In vitro surface characterization of a biological patch fixed with a naturally occurring crosslinking agent

Improved bioavailability and antioxidation of β-carotene-loaded biopolymeric nanoparticles stabilized by glycosylated oat protein isolate

The limited bioavailability of β-carotene hinders its potential application in functional foods, despite its excellent antioxidant properties. Protein-based nanoparticles have been widely used for the delivery of β-carotene to overcome this limitation. However, these nanoparticles are susceptible to environmental stress. In this study, we utilized glycosylated oat protein isolate to prepare nanoparticles loaded with β-carotene through the emulsification-evaporation method, aiming to address this challenge. The results showed that β-carotene was embedded into the spherical nanoparticles, exhibiting relatively high encapsulation efficiency (86.21 %) and loading capacity (5.43 %). The stability of the nanoparticles loaded with β-carotene was enhanced in acidic environments and under high ionic strength. The nanoparticles offered protection to β-carotene against gastric digestion and facilitated its controlled release (95.76 % within 6 h) in the small intestine, thereby leading to an improved in vitro bioavailability (65.06 %) of β-carotene. This improvement conferred the benefits on β-carotene nanoparticles to alleviate tert-butyl hydroperoxide-induced oxidative stress through the upregulation of heme oxygenase-1 and NAD(P)H quinone dehydrogenase 1 expression, as well as the promotion of nuclear translocation of nuclear factor-erythroid 2-related factor 2. Our study suggests the potential for the industry application of nanoparticles based on glycosylated proteins to effectively deliver hydrophobic nutrients and enhance their application.

Review paper: The importance of consideration of collagen cross-links in computational models of collagen-based tissues

Collagen as the main protein in Extra Cellular Matrix (ECM) is the main load-bearing component of fibrous tissues. Nanostructure and architecture of collagen fibrils play an important role in mechanical behavior of these tissues. Extensive experimental and theoretical studies have so far been performed to capture these properties, but none of the current models realistically represent the complexity of network mechanics because still less is known about the collagen's inner structure and its effect on the mechanical properties of tissues. The goal of this review article is to emphasize the significance of cross-links in computational modeling of different collagen-based tissues, and to reveal the need for continuum models to consider cross-links properties to better reflect the mechanical behavior observed in experiments. In addition, this study outlines the limitations of current investigations and provides potential suggestions for the future work.

Redox-Activation of Neutrophils Induced by Pericardium Scaffolds

Implantation of scaffolds causes a local inflammatory response whereby the early recruitment of neutrophils is of great importance not only for fighting the infection, but also for facilitating effective regeneration. We used luminol-dependent chemiluminescence, flow cytometry, ELISA, and confocal microscopy to assess the responses of neutrophils after the exposure to the scaffold-decellularized bovine pericardium (collagen type I) crosslinked with genipin (DBPG). We demonstrated that DBPG activated neutrophils in whole blood causing respiratory burst, myeloperoxidase (MPO) secretion, and formation of neutrophil extracellular trap-like structures (NETs). In addition, we studied platelets, another important player of the immediate immune host response. We found that platelets triggered redox-activation of isolated neutrophils by the pericardium scaffold, and likely participate in the NETs formation. Free radicals generated by neutrophils and hypochlorous acid produced by MPO are potent oxidizing agents which can oxidatively degrade biological structures. Understanding the mechanisms and consequences of redox activation of neutrophils by pericardium scaffolds is important for the development of new approaches to increase the efficiency of tissue regeneration.

A Comparative Study on Two Types of Porcine Acellular Dermal Matrix Sponges Prepared by Thermal Crosslinking and Thermal-Glutaraldehyde Crosslinking Matrix Microparticles

Common commercial porcine acellular dermal matrix (PADM) products take the form of a thin membrane. Given its dense structure, delaying vascularization after implantation remains an issue to be solved. In addition, overlaying multiple sheets to address deep wounds and large tissue defects that are difficult to repair by self-tissues could hinder tissue ingrowth, angiogenesis, and integration. Here, we creatively prepared PADM microparticles through a homogenizing treatment and crosslinked them to ADM sponges by thermal crosslinking (VT-ADM) and thermal-glutaraldehyde crosslinking (GA-ADM). The resulting VT-ADM was thicker than GA-ADM, and both maintained the natural dermal matrix microstructure and thermal stability. The porosity of GA-ADM (mean 82%) was lower than that of VT-ADM (mean 90.2%), but the mechanical strength and hydrophilicity were significantly higher. The two types of ADM sponges showed no obvious difference in cell adhesion and proliferation without cytotoxicity. Furthermore, the human adipose stem cells were co-cultured with ADM sponges which promoted proliferation, tube formation, and migration of endothelial cells, and the GA-ADM group exhibited better migration behavior. There were no markable differences among expressions of pro-angiogenesis genes, including vascular endothelial growth factor, insulin-like growth factor-1, and epidermal growth factor. In a nude mouse model, the VT-ADM and GA-ADM pre-cultured with human adipose stem cells for 1 week in advance were implanted subcutaneously. The VT-ADM and the GA-ADM showed great histocompatibility without local redness, swelling, or necrosis. The vascular density of the local skin flap above the material was visualized using indocyanine green and showed no statistical difference between the two groups. The collagen tissue deposition in the pores and vessel formation within the sponges increased with time. Although VT-ADM had a higher degradation rate in vivo, the integrity of the two scaffolds was preserved. Collectively, the VT-ADM and the GA-ADM retained a natural matrix structure and presented biocompatibility. Thus, the above-mentioned two crosslinking methods for ADM sponges are safe and practicable. The novel ADM sponges with good physicochemical and biological properties are no longer limited to membrane tissue regeneration but could also realize structure remodeling where they act as scaffolds for a soft tissue filler and three-dimensional reconstruction of the tissue with strength requirements.

