Preparation and Characterization of a Novel Decellularized Fibrocartilage “Book” Scaffold for Use in Tissue Engineering

At the tendon-to-bone insertion, there is a unique transitional structure: tendon, non-calcified fibrocartilage, calcified fibrocartilage, and bone. The reconstruction of this special graded structure after defects or damage is an important but challenging task in orthopedics. In particular, reconstruction of the fibrocartilage zone has yet to be successfully achieved. In this study, the development of a novel book-shape scaffold derived from the extracellular matrix of fibrocartilage was reported. Specifically, fibrocartilage from the pubic symphysis was obtained from rabbits and sliced into the shape of a book (dimensions: 10 mm × 3 mm × 1 mm) with 10 layers, each layer (akin to a page of a book) with a thickness of 100-μm. These fibrocartilage “book” scaffolds were decellularized using sequentially 3 freeze-thaw cycles, 0.1% Triton X-100 with 1.5 M KCl, 0.25% trypsin, and a nuclease. Histology and DNA quantification analysis confirmed substantial removal of cells from the fibrocartilage scaffolds. Furthermore, the quantities of DNA, collagen, and glycosaminoglycan in the fibrocartilage were markedly reduced following decellularization. Scanning electron microscopy confirmed that the intrinsic ultrastructure of the fibrocartilage tissue was well preserved. Therefore, the results of this study suggest that the novel “book” fibrocartilage scaffold could have potential applications in tissue engineering.     

Trehalose Maintains Bioactivity and Promotes Sustained Release of BMP-2 from Lyophilized CDHA Scaffolds for Enhanced Osteogenesis In Vitro and In Vivo

Calcium phosphate (Ca-P) scaffolds have been widely employed as a supportive matrix and delivery system for bone tissue engineering. Previous studies using osteoinductive growth factors loaded Ca-P scaffolds via passive adsorption often experience issues associated with easy inactivation and uncontrolled release. In present study, a new delivery system was fabricated using bone morphogenetic protein-2 (BMP-2) loaded calcium-deficient hydroxyapatite (CDHA) scaffold by lyophilization with addition of trehalose. The in vitro osteogenesis effects of this formulation were compared with lyophilized BMP-2/CDHA construct without trehalose and absorbed BMP-2/CDHA constructs with or without trehalose. The release characteristics and alkaline phosphatase (ALP) activity analyses showed that addition of trehalose could sufficiently protect BMP-2 bioactivity during lyophilization and achieve sustained BMP-2 release from lyophilized CDHA construct in vitro and in vivo. However, absorbed BMP-2/CDHA constructs with or without trehalose showed similar BMP-2 bioactivity and presented a burst release. Quantitative real-time PCR (RT-qPCR) and enzyme-linked immunosorbent assay (ELISA) demonstrated that lyophilized BMP-2/CDHA construct with trehalose (lyo-tre-BMP-2) promoted osteogenic differentiation of bone marrow stromal cells (bMSCs) significantly and this formulation could preserve over 70% protein bioactivity after 5 weeks storage at 25°C. Micro-computed tomography, histological and fluorescent labeling analyses further demonstrated that lyo-tre-BMP-2 formulation combined with bMSCs led to the most percentage of new bone volume (38.79% ±5.32%) and area (40.71% ±7.14%) as well as the most percentage of fluorochrome stained bone area (alizarin red S: 2.64% ±0.44%, calcein: 6.08% ±1.37%) and mineral apposition rate (4.13±0.62 µm/day) in critical-sized rat cranial defects healing. Biomechanical tests also indicated the maximum stiffness (118.17±15.02 Mpa) and load of fracture (144.67±16.13 N). These results lay a potential framework for future study by using trehalose to preserve growth factor bioactivity and optimize release profile of Ca-P based delivery system for enhanced bone regeneration.     

Three Dimensional Collagen Scaffold Promotes Intrinsic Vascularisation for Tissue Engineering Applications

Here, we describe a porous 3-dimensional collagen scaffold material that supports capillary formation in vitro, and promotes vascularization when implanted in vivo. Collagen scaffolds were synthesized from type I bovine collagen and have a uniform pore size of 80 μm. In vitro, scaffolds seeded with primary human microvascular endothelial cells suspended in human fibrin gel formed CD31 positive capillary-like structures with clear lumens. In vivo, after subcutaneous implantation in mice, cell-free collagen scaffolds were vascularized by host neovessels, whilst a gradual degradation of the scaffold material occurred over 8 weeks. Collagen scaffolds, impregnated with human fibrinogen gel, were implanted subcutaneously inside a chamber enclosing the femoral vessels in rats. Angiogenic sprouts from the femoral vessels invaded throughout the scaffolds and these degraded completely after 4 weeks. Vascular volume of the resulting constructs was greater than the vascular volume of constructs from chambers implanted with fibrinogen gel alone (42.7±5.0 μL in collagen scaffold vs 22.5±2.3 μL in fibrinogen gel alone; p<0.05, n = 7). In the same model, collagen scaffolds seeded with human adipose-derived stem cells (ASCs) produced greater increases in vascular volume than did cell-free collagen scaffolds (42.9±4.0 μL in collagen scaffold with human ASCs vs 25.7±1.9 μL in collagen scaffold alone; p<0.05, n = 4). In summary, these collagen scaffolds are biocompatible and could be used to grow more robust vascularized tissue engineering grafts with improved the survival of implanted cells. Such scaffolds could also be used as an assay model for studies on angiogenesis, 3-dimensional cell culture, and delivery of growth factors and cells in vivo.     

