Host Response and Neo-Tissue Development during Resorption of a Fast Degrading Supramolecular Electrospun Arterial Scaffold

Off-the-Shelf Synthetic Biodegradable Grafts Transform In Situ into a Living Arteriovenous Fistula in a Large Animal Model

Current vascular access options require frequent interventions. In situ tissue engineering (TE) may overcome these limitations by combining the initial success of synthetic grafts with long-term advantages of autologous vessels by using biodegradable grafts that transform into autologous vascular tissue at the site of implantation. Scaffolds (6 mm-Ø) made of supramolecular polycarbonate-bisurea (PC-BU), with a polycaprolactone (PCL) anti-kinking-coil, are implanted between the carotid artery and jugular vein in goats. A subset is bio-functionalized using bisurea-modified-Stromal cell-derived factor-1α (SDF1α) derived peptides and ePTFE grafts as controls. Grafts are explanted after 1 and 3 months, and evaluated for material degradation, tissue formation, compliance, and patency. At 3 months, the scaffold is resorbed and replaced by vascular neo-tissue, including elastin, contractile markers, and endothelial lining. No dilations, ruptures, or aneurysms are observed and grafts are successfully cannulated at termination. SDF-1α-peptide-biofunctionalization does not influence outcomes. Patency is lower in TE grafts (50%) compared to controls (100% patency), predominantly caused by intimal hyperplasia. Rapid remodeling of a synthetic, biodegradable vascular scaffold into a living, compliant arteriovenous fistula is demonstrated in a large animal model. Despite lower patency compared to ePTFE, transformation into autologous and compliant living tissue with self-healing capacity may have long-term advantages.

Potential Use of Nucleic Acids as a Preceramic Polymer to Synthesize Nanodiamond-Embedded Phosphate Glass for Hard Tissue Engineering

In recent years, nucleic acid has emerged as a versatile molecule that has been strategically used in material synthesis and biomedical applications. Keeping in mind the presence of the phosphate group, a glass former in the nucleic acids, we synthesized a transparent glass-like material by the thermal treatment of nucleic acids (DNA and RNA) at 900 °C at atmospheric pressure. Characterization of this material by transmission electron microscopy, X-ray photoelectron spectroscopy, and confocal fluorescence microscopy suggested the presence of in situ-formed nanodiamonds within the phosphate glass matrix. The molecular structure of glass investigated by X-ray photoelectron and infrared spectroscopy indicated a nearly equal proportion of metaphosphates and smaller phosphate units (pyro- and ortho-phosphate) that form the phosphate glass matrix. Thereafter, in vitro biological experiments showed that the nucleic acid-derived glass was non-toxic and cytocompatible, enhanced extracellular matrix secretion, and increased intracellular alkaline phosphatase activity, with potential application in hard tissue engineering. Our work offers insights into nanodiamond synthesis at atmospheric pressure and proves that nucleic acids could be used as a precursor to making an innovative glass-ceramic biomaterial.

The effect of chronic kidney disease on tissue formation of in situ tissue-engineered vascular grafts

Vascular in situ tissue engineering encompasses a single-step approach with a wide adaptive potential and true off-the-shelf availability for vascular grafts. However, a synchronized balance between breakdown of the scaffold material and neo-tissue formation is essential. Chronic kidney disease (CKD) may influence this balance, lowering the usability of these grafts for vascular access in end-stage CKD patients on dialysis. We aimed to investigate the effects of CKD on in vivo scaffold breakdown and tissue formation in grafts made of electrospun, modular, supramolecular polycarbonate with ureido-pyrimidinone moieties (PC-UPy). We implanted PC-UPy aortic interposition grafts (n = 40) in a rat 5/6th nephrectomy model that mimics systemic conditions in human CKD patients. We studied patency, mechanical stability, extracellular matrix (ECM) components, total cellularity, vascular tissue formation, and vascular calcification in CKD and healthy rats at 2, 4, 8, and 12 weeks post-implantation. Our study shows successful in vivo application of a slow-degrading small-diameter vascular graft that supports adequate in situ vascular tissue formation. Despite systemic inflammation associated with CKD, no influence of CKD on patency (Sham: 95% vs CKD: 100%), mechanical stability, ECM formation (Sirius red⁺, Sham 16.5% vs CKD 25.0%-p:0.83), tissue composition, and immune cell infiltration was found. We did find a limited increase in vascular calcification at 12 weeks (Sham 0.08% vs CKD 0.80%-p:0.02) in grafts implanted in CKD animals. However, this was not associated with increased stiffness in the explants. Our findings suggest that disease-specific graft design may not be necessary for use in CKD patients on dialysis.

