Multiphoton Imaging of Maturation in Tissue Engineering

Donor cell-specific tissue-engineered (TE) implants are a promising therapy for personalized treatment of cardiovascular diseases, but current development protocols lack a stable longitudinal assessment of tissue development at subcellular resolution. As a first step toward such an assessment approach, in this study we establish a generalized labeling and imaging protocol to obtain quantified maturation parameters of TE constructs in three dimensions (3D) without the need of histological slicing, thus leaving the tissue intact. Focusing on intracellular matrix (ICM) and extracellular matrix (ECM) networks, multiphoton laser scanning microscopy (MPLSM) was used to investigate TE patches of different conditioning durations of up to 21 days. We show here that with a straightforward labeling procedure of whole-mount samples (so without slicing into thin histological sections), followed by an easy-to-use multiphoton imaging process, we obtained high-quality images of the tissue in 3D at various time points during development. The stacks of images could then be further analyzed to visualize and quantify the volume of cell coverage as well as the volume fraction and network of structural proteins. We showed that collagen and alpha-smooth muscle actin (α-SMA) volume fractions increased as normalized to full tissue volume and proportional to the cell count, with a converging trend to the final density of (4.0% ± 0.6%) and (7.6% ± 0.7%), respectively. The image analysis of ICM and ECM revealed a developing and widely branched interconnected matrix. We are currently working on the second step, that is, to integrate MPLSM endoscopy into a dynamic bioreactor system to monitor the maturation of intact TE constructs over time, thus without the need to take them out.

A Dexamethasone-Loaded Polymeric Electrospun Construct as a Tubular Cardiovascular Implant

Cardiovascular tissue engineering is providing many solutions to cardiovascular diseases. The complex disease demands necessitating tissue-engineered constructs with enhanced functionality. In this study, we are presenting the production of a dexamethasone (DEX)-loaded electrospun tubular polymeric poly(l-lactide) (PLA) or poly(d,l-lactide-co-glycolide) (PLGA) construct which contains iPSC-CMs (induced pluripotent stem cell cardiomyocytes), HUVSMCs (human umbilical vein smooth muscle cells), and HUVECs (human umbilical vein endothelial cells) embedded in fibrin gel. The electrospun tube diameter was calculated, as well as the DEX release for 50 days for 2 different DEX concentrations. Furthermore, we investigated the influence of the polymer composition and concentration on the function of the fibrin gels by imaging and quantification of CD31, alpha-smooth muscle actin (αSMA), collagen I (col I), sarcomeric alpha actinin (SAA), and Connexin 43 (Cx43). We evaluated the cytotoxicity and cell proliferation of HUVECs and HUVSMCs cultivated in PLA and PLGA polymeric sheets. The immunohistochemistry results showed efficient iPSC-CM marker expression, while the HUVEC toxicity was higher than the respective HUVSMC value. In total, our study emphasizes the combination of fibrin gel and electrospinning in a functionalized construct, which includes three cell types and provides useful insights of the DEX release and cytotoxicity in a tissue engineering perspective.

Bio-Inspired Fiber Reinforcement for Aortic Valves: Scaffold Production Process and Characterization

The application of tissue-engineered heart valves in the high-pressure circulatory system is still challenging. One possible solution is the development of biohybrid scaffolds with textile reinforcement to achieve improved mechanical properties. In this article, we present a manufacturing process of bio-inspired fiber reinforcement for an aortic valve scaffold. The reinforcement structure consists of polyvinylidene difluoride monofilament fibers that are biomimetically arranged by a novel winding process. The fibers were embedded and fixated into electrospun polycarbonate urethane on a cylindrical collector. The scaffold was characterized by biaxial tensile strength, bending stiffness, burst pressure and hemodynamically in a mock circulation system. The produced fiber-reinforced scaffold showed adequate acute mechanical and hemodynamic properties. The transvalvular pressure gradient was 3.02 ± 0.26 mmHg with an effective orifice area of 2.12 ± 0.22 cm2. The valves sustained aortic conditions, fulfilling the ISO-5840 standards. The fiber-reinforced scaffold failed in a circumferential direction at a stress of 461.64 ± 58.87 N/m and a strain of 49.43 ± 7.53%. These values were above the levels of tested native heart valve tissue. Overall, we demonstrated a novel manufacturing approach to develop a fiber-reinforced biomimetic scaffold for aortic heart valve tissue engineering. The characterization showed that this approach is promising for an in situ valve replacement.

Influence of Diameter and Cyclic Mechanical Stimulation on the Beating Frequency of Myocardial Cell-Laden Fibers

Atrioventricular block (AVB) is a severe disease for pediatric patients. The repetitive operations needed in the case of the pacemaker implantation to maintain the electrical signal at the atrioventricular node (AVN) affect the patient's life quality. In this study, we present a method of biofabrication of multi-cell-laden cylindrical fibrin-based fibers that can restore the electrical signal at the AVN. We used human umbilical vein smooth muscle cells (HUVSMCs), human umbilical vein endothelial cells (HUVECs) and induced pluripotent stem cell cardiomyocytes (iPSC-CMs) cultivated either statically or dynamically to mimic the native AVN. We investigated the influence of cell composition, construct diameter and cyclic stretch on the function of the fibrin hydrogels in vitro. Immunohistochemistry analyses showed the maturity of the iPSC-CMs in the constructs through the expression of sarcomeric alpha actinin (SAA) and electrical coupling through Connexin 43 (Cx43) signal. Simultaneously, the beating frequency of the fibrin hydrogels was higher and easy to maintain whereas the concentration of iPSC-CMs was higher compared with the other types of cylindrical constructs. In total, our study highlights that the combination of fibrin with the cell mixture and geometry is offering a feasible biofabrication method for tissue engineering approaches for the treatment of AVB.

Lifelike Transformative Materials for Biohybrid Implants: Inspired by Nature, Driven by Technology

Today's living world is enriched with a myriad of natural biological designs, shaped by billions of years of evolution. Unraveling the construction rules of living organisms offers the potential to create new materials and systems for biomedicine. From the close examination of living organisms, several concepts emerge: hierarchy, pattern repetition, adaptation, and irreducible complexity. All these aspects must be tackled to develop transformative materials with lifelike behavior. This perspective article highlights recent progress in the development of transformative biohybrid systems for applications in the fields of tissue regeneration and biomedicine. Advances in computational simulations and data-driven predictions are also discussed. These tools enable the virtual high-throughput screening of implant design and performance before committing to fabrication, thus reducing the development time and cost of biomimetic and biohybrid constructs. The ongoing progress of imaging methods also constitutes an essential part of this matter in order to validate the computation models and enable longitudinal monitoring. Finally, the current challenges of lifelike biohybrid materials, including reproducibility, ethical considerations, and translation, are discussed. Advances in the development of lifelike materials will open new biomedical horizons, where perhaps what is currently envisioned as science fiction will become a science-driven reality in the future.

Transformative Materials to Create 3D Functional Human Tissue Models In Vitro in a Reproducible Manner

Recreating human tissues and organs in the petri dish to establish models as tools in biomedical sciences has gained momentum. These models can provide insight into mechanisms of human physiology, disease onset, and progression, and improve drug target validation, as well as the development of new medical therapeutics. Transformative materials play an important role in this evolution, as they can be programmed to direct cell behavior and fate by controlling the activity of bioactive molecules and material properties. Using nature as an inspiration, scientists are creating materials that incorporate specific biological processes observed during human organogenesis and tissue regeneration. This article presents the reader with state-of-the-art developments in the field of in vitro tissue engineering and the challenges related to the design, production, and translation of these transformative materials. Advances regarding (stem) cell sources, expansion, and differentiation, and how novel responsive materials, automated and large-scale fabrication processes, culture conditions, in situ monitoring systems, and computer simulations are required to create functional human tissue models that are relevant and efficient for drug discovery, are described. This paper illustrates how these different technologies need to converge to generate in vitro life-like human tissue models that provide a platform to answer health-based scientific questions.

