Herein we demonstrate the ability to fabricate polymeric microtubes with an inner diameter of approximately 3 microm through co-electrospinning of core and shell polymeric solutions. The mechanism by which the core/shell structure is transformed into hollow fibers (microtubes) is primarily based on the evaporation of the core solution through the shell and is described here in detail. Additionally, we present the filling of these microtubes, thus demonstrating their possible use in microfluidics. We also report the incorporation of a protein (green fluorescent protein) within such fibers, which is of interest for sensorics.

Nanotubes have great potential for applications in a rapidly increasing range of fields: catalysis, medicine, pharmacy, pheromone-release systems for crop protection, sensorics, and photonics. [1][2][3][4][5] This is due to their high anisotropy, huge specific surface area that enhances reactivity, high rate of adsorption, and efficiency of transport processes both within and across the nanotube walls. [6] Electrospinning is a process that produces continuous polymer fibers through the action of an external electric field imposed on a polymer solution [7,8] (for a review of this process see previous publications [9][10][11][12] ). To manufacture nanotubes, two fundamentally different approaches have been reported: self-assembly and template-based methods. [1,[13][14][15][16][17][18][19] In the TUFT (tubes by fiber templates) process, electrospun nanofibers themselves are used as templates to produce nanotubes. [14][15][16][17][18][19] The template-based TUFT process for nanotube production consists of three stages: i) the electrospinning of template nanofibers, ii) shell deposition via chemical or physical vapor deposition (CVD or PVD, respectively), and iii) core removal by thermal or chemical means.[1]Recently, a new technique was introduced that allows the co-electrospinning of polymer solutions from a spinneret containing two coaxial capillaries.[20] Using this technique, coelectrospinning of immiscible and miscible pairs of polymer solutions produced nanofibers with core/shell structures. This technique was then used to co-electrospin conjugated polymer nanofibers [21] and to make the PCL/gelatin (PCL: poly(e-caprolactone)) core/shell structure that holds great potential for controlled drug delivery and as a scaffold for tissue engineering. [22] It was possible to co-electrospin almost non-spinnable polymers such as the conducting polymer polyaniline (PAni). [23] Using this same co-electrospinning technique, ceramic sol-gel precursors were added to the shell solutions to create ceramic nanotubes. The core material-a heavy mineral oil-was later extracted with octane.[24]The aim of the present work is to produce hollow carbon nanotubes by co-electrospinning two polymer solutions. The process was carried out in two stages. In the first stage, use was made of the non-solvent effect on one of the polymers to facilitate the creation of a solid interface between the nanofiber's core and shell. In the second stage, the nanofibers were subjected to heat treatment to degrade the core polymer and carbonize the polymer shell. Figure 1 shows a typical pattern of the co-electrospinning process close to the core/shell spinneret. The core-polymer capillary protrudes 0.3 to 1 mm below the shell capillary. As can be seen, the core liquid experiences a sudden increase in diameter upon exiting the capillary tube, which may be attributed to the die swell effect. Below the point of maximum swelling, the outer solution forms a thin shell that attaches to the core-polymer stream. As a result of the induced electric field, both liqu...

Encapsulation of whole microbial cells in microtubes for use in bioremediation of pollutants in water systems was the main focus of this investigation. Coelectrospinning of a core polymeric solution with bacterial cells and a shell polymer solution using a spinneret with two coaxial capillaries resulted in microtubes with porous walls. The ability of the microtube's structure to support cell attachment and maintain enzymatic activity and proliferation of the encapsulated microbial cells was examined. The results obtained show that the encapsulated cells maintain some of their phosphatase, β-galactosidase and denirification activity and are able to respond to conditions that induce these activities. This study demonstrates electrospun microtubes are a suitable platform for the immobilization of intact microbial cells.

