Polymeric Micelles: A Review of Their Synthesis, Characterization, Types & Applications

Many new potential drugs (40% or more) have poor water solubility, making it difficult to administer them parenterally and slowing down the drug development process(1). Polymer science advancements have enabled the design of colloidal systems, such as polymeric micelles, which can accumulate in solid tumors, improve drug loading and therapeutic efficacy, enhance targeting through surface modification. Polymeric micelles consist of hydophobic core, hydrophilic shell (corona), covalently attached blocks or grafts(2). The versatility of micelles produced from amphiphilic copolymers as self-assembled nanostructures (≈10 to 200 nm) has signalled significant advances in biomedical area due to their varying functions and clinical success(3). The core of polymeric micelles acts as a reservoir for hydrophobic bioactives, while the shell provides required colloidal stability. The shell plays an important role in preventing opsonization, protein adsorption and together with the small size of polymeric micelles when accumulated in tissues with leaky vasculature through enhanced permeation and retention effect (EPR). Long circulation of these carriers can be prevented by glomerular filtration(4). There are limited formulation approaches to solubilize poorly water-soluble drugs. Common methods include:

1. Salt formation or pH adjustment (limited to ionizable drugs and risks precipitation)

2. Using cosolvents (e.g., propylene glycol, ethanol) for non-ionizable drugs(1).

Polymeric micelles are generated in an aquatic environment by the self-assembly of amphiphilic block copolymers. They have a nanoscopic, usually spherical, core/shell structure, with the hydrophobic core acting as a microreservoir for the encapsulation of hydrophobic medicines, proteins, or DNA, and the hydrophilic shell interacting with the biological media. The adaptability of the core/shell structure is what distinguishes polymeric micelles from other colloidal delivery techniques. The chemical flexibility of the polymeric micellar structure enables the development of custom made carriers that can be tailored to the physicochemical properties of the incorporated drug, disease pathophysiology, site of drug action, and proposed route of administration(5). Variations in the chemical structure of the core-forming block in polymeric micelles may be used to improve drug encapsulation, enhance micellar stability and control the rate of drug release from the carrier. The chemical structure of the micelle-forming block copolymer may also be modified to change the biological destination of the polymeric micellar carrier, enhance their specificity for an organ or tissue, or make them responsive to an external stimulus, thereby enhancing the targeting efficiency of the drug carrier. To this end, polymeric micellar delivery systems have mostly been designed and used to refine three critical parameters in drug performance: solubility, release and biological distribution(6).

Structure

The structure of polymeric micelles follows and exemplifies the similar structure of micelles proposed as per different miceller theories. It is comprised of a core, which is usually a hydrophobic section while the exterior, which is also known as corona, represents a hydrophilic block of the copolymer structure (Fig. 1). In the past two decades, several different polymers have been reported to play the role as a core or corona with their own added merits which has been utilized extensively in drug delivery and targeting. The following paragraphs would comment on the different types of polymers used for hydrophilic and the hydrophobic block of a polymeric micelle(7).

Figure 1 Polymeric Micelle

Synthesis

In this system, the formation of hydrophobic interactions or hydrogen bonds between the micelle-forming block copolymer and drug provides the basis for the solubilisation and stabilisation of drugs in the polymeric micelles. Polymeric micellar nano-containers may be prepared by the direct addition and incubation of drug with block copolymers in an aqueous environment, only if the block copolymer and the drug are water soluble. The method, however, is not very efficient in terms of drug-loading levels and not feasible for most block copolymer/drug structures. Instead, physical incorporation of drugs into polymeric micelles is usually accomplished through one of the following methods of encapsulation.

A) dialysis method

B) oil/water emulsion method

C) solvent evaporation method

D) freeze-drying method

E) co-solvent evaporation

A) Dialysis method

The dialysis method is carried out through the dissolution of block copolymer and drug in a water-miscible organic solvent (such as N, N-dimethyl formamide) followed by the dialysis of this solution against water. In this method, gradual replacement of the organic solvent with water (i.e., the non-solvent for the core forming block) triggers the self-association of block copolymers and the entrapment of drug in the assembled structures. The semipermeable membrane keeps the micelles inside the dialysis bag, but allows the removal of unloaded free drug from the polymeric micelles. This method has been extensively used for the preparation of polymeric micellar formulations in a laboratory setting but may not suite large-scale production. Incomplete removal of the free drug from the polymeric micellar formulation is another drawback for this method of incorporation.