Self-crosslinked polyvinyl alcohol/cellulose nanofibril cryogels loaded with synthesized aminophosphonates as antimicrobial wound dressings

Microbial infection is the most common obstacle in the wound healing process, leading to wound healing impairment and complications and ultimately increasing morbidity and mortality. Due to the rising number of pathogens evolving resistance to the existing antibiotics used for wound care, alternative approaches are urgently required. In this study, α-aminophosphonate derivatives as antimicrobial agents were synthesized and incorporated into self-crosslinked tri-component cryogels composed of fully hydrolyzed polyvinyl alcohol (PVA-F), partially hydrolyzed polyvinyl alcohol (PVA-P), and cellulose nanofibrils (CNFs). Initially, the antimicrobial activity of four α-aminophosphonate derivatives against selected skin bacterial species was tested and their minimum inhibitory concentration was determined based on which the most effective compound was loaded into the cryogels. Next, the physical and mechanical properties of cryogels with various blending ratios of PVA-P/PVA-F and fixed amounts of CNFs were assessed, and drug release profiles and biological activities of drug-loaded cryogels were analyzed. Assessment of α-aminophosphonate derivatives showed the highest efficacy of a cinnamaldehyde-based derivative (Cinnam) against both Gram-negative and Gram-positive bacteria compared to other derivatives. The physical and mechanical properties of cryogels showed that PVA-P/PVA-F with a 50/50 blending ratio had the highest swelling ratio (1600%), surface area (523 m² g^-1), and compression recoverability (72%) compared to that with other blending ratios. Finally, antimicrobial and biofilm development studies showed that the cryogel loaded with a Cinnam amount of 2 mg (relative to polymer weight) showed the most sustained drug release profile over 75 h and had the highest efficacy against Gram-negative and Gram-positive bacteria. In conclusion, self-crosslinked tri-component cryogels loaded with the synthesized α-aminophosphonate derivative, having both antimicrobial and anti-biofilm formation properties, can have a significant impact on the management of uprising wound infection.

Dual Labeling of Primary Cells with Fluorescent Gadolinium Oxide Nanoparticles

The interest in mesenchymal stromal cells as a therapy option is increasing rapidly. To improve their implementation, location, and distribution, the properties of these must be investigated. Therefore, cells can be labeled with nanoparticles as a dual contrast agent for fluorescence and magnetic resonance imaging (MRI). In this study, a more efficient protocol for an easy synthesis of rose bengal-dextran-coated gadolinium oxide (Gd₂O₃-dex-RB) nanoparticles within only 4 h was established. Nanoparticles were characterized by zeta potential measurements, photometric measurements, fluorescence and transmission electron microscopy, and MRI. In vitro cell experiments with SK-MEL-28 and primary adipose-derived mesenchymal stromal cells (ASC), nanoparticle internalization, fluorescence and MRI properties, and cell proliferation were performed. The synthesis of Gd₂O₃-dex-RB nanoparticles was successful, and they were proven to show adequate signaling in fluorescence microscopy and MRI. Nanoparticles were internalized into SK-MEL-28 and ASC via endocytosis. Labeled cells showed sufficient fluorescence and MRI signal. Labeling concentrations of up to 4 mM and 8 mM for ASC and SK-MEL-28, respectively, did not interfere with cell viability and proliferation. Gd₂O₃-dex-RB nanoparticles are a feasible contrast agent to track cells via fluorescence microscopy and MRI. Fluorescence microscopy is a suitable method to track cells in in vitro experiments with smaller samples.

Biomechanical evaluation of a novel biomimetic artificial intervertebral disc in canine cervical cadaveric spines

Background Context

Cervical disc replacement (CDR) aims to restore motion of the treated level to reduce the risk of adjacent segment disease (ASD) compared with spinal fusion. However, first-generation articulating devices are unable to mimic the complex deformation kinematics of a natural disc. Thus, a biomimetic artificial intervertebral CDR (bioAID), containing a hydroxyethylmethacrylate (HEMA)-sodium methacrylate (NaMA) hydrogel core representing the nucleus pulposus, an ultra-high-molecular-weight-polyethylene fiber jacket as annulus fibrosus, and titanium endplates with pins for primary mechanical fixation, was developed.

Purpose

To assess the initial biomechanical effect of the bioAID on the kinematic behavior of the canine spine, an ex vivo biomechanical study in 6-degrees-of-freedom was performed.

Study Design

A canine cadaveric biomechanical study.

Methods

Six cadaveric canine specimens (C3-C6) were tested in flexion-extension (FE), lateral bending (LB) axial rotation (AR) using a spine tester in three conditions: intact, after C4-C5 disc replacement with bioAID, and after C4-C5 interbody fusion. A hybrid protocol was used where first the intact spines were subjected to a pure moment of ±1 Nm, whereafter the treated spines were subjected to the full range of motion (ROM) of the intact condition. 3D segmental motions at all levels were measured while recording the reaction torsion. Biomechanical parameters studied included ROM, neutral zone (NZ), and intradiscal pressure (IDP) at the adjacent cranial level (C3-C4).

Results

The bioAID retained the sigmoid shape of the moment-rotation curves with a NZ similar to the intact condition in LB and FE. Additionally, the normalized ROMs at the bioAID-treated level were statistically equivalent to intact during FE and AR while slightly decreased in LB. At the two adjacent levels, ROMs showed similar values for the intact compared to the bioAID for FE and AR and an increase in LB. In contrast, levels adjacent to the fused segment showed an increased motion in FE and LB as compensation for the loss of motion at the treated level. The IDP at the adjacent C3-C4 level after implantation of bioAID was close to intact values. After fusion, increased IDP was found compared with intact but did not reach statistical significance.

Conclusion

This study indicates that the bioAID can mimic the kinematic behavior of the replaced intervertebral disc and preserves that for the adjacent levels better than fusion. As a result, CDR using the novel bioAID is a promising alternative treatment for replacing severely degenerated intervertebral discs.

Mechanical characterization of a novel biomimetic artificial disc for the cervical spine

A novel biomimetic artificial intervertebral disc (bioAID) replacement implant has been developed containing a swelling hydrogel representing the nucleus pulposus, a tensile strong fiber jacket as annulus fibrosus and titanium endplates with pins to primarily secure the device between the vertebral bodies. In this study, the design safety of this novel implant was evaluated based on several biomechanical parameters, namely compressive strength, shear-compressive strength, risk of subsidence and device expulsion as well as identifying the diurnal creep-recovery characteristics of the device. The bioAID remained intact up to 1 kN under static axial compression and only 0.4 mm of translation was observed under a compressive shear load of 20 N. No subsidence was observed after 0.5 million cycles of sinusoidal compressive loading between 50 and 225 N. After applying 400 N in antero-posterior direction under 100 N axial compressive preload, approximately 2 mm displacement was found, being within the range of displacements reported for other commercially available cervical disc replacement devices. The diurnal creep recovery behavior of the bioAID closely resembled what has been reported for natural intervertebral discs in literature. Overall, these results indicate that the current design can withstand (shear-compression loads and is able to remain fixed in a mechanical design resembling the vertebral bodies. Moreover, it is one of the first implants that can closely mimic the poroelastic and viscoelastic behavior of natural disc under a diurnal loading pattern.

Bioglues Based on an Elastin-Like Recombinamer: Effect of Tannic Acid as an Additive on Tissue Adhesion and Cytocompatibility

More than 260 million surgical procedures are performed worldwide each year. Although sutures and staples are widely used to reconnect tissues, they can cause further damage and increase the risk of infection. Bioadhesives have been proposed as an alternative to reconnect tissues. However, clinical adhesives that combine strong adhesion with cytocompatibility have yet to be developed. In this study, we explored the production of adhesives based on protein-engineered polymers bioinspired by the sequence of elastin (i.e., elastin-like recombinamers, ELRs). We hypothesized that the combination of polyphenols (i.e., tannic acid, TA) and ELRs would produce an adhesive coacervate (ELR+TA), as reported for other protein polymers such as silk fibroin (SF). Notably, the adhesion of ELR alone surpassed that of ELR+TA. Indeed, ELR alone achieved adhesive strengths of 88.8 ± 33.2 kPa and 17.0 ± 2.0 kPa on porcine bone and skin tissues, respectively. This surprising result led us to explore a multicomponent bioadhesive to encompass the complementary roles of elastin (mimicked here by ELR) and silk fibroin (SF), and subsequently mirror more closely the multicomponent nature of the extracellular matrix. Tensile testing showed that ELR+SF achieved an adhesive strength of 123.3 ± 60.2 kPa on porcine bone and excellent cytocompatibility. To express this in a more visual and intuitive way, a small surface of only 2.5 cm2 was able to lift at least 2 kg of weight. This opens the door for further studies focusing on the ability of protein-engineered polymers to adhere to biological tissues without further chemical modification for applications in tissue engineering.

