Thick PCL Fibers Improving Host Remodeling of PGS-PCL Composite Grafts Implanted in Rat Common Carotid Arteries.

Vasculopathy and the consequential ischemia are major medical challenges. Grafting is an effective treatment to vascular occlusion. However, autologous grafting, despite scarcity, is the only choice for small diameter blood vessels. Synthetic grafts can fill the gap if they can work satisfactorily in arterial circulation. Electrospun polycaprolactone (PCL) sheathed porous poly(glycerol sebacate) (PGS) vascular grafts have good performances in arterial circulation in abdominal aortas and carotid arteries in rats. However, a major issue associated with the graft remodeling in vivo is limited neo-tissue formation inside PCL sheaths. Small pores of PCL sheaths inhibit cell infiltration and migration. To increase porosity of PCL sheaths of PGS-PCL composite grafts, diameters of electrospun PCL fibers are increased. The thick PCL fibers encourage cell migration and elicit a higher degree of CD206+ cells. In addition, some of the CD206+ cells co-express vascular cell markers in the thick-fiber grafts. The thick-fiber grafts also show improved mechanical properties and a higher elastin and collagen content. The data demonstrate the feasibility of improving graft vascular remodeling by increasing PCL fiber diameters and the critical role of CD206+ cells during graft vascular remodeling.

Blending with transition metals improves bioresorbable zinc as better medical implants.

Zinc (Zn) is a new class of bioresorbable metal that has potential for cardiovascular stent material, orthopedic implants, wound closure devices, etc. However, pure Zn is not ideal for these applications due to its low mechanical strength and localized degradation behavior. Alloying is the most common/effective way to overcome this limitation. Still, the choice of alloying element is crucial to ensure the resulting alloy possesses sufficient mechanical strength, suitable degradation rate, and acceptable biocompatibility. Hereby, we proposed to blend selective transition metals (i.e., vanadium-V, chromium-Cr, and zirconium-Zr) to improve Zn's properties. These selected transition metals have similar properties to Zn and thus are beneficial for the metallurgy process and mechanical property. Furthermore, the biosafety of these elements is of less concern as they all have been used as regulatory approved medical implants or a component of an implant such as Ti6Al4V, CoCr, or Zr-based dental implants. Our study showed the first evidence that blending with transition metals V, Cr, or Zr can improve Zn's properties as bioresorbable medical implants. In addition, three in vivo implantation models were explored in rats: subcutaneous, aorta, and femoral implantations, to target the potential clinical applications of bioresorbable Zn implants.

Mir-22-incorporated polyelectrolyte coating prevents intima hyperplasia after balloon-induced vascular injury.

Drug-coated balloons (DCBs) offer potential to deliver drugs to treat coronary lesions but without leaving permanent implants behind. Paclitaxel and sirolimus are anti-proliferation drugs that are commonly used in commercially available DCBs. However, these drugs present significant cytotoxicity concern and low efficacy in vivo. Here, we use microRNA-22 (miR-22) as balloon loaded drugs and polyelectrolyte complexes (PECs) polyethyleneimine/polyacrylic acid (PEI/PAA) as balloon coatings to establish a new DCB system through the ultrasonic spray method. The PEI/PAA forms a stable and thin coating on the balloon, which resulted in a good transfer capacity to the vessel wall both in vitro and in vivo. miR-22 that could modulate smooth muscle cell (SMC) phenotype switching is incorporated into the PEI/PAA coating and shows a sustained release profile. The PEI/PAA/miR-22 coated balloon successfully inhibits intima hyperplasia after balloon-induced vascular injury in a rat model through decreasing proliferative SMCs via the miR-22-methyl-CpG binding protein 2 (MECP2) axis. Our findings indicate that balloons coated with PEI/PAA/miR-22 have great potential to be promising DCBs in the treatment of cardiovascular disease.

Evolution of metallic cardiovascular stent materials: A comparative study among stainless steel, magnesium and zinc.

