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PCL nanofiber crystallinity is significantly increased in all orientations after annealing.

Regenerated silk fibroin (SF) fiber is a multifaceted protein matrix suitable for engineering a wide variety of biological materials. Numerous artificial spinning systems have been developed to mimic the molecular structure and hierarchical properties found in native silks. Here, we show a bioinspired technique that can readily form nanofibers and induce both orientation and structure formation of crystalline β-sheet assemblies seen in natural silk. In this study, electrospun postdrawn SF nanofibers were fabricated using an automated track-drawing (TD) approach for the continuous production of highly aligned protein nanofibers. This one-step postdrawing process simulates the dominant pulling force seen in natural spinning. The mechanical performance of the postdrawn SF nanofibers with a draw ratio of 2 (DR2) via TD exhibited a 115% increase in Young's modulus and an 80% increase in ultimate tensile strength, compared with the undrawn SF fibers after water treatment. It was also determined that the intermolecular β-sheet content in DR2 nanofibers increased by 75%. This contribution led to higher glass-transition and degradation temperatures. These biomimetic fibers with structural hierarchy and mechanical properties may be used to build high-performance load-bearing and directionally propagating structures relevant in biomaterial and sustainable material applications.

This manuscript proposes a continuous and straightforward method for fabricating suspended micro- and nanodiameter polymer fibers using an automated single-step drawing system. Termed track spinning, the system is based on a simple manual fiber drawing process that is automated by using two oppositely rotating tracks. Fibers are continuously spun by direct contact of polymer solution coated tracks followed by mechanical drawing as the distance between the tracks increases. The device can draw single or multifilament arrays of micro- and nanofibers from many kinds of polymers and solvent combinations. To demonstrate, fibers were pulled from polymer solutions containing polyvinyl acetate (PVAc) and polyurethane (PU). Fiber morphology was smooth and uniform, and the diameter was sensitive to draw length and polymer solution/melt properties. Polymer nanofibers with diameters as small as 450 nm and length of 255 mm were produced. The track spinning method is able to form fibers from high viscosity solutions and melts that are not compatible with some other nanofiber fabrication methods. Further, the setup is simple and inexpensive to implement and nozzleless and does not require an electric field or high-velocity jets, and the tracks can be widened and patterned/textured to enhance fiber yield and manufacturing precision.

This paper reports the molecular organization and mechanical properties of electrospun, post-drawn polyacrylonitrile (PAN) nanofibers. Without post-drawing, the polymer chain was kinked and oriented in hexagonal crystalline structures. Immediate post-drawing in the semi-solid state disrupted the crystal structures and chain kink at maximum draw ratio. Structural re-orientation at maximum draw resulted in a 500% increase in Young’s modulus and a 100% increase in ultimate tensile strength. By applying post-drawing to electrospinning it may be possible to obtain PAN fibers and PAN-derived carbon fibers with enhanced mechanical properties compared to available fabrication technologies.

This study reports the effects of post-draw processing on the structural and functional properties of electrospun poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) nanofibers. Previous studies have independently demonstrated the potential of the electrospinning process or the post-drawing process to enhance the piezoelectric properties of PVDF nanofiber, and thin film structures respectively. However, post drawing electrospun nanofibers has been difficult to achieve. To overcome this limitation, a parallel automated track device was implemented to post-draw thousands of individual PVDF-HFP electrospun nanofibers per minute. Relationships were established showing that yield strength, Young's modulus, and piezoelectric output were enhanced with increasing draw ratio. The normalized voltage output of PVDF-HFP nanofibers post-drawn to a draw ratio of 3 increased by four-fold compared to undrawn control. We hypothesize that polymer chain and crystal alignment in the direction of the fiber axis, which were increased by post drawing, resulted in enhanced voltage output of PVDF-HFP nanofibers under mechanical stimulation.

