Silk fibroin based biomimetic artificial extracellular matrix for hepatic tissue engineering applications

Nanofiber-induced hierarchically-porous magnesium phosphate bone cements accelerate bone regeneration by inhibiting Notch signaling

Magnesium phosphate bone cements (MPC) have been recognized as a viable alternative for bone defect repair due to their high mechanical strength and biodegradability. However, their poor porosity and permeability limit osteogenic cell ingrowth and vascularization, which is critical for bone regeneration. In the current study, we constructed a novel hierarchically-porous magnesium phosphate bone cement by incorporating extracellular matrix (ECM)-mimicking electrospun silk fibroin (SF) nanofibers. The SF-embedded MPC (SM) exhibited a heterogeneous and hierarchical structure, which effectively facilitated the rapid infiltration of oxygen and nutrients as well as cell ingrowth. Besides, the SF fibers improved the mechanical properties of MPC and neutralized the highly alkaline environment caused by excess magnesium oxide. Bone marrow stem cells (BMSCs) adhered excellently on SM, as illustrated by formation of more pseudopodia. CCK8 assay showed that SM promoted early proliferation of BMSCs. Our study also verified that SM increased the expression of OPN, RUNX2 and BMP2, suggesting enhanced osteogenic differentiation of BMSCs. We screened for osteogenesis-related pathways, including FAK signaing, Wnt signaling and Notch signaling, and found that SM aided in the process of bone regeneration by suppressing the Notch signaling pathway, proved by the downregulation of NICD1, Hes1 and Hey2. In addition, using a bone defect model of rat calvaria, the study revealed that SM exhibited enhanced osteogenesis, bone ingrowth and vascularization compared with MPC alone. No adverse effect was found after implantation of SM in vivo. Overall, our novel SM exhibited promising prospects for the treatment of critical-sized bone defects.

Biomedical applications of chitosan/silk fibroin composites: A review

Natural polymers have been used in the biomedical fields for decades, mainly derived from animals and plants with high similarities with biomacromolecules in the human body. As an alkaline polysaccharide, chitosan (CS) attracts much attention in tissue regeneration and drug delivery with favorable biocompatibility, biodegradation, and antibacterial activity. However, to overcome its mechanical properties and degradation behavior drawbacks, a robust fibrous protein-silk fibroin (SF) was introduced to prepare the CS/SF composites. Not only can CS be combined with SF via the amide and hydrogen bond formation, but also their functions are complementary and tunable with the blending ratio. To further improve the performances of CS/SF composites, natural (e.g., hyaluronic acid and collagen) and synthetic biopolymers (e.g., polyvinyl alcohol and hexanone) were incorporated. Also, the CS/SF composites acted as slow-release carriers for inorganic non-metals (e.g., hydroxyapatite and graphene) and metal particles (e.g., silver and magnesium), which could enhance cell functions, facilitate tissue healing, and inhibit bacterial growth. This review presents the state-of-the-art and future perspectives of different biomaterials combined with CS/SF composites as sponges, hydrogels, membranes, particles, and coatings. Emphasis is devoted to the biological potentialities of these hybrid systems, which look rather promising toward a multitude of applications.

Silk Fibroin Combined with Electrospinning as a Promising Strategy for Tissue Regeneration

The development of tissue engineering scaffolds is of great significance for the repair and regeneration of damaged tissues and organs. Silk fibroin (SF) is a natural protein polymer with good biocompatibility, biodegradability, excellent physical and mechanical properties and processability, making it an ideal universal tissue engineering scaffold material. Nanofibers prepared by electrospinning have attracted extensive attention in the field of tissue engineering due to their excellent mechanical properties, high specific surface area, and similar morphology as to extracellular matrix (ECM). The combination of silk fibroin and electrospinning is a promising strategy for the preparation of tissue engineering scaffolds. In this review, the research progress of electrospun silk fibroin nanofibers in the regeneration of skin, vascular, bone, neural, tendons, cardiac, periodontal, ocular and other tissues is discussed in detail.

