2. Carbon Dioxide as Supercritical Fluid

SC-CO₂ is presently very popular as a green alternative for several applications including extraction, production of polymers, removal of residual solvents or monomers and formulation of polymeric particles, foams or scaffolds ^[12]. The word “supercritical” defines the state of the solvent, which is a prerequisite for using it in the areas mentioned above. This physical state was already described in 1822 by Cagnaird de la Tour, who observed the disappearance of the boundaries between the liquid and gaseous phases when increasing the temperature far above the boiling point of water ^[13]. The author described the critical pressure and critical temperature concepts, including the critical point (Figure 1). SCFs impart valuable physicochemical properties such as low viscosity, high diffusivity, and high compressibility that are a combination of the liquid and gaseous states of the compound and are largely influenced by a slight change in pressure and temperature. The generation of SCFs is a reversible process; i.e., SCF goes back to liquid or gaseous phase depending on the pressure and temperature of the system. Examples of commonly known SCFs are CO₂, acetone, chlorodifluoromethane, ethanol, hexane, propane, pentane, toluene as well as water. The supercritical conditions for acetone, ethanol, propane, toluene, and water are hard to achieve due to the high pressure and temperature required. Moreover, water in its supercritical state is highly corrosive and harsh ^[14]. On the contrary, CO₂ can achieve its supercritical state at “mild” conditions, T = 31 °C and P = 73 bar. Additionally, it is non-toxic, chemically stable, non-flammable, presents low surface tension and is cost-effective.

Figure 1. Phase diagram of carbon dioxide (CO₂) (not to scale). S, L, and G denote the solid, liquid and gaseous phases respectively. Cp denotes the critical pressure point (31 °C, 7.3 Mpa) and Tp denotes the triple point (−56 °C, 0.5 MPa) for CO₂.

3. Solubility of Polyesters in SC-CO₂

In the light of prior research and studies, SC-CO₂ is an established solvent for small molecules but not for organic polymers. Also, SC-CO₂ is nonpolar and aprotic and hence exhibits certain selective solubility for ionic and polar compounds as well as for higher molar mass polymers ^[15]. The solubility of a polymer in SC-CO₂ depends on its chemical structure as well as on its physicochemical and mechanical properties, e.g., molar mass, end groups (e.g., fluorine, amine), the architecture (branched or linear), T_g and crystallinity. However, it also depends on the experimental conditions, such as pressure, temperature, and the use or not of a co-solvent in the system. The most important parameters influencing the solubility of a polymer in SC-CO₂ are presented in Figure 2. Moreover, the main parameters for the solubility of polyesters are briefly discussed in the following.

Figure 2. Parameters that influence the solubility of a polymer in SC-CO₂ divided into the two main categories “polymer factors” and “experimental factors” with their illustrated subgroups.

Compared to other polymer classes such as fluoropolymers, polyesters such as PLA generally reveal a limited solubility in SC-CO₂, which was explained among other factors by their lower chain flexibility ^[15]. Nevertheless, the solubility of the polyesters can be positively influenced, e.g., by decreasing the molar mass and dispersity of the polymer. It is also worth mentioning that the introduction of terminal groups, such as carboxyl, hydroxyl, acetate, or fluorine groups that modify the interactions between the polymer and the CO₂ molecule, plays an important role in the solubility properties of the polymer. Early study from J. Gregorowicz et al. confirmed the influence of molar masses and terminal groups on the solubility by illustrating the difference of solubility between two different molar mass PLLA polymers with two different terminal groups. The study suggests that for low molar mass systems the solubility is controlled by the end groups rather than by the molar mass ^[16]. The architecture of the polymer and subsequently its free volume plays a vital role in the solubility in SC-CO₂. The free volume refers hereby to the free spaces left in the solid-state of the polymeric chain at the molecular level ^[17]. The free volume of the polymer chain is directly proportional to the polymer chain flexibility and leads to higher solvent diffusion and enhanced solubility in the SC-solvent. Influenced by the architecture, the free volume of a highly branched polymer is higher and, thus, it is more likely to be soluble compared to a linear polymer ^[18][19]. Moreover, the solubility can be positively influenced by hyperbranching, a shorter chain length including the addition of several acetate or fluorine moieties as terminal groups ^[20][21]. Finally, the solubility of a polymer is determined by the density of SC-CO₂, which is dependent on the process parameters and tends to elevate with pressure. With increasing pressure, the polymer solubility is likely to be enhanced.

However, if the polymer remains insoluble in SC-CO₂, even under high-pressure conditions, polar co-solvents such as ethanol or acetone are used. The addition of these solvents contributes to decreasing the difference in the free volume between the polymer and the SCF solvent and, hence, increasing the solubility of the polymer in the system ^[18].

4. SC-CO₂-based formulation of PLA/PLGA particles 

The previous section provided a basic understanding of SC-CO₂-based principles. This section intends to serve as an overview of completed studies related to the production of drug delivery systems consisting of PLA/PLGA-based polymers until recently.  J. W.Tom and P. G. Debenedetti in 1991 introduced the use of SC-CO₂ for the formulation of polyester particles. A recent study used a mixture of dimethyl sulfoxide (DMSO) and CH₂Cl₂ (ratio 1:1 v/v %) within the SAS process for the production of PLLA-coated 5-Fu (5-Fluorouracil).  The obtained size for the coated particles varied from 0.6 to 1.2 µm (at 35 and 50 ◦C respectively) and the drug load ranged from 32 to 42%. The dissolution profile clearly showed a slower release rate from the PLLA-coated 5-Fu particles compared to the micronized particles.

The most applied technique is Supercritical Fluid emulsion extraction was published in 2019 by C. Gimenez-Rota et al. β-carotene was encapsulated together with the anti-oxidants α-tocopherol and rosmarinic acid into PLLA and PLGA particles. Experiments with several different double emulsions and various external phases were shown and the SFEE process was carried out at 80 bar and 37 ◦C throughout all the experiments with a constant liquid-gas ratio of 0.1. The final sizes for the loaded PLLA particles ranged from 0.5 to 1.4 µm with the EE was calculated to be 72% and 62%, respectively. For PLGA, the same droplet sizes (2.1 and 1.2 µm) were tested and particles of 2 and 0.3 µm with an EE of 52% and 62%, respectively, were reported.

The supercritical fluid-based technologies are environmental friendly approaches for the formulation of drug delivery systems. Although the process seems convenient, previous work demonstrates the need for optimization for each polymer system and cannot be universally applicable. SC-CO₂ is the best choice as SCF for the pharmaceutical industry, but the SC-CO₂-based techniques are not yet answered for scale-up processes, e.g., the GMP cost estimation, calculation for estimation of parts of GMP equipment, together with the above-mentioned lack of standard processing conditions. Nevertheless, it is obvious that although the advantages of the SC-CO2-based processes are numerous, many gaps in knowledge still need to be bridged. Thus, rigorous evaluation and detailed investigation are necessary for a broad and successful application of this technology in the near future.