Design, Manufacture, and In vivo Testing of a Tissue Scaffold for Permanent Female Sterilization by Tubal Occlusion

Tensile properties of freeze-cast collagen scaffolds: How processing conditions affect structure and performance in the dry and fully hydrated states

Tensile properties of directionally freeze-cast biopolymer scaffolds are rarely reported, even though they are of interest from a fundamental science perspective and critical in applications such as scaffolds for the regeneration of nerves or when used as ureteral stents. The focus of this study is on collagen scaffolds freeze-cast with two different applied cooling rates (10 °C/min and 1 °C/min) in two freezing directions (longitudinal and radial). Reported are the results of a systematic structural characterization of dry scaffolds by scanning electron microscopy and the mechanical characterization in tension of both dry and fully hydrated scaffolds. Systematic structure-property-processing correlations are obtained for a comparison of the tensile performance of longitudinally and radially freeze-cast collagen scaffolds with their performance in compression. Collated, the correlations, obtained both in tension in this study and in compression for collagen and chitosan in two earlier reports, not only enable the custom-design of freeze-cast biopolymer scaffolds for biomedical applications but also provide new insights into similarities and differences of scaffold and cell-wall structure formation during the directional solidification of "smooth" and "fibrillar" biopolymers.

Use of n-Butyl-2-Cyanoacrylate for Fallopian Tube Embolization via Selective Catheterization in a Rabbit Model: Feasibility Study for Potential Nonsurgical Sterilization

PURPOSE

To determine whether fallopian tube embolization with n-butyl-2-cyanoacrylate (nBCA) administered via a microcatheter in a rabbit model was technically feasible and resulted in short-term tubal occlusion.

MATERIALS AND METHODS

In 10 female New Zealand white rabbits, the 2 cervices were cannulated using a 5-F catheter and hydrophilic guide wire transvaginally. Salpingography confirmed tubal patency bilaterally. A 2.4-F microcatheter was advanced to the distal fallopian tube, and nBCA/ethiodized oil was administered as the microcatheter was withdrawn to fill the length of the tube. A metallic coil was deployed prior to nBCA administration in half of the fallopian tubes. Rabbits were evaluated for tubal occlusion with salpingography at 1 month, followed by euthanasia and histopathologic analysis. Inflammation and fibrosis were graded from 0 (normal) to 3 (severe).

RESULTS

Fallopian tube embolization was technically successful in 17 (85%) of 20 fallopian tubes. Thirteen (76%) of 17 embolized fallopian tubes were occluded at 1 month on salpingography (nBCA only, 7/9; nBCA and coil, 6/8). On histopathologic analysis, direct or indirect evidence of occlusion was observed in 14 (82%) of 17 fallopian tubes. Mild or early fibrosis was observed in 65% of the tubes. The mean inflammation and fibrosis scores for the embolized tubes were 0.62 and 0.94, respectively.

CONCLUSIONS

This pilot study demonstrated that embolization of rabbit fallopian tubes using nBCA administered via a microcatheter is technically feasible and results in occlusion of most fallopian tubes in the short term with minimal inflammation. Investigation of efficacy in preventing pregnancy over the long term is warranted.

Promotion of right ventricular outflow tract reconstruction using a novel cardiac patch incorporated with hypoxia-pretreated urine-derived stem cells

Approximately 25% of patients with congenital heart disease require implantation of patches to repair. However, most of the currently available patches are made of inert materials with unmatched electrical conductivity and mechanical properties, which may lead to an increased risk for arrhythmia and heart failure. In this study, we have developed a novel Polyurethane/Small intestinal submucosa patch (PSP) with mechanical and electrical properties similar to those of the native myocardial tissue, and assessed its feasibility for the reconstruction of right ventricular outflow tract. A right ventricular outflow tract reconstruction model was constructed in 40 rabbits. Compared with commercially available bovine pericardium patch, the PSP patch has shown better histocompatibility and biodegradability, in addition with significantly improved cardiac function. To tackle the significant fibrosis and relatively poor vascularization during tissue remodeling, we have further developed a bioactive patch by incorporating the PSP composites with urine-derived stem cells (USCs) which were pretreated with hypoxia. The results showed that the hypoxia-pretreated bioactive patch could significantly inhibit fibrosis and promote vascularization and muscularization, resulting in better right heart function. Our findings suggested that the PSP patch combined with hypoxia-pretreated USCs may provide a better strategy for the treatment of congenital heart disease.

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A novel cardiac patch (PSP) with mechanical and electrical properties similar to native myocardium.

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PSP patch improved cardiac function in right ventricular outflow tract reconstruction model.

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Hypoxia pretreated USCs combined PSP patch promoted vascularization and inhibited fibrosis.

