Heparin-Chitosan-Coated Acellular Bone Matrix Enhances Perfusion of Blood and Vascularization in Bone Tissue Engineering Scaffolds

In vitro degradation, swelling, and bioactivity performances of in situ forming injectable chitosan-matrixed hydrogels for bone regeneration and drug delivery

Injectable, tissue mimetic, bioactive, and biodegradable hydrogels offer less invasive regeneration and repair of tissues. The monitoring swelling and in vitro degradation capacities of hydrogels are highly important for drug delivery and tissue regeneration processes. Bioactivity of bone tissue engineered constructs in terms of mineralized apatite formation capacity is also pivotal. We have previously reported in situ forming chitosan-based injectable hydrogels integrated with hydroxyapatite and heparin for bone regeneration, promoting angiogenesis. These hydrogels were functionalized by glycerol and pH to improve their mechano-structural properties. In the present study, functionalized hybrid hydrogels were investigated for their swelling, in vitro degradation, and bioactivity performances. Hydrogels have degraded gradually in phosphate-buffered saline (PBS) with and without lysozyme enzyme. The percentage weight loss of hydrogels and their morphological and chemical properties, and pH of media were analyzed. The swelling ratio of hydrogels (55%-68%(wt), 6 h of equilibrium) indicated a high degree of cross-linking, can be suitable for controlled drug release. Hydrogels have gradually degraded reaching to 60%-70% (wt%) in 42 days in the presence and absence of lysozyme, respectively. Simulated body fluid (SBF)-treated hydrogels containing hydroxyapatite-induced needle-like carbonated-apatite mineralization was further enhanced by heparin content significantly.

Bone healing study of alendronate combined with enoxaparin sodium bone cement in rabbits with bone defects

BACKGROUND

To observe the effect of enoxaparin sodium-polymethyl methacrylate (ES-PMMA) bone cement supplemented with alendronate (AN) on bone repair of bone defects in New Zealand rabbits.

METHODS

Twenty-seven New Zealand rabbits were randomly divided into ES/AN, ES-PMMA and PMMA groups, with a total of 27 New Zealand rabbits. The drugs loaded in 40 g bone cement powder were as follows: ES/AN group 8000 AxaIU enoxaparin (ES) and 200 mg alendronate (AN), ES-PMMA group 8000 AxaIU enoxaparin (ES), PMMA group without drugs. A bone defect model with a length of 10 mm and a diameter of 5 mm was made from the left tibia of rabbits, and the prepared bone cement was placed in the tibia defect. At 4 weeks, 8 weeks and 12 weeks after the operation, 3 rabbits in each group were sacrificed, and left tibia samples were collected for histological scoring, HE staining and Masson staining. Bone mineral density and new bone volume were measured by imaging, and the related data were processed by one-way ANOVA and least significance difference (LSD) post hoc test.

RESULTS

(1) Bone mineral density (BMD, mg/mm3) around the bone defect: at the 4th week, BMD in the ES/AN group was higher than that in the PMMA group; at the 8th week, the BMD in the ES/AN group was significantly higher than that in the other two groups; and at the 12th week, the BMD in the ES/AN group was significantly higher than that in the other two groups. (2) New bone volume (BV, mm3): at the 4th week, BV in the ES/AN group was significantly higher than that in the other two groups, BV in the ES/AN group was significantly higher than that in the other two groups at the 8th and 12th weeks, and BV in the ES-PMMA group was higher than that in the PMMA group. (3) Histological score: at the 4th and 8th weeks, the histological score of the ES/AN group was higher than that of the PMMA group, and at the 12th week, the histological score of the ES/AN group was higher than that of the other two groups. (4) Cortical bone thickness (μm): at the 4th, 8th and 12th weeks, the cortical bone thickness in the ES/AN group was higher than that in the other two groups, and the cortical bone thickness in the ES-PMMA group was higher than that in the PMMA group. (5) The percentage of mature area of new bone in the ES/AN group was higher than that in the other two groups at the 4th week, and at the 8th and 12th weeks, the percentage of mature area of new bone in the ES/AN group and ES-PMMA group was significantly higher than that in the PMMA group.