Silk-Based Biomaterials for Designing Bioinspired Microarchitecture for Various Biomedical Applications

Biomaterial research has led to revolutionary healthcare advances. Natural biological macromolecules can impact high-performance, multipurpose materials. This has prompted the quest for affordable healthcare solutions, with a focus on renewable biomaterials with a wide variety of applications and ecologically friendly techniques. Imitating their chemical compositions and hierarchical structures, bioinspired based materials have elevated rapidly over the past few decades. Bio-inspired strategies entail extracting fundamental components and reassembling them into programmable biomaterials. This method may improve its processability and modifiability, allowing it to meet the biological application criteria. Silk is a desirable biosourced raw material due to its high mechanical properties, flexibility, bioactive component sequestration, controlled biodegradability, remarkable biocompatibility, and inexpensiveness. Silk regulates temporo-spatial, biochemical and biophysical reactions. Extracellular biophysical factors regulate cellular destiny dynamically. This review examines the bioinspired structural and functional properties of silk material based scaffolds. We explored silk types, chemical composition, architecture, mechanical properties, topography, and 3D geometry to unlock the body's innate regenerative potential, keeping in mind the novel biophysical properties of silk in film, fiber, and other potential forms, coupled with facile chemical changes, and its ability to match functional requirements for specific tissues.

Innovative electrospun PCL/fibroin/l-dopa scaffolds scaffolds supporting bone tissue regeneration

Poly-caprolactone is one of the most promising biocompatible polymers on the market, in particular for temporary devices that are not subjected to high physiological loads. Even if completely resorbable in various biological environments, poly-caprolactione does not play any specific biological role in supporting tissue regeneration and for this reason has a limited range of possible applications. In this preliminary work, for the first time l-dopa and fibroin have been combined with electrospun poly-caprolactone fibers in order to induce bioactive effects and, in particular, stimulate the proliferation, adhesion and osteoconduction of the polymeric fibers. Results showed that addition of low-molecular weight fibroin reduces the mechanical strength of the fibers while promoting the formation of mineralized deposits, when tested in vitro with KUSA-A1 mesenchymal cells. l-dopa, on the other hand, improved the mechanical properties and stimulated the formation of agglomerates of mineralized deposits containing calcium and phosphorous with high specific volume. The combination of the two substances resulted in good mechanical properties and higher amounts of mineralized deposits formed in vitro.

A Comparative Analysis of the Structure and Biological Properties of Films and Microfibrous Scaffolds Based on Silk Fibroin

A comparative analysis of the structure and biological properties of silk fibroin constructions was performed. Three groups of constructions were obtained: films obtained by casting an aqueous solution of silk fibroin and electrospun microfibrous scaffolds based on silk fibroin, with the addition of 30% gelatin per total protein weight. The internal structures of the films and single fibers of the microfibrous scaffolds consisted of densely packed globule structures; the surface area to volume ratios and volume porosities of the microfibrous scaffolds were calculated. All constructions were non-toxic for cells and provide high levels of adhesion and proliferation. The high regenerative potential of the constructions was demonstrated in a rat full-thickness skin wound healing model. The constructions accelerated healing by an average of 15 days and can be considered to be promising constructions for various tasks of tissue engineering and regenerative medicine.

Solid Organ Bioprinting: Strategies to Achieve Organ Function

The field of tissue engineering has advanced over the past decade, but the largest impact on human health should be achieved with the transition of engineered solid organs to the clinic. The number of patients suffering from solid organ disease continues to increase, with over 100 000 patients on the U.S. national waitlist and approximately 730 000 deaths in the United States resulting from end-stage organ disease annually. While flat, tubular, and hollow nontubular engineered organs have already been implanted in patients, in vitro formation of a fully functional solid organ at a translatable scale has not yet been achieved. Thus, one major goal is to bioengineer complex, solid organs for transplantation, composed of patient-specific cells. Among the myriad of approaches attempted to engineer solid organs, 3D bioprinting offers unmatched potential. This review highlights the structural complexity which must be engineered at nano-, micro-, and mesostructural scales to enable organ function. We showcase key advances in bioprinting solid organs with complex vascular networks and functioning microstructures, advances in biomaterials science that have enabled this progress, the regulatory hurdles the field has yet to overcome, and cutting edge technologies that bring us closer to the promise of engineered solid organs.