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

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

Structure-property-processing correlations of longitudinal freeze-cast chitosan scaffolds for biomedical applications

Needed for the custom-design of longitudinally freeze-cast chitosan scaffolds for biomedical applications are systematic structure-property-processing correlations. Combining mechanical testing in compression with both scanning electron microscopy and semiautomated confocal microscopy for a quantitative structural characterization of fully hydrated chitosan scaffolds, robust correlations were determined. Decreasing the applied cooling rate from 10 °C/min to 0.1 °C/min, the short and long axes of the pore cross-sections, the pore aspect ratio, and the pore area were found to increase from 68.0 μm to 120.5 μm, from 189.2 μm to 401.2 μm, from 2.64 to 3.52, and from 8,922 μm² to 35,596 μm², respectively. Values for the scaffolds' modulus, yield strength, and toughness range from 1,067 kPa to 3,209 kPa, from 37.7 kPa to 75.5 kPa, and from 20.3 kJ/m³ to 35.3 kJ/m³, respectively. Because of additional structural features, such as cell wall stiffening ridges, affecting the mechanical properties, not linear but more complex correlation with modulus, yield strength, and toughness were observed. Contrasting the results of this study with those obtained in an earlier study of dry and fully hydrated collagen scaffolds, we were able to identify features that are important and peculiar to each material system. Highlighted in this study are newly determined robust structure-property-processing correlations as well as processing conditions and features that are critical for the mechanical performance of chitosan and other biopolymer scaffolds made by freeze casting for biomedical applications.

Quantitative evaluation of the in vivo biocompatibility and performance of freeze-cast tissue scaffolds

Quantitative methods are little used for the in vivo assessment of tissue scaffolds to evaluate biocompatibility. To complement current histological techniques, we introduce as a measure of biocompatibility a straightforward, geometric analysis for the quantitative assessment of encapsulation thickness, cross-sectional area, and biomaterial shape. Advantages of this new technique are that it enables, on the one hand, a more complete and objective comparison of scaffolds with differing compositions, architectures, and mechanical properties, and, on the other, a more objective approach to their selection for a given application. In this contribution, we focus on freeze-cast polymeric scaffolds for tissue regeneration and their subcutaneous implantation in mice for biocompatibility testing. Initially, seven different scaffold types are screened. Of these, three are selected for systematic biocompatibility studies based on histopathological criteria: EDC-NHS-crosslinked bovine collagen, EDC-NHS-crosslinked bovine collagen-nanocellulose, and chitin. Geometric models developed to quantify scaffold size, ovalization, and encapsulation thickness are tested, evaluated, and found to be a powerful and objective metric for the in vivo assessment of biocompatibility and performance of tissue scaffolds.

A review of multifunctional nacre-mimetic materials based on bidirectional freeze casting

Nacre has achieved an excellent combination of strength and toughness through its unique brick-and-mortar structure of layered aragonite platelets bonded with biopolymers. Mimicking nacre has been considered as a practical way for the development of high-performance structural composites. Over the past years, many techniques have been developed to fabricate multifunctional nacre-mimetic materials, including freeze casting, layer-by-layer assembly, vacuum filtration, 3D printing and so on. Among them, freeze casting, especially bidirectional freeze casting, as an environmentally friendly and scalable method, has attracted extensive attention recently. In this review, we begin with the introduction and discussion of various fabrication techniques comparing their advantages and disadvantages, focusing on the most recent advances of the bidirectional freeze casting technique. Then, we summarize representative examples of applying the bidirectional freeze casting technique to assemble various building blocks into multifunctional nacre-mimetic materials and their wide applications. At the end, we discuss the future direction of using bidirectional freeze casting to make nacre-mimetic materials.

Multiscale Toughening Mechanisms in Biological Materials and Bioinspired Designs

Biological materials found in Nature such as nacre and bone are well recognized as light-weight, strong, and tough structural materials. The remarkable toughness and damage tolerance of such biological materials are conferred through hierarchical assembly of their multiscale (i.e., atomic- to macroscale) architectures and components. Herein, the toughening mechanisms of different organisms at multilength scales are identified and summarized: macromolecular deformation, chemical bond breakage, and biomineral crystal imperfections at the atomic scale; biopolymer fibril reconfiguration/deformation and biomineral nanoparticle/nanoplatelet/nanorod translation, and crack reorientation at the nanoscale; crack deflection and twisting by characteristic features such as tubules and lamellae at the microscale; and structure and morphology optimization at the macroscale. In addition, the actual loading conditions of the natural organisms are different, leading to energy dissipation occurring at different time scales. These toughening mechanisms are further illustrated by comparing the experimental results with computational modeling. Modeling methods at different length and time scales are reviewed. Examples of biomimetic designs that realize the multiscale toughening mechanisms in engineering materials are introduced. Indeed, there is still plenty of room mimicking the strong and tough biological designs at the multilength and time scale in Nature.

