Role of Anisotropy in Tissue Engineering

Renewable biomass-based aerogels: from structural design to functional regulation

Global population growth and industrialization have exacerbated the nonrenewable energy crises and environmental issues, thereby stimulating an enormous demand for producing environmentally friendly materials. Typically, biomass-based aerogels (BAs), which are mainly composed of biomass materials, show great application prospects in various fields because of their exceptional properties such as biocompatibility, degradability, and renewability. To improve the performance of BAs to meet the usage requirements of different scenarios, a large number of innovative works in the past few decades have emphasized the importance of micro-structural design in regulating macroscopic functions. Inspired by the ubiquitous random or regularly arranged structures of materials in nature ranging from micro to meso and macro scales, constructing different microstructures often corresponds to completely different functions even with similar biomolecular compositions. This review focuses on the preparation process, design concepts, regulation methods, and the synergistic combination of chemical compositions and microstructures of BAs with different porous structures from the perspective of gel skeleton and pore structure. It not only comprehensively introduces the effect of various microstructures on the physical properties of BAs, but also analyzes their potential applications in the corresponding fields of thermal management, water treatment, atmospheric water harvesting, CO2 absorption, energy storage and conversion, electromagnetic interference (EMI) shielding, biological applications, etc. Finally, we provide our perspectives regarding the challenges and future opportunities of BAs. Overall, our goal is to provide researchers with a thorough understanding of the relationship between the microstructures and properties of BAs, supported by a comprehensive analysis of the available data.

Manufacturing and validation of small-diameter vascular grafts: A mini review

The field of small-diameter vascular grafts remains a challenge for biomaterials scientists. While decades of research have brought us much closer to developing biomimetic materials for regenerating tissues and organs, the physiological challenges involved in manufacturing small conduits that can transport blood while not inducing an immune response or promoting blood clots continue to limit progress in this area. In this short review, we present some of the most recent methods and advancements made by researchers working in the field of small-diameter vascular grafts. We also discuss some of the most critical aspects biomaterials scientists should consider when developing lab-made small-diameter vascular grafts.

Design of Chitin Cell Culture Matrices for 3D Tissue Engineering: The Importance of Chitin Types, Solvents, Cross-Linkers, and Fabrication Techniques

This review focuses on factors and the fabrication techniques affecting the microarchitecture of tissue engineering scaffolds from the second most abundant biopolymer, chitin. It emphasizes the unique potentiality of this polymer in tissue engineering (TE) applications and highlights the variables important to achieve tailored scaffold properties. First, we describe aspects of scaffolds' design, and the complex interplay between chitin types, solvent systems, additives, and fabrication techniques to incorporate porosity, with regard to best practices. In the following section, we provide examples of scaffolds' use, with a focus on in vitro cell studies. Finally, an analysis of their biodegradability is presented. Our review emphasizes the potentiality of chitin and the pressing need for further research to overcome existing challenges and fully harness its capabilities in tissue engineering.

Recent Methods for Modifying Mechanical Properties of Tissue-Engineered Scaffolds for Clinical Applications

The limited regenerative capacity of the human body, in conjunction with a shortage of healthy autologous tissue, has created an urgent need for alternative grafting materials. A potential solution is a tissue-engineered graft, a construct which supports and integrates with host tissue. One of the key challenges in fabricating a tissue-engineered graft is achieving mechanical compatibility with the graft site; a disparity in these properties can shape the behaviour of the surrounding native tissue, contributing to the likelihood of graft failure. The purpose of this review is to examine the means by which researchers have altered the mechanical properties of tissue-engineered constructs via hybrid material usage, multi-layer scaffold designs, and surface modifications. A subset of these studies which has investigated the function of their constructs in vivo is also presented, followed by an examination of various tissue-engineered designs which have been clinically translated.

