Tissue compatibility of interfacial polyelectrolyte complexation fibrous scaffold: evaluation of blood compatibility and biocompatibility

Development of a novel thermogelling PEC-based ECM mimicking nanocomposite bioink for bone tissue engineering

Non-union of large bone defects has been an existing clinical problem. 3D extrusion-based bioprinting provides an efficient approach to tackle such problems. This approach enables the use of various biomaterials, cell types and growth factors in developing a superior bone graft that is specific to the defect. In this article, we have designed and printed an ECM mimicking, self-assembled polyelectrolyte complex (PEC) based fibrous bioink using natural polymers like chitosan-polygalacturonic acid (PGA) and other biomaterials - gelatin, laponite and nanohydroxyapatite with a modified 3D printer. The developed bioink possesses a thermo-reversible sol-gel transition at physiological pH and temperature. Here, we demonstrated that post-printing, our fiber-reinforced bioink had significant cell proliferation with cell viability of >80% and negligible cell morbidity. The practicability of developing this self-assembled PEC-based bioink was assessed. Bioink with 4% gelatin (PECHLG4) had optimal printability with a minimal swelling ratio of approximately 3%. The printed scaffold had integrity for a period of 8 days under 0.5 mg/mL lysozyme concentration. We also evaluated the mechanical property of the bioink using compression analysis which gave an elastic modulus of 16 KPa. This combination of natural polymers and nanocomposite, along with a fibrous network of PECs, is itself a novel approach for 3D bioprinting and can be a preliminary proposition for the treatment of large bone defects.

Sustained release of TGF-β3 from polysaccharide nanoparticles induces chondrogenic differentiation of human mesenchymal stromal cells

Medical treatment of certain diseases and biomedical implants are tending to use delivery systems on the nanoscale basis for biologically active factors including drugs (e. g. antibiotics) or growth factors. Nanoparticles are a useful tool to deliver bioactive substances of different chemical nature directly to the site where it is required in the patient. Here we developed three innovative delivery systems based on different polysaccharides in order to induce a sustained release of TGF-β₃ to mediate chondrogenesis of human mesenchymal stromal cells. We were able to encapsulate the protein into nanoparticles and subsequently release TGF-β₃ from these particles. The protein was still active and was able to induce chondrogenic differentiation of human mesenchymal stromal cells.

In vitro and in vivo biocompatibility assessment of free radical scavenging nanocomposite scaffolds for bone tissue regeneration

Bone is the second most transplanted tissue in the world, resulting in increased demand for bone grafts leading to the fabrication of synthetic scaffold grafting alternatives. Fracture sites are under increased oxidative stress after injuries, affecting osteoblast function and hindering fracture healing and remodeling. To counter oxidative stress, free radical scavenging agents, such as cerium oxide nanoparticles, have gained traction in tissue engineering. Toward the goal of developing a functional synthetic system for bone tissue engineering, we characterized the biocompatibility of a porous, bioactive, free radical scavenging nanocomposite scaffold composed of poly(1,8 octanediol-co-citrate), beta-tricalcium phosphate, and cerium oxide nanoparticles. We studied cellular and tissue compatibility utilizing in vitro and in vivo models to assess nanocomposite interactions with both human osteoblast cells and rat subcutaneous tissue. We found the scaffolds were biocompatible in both models and supported cell attachment, proliferation, mineralization, and infiltration. Using hydrogen peroxide, we simulated oxidative stress to study the protective properties of the nanocomposite scaffolds via a reduction in cytotoxicity and recovered mineralization of osteoblast cells in vitro. We also found after implantation in vivo the scaffolds exhibited biocompatible properties essential for successful scaffolds for bone tissue engineering. Cells were able to infiltrate through the scaffolds, the surrounding tissues elicited a minimal immune response, and there were signs of scaffold degradation after 30 days of implantation. After the array of biological characterization, we had confirmed the development of a nanocomposite scaffold system capable of supporting bone-remodeling processes while providing a protective free radical scavenging effect.

