Recent developments in the use of centrifugal spinning and pressurized gyration for biomedical applications

,

adobe construction techniques to improve the load-bearing capabilities of their walls (Quagliarini & Lenci, 2010).With developing technologies and needs, fiber structures have not been limited to being used only as textile and support materials.Today, fiber structures are most commonly used as filtration materials and in biomedical applications.Tissue scaffolds, wound dressings, tissue patches, and surgical sutures are widely used in biomedical applications of fibers, which form the main idea of the aforementioned review.For targeted biomedical application of fibers; architecture (geometry), material, material-active material interaction, production speed and cost come to the fore as the main factors in determining the production technique to be used.
The production mechanisms have also evolved as modern systems are becoming more prevalent.Many methods of fiber spinning have taken advantage of the centrifugal force concept, an inertial force which acts on all objects when viewed in a rotating frame of reference.Centrifugal spinning refers to the use of technology whereby a liquid material, usually a polymer solution, is ejected out of a rapidly rotating spinning head (Chen et al., 2019).Fibers are produced from this method as the centrifugal force from the spinning head overcomes the surface tension of the solution and a jet is formed, this jet subsequently undergoes a stretching process where it dries and is eventually deposited as fibers.
Centrifugal spinning has long been used in industrial processes before it became popular in academic publications.Centrifugal spinning was first described in 1909 by Ernst Pick to spin molten glass in industry (Kambic & Nosé, 1997;Zannini Luz & Loureiro dos Santos, 2022).During the process of producing glass fiber, molten glass is poured into a large metal container comprising of many small uniformly distributed holes on its walls, a spinning head rotates in excess of 3000 rpm and the settled temperature should be $1000 C, causing a constant stream of glass to flow through these orifices (Eichhorn et al., 2009).
More recently however, centrifugal spinning has seen much exposure in academic work for the production of small diameter fibers with various applications in the biomedical realm.In this paper, a brief overview of the basic centrifugal spinning setup will be presented alongside the recent developments in centrifugal spinning-based technologies and their uses in potential biomedical applications.

| CENTRIFUGAL SPINNING
Although small variations of this technology are inevitable, centrifugal spinning is usually centered around a driving motor which can rotate a centrifugal spinneret at varying degrees of speed.The basic overview of centrifugal spinningbased technologies can be seen in Figure 1.A spinneret is usually constructed out of metal and either directly houses a chamber or facilitates the transport of solution into the nozzle.The nozzle is an opening found at the terminals of the spinneret which can be characterized as small open orifices and have the function of extruding the solution through the openings at high speeds.
F I G U R E 1 Diagram showing overview of the basic centrifugal spinning setup.The spinneret may directly contain the reservoir of solution or connect to it, the solution exists the spinneret at high speeds via an exit orifice or a nozzle created through the walls of the spinneret.
When the speed of the spinneret rotation achieves a certain critical state, the centrifugal force becomes sufficient to overcome the surface tension and viscous shear stress of the polymer solution.Upon exceeding this critical state, the solution is expelled from the orifices and an initial jet is formed (Zhiming et al., 2018).
This polymer jet is essential for fiber production and is largely the same mechanism used in electrospinning to produce fibers, although no external electric field is applied (Reneker & Yarin, 2008).The jet can be characterized as a high velocity spray of liquid, which is responsible for the atomization of the solvent, leading to rapid evaporation.As the jet travels out of the spinneret orifices, it undergoes a process of stretching where it moves centrifugally around the axis of rotation and bends to an arch-shape trajectory due to the rotational force (P ar au et al., 2007).As the solvent portion of the jet evaporates, dry fibers are deposited onto the collector, concluding the principle of centrifugal spinning.
Because the evaporation of the solvent plays a vital role in the fiber forming process, the collection distance, the distance between the orifice and the collection walls becomes an important parameter (Divvela et al., 2017).The collection distance does not affect the force of the fiber, but instead provides a time-frame for solvent evaporation where larger distances afford a greater drying time.In turn, a larger collection distance usually results in additional jet elongation, leading to thinner fibers being deposited (Altun et al., 2022).There exists a range of collection distances, specific to the operating conditions and polymer solution properties which can determine the effect of the final fiber morphology.For example, if the collection distance is too high, the fibers could break during their stretching phase.However, if the collection distance is too short, the jet will not undergo sufficient elongation and the solvent will not completely evaporate, leading to thicker fibers being collected (Taghavi & Larson, 2014).
In the use of centrifugal spinning for biomedical applications, two main sources of polymer can be fed into the spinneret.As the polymer must be in liquid form to be manipulated by the centrifugal force, it can be in the form of a melt or dissolved in a volatile solvent.The former is less frequently utilized due to the increased complexity of incorporating a melting step to the process and the added energy intensive nature.The most common method of solution feed for centrifugal spinning is a polymer dissolved in a suitable solvent.
Although other methods of fiber production exist, namely electrospinning, centrifugal spinning and its derivative technologies possess a few key advantages.Firstly, centrifugal spinning represents an easy-to-implement setup which is low cost and contains few steps for production.Centrifugal spinning lacks the application of a direct electric field which could otherwise limit choice in the case of charge-absent polymers (Ahmed et al., 2018).Compared to electrospinning, more viscous polymers can be used to make fibers, as highly viscous solutions lead to problems such as needle clogging (Wang et al., 2012).Perhaps the biggest advantage that centrifugal spinning holds over other technologies, is the potential for industrial scale-up and a high production rate (Zhang & Lu, 2014).

| Advancements in centrifugal spinning
From the available literature before the year 2000, there are no records of centrifugal spinning being widely utilized for biomedical applications.Instead, centrifugal spinning was used to manufacture wool and other fibers for industrial use (Hodgson, 1993).The first appearance of centrifugal spinning for biomedical applications emerges in 2001, when a novel device was used to produce hollow fiber membranes (HFMs) with poly(2-hydroxyethyl methacrylate-co-methyl methacrylate) hydrogel (Luo et al., 2001).Although the resulting HFMs depicted more of a microporous membrane than a nonwoven fibrous structure, this work showed the potential of using centrifugal spinning to produce biomaterials that could be custom-made for specific biomedical applications.Nonwoven fibers later became the de facto standard for use in biomedical applications owing to their vital characteristic of being able to greatly increase the available surface area to volume ratio for surface interactions (Xie et al., 2011).In 2001, nonwoven fibers were produced by centrifugal spinning showcased in a report which did not shed light on the inner workings of the setup, referring to it as "gel-spinning" (Foster et al., 2001).In this work, poly(β-hydroxybutyrate) fibers were generated with an average diameter of between 1 and 15 μm.Poly(β-hydroxybutyrate) belongs to a class of polymers referred to as polyhydroxyalkanoates (PHAs), due to slower lower degradation rates when compared to other biodegradable polymers, PHAs are more useful in fibrous forms where their bulk porosity and increased surface area combined to enchance degradability (Yasin et al., 1990).
The early 2000s proved to be a slow decade for the development and use of centrifugal spinning, where only a small volume of reports where created, and the few that were published, did not focus on the technology itself.Volatile organic compounds (VOCs) have a high vapor pressure at ambient temperature and can have some dangerous effects to human health and the environment (Rumchev et al., 2007).In 2002, a report showed the preparation of a mesoporous silica fiber matrix produced by centrifugal spinning for the removal of potentially harmful VOCs (Chu et al., 2002).In this study a rotational speed of 500, 1000, and 2000 rpm was used to spin the fibers.Higher spinning speeds lead to thinner fiber diameters and at 2000 rpm, the average was 20 μm.They found that after spinning, the fibrous sample showed a sharper x-ray powder diffraction peak, suggesting that the spinning process caused lattice contraction and led to the ordering of the mesoporous material.Furthermore, the study suggests that an increase in spinning rate could lead to an increase in the pore radius, affecting the growth of the mesopore structure.

| Centrifugal spinning biomedical applications
Centrifugal spinning is based on utilizing solvent and melt spinning methods.The most important features to be considered while choosing the method used are the physicochemical parameters of the polymer to be used.Solubility and melting point parameters of the selected polymer are the main properties that should be considered.When it comes to biomedical applications, the parameters to be considered vary depending on the purpose and the requirements of the targeted application.Depending on the type of biomedical application, it is possible to produce materials such as tissue scaffolds, drug delivery systems, and wound dressings by centrifugal spinning (Zannini Luz & Loureiro dos Santos, 2022).The parameters affecting the choice of melt and solvent in centrifugal spinning become more important.For example, in the application of using centrifugal melt spinning (CMS), the melting temperature of the polymer forming the content of the biomaterial should be compatible with the temperature range at which the drug and polymer is stable at.
On the other hand, with the centrifugal solution spinning (CSS) technique, the active ingredient (e.g., drug)-polymer or polymer-polymer pairs must be able to dissolve in the common solvent.The targeted fiber diameter, the porosity and morphology of the resulting mass structure are related to the viscosity and surface tension parameters of the polymer solution used, as well as the rotational speed of the rotation unit (Merchiers et al., 2020).The geometry of the centrifuge unit, the diameters of the orifices, their distance from each other and the distance of the collector unit to the centrifuge unit (collection distance), are the basic device-based parameters that affect fiber production.Considering all these production parameters, centrifugal spinning is an easy, fast, and economical technique to cater to biomedical applications (Zhang & Lu, 2014).

