Paul Ehrlich discovered a medication delivery method that would target directly to diseased cells in the 1990s and this discovery led to the development of targeted drug delivery [1,2]. Spanlastics, a new drug delivery device, contains the medicine in the bilayer's hollow. In 2011, Spanlastics (Span + Elastic) emerged in print [3]. These are very elastic and malleable carriers that resemble transfersomes. The permeability of these deformable vesicular carrier systems is greater than that of medication solutions. The substance is encased in a nonionic surfactant-formed vesicle. Spanlastics are microscopic and incredibly small [4]. These unique nanovesicles are designed to overcome the limitations of liposomes, such as chemical instability. Due to their propensity for oxidative breakdown and varying phospholipid purity, liposomes are chemically unstable. Edge activators give these vesicles their elasticity. A distinct type of vesicular carrier termed spanlastics delivers ocular, oral, topical, nasal and trans ungual medicines site-specifically [5]. The article includes a summary of the preparation method and its current application in addition to some key aspects.         

Salient Characteristics of Spanlastics

• Osmotically active and stable solutes can be trapped by spanlastics materials

• They utilize their bilayer, which supports the release of encased drugs, to release medication in a controlled manner

• They have adaptable structural characteristics, allowing them to be modified to meet specific requirements [6,7]

• By shielding it from a biological environment, spanlastics improve the availability of the drug at the location

Advantages

• Spanlastics are naturally non-immunogenic and biodegradable

• In order to enhance bioavailability relative to traditional medicine, the pharmaceutical product incorporates protective support mechanisms that facilitate its targeted delivery without undergoing significant degradation [8]

• Target Specific: By protecting the drug from the elements and reducing its effect on the targeted place, they improve the restorative performance of medicated particles

• Both osmotically active and stable, they enhance drug entrapment stability

• Surfactants can be handled and stored without particular circumstances

• They can be delivered topically, parenterally, or orally

• In prolonged drug delivery, they are crucial in postponing drug molecules' removal from the bloodstream

• They are very compatible with biological systems and have low toxicity due to the nonionic surfactants present in their structure

• In contrast to niosomal formulations, these vesicular systems have better ocular permeability because of their very elastic and malleable character

• Their function is to carry out a task that depends on their physical position. These vesicles are able to reach the retinal pigment epithelium, vitreous cavity and choroid in the anterior and posterior segments of the eye thanks to their elastic nature and the ability to pass through the corneal membrane. Compared to liposomes, they are chemically more stable

• The following sequence describes the diminishing irritancy of a surfactant: Since cationic >anionic >ampholytic >nonionic, spanlastics composed of nonionic surfactants are non-irritating to the eyes.

• Economical preparation technique

• Raw resources are easily accessible

Spanlastics Classification

Similar to liposomes, they can be categorized according to how many layers they consist of, as shown in the following:

• MLVs are the most often employed type of multi-lamellar vesicles. There are several bilayers in it. Vesicles are roughly 0.5 to 1.0 micron in diameter. It is easy to create and, after being stored for a while, is mechanically stable.

• LUVs (large unilamellar vesicles) have a high aqueous/lipid component ratio, allowing for the entrapment of greater amounts of bioactive compounds [9,10]

•  SUV (small unilamellar vesicles) the most common methods used to create SUVs from multilamellar vesicles include extrusion, French press and sonication

Spanlastics' Components Include

Spanlastics resemble regular liposomes structurally. These resemble Transfersomes, which are very elastic and flexible liposomes. A nonionic surfactant and an edge activator make up the two essential components of spanlastics. Spanlastics is the name given to these vesicles since they are predominantly made of Spans (surfactants) [11,12].