Small Diameter Cell-Free Tissue-Engineered Vascular Grafts: Biomaterials and Manufacture Techniques to Reach Suitable Mechanical Properties

Vascular grafts (VGs) are medical devices intended to replace the function of a blood vessel. Available VGs in the market present low patency rates for small diameter applications setting the VG failure. This event arises from the inadequate response of the cells interacting with the biomaterial in the context of operative conditions generating chronic inflammation and a lack of regenerative signals where stenosis or aneurysms can occur. Tissue Engineered Vascular grafts (TEVGs) aim to induce the regeneration of the native vessel to overcome these limitations. Besides the biochemical stimuli, the biomaterial and the particular micro and macrostructure of the graft will determine the specific behavior under pulsatile pressure. The TEVG must support blood flow withstanding the exerted pressure, allowing the proper compliance required for the biomechanical stimulation needed for regeneration. Although the international standards outline the specific requirements to evaluate vascular grafts, the challenge remains in choosing the proper biomaterial and manufacturing TEVGs with good quality features to perform satisfactorily. In this review, we aim to recognize the best strategies to reach suitable mechanical properties in cell-free TEVGs according to the reported success of different approaches in clinical trials and pre-clinical trials.

In Vivo Evaluation of PCL Vascular Grafts Implanted in Rat Abdominal Aorta

Electrospun tissue-engineered grafts made of biodegradable materials have become a perspective search field in terms of vascular replacement, and more research is required to describe their in vivo transformation. This study aimed to give a detailed observation of hemodynamic and structural properties of electrospun, monolayered poly-ε-caprolactone (PCL) grafts in an in vivo experiment using a rat aorta replacement model at 10, 30, 60 and 90 implantation days. It was shown using ultrasound diagnostic and X-ray tomography that PCL grafts maintain patency throughout the entire follow-up period, without stenosis or thrombosis. Vascular compliance, assessed by the resistance index (RI), remains at the stable level from the 10th to the 90th day. A histological study using hematoxylin-eosin (H&E), von Kossa and Russell–Movat pentachrome staining demonstrated the dynamics of tissue response to the implant. By the 10th day, an endothelial monolayer was forming on the graft luminal surface, followed by the gradual growth and compaction of the neointima up to the 90th day. The intense inflammatory cellular reaction observed on the 10th day in the thickness of the scaffold was changed by the fibroblast and myofibroblast penetration by the 30th day. The cellularity maximum was reached on the 60th day, but by the 90th day the cellularity significantly (p = 0.02) decreased. From the 60th day, in some samples, the calcium phosphate depositions were revealed at the scaffold-neointima interface. Scanning electron microscopy showed that the scaffolds retained their fibrillar structure up to the 90th day. Thus, we have shown that the advantages of PCL scaffolds are excellent endothelialization and good surgical outcome. The disadvantages include their slow biodegradation, ineffective cellularization, and risks for mineralization and intimal hyperplasia.

