Microbubbling and microencapsulation by co-axial electrohydrodynamic atomization

Microbubbles coated with polymers or surfactants have been used in medical imaging for several years as ultrasound contrast agent particles and are now being investigated by researchers as drug and gene delivery vehicles and blood substitutes. Current methods available for the preparation of microbubbles are insufficient as they result in microbubbles with a wide size distribution and as such filtration is necessary before their use. With a view to fill the above demand, a detailed investigation has been carried out in this research to learn the viability of co-axial electrohydrodynamic atomization (CEHDA) technique to prepare microbubbles. The research also focuses on the effects of the process parameters such as flow rates, applied voltage and material parameters such as electrical conductivity, surface tension and viscosity with the objective of preparing polymer or surfactant coated stabilized microbubbles with diameters < 8 μm and with a narrow size distribution. A model glycerol-air system was used so that the CEHDA technique was modified to generate suspensions of microbubbles to a diameter < 8 μm with a narrow size distribution and then to characterise the CEHDA microbubbling process in terms of size and stability with varying process parameters and material parameters. Construction of a parametric plot between the air flow rate and the liquid flow rate was extremely useful in identifying the flow rate regime of air and liquid or suspension or solution for the continuous microbubbling of the system used. With further investigations into the CEHDA microbubbling technique, it was possible to develop strategies, first, to prepare suspensions of stabilized phospholipids-coated microbubbles with a mean diameter of ~ 5 μm and a polydispersivity index of 9%, and second, polymeric microspheres with a mean diameter of 400 nm and a polydispersivity index of 8% using a biocompatible polymer

Electrohydrodynamic Atomisation Driven Design and Engineering of Opportunistic Particulate Systems For Applications in Drug Delivery, Therapeutics and Pharmaceutics

The file attached to this record is the author's final peer reviewed version. The Publisher's final version can be found by following the DOI link.Electrohydrodynamic atomisation (EHDA) technologies have evolved significantly over the past decade; branching into several established and emerging healthcare remits through timely advances in the engineering sciences and tailored conceptual process designs. More specifically for pharmaceutical and drug delivery spheres, electrospraying (ES) has presented itself as a high value technique enabling a plethora of different particulate structures. However, when coupled with novel formulations (e.g. co-flows) and innovative device aspects (e.g., materials and dimensions), core characteristics of particulates are manipulated and engineered specifically to deliver an application driven need, which is currently lacking, ranging from imaging and targeted delivery to controlled release and sensing. This demonstrates the holistic nature of these emerging technologies; which is often overlooked. Parametric driven control during particle engineering via the ES method yields opportunistic properties when compared to conventional methods, albeit at ambient conditions (e.g., temperature and pressure), making this extremely valuable for sensitive biologics and molecules of interest. Furthermore, several processing (e.g., flow rate, applied voltage and working distance) and solution (e.g., polymer concentration, electrical conductivity and surface tension) parameters impact ES modes and greatly influence the production of resulting particles. The formation of a steady cone-jet and subsequent atomisation during ES fabricates particles demonstrating monodispersity (or near monodispersed), narrow particle size distributions and smooth or textured morphologies; all of which are successfully incorporated in a one-step process. By following a controlled ES regime, tailored particles with various intricate structures (hollow microspheres, nanocups, Janus and cell-mimicking nanoparticles) can also be engineered through process head modifications central to the ES technique (single-needle spraying, coaxial, multi-needle and needleless approaches). Thus, intricate formulation design, set-up and combinatorial engineering of the EHDA process delivers particulate structures with a multitude of applications in tissue engineering, theranostics, bioresponsive systems as well as drug dosage forms for specific delivery to diseased or target tissues. This advanced technology has great potential to be implemented commercially, particularly on the industrial scale for several unmet pharmaceutical and medical challenges and needs. This review focuses on key seminal developments, ending with future perspectives addressing obstacles that need to be addressed for future advancement

Electric jet assisted production of micro and nano-scale particles as drug delivery carriers

