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Principles of Electrospinning and the Production of Nanofibers

Anyone should be able to make nanofibers with a high voltage source and a little ingenuity. However it is not easy to obtain a defined fiber diameter or to spin unconventional material. This article deals with the points that need to be considered in such situations.

Electrospinning

Electrospinning (ES) is extensively used for electrostatic fiber production, during which electrostatic forces are used for creating polymer fibers with diameters ranging from 2 nm to several micrometres from polymer solutions or melts.

Recently, this process has attracted a large amount of attention due to its versatility and ability to create fibers on a nanometer scale which is not easy to achieve with other standard technologies.

The key principle of this process is the impact of high voltage on a polymer solution. A Taylor cone is formed on the polymer solution droplet because of the high voltage. In case there is a further increase in the electric field, a charged polymer jet is ejected from the Taylor cone tip.

The polymer solution jet is subjected to an instability process during which the solvent evaporates. Electrostatic forces then attract the polymer fiber to the collector. The resulting layer has a structure of randomly arranged fibers due to the chaotic movement of fibers to the collector.

The ES system includes the following:

• High voltage source
• Spinneret
• A grounded collector

Polymer Solutions

In order to spin fibers, obviously we must have something to spin.Normally solutions can be divided into two groups. However a combination of the polymers is quite common as detailed below:

Synthetic Polymers

Synthetic polymers are more advantageous than natural polymers. They are more cost-effective and can be fine-tuned to a wide range of properties. Synthetic polymers may be non-biodegradable or biodegradable but their biocompatibility is a challenge.

Low concentration polymer PLGA nanoparticles.

High concentration polymer PLGA beads effect.

Natural Polymers

Natural biopolymers in comparison to synthetic polymers are especially suited for biomedical applications to improve their biocompatibility. Natural polymers, such as collagen, gelatin and chitosan, are extracted from the natural extracellular matrix of humans or animals and have their own bioactivity with peptide sequences which can promote the adhesion, proliferation and differentiation of cells.

There are several disadvantages of nanofiber materials made from natural polymers, which include inadequate mechanical properties and short degradation time. Furthermore the conversion of a natural biopolymer into the form of nano- or microfibers through electronic spinning is more difficult than synthetic polymers.

Hyaluronic polymer nanofibers.

Influence of Individual Variables on Process Outcome

Solution selection is only the first step in the work. For highly effective, controlled and reproducible production of nanofiber layers, it is important to regulate and record many other parameters that can influence the process.

Table 1. Key variables for nanofiber production

Influence of the Number of Parameters on the Production Set-up Design

It is obvious from the list of parameters the number of variables that need to be changed in the production process and what needs to be kept in mind while designing equipment for nanomaterial production.

It is essential to make process handling as simple, controllable and measurable (recordable) as possible. A specific solution can be shown in the form of laboratory equipment emphasizing the ability to spin and influence morphology.

The 4SPIN laboratory device can be used for the creation of nanomaterials by electroblowing, electrospraying and electrospinning. Electroblowing is a technique combining electrostatic nanofiber production (electrospinning) with airflow around the spinneret. The tangential forces of the flowing air acting on a drop of mixture contribute to Taylor cone formation and to the creation of the fiber.

The 4SPIN laboratory device for the development of nanomaterials.

In the emitter vicinity, the right climatic conditions favourably influencing the spinning process are also formed. The electroblowing method can successfully spin solutions without using surfactants or other solvent systems, which are generally toxic and therefore unsuitable, for example, for medical applications.

Electrospraying

Electrospraying is a technique used to create materials in the form of small beads with a diameter of tens of nanometres to several micrometres. The mechanism of creating such nanostructures is similar to the principle of electrostatic spinning.

Modular equipment is made of several components with central intuitive and clear control of all settings. Parameters available include:

• Up to 60 kV high voltage
• Automatic collector distancing
• The precise configuration of rotating collector speeds to affect the degree to which nanofiber structures are aligned
• An integrated dispensing system with the possibility of inserting syringes with varying volumes of polymer solution (10, 20 and 30 ml)
• Regulation of the flow velocity and air heating (“electroblowing”)
• Monitoring and measurement of conditions during the process.

Spinneret

Spinnability and nanofiber production throughput are based on the emitter geometry. Emitters are electrodes to which the melt or solution intended for spinning is fed. The emitter system offered with 4SPIN is designed so that nanofiber materials can be prepared simply and also highly productively.

This system is similar to the process for nanomaterial development, where the selected mixture for spinning is first requested and is then spun with greater productivity. During development, there is a gradual progression from the least productive, but most precise needle emitter, to the highly productive multi-jet. The development of materials can be scaled up to pilot operation at this stage.

Multi-jet Emitter.

For easy cleaning and solution replacement, the unit is equipped for the connection of components used in dosing system distribution through a Luer Lock system. The use of disposable components allows parts to be replaced inexpensively and maintenance-free.

Since aggressive solvents are used which are mainly used for the preparation of solutions made from synthetic polymers, three structural materials (POM, PP, PEEK) are selected for unit components that come into direct contact with the spun solution, ensuring maximum chemical resistance, the possibility of radiation sterilization, and FDA approved.

An important safety feature of emitters is the automatic discharge of the unit’s residual charge without the need to use other discharge elements. The device safety in terms of the risk of injury caused by electric shock, is also improved by the all-plastic design, the electronically controlled safety door locks and the strict bonding and grounding of all conductive parts.

Collector

The collector structure has a definite impact on the properties of the resulting material (product). On the other hand, the collector design is always tailored to the requirements such as the size and internal morphology of nanomaterials.

Rotating continual collector.

Rotating patterned collector.

The collectors are conductive electrodes, the fibers, once stiff, settle on their surface. Fibers are gradually deposited in layers, forming nanofiber material. The system of collectors offered with 4SPIN laboratory equipment covers requirements for the preparation of both small and large flat materials, with both random internal and precisely aligned structures.

In case a large surface material needs to be created, the plate collector or slowly rotating drum (which provides the largest surface area of product) can be used. The chaotic motion of flying nanofibers can be tamed, and organized nanofiber layers can be created, by means of either a static or split rotary collector.

Static colector.

Static patterned collector.