Electrospun silk biomaterial scaffolds for regenerative medicine

Control of olfactory ensheathing cell behaviors by electrospun silk fibroin fibers

Olfactory ensheathing cells (OECs), the only glial cell type that normally penetrates the transition zone between the peripheral and central nervous systems, are one of the most promising candidates for cell transplantation in repairing spinal cord injury (SCI). However, we must manipulate and regulate OECs' behavior to make these cells effective in cell transplantation. In the present study, we assessed the response of rat OECs to different variants of nanofibrous silk fibroin mats with regard to cell morphology, adhesion, proliferation, and migration and the related gene and protein expression. Results showed that OECs adhere and spread more easily on Tussah silk fibroin (TSF) fibers than Bombyx mori silk fibroin fibers, resulting in a higher rate of cell proliferation and gene and protein expression, examined by RT-PCR and ELISA. In addition, the morphology of OECs on microfibers is mainly polygonal with short protrusions, whereas the OECs on nanofibers exhibit a bipolar shape with long protrusions that align along the fibers, especially when aligned fibers are employed. Moreover, OECs on silk fibroin nanofibers migrate more efficiently than those on poly-L-lysine (PLL). Based on the experimental results, the morphology, adhesion, spread, gene and protein expression, and migration of OECs could be modulated and regulated by adjusting the contents and structure of silk fibroin nanofibers, shedding light on the control of OECs' behavior in nerve tissue engineering and thus the future therapeutic intervention for nerve repair after injury. This manuscript is published as part of the International Association of Neurorestoratology (IANR) supplement issue of Cell Transplantation.

Electro-spinning/netting: A strategy for the fabrication of three-dimensional polymer nano-fiber/nets

Since 2006, a rapid development has been achieved in a subject area, so called electro-spinning/netting (ESN), which comprises the conventional electrospinning process and a unique electro-netting process. Electro-netting overcomes the bottleneck problem of electrospinning technique and provides a versatile method for generating spider-web-like nano-nets with ultrafine fiber diameter less than 20 nm. Nano-nets, supported by the conventional electrospun nanofibers in the nano-fiber/nets (NFN) membranes, exhibit numerious attractive characteristics such as extremely small diameter, high porosity, and Steiner tree network geometry, which make NFN membranes optimal candidates for many significant applications. The progress made during the last few years in the field of ESN is highlighted in this review, with particular emphasis on results obtained in the author's research units. After a brief description of the development of the electrospinning and ESN techniques, several fundamental properties of NFN nanomaterials are addressed. Subsequently, the used polymers and the state-of-the-art strategies for the controllable fabrication of NFN membranes are highlighted in terms of the ESN process. Additionally, we highlight some potential applications associated with the remarkable features of NFN nanostructure. Our discussion is concluded with some personal perspectives on the future development in which this wonderful technique could be pursued.

Electrospun drug loaded membranes for sublingual administration of sumatriptan and naproxen

Sublingual administration of active pharmaceutical substances is in principle favourable for rapid onset of drug action, ready accessibility and avoidance of first pass metabolism. This administration could prove very useful in the treatment of migraines, thus two frequently used drugs were selected for our study. Sumatriptan succinate, naproxen, and its salt as well as combinations of these were incorporated into nanofibrous membranes via the electrospinning process. DSC measurements proved that the resulted membranes contained non-crystalline drug forms. SEM imaging approved good homogeneity of diameter and shape of the membrane nanofibres. The nanofibrous membranes always showed the rapid and mutually independent release of the tested drugs. The drugs exhibited very high differences in sublingual permeation rates in vitro, but the rates of both substances were increased several times using nanofibrous membranes as the drug carrier in comparison to drug solutions. The released drugs subsequently permeated through sublingual mucosa preferentially as non-ionized moieties. The prepared nanofibrous membranes proved very flexible and mechanically resistant. With their drug load capacity of up to 40% of membrane mass, they could be very advantageous for the formulation of sublingual drug delivery systems.

Biomimetic electrospun nanofibrous structures for tissue engineering

Biomimetic nanofibrous scaffolds mimicking important features of the native extracellular matrix provide a promising strategy to restore functions or achieve favorable responses for tissue regeneration. This review provides a brief overview of current state-of-the-art research designing and using biomimetic electrospun nanofibers as scaffolds for tissue engineering. It begins with a brief introduction of electrospinning and nanofibers, with a focus on issues related to the biomimetic design aspects. The review next focuses on several typical biomimetic nanofibrous structures (e.g. aligned, aligned to random, spiral, tubular, and sheath membrane) that have great potential for tissue engineering scaffolds, and describes their fabrication, advantages, and applications in tissue engineering. The review concludes with perspectives on challenges and future directions for design, fabrication, and utilization of scaffolds based on electrospun nanofibers.

