Microparticles by supercritical fluid processing

Polymer microparticles can join together to form 3D scaffolds [142], alternatively they can be incorporated as particle components of composite scaffolds - the benefit imparted by the microparticles to the scaffold may be release of bioactive signalling molecules (e.g. BMP-2

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[143] or antibiotics to reduce risk of infection post-implantation. There is an abundance of literature surrounding SCF based methods for producing particles (see for example [144–

146]). Many papers deal with production of polymer microparticles while others deal with production of active molecule based nanoparticles, with little cross-over between the two. Most recently polymer-based microparticles in regenerative medicine were reviewed by Oliveira and Mano [147], although with limited discussion of SCF based methods. Sheth et al. [148] provided a recent and comprehensive review of SCF technologies for nanoparticle production in the pharmaceutical industry.

There are three established techniques for particle production using SCFs. These are: • Rapid expansion of supercritical solutions (RESS), where an SCF is saturated with a

substrate and rapidly depressurised through a heated nozzle to form small particles [146].

• Particles from gas-saturated solutions (PGSS), where a liquid or polymer is saturated with an SCF which is then expanded through a nozzle to produce particles.

• Supercritical antisolvent precipitation (SAS), where a liquid solution is contacted with an SCF in which the solvent is miscible and the solute is insoluble, causing supersatu- ration and precipitation of the solute [146].

A schematic overview is provided in Figure 1.14

Many innovative adaptations to these techniques have recently been patented. Fulton et al. [149] transformed the RESS method of producing nanoparticles into a novel coating process. A stream of charged ions was generated that interacted with the RESS produced nanoparticles, which enabled electrostatic deposition. Previous work from the same group had shown that it was extremely difficult to deposit particles in the range 10–500 nm on to a surface from the RESS technique due to their low mass [150]. This transformed process overcame these coating difficulties, avoided the use of organic solvents, and provided a more uniform coating compared to those prepared by a cloud-point precipitation technique.

Recently patented adaptations to the RESS method tackle the challenge of producing aqueous solutions of insoluble materials [151,152]. For example, Sun et al. [151] patented the use of stabilizing agents (that do not form micelles in aqueous solution), mixed with biologically active material (that has little or no solubility in water) in a solvent such as scCO2

to form a non-aqueous solution. Then, using RESS, an aqueous suspension of particles was 37

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Figure 1.14:Established techniques for SCF particle production utilising scCO2. (P = Vessel

pressure and Pc= Critical pressure of CO2). [1]

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achieved where the particles had an average diameter <100 nm. Suspensions of nanoscale materials are treated as equivalent to solutions of the materials when considered for medical applications [153]. Suspensions of nanoparticles were created using established processes such as SCF spray drying, or precipitation by directly collecting the particles created in a media that formed the final formulation [152]. Importantly, this reduced the number of processing steps.

Innovative advances have been made for SAS particle manufacture methods [154–156]. For example, Subramaniam et al. [154] adapted the SAS method to produce coated particles. A high energy gas stream was used to break up the fluid dispersion into extremely small droplets which precipitated into small particles (0.1–10 µm) in the SCF anti-solvent. In the precipitation chamber it was possible to coat a counter current flow of "core" particles with the precipitating nanoparticles. Conventional methods for producing nanoparticles such as micronisation are often unsuitable for less brittle materials, like polymers, for which this method is appropriate. An alternative revision of the SAS process, from a different research team, was the use of a modulator (such as a polyethylene oxide), with a solubility inversely proportional to temperature in aqueous solutions [155]. The modulating agent enabled reduction in particle size and agglomeration, and aqueous solutions of a substrate to be used in the SAS process with scCO2. Particle size and agglomeration were reduced by

Shekunov et al. [156], who developed a method for utilising growth retardant compounds for the SAS technique. Compounds with CO2-philic and CO2-phobic groups such as sugar

acetates were shown to reduce the particle growth rate and prevent agglomeration.

An adaptation of PGSS was made by Chattopadhyay et al. [157] where an emulsion of polymer, wax/lipid, and bioactive compound was created in an SCF environment. The expanded emulsion provided greater control over particle size, morphology, and size dis- tribution than expanded polymer melts have achieved. In contrast, Deschamps et al. [158] produced solid micro or nano particles of active substance by expanding scCO2solutions

of the active substance into chambers under conditions where liquid CO2was present con-

taining a divided solid. This process produced a solid dry product in a stable form (such as a nifedipine/mannitol formulation), avoiding the use of wet size-reducing steps (e.g. ultrasound) that are common for nanoparticle dispersion production.

The atomisation of substances that are insoluble (or poorly soluble) in scCO2has previ-

ously been limited to methods that avoided the use of scCO2. However, Osada et al. [159]

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pioneered a method to atomise substances with poor solubility in scCO2through the com-

bined use of water and scCO2. Compounds or polymers insoluble in scCO2 were reduced to

fine particles with an average size up to one hundred times smaller than their starting size. This was achieved by exposure to scCO2and water in a pressure vessel, followed by reduc-

tion of internal pressure. Whilst this method has been proven to be effective, the mechanisms of the atomisation process have not been fully elucidated by the authors.

A one step purification/harvesting method developed by Tsung et al. [160] used an SCF, or high pressure gas, to remove a solvent from a microparticle dispersion. This method removed the need for lyophilisation or evaporation steps to dry the microparticles. Cas- tor et al. [161] also utilised SCFs as working fluids to purify protein-containing-polymer- microspheres and avoid use of organic solvents. Kaiso et al. [162] employed scCO2 as a

washing agent in their novel process for forming porous polyamide microparticles. The polyamide was dissolved in a cyclic amide, then heated and cooled to form particles; these contained residual cyclic amide that was recovered using scCO2, allowing recycling of the

solvent.

Recent interest has been shown in the use of scCO2 in phase separation processing

[163,164]. Brown et al. [164] patented the use of a phase-separation enhancing agent (PSEA) for particle production. The PSEA enhanced or induced liquid-solid phase separation of active ingredients in aqueous solutions to produce spherical particles. Supercritical fluids were then utilised to remove residual PSEA from the particles. Day et al. [163] employed thermally induced phase separation to produce polymer microparticles with radial pores. Polymer was dissolved in a solvent, such as an SCF, then homogenised, rapidly frozen, and freeze dried. Microspheres with and without a nonporous skin region were formed by this method.