Figure 2 dialysis technique

B) oil/water emulsion method

The oil/water (o/w) emulsion method (Figure 3B) is accomplished by dissolving the drug in a water-immiscible organic solvent (such as chloroform or methylene chloride), followed by the addition of organic phase to aqueous phase under vig orous stirring. The polymer may be dissolved in either organic or aqueous phase. The organic solvent is then removed by evaporation. Solubilisation of doxorubicin (DOX) and indomethacin in poly(ethylene oxide)-b-poly(β-benzyl-L aspartate) (PEO-b-PBLA; Figure 1) micelles by o/w emulsion method has been reported(8).

Figure 2 oil in water emulsion technique

C) Solvent evaporation method

The solvent evaporation approach involves dissolving the drug and polymer in a volatile organic solvent and then evaporating the organic solvent completely, resulting in the development of a polymer/drug film. This film is then vigorously shaken to reconstitute it in an aqueous phase. A solvent evaporation approach was used to successfully encapsulate paclitaxel (PTX) in PEO-b-poly (d,l-lactide) (PEO-b-PDLLA) micelles and amphotericin b in PEO-b-poly(n hexyl stearate l aspartamide) (PEO-b-PHSA). Although the solvent evaporation method of drug loading has advantages over dialysis in terms of scale-up, it can only be used for micelle forming block copolymers with high hydrophilic lipophilic balance values and polymer films that can be easily reconstituted in aqueous media.

Figure 4 solvent evaporation technique

D) Freeze-drying method

The freeze-drying method uses a freeze-dryable organic solvent such as tert-butanol to dissolve the polymer and drug. After that, the solution is combined with water, freeze dried, and reconstituted with isotonic aqueous media. Although this process is pharmaceutically practical for large-scale manufacture, it is limited to block copolymers and drug structures that can be dissolved in tert-butanol. This approach cannot be employed for PEO-containing block copolymers due to PEO's insolubility in tert-butanol. For the encapsulation of PTX (paclitaxel) and its derivatives in poly(n-vinylpyrrolidone)-b-PDLLA (PVP-b-PDLLA), the freeze drying process was used(9).

Figure 5 solvent evaporation technique

E) Co-solvent evaporation method

The co-solvent evaporation method (Figure 3D) involves the drug and polymer being dissolved in a volatile water-miscible organic solvent (co-solvent). Self-assembly and drug entrap ment is then triggered by the addition of aqueous phase (non solvent for the core-forming block) to the organic phase (or vice versa), followed by the evaporation of the organic co-sol vent. Encapsulation of important therapeutic agents such as PTX(10).

Figure 6 Co solvent evaporation method

Characterization

The important parameters that are used for the characterization of polymeric micelles are CMC, and size, shape, and stability of polymeric micelles.

1. Critical micelle concentration

This is the minimum concentration at which polymers align or aggregate in the form of micelle. At this level of concentration the amphiphilic polymers self assemble in aqueous solution in which hydrophobic chains aggregate in the form of core and hydrophilic chains align towards the aqueous environment. Below the CMC the polymeric chains are not able to self-assemble and may collapse. Therefore, CMC is the key parameter to generate stable micelles and will eventu ally provide efficient drug carriers that will not disintegrate while interacting with the blood components before reaching the target site. This physical quantity can be determined by various methods such as surface tension measurement, gel permeation chromatography, and by using fluorescent probes. Among all the above-mentioned methods the fluorescent probe with pyrene is widely used for the determination of CMC.

2. Size and shape of polymeric micelles

The size of micelles plays an important role in drug delivery. The smaller size will minimize the risks of embolism in capillaries as compared with large drug carriers. The size of polymeric micelles depends on several factors such as molecular weight of copolymer, relative proportion of hydrophilic and hydrophobic chain, and aggregation number. The size of polymeric micelles can be determined by various techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM). SEM is widely used for the determination of size and shape of colloidal carriers but needs gold coating. While the new imaging technique, atomic force microscopy, enables the determination of size of polymeric micelles without gold coating. The polydispersity index of prepared micelles can be examined by using quasi elastic light scattering technique. The blue colour observed from the monodisperse micelles on scattering of light indicates good micelle preparation as compared with white coloured observed aggregates. The sphericity of polymeric micelles can be determined by using dynamic light scattering technique(11).

