Processes with supercritical CO2 and encapsulation: expanding possibilities

Supercritical CO2 can be applied not only to obtain extracts suitable for encapsulation but also to other stages of the encapsulation process, such as generating particles, impregnating materials, reducing microbial load, and more.

The search for more sustainable alternatives to address technological and social challenges is no longer a niche trend but a necessity recognised by society and institutions. This is reflected in initiatives such as the Sustainable Development Goals (SDGs) or the European Green Deal, which forms the basis of the Circular Economy Action Plan adopted in 2020. Consumer preferences and industry requirements to meet them include, for example, the acquisition and use of more natural and safe products to improve quality of life, while seeking to make the sensations associated with their use as pleasant as possible. Interest in personal health, as well as the health of other living beings and the environment, supports a growing focus not only on raw materials and derived ingredients but also on the methods used to obtain them. This trend is particularly notable in the food sector but also extends to cosmetics. This explains the increasing interest in protecting fragile bioactive substances so that they retain their functionality (functional extracts, but also microorganisms or cells), or in enhancing their properties to facilitate their use (for instance, reducing their organoleptic impact). Encapsulation—whether on a macroscopic, microscopic, or nanometric scale—can positively contribute to this objective. In this context, supercritical fluid technology, and more specifically supercritical carbon dioxide (SC-CO2), offers numerous opportunities for innovative and sustainable processes.

Supercritical CO2: a multifunctional tool for innovation in encapsulation

In general, SC-CO2 is regarded as the most widely used supercritical fluid in various applications, thanks to its advantageous properties under these specific process conditions (high diffusivity, strong dissolving power, non-toxic, non-flammable, non-explosive, cost-effective, widely available, etc.). The versatility of supercritical CO2 has driven the expansion of research topics and fields over recent decades, as evidenced by the steady growth of scientific and technological publications, particularly the innovations that emerged in 2020.

Focusing on the application of CO2 under supercritical conditions to one of the process stages associated with the encapsulation of various substances, it is possible to highlight some of the most relevant possibilities with examples published in recent months.

Obtaining Extracts for Subsequent Encapsulation

Supercritical CO2 is an ideal extraction agent for non-polar lipophilic substances. For this reason, in addition to its use in degreasing and obtaining protein ingredients, it can also be employed to produce high-quality extracts. Extraction processes using supercritical CO2 are conducted in the absence of oxygen and generally at moderate temperatures, allowing the extracted fractions to retain their properties and offer functional or organoleptic advantages compared to extracts obtained through other technologies, particularly those using organic solvents. In this field, examples of applications that have reached an industrial scale (such as in the cosmetics and fragrance industries) coexist with new research aiming to improve aspects that could enable new applications, such as interactions with other components. A study published in 2020 highlighted the use of supercritical CO2 to obtain an extract from onion skins with antioxidant activity and quercetin as the main flavonoid, as well as its interaction with peptides such as beta-lactoglobulin as potential carriers.

Generation of micro/nanoencapsulated particles in supercritical CO2-assisted processes