New Insights of Scaffolds Based on Hydrogels in Tissue Engineering

In recent years, biomaterials development and characterization for new applications in regenerative medicine or controlled release represent one of the biggest challenges. Tissue engineering is one of the most intensively studied domain where hydrogels are considered optimum applications in the biomedical field. The delicate nature of hydrogels and their low mechanical strength limit their exploitation in tissue engineering. Hence, developing new, stronger, and more stable hydrogels with increased biocompatibility, is essential. However, both natural and synthetic polymers possess many limitations. Hydrogels based on natural polymers offer particularly high biocompatibility and biodegradability, low immunogenicity, excellent cytocompatibility, variable, and controllable solubility. At the same time, they have poor mechanical properties, high production costs, and low reproducibility. Synthetic polymers come to their aid through superior mechanical strength, high reproducibility, reduced costs, and the ability to regulate their composition to improve processes such as hydrolysis or biodegradation over variable periods. The development of hydrogels based on mixtures of synthetic and natural polymers can lead to the optimization of their properties to obtain ideal scaffolds. Also, incorporating different nanoparticles can improve the hydrogel’s stability and obtain several biological effects. In this regard, essential oils and drug molecules facilitate the desired biological effect or even produce a synergistic effect. This study’s main purpose is to establish the main properties needed to develop sustainable polymeric scaffolds. These scaffolds can be applied in tissue engineering to improve the tissue regeneration process without producing other side effects to the environment.

Current methods of collagen cross-linking: Review

This review provides a report on cross-linking methods used for collagen modifications. Collagen materials have attracted significant academic interest due to its biological properties in native state. However, in many cases the mechanical properties and degradation rate should be tailored to especial biomedical and cosmetic applications. In the proposed review paper, the structure, preparation, and properties of several collagen based materials have been discussed in general, and detailed examples of collagen cross-linking methods have been drawn from scientific literature and practical work. Both, physical and chemical methods of improvement of collagenous materials have been reviewed. In the review paper the cross-linking with glutaraldehyde, genipin, EDC-NHS, dialdehyde starch, chitosan, temperature, UV light and enzyme has been discussed. A critical comparison of currently available cross-linking methods has been shown.

The Use of Genipin as an Effective, Biocompatible, Anti-Inflammatory Cross-Linking Method for Nerve Guidance Conduits

A number of natural polymer biomaterial-based nerve guidance conduits (NGCs) are developed to facilitate repair of peripheral nerve injuries. Cross-linking ensures mechanical integrity and desired degradation properties of the NGCs; however, common methods such as formaldehyde are associated with cellular toxicity. Hence, there is an unmet clinical need for alternative nontoxic cross-linking agents. In this study, collagen-based NGCs with a collagen/chondroitin sulfate luminal filler are used to study the effect of cross-linking on mechanical and structural properties, degradation, biocompatibility, and immunological response. A simplified manufacturing method of genipin cross-linking is developed, by incorporating genipin into solution prior to freeze-drying the NGCs. This leads to successful cross-linking as demonstrated by higher cross-linking degree and similar tensile strength of genipin cross-linked conduits compared to formaldehyde cross-linked conduits. Genipin cross-linking also preserves NGC macro and microstructure as observed through scanning electron microscopy and spectral analysis. Most importantly, in vitro cell studies show that genipin, unlike the formaldehyde cross-linked conduits, supports the viability of Schwann cells. Moreover, genipin cross-linked conduits direct macrophages away from a pro-inflammatory and toward a pro-repair state. Overall, genipin is demonstrated to be an effective, safe, biocompatible, and anti-inflammatory alternative to formaldehyde for cross-linking clinical grade NGCs.

Sustained scleral stiffening in rats after a single genipin treatment

Scleral stiffening has been proposed as a therapy for glaucoma and myopia. Previous in vivo studies have evaluated the efficacy of scleral stiffening after multiple treatments with a natural collagen crosslinker, genipin. However, multiple injections limit clinical translatability. Here, we examined whether scleral stiffening was maintained after four weeks following a single genipin treatment. Eyes from brown Norway rats were treated in vivo with a single 15 mM genipin retrobulbar injection, sham retrobulbar injection, or were left naive. Eyes were enucleated either 1 day or four weeks post-injection and underwent whole globe inflation testing. We assessed first principal Lagrange strain of the posterior sclera using digital image correlation as a proxy for scleral stiffness. Four weeks post-injection, genipin treatment resulted in a 58% reduction in scleral strain as compared to controls (p = 0.005). We conclude that a single in vivo injection of genipin effectively stiffened rat sclera for at least four weeks which motivates further functional studies and possible clinical translation of genipin-induced scleral stiffening.

Gelatin-genipin-based biomaterials for skeletal muscle tissue engineering

Skeletal muscle engineering aims at tissue reconstruction to replace muscle loss following traumatic injury or in congenital muscle defects. Skeletal muscle can be engineered by using biodegradable and biocompatible scaffolds that favor myogenic cell adhesion and subsequent tissue organization. In this study, we characterized scaffolds made of gelatin cross-linked with genipin, a natural derived cross-linking agent with low cytotoxicity and high biocompatibility, for tissue engineering of skeletal muscle. We generated gelatin-genipin hydrogels with a stiffness of 13 kPa to reproduce the mechanical properties characteristic of skeletal muscle and we show that their surface can be topographically patterned through soft lithography to drive myogenic cells differentiation and unidirectional orientation. Furthermore, we demonstrate that these biomaterials can be successfully implanted in vivo under dorsal mouse skin, showing good biocompatibility and slow biodegradation rate. Moreover, the grafting of this biomaterial in partially ablated tibialis anterior muscle does not impair muscle regeneration, supporting future applications of gelatin-genipin biomaterials in the field of skeletal muscle tissue repair. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 2763-2777, 2018.