Engineering the microstructure of electrospun fibrous scaffolds by microtopography

Controlling the structure and organization of electrospun fibers is desirable for fabricating scaffolds and materials with defined microstructures. However, the effects of microtopography on the deposition and, in turn, the organization of the electrospun fibers are not well understood. In this study, conductive polydimethylsiloxane (PDMS) templates with different micropatterns were fabricated by combining photolithography, silicon wet etching, and PDMS molding techniques. The fiber organization was varied by fine-tuning the microtopography of the electrospinning collector. Fiber conformity and alignment were influenced by the depth and the slope of microtopography features, resulting in scaffolds comprising either an array of microdomains with different porosity and fiber alignment or an array of microwells. Microtopography affected the fiber organization for hundreds of micrometers below the scaffold surface, resulting in scaffolds with distinct surface properties on each side. In addition, the fiber diameter was also affected by the fiber conformity. The effects of the fiber arrangement in the scaffolds on the morphology, migration, and infiltration of cells were examined by in vitro and in vivo experiments. Cell morphology and organization were guided by the fibers in the microdomains, and cell migration was enhanced by the aligned fibers and the three-dimensional scaffold structure. Cell infiltration was correlated with the microdomain porosity. Microscale control of the fiber organization and the porosity at the surface and through the thickness of the fibrous scaffolds, as demonstrated by the results of this study, provides a powerful means of engineering the three-dimensional structure of electrospun fibrous scaffolds for cell and tissue engineering.

Creation of macropores in electrospun silk fibroin scaffolds using sacrificial PEO-microparticles to enhance cellular infiltration

Electrospun scaffolds are widely used in tissue engineering; however, a common problem is the poor cell infiltration because of the small pore size and tightly packed structure of these fibrous scaffolds. To address this issue, a novel technique was developed to fabricate electrospun silk fibroin (SF) scaffolds with rather macropores and high porosity using electrospraying-generated PEO microparticles as porogen. The morphology and pore size of MPES scaffolds were evaluated by scanning electron microscopy. It was revealed that MPES scaffold had a relatively loose structure with an increase of mean pore size (i.e., approx. 30 μm of MPES vs. approx. 5 μm of traditional electrospun scaffolds (TES) and porosity (i.e., 95% vs. 84% of TES). Culture of mouse 3T3 fibroblast in TES and MPES scaffold revealed that both scaffolds could support cell attachment, spread and proliferation. Yet, cell inflitration in vitro under the static culture condition only occurred in the MPES scaffold. Subcutaneous implantation of scaffolds in rats further confirmed that the tissue ingrowth was more efficient in the MPES scaffold compared to TES scaffold. Thus, the use of PEO microparticles as porogen was a feasible and effective method for creating macroporous electrospun SF scaffold, which provided an alternative to address the limitation of cell infiltration associated with electrospun fibrous scaffold.

Silk fibroin biomaterials for tissue regenerations

Regeneration of tissues using cells, scaffolds and appropriate growth factors is a key approach in the treatments of tissue or organ failure. Silk protein fibroin can be effectively used as a scaffolding material in these treatments. Silk fibers are obtained from diverse sources such as spiders, silkworms, scorpions, mites and flies. Among them, silk of silkworms is a good source for the development of biomedical device. It possesses good biocompatibility, suitable mechanical properties and is produced in bulk in the textile sector. The unique combination of elasticity and strength along with mammalian cell compatibility makes silk fibroin an attractive material for tissue engineering. The present article discusses the processing of silk fibroin into different forms of biomaterials followed by their uses in regeneration of different tissues. Applications of silk for engineering of bone, vascular, neural, skin, cartilage, ligaments, tendons, cardiac, ocular, and bladder tissues are discussed. The advantages and limitations of silk systems as scaffolding materials in the context of biocompatibility, biodegradability and tissue specific requirements are also critically reviewed.

Construction of engineering adipose-like tissue in vivo utilizing human insulin gene-modified umbilical cord mesenchymal stromal cells with silk fibroin 3D scaffolds

We evaluated the use of a combination of human insulin gene-modified umbilical cord mesenchymal stromal cells (hUMSCs) with silk fibroin 3D scaffolds for adipose tissue engineering. In this study hUMSCs were isolated and cultured. HUMSCs infected with Ade-insulin-EGFP were seeded in fibroin 3D scaffolds with uniform 50-60 µm pore size. Silk fibroin scaffolds with untransfected hUMSCs were used as control. They were cultured for 4 days in adipogenic medium and transplanted under the dorsal skins of female Wistar rats after the hUMSCs had been labelled with chloromethylbenzamido-1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (CM-Dil). Macroscopical impression, fluorescence observation, histology and SEM were used for assessment after transplantation at 8 and 12 weeks. Macroscopically, newly formed adipose tissue was observed in the experimental group and control group after 8 and 12 weeks. Fluorescence observation supported that the formed adipose tissue originated from seeded hUMSCs rather than from possible infiltrating perivascular tissue. Oil red O staining of newly formed tissue showed that there was substantially more tissue regeneration in the experimental group than in the control group. SEM showed that experimental group cells had more fat-like cells, whose volume was larger than that of the control group, and degradation of the silk fibroin scaffold was greater under SEM observation. This study provides significant evidence that hUMSCs transfected by adenovirus vector have good compatibility with silk fibroin scaffold, and adenoviral transfection of the human insulin gene can be used for the construction of tissue-engineered adipose.

