Incorporation of growth factor containing Matrigel promotes vascularization of porous PLGA scaffolds

Structural design and mechanical performance of composite vascular grafts

This study reviews the state of the art in structural design and the corresponding mechanical behaviours of composite vascular grafts. We critically analyse surface and matrix designs composed of layered, embedded, and hybrid structures along the radial and longitudinal directions; materials and manufacturing techniques, such as tissue engineering and the use of textiles or their combinations; and the corresponding mechanical behaviours of composite vascular grafts in terms of their physical–mechanical properties, especially their stress–strain relationships and elastic recovery. The role of computational studies is discussed with respect to optimizing the geometrics designs and the corresponding mechanical behaviours to satisfy specialized applications, such as those for the aorta and its subparts. Natural and synthetic endothelial materials yield improvements in the mechanical and biological compliance of composite graft surfaces with host arteries. Moreover, the diameter, wall thickness, stiffness, compliance, tensile strength, elasticity, and burst strength of the graft matrix are determined depending on the application and the patient. For composite vascular grafts, hybrid architectures are recommended featuring multiple layers, dimensions, and materials to achieve the desired optimal flexibility and function for complying with user-specific requirements. Rapidly emerging artificial intelligence and big data techniques for diagnostics and the three-dimensional (3D) manufacturing of vascular grafts will likely yield highly compliant, subject-specific, long-lasting, and economical vascular grafts in the near-future.

Graphic abstract

Bioactive Materials Promote Wound Healing through Modulation of Cell Behaviors

Skin wound repair is a multistage process involving multiple cellular and molecular interactions, which modulate the cell behaviors and dynamic remodeling of extracellular matrices to maximize regeneration and repair. Consequently, abnormalities in cell functions or pathways inevitably give rise to side effects, such as dysregulated inflammation, hyperplasia of nonmigratory epithelial cells, and lack of response to growth factors, which impedes angiogenesis and fibrosis. These issues may cause delayed wound healing or even non-healing states. Current clinical therapeutic approaches are predominantly dedicated to preventing infections and alleviating topical symptoms rather than addressing the modulation of wound microenvironments to achieve targeted outcomes. Bioactive materials, relying on their chemical, physical, and biological properties or as carriers of bioactive substances, can affect wound microenvironments and promote wound healing at the molecular level. By addressing the mechanisms of wound healing from the perspective of cell behaviors, this review discusses how bioactive materials modulate the microenvironments and cell behaviors within the wounds during the stages of hemostasis, anti-inflammation, tissue regeneration and deposition, and matrix remodeling. A deeper understanding of cell behaviors during wound healing is bound to promote the development of more targeted and efficient bioactive materials for clinical applications.

Cell-Seeded Biomaterial Scaffolds: The Urgent Need for Unanswered Accelerated Angiogenesis

One of the most arduous challenges in tissue engineering is neovascularization, without which there is a lack of nutrients delivered to a target tissue. Angiogenesis should be completed at an optimal density and within an appropriate period of time to prevent cell necrosis. Failure to meet this challenge brings about poor functionality for the tissue in comparison with the native tissue, extensively reducing cell viability. Prior studies devoted to angiogenesis have provided researchers with some biomaterial scaffolds and cell choices for angiogenesis. For example, while most current angiogenesis approaches require a variety of stimulatory factors ranging from biomechanical to biomolecular to cellular, some other promising stimulatory factors have been underdeveloped (such as electrical, topographical, and magnetic). When it comes to choosing biomaterial scaffolds in tissue engineering for angiogenesis, key traits rush to mind including biocompatibility, appropriate physical and mechanical properties (adhesion strength, shear stress, and malleability), as well as identifying the appropriate biomaterial in terms of stability and degradation profile, all of which may leave essential trace materials behind adversely influencing angiogenesis. Nevertheless, the selection of the best biomaterial and cells still remains an area of hot dispute as such previous studies have not sufficiently classified, integrated, or compared approaches. To address the aforementioned need, this review article summarizes a variety of natural and synthetic scaffolds including hydrogels that support angiogenesis. Furthermore, we review a variety of cell sources utilized for cell seeding and influential factors used for angiogenesis with a concentrated focus on biomechanical factors, with unique stimulatory factors. Lastly, we provide a bottom-to-up overview of angiogenic biomaterials and cell selection, highlighting parameters that need to be addressed in future studies.

Exogenous FGF-2 prolongs endothelial connection in multilayered human skeletal muscle cell sheet

Angiogenesis is a pressing issue in tissue engineering associated with restoration of blood supply to ischemic tissues and promotion of rapid vascularization of tissue-engineered grafts. Fibroblast growth factor-2 (FGF-2) plays a vital role in processes such as angiogenesis and is an attractive candidate for tissue engineering. While skeletal muscle tissue engineering is established, the role of FGF-2 in endothelial function to promote angiogenesis after transplantation is unclear. Here, a culture system comprising a five-layered sheet of human skeletal muscle cells co-incubated on green fluorescent protein-expressing human umbilical vein endothelial cells (GFP-HUVECs) mimicking in vivo angiogenesis was used to investigate the role of FGF-2 in vascularization of engineered tissues. The basal level of FGF-2 in cultured media of skeletal muscle cell sheets was undetectable. Therefore, cell sheets co-incubated with GFP-HUVECs were exogenously treated with 10 ng/mL FGF-2, and endothelial network formation was evaluated. After prolonged culture, the endothelial network length and connectivity increased following treatment with FGF-2 as compared with control treatment. The numbers of medium and long endothelial networks significantly increased inside the sheet longer than 0.2 and 0.4 cm, respectively, after FGF-2 treatment. Time-lapse microscopy monitoring dynamic endothelial behavior revealed that FGF-2-mediated maintenance of endothelial connection and retardation of endothelial network disconnection after 72 h. The present study suggests the precise role of FGF-2 in maintaining endothelial connection and the extent of the endothelial network in skeletal muscle cell sheets. This understanding can be applied to design in vitro pre-vascularized tissue and graft integration prospects.