Glycosylated-Chitosan Derivatives: A Systematic Review

Chitosan derivatives, and more specifically, glycosylated derivatives, are nowadays attracting much attention within the scientific community due to the fact that this set of engineered polysaccharides finds application in different sectors, spanning from food to the biomedical field. Overcoming chitosan (physical) limitations or grafting biological relevant molecules, to mention a few, represent two cardinal strategies to modify parent biopolymer; thereby, synthetizing high added value polysaccharides. The present review is focused on the introduction of oligosaccharide side chains on the backbone of chitosan. The synthetic aspects and the effect on physical-chemical properties of such modifications are discussed. Finally, examples of potential applications in biomaterials design and drug delivery of these novel modified chitosans are disclosed.

Well-defined monolith morphology regulates cell adhesion and its functions

Hydrophilic epoxy resin-based monoliths were employed as cell culture substrates. The monoliths were made of a porous material with a bicontinuous structure that consisted of a porous channel and a resin skeleton. Monolith disks were prepared with a skinless surface through polymerization-induced spinodal decomposition-type phase separation. The pore sizes, which were well controlled by the polymerization temperature, ranged from 70 to 380 nm. The quantity of protein adsorbed per unit area and the early-stage adhesion of HepG2 cells on the monolith substrates were independent of pore size, meaning they were not affected by surface topology. Long-term cell adhesion, as indicated by adherent cell number and shape, as well as liver-specific gene expression were significantly affected by pore size. In terms of cell shape, number, and gene expression, pores of approximately 200 nm were most suitable for HepG2 cell growth. These results highlight the importance of monolith morphology for use as a cell culture substrate. The well-controlled morphology demonstrated in this work indicates monoliths are capable of supporting growth for various types of cells in a range of applications.

Strategies to Tune Electrospun Scaffold Porosity for Effective Cell Response in Tissue Engineering

Tissue engineering aims to develop artificial human tissues by culturing cells on a scaffold in the presence of biochemical cues. Properties of scaffold such as architecture and composition highly influence the overall cell response. Electrospinning has emerged as one of the most affordable, versatile, and successful approaches to develop nonwoven nano/microscale fibrous scaffolds whose structural features resemble that of the native extracellular matrix. However, dense packing of the fibers leads to small-sized pores which obstruct cell infiltration and therefore is a major limitation for their use in tissue engineering applications. To this end, a variety of approaches have been investigated to enhance the pore properties of the electrospun scaffolds. In this review, we collect state-of-the-art modification methods and summarize them into six classes as follows: approaches focused on optimization of packing density by (a) conventional setup, (b) sequential or co-electrospinning setups, (c) involving sacrificial elements, (d) using special collectors, (e) post-production processing, and (f) other specialized methods. Overall, this review covers historical as well as latest methodologies in the field and therefore acts as a quick reference for those interested in electrospinning matrices for tissue engineering and beyond.

Effect of Silk Fibroin on Cell Viability in Electrospun Scaffolds of Polyethylene Oxide

In this study, a coating from electrospun silk fibroin was performed with the aim to modify the surface of breast implants. We evaluated the effect of fibroin on polymeric matrices of poly (ethylene oxide) (PEO) to enhance cell viability, adhesion, and proliferation of HaCaT human keratinocytes to enhance the healing process on breast prosthesis implantation. We electrospun six blends of fibroin and PEO at different concentrations. These scaffolds were characterized by scanning electron microscopy, contact angle measurements, ATR-FTIR spectroscopy, and X-ray diffraction. We obtained diverse network conformations at different combinations to examine the regulation of cell adhesion and proliferation by modifying the microstructure of the matrix to be applied as a potential scaffold for coating breast implants. The key contribution of this work is the solution it provides to enhance the healing process on prosthesis implantation considering that the use of these PEO⁻fibroin scaffolds reduced (p < 0.05) the amount of pyknotic nuclei. Therefore, viability of HaCaT human keratinocytes on PEO⁻fibroin matrices was significantly improved (p < 0.001). These findings provide a rational strategy to coat breast implants improving biocompatibility.