Understanding the effect of mean pore size on cell activity in collagen-glycosaminoglycan scaffolds

Mean pore size is an essential aspect of scaffolds for tissue-engineering. If pores are too small cells cannot migrate in towards the center of the construct limiting the diffusion of nutrients and removal of waste products. Conversely, if pores are too large there is a decrease in specific surface area available limiting cell attachment. However the relationship between scaffold pore size and cell activity is poorly understood and as a result there are conflicting reports within the literature on the optimal pore size required for successful tissue-engineering. Previous studies in bone tissue-engineering have indicated a range of mean pore sizes (96–150 µm) to facilitate optimal attachment. Other studies have shown a need for large pores (300–800 µm) for successful bone growth in scaffolds. These conflicting results indicate that a balance must be established between obtaining optimal cell attachment and facilitating bone growth. In this commentary we discuss our recent investigations into the effect of mean pore size in collagen-glycosaminoglycan (CG) scaffolds with pore sizes ranging from 85–325 µm and how it has provided an insight into the divergence within the literature.Key words: bone tissue engineering, cell adhesion, collagen, extracellular matrix, pore size, scaffoldThe goal of tissue engineering is to develop cell, construct and living system technologies to restore the structure and functional mechanical properties of damaged or degenerated tissue. While the field of tissue engineering may be relatively new, the idea of replacing tissue with another goes as far back as the 16^th century when an Italian, Gasparo Tagliacozzi (1546–99), Professor of Surgery and Anatomy at the Bologna University, described a nose replacement that he had constructed from a forearm flap in his work “De Custorum Chirurigia per Insitionem” (The Surgery of Defects by Implantation) which was published in 1597. In modern times, the techniques of transplanting tissue from one site to another in the same patient (an autograft) or from one individual to another (transplant or allograft) have been revolutionary and lifesaving. However major problems exist with both techniques. Harvesting autografts is expensive, painful, constrained by anatomical limitations and associated with donor-site morbidity due to infection and hemorrhage. Transplants have serious constraints. The major problem is accessing enough tissue and organs for all of the patients who require them. Transplants are strongly associated with rejection by the patient''s immune system and they are also limited by the potential risks of introducing infection or disease.Tissue engineering was born from the belief that primary cells could be isolated from a patient, expanded in vitro and seeded onto a substrate that could be grafted back into the patient.¹ It provides a biological alternative to transplantations and prosthesis. One of the first scaffolds pioneered for tissue regeneration was synthesized as a graft co-polymer of type I collagen and chondroitin 6-sulphate, a glycosaminoglycan. The development of these scaffolds, which are capable of supporting tissue synthesis when seeded with cells, marks the beginning of the field of tissue engineering.^2,3 Since this early work, there have been rapid advances in bone tissue engineering with the development of porous, biocompatible, three-dimensional scaffolds. Regardless of the application, the scaffold should be biocompatible and imitate both the physical and biological function of the native extracellular matrix (ECM), as the ECM provides a substrate with specific ligands for cell adhesion as well as physical support for cells.⁴ When designing scaffolds for any tissue engineering application, a major consideration is the mean pore size. Scaffolds must be permeable with interconnecting pores to facilitate cell growth, migration and nutrient flow. A previous study demonstrated that permeability increases with increasing pore size due to a reduction in specific surface area.⁵ If pores are too small, cell migration is limited, resulting in the formation of a cellular capsule around the edges of the scaffold. This in turn can limit the distribution of nutrients and removal of waste products resulting in necrotic regions within the construct. Conversely if pores are too large there is a decrease in specific surface area.³ It has been proposed that a reduction in specific surface area reduces the ligand density available for cells to bind to.⁶ Cellular activity is influenced by specific integrin-ligand interactions between cells and surrounding ECM. Initial cell adhesion mediates all subsequent events such as proliferation, migration and differentiation within the scaffold. As a result the mean pore size within a scaffold affects cell adhesion and ensuing proliferation, migration and infiltration. Therefore maintaining a balance between the optimal pore size for cell migration and specific surface area for cell attachment is essential.^4,7In our laboratory we use a composite scaffold fabricated from collagen and a glycosaminoglycan (GAG) for bone tissue engineering applications produced by a lyophilisation (freeze-drying) fabrication process. The first generation of this collagen-GAG (CG) scaffold was originally developed for skin regeneration but has since been applied to a number of other tissue engineering applications, due to its high biological activity and resultant ability to promote cell growth and tissue development.^2,8–12 Originally CG scaffolds were fabricated using a rapid uncontrolled quench process during lyophilisation which resulted in heterogeneous porous scaffolds with a large variation of pore size within certain areas of the scaffold.² When these scaffolds were used in previous studies they were visually examined so that the areas of variation could be avoided resulting in subjective selection of scaffold samples for analysis.⁸ However, an improved lyophilisation technique was later developed which incorporated a constant cooling rate which controlled the formation and growth of ice-crystals thus resulting in CG scaffolds with homogenous pore structures.¹³ The traditional final temperature of freezing used to produce these scaffolds is −40°C; however, further modifications to the lyophilisation process demonstrated that by changing the final temperature of freezing, it is possible to tailor the mean pore size in the scaffolds. This study showed that by varying the temperature of freezing from −40 to −10°C it was possible to produce homogenous CG scaffolds with mean pore sizes ranging from 96–151 µm.⁶A cellular solid is one made up of an interconnecting porous network and cellular solids modeling techniques can be used to describe both mechanical and microstructural (i.e., specific surface area) properties of scaffolds. A cellular solids model utilizing a tetrakaidecahedral unit cell (a 14-sided polyhedron that packs to fill space) was used to determine the effect of mean pore size on specific surface area. Specific surface area can be related to the relative density of a scaffold and using a tetrakaidecahedral unit cell it was possible to model the geometry of the CG scaffolds.