TNF-α-Inhibition Improves the Biocompatibility of Porous Polyethylene Implants In Vivo

Background:

To improve the biocompatibility of porous polyethylene (PPE) implants and expand their application range for reconstructive surgery in poorly vascularized environments, implants were coated with tumor necrosis factor α (TNFα) inhibitor Etanercept. While approved for systemic application, local application of the drug is a novel experimental approach. Microvascular and mechanical integration as well as parameters of inflammation were analyzed in vivo.

Methods:

PPE implants were coated with Etanercept and extracellular matrix (ECM) components prior to implantation into dorsal skinfold chambers of C57BL/6 mice. Fluorescence microscopy analyses of angiogenesis and local inflammatory response were thrice performed in vivo over a period of 14 days to assess tissue integration and biocompatibility. Uncoated implants and ECM-coated implants served as controls.

Results:

TNFα inhibition with Etanercept led to a reduced local inflammatory response: leukocyte-endothelial cell adherence was significantly lowered compared to both control groups (n = 6/group) on days 3 and 14, where the lowest values were reached: 3573.88 leukocytes/mm-2 ± 880.16 (uncoated implants) vs. 3939.09 mm-2 ± 623.34 (Matrigel only) vs. 637.98 mm-2 + 176.85 (Matrigel and Etanercept). Implant-coating with Matrigel alone and Matrigel and Etanercept led to significantly higher vessel densities 7 and 14 days vs. 3 days after implantation and compared to uncoated implants. Mechanical implant integration as measured by dynamic breaking strength did not differ after 14 days.

Conclusion:

Our data show a reduced local inflammatory response to PPE implants after immunomodulatory coating with Etanercept in vivo, suggesting improved biocompatibility. Application of this tissue engineering approach is therefore warranted in models of a compromised host environment.

Electronic supplementary material

The online version of this article (10.1007/s13770-020-00325-w) contains supplementary material, which is available to authorized users.

Free Gingival Graft and Collagen Matrix Revascularization in an Enoral Open Wound Situation

PURPOSE

Vestibuloplasty with free gingival grafting is a frequently performed surgical procedure to generate sufficient keratinized mucosa (KM) around dental implants. Avascular porcine collagen matrices (CM) have been proclaimed to be sufficient substitutes as alternatives to free gingival grafts (FGGs). However, the process of graft integration and vascularization is still incompletely understood.

METHODS

In 18 patients a vestibuloplasty in the lower edentulous jaw situation was performed during implant exposure, either with FGGs from the palate or a porcine CM (mucoderm). Tissue perfusion of the soft tissue grafts was measured using laser-doppler-spectrophotometer intraoperatively and on postoperative days 2, 5, 10, 30 and between days 60 and 90. With graft perfusion expressed by oxygen saturation [SO2%], the relative amount of hemoglobin [rHb], blood flow, and velocity [AU] was detected and compared between groups and the surrounding mucosa.

RESULTS

Healing was uneventful in both groups, with mature KM around dental implants after healing. Blood flow and velocity significantly increased until postoperative day 10, comparable to perfusion values of the surrounded mucosa. Intergroup comparisons revelated no significant differences concerning the flow between CM and FGGs. Oxygen saturation also significantly increased within the first 5 postoperative days in both groups. Hemoglobin content did not show any differences during the investigated period.

CONCLUSIONS

The perfusion mainly progresses within the first postoperative week with only minimal further detectable alterations until the final investigation, comparable in both groups. Although integration of FGGs (revascularized) and the CM (new tissue formation) is biologically different, both transplants show comparable perfusion patterns, leading to sufficient KM.

Angiogenic biomaterials to promote therapeutic regeneration and investigate disease progression

The vasculature is a key component of the tissue microenvironment. Traditionally known for its role in providing nutrients and oxygen to surrounding cells, the vasculature is now also acknowledged to provide signaling cues that influence biological outcomes in regeneration and disease. These cues come from the cells that comprise vasculature, as well as the dynamic biophysical and biochemical properties of the surrounding extracellular matrix that accompany vascular development and remodeling. In this review, we illustrate the larger role of the vasculature in the context of regenerative biology and cancer progression. We describe cellular, biophysical, biochemical, and metabolic components of vascularized microenvironments. Moreover, we provide an overview of multidimensional angiogenic biomaterials that have been developed to promote therapeutic vascularization and regeneration, as well as to mimic elements of vascularized microenvironments as a means to uncover mechanisms by which vasculature influences cancer progression and therapy.

Vascularization strategies for skin tissue engineering

Lack of proper vascularization after skin trauma causes delayed wound healing. This has sparked the development of various tissue engineering strategies to improve vascularization.

Vascularization of tissue-engineered skeletal muscle constructs

Skeletal muscle tissue can be created in vitro by tissue engineering approaches, based on differentiation of muscle stem cells. Several approaches exist and generally result in three dimensional constructs composed of multinucleated myofibers to which we refer as myooids. Engineering methods date back to 3 decades ago and meanwhile a wide range of cell types and scaffold types have been evaluated. Nevertheless, in most approaches, myooids remain very small to allow for diffusion-mediated nutrient supply and waste product removal, typically less than 1 mm thick. One of the shortcomings of current in vitro skeletal muscle organoid development is the lack of a functional vascular structure, thus limiting the size of myooids. This is a challenge which is nowadays applicable to almost all organoid systems. Several approaches to obtain a vascular structure within myooids have been proposed. The purpose of this review is to give a concise overview of these approaches.