| Tissue engineering
Scaffolds used in tissue engineering should have a porous structure that allows cells to attach, adapt and proliferate.The cells should then replace the tissue scaffolds, which degrade in proportion to the proliferation rate of the cells (Zhang et al., 2014).Micro-nano fiber structures are suitable for tissue engineering and have a wide range of applications, due to their adjustable porosity and 3-dimensional network structures.Polycaprolactone (PCL) polymer, which is frequently used in tissue engineering applications, has wide application potential due to its FDA-approved, biocompatible and biodegradable properties (Klicova et al., 2020;Li & Tan, 2014).In a study by Zander, fibers were produced by the CMS and CSS techniques with PCL (Figure 2a).Production parameters, morphological features and interactions with PC12-neuronal cells were comparatively examined in detail (Zander, 2015).Although the melting temperature of PCL is 60 C, it was stated that fiber with smooth morphology could not be obtained below 200 C, as the the viscosity of PCL is very high below this tempurature.Fiber production was achieved at low temperatures, only when the spin speed was increased.However, low yield and heterogeneous fiber diameter distributions were detected.Additionally, the effect of the collection distance at the optimized temperature of 200 C was examined.The collection distances of 100, 120, and 140 mm did not have an effect on the fiber diameter distribution, but when the distance decreased, bead formation in the fibers was observed.
The effect of rotation speed on PCL fiber morphology prepared by melt centrifugal spinning has been examined (Obregon et al., 2016).It has been reported that the fiber formation does not occur below 8000 rpm, fiber production efficiency is high between 14,000 and 18,000 rpm, and bead formation occurs due to the shortening of the solvent evaporation time above a rotation speed of 18,000 rpm.PCL fibers were also prepared using the CSS technique.As a result of optimization studies with different rotation speeds and PCL solution concentrations, it has been stated that increasing the solution concentration and rotation speed reduces the fiber diameter (Figure 2b,c).The lowest fiber diameters produced by the CMS and CSS techniques were 7.05 ± 1.1 and 0.81 ± 0.5 μm, respectively.The fiber diameter distribution obtained by melt centrifugal spinning is approximately 10 times higher than that obtained by solution centrifugal spinning.Cell attachment and viability showed similar results in fibers prepared by both techniques.Since PCL is a biocompatible polymer, no toxic effects were observed (Figure 2d-f).
Tissue engineering studies using CMS and CSS can be exemplified by a multitude of applications due to the inherent advantages of the produced fiber morphologies, and is an area open to development with different polymerapplication combinations.

| Drug delivery
Micro and nanofiber structures have high surface area/volume ratio compared to film surfaces.Therefore, they have higher drug loading and release potential compared to film surfaces.On the other hand, fiber structures produced by the centrifugal spinning technique address the solubility problems of drugs and can provide fiber production with a drug-carrier combination in molten state (Satish & Priya, 2022).For example, oxcarbazepine (OXC) loaded sucrose fibers were produced that could increase the bioavailability of OXC, by utilizing the CMS technique.To further increase the stability of the drug-loaded fibers, they were doped with polyvinylpyrrolidone (PVP), a polymer with a hydrophilic nature (Nasir et al., 2021).The produced sucrose-OXC fibers were compressed into tablets that dissolved in the mouth.The data obtained showed that the prepared sucrose-OXC fiber tablets had a drug loading of 92.7% ± 3%.Tablets produced with CMS offer a fast, easy and economical method with increased bioavailability, both for in vivo and in vitro conditions.
In another example of drug-loaded orally disintegrating tablets prepared with the CSS technique, carvedilol was loaded into hydroxypropyl cellulose (Szab o et al., 2015).Carvedilol is a biopharmaceutical classification system class II drug, which has low solubility but a high permeability.Carvediol was suspended in ethanol and then mixed with hydroxypropyl cellulose prior to spinning.The produced drug loaded fibers were compressed to form orodispersible tablets.The dissolution properties of the carvedilol-hydroxypropyl cellulose fiber tablets compared to the mixture of the compressed tablet showed that, the fiber structure created a pH independent release profile.
Tetracycline loaded PVP-PCL blend fibers were produced by centrifugal spinning as a drug delivery vehicle (Mary et al., 2013).Following production, fibers were kept at vacuo to remove residual organic solvents.The fibers produced were in the sub micrometer range, having a diameter distribution from 311 to 927 nm.The addition of tetracycline increased the fiber diameter compared to the other polymer blends.Tetracycline loaded PVP/PCL fiber showed antibacterial activity against gram-negative (Escherichia coli; E. coli, Pseudomonas aeruginosa; P. aeruginosa) and grampositive bacteria species (Staphylococcus epidermidis, Bacillus megaterium).The antibacterial activity of tetracyclineloaded PVP/PCL fibers and their ease of manufacture also reveal their potential for use as a wound dressing.
Active ingredient loading by centrifugal spinning is not just limited to drugs, but also allows for the loading of growth factors or nanoparticles within fibrous structures.For example, Rampichova et al. were produced platelet functionalised PCL fibers by centrifugal spinning (Rampichov a et al., 2017).Centrifugally spun 3D fiber production followed by platelet functionalisation through simple immersion techniques, stands out as an easy and fast technique for a fiber-based tissue regeneration product.
Centrifugal spinning also enables for the production of core-shell fibers by modifying the sample holder and nozzle geometry of the device with an appropriate design.Core-shell fibers were produced by using the emulsion of oil/water phases of PCL and Pluronic F-68 in different phases (Buzgo et al., 2017).Platelet lyophilizates are loaded in the core hydrophilic structure to prolong the release.The release of platelets increased the proliferation of fibroblast and osteoblast cell populations with improved metabolic activity.
It has been determined that the CMC and CSS techniques can be used as an easy, fast and economical solution in eliminating the disadvantages such as low solubility and low bioavailability of many drugs.Additionally, prolonged and controlled release profiles can be obtained with specified drug-polymer combinations.

| Wound dressings
Wounds can be formed due to a range of reasons reasons such as injuries, surgical procedures, diabetes and pressure sores.In the design of wound dressings, it is of immense importance to develop dressings that absorb exudate and blood leakage, provide tissue regeneration and have antibacterial effects.Fiber wound dressings have always had a wide range of applications, due to their high porosity, fast and cheap production methods, ease of mass production and high loading capacities of drugs, antibacterial agents, and growth factors.Centrifugal spinning is also a technique used in the production of wound dressings.Antibacterial wound dressings consisting of gelatin fibrous mats can be manufactured via centrifugal spinning (Gungor et al., 2021).Silver nitrate was added to the gelatin polymer solution to produce antibacterial centrifugal spun fiber structures.Silver nanoparticle formation was induced by reducing silver nitrate salts in the solution via exposure to ultraviolet light.Produced Ag nanoparticle loaded gelatin fibers were thermally crosslinked at 170 C. The silver-loaded fiber structures were compared with the null sample group, and showed that antibacterial activity was detected in the presence of Ag nanoparticles.Antibacterial activity was determined against both E. coli and Staphylococcus aureus (S. aureus) bacteria species.Gelatin-AG nanoparticle containing fibers showed a high water uptake capacity of 543%, were mechanically durable, showed great antibacterial and air permeable properties, making them a suitable wound dressing material.
In another study conducted with centrifugal spinning, ciprofloxacin (CPF) was loaded on poly(lactic acid)/gelatin blend fibers (Xia et al., 2019).Depending on the increasing CPF concentration from 0 to 12%, fiber diameter sizes increased from 512 to 622 nm.Drug release profiles showed a characteristic burst release effect in the initial stage and prolonged sustained release afterwards.CPF showed up to 36 mm inhibition zone for E. coli and 44 mm for S. aureus, respectively.As a result, it was determined that CPF loaded PLA-gelatin fibers were successfully produced by centrifugal spinning and showed good antibacterial effects.This suggests that the aforementioned fiber structures have a huge potential for use as a wound dressings.
In recent years, while the capacity and features of centrifugal spinning devices have improved, their multi-axis production capacity has also increased.For example, co-axis centrifugal spinning was designed to produce core-shell nanofibers (Z.Li, Mei, et al., 2021).The core-shell structures were composed of carboxylated chitosan (CCS), polyethylene oxide (PEO) polymers-ibuprofen and human epidermal growth factor (hEGF) active agents (Figure 3).The main strategy of the core-shell structure is loading two different agents; ibuprofen and human epidermal growth factor (hEGF), to different layers of fiber constructs with different types of polymers.PEO-hEGF was placed in the core and CCS/PEO blend-ibuprofen in the shell structure.The produced fiber structures were in the sub-micrometer size.Dual drug loaded core-shell fibers showed antibacterial activity against E. coli, S. aureus, and P. aeruginosa bacteria species with similar results with AquacelAg, however, the cell cytotoxicity results showed better cell proliferation.Dual drugloaded core-shell fibers showed antibacterial activity against E. coli, S. aureus, and P. aeruginosa bacterial strains with similar results as the commercial product, AquacelAg.On the other hand, for the cytotoxicity results, in which the effect on cell viability was evaluated, the core-shell fibers loaded with dual drugs showed higher viability.Considering these successful results with dual efficacy antibacterial activity and wound healing properties, these fibers have strong potential for commercialization.
Wound dressings are not only used to cover the wound area, but they can be used to prevent bacterial infection.It can also be used in skin regeneration and skin tissue engineering applications.For this purpose, poly(vinyl alcohol) (PVA)/sodium alginate dialdehyde (ADA)/gelatin(GEL) fibers were produced by centrifugal spinning (Akhtar et al., 2022).Additionally, Cu-Ag doped mesoporous bioactive glass nanoparticles (Cu-Ag MBGNs) were loaded to achieve antibacterial properties on the fibers.Beadless fibers have been successfully produced by centrifugal spinning.ADA-GEL polymers which form a polymer blend structure were cross-linked with the Schiff base reaction, and made into mechanically durable fibers.The antibacterial activity of PVA/ADA-GEL fibers against E. coli, S. aureus bacteria species was also determined and demonstrated.

