Bioactive materials

Electrospun fibers

Electrospinning is the only general technique available for the production of polymeric fibers below the micron scale. In a typical electrospinning set-up, a syringe pump dispenses a polymer melt or solution through a spinneret into a high voltage electric field formed between the spinneret and a grounded plate or collector. The electric field generates a charge build-up within the polymer phase, which causes the solution to adopt a cone-like shape, called Taylor Cone, pointing towards the collector. The fibers form when electrostatic repulsions overcome surface tension and accelerate the polymer liquid towards the grounded collector plate drawing a thin jet of fluid that whips into a fast moving spiral. During its way to the collector, the jet elongates by electrostatic repulsion and the solvent evaporates leaving a solid fibre. This phenomenon results in an intricate mesh of polymeric fibers on the collection plate referred to as mat. Depending on the goal pursued, a number of collector configurations can be used ranging from the simple stationary plate that produces randomly oriented fibre mats to a variety of rotating devices such as rotating drums, disks and mandrels, which allow creating a variety of aligned nanofibers. Rotating collectors are more complex to use because the rotation introduces a mechanical force that plays an important role in determining the degree of fibre anisotropy.

Being quite an old technique, it gained renewed interest in recent years with the increasing demand for nanotechnology and its focus on high surface-to-volume ratio and functionalized materials. The rediscovery of electrospinning comes in parallel with a boost in the number of works reporting composite nanofibers from a rich variety of materials. Nanofibres aligned and arrayed, smooth and porous, flat and randomly oriented, raw and highly functionalized nanofibers as well as forming more complex core/sheath nanostructures have been fabricated in order to cover a continuously growing spectrum of applications. Relatively low cost equipment, simple basic operation and the promising possibility of large-scale nanofiber production, resulted in a rapid development of electrospinning technique with many papers describing laboratory scale applications. During the last years different modes of production of nanofibers have been extensively explored. Alternative geometrical modifications of the basic set-up equipment and a number of post-processing treatments have been proposed for achieving improved electric-field uniformity and enhanced control over inter-fibre positional ordering and intra-fibre molecular alignment. A large variety of materials and solvents have been combined in order to tailor specific properties and functionalities of electrospun products, even if not all these achievements are easily transferable to industrial production. From Polym. Rev., 56, 631, 2016..

[Electrsopun fibers of cellulose acetate (left) and detail of electrospun poly(lactic acid) (right).]

Antimicrobial membranes

Electrospun materials including antimicrobial particles showed a remarkable efficiency in limiting bacterial colonization. Silver, copper and cobalt materials were successfully included in a cellulose aceted, poly(lactica acid) and other membranes. The composite electrospun membranes became less susceptible to bacterial colonization and biofilm formation. Confocal microscopy and quantitative tests for microbial growth assessment confirmed this observation. The comparison between the amount of viable cells and the number of viable for bacterial cultures in contact with metal-loaded membranes showed the existence of viable but non-culturable microorganisms, which is a usual outcome of disifection systems. From Sci. Total Environ., 563–564, 912, 2016, J. Hazard. Mater., 299, 298, 2015 & Chem. Eng. J. 262, 189–197, 2015..

[SEM micrographs of fibers in contact (20 h) with cultures of Sthapilococcus aureus. The fibers ara pure poly(lactic acid) (left) and the same polymer loaded with two different amounts of a cobal-releasing material (4.5% and 6%).] From Chem. Eng. J., 262, 189–197, 2015..

Bioactive materials and surfaces

Controlling cell-material interactions is essential in many expanding applications such as the production of antimicrobial surfaces or biocompatible materials. It has been shown that surface topography and physicochemical properties determine cell adhesion and proliferation. The adhesion of microorganisms on natural or synthetic surfaces is a critical issue in many important fields, such as the fight against human infections and pathogen control during food processing and storage. Microbial adhesion is detrimental when associated with the dissemination of pathogens, but can be also beneficial, for example for the production of wastewater treatment bioreactors or for biopolymer degradation5. Once attached to a surface, bacteria form biofilms consisting of cells immobilised cells embedded in a polymeric matrix of microbial origin. Biofilms are complex biological communities characterised by cells with an altered phenotype that create their own environment. The prevention of biofilms and the enhancement of biocompatibility are closely interconnected goals that require a deep understanding of surface physicochemistry.

We reported a reversible wettability hydrophobic to hydrophilic transition in transparent glass-like carbon films under ultraviolet irradiation when the source emitted in the vacuum ultraviolet. The transition occurred at doses below 5 J/cm2 for devices emitting at 185 nm and was absent when using a 266 nm monochromatic laser source. Hydrophilicity was higher when the films were irradiated in air with high relative humidity and it was almost entirely restored over the following 24 h. Our observations indicated that thetransition was caused by the dissociative adsorption of water molecules leading to the formation of polar surface groups. Over the few hours in which the surface remained hydrophilic under ambient conditions, a rapid colonisation with Escherichia coli took place with extensive biofilm formation. The percent surface colonised increased from 1.30 % ± 0.4 % to a maximum of 51.0 % ± 2.7 % for carbon films irradiated in dry air. Our study demonstrated that vacuum ultraviolet irradiation induces a wettability transition in glass-like carbon films, and that a relatively short ultraviolet dose of 185 nm irradiation render their surfaces highly biocompatible. From RSC Adv., 6, 50278, 2016..

[Live/dead confocal micrographs of E. coli cultured on (a) glass-like carbon film as produced, (b) vacuum preconditioned film, (c) films irradiated in dry air and (d) flims irradiated films in wet air. Green stain marks viable bacteria.] From RSC Adv., 6, 50278, 2016.

Further reading

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Bioactive applications for electrospun fibers, Polym. Rev., 56(4), 631-667, 2016

Antimicrobial activity of poly(vinyl alcohol)-poly(acrylic acid) electrospun nanofibers, Colloids Surf. B: Biointerfaces, 146, 144-151, 2016

Microbial colonization of transparent glass-like carbon films triggered by a temporal radiation-induced hydrophobic to hydrophilic transition, RSC Adv., 6, 50278–50287, 2016

Electrospun cellulose acetate composites containing supported metal nanoparticles for antifungal membranes, Sci. Total Environ., 563–564, 912-920, 2016

Superhydrophillic anti-fouling electrospun cellulose acetate membranes coated with chitin nanocrystals for water filtration, J. Memb. Sci., 510, 238-248, 2016

Antimicrobial electrospun silver-, copper- and zinc-doped polyvinylpyrrolidone nanofibers, J. Hazard. Mater., 299, 298–305, 2015

Antimicrobial metal-organic frameworks incorporated into electrospun fibers, Chem. Eng. J., 262, 189–197, 2015

Antifouling membranes prepared by electrospinning polylactic acid containing biocidal nanoparticles, J. Memb. Sci., 405-406, 134-140, 2012