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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5888_Библиотеки_им_академика_М_И_Перельмана
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32 M. G. Fabiano et al.
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3.2.3 Charged Molecules
Charged molecules (e.g. negatively charged molecules such as dicetyl phosphate and
phosphatidic acid) are added to overcome the aggregation of vesicles by creating
electrostatic repulsions between the individual vesicles (Junyaprasert et al.
Müller et al. 2001). A charge on the niosomes (e.g. a positive charge from
stearylamine or cetylpyridinium chloride) can also alter the in vivo distribution
pattern. It is currently admitted that zeta potentials over ±30 mV are required for
full electrostatic stabilization; potentials between ±5 and 15 mV are in the region
where there is limited flocculation and a zeta potential of between ±3 and 5 mV are
associated with maximum flocculation. Thus, particle aggregation is less likely to
occur for charged particles (high zeta potent ial) due to electrostatic repulsion.
However, this rule cannot strictly be applied to systems, which contain steric
stabilizers, because the adsorption of steric stabilizers will decrease the zeta potential
due to the shift in the shear plane of the particle. It should be noted that the zeta
potential depends on the ions present in the medium (Heurtault et al.
2003).
2008;
3.3 Factors Governing the Niosome Formation
3.3.1 Thermodynamic Features
The assembly into closed bilayers, both in the case of liposomes (Mozafari and
Mortazavi
vesicles are formed only in the presence of specific mixtures of surfactants and
charge-inducing agents. Niosome formation involves the input of energy, for
instance by means of physical agitation, for example, by using the hand-shaking
method (Baillie et al.
(Mozafari et al. 2002, 2007; Mozafari 2005a). In the resulting closed bilayer
structure, hydrophobic parts of the molecule are oriented away from the aqueous
solvent, whereas the hydrophilic head group comes in contact with the aqueous
solvent. The parameters, which play a role in vesicle formation, are the thermodynamic and physi cochemical parameters such as the hydrophilic lipophilic balance
and geometric features of amphiphilic molecule.
The energy required to form vesicles with amphiphilic molecules has three
contributors related to the surface energy, the mechanical energy, and the chemical
potential excess energy.
The association of non-ionic surfactant monomers into vesicles on hydration is a
result of the fact that there exists a high interfacial tension between water and the
hydrocarbon portion (or any other hydrophobic group) of the amphiphile; this high
interfacial energy causes these groups to associate. Simultaneously, the steric,
hydrophilic, and/or ionic repulsion between the head groups ensures that these
groups are in contact with water. These two opposing forces result in a
2005) and niosomes, is not spontaneous. Thermodynamically stable
1985) or heat, for example, by using the heating method
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supramolecular assembly (Uchegbu and Florence 1995). Surfactants form
anisometric micelles, when dissolved in water at low concentrations, while at higher
concentrations (more than 70%), they exist only in lamellar liquid-crystalline phases
(Balzer
1991; Platz et al. 1994). Thermodynamically, the energy associated with the
formation of vesicles (Schukin and Amelina 1979) or the aggregation energy
(Hunter 1991) is the sum of three contributors, namely, the surface energy (E
the mechanical energy (E
chemical potential excess (E
) due to the curvature effect, and the variation of the
ΔP
0
):
Δμ
γ
),
E ¼ E
þ EΔP- E
γ
Δμ
0
ð3:1Þ
In order to find the values for the formation energy, E, using the differential
geometry results (Do Carmo
1976), the three terms may be globally considered and
the energy balance (δ)defined as:
0
where S
is the molar surface area corresponding to the amphiphile head group, γ is
m
the surface tension, and Δμ
2 . Δμ
δ ¼
γ . S
m
0
is the chemical potential. The non-dimensional charac-
ð3:2Þ
ter of δ allows for the comparison of different systems in several experimental
conditions.
Thermodynamically, the self-assembly must contend with a negative entropy
˚
component (-ΔS) and reduction in free energy (-E
) which is only achieved by
the favourable enthalpy (-ΔH ) contribution arising from the establishment of van
der Waals attractions, hydrophobic forces, hydrogen bond formation, and the
screening of electrostatic interactions (Israelachvili
1992).
The formation of small niosomes requires the input of considerable energy and
creates a thermodynamically unfavourable packing status. Aggregation (and/or
fusion) of these vesicles is a mechanism by which the system dissipates the excess
surface energy originating from the distorted molecular packing (Lentz et al. 1985).