| Antibacterial activity
Biomaterials with antibacterial activity are of cruical importance for the prevention or elimination of infection in biomedical applications.Today, hospital-acquired infections are still among the top causes of both morbidity and mortality in healthcare settings.In order to eliminate this important issue, researchers focus on studies that provide antibacterial activity.Biomaterial developed with fast and cheap production methods such as centrifugal spinning areincreasing day by day and are a great way to tackle problems with infections that require fast and economical solutions.In three different studies conducted between the years 2019 and 2021, fiber structures with antibacterial activity were made by centrifugal spinning.The diameter of the fiber structures produced from Nylon 6 was as low as 209 nm.These produced fibers were then dipped into orange juice and their antibacterial activity was determined.Commercial gauze was used as the control group.Although there was no statistically significant difference between the immediate release profiles and the antibacterial activities during the testing period, when the the long-term effectiveness was examined, the Centrifugal spinning unit with the produced fiber structures.Representative image of (b) single layer nanofiber with single nozzle, (c) core-shell nanofiber with co-axial nozzle (Z.Li, Mei, et al., 2021;Permission required).
release profile of the fiber structures was prolonged in proportion to its increased surface area (Akia et al., 2019).It has been determined that the fibers produced not only show antibacterial activity, but also inhibit cell adhesion.The fiber structures may not be suitable as tissue scaffolds, but can be used as wound cleaning wipes that prevent adhesion and inflammation in wounds that are to be cleaned daily.
In a recent study carboxymethyl chitosan/PEO (CMCS/PEO) fibers were produced by centrifugal spinning (C.Li, Huang, et al., 2021).The diameter distribution of the produced fibers were in the micrometer scale.The antibacterial activity increased with the CMCS ratio, a cationic polymer with a potent antibacterial nature.The antibacterial efficiency was determined to be higher for S. aureus compared to E. coli bacteria.The main reason for this, is the difference between the antibacterial action mechanisms of Gram-positive and Gram-negative bacteria species.
In another study, antibacterial fibers were produced through centrifugal spinning by doping PEO solution with silver nanoparticles (PEO/Ag nanoparticles).PEO/Ag nanoparticles fibers were produced at the nanometer scale $240 nm (Hasan et al., 2021).PEO/Ag nanoparticles nanofibers formed 99% and 88% inhibition zones against E. coli and Bacillus cereus bacterial species, respectively.The strong antibacterial activity was predominantly due to the AG nanoparticle content.The centrifugal spinning technique stands out as an easy solution for nanoparticleloading into various polymeric solutions, an approach which allows for the creation of fibers from simple polymer suspensions.

| Comparison with electrospinning
There are many studies in the literature aiming to compare the advantages and disadvantages of electrospinning and centrifugal spinning techniques.The main difference between the two techniques can be defined by the ultimate driving force of the solutions to generate fibers; in electrospinning this is via an applied electric field and in centrifugal spinning, this is the centrifugal force.Variations of the centrifugal spinning technology can have additional driving forces such as temperature for the melt variation and solution blowing for other variations.The production of platelet-loaded core-shell fibers by electrospinning and centrifugal spinning has been investigated (Figure 4) and its effect on cell interactions have been compared in detail (Buzgo et al., 2017).The effect of platelet lyophilizates loaded on 3-dimensional (3D) core-shell fibers prepared with PCL/Pluronic F-68 polymers on fibroblast and keratinocyte cells was also evaluated by the same authors (Vocetkova et al., 2017).However, it was determined that 2-dimensional (2D) structures produced by electrospinning induced cell proliferation and this was more superior than the 3D fibers produced by centrifugal spinning.On the other hand, it has been reported that the fibers produced by both techniques also exhibit an extendedrelease profiles and have the potential to be converted into products with high-scale production and increased widespread application.

| PRESSURIZED GYRATION
In 2013, a novel fiber manufacturing technique, pressurized gyration, based on centrifugal spinning was revealed in an article which demonstrated that this technology could produce large quantities of nanofibres at a high scale (Mahalingam & Edirisinghe, 2013).The name pressurized gyration (PG) is indicative of the method in which fibers are produced, as it marries centrifugal spinning with a variable gas infusion.The general laboratory setup involves a highspeed (up to 36,000 rpm) rotating aluminium vessel (35 mm in height, diameter of 60 mm), which is connected to a gas inlet pipe which supplies an additional pressure into the vessel of up to 0.3 MPa (Dai et al., 2023;Heseltine et al., 2018).An overview of the basic PG laboratory setup is shown in Figure 5.
A polymer solution, typically formed via mixing a polymer with an organic solvent, is placed into the gyration vessel where the high-speed rotation and applied gas pressure causes the fluid to be manipulated into a jet.Just as with centrifugal spinning, the centrifugal force overcomes the surface tension and viscous forces of the solution, in order to cause a jet to be formed.The most significant difference between centrifugal spinning and PG is that the applied gas pressure supplements the centrifugal force in order to contribute to the formation of the polymer jet.The polymer jet, which is the rapid stream of polymer and solvent, causes the speedy evaporation of the solvent, in turn creating polymeric fibers when collected.PG is a single step process like centrifugal spinning, but because it has more operating parameters, it allows for a more customized production of a larger quantity for nano-and microfibres for biomedical applications.

| Advancements in pressurized gyration
Shortly after the first publication mentioning PG, a subsequent article in 2014 showed how PG could be used to spin fibers with composite polymer blends containing naturally occurring biopolymers (Mahalingam et al., 2014).PG was used to spin fibers containing starch and PEO.Compared to electrospinning, which was the most dominant technology for spinning natural nanofibers in the laboratory, PG was not limited by the rheological properties of starch, where the content of amylose/amylopectin contributed to the stiffness and flexibility of the fibers.Solutions of PEO and starch mixtures were spun at a fixed additional gas pressure of 0.1 MPa and at differing rotational speeds (24,000-36,000 rpm).It was found unsurprisingly that the increase in rotational speeds resulted in thinner fiber diameters to be produced with all the tested samples.The addition of starch into the PEO polymer matrix followed a reduction in overall fiber diameter, from about 500 nm for the PEO-only sample and below 300 nm for the PEO:starch combination.The study demonstrated that continuous fibers using PEO-starch mixtures could easily be manufactured by PG and that the fiber diameters are a function of polymer concentration and rotating speed of the processing system.Polysaccharide based fibers are of interest to biomedical research as they pose as suitable structures for many biomedical applications such as scaffolds used in tissue engineering, drug release, artificial organs, wound healing, and vascular grafts (Heinze et al., 2006;Pareta & Edirisinghe, 2006;Sweeney et al., 2014).