Bioengineered silk proteins to control cell and tissue functions

Silks are defined as protein polymers that are spun into fibers by some lepidoptera larvae such as silkworms, spiders, scorpions, mites, and flies. Silk proteins are usually produced within specialized glands in these animals after biosynthesis in epithelial cells that line the glands, followed by secretion into the lumen of the gland prior to spinning into fibers.The most comprehensively characterized silks are from the domesticated silkworm (Bombyx mori) and from some spiders (Nephila clavipes and Araneus diadematus). Silkworm silk has been used commercially as biomedical sutures for decades and in textile production for centuries. Because of their impressive mechanical properties, silk proteins provide an important set of material options in the fields of controlled drug release, and for biomaterials and scaffolds for tissue engineering. Silkworm silk from B. mori consists primarily of two protein components, fibroin, the structural protein of silk fibers, and sericins, the water-soluble glue-like proteins that bind the fibroin fibers together. Silk fibroin consists of heavy and light chain polypeptides linked by a disulfide bond. Fibroin is the protein of interest for biomedical materials and it has to be purified/extracted from the silkworm cocoon by removal of the sericin. Characteristics of silks, including biodegradability, biocompatibility, controllable degradation rates, and versatility to generate different material formats from gels to fibers and sponges, have attracted interest in the field of biomaterials. Cell culture and tissue formation using silk-based biomaterials have been pursued, where appropriate cell adhesion, proliferation, and differentiation on or in silk biomaterials support the regeneration of tissues. The relative ease with which silk proteins can be processed into a variety of material morphologies, versatile chemical functionalization options, processing in water or solvent, and the related biological features of biocompatibility and enzymatic degradability make these proteins interesting candidates for biomedical applications.

Seamless, axially aligned, fiber tubes, meshes, microbundles and gradient biomaterial constructs

A new electrospinning apparatus was developed to generate nanofibrous materials with improved organizational control. The system functions by oscillating the deposition signal (ODS) of multiple collectors, allowing significantly improved nanofiber control by manipulating the electric field which drives the electrospinning process. Other electrospinning techniques designed to impart deposited fiber organizational control, such as rotating mandrels or parallel collector systems, do not generate seamless constructs with high quality alignment in sizes large enough for medical devices. In contrast, the ODS collection system produces deposited fiber networks with highly pure alignment in a variety of forms and sizes, including flat (8 × 8 cm(2)), tubular (1.3 cm diameter), or rope-like microbundle (45 μm diameter) samples. Additionally, the mechanism of our technique allows for scale-up beyond these dimensions. The ODS collection system produced 81.6 % of fibers aligned within 5° of the axial direction, nearly a four-fold improvement over the rotating mandrel technique. The meshes produced from the 9 % (w/v) fibroin/PEO blend demonstrated significant mechanical anisotropy due to the fiber alignment. In 37 °C PBS, aligned samples produced an ultimate tensile strength of 16.47 ± 1.18 MPa, a Young's modulus of 37.33 MPa, and a yield strength of 7.79 ± 1.13 MPa. The material was 300 % stiffer when extended in the direction of fiber alignment and required 20 times the amount of force to be deformed, compared to aligned meshes extended perpendicular to the fiber direction. The ODS technique could be applied to any electrospinnable polymer to overcome the more limited uniformity and induced mechanical strain of rotating mandrel techniques, and greatly surpasses the limited length of standard parallel collector techniques.

Nanocomposite gold-silk nanofibers

Cell-biomaterial interactions can be controlled by modifying the surface chemistry or nanotopography of the material, to induce cell proliferation and differentiation if desired. Here we combine both approaches in forming silk nanofibers (SNFs) containing gold nanoparticles (AuNPs) and subsequently chemically modifying the fibers. Silk fibroin mixed with gold seed nanoparticles was electrospun to form SNFs doped with gold seed nanoparticles (SNF(seed)). Following gold reduction, there was a 2-fold increase in particle diameter confirmed by the appearance of a strong absorption peak at 525 nm. AuNPs were dispersed throughout the AuNP-doped silk nanofibers (SNFs(Au)). The Young's modulus of the SNFs(Au) was almost 70% higher than that of SNFs. SNFs(Au) were modified with the arginine-glycine-aspartic acid (RGD) peptide. Human mesenchymal stem cells that were cultured on RGD-modified SNF(Au) had a more than 2-fold larger cell area compared to the cells cultured on bare SNFs; SNF(Au) also increased cell size. This approach may be used to alter the cell-material interface in tissue engineering and other applications.