Recent advances in 3D printing: vascular network for tissue and organ regeneration

Over the past years, the fabrication of adequate vascular networks has remained the main challenge in engineering tissues due to technical difficulties, while the ultimate objective of tissue engineering is to create fully functional and sustainable organs and tissues to transplant in the human body. There have been a number of studies performed to overcome this limitation, and as a result, 3D printing has become an emerging technique to serve in a variety of applications in constructing vascular networks within tissues and organs. 3D printing incorporated technical approaches allow researchers to fabricate complex and systematic architecture of vascular networks and offer various selections for fabrication materials and printing techniques. In this review, we will discuss materials and strategies for 3D printed vascular networks as well as specific applications for certain vascularized tissue and organ regeneration. We will also address the current limitations of vascular tissue engineering and make suggestions for future directions research may take.

Tissue Engineering in Pediatric Bladder Reconstruction-The Road to Success

Several congenital disorders can cause end stage bladder disease and possibly renal damage in children. The current gold standard therapy is enterocystoplasty, a bladder augmentation using an intestinal segment. However, the use of bowel tissue is associated with numerous complications such as metabolic disturbance, stone formation, urine leakage, chronic infections, and malignancy. Urinary diversions using engineered bladder tissue would obviate the need for bowel for bladder reconstruction. Despite impressive progress in the field of bladder tissue engineering over the past decades, the successful transfer of the approach into clinical routine still represents a major challenge. In this review, we discuss major achievements and challenges in bladder tissue regeneration with a focus on different strategies to overcome the obstacles and to meet the need for living functional tissue replacements with a good growth potential and a long life span matching the pediatric population.

Accelerated Angiogenic Induction and Vascular Integration in a Novel Synthetic Scaffolding Matrix for Tissue Replacement

Purpose

Reduced or delayed neovascularization is a major obstacle with regard to tissue-engineered constructs. The aim of this study was to evaluate the early microvascular response to a novel degradable ε-caprolactone terpolymer matrix.

Methods

ε-caprolactone terpolymer matrices (Suprathel Plus®; Institute of Textile and Process Engineering, Denkendorf, Germany) were implanted into dorsal skinfold chambers of balb/c mice (n=10). Microcirculatory changes were observed by intravital fluorescence microscopy. Scaffolding matrices from PEGT/PBT copolymer were used as controls (n=10).

Results

The formation of de novo vascular networks within both scaffolding matrices was noted throughout the experiment. A vascular ingrowth of perfused microvessels into the matrices up to 600 μm apart from the edge was noted within 10 days of implantation. The earliest signs of neoangiogenesis were visible in ε-caprolactone terpolymer matrices on day 1. In both scaffolds the new developed vessels extended centripetally from the border of the matrices towards the center and anastomosed to form a perfused microvascular network. There was significantly earlier onset of vascularization, increased vascularized area and higher vessel density in ε-caprolactone terpolymer matrices compared to PEGT/PBT copolymer matrices were observed.

Conclusions

The scaffolding matrix from ε-caprolactone terpolymer allowed for an earlier and more intense induction of angiogenesis and displayed the tendency to vascularize more rapidly within a shorter period of time after transplantation compared to PEGT/PBT copolymer scaffolds, thus indicating its potential application for tissue engineering purposes.

3D Printed Structures Filled with Carbon Fibers and Functionalized with Mesenchymal Stem Cell Conditioned Media as In Vitro Cell Niches for Promoting Chondrogenesis

In this study, we present a novel approach towards the straightforward, rapid, and low-cost development of biomimetic composite scaffolds for tissue engineering strategies. The system is based on the additive manufacture of a computer-designed lattice structure or framework, into which carbon fibers are subsequently knitted or incorporated. The 3D-printed lattice structure acts as support and the knitted carbon fibers perform as driving elements for promoting cell colonization of the three-dimensional construct. A human mesenchymal stem cell (h-MSC) conditioned medium (CM) is also used for improving the scaffold's response and promoting cell adhesion, proliferation, and viability. Cell culture results-in which scaffolds become buried in collagen type II-provide relevant information regarding the viability of the composite scaffolds used and the prospective applications of the proposed approach. In fact, the advanced composite scaffold developed, together with the conditioned medium functionalization, constitutes a biomimetic stem cell niche with clear potential, not just for tendon and ligament repair, but also for cartilage and endochondral bone formation and regeneration strategies.

Polymers for 3D Printing and Customized Additive Manufacturing

Additive manufacturing (AM) alias 3D printing translates computer-aided design (CAD) virtual 3D models into physical objects. By digital slicing of CAD, 3D scan, or tomography data, AM builds objects layer by layer without the need for molds or machining. AM enables decentralized fabrication of customized objects on demand by exploiting digital information storage and retrieval via the Internet. The ongoing transition from rapid prototyping to rapid manufacturing prompts new challenges for mechanical engineers and materials scientists alike. Because polymers are by far the most utilized class of materials for AM, this Review focuses on polymer processing and the development of polymers and advanced polymer systems specifically for AM. AM techniques covered include vat photopolymerization (stereolithography), powder bed fusion (SLS), material and binder jetting (inkjet and aerosol 3D printing), sheet lamination (LOM), extrusion (FDM, 3D dispensing, 3D fiber deposition, and 3D plotting), and 3D bioprinting. The range of polymers used in AM encompasses thermoplastics, thermosets, elastomers, hydrogels, functional polymers, polymer blends, composites, and biological systems. Aspects of polymer design, additives, and processing parameters as they relate to enhancing build speed and improving accuracy, functionality, surface finish, stability, mechanical properties, and porosity are addressed. Selected applications demonstrate how polymer-based AM is being exploited in lightweight engineering, architecture, food processing, optics, energy technology, dentistry, drug delivery, and personalized medicine. Unparalleled by metals and ceramics, polymer-based AM plays a key role in the emerging AM of advanced multifunctional and multimaterial systems including living biological systems as well as life-like synthetic systems.

Production of tissue-engineered intestine from expanded enteroids

BACKGROUND

Short bowel syndrome is a life-threatening condition with few solutions. Tissue-engineered intestine (TEI) is a potential treatment, but donor intestine is a limiting factor. Expanded epithelial surrogates termed enteroids may serve as a potential donor source.