Biomimetic modification of poly(vinyl alcohol): Encouraging endothelialization and preventing thrombosis with antiplatelet monotherapy

Poly(vinyl alcohol) (PVA) has shown promise as a biomaterial for cardiovascular application. However, its antifouling properties prevent in vivo endothelialization. This work examined the endothelialization and thrombogenicity of modified PVA with different concentrations of proteins and adhesion peptides: collagen, laminin, fibronectin, GFPGER, YIGSR, and cRGD. Material surface properties were quantified, and the endothelialization potential was determined with human endothelial colony forming cells. Additionally, platelet attachment was assessed in vitro with human platelet rich plasma, and promising samples were tested in an ex vivo shunt model. This well-established arteriovenous shunt model was used with and without clinically-relevant antiplatelet therapies, specifically acetylsalicylic acid (ASA) with and without clopidogrel to examine the minimum necessary treatment to prevent thrombosis. Collagen, laminin, and GFPGER biomolecules increased endothelialization, with GFPGER showing the greatest effect at the lowest concentrations. GFPGER-PVA tubes tested under whole blood did exhibit an increase in platelet (but not fibrin) attachment compared to plain PVA and clinical controls. However, application of ASA monotherapy reduced the thrombogenicity of GFPGER-PVA below the clinical control with the ASA. This work is significant in developing cardiovascular biomaterials-increasing endothelialization potential while reducing bleeding side effects by using an antiplatelet monotherapy, typical of clinical patients. STATEMENT OF SIGNIFICANCE: We modified the endothelialization potential of synthetic, hydrogel vascular grafts with proteins and peptides of the vascular tissue matrix. Cell attachment was dramatically increased with the GFPGER peptide, and while some additional platelet attachment was seen under flow with whole blood, this was completely knocked down using clinical antiplatelet monotherapy. This indicates that long-term patency of this biomaterial could be improved without the associated bleeding risk of multiple platelet therapies.

Submillimeter Diameter Poly(Vinyl Alcohol) Vascular Graft Patency in Rabbit Model

Microvascular surgery is becoming a prevalent surgical practice. Replantation, hand reconstruction, orthopedic, and free tissue transfer procedures all rely on microvascular surgery for the repair of venous and arterial defects at the millimeter and submillimeter levels. Often, a vascular graft is required for the procedure as a means to bridge the gap between native arteries. While autologous vessels are desired for their bioactivity and non-thrombogenicity, the tedious harvest process, lack of availability, and caliber or mechanical mismatch contribute to graft failure. Thus, there is a need for an off-the-shelf artificial vascular graft that has low thrombogenic properties and mechanical properties matching those of submillimeter vessels. Poly(vinyl alcohol) hydrogel (PVA) has excellent prospects as a vascular graft due to its bioinertness, low thrombogenicity, high water content, and tunable mechanical properties. Here, we fabricated PVA grafts with submillimeter diameter and mechanical properties that closely approximated those of the rabbit femoral artery. In vitro platelet adhesion and microparticle release assay verified the low thrombogenicity of PVA. A stringent proof-of-concept in vivo test was performed by implanting PVA grafts in rabbit femoral artery with multilevel arterial occlusion. Laser Doppler measurements indicated the improved perfusion of the distal limb after implantation with PVA grafts. Moreover, ultrasound Doppler and angiography verified that the submillimeter diameter PVA vascular grafts remained patent for 2 weeks without the aid of anticoagulant or antithrombotics. Endothelial cells were observed in the luminal surface of one patent PVA graft. The advantageous non-thrombogenic and tunable mechanical properties of PVA that are retained even in the submillimeter diameter dimensions support the application of this biomaterial for vascular replacement in microvascular surgery.

In vitro and ex vivo hemocompatibility of off-the-shelf modified poly(vinyl alcohol) vascular grafts

Synthetic small diameter vascular grafts with mechanical properties of native arteries, resistance to thrombosis and capacity to stimulate in situ endothelialization are an unmet clinical need. Poly(vinyl alcohol) hydrogel (PVA) is an excellent candidate as a vascular graft due to its tunable mechanical properties. However, the hydrophilicity and bio-inertness of PVA prevents endothelialization in vivo. We hypothesize that the modification of PVA with biomolecules and topographies creates a hemocompatible environment that also enhances bioactivity. PVA modified with fibronectin, RGDS peptide, cyclicRGD (cRGD) peptide, or heparin provided cell-adhesion motifs, which were confirmed by detection of nitrogen through X-ray photoelectron spectroscopy. Protein- and peptide-modified surfaces showed a slight increase in human vascular endothelial cell proliferation over unmodified PVA. With the exception of fibronectin modification, modified surfaces showed in vitro hemocompatibility comparable with unmodified PVA. To further improve bioactivity, cRGD-PVA was combined with gratings and microlens topographies. Combined modifications of 2 μm gratings or convex topography and cRGD significantly improved human vascular endothelial cell viability on PVA. In vitro hemocompatibility testing showed that topography on cRGD-PVA did not significantly trigger an increase of platelet adhesion or activation compared with unpatterned PVA. Using the more physiologically relevant ex vivo hemocompatibility testing, all PVA grafts tested showed similar platelet adhesion to ePTFE and significantly lower platelet accumulation compared to collagen-coated ePTFE grafts. The biochemical and topographical modification of PVA demonstrates excellent hemocompatibility with enhanced bioactivity of PVA, thus highlighting its potential as a vascular graft.