A comparative investigation of Bombyx mori silk fibroin hydrogels generated by chemical and enzymatic cross-linking

Fibroin, the major proteinaceous component of the silk fiber produced by larvae of the domesticated silk moth (Bombyx mori), has been widely investigated as a biomaterial for potential applications in tissue engineering and regenerative medicine. Following sol-gel transition, silk fibroin solutions can generate hydrogels that present certain advantages when employed as biomaterials, especially if they are cross-linked. The subject of this study was the self-cross-linking of silk fibroin through a process induced by the enzyme horseradish peroxidase (HRP) in the presence of hydrogen peroxide, a method only recently proposed and scarcely reported. The hydrogels were prepared either by physical cross-linking, by cross-linking with a natural compound (genipin), or by enzymatic cross-linking. The products were comparatively characterized in regard to their synthesis and background chemical aspects, physical and optical properties, mechanical properties, secondary structure, swelling/deswelling behavior, enzymatic degradation, and compatibility as substrates for cell adhesion and proliferation. The study confirmed the advantages of the HRP-induced cross-linking, which included considerably shorter gelation times, enhanced elasticity of the resulting hydrogels, and improved cytocompatibility. Discrepancies between certain results of this investigation and those reported previously were discussed in detail.

Genipin diffusion and reaction into a gelatin matrix for tissue engineering applications

Genipin is a natural low-toxic cross-linker for molecules with primary amino groups, and its use with collagen and gelatin has shown a great potential in tissue engineering applications. The fabrication of scaffolds with a well-organized micro and macro topology using additive manufacturing systems requires an accurate control of working parameters, such as reaction rate, gelling time, and diffusion constant. A polymeric system of 5% w/v gelatin in PBS with 2 mg/mL collagen solutions in a 1:1 weight ratio was used as template to perform measurements varying genipin concentration in a range of 0.1-1.5% w/w with respect to gelatin. In the first part of this work, the reaction rate of the polymeric system was estimated using a new colorimetric analysis of the reaction. Then its workability time, closely related to the gelling time, was evaluated thanks to rheological analysis: finally, the quantification of static and dynamic diffusion constants of genipin across nonreacting and reacting membranes, made respectively by agarose and gelatin, was performed. It was shown that the colorimetric analysis is a good indicator of the reaction progress. The gelling time depends on the genipin concentration, but a workability window of 40 min guaranteed up to 0.5% w/w genipin. The dynamic diffusion constant of genipin in the proposed polymeric system is in the order of magnitude of 10^-7 . The obtained results indicated the possibility to use the genipin, gelatin, and collagen, in the proposed concentrations, to build well-defined hydrogel scaffolds with both extrusion-based and 3D ink-jet system. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 473-480, 2017.

Genipin crosslinking of cartilage enhances resistance to biochemical degradation and mechanical wear

Collagen crosslinking enhances many beneficial properties of articular cartilage, including resistance to chemical degradation and mechanical wear, but many crosslinking agents are cytotoxic. The purpose of this study was to evaluate the effectiveness of genipin, a crosslinking agent with favorable biocompatibility and cytotoxicity, as a potential treatment to prevent the degradation and wear of articular cartilage. First, the impact of genipin concentration and treatment duration on the viscoelastic properties of bovine articular cartilage was quantified. Next, two short-term (15 min) genipin crosslinking treatments were chosen, and the change in collagenase digestion, cartilage wear, and the friction coefficient of the tissue with these treatments was measured. Finally, chondrocyte viability after exposure to these genipin treatments was assessed. Genipin treatment increased the stiffness of healthy, intact cartilage in a dose-dependent manner. The 15-min crosslinking treatments improved cartilage's resistance to both chemical degradation, particularly at the articular surface, and to damage due to mechanical wear. These enhancements were achieved without sacrificing the low coefficient of friction of the tissue and at a genipin dose that preserved chondrocyte viability. The results of this study suggest that collagen crosslinking via genipin may be a promising preventative treatment to slow the degradation of cartilage.

Nanostructured biomaterials from electrospun demineralized bone matrix: a survey of processing and crosslinking strategies

In the design of scaffolds for tissue engineering biochemical function and nanoscale features are of particular interest. Natural polymers provide a wealth of biochemical function, but do not have the processability of synthetic polymers, limiting their ability to mimic the hierarchy of structures in the natural extracellular matrix. Thus, they are often combined with synthetic carrier polymers to enable processing. Demineralized bone matrix (DBM), a natural polymer, is allograft bone with inorganic material removed. DBM contains the protein components of bone, which includes adhesion ligands and osteoinductive signals, such as important growth factors. Herein we describe a novel method for tuning the nanostructure of DBM through electrospinning without the use of a carrier polymer. This work surveys solvents and solvent blends for electrospinning DBM. Blends of hexafluoroisopropanol and trifluoroacetic acid are studied in detail. The effects of DBM concentration and dissolution time on solution viscosity are also reported and correlated to observed differences in electrospun fiber morphology. We also present a survey of techniques to stabilize the resultant fibers with respect to aqueous environments. Glutaraldehyde vapor treatment is successful at maintaining both macroscopic and microscopic structure of the electrospun DBM fibers. Finally, we report results from tensile testing of stabilized DBM nanofiber mats, and preliminary evaluation of their cytocompatibility. The DBM nanofiber mats exhibit good cytocompatibility toward human dermal fibroblasts (HDF) in a 4-day culture; neither the electrospun solvents nor the cross-linking results in any measurable residual cytotoxicity toward HDF.

Riboflavin-ultraviolet-A-induced collagen cross-linking treatments in improving dentin bonding

OBJECTIVES

To evaluate the collagen cross-linkers, riboflavin-ultraviolet-A (RF/UVA) and glutaraldehyde, with regard to their efficacy in cross-linking the dentinal collagen and improving dentin bonding.

METHODS

Glutaraldehyde and different RF/UVA protocols (0.1%RF/1-minUV, 0.1%RF/2-minUV, and 1%RF/1-minUV) were first evaluated by gel electrophoresis to determine their abilities of collagen cross-linking. The mechanical properties of acid-etched dentin receiving these cross-linking treatments were examined in either dry or wet condition by a nanoindentation test. Fifteen teeth with exposed occlusal dentin received the microtensile bond strength (μTBS) test. The teeth were primed either with RF/UVA or glutaraldehyde, followed by adhesive treatment and composite restorations, and then cut into resin-dentin microbeams. Half of the microbeams received the μTBS test after 24h, and the other half received test after 5000 thermocycles. Nanoleakage at the bond interface was examined under TEM. The alignments of collagen fibrils in the hybrid layers were also defined by an image analysis.