Vesicles are generally designed to be incorporated in complex mixtures. Therefore, it is very important to understand their behaviour in the presence of various
agents. Many organic substances are more or less surfa ce-active; thus, it is reasonable to study the properties of surfactants in the presence of vesicle preparations.
Previous studies have shown that niosomes have a greater resistance to micellar
solubilization than phospholipid vesicles (Lesieu r et al.
Morancais
1996).
1990; Vanlerberghe and
3.3.2 Hydrophile–Lipophile Balance
Analysis of vesicle stru ctures indicates that vesicle formation may depend on
the surfactant balance between hydrophobicity and hydrophilicity (generally
34 M. G. Fabiano et al.
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expressed as an HLB value). The guidance offered by the HLB number is useful in
the evaluation of new classes of compounds for their vesicle-forming ability.
Generally, it is considered that the lipophilic surfactants, with HLB values between
4 and 8, are suitable for preparation of niosomes; however, under certain conditions
hydrophilic surfactants (HLB > 14) may also act as bilayer-forming components. A
crucial factor in this process is the inclusion of other element
membrane bilayer arrangement,
such as cholesterol and participating at an optimal
s, participating in the
concentration (most often), or even the encapsulated lipophilic drug (e.g. curcumin)
(Momekova et al.
2021).
3.3.3 Geometric Features of Amphiphilic Molecule
The geometric feature may be another cruci al factor in determining the type of
aggregate formed in aqueous environments (Israelachvili 1992). To explain the
aqueous behaviour of non-ionic surfactants, a hydrophobic effect was proposed
many years ago, as the essential driver for self-assembly (Tanford
morphologies of the spontaneously formed association colloids may be predicted
with considerable certainty using nominal geometric parameters of the surfactant
molecule: the polar head surface area (a
critical hydrophobic tail length (l
), the hydrophobic tail volume (V ), and the
0
).
c
The critical packing parameter (CPP) has been defined by Israelachvili
(Israelachvili 1992):
1980). The
CPP ¼
V
a
0lc
ð3:3Þ
According to the CPP value, the shape and size of the equilibrium aggregate
would evolve from spherical micelles (CPP ≤ 0.33) to cylindrical micelles
(1/3 ≤ CPP ≤ 0.5), bilayers (0.5 ≤ CPP ≤ 1), or inverse micelles (CPP > 1)
(Fig. 3.2).
3.4 Niosome Preparation
Many approaches exist to prepare niosomes; however, some of these methods are
used to manufacture liposomes and have been adapted/used as such. Differently
from liposomes, the niosome preparation is carried out in the absence of nitrogen as
the surfactant is resistant to air oxidation. Moreover, the elevated temperature used
does not present as an issue, because non-ionic surfactants are more stable than
phospholipids. Many techniques such as the thin film hydration, the organic solvent
injection method, the reverse phase evaporation method, and the bubble method
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Fig. 3.2 The relationship between the CPP and the morphology of self-assembled amphiphilic
molecules
have been previously described for the preparation of niosomes (Apoorva et al.
2012; Uchegbu and Vyas 1998).
Recently, a few innovative methods of niosome preparation have been reported in
the literature and these will be described in this section.
3.4.1 Proniosome
A technique of producing nioso mes involves coating a water-soluble carrier with
non-ionic surfactants (e.g. sucrose stearate and the Span surfactants). The watersoluble carrier consists of a sugar such as maltodextrin, glucose, sorbitol, or lactose
monohydrate. The result of the coating process is a dry formulation in which each
water-soluble particle is covered with a thin film of dry surfactant. This preparation
is known as a ‘proniosome’ preparation (Hu and Rhodes
Proniosome powders may be stored in sealed containers at 4
hydrated immediately before use to yield surfactant vesicles. The niosome dispersions are obtained by hydrating the proniosome preparation with a warm aqueous
phase at a temperature above the phase transition temperature and brief agitation.
However, in this method, a thin surfactant film on the carrier is desirable to facilitate
the hydration of the proniosome powder. In fact, the rate of surfactant application is
controlled so that the powder bed of spray dried sugar does not become overly wet
such that a slurry would form. An important advantage of proniosome-derived
niosomes is the reduction of niosome physical instability such as aggregation,
fusion, and leakage. In addition, the proniosomes are obtained as a dry powder, so
2000).
˚
C and may be
36 M. G. Fabiano et al.
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further processing is possible. To provide convenient unit dosing, the proniosome
powder may be processed to make beads, tablets, or capsules.