Remodeling of inherent antimicrobial nanofiber dressings with melamine-modified fibroin into neoskin

A melamine-modified fibroin was synthesized and fabricated into electrospun nanofiber films with broad-spectrum antimicrobial activity, sustained water retention, and fast reepithelialization and revascularization.

Functional hepatocyte clusters on bioactive blend silk matrices towards generating bioartificial liver constructs

The creation of in vitro functional hepatic tissue simulating micro-environmental niche of native liver is a keen area of research due to its demand in bioartificial liver (BAL) and cell-based tissue engineering. Here, we investigated the potential of novel blend (BA) silk scaffold fabricated by blending mulberry (Bombyx mori, BM) silk fibroin with cell adhesion motif (RGD) rich non-mulberry (Antheraea assamensis, AA) silk fibroin, in generating a functional liver construct. Three-dimensional (3D) porous silk scaffolds (BM, AA and BA) were physico-chemically characterized and functionally evaluated using human hepatocarcinoma cells (HepG2) and primary neonatal rat hepatocytes. The growth and distribution of hepatocytes within the scaffolds were tracked by FESEM, alamar blue proliferation assay and live/dead staining. Hemocompatible BA scaffolds supported the formation of high density hepatocyte clusters, facilitating cell-matrix and cell-cell interactions. Blend scaffolds evinced enhanced liver-specific functions of cultured hepatocytes in terms of albumin synthesis, urea synthesis and cytochrome P450 enzyme activity over 21 days. Subcutaneous implantation of scaffolds demonstrated minimal macrophage infiltration in blend scaffolds. These findings substantiate that the integral property of blend (BA) scaffold offers a befitting environment by influencing spheroidal growth of hepatocytes with enhanced biological activity. Collectively, the present study provides a new 3D bio-matrix niche for growing functional liver cells that would have future prospects in BAL as well as regenerative medicine.

STATEMENT OF SIGNIFICANCE

An end stage liver disease called cirrhosis perturbs the self-healing ability and physiological functions of liver. Due to the scarcity of healthy donors, a functional in vitro hepatic construct retaining the liver-specific functions is in great demand for its prospects in bioartificial liver (BAL) and cell-based tissue engineering. Physicochemical attributes of a matrix influence the behavior of cultured hepatocytes in terms of attachment, morphology and functionality. Mulberry and non-mulberry silk fibroin presents unique amino acid sequence with difference in hydrophobicity and crystallinity. Considering this, the present study focuses on the development of a suitable three-dimensional (3D) bioactive matrix incorporating both mulberry silk fibroin and cell adhesion motif (RGD) rich non-mulberry silk fibroin. Porous silk blend scaffolds facilitated the formation of hepatocyte clusters with enhanced liver-specific functions emphasizing both cell-cell and cell-matrix interactions. Hemocompatibility and integral property of blend scaffolds offers a biological niche for seeding functional liver cells that would have future prospects in biohybrid devices.

Injectable silk fibroin hydrogels functionalized with microspheres as adult stem cells-carrier systems

Hydrogels are good candidate materials for cell delivery scaffolds because they can mimic the physical, chemical, electrical and biological properties of most of the native tissues. In this study, composite biosynthetic hydrogels were produced by combining the bio-functionality of silk fibroin (SF) with the structural versatility of polyethylene-glycol-diacrylated (PEGDa). The formation of a photopolymerizable PEGDa-SF hydrogel (PSFHy) was optimized for 3D-cell culture. Functionalization of the 3D-PSFHy with protein microspheres (MS) was required to increase the porosity and cell-adhesive properties of the material. Cardiac mesenchymal stem cells, which were cultured within the MS-embedding PSFHy, exhibited good viability and expression of proteins that are characteristic of the initial phases of the cardiac muscle differentiation process. Further, the addition of chondroitin sulfate into the scaffolds improved the cell viability. A cell-preconditioning of the scaffold was also performed, suggesting a potential application of these sponge-like scaffolds for analysing the effects of several extracellular microenvironments, produced by different kinds of cells, on the stem cells fate. The results presented herein highlight on the possibility to use the PSFHys functionalized with MS as stem cell-carrier systems with sponge-like properties, potential ultrasound-imaging contrast agents and controlled biochemical factor delivery.