| Medical imaging and diagnostics
The potential to spin polymer regardless of their rheological properties was a notable advantage of PG, coupled with the fact that adding bioactive additions or making composite systems became very simple.PG was used to form proteinÀgold nanoparticle stabilized microbubbles for promising applications in medical imaging and diagnostics (Mahalingam, Raimi-Abraham, et al., 2015).Although PG was created to produce fibers, it was found that other structures such as microbubbles could be formed using it when an appropriate solution such as lysozyme was the feedstock.Compared to the status quo, microfluidics and coaxial electrohydrodynamic atomization, PG benefits from a high production rate due to the high-speed motor and gas supply.In this work, PVA, in combination with a model protein (lysozyme) and gold nanoparticles were used to produce bubbles.The destabilizing centrifugal force and dynamic fluid flow of the PG vessel works on the protein surface tension to form microbubbles at a rapid rate.Solutions of PVA:lysozyme and PVA-lysozyme: gold nanoparticles at different concentrations were spun at different rotational speeds and working pressures, and was found that overall, the bubble diameter decreased at higher speeds and working pressures.Bubbles with an air core and lysosome/gold shell were produced with an overall diameter of up to 250 μm.A parametric plot was created from the data to visualize the parameters that could be altered to transition between the production of fibers to bubbles, this is shown in Figure 6.By incorporating gold nanoparticles on the surface of these bubbles, stability studies showed that morphological change occurred which led to an improvement that enhanced the stability of the PVA-lysozyme: gold microbubbles.Gold and other nanoparticles can be used to fabricate quantum dots, which has become a buzzword in the consumer electronics and scholarly world, these quantum dots have a plethora of real-world uses, but in the biomedical realm, they show great promise as targeted drug delivery vehicles and contrast agents in photoacoustic imaging.PG therefore allows for these sorts of materials to be produced via a low-cost and simple process, that benefits from being a single step process.

| Preceramic fibers
PG was further utilized to produce ceramic fibers from preceramic polymers where solutions containing silicone resin and graphene where spun with a carrier polymer (Mahalingam, Pierin, et al., 2015).Preceramic polymers are attractive precursors in the manufacture of ceramic components as they allow complicated shapes to be easily fabricated.Ceramic fibers in their own right, have attracted much attention due to their superior properties which becomes apparent in a high-surface-area fibrous form.In this work, two silicone resins with different carbon content (poly-methylsilsesquioxane and poly-methyl-phenyl-silsesquioxane), along with graphene filler material was formed into solutions using a polyvinylpyrrolidone polymer matrix.This method took full advantage of PG and the use of a carrier polymer.The produced fibers were pyrolysed, melting off the carrier polymer, but leaving behind ceramic-graphene matrix in the form of fibers (Figure 7).
The work with the ceramic precursor fibers showed that PG was a simple and reliable process to mass produce dense ceramic structures in a high-surface-area fibrous form.The topography of the produced fibers resembled smooth surfaces devoid of surface porosity, and the graphene flakes seemed to be successfully embedded the fibers.The technique also makes it easy to use a binary solvent systems, because unlike with electrospinning, the internal mixing in the gyration pot allows for the fabrication of polymer solution which has not undergone complete dissolution.

| Drug delivery
Small diameter fibers have many different applications in biomedical engineering and can notably be used in pharmaceutical engineering to overcome issues with poor drug solubility in oral formulations.Many techniques can be used to load drug molecules into a polymer matrix, a popular approach is to utilize amorphous solid dispersions.Solid dispersions are a technique where the solubility of a drug is improved by disarranging it's hard to dissolve crystalline structure into an easy to dissolve amorphous form, embedded into a polymer matrix which prevents the drug from returning to its crystalline structure (Baghel et al., 2016).
Drug delivery represents an arm of biomedical engineering which has attracted much attention from multiple different industries due to the practicality and improvement in administering active pharmaceutical ingredients.A modern method of exploiting polymers in drug delivery is to create a solid dispersion, where a drug in its readily soluble form (amorphous) is dispersed within a polymer matrix, to improve the oral solubility of poorly water soluble drugs (da Silva Júnior et al., 2017).Large surface area structures such as beads and fibers have therefore seen increasing utilization as solid dispersions.In 2015, the first reports where published where PG was used to produce an amorphous nanofibre solid dispersion of ibuprofen, a poorly water-soluble drug, using PVP as the polymer matrix (Raimi-Abraham et al., 2015).The production of the PVP-ibuprofen nanofibers proved to be extremely rapid with diameters averaging as little as 1.2 μm, that provided a large surface area for dissolution.Drug-loading of up to 50% (w/w) was achieved demonstrating the massive potential for high deliverable doses with PVP.Dissolution studies performed on the drug-loaded fibers showed a clear increase in dissolution rate compared to just administering the drug alone.Within the fibers at a molecular basis, Ibuprofen was found to be dispersed following analysis via attenuated total reflectance Fourier transform infrared spectroscopy.This study showed promising evidence that PG is capable of being a viable technique for generating drug-loaded fibers for oral administration routes, with enhanced in vitro dissolution rate enhancements.
Whilst oral delivery routes are widely preferred, there are some other routes which benefit from a higher level of bioavailability when considering the circumstances.In 2015, mucoadhesive nanofibers were produced by PG for intra-vaginal delivery (Brako et al., 2015).In this study, different polymer blends were used to understand the effects these had on their mucoadhesive properties; these polymers included sodium carboxymethylcellulose (CMC), PEO, polyacrylic acid (PAA) and sodium alginate.The produced PEO:CMC fibers had an average diameter ranging from as F I G U R E 6 Parametric plot between the rotating speed and the working pressure of PVA-protein solutions showing the onset of bubbling (continuous vertical line).Zone 1, no bubbling; zone 2, no fibers; zone 3, fibers; zone 4, bubbling; and zone 5, fiber formable region for PVA solution only.(Creative commons : Mahalingam, Raimi-Abraham, et al., 2015).little as 161-280 nm, showing that PG could produce extremely thin fibers.Other polymer blends including PEO: alginate, PEO:PAA and PEO:CMC all had an average fiber diameter below 250 nm.The mucoadhesive properties of these polymer blends were evaluated by measuring the force required in detaching away from simulated mucosal environments using atomic force microscopy (AFM).The surface roughness of dried residues taken from the fiber/mucin mixtures was analyzed to determine the extent of dissolution of the fibers within the simulated vaginal mucosal environment, as this could provide an insight into the hydration levels prior to mucoadhesion.From these tests, PEO: alginate and PEO:CMC fibers demonstrated the highest potential for mucoadhesion.
Following on from the initial work carried out on mucoadhesive polymers for vaginal delivery, a paper in 2018 demonstrated the potential of using PG to deliver progesterone intra-vaginally (Brako et al., 2018).Progesterone has many crucial uses in women's health, ranging from hormone replacement therapy to reducing the risk of pre-term birth, by providing an easy to use vaginal route and many more women around the world would benefit (Norman, 2020).In this study, different blends of PEO and CMC containing 5 wt% progesterone drug was spun with PG to see the effects on the mucoadhesive properties.Without any drug loading, the PEO:CMC fibers had an average fiber diameter of 194 nm, but with the addition of progesterone, the average diameter increased to 404 nm for the PEO (13.75 wt %)/CMC (1.25 wt %) polymer blend.Scanning electron micrographs showed a relatively smooth surface with presence of some ridges for the drug-free fibers and a much smoother surface for the progesterone-loaded fibers.The mucoadhesive properties of various progesterone-loaded fibers were measured using a texture analyzer on an artificial cellulose acetate membrane and natural lamb esophagus mucosa.It was found that increasing content of CMC within the fiber lead to a higher detaching force being required.A greater detaching force corresponds to improved mucoadhesion.The study therefore showed that progesterone loaded fibers could successfully be produced by PG with varying levels of muscoadhesion to suit the end use-case scenario.

| Ultrafiltration
Small diameter fibers produced by technologies such as centrifugal spinning and pressurized gyration have the beneficial characteristics of high surface area and porosity, these features make them especially useful as filter material.Hospital acquired infections are a leading cause for concern in healthcare settings and can contribute to thousands of annual deaths, a considerable economic strain (Anaissie et al., 2002).Hospitals currently use "point of use" filters on their air and water supplies with the intention of preventing the spread of infection, whilst these filters may trap microbes, they are not able to always kill them.By incorporating known antibacterial nanoparticles into gyrospun polymeric fibers, mesh escapable of antimicrobial filtration have been produced (U.E. Illangakoon, Mahalingam, Wang, et al., 2017).Two types of antimicrobial nanoparticles produced by The University of Hertfordshire (UK), AMNP1 and AMNP2, which contained various metallic components such as aluminium, calcium, copper, tungsten, and zinc, were integrated into a poly(methylmethacrylate) (PMMA) polymer solution and spun into fibers.
Pressurized gyration was able to form a large yield of fibers which could be attached to a metallic disc and tested for their antimicrobial capabilities against P. aeruginosa, a common hospital acquired infection.Photographs showing these fibers can be seen in Figure 8.The fibers produced where continuous and bead-free, fibers containing AMNP1 had an average diameter of 6 ± 4 μm and fibers containing AMNP had an average diameter of 7 ± 4 μm.The surface of the fibers also contained additional porosity with the presence of nanopores, which further increases the surface area available for filtration.Both AMNP1 and AMNP2 containing fibers showed a bacterial reduction of over 70%, demonstrating the viability of pressurized gyration to produce meshes capable of antimicrobial filtration.