This study examines the effects of electrospun polycaprolactone (PCL) fiber density and strain rate on nanofiber mat mechanical properties. An automated track collection system was employed to control fiber number per mat and promote uniform individual fiber properties regardless of the duration of collection. Fiber density is correlated to the mechanical properties of the nanofiber mats. Young’s modulus was reduced as fiber density increased, from 14,901 MPa for samples electrospun for 30 seconds (717 fibers +/- 345) to 3,615 MPa for samples electrospun for 40 minutes (8,310 fibers +/- 1,904). Ultimate tensile strength (UTS) increased with increasing fiber density, where samples electrospun for 30 seconds resulted in a UTS of 594 MPa while samples electrospun for 40 minutes demonstrated a UTS of 1,250 MPa. An average toughness of 0.239 GJ/m3 was seen in the 30 second group, whereas a toughness of 0.515 GJ/m3 was observed at 40 minutes. The ultimate tensile strain for samples electrospun for 30 seconds was observed to be 0.39 and 0.48 for samples electrospun for 40 minutes. The relationships between UTS, Young’s modulus, toughness, and ultimate tensile strain with increasing fiber density are the result of fiber-fiber interactions which leads to network mesh interactions.

Tissue extracellular matrix (ECM) is a complex material made up of fibrous proteins and ground substance (glycosaminoglycans, GAGs) that are secreted by cells. ECM contains important biological cues that modulate cell behaviors, and it also serves as a structural scaffold to which cells can adhere. For clinical applications, where immune rejection is a constraint, ECM can be processed using decellularization methods intended to remove cells and donor antigens from tissue or organs, while preserving native biological cues essential for cell growth and differentiation. In this study, a decellularized ECM-based composite hydrogel was formulated by using modified GAGs that covalently bind tissue particles. These GAG-ECM composite hydrogels combine the advantages of solid decellularized ECM scaffolds and pepsin-digested ECM hydrogels by facilitating ECM hydrogel formation without a disruptive enzymatic digestion process. Additionally, engineered hydrogels can contain more than one type of ECM (from bone, fat, liver, lung, spleen, cartilage, or brain), at various concentrations. These hydrogels demonstrated tunable gelation kinetics and mechanical properties, offering the possibility of numerous in vivo and in vitro applications with different property requirements. Retained bioactivity of ECM particles crosslinked into this hydrogel platform was confirmed by the variable response of stem cells to different types of ECM particles with respect to osteogenic differentiation in vitro, and bone regeneration in vivo. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 147-159, 2018.

The study reports a nanomanufacturing method that facilitates post-drawing of electrospun polymer nanofibers. Post-drawing is critical in conventional microfiber processing to enhance mechanical strength, but has been difficult to implement with nanofibers. Here, a parallel automated track collector was designed to permit continuous processing of thousands of individual nanofibers per minute under highly controllable drawing conditions. Post-drawing systematically induced macromolecular alignment and altered the chemical bond composition of polycaprolactone nanofibers with increasing draw ratio. The strength and stiffness of fibers with a draw ratio of 4 were enhanced by 7 and 15 times respectively and their mechanical properties exceeded previously reported values for conventional and nanoscale polycaprolactone structures.

Cell and protein arrays have demonstrated remarkable utility in the high-throughput evaluation of biological responses; however, they lack the complexity of native tissue and organs. Here, we describe tissue extracellular matrix (ECM) arrays for screening biological outputs and systems analysis. We spotted processed tissue ECM particles as two-dimensional arrays or incorporated them with cells to generate three-dimensional cell-matrix microtissue arrays. We then investigated the response of human stem, cancer, and immune cells to tissue ECM arrays originating from 11 different tissues, and validated the 2D and 3D arrays as representative of the in vivo microenvironment through quantitative analysis of tissue-specific cellular responses, including matrix production, adhesion and proliferation, and morphological changes following culture. The biological outputs correlated with tissue proteomics, and network analysis identified several proteins linked to cell function. Our methodology enables broad screening of ECMs to connect tissue-specific composition with biological activity, providing a new resource for biomaterials research and translation.