3.4.2 Heating Method (HM)
The heating method developed by Mozafari to produce vesicles involves the hydration of an amphipathic molecule in an aqueous solution containing 3 vol% polyol at
high temperature (Mozafari 2005b). HM-niosomes are prepared as follows: mixtures
of non-ionic surfactant, cholesterol, and/or charge-inducing molecules are added to
an aqueous medium (e.g. buffer or distilled water) in the presence of a glycerol as the
polyol. Glycerol is utilized since it is a water soluble and a physiologically acceptable chemical with the ability to act as a tonicity agent and increase the vesicle
stability by preventing coagulation and sedimentation. The mixture is heated while
stirring (at low shear forces) until ve sicles are formed. This is a non-toxic, scalable,
and one-step method.
3.4.3 Supercritical Carbon Dioxide Fluid (scCO2)
In the last decade, a new method, the supercritical reverse phase evaporation
(scRPE) method, for the preparation of several bilayer vesicles using supercritical
fluids has been developed (Imura et al.
are those fluids which are non-condensabl e and highly dense at temperatures and
pressures beyond the critical values. At the critical point, supercritical fluids have the
density of a liquid and low viscosity but with the with flow property of a gas. Carbon
dioxide is a widely used gas to produce supercritical fluids because of its low critical
temperature (T
¼ 31.1˚C) and pressure (Pc ¼ 73.8 bar). Similar to non-polar
c
solvents, the flow property of CO
temperature. Therefore, supercritical carbon dioxide (scCO
tute organic solvents in the formation of bilayer vesicles with the advantages of
being environmentally friendly, non-toxic, non-inflammable, and inexpensive (Cooper 2000;). The scCO
method efficient ly entraps water-soluble and thermolabile
2
substances in niosomes without using highly toxic organic solvents.
Manosroi et al. (2008a) report that niosomes prepared wi th the scCO
large unilamellar structures with a size range of 100–440 nm. A reduced size
distribution may be obtained by the combination of the scCO
tion or polycarbonate filter extrusion methods.
Because the method is a one-step method and uses no inflammable, volatile, or
toxic organic solvents, scale up of niosome production should be less problematic.
2003; Otake et al. 2001). Supercritical fluids
may be adjusted by changing the pressure or
2
) may be used to substi-
2
method are
2
method with sonica-
2
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3.4.4 Membrane Contactor
The scale up of niosome production, from laboratory scale using a syringe-pump
device to a pilot scale using the membrane contactor module has been reported by
Pham et al. (
An ethanol injection-based method was applied for niosome preparation using the
Shirasu Porous Glass (SPG) membrane contactor.
For the niosome preparation, the required amounts of excipients and active
ingredients were dissolved in ethanol. The ethanolic phase was placed in the
pressurized vessel; under N
the same time was allowed and the organic phase permeated through the pores into
the aqueous phase. Spontaneous niosome formation occurred as soon as the organic
solution was in contact with the aqueous phase. Then, the suspension was stabilized
for 15 minutes with magnetic stirring. The ethanol was removed by rotary evaporation under reduced pressure.
2012).
, the circulation of both aqueous and organic phases at
2
3.4.5 Ball Milling Method
The Ball Milling (BM) method is a promising method for the preparation of
niosomes for encapsulation of poorly soluble drugs with improved drug release
profiles (Loh et al.
reduction, ease of use, low cost, and environmental friendliness. The BM is a
mechanical technique that uses a cylindrical container rolling around its longitudinal
axis. It is partially filled with grinding balls. The energy released from collisions and
friction between the balls and raw material leads to the size reduction of samples to
micro- or nanoscale (Ferreira et al.
for preparing niosomes (Temprom et al.
2015). It has numerous other advantages such as particle size
2020). Recently, this technique has been applied
2022).
3.4.6 Microfluidic Hydrodynamic Focusing
Conventional bulk method of niosome preparation requires bulk mixing of two
liquid phases, which is time-consuming and not well-controlled. Such mixing
conditions often lead to large niosomes with high size polydispersity, thus affecting
the consistency of niosome dosage or imaging quality. The new method of noisome
preparation by microfluidic hydrodynamic focusing (Fig. 3.3) leads to improvements
in the size and size distribution of niosomes, by taking advantage of the rapid and
controlled mixing of two miscible fluids (i.e. alcohol and water) in microchannels.
Different parameters might affect the assembly of niosomes, such as (1) conditions
for the microfluidic mixing, (2) chemi cal structures of the surfactant used (i.e. Span
20, Span 60, and Span 80), and (3) device materials for the microchannel fabrication
38 M. G. Fabiano et al.
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Fig. 3.3 Schematics of
noisome self-assembly by
microfluidic hydrodynamic
focusing. A central stream
containing the surfactant
mixture in isopropyl alcohol
is focused by adjacent
streams of phosphate buffer
in a microfluidic format.