^5,6,14 As a result the specific surface area (SA) per unit volume (V) available for cell adhesion in each of the scaffolds with different mean pore sizes (d) was estimated as: SA/V = 0.718/d(1)This relationship demonstrates that the specific surface area is inversely proportional to the mean pore size. The authors then carried out a simple experiment and seeded the scaffold range with osteoblasts and monitored initial cell adhesion up to 48 h post-seeding. Cell adhesion is the binding of cells to their extracellular environment via specific ligand-integrin interactions. The results demonstrated that cell adhesion decreased with increasing pore size and that the highest levels of cell attachment were found on the scaffolds with the smallest pore size (96 µm). The rationale for this result, as suggested by the authors, was the effect of specific surface area on cell adhesion due to the scaffolds with larger pores having less available specific surface area and thus a lower ligand density for initial cell attachment.^5,6The results of this study conflicted with other studies within the literature which demonstrate a need for larger pores. The relationship between scaffold pore size and cell activity is not fully understood and as a result, over the years there have been conflicting reports on the optimal pore size required for bone tissue engineering. Pores ranging from 20–1,500 µm have been used in bone tissue engineering applications.^15–18 Initial studies demonstrated that the minimum pore size for significant bone growth is 75–100 µm with an optimal range of 100–135 µm.^15,19 Since this early work it has been reported that pores greater than ∼300 µm are essential for vascularisation of constructs and bone ingrowth, while pores smaller than ∼300 µm can encourage osteochondral ossification.^20–22In a very recent study in our laboratory, which utilized improved technical capability of our freeze-drying system and introduced a novel annealing step during lyophilisation, we have been able to further expand the range of mean pore sizes produced in the CG scaffolds from 96–151 µm up to 85–325 µm.²³ We then investigated the effect of this new expanded range of scaffolds on initial cell attachment followed by migration and proliferation by monitoring cellular activity up to 7 days post-seeding (as opposed to 48 h in the earlier study⁶) to see whether the pattern of specific surface area affecting initial cell adhesion as seen in the previous studies would continue as cells proliferated.²⁴The results provide a possible insight into why there are conflicting reports in the literature on the optimal scaffold pore size for bone tissue engineering. A non-linear effect of pore size was seen on cell proliferation over the 7 day incubation period. Scaffolds with the largest pore size of 325 µm facilitated higher cell number at all time points in comparison to the other scaffold types. However, within the lower range of pore sizes there was a small peak in cell number at 24 h and 48 h post-seeding in scaffolds with a mean pore size of 120 µm. This peak disappeared by day 7 (Fig. 1). This peak is consistent with that seen in the earlier study⁶ and can therefore be explained by the effect of pore surface area on cell attachment. Collagen, a natural component of bone ECM, contains binding sites (ligands) that are recognized by specific cell surface receptors (integrins), the main collagen integrins being α₁β₁ and α₂β₁. Based on the interactions between integrins and their corresponding ligands, cells can detect subtle changes in ECM that can influence cell attachment and consequently determine cell proliferation, speed and migration. Our results reflected this within the smaller pore range (85–190 µm) when cell number was presented as a percentage of the cells seeded onto the scaffolds,²⁴ indicating that high specific surface area in scaffolds is important for optimal cell attachment. However, when this range of pore sizes was expanded (85–325 µm) the linear relationship between mean pore size and specific surface area was no longer applicable (Fig. 1) and scaffolds with the largest pores showed the highest cell numbers even though the surface area is lower than that for the other scaffold variants. We propose that the effect of specific surface area is overcome in larger pores by the improved potential for cell migration and proliferation as was seen histologically in scaffolds with 325 µm.Open in a separate windowFigure 1Effect of mean pore size on cell number at each time point. Cell number increases to a small peak 24 h post seeding in scaffolds with a pore size of 120 µm. This peak declines at later time points. Cell number significantly peaks in scaffolds with a mean pore size of 325 µm. *p < 0.001 (reviewed in ref. ²⁴).When seeding three-dimensional scaffolds it is desirable that the cells infiltrate and colonize the scaffold laying down their own ECM. The CG scaffolds are highly porous (∼99%)⁵ and it has previously been shown that cell migration behavior decreases with increasing pore size.²⁶ However, similarly to other studies,⁶ these results were based on limited range of mean pore sizes incubated for less than 48 h. In this study, migration of cells was assessed histologically after 7 days incubation. Cells were observed lining the pores in all scaffolds. However, cell aggregations were seen along the edges of the scaffolds with smaller pore sizes of 85 µm–120 µm limiting the number of cells infiltrating the scaffold (Fig. 2A). Cell aggregations form a “skin” around the outer surface of the scaffold which restricts the diffusion of nutrients and removal of waste from the cells colonizing the center of the scaffold. As the mean pore size increased, cells migrated further away from the edges and in towards the center of the scaffold until cells were seen colonizing the center of the scaffolds with the largest mean pore size of 325 µm (Fig. 2B). An increase in cell number was seen in 120 µm pore size, but the aggregations seen on the surface of these scaffolds compound the hypothesis that this peak was related to initial cell adhesion and the advantages of this pore size were lost with subsequent cell proliferation and migration.Open in a separate windowFigure 2Effect of mean pore size on cell infiltration and distribution CG scaffolds after 7 days. Scaffolds were stained with H&E: (A) 85 µm pore size at x40 magnification, (B) 325 µm pore size at ×40 magnification. Collagen scaffold is stained pink and cell nuclei a deep purple. The arrow indicates cell aggregations along the edges of the scaffold. Aggregations disappeared and cell migration increased with increasing pore size (reviewed in ref. ²⁴).The study²⁴ had a number of limitations. It was not possible to determine the upper pore size limit for cell activity within a CG scaffold. If the pores become too large the mechanical properties of the scaffold will be compromised due to void volume⁷ and as pore size increases further, the specific surface area will eventually reduce to a level that will limit cell adhesion. Furthermore, this study has determined the optimal pore size for MC3T3-E1 pre-osteoblast activity. It has been hypothesised that the optimal pore size will vary with different cell types⁶ and another recent study from our laboratory has demonstrated that mesenchymal stem cells seeded on the smaller range of CG scaffolds and maintained in osteogenic culture for 3 weeks showed improved osteogenesis on the scaffolds with bigger pores²⁵. For this reason it is important to repeat this study with different cell types. However, regardless of these limitations, this paper has demonstrated that mean pore size does affect cell behavior within a scaffold and that subtle changes in pore size can have a significant effect on cell behavior. We also provide an insight into why the literature reports conflicting results on the optimal pore size required for bone tissue engineering, whereby increased specific surface area provided by scaffolds with small pores has a benefi- cial effect on initial cell attachment, but this is overcome by the improved cellular infiltration provided by scaffolds with larger pores suggesting that these scaffolds might be optimal for longer term in vitro culture with the aim of facilitating bone tissue repair.     