Elastomeric Trilayer Substrates with Native-like Mechanical Properties for Heart Valve Leaflet Tissue Engineering

Heart valve leaflets have a complex trilayered structure with layer-specific orientations, anisotropic tensile properties, and elastomeric characteristics that are difficult to mimic collectively. Previously, trilayer leaflet substrates intended for heart valve tissue engineering were developed with nonelastomeric biomaterials that cannot deliver native-like mechanical properties. In this study, by electrospinning polycaprolactone (PCL) polymer and poly(l-lactide-co-ε-caprolactone) (PLCL) copolymer, we created elastomeric trilayer PCL/PLCL leaflet substrates with native-like tensile, flexural, and anisotropic properties and compared them with trilayer PCL leaflet substrates (as control) to find their effectiveness in heart valve leaflet tissue engineering. These substrates were seeded with porcine valvular interstitial cells (PVICs) and cultured for 1 month in static conditions to produce cell-cultured constructs. The PCL/PLCL substrates had lower crystallinity and hydrophobicity but higher anisotropy and flexibility than PCL leaflet substrates. These attributes contributed to more significant cell proliferation, infiltration, extracellular matrix production, and superior gene expression in the PCL/PLCL cell-cultured constructs than in the PCL cell-cultured constructs. Further, the PCL/PLCL constructs showed better resistance to calcification than PCL constructs. Trilayer PCL/PLCL leaflet substrates with native-like mechanical and flexural properties could significantly improve heart valve tissue engineering.

Bioactive cellulose acetate nanofiber loaded with annatto support skeletal muscle cell attachment and proliferation

Electrospinning emerged as a promising technique to produce scaffolds for cultivated meat in function of its simplicity, versatility, cost-effectiveness, and scalability. Cellulose acetate (CA) is a biocompatible and low-cost material that support cell adhesion and proliferation. Here we investigated CA nanofibers, associated or not with a bioactive annatto extract (CA@A), a food-dye, as potential scaffolds for cultivated meat and muscle tissue engineering. The obtained CA nanofibers were evaluated concerning its physicochemical, morphological, mechanical and biological traits. UV-vis spectroscopy and contact angle measurements confirmed the annatto extract incorporation into the CA nanofibers and the surface wettability of both scaffolds, respectively. SEM images revealed that the scaffolds are porous, containing fibers with no specific alignment. Compared with the pure CA nanofibers, CA@A nanofibers showed increased fiber diameter (420 ± 212 nm vs. 284 ± 130 nm). Mechanical properties revealed that the annatto extract induces a reduction of the stiffness of the scaffold. Molecular analyses revealed that while CA scaffold favored C2C12 myoblast differentiation, the annatto-loaded CA scaffold favored a proliferative state of these cells. These results suggest that the combination of cellulose acetate fibers loaded with annatto extract may be an interesting economical alternative for support long-term muscle cells culture with potential application as scaffold for cultivated meat and muscle tissue engineering.

Regenerating dynamic organs using biomimetic patches

The regeneration of dynamic organs remains challenging because they are intrinsically anisotropic and undergo large volumetric deformation during normal or pathological function. This hampers the durability and applicability of regenerative medicine approaches. To address the challenges of organ dynamics, a new class of patches have emerged with anisotropic and auxetic properties that mimic native tissue biomechanics and accommodate volumetric deformation. Here, we outline the critical design, materials, and processing considerations for achieving optimal patch biomechanics according to target pathology and summarize recent advances in biomimetic patches for dynamic organ regeneration. Furthermore, we discuss the challenges and opportunities which, if overcome, would open up new applications in organ regeneration and expedite the clinical translation of patch-based therapeutics.

Poly(L-co-D,L lactic acid-co-Trimethylene Carbonate) 3D printed scaffold cultivated with mesenchymal stem cells directed to bone reconstruction: In vitro and in vivo studies

A recent and quite promising technique for bone tissue engineering is the 3D printing, peculiarly regarding the production of high-quality scaffolds. The 3D printed scaffold strictly provides suitable characteristics for living cells, in order to induce treatment, reconstruction and substitution of injured tissue. The purpose of this work was to evaluate the behavior of the 3D scaffold based on Poly(L-co-D,L lactic acid-co-Trimethylene Carbonate) (PLDLA-TMC), which was designed in Solidworks™ software, projected in 3D Slicer™, 3D printed in filament extrusion, cultured with mesenchymal stem cells (MSCs) and tested in vitro and in vivo models. For in vitro study, the MSCs were seeded in a PLDLA-TMC 3D scaffold with 600 μm pore size and submitted to proliferation and osteogenic differentiation. The in vivo assays implanted the PLDLA-TMC scaffolds with or without MSCs in the calvaria of Wistar rats submitted to 8 mm cranial bone defect, in periods of 8-12 weeks. The results showed that PLDLA-TMC 3D scaffolds favored adherence and cell growth, and suggests an osteoinductive activity, which means that the material itself augmented cellular differentiation. The implanted PLDLA-TMC containing MSCs, showed better results after 12 weeks prior grafting, due the absence of inflammatory processes, enlarged regeneration of bone tissue and facilitated angiogenesis. Notwithstanding, the 3D PLDLA-TMC itself implanted enriched tissue repair; the addition of cells known to upregulate tissue healing reinforce the perspectives for the PLDLA-TMC applications in the field of bone tissue engineering in clinical trials.