The recent growth in portable electronics has sparked a demand for alternative energy sources. Energy harvesters that utilize piezoelectric materials are promising in capturing the mechanical energy from body movement to power portable electronics. This study investigated the characteristics of PVDF-HFP nanofibers created from traditional electrospinning and a novel technique called wet-stretching electrospinning. The solution was initially processed using the traditional method, flat-plate electrospinning, which resulted in a fiber network with random orientations. When performing electrical testing the fibers produced minimal voltage. The solution was then processed utilizing a novel wet-stretching electrospinning technique that allowed for fiber alignment and dynamic stretch ratios. Fibers that underwent this method produced higher voltages than fibers from the traditional electrospinning method. It was observed that fibers processed using the wet-stretching technique with different draw ratios (DR) such as 1 (DR 1) and 2.5 (DR 2.5) showed enhanced piezoelectric properties. This research suggests that the wet-stretched PVDF-HFP nanofibers are better suited for piezoelectric applications than traditionally electrospun nanofibers.Copyright © 2017 by ASME

Energy harvested from human body movement can produce continuous, stable energy to portable electronics and implanted medical devices. The energy harvesters need to be light, small, inexpensive, and highly portable. Here we report a novel biocompatible device made of poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) nanofibers on flexible substrates. The nanofibers are prepared with electrospinning followed by a stretching process. This results in aligned nanofibers with diameter control. The assembled device demonstrates high mechanical-to-electrical conversion performance, with stretched PVDF-HFP nanofibers outperforming regular electrospun samples by more than 10 times. Fourier transform infrared spectroscopy (FTIR) reveals that the stretched nanofibers have a higher β phase content, which is the critical polymorph that enables piezoelectricity in polyvinylidene fluoride (PVDF). Polydimethylsiloxane (PDMS) is initially selected as the substrate material for its low cost, high flexibility, and rapid prototyping capability. Bombyx Mori silkworm silk fibroin (SF) and its composites are investigated as promising alternatives due to their high strength, toughness, and biocompatibility. A composite of silk with 20% glycerol demonstrates higher strength and larger ultimate strain than PDMS. With the integration of stretched electrospun PVDF-HFP nanofibers and flexible substrates, this pilot study shows a new pathway for the fabrication of biocompatible, skin-mountable energy devices.

Biomaterials derived from the decellularization of mature tissues retain biological and architectural features that profoundly influence cellular activity. However, the clinical utility of such materials remains limited as the shape and physical properties are difficult to control. In contrast, scaffolds based on synthetic polymers can be engineered to exhibit specific physical properties, yet often suffer from limited biological functionality. This study characterizes composite materials that present decellularized extracellular matrix (DECM) particles in combination with synthetic nanofibers and examines the ability of these materials to influence stem cell differentiation. Mechanical processing of decellularized tissues yielded particles with diameters ranging from 71 to 334 nm. Nanofiber scaffolds containing up to 10% DECM particles (wt/wt) derived from six different tissues were engineered and evaluated to confirm DECM particle incorporation and to measure bioactivity. Scaffolds containing bone, cartilage, and fat promoted osteogenesis at 1 and 3 weeks compared to controls. In contrast, spleen and lung DECM significantly reduced osteogenic outcomes compared to controls. These findings highlight the potential to incorporate appropriate source DECM nanoparticles within nanofiber composites to design a scaffold with bioactivity targeted to specific applications.

An average human body produces a large amount of energy throughout the day. A significant portion of this energy is utilized as mechanical energy. Body movement such as footfalls and arm swings can produce enough energy to power portable electronics using mechanical-to-electrical energy harvesters. These devices should be small, light, portable, and flexible. Polyvinylidene fluoride (PVDF) has shown a high biocompatibility and is a suitable candidate for energy harvesting applications. Moreover, PVDF can be produced in large quantities while still maintaining a low cost. Electrospinning is a common process used to prepare PVDF nanofibers. Here we introduce a novel technique called wet-stretched electrospinning to further increase the amount of energy generated by the PVDF devices. Our initial results show that the wet-stretched nanofibers outperform the regular PVDF nanofibers by up to 12 times under similar conditions. These promising results suggest that the proposed method has great potential to be utilized as a major improvement from the traditional electrospinning process of PVDF. These findings are significant and are especially pertinent to the field of energy harvesters designed for powering medical devices or wearable systems.Copyright © 2016 by ASME