Nonionic Surfactant

To lower the surface tension among two liquids (the aqueous phase and the oily phase), surface active agents (surfactants) are utilized. A charged group does not exist in the head of a nonionic surfactant. One significant group of nonionic surfactants is the sorbitan alkyl esters (Spans). The organization of Spans into concentric bilayers gives Spanlastics its vesicular structure. The kind of fatty acid connected to the polyoxyethylene sorbitan component of the molecule determines the shape of Spans: Span 80 (monooleate), Span 60 (monostearate), Span 40 (monopalmitate) and Span 20. The stability of the vesicular formulation can be predicted in large part by looking at the types of Span. Vesicles based on Span 80 and Span 40 exhibit significant disruption, aggregation and instability. However, the produced vesicles are more sustainable since Span 60 contains saturated alkyl chains. Span 60's saturated alkyl chains have a lipophilic character that makes it possible for unilamellar or multilamellar matrix vesicles to form. The edge activator would be strengthened by the surfactant's surface activity, lowering the interfacial tension and causing the growth of tiny spanlastic dispersions [13-15].

Edge Activators (EAs)

Such surfactants have a high hydrophilicity (HLB value), making them stand out from the rest of the group. To make the bilayer vesicles more malleable, these single-chain surfactants reduce their interfacial tension. Thus, they enable these vesicles' lipid bilayer membranes to be flexible. EAs frequently generate vesicles with lower particle sizes and more spherical vesicles. The flexibility of the vesicles might be increased by the addition of an EAs (Tween 80), allowing for somewhat bigger vesicles to enter the biological membranes and increasing drug penetration [16].

In addition, these hydrophilic surfactants can, to varying degrees, disrupt packing properties, destabilize vesicular membranes, make them more deformable and generate systems [16].

Ethanol

The characteristics of these nano-vesicular carriers are improved by ethanol. It aids in enhancing drug entrapment and partitioning within the vesicles. Ethanol's capacity to condense on membranes results in a reduction in vesicular membrane thickness, which in turn leads to smaller vesicles. By adjusting the system's net charge towards a negative zeta potential, steric stability is achieved to some extent [17].

Morphology

As shown in Figure 1, spanlastics have concentric bilayers that resemble liposomes. These come in both unilamellar and multilamellar (MLV) varieties. Depending on their size, these may be (SUVs) small unilamellar vesicles (10-100 nm) or (LUVs) large unilamellar vesicles (100-3000 nm). It has been reported that, when comparing MLVs to SUVs with the same lipid content, MLVs had extended retention times. The term "spanlastic systems" refers to spheroid structures composed of amphiphilic molecules that exhibit exceptional properties as a matrix for bioencapsulation [18,19].

Figure 1: Structure of Spanlastics [19]

Mechanism of Spanlastics Penetration

Edge Activators (EAs) induce a reduction in the stability of lipid bilayers, hence enhancing the deformability of vesicles. These vesicles' surfactant induces holes in membranes and other lipid structures and at greater concentrations, it also promotes solubilization (lysis). As a result, elastic vesicles can squeeze through intercellular spaces when a water gradient is present because of the membrane's variable bending energy. According to Figure 2, there are two drug penetration mechanisms:

Figure 2: The Mechanism of Spanlastics Penetration [22]

• The First Mechanism: The intercellular lipid lamellae are altered as a result of the elastic vesicles' interactions and penetration-enhancing functions with the epithelial cell membrane

• The Second Mechanism: When used as drug delivery systems, elastic vesicles allow intact drug-carrying vesicles to breach biological membranes and transit through intercellular spaces [20,21]

Successful passage of these carriers depends on the presence of an osmotic gradient and the vesicle bilayers' extremely stress-dependent flexibility [22].

Procedure for Preparation

Ether Injection: This technique involves slowly injecting surfactant into a warmed 4 mL aqueous phase containing the medication at a temperature of 600 o C while moving at a rate of 25 mL per minute through a 14 gauze needle. The organic solvent will be evaporated from the ether solution by means of a rotary evaporator, which results in the formation of single-layered vesicles [23] (Figure 3).

Figure 3: The Injection Method of Spanlastics Preparation [22]

Sonication

After being sonicated, a prepared aliquot of the drug in an appropriate buffer is additional to the surfactant combination in a 10 ml of glass container. To sonicate the combination, a titanium probe is employed [24].

Method for Shaking Hands

In order to promote the dissolving of surfactants, the first stage entails using an organic solvent, such as ether, chloroform, or benzene. Following this, the solvent is exposed to the process of evaporation inside a vacuum evaporator, using reduced pressure, within a flask with a flat bottom. The surfactant layer expands when it is rehydrated with a drug-containing water solution while being constantly shaken. Swelled amphiphiles finally fold into vesicles, which contain the drug [25].