Understanding and utilizing textile-based electrostatic flocking for biomedical applications

Electrostatic flocking immobilizes electrical charges to the surface of microfibers from a high voltage-connected electrode and utilizes Coulombic forces to propel microfibers toward an adhesive-coated substrate, leaving a forest of aligned fibers. This traditional textile engineering technique has been used to modify surfaces or to create standalone anisotropic structures. Notably, a small body of evidence validating the use of electrostatic flocking for biomedical applications has emerged over the past several years. Noting the growing interest in utilizing electrostatic flocking in biomedical research, we aim to provide an overview of electrostatic flocking, including the principle, setups, and general and biomedical considerations, and propose a variety of biomedical applications. We begin with an introduction to the development and general applications of electrostatic flocking. Additionally, we introduce and review some of the flocking physics and mathematical considerations. We then discuss how to select, synthesize, and tune the main components (flocking fibers, adhesives, substrates) of electrostatic flocking for biomedical applications. After reviewing the considerations necessary for applying flocking toward biomedical research, we introduce a variety of proposed use cases including bone and skin tissue engineering, wound healing and wound management, and specimen swabbing. Finally, we presented the industrial comments followed by conclusions and future directions. We hope this review article inspires a broad audience of biomedical, material, and physics researchers to apply electrostatic flocking technology to solve a variety of biomedical and materials science problems.

Bioscaffold Stiffness Mediates Aerosolized Nanoparticle Uptake in Lung Epithelial Cells

In this study, highly porous, ultrasoft polymeric mats mimicking human tissues were formed from novel polyurethane soft dendritic colloids (PU SDCs). PU SDCs have a unique fibrillar morphology controlled by antisolvent precipitation. When filtered from suspension, PU SDCs form mechanically robust nonwoven mats. The stiffness of the SDC mats can be tuned for physiological relevance. The unique physiochemical characteristics of the PU SDC particles dictate the mechanical properties resulting in tunable elastic moduli ranging from 200 to 800 kPa. The human lung A549 cells cultured on both stiff and soft PU SDC membranes were found to be viable, capable of supporting the air-liquid interface (ALI) cell culture, and maintained barrier integrity. Furthermore, A549 cellular viability and uptake efficiency of aerosolized tannic acid-coated gold nanoparticles (Ta-Au) was found to depend on elastic modulus and culture conditions. Ta-Au nanoparticle uptake was twofold and fourfold greater on soft PU SDCs, when cultured at submerged and ALI conditions, respectively. The significant increase in endocytosed Ta-Au resulted in a 20% decrease in viability, and a 4-fold increase in IL-8 cytokine secretion when cultured on soft PU SDCs at ALI. Common tissue culture materials exhibit super-physiological elastic moduli, a factor found to be critical in analyzing nanomaterial cellular interactions and biological responses.

Architected fibrous scaffolds for engineering anisotropic tissues

Mimicking the native three-dimensional microenvironment is of crucial importance when biofabricating a new healthcare material. One aspect of the native tissue that is often omitted when designing a suitable scaffold is its anisotropy. Not only is matching native mechanical properties important when designing implantable scaffolds or healthcare materials, but matching physiological structure is also important as many cell populations respond differently to fiber orientation. Therefore, novel aligned electrospun scaffolds with varying fiber angles and spacing of bundles were created and mechanically characterized. Through controlling the angle between the fibers in each layer of the scaffold, a range of different physiological anisotropic mechanical properties were achieved that encompasses values found in native tissues. Extrapolation of this mechanical data allowed for any native tissue's anisotropic Young's modulus to be mimicked by electrospinning fibers at a particular angle. These electrospun scaffolds were then incorporated with cell-laden hydrogels to create hybrid structures that contain the benefits of both scaffolding techniques with the ability to encapsulate cells in the hydrogel. To conclude, this study develops a novel bundled fiber scaffold that was architected to yield anisotropic properties matching native tissues.