A cardiovascular stent is a small mesh tube that expands a narrowed or blocked coronary artery. Unfortunately, current stents, regardless metallic or polymeric, still largely fall short to the ideal clinical needs due to late restenosis, thrombosis and other clinical complications. Nonetheless, metallic stents are preferred clinically thanks to their superior mechanical property and radiopacity to their polymeric counterparts. The emergence of bioresorbable metals opens a window for better stent materials as they may have the potential to reduce or eliminate late restenosis and thrombosis. In fact, some bioresorbable magnesium (Mg)-based stents have obtained regulatory approval or under trials with mixed clinical outcomes. Some major issues with Mg include the too rapid degradation rate and late restenosis. To mitigate these problems, bioresorbable zinc (Zn)-based stent materials are being developed lately with the more suitable degradation rate and better biocompatibility. The past decades have witnessed the unprecedented evolution of metallic stent materials from first generation represented by stainless steel (SS), to second generation represented by Mg, and to third generation represented by Zn. To further elucidate their pros and cons as metallic stent materials, we systematically evaluated their performances in vitro and in vivo through direct side-by-side comparisons. Our results demonstrated that tailored Zn-based material with proper configurations could be a promising candidate for a better stent material in the future.

Slow degrading poly(glycerol sebacate) derivatives improve vascular graft remodeling in a rat carotid artery interposition model.

Porous synthetic grafts made of poly (glycerol sebacate) (PGS) can transform into autologous vascular conduits in vivo upon degradation of PGS. A long-held doctrine in tissue engineering is the necessity to match degradation of the scaffolds to tissue regeneration. Here, we tested the impact of degradation of PGS and its derivative in an interposition model of rat common carotid artery (CCA). Previous work indicates a complete degradation of PGS within approximately 2 weeks, likely at the fast end of the spectrum. Thus, the derivation of PGS focuses on delay degradation by conjugating the free hydroxy groups in PGS with a long chain carboxylic acid: palmitic acid, one of the most common lipid components. We evaluated two of the resultant palmitate-PGS (PPGS) in this study: one containing 9% palmitate (9-PPGS) and the other16% palmitate (16-PPGS). 16-PPGS grafts had the highest patency. Ultrasound imaging showed that the lumens of 16-PPGS grafts were similar to CCA and smaller than 9-PPGS and PGS grafts 12 weeks post-operation. Immunohistological and histological examination showed an endothelialized lumens in all three types of grafts within 4 weeks. Inflammatory responses to 16-PPGS grafts were limited to the adventitial space in contrast to a more diffusive infiltration in 9-PPGS and PGS grafts in week 4. Examination of calponin+ and αSMA+ cells revealed that 16-PPGS grafts remodeled into a distinctive bi-layered wall, while the walls of 9-PPGS grafts and PGS grafts only had one thick layer of smooth muscle-like cells. Correspondingly, the expression of collagen III and elastin displayed an identical layered structure in the remodeled 16-PPGS grafts, in contrast to a more spread distribution in 9-PPGS and PGS grafts. All the three types of grafts exhibited the same collagen content and burst pressure after 12 weeks of host remodeling. However, the compliance and elastin content of 16-PPGS grafts in week 12 were closest to those of CCA. Overall, placing the degradation of PGS derived elastomer to a window of 4-12 weeks results in vascular conduits closer to arteries in a rat carotid artery interposition model over a 12-week observation period.

Mechanical Strength, Biodegradation, and in Vitro and in Vivo Biocompatibility of Zn Biomaterials.