In this thesis, the capability of the electrohydrodynamic atomization (EHDA) process for preparing drug delivery carriers consisting of biodegradable polymeric particles with different sizes and shapes was explored. The first part of the thesis describes a detailed investigation of how the size, morphology and shape of the particles generated can be controlled through the operating parameters; specifically the flow rate, applied voltage and the properties of the solutions. Diameter and shape of the particles were greatly influenced by viscosity and applied voltage. The mean size of the particles changed from 340 nm to 4.4 μm as the viscosity increased from 2.5 mPa s to 11 mPa s. Also, using more concentrated polymer solution (30 wt%) and higher applied voltage (above 14 kV) were found to be ideal for promoting chain entanglement and shape transition from spherical to oblong to a more needle-like shape. Estradiol-loaded micro and nanoparticles were produced with mean sizes ranging from 100 nm to 4.5 μm with an encapsulation efficiency ranging between 65% to 75%. The in vitro drug release profiles of the particles started with an initial short burst phase and followed by a longer period characterised by a lower release rate. Two strategies were developed to tailor these profiles. First, ultrasound was explored as a non-invasive method to stimulate “on demand” drug release from carrier particles. Systematic investigations were carried out to determine the effect of various ultrasound exposure parameters on the release rate in particular output power, duty cycle and exposure time. These three exposure parameters were seen to have a significant enhancing effect upon the drug release rate (up to 14%). The second strategy explored was coating the surface of the particles with chitosan and gelatin. This enabled control and reduction of the prominence ‘burst release’ phase without affecting other parts of the release profile. Coating the particle surface with 1 wt% chitosan solution considerably reduces the initial release by 62%, 60% and 42% for PLGA 2 wt%, 5 wt% and 10 wt%, respectivly in the first 72 hours This work demonstrates a powerful method of generating micro and nano drug-loaded polymeric particles, with modified release behaviour and with control over the initial release

Applications of Magnetic Microbubbles for Theranostics

Compared with other diagnostic methods, ultrasound is proven to be a safe, simple, non-invasive and cost-effective imaging technique, but the resolution is not comparable to that of magnetic resonance imaging (MRI). Contrast-enhanced ultrasound employing microbubbles can gain a better resolution and is now widely used to diagnose a number of diseases in the clinic. For the last decade, microbubbles have been widely used as ultrasound contrast agents, drug delivery systems and nucleic acid transfection tools. However, microbubbles are not fairly stable enough in some conditions and are not well administrated distributed in the circulation system. On the other hand, magnetic nanoparticles, as MRI contrast agents, can non-specifically penetrate into normal tissues because of their relatively small sizes. By taking advantage of these two kinds of agents, the magnetic microbubbles which couple magnetic iron oxides nanoparticles in the microbubble structure have been explored. The stability of microbubbles can be raised by encapsulating magnetic nanoparticles into the bubble shells and with the guidance of magnetic field, magnetic microbubbles can be delivered to regions of interest, and after appropriate ultrasound exposure, the nanoparticles can be released to the desired area while the magnetic microbubbles collapse. In this review, we summarize magnetic microbubbles used in diagnostic and therapeutic fields, and predict the potential applications of magnetic microbubbles in the future

Effect of copolymer composition on particle morphology and release behavior in vitro using progesterone

This study was aimed at improving dissolution rate and sustained release of progesterone by varying copolymer composition and polymer: drug ratio of PLGA. Drug-loaded particles were prepared using electrohydrodynamic atomization. The effects of polymer: drug ratio and copolymer composition on particle properties and in vitro drug-release profile were investigated. The physical form of the generated particles was determined via X-ray powder diffraction (XRPD) and Fourier transform infrared spectroscopy (FTIR). Drug release in vitro was found to be dependent on copolymer composition, where the release rate increased with decreased lactide content of PLGA. Particles produced with solutions of copolymer (75:25) had elongated shapes. In general, the obtained results indicated that the prepared microparticles were ideal carriers for oral administration of progesterone offering great potential to improve the dissolution rate of drugs that suffer from low aqueous solubility

Preparation of monodisperse microbubbles in a capillary embedded T-Junction device and the influence of process control parameters on bubble size and stability