Biomolecule gradient in micropatterned nanofibrous scaffold for spatiotemporal release

Controlled molecule release from scaffolds can dramatically increase the scaffold ability of directing tissue regeneration in vitro and in vivo. Crucial to the regeneration is precise regulation over release direction and kinetics of multiple molecules (small genes, peptides, or larger proteins). To this end, we developed gradient micropatterns of electrospun nanofibers along the scaffold thickness through programming the deposition of heterogeneous nanofibers of poly(ε-caprolactone) (PCL) and poly(D,L-lactide-co-glycolide) acid (PLGA). Confocal images of the scaffolds containing fluorophore-impregnated nanofibers demonstrated close matching of actual and designed gradient fiber patterns; thermal analyses further showed their matching in the composition. Using acid-terminated PLGA (PLGAac) and ester-terminated PLGA (PLGAes) to impregnate molecules in the PCL-PLGA scaffolds, we demonstrated for the first time their differences in nanofiber degeneration and molecular weight change during degradation. PLGAac nanofibers were more stable with gradual and steady increase in the fiber diameter during degradation, resulting in more spatially confined molecule delivery from PCL-PLGA scaffolds. Thus, patterns of PCL-PLGAac nanofibers were used to design versatile controlled delivery scaffolds. To test the hypothesis that molecule-impregnated PLGAac in the gradient-patterned PCL-PLGAac scaffolds can program various modalities of molecule release, model molecules, including small fluorophores and larger proteins, were respectively used for time-lapse release studies. Gradient-patterns were used as building blocks in the scaffolds to program simultaneous release of one or multiple proteins to one side or, respectively, to the opposite sides of scaffolds for up to 50 days. Results showed that the separation efficiency of molecule delivery from all the scaffolds with a thickness of 200 μm achieved >88% for proteins and >82% for small molecules. In addition to versatile spatially controlled delivery, micropatterns were designed to program sequential release of proteins. The hierarchically structured materials presented here may enable development of novel multifunctional scaffolds with defined 3D dynamic microenvironments for tissue regeneration.

Developing scaffolds for tissue engineering using the Ca2+-induced cold gelation by an experimental design approach

The Ca(2+)-induced cold gelation technique was found suitable to prepare highly porous biodegradable scaffolds based on bovine serum albumin (BSA) and alpha-casein from bovine milk for tissue engineering. A 2(3) full factorial design was used to study the influence and impact of each factor on the several responses of the scaffolds. In vitro degradation (ID), swelling ratio (SR), porosity (PO), and pore size (PS) as well cytotoxicity (CT) were evaluated and shown to be dependent on the pH of sample preparation and on the amount of BSA and casein present, making these scaffolds tunable structures. Under optimized working conditions (4.19% of BSA, 0.69% of Casein, pH 7.07), the ID attained was 37.97%, the SR observed was 11.87, the PO was 82.11%, the PS measured was 180.63 μm at surface, and 175.91 μm at fracture, whereas maximum cell viability was 84% in comparison to controls. Moreover, the scaffold supported cell adhesion and proliferation. These results, consistent with the prediction by the experimental design approach, support the use of this methodology to develop tunable scaffolds for tissue engineering using the Ca(2+)-induced cold gelation.

Silk constructs for delivery of musculoskeletal therapeutics

Silk fibroin (SF) is a biopolymer with distinguishing features from many other bio- as well as synthetic polymers. From a biomechanical and drug delivery perspective, SF combines remarkable versatility for scaffolding (solid implants, hydrogels, threads, solutions), with advanced mechanical properties and good stabilization and controlled delivery of entrapped protein and small molecule drugs, respectively. It is this combination of mechanical and pharmaceutical features which renders SF so exciting for biomedical applications. This pattern along with the versatility of this biopolymer has been translated into progress for musculoskeletal applications. We review the use and potential of silk fibroin for systemic and localized delivery of therapeutics in diseases affecting the musculoskeletal system. We also present future directions for this biopolymer as well as the necessary research and development steps for their achievement.

Significantly reinforced composite fibers electrospun from silk fibroin/carbon nanotube aqueous solutions

Microcomposite fibers of regenerated silk fibroin (RSF) and multiwalled carbon nanotubes (MWNTs) were successfully prepared by an electrospinning process from aqueous solutions. A quiescent blended solution and a three-dimensional Raman image of the composite fibers showed that functionalized MWNTs (F-MWNTs) were well dispersed in the solutions and the RSF fibers, respectively. Raman spectra and wide-angle X-ray diffraction (WAXD) patterns of RSF/F-MWNT electrospun fibers indicated that the composite fibers had higher β-sheet content and crystallinity than the pure RSF electrospun fibers, respectively. The mechanical properties of the RSF electrospun fibers were improved drastically by incorporating F-MWNTs. Compared with the pure RSF electrospun fibers, the composite fibers with 1.0 wt % F-MWNTs exhibited a 2.8-fold increase in breaking strength, a 4.4-fold increase in Young's modulus, and a 2.1-fold increase in breaking energy. Cytotoxicity test preliminarily demonstrated that the electrospun fiber mats have good biocompatibility for tissue engineering scaffolds.

A photoactivated nanofiber graft material for augmented Achilles tendon repair

BACKGROUND AND OBJECTIVE

Suture repair of Achilles tendon rupture can cause infection, inflammation and scarring, while prolonged immobilization promotes adhesions to surrounding tissues and joint stiffness. Early mobilization can reduce complications provided the repair is strong enough to resist re-rupture. We have developed a biocompatible, photoactivated tendon wrap from electrospun silk (ES) to provide additional strength to the repair that could permit early mobilization, and act as a barrier to adhesion formation.

