metals and polymers together in a hybrid material structure configured in a thin serpentine geometry that yields elastic mechanical properties. Finite element analysis guides optimized choices of layouts in these systems. Regular and synchronous contraction of human induced pluripotent stem cell-derived cardiomyocytes on the cardiac patch and ex vivo studies offer insights into the essential properties and the bio-interface. These results provide additional options in the design of cardiac patches to treat MI and other cardiac disorders.

Engineering multifunctional bioactive citrate-based biomaterials for tissue engineering

Developing bioactive biomaterials with highly controlled functions is crucial to enhancing their applications in regenerative medicine. Citrate-based polymers are the few bioactive polymer biomaterials used in biomedicine because of their facile synthesis, controllable structure, biocompatibility, biomimetic viscoelastic mechanical behavior, and functional groups available for modification. In recent years, various multifunctional designs and biomedical applications, including cardiovascular, orthopedic, muscle tissue, skin tissue, nerve and spinal cord, bioimaging, and drug or gene delivery based on citrate-based polymers, have been extensively studied, and many of them have good clinical application potential. In this review, we summarize recent progress in the multifunctional design and biomedical applications of citrate-based polymers. We also discuss the further development of multifunctional citrate-based polymers with tailored properties to meet the requirements of various biomedical applications.

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Multifunctional bioactive citrate-based biomaterials have broad applications in regenerative medicine.

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Recent advances in multifunctional design and biomedical applications of citate-based polymers are summarized.

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Future challenge of citrate-based polymers in various biomedical applications are discussed.

Coating small-diameter ePTFE vascular grafts with tunable poly(diol-co-citrate-co-ascorbate) elastomers to reduce neointimal hyperplasia

Lack of long-term patency has hindered the clinical use of small-diameter prosthetic vascular grafts with the majority of these failures due to the development of neointimal hyperplasia. Previous studies by our laboratory revealed that small-diameter expanded polytetrafluoroethylene (ePTFE) grafts coated with antioxidant elastomers are a promising localized therapy to inhibit neointimal hyperplasia. This work is focused on the development of poly(diol-co-citrate-co-ascorbate) (POCA) elastomers with tunable properties for coating ePTFE vascular grafts. A bioactive POCA elastomer (@20 : 20 : 8, [citrate] : [diol] : [ascorbate]) coating was applied on a 1.5 mm diameter ePTFE vascular graft as the most promising therapeutic candidate for reducing neointimal hyperplasia. Surface ascorbate density on the POCA elastomer was increased to 67.5 ± 7.3 ng mg-1 cm-2. The mechanical, antioxidant, biodegradable, and biocompatible properties of POCA demonstrated desirable performance for in vivo use, inhibiting human aortic smooth muscle cell proliferation, while supporting human aortic endothelial cells. POCA elastomer coating number was adjusted by a modified spin-coating method to prepare small-diameter ePTFE vascular grafts similar to natural vessels. A significant reduction in neointimal hyperplasia was observed after implanting POCA-coated ePTFE vascular grafts in a guinea pig aortic interposition bypass graft model. POCA elastomer thus offers a new avenue that shows promise for use in vascular engineering to improve long-term patency rates by coating small-diameter ePTFE vascular grafts.

All-trans retinoic acid attenuates the progression of Ang II-induced abdominal aortic aneurysms in ApoE-/-mice

Background

To determine whether all-trans retinoic acid (ATRA) can influence the development of Angiotensin II (Ang II) induced experimental abdominal aortic aneurysms (AAAs).

Methods

Apolipoprotein E knock-out (ApoE^−/−) mice were randomly assigned to 4 groups. Mice in the AAA and ATRA groups underwent continuous subcutaneous Ang II infusion for 28 days to induce AAA, while the Sham and Control groups were infused with saline. Systolic blood pressure was measured by the tail-cuff technique. The Control and ATRA groups received ATRA treatment. Aortic tissue samples were obtained at 28 days after surgery and evaluated by aortic diameter measurement, Western blotting, immunohistochemistry, and hematoxylin-eosin (H&E) and Verhoeff-Van Gieson (EVG) staining.

Results

The abdominal aortic diameter was significantly reduced in the ATRA group compared with the AAA group (3 of 12 (25%) vs 9 of 12 (75%), P < 0.05), and the ATRA group exhibited reduced blood pressure on days 7, 14, and 28. Low expression of angiotensin II receptor type 1 (AT1), matrix metalloproteinase 2 (MMP2), and matrix metalloproteinase 9 (MMP9) and EVG staining revealed a significant reduction in the disruption of elastic fibers in the abdominal aortic tissue of the ATRA group compared to the AAA group. Western blot analysis indicated that protein levels of retinoic acid receptor α (RARα), MMP2, MMP9, and AT1 were dramatically affected by ATRA treatment.

Conclusions

In conclusion, ATRA attenuates the progression of Ang II-induced AAAs, possibly by downregulating MMP2, MMP9, and AT-1 expression.