| Wound healing
Wound healing is a complicated multi-step process which necessitates many specific conditions in order to facilitate proper and timely healing (Broughton 2nd et al., 2006).The most common form of basic wound care is the provision of a bandage, which serves to protect an open wound from immediate trauma.Traditional bandages are limited however, by their inability to stop and prevent the spread of infection, as well as in providing an ideal microenvironment for high quality healing.Bacterial cellulose has been discovered as a biopolymer which offers many desirable wound healing characteristics such as having a significant biocompatibility, biomimetic nature and a high water uptake capability (Ahmed et al., 2020).Whilst electrospinning has been used to produce composite fibers with bacterial cellulose, higher loadings of bacterial cellulose become increasingly difficult to spin (Altun et al., 2019).Furthermore, to achieve fibrous structures which resemble wearable traditional bandages, the technology used must produce a high enough yield and mechanical integrity.In 2018, initial work involving PG-spun bacterial cellulose loaded PMMA fibers was presented (Altun, Aydogdu, et al., 2018).Several weight ratios of bacterial cellulose: PMMA polymer solutions were spun and characterized for their suitability as a wound healing materials.It was found that additions of bacterial cellulose to the PMMA polymer matrix significantly increased the viscosity, making forming more difficult.The gyrospun fibers were produced via a single step and resembled bandages that could be worn directly (Figure 9).
Upon closer inspection, the fibers appeared to be highly beaded (see inset 1 of Figure 9), a common occurrence when the bacterial cellulose concentration is increased.Bacterial cellulose concentrations as high as 10 wt% was achieved in this study.The average fiber diameters ranged from 1.7 to 6.8 μm.As these fibers were intended to be used as bandages, their toxicity was measured by assays involving the human osteosarcoma cell line, Saos-2.By the end of a testing period of 72 h, cell viability studied for most of the bacterial cellulose:PMMA samples indicated no toxicity.However, at higher PMMA concentrations, cell viability reduced, possibly relating to the effect of PMMA in the bandages.Cell migration, in addition to cell proliferation, essential in wound healing microenvironments, was observed on the gyrospun scaffolds.This study opened the doors up to future work on producing viable bandages using pressurized gyration.
A follow up study on producing wound-healing capable bacterial cellulose bandages improved upon the production quality and loading of bacterial cellulose (Altun, Aydogdu Mehmet, et al., 2018).In this study, two new antimicrobial nanoparticles (UHNP-1 and AVNP-2), which contained silver and copper were used in conjunction with PMMA and tested in a co-culture of skin cells (keratinocytes) and pathogenic microbes (S. aureus).A co-culture is a more representative environment where we can assess the possible interactions between the cells and the microbes.Using more advanced techniques such as ultrasonication to improve the solubility of the bacterial cellulose, higher quality fibers where produced (Figure 10).As can be seen by the photographs of the fibers, pressurized gyration was able to produce substantial amounts of fiber in a single operation, which could be used to make multiple bandages, with the incorporation of antimicrobial nanoparticles.Mechanical testing showed them to have sufficient tensile strength (0.99 ± 0.06 MPa for the bacterial cellulose:PMMA UHNP-1 fibers).
In co-culture testing, the cell viability of the tested fibrous samples mirrored that of the control, showing little to no toxicity, even with the addition of antimicrobial nanoparticles in the 24-h test period.The addition of AVNP-2 to the fibers led to a significant reduction in the population of S. aureus and micrographs taken from the surface confirm the lack of bacterial attachment, indicating effective antibacterial action.The works reveals the potential of using pressurized gyration to form composite bandage-like fibers with adequate mechanical and antimicrobial ability.

| Comparisons to electrospinning
Investigation into exploiting small-diameter fibers for pharmaceutical engineering is extensive with thousands of publications using electrospinning to form drug delivery structures (Alavarse et al., 2017;Hu et al., 2014;Shen et al., 2011).Drugs which have poor water solubility, often have great permeability, meaning that if their solubility were to be overcome, they could be administered with ease.Polymeric amorphous solid dispersions are an engineering approach to overcoming such solubility issues by incorporating a dissolved drug into a matrix of a hydrophilic polymer, by spinning the polymer into a high surface area fiber, the drug can easily be dosed orally (Vass et al., 2020;Williams et al., 2013).The drug solubility increasing ability of pressurized gyration spun and electrospun fibers were compared in a study which looked at producing antiviral PVP fibers (Ahmed et al., 2018).
Comparatively, electrospinning produced thinner overall fibers with the addition of drug, lowering the average diameter.For example, itraconazole-loaded PVP fibers produced by electrospinning had an average diameter of 0.94 ± 0.34 μm, compared to 1.60 ± 0.87 μm for the same PG spun counterpart.The morphological features of the drug delivery patches will have a marked effect on the solubility enhancement, although surface area is just one consideration, the porosity and alignment between the fibers can also have a differing influence.The drug-release profiles of the PG and electrospun fibers containing the antiviral drugs itraconazole and amphotericin B are shown in Figure 11.
Both fiber types spun by the two different technologies saw a significant increase in solubility when compared to the same dose of the virgin drug.Itraconazole-loaded electrospun fibers demonstrated a more rapid initial release profile when compared to the PG-spun fibers, however the overall release shortly became stronger on the gyration delivery.For the amphotericin B-loaded PVP fibers, the release profile was again similar, with PG-spun fibers releasing more overall drug, but at a slightly slower initial burst.This work shows the comparison between the two technologies, where the final fiber structure, as well as the fiber diameter, plays a key role in drug kinetics.

| Antimicrobials
Research into graphene and its derivatives has gathered tremendous momentum over the past decade with multiple papers appearing at a frequent pace.Graphene is a single atom layer of graphite and has attracted much attention due to its unique properties pertaining to features such as high thermal and electrical conductivity, hardness and even is antibacterial effect (Geim, 2009).Through investigations into the antimicrobial properties of graphene, a few mechanisms of microbicidal action have been proposed: through the production of reactive oxygen species, via microbial encapsulation and through the physical damage to the microbial membrane (Akhavan & Ghaderi, 2010;Akhavan & Ghaderi, 2012;Hu et al., 2010).The benefit in the latter is that antimicrobial action via the physical rupturing of membrane material is very difficult to overcome by mutation and evolution, leading to a longer-lasting effect.Graphene nanoplatelets, a derivate of graphene which consists of a single layer of sp 2 hybridized carbon atoms was investigated for its antimicrobial ability in fibrous form (Matharu, Porwal, Ciric, & Edirisinghe, 2018).PG was used to form fibers of 2, 4 and 8 wt% Graphene nanoplatelets within a PMMA polymer matrix and spun at maximum speed of 36,000 rpm with an applied additional pressure of 0.2 MPa.The produced fibers appeared to be tubular in nature and contain extensive surface porosity.When increasing the concentration of graphene nanoplatelets, the morphology slightly shifted to contain more beads and the tubular structure slightly flattened.Fibers of 2 and 4 wt% graphene nanoplatelets had an average fiber diameter under 1 μm, but at 8 wt%, the diameter increased to 2.7 μm.The fibers where then tested against E. coli and P. aeruginosa, the results are shown in (Figure 12).The results from the antibacterial testing of the graphene nanoplatelet fibers are very interesting.At 2 and 4 wt% concentrations, there is no bactericidal effect realized by the fibrous mesh, in fact there is a large bacterial growth for both E. coli and P. aeruginosa populations, suggesting that conditions are suitable for microbial propagation.However, at a concentration of 8 wt%, a relatively high loading of graphene nanoplatelet, there is strong antibacterial activity, shown by a cell inactivation percentage of 85% for E. coli and 95% for P. aeruginosa.At higher loadings, the graphene nanoplatelets are more commonly exposed onto the surface of the gyrospun fibers, making them a hostile environment for bacteria and contributing to a reduction in their numbers.This pioneering study shows the feasibility of pressure spinning technologies to produce long-lasting antibacterial meshes for a wide range of biomedical applications.
Known antibacterial metallic nanoparticles such as silver and copper retain the ability to fight pathogenic growth but do so with some toxicity.The novel use of non-widespread elements into fibrous meshes can serve to advance the field.Given the simple nature of PG to produce composite polymeric fibers, tellurium particles were used to produce fibers that were tested for their antibacterial capabilities (Matharu, Charani, Ciric, Illangakoon Upulitha, & Edirisinghe, 2018).Tellurium is a rare metal element that has many ionic states, giving it toxicity against some bacterial species via the reactive oxygen species (Zonaro et al., 2015).Unlike other metals such as copper and silver, tellurium is less explored for its antibacterial capabilities.PG was used to generate a range of tellurium-loaded PMMA fibers from 0 wt% to 4 wt%, using the maximum rotational speed (36,000 rpm), and an applied gas pressure of 0.1 MPa.Upon close inspection, all the tested fibers appeared to be smooth and continuous in morphology, 1 wt% tellurium-PMMA fibers had an average fiber diameter of about 8 μm, with higher loadings of tellurium, the average increased to about 14 μm.Energy-dispersive x-ray spectroscopy (EDX) confirmed the presence of tellurium on the fibers, the images and EDX profiles can be seen in Figure 13.
The fibers where then tested against the growth of Gram-positive (S. aureus) and Gram-negative (E. coli and P. aeruginosa) bacterial species.The control sample consisting of only PMMA, saw a small log reduction of about 0.03 in bacterial growth, this could be due to the brittle nature of PMMA.However, the addition and subsequent concentration increases of tellurium into the fibers saw an exponential increase in the bacterial reduction.At only 1 wt%, there is a significant increase in bacterial numbers, and at 4 wt% a log reduction of about 1.16 is observed.Once again, this work demonstrates the feasibility of using PG to produce functional fibrous meshes for antimicrobial activities, such fibers have enormous potential as point-of-contact antimicrobial filters.