Silk fibroin gelation via non-solvent induced phase separation

Tissue engineering benefits from novel materials with precisely tunable physical, chemical and mechanical properties over a broad range. Here we report a practical approach to prepare Bombyx mori silk fibroin hydrogels using the principle of non-solvent induced phase separation (NIPS). A combination of reconstituted silk fibroin (RSF) and methanol (non-solvent), with a final concentration of 2.5% w/v and 12.5% v/v respectively, maintained at 22 °C temperature turned into a hydrogel within 10 hours. Freeze-drying of this gel gave a foam with a porosity of 88%, a water uptake capacity of 89% and a swelling index of 8.6. The gelation kinetics and the loss tangent of the gels were investigated by rheometry. The changes in the morphology of the porous foams were visualized by SEM. The changes in RSF chemical composition and the relative fraction of its secondary structural elements were analyzed by ATR-FTIR along with Fourier self-deconvolution. And, the changes in the glass transition temperature, specific heat capacity and the relative fraction of crystallinity of RSF were determined by TM-DSC. Data suggested that RSF-water-methanol behaved as a polymer-solvent-non-solvent ternary phase system, wherein the demixing of the water-methanol phases altered the thermodynamic equilibrium of RSF-water phases and resulted in the desolvation and eventual separation of the RSF phase. Systematic analysis revealed that both gelation time and the properties of hydrogels and porous foams could be controlled by the ratios of RSF and non-solvent concentration as well as by the type of non-solvent and incubation temperature. Due to the unique properties we envisage that the herein prepared NIPS induced RSF hydrogels and porous foams can possibly be used for the encapsulation of cells and/or for the controlled release of both hydrophilic and hydrophobic drugs.

Comparative Study of Ultrasonication-Induced and Naturally Self-Assembled Silk Fibroin-Wool Keratin Hydrogel Biomaterials

This study reports the formation of biocompatible hydrogels using protein polymers from natural silk cocoon fibroins and sheep wool keratins. Silk fibroin protein contains β-sheet secondary structures, allowing for the formation of physical cross-linkers in the hydrogels. Comparative studies were performed on two groups of samples. In the first group, ultrasonication was used to induce a quick gelation of a protein aqueous solution, enhancing the ability of Bombyx mori silk fibroin chains to quickly entrap the wool keratin protein molecules homogenously. In the second group, silk/keratin mixtures were left at room temperature for days, resulting in naturally-assembled gelled solutions. It was found that silk/wool blended solutions can form hydrogels at different mixing ratios, with perfectly interconnected gel structure when the wool content was less than 30 weight percent (wt %) for the first group (ultrasonication), and 10 wt % for the second group (natural gel). Differential scanning calorimetry (DSC) and temperature modulated DSC (TMDSC) were used to confirm that the fibroin/keratin hydrogel system was well-blended without phase separation. Fourier transform infrared spectroscopy (FTIR) was used to investigate the secondary structures of blended protein gels. It was found that intermolecular β-sheet contents significantly increase as the system contains more silk for both groups of samples, resulting in stable crystalline cross-linkers in the blended hydrogel structures. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to analyze the samples' characteristic morphology on both micro- and nanoscales, which showed that ultrasonic waves can significantly enhance the cross-linker formation and avoid phase separation between silk and keratin molecules in the blended systems. With the ability to form cross-linkages non-chemically, these silk/wool hydrogels may be economically useful for various biomedical applications, thanks to the good biocompatibility of protein molecules and the various characteristics of hydrogel systems.

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.

Preliminary study of a novel nanofiber-based valve integrated tubular graft as an alternative for a pulmonary valved artery

A tri-leaflet valve integrated tubular scaffold was obtained using a 3D printing mold by TIPS. After testing its valuvalar performance via computational fluid dynamics, the biocompatibility of resultant valve scaffold was evaluated in vivo.