FRR ¼ flow rate;
QB ¼ buffer volumetric
flow rates; QS ¼ alcohol
volumetric flow rate;
Wf ¼ width of the focusing
stream; τmix ¼ diffusive
mixing time (Marianecci
2014)
et al.
(Lo et al. 2010.). The flow rate ratio of solvent/aqueous phase is an important
parameter that strongly affects the particle characteristics and must be optimized in
order to produce vesicles with a defined size and PDI (Ag Seleci et al.
2019).
Microfluidic hydrodynamic focusing also allows for the production of small and
homogeneous cholesterol-free niosomes (Machado et al. 2020).
3.4.7 Vesicle Purification
The hydration of niosome mixtures rarely leads to complete drug encapsulation,
regardless of the drug loading optimization steps taken. It is thus often a requirement
that unencapsulated drug must be removed. Although it may be argued that the use
of systems in which the drug is partially encapsulated in niosomes may eventually
yield systems with a beneficial biphasic biodistribution profile. This drug delivery
system would give an initial burst to initiate therapy followed by a sustained
maintenance dose. The methods used for the removal of unentrapped material
include different techniques as described below.
3.4.7.1 Dialysis
Dialysis is based on the principles of the diffusion and osmosis of solutes and fluid
across a semi-permeable membrane. Several authors (Manconi et al.
Muzzalupo et al. 2005, 2007) have used this technique to purify their vesicle
2002;
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dispersion from unentrapped materials: the prepared niosomes are filled into dialysis
bags and the free drug is dialyzed against buffered saline.
3.4.7.2 Gel Filtration
In order to separate loaded niosomes from unentrapped substances, purified vesicle
dispersion may be obtained by means of gel filtration on Sephadex G75 (Carafa et al.
2002, 2004, 2006; Terzano et al. 2005), G50 (Manconi et al. 2003), or G25
(Tabbakhian et al. 2006) columns.
3.4.7.3 Centrifugation/Ultracentrifugation
To purify niosomes, several authors (Sabareesh et al. 2021) report the use of
centrifugation. This process is based on centrifugal force (g-force) to remove the
unentrapped soluble drug from niosomes and to obtain the largest fraction of vesicles
residing in the pellet according to their size, shape, and density. However, some
particles, extremely small, will not sediment unless subjected to high centrifugal
force (ultracentrifugation). The ultracentrifuge is a special type of centrifuge generating accelerations of 1,000,000 g (Dufes et al. 2004; Guinedi et al. 2005; Bhardwaj
et al. 2020). In this type of separation, repeated washing of the pellets by
resuspending in isotonic solvents and re-pelleting may result in complete removal
of unentrapped drug. At the end of the centrifugation/ultracentrifugation steps, the
pelleted vesicles are resuspended in appropriate buffered solutions.
Several authors combine the centrifugation processes with Sephadex chromatography mini columns (Minicolumn centrifugation method). In the minicolumn centrifugation method, mixtures of vesicles, entrapped and free low molecular weight
solutes are applied to the column bed and inserted in a spin tube. During the
centrifugation, the vesicles run through the column under g-force, while the free
solute is quantitatively retained in the Sephadex bed (Gupta et al.
2004).
This procedure, compared to other purification methods, is applicable to a variety
of solutes and 92–100% recovery is achieved for the niosomes with no dilution of
the niosomal preparation.
Specific centrifugation methods have been used by several authors to remove
unentrapped genetic material from niosomes, such as density gradient centrifugation
(Jain et al.
gation utilizes a specific medium that gradually increases in density from the top to
the bottom of a centrifuge tube. This means that under centrifugal force, particles
will move through the density gradient and stop when the density of the particle
equals the density of the surrounding medium. Ficoll is used to create the density
gradient as it is an inert, high molecular weight, densely branched polysaccharide
that more nearly approximates to a compact sphere. It is especially used for the
2005; Vyas et al. 2005
). As the name implies, density gradient centrifu-
2005; Singh et al.
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isolation of leukocytes from peripheral blood (erythrocytes) and for the separation of
leukocyte types (New 1990).
These methods have their inherent advantages and disadvantages. The choice of
the method must take different factors into account and for industrial purposes, it
may be more worthwhile to concentrate efforts and resources on the achievement of
high levels of drug loading to avoid these separation steps altogether or to consider
systems in which the unentrapped drug may be useful as a specific priming dose.