MATERIALS AND METHODS

To produce TEI from enteroids, crypts were harvested from mice and enteroid cultures established. Enteroids were seeded onto polymer scaffolds using Matrigel or culture medium and implanted in immunosuppressed mice for 4 wk. Histology was analyzed using Periodic acid-Schiff staining and immunofluorescence. Neomucosa was quantified using ImageJ software. To determine whether TEI could be produced from enteroids established from small intestinal biopsies, 2 × 2-mm pieces of jejunum were processed for enteroid culture, enteroids were expanded and seeded onto scaffolds, and scaffolds implanted for 4 wk.

RESULTS

Enteroids in Matrigel produced TEI in 15 of 15 scaffolds, whereas enteroids in medium produced TEI in 9 of 15 scaffolds. Use of Matrigel led to more neomucosal surface area compared to media (10,520 ± 2905 μm versus 450 ± 127 μm, P < 0.05). Histologic examination confirmed the presence of crypts and blunted villi, normal intestinal epithelial lineages, intestinal subepithelial myofibroblasts, and smooth muscle cells. Crypts obtained from biopsies produced an average of 192 ± 71 enteroids. A single passage produced 685 ± 58 enteroids, which was adequate for scaffold seeding. TEI was produced in 8 of 9 scaffolds seeded with expanded enteroids.

CONCLUSIONS

Enteroids can be obtained from minimal starting material, expanded ex vivo, and implanted to produce TEI. This method shows promise as a solution to the limited donor intestine available for TEI production in patients with short bowel syndrome.

In Vivo Assessment of Bone Regeneration in Alginate/Bone ECM Hydrogels with Incorporated Skeletal Stem Cells and Single Growth Factors

The current study has investigated the use of decellularised, demineralised bone extracellular matrix (ECM) hydrogel constructs for in vivo tissue mineralisation and bone formation. Stro-1-enriched human bone marrow stromal cells were incorporated together with select growth factors including VEGF, TGF-β3, BMP-2, PTHrP and VitD3, to augment bone formation, and mixed with alginate for structural support. Growth factors were delivered through fast (non-osteogenic factors) and slow (osteogenic factors) release PLGA microparticles. Constructs of 5 mm length were implanted in vivo for 28 days within mice. Dense tissue assessed by micro-CT correlated with histologically assessed mineralised bone formation in all constructs. Exogenous growth factor addition did not enhance bone formation further compared to alginate/bone ECM (ALG/ECM) hydrogels alone. UV irradiation reduced bone formation through degradation of intrinsic growth factors within the bone ECM component and possibly also ECM cross-linking. BMP-2 and VitD3 rescued osteogenic induction. ALG/ECM hydrogels appeared highly osteoinductive and delivery of angiogenic or chondrogenic growth factors led to altered bone formation. All constructs demonstrated extensive host tissue invasion and vascularisation aiding integration and implant longevity. The proposed hydrogel system functioned without the need for growth factor incorporation or an exogenous inducible cell source. Optimal growth factor concentrations and spatiotemporal release profiles require further assessment, as the bone ECM component may suffer batch variability between donor materials. In summary, ALG/ECM hydrogels provide a versatile biomaterial scaffold for utilisation within regenerative medicine which may be tailored, ultimately, to form the tissue of choice through incorporation of select growth factors.

Prevascularization in tissue engineering: Current concepts and future directions

The survival of engineered tissue constructs during the initial phase after their implantation depends on the rapid development of an adequate vascularization. This, in turn, is a major prerequisite for the constructs' long-term function. 'Prevascularization' has emerged as a promising concept in tissue engineering, aiming at the generation of a preformed microvasculature in tissue constructs prior to their implantation. This should shorten the time period during which the constructs are avascular and suffer hypoxic conditions. Herein, we provide an overview of current strategies for the generation of preformed microvascular networks within tissue constructs. In vitro approaches use cell seeding, spheroid formation or cell sheet technologies. In situ approaches use the body as a natural bioreactor to induce vascularization by angiogenic ingrowth or flap and arteriovenous (AV)-loop techniques. In future, these strategies may be supplemented by the transplantation of adipose tissue-derived microvascular fragments or the in vitro generation of highly organized microvascular networks by means of sophisticated microscale technologies and microfluidic systems. The further advancement of these prevascularization concepts and their adaptation to individual therapeutic interventions will markedly contribute to a broad implementation of tissue engineering applications into clinical practice.

Putative CD133+ melanoma cancer stem cells induce initial angiogenesis in vivo

Tumor angiogenesis is essential for tumor growth and metastasis, and is regulated by a complex network of various types of cells, chemokines, and stimulating factors. In contrast to sprouting angiogenesis, tumor angiogenesis is also influenced by hypoxia, inflammation, and the attraction of bone-marrow-derived cells. Recently, cancer stem cells have been reported to mimic vascularization by differentiating into endothelial cells and inducing vessel formation. In this study, the influence of cancer stem cells on initial angiogenesis was evaluated for the metastatic melanoma cell line D10. Following flow cytometry, CD133+ and CD133- cells were isolated using magnetic cell separation and different cell fractions were transferred to porcine gelatin sponges, which were implanted into the dorsal skinfold chamber of immunocompromised mice. Angiogenesis was analyzed based on microvessel density over a 10-day period using in vivo fluorescence microscopy, and the results were verified using immunohistology. CD133+ D10 cells showed a significant induction of early angiogenesis in vivo, contrary to CD133- D10 cells, unsorted D10 cells, and negative control. Neovascularization was confirmed by visualizing endothelial cells by immunohistology using an anti-CD31 antibody. Because CD133+ cells are rare in clinical specimens and hardly amenable to functional assays, the D10 cell line provides a suitable model to study the angiogenic potential of putative cancer stem cells and the leukocyte-endothelial cell interaction in the dorsal skinfold chamber in vivo. This cancer stem cell model might be useful in the development and evaluation of therapeutic agents targeting tumors.