RESULTS

Gel electrophoresis showed that glutaraldehyde induced strong collagen gelation, while RF/UVA generated milder collagen cross-linking. Glutaraldehyde, 0.1%RF/2-min-UVA, and 1%RF/1-minUV showed higher stiffness compared to untreated and 0.1%RF/1-minUV in wet condition. All the crosslinking treatments improved early μTBS, but 0.1%RF/2-minUVA treatment maintained high μTBS after theromocycles. Under TEM, glutaraldehyde-treated dentin showed dense and enclosed collagen network on the adhesive interface. 0.1%RF/2-minUVA showed the least nanoleakage, and this could be associated with the suspended collagen fibrils in the hybrid layer.

SIGNIFICANCE

0.1%RF/2-minUVA treatment enhanced resin-dentin bond possibly through enhancing the stiffness and maintaining the expanding collagen matrix in the hybrid layer.

Experimental and modeling study of collagen scaffolds with the effects of crosslinking and fiber alignment

Collagen type I scaffolds are commonly used due to its abundance, biocompatibility, and ubiquity. Most applications require the scaffolds to operate under mechanical stresses. Therefore understanding and being able to control the structural-functional integrity of collagen scaffolds becomes crucial. Using a combined experimental and modeling approach, we studied the structure and function of Type I collagen gel with the effects of spatial fiber alignment and crosslinking. Aligned collagen scaffolds were created through the flow of magnetic particles enmeshed in collagen fibrils to mimic the anisotropy seen in native tissue. Inter- and intra- molecular crosslinking was modified chemically with Genipin to further improve the stiffness of collagen scaffolds. The anisotropic mechanical properties of collagen scaffolds were characterized using a planar biaxial tensile tester and parallel plate rheometer. The tangent stiffness from biaxial tensile test is two to three orders of magnitude higher than the storage moduli from rheological measurements. The biphasic nature of collagen gel was discussed and used to explain the mechanical behavior of collagen scaffolds under different types of mechanical tests. An anisotropic hyperelastic constitutive model was used to capture the characteristics of the stress-strain behavior exhibited by collagen scaffolds.

Effects of exogenous glycosaminoglycans on human chondrocytes cultivated on type II collagen scaffolds

Cartilage extracellular matrix (ECM) is composed primarily of type II collagen (COL II) and large, networks of proteoglycans (PGs) that contain glycosaminoglycans such as hyaluronic acid (HA) and chondroitin sulfate (CS). Since cartilage shows little tendency for self-repair, injuries are kept unhealed for years and can eventually lead to further degeneration. During the past decades, many investigations have pursued techniques to stimulate articular cartilage repair or regeneration. The current study assessed the effects of exogenous glycosaminoglycans (GAGs) including CS-A, CS-B, CS-C, heparan sulfate and HA, administration on human chondrocytes in terms of proliferation and matrix synthesis, while the cells were seeded and grown on the genipin-crosslinked collagen type II (COL II) scaffold. DNA content was measured by Hoechst dye intercalation, matrix deposition was evaluated by DMMB dye. Expression of collagen II and aggrecan mRNAs was assessed by RT-PCR, followed by gel electrophoresis. In a 28-day in vitro culture, administration of 5 microg/ml CS-A, 50 microg/ml CS-B, 50 microg/ml CS-C, 5 microg/ml HS, and 500 kDa HA led to significant increase in biosynthesis rate of PGs. Gene expression of aggrecan and collagen II were upregulated by CS-A, CS-C and HA. These results showed considerable relevance of GAGs to the issue of in vitro/ex vivo neo-cartilage synthesis for tissue engineering and regenerative medical applications.

Comparison of the fixative properties of five disinfectant solutions

Following a French circular published in 2001, the use of glutaraldehyde for the disinfection of reusable medical devices was abandoned in favour of non-fixative disinfectants such as peracetic-acid-based solutions. Data published regarding the fixative properties of alternative disinfectants remain contradictory. We compared the effect of repetitive treatments of polytetrafluoroethylene (PTFE) tubes, contaminated by a liquid medium inoculated with Pseudomonas aeruginosa, using five different disinfectant solutions: two peracetic acid solutions (with and without an activator), glutaraldehyde, ortho-phthaldehyde and succine dialdehyde. The results confirmed that repeated treatments of a PTFE tube with a 2% glutaraldehyde solution induce an important accumulation and/or fixation of protein, compared to peracetic-acid-based disinfectants, for which the accumulation and/or fixation of proteins remain low and vary from one formulation to another.

Aglycone geniposidic acid, a naturally occurring crosslinking agent, and its application for the fixation of collagenous tissues

A natural compound, aglycone geniposidic acid (aGSA), originated from the fruits of Gardenia jasminoides ELLIS was used for the fixation of collagenous tissues. The presumed crosslinking reaction mechanism of collagenous tissues with aGSA was inferred by reacting aGSA with a bifunctional amine, 1,6-hexanediamine, using a series of (1)H NMR, FT-IR, and UV/Vis spectra analyses. aGSA reacted with 1,6-hexanediamine by a nucleophilic attack on the olefinic carbon atom at C-2 of deoxyloganin aglycone, followed by opening the dihydropyran ring to form heterocyclic amine compounds. It is inferred that aGSA may form intramolecular and intermolecular crosslinks with a heterocyclic structure within collagen fibers in tissues. The degrees of tissue fixation by aGSA at different pH values were investigated by examining the fixation indices and denaturation temperatures of test samples. It was found that the fixation indices and denaturation temperatures of test samples fixed at neutral or basic pH (pH 7.4 or pH 8.5) were significantly greater than at acidic pH (pH 4.0). The results obtained in this study may be used to elucidate the crosslinking mechanism and optimize the fixation process for developing bioprostheses fixed by aGSA.