CONCLUSION

(1) Enoxaparin sodium bone cement supplemented with alendronate was superior to enoxaparin sodium bone cement and PMMA bone cement in promoting bone repair of tibial bone defects in New Zealand rabbits. (2) Enoxaparin sodium bone cement is superior to PMMA bone cement in promoting bone repair, showing a certain osteogenic potential.

Towards the development of osteochondral allografts with reduced immunogenicity

Nowadays, repair and replacement of hyaline articular cartilage still challenges orthopedic surgery. Using a graft of decellularized articular cartilage as a structural scaffold is considered as a promising therapy. So far, successful cell removal has only been possible for small samples with destruction of the macrostructure or loss of biomechanics. Our aim was to develop a mild, enzyme-free chemical decellularization procedure while preserving the biomechanical properties of cartilage. Porcine osteochondral cylinders (diameter: 12 mm; height: 10 mm) were divided into four groups: Native plugs (NA), decellularized plugs treated with PBS, Triton-X-100 and SDS (DC), and plugs additionally treated with freeze-thaw-cycles of -20 °C, -80 °C or shock freezing in nitrogen (N₂) before decellularization. In a non-decalcified HE stain the decellularization efficiency (cell removal, cell size, depth of decellularization) was calculated. For biomechanics the elastic and compression modulus, transition and failure strain as well as transition and failure stress were evaluated. The -20 °C, -80 °C, and N₂ groups showed a complete decellularization of the superficial and middle zone. In the deep zone cells could not be removed in any experimental group. The biomechanical analysis showed only a reduced elastic modulus in all decellularized samples. No significant differences were found for the other biomechanical parameters.

The Auxiliary Role of Heparin in Bone Regeneration and its Application in Bone Substitute Materials

Bone regeneration in large segmental defects depends on the action of osteoblasts and the ingrowth of new blood vessels. Therefore, it is important to promote the release of osteogenic/angiogenic growth factors. Since the discovery of heparin, its anticoagulant, anti-inflammatory, and anticancer functions have been extensively studied for over a century. Although the application of heparin is widely used in the orthopedic field, its auxiliary effect on bone regeneration is yet to be unveiled. Specifically, approximately one-third of the transforming growth factor (TGF) superfamily is bound to heparin and heparan sulfate, among which TGF-β1, TGF-β2, and bone morphogenetic protein (BMP) are the most common growth factors used. In addition, heparin can also improve the delivery and retention of BMP-2 in vivo promoting the healing of large bone defects at hyper physiological doses. In blood vessel formation, heparin still plays an integral part of fracture healing by cooperating with the platelet-derived growth factor (PDGF). Importantly, since heparin binds to growth factors and release components in nanomaterials, it can significantly facilitate the controlled release and retention of growth factors [such as fibroblast growth factor (FGF), BMP, and PDGF] in vivo. Consequently, the knowledge of scaffolds or delivery systems composed of heparin and different biomaterials (including organic, inorganic, metal, and natural polymers) is vital for material-guided bone regeneration research. This study systematically reviews the structural properties and auxiliary functions of heparin, with an emphasis on bone regeneration and its application in biomaterials under physiological conditions.