3D Differentiation of Neural Stem Cells in Macroporous Photopolymerizable Hydrogel Scaffolds

Neural stem/progenitor cells (NSPCs) are the stem cell of the adult central nervous system (CNS). These cells are able to differentiate into the major cell types found in the CNS (neurons, oligodendrocytes, astrocytes), thus NSPCs are the mechanism by which the adult CNS could potentially regenerate after injury or disorder. Microenviromental factors are critical for guiding NSPC differentiation and are thus important for neural tissue engineering. In this study, D-mannitol crystals were mixed with photocrosslinkable methacrylamide chitosan (MAC) as a porogen to enhance pore size during hydrogel formation. D-mannitol was admixed to MAC at 5, 10 and 20 wt% D-mannitol per total initial hydrogel weight. D-mannitol crystals were observed to dissolve and leave the scaffold within 1 hr. Quantification of resulting average pore sizes showed that D-mannitol addition resulted in larger average pore size (5 wt%, 4060±160 µm², 10 wt%, 6330±1160 µm², 20 wt%, 7600±1550 µm²) compared with controls (0 wt%, 3150±220 µm²). Oxygen diffusion studies demonstrated that larger average pore area resulted in enhanced oxygen diffusion through scaffolds. Finally, the differentiation responses of NSPCs to phenotypic differentiation conditions were studied for neurons, astrocytes and oligodendrocytes in hydrogels of varied porosity over 14 d. Quantification of total cell numbers at day 7 and 14, showed that cell numbers decreased with increased porosity and over the length of the culture. At day 14 immunohistochemistry quantification for primary cell types demonstrated significant differentiation to the desired cells types, and that total percentages of each cell type was greatest when scaffolds were more porous. These results suggest that larger pore sizes in MAC hydrogels effectively promote NSPC 3D differentiation.     