Thermoresponsive nanofibers loaded with antimicrobial α-aminophosphonate-o/w emulsion supported by cellulose nanocrystals for smart wound care patches

Long-term topical application of antibiotics on wounds has led to the emergence of drug-resistant bacterial infections. Antibiotic incorporation into the wound dressing requires enormous advancement of the field to ensure that the needed dose is released when the infection arises. This study synthesized a series of antimicrobial α-aminophosphonate derivatives, and the most effective compound was incorporated into thermoresponsive wound dressing patches. Wound dressing mats were fabricated by needleless electrospinning, and the resultant nanofiber mats were coated with a thermoresponsive eicosane/cellulose nanocrystals o/w system loaded with active α-aminophosphonate derivatives. Chemical, physical, thermal, and antimicrobial properties of the wound dressings were characterized wound dressings. Using SEM analysis, Nanofibers spun with 20 % w/v solutions were selected for drug-emulsion loading since they showed lower diameters with higher surface area. Furthermore, the drug-emulsion coating on the electrospun dressings improved the hydrophilicity of the wound dressings, and the thermoresponsive behavior of the mats was proved using differential scanning calorimetry data. Finally, the drug-loaded electrospun meshes were found active against tested microorganisms, and clear inhibition zones were observed. In conclusion, this novel approach of synthesizing a new family of antimicrobial molecules and their incorporation into nanofibers from renewable sources exhibits great potential for smart and innovative dressings.

Long-Term Degradation Assessment of a Polyurethane-Based Surgical Adhesive-Assessment and Critical Consideration of Preclinical In Vitro and In Vivo Testing

Tissue adhesives constitute a great possibility to improve conventional wound closure. In contrast to sutures, they enable nearly immediate hemostasis and can prevent fluid or air leaks. In the present study, a poly(ester)urethane-based adhesive was investigated which already proved to be suitable for different indications, such as reinforcing vascular anastomosis and sealing liver tissue. Using in vitro and in vivo setups, the degradation of the adhesives was monitored over a period of up to 2 years, to evaluate long-term biocompatibility and determine degradation kinetics. For the first time, the complete degradation of the adhesive was documented. In subcutaneous locations, tissue residues were found after 12 months and in intramuscular locations, tissue degradation was complete after about 6 months. A detailed histological evaluation of the local tissue reaction revealed good biocompatibility throughout the different degradation stages. After full degradation, complete remodeling to physiological tissue was observed at the implant locations. In addition, this study critically discusses common issues related to the assessment of biomaterial degradation kinetics in the context of medical device certification. This work highlighted the importance and encouraged the implementation of biologically relevant in vitro degradation models to replace animal studies or at least reduce the number of animals in preclinical testing prior to clinical studies. Moreover, the suitability of frequently used implantation studies based on ISO 10993-6 at standard locations was critically discussed, especially in light of the associated lack of reliable predictions for degradation kinetics at the clinically relevant site of implantation.

Influence of Aerosolization on Endothelial Cells for Efficient Cell Deposition in Biohybrid and Regenerative Applications

The endothelialization of gas exchange membranes can increase the hemocompatibility of extracorporeal membrane oxygenators and thus become a long-term lung replacement option. Cell seeding on large or uneven surfaces of oxygenator membranes is challenging, with cell aerosolization being a possible solution. In this study, we evaluated the endothelial cell aerosolization for biohybrid lung application. A Vivostat^® system was used for the aerosolization of human umbilical vein endothelial cells with non-sprayed cells serving as a control. The general suitability was evaluated using various flow velocities, substrate distances and cell concentrations. Cells were analyzed for survival, apoptosis and necrosis levels. In addition, aerosolized and non-sprayed cells were cultured either static or under flow conditions in a dynamic microfluidic model. Evaluation included immunocytochemistry and gene expression via quantitative PCR. Cell survival for all tested parameters was higher than 90%. No increase in apoptosis and necrosis levels was seen 24 h after aerosolization. Spraying did not influence the ability of the endothelial cells to form a confluent cell layer and withstand shear stresses in a dynamic microfluidic model. Immunocytochemistry revealed typical expression of CD31 and von Willebrand factor with cobble-stone cell morphology. No change in shear stress-induced factors after aerosolization was reported by quantitative PCR analysis. With this study, we have shown the feasibility of endothelial cell aerosolization with no significant changes in cell behavior. Thus, this technique could be used for efficient the endothelialization of gas exchange membranes in biohybrid lung applications.

Influence of polypropylene mesh degradation on tissue inflammatory reaction

Polypropylene degradation in vivo appears as mesh surface cracking and peeling. This aging process of the mesh, resulting in the lack of bio-stability, contradicts the requirement of biocompatibility. However, to date, it is still not clearly established how much this mesh degradation influences the local tissue response with subsequent clinical consequences. This study aims to find out whether mesh degradation is correlated with elevated inflammatory tissue reaction through analyzing 100 human PP meshes explanted from the pelvic floor. A degradation classification method, based on standard pathological H&E stained slides of the explanted mesh via light microscope, was developed to classify the mesh degradation into four classes (no, mild, moderate and severe degradation). The peri-filamentary tissue inflammatory reaction was analyzed by scoring the expression of the most common cell markers for the innate immune reaction: CD68 as marker for macrophage, CD86 for M1 subtype, CD163 for M2 subtype, CD3 for T-lymphocyte and CD15 for neutrophil granulocytes. The correlation between immune cell expression, degradation classification and time of implantation of the meshes are evaluated with Spearman-Rho-Test. Mesh degradation worsens significantly (p < .001) with longer time of implantation. The increasing tendency of CD68 expression by mesh with higher degradation class indicates that the number of macrophages increases with worsening mesh degradation. The significantly increased expression of CD163 and CD3 cell by severely degraded mesh demonstrate the increased number of M2 and T-Lymphocyte when mesh degradation becomes severe. None of the inflammatory cells show the usual declining expression with longer time of implantation. The result of this study suggests that the degradation of PP mesh results in an elevated local inflammatory reaction in female pelvic floor. A material with better bio-stability for mesh implant in pelvic floor is required.

Effervescent Atomizer as Novel Cell Spray Technology to Decrease the Gas-to-Liquid Ratio

Cell spraying has become a feasible application method for cell therapy and tissue engineering approaches. Different devices have been used with varying success. Often, twin-fluid atomizers are used, which require a high gas velocity for optimal aerosolization characteristics. To decrease the amount and velocity of required air, a custom-made atomizer was designed based on the effervescent principle. Different designs were evaluated regarding spray characteristics and their influence on human adipose-derived mesenchymal stromal cells. The arithmetic mean diameters of the droplets were 15.4−33.5 µm with decreasing diameters for increasing gas-to-liquid ratios. The survival rate was >90% of the control for the lowest gas-to-liquid ratio. For higher ratios, cell survival decreased to approximately 50%. Further experiments were performed with the design, which had shown the highest survival rates. After seven days, no significant differences in metabolic activity were observed. The apoptosis rates were not influenced by aerosolization, while high gas-to-liquid ratios caused increased necrosis levels. Tri-lineage differentiation potential into adipocytes, chondrocytes, and osteoblasts was not negatively influenced by aerosolization. Thus, the effervescent aerosolization principle was proven suitable for cell applications requiring reduced amounts of supplied air. This is the first time an effervescent atomizer was used for cell processing.

From In Vitro to Perioperative Vascular Tissue Engineering: Shortening Production Time by Traceable Textile-Reinforcement

Background:

The production of tissue-engineered vascular graft (TEVG) usually involves a prolonged bioreactor cultivation period of up to several weeks to achieve maturation of extracellular matrix and sufficient mechanical strength. Therefore, we aimed to substantially shorten this conditioning time by combining a TEVG textile scaffold with a recently developed copolymer reinforced fibrin gel as a cell carrier. We further implemented our grafts with magnetic resonance imaging (MRI) contrast agents to allow the in-vitro monitoring of the TEVG’s remodeling process.

Methods:

Biodegradable polylactic-co-glycolic acid (PLGA) was electrospun onto a non-degradable polyvinylidene fluoride scaffold and molded along with copolymer-reinforced fibrin hydrogel and human arterial cells. Mechanical tests on the TEVGs were performed both instantly after molding and 4 days of bioreactor conditioning. The non-invasive in vitro monitoring of the PLGA degradation and the novel imaging of fluorinated thermoplastic polyurethane (¹⁹F-TPU) were performed using 7T MRI.