Polymer nanofibers are favorable for tissue engineering scaffolds because of their high surface-to-volume ratio and biomimicry of the extracellular matrix. Random and uniaxially oriented polymer nanofibers are easily fabricated by conventional electrospinning techniques; however, control over fiber organization within nanofiber structures is limited when they are collected directly from an electrospinning jet. The regenerative medicine applications of electrospun scaffolds could be expanded by developing assembly methods that allow better control of fiber organization. Here, a novel technique is presented that utilizes parallel automated tracks to orient and collect nanofibers from an electrospinning jet. The stabilized fibers are then subsequently assembled into desirable structures. It is difficult to assemble complex structures directly from an electrospinning jet because of high electrical charge and velocities, so this technology adds an intermediate step where nanofibers are immobilized on automated tracks. The result is a continuous steady-state delivery of static stabilized nanofibers that provides a unique and promising platform for automated post processing into useful nanofiber structures. This technique also allows for an indefinite amount of time, as determined by design parameters, for fibers to dry or cool before they contact other nanofibers in the collection site, thus eliminating potential for fiber-to-fiber adhesions even with slow evaporating solvents or high-temperature melts. To demonstrate potential in regenerative medicine applications, several nanofiber structures were fabricated, including: 2D structures with well-controlled fiber density; 3D loosely assembled aligned nanofiber structures with good cell penetration properties; and, complex layer-by-layer 3D aligned fiber structures assembled by integration with post-processing techniques.

Electrospun nanofibers possess unique qualities such as nanodiameter, high surface area to volume ratio, biomimetic architecture, and tunable chemical and electrical properties. Numerous studies have demonstrated the potential of nanofibrous architecture to direct cell morphology, migration, and more complex biological processes such as differentiation and extracellular matrix (ECM) deposition through topographical guidance cues. These advantages have created great interest in electrospun fibers for biomedical applications, including tendon and ligament repair. Electrospun nanofibers, despite their nanoscale size, generally exhibit poor mechanical properties compared to larger conventionally manufactured polymer fiber materials. This invites the question of what role electrospun polymer nanofibers can play in tendon and ligament repair applications that have both biological and mechanical requirements. At first glance, the strength and stiffness of electrospun nanofiber grafts appear to be too low to fill the rigorous loading conditions of these tissues. However, there are a number of strategies to enhance and tune the mechanical properties of electrospun nanofiber grafts. As researchers design the next-generation electrospun tendon and ligament grafts, it is critical to consider numerous physiologically relevant mechanical criteria and to evaluate graft mechanical performance in conditions and loading environments that reflect in vivo conditions and surgical fixation methods.

Introduction Polymer nanofibers have several properties that make them an extremely promising material in regenerative medicine. These favorable properties are derived from their high surface-area-to-volume ratios, their size relationship to that of cells, and their geometric similarity to natural extracellular matrix (ECM) fibers such as collagen. The literature in the field of tissue engineering generally defines nanofibers as those with diameters less than 1000 nm, while fibers larger than that are described as microfibers. The high surface-area-to-volume ratio of nanofibers allows them to interact with biomolecules at very high efficiency. In addition, nanofibers and microfibers are valuable in tissue engineering because their size is suitable for assembling complex three-dimensional (3D) architectures that can be perceived and populated by cells. It has become increasingly apparent that cell behaviors are highly dependent on the physical environment. Substrate microstructure cues such as size, orientation, and dimensionality modulate cell behaviors ranging from attachment and morphology to differentiation and ECM production. Specifically, it has been shown that polymer nanofibrous structures can improve cell attachment; increase cell viability, proliferation, and ECM production; and predictably push cells toward specific morphologies and differentiation paths [1]. Researchers have been exploring ways of designing polymer nanofiber scaffolds that elicit desired cell responses for specific tissue engineering applications. The vast majority of these designs start with the electrospinning fabrication method. The electrospinning method is a simple and relatively inexpensive process, yet demonstrates amazing versatility in terms of the types of fibers and structures that can be fabricated. This chapter will introduce the basics of electrospinning and describe in detail strategies to make nanofiber structures for tissue engineering applications. These strategies will be focussed on electrospinning fibers with desired biofunctionality, electrospinning fiber arrays with uniaxial alignment, and methods of assembling individual fibers into 3D scaffolds conducive to cell population. It is our hope that the tools presented here can be used to design better electrospun scaffolds for applications in regenerative medicine.