Systematic investigation of porogen size and content on scaffold morphometric parameters and properties

A systematic investigation of tissue engineering scaffolds prepared by salt leaching of a photopolymerized dimethacrylate was performed to determine how the scaffold structure (porosity, pore size, etc.) can be controlled and also to determine how the scaffold structure and the mechanical properties are related. Two series of scaffolds were prepared with (1) the same polymer-to-salt ratio but different salt sizes (ranging from average size of 100 to 390 microm) and (2) the same salt size but different polymer-to-salt ratios (ranging from salt mass of 70 to 90%). These scaffolds were examined to determine how the fabrication parameters affected the scaffold morphometric parameters and corresponding mechanical properties. Combined techniques of X-ray microcomputed tomography (microCT), mercury porosimetry, and gravimetric analysis were used to determine the scaffold parameters, such as porosity, pore size, and strut thickness and their size distributions, and pore interconnectivity. Scaffolds with porosities ranging from 57% to 92% (by volume) with interconnected structures could be fabricated using the current technique. The porosity and strut thickness were subsequently related to the mechanical response of the scaffolds, both of which contribute to the compression modulus of the scaffold. The current study shows that the structure and properties of the scaffold could be tailored by the size and the amount of porogen used in the fabrication of the scaffold.     

Anti-biofouling 3D porous systems: the blend effect of oxazoline-based oligomers on chitosan scaffolds

The production, characterization and anti-biofouling activity of 3D porous scaffolds combining different blends of chitosan and oxazoline-based antimicrobial oligomers is reported. The incorporation of ammonium quaternized oligo(2-oxazoline)s into the composition of the scaffold enhances the stability of the chitosan scaffold under physiological conditions as well as its ability to repel protein adsorption. The blended scaffolds showed mean pore sizes in the range of 18–32?μm, a good pore interconnectivity and high porosity, as well as a large surface area, ultimate key features for anti-biofouling applications. Bovine serum albumin (BSA) adhesion profiles showed that the composition of the scaffolds plays a critical role in the chitosan–oligooxazoline system. Oligobisoxazoline-enriched scaffolds (20%?w/w, CB8020) decreased protein adsorption (BSA) by up to 70%. Moreover, 1?mg of CB8020 was able to kill 99.9% of Escherichia coli cells upon contact, demonstrating its potential as promising material for production of tailored non-fouling 3D structures to be used in the construction of novel devices with applications in the biomedical field and water treatment processes.     

Porous Allograft Bone Scaffolds: Doping with Strontium

Strontium (Sr) can promote the process of bone formation. To improve bioactivity, porous allograft bone scaffolds (ABS) were doped with Sr and the mechanical strength and bioactivity of the scaffolds were evaluated. Sr-doped ABS were prepared using the ion exchange method. The density and distribution of Sr in bone scaffolds were investigated by inductively coupled plasma optical emission spectrometry (ICP-OES), X-ray photoelectron spectroscopy (XPS), and energy-dispersive X-ray spectroscopy (EDS). Controlled release of strontium ions was measured and mechanical strength was evaluated by a compressive strength test. The bioactivity of Sr-doped ABS was investigated by a simulated body fluid (SBF) assay, cytotoxicity testing, and an in vivo implantation experiment. The Sr molar concentration [Sr/(Sr+Ca)] in ABS surpassed 5% and Sr was distributed nearly evenly. XPS analyses suggest that Sr combined with oxygen and carbonate radicals. Released Sr ions were detected in the immersion solution at higher concentration than calcium ions until day 30. The compressive strength of the Sr-doped ABS did not change significantly. The bioactivity of Sr-doped material, as measured by the in vitro SBF immersion method, was superior to that of the Sr-free freeze-dried bone and the Sr-doped material did not show cytotoxicity compared with Sr-free culture medium. The rate of bone mineral deposition for Sr-doped ABS was faster than that of the control at 4 weeks (3.28±0.23 µm/day vs. 2.60±0.20 µm/day; p<0.05). Sr can be evenly doped into porous ABS at relevant concentrations to create highly active bone substitutes.     

The Osteogenic Potential of Mesoporous Bioglasses/Silk and Non-Mesoporous Bioglasses/Silk Scaffolds in Ovariectomized Rats: In vitro and In vivo Evaluation