Recent Advances in Microfluidically Spun Microfibers for Tissue Engineering and Drug Delivery Applications

In recent years, the unique and tunable properties of microfluidically spun microfibers have led to tremendous advancements for the field of biomedical engineering, which have been applied to areas such as tissue engineering, wound dressing, and drug delivery, as well as cell encapsulation and cell seeding. In this article, we analyze the most recent advances in microfluidics and microfluidically spun microfibers, with an emphasis on biomedical applications. We explore in detail these new and innovative experiments, how microfibers are made, the experimental purpose of making microfibers, and the future work that can be done as a result of these new types of microfibers. We also focus on the applications of various materials used to fabricate microfibers, as well as their many promises and limitations.

Hydrothermal-Assisted Synthesis and Stability of Multifunctional MXene Nanobipyramids: Structural, Chemical, and Optical Evolution

Recent advancements in two-dimensional materials have brought MXene (Ti₃C₂) into attention due to its exciting properties as a very promising material for various applications. In this work, we report a novel Ti₃C₂ nanobipyramid (Ti₃C₂ NB) structure obtained through a three-step process involving exfoliation, delamination, and subsequent hydrothermal treatment. The morphological and textural properties at each step of synthesis were studied using an array of experimental techniques such as transmission electron microscopy, scanning electron microscopy, and atomic force microscopy and the chemical properties through X-ray diffraction, Raman, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy analysis. The Ti₃C₂ NBs exhibit fluorescence with an excitation-dependent emission. Further, the effect of temperature and pH on the fluorescence was also investigated, which opens up its scope in bioanalytical applications. Ti₃C₂ NBs showed a ∼43% increase in photoluminescence intensity from pH 3 to 11 while a ∼38% increase with the temperature from 20 to 80 °C. Usually, MXenes are highly susceptible to oxidation, but the Ti₃C₂ NBs were found to be chemically and optically stable even after 30 days. Bestowed with good hydrophilicity, the material exhibited high biocompatibility on the mouse fibroblast cell line L929. Further, L929 cells also showed good cellular adhesion on a Ti₃C₂ NB-modified glass substrate. These properties pave a way for its multifunctional ability as a sensor for pH and temperature as well as bioimaging.

Silkworm and spider silk electrospinning: a review

Issues of fossil fuel and plastic pollution are shifting public demand toward biopolymer-based textiles. For instance, silk, which has been traditionally used during at least 5 milleniums in China, is re-emerging in research and industry with the development of high-tech spinning methods. Various arthropods, e.g. insects and arachnids, produce silky proteinic fiber of unique properties such as resistance, elasticity, stickiness and toughness, that show huge potential for biomaterial applications. Compared to synthetic analogs, silk presents advantages of low density, degradability and versatility. Electrospinning allows the creation of nonwoven mats whose pore size and structure show unprecedented characteristics at the nanometric scale, versus classical weaving methods or modern techniques such as melt blowing. Electrospinning has recently allowed to produce silk scaffolds, with applications in regenerative medicine, drug delivery, depollution and filtration. Here we review silk production by the spinning apparatus of the silkworm Bombyx mori and the spiders Aranea diadematus and Nephila Clavipes. We present the biotechnological procedures to get silk proteins, and the preparation of a spinning dope for electrospinning. We discuss silk's mechanical properties in mats obtained from pure polymer dope and multi-composites. This review highlights the similarity between two very different yarn spinning techniques: biological and electrospinning processes.