Composite scaffolds for osteochondral repair obtained by combination of additive manufacturing, leaching processes and hMSC-CM functionalization

Articular repair is a relevant and challenging area for the emerging fields of tissue engineering and biofabrication. The need of significant gradients of properties, for the promotion of osteochondral repair, has led to the development of several families of composite biomaterials and scaffolds, using different effective approaches, although a perfect solution has not yet been found. In this study we present the design, modeling, rapid manufacturing and in vitro testing of a composite scaffold aimed at osteochondral repair. The presented composite scaffold stands out for having a functional gradient of density and stiffness in the bony phase, obtained in titanium by means of computer-aided design combined with additive manufacture using selective laser sintering. The chondral phase is obtained by sugar leaching, using a PDMS matrix and sugar as porogen, and is joined to the bony phase during the polymerization of PDMS, therefore avoiding the use of supporting adhesives or additional intermediate layers. The mechanical performance of the construct is biomimetic and the stiffness values of the bony and chondral phases can be tuned to the desired applications, by means of controlled modifications of different parameters. A human mesenchymal stem cell (h-MSC) conditioned medium (CM) is used for improving scaffold response. Cell culture results provide relevant information regarding the viability of the composite scaffolds used.

Comparison of acellular nerve allograft modification with Schwann cells or VEGF

BACKGROUND

Individual contributions of exogenous Schwann cells (SCs) and vascular endothelial growth factor (VEGF) were evaluated in acellular nerve allografts (ANAs). ANA processing removes SCs and vasculature, likely contributing to reduced regeneration compared to autografts. Exogenous SCs may improve the regenerative microenvironment, and VEGF has been shown to stimulate angiogenesis. Replacing these components in ANAs may improve regeneration.

METHODS

A rat sciatic nerve transection model was used to study 20-mm grafts. Four graft types were studied: (1) isograft, (2) ANA, (3) ANA-SCs, and (4) ANA-VEGF. After 10 weeks in vivo, the midgraft and distal nerve to the grafts were analyzed for axonal regeneration using histomorphometry to assess total myelinated axon counts, density, width, and percent neural tissue.

RESULTS

The most axons in the distal nerve were regenerated in the isograft followed by the ANA- SC group, with 9171 ± 1822 and 7103 ± 1576 regenerated axons respectively. Both the ANA and ANA-VEGF groups had significantly fewer regenerated axons compared to the isograft (p < 0.05) with 5225 ± 2994 and 5709 ± 2657 regenerated axons, respectively. The ANA and ANA-VEGF groups also had significantly reduced fiber density and percent nerve compared to the isograft; the isograft and ANA-SC groups were not significantly different (p < 0.05).

CONCLUSIONS

These results show that SCs improve axonal regeneration in a 20 mm ANA to a greater extent than VEGF. VEGF treatment showed a trend toward increased axonal regeneration but was not significantly different compared to the untreated ANA. The role of VEGF may be clearer in longer grafts where ischemia is a greater factor.

Induction of Angiogenesis by Matrigel Coating of VEGF-Loaded PEG/PCL-Based Hydrogel Scaffolds for hBMSC Transplantation

hBMSCs are multipotent cells that are useful for tissue regeneration to treat degenerative diseases and others for their differentiation ability into chondrocytes, osteoblasts, adipocytes, hepatocytes and neuronal cells. In this study, biodegradable elastic hydrogels consisting of hydrophilic poly(ethylene glycol) (PEG) and hydrophobic poly(ε-caprolactone) (PCL) scaffolds were evaluated for tissue engineering because of its biocompatibility and the ability to control the release of bioactive peptides. The primary cultured cells from human bone marrow are confirmed as hBMSC by immunohistochemical analysis. Mesenchymal stem cell markers (collagen type I, fibronectin, CD54, integrin1β, and Hu protein) were shown to be positive, while hematopoietic stem cell markers (CD14 and CD45) were shown to be negative. Three different hydrogel scaffolds with different block compositions (PEG:PCL=6:14 and 14:6 by weight) were fabricated using the salt leaching method. The hBMSCs were expanded, seeded on the scaffolds, and cultured up to 8 days under static conditions in Iscove's Modified Dulbecco's Media (IMDM). The growth of MSCs cultured on the hydrogel with PEG/PCL= 6/14 was faster than that of the others. In addition, the morphology of MSCs seemed to be normal and no cytotoxicity was found. The coating of the vascular endothelial growth factor (VEGF) containing scaffold with Matrigel slowed down the release of VEGF in vitro and promoted the angiogenesis when transplanted into BALB/c nude mice. These results suggest that hBMSCs can be supported by a biode gradable hydrogel scaffold for effective cell growth, and enhance the angiogenesis by Matrigel coating.

Locally applied macrophage-activating lipopeptide-2 (MALP-2) promotes early vascularization of implanted porous polyethylene (Medpor®)

Porous polyethylene (Medpor®) is frequently used in craniofacial reconstructive surgery. Rapid vascularization of the biomaterial crucially contributes to its adequate incorporation without complications. Macrophage-activating lipopeptide-2 (MALP-2) is a toll-like receptor (TLR)-2/6 agonist with pro-angiogenic properties. Herein we analyzed whether local single-shot application of MALP-2 improves the angiogenic host tissue response to Medpor®. Medpor® (3 mm×3 mm×0.25 mm) was implanted into dorsal skinfold chambers of BALB/c mice topically exposed to different MALP-2 doses (0.1 and 0.5 μg) or vehicle (control). The vascularization of the implants and the inflammatory foreign body reaction was analyzed using intravital fluorescence microscopy, histology and immunohistochemistry over 14 days. MALP-2 treatment dose-dependently improved the vascularization of Medpor®, as indicated by a significantly higher functional microvessel density at the border and center of the implants when compared to controls. This was associated with a temporary increase of adherent leukocytes in host tissue venules during the first 3 days after implantation. At day 14, implants in MALP-2-treated chambers were surrounded by granulation tissue, which exhibited a significantly higher density of CD31-positive microvessels and number of F4/80-positive macrophages when compared to controls. Additional biomaterial-free chambers did not show any signs of angiogenesis when treated with MALP-2. This indicates that locally applied MALP-2 effectively stimulates the early vascularization of Medpor® without inducing any local or systemic side effects. Accordingly, this easy approach may further improve the rapid incorporation of this biomaterial at the implantation site.