Results:

After 4 days of close loop bioreactor conditioning, 617 ± 85 mmHg of burst pressure was achieved, and advanced maturation of extracellular matrix (ECM) was observed by immunohistology, especially in regards to collagen and smooth muscle actin. The suture retention strength (2.24 ± 0.3 N) and axial tensile strength (2.45 ± 0.58 MPa) of the TEVGs achieved higher values than the native arteries used as control. The contrast agents labeling of the TEVGs allowed the monitorability of the PLGA degradation and enabled the visibility of the non-degradable textile component.

Conclusion:

Here, we present a concept for a novel textile-reinforced TEVG, which is successfully produced in 4 days of bioreactor conditioning, characterized by increased ECM maturation and sufficient mechanical strength. Additionally, the combination of our approach with non-invasive imaging provides further insights into TEVG’s clinical application.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13770-022-00482-0.

Biohybrid elastin-like venous valve with potential for in situ tissue engineering

Chronic venous insufficiency (CVI) is a leading vascular disease whose clinical manifestations include varicose veins, edemas, venous ulcers, and venous hypertension, among others. Therapies targeting this medical issue are scarce, and so far, no single venous valve prosthesis is clinically available. Herein, we have designed a bi-leaflet transcatheter venous valve that consists of (i) elastin-like recombinamers, (ii) a textile mesh reinforcement, and (iii) a bioabsorbable magnesium stent structure. Mechanical characterization of the resulting biohybrid elastin-like venous valves (EVV) showed an anisotropic behavior equivalent to the native bovine saphenous vein valves and mechanical strength suitable for vascular implantation. The EVV also featured minimal hemolysis and platelet adhesion, besides actively supporting endothelialization in vitro, thus setting the basis for its application as an in situ tissue engineering implant. In addition, the hydrodynamic testing in a pulsatile bioreactor demonstrated excellent hemodynamic valve performance, with minimal regurgitation (<10%) and pressure drop (<5 mmHg). No stagnation points were detected and an in vitro simulated transcatheter delivery showed the ability of the venous valve to withstand the implantation procedure. These results present a promising concept of a biohybrid transcatheter venous valve as an off-the-shelf implant, with great potential to provide clinical solutions for CVI treatment.

Towards technically controlled bioreactor maturation of tissue-engineered heart valves

Bioreactors are important tools for the pre-conditioning of tissue-engineered heart valves. The current state of the art mostly provides for timed, physical and biochemical stimulation in the bioreactor systems according to standard protocols (SOP). However, this does not meet to the individual biological variability of living tissue-engineered constructs. To achieve this, it is necessary to implement (i) sensory systems that detect the actual status of the implant and (ii) controllable bioreactor systems that allow patient-individualized pre-conditioning. During the maturation process, a pulsatile transvalvular flow of culture medium is generated within the bioreactor. For the improvement of this conditioning procedure, the relationship between the mechanical and biochemical stimuli and the corresponding tissue response has to be analyzed by performing reproducible and comparable experiments. In this work, a technological framework for maturation experiments of tissue-engineered heart valves in a pulsating bioreactor is introduced. The aim is the development of a bioreactor system that allows for continuous control and documentation of the conditioning process to increase reproducibility and comparability of experiments. This includes hardware components, a communication structure and software including online user communication and supervision. Preliminary experiments were performed with a tissue-engineered heart valve to evaluate the function of the new system. The results of the experiment proof the adequacy of the setup. Consequently, the concept is an important step for further research towards controlled maturation of tissue-engineered heart valves. The integration of molecular and histological sensor systems will be the next important step towards a fully automated, self-controlled preconditioning system.

Bioprinting-Associated Shear Stress and Hydrostatic Pressure Affect the Angiogenic Potential of Human Umbilical Vein Endothelial Cells

Bioprinting-associated shear stress and hydrostatic pressure can negatively affect the functionality of dispensed cells. We hypothesized that these mechanical stimuli can potentially affect the angiogenic potential of human umbilical vein endothelial cells (HUVECs). A numerical simulation model was used to calculate the shear stress during microvalve-based droplet ejection. The impact of different levels of applied pressure and the resulting shear stress levels on the angiogenic potential of HUVECs was investigated after up to 14 days of cultivation. In vitro results showed that bioprinting-associated stress not only has short-term but also long-term effects. The short-term viability results indicate a 20% loss in post-printing cell viability in samples printed under the harshest conditions compared to those with the lowest shear stress level. Further, it was revealed that even in two-dimensional culture, HUVECs were able to form a capillary-like network organization regardless of bioprinting pressure. In three-dimensional culture experiments; however, the HUVECs printed at 3 bar were not able to form tubular structures due to their exposure to high shear stress levels. In conclusion, this study provides new insights into how the bioprinting process should be conducted to control printing-associated shear stress and hydrostatic pressure to preserve the functionality and angiogenetic potential of HUVEC.

Suitability of Bronchoscopic Spraying for Fluid Deposition in Lower Airway Regions: Fluorescence Analysis on a Transparent In Vitro Airway Model

Introduction: Bronchoscopic spraying has potential for the application of therapeutic drugs in distal regions of the lung by bypassing the upper airways. However, there is a lack of understanding about the underlying fluid transport phenomena that are responsible for the intrapulmonary propagation of applied liquid. Methods: By using a transparent airway model, this study provides first experimental insights into relevant transport phenomena of bronchoscopic spraying. Furthermore, the penetration depth of the application is quantitatively evaluated. Laser-induced fluorescence is used to analyze fluid propagation in the transparent channels. Potential influencing factors such as the positioning in different airways, application number, breathing pattern, and lung obstructions are varied within this study to determine their influence on liquid deposition. Findings: This study shows that the method of bronchoscopic spraying allows the application of liquid in distal regions of the airway model. The position of the bronchoscope is a key influencing factor in increasing the penetration depth. We found that fluid transport along the distal airways essentially occurs by the film and plug flow phenomenon during application, which is similar to the transport mechanisms during instillation. Liquid plugs in lower airways are responsible for the reorganization of liquid during proximal movements and thereby influence the penetration depth in subsequent applications.

Bone Marrow Derived Mesenchymal Stromal Cells Promote Vascularization and Ciliation in Airway Mucosa Tri-Culture Models in Vitro

Patients suffering from irresectable tracheal stenosis often face limited treatment options associated with low quality of life. To date, an optimal tracheal replacement strategy does not exist. A tissue-engineered tracheal substitute promises to overcome limitations such as implant vascularization, functional mucociliary clearance and mechanical stability. In order to advance a tracheal mucosa model recently developed by our group, we examined different supporting cell types in fibrin-based tri-culture with primary human umbilical vein endothelial cells (HUVEC) and primary human respiratory epithelial cells (HRE). Bone marrow-derived mesenchymal stromal cells (BM-MSC), adipose-derived mesenchymal stromal cells (ASC) and human nasal fibroblasts (HNF) were compared regarding their ability to promote mucociliary differentiation and vascularization in vitro. Three-dimensional co-cultures of the supporting cell types with either HRE or HUVEC were used as controls. Mucociliary differentiation and formation of vascular-like structures were analyzed by scanning electron microscopy (SEM), periodic acid Schiff’s reaction (PAS reaction), two-photon laser scanning microscopy (TPLSM) and immunohistochemistry. Cytokine levels of vascular endothelial growth factor (VEGF), epidermal growth factor (EGF), interleukin-6 (IL6), interleukin-8 (IL8), angiopoietin 1, angiopoietin 2, fibroblast growth factor basic (FGF-b) and placenta growth factor (PIGF) in media supernatant were investigated using LEGENDplex™ bead-based immunoassay. Epithelial morphology of tri-cultures with BM-MSC most closely resembled native respiratory epithelium with respect to ciliation, mucus production as well as expression and localization of epithelial cell markers pan-cytokeratin, claudin-1, α-tubulin and mucin5AC. This was followed by tri-cultures with HNF, while ASC-supported tri-cultures lacked mucociliary differentiation. For all supporting cell types, a reduced ciliation was observed in tri-cultures compared to the corresponding co-cultures. Although formation of vascular-like structures was confirmed in all cultures, vascular networks in BM-MSC-tri-cultures were found to be more branched and extended. Concentrations of pro-angiogenic and inflammatory cytokines, in particular VEGF and angiopoietin 2, revealed to be reduced in tri-cultures compared to co-cultures. With these results, our study provides an important step towards a vascularized and ciliated tissue-engineered tracheal replacement. Additionally, our tri-culture model may in the future contribute to an improved understanding of cell-cell interactions in diseases associated with impaired mucosal function.