Hydroxyapatite-Integrated, Heparin- and Glycerol-Functionalized Chitosan-Based Injectable Hydrogels with Improved Mechanical and Proangiogenic Performance

The investigation of natural bioactive injectable composites to induce angiogenesis during bone regeneration has been a part of recent minimally invasive regenerative medicine strategies. Our previous study involved the development of in situ-forming injectable composite hydrogels (Chitosan/Hydroxyapatite/Heparin) for bone regeneration. These hydrogels offered facile rheology, injectability, and gelation at 37 °C, as well as promising pro-angiogenic abilities. In the current study, these hydrogels were modified using glycerol as an additive and a pre-sterile production strategy to enhance their mechanical strength. These modifications allowed a further pH increment during neutralisation with maintained solution homogeneity. The synergetic effect of the pH increment and further hydrogen bonding due to the added glycerol improved the strength of the hydrogels substantially. SEM analyses showed highly cross-linked hydrogels (from high-pH solutions) with a hierarchical interlocking pore morphology. Hydrogel solutions showed more elastic flow properties and incipient gelation times decreased to just 2 to 3 min at 37 °C. Toluidine blue assay and SEM analyses showed that heparin formed a coating at the top layer of the hydrogels which contributed anionic bioactive surface features. The chick chorioallantoic membrane (CAM) assay confirmed significant enhancement of angiogenesis with chitosan-matrixed hydrogels comprising hydroxyapatite and small quantities of heparin (33 µg/mL) compared to basic chitosan hydrogels.

A Biohybrid Material With Extracellular Matrix Core and Polymeric Coating as a Cell Honing Cardiovascular Tissue Substitute

Objective

To investigate the feasibility of a hybrid material in which decellularized pericardial extracellular matrix is functionalized with polymeric nanofibers, for use as a cardiovascular tissue substitute.

Background

A cardiovascular tissue substitute, which is gradually resorbed and is replaced by host's native tissue, has several advantages. Especially in children and young adults, a resorbable material can be useful in accommodating growth, but also enable rapid endothelialization that is necessary to avoid thrombotic complications. In this study, we report a hybrid material, wherein decellularized pericardial matrix is functionalized with a layer of polymeric nanofibers, to achieve the mechanical strength for implantation in the cardiovascular system, but also have enhanced cell honing capacity.

Methods

Pericardial sacs were decellularized with sodium deoxycholate, and polycaprolactone-chitosan fibers were electrospun onto the matrix. Tissue-polymer interaction was evaluated using spectroscopic methods, and the mechanical properties of the individual components and the hybrid material were quantified. In-vitro blood flow loop studies were conducted to assess hemocompatibility and cell culture methods were used to assess biocompatibility.

Results

Encapsulation of the decellularized matrix with 70 μm thick matrix of polycaprolactone-chitosan nanofibers, was feasible and reproducible. Spectroscopy of the cross-section depicted new amide bond formation and C–O–C stretch at the interface. An average peel strength of 56.13 ± 11.87 mN/mm2 was measured, that is sufficient to withstand a high shear of 15 dynes/cm2 without delamination. Mechanical strength and extensibility ratio of the decellularized matrix alone were 18,000 ± 4,200 KPa and 0.18 ± 0.03% whereas that of the hybrid was higher at 20,000 ± 6,600 KPa and 0.35 ± 0.20%. Anisotropy index and stiffness of the biohybrid were increased as well. Neither thrombus formation, nor platelet adhesion or hemolysis was measured in the in-vitro blood flow loop studies. Cellular adhesion and survival were adequate in the material.

Conclusion

Encapsulating a decellularized matrix with a polymeric nanofiber coating, has favorable attributes for use as a cardiovascular tissue substitute.

Effect of heparinization on promoting angiogenesis of decellularized kidney scaffolds

Native decellularized extracellular matrix provides an adequate platform for tissues and organs and promotes the development of organogenesis and tissue remodeling. However, thrombosis poses a great challenge that hinders the transplantation for a substantial organ in vivo. Therefore, anticoagulation and re-reendothelialization of organ biological scaffolds are the primary concerns to be addressed before orthotopic transplantation. Herein, a heparinized decellularized kidney scaffold (HEP-DKSs) was prepared using end-point attachment technology, followed by binding the vascular endothelial growth factor (VEGF) to greatly improve the hemocompatibility and angiogenesis of DKSs. Based on the anticoagulant, co-culture of human umbilical vein endothelial cells, and subcapsular transplantation of kidney experiments, HEP-VEGF-DKSs are shown to reduce platelet adhesion, which is crucial for subsequent vascularization and slow release of heparin and VEGF, suggesting its ability of improve neovascularization. Taken together, these data indicated an optimal anticoagulation function of HEP-VEGF-DKSs and the potential of vascularization for regeneration of whole decellularized kidney.