STATEMENT OF SIGNIFICANCE

New synthetic small diameter vascular grafts with mechanical properties, blood-clot resistance and endothelial lining mimicking native arteries remains an unresolved critical clinical need. We aim to achieve this by modifying the mechanically-tunable poly(vinyl alcohol) hydrogel (PVA) vascular graft with both biochemical and biophysical cues in the lumenal surface. PVA modified with cyclic RGD peptide and ordered micrometer-sized topography showed low platelet adhesion in both a rabbit in vitro and baboon ex vivo blood compatibility assay. Modified PVA also exhibited significant enhancement of human vascular endothelial cell viability and proliferation in vitro. The readily available, modified PVA grafts are the first to show biophysical and biochemical modification in a three-dimensional scaffold with hemocompatibility, biofunctionality and excellent potential for clinical application.

Composite Scaffolds of Interfacial Polyelectrolyte Fibers for Temporally Controlled Release of Biomolecules

Various scaffolds used in tissue engineering require a controlled biochemical environment to mimic the physiological cell niche. Interfacial polyelectrolyte complexation (IPC) fibers can be used for controlled delivery of various biological agents such as small molecule drugs, cells, proteins and growth factors. The simplicity of the methodology in making IPC fibers gives flexibility in its application for controlled biomolecule delivery. Here, we describe a method of incorporating IPC fibers into two different polymeric scaffolds, hydrophilic polysaccharide and hydrophobic polycaprolactone, to create a multi-component composite scaffold. We showed that IPC fibers can be easily embedded into these polymeric structures, enhancing the capability for sustained release and improved preservation of biomolecules. We also created a composite polymeric scaffold with topographical cues and sustained biochemical release that can have synergistic effects on cell behavior. Composite polymeric scaffolds with IPC fibers represent a novel and simple method of recreating the cellular niche.

In vivo bioactivity of rhBMP-2 delivered with novel polyelectrolyte complexation shells assembled on an alginate microbead core template

Electrostatic interactions between polycations and polyanions are being explored to fabricate polyelectrolyte complexes (PEC) that could entrap and regulate the release of a wide range of biomolecules. Here, we report the in vivo application of PEC shells fabricated from three different polycations: poly-l-ornithine (PLO), poly-l-arginine (PLA) and DEAE-dextran (DEAE-D) to condense heparin on the surface of alginate microbeads and further control the delivery of recombinant human bone morphogenetic protein 2 (rhBMP-2) in spinal fusion application. We observed large differences in the behavior of PEC shells fabricated from the cationic polyamino acids (PLO and PLA) when compared to the cationic polysaccharide, DEAE-D. Whereas DEAE-D-based PEC shells eroded and released rhBMP-2 over 2 days in vitro, PLO- and PLA-based shells retained at least 60% of loaded rhBMP-2 after 3 weeks of incubation in phosphate-buffered saline. In vivo implantation in a rat model of posterolateral spinal fusion revealed robust bone formation in the PLO- and PLA-based PEC shell groups. This resulted in a significantly enhanced mechanical stability of the fused segments. However, bone induction and biomechanical stability of spine segments implanted with DEAE-D-based carriers were significantly inferior to both PLO- and PLA-based PEC shell groups (p<0.01). From these results, we conclude that PEC shells incorporating native heparin could be used for growth factor delivery in functional bone tissue engineering application and that PLA- and PLO-based complexes could represent superior options to DEAE-D for loading and in vivo delivery of bioactive BMP-2 in this approach.