STUDY DESIGN/MATERIAL AND METHODS

ES nanofiber mats were prepared by electrospinning. New Zealand white rabbits underwent surgical transection of the Achilles tendon and repair by: (a) SR: standard Kessler suture + epitendinous suture (5-0 vicryl). (b) ES/PTB: a single stay suture and a section of ES mat, stained with 0.1% Rose Bengal (RB), wrapped around the tendon and bonded with 532 nm light (0.3 W/cm(2) , 125 J/cm(2) ). (c) SR + ES/PTB: a combination of (a) and (b). Gross appearance, extent of adhesion formation and biomechanical properties of the repaired tendon were evaluated at Days 7, 14, or 28 post-operatively (n = 8 per group at each time point).

RESULTS

Ultimate stress (US) and Young's modulus (E) in the SR group were not significantly different from the ES/PTB group at Days 7 (US, P = 0.85; E, P = 1), 14 (US, P = 0.054; E, P = 1), and 28 (US, P = 0.198; E, P = 0.12) post-operatively. Adhesions were considerably greater in the SR group compared to the ES/PTB group at Days 7 (P = 0.002), 14 (P < 0.0001), and 28 (P < 0.0001). The combination approach of SR + ES/PTB gave the best outcomes in terms of E at 7 (P < 0.016) and 14 days (P < 0.016) and reduced adhesions compared to SR at 7 (P < 0.0001) and 14 days (P < 0.0001), the latter suggesting a barrier function for the photobonded ES wrap.

CONCLUSION

Photochemical sealing of a ES mat around the tendon repair site provides considerable benefit in Achilles tendon repair. Lasers Surg. Med. 44: 645-652, 2012. © 2012 Wiley Periodicals, Inc.

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.

Reduction of inflammatory responses and enhancement of extracellular matrix formation by vanillin-incorporated poly(lactic-co-glycolic acid) scaffolds

Vanillin is one of the major components of vanilla, a commonly used flavoring agent and preservative and is known to exert potent antioxidant and anti-inflammatory activities. In this work, vanillin-incorporated poly(lactic-co-glycolic acid) (PLGA) films and scaffolds were fabricated to evaluate the effects of vanillin on the inflammatory responses and extracellular matrix (ECM) formation in vitro and in vivo. The incorporation of vanillin to PLGA films induced hydrophilic nature, resulting in the higher cell attachment and proliferation than the pure PLGA film. Vanillin also reduced the generation of reactive oxygen species (ROS) in cells cultured on the pure PLGA film and significantly inhibited the PLGA-induced inflammatory responses in vivo, evidenced by the reduced accumulation of inflammatory cells and thinner fibrous capsules. The effects of vanillin on the ECM formation were evaluated using annulus fibrous (AF) cell-seeded porous PLGA/vanillin scaffolds. PLGA/vanillin scaffolds elicited the more production of glycosaminoglycan and collagen than the pure PLGA scaffold, in a concentration-dependent manner. Based on the low level of inflammatory responses and enhanced ECM formation, vanillin-incorporated PLGA constructs make them promising candidates in the future biomedical applications.

Blood compatibility of polymers derived from natural materials

Several polymers derived from natural materials are effective for tissue engineering or drug delivery applications, due to specific properties, such as biocompatibility, biodegradability, and structural activity. Their blood compatibility needs to be carefully evaluated to avoid thrombosis and other material-related adverse events in the hematic environment. We compared the surface properties and blood compatibility of protein and polysaccharide polymers, including fibroin, gelatin, and chitosan. Both fibroin and chitosan showed good hemocompatibility, with low platelet adhesion and spreading. Chitosan induced strong interactions with plasma proteins, especially with albumin. It was hypothesized that surface passivation by albumin inhibited the adsorption of other procoagulant and proadhesive proteins on chitosan and fibroin films, which limited platelet spreading. However, the significant and rapid polymer swelling encouraged protein entrapment within the soft, gelatin films, inducing higher platelet adhesion and activation. Thrombin generation assay confirmed the higher blood compatibility of chitosan and fibroin with regard to clotting.

Artificial egg stalks made of a recombinantly produced lacewing silk protein

Rigid threads: Lacewings protect their eggs from predators by laying them on small stalks (see picture). The stalks have good mechanical properties and, unlike most other silks, a cross β structure. An artificial egg stalk was produced using a designed recombinant variant of a sequenced lacewing egg stalk protein, and it attained 90 % of the tensile strength of a natural egg stalk.

Emerging technologies in medical applications of minimum volume vitrification

Cell/tissue biopreservation has broad public health and socio-economic impact affecting millions of lives. Cryopreservation technologies provide an efficient way to preserve cells and tissues targeting the clinic for applications including reproductive medicine and organ transplantation. Among these technologies, vitrification has displayed significant improvement in post-thaw cell viability and function by eliminating harmful effects of ice crystal formation compared to the traditional slow freezing methods. However, high cryoprotectant agent concentrations are required, which induces toxicity and osmotic stress to cells and tissues. It has been shown that vitrification using small sample volumes (i.e., <1 µl) significantly increases cooling rates and hence reduces the required cryoprotectant agent levels. Recently, emerging nano- and micro-scale technologies have shown potential to manipulate picoliter to nanoliter sample sizes. Therefore, the synergistic integration of nanoscale technologies with cryogenics has the potential to improve biopreservation methods.