Accelerating the early angiogenesis of tissue engineering constructs in vivo by the use of stem cells cultured in matrigel

In tissue engineering research, generating constructs with an adequate extent of clinical applications remains a major challenge. In this context, rapid blood vessel ingrowth in the transplanted tissue engineering constructs is the key factor for successful incorporation. To accelerate the microvascular development in engineered tissues, we preincubated osteoblast-like cells as well as mesenchymal stem cells or a combination of both cell types in Matrigel-filled PLGA scaffolds before transplantation into the dorsal skinfold chambers of balb/c mice. By the use of preincubated mesenchymal stem cells, a significantly accelerated angiogenesis was achieved. Compared with previous studies that showed a decisive increase of vascularization on day 6 after the implantation, we were able to halve this period and achieve explicitly denser microvascular networks 3 days after transplantation of the tissue engineering constructs. Thereby, the inflammatory host tissue response was acceptable and low, comparable with former investigations. A co-incubation of osteoblast-like cells and stem cells showed no additive effect on the density of the newly formed microvascular network. Preincubation of mesenchymal stem cells in Matrigel is a promising approach to develop rapid microvascular growth into tissue engineering constructs. After the implantation into the host organism, scaffolds comprising stem cells generate microvascular capillary-like structures exceptionally fast. Thereby, transplanted stem cells likely differentiate into vessel-associated cells. For this reason, preincubation of mesenchymal stem cells in nutrient solutions supporting different steps of angiogenesis provides a technique to promote the routine use of tissue engineering in the clinic.

Effects of silk fibroin fiber incorporation on mechanical properties, endothelial cell colonization and vascularization of PDLLA scaffolds

Attainment of functional vascularization of engineered constructs is one of the fundamental challenges of tissue engineering. However, the development of an extracellular matrix in most tissues, including bone, is dependent upon the establishment of a well developed vascular supply. In this study a poly(d,l-lactic acid) (PDLLA) salt-leached sponge was modified by incorporation of silk fibroin fibers to create a multicomponent scaffold, in an effort to better support endothelial cell colonization and to promote in vivo vascularization. Scaffolds with and without silk fibroin fibers were compared for microstructure, mechanical properties, ability to maintain cell populations in vitro as well as to permit vascular ingrowth into acellular constructs in vivo. We demonstrated that adding silk fibroin fibers to a PDLLA salt-leached sponge enhanced scaffold properties and heightened its capacity to support endothelial cells in vitro and to promote vascularization in vivo. Therefore refinement of scaffold properties by inclusion of materials with beneficial attributes may promote and shape cellular responses.

Three-dimensional printing of soy protein scaffolds for tissue regeneration

Fabricating three-dimensional (3D) porous scaffolds with controlled structure and geometry is crucial for tissue regeneration. To date, exploration in printing 3D natural protein scaffolds is limited. In this study, soy protein slurry was successfully printed using the 3D Bioplotter to form scaffolds. A method to verify the structural integrity of resulting scaffolds during printing was developed. This process involved measuring the mass extrusion flow rate of the slurry from the instrument, which was directly affected by the extrusion pressure and the soy protein slurry properties. The optimal mass flow rate for printing soy slurry at 27°C was 0.0072±0.0002 g/s. The addition of dithiothreitol to soy slurries demonstrated the importance of disulfide bonds in forming solid structures upon printing. Resulting Bioplotted soy protein scaffolds were cured using 95% ethanol and post-treated using dehydrothermal treatment (DHT), a combination of freeze-drying and DHT, and chemical crosslinking using 1-ethyl-3-(3 dimethylaminopropyl)carbodiimide (EDC) chemistry. Surface morphologies of the different treatment groups were characterized using scanning electron microscopy. Scaffold properties, including relative crosslink density, mass loss upon rinsing, and compressive modulus revealed that EDC crosslinked scaffolds were the most robust with moduli of approximately 4 kPa. Scaffold geometry (45° and 90° layer rotations) affected the mechanical properties for DHT and EDC crosslinked scaffolds. Seeding efficiency of human mesenchymal stem cells (hMSC) was highest for nontreated and thermally treated scaffolds, and all scaffolds supported hMSC viability over time.

Vitalization of porous polyethylene (Medpor®) with chondrocytes promotes early implant vascularization and incorporation into the host tissue

Porous polyethylene (Medpor(®)) is frequently used in craniofacial reconstructive surgery. The successful incorporation of this alloplastic biomaterial depends on adequate vascularization. Here, we analyzed whether the early vascularization of porous polyethylene can be accelerated by vitalization with human chondrocytes. For this purpose, small polyethylene samples were coated with platelet-rich plasma (PRP) or a suspension of PRP and human chondrocytes. Uncoated polyethylene samples served as controls. Subsequently, the samples were implanted into the dorsal skinfold chamber of CD-1 nude mice to repetitively analyze their vascularization and biocompatibility by means of intravital fluorescence microscopy. PRP-chondrocyte-coated polyethylene exhibited an accelerated and improved vascularization when compared with the other two groups. This was indicated by a significantly higher functional capillary density of the microvascular network developing around the implants. Moreover, a leukocyte-endothelial cell interaction was found in a physiological range at the implantation site of all three groups, demonstrating that the vitalization with PRP and chondrocytes did not affect the good biocompatibility of the alloplastic material. Additional histological, immunohistochemical, and in situ hybridization analyses revealed that the chondrocytes formed a bioprotective tissue layer, which prevented the accumulation of macrophages and foreign body giant cells on the polyethylene surface. These findings clearly indicate that vitalization of polyethylene with chondrocytes promotes early implant vascularization and incorporation into the host tissue and, thus, may be a promising approach that prevents postoperative complications such as implant extrusion, migration, and infection.