| Infusion gyration
Given the success of pressured gyration and the simplicity in which it operates, further derivatives of the technology have been developed and explored.The first development of the PG device came in the form a sister technology coined infusion gyration.The premise of this technology is to allow for a variable flow rate into the gyration pot, combining both centrifugal spinning and polymer infusion (Figure 14) (Zhang et al., 2015).
The infusion gyration technique can be used to produce polymeric fibers for a wide range of biomedical applications, in its first appearance, it was used to produce nanofibers integrated with genetically engineered proteins.The gold-binding dodecapeptide, Au-BP2, together with gold nanoparticles were incorporated into 10 wt% PEO.The solution containing the green solvent phosphate buffer saline, was spun into nanofibers at different polymer flow rates of 600-5000 μL/min.The resulting images of the fibers can be seen in Figure 15.The produced fibers displayed very fine diameters, ranging from an average of 117-216 nm.
Generally, an increase in flow rate led to a slight increase in fiber diameter, but more uniform fiber production was achieved when there was a suitable polymer flow rate.The control over the flow rate becomes especially important with polymers with higher viscosities, which generally produce substantially thicker fibers.It was also found that at higher infusion rates, the yield also increases accordingly.The fibers where then characterized by several techniques to confirm the protein uptake of the fibers, the engineered proteins were bound to the gold nanoparticles, as confirmed by the fluorescent activity of the fibers.The protein-incorporated nanofibers where able to maintain their biological function even in different buffer conditions.The work showed how infusion gyration could be used to produce functional polymeric nanofibers with the incorporation of important bioactive materials such as genetically modified proteins.One downside however of the technology was that it did not incorporate an applied gas infusion, something that hugely benefits fiber production via pressured gyration.

| Melt gyration
In 2016, another use of pressurized gyration was shown, in its ability to produce polymeric fibers without the use of a solvent.The technique of creating a polymer melt from a thermoplastic and subsequently forming fibers was shown in a technique later called melt-pressurized gyration (Figure 16).PCL, one of the most extensively used biocompatible and biodegradable polymers for biomedical applications, was spun into fibrous scaffolds for tissue engineering applications (Xu et al., 2016).
PCL pellets were loaded into the gyration vessel, where a temperature of up to 600 C could be applied via a heat gun and monitored via a thermocouple.To assess the difference in product formation at different applied temperatures, the melt-pressurized gyration process was used to form fibers using the maximum rotation speed and various applied pressures at temperatures of 95, 125, 155 and 200 C. PCL has a melting point of around 60 C, and the polymer was loaded with silver nanoparticles, to offer a solvent-free alternative to a tissue engineering scaffold.Fibers where successfully formed using the technique where it was found that the fiber diameter decreased with increasing temperatures, the images of these fibers can be seen in Figure 17.The thinnest fibers were achieved at the highest recorded temperature of 200 C and at the highest recorded applied pressure of 0.2 MPa.Higher temperatures allow for the molten polymer to remain in liquid state for longer, meaning that there is additional time afforded to jet elongation, and the polymer is easier to manipulate due to its lower viscosity.Also, depending on the conditions of the spinning, different surface morphologies where observed.For example, at 105 C, the surfaces of the produced fibers appeared smooth, but at 200 C, surface features such as particle formation and grooves were noted.
The silver-loaded PCL scaffolds produced by melt gyration where then assessed for their antibacterial effectiveness against E. coli and P. aeruginosa.Against E. coli, all of the silver containing scaffolds gave a near-identical antibacterial rate response of about 100%, indicating a very strong bactericidal ability (Figure 18).However, against P. aeruginosa, the antibacterial ability reduced with increasing temperature, suggesting that the differences in the surface topography of the fibers played a role in allowing more bacteria to survive.

| Pressure-coupled-infusion gyration
Generating fibers using a controllable polymer flow rate has proved to be a viable method in improving the product quality of centrifugal spinning, however what makes pressurized gyration unique is that it further incorporates an inlet gas pressure which is a supplementary driving force in the production of fine fibers.A system which incorporates pressurized gyration and infusion gyration has been developed named pressure-coupled-infusion-gyration (PCIG) for the creation of functional nanofibres (Hong et al., 2017).The setup consists of the usual high-pressure nitrogen gas infusion and the high-speed motor, but additionally incorporates a polymer pump system into a T-junction, that can provide a polymer flow rate of up to 5000 μL/min (Figure 19).In this setup, there are additional operating parameters which allow for more control over the final fiber morphology.
To test the validity of combining an infusion of gas and polymer into a centrifugal spinning setup, fibers were formed by PCIG and subsequently characterized.PEO polymer solutions were prepared in 5 differing concentrations (3, 5, 10, 15 and 21 wt%), and spun under different operating parameters.In PCIG, the infusion of polymer solution adds a hydrostatic pressure to the PG system which creates an additional driving force to the polymer jetting process.Generally speaking, the increase in polymer flow rate, above the critical minimum, leads to increases in the average fiber diameter.For 10 wt% PEO solutions, which was deemed to be the optimal solution for spinning, the thinnest fibers were achieved using a flow rate of 500 μL/min, with an average diameter of only 112 nm and the thickest at 5000 μL/min, where the average was 206 nm.Increases in working pressure led to decreases in the average fiber diameter, as is expected with a water-soluble polymer system.10 wt% PEO fibers spun at different working pressures and flow rates can be seen in Figure 20.
At high working pressures and low polymer infusion rates, thin continuous fibers with narrow diameter distributions can be produced.At a working pressure of 3 Â 10 5 Pa, maximum rotation speed and a polymer infusion rate of F I G U R E 1 5 SEM images, diameter distributions and fluorescence micrographs of protein-integrated fibers produced at flow rates of (a) 500 μL/min (b) 1000 μL/min, (c) 2000 μL/min, (d) 3000 μL/1 (e) 4000 μL/min, (f) 5000 μL/min, at a fixed maximum rotation speed.(Creative commons: Zhang et al., 2015).3000 μL/min, PEO fibers with an average diameter of 92 nm were produced, which rivals even electropun fibers.At 10 wt%, the fibers were smooth, bead-free and highly aligned.It was found that using PCIG, the solution properties of the polymer had a greater effect on surface features as the additional operating parameters allowed for greater product customizability.Beaded nanofibers were produced when the polymer solution was too low (5 wt%), indicating that in this case, the bead-on-string morphology could be due to the transition between particle to fiber.Mathematical modeling has also be used to predict some of the behaviors of PCIG, showing that it is a simple and predictable method in the generation of nanofibres for use in a wide range of biomedical applications (Hong et al., 2018(Hong et al., , 2019)).