Preparation and characterization of biomimetic silk fibroin/chitosan composite nanofibers by electrospinning for osteoblasts culture

In this study, we have successfully fabricated electrospun bead-free silk fibroin [SF]/chitosan [CS] composite nanofibers [NFs] covering the whole range of CS content (0%, 25%, 50%, 75%, and 100%). SF/CS spinning solutions were prepared in a mixed solvent system of trifluoroacetic acid [TFA] and dichloromethane. The morphology of the NFs was observed by scanning electron microscope, and the average fiber diameter ranges from 215 to 478 nm. Confocal laser scanning microscopy confirms the uniform distribution of SF and CS within the composite NFs. To increase biocompatibility and preserve nanostructure when seeded with cells in culture medium, NFs were treated with an ethanol/ammonia aqueous solution to remove residual TFA and to change SF protein conformation. After the chemical treatment, SF/CS NFs could maintain the original structure for up to 54 days in culture medium. Properties of pristine and chemically treated SF/CS NFs were investigated by Fourier transform infrared spectroscopy [FT-IR], X-ray diffraction [XRD], and thermogravimetry/differential scanning calorimetry [TG/DSC]. Shift of absorption peaks in FT-IR spectra confirms the conformation change of SF from random coil to β-sheet by the action of ethanol, which is also consistent with the SF crystalline diffraction patterns measured by XRD. From TG/DSC analysis, the decomposition temperature peaks due to salt formation from TFA and protonated amines disappeared after chemical treatment, indicating complete removal of TFA by binding with ammonium ions during the treatment. This was also confirmed with the disappearance of F1s peak in X-ray photoelectron spectroscopy spectra and disappearance of TFA salt peaks in FT-IR spectra. The composite NFs could support the growth and osteogenic differentiation of human fetal osteoblastic [hFOB] cells, but each component in the composite NF shows distinct effect on cell behavior. SF promotes hFOB proliferation while CS enhances hFOB differentiation. The composite SF/CS NFs will be suitable for bone tissue engineering applications by choosing a suitable blend composition.PACS: 87.85.jf; 87.85.Rs; 68.37.Hk.

Mechanisms and control of silk-based electrospinning

Silk fibroin (SF) nanofibers, formed through electrospinning, have attractive utility in regenerative medicine due to the biocompatibility, mechanical properties, and tailorable degradability. The mechanism of SF electrospun nanofiber formation was studied to gain new insight into the formation and control of nanofibers. SF electrospinning solutions with different nanostructures (nanospheres or nanofilaments) were prepared by controlling the drying process during the preparation of regenerated SF films. Compared to SF nanospheres in solution, SF nanofilaments had better spinnability with lower viscosity when the concentration of silk protein was below 10%, indicating a critical role for SF morphology, and in particular, nanostructures, for the formation of electrospun fibers. More interesting, the diameter of electrospun fibers gradually increased from 50 to 300 nm as the concentration of SF nanofilaments in the solution increased from 6 to 12%, implying size control by simply adjusting SF nanostructure and concentration. Aside from process parameters investigated in previous studies, such as SF concentration, viscosity, and electrical potential, the present mechanism emphasizes significant influence of SF nanostructure on spinnability and diameter control of SF electrospun fibers, providing a controllable option for the preparation of silk-based electrospun scaffolds for biomaterials, drug delivery, and tissue engineering needs.

Influence of post-treatment with 75% (v/v) ethanol vapor on the properties of SF/P(LLA-CL) nanofibrous scaffolds

In order to improve the water-resistant ability of silk fibroin (SF) and SF/P(LLA-CL) blended nanofibrous scaffolds for tissue engineering applications, 75% (v/v) ethanol vapor was used to post-treat electrospun nanofibers. SEM indicated that the treated SF and SF/P(LLA-CL) nanofibrous scaffolds maintained a nanofibrous structure and possessed good water-resistant ability. Characterization of ¹³C CP-MAS NMR clarified that 75% (v/v) ethanol vapor could induce SF conformation from random coil or α-helix to β-sheet. Although the water contact showed that treated SF/P(LLA-CL) blended nanofibrous scaffolds were hydrophobic, the water uptake demonstrated that their hydrophilicity was greatly superior to those of pure P(LLA-CL) nanofibrous scaffolds. Furthermore, the treated SF/P(LLA-CL) nanofibrous scaffolds, both in dry state and wet state, could retain good mechanical properties. Therefore, 75% (v/v) ethanol vapor treatment might be an ideal method to treat SF and SF/P(LLA-CL) nanofibrous scaffolds for biomedical applications.