Extrusion Method

To form a thin coating, surfactant and diacetyl phosphate are mixed and evaporated in a rotating vacuum evaporator. To ensure consistency, the drug solution is rehydrated with an aqueous drug solution before being extruded through a polycarbonate membrane (mean pore size: 0.1 millimetre) in a series of up to eight passes [25].

Method of Micro Fluidization

Two fluidized streams, one holding a drug and the other a surfactant, contact at very high speeds in carefully specified microchannels inside the interaction chamber, ensuring that the energy input to the system relics in the area of spanlastic formulations. This is referred to as the submerged jet principle. The results include improved homogeneity, lower size and repeatability in the formulation [26].

Spanlastics' Physico-Chemical Properties Are Affected by:

• Membrane Additives: In addition to the principal surfactant and medication, adding other additives to the formulation can improve the stability of spanlastics. Numerous additives affect the membrane stability, shape and permeability of vesicles; for instance, adolescents make produced vesicles more flexible so they can more easily penetrate a specific location [27]

• Hydration Temperature: Hydration temperature has an impact on both shape and size. The system's temperature fluctuation has an impact on how the vesicles are assembled. Temperature changes may also cause vesicle shape to vary. Polyhedral vesicles composed of a mixture of C16G2 and solulan C24 (in a ratio of 91:9) undergo formation at a temperature of 25 °C. The vesicles undergo a transition into a spherical shape when exposed to a temperature of 45 °C. Additionally, a decrease in temperature from 55 to 49 °C leads to the aggregation of smaller spherical nano-vesicles into a cluster [28]

• Drug Characteristics: Medication entrapment effectiveness can be influenced by the molecular weight, chemical make-up, hydrophilicity, lipophilicity and Hydrophilic-Lipophilic Balance (HLB) value of the medication. Drug entrapment may cause vesicle size to grow. The vesicle size is raised even more by the surfactant bilayer pushing away the drug particle. This is most likely because the drug particle interacts with the surfactant head group, which raises the charge on the polymer and makes it charge higher [29]

• Content and Surfactant Type: The HLB value of surfactants like span 85 (HLB 1.8) to span 20 (HLB 8.6) rises along with the mean size of the vesicles. It could be because the surfactant's surface free energy drops as its hydrophilicity increases. Alkyl chains are existing in a well-ordered form in a gel state, but the bilayer structure is further disordered in a liquid state. The temperature of the gel-liquid phase Transition (TC) is used to describe lipids and surfactants. For instance, spans with a higher TC have a greater entrapment efficiency because phase transition plays a role in this process. The HLB value affects the spanlastics' trapping efficiency. For instance, the entrapment efficiency is great at an HLB value of 8.6. However, formulations with HLB values of 14 to 17 are not advised [30]

• Surfactant structure: The geometry of the vesicles designed during formulation is influenced by the critical packing parameter (CPP). The geometry of vesicle may be predicted using CPP. The CPP value may be calculated using this equation [31]:

CPP = V/ IC*A

Where

CPP        : The critical packing parameter

V             : Hydrophobic group volume

IC            : The critical hydrophobic group length 

A             : The area of the hydrophilic head group

The following ways in which CPP is useful for predicting vesicle structure include: 

• Spherical micelles generated if CPP equal ½

• If CPP between½ and 1, bilayer micelles are generated

• If CPP >1, inverted micelles develop

• Osmotic Stress Resistance: When the hypertonic salt solution is used in the formulation of spanlastics, vesicle diameter reduces. In hypotonic salt, the spanlastics vesicles release slowly and grow little due to fluid elution obstruction, then rapidly due to osmotic stress releasing their structural parts [32]

• Preparation Method: Spanlastics preparation techniques like handshaking, ether injection and sonication significantly reduce the quality of the final formulation. For instance, vesicles created with ether injection are smaller than vesicles created through the handshaking approach. The vesicles produced by the hand-shaken approach can be reduced by hydrating the mixture above and then vortexing [33,34]

In-Vivo Spanlastics' Actions

Nano-vesicles and in-vivo spanlastics have been determined to be equivalent and their dispersion patterns are similar to those of colloidal drug delivery systems. Because of the natural vectoring process, these elements have a noticeably high level of disposition in life. The pattern of drug disposal from circulation changes when bigger vesicles are retained in the alveolar region of the lungs due to retention or maybe phagocytic activity. Smaller vesicles may enter the spleen more readily and pass through the sinusoidal epithelium [34,35].