Evaluation of the long-term results of vascular anastomosis using polyurethane adhesive and shape-memory stent in the rat carotid artery model

INTRODUCTION

In free flaps, 5%-10% of complications are related to failure of sutured vascular anastomoses. Adhesive-based microvascular anastomoses are potential alternatives but are associated with failure rates of 70% in research studies. VIVO is a new adhesive with slow biodegradation within 6 months that has shown a 100% patency rate in research studies over 2 h observation time but long-term patency has not been evaluated. The authors hypothesize that VIVO will enable a reliable microvascular procedure comparable to sutured anastomoses over a 28-day period.

MATERIALS AND METHODS

The right common carotid artery of 60 male Sprague Dawley rats, ~450 g, were used for microvascular end-to-end anastomosis. VIVO was applied with reduced sutures with a temporary catheter in one group and in the other with a custom-shaped memory stent. Anastomoses with eight interrupted sutures served as control. All groups were n = 20. Anastomosis time and bleeding were recorded for each procedure. Doppler flowmetry was performed 20 min, 1, 10, and 28 days postoperatively. Postmortem toluidine staining was used for semi-quantitative analysis of stenosis, thrombosis, necrosis, and aneurysm formation by histologic evaluation.

RESULTS

No occlusion was detected 20 min and 1 day postoperative, and after 28 days of observation in all anastomoses. The anastomosis time of the VIVO with catheter group was about 32% significantly faster than the VIVO with stent group. In the VIVO group with stent, the bleeding time was ~80% shorter than in the control group with 2.1 ± 0.3 and VIVO with catheter 2.0 ± 0.5 (p ≤ .001 each). Minor and nonsignificant stent-associated thrombus formation and stent-typical intraluminal stenosis were detected exclusively in the VIVO with stent group.

CONCLUSION

Within the limitations of a rat study, the use of VIVO in anastomosis showed promising results. VIVO with catheter was found to be advantageous.

Novel Elastic Threads for Intestinal Anastomoses: Feasibility and Mechanical Evaluation in a Porcine and Rabbit Model

Gastrointestinal anastomoses are an important source of postoperative complications. In particular, the ideal suturing material is still the subject of investigation. Therefore, this study aimed to evaluate a newly developed suturing material with elastic properties made from thermoplastic polyurethane (TPU); Polyvinylidene fluoride (PVDF) and TPU were tested in two different textures (round and a modified, “snowflake” structure) in 32 minipigs, with two anastomoses of the small intestine sutured 2 m apart. After 90 days, the anastomoses were evaluated for inflammation, the healing process, and foreign body reactions. A computer-assisted immunohistological analysis of staining for Ki67, CD68, smooth muscle actin (SMA), and Sirius red was performed using TissueFAXS. Additionally, the in vivo elastic properties of the material were assessed by measuring the suture tension in a rabbit model. Each suture was tested twice in three rabbits; No major surgical complications were observed and all anastomoses showed adequate wound healing. The Ki67+ count and SMA area differed between the groups (F (3, 66) = 5.884, p = 0.0013 and F (3, 56) = 6.880, p = 0.0005, respectively). In the TPU-snowflake material, the Ki67+ count was the lowest, while the SMA area provided the highest values. The CD68+ count and collagen I/III ratio did not differ between the groups (F (3, 69) = 2.646, p = 0.0558 and F (3, 54) = 0.496, p = 0.686, respectively). The suture tension measurements showed a significant reduction in suture tension loss for both the TPU threads; Suturing material made from TPU with elastic properties proved applicable for intestinal anastomoses in a porcine model. In addition, our results suggest a successful reduction in tissue incision and an overall suture tension homogenization.

Monitoring the Remodeling of Biohybrid Tissue-Engineered Vascular Grafts by Multimodal Molecular Imaging

Tissue-engineered vascular grafts (TEVGs) with the ability to grow and remodel open new perspectives for cardiovascular surgery. Equipping TEVGs with synthetic polymers and biological components provides a good compromise between high structural stability and biological adaptability. However, imaging approaches to control grafts' structural integrity, physiological function, and remodeling during the entire transition between late in vitro maturation and early in vivo engraftment are mandatory for clinical implementation. Thus, a comprehensive molecular imaging concept using magnetic resonance imaging (MRI) and ultrasound (US) to monitor textile scaffold resorption, extracellular matrix (ECM) remodeling, and endothelial integrity in TEVGs is presented here. Superparamagnetic iron-oxide nanoparticles (SPION) incorporated in biodegradable poly(lactic-co-glycolic acid) (PLGA) fibers of the TEVGs allow to quantitatively monitor scaffold resorption via MRI both in vitro and in vivo. Additionally, ECM formation can be depicted by molecular MRI using elastin- and collagen-targeted probes. Finally, molecular US of α_v β₃ integrins confirms the absence of endothelial dysfunction; the latter is provocable by TNF-α. In conclusion, the successful employment of noninvasive molecular imaging to longitudinally evaluate TEVGs remodeling is demonstrated. This approach may foster its translation from in vitro quality control assessment to in vivo applications to ensure proper prostheses engraftment.

A polyurethane-based surgical adhesive for sealing blood vessel anastomoses-A feasibility study in pigs

Peri- and postoperative anastomotic leakage from blood vessel anastomosis is a common and potentially life-threatening complication. As an adjunctive therapy providing an additional layer of safety, a new biodegradable, polyurethane-based adhesive was developed. It consists of two components: an isocyanate-functionalized prepolymer and an amino-based curing agent. The adhesive was investigated in a porcine animal model to seal sutured blood vessel anastomoses of arteries, veins, aortas and prosthetic aortic graft replacements. The material-determined properties of the adhesive like viscosity, processing and polymerization time as well as bonding strength were well suited for this application. The adhesive stopped perioperative suture-line bleedings and stayed on all anastomoses until sacrifice. Hematological and serological inflammation marker assessments were unobtrusive. The histological evaluation showed a mild to moderate local tissue reaction to the adhesive constituting a physiological, non-adverse tissue-biomaterial interaction. The adhesive did not interfere with vascular wound healing. The adhesive demonstrated to be suitable to improve the outcome of cardiovascular surgeries by securing the classical sutured anastomoses in a fast, easy and safe manner. However, further studies are required to quantitatively evaluate efficacy in terms of anastomotic leakage prevention as well as long-term tissue compatibility and degradation.

Biodegradation and Immunological Parameters of Polyurethane-based Tissue Adhesive in Arterial Microvascular Anastomoses - a Long-term in Vivo Study

In microsurgical anastomosis, non-synthetic fibrin-based adhesives have predominantly shown superior properties to synthetic cyanoacrylates, but they have hardly any clinical application. This study aimed to investigate the local and systemic effects of synthetically produced biodegradable adhesive VIVO when used in microsurgical anastomosis. VIVO was used in two different anastomosis procedures in the common carotid artery in a rat model: VIVO in addition to a temporary catheter (VIVO TC) and VIVO with a custom-shaped memory nitinol stent (VIVO SM). Conventionally sutured anastomoses served as controls (C). Tissue response was assessed by in vivo fluorescence imaging and histological examination. The systemic effects of biodegradation were measured using hematologic parameters and serum levels of transaminase activity and lactate dehydrogenase. Finally, the degree of local adhesion of the different anastomotic procedures was evaluated. Fluorescence imaging showed reduced inflammatory blood flow in the VIVO TC group. Histological analysis of the anastomosed vessels also revealed significantly more inflammation in C than in the two adhesive groups. The severity of VIVO adhesions proved acceptable, and no histotoxic effects of VIVO were detected. The data demonstrated that the synthetic tissue adhesive VIVO is a reliable and- compared to sutures-tissue-friendly adhesive for microsurgical anastomoses. This article is protected by copyright. All rights reserved.