Carbon nanotubes and their polymeric composites: the applications in tissue engineering

Carbon nanotubes (CNTs), with unique graphitic structure, superior mechanical, electrical, optical and biological properties, has attracted more and more interests in biomedical applications, including gene/drug delivery, bioimaging, biosensor and tissue engineering. In this review, we focus on the role of CNTs and their polymeric composites in tissue engineering applications, with emphasis on their usages in the nerve, cardiac and bone tissue regenerations. The intrinsic natures of CNTs including their physical and chemical properties are first introduced, explaining the structure effects on CNTs electrical conductivity and various functionalization of CNTs to improve their hydrophobic characteristics. Biosafety issues of CNTs are also discussed in detail including the potential reasons to induce the toxicity and their potential strategies to minimise the toxicity effects. Several processing strategies including solution-based processing, polymerization, melt-based processing and grafting methods are presented to show the 2D/3D construct formations using the polymeric composite containing CNTs. For the sake of improving mechanical, electrical and biological properties and minimising the potential toxicity effects, recent advances using polymer/CNT composite the tissue engineering applications are displayed and they are mainly used in the neural tissue (to improve electrical conductivity and biological properties), cardiac tissue (to improve electrical, elastic properties and biological properties) and bone tissue (to improve mechanical properties and biological properties). Current limitations of CNTs in the tissue engineering are discussed and the corresponded future prospective are also provided. Overall, this review indicates that CNTs are promising “next-generation” materials for future biomedical applications.

Bioengineering a pre-vascularized pouch for subsequent islet transplantation using VEGF-loaded polylactide capsules

The effectiveness of cell transplantation can be improved by optimization of the transplantation site. For some types of cells that form highly oxygen-demanding tissue, e.g., pancreatic islets, a successful engraftment depends on immediate and sufficient blood supply. This critical point can be avoided when cells are transplanted into a bioengineered pre-vascularized cavity which can be formed using a polymer scaffold. In our study, we tested surface-modified poly(lactide-co-caprolactone) (PLCL) capsular scaffolds containing the pro-angiogenic factor VEGF. After each modification step (i.e., amination and heparinization), the surface properties and morphology of scaffolds were characterized by ATR-FTIR and XPS spectroscopy, and by SEM and AFM. All modifications preserved the gross capsule morphology and maintained the open pore structure. Optimized aminolysis conditions decreased the Mw of PLCL only up to 10% while generating a sufficient number of NH2 groups required for the covalent immobilization of heparin. The heparin layer served as a VEGF reservoir with an in vitro VEGF release for at least four weeks. In vivo studies revealed that to obtain highly vascularized PLCL capsules (a) the optimal VEGF dose for the capsule was 50 μg and (b) the implantation time was four weeks when implanted into the greater omentum of Lewis rats; dense fibrous tissue accompanied by vessels completely infiltrated the scaffold and created sparse granulation tissue within the internal cavity of the capsule. The prepared pre-vascularized pouch enabled the islet graft survival and functioning for at least 50 days after islet transplantation. The proposed construct can be used to create a reliable pre-vascularized pouch for cell transplantation.

Preparation and biological properties of silk fibroin/nano-hydroxyapatite/graphene oxide scaffolds with an oriented channel-like structure

A novel SF/nHAp/GO hybrid scaffold with oriented channel-like structure in bone tissue engineering.

Effect of Mold Geometry on Pore Size in Freeze-Cast Chitosan-Alginate Scaffolds for Tissue Engineering

Freeze-casting is a popular method to produce biomaterial scaffolds with highly porous structures. The pore structure of freeze-cast biomaterial scaffolds is influenced by processing parameters but has mostly been controlled experimentally. A mathematical model integrating Computational Fluid Dynamics with Population Balance Model was developed to predict average pore size (APS) of 3D porous chitosan-alginate scaffolds and to assess the influence of the geometrical parameters of mold on scaffold pore structure. The model predicted the crystallization pattern and APS for scaffolds cast in different diameter molds and filled to different heights. The predictions demonstrated that the temperature gradient and solidification pattern affect ice crystal nucleation and growth, subsequently influencing APS homogeneity. The predicted APS compared favorably with APS measurements from a corresponding experimental dataset, validating the model. Sensitivity analysis was performed to assess the response of the APS to the three geometrical parameters of the mold: well radius; solution fill height; and spacing between wells. The pore size was most sensitive to the distance between the wells and least sensitive to solution height. This validated model demonstrates a method for optimizing the APS of freeze-cast biomaterial scaffolds that could be applied to other compositions or applications.