Development of Poly(1,8-octanediol-co-citrate-co-ascorbate) Elastomers with Enhanced Ascorbate Performance for Use as a Graft Coating to Prevent Neointimal Hyperplasia

Small-diameter expanded polytetrafluoroethylene (ePTFE) graft surfaces have poor long-term patency due to limited endothelial cell (EC) coverage and anastomotic intimal hyperplasia. Multifunctional elastomers that coat the ePTFE graft surface to promote EC adhesion while simultaneously inhibiting intimal hyperplasia are highly desirable. Poly(diol-co-citrate) (PDC), a thermoset elastomer, is biodegradable, biocompatible, and mimics vascular mechanical properties. Engineering antioxidant components into PDC polymeric structures improves biocompatibility by attenuating oxidative stress yet is limited by bioavailability. Herein, we develop a new ascorbate protection and deprotection strategy (APDS) for loading bioactive ascorbic acid into the structure of PDC elastomers to improve poly(1,8-octanediol-co-citrate-co-ascorbate) (POCA) prepolymer ascorbate activity. Elastomers cured from APDS POCA prepolymers provide twice the active ascorbate sites on the elastomer surface (35.19 ± 1.64 ng mg-1 cm-2) versus unprotected POCA (Un.POCA, 18.31 ± 0.97 ng mg-1 cm-2). APDS POCA elastomers displayed suitable mechanical properties for vascular graft coating [Young's modulus (2.15-2.61 MPa), elongation (189.5-214.6%) and ultimate tensile strength (2.73-3.61 MPa)], and superior surface antioxidant performance through 1,1-diphenyl-2-picrylhydrazyl free radical scavenging and lipid peroxidation inhibition as compared to poly(1,8-octanediol-co-citrate) (POC) and Un.POCA. Hydrolytic degradation of APDS POCA occurred within 12 weeks under physiological conditions with a mass loss of 25.8 ± 3.4% and the degradation product retaining ascorbate activity. APDS POCA elastomer surfaces supported human aortic endothelial cell proliferation while inhibiting human aortic smooth muscle cell proliferation in vitro. APDS POCA elastomer surfaces displayed superior decomposition of S-nitrosothiols compared to POC and Un.POCA. Taken together, these findings indicate the potential of APDS POCA elastomers to serve as bioactive, therapeutic coatings that enhance the long-term patency of small diameter ePTFE grafts.

Full biodegradable elastomeric nanocomposites fabricated by chitin nanocrystal and poly(caprolactone-diol citrate) elastomer

Chitin nanocrystal is a biocompatible and biodegradable nanofiller, with great potential in enhancing the mechanical and biological properties of polymers. Poly(caprolactone-diol citrate) is a kind of citrate-based biodegradable elastomer prepared by an additive-free melt polycondensation of polycaprolactone-diol and citric acid coupled with subsequent thermocuring. Here, a facile casting/evaporation method was utilized to prepare full biodegradable poly(caprolactone-diol citrate)/chitin nanocrystal nanocomposites, and their structure and properties were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, uniaxial tensile test, dynamic mechanical analysis, surface wettability and swelling analysis, thermogravimetric analysis, in vitro degradation, and cytocompatibility test. The results showed the chitin nanocrystals were uniformly distributed in the poly(caprolactone-diol citrate) matrix; with increasing chitin nanocrystal loading, the tensile modulus and strength significantly increased; furthermore, the incorporation of chitin nanocrystals endowed the poly(caprolactone-diol citrate) with more hydrophilicity, lower swelling in phosphate buffered saline solution, slow degradation rate, and greatly improved cytocompatibility. Thus, the chitin nanocrystal was a good bio-based nanofiller that could be used to tune the properties of poly(caprolactone-diol citrate) degradable bioelastomer.

Polyester elastomers for soft tissue engineering

Polyester elastomers are soft, biodegradable and biocompatible and are commonly used in various biomedical applications, especially in tissue engineering. These synthetic polyesters can be easily fabricated using various techniques such as solvent casting, particle leaching, molding, electrospinning, 3-dimensional printing, photolithography, microablation etc. A large proportion of tissue engineering research efforts have focused on the use of allografts, decellularized animal scaffolds or other biological materials as scaffolds, but they face the major concern of triggering immunological responses from the host, on top of other issues. This review paper will introduce the recent developments in elastomeric polyesters, their synthesis and fabrication techniques, as well as their application in the biomedical field, focusing primarily on tissue engineering in ophthalmology, cardiac and vascular systems. Some of the commercial and near-commercial polyesters used in these tissue engineering fields will also be described.

A thermoresponsive, citrate-based macromolecule for bone regenerative engineering

There is a need in orthopaedic and craniomaxillofacial surgeries for materials that are easy to handle and apply to a surgical site, can fill and fully conform to the bone defect, and can promote the formation of new bone tissue. Thermoresponsive polymers that undergo liquid to gel transition at physiological temperature can potentially be used to meet these handling and shape-conforming requirements. However, there are no reports on their capacity to induce in vivo bone formation. The objective of this research was to investigate whether the functionalization of the thermoresponsive, antioxidant macromolecule poly(poly-ethyleneglycol citrate-co-N-isopropylacrylamide) (PPCN), with strontium, phosphate, and/or the cyclic RGD peptide would render it a hydrogel with osteoinductive properties. We show that all formulations of functionalized PPCN retain thermoresponsive properties and can induce osteodifferentiation of human mesenchymal stem cells without the need for exogenous osteogenic supplements. PPCN-Sr was the most osteoinductive formulation in vitro and produced robust localized mineralization and osteogenesis in subcutaneous and intramuscular tissue in a mouse model. Strontium was not detected in any of the major organs. Our results support the use of functionalized PPCN as a valuable tool for the recruitment, survival, and differentiation of cells critical to the development of new bone and the induction of bone formation in vivo. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 1743-1752, 2018.

Coordination-driven multilayer of phosvitin-polyphenol functional nanofibrous membranes: antioxidant and biomineralization applications for tissue engineering

The layer-by-layer (LBL) deposition technique has been widely used to decorate the nanofibers formed from polymer pairs with complementary functional groups.