3. Differential scanning calorimetry

The glass and melting point temperature and enthalpies of nano micelles can be determined by differential scanning calorimetry (DSC), which provides information about the nature and speciation of the crystallinity within nano micelles. The polymeric micelles are analyzed by this technique to study the various states of the core and the extent of interactions between drugs and polymers.

4. Nuclear magnetic resonance

The structure and composition of synthesized polymers used to produce the polymeric micelles can be determined by nuclear magnetic resonance (NMR) technique. The number average molecular weight of the polymers can also be determined by this technique(12).

5. Stability of polymeric micelles

The stability of polymeric micelles is also an important component in drug delivery. The polymeric micelles on intravenous injection face different environmental changes such as dilution, exposure to pH and salt changes, and contact with various proteins and cells. Under these circumstances polymeric micelles should be capable of releasing drugs at the target site and preventing dis integration before reaching the target. The stability of polymeric micelles largely depends on the length of a hydrophobic segment and the tendency to dissociate is related to the composition and cohesion of the hydrophobic core(11).

Advantages and applications

Polymer-based nano systems are engaging platforms in different targeted therapies. The increased interest in integrating nanotechnology with cancer diagnosis and treatment has led to the development of various tumour-targeting nanoparticles for cancer applications.

1. Polymeric Micelles as Versatile Drug Delivery Carriers

Amphiphilic copolymers are highly versatile components of drug delivery systems. Tuning the hydrophilic and the hydrophobic blocks allow obtaining a variety of self-assembled structures with the different capabilities of hosting drugs, enhancing their apparent solubility and stability, crossing biological barriers, and delivering the drugs to the right site. Also, some block copolymers have been shown to act as active molecules that are able to enhance the therapeutic effects of the drugs. The combination of block copolymers with cyclodextrins expands, even more, the spectrum of applications in the drug delivery field. Poly(pseudo)rotaxane formation enables fine-tuning of the rheological properties of the formulations and the drug release kinetics(13).

2. Acid-Sensitive Polymeric Micelles

There are a number of pH gradients that exist in normal and pathophysiological states inside the body. Acid-sensitive or pH-sensitive polymeric micelles exploit these differences in pH for drug targeting. In tumours and inflammatory tissues, a mildly acidic pH is encountered (pH approx. 6.8). This is a slightly low value as compared with the pH of blood and normal tissues (pH approx. 7.4). Micelles can also be taken up into the cell by the process of endocytosis and may as well enter cell organelles as endosomes, lysosomes, etc. The pH 65 value inside these organelles is nearly 5.5. This has served as the basis for the development of pH-sensitive polymeric micelles. e.g., negatively charged oligo/poly (nucleic acids) can be delivered intracellularly by complexing them with cationic polymers. Once into endosomes, these are deprotonated causing disruption of endosomal membrane and releasing nucleic acids in the cytosol. Two main approaches that have been used for developing pH sensitive systems are: involvement of a titrable group into the copolymer, and inclusion of labile linkages that are destabilized in acidic conditions. Incorporation of titrable groups such as amines, carboxylic acids into the backbone of the copolymer leads to an alteration of the solubility of the polymer upon protonation. This in effect may disrupt the micellar structure. Inclusion of acid-labile linkages, such as benzoic imine linkage, in polymeric structures has shown to cause change in micellar integrity or complete destruction of the micellar structure when these polymers encounter low-pH environment(14).

3. Solubilization

When surfactant unimers combine to form a micelle, the central micelle provides a relatively hydrophobic micro-environment. This somewhat nonpolar hydrophobic core is more suitable for less soluble chemicals than the bulk watery phase. Solubilization refers to the process by which weakly soluble molecules are integrated into local habitats capable of higher solubility in solution. Solubilization of poorly water-soluble chemicals by block copolymer micelles can enable for the formulation and distribution of difficult molecules that would else unsafe or ineffective. (15)

4. Inherent Size

A multitude of benefits stem from the intrinsic size of surfactant micelles. First of all, because to its minuscule size, the micelle may be easily sterile filtered before being packaged or administered. The dimensions of micelles may influence their eventual biological destiny. Perhaps more importantly, the result of drug cargo encapsulation can be affected by the size of micelles. It is theoretically possible to avoid certain processes of clearance from the body by using dynamic diameters between 20 and 80 nm. Smaller particles less than 5–10 nm are quickly removed from the body following systemic injection by extravasation or renal clearance, but larger particles are not. Even at 10-70 nm, particles can potentially distribute tissue effectively since they are tiny enough to enter bodily capillaries. (16) This ideal range of sizes is commonly seen in micelles. In order to evade being mechanically filtered via splenic inter-endothelial cell slits and then cleared by phagocytic cells, long-circulating particles need also have a size of less than 200 nm. A micelle that is undamaged typically has a total estimated mass of 105–106 g/mol. With bigger particles being cleared more slowly, the renal molecular weight limit for clearance is gradient. Comparatively, the typical renal limit for efficient clearance is between 40 and 55 Kda, reaching a maximum of about 70 Kda. This size cut-off is obviously less than the mass of the majority of micelles. However, it is rare to find surfactant unimers having mol. weights higher than 20 kda. (17)