Low-temperature electrospun silk scaffold for in vitro mucosal modeling

Electrospinning is often used to create scaffolding as a biomimetic of the extracellular matrix of tissues. A frequent limitation of this technique for three-dimensional tissue modeling is poor cell infiltration throughout the void volume of scaffolds. Here, we generated low-temperature electrospun silk scaffolds and compared these with conventional electrospun silk scaffolds in terms of mechanical properties, void volume, cell infiltration, cell viability, and potential to support mucosal models under three-dimensional culture conditions. Low-temperature electrospun silk scaffolds supported fibroblast attachment and infiltration throughout the volume of the scaffolds, while conventional electrospun scaffolds exhibited limited cell infiltration with fibroblasts attaching exclusively to the seeding surface of the scaffolds. The porosity of low-temperature electrospun scaffolds was 93% compared with 88% of conventional electrospun silk scaffolds. Uniaxial tensile testing showed a 3.5-fold reduction in strength of low-temperature electrospun silk compared with the conventional in terms of peak stress and modulus but no significant change in strain at break. Mucosal modeling with fibroblast-keratinocyte or fibroblast-carcinoma cocultures showed similar results, with cell infiltration occurring only in low-temperature electrospun scaffolds. Cell viability was confirmed using live/dead staining after 21 days in culture. Furthermore, low-temperature electrospun silk scaffolds were able to support keratinocyte differentiation, as judged by involucrin immunoreactivity. The low-temperature electrospun silk scaffold that we have developed eliminates the limitation of electrospun silk scaffolds in terms of cell infiltration and, therefore, can potentially be used for a wide range of tissue engineering purposes ranging from in vitro tissue modeling to in vivo tissue regeneration purposes.

Growth factor delivery systems and repair strategies for damaged peripheral nerves

Artificial nerve conduits (NCs) are, in certain cases, instrumental for repairing damaged peripheral nerves, although therapeutic efficacy remains often suboptimal. Considerable efforts have been made to improve the therapeutic performance of NCs. This article reviews recent developments in NC-technology for peripheral nerve regeneration with a main focus on growth factors delivery systems and repair strategies. Commonly used materials for NC fabrication include collagen, silk fibroin, and biodegradable aliphatic polyesters. The basic NC structure, i.e., a hollow tube, can be manufactured by diverse methods: spinning mandrel technology, sheet rolling, injection-molding, freeze-drying, and electro-spinning. Polymeric and cellular delivery systems for growth factors can be integrated in the NC wall or within luminal structures such as gels, fibers, or biological materials providing binding affinity for the bioactive compounds. NCs with sustained growth factor delivery generally improve significantly the axonal outgrowth in nerve defect models, although restoration of sensory and motor functions remains inferior to that achieved with autologous nerve grafts. To improve therapeutic outcomes, further biofunctionalization of NCs will be needed, i.e., adjusting degradation kinetics of NC scaffolding to be compatible with axonal regeneration; delivering multiple growth factors at individually optimized kinetics; incorporating modality specific glial cells and furnishing NCs with guiding nanostructures.

Multifunctionalized electrospun silk fibers promote axon regeneration in central nervous system

The repair of central nerves remains a major challenge in regenerative neurobiology. Regenerative guides possessing critical features such as cell adhesion, physical guiding and topical stimulation are needed. To generate such a guide, silk protein materials are prepared using electrospinning. The silk is selected for this study due to its biocompatibility and ability to be electrospun for the formation of aligned biofunctional nanofibers. The addition of Brain Derived Neurotrophic Factor (BDNF), Ciliary Neurotrophic Factor (CNTF) or both to the electrospun fibers enable enhanced function without impact to the structure or the surface morphology. Only a small fraction of the loaded growth factors is released over time allowing the fibers to continue to provide these factors to the cells for extended periods of time. The entrapped factors remain active and available to the cells as rat retinal ganglion cells (RGCs) exhibit longer axonal growth when in contact with the biofunctionalized fibers. Compare to non-functionalized fibers, the growth of neurites increased 2 fold on fibers containing BDNF, 2.5 fold with fibers containing CNTF and by almost 3-fold on fibers containing both factors. The results demonstrate the potential of aligned and functionalized electrospun silk fibers to promote nerve growth in the central nervous system, underlying the great potential of complex biomaterials in neuroregenerative strategies following axotomy and nerve crush traumas.

Production, structure and in vitro degradation of electrospun honeybee silk nanofibers

Honeybees produce silken cocoons containing four related fibrous proteins. High levels of each of the honeybee silk proteins can be produced recombinantly by fermentation in Escherichia coli. In this study we have used electrospinning to fabricate a single recombinant honeybee silk protein, AmelF3, into nanofibers of around 200 nm diameter. Infrared spectroscopy found that the molecular structure of the nanofibers was predominantly coiled coil, essentially the same as native honeybee silk. Mats of the honeybee nanofibers were treated with methanol or by water annealing, which increased their β-sheet content and rendered them water insensitive. The insoluble mats were degraded by protease on a time scale of hours to days. The protease gradually released proteins from the solid state and these were subsequently rapidly degraded into small peptides without the accumulation of partial degradation products. Cell culture assays demonstrated that the mats allowed survival, attachment and proliferation of fibroblasts. These results indicate that honeybee silk proteins meet many prerequisites for use as a biomaterial.