Establishment of a Pre-vascularized 3D Lung Cancer Model in Fibrin Gel-Influence of Hypoxia and Cancer-Specific Therapeutics

Lung cancer is the most frequently diagnosed cancer worldwide and the one that causes the highest mortality. In order to understand the disease and to develop new treatments, in vitro human lung cancer model systems which imitate the physiological conditions is of high significance. In this study, a human 3D lung cancer model was established that features the organization of a tumor with focus on tumor angiogenesis. Vascular networks were formed by co-culture of human umbilical vein endothelial cells and adipose tissue-derived mesenchymal stem cells (ASC) for 14 days in fibrin. A part of the pre-vascularized fibrin gel was replaced by fibrin gel containing lung cancer cells (A549) to form tri-cultures. This 3D cancer model system was cultured under different culture conditions and its behaviour after treatment with different concentrations of tumor-specific therapeutics was evaluated. The evaluation was performed by measurement of metabolic activity, viability, quantification of two-photon laser scanning microscopy and measurement of the proangiogenic factor vascular endothelial growth factor in the supernatant. Hypoxic conditions promoted vascularization compared to normoxic cultured controls in co- and tri-cultures as shown by significantly increased vascular structures, longer structures with a higher area and volume, and secretion of vascular endothelial growth factor. Cancer cells also promoted vascularization. Treatment with 50 µM gefitinib or 50 nM paclitaxel decreased the vascularization significantly. VEGF secretion was only reduced after treatment with gefitinib, while in contrast secretion remained constant during medication with paclitaxel. The findings suggest that the herein described 3D lung cancer model provides a novel platform to investigate the angiogenic potential of cancer cells and its responses to therapeutics. Thus, it can serve as a promising approach for the development and patient-specific pre-selection of anticancer treatment.

FRESH bioprinting technology for tissue engineering - the influence of printing process and bioink composition on cell behavior and vascularization

The rapid and tailored biofabrication of natural materials is of high interest for the field of tissue engineering and regenerative medicine. Scaffolds require both high biocompatibility and tissue-dependent mechanical strength to function as basis for tissue-engineered implants. Thus, natural hydrogels such as fibrin are promising but their rapid biofabrication remains challenging. Printing of low viscosity and slow polymerizing solutions with good spatial resolution can be achieved by freeform reversible embedding of suspended hydrogels (FRESH) bioprinting of cell-laden natural hydrogels. In this study, fibrin and hyaluronic acid were used as single components as well as blended ink mixtures for the FRESH bioprinting. Rheometry revealed that single materials were less viscous than the blended bioink showing higher values for viscosity over a shear rate of 10-1000 s^-1. While fibrin showed viscosities between 0.1624 and 0.0017 Pa·s, the blended ink containing fibrin and hyaluronic acid were found to be in a range of 0.1-1 Pa·s. In 3D vascularization assays, formation of vascular structures within the printed constructs was investigated indicating that the printing process did not harm cells and allowed formation of vasculature comparable to moulded control samples. Best values for vascularization were achieved in bioinks consisting of 1.0% fibrin-0.5% hyaluronic acid. The vascular structure area and length were three times higher compared to other tested bioinks, and structure volume as well as number of branches revealed almost four times higher values. In this study, we combined the benefits of the FRESH printing technique with in vitro vascularization, showing that it is possible to achieve a mechanically stable small-scale hydrogel construct incorporating vascular network formation.

Microvascular anastomosis techniques using the medical adhesive VIVO and expandable micro-stents in a rat carotid artery model

BACKGROUND

Sutured anastomosis remains the gold standard in microvascular surgery. The procedure is not free of complications and is a time-consuming operation requiring a high level of experience. The aim of this study was to develop new methods for a stable, faster, and safer anastomosis using a novel biodegradable adhesive, VIVO, and a custom-made microvascular stent.

METHODS

The VIVO medical adhesive was used for a total of 30 anastomoses in rats in the right carotid artery: 15 anastomoses were performed with a temporary intraluminal catheter, VIVO, and reduced sutures (VIVO + TC). A further 15 anastomoses were performed with nitinol stents, VIVO, and reduced sutures (VIVO + SM). Sutured anastomoses served as controls (C) and were performed on the left carotid arteries of the 30 rats. Operation and bleeding times were assessed, and patency was evaluated by Doppler flowmetry and indocyanine green (ICG) angiography. Subsequently, the anastomoses were evaluated histopathological.

RESULTS

The overall patency was recorded as 100% in all groups. No thrombosis or circulatory disturbance was found. Compared to C and VIVO + SM, VIVO + TC proved to be significantly less traumatic, less demanding, and time-saving. The sealing properties of VIVO lead to shorter bleeding times and less oozing. In contrast, VIVO + SM proved to be the most technically demanding and time-consuming procedure.

CONCLUSION

The success of a microvascular sutured anastomosis is determined by a short ischemic interval. Compared to sutured anastomosis, VIVO + TC showed ease of use as well as shorter time taken for anastomosis, less trauma, and lower blood loss. More long-term studies on the functions, biological interactions, and survival rates of glue-based anastomoses need to be initiated.

Drug-Eluting Medical Textiles: From Fiber Production and Textile Fabrication to Drug Loading and Delivery

Drug-eluting medical textiles have recently gained great attention to be used in different applications due to their cost effectiveness and unique physical and chemical properties. Using various fiber production and textile fabrication technologies, fibrous constructs with the required properties for the target drug delivery systems can be designed and fabricated. This review summarizes the current advances in the fabrication of drug-eluting medical textiles. Different fiber production methods such as melt-, wet-, and electro-spinning, and textile fabrication techniques such as knitting and weaving are explained. Moreover, various loading processes of bioactive agents to obtain drug-loaded fibrous structures with required physicochemical and morphological properties, drug delivery mechanisms, and drug release kinetics are discussed. Finally, the current applications of drug-eluting fibrous systems in wound care, tissue engineering, and transdermal drug delivery are highlighted.

Bioengineered percutaneous heart valves for transcatheter aortic valve replacement: a comparative evaluation of decellularised bovine and porcine pericardia

Glutaraldehyde-treated, surgical bioprosthetic heart valves undergo structural degeneration within 10-15 years of implantation. Analogous preliminary results were disclosed for percutaneous heart valves (PHVs) realized with similarly-treated tissues. To improve long-term performance, decellularised scaffolds can be proposed as alternative fabricating biomaterials. The aim of this study was to evaluate whether bovine and porcine decellularised pericardia could be utilised to manufacture bioengineered percutaneous heart valves (bioPHVs) with adequate hydrodynamic performance and leaflet resistance to crimping damage. BioPHVs were fabricated by mounting acellular pericardia onto commercial stents. Independently from the pericardial species used for valve fabrication, bioPHVs satisfied the minimum hydrodynamic performance criteria set by ISO 5840-3 standards and were able to withstand a large spectrum of cardiac output conditions, also during extreme backpressure, without severe regurgitation, especially in the case of the porcine group. No macroscopic or microscopic leaflet damage was detected following bioPHV crimping. Bovine and porcine decellularized pericardia are both suitable alternatives to glutaraldehyde-treated tissues. Between the two types of pericardial species tested, the porcine tissue scaffold might be preferable to fabricate advanced PHV replacements for long-term performance. CONDENSED ABSTRACT: Current percutaneous heart valve replacements are formulated with glutaraldehyde-treated animal tissues, prone to structural degeneration. In order to improve long-term performance, bovine and porcine decellularised pericardia were utilised to manufacture bioengineered replacements, which demonstrated adequate hydrodynamic behaviour and resistance to crimping without leaflet architectural alteration.

Endoscopic atomization of mesenchymal stromal cells: in vitro study for local cell therapy of the lungs

BACKGROUND AIMS

Cell-based therapies of pulmonary diseases with mesenchymal stromal cells (MSCs) are increasingly under experimental investigation. In most of these, MSCs are administered intravenously or by direct intratracheal instillation. A parallel approach is to administer the cells into the lung by endoscopic atomization (spraying). In a previous study, the authors developed a flexible endoscopic atomization device that allows administration of respiratory epithelial cells in the lungs with high survival.

METHODS

In this study, the authors evaluated the feasibility of spraying MSCs with two different endoscopic atomization devices (air and pressure atomization). Following atomization, cell viability was evaluated with live/dead staining. Subsequent effects on cytotoxicity, trilineage differentiation and expression of MSC-specific markers as well as on MSC metabolic activity and morphology were analyzed for up to 7 days.

RESULTS

MSC viability immediately after spraying and subsequent metabolic activity for 7 days was not influenced by either of the devices. Slightly higher cytotoxicity rates could be observed for pressure-atomized compared with control and air-atomized MSCs over 7 days. Flow cytometry revealed no changes in characteristic MSC cell surface marker expression, and morphology remained unchanged. Standard differentiation into osteocytes, chondrocytes and adipocytes was inducible after atomization.