Morphological and biomechanical characterization of immature and mature nasoseptal cartilage

Nasoseptal cartilage has been assumed to be isotropic, unlike the well-defined zonal organization of articular cartilage attributed to postnatal biomechanical loading. We know from clinical experience that malrotation of surgical nasoseptal cartilage grafts can lead to increased graft absorption. Other studies have also suggested directionally dependent compressive stiffness suggesting anisotropy, but morphological investigations are lacking. This study characterizes immature and mature native bovine nasoseptal cartilage using a combination of immunohistochemistry, biomechanical testing and structural imaging. Our findings indicate that there is extensive postnatal synthesis and reorganization of the extracellular matrix in bovine nasoseptal cartilage, independent of joint loading forces responsible for articular cartilage anisotropy. Immature nasoseptal cartilage is more cellular and homogenous compared to the zonal organization of cells and extracellular matrix of mature cartilage. Mature samples also exhibited greater glycosaminoglycan content and type II collagen fibre alignment compared to immature cartilage and this correlates with greater compressive stiffness. Engineered neocartilage often consists of immature, isotropic, homogenous tissue that is unable to meet the functional and mechanical demands when implanted into the native environment. This study demonstrates the importance of anisotropy on biomechanical tissue strength to guide future cartilage tissue engineering strategies for surgical reconstruction.

Optimizing the alignment of thermoresponsive poly(N-isopropyl acrylamide) electrospun nanofibers for tissue engineering applications: A factorial design of experiments approach

Thermoresponsive polymers, such as poly(N-isopropyl acrylamide) (PNIPAM), have been identified and used as cell culture substrates, taking advantage of the polymer's lower critical solution temperature (LCST) to mechanically harvest cells. This technology bypasses the use of biochemical enzymes that cleave important cell-cell and cell-matrix interactions. In this study, the process of electrospinning is used to fabricate and characterize aligned PNIPAM nanofiber scaffolds that are biocompatible and thermoresponsive. Nanofiber scaffolds produced by electrospinning possess a 3D architecture that mimics native extracellular matrix, providing physical and chemical cues to drive cell function and phenotype. We present a factorial design of experiments (DOE) approach to systematically determine the effects of different electrospinning process parameters on PNIPAM nanofiber diameter and alignment. Results show that high molecular weight PNIPAM can be successfully electrospun into both random and uniaxially aligned nanofiber mats with similar fiber diameters by simply altering the speed of the rotating mandrel collector from 10,000 to 33,000 RPM. PNIPAM nanofibers were crosslinked with OpePOSS, which was verified using FTIR. The mechanical properties of the scaffolds were characterized using dynamic mechanical analysis, revealing an order of magnitude difference in storage modulus (MPa) between cured and uncured samples. In summary, cross-linked PNIPAM nanofiber scaffolds were determined to be stable in aqueous culture, biocompatible, and thermoresponsive, enabling their use in diverse cell culture applications.

Cell Morphology on Poly(methyl methacrylate) Microstructures as Function of Surface Energy

Whilst the significance of substrate topography as a regulator of cell function is well established, a systematic analysis of the principles underlying this is still unavailable. Here we evaluate the hypothesis that surface energy plays a decisive role in substrate-mediated modulation of cell phenotype by evaluation of cell behaviour on synthetic microstructures exhibiting pronounced differences in surface energy. These microstructures, specifically cubes and walls, were fabricated from a biocompatible base polymer, poly(methyl methacrylate), by variotherm injection molding. The dimensions of the cubes were 1 μm x 1 μm x 1 μm (height x width x length) with a periodicity of 1:1 and 1:5 and the dimensions of the walls 1 μm x 1 μm x 15 mm (height x width x length) with a periodicity of 1:1 and 1:5. Mold inserts were made by lithography and electroplating. The surface energy of the resultant microstructures was determined by static contact angle measurements. Light scanning microscopy of the morphology of NT2/D1 and MC3T3-E1 preosteoblast cells cultured on structured PMMA samples in both cases revealed a profound surface energy dependence. “Walls” appeared to promote significant cell elongation, whilst a lack of cell adhesion was observed on “cubes” with the lowest periodicity. Contact angle measurements on walls revealed enhanced surface energy anisotropy (55 mN/m max., 10 mN/m min.) causing a lengthwise spreading of the test liquid droplet, similar to cell elongation. Surface energy measurements for cubes revealed increased isotropic hydrophobicity (87° max., H₂O). A critical water contact angle of ≤ 80° appears to be necessary for adequate cell adhesion. A “switch” for cell adhesion and subsequently cell growth could therefore be applied by, for example, adjusting the periodicity of hydrophobic structures. In summary cell elongation on walls and a critical surface energy level for cell adhesion could be produced for NT2/D1 and MC3T3-E1 cells by symmetrical and asymmetrical energy barrier levels. We, furthermore, propose a water-drop model providing a common physicochemical cause regarding similar cell/droplet geometries and cell adhesion on the investigated microstructures.