Materials fabrication from Bombyx mori silk fibroin

Silk fibroin, derived from Bombyx mori cocoons, is a widely used and studied protein polymer for biomaterial applications. Silk fibroin has remarkable mechanical properties when formed into different materials, demonstrates biocompatibility, has controllable degradation rates from hours to years and can be chemically modified to alter surface properties or to immobilize growth factors. A variety of aqueous or organic solvent-processing methods can be used to generate silk biomaterials for a range of applications. In this protocol, we include methods to extract silk from B. mori cocoons to fabricate hydrogels, tubes, sponges, composites, fibers, microspheres and thin films. These materials can be used directly as biomaterials for implants, as scaffolding in tissue engineering and in vitro disease models, as well as for drug delivery.

Will silk fibroin nanofiber scaffolds ever hold a useful place in Translational Regenerative Medicine?

Presently, some view silk fibroin-based biomaterials as obsolete, being outperformed by a host of newly discovered biomaterials. But several lines of evidence support the notion that silk fibroin proteins, especially those from B. mori and spiders and their recombinant forms, particularly in the form of electrospun nanofiber scaffolds, still represent promising tools for human tissue engineering/regeneration. Inevitably, the allure of recently reported biomaterials turns away many scientists and resources from the aim of more deeply elucidating the biological interactions of the various kinds of silk fibroin nanofiber scaffolds in vivo. But, even the biological features of newly reported biomaterials are not investigated in adequate depth. Hence, collaborative efforts among biomaterialists, biomedical experts, and private firms must be undertaken on a much greater scale than hitherto done to assess the real usefulness of silk fibroin proteins, thereby allowing or denying their useful introduction into the fields of Translational Regenerative Medicine.

Mammalian cell viability in electrospun composite nanofiber structures

Incorporation of mammalian cells into nanofibers (cell electrospinning) and multilayered cell-nanofiber structures (cell layering) via electrospinning are promising techniques for tissue engineering applications. We investigate the viability of 3T3-L1 mouse fibroblasts after incorporation into poly(vinyl alcohol) nanofibers and multilayering with poly(caprolactone) nanofibers and analyze the possible factors that affect cell viability. We observe that cells do not survive cell electrospinning but survive cell layering. Assessing the factors involved in cell electrospinning, we find that dehydration and fiber stretching are the main causes of cell death. In cell layering, the choice of solvent is critical, as residual solvent in the electrospun fibers could be detrimental to the cells.

Stem cell-based meniscus tissue engineering

Knee meniscus, a fibrocartilaginous tissue, is characterized by heterogeneity in extracellular matrix (ECM) and biomechanical properties, and critical for orthopedic stability, load transmission, shock absorption, and stress distribution within the knee joint. Most damage to the meniscus cannot be effectively healed by the body due to its partial avascular nature; thus, damage caused by injury or age impairs normal knee function, predisposing patients to osteoarthritis. Meniscus tissue engineering offers a possible solution to this problem by generating replacement tissue that may be implanted into the defect site to mimic the function of natural meniscal tissue. To address this need, a multiporous, multilamellar meniscus was formed using silk protein scaffolds and stem cells. The silk scaffolds were seeded with human bone marrow stem cells and differentiated over time in chondrogenic culture in the presence of transforming growth factor-beta 3 to generate meniscus-like tissue in vitro. High cellularity along with abundant ECM leading to enhanced biomechanics similar to native tissue was found. Higher levels of collagen type I and II, sulfated glycosaminoglycans along with enhanced collagen 1-α1, aggrecan, and SOX9 gene expression further confirmed differentiation and matured cell phenotype. The results of this study are a step forward toward biomechanically competent meniscus engineering, reconstituting both form and function of the native meniscus.

A synthetic polypeptide electrospun biomaterial

Fiber mats of a synthetic anionic copolypeptide of l-glutamic acid and l-tyrosine (PLEY) have been produced by electrospinning, and physical, chemical, and biological properties of the fibers have been characterized in vitro. Fibers were obtained from polymer dissolved in water at concentrations of 20-60% (w/v) but not below this range. Applied voltage and spinneret-collector distance were also found to influence polymer spinnability. Oriented fibers were obtained by changing the geometry of the collector. Fiber diameter was measured by scanning electron microscopy (SEM). A common chemical reagent was used to cross-link polymers postspinning. Fiber solubility in aqueous solution varied as a function of cross-linking time. Cationic polypeptides labeled with a fluorescent dye became noncovalently associated with cross-linked fibers, enabling visualization by fluorescence microscopy. Spectroscopy provided information on polymer chain conformation in solution and in fibers. Degradation of cross-linked fibers by different proteases has been demonstrated. Fibroblasts were cultured on cross-linked fiber mats to test basic cytocompatibility. Synthetic polypeptide fiber mats may be useful in applications in medicine, biotechnology, and other areas.