A novel polymer for potential use in a trileaflet heart valve

A novel polyolefin, poly(styrene-b-isobutylene-b-styrene) (Quatromer), is being proposed as a viable polymer for use in trileaflet heart valves because of its oxidative stability. The current study was designed to assess the polymer's hemocompatibility and mechanical durability. Mechanical characterization included static tensile tests and dynamic tension-tension and bending fatigue tests, where the properties of isotropic and composite (polypropylene (PP) embedded) Quatromer specimens were compared with those of a polyurethane (PUR) approved for cardiovascular applications. It was found that by embedding PP fibers into the Quatromer matrix, the tensile and fatigue properties of the polymer could be improved, making them comparable, if not better than the PUR. The thrombotic potential of Quatromer was compared with the PUR, glutaraldehyde-fixed porcine valve material, and a positive and negative control by measuring platelet deposition with radiolabeled platelets in a parallel plate flow configuration. The porcine valve material was found to have significantly higher platelet deposition under all flow regimes, while no significant difference existed between Quatromer and PUR. In conclusion, Quatromer is shown to have suitable hemocompatibility and mechanical durability for use in polymer trileaflet heart valves, and fiber reinforcement can effectively be used to tailor the mechanical properties.

Mesothelium regeneration on acellular bovine pericardia loaded with an angiogenic agent (ginsenoside Rg1) successfully reduces postsurgical pericardial adhesions

OBJECTIVE

Our objective was to reduce postsurgical pericardial adhesions with porous acellular bovine pericardia loaded with ginsenoside Rg1, an angiogenic agent isolated from Panax ginseng (the Acellular/Rg1 patch).

METHODS

The acellular/Rg1 patch was used as a substitute to repair a defect created in the pericardium of a rabbit model. A commercially available expanded polytetrafluoroethylene patch, the cellular pericardium (the cellular patch), and the acellular pericardium without loading Rg1 (the acellular patch) were used as controls. The implanted samples were retrieved at 1 and 3 months after surgery (n = 5 per group at each time point).

RESULTS

It was found that each side of the implanted patch could be remesothelialized provided that regeneration of neo-tissue fibrils occurred initially on its surfaces. Because remesothelialization did not take place on the surfaces of the expanded polytetrafluoroethylene and cellular patches, moderate to severe adhesions to the lung and epicardium were clearly observed. As compared with the cellular patch, the acellular patch significantly reduced postsurgical pericardial adhesions, especially on its lung side, as a result of remesothelialization. In the presence of Rg1, a faster remesothelialization was observed on each side of the acellular/Rg1 patch. Therefore, the acellular/Rg1 patch was free of any adhesions to the lung; however, there was still a filmy adhesion to the epicardium observed in 3 of the 5 studied animals at 3 months after surgery, due to incomplete remesothelialization.

CONCLUSIONS

The acellular/Rg1 patch effectively repaired pericardial defects in rabbits and successfully reduced the formation of pericardial adhesions.

Porous acellular bovine pericardia seeded with mesenchymal stem cells as a patch to repair a myocardial defect in a syngeneic rat model

A patch is often mandatory to repair myocardial defects; however, currently available patches lack the possibility of regeneration. To overcome this limitation, a porous acellular bovine pericardium seeded with BrdU-labeled mesenchymal stem cells (MSCs) was prepared (the MSC patch) to repair a surgically created myocardial defect in the right ventricle of a syngeneic rat model. The bovine pericardium before cell extraction was used as a control (the Control patch). The implanted samples were retrieved at 4- and 12-week postoperatively (n=5 per group at each time point). At retrieval, no aneurysmal dilation of the implanted patches was seen for both studied groups. No apparent tissue adhesion was observed for the MSC patch throughout the entire course of the study, while for the Control patch, two out of the five studied animals at 12-week postoperatively had a filmy adhesion to the chest wall. On the inner (endocardial) surface, intimal thickening was observed for both studied groups; however, no thrombus formation was found. Intact layers of endothelial and mesothelial cells were identified on the inner and outer (epicardial) surfaces of the MSC patch. Smooth muscle cells together with neo-muscle fibers, neo-glycosaminoglycans and neo-capillaries were observed within the pores of the MSC patch. Some cardiomyocytes, which stained positively for BrdU and alpha-sacromeric actin, were observed in the MSC patch, indicating that the implanted MSCs can engraft and differentiate into cardiomyocytes. Additionally, a normality of the local electrograms on the epicardial surface of the MSC patch was observed. In contrast, no apparent tissue regeneration was observed for the Control patch throughout the entire course of the study, while only abnormal electrogram signals were seen on its epicardial surface. In conclusion, the MSC patch may preserve the structure of the ventricular wall while providing the potential for myocardial tissue regeneration.

Some physical and microstructural properties of genipin-crosslinked gelatin–maltodextrin hydrogels

The physical properties and microstructure of gelatin-maltodextrin hydrogels fixed with genipin (GP) were investigated as a function of pH (3-7), maltodextrin (MD) (0-9%, w/w) and GP (0-10 mM levels), at a constant gelatin (G) concentration (10%, w/w). Network strength (elastic modulus, E) and swelling behavior were characterized by large deformation testing and by swelling index (SI). In general, network strength increased and swelling decreased at higher pH, MD and GP levels, except at pH 3, where E was independent of the GP concentration until approximately 7.5 mM, above which it declined. Confocal scanning laser microscopy (CLSM) images showed phase separation to be suppressed at pH 3, whereas at pH 7, separation into a self-similar dispersed phase was apparent. Overall, the judicious use of GP to crosslink G was an appropriate means of kinetically trapping MD within the gelatin network.

Tissue regeneration observed in a porous acellular bovine pericardium used to repair a myocardial defect in the right ventricle of a rat model

OBJECTIVE

Nonliving synthetic materials have been widely used to repair myocardial defects; however, material-related failures do occur. To overcome these problems, an acellular bovine pericardium with a porous structure fixed with genipin (the AGP patch) was developed.

METHODS

The AGP patch was used to repair a surgically created myocardial defect in the right ventricle of a rat model. A commercially available expanded polytetrafluoroethylene (e-PTFE) patch was used as a control. At retrieval, a computerized mapping system was used to acquire local epicardial electrograms of each implanted sample, and the appearance of each retrieved sample was grossly examined. The retrieved samples were then processed for histologic examination.