Mechanically Strong Silica-Silk Fibroin Bioaerogel: A Hybrid Scaffold with Ordered Honeycomb Micromorphology and Multiscale Porosity for Bone Regeneration

Due to the synergic feature of individual components in hybrid (nano)biomaterials, their application in regenerative medicine has drawn significant attention. Aiming to address all the current challenges of aerogel as a potent scaffold in bone tissue engineering application, we adopted a novel synthesis approach to synergistically improve the pore size regime and mechanical strength in the aerogel. The three-dimensional aerogel scaffold in this study has been synthesized through a versatile one-pot aqueous-based sol-gel hybridization/assembly of organosilane (tetraethyl orthosilicate) and silk fibroin (SF) biopolymer, followed by unidirectional freeze-casting of the as-prepared hybrid gel and supercritical drying. The developed ultralight silica-SF aerogel hybrids demonstrated a hierarchically organized porous structure with interesting honeycomb-shaped micromorphology and microstructural alignment (anisotropy) in varied length scales. The average macropore size of the hybrid aerogel lied in ∼0.5-18 μm and was systematically controlled with freeze-casting conditions. Together with high porosity (91-94%), high Young's modulus (∼4-7 MPa, >3 order of magnitude improvement compared to their pristine aerogel counterparts), and bone-type anisotropy in the mechanical compressive behavior, the silica-SF hybrid aerogel of this study acted as a very competent scaffold for bone tissue formation. The results of in vitro assessments revealed that the silica-SF aerogel is not only cytocompatible and nonhemolytic but also acted as an open porous microenvironment to trigger osteoblast cell attachment, growth, and proliferation on its surface within 14 days of incubation. Moreover, to support the in vitro results, in vivo bone formation within the aerogel implant in the bone defect site was studied. The X-ray radiology and microcomputed tomography analyses confirmed that a significant new bone tissue density formed in the defect site within 25 days of implantation. Also, in vivo toxicology studies showed a zero-toxic impact of the aerogel implant on the blood biochemical and hematological parameters. Finally, the study clearly shows the potential of aerogel as a bioactive and osteoconductive open porous cellular matrix for a successful osseointegration process.

Cardiomyogenic differentiation of human bone marrow-derived mesenchymal stem cell spheroids within electrospun collagen nanofiber mats

Collagen is the major structural protein in myocardium and contributes to tissue strength and integrity, cellular orientation, and cell-cell and cell-matrix interactions. Significant post-myocardial infarction related loss of cardiomyocytes and cardiac tissue, and their subsequent replacement with fibrous scar tissue, negatively impacts endogenous tissue repair and regeneration capabilities. To overcome such limitations, tissue engineers are working toward developing a 3D cardiac patch which not only mimics the structural, functional, and biological hierarchy of the native cardiac tissue, but also could deliver autologous stem cells and encourage their homing and differentiation. In this study, we examined the utility of electrospun, randomly-oriented, type-I collagen nanofiber (dia = 789 ± 162 nm) mats on the cardiomyogenic differentiation of human bone marrow-derived mesenchymal stem cells (BM-MSC) spheroids, in the presence or absence of 10 μM 5-azacytidine (aza). Results showed that these scaffolds are biocompatible and enable time-dependent evolution of early (GATA binding protein 4: GATA4), late (cardiac troponin I: cTnI), and mature (myosin heavy chain: MHC) cardiomyogenic markers, with a simultaneous reduction in CD90 (stemness) expression, independent of aza-treatment. Aza-exposure improved connexin-4 expression and sustained sarcomeric α-actin expression, but provided only transient improvement in cardiac troponin T (cTnT) expression. Cell orientation and alignment significantly improved in these nanofiber scaffolds over time and with aza-exposure. Although further quantitative in vitro and in vivo studies are needed to establish the clinical applicability of such stem-cell laden collagen nanofiber mats as cardiac patches for cardiac tissue regeneration, our results underscore the benefits of 3D milieu provided by electrospun collagen nanofiber mats, aza, and spheroids on the survival, cardiac differentiation and maturation of human BM-MSCs. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 3303-3312, 2018.