Marker-Independent Monitoring of in vitro and in vivo Degradation of Supramolecular Polymers Applied in Cardiovascular in situ Tissue Engineering

The equilibrium between scaffold degradation and neotissue formation, is highly essential for in situ tissue engineering. Herein, biodegradable grafts function as temporal roadmap to guide regeneration. The ability to monitor and understand the dynamics of degradation and tissue deposition in in situ cardiovascular graft materials is therefore of great value to accelerate the implementation of safe and sustainable tissue-engineered vascular grafts (TEVGs) as a substitute for conventional prosthetic grafts. In this study, we investigated the potential of Raman microspectroscopy and Raman imaging to monitor degradation kinetics of supramolecular polymers, which are employed as degradable scaffolds in in situ tissue engineering. Raman imaging was applied on in vitro degraded polymers, investigating two different polymer materials, subjected to oxidative and enzymatically-induced degradation. Furthermore, the method was transferred to analyze in vivo degradation of tissue-engineered carotid grafts after 6 and 12 months in a sheep model. Multivariate data analysis allowed to trace degradation and to compare the data from in vitro and in vivo degradation, indicating similar molecular observations in spectral signatures between implants and oxidative in vitro degradation. In vivo degradation appeared to be dominated by oxidative pathways. Furthermore, information on collagen deposition and composition could simultaneously be obtained from the same image scans. Our results demonstrate the sensitivity of Raman microspectroscopy to determine degradation stages and the assigned molecular changes non-destructively, encouraging future exploration of this techniques for time-resolved quality assessment of in situ tissue engineering processes.

Immuno-regenerative biomaterials for in situ cardiovascular tissue engineering - Do patient characteristics warrant precision engineering?

In situ tissue engineering using bioresorbable material implants - or scaffolds - that harness the patient's immune response while guiding neotissue formation at the site of implantation is emerging as a novel therapy to regenerate human tissues. For the cardiovascular system, the use of such implants, like blood vessels and heart valves, is gradually entering the stage of clinical translation. This opens up the question if and to what extent patient characteristics influence tissue outcomes, necessitating the precision engineering of scaffolds to guide patient-specific neo-tissue formation. Because of the current scarcity of human in vivo data, herein we review and evaluate in vitro and preclinical investigations to predict the potential role of patient-specific parameters like sex, age, ethnicity, hemodynamics, and a multifactorial disease profile, with special emphasis on their contribution to the inflammation-driven processes of in situ tissue engineering. We conclude that patient-specific conditions have a strong impact on key aspects of in situ cardiovascular tissue engineering, including inflammation, hemodynamic conditions, scaffold resorption, and tissue remodeling capacity, suggesting that a tailored approach may be required to engineer immuno-regenerative biomaterials for safe and predictive clinical applicability.

Effectiveness of cell adhesive additives in different supramolecular polymers

Supramolecular motifs in elastomeric biomaterials facilitate the modular incorporation of additives with corresponding motifs. The influence of the elastomeric supramolecular base polymer on the presentation of additives has been sparsely examined, limiting the knowledge of transferability of effective functionalization between polymers. Here it was investigated if the polymer backbone and the additive influence biomaterial modification in two different types of hydrogen bonding supramolecular systems, that is, based on ureido-pyrimidinone or bis-urea units. Two different cell-adhesive additives, that is, catechol or cyclic RGD, were incorporated into different elastomeric polymers, that is, polycaprolactone, priplast or polycarbonate. The additive effectiveness was evaluated with three different cell types. AFM measurements showed modest alterations on nano-scale assembly in ureido-pyrimidinone materials modified with additives. On the contrary, additive addition was highly intrusive in bis-urea materials. Detailed cell adhesive studies revealed additive effectiveness varied between base polymers and the supramolecular platform, with bis-urea materials more potently affecting cell behavior. This research highlights that additive transposition might not always be as evident. Therefore, additive effectiveness requires re-evaluation in supramolecular biomaterials when altering the polymer backbone to suit the biomaterial application.