Sterilization and disinfection methods for decellularized matrix materials: Review, consideration and proposal

Sterilization is the process of killing all microorganisms, while disinfection is the process of killing or removing all kinds of pathogenic microorganisms except bacterial spores. Biomaterials involved in cell experiments, animal experiments, and clinical applications need to be in the aseptic state, but their physical and chemical properties as well as biological activities can be affected by sterilization or disinfection. Decellularized matrix (dECM) is the low immunogenicity material obtained by removing cells from tissues, which retains many inherent components in tissues such as proteins and proteoglycans. But there are few studies concerning the effects of sterilization or disinfection on dECM, and the systematic introduction of sterilization or disinfection for dECM is even less. Therefore, this review systematically introduces and analyzes the mechanism, advantages, disadvantages, and applications of various sterilization and disinfection methods, discusses the factors influencing the selection of sterilization and disinfection methods, summarizes the sterilization and disinfection methods for various common dECM, and finally proposes a graphical route for selecting an appropriate sterilization or disinfection method for dECM and a technical route for validating the selected method, so as to provide the reference and basis for choosing more appropriate sterilization or disinfection methods of various dECM.

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Asepsis is the prerequisite for the experiment and application of biomaterials.

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Sterilization or disinfection affects physic-chemical properties of biomaterials.

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Mechanism, advantages and disadvantages of sterilization or disinfection methods.

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Factors influencing the selection of sterilization or disinfection methods.

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Selection of sterilization or disinfection methods for decellularized matrix.

Fabrication of Biomolecule-Loaded Composite Scaffolds Carried by Extracellular Matrix Hydrogel

Fabrication of multifunctional scaffolds with biomimicking physical and biological signals play an important role in enhancing tissue regeneration. Multifunctional features come from the composite scaffold with various bioactive molecules. However, simple, biocompatible, and controllable hybridization strategy is still lacking. In this study, we leverage naturally derived extracellular matrix (ECM) as chemically controllable hydrogel carrier to effectively load functional biomolecules. The use of ECM hydrogel takes advantage of both native functionality of ECM components and tunability of hydrogel in controlling release of loaded molecules. As a proof of concept, porous acellular bone scaffold was selected as the solid pristine scaffold to be composited with BMP-2 and VEGF, which are loaded by spinal cord-derived ECM (SC-ECM) hydrogel. Crosslinking degree of SC-ECM hydrogel is tuned by changing genipin concentration, which renders the control over release kinetics of BMP-2 and VEGF. The mechanical strength of scaffold maintained after hybridization and is not significantly decreased in wet condition. In vitro evaluations of scaffolds cocultured with osteoblasts and mesenchymal stem cells (MSCs) demonstrate the biocompatible and bioactive features resulting from the composite scaffolds. Evidenced by alkaline phosphatase test, immunofluorescence, and real-time polymerase chain reaction, differentiation of MSCs towards osteoblast lineage is significantly enhanced by composite scaffolds. Therefore, our strategy in fabricating composite scaffold enabled by biomolecule-loaded ECM hydrogel holds great promise in regenerating diverse tissue types by appropriate combinations of solid pristine scaffolds, ECM, and bioactive molecules. Impact statement We developed a bioactive molecule (e.g., growth factor, protein) loading method using extracellular matrix hydrogel as a carrier. It brings a new strategy to fabricate composite scaffolds with unique biofunctions.