Values and property charts for anisotropic freeze-cast collagen scaffolds for tissue regeneration

Presented in this article are systematic microstructural and mechanical property data for anisotropic collagen scaffolds made by freeze casting. Three applied cooling rates (10 °C/min, 1 °C/min, 0.1 °C/min) and two freezing directions (longitudinal and radial) were used during scaffold manufacture. Utilizing a semi-automated image analysis technique applied to confocal micrographs of fully hydrated scaffolds, pore area, long and short pore axes, and pore aspect ratio were determined. Compression testing was performed to determine scaffold modulus, yield strength, and toughness.

Freeze-casting porous chitosan ureteral stents for improved drainage

As a new strategy for improved urinary drainage, in parallel to the potential for additional functions such as drug release and self-removal, highly porous chitosan stents are manufactured by radial, bi-directional freeze-casting. Inserting the porous stent in to a silicone tube to emulate its placement in the ureter shows that it is shape conforming and remains safely positioned in place, also during flow tests, including those performed in a peristaltic pump. Cyclic compression tests on fully-hydrated porous stents reveal high stent resilience and close to full elastic recovery upon unloading. The drainage performance of the chitosan stent is evaluated, using effective viscosity in addition to volumetric flow and flux; the porous stent's performance is compared to that of the straight portion of a commercial 8 Fr double-J stent which possesses, in its otherwise solid tube wall, regularly spaced holes along its length. Both the porous and the 8 Fr stent show higher effective viscosities, when tested in the silicone tube. The performance of the porous stent improves considerably more (47.5%) than that of the 8 Fr stent (30.6%) upon removal from the tube, illustrating the effectiveness of the radially aligned porosity for drainage. We conclude that the newly-developed porous chitosan ureteral stent merits further in vitro and in vivo assessment of its promise as an alternative and complement to currently available medical devices. STATEMENT OF SIGNIFICANCE: No papers, to date, report on porous ureteral stents, which we propose as a new strategy for improved urinary drainage. The highly porous chitosan stents of our study are manufactured by radial, bi-directional freeze casting. Cyclic compression tests on fully-hydrated porous stents revealed high stent resilience and close to full recovery upon unloading. The drainage performance of the chitosan is evaluated, using effective viscosity in addition to volumetric flow and flux, and compared to that of the straight portion of a commercial 8 Fr double-J stent. The performance of the porous stent improves considerably more (47.5%) than that of the 8 Fr stent (30.6%) upon removal from the tube, illustrating the effectiveness of the radially aligned porosity for drainage. While further studies are required to explore other potential benefits of the porous stent design such as antimicrobial behavior, drug release, and biodegradability, we conclude that the newly-developed porous chitosan ureteral stent has considerable potential as a medical device.

Anisotropic freeze-cast collagen scaffolds for tissue regeneration: How processing conditions affect structure and properties in the dry and fully hydrated states

Few systematic structure-property-processing correlations for directionally freeze-cast biopolymer scaffolds are reported. Such correlations are critical to enable scaffold design with attractive structural and mechanical cues in vivo. This study focuses on freeze-cast collagen scaffolds with three different applied cooling rates (10, 1, and 0.1 °C/min) and two freezing directions (longitudinal and radial). A semi-automated approach for the structural characterization of fully hydrated scaffolds by confocal microscopy is developed to facilitate an objective quantification and comparison of structural features. Additionally, scanning electron microscopy and compression testing are performed longitudinally and transversely. Structural and mechanical properties are determined on dry and fully hydrated scaffolds. Longitudinally frozen scaffolds have aligned and regular pores while those in radially frozen ones exhibit greater variations in pore geometry and alignment. Lamellar spacing, pore area, and cell wall thickness increase with decreasing cooling rate: in longitudinally frozen scaffolds from 25 µm to 83.5 µm, from 814 µm² to 8452 µm², and from 4.21 µm to 10.4 µm, and in radially frozen ones, from 69 µm to 116 µm, from 7679 µm² to 25,670 µm², and from 6.18 µm to 13.6 µm, respectively. Both longitudinally and radially frozen scaffolds possess higher mechanical property values, when loaded parallel rather than perpendicular to the ice-crystal growth direction. Modulus and yield strength range from 779 kPa to 4700 kPa and from 38 kPa to 137 kPa, respectively, as a function of cooling rate and freezing direction. Collated, the correlations obtained in this study enable the custom-design of freeze-cast collagen scaffolds, which are ideally suited for a large variety of tissue regeneration applications.