Implementation of Pericytes in Vascular Regeneration Strategies

For the survival and integration of complex large-sized tissue-engineered (TE) organ constructs that exceed the maximal nutrients and oxygen diffusion distance required for cell survival, graft (pre)vascularization to ensure medium or blood supply is crucial. To achieve this, the morphology and functionality of the microcapillary bed should be mimicked by incorporating vascular cell populations, including endothelium and mural cells. Pericytes play a crucial role in microvascular function, blood vessel stability, angiogenesis, and blood pressure regulation. In addition, tissue-specific pericytes are important in maintaining specific functions in different organs, including vitamin A storage in the liver, renin production in the kidneys and maintenance of the blood-brain-barrier. Together with their multipotential differentiation capacity, this makes pericytes the preferred cell type for application in TE grafts. The use of a tissue-specific pericyte cell population that matches the TE organ may benefit organ function. In this review, we provide an overview of the literature for graft (pre)-vascularization strategies and highlight the possible advantages of using tissue-specific pericytes for specific TE organ grafts. Impact statement The use of a tissue-specific pericyte cell population that matches the tissue-engineered (TE) organ may benefit organ function. In this review, we provide an overview of the literature for graft (pre)vascularization strategies and highlight the possible advantages of using tissue-specific pericytes for specific TE organ grafts.

Transcatheter-Delivered Expandable Bioresorbable Polymeric Graft With Stenting Capacity Induces Vascular Regeneration

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We designed a transcatheter balloon-expandable resorbable vascular graft with support capacity.

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After 2 months in vivo, grafts show native-like tissue reconstruction with endoluminal elastin.

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The concept convenes regenerative grafting, minimally invasive delivery, and clinical stenting.

As the next step in the translation of vascular tissue engineering, this study uniquely combines transcatheter delivery and in situ tissue regeneration using a novel bioresorbable electrospun polymer graft that can be implanted minimally invasively. Once delivered inside a small-diameter vessel, the electrospun microstructure supports the vessel wall, facilitates cellular infiltration, and guides organized tissue formation.

Inconsistency in Graft Outcome of Bilayered Bioresorbable Supramolecular Arterial Scaffolds in Rats

There is a continuous search for the ideal bioresorbable material to develop scaffolds for in situ vascular tissue engineering. As these scaffolds are exposed to the harsh hemodynamic environment during the entire transformation process from scaffold to neotissue, it is of crucial importance to maintain mechanical integrity and stability at all times. Bilayered scaffolds made of supramolecular polycarbonate-ester-bisurea were manufactured using dual electrospinning. These scaffolds contained a porous inner layer to allow for cellular infiltration and a dense outer layer to provide strength. Scaffolds (n = 21) were implanted as an interposition graft into the abdominal aorta of male Lewis rats and explanted after 1, 3, and 5 months in vivo to assess mechanical functionality and neotissue formation upon scaffold resorption. Results demonstrated conflicting graft outcomes despite homogeneity in the experimental group and scaffold production. Most grafts exhibited adverse remodeling, resulting in aneurysmal dilatation and calcification. However, a few grafts did not demonstrate such features, but instead were characterized by graft extension and smooth muscle cell proliferation in the absence of endothelium, while remaining patent throughout the study. We conclude that it remains extremely difficult to anticipate graft development and performance in vivo. Next to rational mechanical design and good performance in vitro, a thorough understanding of the mechanobiological mechanisms governing scaffold-driven arterial regeneration as well as potential influences of surgical procedures is warranted to further optimize scaffold designs. Careful analysis of the differences between preclinical successes and failures, as is done in this study, may provide initial handles for scaffold optimization and standardized surgical procedures to improve graft performance in vivo. Impact statement In situ vascular tissue engineering using cell-free bioresorbable scaffolds is investigated as an off-the-shelf option to grow small caliber arteries inside the body. In this study, we developed a bilayered electrospun supramolecular scaffold with a dense outer layer to provide mechanical integrity and a porous inner layer for cell recruitment and tissue formation. Despite homogenous scaffold properties and mechanical performance in vitro, in vivo testing as rat aorta interposition grafts revealed distinct graft outcomes, ranging from aneurysms to functional arteries. Careful analysis of this variability provided valuable insights into materials-driven in situ artery formation relevant for scaffold design and implantation procedures.