Blood-Compatible Polymer for Hepatocyte Culture with High Hepatocyte-Specific Functions toward Bioartificial Liver Development

The development of bioartificial liver (BAL) is expected because of the shortage of donor liver for transplantation. The substrates for BAL require the following criteria: (a) blood compatibility, (b) hepatocyte adhesiveness, and (c) the ability to maintain hepatocyte-specific functions. Here, we examined blood-compatible poly(2-methoxyethyl acrylate) (PMEA) and poly(tetrahydrofurfuryl acrylate) (PTHFA) (PTHFA) as the substrates for BAL. HepG2, a human hepatocyte model, could adhere on PMEA and PTHFA substrates. The spreading of HepG2 cells was suppressed on PMEA substrates because integrin contribution to cell adhesion on PMEA substrate was low and integrin signaling was not sufficiently activated. Hepatocyte-specific gene expression in HepG2 cells increased on PMEA substrate, whereas the expression decreased on PTHFA substrates due to the nuclear localization of Yes-associated protein (YAP). These results indicate that blood-compatible PMEA is suitable for BAL substrate. Also, PMEA is expected to be used to regulate cell functions for blood-contacting tissue engineering.

Response of human mesenchymal stem cells to intrafibrillar nanohydroxyapatite content and extrafibrillar nanohydroxyapatite in biomimetic chitosan/silk fibroin/nanohydroxyapatite nanofibrous membrane scaffolds

Incorporation of nanohydroxyapatite (nHAP) within a chitosan (CS)/silk fibroin (SF) nanofibrous membrane scaffold (NMS) may provide a favorable microenvironment that more closely mimics the natural bone tissue physiology and facilitates enhanced osteogensis of the implanted cell population. In this study, we prepared pristine CS/SF NMS, composite CS/SF/nHAP NMS containing intrafibrillar nHAP by in situ blending of 10% or 30% nHAP before the electrospinning step, and composite CS/SF/nHAP NMS containing extrafibrillar nHAP by depositing 30% nHAP through alternative soaking surface mineralization. We investigated the effect of the incorporation of HAP nanoparticles on the physicochemical properties of pristine and composite NMS. We confirmed the presence of ~30 nm nHAP in the composite nanofibrous membranes by thermogravimetry analysis (TGA), X-ray diffraction (XRD), and scanning electron microscopy (SEM), either embedded in or exposed on the nanofiber. Nonetheless, the alternative soaking surface mineralization method drastically influenced the mechanical properties of the NMS with 88% and 94% drop in Young’s modulus and ultimate maximum stress. Using in vitro cell culture experiments, we investigated the effects of nHAP content and location on proliferation and osteogenic differentiation of human bone marrow mesenchymal stem cells (hMSCs). The proliferation of hMSCs showed no significant difference among pristine and composite NMS. However, the extent of osteogenic differentiation of hMSCs was found to be positively correlated with the content of nHAP in the NMS, while its location within the nanofiber played a less significant role. In vivo experiments were carried out with hMSCs seeded in CS/SF/30%nHAP NMS prepared by in situ blending and subcutaneous implantation in nude mice. Micro-computed tomography images as well as histological and immunohistochemical analysis of the retrieved hMSCs/NMS construct 1 and 2 months postimplantation indicated that NMS had the potential for bone regeneration and can be suggested as a promising scaffold for bone tissue engineering.

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.

Cellulose nanowhiskers and fiber alignment greatly improve mechanical properties of electrospun prolamin protein fibers

