Melt electrospinning is a processing technique to produce fibrous structures from polymer melts for applications that include

tissue engineering Tissue engineering is a biomedical engineering discipline that uses a combination of Cell (biology), cells, engineering, Materials science, materials methods, and suitable biochemistry, biochemical and physicochemical factors to restore, maintai ...

textiles Textile is an umbrella term that includes various fiber-based materials, including fibers, yarns, filaments, threads, different fabric types, etc. At first, the word "textiles" only referred to woven fabrics. However, weaving is not the ...

and

filtration Filtration is a physical separation process that separates solid matter and fluid from a mixture using a ''filter medium'' that has a complex structure through which only the fluid can pass. Solid particles that cannot pass through the filter ...

. In general,

electrospinning Electrospinning is a fiber production method that uses Electrostatics, electric force to draw charged threads of polymer solutions or polymer melts up to fiber diameters in the order of some hundred nanometers. Electrospinning shares characterist ...

can be performed using either polymer melts or polymer solutions. However, melt electrospinning is distinct in that the collection of the fiber can very focused; combined with moving collectors, melt electrospinning writing is a way to perform

3D printing 3D printing or additive manufacturing is the Manufacturing, construction of a three-dimensional object from a computer-aided design, CAD model or a digital 3D modeling, 3D model. It can be done in a variety of processes in which material is ...

. Since volatile solvents are not used, there are benefits for some applications where solvent toxicity and accumulation during manufacturing are a concern.

History

The first description of melt electrospinning was by Charles Norton in a patent approved in 1936. After this first discovery, it wasn't until 1981 that melt electrospinning was described as part of a three-paper series. A meeting abstract on melt electrospinning in a

vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often dis ...

was published by Reneker and Rangkupan 20 years later in 2001. Since this scientific publication in 2001, there have been regular articles on melt electrospinning, including reviews on the subject.Hutmacher DW & Dalton PD (2011) Melt Electrospinning. Chem Asian J, 6, 44-5. In 2011, melt electrospinning combined with a translating collector was with proposed as a new class of

.Brown TD, Dalton PD, Hutmacher DW. (2011) Direct Writing by Way of Melt Electrospinning. Advanced Materials, 23, 5651-57.

Principles

The same physics of electrostatic fiber drawing apply to melt electrospinning. What differs are the physical properties of the polymer melt, compared to a polymer solution. When comparing polymer melts and polymer solutions, the former are normally more

viscous The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inter ...

than polymer solutions, and elongated electrified jets have been reported.Dalton PD, Grafahrend D, Klinkhammer K, Klee D, Möller M (2007) Electrospinning of polymer melts: phenomenological observations. Polymer, 48, 6823-6833. The molten electrified jet also requires cooling to solidify, while solution electrospinning relies on

evaporation Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when humidi ...

. While melt electrospinning typically results in micron diameter fibers, the path of the electrified jet in melt electrospinning can be predictable.Dalton PD, Vaquette C, Farrugia B, Dargaville TR, Brown TD, Hutmacher DW. (2013) Electrospinning and Additive manufacturing: converging technologies. Biomater Sci, 1, 171.

Parameters

Temperature

A minimum temperature is needed to ensure a molten polymer, all the way to the tip of the spinneret. Spinnerets have a relatively short length, compared to solution electrospinning.

Flow Rate

The most significant parameter for controlling the fiber diameter is the flow rate of the polymer to the spinneret - in general, the higher the flow rate, the larger the fiber diameter. While reported flow rates are low, all of the fluid electrospun is collected, unlike solution electrospinning where a great part of the

solvent A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...

is evaporated.

Molecular Weight

The

molecular weight A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...

is important as to whether the polymer can be melt electrospun. For linear homogeneous polymers, a low molecular weight (below 30,000g/mol) can result in broken and poor quality fibers.Dalton PD, Calvet J-L, Mourran A, Klee D, Möller M (2006) Melt Electrospinning of poly(ethylene oxide-block-ε-caprolactone). Biotechnol J, 1, 998-1006. For high molecular weights (above 100,000 g/mol), the polymer can be very difficult to flow through the spinneret. Many melt electrospun fibers reported use molecular weights between 40,000 and 80,000 g/mol or are blends of low and high molecular weight polymers.

Voltage

Modifying the

voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...

does not greatly effect the resulting fiber diameter, however it has been reported that an optimum voltage is needed to make high quality and consistent fibers. Voltages from as low as 0.7kV up to 60kV have been used to melt electrospin.R. J. Deng, Y. Liu, Y. M. Ding, P. C. Xie, L. Luo, W. M. Yang, Journal of Applied Polymer Science 2009, 114, 166.

Apparatus

Different melt electrospinning machines have been built, with some mounted vertically and some horizontally. The approach to heating the polymer does vary and includes electrical heaters, heated air and circulating heaters. One approach to melt electrospinning is pushing a solid polymer filament into a

laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fir ...

, which melts and is electrospun.

Polymers

Polymers exhibiting a melting point or

glass transition temperature The glass–liquid transition, or glass transition, is the gradual and reversible transition in amorphous materials (or in amorphous regions within semicrystalline materials) from a hard and relatively brittle "glassy" state into a viscous or rubb ...

(Tg) are required for melt electrospinning, excluding thermosets (such as

bakelite Polyoxybenzylmethylenglycolanhydride, better known as Bakelite ( ), is a thermosetting phenol formaldehyde resin, formed from a condensation reaction of phenol with formaldehyde. The first plastic made from synthetic components, it was developed ...

) and biologically derived polymers (such as

collagen Collagen () is the main structural protein in the extracellular matrix found in the body's various connective tissues. As the main component of connective tissue, it is the most abundant protein in mammals, making up from 25% to 35% of the whole ...