CONCLUSIONS

In the literature, a minimal survival of 50% was previously defined as the cutoff value for successful cell atomization. This is easily met with both of the authors' devices, with more than 90% survival. Thus, there is a potential role for atomization in intrapulmonary MSC-based cell therapies, as it is a feasible and easily utilizable approach based on clinically available equipment.

Advances in Engineering Venous Valves: The Pursuit of a Definite Solution for Chronic Venous Disease

Native venous valves enable proper return of blood to the heart. Under pathological conditions (e.g., chronic venous insufficiency), venous valves malfunction and fail to prevent backward flow. Clinically, this can result in painful swelling, varicose veins, edema, and skin ulcerations leading to a chronic wound situation. Surgical correction of venous valves has proven to drastically reduce these symptoms. However, the absence of intact leaflets in many patients limits the applicability of this strategy. In this context, the development of venous valve replacements represents an appealing approach. Despite acceptable results in animal models, no venous valve has succeeded in clinical trials, and so far no single prosthetic venous valve is commercially available. This calls for advanced materials and fabrication approaches to develop clinically relevant venous valves able to restore natural flow conditions in the venous circulation. In this study, we critically discuss the approaches attempted in the last years, and we highlight the potential of tissue engineering to offer new avenues for valve fabrication. Impact statement Venous valves prosthesis offer the potential to restore normal venous flow, and to improve the prospect of patients that suffer from chronic venous disease. Current venous valve replacements are associated with poor outcomes. A deeper understanding of the approaches attempted so far is essential to establish the next steps toward valve development, and importantly, tissue engineering constitutes a unique toolbox to advance in this quest.

Cross-section modified and highly elastic sutures reduce tissue incision and show comparable biocompatibility: in-vitro and in-vivo evaluation of novel thermoplastic urethane surgical threads

Surgical sutures are indispensable for a vast majority of operative procedures. An ideal suture is characterized by high tissue compliance without cutting into the mended tissue and optimal biocompatibility. Therefore, we assessed these mechanical and biological properties for novel elastic thermoplastic polyurethane (TPU) and cross-sectional modified "snowflake" sutures. Circular and "snowflake"-shaped TPU threads were manufactured and compared to similar surface modified polyvinylidene fluoride (PVDF) sutures. Regular PVDF sutures were used as the control group. Single-axis tensile test with and without gelatinous tissue surrogates were performed to evaluate the suture incision into the specimens. Biocompatibility was evaluated by subcutaneous implantation (n = 18) in rats for 7 and 21 days. Histology and immunohistology was conducted for assessment of the foreign body reaction. Regular and modified TPU threads showed a significant reduction of incision into the tissue surrogates compared to the control. Both TPU sutures and the modified PVDF sutures achieved comparable biocompatibility versus regular PVDF threads. Detailed histology revealed novel tissue integration into the notches of the surface modified sutures, we termed this newly shaped granuloma "intrafilamentous" granuloma. Elastic TPU threads showed a significant reduction of tissue surrogate incision and suture tension loss. Biocompatibility did not significantly differ from standard PVDF. Histology demonstrated tissue ingrowth following the surface modification of the suture referred to as "intrafilamentous" granuloma. Further in vivo studies are required to illuminate the exact potential of the new sutures to optimize intestinal anastomosis.

Warp-Knitted Spacer Fabrics: A Versatile Platform to Generate Fiber-Reinforced Hydrogels for 3D Tissue Engineering

Mesenchymal stem cells (MSCs) possess huge potential for regenerative medicine. For tissue engineering approaches, scaffolds and hydrogels are routinely used as extracellular matrix (ECM) carriers. The present study investigated the feasibility of using textile-reinforced hydrogels with adjustable porosity and elasticity as a versatile platform for soft tissue engineering. A warp-knitted poly (ethylene terephthalate) (PET) scaffold was developed and characterized with respect to morphology, porosity, and mechanics. The textile carrier was infiltrated with hydrogels and cells resulting in a fiber-reinforced matrix with adjustable biological as well as mechanical cues. Finally, the potential of this platform technology for regenerative medicine was tested on the example of fat tissue engineering. MSCs were seeded on the construct and exposed to adipogenic differentiation medium. Cell invasion was detected by two-photon microscopy, proliferation was measured by the PrestoBlue assay. Successful adipogenesis was demonstrated using Oil Red O staining as well as measurement of secreted adipokines. In conclusion, the given microenvironment featured optimal mechanical as well as biological properties for proliferation and differentiation of MSCs. Besides fat tissue, the textile-reinforced hydrogel system with adjustable mechanics could be a promising platform for future fabrication of versatile soft tissues, such as cartilage, tendon, or muscle.

Fabrication of blood-derived elastogenic vascular grafts using electrospun fibrinogen and polycaprolactone composite scaffolds for paediatric applications

The development of tissue-engineered vascular grafts (TEVGs) for paediatric applications must consider unique factors associated with this patient cohort. Although the increased elastogenic potential of neonatal cells offers an opportunity to overcome the long-standing challenge of in vitro elastogenesis, neonatal patients have a lower tolerance for autologous tissue harvest and require grafts that exhibit growth potential. The purpose of this study was to apply a multipronged strategy to promote elastogenesis in conjunction with umbilical cord-derived materials in the production of a functional paediatric TEVG. An initial proof-of-concept study was performed to extract fibrinogen from human umbilical cord blood samples and, through electrospinning, to produce a nanofibrous fibrinogen scaffold. This scaffold was seeded with human umbilical artery-derived smooth muscle cells (hUASMCs), and neotissue formation within the scaffold was examined using immunofluorescence microscopy. Subsequently, a polycaprolactone-reinforced porcine blood-derived fibrinogen scaffold (isolated using the same protocol as cord blood fibrinogen) was used to develop a rolled-sheet graft that employed topographical and biochemical guidance cues to promote elastogenesis and cellular orientation. This approach resulted in a TEVG with robust mechanical properties and biomimetic arrangement of extracellular matrix (ECM) with rich expression of elastic fibre-related proteins. The results of this study hold promise for further development of paediatric TEVGs and the exploration of the effects of scaffold microstructure and nanostructure on vascular cell function and ECM production.

Extracellular Vesicles-Loaded Fibrin Gel Supports Rapid Neovascularization for Dental Pulp Regeneration

Rapid vascularization is required for the regeneration of dental pulp due to the spatially restricted tooth environment. Extracellular vesicles (EVs) released from mesenchymal stromal cells show potent proangiogenic effects. Since EVs suffer from rapid clearance and low accumulation in target tissues, an injectable delivery system capable of maintaining a therapeutic dose of EVs over a longer period would be desirable. We fabricated an EV-fibrin gel composite as an in situ forming delivery system. EVs were isolated from dental pulp stem cells (DPSCs). Their effects on cell proliferation and migration were monitored in monolayers and hydrogels. Thereafter, endothelial cells and DPSCs were co-cultured in EV-fibrin gels and angiogenesis as well as collagen deposition were analyzed by two-photon laser microscopy. Our results showed that EVs enhanced cell growth and migration in 2D and 3D cultures. EV-fibrin gels facilitated vascular-like structure formation in less than seven days by increasing the release of VEGF. The EV-fibrin gel promoted the deposition of collagen I, III, and IV, and readily induced apoptosis during the initial stage of angiogenesis. In conclusion, we confirmed that EVs from DPSCs can promote angiogenesis in an injectable hydrogel in vitro, offering a novel and minimally invasive strategy for regenerative endodontic therapy.

Choosing the Right Differentiation Medium to Develop Mucociliary Phenotype of Primary Nasal Epithelial Cells In Vitro

In vitro differentiation of airway epithelium is of interest for respiratory tissue engineering and studying airway diseases. Both applications benefit from the use of primary cells to maintain a mucociliated phenotype and thus physiological functionality. Complex differentiation procedures often lack standardization and reproducibility. To alleviate these shortfalls, we compared differentiation behavior of human nasal epithelial cells in four differentiation media. Cells were differentiated at the air-liquid interface (ALI) on collagen-coated inserts. Mucociliary differentiation status after five weeks was analyzed by electron microscopy, histology and immunohistochemistry. The amount of ciliation was estimated and growth factor concentrations were evaluated using ELISA. We found that retinoic-acid-supplemented mixture of DMEM and Airway Epithelial Cell Growth Medium gave most promising results to obtain ciliated and mucus producing nasal epithelium in vitro. We discovered the balance between retinoic acid (RA), vascular endothelial growth factor (VEGF), epidermal growth factor (EGF) and fibroblast growth factor β (FGF-β) to be relevant for differentiation. We could show that low VEGF, EGF and FGF-β concentrations in medium correspond to absent ciliation in specific donors. Therefore, our results may in future facilitate donor selection and non-invasive monitoring of ALI cultures and by this contribute to improved standardization of epithelial in vitro culture.