The design of mat-like scaffolds slow-releasing bone morphogenetic protein-2 (BMP-2) retaining bone regeneration functions has been a major challenge in tissue engineering. This study aimed to develop core-shell fiber scaffolds releasing BMP-2 to support bone regeneration. BMP-2 was incorporated in an aqueous core solution of poly(ethylene oxide), whereas the shell solution was made of polycaprolactone blended with poly(ethylene glycol). This blending induced pores in the shell, which pronouncedly affected the movement of proteins out of the fibers. BMP-2 release profiles were monitored. In vitro bioactivity of BMP-2 released from the scaffolds was assessed using human mesenchymal stem cells by measuring alkaline phosphatase activity. Bone regeneration capabilities were demonstrated by implanting the BMP-2-embedded scaffolds in rat cranial defect model followed by micro-computed tomography analysis. The degree of fiber's shell porosity, highly correlative with the slow- and fast-release patterns of BMP-2, were found to be dependent on the relative amount of poly(ethylene glycol) within the shell. In vitro assays of scaffolds manifesting the slow-release pattern have revealed significant (∼9-fold) increase in alkaline phosphatase activity, compared to fast BMP-2 releasing scaffolds. Likewise, in vivo studies have revealed significant bone regeneration in cranial defects of scaffold implants with recombinant human BMP-2 with slow-release pattern.

Biomimetic scaffolds generally aim at structurally and compositionally imitating native tissue, thus providing a supportive microenvironment to the transplanted or recruited cells in the tissue. Native decellularized porcine extracellular matrix (ECM) is becoming the ultimate bioactive material for the regeneration of different organs. Particularly for cardiac regeneration, ECM is studied as a patch and injectable scaffolds, which improve cardiac function, yet lack reproducibility and are difficult to control or fine‐tune for the desired properties, like most natural materials. Seeking to harness the natural advantages of ECM in a reproducible, scalable, and controllable scaffold, for the first time, a matrix that is produced from whole decellularized porcine cardiac ECM using electrospinning technology, is developed. This unique electrospun cardiac ECM mat preserves the composition of ECM, self‐assembles into the same microstructure of cardiac ECM ,and ,above all, preserves key cardiac mechanical properties. It supports cell growth and function, and demonstrates biocompatibility in vitro and in vivo. Importantly, this work reveals the great potential of electrospun ECM‐based platforms for a wide span of biomedical applications, thus offering the possibility to produce complex natural materials as tailor‐made, well‐defined structures.

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Enzymes were directly encapsulated in microtubes fabricated by coelectrospinning. Coelectrospinning makes it possible to separate between an organic phase, which may be harmful to enzyme activity, and an aqueous one. In this report, enzymes were incorporated in an aqueous core solution of poly(ethylene oxide) (PEO) while the shell solution was made of polycaprolactone (PCL) dissolved in a mixture of chloroform and dimethylformamide (DMF). Indeed, this separation was found to preserve the enzymatic activities of two enzymes during and after electrospinning, as ascertained when the fiber mats were placed in an aqueous assay environment. The shell morphology was altered by blending PCL with poly(ethylene glycol) (PEG). This blending induced pores in the shell which in turn pronouncedly affected the movement of molecules into or out of the fibers. An enzyme of about 80 kDa, alkaline phosphatase (AP), diffused through the pores into the surrounding medium. The fibers can thus act as release devices. However, a larger enzyme, β-galactosidase (465 kDa), remained in the fibers without any leaching. Enzyme assays showed that the substrates of both enzymes diffused efficiently into the fibers and were cleaved by the encapsulated enzyme, and the reaction product then diffused outside. With encapsulated β-galactosidase, the fiber acted as an efficient enzymatic microreactor and the relative enzymatic activity reached a level of about 50%.

The final form of tubular nanofibres produced by the co-electrospinning of two solutions (core/shell) is largely determined by the kinetics governing the buckling phenomenon. The buckling mechanism involves the evaporation of the core solution through a solidified shell resulting in a pressure difference across the fibre shell. Buckling can take place when the pressure drop across the fibre shell exceeds a critical value. In this work the physical conditions leading to fibre buckling are analysed from a kinetic point of view. A time interval, Δt, during which buckling may occur, is introduced as a single criterion determining the buckling probability. Different core/shell systems were spun by varying the surface tension, viscosity, flow rate, electric field and the diffusion coefficient of the core solvent through the fibre shell. The imaged as-spun nanofibres were analysed statistically to determine the buckling probability, and the corresponding Δt values were calculated using the values of the spinning parameters. The obtained data were fitted with an exponential distribution function which afforded determination of the characteristic time to buckling, tb. The results provide a means of predicting the buckling of tubular nanofibres. In particular, one can conclude that the dominant parameter determining the final form of the as-spun tubular nanofibres is the diffusion coefficient of the core solvent through the fibre shell.