In-Situ Forming pH and Thermosensitive Injectable Hydrogels to Stimulate Angiogenesis: Potential Candidates for Fast Bone Regeneration Applications

Biomaterials that promote angiogenesis are required for repair and regeneration of bone. In-situ formed injectable hydrogels functionalised with bioactive agents, facilitating angiogenesis have high demand for bone regeneration. In this study, pH and thermosensitive hydrogels based on chitosan (CS) and hydroxyapatite (HA) composite materials loaded with heparin (Hep) were investigated for their pro-angiogenic potential. Hydrogel formulations with varying Hep concentrations were prepared by sol-gel technique for these homogeneous solutions were neutralised with sodium bicarbonate (NaHCO3) at 4 °C. Solutions (CS/HA/Hep) constituted hydrogels setting at 37 °C which was initiated from surface in 5-10 minutes. Hydrogels were characterised by performing injectability, gelation, rheology, morphology, chemical and biological analyses. Hydrogel solutions facilitated manual dropwise injection from 21 Gauge which is highly used for orthopaedic and dental administrations, and the maximum injection force measured through 19 G needle (17.191 ± 2.296N) was convenient for manual injections. Angiogenesis tests were performed by an ex-ovo chick chorioallantoic membrane (CAM) assay by applying injectable solutions on CAM, which produced in situ hydrogels. Hydrogels induced microvascularity in CAM assay this was confirmed by histology analyses. Hydrogels with lower concentration of Hep showed more efficiency in pro-angiogenic response. Thereof, novel injectable hydrogels inducing angiogenesis (CS/HA/Hep) are potential candidates for bone regeneration and drug delivery applications.

Decellularized human bone as a 3D model to study skeletal progenitor cells in a natural environment

There has been an increasing interest in exploring naturally derived extracellular matrices as an material mimicking the complexity of the cell microenvironment in vivo. Bone tissue-derived decellularized constructs are able to preserve native structural, biochemical, and biomechanical cues of the tissue, therefore providing a suitable environment to study skeletal progenitor cells. Particularly for bone decellularization, different methods have been reported in the literature. However, the used methods critically affect the final ultrastructure and surface chemistry as well as the decellularization efficiency, consequently causing complications to draw conclusions and compare results in between studies. In this chapter, an optimized protocol for the preparation of human bone derived scaffolds is described, including processing techniques and further characterization methods, which allow the final construct to be recognized as a major platform for bone therapeutic and/or diagnostic applications.

Enzymatic in situ formed hydrogel from gelatin-tyramine and chitosan-4-hydroxylphenyl acetamide for the co-delivery of human adipose-derived stem cells and platelet-derived growth factor towards vascularization

An injectable, in situ forming hydrogel system capable of co-delivering human adipose-derived stem cells (hADSC) and platelet-derived growth factor (PDGF) was investigated as a new system for tissue engineering, envisaged to support vascularization. The system consists of tyramine-conjugated gelatin and hydroxyphenyl acetamide chitosan derivative. Both are soluble and stable at physiologic conditions, which is a key factor for retaining viable cells and active growth factor. In situ gelation involved enzymatic crosslinking using horseradish peroxidase as a catalyst and hydrogen peroxide as an oxidant. Gel formation occurred within 30-90 s by controlling the concentration of polymers. PDGF release showed adequate release kinetics within the intended period of time and hADSC showed good compatibility with the hydrogel formulation based on the in vitro assay and subcutaneous implantation into BALB/c-nu/nu nude female mice. Immunohistochemical analysis confirmed viability of delivered hADSC. Histological analysis showed no immune reaction and confirmed blood vessel formation. The results implicate the hydrogel as a promising delivery vehicle or carrier of both cell and growth factor, which support vascularization for tissue engineering applications.

Decellularized Bone Matrix Scaffold for Bone Regeneration

Decellularized bone matrix is gaining a lot of attention as implantable biomaterials and/or biological scaffolds for bone tissue repair, and shows good clinical performance. This chapter describes the processing techniques and characterization protocols of decellularized bone. For the applications of the decellularized bone scaffold in promoting bone repair and regeneration, we discuss some of the current advances, and highlight the advantages and disadvantages of these scaffolds. Fabrication and application of the hydrogel derived from decellularized bone for bone tissue engineering are also presented.