3.5 Niosome Characterization
A rati onal characterization of nanosystems is needed for the control of the quality of
the product, which is a prerequisite for the development and clinical applications
(Khosravi-Darani et al.
importance on their in vivo performance, parameters such as morphology, size,
polydispersity index, number of lamellae, zeta potential, bilayer fluidity, composition, encapsulation efficiency, carrier-bioactive interaction, and chemical stability
must be evaluated.
2007). Having direct impact on the stability and critical
3.5.1 Vesicle Size
Determination of nanocarrier size distribution is a necessary quality control assay
because of the importance of this parameter on the physical properties and stability
of the formulations (Goren et al. 1990). Size distribution, along with composition,
defines plasma pharmacokinetics, biodistr ibution, and the stability of the
nanocarriers and their associated/entrapped substances in plasma and other organs
(Barenholz and Amselem 1993).
Niosomes are usually spherical and so their diameter may be determined using
microscopy or dynamic light scatt ering (DLS or photon correlation
microscopy, PCS).
DLS provides cumulative average informat ion of the size of a large number of
particles simultaneously and it is useful for the determination of particle size
distribution (Manosroi et al.
valuable information on the homogeneity of the vesicular suspension (Girigoswami
et al.
2006; Muzzalupo et al. 2008). A single sharp peak in the DLS profile implies
the existence of a single population of vesicles. The polydispersity index (PI) is of
critical importance to in vivo performance: PI values of less than 0.4 are indicative of
a monodisperse sample population. One problem in size determination with DLS is
that it does not provide information on the shape of the nanosystem and it assumes
any aggregation of more than one particle as one single particle.
2008a; Vangala et al. 2006). Moreover, DLS provides
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Nanoparticle Tracking Analysis (NTA) utilizes the properties of both light
scattering and Brownian motion in order to obtain the nanoparticle size distribution
of samples in liquid suspension (Foreman-Ortiz et al. 2022; Manrique et al. 2020).
The microscopic approach is commonly used to characterize the structure/morphology/geometry of the nanocarriers. The microscope studies give us a qualitative
idea about the size and shape of the particles formed in various surfactant/cholesterol
systems.
Electron microscopy techniques have been widely used to measure the size and
the size distribution of the particles. Several electron microscopy techniques may be
employed for nanocarrier research. These include: scanning electron microscopy
(SEM) (Abd-Elbary et al.
2022; Hashemi et al. 2022), transmission electron microscopy (TEM) (Abd-Elbary
et al. 2008; Manconi et al. 2002; Manosroi et al. 2008b; Muzzalupo et al. 2008;
Hanieh et al. 2021), negative-stained transmission electron microscopy (NS-TEM)
(Liu and Guo 2007a, b), freeze fracture transmission electron microscopy (FF-TEM)
(Carafa et al. 2004, 2006; Manosroi et al. 2008a), and cryo-transmission electron
microscopy (cryo-TEM). Atomic force microscopy (AFM) (Cortesi et al. 2007;
Rinaldi et al.
The scanning tunnelling microscope (STM), used in 1982 by Binnig and
co-workers (Binnig et al. 1982), is also a powerful tool for the study of micro- and
nanoscale structures. Compared with the other types of microscopies, STM has
unique characteristics, including: (1) ultra-high resolution down to atomic dimensions; (2) three-dim ensional images with very high resolution especially in the
vertical direction; (3) a variety of operating conditions, such as in vacuum, air, and
liquids; (4) an observation range spanning from micrometres to angstroms; (5) the
ability to do tunnelling spectroscopy. Furthermore, STM, in particular, is very useful
in determining the bilayer thickness of liposomes and niosomes (Zareie et al. 1997).
Microscopy techniques sometimes produce artefacts and hence it will be useful to
combine different techniques to obtain reliable results.
2020) is also a useful technique.
2008; Blazek-Welsh and Rhodes 2001; Abootorabi et al.
3.5.2 Zeta Potential and Surface Properties
The zeta potential value is important to vesicle stability and vesicle in vivo fate.
Several authors confirmed the importance of zeta potential measurements to assess
vesicle formation, to study drug/vesicle interaction and formulation stability (del
Burgo et al. 2007; Carafa et al. 2004, 2006; Liu et al. 2007).
The zeta potential is a critical parameter to be taken carefully into account in the
preparation of niosomes (Sennato et al.
2008; Agrati et al. 2011).
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