High-pressure CO2 technology, particularly in its supercritical state, is among the available alternatives for producing small-sized particles (micronisation) or generating encapsulates of specific substances coated by others, especially on a micrometric scale. Processes focused on generating small-sized particles that leverage the properties of SC-CO2 are numerous and varied, with this application area representing one of the most cutting-edge trends in technology due to the extensive opportunities and many technological challenges it presents, depending on the specific needs of each case. The characteristics of the substances to be encapsulated, the type of coating or the final application of the encapsulate, the purpose of the encapsulation, and other factors determine the selection of the most appropriate technology. It is therefore essential to analyse the alternatives offered by CO2-based processes that could be employed, taking into account these factors and the current state of the technology (considering not only technological advances but also their level of development and intellectual property protection). Based on this analysis, it is possible to eliminate options that do not meet the requirements and/or narrow down the most promising ones, as well as identify the path necessary for the development of specific concrete solutions. To simplify the role of supercritical CO2 in encapsulation processes, it is worth noting that in some process types, the solubility of the substances involved in SC-CO2 is exploited, generating particles by altering process conditions to others where these substances cease to be soluble. Conversely, other process options use SC-CO2 as an anti-solvent, forcing particle generation by reducing the solubility of the substances in the initial solutions due to the intervention of this fluid. The list of processes and variants using SC-CO2 in different roles is extensive [some of the most recognised: RESS (Rapid Expansion of Supercritical Solutions), GAS (Gas Anti-Solvent processes), SAS (Supercritical Anti-Solvent), PCA (Precipitation with Compressed Anti-Solvent Fluid), PGSS (Particles from Gas-Saturated Solutions), DELOS (Depressurisation of an Expanded Liquid Organic Solution); SFEE (Supercritical Fluid Extraction of Emulsions), ASES (Aerosol Solvent Extraction System), etc.] and continues to grow, demonstrating the interest in this technology. In all types of processes, the final result depends not only on the associated mechanisms and phenomena but also on the experimental systems and devices used. Consequently, research and publications in this field also include innovations related to key elements of the installations (e.g., nozzles, particle collection systems, mixing systems, control systems, etc.). Another technological possibility lies in the high diffusivity of supercritical CO2, which facilitates penetration through the structure of materials. This capability, combined with the solubilisation of substances, supports impregnation processes that confer additional functionality to materials. Thus, in addition to other impregnation applications, CO2 can be used to incorporate substances into particles generated by other techniques, resulting in encapsulates or microencapsulates, depending on the characteristics of the support materials into which the substances are integrated. For example, a study reported the supercritical impregnation of soy microparticles, previously generated by another technique, with chia oil. This process allows the oil to be protected against oxidation and ensures controlled release under conditions similar to those of the gastrointestinal tract, which is essential for its absorption.

Post-treatment of encapsulated particles using other technological alternatives: removal of solvent residues, drying, sterilisation of sensitive materials

The technological alternatives for microencapsulation are highly diverse, and some processes involve the use of organic solvents, which often leave residual traces in the final product. If this is undesirable, aside from considering a different encapsulation method, a post-treatment process can be applied to the microcapsules. One such example, published in 2020, involved researchers using supercritical CO2 to remove dichloromethane residues from PLGA microspheres containing risperidone, achieving up to 99% removal.

In other cases, encapsulation involves highly delicate substances, requiring suitable pre-treatment to ensure safe final use without compromising the material itself. A recent example illustrates the convergence between biomedical and process research for the development of a new material intended for disease treatment. This published work focuses on the potential of cell encapsulation, combined with the development of cells designed to respond specifically to disease treatment, such as type 1 diabetes. The researchers have concentrated on developing nanoporous membranes based on high-water-content polyurethane hydrogels. These are subsequently treated to remove cytotoxic substances and achieve a sufficient level of sterility for future implantation into the body, allowing the cells to excrete substances. Specifically, they reported that supercritical CO2 treatment for 2 hours was sufficient to reach a SAL (sterility assurance level) of 10^(-6). The developed nanoporous membranes were considered suitable for cell encapsulation in macroscale devices for use in implants, with future studies aimed at further exploring and advancing in this direction. AINIA brings together expertise and know-how in both supercritical fluid technology and encapsulation, supporting its activities in leveraging the opportunities offered by supercritical CO2 in the encapsulation field. AINIA has more than 25 years of experience in developing and scaling up sustainable and efficient solutions based on supercritical CO2 processes, from pilot scale to industrial level at ALTEX, as well as over 15 years of experience in the development and scaling up of various microencapsulation processes. In 2020, progress continued in research on supercritical CO2 processes for the encapsulation of substances using different technological alternatives, expanding and strengthening available capabilities. Key elements were identified for integration into several process alternatives, and the most suitable ones were selected for the acquisition and installation of new experimental devices.

These advances have been developed as part of research lines co-funded by the IVACE (Valencian Institute for Business Competitiveness), within the framework of the collaboration agreement with AINIA to develop R&D&I activities that can be transferred to the industrial sector.