| Microbubble generation with pressurized gyration
Although the majority of the work carried out regarding pressurized gyration involved the production of fibers, other polymeric structures can also be produced using this technique.In 2014, PG was explored for its ability to form microbubbles which has applications in medical imaging and drug delivery (Mahalingam, Raimi-Abraham, et al., 2015).Unlike centrifugal spinning alone which cannot generate bubbles, PG has the added benefit of an applied gas pressure which aids in the bubble pinch-off process which can produce micro-sized bubbles.Because of their gas filled core, microbubbles have established use in applications such as in diagnostics, as ultrasound contrast agents, this is because bubbles can be readily compressed, absorbing and reradiating sound energy as well as simply reflecting it (Blomley et al., 2001).
In order to produce microbubbles, polyvinyl alcohol along with lysozyme and gold nanoparticles were used to form a solution.This polymer solution was then spun by PG at various rotation speeds and applied gas pressures to observe the effect of the operating parameters on bubble formation.Bubbles were successfully produced using PG and images of these can be seen in Figure 21.
Microbubbles up to 250 μm in diameter were produced and the gas core can be clearly seen, indicating that the bubble formation is aided by the applied gas pressure of the PG setup.Reduction in the bubble diameter was observed during increases in rotational speed, for the lysozyme solution an increase from 10,000 to 36,000 rpm resulted in a reduction from 136 to 95 μm.The produced microbubbles had a spherical morphology, even at higher rotational speeds.The applied gas pressure had the most dramatic reduction effect on the bubble diameter where an increase from 0.1 to 0.3 MPa saw a near halving of diameter from about 50 to 25 μm.The microbubble diameter could further be tailored by altering the nanoparticle content, something of which is especially easy in gyration-based spinning techniques.
To continue the investigation into microbubble formation using PG, a follow-up study showed the feasibility of producing PVA-Lysozyme microbubbles for potential biosensing applications (Mahalingam, Xu, et al., 2015).In this study, microbubbles up to 250 μm in diameter were produced by PG using a succession of PVA-lysozyme-gold nanoparticle solutions, as shown in Figure 22.It was confirmed that the bubble diameter could be reliably reduced by increasing the rotational speed and applied gas pressure.The average microbubble diameter was reduced from $100 to $50 μm when the rotational speed was increased from 10,000 to 36,000 rpm.The PVA-lysozyme microbubbles were also tested for their antibacterial activity against the Gram-negative E. coli microorganism, where an increase in gold nanoparticle content in the microbubbles corresponded to an increase in antibacterial activity (73% average antibacterial activity for the gold-loaded bubbles and 59% activity for the bare protein bubbles).By producing these materials in bubble form, the interactions between the positively charged lysozyme can contribute to enhanced antibacterial activity.
To assess their suitability in biosensing applications, the microbubbles were conjugated with alkaline phosphatase and investigated for their detection of paraoxon, a cholinesterase inhibitor.The conjugated microbubbles were able to detect paraoxon at concentrations as low as 0.5 ppm, which demonstrated superior sensitivity than other bioassays.The work with microbubbles effectively establishes the main differences between a centrifugal-only based fiber manufacturing production system and one which incorporates additional driving forces such as an applied gas pressure and polymer infusion, that is, PG.

| Core-sheath fiber production
In producing fibers for biomedical applications, generating novel morphologies becomes an attractive feat as every new structure opens up a whole range of new possible applications.Recently, fibers consisting of a core and a sheath, dubbed core-sheath fibers, have attracted a lot of attention for various applications from wound healing to nerve tissue engineering (Gao et al., 2018;Zhang et al., 2014).Most commonly, these fibers are manufactured using a modified electrospinning setup which comprises of a metallic nozzle encompassed within another, to produce concentric fiber production (Yoon et al., 2018).Electrospinning offers a wide choice in the production of core-sheath fibers, but the exploration of pressure-driven methods to achieve such fibers will always serve to further the available choices.In 2020, such a device, based on pressurized gyration was created which was able to form core-sheath fibers made of two different polymers (Mahalingam et al., 2020).
The original core-sheath pressurized gyration device had an apparatus consisting of a twin-reservoir aluminium vessel containing an outer diameter of 100 mm and an inner diameter of 80 mm.Both the inner and outer vessel had a total volume of 20 mL where there existed two concentric capillaries measuring 1.6 mm for the outer and 0.8 mm for the inner diameter.This core-sheath PG setup was also capable of providing a gas infusion pressure of up to 0.3 MPa and a rotational speed of up to 6000 rpm.To showcase the production of core-sheath fibers, PEO and PVP polymer solutions were fed into the twin reservoirs, spun at various conditions and the products where subsequently characterized.
It was found that at lower rotational speeds, beaded fibers would be collected and at higher speeds, the bead density would decrease.This is one benefit of designing a light-weight aluminium vessel where high rotational speeds can easily be achieved.Images of fibers produced at a constant applied pressure of 0.1 MPa but at different rotational speeds can be seen in Figure 23.
At a rotational speed of 6000 rpm and at no additional pressure, the fibers produced has an average diameter of 625 nm, this reduced to 430 nm in the presence of an applied gas pressure of 0.1 MPa.Overall, the fibers showed a pore-free and smooth topography with a highly unidirectionally aligned morphology.In order to visualize the internal structure of the core-sheath fibers, focused ion beam imaging was utilized, micrographs showing the cross-section images of the fibers can be seen in Figure 24.(Creative commons : Mahalingam, Raimi-Abraham, et al., 2015).
It is apparent from the images that the core of the polymer is separate from the sheath material.In the core-sheath vessel, the two solutions are separated by thin walls and interaction does not occur until they exit the orifices.The solution belonging to the sheath reservoir experiences greater centrifugal force than the core solution due to the greater radius of the outer vessel and consequently acts as the propeller of the core solution when it reaches the orifices.
The core-sheath pressurized gyration device, also known as co-axial gyro-spinning and was later used to produce core-sheath fibrous scaffolds for bone tissue engineering (Mahalingam et al., 2021).By utilizing this technique, fibrous scaffolds could be produced with two different polymeric phases with suitable properties as a tissue engineering scaffold and this can further be incorporated with bioactive additions such as loading it with nano-hydroxyapatite.The two phases could work in tandem to provide the necessary factors for regeneration such as osseointegration and angiogenesis cues, whilst also sustaining an antimicrobial action.
Additionally, core-sheath gyro-spun fibers have been produced for use in drug delivery applications where the use of two different polymer types provides control over the release profiles (Majd et al., 2022).In this study, antibiotic tetracycline hydrochloride (TEHCL) was embedded into the hydrophilic polymeric core made of PVP and also into the hydrophobic sheath made of PCL.The drug release of the core-sheath fibers where then compared with TEHCL-loaded PCL fibers without a separate polymer core.Confocal microscopy images show one of the PCL:PVP(TEHCL) fiber strands, with the PCL sheath material and PVP core in distinct phases Figure 25.The core-sheath fibers were produced using the maximum available rotational speed of 6000 rpm and at an applied gas pressure of 0.1 and 0.2 MPa.At an F I G U R E 2 2 PVA-lysozyme microbubble size distribution and corresponding optical micrograph obtained at rotational speeds (rpm) of: (a) 10,000, (b) 24,000, and (c) 36,000.In all instances the working pressure was 0.2 Â 10 5 Pa.(Creative commons: Mahalingam, Xu, et al., 2015).applied gas pressure of 0.1 MPa, the PCL:PVP(TEHCL) fibers had an average diameter of 5.0 ± 1.4 μm and showed excellent unidirectional alignment, increasing the gas pressure to 0.2 MPa, saw a size reduction and uniformity improvement to 4.1 ± 1.1 μm.The surface of the produced core-sheath fibers contained many nanopores as a result of using a volatile solvent such as chloroform, which would provide rapid evaporation and the formation of microdroplets, leading to the formation of surface nanopores (E.U. Illangakoon, Mahalingam, Matharu, & Edirisinghe, 2017).
The drug release kinetics of the PCL:PVP(TEHCL) fibers were compared to TEHCL-loaded PCL fibers.Compared to the PCL-only fibers, the core-sheath fibers demonstrated a much more subdued burst profile, releasing a lower percentage of drug in the tested 168-hour time period.This indicates that the drug-release profile is more akin to a sustained release model and that the technique of incorporating a drug in both the hydrophobic sheath and hydrophilic core can be exploited to offer therapeutic delivery with more control over dosing.The coaxial gyration technique therefore offers a viable alternative to electric-field-free methods of core-sheath fiber generation, with the access to massscale and cost-effective production.
As much work has gone into the development of gyrospinning technologies, the technique continues to be upgraded in order to improve production and suitability for mass production.In continuing with the theme of core-sheath capable nanofibre production, a novel device which improves upon all aspects of the original coaxial pressurized gyration device has been designed and manufactured (Alenezi et al., 2021).The detailed schematics of this novel device can be found in Figure 26, compared to the original core-sheath assembly, the new device is capable of achieving faster rotational speeds and had an improved reservoir design for producing higher quality fibers.In order to assess the production quality of this device, several polymers were used to produce core-sheath fibers, namely PEO, PCL and polylactic acid (PLA).
By utilizing focused ion beam electron microscopy, cross sectional images of the core-sheath fibers were attained, which visibly shows the difference between the separate sheath and core material Figure 27.The images show that the functional layer of sheath material is considerably thinner than the area of the core, allowing for more economical utilization of active functional sheath material.
Core-sheath fibers containing PLA as the sheath material and PEO as the core, showed the thinnest diameters, at an average of 529 nm.The results shown in this study demonstrates the potential of utilizing the base technique of  pressurized gyration in order to continue developing systems that push the boundaries when it comes to the production of polymeric nanostructures.