Silk-based scaffolds have been introduced to bone tissue regeneration for years, however, their local therapeutic efficency in bone metabolic disease condition has been seldom reported. According to our previous report, mesoporous bioactive glass (MBG)/silk scaffolds exhibits superior in vitro bioactivity and in vivo osteogenic properties compared to non-mesoporous bioactive glass (BG)/silk scaffolds, but no information could be found about their efficiency in osteoporotic (OVX) environment. This study investigated a biomaterial-based approach for improving MSCs behavior in vitro, and accelerating OVX defect healing by using 3D BG/silk and MBG/silk scaffolds, and pure silk scaffolds as control. The results of SEM, CCK-8 assay and quantitative ALP activity showed that MBG/silk scaffolds can improve attachment, proliferation and osteogenic differentiation of both O-MSCs and sham control. In vivo therapeutic efficiency was evaluated by μCT analysis, hematoxylin and eosin staining, safranin O staining and tartrate-resistant acid phosphatase, indicating accelerated bone formation with compatible scaffold degradation and reduced osteoclastic response of defect healing in OVX rats after 2 and 4 weeks treatment, with a rank order of MBG/silk > BG/silk > silk group. Immunohistochemical markers of COL I, OPN, BSP and OCN also revealed that MBG/silk scaffolds can better induce accelerated collagen and non-collagen matrix production. The findings of this study suggest that MBG/silk scaffolds provide a better environment for cell attachment, proliferation and differentiation, and act as potential substitute for treating local osteoporotic defects.     

Kinetics of Diffusion and Convection in 3.2-Å Pores: Exact Solution by Computer Simulation

The kinetics of transport in pores the size postulated for cell membranes has been investigated by direct computer simulation (molecular dynamics). The simulated pore is 11 Å long and 3.2 Å in radius, and the water molecules are modeled by hard, smooth spheres, 1 Å in radius. The balls are given an initial set of positions and velocities (with an average temperature of 313° K) and the computer then calculates their exact paths through the pore. Two different conditions were used at the ends of the pore. In one, the ends are closed and the balls are completely isolated. In the other, the ball density in each end region is fixed so that a pressure difference can be established and a net convective flow produced. The following values were directly measured in the simulated experiments: net and diffusive (oneway) flux; pressure, temperature, and diffusion coefficients in the pore; area available for diffusion; probability distribution of ball positions in the pore; and the interaction between diffusion and convection. The density, viscosity, and diffusion coefficients in the bulk fluid were determined from the theory of hard sphere dense gases. From these values, the “equivalent” pore radius (determined by the same procedure that is used for cell membranes) was computed and compared with the physical pore radius of the simulated pore.     

Microfluidic Fabrication of Polymeric and Biohybrid Fibers with Predesigned Size and Shape

A “sheath” fluid passing through a microfluidic channel at low Reynolds number can be directed around another “core” stream and used to dictate the shape as well as the diameter of a core stream. Grooves in the top and bottom of a microfluidic channel were designed to direct the sheath fluid and shape the core fluid. By matching the viscosity and hydrophilicity of the sheath and core fluids, the interfacial effects are minimized and complex fluid shapes can be formed. Controlling the relative flow rates of the sheath and core fluids determines the cross-sectional area of the core fluid. Fibers have been produced with sizes ranging from 300 nm to ~1 mm, and fiber cross-sections can be round, flat, square, or complex as in the case with double anchor fibers. Polymerization of the core fluid downstream from the shaping region solidifies the fibers. Photoinitiated click chemistries are well suited for rapid polymerization of the core fluid by irradiation with ultraviolet light. Fibers with a wide variety of shapes have been produced from a list of polymers including liquid crystals, poly(methylmethacrylate), thiol-ene and thiol-yne resins, polyethylene glycol, and hydrogel derivatives. Minimal shear during the shaping process and mild polymerization conditions also makes the fabrication process well suited for encapsulation of cells and other biological components.     

Elastic Torques about Membrane Edges: A Study of Pierced Egg Lecithin Vesicles

The shape of mechanically pierced giant vesicles is studied to obtain the elastic modulus of Gaussian curvature of egg lecithin bilayers. It is argued that such experiments are governed by an apparent modulus, ¯κ_app, not the true modulus of Gaussian curvature, ¯κ. A theory of ¯κ_app is proposed, regarding the pierced bilayer vesicle as a closed monolayer vesicle. The quantity measured, i.e. ¯κ_app/κ, where κ is the rigidity, agrees satisfactorily with the theory. We find ¯κ_app = -(1.9 ± 0.3) · 10^-12 erg (on the basis of κ = (2.3 ± 0.3) · 10^-12 erg). The result may have implications for bilayer fusion.     

Self-fitting shape memory polymer foam inducing bone regeneration: A rabbit femoral defect study

Although tissue engineering has been attracted greatly for healing of critical-sized bone defects, great efforts for improvement are still being made in scaffold design. In particular, bone regeneration would be enhanced if a scaffold precisely matches the contour of bone defects, especially if it could be implanted into the human body conveniently and safely. In this study, polyurethane/hydroxyapatite-based shape memory polymer (SMP) foam was fabricated as a scaffold substrate to facilitate bone regeneration. The minimally invasive delivery and the self-fitting behavior of the SMP foam were systematically evaluated to demonstrate its feasibility in the treatment of bone defects in vivo. Results showed that the SMP foam could be conveniently implanted into bone defects with a compact shape. Subsequently, it self-matched the boundary of bone defects upon shape-recovery activation in vivo. Micro-computed tomography determined that bone ingrowth initiated at the periphery of the SMP foam with a constant decrease towards the inside. Successful vascularization and bone remodeling were also demonstrated by histological analysis. Thus, our results indicate that the SMP foam demonstrated great potential for bone regeneration.     