The main goal for this work was to produce microbubbles for a wide range of applications with sizes ranging between 10 to 300 μm in a capillary embedded Tjunction device. Initially the bubble formation process was characterized and the factors that affected the bubble size; in particular the parameters that reduce it were determined. In this work, a polydimethylsiloxane (PDMS) block (100 x 100 x 10 mm3) was used, in which the T-shaped junction was created by embedded capillaries of fixed outer diameter. The effect of the inner diameter was investigated by varying all the inlet and outlet capillaries’ inner diameter at different stages. In addition, the effect of changes in the continuous phase viscosity and flow rate (Ql) as well as the gas pressure (Pg) on the resulting bubble size was studied. Aqueous glycerol solutions were chosen for the liquid phase, as they are widely used in experimental studies of flow phenomena and provide a simple method of varying properties through dilution. In addition, the viscosity could be varied without significantly changing the surface tension and density of the solutions. The experimental data were then compared with empirical data derived from scaling models proposed in literature, which is widely used and accepted as a basis of comparison among investigators. While the role of liquid viscosity was investigated by these authors, it was not directly incorporated in the scaling models proposed and therefore the effect of viscosity was also studied experimentally. It was found that bubble formation was influenced by both the ratio of liquid to gas flow rate and the capillary number. Furthermore, the effect of various surfactant types and concentrations on the bubble formation and stability were investigated. Preliminary studies with the current T-junction set-up indicated that producing microbubbles with size ranging from 50-300 μm was achievable. Subsequently, the study progressed to optimise the junction to produce smaller bubbles (~ 20 μm) by directly introducing an electric field to the T-junction set-up and assisting the bubble breakup with the combination of microfluidic and electrohydrodynamic focusing techniques. Finally, in this thesis, a novel method that combines microfluidics with electrohydrodynamic (EHD) processing to produce porous BSA scaffolds from microbubble templates with functional particles and/or fibres incorporated into the scaffolds’ structure is presented

Experimental and computational analysis of bubble generation combining oscillating fields and microfluidics

Microbubbles generated by microfluidic techniques have gained substantial interest in various fields such as food engineering, biosensors and the biomedical field. Recently, T-Junction geometries have been utilised for this purpose due to the exquisite control they offer over the processing parameters. However, this only relies on pressure driven flows; therefore bubble size reduction is limited, especially for very viscous solutions. The idea of combining microfluidics with electrohydrodynamics has recently been investigated using DC fields, however corona discharge was recorded at very high voltages with detrimental effects on the bubble size and stability. In order to overcome the aforementioned limitation, a novel set-up to superimpose an AC oscillation on a DC field is presented in this work with the aim of introducing additional parameters such as frequency, AC voltage and waveform type to further control bubble size, capitalising on well documented bubble resonance phenomena and properties. Firstly, the effect of applied AC voltage magnitude and the applied frequency were investigated. This was followed by investigating the effect of the mixing region and electric field strength on the microbubble diameter. A capillary embedded T-junction microfluidic device fitted with a stainless steel capillary was utilised for microbubble formation. A numerical model of the T-Junction was developed using a computational fluid dynamics-based multiphysics technique, combining the solution of transport equations for mass and momentum (Navier-Stokes Equations), a Volume of Fluid algorithm for tracking the gas-liquid interfaces, and a Maxwell Equations solver, all in a coupled manner. Simulation results were attained for the formation of the microbubbles with particular focus on the flow fields along the detachment of the emerging bubble. Experimental results indicated that frequencies between 2-10 kHz have a pronounced effect on the bubble size, whereas elevated AC voltages of 3-4 〖kV〗_(P-P) promoted bubble elongation and growth. It was observed that reducing the mixing region gap to 100 μm facilitated the formation of smaller bubbles due to the reduction of surface area, which increases the shear stresses experienced at the junction. Reducing the tip-to-collector distance causes a further reduction in the bubble size due to an increase in the electric field strength. Computational simulations suggest that there is a uniform velocity field distribution along the bubble upon application of a superimposed field. Microbubble detachment is facilitated by the recirculation of the dispersed phase. A decrease in velocity was observed upstream as the gas column occupies the junction suggesting the build-up in pressure, which corresponds to the widely reported ‘squeezing regime’ before the emerging bubble breaks off from the main stream. The novel set-up described in this work provides a viable processing methodology for preparing microbubbles that offers superior control and precision. In conjunction with optimised processing parameters, microbubbles of specific sizes can be generated to suit specific industrial applications

Fabrication of Porous Particulate Scaffolds Using Electrohydrodynamics and Thermally Induced Phase Separation for Biomedical Engineering Applications