In vivo vascularization of MSC-loaded porous hydroxyapatite constructs coated with VEGF-functionalized collagen/heparin multilayers

Rapid and adequate vascularization is vital to the long-term success of porous orbital enucleation implants. In this study, porous hydroxyapatite (HA) scaffolds coated with vascular endothelial growth factor (VEGF)-functionalized collagen (COL)/heparin (HEP) multilayers (porosity 75%, pore size 316.8 ± 77.1 μm, VEGF dose 3.39 ng/mm³) were fabricated to enhance vascularization by inducing the differentiation of mesenchymal stem cells (MSCs) to endothelial cells. The in vitro immunofluorescence staining, quantitative real-time polymerase chain reaction (qRT-PCR), and western blotting results demonstrated that the expression of the endothelial differentiation markers CD31, Flk-1, and von Willebrand factor (vWF) was significantly increased in the HA/(COL/HEP)₅/VEGF/MSCs group compared with the HA/VEGF/MSCs group. Moreover, the HA/(COL/HEP)₅ scaffolds showed a better entrapment of the MSCs and accelerated cell proliferation. The in vivo assays showed that the number of newly formed vessels within the constructs after 28 d was significantly higher in the HA/(COL/HEP)₅/VEGF/MSCs group (51.9 ± 6.3/mm²) than in the HA (26.7 ± 2.3/mm²) and HA/VEGF/MSCs (38.2 ± 2.4/mm²) groups. The qRT-PCR and western blotting results demonstrated that the HA/(COL/HEP)₅/VEGF/MSCs group also had the highest expression of CD31, Flk-1, and vWF at both the mRNA and protein levels.

Cell-free scaffolds with different stiffness but same microstructure promote bone regeneration in rabbit large bone defect model

To promote bone healing, bone repair biomaterials are increasingly designed to incorporate growth factors. However, the impact of matrix mechanics of cell-free scaffold independent of microstructure on the osteogenic differentiation of endogenous osteoprogenitor cells orchestrating bone repair and regeneration remains not to be fully understood. In our recent study, three-dimensional (3D) scaffolds with different stiffness but same microstructure have been successfully fabricated by coating decellularized bone with collagen/hydroxyapatite (HA) mixture with different collagen rations. It has been demonstrated that the scaffold with optimal stiffness can induce the osteogenic differentiation of MSCs in vitro and in the subcutaneous tissue. The present in vivo study further investigated the repair efficiency of these scaffolds in a rabbit radius with a critical-sized segmental defect model and its potential mechanism. Micro-computed tomography (μ-CT), X-ray and histological analysis were carried out to evaluate the repair capacity of these scaffolds. The results demonstrated that the cell-free scaffold with optimal stiffness incorporation of endogenous osteoprogenitor cells significantly promoted the repair and reconstruction quality of mass bone defect. One of the crucial mechanisms was that hypoxia and stromal cell-derived factor-1α (SDF-1α) mediated mesenchymal stem cells (MSCs) migration by which matrix mechanics exerted influence on bone fracture healing. These findings suggested that only modulating the matrix stiffness of cell-free scaffold can be one of the most attractive strategies for promoting the progression of bone healing.