). Polymers melt electrospun so far include: #

Polycaprolactone Polycaprolactone (PCL) is a biodegradable polyester with a low melting point of around 60 °C and a glass transition temperature of about −60 °C. The most common use of polycaprolactone is in the production of speciality polyure ...

Brown TD, Slotosch A, Thibaudeau L, Taubenberger A, Loessner D, Vaquette C, Dalton PD, Hutmacher DW. (2012) Design and fabrication of tubular scaffolds by direct writing in a melt electrospinning mode. Biointerphases, 7, 13, DOI 10.1007/s13758-011-0013-7. #

Polylactic acid Polylactic acid, also known as poly(lactic acid) or polylactide (PLA), is a thermoplastic polyester with backbone formula or , formally obtained by condensation of lactic acid with loss of water (hence its name). It can also be prepared by rin ...

Poly(lactide-co-glycolide) PLGA, PLG, or poly(lactic-''co''-glycolic acid) is a copolymer which is used in a host of Food and Drug Administration (FDA) approved therapeutic devices, owing to its biodegradability and biocompatibility. PLGA is synthesized by means of ring-op ...

Kim SJ, Jang DH, Park WH, Min BM (2010) Fabrication and characterization of 3-dimensional PLGA nanofiber/microfiber composite scaffolds. Polymer, 51, 1320-7 #

Poly(methyl methacrylate) Poly(methyl methacrylate) (PMMA) belongs to a group of materials called engineering plastics. It is a transparent thermoplastic. PMMA is also known as acrylic, acrylic glass, as well as by the trade names and brands Crylux, Plexiglas, Acrylite, ...

Polypropylene Polypropylene (PP), also known as polypropene, is a thermoplastic polymer used in a wide variety of applications. It is produced via chain-growth polymerization from the monomer propylene. Polypropylene belongs to the group of polyolefins and ...

L. Larrondo, R. S. J. Manley, Journal of Polymer Science Part B-Polymer Physics 1981, 19, 909. #

Polyethylene Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic. It is a polymer, primarily used for packaging ( plastic bags, plastic films, geomembranes and containers including bo ...

# Poly(caprolactone-block-ethylene glycol) #

Polyurethane Polyurethane (; often abbreviated PUR and PU) refers to a class of polymers composed of organic chemistry, organic units joined by carbamate (urethane) links. In contrast to other common polymers such as polyethylene and polystyrene, polyurethan ...

These polymers are examples of the most used polymers, and a more comprehensive list can be found elsewhere.

Uses

Potential applications of melt electrospinning mirror that of solution electrospinning. Not using solvents to process a polymer assists in tissue engineering applications where solvents are often toxic. Additionally, some polymers such as polypropylene or polyethylene are not readily dissolved, so melt electrospinning is one approach to electrospin them into fibrous material.

Tissue Engineering

Melt electrospinning is used to process biomedical materials for tissue engineering research. Volatile solvents are often toxic so avoiding solvents has benefits in this field. Melt electrospun fibers were used as part of a "bimodal

tissue scaffold Tissue engineering is a biomedical engineering discipline that uses a combination of cells, engineering, materials methods, and suitable biochemical and physicochemical factors to restore, maintain, improve, or replace different types of biolog ...

", where both micron-scale and nano-scale fibers were deposited simultaneously. Scaffolds made via melt electrospinning can be fully penetrated with cells, which in turn produce

extracellular matrix In biology, the extracellular matrix (ECM), also called intercellular matrix, is a three-dimensional network consisting of extracellular macromolecules and minerals, such as collagen, enzymes, glycoproteins and hydroxyapatite that provide stru ...

within the scaffold.Farrugia B, Brown TD, Hutmacher DW, Upton Z, Dalton PD, Dargaville TR. (2013) Dermal fibroblast infiltration of poly(ε–caprolactone) scaffolds fabricated by melt electrospinning in a direct writing mode. Biofabrication 5, 025001.

Drug Delivery

Melt electrospinning is also capable to formulate drug-loaded fibers for

drug delivery Drug delivery refers to approaches, formulations, manufacturing techniques, storage systems, and technologies involved in transporting a pharmaceutical compound to its target site to achieve a desired therapeutic effect. Principles related to d ...

. It is a promising new formulation technique in the field of pharmaceutical technology to prepare amorphous solid dispersions or

solid solution A solid solution, a term popularly used for metals, is a homogenous mixture of two different kinds of atoms in solid state and have a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The word ...

s with enhanced or controlled drug dissolution because it can combine the advantages of melt extrusion (e.g. solvent-free, effective amorphization, continuous process) and solvent-based

(increased surface area).

Melt Electrospinning Writing

The electrified molten jet created via melt electrospinning has a more predictable path, and polymer fibers can be deposited accurately onto the collector. When the collector is moved at sufficient speed (referred to as the critical translation speed), straight melt electrospun fibers can be deposited in a layer upon layer approach. This enables for the fabrication of complex, well-ordered structures. In this respect melt electrospinning writing (MEW) can be considered a class of

. Melt electrospinning writing has been performed using either a translating flat surface or a rotating cylinder/mandrel. Most polymers that can be melt-electrospun can also be written assuming the parameters can be tuned in such a way as to produce a stable jet. Piezoelectric polymers such as polyvinylidene difluoride (PVDF) have also been shown to be processable via MEW, opening up potential applications in 3d printed sensors, soft robotics, and further applications in

biofabrication Biofabrication is a branch of biotechnology specialising in the research and development of biologically engineered processes for the automated production of biologically functional products through bioprinting or bioassembly and subsequent ti ...

References

{{Reflist Polymer physics