Zn-based biomaterials have emerged as promising new types of bioresorbable metallics applicable to orthopedic devices, cardiovascular stents, and other medical applications recently. Compared to other degradable metallic biomaterials (i.e., Mg- or Fe-based), Zn biomaterials have a more appropriate corrosion rate without hydrogen gas evolution. Here, we evaluated the potential of Zn-based metallics as medical implants, both in vitro and in vivo, alongside a standard benchmark Mg alloy, AZ31. The mechanical properties of the pure Zn were not strong enough but were significantly enhanced (microhardness > 70 kg/mm2, strength > 220 MPa, elongation > 15%) after alloying with Sr or Mg (1.5 at. %), surpassing the minimal design criteria for load-bearing device applications. The corrosion rate of Zn-based biomaterials was about 0.4 mm/year, significantly slower than that of AZ31. The measured cell viability and proliferation of three different human primary cells fared better for Zn-based biomaterials than AZ31 using both direct and indirect culture methods. Platelet adhesion and activation on Zn-based materials were minimal, significantly less than on AZ31. The hemolysis ratio of red cells (<0.5%) after incubation with Zn-based materials was also well below the ISO standard of 5%. Moreover, Zn-based biomaterials promoted stem cell differentiation to induce the extracellular matrix mineralization process. In addition, in vivo animal testing using subcutaneous, bone, and vascular implantations revealed that the acute toxicity and immune response of Zn-based biomaterials were minimal/moderate, comparable to that of AZ31. No extensive cell death and foreign body reactions were observed. Taken together, Zn-based biomaterials may have a great potential as promising candidates for medical implants.

Optimization of a polydopamine (PD)-based coating method and polydimethylsiloxane (PDMS) substrates for improved mouse embryonic stem cell (ESC) pluripotency maintenance and cardiac differentiation.

Myocardiocyte derived from pluripotent stem cells, such as induced pluripotent stem cells (iPSCs) and embryonic stem cells (ESCs), is a promising cell source for cardiac tissue engineering. Combined with microfluidic technologies, a heart-on-a-chip is very likely to be developed and function as a platform for high throughput drug screening. Polydimethylsiloxane (PDMS) silicone elastomer is a widely-used biomaterial for the investigation of cell-substrate interactions and biochip fabrication. However, the intrinsic PDMS surface hydrophobicity inhibits cell adhesion on the PDMS surface, and PDMS surface modification is required for effective cell adhesion. Meanwhile, the formulation of PDMS also affects the behaviors of the cells. To fabricate PDMS-based biochips for ESC pluripotency maintenance and cardiac differentiation, PDMS surface modification and formulation were optimized in this study. We found that a polydopamine (PD) with gelatin coating greatly improved the ESC adhesion, proliferation and cardiac differentiation on its surface. In addition, different PDMS substrates varied in their surface properties, which had different impacts on ESCs, with the 40 : 1 PDMS substrate being more favorable for ESC adhesion and proliferation as well as embryoid body (EB) attachment than the other PDMS substrates. Moreover, the ESC pluripotency was best maintained on the 5 : 1 PDMS substrate, while the cardiac differentiation of the ESCs was optimal on the 40 : 1 PDMS substrate. Based on the optimized coating method and PDMS formulation, biochips with two different designs were fabricated and evaluated. Compared to the single channels, the multiple channels on the biochips could provide larger areas and accommodate more nutrients to support improved ESC pluripotency maintenance and cardiac differentiation. These results may contribute to the development of a real heart-on-a-chip for high-throughput drug screening in the future.

Murine pluripotent stem cells derived scaffold-free cartilage grafts from a micro-cavitary hydrogel platform.