Spanlastics Characteristics

The Efficiency of Entrapment (EE): It is described as the percentage of medicine that the spanlastics successfully catch. The following formula is used to calculate entrapment efficiency [36-38]:

EE = Amount of drug entrapped/Added Amount * 100

To determine the degree of entrapment, the unentrapped substance is first separated using an appropriate technique (such as a centrifugation procedure). After separating the solution, the liquid precipitate is collected. After collection, the supernatant is diluted as necessary and its concentration is determined using an appropriate method, as specified in the drug's monograph. The Entrapment Efficiency (EE) and produce of spanlastics are affected not only by the production process but also by the physicochemical features of a material. The process of producing spanlastics and including tweens, which enhance their impermeability, have an impact on several factors, including the double layers’ number, size and distribution of vesicles, the efficiency of aqueous phase entrapment and permeability of vesicle membranes. Bhaskaran and Lakshmi's study revealed that the transmembrane pH gradient technique had a greater Encapsulation Efficiency (EE) compared to other approaches, such as the ether injection method and the film hydration method. There may be an increase in water absorption across the double layer if a net charge, positive or negative, is present throughout this process. As a result of this hydration, the number of loaded molecules with hydrophilic properties increases relative to their uncharged vesicles, which are likely to be found both inside the bilayer and in the center of aggregated formations.

The Morphology Test by Different Ways

• The size, form and lamellarity of spanlastics are studied using Transmission Electron Microscopy (TEM). In summary, a suspension is created and then an adequate quantity of 1% phosphotungstic acid is applied. When applying a drop of the mixture to a carbon-coated grid and allowing the excess to drain out, the grid was examined and images were obtained under the proper magnification with a TEM when it had completely dried (Philips TEM) [38,39]

• Microscopy using Freeze Fractured Film: The type of drug entrapped, the type of medication utilized and the type of surfactant were found to affect the size and form of spanlastics. In order to measure a vesicle, it must first be frozen and thawed many times before being examined using a freeze-fractured electron microscope. Typically, liquid propane is used at low pressure to cry fix the vesicular suspension (glycol may be used as a cryoprotectant). For the fracturing of cryofixed vesicles, a specific angle is adopted. The completed surface is then 45°-shadowed with vaporized platinum or carbon. The replica is strengthened by the carbon coating employed in this procedure. After being cleaned, the replica is inspected and studied using TEM [40]

• Optically guided microscopy: The observation of size and shape is another use of this method. For determining particle size, around 100 spanlastics are employed. This methodology involves the measurement of the stage micrometer and eyepiece micrometer to determine the size of the formulation. To determine the size distribution, mean surface diameter and mass distribution of spanlastics, laser-based master sizes are employed. Using the Malvern zeta sizer in a Dynamic Light Scattering (DLS) investigation, the size distribution, mean diameter and zeta potential are determined [41]

Study of In-Vitro Release

In this research, the dialysis membrane technique is often used. A little amount of spandex is stuffed into a dialysis bag and the ends are knotted. Dialysis bags are immersed in dissolved substances at 37 degrees Celsius and agitated with a magnetic stirrer. At regular intervals, a solution sample is removed from the beaker and replaced with new dissolving media. The samples were examined for drug concentration at a certain wavelength as described in the relevant drug's monograph [42,43].