Impact of Additives on Mechanical Properties of Supramolecular Electrospun Scaffolds

The mechanical properties of scaffolds used for mechanically challenging applications such as cardiovascular implants are unequivocally important. Here, the effect of supramolecular additive functionalization on mechanical behavior of electrospun scaffolds was investigated for one bisurea-based model additive and two previously developed antifouling additives. The model additive has no effect on the mechanical properties of the bulk material, whereas the stiffness of electrospun scaffolds was slightly decreased compared to pristine PCL-BU following the addition of the three different additives. These results show the robustness of supramolecular additives used in biomedical applications, in which mechanical properties are important, such as vascular grafts and heart valve constructs.

Imaging the In Vivo Degradation of Tissue Engineering Implants by Use of Supramolecular Radiopaque Biomaterials

For in situ tissue engineering (TE) applications it is important that implant degradation proceeds in concord with neo-tissue formation to avoid graft failure. It will therefore be valuable to have an imaging contrast agent (CA) available that can report on the degrading implant. For this purpose, a biodegradable radiopaque biomaterial is presented, modularly composed of a bisurea chain-extended polycaprolactone (PCL2000-U4U) elastomer and a novel iodinated bisurea-modified CA additive (I-U4U). Supramolecular hydrogen bonding interactions between the components ensure their intimate mixing. Porous implant TE-grafts are prepared by simply electrospinning a solution containing PCL2000-U4U and I-U4U. Rats receive an aortic interposition graft, either composed of only PCL2000-U4U (control) or of PCL2000-U4U and I-U4U (test). The grafts are explanted for analysis at three time points over a 1-month period. Computed tomography imaging of the test group implants prior to explantation shows a decrease in iodide volume and density over time. Explant analysis also indicates scaffold degradation. (Immuno)histochemistry shows comparable cellular contents and a similar neo-tissue formation process for test and control group, demonstrating that the CA does not have apparent adverse effects. A supramolecular approach to create solid radiopaque biomaterials can therefore be used to noninvasively monitor the biodegradation of synthetic implants.

Combinatorial functionalization with bisurea-peptides and antifouling bisurea additives of a supramolecular elastomeric biomaterial

The bioactive additive toolbox to functionalize supramolecular elastomeric materials expands rapidly. Here we have set an explorative step toward screening of complex combinatorial functionalization with antifouling and three peptide-containing additives in a bisurea-based supramolecular system. Thorough investigation of surface properties of thin films with contact angle measurements, X-ray photoelectron spectroscopy and atomic force microscopy, was correlated to cell-adhesion of endothelial and smooth muscle cells to apprehend their respective predictive values for functional biomaterial development. Peptides were presented at the surface alone, and in combinatorial functionalization with the oligo(ethylene glycol)-based non-cell adhesive additive. The bisurea-RGD additive was cell-adhesive in all conditions, whereas the endothelial cell-specific bisurea-REDV showed limited bioactive properties in all chemical nano-environments. Also, aspecific functionality was observed for a bisurea-SDF1α peptide. These results emphasize that special care should be taken in changing the chemical nano-environment with peptide functionalization. © 2019 The Authors. Journal of Polymer Science Part B: Polymer Physics published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 1725-1735.

In vitro and preclinical characterisation of compressed, macro-porous and collagen coated poly-ε-caprolactone electro-spun scaffolds

Low in macro-porosity electro-spun scaffolds are often associated with foreign body response, whilst macro-porous electro-spun scaffolds have low mechanical integrity. Herein, compressed, macro-porous and collagen (bovine Achilles tendon and human recombinant) coated electro-spun poly-ε-caprolactone scaffolds were developed and their biomechanical, in vitro and in vivo properties were assessed. Collagen coating, independently of the source, did not significantly affect the biomechanical properties of the scaffolds. Although no significant difference in cell viability was observed between the groups, collagen coated scaffolds induced significantly higher DNA concentration. In vivo, no signs of adverse tissue effect were observed in any of the groups and all groups appeared to equally integrate into the subcutaneous tissue. It is evidenced that macro-porous poly-ε-caprolactone electro-spun meshes with adequate mechanical properties and acceptable host response can be developed for biomedical applications.