By means of appropriate cell type and scaffold, tissue-engineering approaches aim to construct grafts for cartilage repair. Pluripotent stem cells especially induced pluripotent stem cells (iPSCs) are of promising cell candidates due to the pluripotent plasticity and abundant cell source. We explored three dimensional (3D) culture and chondrogenesis of murine iPSCs (miPSCs) on an alginate-based micro-cavity hydrogel (MCG) platform in pursuit of fabricating synthetic-scaffold-free cartilage grafts. Murine embryonic stem cells (mESCs) were employed in parallel as the control. Chondrogenesis was fulfilled using a consecutive protocol via mesoderm differentiation followed by chondrogenic differentiation; subsequently, miPSC and mESC-seeded constructs were further respectively cultured in chondrocyte culture (CC) medium. Alginate phase in the constructs was then removed to generate a graft only comprised of induced chondrocytic cells and cartilaginous extracellular matrix (ECMs). We found that from the mESC-seeded constructs, formation of intact grafts could be achieved in greater sizes with relatively fewer chondrocytic cells and abundant ECMs; from miPSC-seeded constructs, relatively smaller sized cartilaginous grafts could be formed by cells with chondrocytic phenotype wrapped by abundant and better assembled collagen type II. This study demonstrated successful creation of pluripotent stem cells-derived cartilage/chondroid graft from a 3D MCG interim platform. By the support of materials and methodologies established from this study, particularly given the autologous availability of iPSCs, engineered autologous cartilage engraftment may be potentially fulfilled without relying on the limited and invasive autologous chondrocytes acquisition. STATEMENT OF SIGNIFICANCE: In this study, we explored chondrogenic differentiation of pluripotent stem cells on a 3D micro-cavitary hydrogel interim platform and creation of pluripotent stem cells-derived cartilage/chondroid graft via a consecutive procedure. Our results demonstrated chondrogenic differentiation could be realized on the platform via mesoderm differentiation. The mESCs/miPSCs derived chondrocytic cells were further cultured to finally generate a pluripotent stem cells-derived scaffold-free construct based on the micro-cavitary hydrogel platform, in which alginate hydrogel could be removed finally. Our results showed that miPSC-derived graft could be formed by cells with chondrocytic phenotype wrapped by abundant and assembled collagen type II. To our knowledge, this study is the first study that initials from pluripotent stem cell seeding on 3D scaffold environment and ends with a scaffold-free chondrogenic micro-tissue. By the support of materials and methodologies established from this study, engineered autologous iPSC-derived cartilage engraftment may be potentially developed instead of autologous chondrocytes grafts that have limited source.

A mussel-inspired double-crosslinked tissue adhesive intended for internal medical use.

It has been a great challenge to develop aldehyde-free tissue adhesives that can function rapidly and controllably on wet internal tissues with fine adhesion strength, sound biocompatibility and degradability. To this end, we have devised a mussel-inspired easy-to-use double-crosslink tissue adhesive (DCTA) comprising a dopamine-conjugated gelatin macromer, a rapid crosslinker (namely, Fe(3+)), and a long-term acting crosslinker (namely, genipin). As a mussel-inspired gluing macromer, dopamine is grafted onto gelatin backbone via an one-step reaction, the catechol groups of which are capable of performing strong wet adhesion on tissue surfaces. By addition of genipin and Fe(3+), the formation of catechol-Fe(3+) complexation and accompanying spontaneous curing of genipin-primed covalent crosslinking of gluing macromers in one pot endows DCTA with the double-crosslink adhesion mechanism. Namely, the reversible catechol-Fe(3+) crosslinking executes an controllable and instant adhesive curing; while genipin-induced stable covalent crosslinking promises it with long-term effectiveness. This novel DCTA exhibits significantly higher wet tissue adhesion capability than the commercially available fibrin glue when applied on wet porcine skin and cartilage. In addition, this DCTA also demonstrates fine elasticity, sound biodegradability, and biocompatibility when contacting in vitro cultured cells and blood. In vivo biocompatibility and biodegradability are checked and confirmed via trials of subcutaneous implantation in nude mice model. This newly developed DCTA may be a highly promising product as a biological glue for internal medical use including internal tissue adhesion, sealing, and hemostasis. STATEMENT OF SIGNIFICANCE: There is a great demand for ideal tissue adhesives that can be widely used in gluing wet internal tissues. Here, we have devised a mussel-inspired easy-to-use double-crosslink tissue adhesive (DCTA) that meets the conditions as an ideal tissue adhesive. It is composed of gelatin-dopamine conjugates - a gluing macromer, Fe(3+) - a rapid crosslinker, and genipin - a long-term acting crosslinker. This DCTA is constructed with a novel complexation-covalent double-crosslinking principle in one pot, in which the catechol-Fe(3+) crosslinking executes a controllable and instant adhesive curing, at the same time, genipin-induced covalent crosslinking promises it with long-term effectiveness in physiology conditions. This novel DCTA, with excellent wet tissue adhesion capability, fine elasticity, sound biodegradability, and biocompatibility, is a promising biological glue for internal medical use in surgical operations.