3D Scaffolds with Different Stiffness but the Same Microstructure for Bone Tissue Engineering

A growing body of evidence has shown that extracellular matrix (ECM) stiffness can modulate stem cell adhesion, proliferation, migration, differentiation, and signaling. Stem cells can feel and respond sensitively to the mechanical microenvironment of the ECM. However, most studies have focused on classical two-dimensional (2D) or quasi-three-dimensional environments, which cannot represent the real situation in vivo. Furthermore, most of the current methods used to generate different mechanical properties invariably change the fundamental structural properties of the scaffolds (such as morphology, porosity, pore size, and pore interconnectivity). In this study, we have developed novel three-dimensional (3D) scaffolds with different degrees of stiffness but the same 3D microstructure that was maintained by using decellularized cancellous bone. Mixtures of collagen and hydroxyapatite [HA: Ca10(PO4)6(OH)2] with different proportions were coated on decellularized cancellous bone to vary the stiffness (local stiffness, 13.00 ± 5.55 kPa, 13.87 ± 1.51 kPa, and 37.7 ± 19.6 kPa; bulk stiffness, 6.74 ± 1.16 kPa, 8.82 ± 2.12 kPa, and 23.61 ± 8.06 kPa). Microcomputed tomography (μ-CT) assay proved that there was no statistically significant difference in the architecture of the scaffolds before or after coating. Cell viability, osteogenic differentiation, cell recruitment, and angiogenesis were determined to characterize the scaffolds and evaluate their biological responses in vitro and in vivo. The in vitro results indicate that the scaffolds developed in this study could sustain adhesion and growth of rat mesenchymal stem cells (MSCs) and promote their osteogenic differentiation. The in vivo results further demonstrated that these scaffolds could help to recruit MSCs from subcutaneous tissue, induce them to differentiate into osteoblasts, and provide the 3D environment for angiogenesis. These findings showed that the method we developed can build scaffolds with tunable mechanical properties almost without variation in 3D microstructure. These preparations not only can provide a cell-free scaffold with optimal matrix stiffness to enhance osteogenic differentiation, cell recruitment, and angiogenesis in bone tissue engineering but also have significant implications for studies on the effects of matrix stiffness on stem cell differentiation in 3D environments.

Layer-by-layer heparinization of decellularized liver matrices to reduce thrombogenicity of tissue engineered grafts

Background:

Tissue–engineered liver grafts may offer a viable alternative to orthotopic liver transplantation and help overcome the donor organ shortage. Decellularized liver matrices (DLM) have a preserved vasculature and sustain hepatocellular function in culture, but graft survival after transplantation remains limited due to thrombogenicity of the matrix.

Aim:

To evaluate the effect of heparin immobilization on DLM thrombogenicity.

Methods:

Heparin was immobilized on DLMs by means of layer-by-layer deposition. Grafts with 4 or 8 bilayers and 2 or 4 g/L of heparin were recellularized with primary rat hepatocytes and maintained in culture for 5 days. Hemocompatibility of the graft was assessed by ex vivo diluted whole-blood perfusion and heterotopic transplantation.

Results:

Heparin was deposited throughout the matrix and the heparin content in the graft was higher with increasing number of bilayers and concentration of heparin. Recellularization and in vitro albumin and urea production were unaffected by heparinization. Resistance to blood flow during ex vivo perfusion was lower with increased heparinization and, macroscopically, no clots were visible in grafts with 8 bilayers. Following transplantation, flow through the graft was limited in all groups. Histological evidence of thrombosis was lower in heparinized DLMs, but transplantation of DLM grafts was not improved.

Conclusions:

Layer-by-layer deposition of heparin on a DLM is an effective method of immobilizing heparin throughout the graft and does not impede recellularization or hepatocellular function in vitro. Thrombogenicity during ex vivo blood perfusion was reduced in heparinized grafts and optimal with 8 bilayers, but transplantation remained unsuccessful with this method.