Tissue Distribution/In Vivo Study

Using the appropriate animal models, the tissue distribution profile has been researched. Three groups of healthy albino rats weighing between 100 and 150 grams each were employed by Bhaskaran and Lakshmi to create a tissue distribution profile. Free spanlastics injections were administered to the first group as a control, whereas free drug injections were administered to the second group. The formulation was used to treat the third group. After the animals were sacrificed, several organs were extracted, including the heart, kidney, spleen, liver and lungs. The organs were homogenized and centrifuged after being cleaned with phosphate buffer (pH 7.4). A suitable approach was utilized to determine the drug content of the supernatant that was thus collected. Similar to that, this study by Jadon et al. employed male albino rats. After administering both free and drug-entrapped spanlastics, the amount of the drug in the plasma was measured. Three groups of five were formed from the total number of animals. The first group received a placebo injection of phosphate-buffered saline (pH 7.4), whereas the second and third groups received pure medicine and nanovesicles containing the treatment orally. Blood samples were obtained after predetermined time intervals, centrifuged and immediately frozen and then HPLC was used to analyze them [43-45].

Application of Spanlastics

Nano-vesicles, initially used in cosmetics, are now a popular medicine delivery method. Spandex is an effective method of drug delivery because it can entrap both hydrophilic (lipophilic) and hydrophobic (lipophobic) molecules. The nano-vesicle system has already been developed for numerous pharmaceuticals, including doxorubicin, vaccines, insulin and siRNA, among others. They are conveniently delivered in a number of different ways, including transdermally [46], ocular [47,48], orally and intravenously [49]. Some applications of this nano-vascular medication delivery technology include the following:

Chemical Medications

Nano-vesicles are frequently utilized as carriers for numerous chemical medications due to their most beneficial characteristics. These vesicles have a hydrophobic exterior and a hydrophilic interior, making it easy to transfer the correct molecules onto them. Because it is easy to put two distinct kinds of medications into these vesicles, they could be employed as a co-delivery system. From a formulation standpoint, these vesicles are biocompatible, low-toxic, biodegradable, stable, affordable and simple to store. For instance, Carvediol, a chemical medication, is frequently prescribed to treat arterial disease and congestive cardiac failure. However, first-pass metabolism and a brief half-life have a limit on systemic availability [49]. Nano-vesicles are the best delivery method for this therapy because they protect the medication from degradation, optimize the release profile and improve first-pass digestion. The formulation, which had the maximum encapsulation effectiveness 77.7% and vesicle sizes of approximately 167 nm, was made using the film hydration process. The use of several nano-vesicle variants has potential for cancer therapy owing to their reduced size, resulting in improved permeability and extended retention time inside tumor tissue [50].

Proteins and Peptides

Although peptides and proteins like bacitracin and insulin have significant therapeutic value, their low bioavailability and instability during administration and storage severely restrict their practical use. The use of the nano-vesicular system has shown its superiority as a viable option for mitigating this issue. Moreover, these formulations facilitate the process of vaccination delivery [51,52].

For instance, researched the pharmacokinetics of a nano-vesicular insulin formulation and tested it in diabetic rats after oral administration. The composition of the medication was tested in Simulated Intestinal Fluid (SIF) and Simulated Gastric Fluid (SGF). The results showed that the formulation increased bioavailability and protected against degradation [53,54].

9-deglycinamide 8-arginine vasopressin (DGAVP), which was studied [55], is another instance of a peptide or protein being successfully delivered orally. The vesicular formulation containing drug solution was tested in vitro and it showed improved bioavailability. A number of diseases can be effectively treated using vaccines, but issues with safety and efficacy constrain their use. So, one option to prevent this degradation is to create nano-vesicles using nonionic surfactants [56].

Gene Therapy

Despite its great power, the delivery issue with gene therapy limits its clinical applicability. The nano-vesicular approach, on the other hand, is now being investigated to change the formulations. One example is DNA encoding [56]. Other: This study aims to investigate various uses of sodium stibogluconate via a series of experiments. The efficacy of no-vesicles was shown to be superior to that of a sodium stibogluconate solution in combating the parasite in the liver, spleen and bone marrow [57,58].

Creating innovative Spanlastics surfactant-based vesicles allows for the targeted delivery of drugs without the need for continuous dosing. Pharmaceutical formulations are confronted with challenges related to insolubility, instability, limited bioavailability and rapid disintegration. There is a potential for spanlastics to represent a significant advancement in the field of nano vesicular drug delivery technology. These vesicular systems may be used to achieve targeted effects in lipophilic and hydrophilic medicines. At present, pharmaceutical substances are administered using this methodology to target the middle ear, nasal passageways, trans-ungual area, oral cavity and ocular region.

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