The degradation and performance of electrospun supramolecular vascular scaffolds examined upon in vitro enzymatic exposure

To maintain functionality during in situ vascular regeneration, the rate of implant degradation should be closely balanced by neo-tissue formation. It is unknown, however, how the implant's functionality is affected by the degradation of the polymers it is composed of. We therefore examined the macro- and microscopic features as well as the mechanical performance of vascular scaffolds upon in vitro enzymatic degradation. Three candidate biomaterials with supramolecularly interacting bis-urea (BU) hard blocks ('slow-degrading' polycarbonate-BU (PC-BU), 'intermediate-degrading' polycarbonate-ester-BU (PC(e)-BU), and 'fast-degrading' polycaprolactone-ester-BU (PCL-BU)) were synthesized and electrospun into microporous scaffolds. These materials possess a sequence-controlled macromolecular structure, so their susceptibility to degradation is tunable by controlling the nature of the polymer backbone. The scaffolds were incubated in lipase and monitored for changes in physical, chemical, and mechanical properties. Remarkably, comparing PC-BU to PC(e)-BU, we observed that small changes in macromolecular structure led to significant differences in degradation kinetics. All three scaffold types degraded via surface erosion, which was accompanied by fiber swelling for PC-BU scaffolds, and some bulk degradation and a collapsing network for PCL-BU scaffolds. For the PC-BU and PC(e)-BU scaffolds this resulted in retention of mechanical properties, whereas for the PCL-BU scaffolds this resulted in stiffening. Our in vitro study demonstrates that vascular scaffolds, electrospun from sequence-controlled supramolecular materials with varying ester contents, not only display different susceptibilities to degradation, but also degrade via different mechanisms. STATEMENT OF SIGNIFICANCE: One of the key elements to successfully engineer vascular tissues in situ, is to balance the rate of implant degradation and neo-tissue formation. Due to their tunable properties, supramolecular polymers can be customized into attractive biomaterials for vascular tissue engineering. Here, we have exploited this tunability and prepared a set of polymers with different susceptibility to degradation. The polymers, which were electrospun into microporous scaffolds, displayed not only different susceptibilities to degradation, but also obeyed different degradation mechanisms. This study illustrates how the class of supramolecular polymers continues to represent a promising group of materials for tissue engineering approaches.

Macrophage-Driven Biomaterial Degradation Depends on Scaffold Microarchitecture

In situ tissue engineering is a technology in which non-cellular biomaterial scaffolds are implanted in order to induce local regeneration of replaced or damaged tissues. Degradable synthetic electrospun scaffolds are a versatile and promising class of biomaterials for various in situ tissue engineering applications, such as cardiovascular replacements. Functional in situ tissue regeneration depends on the balance between endogenous neo-tissue formation and scaffold degradation. Both these processes are driven by macrophages. Upon invasion into a scaffold, macrophages secrete reactive oxygen species (ROS) and hydrolytic enzymes, contributing to oxidative and enzymatic biomaterial degradation, respectively. This study aims to elucidate the effect of scaffold microarchitecture, i.e., μm-range fiber diameter and fiber alignment, on early macrophage-driven scaffold degradation. Electrospun poly-ε-caprolactone-bisurea (PCL-BU) scaffolds with either 2 or 6 μm (Ø) isotropic or anisotropic fibers were seeded with THP-1 derived human macrophages and cultured in vitro for 4 or 8 days. Our results revealed that macroph age-induced oxidative degradation in particular was dependent on scaffold microarchitecture, with the highest level of ROS-induced lipid peroxidation, NADPH oxidase gene expression and degradation in the 6 μm Ø anisotropic group. Whereas, biochemically polarized macrophages demonstrated a phenotype-specific degradative potential, the observed differences in macrophage degradative potential instigated by the scaffold microarchitecture could not be attributed to either distinct M1 or M2 polarization. This suggests that the scaffold microarchitecture uniquely affects macrophage-driven degradation. These findings emphasize the importance of considering the scaffold microarchitecture in the design of scaffolds for in situ tissue engineering applications and the tailoring of degradation kinetics thereof.