Relevance for patients:

Tissue engineered liver grafts may offer a viable solution to dramatic shortages in donor organs

A polymer-extracellular matrix composite with improved thromboresistance and recellularization properties

Organ engineering using decellularized scaffolds is a potential long-term solution to donor organ shortage. However, this technology is severely limited by small vessel thrombosis due to incompletely recellularized vessels, resulting in exposure of extracellular matrix (ECM) components to platelets and clotting factors in flowing blood. To address this limitation, we designed a polymer-ECM composite and demonstrated its potential to reduce thrombosis and facilitate re-endothelialization in a vascular graft model. Rat aortas were decellularized using a sequential combination of weak detergents followed by a nuclease treatment that resulted in 96.5±1.3% DNA removal, while ECM components and mechanical properties were well maintained. A biodegradable and biocompatible elastomer poly(1,8 octanediol citrate) (POC, 1wt.%) was infused throughout the ECM at mild conditions (37°C and 45°C) and was functionalized with heparin using carbodiimide chemistry. The polymer-ECM composite significantly reduced platelet adhesion (67.4±8.2% and 82.7±9.6% reduction relative to untreated ECM using one of two processing temperatures, 37°C or 45°C, respectively); inhibited whole blood clotting (85.9±4.3% and 87.0±11.9% reduction relative to untreated ECM at 37°C or 45°C processing temperature, respectively); and supported endothelial cell-and to a lesser extent smooth muscle cell-adhesion in vitro. Taken together, this novel POC composite may provide a solution for thrombosis of small vessel conduits commonly seen in decellularized scaffolds used in tissue engineering applications.

Biomimetic construction of large engineered bone using hemoperfusion and cyto-capture in traumatic bone defect

Due to lack of blood vessel systems, only a few tissues, such as skin, cartilage, and cornea, have been successfully constructed in vivo. Anticoagulative scaffolds have been used in drug-eluting stent systems both in animal studies and clinical therapies, as in the medicinal leech therapy used to salvage venous-congested microvascular free flaps improved perfusion inspired us to tackle this hurdle in bone tissue engineering. We hypothesize that a combination of bone marrow as the blood supply and a heparin/chitosan-coated acellular bone matrix that acts like hirudin, together with a vacuum-assisted closure therapy system, would provide blood perfusion to the scaffold. Using these methods, a biomimetically engineered bone construct would facilitate clinical translation in bone tissue engineering and offer new therapeutic strategies for reconstructing large bone defects if the hypothesis proves to be practical.

Chitosan-heparin polyelectrolyte multilayers on cortical bone: periosteum-mimetic, cytophilic, antibacterial coatings

Cortical bone allografts suffer from high rates of failure due to poor integration with host tissue, leading to non-union, fracture, and infection following secondary procedures. Here, we report a method for modifying the surfaces of cortical bone with coatings that have biological functions that may help overcome these challenges. These chitosan-heparin coatings promote mesenchymal stem cell attachment and have significant antibacterial activity against both S. aureus and E. coli. Furthermore, their chemistry is similar to coatings we have reported on previously, which effectively stabilize and deliver heparin-binding growth factors. These coatings have potential as synthetic periosteum for improving bone allograft outcomes.

Synthesis of biomimetic segmented polyurethanes as antifouling biomaterials

Controlling the non-specific adsorption of proteins, cells and bacteria onto biomaterial surfaces is of crucial importance for the development of medical devices with specific levels of performance. Among the strategies pursued to control the interactions between material surfaces and biological tissues, the immobilization of non-fouling polymers on biomaterial surfaces as well as the synthesis of the so-called biomimetic polymers are considered promising approaches to elicit specific cellular responses. In this study, in order to obtain materials able to prevent infectious and thrombotic complications related to the use of blood-contacting medical devices, heparin-mimetic segmented polyurethanes were synthesized and fully characterized. Specifically, sulfate or sulfamate groups, known to be responsible for the biological activity of heparin, were introduced into the side chain of a carboxylated polyurethane. Due to the introduction of these groups, the obtained polymers possessed a higher hard/soft phase segregation (lower glass transition temperatures) and a greater hydrophilicity than the pristine polymer. In addition, the synthesized polymers were able to significantly delay the activated partial thromboplastin time, this increased hemocompatibility being related both to polymer hydrophilicity and to the presence of the -SO3H groups. This last feature was also responsible for the ability of these biomimetic polymers to prevent the adhesion of a strain of Staphylococcus epidermidis.