Review paper: Critical Issues in Tissue Engineering: Biomaterials, Cell Sources, Angiogenesis, and Drug Delivery Systems

Tissue engineering is a newly emerging biomedical technology, which aids and increases the repair and regeneration of deficient and injured tissues. It employs the principles from the fields of materials science, cell biology, transplantation, and engineering in an effort to treat or replace damaged tissues. Tissue engineering and development of complex tissues or organs, such as heart, muscle, kidney, liver, and lung, are still a distant milestone in twenty-first century. Generally, there are four main challenges in tissue engineering which need optimization. These include biomaterials, cell sources, vascularization of engineered tissues, and design of drug delivery systems. Biomaterials and cell sources should be specific for the engineering of each tissue or organ. On the other hand, angiogenesis is required not only for the treatment of a variety of ischemic conditions, but it is also a critical component of virtually all tissue-engineering strategies. Therefore, controlling the dose, location, and duration of releasing angiogenic factors via polymeric delivery systems, in order to ultimately better mimic the stem cell niche through scaffolds, will dictate the utility of a variety of biomaterials in tissue regeneration. This review focuses on the use of polymeric vehicles that are made of synthetic and/or natural biomaterials as scaffolds for three-dimensional cell cultures and for locally delivering the inductive growth factors in various formats to provide a method of controlled, localized delivery for the desired time frame and for vascularized tissue-engineering therapies.

PLGA/hydrogel biopapers as a stackable substrate for printing HUVEC networks via BioLP

Two major challenges in tissue engineering are mimicking the native cell-cell arrangements of tissues and maintaining viability of three-dimension (3D) tissues thicker than 300 µm. Cell printing and prevascularization of engineered tissues are promising approaches to meet these challenges. However, the printing technologies used in biofabrication must balance the competing parameters of resolution, speed, and volume, which limit the resolution of thicker 3D structures. We suggest that high-resolution conformal printing techniques can be used to print 2D patterns of vascular cells onto biopaper substrates which can then be stacked to form a thicker tissue construct. Towards this end we created 1 cm × 1 cm × 300 µm biopapers to be used as the transferable, stackable substrate for cell printing. 3.6% w/v poly-lactide-co-glycolide was dissolved in chloroform and poured into molds filled with NaCl crystals. The salt was removed with DI water and the scaffolds were dried and loaded with a Collagen Type I or Matrigel. SEM of the biopapers showed extensive porosity and gel loading throughout. Biological laser printing (BioLP) was used to deposit human umbilical vein endothelial cells (HUVEC) in a simple intersecting pattern to the surface of the biopapers. The cells differentiated and stretched to form networks preserving the printed pattern. In a separate experiment to demonstrate "stackability," individual biopapers were randomly seeded with HUVECs and cultured for 1 day. The mechanically stable and viable biopapers were then stacked and cultured for 4 days. Three-dimensional confocal microscopy showed cell infiltration and survival in the compound multilayer constructs. These results demonstrate the feasibility of stackable "biopapers" as a scaffold to build 3D vascularized tissues with a 2D cell-printing technique.

Endothelialized biomaterials for tissue engineering applications in vivo

Rebuilding tissues involves the creation of a vasculature to supply nutrients and this in turn means that the endothelial cells (ECs) of the resulting endothelium must be a quiescent non-thrombogenic blood contacting surface. Such ECs are deployed on biomaterials that are composed of natural materials such as extracellular matrix proteins or synthetic polymers in the form of vascular grafts or tissue-engineered constructs. Because EC function is influenced by their origin, biomaterial surface chemistry and hemodynamics, these issues must be considered to optimize implant performance. In this review, we examine the recent in vivo use of endothelialized biomaterials and discuss the fundamental issues that must be considered when engineering functional vasculature.

A novel composite construct increases the vascularization potential of PEG hydrogels through the incorporation of large fibrin ribbons

Developing a mechanism to vascularize tissue-engineered constructs is imperative for transplant function and integration, particularly when delivering hypoxia-sensitive tissues, such as pancreatic islets. Previous efforts have focused on bulk modifications of scaffold materials rendering the entire construct permissive to vessel penetration or the formation of a porous structure where vessels can infiltrate the empty spaces. Here, we describe a novel construct composed of large fibrin ribbons encapsulated within a poly(ethylene glycol) (PEG) hydrogel. The PEG/fibrin ribbon composite scaffold facilitates coculture of adhesive and nonadhesive cell types, thus providing closely neighboring environments with distinct material properties specific to the needs of two clinically relevant cell populations. This advantage is demonstrated here by the successful coculture of pancreatic islets in the PEG component and vessel-forming endothelial cells in entrapped fibrin ribbons. Transplanted endothelial cells can form anastomosies with host vasculature, suggesting that our cocultures may lead to more rapid scaffold vascularization. Additionally, we show that surface-seeded endothelial cells form multicellular projections that migrate into nonadhesive PEG hydrogels along permissive fibrin ribbons, further demonstrating composite construct vascularization potential. Distribution of large fibrin ribbons throughout PEG hydrogels provide a potential mechanism for vascularization of a well-established biomaterial without inherently changing its desirable properties.