5. Sustained Release

The capacity of polymeric micelles to gradually release a molecule over time, for example, as a molecule is moving through the circulation, is another potential advantage. For a micelle to exhibit sustained drug release properties, a number of attributes must be present. The micelle has to be dilution stable first. Micelles that exhibit dilution stability need either a high degree of thermodynamic balance, as shown by a low CMC, or a degree of kinetic equilibrium derived from a very viscous, low chain mobility core. These physical properties were previously mentioned. High levels of kinetic and thermodynamic stability will be excellent for the best prospects. Moreover, the micelle needs to be able to stop active molecules from diffusing out of the core into the surrounding biological milieu if the cargo is physically enclosed. Precisely small diffusion coefficients between 10-16 and 10-18 cm2/sec are necessary for medications physically entrapped in micellar cores to be eligible for sustained release. Diffusion coefficients so small are essential. (18)

Table 1: Polymeric micellar delivery systems in clinical trials.

Release of drug from micelles

Drug diffusion or micelle disintegration can both lead to drug release from polymeric micelles. Micelles need to have high thermodynamic and kinetic balance to prevent uncontrollable drug release during delivery. As a result, a recent research devised and addressed many physicochemical strategies for either maintaining the encapsulated medication in the micellar core or preventing fast system disaggregation. For instance, it is widely known that a reduction in CMC may be achieved by lengthening the hydrophobic region of unimer block copolymers connected with a lipid molecule. (19) Additional methods encompass the hydrophobic block's functionalization. To monitor drug release just complying the bond breaks, the polymer and drug can form a conjugate by cross-linking the micelle's core. The structural balance of micelles should be studied under bio relevant circumstances because proteins from intracellular fluids or plasma can be absorbed on their surface. This creates the so-called protein corona, which alters the physiological reply of nano-transporters in terms of toxicity, metabolism, cellular absorption, and bio distribution by partially hiding the outer shell's functional groups. It has been shown that serum proteins in particular, which promote micelle rupture or aggregation, are crucial for micelle stability. (20)

Different types of polymeric micelles:

Polymeric micelles can be divided into three primary groups based on the kind of intermolecular forces that control the separation of the core segment from the aqueous environment: those that are produced by hydrophobic interactions, those that are produced by electrostatic interactions (polyion complex micelles), & those that are produced by metal complexation.
Traditional the core segment and the corona area interact in the aqueous environment to produce these micelles. Hydrophobic interactions cause poly (ethylene oxide)-b-poly (propylene oxide)-b-poly (ethylene oxide), one of the most basic amphiphilic block copolymers, to form micelles (21).  Complex Micelles of Polyion Polymeric micelles can also form through electrostatic interactions between two oppositely charged moieties, such as polyelectrolytes. The addition of oppositely charged polymers to the solution can allow them to enter the micelle's corona and create a polyionic micelle. Polyion complex micelles (PICMs) are the name given to such micelles. The form and size of the charged micelle coronas are governed by the van der Waals force of interaction and electrostatic forces. Simple synthesis pathways, easy self-assembly in aqueous media, structural stability, high drug loading capacity, and extended blood circulation are some of the unusual characteristics of PICMs. Since no organic solvents are used in the micelle preparation process, the accompanying negative effects caused by the remaining organic solvents are eliminated. Through electrostatic, hydrophobic, and hydrogen bonding interactions, the core of PICMs can entrap a variety of therapeutic agents, including hydrophilic and hydrophobic chemicals, metal complexes, and charged macromolecules. These agents can then be released upon receiving the appropriate trigger. These factors make the PICMs highly promising for drug release, particularly for the delivery of charged medications in conjunction with DNA, enzymes, and antisense oligonucleotides (22, 23).

Table 2: Type of polymer and their representation

A: Hydrophillic unit, B: Hydrophobic unit

Table 3: Structures of micelle-forming copolymers.