| Sustainable hybrid fiber production
The future of fiber production is to move to more sustainable and environmentally friendly methods and practices.Polymers with desirable properties often require the use of harsh organic solvents which can cause damage human health, aswell as to ecology.Utilizing natural polymers and those that do not require harsh solvents will lead to more sustainability in the field.One particular polymer, cyclodextrin, has been spun with PG and electrospinning to form a highly sustainable and environmentally friendly hybrid structure for various biomedical applications (Kelly et al., 2022).
Cyclodextrins are a family of highly water-soluble polymers that show excellent biocompatibility and are regularly used by the pharmaceutical industry to increase the bioavailability of poorly water-soluble drug formulations.They also have use in tissue engineering applications due to their highly specific cavity structure that provides them the ability to be highly specific in capturing various organic molecules (Liu et al., 2020).By employing PG and electrospinning, pure cyclodextrin dissolved in only water was formed into a hybrid "supermat" containing a PG-spun mechanical support, with an ultra-fine electrospun surface mesh (Figure 28).
As discussed in this article, different manufacturing techniques have their unique strengths and weakness.By combining the use of PG and electrospinning and leveraging their separate advantages, novel structures can be formed to allow for new approaches to biomimetic materials for biomedical applications.

| FUTURE PERSPECTIVES AND CONCLUDING REMARKS
Centrifugal spinning, a technology which has allowed for the production of vital polymeric microstructures, has benefitted the biomedical field immensely.It is a technology which continues to improve and advance in order to match the demands put in place by various healthcare requirements; drug delivery and biosensing in particular.Owing to its popularity, pressurized gyration has piggybacked on the success of using such a technique which leverages a centrifugal force to rapidly generate thin fibers with a high surface area.In its own right however, pressurized gyration and its sister technologies are a separate class of fiber manufacturing due to the incorporation of an applied gas pressure which significantly alters the internal kinetic behavior of the polymer solution.A recent study into the effects of solvent and pressure on the morphology of produced fibers with centrifugal spinning and pressurized gyration found that the applied pressure can significantly alter the structure of the polymeric products (Altun et al., 2022).In an example using PCL as the polymer and tetrahydrofuran as the solvent, only beads were achieved with centrifugal spinning, whereas fibers were produced at applied gas pressures of 0.1-0.3MPa.The applied gas pressure of the PG vessel provides an additional driving force during fiber production which is absent in centrifugal spinning alone, this driving force is responsible for further elongation of the polymer jet, leading to thinner fibers which are deposited.We see from the work presented, that centrifugal spinning and the many manufacturing techniques based on it, have contributed immensely to the advancement of many biomedical applications.For example, in drug delivery, solid dispersions can be produced rapidly to meet the demands of the industry, whilst also providing the ability to entrap a larger number of active pharmaceutical ingredients.Given the simplicity of such techniques to load important antimicrobial agents to polymer solutions, centrifugal spinning and pressurized gyration have contributed to the improved production of polymeric antimicrobial scaffolds which can be used in filtration, tissue engineering and wound healing applications.
Compared to conventional centrifugal spinning setups, pressurized gyration is able to achieve higher speeds, due to its simpler design which can easily be altered to incorporate faster motors and lighter materials.The rotational speed of such technologies is a vital factor in determining the overall size of the fibrous products, as well as the suitability for mass scale-up productions.Both centrifugal spinning and pressurized gyration are desirable in biomedical applications due to their ability to rapidly produce polymeric fibers, especially when compared to alternatives such as electrospinning, which often uses low polymer infusion rates and is limited in scale-up due to the nature of singlenozzle designs (Mehta et al., 2021).Pressurized gyration has shown that it is possible to produce microbubbles for medical imaging and diagnostic applications, utilizing the fast action of rotational speed and applied gas pressure.Although there is much work to be done to improve the uniformity of the bubbles produced, the potential to make such structures opens the door of possibilities for materials in healthcare.
Another benefit that PG enjoys is in its design simplicity, allowing it to be deployable in any laboratory setting whilst being as economical as possible.The vessel itself is made of a lightweight aluminium construction which can be produced cheaply and allow for high rotational speeds.Many centrifugal spinning technologies use propriety fittings and connections which make them more expensive and more difficult to replace.We have seen many sister technologies formed from the simplicity of pressurized gyration such as infusion gyration, melt gyration and coaxial pressurized gyration.The technology remains simple to modify and therefore encourages continuous development.The parts are simple to replace and can be easily ordered from a range of suppliers, avoiding possible supply chain issues.
The future of centrifugal spinning and pressure-based centrifugal spinning technologies such as pressurized gyration will be to continue pushing boundaries in the type of polymeric products they can produce.For example, much work has gone into producing better coaxial-capable gyration systems which can produce core-sheath fibers of higher quality, something which is especially difficult to produce at large scale.Furthermore, development is going into multilayered fibers consisting of more than one core or sheath, which will allow for a more diverse range of uses in applications such as tissue engineering and drug-delivery.In order to scale to multi-layered systems however, more work must be uncovered about the physics of core-sheath capable gyration systems, particularly in the fluid dynamics of the polymer solution and the physics behind the action of the multiple reservoirs.Core-sheath structures are also more effective with a high strength and tough core to maintain the mechanical robustness of the fibers, while the thin sheath makes it functionally active, for example, for making antimicrobial mask fibers (Huang et al., 2022).
Recently, other novel modifications to the pressurized gyration models have taken place, namely a technology called "nozzle pressurized gyration" which uses protruding guided nozzles to direct the emerging polymer jet in order to generate fibers with a greater degree of uniaxial uniformity and alignment (Dai et al., 2022).Using modified methods, we can aim to generate novel polymeric structures such as hollow-fibers, ribbon-like fibers and other nanomaterials.Novel morphological cues have tremendous suitability in certain tissue engineering applications where some cell niches respond to specific stimuli, able to be mimicked by polymeric fibers.
One particular focus on the development of these centrifugal-based fiber manufacturing techniques is seeing the transition to industrial scale mass production.Due to the use of high-speed motors, both centrifugal spinning and pressurized gyration, have the ability to produce masses of fiber in a short time.For true, high-scale production to take place, there must be a level of automation, which eliminates the batch-processing nature, which is common in many laboratory settings.It can therefore be suggested that the field investigates the incorporation of some sort of robotic assembly technique in order to match the high demands of such fibrous products needed by many biomedical applications.

F
I G U R E 4 Schematic diagram of (a) core-shell fiber production process of oil/water emulsion system by centrifugal spinning (b) centrifugal spinning set-up (c) Orifices and sample holder design of centrifugal spinning (d) Orifices with aggregated droplets interaction (Buzgo et al., 2017; Permission required).

5
Diagram showing overview of the basic pressurized gyration spinning setup.

F
I G U R E 9 Photograph showing the as-spun PMMA:bacterial cellulose fibers, suitable for a bandage.(1) inset showing beaded nature of the fibers.(original figure)

F
I G U R E 1 6 Diagram of the experimental setup of melt-pressurized gyration process.

F
I G U R E 1 7 SEM images of PCL fibers obtained at: (a) 95, (b) 105, (c) 125, (d) 155, (e) 200 C. Insets show high surface topography of a single corresponding fiber strand.Scale bars for the inset images are 20 μm.(Creative commons: Xu et al., 2016).F I G U R E 1 8 Antibacterial rates against (a) E. coli and (b) P. aeruginosa for the different samples of PCL fiber mats.(Creative commons: Xu et al., 2016).F I G U R E 1 9 Schematic diagram illustrating the pressure coupled infusion gyration equipment.

F
I G U R E 2 1 (a) Graph showing the effect of rotation speed on bubble diameter, at a working pressure of 0.2 MPa.(b) Optical micrograph of microbubbles generated at 10000 rpm and 0.2 MPa.(c) Graph showing the effect of the working pressure on bubble diameter, at a rotating speed of 36,000 rpm.(d) Optical micrograph of microbubbles processed at 36000 rpm and 0.1 MPa.NP indicates nanoparticle.

F
I G U R E 2 3 Scanning electron micrographs of core-sheath nanofibres obtained at a constant working pressure of 0.1 MPa at: (a) 2000 rpm (b) 4000 rpm (c) 6000 rpm.Insets show the corresponding fiber size distributions of core-sheath nanofibres for each case.(Creative commons: Mahalingam et al., 2020).

F
I G U R E 2 5 Confocal microscopy image of a core-sheath PCL:PVP(TEHCL) fiber strand, Rhodamine B (gray) illustrating the sheath and Acriflavine hydrochloride (purple) illustrating the core material.(Creative commons: Majd et al., 2022).

F
I G U R E 2 8 (a) Image of the cyclodextrin PG-spun and electrospun "super-mat" consisting of a PG-spun cyclodextrin base and electrospun exterior, (b) scanning electron microscopy image of the "super-mat" under high magnification showing the interface between the electrospun and the PG-spun cyclodextrin fibers, thicker gyration fibers have been highlighted with dotted lines.((Kelly et al., 2022) (permission may be required)).