Biocompatibility of porous polyethylene implants tissue-engineered by extracellular matrix and VEGF

Rapid ingrowth of blood vessels and low inflammatory response are considered major prerequisites for successful implantation of biomaterials in reconstructive surgery. Aim of the present study was to evaluate whether tissue-engineered porous polyethylene (PPE) implants providing extracellular matrix components (ECM) and vascular endothelial growth factor (VEGF) in vivo improve microvascular ingrowth and mechanical integration with regard to initial inflammatory responses. PPE implants (3 x 3 x 0.1 mm(3), pore size approximately 100-200 microm) were tissue-engineered by incorporation of ECM components (GFR-Matrigel) adding recombinant murine VEGF (1 microg/mL) and grafted into dorsal skinfold chamber preparations of C57BL/6 mice. Control animals received uncoated implants or implants coated with ECM components alone (n = 6 per group). Using in vivo fluorescence microscopy angiogenic activity and inflammatory leukocyte-endothelial cell interactions were analyzed for 2weeks. Finally, mechanical integration was quantified by measurement of dynamic desintegration strengths at the host-implant border. Functional vessel density, red blood cell velocity, and vessel diameters increased continuously in all groups indicating that rapid microvascular integration of PPE occurred even without incorporation of ECM or VEGF. However, a transient initial inflammatory response with increased leukocyte-endothelial cell adherence on day 7 in uncoated control implants was efficiently reduced by incorporation of ECM and VEGF. Measurement of dynamic breaking strengths revealed no significant differences between the groups although there was a tendency to improved mechanical integration in tissue-engineered implants. Therefore, novel tissue- engineered constructs of PPE implants providing ECM and VEGF in high local concentrations can increase biocompatibility especially under unfavorable conditions for implantation.

Nanofibrous architecture of silk fibroin scaffolds prepared with a mild self-assembly process

Besides excellent biocompatibility and biodegradability, a useful tissue engineering scaffold should provide suitable macropores and nanofibrous structure, similar to extracellular matrix (ECM), to induce desired cellular activities and to guide tissue regeneration. In the present study, a mild process to prepare porous and nanofibrous silk-based scaffolds from aqueous solution is described. Using collagen to control the self-assembly of silk, nanofibrous silk scaffolds were firstly achieved through lyophilization. Water annealing was used to generate insolubility in the silk-based scaffolds, thereby avoiding the use of organic solvents. The nano-fibrils formed in the silk-collagen scaffolds had diameters of 20-100 nm, similar with native collagen in ECM. The silk-collagen scaffolds dissolved slowly in PBS solution, with about a 28% mass lost after 4 weeks. Following the dissolution or degradation, the nanofibrous structure inside the macropore walls emerged and interacted with cells directly. During in vitro cell culture, the nanofibrous silk-collagen scaffolds containing 7.4% collagen demonstrated significantly improved cell compatibility when compared with salt-leached silk scaffolds and silk-collagen scaffolds containing 20% collagen that emerged less nano-fibrils. Therefore, this new process provides useful scaffolds for tissue engineering applications. Furthermore, the process involves all-aqueous, room temperature and pressure processing without the use of toxic chemicals or solvents, offering new green chemistry approaches, as well as options to load bioactive drugs or growth factors into process.

Vascularization--the conduit to viable engineered tissues

Long-term viability of thick three-dimensional engineered tissue constructs is a major challenge. Addressing it requires development of vessel-like network that will allow the survival of the construct in vitro and its integration in vivo owing to improved vascularization after implantation. Resulting from work of various research groups, several approaches were developed aiming engineered tissue vascularization: (1) embodiment of angiogenesis growth factors in the polymeric scaffolds for prolonged release, (2) coculture of endothelial cells with target tissue cells and angiogenesis signaling cells, (3) use of microfabrication methods for creating designed channels for allowing nutrients to flow and/or for directing endothelial cells attachment, and (4) decellularization of organs and blood vessels for creating extracellular matrix. A synergistic effect is expected by combining several of these approaches as already demonstrated in some of the latest studies. Current paper reviews the progress in each approach and recent achievements toward vascularization of engineered tissues.

In vitro and in vivo evaluation of a novel nanosize hydroxyapatite particles/poly(ester-urethane) composite scaffold for bone tissue engineering

Scaffolds for bone tissue engineering should provide an osteoconductive surface to promote the ingrowth of new bone after implantation into bone defects. This may be achieved by hydroxyapatite loading of distinct scaffold biomaterials. Herein, we analyzed the in vitro and in vivo properties of a novel nanosize hydroxyapatite particles/poly(ester-urethane) (nHA/PU) composite scaffold which was prepared by a salt leaching-phase inverse process. Microtomography, scanning electron microscopy and X-ray spectroscopy analyses demonstrated the capability of the material processing to create a three-dimensional porous PU scaffold with nHA on the surface. Compared to nHA-free PU scaffolds (control), this modified scaffold type induced a significant increase in in vitro adsorption of model proteins. In vivo analysis of the inflammatory and angiogenic host tissue response to implanted nHA/PU scaffolds in the dorsal skinfold chamber model indicated that the incorporation of nHA particles into the scaffold material did not affect biocompatibility and vascularization when compared to control scaffolds. Thus, nHA/PU composite scaffolds represent a promising new type of scaffold for bone tissue engineering, combining the flexible material properties of PU with the advantage of an osteoconductive surface.

Inosculation: connecting the life-sustaining pipelines

Recent progress in engineering microvascular networks in vitro and in vivo offers exciting opportunities to create tissue constructs with preformed blood vessels, which are rapidly blood perfused by developing interconnections to the preexisting blood vessels of the host tissue after implantation. This process, termed as inosculation, is well known from the revascularization of various tissue grafts, such as transplanted skin, nerves, or bone. It is characterized by the close interaction of the implant's preformed microvascular network and the host microvasculature. The sprouting angiogenic activity of both counterparts determines whether inosculation takes place internally within the implant or externally within the surrounding host tissue. Successful inosculation involves vascular remodeling as well as infiltration of inflammatory cells and stem cells. With the use of sophisticated in vitro and in vivo models, more detailed analysis of regulatory mechanisms of inosculation will help to develop novel strategies, aiming at further accelerating the establishment of a life-sustaining blood supply to implanted tissue constructs.