RESULTS

The amplitude of local electrograms on the AGP patch increased significantly with increasing implantation duration, whereas only low-amplitude electrograms were observed on the e-PTFE patch throughout the entire course of the study. No aneurysmal dilation of the implanted patches was seen for either studied group. Additionally, no tissue adhesion was observed on the outer (epicardial) surface of the AGP patch, whereas a moderate tissue adhesion was observed on the e-PTFE patch. On the inner (endocardial) surface, intimal thickening was observed for both studied groups; however, no thrombus formation was found. Intact layers of endothelial and mesothelial cells were identified on the inner and outer surfaces of the AGP patch, respectively. At 4 weeks postoperatively, smooth muscle cells, together with neomuscle fibers (with a few neocollagen fibrils), neoglycosaminoglycans, and neocapillaries, were observed to fill the pores in the AGP patch, an indication of tissue regeneration. These observations were more pronounced at 12 weeks postoperatively. In contrast, no apparent tissue regeneration was observed in the e-PTFE patch.

CONCLUSION

The present study indicated that the AGP patch holds promise to become a suitable patch for surgical repair of myocardial defects.

A novel drug-eluting stent spray-coated with multi-layers of collagen and sirolimus

In the study, a novel drug-eluting stent for treating the coronary arterial stenosis was developed. Using a spray-coating method, aqueous bovine type I collagen and sirolimus were coated layer-by-layer alternatively onto the surface of a metallic stent and a topcoat of collagen was used as a barrier to control drug release. To prevent dissolution of the collagen matrices, the spray-coated collagen was further crosslinked by genipin, a naturally occurring crosslinking agent. The results obtained in the atomic force microscopy (AFM) examination suggested that the spray-coated collagen was tightly adhered to the surface of the stent. Additionally, the collagen coating was demonstrated by the scanning electron microscopy (SEM) to be sufficiently flexible to allow balloon expansion of the stent without cracking or peeling from the wire. The resistance against enzymatic degradation and the hemocompatibility of the collagen matrices increased significantly as their degree of crosslinking increased. All the studied sirolimus-loaded stents exhibited a nearly linear sustained-release profile (except at the end stage of release) with no significant burst releases. It was found that a topcoat of collagen on the collagen/sirolimus coated stent did slow down the release of sirolimus to some extent. Additionally, the number of layers of collagen/sirolimus coated significantly affected the duration of sirolimus released. Furthermore, the sustained-release duration of sirolimus was proportional to the actual amount of drug loaded on the stent. The aforementioned results indicated that the drug-eluting stent developed had a tightly adhered collagen coating and can be used as a drug reservoir to sustain release of sirolimus.

Peritoneal Regeneration Induced by an Acellular Bovine Pericardial Patch in the Repair of Abdominal Wall Defects

BACKGROUND

This study was to evaluate the feasibility of using an acellular bovine pericardium fixed with genipin (AGP) to repair an abdominal wall defect created in a rat model.

MATERIALS AND METHODS

The glutaraldehyde-fixed acellular pericardium (AGA), the genipin-fixed cellular pericardium (GP), and a commercially available polypropylene mesh were used as controls.

RESULTS

Gross examination at 3-month post-operatively revealed that dense adhesions to the visceral organs were observed for the polypropylene mesh and the AGA patch, while a filmy to dense adhesion was seen for the GP patch. In contrast, no adhesion to the visceral organs was observed for the AGP patch. Histologically, inflammatory cells were found mainly surrounding the GP patch. In contrast, host cells (inflammatory cells, fibroblasts, and neo-capillaries) were able to infiltrate into the AGA and AGP patches. Unlike the AGA patch, the AGP patch retrieved at 1-month post-operatively became well integrated with the host tissue near the suture line. Additionally, there were some mesothelial cells, identified by the van Gieson stain, observed on the AGP patch. At 3-month post-operatively, a neo-peritoneum was observed on the AGP patch. The neo-peritoneum consisted of organized vascularized connective tissues covered by an intact layer of mesothelial cells. The calcium contents of the polypropylene mesh and the AGA patch increased significantly at 3-month post-operatively, while those of the GP and AGP patches stayed minimal throughout the entire course of the study.

CONCLUSIONS

The results obtained in the study revealed that the AGP patch effectively repaired abdominal wall defects in rats and successfully prevented the formation of post-surgical abdominal adhesions.

In vivo and mechanical properties of peritoneum/fascia as a novel arterial substitute

OBJECTIVE

This study evaluates the efficacy of bovine peritoneum/fascia as an arterial substitute.

METHODS AND OUTCOME MEASURES

Twelve dogs underwent bilateral femoral artery patch angioplasty with a glutaraldehyde-fixed bovine peritoneal/fascial patch (PFA patch) on one side and polyester patch on the contralateral side. Arteriograms were performed just before vessel harvest at 1 and 6 months, and vessels were evaluated for aneurysms and inflammation. Histologic analysis included intima area, media thickness, and lumen area. Immunofluorescence for CD 34 and Factor VIII was done to evaluate endothelialization and alpha-actin for smooth muscle cell growth. Mechanical strength testing was evaluated in separate PFA patches and compared independently to a commercially available bovine pericardial patch and polyester patch.

RESULTS

All vessels examined at both 1 and 6 months were patent with no arteriographic evidence of stenosis. There was no evidence of aneurysm formation in any vessel and no difference between groups in inflammatory reaction. One polyester patch at 1 month developed an infection. Microscopic evaluation of experimental vessels revealed no difference between groups in intima area at 1 month (2.1 +/- 1.2 vs 2.2 +/- 1.2 mm 2 ; P = .5) and at 6 months (1.81 +/- 1.2 vs 1.9 +/- 1.2 mm 2 ; P = .5). There was no difference in media thickness, but the PFA patch group had a greater lumen area at 1 month (8.8 +/- 2.9 vs 9.8 +/- 3.0 mm 2 ; P = .02) and 6 months (10.5 +/- 4.2 vs 11.7 +/- 5.6 mm 2 ; P = .02). Immunofluorescence for CD34 and Factor VIII demonstrated complete re-endothelialization of all patches. The polyester patch had a chronic inflammatory response, but not the PFA patch. Mechanical strength testing demonstrated that compared to pericardium, the PFA patch had superior ( P < .05) failure tension, stiffness, and suture pull-out strength, whereas extensibility, fatigue tension, relax slope, and creep tests were not different. Polyester demonstrated superior suture pull-out, stiffness, relax slope, and failure strain ( P < .05), but it was not different in failure tension and extensibility than the PFA patch. However, the PFA patch had significantly less creep (0.25 +/- 0.25 vs 4.92 +/- 0.84; P < .01).