While mattress sutures are commonly used to secure annuloplasty rings during mitral valve repair, the use of a flexible ring secured with a running polypropylene suture has recently been advocated. The study aim was to assess the separation tensions of semi-rigid and flexible rings using mattress and running suture techniques in an in-vitro static load model.Semi-rigid and flexible annuloplasty rings were sutured with either mattress or running sutures (RM, RR, FM, FR) in four groups, of eight hearts each. Twelve additional sutures were passed through the surfaces of each ring to fix the preparation to a testing machine. In a fifth (control) group the mattress sutures securing a flexible ring (FMS) were connected directly to the machine. Each preparation was subjected to progressive axially directed (base-apex) tension until ring-tissue separation occurred.The first major decrease in tension (defined asor = 10 N inor = 1.5 s) typically occurred with the separation of at least three adjacent sutures. These starting tensions (N) were: FMS 117 +/- 32.6, RR 131.7 +/- 30.5, RM 137.4 +/- 35.3, FM 152.1 +/- 32.3, and FR 213.2 +/- 30.5. The magnitudes of tension decrease with separation (and percentage of starting tensions) were: FMS 25.4 (21.2%), RR 26.8 (17.6%), RM 28.9 (21.6%), FM 24.6 (17.6%), and FR 22.5 (10.8%). The FR group required more tension to separate than the other groups (p0.001), but had a lower magnitude of force drop at dehiscence.Flexible rings secured with a running suture required more force to separate than other ring-suture combinations. The lower magnitude of force drop in this group indicated a better tension distribution than in the other groups. Semi-rigid rings separated with a lower force, and had larger drops in tension, regardless of the suture technique used.

Polymer nanofibers exhibit properties that make them a favorable material for the development of tissue engineering scaffolds, filtration devices, sensors, and high strength lightweight materials. Electrospinning is a versatile method commonly used to manufacture polymer nanofibers. Collection of electrospun nanofibers across two parallel plates is a technique useful for creating nanofiber structures because it allows for the collection of linearly oriented individual nanofiber arrays and these arrays can be easily transferred to other substrates or structures. It is of importance to have some understanding of the capabilities of this collection method, such as the maximum length of fibers that can be collected across two parallel plates. The effect of different electrospinning parameters on maximum fiber length, average fiber diameter, diameter uniformity, and fiber quality was explored. It was shown that relatively long continuous polycaprolactone (PCL) nanofibers with average diameters from approximately 350 nm to 1 µm could be collected across parallel plates at lengths up to 35–50 cm. Experimental results lead to the hypothesis that even longer continuous nanofibers over 50 cm could be collected if the size of the parallel plates were increased. Extending the maximum fiber length that can be collected across parallel plates could expand the applications of electrospinning. Polymer solution concentration, plate size, and applied voltage were all shown to have varying effects on maximum fiber length, fiber diameter, and fiber uniformity.

Extracellular matrices and degradable nanofibers are two very promising materials in the field of tissue engineering; however both of these structures face limitations as tissue engineering scaffolds. Extracellular matrices, such as collagen, gelatin, and laminin, have excellent biocompatibility and allow cell in growth and survival, but structural weakness makes them difficult to handle and greatly limits their uses. Degradable nanofibers support cell attachment and can provide structural support and directional guidance, but individual degradable nanofibers are fragile and have a tendency to form dense fiber bundles which limit cell penetration into the spaces between the nanofibers, especially in the case of aligned nanofibers. To overcome these difficulties, degradable loose nanofibers were embedded in protein matrix in an attempt to fabricate a hybrid scaffold with improved properties, such as improved strength, guidance, spacing among nanofibers, etc. Polycaprolactone (PCL) was used as a model material for degradable nanofibers. Gelatin was employed as a model protein for matrix structure formation. Thin hybrid films (average thickness = 2.78 um) were fabricated by wetting the loose aligned undirectional nanofiber arrays or loose aligned bi-directional nanofiber grids with a gelatin aqueous solution, which also allows for live cell loading into the nanofiber-protein composite if cell are premixed with protein solution or on the surface of the films. Gelatin film alone without nanofiber reinforcement is difficult to handle due to the weakness of the thin membrane. Gelatin films with a fiber density as low as 3% v/v were structurally robust enough for handling, and manipulation into complex shapes. Mechanical testing confirmed that the addition of nanofibers enhanced the strength of gelatin films, in both dry and hydrated state. In vitro testing confirmed that nanofiber reinforced films were biocompatible and provided cells with directional guidance. Results demonstrate the promise of gelatin/PCL nanofiber composites as a tissue scaffolding material.