3D scaffolds for brain tissue regeneration: architectural challenges

Critical analysis of experimental studies on 3D scaffolds for brain tissue engineering.

Polymer scaffolds with no skin-effect for tissue engineering applications fabricated by thermally induced phase separation

Thermally induced phase separation (TIPS) based methods are widely used for the fabrication of porous scaffolds for tissue engineering and related applications. However, formation of a less-/non-porous layer at the scaffold's outer surface at the air-liquid interface, often known as the skin-effect, restricts the cell infiltration inside the scaffold and therefore limits its efficacy. To this end, we demonstrate a TIPS-based process involving the exposure of the just quenched poly(lactide-co-caprolactone):dioxane phases to the pure dioxane for a short time while still being under the quenching strength, herein after termed as the second quenching (2Q). Scanning electron microscopy, mercury intrusion porosimetry and contact angle analysis revealed a direct correlation between the time of 2Q and the gradual disappearance of the skin, followed by the widening of the outer pores and the formation of the fibrous filaments over the surface, with no effect on the internal pore architecture and the overall porosity of scaffolds. The experiments at various quenching temperatures and polymer concentrations revealed the versatility of 2Q in removing the skin. In addition, the in vitro cell culture studies with the human primary fibroblasts showed that the scaffolds prepared by the TIPS based 2Q process, with the optimal exposure time, resulted in a higher cell seeding and viability in contrast to the scaffolds prepared by the regular TIPS. Thus, TIPS including the 2Q step is a facile, versatile and innovative approach to fabricate the polymer scaffolds with a skin-free and fully open porous surface morphology for achieving a better cell response in tissue engineering and related applications.

Understanding cell homing-based tissue regeneration from the perspective of materials

The triad of cell homing-based tissue engineering.

Patterned electrospun nanofiber matrices via localized dissolution: potential for guided tissue formation

With the assistance of an ink-jet printer, solvent (the "ink") can be controllably and reproducibly printed onto electrospun nanofiber meshes (the "paper") to generate various micropatterns and subsequently guide distinct cellular organization and phenotype expression. In combination with the nanofiber-assisted layer-by-layer cell assembly, the patterned electrospun meshes will define an instructive microenvironment for guided tissue formation.

Dip TIPS as a facile and versatile method for fabrication of polymer foams with controlled shape, size and pore architecture for bioengineering applications

The porous polymer foams act as a template for neotissuegenesis in tissue engineering, and, as a reservoir for cell transplants such as pancreatic islets while simultaneously providing a functional interface with the host body. The fabrication of foams with the controlled shape, size and pore structure is of prime importance in various bioengineering applications. To this end, here we demonstrate a thermally induced phase separation (TIPS) based facile process for the fabrication of polymer foams with a controlled architecture. The setup comprises of a metallic template bar (T), a metallic conducting block (C) and a non-metallic reservoir tube (R), connected in sequence T-C-R. The process hereinafter termed as Dip TIPS, involves the dipping of the T-bar into a polymer solution, followed by filling of the R-tube with a freezing mixture to induce the phase separation of a polymer solution in the immediate vicinity of T-bar; Subsequent free-drying or freeze-extraction steps produced the polymer foams. An easy exchange of the T-bar of a spherical or rectangular shape allowed the fabrication of tubular, open- capsular and flat-sheet shaped foams. A mere change in the quenching time produced the foams with a thickness ranging from hundreds of microns to several millimeters. And, the pore size was conveniently controlled by varying either the polymer concentration or the quenching temperature. Subsequent in vivo studies in brown Norway rats for 4-weeks demonstrated the guided cell infiltration and homogenous cell distribution through the polymer matrix, without any fibrous capsule and necrotic core. In conclusion, the results show the "Dip TIPS" as a facile and adaptable process for the fabrication of anisotropic channeled porous polymer foams of various shapes and sizes for potential applications in tissue engineering, cell transplantation and other related fields.