CONCLUSIONS

The PFA patch has similar clot-resistant properties to polyester and is superior to the pericardial patch in mechanical strength. It is a promising endothelial alternative for not only arterial patches but other vascular products.

CLINICAL RELEVANCE

The search for an artificial, thromboresistant, and intimal hyperplasia resistant interface between blood and native blood vessels still continues. This study demonstrates the feasibility and proof of concept of the peritoneum's clot-resistant properties. When adding the underlying fascia, it serves as an ideal arterial patch. Other studies are underway evaluating its feasibility as a bypass graft and a "drug coated"-like stent lining.

Biodegradable poly(ethylene glycol) hydrogels crosslinked with genipin for tissue engineering applications

In this study amino-terminated poly(ethylene glycol) (PEG-diamine) hydrogels were crosslinked with genipin, a chemical naturally derived from the gardenia fruit. Dissolution, swelling, and PEG-genipin release properties were determined. The dissolution studies indicated that the hydrogels are water soluble, and that the dissolution rate was concentration, mass, and temperature dependent. The dissolution rates are easily tailored from 3 min to >100 days. The PEG-genipin release study indicated that the greatest release occurs within the first 24 h of immersion in water, and that incubation at 37 degrees C elicits a greater initial release than samples incubated at room temperature for all genipin concentrations. Through scanning electron microscopy it was observed that the hydrogels are porous, and surface morphology changes before and after swelling. Furthermore, smooth muscle cell (SMC) adhesion studies indicated that the PEG-genipin hydrogel is a suitable substrate for SMC seeding. Overall, the results of these studies indicate that PEG-genipin hydrogels may provide potential scaffolding for a variety of tissue engineering applications.

Acellular bovine pericardia with distinct porous structures fixed with genipin as an extracellular matrix

A cell extraction process was employed to remove the cellular components from bovine pericardia. Various porous structures of the acellular tissues were then created, using acetic acid and collagenase, and subsequently fixed with genipin. The biological response and tissue regeneration pattern for each studied group were evaluated in a growing rat model. One month postoperatively, fibroblasts, neoconnective tissue fibrils, and neocapillaries were observed in the acellular, acetic acid-treated, and collagenase-treated tissues to fill the pores within the implanted samples, indicating that these tissue samples were being regenerated. The neoconnective tissue fibrils were identified to be neocollagen fibrils and neoglycosaminoglycans. On the other hand, no tissue regeneration was observed in the cellular tissue throughout the entire course of the study; tissue regeneration was limited to the outer most layer of the acellular tissue. In contrast, the areas of tissue regeneration in the acetic acid-treated and collagenase-treated tissues were expanded with increasing duration of implantation. However, 1 year postoperatively there were still numerous inflammatory cells observed in the acetic acid-treated tissue, whereas inflammatory cells in the collagenase-treated tissue had almost disappeared. These results indicated that tissue regeneration patterns within acellular tissues were significantly affected by their porous structures.

Effects of crosslinking degree of an acellular biological tissue on its tissue regeneration pattern

It was reported that acellular biological tissues can provide a natural microenvironment for host cell migration and may be used as a scaffold for tissue regeneration. To reduce antigenicity, biological tissues have to be fixed with a crosslinking agent before implantation. As a tissue-engineering scaffold, it is speculated that the crosslinking degree of an acellular tissue may affect its tissue regeneration pattern. In the study, a cell extraction process was employed to remove the cellular components from bovine pericardia. The acellular tissues then were fixed with genipin at various known concentrations to obtain varying degrees of crosslinking. It was shown in the in vitro degradation study that after fixing with genipin, the resistance against enzymatic degradation of the acellular tissue increased significantly with increasing its crosslinking degree. In the in vivo subcutaneous study, it was found that cells (inflammatory cells, fibroblasts, endothelial cells, and red blood cells) were able to infiltrate into acellular tissues. Generally, the depth of cell infiltration into the acellular tissue decreased with increasing its crosslinking degree. Infiltration of inflammatory cells was accompanied by degradation of the acellular tissue. Due to early degradation, no tissue regeneration was observed within fresh (without crosslinking) and the 30%-degree-crosslinking acellular tissues. This is because the scaffolds provided by these two samples were already completely degraded before the infiltrated cells began to secrete their own extracellular matrix. In contrast, tissue regeneration (fibroblasts, neo-collagen fibrils, and neo-capillaries) was observed for the 60%- and 95%-degree-crosslinking acellular tissues by the histological examination, immunohistological staining, transmission electron microscopy, and denaturation temperature measurement. The 95%-degree-crosslinking acellular tissue was more resistant against enzymatic degradation than its 60%-degree-crosslinking counterpart. Consequently, tissue regeneration was limited in the outer layer of the 95%-degree-crosslinking acellular tissue throughout the entire course of the study (1-year postoperatively), while tissue regeneration was observed within the entire sample for the 60%-degree-crosslinking acellular tissue. In conclusion, the crosslinking degree determines the degradation rate of the acellular tissue and its tissue regeneration pattern.

A genipin-crosslinked gelatin membrane as wound-dressing material: in vitro and in vivo studies

A naturally occurring crosslinking agent (genipin) was used in this study to crosslink gelatin hydrogel to develop a wound-dressing membrane. The study was to investigate the in vitro characteristics of the genipin-crosslinked gelatin membrane. The glutaraldehyde-crosslinked counterpart, at a similar crosslinking degree, was used as control. Additionally, an in vivo experiment was undertaken to study the wound healings covered with the glutaraldehyde- and genipin-crosslinked dressings in a rat model. The in vitro results obtained suggested that crosslinking of gelatin membranes with glutaraldehyde or genipin may produce distinct crosslinking structures. The differences in crosslinking structure can significantly affect the mechanical property, water-vapor-transmission rate, swelling ratio, degradation against enzyme and cellular compatibility of the crosslinked membranes. In the in vivo study, it was found that the degree of inflammatory reaction for the wound treated with the genipin-crosslinked dressing was significantly less severe than that covered with the glutaraldehyde-crosslinked dressing throughout the entire course of the study. Additionally, the healing rate for the wound treated with the genipin-crosslinked dressing was notably faster than its glutaraldehyde-crosslinked counterpart.