Abstract The availability of forming technologies able to mass produce porous polymeric microspheres with diameters ranging from 150 to 300 µm is significant for some biomedical applications where tissue augmentation is required. Moreover, appropriate assembly of microspheres into scaffold is an important challenge to enable direct usage of the scaffolds in chronic wound treatments. In this thesis, the feasibility of the electrohydrodynamic (EHD) atomization forming combined with thermally induced phase separation (TIPS) for production of such drug delivery carriers, using biodegradable polymers (poly (lactic-co-glycolic acid) and poly (ε-caprolactone)) was explored. To achieve this goal, the first part of the thesis describes comprehensive parametric mode mappings of the diameter distribution profiles of the microspheres obtained over a broad range of key processing parameters and correlating this with the material parameters of five different polymer solutions of various concentrations. Based on the mode mapping studies, combination of poly (lactic-co-glycolic acid) (PLGA) and dimethyl carbonate (DMC) was found to be ideal for generating the microspheres within the targeted diameter range (150-300 µm). Surface porosity was achieved by electrospraying the PLGA/DMC solution and collecting the required size of the polymer particles in liquid nitrogen followed by lyophilisation. The second aim of this thesis was the in vitro release studies. In order to conduct this part of the study, the single needle and co-axial needle EHD/TIPS methods were used to generate the dye loaded microspheres of the required size. Three different dyes (Erythrosin B, Pyronin B and Reichardt’s) were selected as model drugs to be encapsulated separately in the produced microspheres. The purpose of selecting three different dyes was to have a prediction on the release profile of immunosuppressants with high toxicity used for treatment of chronic wounds such as perianal fistulae. The in vitro release studies showed that the dyes were released with the high initial burst release phase in 3.5-5.5 hours followed by a long and sustained release phase (in 30-360 hours). Systematic investigations using different external stimuli such as temperature, fresh media and sonication exposure was also carried out to observe their effects on the release rate of the encapsulated materials from the produced microspheres. The results acquired from the in vitro release studies showed that the temperature variations and the sonication with different frequencies have significant effects on the release rates of the incorporated materials from the polymeric microspheres. Moreover, the results demonstrated that the products collected by the single needle EHD/TIPS method is more capable of releasing the payload in a longer period of time with more sustained manner compared to their counterparts obtained from the co-axial needle method

Development and characterization of co-substituted hydroxyapatites for biomedical applications via advanced electrohydrodynamic processing

Hydroxyapatite (HA) is a well-known bioactive material for biomedical applications, particularly for bone graft and implant coatings. Substitutions with metal ions in HA can provide add-on functions, such as magnetic property by adding Fe3+ and Mn2+ and antimicrobial property by adding Cu2+ and Zn2+. Electrohydrodynamic atomization (EHDA) has been developed for coating using nanosuspensions. The stability of Taylor cone in EHD jetting was improved by a modification of nozzle geometry. Spherical-nozzles introduced were able to overcome the instability caused by frequent spray-mode switches, which interferes coating processing. As a result, the uniformity of the deposition was improved and 10-fold increase in the through-put of processing of HA nanosuspensions was achieved. Importantly, a stable cone-jet could be achieved with a water-ethanol ratio up to 4:1, which was significantly improved from 5% aqueous solution with conventional nozzle and extended the range of processable materials. Iron and manganese co-substituted HA (Fe-MnHA) has been synthesized to combine magnetic property with bioactivity. An analysis of the deposition of nanoparticles (NPs) using template-assisted electrohydrodynamic atomization (TAEA) under magnetic field showed that Fe-MnHA NPs was highly responsive in comparison with mono-substituted HAs, magnetic iron oxide nanoparticles (IONPs) and IONPs-HA mixture (50/50wt%). The in vitro bioactivity of Fe-MnHA was in the same level as HA but higher than IONPs and their mixture in simulated body fluid (SBF) testing. Therefore, Fe-MnHA balanced the magnetic property and bioactivity, and may have a great potential in biomedical applications, from cell guidance, contrast agents for medical imaging to cancer therapy. Multifunctional copper and zinc co-substituted HA (Cu-ZnHA) has been formulated to combine bioactivity with antimicrobial ability. Flow cytometry and crystal violet assays indicated its effective antibacterial and antibiofilm ability against E. coli and S. aureus. At the lowest metallic content (0.005mol%), Cu-ZnHA was not only antibacterial, but also showed a high biocompatibility by supporting the growth of human osteoblast-like MG63 cells. Therefore, Cu-ZnHA is a promising material for developing multifunctional implant coatings