Formation of cyclic structures in the cationic ring-opening polymerization of 1,3-dioxolane

The cationic ring-opening polymerization of acetals is prone to cyclization of the polymer chains. This is also the case for the polymerization of 1,3-dioxolane. Literature states that this cyclization can be reduced by applying the Active Monomer mechanism, at least if no competition with the Active Chain End mechanism occurs. In this work, a detailed characterization of the different distributions resulting from the cationic ring-opening polymerization of 1,3-dioxolane via the Active Monomer mechanism is made by a combination of gel permeation chromatography, ¹H NMR, and for the first time by matrix assisted laser desorption/ionization time of flight mass spectrometry. The influence of monomer addition speed, catalyst to initiator ratio and solvent were studied on both kinetics and composition of the product. Furthermore, it was found that increasing the conversion and monomer to initiator ratios leads to an increased amount of cyclic structures and to broader distributions, in correspondence with the Jacobson–Stockmayer theory. Furthermore, ion trapping experiments using ³¹P NMR provide insights into the actual reaction mechanism. Finally, purification of the products after the reactions led to a reduction of the cyclic fraction.

Elucidating the mechanism of the cationic ring-opening polymerization of dioxolane using gel permeation chromatography, matrix assisted laser desorption/ionization time of flight mass spectrometry and ³¹P NMR.

Influence of Different Cell Types and Sources on Pre-Vascularisation in Fibrin and Agarose-Collagen Gels

Vascularisation is essential for the development of tailored, tissue-engineered organs and tissues due to diffusion limits of nutrients and the lack of the necessary connection to the cardiovascular system. To pre-vascularize, endothelial cells and supporting cells can be embedded in the scaffold to foster an adequate nutrient and oxygen supply after transplantation. This technique is applied for tissue engineering of various tissues, but there have been few studies on the use of different cell types or cells sources. We compare the effect of supporting cells from different sources on vascularisation. Fibrin gels and agarose-collagen hydrogels were used as scaffolds. The supporting cells were primary human dermal fibroblasts (HDFs), human nasal fibroblasts (HNFs), human mesenchymal stem cells from umbilical cord’s Wharton’s jelly (WJ MSCs), adipose-derived MSCs (AD MSCs) and femoral bone marrow-derived MSCs (BM MSCs). The tissue constructs were incubated for 14 days and analyzed by two-photon laser scanning microscopy. Vascularisation was supported by all cell types, forming branched networks of tubular vascular structures in both hydrogels. In general, fibrin gels present a higher angiogenic promoting environment compared to agarose-collagen hydrogels and fibroblasts show a high angiogenic potential in co-culture with endothelial cells. In agarose-collagen hydrogels, vascular structures supported by AD MSCs were comparable to our HDF control in terms of volume, area and length. BM MSCs formed a homogeneous network of smaller structures in both hydrogels. This study provides data toward understanding the pre-vascularisation properties of different supporting cell types and sources for tissue engineering of different organs and tissues.

EndOxy: Dynamic Long-Term Evaluation of Endothelialized Gas Exchange Membranes for a Biohybrid Lung

In the concept of a biohybrid lung, endothelial cells seeded on gas exchange membranes form a non-thrombogenic an anti-inflammatory surface to overcome the lacking hemocompatibility of today’s oxygenators during extracorporeal membrane oxygenation. To evaluate this concept, the long-term stability and gas exchange performance of endothelialized RGD-conjugated polydimethylsiloxane (RGD-PDMS) membranes was evaluated. Human umbilical vein endothelial cells (ECs) were cultured on RGD-PDMS in a model system under physiological wall shear stress (WSS) of 0.5 Pa for up to 33 days. Gas exchange performance was tested with three biological replicates under elevated WSS of 2.5 Pa using porcine blood adjusted to venous values following ISO 7199 and blood gas analysis. EC morphology was assessed by immunocytochemistry (n = 3). RGD-PDMS promoted endothelialization and stability of endothelialized membranes was shown for at least 33 days and for a maximal WSS of 2.5 Pa. Short-term exposure to porcine blood did not affect EC integrity. The gas transfer tests provided evidence for the oxygenation and decarboxylation of the blood across endothelialized membranes with a decrease of transfer rates over time that needs to be addressed in further studies with larger sample sizes. Our results demonstrate the general suitability of RGD-PDMS for biohybrid lung applications, which might enable long-term support of patients with chronic lung failure in the future.

Small Caliber Compliant Vascular Grafts Based on Elastin-Like Recombinamers for in situ Tissue Engineering

Vascular disease is a leading cause of death worldwide, but surgical options are restricted by the limited availability of autologous vessels, and the suboptimal performance of prosthetic vascular grafts. This is especially evident for coronary artery by-pass grafts, whose small caliber is associated with a high occlusion propensity. Despite the potential of tissue-engineered grafts, compliance mismatch, dilatation, thrombus formation, and the lack of functional elastin are still major limitations leading to graft failure. This calls for advanced materials and fabrication schemes to achieve improved control on the grafts' properties and performance. Here, bioinspired materials and technical textile components are combined to create biohybrid cell-free implants for endogenous tissue regeneration. Clickable elastin-like recombinamers are processed to form an open macroporous 3D architecture to favor cell ingrowth, while being endowed with the non-thrombogenicity and the elastic behavior of the native elastin. The textile components (i.e., warp-knitted and electrospun meshes) are designed to confer suture retention, long-term structural stability, burst strength, and compliance. Notably, by controlling the electrospun layer's thickness, the compliance can be modulated over a wide range of values encompassing those of native vessels. The grafts support cell ingrowth, extracellular matrix deposition and endothelium development in vitro. Overall, the fabrication strategy results in promising off-the-shelf hemocompatible vascular implants for in situ tissue engineering by addressing the known limitations of bioartificial vessel substitutes.

A bench-top molding method for the production of cell-laden fibrin micro-fibers with longitudinal topography

Tissue-engineered constructs have great potential in many intervention strategies. In order for these constructs to function optimally, they should ideally mimic the cellular alignment and orientation found in the tissues to be treated. Here we present a simple and reproducible method for the production of cell-laden pure fibrin micro-fibers with longitudinal topography. The micro-fibers were produced using a molding technique and longitudinal topography was induced by a single initial stretch. Using this method, fibers up to 1 m in length and with diameters of 0.2-3 mm could be produced. The micro-fibers were generated with embedded endothelial cells, smooth muscle cell/fibroblasts or Schwann cells. Polarized light and scanning electron microscopy imaging showed that the initial stretch was sufficient to induce longitudinal topography in the fibrin gel. Cells in the unstretched control micro-fibers elongated randomly in both the floating and encapsulated environments, whereas the cells in the stretched micro-fibers responded to the introduced topography by adopting a similar orientation. Proof of concept bottom-up tissue engineering (TE) constructs are shown, all displaying various anisotropic organization of cells within. This simple, economical, versatile and scalable approach for the production of highly orientated and cell-laden micro-fibers is easily transferrable to any TE laboratory.

Comparison of Covered Laser-cut and Braided Respiratory Stents: From Bench to Pre-Clinical Testing

Lung cancer patients often suffer from severe airway stenosis, the symptoms of which can be relieved by the implantation of stents. Different respiratory stents are commercially available, but the impact of their mechanical performance on tissue responses is not well understood. Two novel laser-cut and hand-braided nitinol stents, partially covered with polycarbonate urethane, were bench tested and implanted in Rhön sheep for 6 weeks. Bench testing highlighted differences in mechanical behavior: the laser-cut stent showed little foreshortening when crimped to a target diameter of 7.5 mm, whereas the braided stent elongated by more than 50%. Testing also revealed that the laser-cut stent generally exerted higher radial resistive and chronic outward forces than the braided stent, but the latter produced significantly higher radial resistive forces at diameters below 9 mm. No migration was observed for either stent type in vivo. In terms of granulation, most stents exerted a low to medium tissue response with only minimal formation of granulation tissue. We have developed a mechanical and in vivo framework to compare the behavior of different stent designs in a large animal model, providing data, which may be employed to improve current stent designs and to achieve better treatment options for lung cancer patients.