Electrospun fibers from natural polymers must possess appropriate mechanical properties if they are to be functional in numerous applications. In this research, two convenient physical approaches were applied to reinforce the assembled hordein/zein electrospun nanofabrics: incorporation of surface-modified cellulose nanowhiskers (SCN) and fiber alignment. The mechanical properties and stability of the modified fibers were tested in relation to fiber morphology and structure as characterized by scanning electron microscopy, transmission electron microscopy, Fourier-transform infrared spectroscopy, and Raman spectroscopy. SCN modified by quaternary ammonium salt were well-dispersed in hordein/zein networks, leading to fibers with significantly improved mechanical properties and water resistance. With the addition of 3 wt % SCN, the tensile strength and Young's modulus of hordein/zein fibers increased from 4.36 ± 0.29 to 7.79 ± 0.36 MPa and from 195.80 ± 13.02 to 396.64 ± 18.33 MPa, respectively, and the elongation at break was retained because of the formation of a percolating network of SCN. The alignment of electrospun fibers strengthened the hordein/zein nanofabrics in both tangential and normal directions to 17.26 ± 1.41 and 14.02 ± 0.74 MPa, respectively, by not only altering the piling up pattern, but also by promoting phase separation and improved interactions. When applying both of the reinforcing methods, the tensile strength of hordein/zein fibers was further enhanced to 21.99 ± 1.19 MPa, stronger than that of cancellous bones (5-10 MPa). All the reinforced fibers exhibited a reduced burst effect in phosphate-buffered saline (PBS) while releasing the incorporated bioactive molecule in a controlled manner. These physically reinforced prolamin protein fibers possessed significantly improved mechanical properties and may have potential to be used as tissue engineering scaffold materials or natural delivery systems for biomedical applications.

Advancements in electrospinning of polymeric nanofibrous scaffolds for tissue engineering

Polymeric nanofibers have potential as tissue engineering scaffolds, as they mimic the nanoscale properties and structural characteristics of native extracellular matrix (ECM). Nanofibers composed of natural and synthetic polymers, biomimetic composites, ceramics, and metals have been fabricated by electrospinning for various tissue engineering applications. The inherent advantages of electrospinning nanofibers include the generation of substrata with high surface area-to-volume ratios, the capacity to precisely control material and mechanical properties, and a tendency for cellular in-growth due to interconnectivity within the pores. Furthermore, the electrospinning process affords the opportunity to engineer scaffolds with micro- to nanoscale topography similar to the natural ECM. This review describes the fundamental aspects of the electrospinning process when applied to spinnable natural and synthetic polymers; particularly, those parameters that influence fiber geometry, morphology, mesh porosity, and scaffold mechanical properties. We describe cellular responses to fiber morphology achieved by varying processing parameters and highlight successful applications of electrospun nanofibrous scaffolds when used to tissue engineer bone, skin, and vascular grafts.

Cryogenic electrospinning: proposed mechanism, process parameters and its use in engineering of bilayered tissue structures

BACKGROUND

Conventional electrospun scaffolds have very small pores, thus limiting cellular infiltration, tissue ingrowth and vascularization in tissue engineering applications. The cryogenic electrospinning process overcame the small pore size constraints found in conventional electrospun scaffolds.

AIM

The aim of this paper is to propose a mechanism for cryogenic electrospinning and how scaffold pore size can be controlled.

MATERIALS & METHODS

We studied the roles of ice crystals in controlling the pore size of cryogenic electrospun scaffolds (CES). Based on this understanding, we have successfully fabricated a bilayered scaffold with distinctly different pore sizes.

RESULTS

Our study showed that CES pore size was dependent on the structure of the frost layer formed and hence the factors affecting ice deposition. The bilayered scaffold was able to support the coculture of human dermal fibroblasts and keratinocytes.

CONCLUSION

The larger pores of CES add versatility to the use of electrospun scaffolds in tissue engineering applications.

Silk fibroin in tissue engineering

Tissue engineering (TE) is a multidisciplinary field that aims at the in vitro engineering of tissues and organs by integrating science and technology of cells, materials and biochemical factors. Mimicking the natural extracellular matrix is one of the critical and challenging technological barriers, for which scaffold engineering has become a prime focus of research within the field of TE. Amongst the variety of materials tested, silk fibroin (SF) is increasingly being recognized as a promising material for scaffold fabrication. Ease of processing, excellent biocompatibility, remarkable mechanical properties and tailorable degradability of SF has been explored for fabrication of various articles such as films, porous matrices, hydrogels, nonwoven mats, etc., and has been investigated for use in various TE applications, including bone, tendon, ligament, cartilage, skin, liver, trachea, nerve, cornea, eardrum, dental, bladder, etc. The current review extensively covers the progress made in the SF-based in vitro engineering and regeneration of various human tissues and identifies opportunities for further development of this field.