Biomimetic approaches to tendon repair

The linear organization of collagen fibers in tendons results in optimal stiffness and strength at low strains under tensile load. However, this organization makes repairing ruptured or lacerated tendons extremely difficult. Current suturing techniques to join split ends of tendons, while providing sufficient mechanical strength to prevent gapping, are inadequate to carry normal loads. Immobilization protocols necessary to restore tendon congruity result in scar formation at the repair site and peripheral adhesions that limit excursion. These problems are reviewed to emphasize the need for novel approaches to tendon repair, one of which is the development of biomimetic tendons. The objective of the empirical work described here was to produce biologically-based, biocompatible tendon replacements with appropriate mechanical properties to enable immediate mobilization following surgical repair. Nor-dihydroguaiaretic acid (NDGA), a di-catechol from creosote bush, caused a dose dependent increase in the material properties of reconstituted collagen fibers, achieving a 100-fold increase in strength and stiffness over untreated fibers. The maximum tensile strength of the optimized NDGA treated fibers averaged 90 MPa; the elastic modulus of these fibers averaged 580 MPa. These properties were independent of strain rates ranging from 0.60 to 600 mm/min. Fatigue tests established that neither strength nor stiffness were affected after 80 k cycles at 5% strain. Treated fibers were not cytotoxic to tendon fibroblasts. Fibroblasts attached and proliferated on NDGA treated collagen normally. NDGA-fibers did not elicit a foreign body response nor did they stimulate an immune reaction during six weeks in vivo. The fibers survived 6 weeks with little evidence of fragmentation or degradation. The polymerization scheme described here produces a fiber-reinforced NDGA-polymer with mechanical properties approaching an elastic solid. The strength, stiffness and fatigue properties of the NDGA-treated fibers are comparable to those of tendon. These fibers are biocompatible with tendon fibroblasts and elicit little rejection or antigenic response in vivo. These results indicate that NDGA polymerization may provide a viable approach for producing collagenous materials that can be used to bridge gaps in ruptured or lacerated tendons. The tendon-like properties of the NDGA-fiber would allow early mobilization after surgical repair. We predict that timely loading of parted tendons joined by this novel biomaterial will enhance mechanically driven production of neo-tendon by the colonizing fibroblasts and result in superior repair and rapid return to normal properties.

In vivo biocompatibility and degradability of a novel injectable-chitosan-based implant

A novel injectable-chitosan-based delivery system with low cytotoxicity was fabricated in the study. The chitosan microspheres with small particle size, low crystallinity and good sphericity were prepared by a spray-drying method followed by treating with a crosslinker. In the study, a naturally occurring crosslinking reagent (genipin), which has been used in herbal medicine and in the production of food dyes, was used to crosslink the chitosan microspheres. The glutaraldehyde-crosslinked counterparts were used as a control. Histological study of the genipin-crosslinked chitosan microspheres injected intramuscularly into the skeletal muscle of a rat model showed a less inflammatory reaction than its glutaraldehyde-crosslinked counterparts. The results of the scanning electron microscopic examination indicated that the glutaraldehyde-crosslinked chitosan microspheres retrieved at 12-week postoperatively were already degraded into a loose and porous structure. However, the degradation of the genipin-crosslinked chitosan microspheres was not significant after 20 weeks of implantation. The results of the study demonstrated that the genipin-crosslinked chitosan microspheres have a superior biocompatibility and a slower degradation rate than the glutaraldehyde-crosslinked chitosan microspheres. Accordingly, the genipin-crosslinked chitosan microspheres may be a suitable polymeric carrier for long-acting injectable drug delivery.

Biocompatibility of NDGA-polymerized collagen fibers. II. Attachment, proliferation, and migration of tendon fibroblasts in vitro

The material properties of collagen fibers polymerized with nordihydroguaiaretic acid (NDGA) are equivalent to native tendon, suggesting that NDGA crosslinking may provide a viable approach to stabilizing collagenous materials for use in repairing ruptured, lacerated, or surgically transected fibrous tissues, such as tendons and ligaments (Koob & Hernandez, Biomaterials, in press). The present study evaluated the biocompatibility of these fibers with cultured bovine tendon fibroblasts. Fibroblast attachment, migration, and proliferation on NDGA-crosslinked materials were compared to those on prepolymerized type I tendon collagen constructs as well as on tissue-culture-treated plastic. Fibroblast attachment on NDGA-crosslinked collagen fibrils was equivalent to attachment on plates coated with collagen alone. Over a period of 8 days in culture, attached fibroblasts proliferated on NDGA-crosslinked collagen at a rate identical to that of fibroblasts attached to native collagen. In order for the biomaterial effectively to bridge gaps in fibrous tissues, fibroblasts must be able to migrate and replicate on the bridging fiber. Control and crosslinked fibers were inserted in calf tendon explants, with a portion of the fiber extending out of the sectioned end of the tendon. Explants were cultured for 9 weeks, and the number of cells was measured at weekly intervals. Cells appeared on the fibers after 1 week of culture. By 2 weeks, cells had colonized the entire fiber. The number of cells continued to increase throughout the 9 weeks in culture, forming a layer several cells thick. Histologic analysis indicated that the fibroblasts populating the fibers appeared to originate in the epitenon. There was no difference in the rate of fibroblast migration and replication, nor in the ultimate number of colonizing cells, between control collagen fibers and NDGA-crosslinked fibers. NDGA-crosslinked fibers may provide a means of bridging gaps in ruptured, lacerated, or surgically transected tendons by providing a mechanically competent scaffold on which tendon fibroblasts can migrate, attach, and proliferate.