The biocompatibility of calcium phosphate cements containing alendronate-loaded PLGA microparticles in?vitro

The composite of poly-lactic-co-glycolic acid (PLGA) and calcium phosphate cements (CPC) are currently widely used in bone tissue engineering. However, the properties and biocompatibility of the alendronate-loaded PLGA/CPC (APC) porous scaffolds have not been characterized. APC scaffolds were prepared by a solid/oil/water emulsion solvent evaporation method. The morphology, porosity, and mechanical strength of the scaffolds were characterized. Bone marrow mesenchymal stem cells (BMSCs) from rabbit were cultured, expanded and seeded on the scaffolds, and the cell morphology, adhesion, proliferation, cell cycle and osteogenic differentiation of BMSCs were determined. The results showed that the APC scaffolds had a porosity of 67.43 ± 4.2% and pore size of 213 ± 95 µm. The compressive strength for APC was 5.79 ± 1.21 MPa, which was close to human cancellous bone. The scanning electron microscopy, cell counting kit-8 assay, flow cytometry and ALP activity revealed that the APC scaffolds had osteogenic potential on the BMSCs in vitro and exhibited excellent biocompatibility with engineered bone tissue. APC scaffolds exhibited excellent biocompatibility and osteogenesis potential and can potentially be used for bone tissue engineering.     

Cryo-EM structure of the heptameric calcium homeostasis modulator 1 channel

Calcium homeostasis modulator 1 (CALHM1) is a voltage- and Ca²⁺-gated ATP channel that plays an important role in neuronal signaling. However, as the previously reported CALHM structures are all in the ATP-conducting state, the gating mechanism of ATP permeation is still elusive. Here, we report cryo-EM reconstructions of two Danio rerio CALHM1 heptamers with ordered or flexible long C-terminal helices at resolutions of 3.2 Å and 2.9 Å, respectively, and one D. rerio CALHM1 octamer with flexible long C-terminal helices at a resolution of 3.5 Å. Structural analysis shows that the heptameric CALHM1s are in an ATP-nonconducting state with a central pore diameter of approximately 6.6 Å. Compared with those inside the octameric CALHM1, the N-helix inside the heptameric CALHM1 is in the “down” position to avoid steric clashing with the adjacent TM1 helix. Molecular dynamics simulations show that as the N-helix moves from the “down” position to the “up” position, the pore size of ATP molecule permeation increases significantly. Our results provide important information for elucidating the mechanism of ATP molecule permeation in the CALHM1 channel.     

Fabrication of individual scaffolds based on a patient-specific alveolar bone defect model

Fabricating individualized tissue engineering scaffolds based on the three-dimensional shape of patient bone defects is required for the successful clinical application of bone tissue engineering. However, there are currently no reported studies of individualized bone tissue engineering scaffolds that truly reproduce a patient-specific bone defect. We fabricated individualized tissue engineering scaffolds based on alveolar bone defects. The individualized poly(lactide-co-glycolide) and tricalcium phosphate composite scaffolds were custom-made by acquiring the three-dimensional model through computed tomography, which was input into the computer-aided low-temperature deposition manufacturing system. The three-dimensional shape of the fabricated scaffold was identical to the patient-specific alveolar bone defects, with an average macropore diameter of 380 μm, micropore diameters ranging from 3 to 5 μm, and an average porosity of 87.4%. The mechanical properties of the scaffold were similar to adult cancellous bone. Scaffold biocompatibility was confirmed by attachment and proliferation of human bone marrow mesenchymal stem cells. Successful realization of individualized scaffold fabrication will enable clinical application of tissue-engineered bone at an early date.     

Human Urine Derived Stem Cells in Combination with β-TCP Can Be Applied for Bone Regeneration

Bone tissue engineering requires highly proliferative stem cells that are easy to isolate. Human urine stem cells (USCs) are abundant and can be easily harvested without using an invasive procedure. In addition, in our previous studies, USCs have been proved to be able to differentiate into osteoblasts, chondrocytes, and adipocytes. Therefore, USCs may have great potential and advantages to be applied as a cell source for tissue engineering. However, there are no published studies that describe the interactions between USCs and biomaterials and applications of USCs for bone tissue engineering. Therefore, the objective of the present study was to evaluate the interactions between USCs with a typical bone tissue engineering scaffold, beta-Tricalcium Phosphate (β-TCP), and to determine whether the USCs seeded onto β-TCP scaffold can promote bone regeneration in a segmental femoral defect of rats. Primary USCs were isolated from urine and seeded on β-TCP scaffolds. Results showed that USCs remained viable and proliferated within β-TCP. The osteogenic differentiation of USCs within the scaffolds was demonstrated by increased alkaline phosphatase activity and calcium content. Furthermore, β-TCP with adherent USCs (USCs/β-TCP) were implanted in a 6-mm critical size femoral defect of rats for 12 weeks. Bone regeneration was determined using X-ray, micro-CT, and histologic analyses. Results further demonstrated that USCs in the scaffolds could enhance new bone formation, which spanned bone defects in 5 out of 11 rats while β-TCP scaffold alone induced modest bone formation. The current study indicated that the USCs can be used as a cell source for bone tissue engineering as they are compatible with bone tissue engineering scaffolds and can stimulate the regeneration of bone in a critical size bone defect.     