Microfabricated scaffold-guided endothelial morphogenesis in three-dimensional culture

Morphogenesis is a fundamental process by which new blood vessels are formed during angiogenesis. The ability to control angiogenesis would lead to improvements in tissue engineering constructions; indeed, the study of angiogenesis has numerous clinical applications, for example, in the investigation of metastatic cancer, peripheral and coronary vascular disease, and wound healing. Conventional in vitro organotypic cell culture approaches to these studies are limited primarily by their reliance on microvascular vessel formation through a random process of morphogenesis that lacks the spatial reproducibility and orientation needed for high-throughput drug testing. We have developed a bioreactor system for scaffold-guided tubulogenesis coupled with 3-D organotypic culture to spatially control vessel formation and its orientation. To create microchannels to guide microvessel formation, we fabricated rigid scaffolds using photolithography and light curing epoxy, and soft scaffolds formed by a polydimethylsiloxane (PDMS) stamp directly into collagen. Scaffolds seeded with dermal microvascular endothelial cells were placed between gelled layers of collagen containing dermal fibroblasts within a Transwell filter system and cultured for up to 2 weeks to allow for vessel maturation. Morphological analysis of thin tissue sections following standard histology and immunohistochemical detection of endothelial cells, fibroblasts, and basement membrane confirmed vessel formation along the microchannel walls with either scaffold. This system may also provide a means to explore revascularization within decellularized extracellular matrices, the culture of microvessel networks with controlled geometries, and possibly the spatial guidance of angiogenesis for interfacing with an external microfluidic supply network. As a new tool for guided angiogenesis, our approach introduces new possibilities for identification of anti-angiogenic therapeutics.

Development and characterization of acellular porcine corneal matrix using sodium dodecylsulfate

PURPOSE

The objective of this study was to produce a porcine corneal acellular matrix (ACM) and assess its possibility for biomedical applications.

METHODS

Porcine corneas were treated with various concentrations of sodium dodecylsulfate for different lengths of time. Optimal conditions for processing the ACM were noted regarding removal of all cellular components and retention of the spatial arrangement of the corneal stroma. The physical characteristics (including water absorption and light transmittance), biomechanics, and cytotoxicity of the ACM were also found to be conserved. Subsequently, ACM was transplanted into the interlaminar stroma of rabbit corneas. The transparency and structures of the collagen fibers were determined.

RESULTS

By immersing corneal tissues in isotonic buffer containing 0.1% sodium dodecylsulfate for 7 hours, we were able to produce an ACM whose cells were completely removed, without disrupting collagen layer structure. Although water absorption and light transmittance of the ACM decreased when compared with natural corneal stroma, ACM showed similar biomechanical properties and biocompatibility as natural ones. After xenotransplantation into rabbit corneal stromal layers for 4 weeks, both ACM and rabbit corneas showed complete transparency. Almost 1 year postoperatively, the corneas remained transparent with regular stromal structures and ACM appeared stable in situ without deliquescence or immunological rejection.

CONCLUSIONS

A simple and valid method to produce decellularized corneal matrix has been successfully developed. These acellular matrices similar to natural corneas in structure, strength, and transparency have tremendous potential for corneal transplantation as ideal implants for donors and for tissue engineering applications as suitable scaffolds.

The use of a polyelectrolyte fibrous scaffold to deliver differentiated hMSCs to the liver

Liver transplantation as a therapy for liver failure is often hampered by a shortage of donor tissue. The delivery of liver-differentiated human mesenchymal stem cells (hMSCs) is a potential therapy to aid in liver regeneration. In this study, an RGD-modified chitosan-alginate polyelectrolyte complex (PEC) fibrous non-woven scaffold was employed to deliver differentiated hMSCs in vivo. Bone marrow-derived hMSCs were differentiated in vitro by a combination of extracellular matrix (ECM) and conditioned medium and seeded onto the RGD-modified chitosan-alginate fibrous scaffolds. The cell/scaffold construct was then implanted into the livers of a rat model, where 70% of the liver had been removed. Post-implantation analysis of the cell/scaffold constructs showed positive periodic acid-Schiff (PAS) staining for glycogen, and expression of the hepatic markers, AFP, CK19, CK18, albumin, HNF-3beta and MRP-2 by immunofluorescence labeling. In addition, human albumin was detectable in the rat serum by spot blot. These findings demonstrated that the RGD-modified chitosan-alginate fibrous scaffold was useful for delivering transdifferentiated hMSCs into the liver and maintaining the differentiated phenotype of the cells.