Many types of tissue in the body, such as nerve, muscle, tendon, ligament, bone, and blood vessels, rely on a highly organized microstructure in order to impart their desired functionality. Cell and extracellular matrix (ECM) alignment in these tissues allows for increased mechanical strength and cell communication. In tissue engineering, aligned polymer nanofibers can be used to take on the role of natural ECM fibers in order to provide mechanical strength, sites for cell attachment, and modulation of cell behavior via morphological cues. A wide variety of physical and electrostatic techniques are available for assembly of aligned nanofiber structures, and many of these structures have been evaluated as tissue engineering scaffolds. It is widely understood that aligned microstructure induces an aligned morphology in most cell types, but aligned nanofibrous topography also influences other cell behaviors such as differentiation, gene expression, and ECM deposition. With a greater understanding of aligned nanofiber scaffold fabrication techniques, and cell interactions with these scaffolds, researchers may be able to overcome current challenges and develop better strategies for regenerating aligned tissues.

The filtering unit of the kidney, the glomerulus, contains capillaries whose walls function as a biological sieve, the glomerular filtration barrier. This comprises layers of two specialised cells, glomerular endothelial cells (GEnC) and podocytes, separated by a basement membrane. Glomerular filtration barrier function, and dysfunction in disease, remains incompletely understood, partly due to difficulties in studying the relevant cell types in vitro. We have addressed this by generation of unique conditionally immortalised human GEnC and podocytes. However, because the glomerular filtration barrier functions as a whole, it is necessary to develop three dimensional co-culture models to maximise the benefit of the availability of these cells. Here we have developed the first two tri-layer models of the glomerular capillary wall. The first is based on tissue culture inserts and provides evidence of cell-cell interaction via soluble mediators. In the second model the synthetic support of the tissue culture insert is replaced with a novel composite bioartificial membrane. This consists of a nanofibre membrane containing collagen I, electrospun directly onto a micro-photoelectroformed fine nickel supporting mesh. GEnC and podocytes grew in monolayers on either side of the insert support or the novel membrane to form a tri-layer model recapitulating the human glomerular capillary in vitro. These models will advance the study of both the physiology of normal glomerular filtration and of its disruption in glomerular disease.

Bioengineering of living heart valve substitute suitable for implantation into pediatric patients with heart valve diseases is an unsolved challenge in cardiovascular tissue engineering. In order to identify desirable material properties of such construct, the biomechanical, histological, histochemical and biochemical properties of fetal, perinatal and adult porcine heart valve leaflets have been systematically investigated. It have been shown that increasing in heart valve stiffness during ontogenesis correlates with accumulation of collagen type 1 and increasing level of collagen cross-linking. Tissue spheroids biofabricated from human fat tissue derived stem cells have been employed to engineer leaflet-like construct on compliant electrospun scaffold using self-assembly or tissue fusion approach. Incubation of tissue engineered heart valve leaflet construct with TGFs as well as periostin transfection significantly increased stiffness of tissue engineered heart valve leaflet. These data demonstrated that the heart valve can be biofabricated by tissue fusion or self-assembly of closely placed tissue spheroids and that chemically and genetically induced accelerated tissue maturation and collagen deposition can enhance mechanical properties of tissue engineered leaflet. Further optimization of accelerated tissue maturation technologies could eventually lead to development of living autologous human heart valve tissue engineered construct with natural- like biomechanical properties suitable for pediatric cardiac patients.