A multiscale computational fluid dynamics approach to simulate the micro-fluidic environment within a tissue engineering scaffold with highly irregular pore geometry

Mechanical stimulation can regulate cellular behavior, e.g., differentiation, proliferation, matrix production and mineralization. To apply fluid-induced wall shear stress (WSS) on cells, perfusion bioreactors have been commonly used in tissue engineering experiments. The WSS on cells depends on the nature of the micro-fluidic environment within scaffolds under medium perfusion. Simulating the fluidic environment within scaffolds will be important for gaining a better insight into the actual mechanical stimulation on cells in a tissue engineering experiment. However, biomaterial scaffolds used in tissue engineering experiments typically have highly irregular pore geometries. This complexity in scaffold geometry implies high computational costs for simulating the precise fluidic environment within the scaffolds. In this study, we propose a low-computational cost and feasible technique for quantifying the micro-fluidic environment within the scaffolds, which have highly irregular pore geometries. This technique is based on a multiscale computational fluid dynamics approach. It is demonstrated that this approach can capture the WSS distribution in most regions within the scaffold. Importantly, the central process unit time needed to run the model is considerably low.

     

3D printed PCLA scaffold with nano‐hydroxyapatite coating doped green tea EGCG promotes bone growth and inhibits multidrug‐resistant bacteria colonization

Objectives3D‐printing scaffold with specifically customized and biomimetic structures gained significant recent attention in tissue engineering for the regeneration of damaged bone tissues. However, constructed scaffolds that simultaneously promote bone regeneration and in situ inhibit bacterial proliferation remains a great challenge. This study aimed to design a bone repair scaffold with in situ antibacterial functions.Materials and MethodsHerein, a general strategy is developed by using epigallocatechin‐3‐gallate (EGCG), a major green tea polyphenol, firmly anchored in the nano‐hydroxyapatite (HA) and coating the 3D printed polymerization of caprolactone and lactide (PCLA) scaffold. Then, we evaluated the stability, mechanical properties, water absorption, biocompatibility, and in vitro antibacterial and osteocyte inductive ability of the scaffolds.ResultsThe coated scaffold exhibit excellent activity in simultaneously stimulating osteogenic differentiation and in situ resisting methicillin‐resistant Staphylococcus aureus colonization in a bone repair environment without antibiotics. Meanwhile, the prepared 3D scaffold has certain mechanical properties (39.3 ± 3.2 MPa), and the applied coating provides the scaffold with remarkable cell adhesion and osteogenic conductivity.ConclusionThis study demonstrates that EGCG self‐assembled HA coating on PCLA surface could effectively enhance the scaffold''s water absorption, osteogenic induction, and antibacterial properties in situ. It provides a new strategy to construct superior performance 3D printed scaffold to promote bone tissue regeneration and combat postoperative infection in situ.

Schematic diagram of the 3D polymerization of caprolactone and lactide (PCLA) coated scaffold containing epigallocatechin‐3‐gallate (EGCG)‐modified nano‐HA as an artificial bone matrix with biphasic function to efficiently promote the growth of osteoblasts and inhibit methicillin‐resistant Staphylococcus aureus colonization in the bone repair microenvironment. PCLA/KH‐HA‐EGCG exhibited satisfactory antibacterial properties and leads to significant osteoinduction and osteogenic differentiation in osteoblasts cells, achieving a high‐efficient bone repair effect.     

Flow rates in perfusion bioreactors to maximise mineralisation in bone tissue engineering in vitro

In bone tissue engineering experiments, fluid-induced shear stress is able to stimulate cells to produce mineralised extracellular matrix (ECM). The application of shear stress on seeded cells can for example be achieved through bioreactors that perfuse medium through porous scaffolds. The generated mechanical environment (i.e. wall shear stress: WSS) within the scaffolds is complex due to the complexity of scaffold geometry. This complexity has so far prevented setting an optimal loading (i.e. flow rate) of the bioreactor to achieve an optimal distribution of WSS for stimulating cells to produce mineralised ECM. In this study, we demonstrate an approach combining computational fluid dynamics (CFD) and mechano-regulation theory to optimise flow rates of a perfusion bioreactor and various scaffold geometries (i.e. pore shape, porosity and pore diameter) in order to maximise shear stress induced mineralisation. The optimal flow rates, under which the highest fraction of scaffold surface area is subjected to a wall shear stress that induces mineralisation, are mainly dependent on the scaffold geometries. Nevertheless, the variation range of such optimal flow rates are within 0.5–5 mL/min (or in terms of fluid velocity: 0.166–1.66 mm/s), among different scaffolds. This approach can facilitate the determination of scaffold-dependent flow rates for bone tissue engineering experiments in vitro, avoiding performing a series of trial and error experiments.